Effect of Deposition Conditions on the Structural and Mechanical Properties of Poly SiGe
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Effect of Deposition Conditions on the Structural and Mechanical Properties of Poly SiGe Sherif Sedky1,2, Ann Witvrouw1, Matty Caymax1, Annelies Saerens3, Paul Van Houtte3 1 IMEC, Kapeldreef 75, B3001 Leuven (Belgium) 2 Dep. of Engineering Physics, Faculty of Engineering, Cairo University, 12211 Giza, (Egypt) 3 Dep. of Metallurgy and Materials Engineering Katholieke Universiteit Leuven, Leuven (Belgium) 1. INTRODUCTION MicroElectroMechanical Systems (MEMS) are used in a wide variety of applications such as accelerometers [1], gyroscopes [2], infrared detectors [3],…etc. For high volume applications, fabrication costs can be possibly reduced by monolithic integration of MEMS with the driving electronics. The easiest approach for monolithic integration is post processing MEMS on top of the driving electronics, as this does not introduce any change into standard fabrication processes used for realizing the driving electronics. On the other hand, post processing imposes an upper limit on the fabrication temperature of MEMS in order to avoid any damage or degradation in the performance of the driving electronics. Polycrystalline silicon (Poly Si) has been widely used for MEMS applications [4], but the main disadvantage of this material is that it requires a high processing temperature (higher than 800°C [5]) to achieve the desired physical properties. In particular, a low tensile stress is needed for MEMS. Polycrystalline silicon germanium (Poly SiGe) seems to be an attractive alternative to poly Si as it has similar properties, while the presence of germanium reduces its melting point. Hence, the desired physical properties are expected to be realized at lower temperature. Depending on the germanium concentration and the deposition pressure, the transition temperature from amorphous to polycrystalline can be reduced to 450°C [6], or even lower, compared to 580°C for LPCVD poly Si. Also, the residual mechanical stress in poly SiGe is lower than that in poly Si [7]. The main objective of this paper is to demonstrate the possibility of reducing the deposition temperature of poly SiGe to a limit which is compatible with CMOS post processing. At the same time, the physical properties of the deposited layers should be suitable for MEMS. To achieve this goal, we analyze the effect of decreasing the deposition temperature of poly SiGe from 625°C to 500°C on the growth rate, the germanium concentration and the mechanical properties of the deposited films. 2. SAMPLE PREPARATION Poly SiGe has been deposited on eight-inch wafers using chemical vapor deposition (CVD) in an ASM EPSILON 2000 epi reactor [7]. The deposition pressure has been varied from atmospheric pressure (AP) to a reduced pressure (RP) of 10 Torr. 1% germane in hydrogen has been used as the germanium gas source, whereas, the silicon gas source is either silane or dichlorosilane. Germane flow rate has been fixed to 200 sccm whereas the flow rate of the silicon gas source has been adjusted to yield a germanium concentration varying from 15% to 50% as determined by Rut
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