Shallow trench isolation for thin silicon/silicon-on-insulator substrates by utilizing polysilicon

Abstract

A method for forming an isolation trench in a silicon or silicon-on-insulator substrate is described in which a trench is formed in the semiconductor structure (containing a multiple layer structure of Si, SiO , and SiN layers) and an undoped polysilicon layer is deposited on the bottom and sidewalls of the trench and on the surface of the region adjacent to the trench. A substantial portion of the trench is left unfilled by the undoped polysilicon layer deposited. The polysilicon layer is thermally oxidized to form a thermal oxide that fills the trench and thereby avoids forming a birds-beak formation of the thermal oxide above the sidewalls of the trench. The isolation structure may be planarized by either removing the polysilicon layer from the region adjacent to the trench before oxidation or later removing the oxide from the SiN layer and adjusting height of the oxide in the trench. Alternatively, either a doped polysilicon layer or a doped SiO layer may be formed above the silicon nitride layer before the undoped polysilicon layer is deposited. In this case, the isolation structure is heat treated prior to oxidization to drive dopants from the doped layer into the undoped polysilicon layer, thereby forming a secondarily doped polysilicon layer from the undoped polysilicon layer. The doped layers are then removed by selective wet etching and expose the silicon nitride layer prior to oxidation.