Forward bias enhanced channel hot electron injection for low-level programming improvement in multilevel flash memory

Abstract

Low-voltage programmed levels are hard to achieve in multilevel Flash memory using staircase CHEI (channel hot electron injection) programming. The reasons are that low-level programming marginally deviates from the linear relation between threshold voltage V-TH and control gate voltage V-CG. Forward bias enhancement of CHEI is proposed to overcome this drawback. It is demonstrated that the new technique creates a linear relation between V-TH and V-CG, validated down to a critical VCG that is at least I V lower than traditional CHEI. Through extensive measurements, it is further argued that the most suitable magnitude of forward bias is 0.5 V since (i) it produces the lowest program level of 1.4 V; and (ii) higher biases cause not only large current consumption but also worsened drain disturb performance in NOR array configuration. The corresponding linear relation with the unity slope is maintained after 1 05 program/erase cycling.