MEMS developments in China
11/01/1997
MEMS developments in China
Quanbo Zou, Huikai Xie, Changqing Zhan, Litian Liu, Institute of Microelectronics, Tsinghua University, Beijing, China
Microelectromechanical systems (MEMS) production is a fast-growing cross-disciplinary field with diverse applications in such areas as microelectronics, mechanics, automatic control, biology, medical science, and chemical engineering. In this article, developments and processing are briefly described for several typical MEMS devices: microelectrostatic motor, micropump, microphone, and micromachined gyroscope.
The rapid development of microelectronics technology has not only revolutionized computer/communication technology, but has also gradually permeated into other fields such as mechanics, biology, and robotics to create the rising high-tech field of MEMS. Using modified microelectronics technology, with single-crystal silicon as the substrate material, MEMS processing creates integrated sensors and actuators with 3-D or quasi 3-D structures. Unique fabrication steps include silicon bulk micromachining; surface micromachining; lithography, electroplating, and molding (LIGA) and electroplating; and microelectric discharge machining (EDM).
An important MEMS milestone in the early 1980s was the successful fabrication of a silicon-based, microelectrostatic motor (120-?m rotor dia., 600 rpm rotating speed) at the University of California at Berkeley [1]. Since then, MEMS have grown to be a hot subject of worldwide research. In 1989, the rotating speed of a silicon micromotor at MIT reached 10,000 rpm [2]. In 1991, researchers at the University of Wisconsin at Madison used LIGA to develop a nickel magnetic micromotor, with a maximum rotating speed of 33,000 rpm and output torque of 3 ? 10-9 N-m [3].
Japan, Germany, France, and The Netherlands have invested large amounts of manpower and funds into MEMS R&D, and have successfully developed many devices such as a microwobble motor, linear motor, micropump, microvalve, AFM probe, and microgyroscope. Recently, the creation of microcontrol systems, microgas chromatographs, and digital micromirror devices, indicates that the MEMS field has gradually advanced from device development into systems integration.
China did not begin studying MEMS until the end of the 1980s, but the government paid substantial attention to the field when development finally started. Financial support came from National Natural Science Funds as well as key investment from various ministries, and MEMS were listed in the "National Climbing" plan. The Defense Science Engineering Organization founded the MEMS Engineering Center.
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Figure 1. Wet anisotropic etches create different structures depending on the crystallographic orientation of the original single-crystal-silicon substrate: a) (100) silicon ends up with pyramidal pits; and b) (110) silicon results in perpendicular walls.
In 1992, the Institute of Microelectronics at Tsinghua University manufactured a silicon-based microelectrostatic motor [4, 5] with a 13,000-rpm rotational speed and an integrated, on-chip, speed-testing sensor. Meanwhile, studies were conducted on silicon pressure sensors, accelerometers, micropumps, microphones, and microgyros. Several groups conducted research on micropiezoelectric motors, measurement analyzers of motor-moving parameters, poly-Si turbines, and nickel microgears. Fudan University, Southeastern University, Zhejiang University, Chongqing University, and the Electric Institute of the Chinese Science Academy also achieved notable MEMS research results.
MEMS technology is advancing at an amazing speed, with a potential significance comparable to the development of the transistor. Some researchers predict that MEMS will strongly influence medicine, industry, agriculture, and biology in the next 10 years. Beginning with MEMS processing, we introduce several MEMS devices completed or being developed by us.
Micromachining technology
Although based on microelectronics technology, MEMS processing requires many special 3-D structural machining techniques in addition to regular IC (2-D, planar) technology. The efforts of many researchers for many years produced substantial achievements in 3-D fabrication technology, including the following: silicon bulk micromachining, surface micromachining, high aspect ratio reactive ion etching (RIE), and LIGA and electroplating.
Silicon bulk micromachining. This technique is based on the anisotropic etching of silicon. With the proper choice of etchants, the etching rate of the crystalline silicon substrate`s (111) surface is one or two orders of magnitude lower than that of the (100) and (110) surfaces. Regular inverse pyramids can be etched in (100) silicon wafers, and right-angled walls can be formed on (110) wafers (Fig. 1).
