Chain Scissionable Resists Based on Poly(acetal) Motif for Extreme 

Ultraviolet Lithography 
 

 

 

 

 

 

 

 

A Dissertation 

Presented to the Faculty of the Graduate School 

of Cornell University 

In Partial Fulfillment of the Requirements for the Degree of 

Doctor of Philosophy 

 
 
 

 

 

  

 
 
 

  

 

By

Jingyuan Deng

December 2022



 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

© 2022 Jingyuan Deng 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 



i 

ABSTRACT 

Chain Scissionable Resists Based on Poly(acetal) for Extreme Ultraviolet Lithography 

Jingyuan Deng, Ph.D. 

Cornell University, 2022 

Scissionable polymers are polymers that will depolymerize under different stimuli 

including acid, base, and free radicals. These polymers have been investigated in the 

development of photoresists and other degradable materials. This work focuses on the 

poly(phthalaldehyde), PPA, family of scissionable polymers. The PPA backbone consists 

of acetal linkages that are very sensitive to acids. Upon exposure to acids, the polymer 

chain depolymerizes to its corresponding monomers. This depolymerization behavior 

makes PPAs excellent candidates as photoresist materials. Several new architectures are 

being explored. For example, PPAs with tethered photoacid generators (PAG)s, which 

release acid upon irradiation, depolymerize upon exposure followed by a post exposure 

bake step. The depolymerized monomers in exposed areas could be easily removed using 

appropriate organic solvents while the unexposed areas remain unchanged. Therefore, 

both unsubstituted and substituted PPAs may equally serve as a positive tone photoresist. 

This study focuses on the development of low exposure dose, sensitive PPA photoresists, 

which do not suffer from materials stochastic issues related to non-uniformities at 

nanoscale present in multi-component systems for extreme ultraviolet lithography 

(EUVL). In order to improve the lithographic performance of the PPA photoresists, the 

structure of the polymer backbone as well as PAGs are being investigated and tailored 



ii 

for EUVL. Aryl sulfonates, imidosulfonates, and iminosulfonates were prepared as non-

ionic PAGs for PPA photoresists. The steric and electronic nature of these sulfonate 

PAGs can be easily tuned to optimize acid generation efficiency and their compatibility 

with a polymer photoresist matrix. All synthesis, characterization, and lithographic 

evaluation will be presented. 

 

 

 

 

 

 

 

 

 

 

 

 

 



iii 

BIOGRAPHICAL SKETCH 

Jingyuan Deng was born in Zhengzhou, Henan, China in October 1993. He believes 

“justice without force is powerless; force without justice is tyrannical” and started 

practicing Kyokushi Karate at the age of 16. He attended Nagoya University in 2012 and 

graduated with a B.E. in chemistry. After graduating from Nagoya University, he moved 

to the University of Tokyo to study organometallic and polymer chemistry under the 

supervision of Kyoko Nozaki. Upon completing his M.E. from the University of Tokyo, 

Jingyuan moved to Cornell University to pursue a doctorate degree in 2018. At Cornell 

University, he developed photoresist materials for EUV lithography in collaboration with 

Intel Corporation. He finished his Ph.D. degree in 2022 and decided to further pursue his 

career in semiconductor industry.  

 

 

 

 

 

 

 

 



iv 

 

 

 

 

 

 

 

This dissertation is dedicated to my mother and father, who taught me the meaning of 

love, persistence, and life. 

 

 

 

 

 

 

 

 



v 

ACKNOWLEDGEMENTS 

First, I would like to express my heartfelt gratitude to my advisor, Prof. Christopher Ober 

for providing funding and space for me to conduct research in his group. I would also like 

to thank my committee members, Prof. Brett Fors and Prof. Frank Schroeder for their 

helpful discussions. Prof. Nozomi Ando encouraged me to face the challenges and 

difficulties in graduate school at the beginning of my Ph.D. study. Without her kind 

encouragement, it was impossible for me to complete a Ph.D. degree.  

I would like to thank my collaborators from Intel Corporation, Dr. Grant Kloster, Dr. 

Patrick Theofanis, and Dr. Marie Krysak. In particular, I would like to express my 

gratitude to Dr. James Blackwell for very helpful discussion and also helping with various 

aspects of my research. In addition, I greatly appreciate Prof. Denbeaux’s help from 

SUNY POLY institute on EUV exposures.  

Being a graduate student in chemistry conducting research in materials science at Cornell 

has been a unique experience to me. The best part of this experience was about meeting 

a variety of people. Dr. Atsushi Ozaki, a visiting scholar from Tokyo University of 

Agriculture and Technology helped me tremendously when I was a first year graduate 

student. In my second year, I met Yusuke Otsubo, a visiting scholar from JSR Corporation 

and learned interesting industrial knowledge in photoresist materials from him. In my 

third year, I met my coworker, Sean Bailey, who later became my best amigo and deeply 

influenced me as a researcher as well as a person.  

Other than people at Cornell, I have many more to express my appreciation. I thank Prof. 

Nozaki for encouraging me to pursue happiness, and Prof. Shintani for inspiring me to be 



vi 

    

    

         

 

Last but not least, I would like to thank my Kyokushin Karate shihan, Hitoshi Kiyama 

for teaching me how to cope with adversity in my life. I will never be flattered by power 

and I will never give into violence.  

      

           

       

   

 

 

 

 

 

 

 

 

a better chemist. I thank Dr. Falk Seidel for various discussions and support. Kimihiro 

Nakamura, Satoko Takaoka, Hina Yasuda, and Chihiro Kengoyama were my peers back 

at the University of Tokyo. The time we spent together makes me stronger and 

fearless.

My favorite philosopher, Arthur Schopenhauer wrote “Our happiness lies entirely outside 

us; for it exists only in the heads of others” in his essays, The Wisdom of Life. I do not 

believe in imaginary happiness. I will be patient and courageous throughout my life. 

Hopefully, at the end of my journey, I can make my life with less suffering.



vii 

TABLE OF CONTENTS 

 
CHAPTER 1: INTRODUCTION 

Abstract.............................................................................................................................1  

1.1 Introduction................................................................................................................2 

1.1.1 Chemical Amplification 

1.1.2 Photoacid Generators (PAGs) 

1.2 Self-Immolative Polymers.........................................................................................11  

1.2.1 Poly(phthalaldehyde)s  

1.2.2 Poly(olefin sulfone)s 

1.3 EUV Lithography......................................................................................................25 

1.3.1 Development of EUV Technology 

1.3.2 Resist Materials for EUV Lithography 

1.4 Summary...................................................................................................................34 

1.5 References.................................................................................................................35 

 

CHAPTER 2: REVIEW OF ESSENTIAL USE OF PHOTOACID GENERATORS IN 

LITHOGRAPHIC PATTERNING AND PROCESSING 

Abstract...........................................................................................................................39  

2.1 Introduction..............................................................................................................40  

2.2 Photoacid Generators...............................................................................................44  

2.3 Ionic Photoacid Generators......................................................................................49  

2.4 Nonionic Covalent Photoacid Generators................................................................52 

2.5 Polymer-Bound PAGs...............................................................................................55  



viii 

2.6 Alternatives to Fluorinated PAGs ...........................................................................57 

2.7 Conclusions..............................................................................................................63 

Acknowledgements.........................................................................................................64  

2.8 References.................................................................................................................65  

 

CHAPTER 3: TWO COMPONENT PHOTORESIST SYSTEM BASED ON 

POLY(ACETAL) MOTIF FOR EUV LITHOGRAPHY 

Abstract...........................................................................................................................68  

3.1 Introduction..............................................................................................................69  

3.2 Experimental............................................................................................................72  

3.2.1 General Procedures 

3.2.2 Syntheses and Characterizations of Monomers, Polymers, and PAGs  

3.2.3 TGA Analysis of Polymers 

3.2.4 SEM Characterization of Patterns for DUV Lithography  

3.2.5 MS Experiment on EUV Exposed Samples  

3.2.6 Outgassing Experiment  

3.2.7 Lithographic Performance: Linear vs Cyclic 

3.2.8 Supplemental EUV Contrast Curves 

3.3 Results and Discussion............................................................................................93  

3.3.1 Synthesis of Monomers, Polymers, and PAGs 

3.3.2 DUV Exposure 

3.3.3 EUV Exposure   

3.3.4 Mechanistic Study  

3.3.5 DFT Calculations 



ix 

3.4 Conclusions............................................................................................................116 

Acknowledgements.......................................................................................................117  

3.5 References...............................................................................................................117  

 

CHAPTER 4: END-CAP ENABLED SELF-IMMOLATIVE PHOTORESISTS BASED 

ON POLY(ACETAL) MOTIF FOR EUV LITHOGRAPHY 

Abstract........................................................................................................................129  

4.1 Introduction...........................................................................................................130  

4.2 Experimental..........................................................................................................131  

4.2.1 General Procedures  

4.2.2 Syntheses and Characterizations of Monomers, and Polymers 

4.2.3 TGA Analysis of Polymers 

4.3 Results and Discussion..........................................................................................138  

4.3.1 Photodegradation under UV Irradiation 

4.3.2 EUV Exposure 

4.4 Conclusions............................................................................................................146 

Acknowledgements.......................................................................................................146  

4.5 References..............................................................................................................147  

 

CHAPTER 5: SINGLE COMPONENT PHOTORESIST SYSTEM BASED ON 

POLY(ACETAL) MOTIF FOR EUV LITHOGRAPHY 

Abstract.........................................................................................................................153  

5.1 Introduction............................................................................................................154  

5.2 Experimental...........................................................................................................159  



x 

5.2.1 General Procedures 

5.2.2 Syntheses and Characterizations of Monomers, Polymers, and PAGs 

5.2.3 TGA Analysis of Polymers  

5.2.4 NMR Experiments on PAG 1  

5.2.5 MS Experiments on PAG 1 and PAG 2 

5.2.6 MS Experiments on EUV Exposed Samples 

5.2.7 Relative Stereochemistry Determination of Diacetal Products 

5.2.8 AFM Film Roughness Measurements 

5.2.9 NMR Experiments for Copolymer Ratio Determination 

5.2.10 Suzuki-Miyaura Reaction Condition Screening 

5.2.11 Gamma Calculation from EUV Contrast Curves 

5.2.12 Polymer Ceiling Temperature Calculations 

5.3 Results and Discussion............................................................................................210  

5.3.1 PAG Design  

5.3.2 Synthesis of PAG Tethered Monomers and Polymers 

5.3.3 EUV Exposure 

5.3.4 DFT Calculations 

5.4 Conclusions............................................................................................................230 

Acknowledgements.......................................................................................................231  

5.5 References..............................................................................................................232  

 

CHAPTER 6: CONCLUSIONS AND FUTURE WORK 

 



xi 

LIST OF FIGURES 

CHAPTER 1 

Figure 1. 1 Flowchart for patterning process.                                 3 

Figure 1. 2 Flowchart for patterning process showing different tools.               4 

Figure 1. 3 The concept of chemical amplification.                             8 

Figure 1. 4 Chemical structures of APEX and ESCAP.                          9 

Figure 1. 5 Linear and cyclic poly(phthalaldehyde)s.                           13 

Figure 1. 6 End-capped poly(phthalaldehyde)s.                               15 

Figure 1. 7 Pendent groups that have been successfully incorporated into the poly(olefin 

sulfone) backbone.                                                     17 

Figure 1. 8 Degradation Mechanism of Poly(olefin sulfone)s (a) Acid (b) Free-Radical 

(c) Base.                                                             19 

Figure 1. 9 Structures Poly(vinyl butyl carbonate sulfone)s.                      20 

Figure 1. 10 SEM image of polymer with [(o-nitrobenzyl)oxy]carbonyl group film after 

UV irradiation at 100 mJ/cm2.23                                           24 

Figure 1. 11 Scheme showing the trade-off among resolution, LER, and sensitivity.  27 

Figure 1. 12 Planar molecular glass resists (R is a protecting group).               30 

Figure 1. 13 Cyclic molecular glass resists (R is a protecting group).               31 

Figure 1. 14 Hybrid organic-inorganic nanoparticle resist for EUV lithography.      33 

 

CHAPTER 2 

Figure 2. 1 Representative ionic PAGs: 1, 2, 3, 4 Sulfonium PAG and 5, 6 Iodonium PAG. 

