Fabrication of Thick Free-Standing Lightly-Doped n-Type 4H-SiC Wafers

Abstract
In this work, we have developed a selective wet etching technique for n+-SiC substrate using electrochemical etch process. A mixture of hydrofluoric acid and hydrogen peroxide was used as an electrolyte and the etch rates exceeding 200 μm per hour at the current density of 50 mAcm-2 was achieved. This process is highly selective and the etching process stops at the interface of n+ SiC substrate and n-SiC epi layer. Using this process, we have successfully fabricated 180 to 250 μm thick 4’’ n-SiC epitaxial free standing wafers by separating them from a 350 μm thick n+ SiC substrate. After the substrate is completely removed, free standing wafer is characterized for wafer bow and minority carrier lifetime, using both Si-face and C-face. The wafer bow was reduced from 40 μm to 20 μm after the substrate removal. It was found that the process of removing the substrate does not introduce any extra damage to the wafer as far as the lifetime is concerned. The hole lifetime measured by microwave photoconductivity decay technique was unchanged at around 2 μsec, measured from both Si-face and C-face. These results are very promising and open up many avenues for many device applications where lightly doped free standing epitaxial semiconductor thin film is needed.