Complex 3-D structures are realized by integrating bulk micromachining with lithographic patterning, multilayer etch-stop masks, and sacrificial layers. Such structures are widely used in pressure sensors, silicon accelerometers, AFM probes, and microgyroscopes. For example, bulk etching can remove the majority of material from the backside of a die to leave a thin silicon membrane for pressure sensor applications.
Typical anisotropic silicon etchants include KOH/H2O, ethylene-diamine-pyrocatehol-water (C2H8N2/C6H6O2/H2O), and N2H4/H2O. KOH/H2O is often preferred because of its low temperature, simple high quality, and easily controlled process.
During silicon bulk etching, one of the greatest challenges is the precise control of any required silicon membrane thicknesses. The simplest and the most imprecise method is to stop the etch at a predetermined time. Vastly greater control can be realized with techniques such as electrochemical etching with a buried P+ etch-stop layer, and optical end-pointing (since thin silicon membranes, on the order of 10-?m thick, are partially transmissive to visible light).
Surface micromachining. Also called surface sacrificial-layer etching, this technique is based on the large differences in etch rate between different materials. After multilayer material deposition and selective patterning, the structure is exposed to an etchant and one material is removed while other layers remain.
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Figure 2. Sacrificial oxide layers are removed by wet HF dips to leave poly-Si surface-micromachined structures
The simplest surface micromachining process creates poly-Si structures using liquid HF and sacrificial SiO2 layers (Fig. 2). Since the etching rate of poly-Si and single-crystal-Si is much slower than that of SiO2 in HF, the SiO2 layer will be removed to leave the poly-Si beam and membrane behind (Fig. 2, bottom). This technique has created silicon-based, microelectrostatic motors, polysilicon sensors, resonators, single-chip microphones, and microturnover structures. Often used sacrificial materials include: LPCVD SiO2, phosphosilicate glass, Al, and polymethylmethacrylate.
High aspect ratio RIE. RIE systems produce highly directional plasmas with both chemical and physical components. Anisotropic etching is achieved when physical reactions dominate. Klaassen et al. of Stanford University presented new achievements in RIE technology at the international conference Transducers `95, in which reaction chamber temperatures as low as -100?C "froze out" the chemical reactions to obtain a 300-?m etching depth with >15:1 aspect ratio [6].
LIGA. LIGA is a 3-D micromachining technique invented by German researchers in the mid-1980s. Three basic processes are involved: deep synthesis radiation photolithography, electroplating, and module forming. The unique feature of LIGA is the method`s ability to produce >200:1 aspect ratio structures, greater than all other machining methods.
Complementary machining. Several other complementary techniques can be applied to MEMS fabrication: EDM, electroplating, chemical plating, micro-ultrasonic machining, and laser machining. All show promise in 3-D machining, thick-film deposition, the creation of complex structures, and nonsilicon material machining.
MEMS devices
Research at our institute on silicon-integrated pressure sensors and other MEMS processes started at the end of the 1980s. A MEMS R&D center is gradually being constructed at the institute, with capabilities in design, simulation, fabrication, and testing. Our group made great progress in numerous MEMS technologies, including microelectrostatic motors, microphones, micropumps, microturnover structures, microgyroscopes, and micro-accelerometers.
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Figure 3. Top-view schematic of an electrostatic MEMS motor.
Poly-Si microelectrostatic motor. The device is a poly-Si motor created with surface-micromachining techniques. The
diameter of the rotors is 100-120 ?m; poly-Si film thickness is 4 ?m; the spacing of the rotor and stator is 2.0-2.5 ?m; and the initial driving voltage is 49 V (Fig. 3). The rotating speed of the motor is 600 rpm.
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Figure 4. Process sequences required to produce an electrostatic MEMS motor.
