ANOMALOUS NARROW CHANNEL-EFFECT IN TRENCH-ISOLATED BURIED-CHANNEL P-MOSFETS

An anomalous threshold voltage dependence on channel width measured on 0.25 mu m groundrule trench-isolated buried-channel p-MOSFET's is reported here. As the channel width is reduced, the magnitude of the threshold voltage first decreases before the onset of the expected sharp rise in Vt for widths narrower than 0.4 mu m. Modeling shows that a ''boron puddle'' is created near the trench bounded edge as a result of transient enhanced diffusion (TED) during the gate oxidation step. TED is governed by interstitials produced by a deep phosphorus implant, used for latchup suppression, diffusing towards the trench sidewall and top surface of the device. The presence of the ''boron puddle'' imposes a penalty on the off-current of narrow devices. A solution for minimizing the ''boron puddle'' is demonstrated with simulations, confirmed by measurements.