Dong-il Dan Cho

Surface/bulk micromachined single-crystalline-silicon micro-gyroscope

A single-crystalline-silicon micro-gyroscope is fabricated in a single wafer using the recently developed surface/bulk micromachining (SBM) process. The SBM technology combined with deep silicon reactive ion etching allows fabricating accurately defined single-crystalline-silicon high-aspect-ratio structures with large sacrificial gaps, in a single wafer. The structural thickness of the fabricated micro-gyroscope is 40 /spl mu/m, and the sacrificial gap is 50 /spl mu/m. For electrostatic actuation and capacitive sensing of the developed gyroscope, a new isolation method which uses sandwiched oxide, polysilicon, and metal films, is developed in this paper. This triple-layer isolation method utilizes the excellent step coverage of low-pressure chemical vapor deposition polysilicon films, and thus, this new isolation method is well suited for high-aspect-ratio structures. The thickness of the additional films allows controlling and fine tuning the stiffness properties of underetched beams, as well as the capacitance between electrodes. The noise-equivalent angular-rate resolution of the SBM-fabricated gyroscope is 0.01/spl deg//s, and the bandwidth is 16.2 Hz. The output is linear to within 8% for a /spl plusmn/20/spl deg//s range. Work is currently underway to improve these performance specifications.

Why is (111) Silicon a Better Mechanical Material for MEMS

In this paper, we explain the mechanical properties of single-crystalline silicon with respect to deflectional and torsional motions. Young’s modulus, Poisson’s ratio, and shear modulus are isotropic on silicon (111), whereas the variations on silicon (100) and (110) are quite significant. We newly derive formulae for bulk shear modulus of silicon (100), (110) and (111) and show that bulk shear modulus differs from conventional shear modulus because of the anisotropic characteristics of single-crystal silicon. Furthermore, we show that the bulk shear modulus (which governs torsional motion) varies minimally on silicon (111), with respect to crystallographic directions, as compared to silicon (100) and (110).

A rotational micro biopsy device for the capsule endoscope

Recently the technology of capsule endoscopy has developed dramatically and many researchers are making efforts to combine surgical function into the capsule type endoscope. In this paper, the micro biopsy module which is a part of the capsule endoscope is proposed. The proposed module is less than 2 mm in thickness and has a diameter of 10 mm. It consists of a trigger with paraffin block, rotational tissue-cutting razor with a torsion spring and controller. This module makes it possible for the capsule endoscope to obtain a sample tissue inside the small intestine which can not be reached by a conventional biopsy device. Through dedicated experiments, tissue samples were successfully extracted using the proposed biopsy module and the cells in samples were extracted and tested by a microscope.

Roughened polysilicon for low impedance microelectrodes in neural probes

A polysilicon roughening process is developed to reduce the interface impedance of microelectrodes of neural chips. In developing micromachined neural interface systems, one of the basic requirements is to reduce the interface impedance of microelectrodes, because the neuronal signals generally have a very small amplitude and the increased impedance can cause the charge transfer capability of microelectrodes to decrease. The developed process involves forming metal microelectrodes on top of a low pressure chemical vapor deposition (LPCVD) polysilicon film, which is deposited on top of a heavy-phosphorous-content phosphosilicate glass film. The phosphorous inhibits LPCVD polysilicon nucleation and results in very large grains, and hence, very rough film surfaces. This process significantly increases the effective surface area, and the interface impedance can be significantly reduced without increasing the physical size of microelectrodes. By using this process, the interface impedance is significantly lowered. The impedances of conventional gold microelectrodes and the microelectrodes developed in this paper are measured and compared by using a scanning electron microscope, an atomic force microscopy and an impedance spectroscopy system. Experimental results show approximately 50 times lower interface impedance for the developed method.

A novel microactuator for microbiopsy in capsular endoscopes

This paper presents a LiGA (a German acronym for lithographie, galvanoformung, abformung) process based microactuator to be used for microbiopsy in capsular endoscopes. This microactuator is designed to be integrated into a capsular endoscope and to extract tissue samples inside the small gastrointestine which a conventional endoscope cannot reach. The proposed microactuator was fabricated as a cylindrical shape of diameter 10 mm and length 1.8 mm. This actuator consists of three parts: a microbiopsy part with a microspike, an actuating part with a torsion spring and a triggering part with a shape memory alloy (SMA) heating wire and polymer string. In order to extract sample tissue, a microspike in the developed actuator moves forward and backward using the slider-crank mechanism. For low power consumption triggering, a polymer-melting scheme was applied. The SMA heating wire consumed approximately 1.5 V × 160 mA × 1 second (66.67 µWH) for triggering. The precise components of the microactuator were fabricated using the LiGA process in order to overcome the limitations in accuracy of conventional precision machining. The developed microactuator was evaluated by extracting tissue samples from the small intestine of a pig ex vivo, and examining the tissue with hematoxylin and eosin (H&E) staining protocol. The experimental tests demonstrated that the developed microactuator with microspike successfully extracted tissue samples from the pigs small intestines.

A novel 3D process for single-crystal silicon micro-probe structures

A new fabrication method for a three-dimensional (3D), single-crystal silicon micro-probe structure is developed. A probe card structure requires tips that are at least 50 μm tall on cantilevers thick enough to withstand a few mN of force as well as 50 μm of tip bending. The cantilever structure also must be able to move at least 50 μm of vertical motion, requiring a large sacrificial gap. The developed 3D fabrication method is based on the surface/bulk micromachining technology, which can fabricate released, high aspect ratio, single-crystal silicon microstructures with high yield using (111) silicon.