Micromotor fabrication is relatively simple (Fig. 4). The process starts with an n-type (100) silicon wafer as the substrate. A 1-?m thick, thermally grown SiO2 and a 1.4-?m thick, low-temperature oxide (LTO) are formed, resulting in a 2.4-?m thick sacrificial layer. After the deposition of a 2.4-?m thick poly-Si film, the wafers are diffused with phosphorus and annealed. Then 100-nm thick thermal oxide (SiO2) is grown on the poly-Si film as part of the masking material of poly-Si dry etching. The poly-Si film is patterned with positive resist and etched using RIE to form the rotors and stators.
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Figure 5. SEM of an electrostatic MEMS motor capable of 600-rpm operation at 176 V.
The minimum feature size, the gap between the rotor and stator, can be very small since the rotor and stator are of the same layer. The inner edge of the rotor must be smooth after etching, otherwise excessive friction will stop the rotation.
The rest of the structure is formed in a similar manner. LPCVD and RIE are used to fabricate poly-Si components. LTO sacrificial layers are removed in wet HF. Afterwards, high-density phosphorus diffusion and high-temperature annealing reduce any residual stress in the thin films. Etching the whole wafer in 40% HF for 8 min releases the poly-Si rotor, while the stators cannot be released, since their areas are much larger than those of the rotors.
Additional treatments are necessary before testing, including H2SO4 and H2O2 treatment and a short etch in wet HF. After rinsing with cool deionized water, N2 gas dries the final structure (Fig. 5). The motor successfully rotates when it is supplied with electric power in a dry-air ambient.
When the applied voltage surpasses 49 V, some of the motors start to rotate. The maximum measured rotation speed is 600 rpm at 176 V. Occasional stops of the rotation occur; normal rotation will come again after adjusting the frequency or the amplitude of the driving signal.
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Figure 6. Functional schematic of a microgyroscope.
Microgyroscope. Gyroscopes are used in aerospace, navigation, and automotive applications for motion control. Traditional gyroscopes have the following disadvantages: large volume, difficult fabrication, high expense, and difficulty in making low-volume measurements. Using micromachining and IC fabrication, micromachined gyroscopes reduce complete system size to no more than 1 cm. MIT`s Draper Laboratory integrated microgyros and process circuits, resulting in a micro-inertia-measurement-unit measuring 2 ? 2 ? 0.5 cm and weighing 5 gm, with a shifting error <10?/hr.
With two groups of movable mass-bosses and two groups of fixed comb structures (connected to the glass or silicon substrate), the movable structures vibrate when AC voltage is applied (Fig. 6). Resonance occurs when the natural frequency of the moving structure approaches the frequency of the exciting signal. Changes in capacitance across the gap between two electrodes can be correlated to angular acceleration.
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Figure 7. Process flow for the fabrication of a microgyroscope.
We fabricated the key part of the microgyroscope - the comb structure (Fig. 7). Deep diffusion of phosphorus (10-?m junction depth) on the front side of p-type silicon wafers begins the process. Oxidation and LPCVD Si3N4 form masks for KOH etching. After KOH etching (about 400 min), aluminum is evaporated on the front side of the wafer and patterned. Patterning of the frame and the resonating structures, and etching through the silicon membrane is accomplished by RIE. The major feature of the process is the simple fabrication using pn-junction insulation technique.
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Figure 8. Cross-sectional schematic of a microfluid pump. The two cantilevers on the bottom function as valves, while motion of the top membrane displaces fluid.
Micropump with integrated driving structure [7]. The microfluid pump developed by our institute is fabricated by silicon micromachining (Fig. 8). The membrane is driven by the bimetallic effect between silicon and aluminum. The operation voltage is 5 V. With simply fabricated, small silicon cantilevers as inlet and outlet valves, the pump can work at a relatively low voltage. The whole size of the device is 6 ? 6 ? 1 mm, and the 20-?m thick membrane area is 4 ? 4 mm. The valve size is 400 ? 400 ?m, and the 1-?m thick cantilever is 1.25 ? 1 mm. The maximum flow is 44 ?liters/min and the pressure difference between inlet and outlet can be as high as 110 cm-Hg.
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Figure 9. Schematic of a micromachined condenser microphone. The corrugation in the membrane improves the device`s response.
