TW200514168A - Structure having silicon thin film with changing grain size and method for forming the same by thermal process - Google Patents

Abstract

Roughly described, a silicon layer transitions from polysilicon at one surface to amorphous silicon at the opposite surface. The transition can e monotonic, and can e either continuous or it can change abruptly from polysilicon to amorphous silicon. If such a layer is formed as the floating gate of a floating transistor structure, the larger grain structure adjacent to the tunnel dielectric layer reduces the formation of a tip (protrusion) and thus reduces leakage. On the other hand, the smaller grain structure adjacent to the gate dielectric layer produces a smooth, more uniform gate dielectric layer. The polysilicon-to-amorphous silicon transistor can be fabricated with a temperature profile that favors polysilicon formation at the start of floating gate deposition, and transitions during deposition to a temperature that favors amorphous silicon deposition at the end of floating gate deposition.