Reliability of Amorphous Metallizations for GaAs FETS

Amorphous W-Si, TiW-Si and Ni-Si diffusion barriers have been developed for GaAs field effect transistors to achieve improved reliability of ohmic contacts and Schottky barriers, particularly in high temperature ambients. The amorphous films were formed by a new method in which alternate layers of 500A° (W, TiW or Ni) and 300A° Si were sputter deposited and subsequently annealed just below the glass transition temperature Tg (~ 500°C) and rapidly cooled. Electron channeling was used to determine the amorphous nature of the film after 4 hr. anneals at Tg. Amorphous regions were present in the deposited films due to the rapid quenching process of sputter deposition. After annealing at 500°C for 4 hours, the films were completely amorphous with no evidence of crystallization. High magnification SEM (20,000X) examination of the films after annealing revealed smooth and continuous surfaces with no evidence of grain boundaries. Diffusion along grain boundaries between Au and GaAs in Au/W-Si/GaAs structures is thus virtually eliminated, thereby minimizing both electromigration and contact reactions. Interdiffusion in Au/W-Si/GaAs and Au/TiW-Si/GaAs layers was studied by Auger electron spectroscopy sputter profiling techniques. No interdiffusion of Au or GaAs was observed after 8 hour anneals at 450°C. This is a significant improvement over previous polycrystalline diffusion barriers (e.g., TiPt) which interdiffuse after 2-3 hours at 350°C. Two micron gate (small signal) FETs were fabricated with amorphous TiW-Si gates and AuGeNi ohmic contacts. Accelerated life tests were carried out at 300°C.