Advanced wet etch bulk micromachining in {100} silicon wafers
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1222-DD05-04
Advanced wet etch bulk micromachining in {100} silicon wafers Prem Pal and Kazuo Sato Department of Micro-Nano Systems Engineering Nagoya University, Nagoya 464-8603, Japan ABSTRACT In this work we have developed novel microfabrication processes using wet anisotropic etchants to perform advanced bulk micromachining in {100}Si wafers for the realization of microelectromechanical systems (MEMS) structures with new shapes. The etching is performed in two steps in pure and Triton-X-100 [C14H22O(C2H4O)n, n = 9-10] added 25 wt% tetramethyl ammonium hydroxide (TMAH) solutions. The local oxidation of silicon (LOCOS) is attempted after the first anisotropic etching step in order to protect the exposed silicon. Two types of structures (fixed and freestanding) are fabricated. The fixed structures contain perfectly sharp corners and edges. Thermally grown silicon dioxide (SiO2) is used for the fabrication of freestanding structures. Present research is an approach to fabricate advanced MEMS structures, extending the range of 3D structures fabricated by silicon wet anisotropic etching. INTRODUCTION Bulk micromachining based on wet anisotropic etchants (e.g. KOH, TMAH, etc.) is widely used for the realization of both fixed (e.g. cavities or grooves) and freestanding (e.g. cantilever beams) structures. Most complementary metal oxide semiconductor (CMOS) devices and the micromachined structures applied in MEMS/MOEMS are fabricated in {100}Si wafers. Other orientations are used for specific applications. In the case of {100}Si, etched profiles of the mask patterns bounded by edges resemble their original shapes, while other types of mask patterns comprising non- edges, convex corners and curved edges encounter undercutting at these edges and corners. Extension of etching time always leads to sharp concave corners bounded by {111} planes. In order to obtain exact imaging of arbitrary shape mask pattern in {100}Si surface by wet etching, the etch rates of non-{100}Si planes should be negligible in comparison to that of {100}Si. Previously, in order to extend the range of 3D structures in {100}Si by wet etching, several techniques based on double side etching [1], wafer bonding [2], two-step etching [3], P+ silicon etch stop [4], electrochemical etching [5], and unconventional wafer orientations such as {113}, {112}, {335}, {557}, {331}, etc. [6], have been presented. Although numerous wet anisotropic etchants have been and are used currently, TMAH solutions have gained wide popularity due to their high selectivity with silicon dioxide, and CMOS process compatibility. Recent studies on the effect of the addition of surfactant in TMAH on the etching characteristics has opened new possibilities to develop advanced structures as the etching characteristics of TMAH+surfactant is very different from pure TMAH, particularly in terms of undercutting at convex corners [7-9]. A very small amount (0.001-0.1% v/v) of surfactant in TMAH suppresses the undercutting to a significantly low level. In this work, wet anisotropic etching ba
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