46 



xii 

Figure 2. 2 Representative nonionic covalent PAGs.                           47 

Figure 2. 3 Proposed photolysis mechanism for diaryliodonium salt under DUV exposure. 

50 

Figure 2. 4 Proposed photolysis mechanism for triarylsulfonium salt.             51 

Figure 2. 5 Photoacid generation mechanism for aryl sulfonate esters.             53 

Figure 2. 6 Photoacid generation mechanism for iminosulfonates and imidosulfonates. 54 

Figure 2. 7 Examples of polymer-bound PAGs. (a) Single CF2 unit next to sulfonate21 

and (b) single CF2 unit next to sulfonate in a structure that falls apart on exposure; groups 

(R1, and R2) not specified groups while R3 is a linking group.                   56 

Figure 2. 8 Chemical structure for non-PFAS covalent PAGs: (a) nitrobenzyl ester and 

(b) terarylene backbone-based PAG.                                       58 

Figure 2. 9 Chemical structures of untraditional ionic PAGs: (a) 2-

phenoxytetrafluoroethane sulfonate PAG and (b) pentacyanocyclopentadienide PAGs. 58 

Figure 2. 10 Chemical structures for natural products-based PAGs.                62 

 

CHAPTER 3 

Figure 3. 1 TGA measurement of cPPA polymers. 5% weight loss at 128 ˚C.        84 

Figure 3. 2 TGA measurement of Br-lPPA polymers. 5% weight loss at 178 ˚C.     85 

Figure 3. 3 1:1 Line space pattern with feature size 512 nm observed under SEM after 

exposure of 50 mJ/cm2 deep UV (248nm) exposure using PAG 3.                86 

Figure 3. 4 MS showing compound with molecular formula: C11H7NO + H+       87 

Figure 3. 5 MS showing compound with molecular formula: C8H5BrO2 + H+      88 

Figure 3. 6 MS showing compound with molecular formula: C13H6F3NO5S + H+. The 

last small peak without labeling is m/z=397.99487.                            89 



xiii 

Figure 3. 7 MS spectrum showing outgassing species of P1 under EUV radiation.    90 

Figure 3. 8 Mid-UV exposure: a) Patterns observed using Br-cPPA. b) Patterns observed 

using Br-lPPA.                                                       91 

Figure 3. 9 EUV contrast curves for PAG 3–9 shown in logarithmic scale.          92 

Figure 3. 10 Chemical structures of PPA derivatives.                           94 

Figure 3. 11 chemical structures of PAG 1 to 9.                               95 

Figure 3. 12 DUV contrast curves for PAG 1–4.                               99 

Figure 3. 13 EUV contrast curves for PAG 3–9.                              101 

Figure 3. 14 GPC chromatogram of polymer resist (blue plot), EUV sample (red plot), 

and Br-oPA monomer (black plot).                                        104 

Figure 3. 15 PAG degradation mechanism induced by EUV photons.             105 

Figure 3. 16 Proposed decomposition mechanism of PAG 9 under EUV radiation.   107 

Figure 3. 17 FOD plots at σ = 0.005 e Bohr–3 (FT-B97M-V/def2-TZVPP (Tel = 5000 K) 

level) for neutral and radical anions of PAG 3 to 8 (FOD in yellow).              110 

Figure 3. 18 Potential energy surface along N–O bond calculated at FT-TPSS-D4/def2-

SVP (Tel=5000K) level for PAG 4, 6, 7, and 8.                               114 

Figure 3. 19 Empirical relationship between VEA corrected activation energy and 

photospeed.                                                          115 

 

CHAPTER 4 

Figure 4. 1 TGA measurement of Br-lPPA-OTf polymers.                      136 

Figure 4. 2 TGA measurement of Naph-lPPA-OTf polymers.                   137 

Figure 4. 3 1H NMR spectra of 2 a) before UV irradiation b) after UV irradiation for 30 

minutes c) after UV irradiation for 30 minutes and heated at 110 ˚C for 5 minutes.  140 



xiv 

Figure 4. 4 1H NMR spectra of 3 a) before UV irradiation b) after UV irradiation for 30 

minutes c) after UV irradiation for 30 minutes and heated at 110 ˚C for 5 minutes. The 

embedded picture showed the change of color before and after the photodegradation 

experiment.                                                          141 

Figure 4. 5 EUV contrast curve for 1 to 3.                                  143 

 

CHAPTER 5 

Figure 5. 1 a) Linear PPA with PAG blending. b) PPA with photoactive end-cap. c) 

Functionalized cyclic PPA with PAG tethering.                              158 

Figure 5.2. DART-MS spectrum taken on PAG 1 48 h after isolation (positive mode).185 

Figure 5.3. DART-MS spectrum taken on PAG 1 48 h after isolation (negative mode).185 

Figure 5.4. DART-MS spectrum taken on PAG 2 (positive mode).               186 

Figure 5.5. DART-MS spectrum taken on photoresist formulation containing PAG 2 after 

EUV exposure (positive mode).                                          187 

Figure 5.6. DART-MS spectrum taken on photoresist formulation containing PAG 2 after 

EUV exposure (negative mode).                                          188 

Figure 5.7. DART-MS spectrum taken on photoresist formulation containing PAG 2 after 

EUV exposure (negative mode).                                          188 

Figure 5.8. Cis and trans conformation of 4-bromophthalaldehyde diacetal.        189  

Figure 5.9. Structures showing atom numbering for 4-boronic acid pinacol ester (Bpin) 

phthalaldehyde diacetal cis and trans isomers.                               190 

Figure 5.10. Full 2D NOESY spectrum of 4-Bpin phthalaldehyde diacetal containing both 

diastereomers in DMSO-d6.                                             191  



xv 

Figure 5.11. 2D NOESY spectrum of 4-Bpin phthalaldehyde diacetal containing both 

diastereomers in DMSO-d6 with display trimmed to areas of interest. Numbers in blue 

and black indicate assignments for the cis and trans isomers, respectively.         192 

Figure 5.12. Expansion of 2D NOESY of 4-Bpin phthalaldehyde diacetal containing both 

diastereomers in DMSO-d6 showing the methoxy to acetal NOE correlations. Lines and 

numbers in blue and black indicate assignments for the cis and trans isomers, respectively.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         

Figure 5.13. 2D 13C-coupled 1H/13C HSQC-NOESY spectrum of 4-bromophthalaldehyde 

diacetal containing both diastereomers in DMSO-d6. HSQC correlations are displayed 

with a single blue contour level and NOE correlations are contoured in red.         194 

Figure 5.14. Structures showing atom numbering for 4-bromophthalaldehyde diacetal 

used in this discussion.                                                 195 

Figure 5.15. Figure showing the magnetization transfer pathway for homonuclear 

NOESY and 1H/13C HSQC-NOESY experiments.                          196 

Figure 5.16. Expansion of the 2D, 13C-coupled 1H/13C HSQC-NOESY spectrum of 4-

bromophthalaldehyde diacetal containing both diastereomers in DMSO-d6 showing NOE 

correlations between methoxy groups of the cis isomer. Red and blue contours represent 

NOE and HSQC correlations, respectively.                                 199  

Figure 5.17. Chemical structures of formulation 1.                            199 

Figure 5.18. Chemical structure of formulation 2.                             199 

Figure 5.19. AFM film roughness measurements for formulation 1.               199 

Figure 5.20. AFM film roughness measurements for formulation 2.              200 

Figure 5.21. 1H NMR spectrum showing the monomer ratio in PA 3 after complete 

degradation in CDCl3.                                                 201 



xvi 

Figure 5.22. 1H NMR spectrum showing the monomer ratio in PA 4 after complete 

degradation in CDCl3.                                                 202  

Figure 5.23. 1H NMR spectrum showing the monomer ratio in PA 5 after complete 

degradation in CDCl3.                                                 203 

Figure 5.24. Chemical structures of ligands used in Suzuki-Miyaura reaction condition 

screening.                                                           205 

Figure 5.25. Gamma calculation for resists PA 4 and 5 from EUV contrast curves in 

logarithmic scale.                                                     206 

Figure 5.26. PAG tethered PPAs.                                         207 

Figure 5.27. ΔS calculation for the model reaction.                           208 

Figure 5.28. Tc calculation for PAS 1.                                     208  

Figure 5.29. Tc calculation for PAS 2.                                     208 

Figure 5.30. Tc calculation for PAS 3.                                     209  

Figure 5.31. Tc calculation for PAS 4.                                     209 

Figure 5.32. EUV contrast curve for PA 1 to PA 6.                           223 

Figure 5.33. Model Molecules SC 1 to 3 for DFT Calculations.                 226 

Figure 5.34. FOD plots at σ = 0.005 e Bohr–3 (FT-B97M-V/def2-TZVPP (Tel = 5000 K) 

level) for radical anions of SC 1 to 3 (FOD in yellow).                        227 

Figure 5.35. a) PAGs are tethered with the resist, the photogenerated acid after exposure 

is confined in vicinity of the reactive polymer backbone with smaller diffusion radius. b)  

PAGs are blended with the resist, the photogenerated acid after exposure diffuses with 

larger diffusion radius.                                                 229 

 

 



xvii 

LIST OF TABLES 

Table 3. 1 Table summarizing results of EA, photospeed, bond dissociation energy (BDE), 
and ΔG‡ for PAG 3 to 8. aFor PAG 3 and 5, it is the C–O BDE.                  109 
 
Table 4. 1 Calculated values of VEA and BDE for SEC 1 and 5.                 146 
 
Table 5.1. Suzuki-Miyaura reaction condition screening results.                 205 
 
Table 5.2. Results of Tc calculations.                                      210 
 
Table 5.3. Results of surface roughness measurements.                        226 
 
Table 5.4. Table summarizing results of VEA, BDE, and photospeed for SC 1–3 and PAGs 
1–4.                                                                228 
 

 

 

 

 

 

 

 

 

 

 

 



1 

CHAPTER 1 
 

INTRODUCTION 
 

Abstract  

The semiconductor industry continues to make progress on patterning technology in the 

nanometer dimension, enabling the fabrication of advanced electronic products. High 

resolution lithography is the key process for high volume manufacturing (HVM) of such 

semiconductor devices. Photoresist materials innovation must keep up with the patterning 

technology revolution. The semiconductor industry currently utilizes extreme ultraviolet 

(EUV) technology for HVM of sub–7 nm resolution processing and beyond. This chapter 

will discuss the process of lithography and the development of photoresist materials with 

emphasis on chain scissionable polymers and EUV lithography.  

 

 

 

 

 
 

 

 

 

 

 

 

 

 

 



2 

1.1 Introduction  

  Photolithography has been employed for creating patterns in nanometer resolution on 

thin films for fabrication of advanced structures and devices in the semiconductor industry. 

The advancements in semiconductor processing have improved the performance of 

semiconductor devices dramatically in the past decades. This dramatic improvement was 

made possible by reducing the minimum feature size on chips, with said miniaturization 

which is driven by the famous “Moore’s law”.1 According to “Moore’s law”, the circuit 

density (number of transistors in a dense integrated circuit) doubles approximately every 

two years. In order to meet the denser fabrication expectations set by “Moore’s law”, an 

urgent revolution is needed for both lithography tools and photoresist materials.  

