Bisazide sensitizer

Figure 9. Photochemical transformations of a bisazide sensitizer in negative photoresists based on cyclized polyisoprene.

Iwayanagi (40) recently extended this system into the mid-UV range by changing the sensitizer to an aromatic monoazide compound (4-azidochalcone). Insolubilization of this mid-UV resist does not result in a cross-linked matrix as occurs with the bisazide sensitized MRS. The primary reaction appears to be insertion of the reactive nitrene into a C-H bond on the ring, forming a secondary amine. 

A different approach has been used by Tsuda, Nakane and their collaborators (138,139) who showed that mixtures of poly(methylisopropenyl ketone) (PMIPK) containing a bisazide sensitizer such as 4,4 -diazodiphenyl sulfide or 4-methyl-2,6-di(4-azidobenzylidene) cyclohexanone function as negative dry-developable resists. It is claimed that the bisazide... 

Cyclized polyisoprene has been used as a photoresist by being sensitized with bisazides(1-3). Recently, H.Harada et al. have reported that a partially cyclized 1,2-polybutadiene showed good properties as a practical photoresist material in reproducing submicron patterns (U ). S.Shimazu et al, have studied the photochemical cleavage of 2,6-di(h -azidobenzal)cyclohexanone in a cyclized polyisoprene rubber matrix, and have reported that the principal photoreaction is the simultaneous cleavage of the both azido groups by absorption of a single photon with a U3% quantum yield(5 ). Their result does not support the biphotonic process in the photolysis of bisazide proposed by A.Reiser et al.(6 ). 

Negative Resists for Deep UV. There has been considerable effort recently devoted to the design of negative resists for deep-UV application. Iwayanagi and co-workers (37) have reported on the properties of resists composed of cyclized polyisoprene and several bisazides whose absorption maxima lie within the deep-UV region. Since the sensitizers do not absorb in the visible region they are referred to as "white resists" and are claimed to be 60-450 times more sensitive than PMMA. One of these resist is available commercially as... 

Several bisazides with absorption maxima between 240 and 290 nm have been examined as DUV sensitizers for cyclized polyisoprene (76). The sensitivity of DUV resist formulated with cyclized polyisoprene and 1 wt % of 3,3 -bisazidophenyl sulfone (structure 3.10) is 75 times higher than that of PMMA. These azide DUV resists suffer from the resolution limit imposed by low contrast and the swelling phenomenon, which is as expected from the preceding discussion. 

Typical resists include cyclized polyisoprene with a photosensitive crosslinking agent (ex bisazide) used in many negative photoresists, novolac resins with diazoquinone sensitizers and imidazole catalysts for positive photoresists, poly(oxystyrenes) with photosensitizers for UV resists, polysilanes for UV and X-ray resists, and polymethacrylates and methacrylate-styrenes for electron-beam resists (Clegg and Collyer, 1991). Also note the more recent use of novolac/diazonaphthoquinone photoresists for mid-UV resists for DRAM memory chips and chemically amplified photoacid-catalysed hydroxystyrene and acrylic resists for deep-UV lithography (Choudhury, 1997). 

A comparative study of polystyrene (PS) with bis(perfluorophenyl) azides 1-2 and the corresponding non-fluorinated bisazides 3-4 as deep-UV resists is reported. Inclusion of as low as 1.2 wt-% of 1 in PS led to 70% retention of thickness after photolysis and development. PS containing 2.4 wt-% of 1 is > 100 times more sensitive as a deep-UV negative resist than PS itself. The presence of 1 in PS also increased the contrast of the resist. On a molar basis, 1 was about 10 times as effective as non-fluorinated bisazide 3 in cross-linking PS while 2 was about 6 times as effective as 4. PS containing 2.4 wt-% of 1 was found to have a deep-UV sensitivity of 5-10 mJ cm and resolution of about 0.5 /tm. 

Pol ene (PS) is a negative deep UV (25) and electron beam resist showing high resolution (24) but low sensitivity (25). Apparentty no attempt has been made to improve the sensitivity of PS by adcUtion of a bisazide. We expected that addition of bis-PFPA to PS should result in a deep-UV resist with increased sensitivity. Thus, various amounts of bis-PFPA 1 or 2 and the corresponding nonfluorinated bisazide 3 or 4 were separately added to PS. The resist solutions were then spin-coated on NaQ discs, baked, photolyzed and developed. The intensity of the IR CH stretching absorption at 2924 cm" before and after development was used to estimate the retention of film thidcness. 

In conclusion, we have demonstrated that bis-PFPAs 1-2 are about 10 times better than the corresponding non-fluorinated bisazides 3-4 in cross-linking polystyrene. Utilization of the highly efficient cross-linking agents 1-2 significantly reduces the required amounts of cross-linkers in PS-based resists and increases the sensitivities of the resists. 

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