Real-time process control to prevent CD variation induced by post exposure delay

Abstract

One of the major problems for DUV resists is linewidth change owing to Post Exposure Delay (PED). Linewidth is mainly induced by acid diffusion during exposure and baking. Based on the mechanism of the neutralization of organic base and photogenerated acid, a model had been generated in our previous study to describe the linewidth variation for different FED times. The derived equation can calculate the minimum elapse time, which will cause linewidth variation to exceed the specification of a specific CD. This work concludes that the smaller CD received a higher percentage of CD variation under FED for an isolated line pattern. Therefore, the minimum acceptable time for the smallest CD can be obtained based on +/- 10% of the nominal CD. When a track alarm occurs and wafer processing stops, wafers between exposure and post exposure bake (PEB) will suffer a CD variation due to FED. The start and end time of each processing wafer is traced herein, and the time interval between one end time and the following start time is calculated via an automatic control system If the time interval exceeds the value set herein, an automatic control system will hold this lot and send an error message to the engineer. Based on the in-line configuration of stepper and track, seven wafers must be reworked when FED occurs. By employing the model and automatic control system established herein, linewidth variation induced by FED can be prevented.