  Lithography originated as a planographic printmaking process where a feature is drawn 

onto a flat stone through a chemical reaction. Modern lithography refers to a patterning 

process in which a feature from a photomask is transferred onto a silicon wafer. This 

patterning process utilizes photoresist materials, which react with radiation sources (such 

as UV light) to cause a solubility switch. The process begins with spin coating a 

photoresist solution onto a silicon wafer. The wafer is then soft baked to remove residual 

solvent from the film. During the exposure process, radiation is passed through a 

photomask, transferring the feature to the film. After exposure, the silicon wafer is baked 

at elevated temperature to accelerate the chemical transformation, causing the solubility 

in the exposed area on the film to switch. The coated silicon wafer is then immersed into 

a developer, dissolving the exposed area of a positive tone photoresist, or the unexposed 

portion of a negative tone photoresist. As the last step, the feature on the photomask can 

be successfully transferred onto the silicon wafer by a process called etching. 



3 

 

 

 

 

Figure 1. 1 Flowchart for patterning process.  
 

  

 

Prepared silicon wafer
Spin Coating
& Soft bake

Photoresist film

Exposure

Radiation

Mask

Post-exposure 
bake 

Remove mask

Development
Pattern



4 

 

 

 

 

 

 

Figure 1. 2 Flowchart for patterning process showing different tools. 
 

 

 

 

Step 1: Sample preparation: polymer, PAG in solvent  

Step 2: Spin Coating  

Step 3: Lithography  

Step 4: Spectroscopy &  
Characterization  

254nm ABM Contact Lithography

248nm ASML DUV Stepper

JEOL Ebeam Lithography Tool

AFM

Profilometer

Ellipsometer

SEM

1

2

3

4

EUV Exposure Tool



5 

The flowchart for patterning process is shown in (Figure 1. 1).  

  Different exposure wavelengths necessitate the employment of different tools. For 

example, deep-UV exposure may involve the usage of a 254 nm ABM contact aligner or 

a 248 nm ASML DUV stepper. For patterning characterization, atomic force microscopy 

(AFM) and scanning electron microscope (SEM) are popular tools to use as shown in 

(Figure 1. 2). 

  Several parameters must be accounted for when designing photoresist materials. The 

first parameter is the solubility of the photoresist material in a casting solvent. Casting 

solvents used in semiconductor industry usually have high boiling points and high 

viscosity, such as propylene glycol monomethyl ether acetate (PGMEA) and γ-

butyrolactone (GBL). Photoresist materials must be completely soluble in the casting 

solvent. The second relevant parameter is film formation, meaning that the photoresist 

materials need to be able to form an amorphous film and show good adhesion to the 

hydrophilic silicon wafer.2 Polymers are excellent candidates for photoresist materials as 

they are usually amorphous, and their polarity can be easily tailored though post-

polymerization functionalization reactions. Thirdly, to ensure the photons from radiation 

are not wasted, the photoresist materials must show high absorption at the wavelength of 

the applied radiation. Fourthly, photoresist materials must exhibit high glass transition 

temperatures (Tg) as a low Tg will limit the processing window since the pattern will 

collapse if the post-exposure bake temperature is higher than Tg. Lastly, the photoresist 

materials must show sufficient resistance towards the etching process. The relative 

etching resistance can be roughly estimated by the Ohnishi parameter.3  



6 

𝑉 ∝
𝑁

𝑁! − 𝑁"
 

In the above expression, V represents the etching rate, N represents the represents the total 

number of atoms in the molecule, Nc represents the total number of carbon atoms, and No 

represents the total number of oxygen atoms. According to the Ohnishi parameter, etching 

rate is inversely proportional to the effective carbon content in the photoresist materials. 

One of the most important implications of this fact is that the inclusion of ring structures 

in the photoresist material can significantly increase etching resistance by increasing the 

relative amount of carbon content.4 In addition to the five central parameters discussed, 

consideration must be given to storage and thermal stability, sensitivity, and cost of the 

material. We focus on sensitivity in the next section. 

 

 

 

 

 

 

 

 

 



7 

1.1.1 Chemical Amplification  

Sensitivity is one of the most critical parameters of a photoresist material as it is 

directly proportional to wafer throughput and chips’ manufacturing expenditure. In order 

to further enhance the sensitivity of photoresist materials, Ito, Wilson, and Fréchet 

proposed the concept of chemical amplification.5 Essentially, chemical amplification 

refers to an acid catalyzed deprotection reaction as shown in (Figure 1. 3). The photoresist 

material contains an acid-labile functional group, which is often the tert-

Butyloxycarbonyl (tBoc) protecting group shown in Figure 1. 3. It is mixed with a 

photoacid generator (PAG), which generates acid upon exposure. The generated acid 

causes tBoc protecting group to deprotect, which transforms the photoresist material from 

lipophilic to hydrophilic. This solubility switch enables the exposed area to be dissolved 

in the basic aqueous developer, tetramethylammonium hydroxide (TMAH), while 

keeping the unexposed area unaltered. As shown in figure Figure 1. 3, one proton 

catalyzes a cascade of deprotecting reactions, enabling one photon to trigger numerous 

chemical transformations. The rate of this catalysis reaction is further accelerated by the 

postexposure bake procedure at elevated temperature.  

  Even though chemically amplified resists (CARs) are based on the first developed CAR 

system poly(4-hydroxystyrene) (PHOST) as shown in Figure 1. 3,5 other improved 

version of PHOST exist, such as environmentally stable chemically amplified photoresist 

(ESCAP) and APEX shown in (Figure 1. 4).6-7  

  CAR systems have served as the cornerstone for advanced photolithography and are 

still evolving today. However, chemically amplified resists have become overly 

complicated, causing resist design to be painted into a corner. A new architecture for  



8 

 

 

 

 

Figure 1. 3 The concept of chemical amplification. 

 

 

 

 

n

O O

O

n

OH

Lipophilic
Hydrophilic

H

Acid Catalyzed Deprotection



9 

 

 

 

 

 

 

 

Figure 1. 4 Chemical structures of APEX and ESCAP. 

 

 

 

 

 

O
O

OH

APEX

n m

O O

O

n

OH

m

ESCAP

V



10 

photoresist materials is urgently needed. 

1.1.2 Photoacid Generators (PAGs) 

  Photoacid generators are compounds capable of generating acids upon exposure to 

radiation.8 These compounds were used for photocationic polymerizations for various 

types of monomers, including epoxys, vinyl ethers, etc.9 After Ito, Wilson, and Fréchet 

proposed the concept of chemical amplification in 1980s, wide varieties of PAGs have 

been designed specifically for photolithography as PAGs could be used to generate acid 

upon exposure to light, resulting in the removal of the protecting groups on the polymer 

resin to cause a solubility switch.  

  There are two groups of PAGs: ionic and covalent. The ionic PAGs are onium salts, 

with triarylsulfonium or diaryliodonium triflate salts most commonly used. These ionic 

PAGs are capable of generating strong acids with excellent quantum yield. Covalent 

PAGs are sulfonate esters or sulfonyl compounds, which are capable of generating 

sulfonic acids or sulfinic acids upon exposure to light. The detailed chemical structures 

for both ionic and covalent PAGs and photodegradation mechanism will be presented in 

chapter 2.  

 

 

 

 



11 

1.2 Self-Immolative Polymers. 

  Self-immolative polymers are polymers that depolymerize end-to-end under different 

stimuli including acid, base, and free radicals.10 They have been studied intensively in 

recent decades because of their unique depolymerization behavior. These stimuli-

responsive materials are critical to the development of transient electronics, 

photolithography, sensors, and drug delivery.11 There are two different ways of triggering 

depolymerization of self-immolative polymers. One is through end-cap cleavage, and the 

other is through backbone cleavage. The depolymerization process can be either 

reversible or irreversible. Reversible self-immolative polymers depolymerize back to the 

monomers that they were synthesized from. Therefore, the monomers can be 

repolymerized to generate the polymers. Irreversible self-immolative polymers 

depolymerize to form molecules that are different from the monomers that they were 

synthesized from. In this section, I will introduce two different self-immolative polymers, 

poly(phthalaldehyde)s and poly(olefin sulfone)s, two reversible self-immolative 

polymers. Reversible self-immolative polymers possess low ceiling temperatures (Tc), 

which is defined by the equation below: 

𝑇! =
Δ𝐻
Δ𝑆

 

Δ𝐻	is the change of enthalpy of the polymerization reaction, and Δ𝑆 is the change of 

entropy of the polymerization reaction. Self-immolative polymers with high molecular 

weight can only be synthesized below the ceiling temperature. These polymers are usually 

kinetically trapped by either end-capping or cyclization below the ceiling temperature to 

prevent depolymerization from happening at temperature higher than the ceiling 



12 

temperature. However, when the end-cap is removed or the backbone is cleaved, the 

polymer will depolymerize. The self-immolative polymer I have studied in my Ph.D. 

career, poly(phthalaldehyde) (PPA), has been previously used in high resolution 

photolithography as, for example, mask making.12 The PPA backbone consists of acetal 

linkages that are very sensitive to acids. Upon exposure to acids, the polymer chain 

depolymerizes to its corresponding monomers. 

1.2.1 Poly(phthalaldehyde)s 

  Poly(phthalaldehyde)s are synthesized through polymerization reactions from the 

dialdehyde monomer, o-phthalaldehyde. There are two topologies reported from literature 

on poly(phthalaldehyde)s, cyclic and linear (Figure 1. 5).12 Cyclic poly(phthalaldehyde)s 

are synthesized through cationic polymerizations using Lewis acid catalyst including 

BF3·OEt2, TiCl4, and SnCl4.13 Linear poly(phthalaldehyde)s are synthesized through 

anionic polymerizations with end-caps using anionic initiators such as t-BuOLi, sodium 

with naphthalene or benzophenone, etc.14 Poly(phthalaldehyde)s found applications in 

photolithography in 1980s by Ito, Wilson and coworkers for dry developing resist 

materials.5 Poly(phthalaldehyde)s plus PAG formulations showed high sensitivity under 

exposure of UV light. However, the project tumbled at IBM as the depolymerization 

product, which is the monomer o-phthalaldehyde, is too volatile and contaminates 

expensive optics inside of the exposure tools. Based on this unique feature of 

poly(phthalaldehyde)s, Moore and coworkers developed transient electronics for free-

standing transistor arrays.11 Depolymerization of poly(phthalaldehyde)s could also be 

triggered mechanically.15 Moore and coworkers discovered that depolymerization of 

poly(phthalaldehyde)s with molecular weight above 30 kg/mol could be triggered by  



13 

 

 

 

 

 

 

Figure 1. 5 Linear and cyclic poly(phthalaldehyde)s. 

 

 

 

 

 

OO

O

OO O

R

R

R

n
OO O R2R1

n

R

Linear Cyclic



14 

mechanical forces applied using pulsed ultrasound.  

Depolymerization of linear poly(phthalaldehyde)s can also be triggered by removing 

the end-caps. Phillips and coworkers synthesized poly(phthalaldehyde)s with different 

end-caps that react with specific chemical stimuli (Figure 1. 6).16 Tert-butyldimethylsilyl 

end-capped poly(phthalaldehyde)s react with fluoride ions, and allyl carbonate end-

capped poly(phthalaldehyde)s react with Pd(0) species.  

A substantial amount of research and significant advancements have been achieved on 

unsubstituted poly(phthalaldehyde)s. However, functionalized poly(phthalaldehyde)s are 

rarely reported due to synthetic challenges.17 I developed a new synthetic methodology 

for rapid and efficient synthesis of functionalized poly(phthalaldehyde)s. Details will be 

presented in chapter 5.  

 

 

 

 

 

 

 

 



15 

 

 

 

 

 

 

Figure 1. 6 End-capped poly(phthalaldehyde)s. 

