Patent ID: 6979632

Claim:
A fabrication method for a thin-film semiconductor device comprising: forming a semiconductor film comprising silicon to become a source region, a drain region, and a channel forming region located between the source region and the drain region, the semiconductor film formed over a substrate; crystallizing the semiconductor film by irradiating laser light to the semiconductor film; forming a silicon oxide film on depressions and protrusions of the semiconductor film; first etching the silicon oxide film formed on the protrusions of the semiconductor film to expose at least part of the protrusions of the semiconductor film and to leave behind a part of the silicon oxide film located on at least the depressions; second etching the exposed part of the protrusions of the semiconductor film selectively with an etchant; and forming a gate electrode over the etched semiconductor film with an insulating film there between; wherein, after the first etching, a top surface of the part of the silicon oxide film located on at least the depressions is closer to the substrate than a top surface of the exposed part of the protrusions, wherein the part of the silicon oxide film located on at least the depressions is used as a mask to obtain a smoothed surface of the semiconductor film, wherein a height difference of the smoothed surface of the semiconductor film is 100 Å or less, and wherein an etching rate of the semiconductor film is higher than that of the silicon oxide film with respect to said etchant.