Laser-induced Sub-millisecond Structural Formation Kinetics in Block Copolymers
Jacobs, Alan Gregory
Block copolymer (BCP) self-assembly has found broad use in applications ranging from nanocomposites to nanolithography by exploiting the precise control of nanoscale order possible over macroscopic length scales. One application garnering significant attention for commercialization uses this nanoscale order to augment current photolithography patterning to achieve sub-20 nm features through directed self assembly (DSA). Extending current lithography to these smaller length scales is critical to enable cost-effective next-generation semiconductor devices, furthering technological progress and maintaining the pace of Moore's law. As with many of these applications, DSA utilizes BCPs starting from deeply metastable states. Detail of the initial phase segregation process, structure formation, and refinement are critical to device function, efficacy, and yield. However, understanding of this initial phase segregation from deeply metastable states, especially the temporal evolution, is currently lacking. This ignorance stems in part from both a difficulty in experimentally measuring the short time structural response of polymers, and on the computational difficulty in modeling large enough systems at high fidelity over molecular timescales. Furthermore, for DSA, the anneal must achieve a near perfectly aligned equilibrium structure. The timescale required, and thus the cost, to reach the fully aligned state is dependent upon kinetic pathways, especially past any potential trapped defect states. Laser spike annealing (LSA) can achieve high temperatures for short durations allowing investigation of potential process windows in the microsecond to millisecond time scales. In this work, a CO2 gas laser (120 W, wavelength=10600 nm) and a solid state diode laser (250 W, wavelength=980 nm), were used to achieve peak temperatures up to ~1000 degrees C on time scales from 0.05 ms to 10 ms. Additionally, high throughput experiments of the lateral gradient LSA (lgLSA) method were used to fully explore these time and temperature regimes. This has enabled exploration of a previously inaccessible temperature regime and the determination of kinetic parameters that potentially offers access to new processing regimes and resulting structures. For these short duration anneals, it is shown that the thermal stability of typical organic materials is extended by over 450 degrees C compared to hot plate limits. This stability was quantified using Arrhenius kinetics with activation enthalpies ranging between 0.6 and 1.2 eV. The activation energies appear to scale with the primary (backbone) bond formation energy and inversely with the bond polarity. This extended thermal stability was exploited to probe the self-assembly kinetics of cylinder forming poly(styrene-block-methyl methacrylate) (PS-b-PMMA, 54 kg/mol, fraction PS=0.67) by annealing at temperatures up to 550 degrees C for timescales from 0.25 ms to 10 ms with heating and cooling rates in excess of 10^6 K/s. Segregation kinetics were quantified by X-ray scattering (micro-GISAXS) and electron microscopy (SEM), resulting in kinetic phase maps that describe the phase segregation behavior. The onset of phase segregation and of disordering were found to be kinetically suppressed for times below 1 ms, exceeding the expected transition temperatures by 70 degrees C at 0.25 ms. This is shown to be consistent with the diffusion behavior on these timescales. These BCP segregation kinetics control the ordering and templating required for DSA lithography. High temperature LSA near the order-disorder transformation temperature (TODT) was explored as a means to reduce the segregation driving force, increase polymer mobility, and ultimately reduce defectivity by allowing polymer alignment to the directing template with higher fidelity. Hot plate and/or LSA alone result in films with high defectivity. However, an LSA anneal first to establish the initial segregation, followed by a conventional hot plate anneal, can reduce the defectivity by >80%. This is believed to this reflect nanoscale 3-d ordering in the BCP and interactions with the directing template during the very short LSA near the ODT. This demonstrated defectivity reduction only highlights the need for better understanding of BCP phase segregation kinetics from deeply metastable states, which ultimately will advance our ability to rationally design processing for improved efficacy. While PS-b-PMMA is an important model system, significant opportunities lie in exploring other systems with varying chemistries and glass transition temperatures, especially for highly incompatible systems where disordering is not typically observed thermally. Beyond studying polymer behavior, short duration annealing, and kinetic suppression of structural motifs, presents opportunities for spatially resolved chemistry and other novel applications.
Materials Science; Block Copolymers; Kinetics; Laser; Laser Spike Annealing; Phase Segregation; Self Assembly
Thompson, Michael Olgar
Ober, Christopher Kemper; Pollock, Clifford Raymond; Wiesner, Ulrich B.
Materials Science and Engineering
Ph. D., Materials Science and Engineering
Doctor of Philosophy
Attribution-NonCommercial-NoDerivatives 4.0 International
dissertation or thesis
Except where otherwise noted, this item's license is described as Attribution-NonCommercial-NoDerivatives 4.0 International