Polymer structure effect on dissolution characteristics and acid diffusion in chemically amplified deep ultraviolet resists J.We propose a general model for latent image formation in chemically amplified resists. The model is based on a moving boundary acid transport concept that incorporates transient free volume generation and densification. It is based on experimental observation of negligible acid diffusion in polyhydroxysterene below T g . The model offers insight into the post exposure bake ͑PEB͒ reaction mechanism that governs the relief image formation in chemically amplified resists. During PEB, there is a thermally induced deprotection catalyzed by the photogenerated acid that produces volatile by-products thereby generating free volume in the resist polymer. The free volume enhances local diffusivity of the acid. The rapid loss of the volatile products is followed by relaxation of the polymer matrix which eliminates the transient free volume and densifies the polymer. The densified polymer inhibits the diffusion of any acid trapped in the deprotected sites. We present cases where the model reduces to Fickean and case II type reaction driven diffusion models under some simplifying assumptions. The model was implemented in simulation tools for resist models to simulate one-dimensional and two-dimensional profiles. The results imply that the relief image formation depends strongly on both the mechanical the chemical properties of the resist. This model provides new directions for resist process optimization.
We present recent modeling work aimed at understanding the influence of structural changes in photoacid generators ͑PAGs͒ on acid generation efficiency, deprotection efficiency, and photoacid diffusion in 193 nm chemically amplified resists. An analytical model for the postexposure bake process is used to study the reaction and diffusion properties of the various acids generated by the PAGs. Fourier transfer infrared spectroscopy is used to monitor the generation of photoacid during exposure. Resist thickness loss after PEB as a function of exposure dose is related to the deprotection extent to extract the reaction rate parameters. The effects of the acid size and boiling point on process latitude, line end shortening, and line edge roughness are presented. Analytical model predictions of process latitude and line end shortening are also presented and compared to experimental data. In this study, the photogenerated acid with the smallest molar volume and highest boiling point temperature gave the best overall lithographic performance.
This paper presents a methodology for modeling and simulating line-end shortening (LES) effects in deepUv photoresist and calibrating the simulation with experimental data. A reaction/diffusion model is first calibrated using large area dose-to-clear versus bake time data and thickness loss from a top-to-top bake experiment. SPLAT and STORM are linked to simulate the exposure and post exposure bake processes of chemically amplified resist. A threshold model is then applied to determine the line-end shortening effects.Verification experiments were conducted on two resists, APEX-E and UVIIHS, for several types of geometric features, and the resist model parameters were fine tuned using the Method of Feasible Direction. The measurement of LES agrees quite well when the simulation using an exponential diffusion model of post exposure bake is used. The fine tuning reduced the RMS error to below the noise level in the experimental data and improve the accuracy in predicting LES to 10% of feature size.
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