Effect of cell size reduction on the threshold voltage of UMOSFETs

Abstract

UMOSFET on-resistances have been dramatically improved in recent decades with the miniaturization of cell size by innovations in the fabrication process. However, with miniaturization, failure in the gate oxide, large deviations in the threshold voltage and reductions in avalanche capability have emerged as design problems for mass production. In particular, threshold voltage increase have appeared with the introduction of a trench source contact. The source contact trench and MOS gate trench are fabricated next to each other with a narrow silicon mesa region, and the voltage increase appear when the silicon mesa width becomes narrower than 80 nm. So far, it appears that the P+ layer dopant in the contact sidewall diffuses toward the gate oxide and the channel doping increases, which causes the increase in the threshold voltage Vth. We analyzed the distribution of Vth at the wafer level/shot level and found for the first time that the increase in Vth is caused by the punch-through effect from the channel depletion layer to the contact P+ layer, and thus, sidewall dopant diffusion will not affect the increase in Vth. We established an analytical model for the increase in Vth. Our model showed that for the field plate type UMOSFET with a shorter gate contact length, not only is there a increase in Vth, but also it is difficult to control Vth using the conventional channel implantation method.