William N. Carr

A monolithic variable inductor network using microrelays with combined thermal and electrostatic actuation

An adjustable inductor which is digitally controlled by microrelays has been made using combined surface and bulk micromaching technology. The microrelays were fabricated using a TaSi2/SiO2 bimorph cantilever beam, a gold-to-gold electrical contact, aluminum as sacrificial layer and a combined thermal and electrostatic actuation mechanism. The silicon substrate under the inductor region was etched out to reduce the parasitic oxide capacitors and the eddy current power loss in the substrate semiconductor bulk. Sixteen different inductance values ranging from 2.5 nH to 324.8 nH were obtained using a planar rectangular spiral coil and four microrelays. The minimum self-resonant frequency is 1.9 GHz. The lowest measured combined thermal power and electrostatic voltage for the actuation of microrelays are 8.0 mW and 20 V, respectively. The highest operation frequency of microrelays is 10 kHz limited by the mechanical self-resonance. The measured contact resistance typically ranges from 0.6 ohms to 0.8 ohms. The dimensions of the chip measure 3150×930 µm2.

A micro variable inductor chip using MEMS relays

An adjustable inductor which is digitally controlled by microrelays has been made using combined surface and bulk micromachining technology. The microrelays were fabricated using a TaSi/sub 2//SiO/sub 2/ bimorph cantilever beam, a gold-to-gold electrical contact, aluminum as sacrificial layer, and a combined electrostatic and thermal actuation mechanism. The silicon substrate underneath the inductor region was etched out to reduce the substrate eddy current loss. Sixteen different inductance values ranging from 2.5 nH to 324.8 nH were obtained using four microrelays. The minimum self-resonant frequency is 1.9 GHz. The lowest measured thermal power and electrostatic voltage for the combined actuation of microrelays are 8.0 mW and 20 V, respectively. The measured contact resistance is typically 0.6 to 0.8 ohms.

Characterization of highly doped n- and p-type 6H-SiC piezoresistors

Highly doped (/spl sim/2/spl times/10/sup 19/ cm/sup -3/) n- and p-type 6H-SiC strain sensing mesa resistors configured in Wheatstone bridge integrated beam transducers were investigated to characterize the piezoresistive and electrical properties. Longitudinal and transverse gauge factors, temperature dependence of resistance, gauge factor (GF), and bridge output voltage were evaluated. For the n-type net doping level of 2/spl times/10/sup 19/ cm/sup -3/ the bridge gauge factor was found to be 15 at room temperature and 8 at 250/spl deg/C. For this doping level, a TCR of -0.24%//spl deg/C and -0.74%//spl deg/C at 100/spl deg/C was obtained for the n- and p-type, respectively. At 250/spl deg/C, the TCR was -0.14%//spl deg/C and -0.34%//spl deg/C, respectively. In both types, for the given doping level, impurity scattering is implied to be the dominant scattering mechanism. The results from this investigation further strengthen the viability of 6H-SiC as a piezoresistive pressure sensor for high-temperature applications.

A surface micromachined latching accelerometer

This paper presents a surface micromachined mechanically-latching accelerometer that is used as a peak reading shock recorder covering the range from tens to thousands of G. This new type of accelerometer consists of a seismic-mass-loaded cantilever beam and an array of circularly distributed notches. The cantilever is latched with increasing acceleration into successive notches to provide a stored measure of peak acceleration. The threshold acceleration can then be read out by electrical or optical techniques. A closed form analytical solution of the acceleration sensitivity has been derived and compared with the experimental data. The model is confirmed experimentally for several acceleration levels. The structures are compatible with standard CMOS processing.

Design, fabrication and testing of a micromachined thermo-optical light modulator based on a vanadium dioxide array

In this paper, a thermal-optical light modulator was developed based on a surface micromachined vanadium dioxide (VO2) array. VO2 thin film undergoes a reversible semiconductor-to-metal phase transition at about 68 °C, which is accompanied by an abrupt transformation from a transparent semiconductor phase at low temperature to a reflective metallic state at high temperature. To exploit this phase-transition related thermo-optical switching, low-thermal-mass pixels with long and thin supporting legs were surface micromachined above a glass substrate to achieve good thermal isolation between pixels and ensure fast enough switching speed. The pixel design was optimized by thermal and optical simulations. Active VO2 thin film was fabricated by e-beam evaporation of vanadium metal film followed by oxidation. The deposited VO2 film exhibits a grain structure and undergoes a phase transition at 65 °C with about 15 °C hysteresis. A surface micromachining process was developed to realize a light modulator with 64 × 64 pixels. The light switching and modulation ability of the VO2 array was experimentally tested and demonstrated. Further study shows that the surface micromachining process has no degrading effect on the optical property of VO2 film.

