Threshold voltage instabilities of fresh flash memory devices caused by plasma charging

Abstract

The effects of plasma charging on the threshold voltage shifts for 0.5 μm fresh flash memory devices during threshold voltage measurements, were first reported in this work. It was discovered that threshold voltages (Vth) obtained from fresh flash memory devices (after UV-erased and non-programmed), using standard test measurement conditions (Vg:0 to +6 V; Vdd:0.1 V; Vsub & Vss:0 V or ground) were found to be oscillating and erratic. No repeatable and measurable Vths can be retrieved from the distributed cells across a 200 mm wafer. In addition, rather than obtaining a higher cell drain current (≈ the order of a micro ampere), cell drain currents of nano and pico ampere range were detected instead. A reduction in charge coupling of the device seems to occur during the Vth measurement step, which accounts for the decrease in the cell drain current, as well as for the unaccountability of Vth. Positive charges present in the plasma, are believed to be trapped within the device. The movement of these embedded charges across the interpoly dielectric is suggested to be responsible for the instability of measured Vths.