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Retinal prostheses are biomedical devices that directly utilize electrical stimulation to create an artificial vision to help patients with retinal diseases such as retinitis pigmentosa. A major challenge in the microelectrode array (MEA) design for retinal prosthesis is to have a close topographical fit on the retinal surface. The local retinal topography can cause the electrodes in certain areas to have gaps up to several hundred micrometers from the retinal surface, resulting in impaired, or totally lost electrode functions in specific areas of the MEA. In this manuscript, an MEA with dynamically controlled electrode positions was proposed to reduce the electrode-retina distance and eliminate areas with poor contact after implantation. The MEA prototype had a polydimethylsiloxane and polyimide hybrid flexible substrate with gold interconnect lines and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate electrodes. Ring shaped counter electrodes were placed around the main electrodes to measure the distance between the electrode and the model retinal surface in real time. The results showed that this MEA design could reduce electrode-retina distance up to $100~\mu \text{m}$ with 200 kPa pressure. Meanwhile, the impedance between the main and counter electrodes increased with smaller electrode-model retinal surface distance. Thus, the change of electrode-counter electrode impedance could be used to measure the separation gap and to confirm successful electrode contact without the need of optical coherence tomography scan. The amplitude of the stimulation signal on the model retinal surface with originally poor contact could be significantly improved after pressure was applied to reduce the gap.

A low-profile and high-gain Gaussian beam antenna (GBA) operating at 1 THz is demonstrated for the first time. Imprint and dry etching technologies in silicon are employed. A complementary antenna feed based on the magnetoelectric dipole is proposed for enhancing the radiation characteristics of the antenna. The microfabrication technologies are compatible with the Si-based integrated circuit manufacturing process. The terahertz (THz) antenna is realized with over 20 dBi in antenna gain. With high-precision fabrication technologies, a highly efficient THz GBA with smooth morphology and much lower profile than conventional horn and lens antennas is developed. Moreover, the antenna has the characteristic of low sidelobe levels which is advantageous in many wireless applications.

We investigated a dielectric transmission line consisting of three dielectric substrate layers for millimeter-wave (mmW) applications. This line is referred to as a narrowed dielectric microstrip line (N-DML). The effective dielectric constant method is used to analyze the propagation characteristics of the N-DML. The result of the analysis of the normalized phase constant is consistent with the simulation. For demonstration, two J -band N-DML prototypes of different lengths with transitions to rectangular waveguides were designed, fabricated, and measured. Good agreement between the simulated and measured results is observed. In particular, due to its nonmetal structure, the N-DML shows a low average attenuation constant of only 0.52 dB/cm over 220-280 GHz in the experiment. Due to its good performance, ease of fabrication, low loss, and low cost, the N-DML is a suitable candidate for various mmW applications.

A wideband THz H-plane dielectric horn antenna based on silicon (Si) technology is proposed in this paper. The antenna can be integrated with the planar structure circuit and the dielectric ridge waveguide. To fabricate the proposed antenna, the deep reactive ion etching high-resistivity Si fabrication process is used. The size of the proposed antenna is $3.13 \times 4 \times 0.1$ mm3. The operating frequency of the antenna ranges from 750 to 1000 GHz, which corresponds to a fractional impedance bandwidth of 28.6%. The antenna has a narrow beamwidth in the H-plane and a high gain. To test this antenna, the characterization of the metal waveguide diagonal horn for measurement is analyzed. Then the non-contact measurement method is applied to measure the designed dielectric horn. The simulated radiation efficiency of the antenna is higher than 80% while the measured gain of the antenna is larger than 8 dBi. Measured H-plane radiation patterns from the proposed antenna are presented and show reasonable agreement with the simulated results.

A low-loss dielectric microstrip line (DML) integrated circuit based on silicon (Si) technology is proposed for THz applications in this paper. Using the DML, a coupler and a crossover are designed. In the proposed technology, all THz passive components are made of high-resistivity silicon on insulator (SOI) wafer. To fabricate the proposed transmission line and its components, we developed a high-precision fabrication process for the SOI wafer. A non-contact measurement technology is used to test the fabricated samples. The measured loss per wavelength of DML ranges from 0.0082 to 0.042 dB/λ over 750–925 GHz. The measured isolation of the crossover is 25.45 ± 5.54 dB, and the measured coupler factor of the coupler is –13.22 ± 3.23 dB.

