Site: Hitachi Center for Materials Processing Technology
Mechanical Engineering Research Laboratory
502 Kandatsu, Tsuchiura
Ibaraki 300, Japan
Date Visited: September 29, 1993
Report Author: K.D. Wise
Dr. Toshihiro Yamada Head, Center for Materials Processing
Dr. Kazuo Sato Senior Research Scientist, Center for Materials Processing Technology
Dr. Toshihiro Yamada welcomed the JTEC team to Hitachi and explained the general organization of the Center for Materials Processing Technology and its involvement in MEMS. Four operating units are involved in the center. They deal with: (1) surface modification, welding, and microbonding; (2) microassembly, ultrafine particle applications, and fine pitch bonding; (3) micromachining and precision forming; (4) and design, manufacturing, and assembly. The ultrafine particles are 0.1 mm silicon dioxide and have been applied to enhance images by creating uniform dispersion patterns. Dr. Kazuo Sato then described the work in the third unit in greater detail, and provided reprints of recent articles.
He first described his work on modeling silicon etch processes and its application to a number of new micromachined device structures (Sato, Koide, and Tanaka 1989; Koide, Sato, and Tanaka 1991). The program is currently functional in 2-D and is being extended to 3-D modeling. In order to obtain the etching coefficients as a function of temperature in all of the silicon crystal planes, hemispherical protrusions and recesses were machined in thick silicon segments and etched for various times in KOH-based etching solutions at temperatures from 40°C to 78°C. The resulting surfaces were then carefully probed to measure the resulting etch rates, which were entered in the program. The first application of this data was in a bulk micromachined accelerometer (Koide, Sato, Suzuki, and Miki 1992), which is intended for automotive systems and will be a product next year. It is a 3.2 mm x 5 mm die built on a symmetrical cantilever etched from both sides of the die. The silicon die is bonded between two glass plates to form a differential capacitor. The symmetrical design eliminates most off-axis sensitivity. The device is mounted with hybrid electronics on a ceramic board about 1 cm on a side. Electrostatic self-test is included. The device is a good example of how a detailed knowledge of bulk etching characteristics can be used with computer-assisted mask optimization to realize a complex device from a rather simple process.
A second project was an atomic force microscope tip formed by using a 200 mm long cantilever whose tip angles over a silicon recess and is realized along the side wall (Hosaka et al. 1992). The tips are sharpened to 20 nm using focused ion-beam technology, and position is read out optically. The probe is made of silicon dioxide and for magnetic imaging is coated with a thin film of cobalt. A third project involved the use of isotropic silicon etching to create acoustic lenses (Hashimoto et al. 1993) for scanning acoustic microscopes from 4 to 6 mm-thick silicon stock wafers. Cavities 80 mm in diameter were etched with a sphericity of better than 0.2 mm, and the edges of the wafer were then beveled back by grinding. The devices could image 1 mm lines and spaces at 944 MHz. A variety of devices have been realized.
The microvalve presented at Transducers '93 (Shikida et al. 1993; Sato and Shikida 1992), based on the electrostatic attraction of a thin silicon membrane held between two plates (one containing an orifice), was described. The film was permalloy, chosen because of its availability and mechanical characteristics. The current device measures 25 mm x 25 mm, but could be miniaturized considerably. It was operated at 70 V. The device is innovative and impressive in that it operates across a gap of 2.5 mm and yet seals well, apparently without sticking. It is aimed at gas flow control at low pressures as in semiconductor manufacturing. A silicone resin is used to hold and space the two wafers.
Dr. Sato then described work on systems for cell sorting and blood studies that they have developed (Sato et al. 1990; Kikuchi et al. 1992). In the former project, a matrix of 1,584 microchambers is formed in a silicon wafer. Techniques for loading each of these chambers (en-batch) with paired cells have been developed using suction applied through small holes in the back of the chambers, and the cells can then be fused using electrical pulses applied from electrodes imbedded in the chambers. The latter project (Sato and Shikida 1992) produced interesting data on fluid flow in small micromachined microchannels. For example, the flow in a V-groove approximately 8 mm wide across the top was equivalent to about 60 pl/sec/mm at a pressure of 10 cmH(¯2)O.
The Hitachi work under the national (MITI) micromachine program is aimed at a miniature hydraulic rotor pump produced using electron discharge machining. It has produced pressures to 10 atm. In the laboratory, the panel observed an interesting system for low-temperature bonding based on the use of an argon ion-beam cleaning of the surfaces followed by joining. For example, the system has been used to bond piezoelectric ceramics to FeNiCo alloys with a InPb intermediate at 120°C. The system has been used to remove surface oxides from metals, allowing metal-to-metal seals, and is now being tried with silicon-to-silicon structures. Such a system could be of great benefit in the creation of a wide variety of microstructures.
There was considerable discussion of MEMS challenges. Hitachi has not gotten into LIGA even though the company has strong interest in it. Problems cited included a lack of polymer knowledge and lack of an X-ray source (and the associated up-front investment required). Hitachi also was concerned about the patent situation in this area. It was agreed that there is a real need for batch-fabricated and batch-assembled millimechanics, with the feeling that "batch will have more impact than miniaturization." Most of the actuators discussed were electrostatically-driven; the electronics for most devices were hybrid, and it did not appear that Hitachi was strongly pursuing monolithic integration of readout circuitry directly on the transducer chips.
Hashimoto, H., S. Tanaka, K. Sato, I. Ishikawa, S. Kato, and N. Chubachi. 1993. "Chemical Isotropic Etching of Single-Crystal Silicon for Acoustic Lens of Scanning Acoustic Microscope." Japan J. Appl. Phys. 32, May: 2543-2546.
Hosaka, S., A. Kikukawa, Y. Honda, H. Koyanagi, and S. Tanaka. 1992. "Simultaneous Observation of 3-Dimensional Magnetic Stray Field and Surface Structure Using New Force Microscope." Japan J. Appl. Phys. 31, July: L904-907.
Kikuchi, Y., K. Sato, H. Ohki, and T. Kaneko. 1992. "Optically Accessible Microchannels Formed in a Single-Crystal Silicon Substrate for Studies of Blood Rheology." Microvascular Research. 44: 226-240.
Koide, A., K. Sato, and S. Tanaka. 1991. "Simulation of Two-Dimensional Etch Profile of Silicon during Orientation-Dependent Anisotropic Etching." Proc. IEEE MEMS '91. Pp. 216-220.
Koide, A., K. Sato, S. Suzuki, and M. Miki. 1992. "Multistep Anisotropic Etching Process for Producing 3-D Accelerometers." Digest 11th Sensor Symposium. Pp. 23-26.
Sato, K., A. Koide, and S. Tanaka. 1989. "Measurement of Anisotropic Etching Rate of Single-Crystal Silicon to the Complete Orientation." Digest JIEE Technical Meeting on Micromachining and Micromechatronics. IIC-89-30, pp. 9-17.
Sato, K., and M. Shikida. 1992. "Electrostatic Film Actuator with a Large Vertical Displacement." Digest IEEE MEMS '92. Pp. 1-5.
Sato, K., Y. Kawamura, S. Tanaka, K. Uchida, and H. Kohida. 1990. "Individual and Mass Operation of Biological Cells Using Micromechanical Silicon Devices." Sensors and Actuators. A21-23: 948-953.
Shikida, M., K. Sato, S. Tanaka, Y. Kawamura, and Y. Fujisaki. 1993. "Electrostatically-Actuated Gas Valve with Large Conductance." Digest Int. Conf. on Solid-State Sensors and Actuators. Pp. 94-97.