A Non-Contact-Type RF MEMS Switch for 24-GHz Radar Applications

This paper presents the design, fabrication, and measurement results for a novel non-contact-type radio-frequency (RF) microelectromechanical systems switch for 24-GHz radar applications. The proposed switches are free from unavoidable microwelding and stiction problems in other contact types, which in turn guarantee high reliability and long lifetime. The developed switch is a capacitive shunt type using a variation of the capacitance between the signal line and ground lines. The capacitance is precisely regulated by comb-drive actuators. This concept is simple, but the design requires a large and precise mechanical motion. In addition, for a low-loss switch structure, an air bridge with a large air gap is required. Therefore, the selective silicon-on-insulator process, based on the sacrificial bulk micromachining process, is designed for this switch fabrication. First, large insulating supports are fabricated 60 ¿m below the wafer surface, and then, the RF switch is fabricated on these insulating supports. The measured actuation voltage is 25 V, and the actuation stroke is 25 ¿m. The switching times are 8 ms in the off to the on state and 200 ¿s in the on to the off state. In the RF characteristic measurements, the insertion loss without the long coplanar waveguide line loss is 0.29 dB and the isolation is 30.1 dB at 24 GHz. The bandwidth is relatively narrow, and the isolation is 25 dB or better in the range of 23.5-29 GHz. The reliability test for the switch was performed 109 times with 18-mW RF power. We observed no mechanical failure or RF performance degradation. A power handling capacity of 0.9 W with a hot-switch condition was achieved.

Barbed micro-spikes for micro-scale biopsy

Single-crystal silicon planar micro-spikes with protruding barbs are developed for micro-scale biopsy and the feasibility of using the micro-spike as a micro-scale biopsy tool is evaluated for the first time. The fabrication process utilizes a deep silicon etch to define the micro-spike outline, resulting in protruding barbs of various shapes. Shanks of the fabricated micro-spikes are 3 mm long, 100 ?m thick and 250 ?m wide. Barbs protruding from micro-spike shanks facilitate the biopsy procedure by tearing off and retaining samples from target tissues. Micro-spikes with barbs successfully extracted tissue samples from the small intestines of the anesthetized pig, whereas micro-spikes without barbs failed to obtain a biopsy sample. Parylene coating can be applied to improve the biocompatibility of the micro-spike without deteriorating the biopsy function of the micro-spike. In addition, to show that the biopsy with the micro-spike can be applied to tissue analysis, samples obtained by micro-spikes were examined using immunofluorescent staining. Nuclei and F-actin of cells which are extracted by the micro-spike from a transwell were clearly visualized by immunofluorescent staining.

Surface/bulk micromachining (SBM) process and deep trench oxide isolation method for MEMS

This paper presents a new method for micromachining released structures with single crystal silicon, as well as a new method for electrically isolating the released structures with a deep trench oxide. The developed surface/bulk micromachining (SBM) process utilizes (111) silicon wafers. The structural patterns are defined using a reactive ion etcher. Then, the patterns as well as sidewalls are passivated with an oxide film, and bare silicon is exposed at desired areas. The exposed bare silicon is further reactive ion etched, which defines sacrificial gap dimensions. By undercutting the exposed bulk silicon sidewalls in an aqueous alkaline etchant, cleanly released microstructures can be fabricated. For sensing or electrostatic actuation of released microstructures, a deep trench oxide isolation method is developed and applied successfully to actuate a comb-drive actuator. The developed SBM process with deep trench oxide isolation allows fabricating single crystal silicon MEMS with a low parasitic capacitance.

In Vitro Biocompatibility of Various Polymer-Based Microelectrode Arrays for Retinal Prosthesis

PURPOSE The purpose of our study is to evaluate the biocompatibility of various polymers used as microelectrode arrays (MEAs) in retinal prostheses through in vitro cytotoxicity testing following a standardized METHOD METHODS Three types of polymer-based MEAs were examined: silicone-based platinum, polyimide-based gold and liquid crystal polymer (LCP)-based gold MEAs. The silicone/platinum MEAs were fabricated by a Nd:YAG laser, polyimide/gold MEAs by a semiconductor manufacturing technique, and LCP/gold MEAs by laser micromachining and thermal-bonding process. All experimental procedures followed the International Organization for Standardization (ISO) 10993-5. To obtain the extracts of specimens, 4 g of each type of MEA were eluted by culture media, MEM, for 24 hours. Then, several diluents of extracts, including the original extracts, were applied to a cultured-cell monolayer, L929 fibroblasts. The morphologic changes of cells were analyzed by microscope after 24 and 48 hours of incubation. The quantitative evaluations of cell viability were performed by MTT assay after 24 hours of incubation. RESULTS The microscopic evaluations revealed that extracts from polymer-based MEAs did not induce morphologic changes or reduction of cells compared with control irrespective of concentrations of extracts. The MTT assay showed high viability values of approximately 80 to 130% regardless of diluted ratio of extracts from polymer-based MEAs. None of the polymers demonstrated a significant reduction of cell viability when compared with control. CONCLUSIONS All types of polymer-based MEAs, including silicone/platinum, polyimide/gold, and LCP/gold MEAs, meet the criteria of biocompatibility guided by international standards, ISO 10993-5.