Composite Contact Metallization on SiC for High Temperature Applications in Air
Abstract
Composite ohmic and Schottky contacts fabricated on 4H-SiC for applications in air at 350^oC are described and evaluated in this study. The ohmic contacts were fabricated on p+ implanted and n+ epitaxial materials, while the Schottky contacts were on n- epilayers. These contacts were protected from the harsh environment with the following metallization layers: Ta-Si-N served as diffusion/oxidation barrier, Pt-N sputter-deposited at 250^oC promoted adhesion between the diffusion barrier layer and the AAu cap layer and it also act as barrier to Au diffusion. A gold cap layer was deposited for the purpose of wirebonding to the device or die attach to a carrier. The composite contact's electrical and physical characteristics were monitored as a function of annealing time in air at 350^oC. For the ohmic contact, characteristics such as the specific contact resistance and SiC sheet resistance determined from TLM measurements were monitored with time of anneal. Contact parameters of nickel ohmic contact on heavily implanted p+ 4H-SiC materials were very stable up to 4000 hours at 350^oC in air. 70-30 wt% Al-Ti ohmic contacts on implanted p+, 80-20 wt% Ni-Cr and Ni ohmic contacts on epitaxial n+ 4H-SiC were also studied. Current density - voltage and capacitance - voltage characteristics of nickel silicide and Ta-Si-N Schottky contacts were studied as a function of annealing time in air. Parameters such as the barrier height and ideality factor of the devices were monitored and found to be stable as the devices were annealed in air. Adhesion characteristics of the metallization stack (barrier layer, Pt layer and Au layer) on thermal oxides, Schottky contacts and ohmic contacts were investigated in this study. All the contacts have good adhesion to the metallization as fabricated except for the nickel ohmic contacts that needed Ar ion cleaning to improve its adhesion to the metallization stack. In all cases, inter-diffusion within the metallization and oxidation of the contacts were monitored with RBS (Rutherford backscattering spectrometery) and AES (Auger electron spectroscopy).