<title>Photoresist bake conditions and their effects on lithography process control</title>

Abstract

Bake conditions for three conunercially available positive photoresists have been varied to investigate their effects on commonly monitored photolithography responses, including critical dimension uniformity, exposure latitude, and contrast.The structure of the photoresists varied from ones with photoactive compounds wholly bonded to the resin to one whose photoactive compound is free in solution. Both pre-and post-exposure bake conditions were varied, and effects of minimum versus maximum thin film coupling conditions were studied. Of particular interest is the control and modification of the spatial distribution of the photoactive compound and associated standing waves in the developed profiles by use of bake conditions.To quantify the true impact of standing waves on process control, we present data of CD distributions as a function of resist thickness for the various sensitizer systems. Standard, vendor recommended processes are compared with altered bake processes that yield visible reduction in the amplitude of the standing wave. Further data on exposure latitude at minimum and maximum film coupling are provided. To ensure sufficient accuracy, all CD measurements are made by Prometrix EM-i electrical linewidth measurements of etched polysilicon.A standard, no post-exposure bake process with standing waves in the profile yields a maximum to minimum CD delta of 0.082 microns, which compares with a standard post-exposure bake process with no standing waves having the same CD delta of 0.074 microns. The use of a postexposure bake in the former case, including aggressive temperatures up to 120C, worsens the CD delta to 0.15 microns.