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Monolithic Integration Of Junction Field Effect Transistor And Nanoelectromechanical Systems
The mechanical motion of most NEMS/MEMS devices has to be transduced to electrical domain by using active or passive components. In passive transduction, resistors, capacitors and inductors are used to sense the motional current which is then converted to voltage. In active sensing, transistors are also used for the conversion process. Since transistors can offer enhanced gain through transconductance, they can increase small signals into larger signals that can be less susceptible to systematic and innate noise sources. The active components can be integrated into the NEMS device either by monolithic integration or through a two chip solution. In monolithic integration, both the active device and the NEMS devices are fabricated on the same substrate, using short thin film interconnects, minimizing parasitics. In the two-chip solution, the active and NEMS components are fabricated on separate wafers and the individual dices are wire-bonded, or flip chip bonded which can have higher parasitics and generate mismatches in the system. One of the goals of this thesis is to monolithically integrate JFETs into N/MEMS components to enhance signal transduction. The dissertation begins with the characterization of an SOI pre-biased NEMS electrostatic switch with a pre-biased voltage of 54.8 V and a switching voltage as low as 300 [mu]V. The contact resistance of the switch was 4.3 MΩ due to the Si-to-Si contact used in the switch. Later, to reduce the contact resistance, MoSi2 was used as a iv structural layer and Cr and Pt were sputtered on the switch to produce Pt-to-Pt contact. The measured contact resistance was reduced to 1 KΩ. A Junction Field Effect Transistor (JFET) was integrated into the switches to enable the sensing of the displacement of the moving structure. The JFETs had a pinch-off voltage of -19 V (at VDS=10 V) and a transconductance parameter of 1.9 mA/V2 (at VDS=10 V). These JFETs were monolithically integrated into the switch to minimize parasitics. The JFET was then incorporated into a nanoscale multiple-tip prober which was used for atomic imaging of Highly Ordered Pyrolytic Graphite (HOPG) as well as performing conductance measurements of HOPG. The JFET along with capacitive sensing was used to sense the motion of the movable tip. The resonating tip had a resonance frequency of 293 kHz and the tip radius of <50 nm. Currently, commercial Scanning Probe Microscopes (SPM) such as STM and AFM use a single tip for scanning which limits its use to static electrical measurements. This dissertation presents the development of a novel SPM that uses the multiple tips for scanning and performing dynamic transport measurements. v
JFET; NEMS; INTEGRATION
Tiwari, Sandip; Molnar, Alyosha Christopher; Pollock, Clifford Raymond
Ph.D. of Electrical Engineering
Doctor of Philosophy
dissertation or thesis