HOT-CARRIER RELIABILITY OF TRENCH TRANSISTOR.

To compare the hot-carrier reliability, trench and planar transistors were stressed at the same peak substrate current, 10 mu A/ mu m. The initial number of surface states for the trench transistor (5. 2 multiplied by 10 **1**0/cm**2) is higher than that of the planar transistor (2. 5 multiplied by 10**1**0/cm**2). However, it is found after 5000-s**2 stress that both trench transistor and planar transistor had surface state densities of 6. 5 multiplied by **1**0/cm**2. Also the channel current degradation of trench transistor (1%) is much less than that of planar transistor (12%). To make sure the asymmetric junction depths do not contribute this effect, stress testing was performed both in forward and reverse mode. The results are similar, indicating that junction depth difference is not the reason. This indicates that trench oxide is more hot-carrier resistant than planar oxide. This results because trench oxide is grown under compressive stress. Thus, the higher-stressed oxide at the bottom should be more degradation-resistant. This is supported by observation. The oxide quality appears to be higher at the bottom. Experiments are under way to compare oxides grown under compressive stress and tensile stress.