Mechanical Properties and Reliability of Amorphous vs. Polycrystalline Silicon Thin Films
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Mechanical Properties and Reliability of Amorphous vs. Polycrystalline Silicon Thin Films Joao Gaspar1, Oliver Paul1, Virginia Chu2, and Joao Pedro Conde2,3 1 Dept. Microsystems Eng. (IMTEK), University of Freiburg, Freiburg, 79110, Germany 2 INESC Microsistemas e Nanotecnologias, Lisbon, 1000-029, Portugal 3 Dept. Chemical and Biological Eng., Instituto Superior Tecnico, Lisbon, 1049-001, Portugal
ABSTRACT This paper presents the mechanical characterization of both elastic and fracture properties of thin silicon films from the load-deflection response of membranes, also known as the bulge test. Properties extracted include the plane-strain modulus, prestress, fracture strength and Weibull modulus. Diaphragms made of low-temperature, hydrogenated amorphous and nanocrystalline silicon films (a-Si:H and nc-Si:H, respectively) deposited by plasma enhanced chemical vapor deposition (PECVD) and, for comparison, membranes composed of hightemperature polycrystalline silicon (poly-Si) deposited by low pressure chemical vapor deposition (LPCVD) have been fabricated and characterized. The structures are bulged until failure occurs. From the stress profiles in the diaphragms at fracture, the brittle material strength is analyzed using Weibull statistics. The bulge setup is fully automated for the sequential measurement of several membranes on a substrate realizing the high-throughput acquisition of data under well controlled conditions. A comprehensive study of the mechanical properties of low-temperature silicon films as a function of deposition parameters, namely substrate temperature, RF power, hydrogen dilution and doping, is presented. INTRODUCTION Microelectromechanical systems (MEMS) use planar microelectronics fabrication techniques to produce 3D structures with electronic and mechanical functionality. MEMS sensors and actuators can be based on a variety of different physical, chemical and biological principles [1]. Most MEMS devices are fabricated using bulk micromachining of crystalline silicon (c-Si) substrates or surface micromachining of poly-Si films, requiring processing temperatures in the range of 550-1100ºC [2]. Thin-film MEMS use thin-film technologies. Advantage has been taken of the low temperatures at which some of thin films can be deposited. Low temperature processing makes it possible to use a wide variety of substrates such as glass, plastic or stainless steel sheet. Another important feature of thin-film technologies is that the film properties (optical, mechanical, optoelectronic and chemical) can be tuned by varying the deposition conditions. Low-temperature thin-film MEMS may even be CMOS-compatible, allowing their integration with control electronics as part of a backend process. Thin-film MEMS devices based on a-Si:H have been reported [3-6]. The characterization of the mechanical properties of thin films used in MEMS is of great importance since these determine device functionality and performance. Parameters such as the plane-strain modulus Eps and prestress σ0 can be extracted f
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