 

 

 

 

 

 

 

OO O R
n

O

O

OO O R
nSi

Allyl carbonate

Tert-butyldimethylsilyl



16 

1.2.2 Poly(olefin sulfone)s 

Poly(olefin sulfone) is known as one of the self-immolative polymers that could 

depolymerize under different stimuli. Once the stimulus is applied, the cleavage at the 

relatively weak C–S bond on the polymer backbone will initiate the depolymerization 

process, which releases gaseous SO2 and the corresponding vinyl monomer. This 

depolymerization process is reversible. If the depolymerized monomers can be collected, 

it is possible to polymerize them again, which gives the possibility for sustainable 

chemistry. 

  Poly(olefin sulfone) is synthesized from SO2 and vinyl monomers through free radical 

polymerization using initiators such as tert-butyl hydroperoxide at –78 ˚C with excellent 

functional group tolerance as shown in (Scheme 1. 1). 

 

Scheme 1. 1 Copolymerization of olefins with SO2. 

Other than tert-butyl hydroperoxide, benzoyl peroxide and diethyl ether peroxides could 

also be used as polymerization initiators. General olefin monomers such as 1-butene, 1-

pentene, 1-hexene, cyclopentene, and cyclohexene could be used as the vinyl monomers. 

Poly(olefin sulfone) synthesized by a free radial pathway is an alternating copolymer, 

consisting of the vinyl monomer and sulfur dioxide in 1 to 1 ratio.18 In the past decades, 

a wide variety of vinyl monomers has been successfully incorporated into the polymer 

backbone and some of the pendent group examples are shown in (Figure 1. 7).  

R
–78 °C

SO2

tBuOOH S

O

O
R

n



17 

 

 

 

 

 

 

Figure 1. 7 Pendent groups that have been successfully incorporated into the poly(olefin 
sulfone) backbone. 

 

 

 

 

 

 

O

O
O

O

O

O

O

O

Cl

O O

O

O O

O

N
H

O

O
O

O

Pendent Groups (R)

a b c d e

h

f

i j kg

N3



18 

There are 3 different pathways which will lead to polymer degradation: acid, free radical, 

and base. Degradation mechanisms are shown in (Figure 1. 8). Among these degradation 

mechanisms, degradation under basic conditions has been studied extensively because of 

the controllability of this process. The most acidic protons on the polymer backbone are 

the ones on the carbon atoms adjacent to SO2 groups, which is a very strong electron-

withdrawing group. Under basic conditions, one of these protons will react with base and 

the whole polymer chain is depolymerized back to the vinyl monomer and SO2.19   

  Poly(olefin sulfone)s with tunable thermal stability has been reported by Jeffrey Moore 

and coworkers in 2015. A series of poly(vinyl butyl carbonate sulfone)s has been 

synthesized with tert-, sec-, iso-, and n-butyl groups on the carbonate moiety (Figure 1. 

9) and their thermal stability was evaluated by thermogravimetric analysis (TGA).20 

According to the literature, the thermal degradation of poly(alkyl olefin sulfone)s goes 

through a concerted β-elimination mechanism with a 5-membered ring transition state.21 

The stability of the polymer backbone is significantly affected by sterically bulky tert-

butyl carbonate groups. As expected, the less bulky the side group is, the more stable the 

polymer backbone will be. According to TGA analysis, P1 has the lowest degradation 

temperature, 91˚C. P2, which has a sec-butyl groups on the carbonate moiety showed 

higher degradation temperature, 167 ˚C. P3 and P4 with iso-, and n-butyl groups on the 

carbonate moiety showed the highest degradation temperature, which is 213 ˚C. From the 

observed trend, less substituted β-carbon on the carbonate moiety increased the thermal 

stability of the resulting polymer.  

Researchers also investigated the possibility of using poly(olefin sulfone)s as 

photoresist materials. A poly(olefin sulfone) (POS) tethered or mixed with a photobase  



19 

 

 

 

 

 

 

Figure 1. 8 Degradation Mechanism of Poly(olefin sulfone)s (a) Acid (b) Free-Radical 
(c) Base. 

 

 

 

 

 

S

R

O

O

H

S

R

O

OH

H

H
R

+
H– S

O

OH
S
O

O

+
R

S

R

O O

S

R

O O

S
O O

S

R

O OR

+
R

+ S
O

O

S
O O

H

R

S
O O

S

R

O O

Nu

S
O O R

S
O

S

R

O O

O
+

R
+ S

O

O

a)

b)

c)



20 

 

 

 

 

 

 

Figure 1. 9 Structures Poly(vinyl butyl carbonate sulfone)s. 

 

 

 

 

 

O O

O

S
O O

n

O O

O

S
O O

n

O O

O

S
O O

n

O O

O

S
O O

n

P1 P2

P3 P4



21 

generator (PBG), which releases base upon irradiation, depolymerizes upon irradiation 

followed by post exposure bake as shown in (Scheme 1. 2).22  

 

Scheme 1. 2 Depolymerization of POS with PBG. 
 

Sasaki and coworkers synthesized poly(olefin sulfone) with photobase generators, 

including oxime urethane group and [(o-nitrobenzyl)oxy]carbonyl group incorporated 

into the main polymer backbone.23 These photobase generators could release primary- or 

secondary-amino groups through decarboxylation process as shown in (Scheme 1. 3). 

 

Scheme 1. 3 Structures of poly(olefin sulfone)s with pendant photobase generators and 
the photochemical reactions of the pendant group. 

 

In order to evaluate these materials’ lithographic performance, DMSO was used as the 

solvent to dissolve the polymer materials, and the film was made by spin-coating the 

solution onto a glass plate. The coated glass plates were then irradiated with UV light. 

With the exposure dosage of 1 J/cm2, about 7% of the oxime urethane groups transformed 

S

O

O n

PBG
Irradiation

S

O

O n

Base
Heat

S
O

O

+
R

S

CH2

O O

HC
H2
C

H
N C O N

O CH3
Irradiation

S

CH2

O O

HC
H2
C NH2

+ CO2
N

CH3
N

H3C

+

S

CH2

O O

HC
H2
C

H
N C O

O

N C O
H2
C

O
O2N

Irradiation

S

CH2

O O

HC
H2
C

H
N C O

O

NH + CO2 + HC

O2N
O



22 

to primary amino groups through photolysis. With the same exposure dosage, about 16% 

of the [(o-nitrobenzyl)oxy]carbonyl groups transformed to secondary amino groups 

through an internal redox reaction. Therefore, the [(o-nitrobenzyl)oxy]carbonyl group 

exhibited the highest photosensitivity. After exposure with UV light, post exposure bakes 

at 130 °C for 60 s were employed to further accelerate the depolymerization process. 

Finally, the glass plates were washed with 0.012 M HCl for 30 s to wash away the 

depolymerized monomers and the residual amines. The SEM image of a 40 μm line and 

space pattern is shown in (Figure 1. 10).  

In 2011, Whittaker and coworkers prepared a series of polymers that constitute 

poly(olefin sulfone) backbone and poly(methyl methacrylate) (PMMA) side chains for 

extreme ultraviolet lithography. The poly(olefin sulfone) backbone depolymerizes rapidly 

upon exposure to EUV photons, and PMMA side chains were designed to increase the 

overall stability of the polymer structure.24 Atom transfer radical polymerization (ATRP) 

was utilized to synthesize PMMA macromonomers with olefin group chain end. 

Polymerizing the PMMA macromonomers with sulfur dioxide and pentene to form the 

poly(olefin sulfone) backbone generates the final material as shown in (Scheme 1. 4). 

Synthesized polymers were exposed to EUV photons to create line and space pattern. The 

mixture of isopropyl alcohol (IPA) and methylisobutylketone (MIBK) with a ratio of 

90/10 was used as the developer to remove depolymerized monomers. AFM was used to 

characterize the resulting patterns. From AFM images, 30 nm line and space patterns were 

created. However, bridging and pattern collapse could be seen.  

 

 



23 

 

 

 

 

 

Scheme 1. 4 Synthesis of allyl-terminated PMMA macromers and poly(1-pentene-co-
PMMA sulfone)s with a comb architecture. 

 

 

 

 

O
Br

O
+

O

O Cu(I)Br

Toluene
90 °C

O

O

Br

OO
=

–78 °C

SO2

tBuOOH

1-pentene

S
O

O
S

S

O

O
O

O



24 

 

 

 

 

 

Figure 1. 10 SEM image of polymer with [(o-nitrobenzyl)oxy]carbonyl group film after 
UV irradiation at 100 mJ/cm2.23 

 

 

 

 

 

 



25 

1.3 EUV Lithography 

Extreme ultraviolet (EUV) lithography, which employs a 13.5 nm wavelength light 

source, is used to create patterns with a resolution below 10 nm.  

According to “Moore’s law”, the circuit density (number of transistors in a dense 

integrated circuit) doubles approximately every two years. In order to meet denser 

fabrication expectations set by “Moore’s law”, an urgent revolution is needed for both 

lithography tools and photoresist materials. For optical lithography, the feature size 

(resolution) is dictated by Rayleigh’s equation: 

𝑅 = 𝑘
𝜆
𝑁𝐴 

where R is the resolution limit of the feature. The Rayleigh coefficient k is governed by 

the processing conditions, and λ represetns the optical wavelength. NA is the numerical 

aperture, which is determined by the imaging system including the optical lens and media.  

 

1.3.1 Development of EUV Technology 

In order to print smaller features, according to Rayleigh’s equation, the optical 

wavelength must be shorter, numerical aperture must be higher, or Rayleigh coefficient 

k1 must be smaller. In the semiconductor industry, the optical wavelength for 

photolithography has shortened from 436 nm (visible g-line), 365 nm (ultraviolet i-line), 

248 nm (KrF laser) and 193 nm (ArF laser). The numerical aperture can be increased by 

changing the media with higher refractive index. This has led to the advent of 193 nm 



26 

immersion lithography. The Rayleigh coefficient k could be deceased via multiple 

patterning technology. By combining multiple cycles of exposures and etching, feature 

density can be enhanced even without shorter optical wavelength exposure source or a 

medium with higher refractive index. With multiple patterning technology, it is possible 

to achieve features with 10 nm size. However, for feature sizes smaller than 10 nm, 

multiple patterning technology becomes too costly and complicated in processing. These 

constraints have motivated the semiconductor industry to develop EUV technology for 

sub 10 nm resolution lithography.  

Currently, the leading semiconductor corporations have accomplished the fabrication 

of 7 nm and 5 nm nodes utilizing EUV technology and are working on the fabrication of 

3 nm and 2 nm nodes.  

 

1.3.2 Resist Materials for EUV Lithography 

The performance of a photoresist material is evaluated by several parameters including 

line edge roughness (LER), resolution, sensitivity (photospeed), outgassing, photo-

absorption, and etching resistance. In the era of chemically amplified resists (CARs), the 

trade-off among resolution, LER, and sensitivity (RLS trade-off) (the so called “triangle 

of death”) is infamous (shown in Figure 1. 11).25 The RLS trade-off prevents the 

optimization of resolution, sensitivity, and LER at the same time. A photoresist material 

that has high sensitivity and resolution must subsequently show large LER values. The 

simplified notation for RLS trade-off, known as the Z-parameter,26 is shown in the 

equation below: 



27 

 

 

 

 

 

Figure 1. 11 Scheme showing the trade-off among resolution, LER, and sensitivity. 

 

 

 

Resolution 

LWR

Sensitivity
LER due to photon shot noise

The most difficult technical 
requirement for EUV resist is 
simultaneous improvement in 
resolution, LWR, and sensitivity (RLS).