The design, fabrication, and characterization of a multiple-stage Si microfabricated preconcentrator-focuser (/spl mu/PCF) for a micro gas chromatography (/spl mu/GC) system that can provide real-time quantification and identification of complex organic vapor mixtures are presented. The /spl mu/PCF consists of a Si microheater loaded with Carbopack B, Carbopack X, and Carboxen 1000 carbon adsorbent granules, and a Si micromachined cover plate. Deep reactive ion etching is utilized to produce mechanically robust fluidic interconnection adapters hermetically sealed to fused silica capillary tubing for connection to the other components in the /spl mu/GC. This three-stage device is designed to capture compounds spanning up to 4 orders of magnitude in volatility. The dead volume, thermal mass, heating efficiency, and pressure drop of the three-stage /spl mu/PCF are improved significantly over its single-stage /spl mu/PCF predecessor. We demonstrate the successful capture, desorption, and high-resolution chromatographic separation of a mixture of 30 common organic vapors using our three-stage /spl mu/PCF in a conventional GC system. The peak width at half height is

A vertical actuator fabricated using a trench-refilled-with-polysilicon (TRiPs) process technology and employing an array of vertical oriented comb electrodes is presented. This actuator structure provides a linear drive to deflection characteristic and a large throw capability which are key features in many sensors, actuators and micromechanisms. The actuation principle and relevant theory is developed, including FastCap simulations for theoretical verification. Design simplifications have been suggested that enable one to use parallel plate analytical expressions which match simulation results with /spl sim/5.6% error. Several actuators were designed and fabricated using the 7-mask TRiPs technology with calculated drive voltages as low as 45 V producing 10 /spl mu/m of deflection. The actuators employed a mechanical structure that was 18 /spl mu/m tall using a polysilicon layer 1.5 /spl mu/m thick and occupying a total area of 750 /spl mu/m by 750 /spl mu/m. The actuators were successfully tested electrostatically and several microns of deflection were observed.

The design, fabrication, and testing of a preconcentrator-focuser (PCF), consisting of a thick micromachined Si heater packed with a small quantity of a granular adsorbent material are described. The PCF is developed to capture and concentrate vapors for subsequent focused thermal desorption and analysis in a micro gas chromatograph. The microheater contains an array of high-aspect-ratio, etched-Si heating elements, 520 /spl mu/m (h)/spl times/50 /spl mu/m (w)/spl times/3000 /spl mu/m (l), bounded by an annulus of Si and thermally isolated from the remaining substrate by an air gap. This structure is sandwiched between Pyrex glass plates with inlet/outlet ports that accept capillary tubes for sample flow and is sealed by anodic bonding (bottom) and rapidly annealed glass/metal/Si solder bonding (top). The large microheater surface area allows for high adsorption capacity and efficient, uniform thermal desorption of vapors captured on the adsorbent within the structure. The adsorbent consists of roughly spherical granules, /spl sim/200 /spl mu/m in diameter, of a high-surface-area, graphitized carbon. Key design considerations, fabrication technologies, and results of performance tests are presented with an emphasis on the thermal desorption characteristics of several representative volatile organic compounds as a function of volumetric flow rates and heating rates. Preconcentration factors as high as 5600 and desorbed peak widths as narrow as 0.8 s are achieved from 0.25-L samples of benzene at modest heating rates. The effects of operating variables on sensitivity, chromatographic resolution, and detection limits are assessed. Testing of this PCF with a micromachined separation column and integrated sensor array is discussed briefly.

