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

A method for forming an isolation trench in a semiconductor structure on a silicon-on-insulator substrate comprising;

• forming the semiconductor structure to be a multiple layer structure including a silicon layer, a pad oxide layer above the silicon layer, a silicon nitride layer above the pad oxide layer, and one of a doped polysilicon layer and a doped SiO2, layer above the silicon nitride layer
• forming a trench having a bottom and sidewalk in the semiconductor structure;
• depositing an undoped polysilicon layer on the bottom and the sidewalk of the trench and on a surface of a region of the semiconductor structure adjacent to the trench, the undoped polysilicon layer being deposited such that a substantial portion of the trench is left unfilled;
• heat treating the region adjacent to the trench to drive dopants from the one of the doped polysilicon layer and the doped SiO2 layer into the undoped polysilicon layer thereby forming a secondarily doped polysilicon layer from the undoped polysilicon layer
• wet etching the secondarily doped polysilicon layer and the one of the doped polysilicon layer and the doped SiO2 layer to remove the secondarily doped polysilicon layer and the one of the doped polysilicon layer and the doped SiO2, layer and to expose the silicon nitride layer and
• subsequently oxidizing the polysilicon layer 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 and avoids reduction in active device area in said adjacent semiconductor region.

A method for forming an isolation trench in a semiconductor structure on a silicon substrate comprising:

• forming the semiconductor structure to be a multiple layer structure including a pad oxide layer above the silicon layer, and a silicon nitride layer above the pad oxide layer;
• forming a trench having a bottom and sidewalls in the semiconductor structure;
• depositing an undoped polysilicon layer on the bottom and the sidewalls of the trench and on a surface of a region of the semiconductor structure adjacent to the trench, the undoped polysilicon layer being deposited such that a substantial portion of the trench is left unfilled;
• forming one of a doped polysilicon layer and a doped SiO2 layer above the silicon nitride layer;
• heat treating the region adjacent to the trench prior to oxidizing to drive dopants from the one of the doped polysilicon layer and the doped SiO2 layer into the undoped polysilicon layer, thereby forming a secondarily doped polysilicon layer from the undoped polysilicon layer;
• wet etching the secondarily doped polysilicon layer and the one of the doped polysilicon layer and the doped SiO2 layer to remove the secondarily doped polysilicon layer and the one of the doped polysilicon layer and the doped SiO2 layer and to expose the silicon nitride layer;
• and
• oxidizing the polysilicon layer 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.