(1) RLS trade off problem

There are two important problems in EUV resists

(２) Photon shot noise problem 

E UVL Workshop 2017



28 

𝑍 = 𝑅# × 𝐿$ × 𝐷 

R stands for resolution, L is the LER, and D is the minimum dosage required to completely 

remove the resist film from the silicon wafer. The smaller the Z value, the better the 

overall performance of the resist material is.  

  Poly(4-hydroxystyrene) and its derivatives were commonly used as CAR type 

photoresist materials for deep UV lithography.27 In the past decades, they have been 

tailored to EUV technology. The semiconductor industry has realized 12 nm patterning 

with exposure dosage of 30 mJ/cm2 using CAR based photoresist materials. However, 

considering low EUV photon absorption of carbon and oxygen atoms, continuing use of 

CAR based photoresist materials for sub 10 nm pattering utilizing EUV technology is not 

ideal as the photon density from EUV radiation is much lower compared to that in deep 

UV radiation with the same dosage. In order to circumvent this obstacle, researchers have 

synthesized fluorinated derivatives of poly(4-hydroxystyrene) polymers and they showed 

more than doubled EUV absorption coefficient.28 Another issue associated with CAR 

based photoresist materials is the control of stochastics. The PAG molecules are usually 

blended with polymer matrix in CARs, which results in an inhomogeneous distribution 

of PAGs and polymers. In order to minimize this inhomogeneity, researchers synthesized 

polymers with PAG molecules covalently bound to the polymer backbone.27 The PAG 

tethered CARs showed better LER, resolution and sensitivity compared to the PAG 

blended CARs.  

  As the feature size becomes smaller and smaller, LER becomes a significant challenge. 

Photoresist materials based on polymer materials are polydisperse having uneven 

distribution of molecular weights, which exacerbates the stochastics issues.29 Photoresist 



29 

materials based on molecular glasses have garnered increasing attention as it consists of 

monodisperse small molecules that form a glassy amorphous state at room temperature. 

Molecular glass resists have high glass transition temperature, which is beneficial for thin 

film formation. The monodisperse building blocks of molecular glass resists are 

beneficial for patterning with higher resolution and lower LER values. High thermal 

stability of molecular glass resists allows higher processing temperature for PAB and PEB.  

  Ober and coworkers proposed the design guidelines for designing molecular glass 

resists.30 They can be categorized into planar and branched molecular glass based on 

phenolic and phenylbenzene derivatives (Figure 1. 12),31 and cyclic molecular glass based 

on calix[n]arene and calix[4]resorcinarene derivatives (Figure 1. 13).32 Ober and 

coworkers have studied the structure-property relationship on poly(4-hydroxystyrene) 

derived molecular glass resists and discovered that the glass transition temperature of 

these resists increases with increasing size of the molecules.33 Henderson and coworkers 

have synthesized epoxide functionalized molecular glass resists for e-beam lithography.34 

Line space patterning with 35 nm half-pitch resolution and exposure dosage 20 μC/cm2 

was achieved. Molecular glass resists derived from calix[n]arene core has also been 

intensively studied. Ober and coworkers synthesized t-BOC protected C-4-

hydroxyphenyl-calix[4]resorcinarenes derivatives as positive tone molecular glass resists 

for EUV lithography. Line space patterning with 30 nm half-pitch resolution, exposure 

dosage 22 mJ/cm2 was well resolved.32  

In recent years, photoresist materials based on hybrid organic-inorganic architecture 

have attracted much attention from both the academia and the semiconductor industry. In 

2010, Ober and coworkers developed organic ligand stabilized HfO2 nanoparticles that  



30 

 

 

 

Figure 1. 12 Planar molecular glass resists (R is a protecting group). 
 
 
 
 
 

OR

RO OR

OR

RO OR

RO OR

RO

OR OR

OR

RO

RO

OR

OR

OR

RO OR

OR

OR

RO

OR
OR

OR

OR
RO

RO



31 

 
 

 

Figure 1. 13 Cyclic molecular glass resists (R is a protecting group). 
 

 

RO OR

RO

RO

RO OR

OR

OR

RO OR

RO

RO

RO OR

OR

OR

RO OR

RO

RO

RO OR

OR

OR

RO OR

ORRO

RO OR

RO

RO

RO OR

OR

OR

RO OR

RO

RO

RO OR

OR

OR

O O

OO

RO OR

RO

RO

RO OR

OR

OR



32 

are readily miscible with polymer photoresist materials for 193 nm lithography.35 These 

high refractive index and high transparency HfO2 nanocomposites showed higher 

resistance to plasma etching processes and were able to resolve 200 nm line/space 

patterns under e-beam radiation. In later research, Ober and coworkers have found that 

by coordinating photo-switchable ligands directly to the metal oxide core renders the 

metal complex a photoresist material (Figure 1. 14).36-37 Methacrylic acid was used as the 

photo-switchable ligand that coordinates to a hafnium/zirconium oxide core. When mixed 

with a photoradical initiator or PAG, this metal oxide nanoparticle could be used as a 

dual-tone photoresist materials for EUV lithography. Zirconium oxide based resist 

showed higher sensitivity under EUV exposure. Line space patterning with 26 nm half-

pitch resolution and exposure dosage 4.2 mJ/cm2 was achieved.38 The mechanism for 

triggering the solubility switch of these metal oxide based photoresists is not fully 

understood. Li, Giannelis and coworkers observed that there was a significant change in 

nanoparticle sizes before and after exposure due to ligand exchange, which might 

contribute to the solubility difference.39 As these metal oxide based photoresists show 

high EUV absorption as well as high etching resistance, researchers developed other 

organometallic photoresists based on different metals including tin, titanium, antimony, 

bismuth, and palladium.40-41 

 

 

 

 



33 

 

 

 

 

 

 

 

Figure 1. 14 Hybrid organic-inorganic nanoparticle resist for EUV lithography.  

 

 

 

 

 



34 

1.4 Summary 

The semiconductor industry continues to make progress on patterning technology in 

nanometer dimension, which enables the fabrication of advanced electronic products. 

EUV technology has become the most reliable choice for high volume manufacturing for 

sub-10 nm node. In order to take full advantage of EUV technology, novel photoresist 

materials must be developed. For CAR based photoresist materials, as PAG concentration 

is limited by the RLS-trade off, it is challenging to design resist materials with high 

resolution and sensitivity but low LER. Molecular glass based resists, based on acid 

catalyzed deprotection mechanism, possess low sensitivity in general. Metal oxide based 

resists show great resolution, sensitivity, LER, and high etching resistance. However, the 

mechanism for the solubility switch is not fully understood, which may hinder the 

development of these resists.  

Chain scissionable photoresist have garnered increasing attention in recent years. As 

discussed in 1.2.2, researchers studied the thermal stability of poly(olefin sulfone)s and 

attempted to utilize them as photoresist materials for EUV lithography. However, these 

materials suffer from narrow substrate scope, dark loss, severe outgassing, and low 

sensitivity issues. Eventually, they were abandoned as photoresist materials. As discussed 

in 1.2.1, Ito, Wilson and coworkers attempted to use poly(phthalaldehyde)s as dry 

developing resist materials for photolithography in 1980s.5 However, the project failed 

as the depolymerization product, o-phthalaldehyde was too volatile and contaminated 

expensive optics inside of the exposure tools. These issues could be solved by 

synthesizing phthalaldehyde derivatives with higher thermal stability, which will be 

presented in chapter 3. 



35 

1.5 References 

1. Bondyopadhyay, P. K., Moore's law governs the silicon revolution. Proceedings 
of the Ieee 1998, 86 (1), 78-81. 

2. Kim, J.-B.; Yun, H.-J.; Kwon, Y.-G.; Lee, B.-W., Adhesion-promoted 
copolymers for 193-nm photoresists without cross-linking during lithographic process. 
Journal of Photopolymer Science and Technology 2000, 13 (4), 629-634. 

3. Gokan, H.; Esho, S.; Ohnishi, Y., Dry Etch Resistance of Organic Materials. 
Journal of the Electrochemical Society 1983, 130 (1), 143-146. 

4. Kunz, R.; Palmateer, S.; Forte, A.; Allen, R.; Wallraff, G.; Di Pietro, R.; Hofer, 
D., Limits to etch resistance for 193-nm single-layer resists. SPIE: 1996; Vol. 2724. 

5. Ito, H.; Willson, C. G., Chemical amplification in the design of dry developing 
resist materials. Polymer Engineering & Science 1983, 23 (18), 1012-1018. 

6. Breyta, G.; Hofer, D. C.; Ito, H.; Seeger, D.; Petrillo, K.; Moritz, H.; Fischer, T., 
The Lithographic Performance and Contamination Resistance of a New Family of 
Chemically Amplified DUV Photoresists. Journal of Photopolymer Science and 
Technology 1994, 7 (3), 449-460. 

7. Poppe, J.; Neureuther, A., Improved performance of Apex-E photoresist with the 
application of the electric field enhanced PEB. SPIE: 2004; Vol. 5376. 

8. Martin, C. J.; Rapenne, G.; Nakashima, T.; Kawai, T., Recent progress in 
development of photoacid generators. Journal of Photochemistry and Photobiology C: 
Photochemistry Reviews 2018, 34, 41-51. 

9. Decker, C.; Moussa, K., Kinetic study of the cationic photopolymerization of 
epoxy monomers. Journal of Polymer Science Part A: Polymer Chemistry 1990, 28 (12), 
3429-3443. 

10. Yardley, R. E.; Kenaree, A. R.; Gillies, E. R., Triggering Depolymerization: 
Progress and Opportunities for Self-Immolative Polymers. Macromolecules 2019, 52 (17), 
6342-6360. 

11. Hernandez, H. L.; Kang, S.-K.; Lee, O. P.; Hwang, S.-W.; Kaitz, J. A.; Inci, B.; 
Park, C. W.; Chung, S.; Sottos, N. R.; Moore, J. S.; Rogers, J. A.; White, S. R., Triggered 
Transience of Metastable Poly(phthalaldehyde) for Transient Electronics. Advanced 



36 

Materials 2014, 26 (45), 7637-7642. 

12. Wang, F.; Diesendruck, C. E., Polyphthalaldehyde: Synthesis, Derivatives, and 
Applications. Macromol Rapid Commun 2018, 39 (2). 

13. Aso, C.; Tagami, S., Cyclopolymerization of o-phthalaldehyde. Journal of 
Polymer Science Part B: Polymer Letters 1967, 5 (3), 217-220. 

14. Aso, C.; Tagami, S., Polymerization of Aromatic Aldehydes. III. The 
Cyclopolymerization of Phthaldehyde and the Structure of the Polymer. Macromolecules 
1969, 2 (4), 414-419. 

15. Diesendruck, C. E.; Peterson, G. I.; Kulik, H. J.; Kaitz, J. A.; Mar, B. D.; May, 
P. A.; White, S. R.; Martínez, T. J.; Boydston, A. J.; Moore, J. S., Mechanically triggered 
heterolytic unzipping of a low-ceiling-temperature polymer. Nature Chemistry 2014, 6 
(7), 623-628. 

16. Seo, W.; Phillips, S. T., Patterned Plastics That Change Physical Structure in 
Response to Applied Chemical Signals. Journal of the American Chemical Society 2010, 
132 (27), 9234-9235. 

17. Lutz, J. P.; Davydovich, O.; Hannigan, M. D.; Moore, J. S.; Zimmerman, P. M.; 
McNeil, A. J., Functionalized and Degradable Polyphthalaldehyde Derivatives. Journal 
of the American Chemical Society 2019, 141 (37), 14544-14548. 

18. Cais, R. E.; O'Donnell, J. H.; Bovey, F. A., Copolymerization of Styrene with 
Sulfur Dioxide. Determination of the Monomer Sequence Distribution by Carbon-13 
NMR. Macromolecules 1977, 10 (2), 254-260. 

