Catalytic chain length

The resist films were irradiated with the same incident doses used for the determination of acid content. A postbake was performed at 100°C for one minute. The carbonyl absorption was measured before and after irradiation and postbake. The difference was used to determine the number of t-BOC groups removed for a given weight of resist film. By dividing the number of t-BOC groups by the amount of photogenerated acid, the catalytic chain length was determined (Table II). 

For the incident doses studied, the catalytic chain length varies from about 800 to 1100. The lowest numbers were observed for the lowest and highest doses. At 2.0 mJ/cm2 incident dose the catalytic chain begins to drop off as t-BOC group removal starts to approach completion. 

Table II. Catalytic Chain Length for Acid Catalyzed t-Butylcarhonate...

Absorbed Dose (mJ/cm2)a Loss of Carbonyl (%) I X1SS of Carbonyl (mmol) Acid Produced11 (mmol) Catalytic Chain length ... 

Acid-catalyzed photoresist films acid diffusion, 35 acid generation, 303233/341 advantages, 28 catalytic chain length, 3435r development of classes of cationic photoinitiators, 28 experimental procedure, 35-36 generation mechanism from irradiation of triphenylsulfonium salts, 28-29 merocyanine dye method for acid analysis, 30,31/33/... 

Sensitivity (photospeed) Catalytic chain length for acidolysis, efficiency of acid generation, acid strength, protective group chemistry... 

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.

In each system, the primary photo-event is dissociation of the cationic photoinitiator to produce an acid. This reaction proceeds with a quantum efficiency that is characteristic of the particular initiator. The photogenerated acid then interacts with a carefully chosen polymer matrix to initiate a chain reaction, or acts as a catalyst, such that a single molecule of photogenerated acid serves to initiate a cascade of bond making or breaking reactions. The effective quantum efficiency of the overall process is the product of the photolysis reaction efficiency times the length of the chain reaction (or the catalytic chain length). This multiplicative response constitutes... 

Acid detection, quantification, and catalytic chain length... 

ACID DETECTION, QUANTIFICATION, AND CATALYTIC CHAIN LENGTH... 

Catalytic chain length refers to the average number of amplification chemical events caused hy one acid molecule. 

One approach to provide resists with high sensitivity and contrast involves the principle of chemical amplification (CA) (6-8), The amplification effect is achieved by employing a photo generated acid as a catalyst to carry out a cascade of chemical reactions in the resist film. Catalytic chain lengths of >1000 have been reported in the literature (9,10), However, the high catalytic chain lengths also enhance the resist s sensitivity to airborne basic contaminants and basic moieties on the substrate (11,12), In a positive tone resist such deactivation of the photoacid by airborne or substrate bound basic contaminants results in T-tops or foot formation respectively. 

Processing Characteristics of CA Resists. While the increased radiation sensitivity afforded by CA resists offers enormous practical advantages, it brings the need for far more stringent control of the lithographic process. Any factor that influences the course of the catalytic reaction will have an effect that is amplifled by a factor equal to the catalytic chain length. 

Compound 3 and 2,6-dinitrobenzyl 4-nitrobenzenesulfonate 5 are two examples of PAG s which have been successfully used in chemicalty amplified r-BOC based resist systems (2). Table 1 shows a comparison of the absorbance at 248 nm of a 6 mole% solutions of these materials and 1 and 2 in acetonitrile. As can be seen the absorbance in solution of compounds 1 and 2 are somewhat lower than that of 3 and considerably lower than that of 2,6-dinitroben 4-nitroben-zenesulfonate 5. Table 1 also shows the absorbance per fim (ABS//u,m) of 1, 2 and 3 in a PTBSS polymer matrix. Since the ABS//tm of 1 and 2 are similar to 3 and the same acid is liberated in all three PAG s, it was eiq>ected (based upon our previously established relationship between the lithographic sensitivity and the product of the ABS/ju,m, , and catalytic chain length) (9) that the sensitivities of 1 and 2 should depend primarity on the relative values of the quantum yields. 

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