Electrical and Structural Properties of Ohmic Contacts of SiC Diodes Fabricated on Thin Wafers

Abstract

New generations of SiC power devices require to be fabricated on very thin substrates, in order to significantly reduce the series resistance of the device. The role of thinning process on the formation of backside ohmic contact has been investigated in this work. Three different mechanical grinding processes have been adopted, resulting in different amounts of defectivity and surface roughness values. An excimer UV laser has been used to form a Ni-silicide based ohmic contact on the backside of the wafers. The reacted layer has been studied by means of Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD) analyses, as a function of grinding process parameters and laser annealing conditions. The ohmic contact has been evaluated by measuring the Sheet Resistance (Rs) of silicided layers and the Vf at nominal current of Schottky Barrier Diode (SBD) devices, fabricated on 150 mm-diameter 4H-SiC wafers. A strong relationship has been found between the crystal damage, induced by thinning process, and the structural, morphological and electrical properties of silicided ohmic contact, formed by UV laser annealing, revealing that the silicide reaction is moved forward, at fixed annealing conditions, by the increasing of crystal defectivity and surface roughness of SiC.