Photoacid generator/protecting groups

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... 

Figure 8. Acid catalyzed thermolysis of the t-BOC protected copolymer is responsible for the change in solubility. The quantum efficiency for generation of the phenolic is the product of the efficiency of photoacid generation and the catalytic chain length. Exposure generates a local concentration of acid. Subsequent heating to a temperature below that at which uncatalyzed thermolysis occurs allows local acidolysis of the t-BOC protecting group.

Figure 17.10 shows the process sequence of the hydrophilic overlayer (HOL) process. First, a chemically amplified or non-chemically amplified positive-tone photoresist comprising hydrophobic polymer and appropriate PAG is coated to a nominal thickness on an appropriate substrate such as a silicon wafer, followed by a soft bake to dry out the nonaqueous solvent. Next, the photoresist film is exposed to radiation of appropriate wavelength to generate photoacid from the PAG. Then the exposed film is again baked (called PEB) at the standard temperature to enhance the diffusion of the photoacid and thermolysis of the acid-labile protecting groups of the polymers. 

Acidic a protons on the ketone group show smooth protection and proton reproduction.33 Then a poly(OCMA-co-AdMA) resist was prepared for a superior CA resist. The structure of the copolymer is shown in Figure 2.14. The copolymer is sensitized by TPSF as PAG and dissolved in cyclohexanone as a resist solution. Protected OCMA is dissociated by acid H+ as shown in Figure 2.15. Methyl groups were introduced into poly(OCMA-co-AdMA) to obtain a more sensitive CA resist. Figure 2.16 shows copolymer-introduced methyl groups. The resist was formulated with 2 wt% TPSF as a photoacid generator. The resist exhibited L/S 200 nm. 

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