Poly lithographic sensitivity

Table VI. Lithographic Sensitivity as a Function of the Amount and Position of Chlorine in Poly(styrene) Polymers...

Table VI summarizes the lithographic characteristics of PCMS, CMS, aM-CMS, chlorinated PMS and P-p-MS for polymers with comparable molecular weight and chlorine content. Chlorinated PMS and P-p-MS are as sensitive as PCMS but exhibit better contrast, especially chlorinated P-p-MS. Only the chloromethylated poly-a-methylstyrene shows similar contrast but has much lower sensitivity.

Polysiloxanes. The initial reports of the utility of polysiloxanes for lithographic applications spurred several research groups to further investigate this class of materials. The problem of low Tg was addressed by preparing chloromethylated poly(diphenylsiloxane) 16, 17). More recently, poly(sil-sesquioxanes) 18, 19) have been reported as sensitive, negative, e-beam, ion-beam, and UV resists. These soluble, ladder -type polymers prepared by the hydrolysis of substituted chloro- and alkoxysilanes are high-Tg materials (150 °C) with high silicon contents. 

Combining the lithographic and etch mask functions into a single polymer can be a major challenge, especially for deep-UV lithography. The latitude in resist design is limited, because at least 10 and preferably 15 wt % of the polymer structure must be reserved for silicon. A few materials, like silicon-substituted poly(methyl methacrylates) (6) and polysilanes (7, 8), have been used as positive two-layer resists for deep-UV lithography, but these materials suffer from either poor to moderate sensitivities to deep-UV radiation or an excessive absorption in the UV that limits exposure depth in the resist layer. 

The structure of itaconic acid bears some resemblance to methacryllc acid. While It Is know that methacryllc acid and methacrylic anhydride do enhance sensitivity when copolymerized with methyl methacrylate, some of the Increase In speed may be due to porosity (7,12). The gas formation may cause a higher rate of dissolution then can be attributed to chain scission alone. Pittman and co-workers have reported G(s) values for poly(methacryllc anhydride) of 0.4 (10) and 1.8 and 2.9 (14) based on gamma radiation experiments. Hiroaka (12) measured G(s) by gas evolution on Irradiation of films with electron beams and established values In the ration 1 2 6 for the methyl methacrylate. A terpolymer with the three components In the molar ratio of 70 15 15 (same three monomers) was selected by Moreau et al (13) on the basis of complete lithographic evaluation. The speed Is 4 to lOx that of PMMA. As aforementioned, an Increase In G(s) may be only partially responsible. 

The first step in the exposure mechanism is the loss of an electron from the molecule (Scheme 7.24)/ probably from one of the 2p, orbitals of sulfur this results in a vacancy that is quickly filled from a higher orbital. The lost electron appears eventually in a o- (S—C) bonding orbital, effectively transforaiing the C—S into a weak one-electron bond that can be broken by thermal energy or by excess kinetic energy released in the ionization event. Consequently, the scission yield of poly(afkene sulfones) is very high, which makes these resists very sensitive, both for electron-beam (less than 1 p,C/cm at 10 kV) and for DUV (sensitivity of 5 mJ/cm at 185 nm) lithographic exposures. ... 

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