Shadowing of lightly doped drain implants due to gate etch profiles and implanter configurations

Abstract

Shadowing of lightly doped drain (LDD) implants at the gate edge can cause shifts in effective electrical channel length (Leff), drive current (Ids) and transistor asymmetry. Process integration of gate etch and LDD implant processing and equipment in Motorola has led to the discovery of a unique type of implant shadowing. The increased vertical profile of the gate etch on modern single wafer etch systems obviously increases the potential to shadow implants and some amount of shadowing is not detrimental to the devices. In LDD processes which both the N-type and P-type LDD implants are masked separately, the gate etch profile and implant angle and rotation will determine the amount of shadowing for each transistor orientation. In processes that use a blanket N-LDD and a masked P-LDD adjusted to compensate for the N-dopant in the P-channel devices, an interaction between different implanter rotations has been shown to drastically increase the shadowing affects and leave uncompensated N-LDD dopant. It is necessary to carefully match the implant angles and rotations for proper performance of these LDD structures.