A semiconductor fabrication method is provided for fabricating a shallow-trench isolation (STI) structure with a rounded corner in integrated circuits through a rapid thermal process (RTP). In the fabrication of the STI structure, a sharp corner is often undesirably formed. This sharp corner , if not eliminated, causes the occurrence of a leakage current when the resultant IC device is in operation that significantly degrades the performance of the resultant IC device. To eliminate this sharp corner , an RTP is performed at a temperature of above 1,100.degree. C., which temperature is higher than the glass transition temperature of the substrate, for about 1 to 2 minutes. The result is that the surface of the substrate is oxidized into an sacrificial oxide layer and the sharp corner is deformed into a rounded shape with a larger convex radius of curvature. This allows the problems arising from the existence of the sharp corner to be substantially eliminated. Compared to the prior art, this method not only is more simplified in process, but also allows a considerable saving in thermal budget, which makes this method more cost-effective to implement than the prior art.