Suresh Santhanam

Laminated high-aspect-ratio microstructures in a conventional CMOS process

Abstract Electrostatically actuated microstructures with high-aspect-ratio laminated-beam suspensions have been fabricated using a 0.8 μm three-metal CMOS process followed by a sequence of three maskless dry-etching steps. Laminated structures are etched of the CMOS silicon oxide, silicon nitride, and aluminum layers. The key to the process is the use of the CMOS metallization as an etch-resistant mask to define the microstructures. A minimum beam width of 1.2 μm, gap of 1.2 μm, and maximum beam thickness of 4.8 μm are obtained. These structural features will scale in size as the CMOS technology improves. The laminated material has an effective Youngs modulus of 61 GPa, an effective residual stress of 69 MPa, and a residual strain gradient of 2 × 10 −4 μm −1 . Multi-conductor electrostatic micromechanisms, such as self-actuating springs, x−y microstages, and nested comb-drive lateral resonators, are successfully produced. A self-actuating spring is a lateral electrostatic microactuator without a stator that is insensitive to out-of-plane curl. A spring 107 μm wide by 109 μm long excited by an 11 V a.c. signal has a measured resonance amplitude of 1 μm at 14.9 kHz. Finite-element simulation using the extracted value of Youngs modulus predicts the resonance frequencies of the springs to within 7% of the measured values.

Atmospheric transport and deposition of trace elements onto the Greenland Ice Sheet

Airborne particles of diameter > 0.4 μm reaching Dye 3, Greenland during April–May 1983 were highly variable in size and concentration from day to day. Five-day backward air mass trajectories suggest the importance of long-range transport from more northerly latitudes on days with high concentrations; particle sizes were larger on these days. Lower concentrations and smaller particle sizes were associated with transport from the south. It is inferred that Dye 3 may receive material emitted from Eurasian sources and transported over the Pole, similar to inferences for more northern Arctic sites. Elemental analysis of individual particles showed an abundance of crustal material, with many particles also containing sulfur. Bulk chemical analyses of airborne particles and fresh snow, collected during three snowstorms where ice nucleation dominated, provided data which were used to estimate mass-basis scavenging ratios. Average scavenging ratios were in the range ~1000–2000 for the crustal elements Al, Fe, K, Mg, Mn, and Na. Similar values were observed for Cd, Cu and NO3−. The corresponding ratios for Pb and SO42− averaged less than 200. These ratios were used with precipitation rate data to estimate wet deposition velocities in the order of ~2 cm s−1 for the first nine species, and ~0.2 cm s−1 for Pb and SO42−. Comparing fresh and older surface snow concentrations gave an average dry deposition velocity of roughly 0.2 cm s−1 for the crustal elements, with the small fraction of large particles (~5–10 μm) dominating deposition; much smaller values were associated with the remaining species. When used with other data in the literature, the results of this study suggest that total deposition velocities of Pb and SO42− may be as small as 0.05 cm s−1 in relatively dry regions of the Arctic.

Single-chip computers with microelectromechanical systems-based magnetic memory (invited)

This article describes an approach for implementing a complete computer system (CPU, RAM, I/O, and nonvolatile mass memory) on a single integrated-circuit substrate (a chip)—hence, the name “single-chip computer.” The approach presented combines advances in the field of microelectromechanical systems (MEMS) and micromagnetics with traditional low-cost very-large-scale integrated circuit style parallel lithographic manufacturing. The primary barrier to the creation of a computer on a chip is the incorporation of a high-capacity [many gigabytes (GB)] re-writable nonvolatile memory (in today’s terminology, a disk drive) into an integrated circuit (IC) manufacturing process. This article presents the following design example: a MEMS-based magnetic memory that can store over 2 GB of data in 2 cm2 of die area and whose fabrication is compatible with a standard IC manufacturing process.

Robust gold nanoparticles stabilized by trithiol for application in chemiresistive sensors

The use of gold nanoparticles coated with an organic monolayer of thiol for application in chemiresistive sensors was initiated in the late 1990s; since then, such types of sensors have been widely pursued due to their high sensitivities and reversible responses to volatile organic compounds (VOCs). However, a major issue for chemical sensors based on thiol-capped gold nanoparticles is their poor long-term stability as a result of slow degradation of the monothiol-to-gold bonds. We have devised a strategy to overcome this limitation by synthesizing a more robust system using Au nanoparticles capped by trithiol ligands. Compared to its monothiol counterpart, the new system is significantly more stable and also shows improved sensitivity towards different types of polar or non-polar VOCs. Thus, the trithiol-Au nanosensor shows great promise for use in real world applications.

