Baking post-exposure

A.irbome Basic Chemical Contamination. A critical, and at-first pu22ling problem, was encountered during early manufacturing trials of CA resists. Sporadically, severely distorted resist profiles would be formed in positive-tone CA resists, displaying what seemed to be a cap on the upper surface of the resist image (Fig. 26). In severe cases this cap or T-top would appear as a kin or cmst over the entire wafer surface that prevented development of the pattern. The magnitude of the effect varied dramatically between laboratories and appeared to grow more severe as the time interval between exposure and post-exposure bake was increased. 

Three approaches have been identified that reduce susceptibility of CA resists to airborne contamination. In the first, process engineering changes such as the addition of special activated carbon filters to the environmental chambers surrounding the exposure tools (76,79), overcoating the resist with a soluble protective film to isolate the resist from the environment (77,80,81), or modifications of the process flow to minimize the time interval between exposure and post-exposure bake have been shown to improve CA resist processibibty. 

The process control of the post-exposure bake that is required for chemically amplified resist systems deserves special attention. Several considerations are apparent from the previous fundamental discussion. In addition for the need to understand the chemical reactions and kinetics of each step, it is important to account for the diffusion of the acid. Not only is the reaction rate of the acid-induced deprotection controlled by temperature but so is the diffusion distance and rate of diffusion of acid. An understanding of the chemistry and chemical kinetics leads one to predict that several process parameters associated with the PEB will need to be optimized if these materials are to be used in a submicron lithographic process. Specific important process parameters include ... 

Time between exposure and post-exposure bake... 

The formulated resist composition is then spin coated on silicon wafers. The resist coating layer is exposed through a photomask at 193 nm, and then the exposed coating layers are post-exposure baked at 110°C. The coated wafers are then treated with a diluted aqueous tetramethylammonium hydroxide solution to develop the imaged resist layer and provide a relief image (61). 

The photoresist SU-8 has been used directly to fabricate microstructures. In order to create 400-pm-high micropillars, a 400-pm-thick SU-8 layer was used. To reduce the risk of bubbles and to ensure uniformity in the SU-8 layer, two layers of 200-pm SU-8 were consecutively deposited, each with a post-exposure soft-bake step. The exposed SU-8 layer was developed using propylene glycol methyl ether acetate [236]. 

In the photolithographic process, the functional layer is covered by a photoresist film. State-of-the-art circuits are fabricated with chemically amplified photoresists consisting of a polymer with an acid-labile pendant protection group, photoacid generator molecules (PAG), and additional additives [2], Upon exposure to UV radiation through a patterned mask, the PAG is decomposed generating a low concentration of acid. In a post-exposure bake... 

Toukhy, M.A. Hansen, S.G. Influence of post exposure bake on resist contrast. Proc. SPIE 1994, 2195, 640-651. 

Infrared studies of the radiation-induced degradation of PMPS by Bowden et al. (19) supports the oligomerization process and also shows that the oligomers can be removed by post-exposure baking. These effects have not been seen for other poly (olefin sulfone)s (2.3). Figure 8 and Figure 9 show the yield versus dose curves for irradiation of poly(l-butene sulfone) and poly (cyclohexene sulfone) respectively (20). No comparable shift of the S02/olefin ratio towards unity is observed in the radiolysis of these polymers. 

A cresol novolac resin has been synthesized which exhibits a much greater dissolution-inhibiting effect than in various commercially available novolac or phenolic resins. Using this resin, a positive electron beam resist was prepared and its exposure characteristics were examined. A tetramethylammonium hydroxide aqueous solution was used as the developer. The sensitivity reaches 3xl0 6 C/cm2 without post-exposure baking. It was found that the sensitivity to double exposure was much higher than that to single exposure with the same total dose. A similar phenomenon was also... 

Exposure characteristics for NPR with use of the cresol novolac are shown in Figure 3. This NPR contains 12% PMPS agaist the resin weight. Bowden and his coworkers showed that post-exposure baking increases the sensitivity of NPR(10). However, with the present NPR, the post-exposure baking does not remarkably affect sensitivity. An example of the fine pattern for this NPR as obtained by electron beam delineation is shown in Figure 4. 

Walker, Reduction of photoresist standing wave effects by post exposure bake, IEEE Trans. Electron Dev. ED-22(7), 464 466 (1975). 

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