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 metal-oxide-silicon field-effect transistor (n-UMOSFET) and a device cell comprising several n-UMOSFETs. The geometrical parameters investigated included 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 mobility in the channel; however, the effects of the geometry on the characteristics of the isolated device or device cell are manifested on the spreading resistance of the drain end. Trench processing, in the form of trench etching, trench cleaning and subsequent gate-oxide growth, is observed to primarily influence the n-UMOSFET's immunity to electrical stress, which is studied using charge pumping current and gate-oxide breakdown measurements. It is shown that the gate-oxide edge adjacent to the drain and the oxide/silicon interface overlapping the drain are the regions most susceptible to degradation by Fowler-Nordheim stress. These observations coupled with results from scanning electron microscopy suggest that the gate-oxide growth non-uniformity as well as its condition at the trench comers are the key factors in determining the n-UMOSFET's reliability.