Metal induced crystallization of SiGe at 370°C for monolithically integrated MEMS applications

Abstract

Over the last decade SiGe has been proposed as a structural material for low thermal budget microelectromechanical systems (MEMS) that can be post-processed on top of standard CMOS driving and controlling electronics [1-6]. There are several ways to decrease the deposition temperature of SiGe and at the same time preserve the desired physical properties for MEMS as low electrical resistivity, high quality factor, economical growth rate and low mean stress and strain gradient. The conventional approach to reduce the crystallization thermal budget is to increase the germanium content to 60%, or more, using conventional Low Pressure Chemical Vapor Deposition (LPCVD) [1-3]. In this case highly conductive polycrystalline films can be realized, but the strain gradient is relatively high. This can be eliminated by furnace annealing at 450°C [2], which might introduce damage to the underlying circuits such as in the case of Cu/low k CMOS. This problem can be alleviated using excimer pulsed laser annealing [7, 8], which has been attractive for low thermal budget applications such as thin film transistors (TFT) [9], solar cells fabricated on glass substrates [10] and for monolithic integration of MEMS devices on top of standard driving electronics using SiGe as an active material [8, 11]. Also the use of hydrogenated microcrystalline SiGe allows for a low thermal budget [12]. In addition, metal induced crystallization has recently been proposed to enhance the crystallization of silicon at temperatures as low as 500°C, and the realized devices had outstanding performance compared to those employing conventional solid-phase crystallization [13]. This technique enhances crystallization by two methods. First, it has been observed that depositing SiGe on top of a thin Al or Ni layer, has a polycrystalline micro-structure close to the metal/ SiGe interface [11]. Annealing this film for a long period (is determined by the annealing temperature), results in metal diffusion and a subsequent crystallization of the film. Finally, when the metal is diffused completely through out the film, it can be etched away. The main disadvantage of this approach is that the mean stress is highly compressive and this might affect the functionality of surface micromachined structures [13].