A lateral accelerometer has been designed, simulated, and fabricated using a 3-mask high-aspect ratio technology. Electron beam lithography and high-density plasma etching in an inductively coupled plasma source enabled aspect ratios >30 to be achieved. This makes possible beams with very small spring constants. Combining the ability to measure very small displacement of a proof mass due to narrow capacitive gaps between comb fingers, a highly sensitive accelerometer can be obtained. The fabricated accelerometer with 1 /spl mu/m beams and 0.2 /spl mu/m comb gaps had a spring constant of 0.127 N/m, which is close to the calculated values of 0.146 N/m. Based on the capacitance measurements, the accelerometer sensitivity is calculated to be 6.3 fF/g. Reducing the beam width to 0.4 /spl mu/m lowered the spring constant to 0.03 N/m, and an improved equivalent sensitivity of 79.2 fF/g is calculated. The minimum detectable acceleration is on the order of a few microgravity over a range of hundreds of gravities.

A microcavity surface-emitting coherent electroluminescent device operating at room temperature under pulsed current injection is described. The microcavity is formed by a single defect in the center of a 2-D photonic crystal consisting of a GaAs-based heterostructure. The gain region consists of two 70-/spl Aring/ compressively strained In/sub 0.15/Ga/sub 0.85/As quantum wells, which exhibit a spontaneous emission peak at 940 nm. The maximum measured output power from a single device is 14.4 /spl mu/W. The near-field image of the output resembles the calculated TE mode distribution in a single defect microcavity. The measured far-field pattern indicates the predicted directionality of a microcavity light source. The light-current characteristics of the device exhibit a gradual turn-on, or a soft threshold, typical of single- or few-mode microcavity devices. Analysis of the characteristics with the carrier and photon rate equations yields a spontaneous emission factor /spl beta//spl ap/0.06.

A novel self-aligned process was developed to fabricate gated Si field emission devices. At a gate voltage of 100 V, the emission current from an array of 100 tips increased from 283 to 460 /spl mu/A and the turn-on voltage decreased from 31 to 21 V after H/sub 2/ plasma passivation using an inductively coupled plasma (ICP) source for 2 min. The improvements correspond to a 1.28-eV reduction in the effective work function of the emitters and the instability of the emission current decreased from /spl plusmn/1,25 to /spl plusmn/0.25% after H/sub 2/ plasma passivation. Emitter tips were also coated with Mo silicide and HfC. The emission current increased from 230 /spl mu/A for uncoated emitters to 268 /spl mu/A for emitters coated with Mo silicide and 389 /spl mu/A for emitters coated with HfC. The turn-on voltage decreased from 50 to 41 and 25 V while the breakdown voltage increased from 126 to 129 and 143 V when Mo silicide and HfC were used for coating, respectively, which correspond to reductions of 0.95 and 2.23 eV, respectively, in the effective work function of the emitters. Single emitter tips have similar emission characteristics as high-density field emitter arrays, indicating excellent emission uniformity from the arrays.

A simple process has been developed which combines thick single-crystal Si micromechanical devices with a bipolar complimentary metal-oxide-semiconductor (BiCMOS) integrated circuit process. This merged process allows the integration of Si mechanical resonators as thick as 11 /spl mu/m with any integrated circuit process with the addition of only a single masking step. The process does not require the use of Si on insulator wafers or any type of wafer bonding. The Si resonators were etched in an inductively coupled plasma source which allowed deep trenches to be fabricated with high aspect ratios and smooth sidewall surfaces. Clamped-clamped beam Si resonators that were 500 /spl mu/m long, 5 /spl mu/m wide, and 11 /spl mu/m thick have been fabricated and tested. A typical resonator had a resonance frequency of 28.9 kHz and a maximum amplitude of vibration at resonance of 4.6 /spl mu/m in air. The average measured resonance frequency across a 4-in-diameter Si wafer was within 0.5% of that predicted by theory. Working NMOS transistors were fabricated and tested on the same chip as the resonator with measured threshold voltages of 0.6 V and an output conductance of 2.0/spl times/10/sup -5/ /spl Omega//sup -1/ for a gate voltage of 4 V.

