Optimizing CD uniformity by total PEB cycle temperature control on track equipment

Abstract

Traditionally, the stable-state temperature uniformity of resist post-exposure bake (PEB) units is considered major contributor to within wafer critical dimension (CD) uniformity. However, it has been realized that continued improvements in CD uniformity requires across-the-wafer temperature uniformity for the entire PEB cycle. The wafer temperature uniformity of the PEB heating and chilling cycle contribution to CD uniformity has been characterized in detail. When a silicon wafer is first placed on a traditional hot plate the within wafer temperature variance can be several degrees Celsius before the wafer reaches equilibrium at the set-point temperature target. A completely new hot plate design has been conceptualized and produced to attain thermal uniformity of the process wafer for the entire PEB cycle. This new PEB hot plate design was modeled after thermal transfer principle of a heat pipe to attain across-the-wafer uniformity during wafer heating and stable-state PEB. The new PEB plate design also allows to quickly change the PEB process temperature set-point up or down. Characterization has been completed to detail the importance of this new PEB plate design in attaining within wafer CD uniformity on single-digit nanometer level for KrF and ArF photolithography resist processing on track equipment.