Single-chip condenser microphone. A simple microphone can be fabricated with bulk-silicon and surface-micromachining techniques (Fig. 9). The diaphragm is a corrugated, composite film structure, while the backplate uses high-stiffness silicon. The corrugated diaphragm greatly reduces the bad effects of residual thin-film stress to increase the mechanical sensitivity of the diaphragm and improve reproducibility and stability. The composite film reduces the initial stress of the diaphragm and thus reduces the mechanical stiffness of the diaphragm. Single-chip fabrication greatly increases the production efficiency.
The basic parameters and performance of the microphone are as follows:
diaphragm size - 1 ? 1 mm,
chip size - 1.6 ? 1.6 mm,
microphone capacitance - 5 pF,
sensitivity - 8-14 mV/Pa, and
frequency bandwidth - ~10 kHz.
Micro-accelerometer. High precision micro-accelerometers have wide applications in automobile, aerospace, and other industrial fields. The piezoresistive silicon accelerometer developed by JiShou University and our institute is commercially available. The sensor relies on a novel spring-leaf structure. The fabrication is simple and the sensitivity can be 1.6 mV/g. The sensor chip size is 1.2 ? 1.2 ? 0.35 mm. The whole size of the package is 16 ? 16 ? 18 mm, with a resonance frequency of not less than 1500 Hz.
Another type of capacitance accelerometer developed by our institute and the Department of Precise Instruments and Production has a chip size of 7.8 ? 4 ? .35 mm. The capacitance of the sensor is 35 pF, and the resonance frequency is 600 Hz. This device uses 3-D chip packaging and differential capacitance output, with good overload protection ability.
Besides the above-mentioned MEMS devices, we have some research results on silicon-integrated pressure sensors, microturnover devices [8], and general micromachining technologies.
Conclusion
Many basic theoretical problems need to be solved, including those in microfluid mechanics and microfriction studies, to further MEMS development. Improvements are also needed in design and test.
Since MEMS have shown great potential in diverse applications, both the governments and the industries of many western countries have focused R&D resources on the field. In the US, there are already many companies involved in MEMS work, including IBM, Ford, IC Sensors, and Nova Sensors. In Japan, Hitachi and others have started MEMS R&D projects. In China, several MEMS projects are underway, and results have been published in international journals and conference papers. n
References
1. L.S. Fan, Y.C. Tai, R.S. Muller, "IC-processed Electrostatic Micro-Motors," Tech. Digest, p. 666-669, IEEE International Electron Devices Meeting, San Francisco, CA, Dec. 11-14, 1988.
2. M. Mehregany et al., "A Study of Three Microfabricated Variable Capacitance Motors," Sensors and Actuators, A21-23, p.173-179, 1990.
3. H. Guckel et al., "Fabrication and Testing of the Planar Magnetic Micromotor," Journal of Micromechanics and Microengineering, p. 135-138, Sept. 1991.
4. X. Sun, Z. Li, L. Liu, "An Improved Structural Silicon-based Micro-Electrostatic Motor," Chinese Journal of Semicon., Vol. 4, No. 7, p. 453-454, 1993.
5. X. Sun et al., "A Silicon-based Integrated Micro-wobble-motor," Chinese Journal of Semiconductors, Vol. 16, No. 2, p. 149-152, 1995.
6. E. H. Klaassen et al., "Silicon Fusion Bonding and Deep Reactive Ion Etching: A New Technology for Microstructure," Transducers, 95, p. 556-559, Stockholm, Sweden, June 25-29, 1995.
7. C. Zhan, L. Liu, P. Qian, STC` 95, 1995.5, p. 201-203, Shanghai.
8. H. Xie, X. Sun, L. Liu, "Silicon-based Micro Turn-Over Devices," 9th National Conf. on Integrated Circuits and Silicon Materials, 1995.1, p. 480-483, Xian.
Litian Liu is a professor at the Institute of Microelectronics, Tsinghua University, Beijing, China, where he mentored the other authors. He can be contacted at ph 8610-6278-5486 or 8610-6278-2712.