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The effect of varying time and temperature profile at the PEB step on a 250 nm isolated line is studied for an Acetal and an ESCAP type Deep UV (DUV) resist. Experimental studies on the Acetal resist resulted in very non-linear Critical Dimension (CD) sensitivities with CD variation largest for low Post Expose Bake (PEB) temperature and short PEB time. A global CD model was created by fitting experimental data to a first order kinetics equation. An effective activation energy of 50 Kcal/mole was obtained for the Acetal resist whereas PROLITH simulations for an ESCAP type resist gave 69 Kcal/mole. These results are consistent with the well-documented diffusion- controlled deprotection reaction taking place during the PEB process. The global CD model was then used to investigate the impact of transient and steady-state temperature profiles on CD control. In order to achieve this goal, actual 2 dimensional wafer temperature profiles were input to the global CD model and PEB-induced CD variation was calculated during each sampling period of the 17 temperature readings across the wafer as a function of time. The resultant time- evolution of the PEB-induced CD variation was used to infer the relative importance of the transient and steady-state component of temperature profiles. At the low PEB temperature of 90 degrees Celsius for the Acetal resist, transient effects dominate for a nominal 90 second process. Slower deprotection reaction yields large CD's at 356 nm and predicted PEB-induced CD variation of 6.9 nm. At 100 degrees Celsius, transient effects are less prevalent but still present. Tight steady- state and transient temperature uniformity along with wafer- wafer temperature profile repeatability and stringent control of process timing and delays are important. At 110 degrees Celsius, transient effects are dominant early in the process and they are completed within about 50 seconds in this model. However as photoacid is lost to competing reactions and the time necessary for deprotection is extended, even in this case transients could still play a part in final CD results. In general, both mean CD and CD variation results improve with increasing PEB temperature for a 90 second PEB time. PROLITH simulations using ESCAP resist parameters and actual measured CD profiles both support these model predictions. Improved resist formulations along with advanced photoresist processing tool layout, wafer-handler scheduler and PEB module designs are all necessary ingredients for minimum PEB-induced CD variation.
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