Uniformity Length Research Articles

Abstract: Positive photoresist masking properties of high energy, high dose phosphorous-ion implants and their influence on MOS FET device characteristics

The use of a positive photoresist (PR) as a phosphorus-ion implant mask for defining the source and drain of a MOS FET suffers from outgassing and deformation of the resist when high energy ions (150 keV) and a high dose (5×1015 P+/cm2) are involved. This deformation affects the dose uniformity and MOS FET channel length. Here we compare PR masks with those made up of a baked mixture of photoresist and a thermal free radical (PR+TFR) as suggested by Kluge and Elie.1 During the preimplant bake the thermoplastic deformation of the PR+TFR mask is significantly lower than that of the PR mask, but there is no significant difference between the deformation of the two masks during implantation providing the preimplantation bakes are the same. We have made MOS FET’s with channel length as defined by the mask (Lmask) varying from 0.15 to 1.600 mil(3.8 to 4 μm) and have visually measured Lmask following the preimplantation bake. By measuring the MOS FET characteristics (in the linear region) we can determine the effective channel length and compare this with Lmask over a wide range of values. The deformation of the resist can be reduced by using thinner resist and be made negligible by substituting low temperature vacuum baking (prior to implantation) for the conventional high temperature air bake.