Low-Temperature LPCVD MEMS Technologies
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Low-Temperature LPCVD MEMS Technologies Roger T. Howe1 and Tsu-Jae King Dept. of EECS and BSAC1, University of California at Berkeley, Berkeley, CA 94720-1770, U.S.A. ABSTRACT This paper describes recent research on LPCVD processes for the fabrication of high-quality micro-mechanical structures on foundry CMOS wafers. In order to avoid damaging CMOS electronics with either aluminum or copper metallization, the MEMS process temperatures should be limited to a maximum of 450oC. This constraint rules out the conventional polycrystalline silicon (poly-Si) as a candidate structural material for post-CMOS integrated MEMS. Polycrystalline silicon-germanium (poly-SiGe) alloys are attractive for modular integration of MEMS with electronics, because they can be deposited at much lower temperatures than poly-Si films, yet have excellent mechanical properties. In particular, in-situ doped p-type poly-SiGe films deposit rapidly at low temperatures and have adequate conductivity without post-deposition annealing. Poly-Ge can be etched very selectively to Si, SiGe, SiO2 and Si3N4 in a heated hydrogen peroxide solution, and can therefore be used as a sacrificial material to eliminate the need to protect the CMOS electronics during the MEMSrelease etch. Low-resistance contact between a structural poly-SiGe layer and an underlying CMOS metal interconnect can be accomplished by deposition of the SiGe onto a typical barrier metal exposed in contact windows. We conclude with directions for further research to develop poly-SiGe technology for integrated inertial, optical, and RF MEMS applications.
INTRODUCTION A core technology for micro-electromechanical systems (MEMS) is surface micromachining, the selective etching of a patterned thin-film stack to form suspended microstructures [1]. Starting in the early 1980’s, low-pressure chemical vapor deposition (LPCVD) has been extensively used for depositing the structural and sacrificial layers in surface micromachining processes. The combination of LPCVD polycrystalline silicon (poly-Si) as a structural material and LPCVD silicon dioxide (SiO2) as a sacrificial material has been extensively investigated and commercialized. Multiple structural layers allow for versatility in MEMS design. To date, the most sophisticated surface micromachining process is the SUMMiT-V foundry process developed by Sandia National Laboratories, which uses four sacrificial oxide and four structural poly-Si layers, with a fifth poly-Si layer for interconnect [2]. The success of poly-Si MEMS technology is owing to the nature of the poly-Si LPCVD process, as well as to the electrical and mechanical properties of poly-Si. LPCVD is a wellunderstood technique that yields films with properties that are relatively insensitive to the process tool. LPCVD is an extremely conformal process, which is important for the reliable fabrication of simple MEMS structures, such as beam anchors. Conformal deposition is also essential for making bearings and hinges with tight tolerances. These mechanical elements are
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