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 rep orted here. As the channel width is reduced, the magnitude of the thre shold voltage first decreases before the onset of the expected sharp r ise in Vt for widths narrower than 0.4 mu m. Modeling shows that a ''b oron puddle'' is created near the trench bounded edge as a result of t ransient enhanced diffusion (TED) during the gate oxidation step. TED is governed by interstitials produced by a deep phosphorus implant, us ed for latchup suppression, diffusing towards the trench sidewall and top surface of the device. The presence of the ''boron puddle'' impose s a penalty on the off-current of narrow devices. A solution for minim izing the ''boron puddle'' is demonstrated with simulations, confirmed by measurements.