Improved Reliability Performances of SONOS-Type Devices Using Hot-Hole Erase Method by Novel Negative FN Operations

Several novel negative Fowler-Nordheim (FN) operations for hot-hole-erased SONOS-type devices are studied. By using p⁺-poly gate instead of n⁺-poly gate, gate injection is greatly suppressed, and the device shows self-convergent V_T after -FN. By using this self-converging property, -FN operation can be applied to both erased and programmed states in various sequences/algorithms. In the erase state, a short -FN channel erase called "soft erase" can be performed after hot-hole sector erase. It provides a strong electrical annealing effect to recover the hot-hole damages. On the other hand, "refill" is a shorter version of the soft erase method that expels shallow-level electrons and replaces them with deeper level ones. A spectrum blue shift model is proposed to explain the shifting of the trapped-electron energy distribution to a bluer (deeper) spectrum during refill. Various soft erase and refill conditions are examined to study the charge loss mechanism. The experimental results exhibit superior retention by combining the suitable soft erase and refill conditions. We find that the vertical charge losses measured by V_G -accelerated retention test at room temperature (25 degC) are highly correlated to that measured at high temperature baking (150 degC), and a reliability model is proposed to explain these mechanisms.