19. Shinoda, T.; Nishiwaki, T.; Inoue, H., Decomposition of poly(4-hydroxystyrene 
sulfone) in alkaline aqueous solutions. Journal of Polymer Science Part A: Polymer 
Chemistry 2000, 38 (15), 2760-2766. 

20. Lee, O. P.; Lopez Hernandez, H.; Moore, J. S., Tunable Thermal Degradation of 
Poly(vinyl butyl carbonate sulfone)s via Side-Chain Branching. ACS Macro Letters 2015, 
4 (7), 665-668. 

21. Cubbage, J. W.; Vos, B. W.; Jenks, W. S., Ei Elimination:  An Unprecedented 
Facet of Sulfone Chemistry. Journal of the American Chemical Society 2000, 122 (20), 
4968-4971. 

22. Suyama, K.; Shirai, M., Photobase generators: Recent progress and application 



37 

trend in polymer systems. Progress in Polymer Science 2009, 34 (2), 194-209. 

23. Yaguchi, H.; Sasaki, T., Photoinduced Depolymerization of Poly(olefin sulfone)s 
Possessing Photobase Generating Groups in the Side Chain. Macromolecules 2007, 40 
(26), 9332-9338. 

24. Lawrie, K. J.; Blakey, I.; Blinco, J. P.; Cheng, H. H.; Gronheid, R.; Jack, K. S.; 
Pollentier, I.; Leeson, M. J.; Younkin, T. R.; Whittaker, A. K., Chain scission resists for 
extreme ultraviolet lithography based on high performance polysulfone-containing 
polymers. Journal of Materials Chemistry 2011, 21 (15). 

25. Mojarad, N.; Gobrecht, J.; Ekinci, Y., Beyond EUV lithography: a comparative 
study of efficient photoresists' performance. Scientific Reports 2015, 5 (1), 9235. 

26. Higgins, C. D.; Szmanda, C. R.; Antohe, A.; Denbeaux, G.; Georger, J.; Brainard, 
R. L., Resolution, Line-Edge Roughness, Sensitivity Tradeoff, and Quantum Yield of 
High Photo Acid Generator Resists for Extreme Ultraviolet Lithography. Japanese 
Journal of Applied Physics 2011, 50, 036504. 

27. Itani, T.; Kozawa, T., Resist Materials and Processes for Extreme Ultraviolet 
Lithography. Japanese Journal of Applied Physics 2013, 52 (1R), 010002. 

28. Yamamoto, H.; Kozawa, T.; Tagawa, S.; Yukawa, H.; Sato, M.; Onodera, J., 
Enhancement of Acid Production in Chemically Amplified Resist for Extreme Ultraviolet 
Lithography. Applied Physics Express 2008, 1, 047001. 

29. Patsis, G. P.; Gogolides, E., Effects of model polymer chain architectures and 
molecular weight of conventional and chemically amplified photoresists on line-edge 
roughness. Stochastic simulations. Microelectronic Engineering 2006, 83 (4), 1078-1081. 

30. De Silva, A.; Felix, N. M.; Ober, C. K., Molecular Glass Resists as High-
Resolution Patterning Materials. Advanced Materials 2008, 20 (17), 3355-3361. 

31. De Silva, A.; Ober, C. K., Hydroxyphenylbenzene derivatives as glass forming 
molecules for high resolution photoresists. Journal of Materials Chemistry 2008, 18 (16), 
1903-1910. 

32. Chang, S. W.; Ayothi, R.; Bratton, D.; Yang, D.; Felix, N.; Cao, H. B.; Deng, H.; 
Ober, C. K., Sub-50 nm feature sizes using positive tone molecular glass resists for EUV 
lithography. Journal of Materials Chemistry 2006, 16 (15), 1470-1474. 

33. De Silva, A.; Lee, J.-K.; André, X.; Felix, N. M.; Cao, H. B.; Deng, H.; Ober, C. 



38 

K., Study of the Structure−Properties Relationship of Phenolic Molecular Glass Resists 
for Next Generation Photolithography. Chemistry of Materials 2008, 20 (4), 1606-1613. 

34. Lawson, R. A.; Lee, C.-T.; Yueh, W.; Tolbert, L.; Henderson, C. L., Epoxide 
functionalized molecular resists for high resolution electron-beam lithography. 
Microelectronic Engineering 2008, 85 (5-6), 959-962. 

35. Bae, W. J.; Trikeriotis, M.; Sha, J.; Schwartz, E. L.; Rodriguez, R.; Zimmerman, 
P.; Giannelis, E. P.; Ober, C. K., High refractive index and high transparency HfO2 
nanocomposites for next generation lithography. Journal of Materials Chemistry 2010, 
20 (25), 5186-5189. 

36. Trikeriotis, M.; Bae, W. J.; Schwartz, E.; Krysak, M.; Lafferty, N.; Xie, P.; Smith, 
B.; Zimmerman, P.; Ober, C.; Giannelis, E., Development of an inorganic photoresist for 
DUV, EUV, and electron beam imaging. SPIE: 2010; Vol. 7639. 

37. Krysak, M.; Trikeriotis, M.; Schwartz, E.; Lafferty, N.; Xie, P.; Smith, B.; 
Zimmerman, P.; Montgomery, W.; Giannelis, E.; Ober, C., Development of an inorganic 
nanoparticle photoresist for EUV, e-beam, and 193nm lithography. SPIE: 2011; Vol. 7972. 

38. Trikeriotis, M.; Krysaki, M.; Chung, Y. S.; Ouyang, C.; Cardineau, B.; Brainard, 
R.; Ober, C. K.; Giannelis, E. P.; Cho, K., Nanoparticle photoresists from HfO2 and ZrO2 
for EUV patterning. Journal of Photopolymer Science and Technology 2012, 25 (5), 583-
586. 

39. Li, L.; Chakrabarty, S.; Spyrou, K.; Ober, C. K.; Giannelis, E. P., Studying the 
Mechanism of Hybrid Nanoparticle Photoresists: Effect of Particle Size on 
Photopatterning. Chemistry of Materials 2015, 27 (14), 5027-5031. 

40. Toriumi, M.; Sato, Y.; Koshino, M.; Suenaga, K.; Itani, T., Metal resist for 
extreme ultraviolet lithography characterized by scanning transmission electron 
microscopy. Applied Physics Express 2016, 9 (3), 031601. 

41. Passarelli, J.; Murphy, M.; Del Re, R.; Sortland, M.; Hotalen, J.; Dousharm, L.; 
Fallica, R.; Ekinci, Y.; Neisser, M.; Freedman, D.; Brainard, R., Organometallic 
carboxylate resists for extreme ultraviolet with high sensitivity. Journal of 
Micro/Nanolithography, MEMS, and MOEMS 2015, 14 (4), 043503. 

 

 



39 

CHAPTER 2 
 

REVIEW OF ESSENTIAL USE OF PHOTOACID GENERATORS IN 
LITHOGRAPHIC PATTERNING AND PROCESSING 

(Published as: Ober, C.; Kafer, F.; Deng, J. Review of essential use of fluorochemicals in lithographic patterning and semiconductor 

processing. J. Micro/Nanopatterning, Mater., Metrol. 2022, 21, 010901.) 

Abstract  

We identify and describe categories of fluorochemicals used to produce advanced 

semiconductors within the lithographic patterning manufacturing processes. Topics 

discussed include the per- and polyfluoroalkyl substance (PFAS) materials used and their 

necessary attributes for successful semiconductor manufacturing, consisting of photoacid 

generators, fluorinated polyimides, poly(benzoxazole)s, antireflection coatings, topcoats, 

and embedded barrier layers, fluorinated surfactants, and materials for nanoimprint 

lithography. In particular, an explanation is given of the particular function that these 

PFAS materials contribute. It is noted that in almost all cases fluorine-free alternatives 

are very unlikely to provide the essential properties present in PFAS systems. 

Nonfluorinated alternative compounds are discussed where available. 

 

Author Contribution  

C.K.O designed the review. J.D. contributed to the PAG section in this review. F. K. 

contributed to the other sections.  

 
 

 

 

 

 

 



40 

2.1 Introduction  

  The use of fluorochemicals in lithography and semiconductor patterning plays a critical 

role in the success of semiconductor technology. The addition of small quantities of 

fluorinated mate- rials enables patterning capabilities that are otherwise not possible to 

achieve, and this leads to superior device performance. The compact size of the fluorine 

atom and its strong electron- withdrawing characteristics make it stand out in the periodic 

table and gives fluorocarbon mate- rials unique properties, unmatched by other chemical 

compounds. Fluorochemicals have found use in semiconductor processing for good 

technical reasons. 

1. The presence of fluorine near acidic groups can convert them from an acid to a 

superacid, an essential characteristic for photoacid generators (PAGs) needed in advanced 

photoresists. 

2. Fluorocarbon materials have low surface energy characteristics and act as superior 

barrier layers (including water repellence), which provide useful properties in 

photoresists and in antireflection coatings used in immersion lithography while also 

providing excellent release properties because they do not adhere strongly to other 

materials. 

3. Fluorinated materials have unique solubility characteristics and can prevent 

intermixing between layers in a complex system such as an antireflection coating. 

Fluorinated materials are both hydrophobic and oleophobic and thus have reduced or no 

miscibility with essentially all fluorine-free classes of polymers. 

4. Fluoropolymers have a low refractive index compared with any material except air and 



41 

provide useful optical properties in photoresists and antireflection coatings. 

5. They possess low dielectric constant and are especially good electrical insulators, an 

important feature when polyimides are patterned and retained in the final device. This 

document provides a systematic overview of the photolithography process and key 

fluorinated materials involved, provides insight into performance requirements, and 

describes why fluorinated chemicals help achieve needed characteristics. 

Photolithography, a critical process step in the production of a semiconductor, uses a 

photoresist to transfer a pattern. The primary component of a photoresist is a 

photopolymer whose solubility will be changed upon exposure to short wavelength 

radiation. In addition, the photo- resist contains a deposition solvent and several small-

molecule compounds. The desired solubility change must be great enough that a 

developer (a solvent that removes the unwanted region of a resist pattern) does not swell 

the remaining photoresist. The development process must be able to discriminate between 

exposed and unexposed regions as small as a few nanometers in size. The unremoved 

photoresist must protect the underlying substrate from the next process steps in 

semiconductor manufacturing. Each stage in the process must be virtually perfect with 

yields well above 99%, because there may be hundreds of process steps used to 

manufacture each advanced semiconductor device. Without those very high yields, 

semiconductor manufacturing would fail. 

  The basic lithography process used globally today for advanced semiconductor 

manufacturing and the foreseeable future employs chemically amplified photoresists. 

Chemical amplification was a key invention needed to overcome the challenge of limited 

light sources but was also found to provide superior patterning performance. In such resist 



42 

systems a photopolymer that contains acid cleavable protecting groups is combined with 

a photoactive compound, such as a PAG. In its native state, the photoresist polymer with 

protecting groups is soluble in organic solvents. Upon exposure to UV radiation, the PAG 

releases acid. Frequently, a subsequent post- exposure bake (PEB) step leads to the acid-

catalyzed removal of protecting groups, thereby transforming the hydrophobic 

photopolymer into one that is soluble in an aqueous base developer. The single photon of 

light needed to release one acidic proton is “amplified” by the more efficient acid-

catalyzed deprotection process. By transforming the solubility of the photoresist, a high 

contrast patterning process needed in semiconductor manufacturing becomes possible. 

The combination of photoresist polymer and PAG to make the photoresist system is an 

essential part of this process and fluorination in the PAG provides the high acidity 

necessary for chemical amplification to work and will be described subsequently. 

