High-temperature stable metallization systems for SiC applications

Today, the development of microelectromechanical systems (MEMS) targets new areas of application such as high temperature, radiation, aggressive media, and other hostile environments, which extend beyond the capabilities of the well-established silicon and GaAs technology. Typical applications include automotive electronics, aircraft and space applications, process control in materials engineering, applications in the oil and gas industry, as well as military applications and nuclear safety. Silicon carbide is considered to be the most promising semiconductor for harsh-environment applications. However, because of the unavailability of reliable contacts, interconnects, and packages, its broad application is still limited. This thesis focused on developing a long-term high-temperature stable metallization scheme for SiC sensor applications operating in the 500°C range. A new concept for metallization systems with extended high-temperature stability was developed. The basic idea is to ensure long-term high-temperature stability by minimizing the driving force for reactions within the metallization stack. Tungsten disilicide (WSi2) and tantalum disilicide (TaSi2) contact layers were tested in combination with amorphous ternary WxSiyNz or TaxSiyNz diffusion barriers and platinum interconnect layers. The optimum metallization system exhibits excellent high-temperature stability up to 500h at 500°C. Neither barrier crystallization nor interdiffusion takes place, as confirmed by XRD and AES analysis. The maximum variation of contact resistivity to the 6H-SiC substrate was measured to be as low as 0.9%. The results published in this thesis demonstrate, for the very first time, a metallization system which is both electrically and thermodynamically long-term stable in the 500°C temperature range. Feasibility and potential of the developed metallization for practical applications are demonstrated by applying the final metallization system to a 6H-SiC piezoresistive high-temperature pressure sensor. The presented wafer-level sensor fabrication process demonstrates a viable way for batch fabrication of SiCbased high-temperature and harsh-environment sensor devices.