The dependence of UMOSFET characteristics and reliability on geometry and processing

We have examined the impact of trench processing and trench and device cell geometries on the characteristics of a single n-channel U-shaped trench me tal-oxide-silicon field-effect transistor (n-UMOSFET) and a device cell com prising several n-UMOSFETs. The geometrical parameters investigated include d the trench depth and width, the trench cross-section and the device cell pitch. We have found out that the geometry does not affect the electron mob ility in the channel; however, the effects of the geometry on the character istics of the isolated device or device cell are manifested on the spreadin g resistance of the drain end. Trench processing, in the form of trench etc hing, trench cleaning and subsequent gate-oxide growth, is observed to prim arily influence the n-UMOSFET's immunity to electrical stress, which is stu died using charge pumping current and gate-oxide breakdown measurements. It is shown that the gate-oxide edge adjacent to the drain and the oxide/sili con interface overlapping the drain are the regions most susceptible to deg radation by Fowler-Nordheim stress. These observations coupled with results from scanning electron microscopy suggest that the gate-oxide growth non-u niformity as well as its condition at the trench corners are the key factor s in determining the n-UMOSFET's reliability.