Fabrication method for increasing bottom gate oxide thickness of a power transistor and trench gate structure formed by using the same

A fabrication method for increasing bottom gate oxide thickness of a power transistor, applicable to an U-metal-oxide-semiconductor field-effect transistor (UMOSFET), the fabrication method comprising:

• providing a semiconductor substrate and forming a drift region on the semiconductor substrate;
• forming a first heavily doped region in the drift region;
• forming a second heavily doped region and a third heavily doped region respectively on opposite sides of the first heavily doped region, and providing a well region between the first heavily doped region, the second heavily doped region, the third heavily doped region and the drift region;
• depositing a hard mask layer on the first heavily doped region, the second heavily doped region and the third heavily doped region, and using a lithography process to form a trench;
• depositing a gate oxide layer along the trench, such that the gate oxide layer covers at least two opposite sidewalls and a bottom of the trench;
• forming two polysilicon sidewalls on the two opposite sidewalls of the trench, wherein the two polysilicon sidewalls cover the gate oxide layer on the two opposite sidewalls and are disposed on the gate oxide layer on the bottom of the trench, each of the two polysilicon sidewalls is disposed corresponding to each of the two opposite sidewalls of the trench;
• forming a barrier layer on a side surface of each of the two polysilicon sidewalls, such that the barrier layer covers the side surface of each of the two polysilicon sidewalls, the barrier layer and the two polysilicon sidewalls are commonly disposed on the gate oxide layer on the bottom of the trench;
• removing the gate oxide layer underneath each of the two polysilicon sidewalls and the barrier layer such that each of the two polysilicon sidewalls and the barrier layer are dangling and a vacancy is formed at the bottom of the trench;
• and
• performing a thermal oxidation process to form a thick oxide layer after the two polysilicon sidewalls are oxidized and the thick oxide layer fills the vacancy at the bottom of the trench such that the trench is partially filled.