Elimination of the "Birds Beak" in trench MOS-gate power semiconductor devices

In the conventional fabrication process of trench MOS-gate power devices, the isolation of polysilicon gate from the source metal is achieved by the local oxidation of polysilicon within the refilled trenches. This isolation scheme results in the formation of "Birds Beak" due to the unwanted oxidation of the silicon at the corners near mouth of the trenches. The formation of the birds beak imposes many critical design constraints on the fabrication of trench MOS-gate power devices. Fabrication process steps required to eliminate the birds beak and overcome these constraints are described in this paper. Trench MOS-gate power devices fabricated using the birds beak "free" gate isolation process have the highest channel density. The elimination of the birds beak also simplifies the fabrication of all the self-aligned and triple diffused trench MOS-gate power devices.