Onium salt sensitizer

Ito and coworkers (86-87) recently described a new DUV resist system that has high sensitivity, does not suffer swelling during development, and may be used either as a positive or negative system. The new resist is based on a formulation consisting of t-butyloxycarbonyl (t-BOC) protected poly(vinylphenol) 88 (Figure 50) and an onium salt sensitizer. Exposure... 

Scheme 10.13 Possible initiation routes in the cationic polymerization of monomer M. Initiating system onium salt/sensitizer.

The solubHity properties of the PAG itself can play an important role in the overaH resist performance as weU (50). SolubHity differences between the neutral onium salt and the acidic photoproducts can be quite high and wHl affect the resist contrast. In fact onium salts can serve as dissolution inhibitors in novolac polymers, analogous to diazonaphthoquinones, even in the absence of any acid-sensitive chemical function (51). 

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.

While "conventional positive photoresists" are sensitive, high-resolution materials, they are essentially opaque to radiation below 300 nm. This has led researchers to examine alternate chemistry for deep-UV applications. Examples of deep-UV sensitive dissolution inhibitors include aliphatic diazoketones (61-64) and nitrobenzyl esters (65). Certain onium salts have also recently been shown to be effective inhibitors for phenolic resins (66). A novel e-beam sensitive dissolution inhibition resist was designed by Bowden, et al a (67) based on the use of a novolac resin with a poly(olefin sulfone) dissolution inhibitor. The aqueous, base-soluble novolac is rendered less soluble via addition of -10 wt % poly(2-methyl pentene-1 sulfone)(PMPS). Irradiation causes main chain scission of PMPS followed by depolymerization to volatile monomers (68). The dissolution inhibitor is thus effectively "vaporized", restoring solubility in aqueous base to the irradiated portions of the resist. Alternate resist systems based on this chemistry have also been reported (69,70). 

A nonionic, non-volatile photoactive acid generator, 2,6-dinitrobenzyl tosylate has been recently reported and shown to be effective in chemically amplified resist systems (10). This ester is a nonionic compound that has a much wider range of solubility in matrix polymers and does not contain undesirable inorganic elements. While it is known to exhibit a lower sensitivity to irradiation than the onium salt materials, many structural variations can be produced to precisely vary the acid properties of the molecule and to control the diffusion of the AG in the polymer matrix (11). 

The lithographic performance, as measured by sensitivity and contrast for the TBS, TBMS and TBSS polymers containing two onium salt AG systems and the tosylate AG is given in Table V. The absorbance of these films at 248nm was -0.3 im-1 in each case. 

Deep-UV resists comprised of matrix polymers and a 2,6-dinitrobenzyl tosylate photoactive acid generator have been described and compared to previously reported onium salt systems. Although these resists exhibited lower sensitivity than onium salt-based materials, the contrast and processibility are superior. The use of a matrix polymer capable of radiation-induced chain scission improves the sensitivity and allows the 2,6-dinitrobenzyl tosylate acid generator to more nearly... 

Onium salt-mediated couplings of /V-alkoxycarbonylcysteines and serines. Activated forms of these compounds are particularly sensitive to the tertiary amines required to effect the couplings (see Section 8.1). 

Onium salt-mediated couplings to the amino group of a peptide having an esterified carboxy-terminal cysteine residue. Esters of cysteine are sensitive to the base (see Section 8.1). 

The recent development of several new classes of practical photoinitiators for cationic polymerization has now made it possible to utilize this chemistry in a number of ways to produce highly sensitive photoresists (142-144). The facile synthesis of onium salts (I-III)... 

Initiators based on halonium and sulfonium salts are used commercially in various microlithographic processes and in the coating industry. Onium salts were developed commercially as photoinitiators due to the lower sensitivity of cationic polymerizations to oxygen compared to radical polymerizations. Aromatic halonium and sulfonium salts with complex anions such as SbF6, AsF6 and BF4- do not initiate cationic polymerizations spontaneously, but must be activated by UV irradiation. 

Onium salt photoinitiators have strong absorption bands in the deep UV region, but their sensitivity can be extended to longer wave lengths Xriaryj. 

This increased sensitivity is believed due to the fact that the onium salt produces a much stronger acid, in this case HSbF. Another contributing factor could be participation of free radicals, formed during irradiation of the onium salt. To test this hypothesis, experiments were performed with a latent UV-deblockable sulfonic acid. This compound produces both PTSA and free radicals when it is irradiated. Although the acid produced was PTSA, the curing result was consistent with the result from the onium salt experiment. These experiments indicates it is the free radicals which are effective in crosslinking the matrix. However, it may also just be a solubility effect, e.g. catalysts II and III may be simply more soluble in the MAGME-polymers than PTSA. Further experimentation is needed to determine whether it is a solubility effect or participation of free radicals that explains the low sensitivity of PTSA. In the experiments with pure PTSA, no increase in sensitivity was observed when the PTSA concentration was increased above 0.3J w/w. 

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