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

The effects of plasma charging on the threshold voltage shifts for 0.5 μm fresh flash memory devices during threshold voltage measurements are reported in this work. It was discovered that threshold voltages (V_th) obtained from fresh flash memory devices (after UV-erased and nonprogrammed), using standard test measurement conditions (V_g=0 to +6 V; V_dd=0.1 V; V_sub and V_ss=0 V or ground) were found to be oscillating and erratic. No repeatable and measurable V_ths 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 μA), cell drain currents of nA and pA range were detected instead. A reduction in device charge coupling seems to occur during the V_th measurement step, which accounts for the decrease in the cell drain current, as well as for the unaccountability of V_th. 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 V_ths