  One of the special features of the C─F bond is its strength compared with the C─C 

bond due to the electron-withdrawing power of the fluorine atom. This attribute is the 

basis of many of the technical benefits of fluorinated materials in semiconductor 

processing but leads to its chemical stability and environmental persistence. Fluorination 

brings specific improvement in performance, and its targeted incorporation can minimize 

the quantities of material needed to achieve that performance. Such aspects are discussed 

in the context of PAGs. Thus, despite the remarkable performance improvement in many 

aspects of the lithographic process provided by fluorochemicals (PFAS) that makes 

possible the semiconductor revolution with its benefit to society, the large and growing 

environmental and societal concerns surrounding PFAS may counterbalance the positive 

technological benefits of these materials. The reader is referred to a discussion of such 

PFAS concerns in a well-written review article, but photolithography chemicals are 



43 

largely glossed over.1 Going forward, due to environmental and regulatory concerns, 

performance equivalent alternatives for many of these applications still need to be 

identified and this will be a major research challenge. 

Based on concerns regarding the high persistence, bioaccumulation potential, and 

potential toxicity of PFAS studied to date, it has been suggested that the use of PFAS be 

limited to essential uses only. 

  We discuss whether viable alternatives exist for each of these applications and the 

characteristics that must be achieved to find an alternative compound where none 

currently exists. Finally, we apply the essential use concept described by Cousins et al.2 

to show that these compounds should be considered essential for certain processes in 

semiconductor manufacturing (i.e., photolithography and patterning) because they 

provide for vital functions and are currently without established alternatives. The prior 

paper did an excellent job of discussing different aspects of PFAS use. In this paper, we 

focus our discussion of essential use as “necessary for highly important purposes in 

semiconductor manufacturing for which alternatives are not yet established.” We describe 

the many uses and unique properties of PFAS chemicals, which in our opinion justifies 

their current use as essential in microelectronics manufacturing and for which alternatives 

have not yet been adequately identified. This paper is not intended to be an extensive 

listing of every example of fluorochemical used in photolithography but does attempt to 

explain strategies and classes of material used in the manufacturing of semiconductors. 

 

 



44 

2.2 Photoacid Generators  

PAGs are photoactive compounds that generate acids upon exposure to high-energy light 

[deep ultraviolet (DUV) or extreme ultraviolet (EUV)]. These photoactive compounds 

were originally used for applications in photopolymerization in the early 1960s.3 After 

the introduction of chemically amplified resists (CARs) in the 1980s, they have been used 

in semiconductor manufacturing as key components in advanced photoresists. It is 

important to understand that the process of chemical amplification requires a very strong 

acid in the PAG to function well. PAGs are now highly evolved with over 40 years of in-

depth research and development for photoresist applications. A positive tone resist 

polymer after deprotection, for example, contains weak acid groups that will act to buffer 

(weaken) the acidity of the deprotection process. Without the presence of the strong 

fluorosulfonic (or stronger) acid, the catalyzed deprotection process will be less efficient 

or may not even occur. Sulfonate anions without fluorination have repeatedly been shown 

to be inadequate for use ineffective 193 nm chemically amplified photoresists and this is 

well known in the photoresist community. The unique characteristics of fluorine (noted 

below), which lead to very strong proton donation by fluorinated sulfonic acids, are 

essential in CARs. This intrinsic benefit of fluorinated acids makes it extremely difficult 

to eliminate the use of fluorinated acids whilst retaining the key performance 

characteristics of CARs needed for advanced photolithography in microelectronics 

manufacturing. Other attributes of a PAG that depend less on the acid and more on the 

chromophore include quantum yield at the wavelength of use, the sensitivity of the overall 

resist formulation (e.g., 15 to 60 mJ⁄cm2), miscibility in the resist matrix, thermal and 

hydrolytic stability and shelf life of the photoresist, solubility in aqueous base developer 

for positive tone develop or organic solvent for negative tone development followed by 



45 

removal in the resist strip operation. In general, PAGs are divided into two categories: 

either ionic or covalent (nonionic) structures. As the name suggests, ionic PAGs consist 

of two portions: a cation and an anion. In addition, covalent PAGs are uncharged, 

nonpolar com- pounds that are constructed of covalent bonds but are generally less 

sensitive and therefore less effective than ionic PAGs. The availability of both ionic and 

covalent PAGs offers process flexibility. In some cases, the presence of ionic groups may 

lead to storage instability of the photoresist mixture or the inhomogeneous distribution of 

photoactive compounds in the photoresist, thus making a nonionic PAG necessary. 

However, most photoresist compositions that are used in semiconductor manufacturing 

employ ionic PAGs because of their greater sensitivity. Examples of PAGs are shown in 

Figure 2. 1. In either case, a fluorinated sulfonic acid would be used to make an effective 

PAG. The photoefficiency difference between ionic and covalent PAGs, which leads to 

higher quantum yields in the ionic PAG is controlled by the cation.4 The low diffusivity 

and high strength of the acid resulting from the photolysis of the cation are controlled by 

the resulting accompanying fluorosulfonate anion. These anions are used in virtually all 

current commercial photo- resists. Limited diffusivity is important to achieving high-

resolution patterns because excess diffusion of the PAG has been shown to limit the 

resolution of the images produced in a CAR. While aromatic sulfonic esters are shown in 

some nonionic PAGs described in Figure 2. 2, the strength of the resulting sulfonic acid 

after photolysis is not as high as the ionic PAGs with fluorinated sulfonate anions. 

Covalent PAGs do not suffer from the sorts of phase separation, low miscibility, and dark 

loss (the dissolution of unexposed photoresist) issues that may occur in ionic PAG-

containing resist formulations, but the quantum yield of photoacid generation is generally 

lower for covalent PAGs so this and other factors drive the ultimate choice of PAG.5  



46 

 

 

 

   

 

Figure 2. 1 Representative ionic PAGs: 1, 2, 3, 4 Sulfonium PAG and 5, 6 Iodonium PAG. 
 

 

 

 

 

O O F
F

S OO

O

S

S

O

O

O
F

F
F S

1

2

O

N

O

I
PF6

Br

5

O
S

FF

F F
O

O O
S

3

S
O

O O

F F

FF

F F

F
F

F
S

4

I

S
O

O O

F F

FF

F F

F
F

F

6



47 

 

 

 

 

 

 

 

 

Figure 2. 2 Representative nonionic covalent PAGs. 

 

 

 

 

 

 

O

N

O

O
S

O

O

F F

FF

F F
F
F

F

9

S
O

O O

O

S
O

O O

7 8



48 

In order to increase the acidity of the photoacid, perfluorinated methylene units may be 

placed next to the sulfonate group in both ionic and covalent PAGs. The polarization 

present in the C─F bond due to the electronwithdrawing character of fluorine stabilizes 

the acid anion and makes the acid stronger. A sulfonic acid such as methane sulfonic acid 

has a pKa of −2 (already a strong acid) but trifluoromethyl sulfonic acid (triflic acid) 

has a pKa of −14. Any induction effect is significantly smaller after two or three CF2 

units, so the relative benefit of fluorination is significantly reduced as the neighboring 

CF2 units are further away from the acid group. The original choice of longer sequence 

perfluorinated sulfonates (six or more) has not been explained in patents or the literature 

but was likely due to the effectiveness of the resulting PAG, the reduced diffusivity 

because it is a larger molecule, its availability, and the lack of volatility in this material. 

For example, the volatility of the small triflate anion limits its use in a production 

photoresist PAG because the resulting concentration gradients in such photoresist films 

harm performance. However, shorter CF2 segments (1 or 2) next to the anion and 

connected to other units of higher mass have been shown to make effective PAGs. Finally, 

the diffusivity of the PAG will affect pattern resolution (less diffusion enhances 

resolution) and can be addressed by the use of a higher molar mass PAG/acid and even 

covalent attachment of the PAG to the photoresist polymer itself. Although actively used 

in some applications, triflic acid is not always a useful component in a PAG since it may 

have significant deficiencies when used in a very high-resolution CAR system; it is 

volatile and may evaporate during the PEB step leading to composition gradients that are 

detrimental to image resolution and it readily diffuses during annealing, which may, in 

turn, lead to pattern degradation from deprotection chemistry occurring in unexposed 

areas, effectively reducing image contrast and disrupting pattern formation.  



49 

2.3 Ionic Photoacid Generators 

Most ionic PAGs used in lithography are onium salt derivatives. Such ionic compounds 

consist of an onium moiety as the cation and sulfonate groups as the anion. Upon exposure, 

photolysis occurs and photoacid is formed. The quantum yield of the photoacid is directly 

impacted by the cation fragment. The acidity of the generated photoacid as noted above 

is controlled by the anionic fragment in the PAG (usually a fluorinated sulfonic acid). 

The rate of photoacid release is controlled by both cation and anion. ionic PAGs are 

generally composed of either diaryliodonium or triarylsulfonium photoactive units to 

form a salt with an appropriate anion. Triarylsulfonium PAGs usually have longer shelf 

life compared with diaryliodonium salt. However, a diaryliodonium salt has higher 

absorptivity in particular for next-generation 13.5-nm wavelength EUV photons. The 

mechanism of photolysis of diaryliodonium salt and triarylsulfonium salts6 has been 

studied extensively. Reported photolysis mechanisms for diaryliodonium salt and 

triarylsulfonium salts are shown in Figure 2. 3 and Figure 2. 4, respectively.7 The quantum 

yield of the photoacid is directly impacted by the cation fragment. The acidity of the 

generated photoacid as noted above is controlled by the anionic fragment in the PAG 

(usually a fluorinated sulfonic acid). The rate of photoacid release is controlled by both 

cation and anion. In Figure 2. 3, the energy required to cleave the aromatic C (sp2) and 

iodine bond is somewhat higher compared with the energy required to promote bond 

cleavage between the aromatic C (sp2) and sulfur bond. Generally, the sulfonium PAG 

family is more widely used than iodonium PAGs considering its greater sensitivity and 

longer shelf life when used in either DUV or EUV lithography. Solid state studies at 193, 

248, and 266 nm exposures reveal additional products including in all cases, two 

previously unreported triphenyl sulfonium photoproducts, triphenylene, and  



50 

 

 

 

 

 

 

 

Figure 2. 3 Proposed photolysis mechanism for diaryliodonium salt under DUV 
exposure. 

 

 

 

 

I
A

hv I
A I

A

+

RH

R

I

+ + AH



51 

 

 

 

 

 

 

 

Figure 2. 4 Proposed photolysis mechanism for triarylsulfonium salt. 

  

 

 



52 

dibenzothiophene. 

2.4 Nonionic Covalent Photoacid Generators 

Although ionic PAGs have higher sensitivity in lithographic applications, they may be 

less soluble and more prone to phase separation in photoresist formulations. It is worth 

recalling that the PAG is needed to generate acid in the exposed regions to deprotect the 

photoresist and thereby change its solubility. Uniform distribution of a PAG is an essential 

attribute to excellent performance in a photoresist. Detrimental interaction between ionic 

structures in a photoresist and an ionic PAG may also occur in future resist materials.8 To 

overcome such issues, covalent PAGs may be attractive alternatives. In general, covalent 

PAGs are derivatives of aryl sulfonates,9 iminosulfonates,10 and imidosulfonates.11 

Arylsulfonate esters can be easily synthesized from phenol and sulfonyl chlorides. A 

similar effort to create fluorinated sulfonate ester-containing covalent PAGs has not taken 

place because such PAGs have not been as effective in photoresist applications. The 

photoacid generation mechanism is proposed based on the nonfries photolytic ArO─S 

bond cleavage (pathway A) or pseudofries rearrangement (pathway B), which is more 

likely to occur for electron-rich aryl sulfonates as shown in Figure 2. 5.12-13 

  It is worth noting that in pathway A, in the presence of oxygen and water, stronger 

sulfonic acid is generated. In the absence of oxygen, weaker sulfurous acid is produced. 

