Anomalous punchthrough in ULSI buried-channel MOSFETs

It is shown that punchthrough (pt) in very short buried-channel P-MOSFETs cannot be suppressed by diminishing the p-channel thickness t_c. This is because of the drain-induced channel enlargement (DICE) effect, which switches on and becomes the dominant pt mechanism for very short channels. The DICE effect is independent of t_c, and therefore the DICE-related pt component flows even with t_c to 0, so that strategies other than channel thinning are needed for the pt to be suppressed. A new implant step, called the retrograde implant, together with the LDD (lightly doped drain) structure, is shown to be able to suppress the DICE effect and thereby shift the limit of pt-immune BC-P-MOSFETs from 0.6 mu m down to 0.3 mu m. The pt encountered below a channel length of 0.3 mu m has been found to result from a pure DIBL effect rather than an incomplete DICE effect suppression. As a result, a further gain in channel shortening (without pt) would require the channel to be even thinner than 0.04 mu m.