The effects of polysilicon dopant depletion and Fowler-Nordheim tunnelling on the characteristics of N+ polysilicon-oxide-silicon capacitors

Abstract

High-frequency and quasi-static capacitance measurements are made on both WSix polycided and non-polycided metal-oxide-semiconductor (MOS) capacitors, fabricated on n- or p-well silicon. The thickening of the gate oxide and the depletion of dopants from the N+ polysilicon gate, following the polycidation process, are observed. The role of minority carriers in the polysilicon, under reverse bias, is demonstrated through frequency and temperature measurements. Deep depletion is observed in the silicon well, when the silicon is swept into reverse bias, at a rate as low as 10 mV s-1. This is attributed to an insufficient generation of minority carriers, due to the limited volume in the shallow well. A sharp decrease in both high-frequency and quasi-static capacitances at an oxide field >6 MV cm-1 is observed for capacitors on the p-well silicon. This observation is attributed to significant Fowler-Nordheim tunnelling of inversion carriers, limiting the formation of the inversion layer and thereby causing the silicon substrate to enter deep depletion.