Effect of PEB temperature profile on CD for DUV resists

Abstract

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 1 7 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°C 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°C, 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 I 10°C, transient effects are dominant early in the process and they are completed within about 50seconds 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.

An inverse heat transfer method to provide near-isothermal surface for disc heaters used in microlithography

“…The present problem can also be solved in a steady state domain for the following reasons. Although transient effects could affect the CD uniformity to an extent [11,12], the photo-chemistry and hence the CD accuracy is more sensitive only after the wafer reaches the amplification threshold of T SET [10]. Hence the 120 s baking process is done in two phases.…”

“…Because of the negligible wafer thickness ($1 mm) when compared to the heater thickness (30 mm, for 'thin' bake plates [10], which is used in this study), simulation studies [23] have shown that the thermal resistance of the air in the proximity gap (see Fig. 1(a)) allows the lateral conduction in the wafer, improving the temperature uniformity of the wafer.…”

“…This in turn depends on the spatial temperature uniformity on the thin ($1 mm) wafer surface, which is a direct imprint of the temperature uniformity of the top surface of the baking heater ( Fig. 1(a)) of larger thermal mass [10,6]. This is true only when the natural convection effect from the top and side of the wafer substrate considered in [11][12][13] are eliminated.…”