Lateral growth control by thickness spatial modulation of amorphous silicon film

Abstract

Grain size control in excimer laser crystallization has stimulated a growing activity aiming to locate large silicon grains in the thin-film transistor (TFT) channel. We present a new technique that can be applied to control the lateral growth (LG), by using thickness spatial modulation, and that can be used to the fabrication of high performance polysilicon TFTs. In particular LG can be triggered when thinner regions are completely melted and thicker regions are partially melted. Thicker regions are formed by a two layer structure of doped and undoped material while thinner regions are obtained by removing the doped layer. In this way the thicker regions can be used as source–drain regions of the device while the thinner regions, where large grains are formed, can be used as channel regions. In order to control the LG, different doped and undoped layer thickness have been investigated. The structures have been irradiated at different laser energy densities and we observed that the thicker regions became highly conductive (∼70 Ω/□), due to doping activation and redistribution induced by the melt/regrowth process, while triggering of the LG in the thinner region has been observed within an opportune energy density window. As the LG extension is limited at approximately 1 μm, active channel region ⩽2 μm could be formed. The proposed process is very attractive for polysilicon TFTs as LG-control and doping activation can be simultaneously achieved.