Laminated, sacrificial-poly MEMS technology in standard CMOS

Abstract We present a micro-machining technology which enables MEMS fabrication on standard CMOS with very dense electrical/mechanical integration. Micro-mechanical structures are fabricated alongside circuits on standard 0.5-μm 3-metal CMOS dice using four maskless dry-etch steps. The resulting laminated beams are made of CMOS metal and dielectric layers. Multi-conductor mechanical structures can carry multiple signals for actuation and sensing. Narrow vertical and lateral gaps enable efficient XYZ electrostatic actuation and fine capacitive position sensing. Experiments show that mechanical structures can be integrated 1.5 μm away from signal-processing circuits, resulting in very low parasitics, good system performance, and small die size.

Design, fabrication, switching, and optical characteristics of new magneto‐optic spatial light modulator

A new reflected mode magneto‐optic spatial light modulator (R‐MOSLM) has been developed for miniature optical correlators and computers. A factor of 4 improvement in pixel switching sensitivity, compared to the conventional transmission mode magneto‐optic spatial light modulator, has been achieved by the use of narrower drive lines, and burying the conductor into the film. A factor of 3 higher resolution and a factor of 2 higher optical efficiency have also been achieved by the use of smaller pixels and narrower pixel gaps. The smaller pixels and improved switching sensitivity permit an order of magnitude reduction in optical path length and increase in frame rate, respectively. The progress that has been made in the design of the R‐MOSLM, issues concerning its fabrication, a comparison by finite element analysis of field modeling to experimentally determined current requirements to drive individual lines, and some optical characteristics are discussed.

Micromachined high-Q inductors in 0.18 /spl mu/m Cu interconnect low-K CMOS

Spiral inductors fabricated in a 0.18 /spl mu/m 6-level copper interconnect low-K dielectric process are described. A post-CMOS maskless micromachining process compatible to copper interconnect and low-k dielectric CMOS has been developed to create inductors suspended 100 /spl mu/m above the substrate with sidewall oxide removed. Such inductors have higher quality factors as substrate losses are eliminated by silicon removal and have higher self-resonant frequency due to removal of inter-turn dielectrics. Micromachined inductors have the potential to extend the useful operational frequency range of CMOS RF circuits. Quality factors of greater than 7 were obtained at 5.5 GHz for inductors with silicon undercut and inter-turn oxide removed, compared to a Q of 4 for inductors having only their inter-turn oxide removed but without silicon undercut.

Electrocaloric characterization of a poly(vinylidene fluoride- trifluoroethylene-chlorofluoroethylene) terpolymer by infrared imaging

The electrocaloric effect in thin films of a poly(vinylidene fluoride-trifluoroethylene chlorofluoroethylene) terpolymer (62.6/29.4/8 mol. %, 11–12 μm thick) is directly measured by infrared imaging at ambient conditions. The adiabatic temperature change is estimated to be 5.2 K for an applied electric field of 90 V/μm. The temperature change is independent of the operating frequency in the range of 0.03–0.3 Hz and is stable over a testing period of 30 min. Application of this terpolymer is promising for micro-scale refrigeration.

Active CMOS-MEMS AFM-like conductive probes for field-emission assisted nano-scale fabrication

In this paper, a method of fabricating active CMOS-MEMS AFM-like conductive probes with nanometer-sized probe tips for Tip-directed Field-emission Assisted Nanofabrication (TFAN) is reported. We envision an approach using tip-directed chemical vapor deposition (CVD) for the deposition of Si nanowires (SNWs), using field-emitted electrons to locally crack adsorbed precursors (silane/disilane) through direct momentum transfer. This approach will enable control of nanowire geometry during fabrication. The designed active probe consists of three key parts: a thermal bimorph structure actuator, a capacitive sensor and a field emission (FE) probe tip. Unlike the past Spindt process for FE tip fabrication, a single-step probe tip fabrication process on CMOS-MEMS chips is introduced via electron-beam lithography and evaporation technique using PMMA as resist. The mechanism and key growth conditions for Spindt-type tip formation are investigated.

Volatile organic compound discrimination using nanostructured polythiophene sensors

New synthesis methods have allowed us to make many semiconducting polythiophenes polymers with different side and end groups. Also, co-polymers combining a polythiophene chain attached to another polymer chain were synthesized. This design freedom brings a new dimensionality to the sensing properties of the materials. Single chip micro sensor resistor arrays, utilizing multiple polymers, were fabricated and then tested in an automated system. The sensors demonstrated ppm level sensitivity to various volatile organic compounds (VOCs) including both polar and non-polar materials. Polymers with different chemical structures show strong selectivity to different VOCs. By applying pattern recognition algorithms, the sensor response clearly discriminates between the tested VOCs allowing us to conjecture as to the role molecular modification have in determining response to specific VOCs