Lateral vacuum microelectronic logic gate design

A detailed calculation of static electric fields and electron trajectories is used for designs of vacuum microelectronic NOR and NAND logic gates based on a wedge-shaped field emission cathode. A basic pentode device that permits a compact physical layout has lateral electron trajectories and a field emission wedge-shaped cathode. Electric fields at the cathode surface are calculated using a special zooming technique based on a 1600-point spatial matrix. Cathode current is calculated using the Fowler-Nordheim equation form but with constants adjusted to match available experimental data. The gates modeled are direct-coupled-vacuum-microelectronic logic (DCVML) capable of large fan-in and fan-out. The basic pentode device modeled with a 1 mu m cathode-to-extraction grid spacing has total layout dimension of 17 mu m. The pentode device geometry greatly reduces a major problem of high grid current by making use of two deflector electrodes. The deflectors also provide trajectory confinement to reduce the device size.

Transformation of hydrogen trapped onto microbubbles into H platelet layer in SI

Features of a process of delamination of a crystalline silicon layer from a silicon wafer along a hydrogen platelet layer formed by r.f. plasma hydrogenation are described. The process involves first making a buried layer of nuclei for hydrogen platelets. Ion implantation of inert or low-soluble gases is used to form the layer. The nuclei are microbubbles that appear along the Rp plane of implanted ions. Results for argon are presented. Wafers implanted with a dose of 1015 cm−2 are then hydrogenated with an r.f. plasma. During hydrogenation, atomic hydrogen diffuses into the silicon wafer and collects onto internal surfaces of the microbubbles. Then the hydrogen increases the internal surface of the microbubbles by growing platelet-type extensions to the microbubbles. The extensions grow preferentially along the buried-layer plane. A silicon layer above the layer of grown platelets was delaminated through a pre-bonding/cut/post-bonding sequence as in a standard layer-transfer process. The plasma hydrogenation of the trap layer may be used as a step in a process of fabricating of SOI wafers with a very thin top crystalline silicon layer. Also, implant doses needed to form the microbubble trap layer are much lower than doses of direct implantation of hydrogen in the layer-transfer process.

Electrical characterization of annealed Ti/TiN/Pt contacts on N-type 6H-SiC epilayer

We report results of the electrical characteristics of in vacuo deposited Ti/TiN/Pt contact metallization on n-type 6H-SiC epilayer as function of impurity concentration in the range of 3.3/spl times/10/sup 17/ cm/sup -3/ to 1.9/spl times/10/sup 19/ cm/sup -3/. The as-deposited contacts are rectifying, except for the highly doped sample. Only the lesser doped remains rectifying after samples are annealed at 1000/spl deg/C between 0.5 and 1 min in argon. Bulk contact resistance ranging from factors of 10/sup -5/ to 10/sup -4/ /spl Omega/-cm/sup 2/ and Schottky barrier height in the range of 0.54-0.84 eV are obtained. Adhesion problems associated with metal deposition on pre-processed titanium is not observed, leading to excellent mechanical stability. Auger electron spectroscopy (AES) reveals the out diffusion of Ti-Si and agglomeration of Ti-C species at the epilayer surface. The contact resistance remains appreciably stable after treatment in air at 650/spl deg/C for 65 h. The drop in SBH and the resulting stable contact resistance is proposed to be associated with the thermal activation of TiC diffusion barrier layer on the 6H-SiC epilayer during annealing.

A new isolation method for single crystal silicon MEMS and its application to z-axis microgyroscope

This paper presents a new method for electrically isolating the released high-aspect ratio single crystal silicon MEMS structures. In this method, horizontal dielectric layers are implanted at arbitrary depths in any desired region of a wafer, using the Sacrificial Bulk Micromachining (SBM) process. A z-axis microgyroscope is fabricated by the proposed method. The measured noise-equivalent angular rate resolution is 0.0074/spl deg//sec, the input range is larger than /spl plusmn/ 50/spl deg//sec, and the measured bandwidth is 7.3 Hz. The proposed method achieves electrical isolation with excellent mechanical stability, and is free from the footing phenomenon.

The first sub-deg/hr bias stability, silicon-microfabricated gyroscope

The first sub-deg/hr bias stability gyroscope is fabricated in single crystal silicon using the SBM (sacrificial bulk micromachining) process. The quadrature error is a major concern in MEMS gyroscopes for high performance. To minimize the quadrature error, the fabricated gyroscope has a very flat bottom surface, which gives a highly symmetrical proof mass, and springs, which, in turn, provide high performance levels with significantly reduced quadrature error. The fabricated gyroscope has a bandwidth of 58 Hz, and 4-hr bias stability of 0.3 deg/hr.