Anisotropic etching rates of single-crystal silicon for TMAH water solution as a function of crystallographic orientation

doi:10.1016/S0924-4247(98)00271-4

Sensors and Actuators A: Physical

Volume 73, Issues 1–2, 9 March 1999, Pages 131-137

https://doi.org/10.1016/S0924-4247(98)00271-4 Get rights and content

Abstract

We evaluated orientation dependence in the etching rate of single-crystal silicon for tetramethyl-ammonium-hydroxide (TMAH) water solutions. Etching rates for a number of crystallographic orientations were measured for a wide range of etching conditions, including TMAH concentrations of 10–25% and temperatures of 70–90°C. We found significantly different characteristics from those for KOH water solutions. Firstly, different types of orientation dependence in etching rate were found around (111) between TMAH and KOH. This means the bonding energy of the silicon crystal lattice is not a single factor that dominates orientation dependence, and there exist different etching mechanisms for the two etchants. Secondly, effects of the circulation of etchants on the etching rates were not negligible in TMAH in contrast to KOH system.

Introduction

Anisotropic chemical etching has long been used for fabricating microstructures such as diaphragms and cantilevers on a silicon wafer. The demand for more complicated 3-D microstructures on a silicon chip is increasing in various applications such as ink-jet printing devices and microfluidic systems. We think it is necessary to know the etch rates for a number of crystallographic orientations, in order to fabricate 3-D microstructures whose profiles are composed of a number of facets having different orientations. When the etching rate is known as a function of orientation, etchant, and etching conditions, one can simulate the etched product's shape using a variety of mask patterns, process conditions, and multiple process steps 1, 2. Thus, we started to construct an etching rate database with KOH water solutions, and reported that the orientation dependence varies according to the KOH concentration and etching temperature [3]. In this paper, we further investigated the orientation dependence in etching rate with TMAH water solutions.

TMAH water solution is gradually being introduced into industries, regardless of its expense, taking the place of KOH solutions. This is because: (1) it hardly attacks silicon dioxide film as an etching mask, (2) it does not contain ions harmful for electric circuits which are integrated on the same chip with mechanical structures. Tabata et al. [4]have reported the etch rates of (100), (110), and (111) silicon for TMAH solutions. However, the orientation dependence was not yet clear for a number of other orientations; nor was the influence of etching conditions on the orientation dependence. We evaluated the orientation-dependent etching rates for a number of crystallographic orientations and compared the results with those of the KOH system.

Section snippets

Measurement scheme

We evaluated the orientation-dependent etching rates for TMAH water solutions by applying the same measurement scheme as we did with the KOH system [3]. We used a solid hemispherical specimen of single-crystal silicon as shown in Fig. 1. The radius of the hemisphere was 22 mm, and its sphericity was less than 10 μm. The surface was polished into a mirror. The surface roughness was 0.005–0.007 μm in Ra. All crystallographic orientations appeared on the hemispherical surface. Measuring the

Orientation dependence in etching rate

The measured etch rate distributions are shown in contour maps in Fig. 3 as projections of the hemispheres. According to the symmetry of the crystal lattice, the plots are folded into a quarter of a circle. All of the orientations appear in this quarter circle. Three perpendicular (100) planes appear at the origin and the end of the X, Y axes. Three (110) planes are located in the middle of the X, Y axes and on the periphery of the circle. The (111) is in the middle of the quarter circle. The

Application to etch profile analysis

We applied the measured etch rate results as a function of orientation to the etch profile analysis. We used an etching simulation system MICROCAD [6]that analyzes three dimensional etching profiles based on Wulff–Jaccodine's method. We input all the measured etch rates data to the system, and prosecuted etch profile calculation of a deep groove on a (110) wafer. The calculated groove profiles are shown in Fig. 8 comparing the results between TMAH and KOH solutions. The bottom of the groove

Conclusion

Measurements were carried out on the etch rates of single-crystal silicon for TMAH/water solutions, as a function of crystallographic orientation using hemispherical specimens and deep grooves on (110) wafers. The results are summarized as follows.

(a) It was observed that the orientation dependence changes according to changes in etching temperature and also in TMAH concentration.

(b) Orientation dependence was quite different around (111) for TMAH and KOH solutions. This fact suggests that the

Acknowledgements

This work was supported by a grant-in-aid for Scientific Research (B) No. 08455045 from the Ministry of Education, Science and Culture of the Japanese government. It was also supported by the Micromachining-Process Consortium organized by Fuji Research Institute. The silicon ingots were the donation of Sumitomo Sitix. The authors express their thanks for all of this support.

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Kazuo Sato was born in 1948 in Yokohama, Japan. He received his bachelor's degree (Mechanical Engineering) from Yokohama National University in 1970, and PhD degree from the University of Tokyo in 1982. In 1970, he joined Hitachi, Tokyo. He has been engaged in micromachining technologies and their applications since 1983. Since 1994, he has been a Professor at Nagoya University. Dr. Sato is a member of the Japan Society of Mechanical Engineers, the Japan Society for Precision Engineering, the Institute of Electrical Engineers of Japan, and the Japan Society for Technology of Plasticity.
Mitsuhiro Shikida was born in 1964 in Chiba, Japan. He received his BS (Electrical Engineering) and MS degrees from Seikei University, Tokyo, in 1988 and 1990, respectively, and PhD degree from Nagoya University in 1998. From 1990 to 1995, he was employed at Hitachi, Tokyo. In 1995 he joined the Department of Micro System Engineering at Nagoya University, as a Research Associate. He has been an Assistant Professor since 1998. His research interests include microactuators, microfabrication, and micromechanical structures. Dr. Shikida is a member of the Institute of Electrical Engineers of Japan.
Takashi Yamashiro was born in Hiroshima, Japan, in 1973. He earned his BS degree (Mechanical Engineering) and MS degree (Micro System Engineering) from Nagoya University, Japan, in 1996 and 1998, respectively. He is currently working at Yamaha. His research interests include microfabrication and etching simulation for microstructures.
Kazuo Asaumi was born in Kamagaya, Japan, in 1967. He obtained his BS degree (School of Science and Technology) from Waseda University, Japan, in 1991. He is currently working at Fuji Research Institute, as a researcher. His research interests include supercomputing technology and system engineering.
Yasuroh Iriye was born in Tokyo, Japan, in 1958. He received his BS and MS degrees (Department of Civil Engineering) from Chuo University, Tokyo, Japan, in 1982 and 1984, respectively. He is currently working at Fuji Research Institute as a chief researcher. His research interests include supercomputing technology and system engineering. Mr. Iriye is a member of the Japan Society of Applied Physics and the Japan Society for Industrial and Applied Mathematics.
Masaharu Yamamoto was born in Aichi, Japan, in 1954. He received his BS degree (Department of Electrical Engineering) from Nagoya Institute of Technology, Nagoya, Japan, in 1978. He is currently working at the IRI Aichi Prefectural Government as a chief researcher. His research interest includes precise measurement.

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