Patent ID: 7816209

Claim:
A method for fabricating a semiconductor device, the method comprising: forming a bulb type recess in a substrate, wherein the bulb type recess comprises an upper portion and a lower portion, the lower portion being wider than the upper portion; forming an insulation layer over the substrate including a surface of the bulb type recess; depositing a columnar polysilicon layer over the insulation layer, wherein the columnar polysilicon layer fills the bulb type recess, the columnar polysilicon layer being deposited at a temperature ranging from approximately 650° C. to approximately 800° C. and predominantly comprised of columnar-shaped grains having an initial grain size and crystal structure; and performing a thermal treatment process, wherein the initial grain sizes and crystal structure of the columnar-shaped grains are substantially maintained at a constant after the thermal treatment process is performed such that a seam formed in the columnar polysilicon layer in the lower portion of the bulb type recess does not move during the thermal treatment process.