A frontside-release etch-diffusion process has been developed to create released single-crystalline Si microstructures without the need for wafer bonding. This frontside-release process is simple and requires only a single mask. A deep dry etch in an electron cyclotron resonance source is used to define the structures, followed by a short boron diffusion to convert them to p/sup ++/ Si. A short etch in ethylenediamine pyrocatechol (EDP) is then used to undercut and release the structures from the frontside of the Si wafer. The structures are isolated from the substrate using a reverse-biased p/sup ++//n junction. Since the structures have a high aspect ratio, beams longer than 1 mm can be released without sticking to the substrate, and thick resonators are flat with no bending due to stresses. Resonant microstructures with thicknesses ranging from 10 to 55 /spl mu/m thick have been fabricated using this process and their resonant frequency has been measured. For typical clamped-clamped beam resonators that were 24 /spl mu/m thick, 5 /spl mu/m wide, and 400 /spl mu/m long, with 2-/spl mu/m comb gaps, a resonant frequency of 90.6 kHz and a quality factor of 362 were measured in air.

Arrays of bulk-micromachined high-aspect-ratio vertical Si mirrors were designed, fabricated, and characterized for optical switching applications. These 50-/spl mu/m-tall vertical mirrors were fabricated by the deep-etch shallow-diffusion process. The released Si mirrors have smooth sidewalls with 5-nm surface roughness. An electrostatic comb drive applied to actuate the mirrors which were supported by folded or serpentine beams. For 800-/spl mu/m-long, 3-/spl mu/m-wide, and 50-/spl mu/m-thick folded suspension beams, a lateral mirror movement of 34 /spl mu/m was achieved by a driving voltage of 30 V. Resonant frequency of 987 Hz was obtained for similar devices at atmospheric pressure. There are tradeoffs between mirror displacement and resonant frequency. The mirror displacement increases with beam length (BL) and decreases with beam width (BW). However, the resonant frequency increases with BW, but decreases with BL, which is in agreement with the theoretical calculations. These vertical Si micromirrors were coated with Au to increase the reflectivity. Magnetron sputtering of Au at low pressure was applied to achieve uniform and smooth sidewall coverage. Optical measurement on 50-/spl mu/m-tall Au-coated Si mirrors showed a reflectivity of 85% (-0.71 dB). Dynamic response of the Si micromirror switches was measured, and the switching function was successfully demonstrated at a frequency up to 1.2 KHz.

A novel etch-diffusion process is developed for fabricating high-aspect-ratio Si structures for microsensors. This is accomplished by first dry etching narrow gap Si microstructures using an electron cyclotron resonance (ECR) source, followed by a shallow B diffusion to fully convert the etched microstructures to p/sup ++/ layer. Microstructures up to 40 /spl mu/m deep with 2-/spl mu/m-wide gaps were etched with a Cl/sub 2/ plasma generated using the ECR source. Vertical profile and smooth morphology were obtained at low pressure. A shallow B diffusion at 1175/spl deg/C for 5.5 h. was then carried out to convert the 40-/spl mu/m-thick resonant elements to p/sup ++/ layer. A second dry etching step was used to remove the thin p/sup ++/ layer around the bottom of the resonant elements, followed by bonding to glass and selective wet etch. Released high-aspect-ratio Si microsensors with thicknesses of 35 /spl mu/m have been demonstrated. At atmospheric pressure, only 5 V/sub dc/ driving voltage is needed for 2.5 /spl mu/m vibration amplitude, which is less than the 10 V/sub dc/ required to drive 12-/spl mu/m-thick resonators fabricated by conventional dissolved wafer process.

A dry planarization process utilizing plasma-enhanced chemical vapor deposition (PECVD) of amorphous carbon (a-C:H) films has been developed. The degree of planarization obtained for these films is better than that of many conventional spun-on polymers. Carbon films 2.5 mu m thick have been shown to planarize 1.5- mu m-deep, 400- mu m-wide trenches to within 0.2 mu m. The deposition process can be carried out at room temperature with low ion bombardment energy (10 V) and fast deposition rate (300 nm/min). The planarization layers have been used in conjunction with both wet and dry deposited inorganic imaging layers in bilayer resist schemes to form submicrometer patterns. >