Faisal K. Chowdhury

Novel single-device “XOR” AND “AND” gates for high speed, very low power LSI mechanical processors

We discuss novel functional MEMS/NEMS structures that enable implementation of universal logic gates such as XOR, AND, NAND, NOT, etc. in a single device instead of using 6–14 individual switches used in CMOS. By reducing the number of devices, our approach improves yield, reproducibility and speed and simplifies implementation of circuits such as adders and multiplexers. We show the feasibility of this novel approach through fabrication and testing of XOR, AND and related circuits such as a 2 bit full adder and multiplexer. The XOR gates with ∼1.5V turn-on voltage at 50 MHz with >109 cycles of reliable operations are reported. We also discuss the operation of XOR without deterioration at high temperature and in 90 kW ionizing radiation for 120 minutes.

Micro-plasma field-effect transistors

We designed, fabricated and tested new microplasma FET (MOPFET) devices that operate inside RF helium plasma that generated at atmospheric pressure. Unlike normal FETs, micro-plasma FETs uses electrons and ions as carriers. It has unique advantages over normal FETs in extreme conditions at high temperature and ionizing radiation in space and in a nuclear event. It also has potential applications in combustion engine sensors and diagnostic circuits. MOPFET can potentially operate with very few ions and have the additional potential of producing nano-scale switches and amplifiers. The plasma for our MOPFET was separately generated and sustained using an RF plasma source. Thus, for the first time we achieved small voltage (5-10V) plasma switches and amplifiers. We have developed concentrated and distributed plasma sources suitable for different sizes of integrated MOPFET circuits.

MEMS-based hemispherical resonator gyroscopes

This paper introduces a fabrication technique that uses planar MEMS micromachining processes to produce hemispherical resonating shells for gyroscopes. The hemispheres exhibit a quality factor in excess of 20,000 with resonant frequencies in the range of 20 kHz for the 4-node wineglass mode. The fabrication process enables production of almost perfect hemispheres (less than 1% asphericity near the pedestal) with an average surface roughness of 5nm. The high degree of sphericity contains the relative frequency mismatch Δf/f between the two degenerate modes to 0.02%. Simplicity of the fabrication process and the successful testing of the drive/sense mechanism in the resonator make it a good candidate for use as gyroscopes.

Fabrication and testing of hemispherical MEMS wineglass resonators

This work focuses on understanding the behavior of 3D hemispherical shells operating in wineglass resonance mode through finite element modeling (FEM). Fabrication of the hemispherical shells was done using micromachining technique. The quality factor of the device was in excess of 10,000 when operated in 50mT vacuum. The shell showed better than 95% sphericity and had an rms surface roughness of ~5nm. The separation in the degenerate frequencies of 4-node wineglass resonance was 5 Hz at a resonant frequency of 22 kHz. Modeling of the device behavior relates the frequency mismatch to the asymmetry in the shell and quantifies it.

Non-intrusive electric power sensors for smart grid

An electric power sensor that measures near-field voltage and current waveforms through the insulation layer on a power cord is presented. To measure the line current, we examined Hall, giant magneto-resistive (GMR) and inductive sensors and found that for sensing 60 Hz current through its magnetic field, the inductive probe resulted in the best performance. To measure the voltage waveform, we developed a near-field electric dipole antenna that consisted of two strips of copper approximately 3 mm long. The voltage and current sensors were then calibrated and uncertainties due to the placement of the sensors over power cords were determined. A method was developed to enable the power sensor to perform auto calibration to estimate miss-alignments between the sensors and the wires in the cord. Power measurement accuracy of better than 5% was achieved.

Microplasma field effect transistors

We report, for the first time, a new class of microplasma FET (MOPFET) devices that operate at atmospheric pressure helium. MOPFET are similar to MOSFETs but instead of using electrons and holes in semiconducting active channels, they use ions and electrons in gaseous channels to control channel conduction. Their applications include processing devices for very high temperature and high ionizing radiation environment and they may find use in engine sensors and diagnostic circuits. The plasma for our devices is supplied by an RF probe operating at frequencies ranging from 400 MHz to 700 MHz. DC and RF microplasmas are widely used in displays, medical applications, optical spectroscopic analysis, nanomaterial synthesis, etc.

Investigation of contact resistance evolution of Ir, Pt, W, Ni, Cr, Ti, Cu and Al over repeated hot-contact switching for NEMS switches

This article reports on the evolution of contact resistance (Rc) of metal contacts over 100,000 cycles. A contact-mode atomic force microscope connected to a current versus voltage (I-V) measurement system was used and successive I-V measurements between a Cr-coated AFM conducting tip and Ir, Pt, W, Ni, Cr, Ti, Cu or Al thin-film metals on silicon nitride coated silicon in a nitrogen ambient were carried out. Adhesion forces between the samples and the conducting AFM tip was also measured. The best cyclic I-V performers were Ir, Pt, W & Ti. The trend in changing Rc seen in Ir, Pt and W are similar and can be attributed to factors such as their high Youngs modulus, high melting temperatures and high density and low adhesion forces. Ti yeilded the best I-V behavior where contact resistance improved slightly as a function of cycling. A relationship between adhesion forces and defect generation in the contact region is observed and the results of both sets of experiments are detailed here.

Fabrication and characterization of 3D micro-plasma field effect transistors

We report the design, fabrication and electrical characterization of novel three dimensional Micro-plasma Field Effect Transistor (MOPFET) devices that operate inside atmospheric RF helium plasma. Current versus voltage characterization of MOPFETs are demonstrated in this paper. High transconductance and reliable gate control over Ids can be obtained from coplanar and 3D MOPFETs. COMSOL simulation is used here to demonstrate the operation principles of MOPFETs. For micro-plasma FETs electrons and ions act as charge carriers to conduct current, instead of electrons and holes, as in, traditional semiconductor FETs. Therefore, MOPFETs have advantages over semiconductor FETs in extreme conditions, such as at high temperatures and in high ionizing radiation such as applications in space exploration and in distressed nuclear reactors.

Fabrication of localized plasma gold-tip nanoprobes with integrated microchannels for direct-write nanomanufacturing

We present the microfabrication and characterization of an AFM-tip like device with integrated gas delivery microchannel for the generation of localized microplasmas. The device plasma is generated within a submicron region around its tip for direct-write micro and nanofabrication. The device is fabricated by forming a tall, sharp micromolded gold tip in a KOH etched inverted pyramid followed by thermo-compression bonding and consecutive tip transfer, microfluidic channel patterning and formation of supporting cantilever beam. The tall tip overcomes the height problems of previous designs. Preliminary experiments have been carried out demonstrating the generation of localized microplasma at atmospheric conditions with 1,000V AC stimulation. By mounting the device to a commercialized AFM station and operated in tapping mode, imaging with the same device has also been demonstrated.

High enhancement SERS substrates created using DEP-DLA & annealing Au-W

Techniques of preparing reliable surface enhanced Raman spectroscopy (SERS) substrates with high enhancement factors (EF) in the range of ×106−109 using dielectrophoresis (DEP) of Au nanoparticles is presented. Diffusion limited aggregation (DLA) of Au NPs, here, gives rise to multi-branched fractal geometry that also incorporates the analyte and results in a large density of hot spots. The second substrate was prepared by annealing a thin film of Au on W to form nanometric structures with Au separated from W by a thin (∼40nm) tungsten oxide layer. The benefit of using DEP-DLA is that it enables controlled assembly of NPs in real time. SERS of thymine and intensive surface studies using AFM and SEM were carried out.