Iminosulfonates and imidosulfonates have similar chemical structures with the N─O 

bond undergoing homolytic cleavage upon irradiation to generate sulfonyloxy radicals, 

which subsequently capture hydro gens from nearby molecules to afford the 

corresponding sulfonic acid as shown in Figure 2. 6. 



53 

 

 

 

 

 

 

 

Figure 2. 5 Photoacid generation mechanism for aryl sulfonate esters. 

 

 

 

 

 

Ar
S
O

O O
Irradiation Ar

S
O

O O

Ar
S
O

O O
+

H2O / O2 Ar
S
O

O HO
+

OH

(1) –SO2
Ar(2) H2O + H2SO3

Photoacid Generation

Ar
S
O

O O
Irradiation Ar

S
O

O

+
O

Ar
S
O

O OH

Photoacid Generation

Pathway A

Pathway B



54 

 

 

 

 

 

 

Figure 2. 6 Photoacid generation mechanism for iminosulfonates and imidosulfonates. 

 

 

 

 

 

 

N O S

O

O

R

N O S

O

O

R

O

O
Irradiation

O S

O

O

R HO S

O

O

R

Photoacid Generation

Iminosulfonates

Imidosulfonates



55 

2.5 Polymer-Bound PAGs 

One approach to increasing the resolution to photolithography is to employ PAG that is 

incorporated into the photoresist polymer structure. It has the advantage of making the 

distribution more uniform and at the same time limits the diffusivity of the sulfonate anion 

since it is bound to the photoresist polymer. Resolution is set in part by the diffusivity of 

the PAG in the photoresist formulation, which is associated with the size of the molecule. 

The smaller the anion, the farther the photogenerated proton can diffuse in a given time. 

If the PAG acid diffuses too broadly then deprotection of the photoresist takes place in 

unwanted regions and makes the pattern larger, less precise, and “blurry.” These pattern 

irregularities are characterized in terms of line edge roughness, line width roughness, and 

critical dimension uniformity. Examples of bound-PAG structures have been reported and 

two are described below shown in Figure 2. 7.14 This strategy also lowers concerns about 

“stochastics,” i.e., the chemical heterogeneity of a photoresist mixture at the dimensions 

of the pattern are thought to also contribute to the limit of resolution of today’s most 

advanced lithographic processes. Upon exposure, the fluorosulfonate group becomes 

protonated, catalyzes deprotection of the rest of the photoresist chain, but the strongly 

acidic proton cannot diffuse broadly because it remains near the anion bound to the 

polymer chain and thereby forms higher resolution patterns. By attaching the same 

number of PAG units to each polymer chain, then the PAG is uniformly distributed 

throughout the photoresist film. This strategy is being seriously considered for future 

generations of photoresists, particularly for use in EUV lithography.15 These examples 

share several common features, including the attachment of the anion to the polymer 

backbone. Since many CAR photoresists are based on (meth) acrylates, examples 

reported for 193 nm (DUV) resists possess a sulfonate anion and an adjacent CF2 unit,  



56 

 

 

 

 

Figure 2. 7 Examples of polymer-bound PAGs. (a) Single CF2 unit next to sulfonate21 and 
(b) single CF2 unit next to sulfonate in a structure that falls apart on exposure; groups (R1, 
and R2) not specified groups while R3 is a linking group.  

 

 

 



57 

which then is connected to the methacrylate monomer through an ester linkage. While it 

has not been established if one or two CF2 units are needed to produce sufficiently strong 

anion, this example demonstrates one approach and good prospects for polymer-bound 

PAGs. 

2.6 Alternatives to Fluorinated PAGs 

PAGs other than iodonium and sulfonium units as well as those that do not contain 

traditional PFAS have also been studied for use in photolithography. To be used 

successfully in a CAR photoresist, the resulting acid must be as acidic as a 

perfluorosulfonic acid, lack volatility so that it does not evaporate during the PEB step, 

and in the next generation photoresists possess minimum diffusivity (to enable high-

resolution pattern formation). The PAG-resist combination should have a sensitivity in 

the range between 10 and 75 mJ⁄cm2 under exposure conditions i.e., the source 

wavelength and tool-specific settings. Some new photoresists attach the PAG directly to 

the photopolymer chain to both limit diffusion and deal with issues of stochastic 

variations that may be present in photoresists consisting of mixtures of polymer and 

photoactive molecules. Nontraditional PFAS Covalent PAGs: Nitrobenzyl esters have 

found some application in DUVL and may be extendable to EUV lithography. Such 

molecules can generate photoacid upon irradiation through o-nitrobenzyl rearrangement 

to generate nitrobenzaldehyde and a sulfonic acid such as triflic acid shown in Figure 2. 

8. The chemical structure is shown in Figure 2.8 (a). The terarylene skeleton-based self-

contained PAG is another potential candidate for some applications. The photoacid 

generation is triggered by the 6π-electro-cyclization reaction of photochromic triangular 

terarylenes.16 The chemical structure is shown in Figure 2.8 (b). Similarly, a triflate ester  



58 

 

 

 

 

 

 

Figure 2. 8 Chemical structure for non-PFAS covalent PAGs: (a) nitrobenzyl ester and 
(b) terarylene backbone-based PAG. 

 

 

 

 

 

 

NO2

OTf

a

S N

S

S
N

OTf

Ph

Ph
b



59 

is used in the reported structure to release triflic acid upon exposure. While these and 

other structures can be used to demonstrate PAG concepts, they are unlikely to be as 

useful in new high-resolution photoresist systems because they use triflate groups. 

Alternative acids may be used to make more suitable PAGs from the moieties in Figure 

2.8. Should a useful PAG be produced from these types of photoactive structures the 

resulting sulfonic acid will need to be less volatile and less mobile in the polymer film. A 

higher molar mass, much less volatile, lower diffusivity anion might work well with these 

materials in a functioning photoresist system. Nontraditional PFAS Ionic PAGs: Ionic 

PAGs derived from 2-phenoxytetrafluoroethane sulfonate were introduced by Ober and 

coworkers in 2007.17 This PAG was tested under e-beam and EUV radiation and showed 

high sensitivity, resolution, and acceptably low line edge deviations. The use of such a 

fluorosulfonic acid has the advantage that it limits fluorine content yet produces a very 

strong acid with both limited volatility and diffusivity by placing a CF2 group next to the 

acid group. Such an approach (discussed more below) can be used to minimize fluorine 

incorporation while placing this structure where it is most valuable. Its chemical structure 

is shown in Figure 2. 9. This PAG was tested for its environmental degradation and its 

effect on bacterial populations when first reported and found to be benign under the rules 

of that time.  

The good lithographic results suggest that shorter fluorinated segments (two or 

possibly one CF2 unit adjacent to the sulfonic acid) may make useful ionic PAGs. It 

should be noted that the building blocks for sulfonic acids with one CF2 are the subject 

of experimental studies. The pentacyanocyclopentadienide PAG is another potential ionic 

PAG candidate in some applications. Its lithographic performance was demonstrated by 

Varanasi and coworkers in 2010, and it stands out for the amount of publicity it received.18 



60 

 

 

 

 

 

 

Figure 2. 9 Chemical structures of untraditional ionic PAGs: (a) 2-
phenoxytetrafluoroethane sulfonate PAG and (b) pentacyanocyclopentadienide PAGs. 

 

 

 

 

 

 

 

O
S

FF

F F
O

O O
S

a

S

CN
NC

CN

CN
NC

b



61 

The chemical structure is shown in Figure 2. 9 (b).  

  While announced in 2010 as part of IBM’s efforts to reduce Perfluorooctanoic acid 

(PFOA) in its manufacturing process, to the best of our knowledge, this PAG was not 

commercialized. Finally, PAGs based on glucose or other natural products have been 

explored. These PAGs were demonstrated to be functional materials for some high-

resolution photoresist applications enabling sub-100nm features using ArF laser and e-

beam lithography. Moreover, these PAGs showed successful microbial degradation to 

smaller molecular units under aerobic conditions. The chemical structures are shown in 

Figure 2. 10.  

  Such studies revealed the successful biodegradation of these PFAS units to smaller 

oxidized components as well as low bacterial cytotoxicity14, 19 of the photoactive 

sulfonium subunit. In general, the anionic units underwent biodegradation using sludge 

from a local municipal waste water treatment plant. The sugar or cholesterol groups 

appeared to degrade easily leaving only a short, fluorinated acid residue. An advantage 

of these structures is that the residues retain polar functional groups and are therefore 

more hydrophilic than PFOS/PFAS units. This makes them less likely to accumulate in 

fatty tissues, but further studies are needed to identify any bioaccumulation characteristics. 

The photoactive cation unit but not the fluorinated anion was generally found to be 

cytotoxic to the bacteria. Importantly, the short, fluorinated segment enabled the 

formation of a high-performance PAG that could be subjected to successful biological 

degradation. 

  More recently, patents have appeared that describe the goal of which is intended to 

deliver strong PAG performance and minimize the size of the fluorinated unit in the  



62 

 

 

 

 

 

Figure 2. 10 Chemical structures for natural products-based PAGs. 

 

 

 

 

 



63 

fluorosulfonic acid or eliminate it entirely. These patents claim excellent lithographic 

performance. These and other patents describe PAGs with shorter fluorinated segments, 

some of which are designed to fall into small molecular pieces. To assess their viability 

as alternative PAGs their performance characteristics (sensitivity, acid strength, and 

diffusivity) and environmental characteristics (fluorine content, degradation products, 

and toxicity) will need to be assessed.  

 

2.7 Conclusions 

The use of fluorination in PAGs is to enhance the acidity (make pKa ≪ 1) of the acid 

produced in the region of exposure of a photoresist. The formation of acid to induce a 

solubility change is the critical step in today’s chemically amplified photoresists, the 

workhorse family of photoresists that enable the production of the vast majority of semi- 

conductors. The presence of a fluorinated unit adjacent to the sulfonic acid gives the acid 

its ability to efficiently release a proton that reacts with the resist polymer to create a 

solubility switch. Subsequent development forms a pattern in the photoresist. Today there 

is no effective alternative to a fluorinated sulfonic acid and this situation applies to 

chemically amplified photoresists across all wavelengths of lithography from 248 nm to 

EUV. Efforts to reduce the amount of fluorination in a PAG molecule have been 

demonstrated, but a survey of the current literature has not shown that complete 

elimination of fluorination has produced a successful alternative. However, it is very 

likely that fluorine- free alternatives, which perform equally well and can easily take the 

place of the fluorinated compounds used today, will be more widely used, and developed 

in the coming years. Fluorinated polyimides use the presence of a fluorinated unit to 



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improve the dielectric constant of the material and make it a better insulator while 

retaining excellent thermal stability. This combination of characteristics has not been 

effectively achieved by alternate means. 

Fluorinated materials play a useful and often essential role in many aspects of 

semiconductor processing. These fluorochemicals are employed as com- ponents of PAGs, 

as components of photoresists, as elements of high-temperature polymers, and as 

ingredients in ARCs, BARCs, and as topcoats, frequently satisfying the “essential use” 

criterion. However, there is a strong societal interest in eliminating their use, and 

“essential use” is a stopgap situation in which replacements are actively sought. The 

“essential use” concept expects that PFAS uses considered essential today should be 

continually reviewed for potential removal or replacement by new technologies and be 

targeted by innovation toward alternatives. The concept does not support long-term and 

large-scale remediation technologies to justify the ongoing use of PFAS chemicals. Thus, 

the challenges going forward are to find a means to replace PFAS components that 

achieve or surpass today’s current performance characteristics in the following current 

and possible future lithography systems. 

Acknowledgements 

The authors gratefully acknowledge the Semiconduct