Poly-SiGe, a superb material for MEMS
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Poly-SiGe, a superb material for MEMS Ann Witvrouw1, Maria Gromova1, Anshu Mehta1, Sherif Sedky1,2, Piet De Moor 1, Kris Baert1, Chris van Hoof1 1 IMEC, Kapeldreef 75, B3001 Leuven (Belgium) 2 Physics Department, The American University in Cairo, P.O. Box 2511 Cairo, Egypt ABSTRACT In this overview article several MEMS applications of poly-SiGe are discussed: thermal applications, the application as a capping layer for MEMS wafer-level packaging and the use as MEMS structural layer for processing MEMS devices on top of CMOS. For all these applications also different deposition processes have been developed: chemical vapor deposition at reduced pressure (RPCVD), at low pressure (LPCVD) and with plasma enhancement (PECVD). Special techniques to reduce the processing temperature to the absolute minimum are reviewed as well: the use of hydrogenated microcrystalline SiGe, of metal-induced crystallization and of laser annealing. The latter methods are important when one wants to process SiGe MEMS above advanced CMOS with low-permittivity dielectrics. 1. INTRODUCTION Micro-electromechanical systems (MEMS) such as infrared detectors, accelerometers, gyroscopes, … are increasingly used. One of the standard processing techniques used for producing MEMS is surface micromachining. In surface micromachining a sacrificial layer is etched away underneath a structural layer to create freestanding structures above a substrate. Polycrystalline silicon (poly-Si) is widely used as MEMS structural layer (PolyMUMPs, Sandia SUMMiT, thick poly-Si of Bosch, integrated poly-Si of Analog Devices...) [1-4] as this material has a high Young’s modulus, a high yield strength (see Table 1) and it is a semiconductor that can be doped. Moreover a low stress and stress gradient, as is required for surface micromachining applications, are also achievable for this material. However, in order to achieve the latter and to activate the dopants, a high processing temperature (> 800°C) is required. While this is no problem when only a sacrificial Si-oxide layer and/or other poly-Si layers are already deposited on the substrate, the high thermal budget required might become a problem when also other devices are present on the substrate (e.g. in packaging applications) or when integration with CMOS (e.g. when processing MEMS on top of CMOS) is envisioned. Poly-SiGe is an attractive alternative to poly-Si as it has similar properties, while, as shown in literature and as will be demonstrated in this article, the desired electrical and mechanical properties can be realized at a temperatures as low as 400 –450 ºC [5-11]. Moreover, poly SiGe has a 5 times lower thermal conductivity compared to poly Si, making it an interesting material for thermopiles, bolometers etc... [12-16]. In this article three different MEMS applications of poly-SiGe will be discussed: thermal applications, the application as a capping layer for MEMS zero-level packaging and the use as MEMS structural layer for processing MEMS devices on top of standard CMOS. The multitude of possi
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