Mid- and Deep-UV Photoresists

However, the most important conclusion of this work is that the polymers show the same relative sensitivity to synchrotron radiation as they do to mid and deep UV. This has important significance in the design of photoresists for use with soft X-ray sources. 

Conventional positive photoresists only have limited utility in the mid-UV and deep-UV range. While it appears feasible to modify the structure of the o-quinonediazides to optimize their absorption characteristics in the mid-UV range, (8,9) or even to use appropriate sensitizers for conventional resists,... 

A mixture of three isomeric cresols is used in a commercially available cresol-formaldehyde Novolak resin. This mixed Novolak resin, Varcum resin (12), provides adequate properties as a host resin for near-UV- and mid-UV-photoresist applications. Gipstein and his co-workers prepared pure cresol-formaldehyde Novolak resin from each isomeric cresol and compared their spectroscopic and resist characteristics (13). Their data on the UV-absorption spectra of each cresol-formaldehyde Novolak resin together with the commercially available Varcum resin are as follows the absorbances of 0.2 jim thick Novolak films at 250 nm are 0.165(Varcum), 0.096(o-cresol), 0.092(m-cresol), and 0.055(p-cresol). The so-called "window" in the UV absorption at around 250 nm is a maximum with the p-cresol-formaldehyde Novolak resin, while the other isomeric cresol and formaldehyde Novolak resins yielded similar UV absorptions at this wavelength. The smallest UV absorption at 254 nm is an advantage for the p-cresol-formaldehyde Novolak when the resin is used for a deep UV photoresist with a suitable photoactive compound (14). 

The imaging properties of PVTMSK were studied by spin coating 350-nm-thick films on silicon wafers or on silicon wafers precoated with a 1.5-(xm-thick layer of hard-baked photoresist, exposing them to mid- or deep-UV radiation through a chromium-on-quartz lithographic mask, and developing the pattern as described earlier. This scheme was used to test the intended application of PVTMSK as an imaging material for two-layer resist applications. The densest patterns resolved were composed of l-(xm coded lines and spaces. 

These polymers are of potential interest as photoresists. Accordingly, their photochemistry was also studied in very thin (1-4 jtm) films which were spin-coated onto polished salt plates. After irradiation in a standard xenon arc photoilluminator, the loss of carbonyl could be determined from FTIR measurements. Experiments were carried out both at 254 nm (deep-UV) and 313 nm (mid-UV). Typical rate curves are shown in Figures 2 and 3. The... 

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). 

Novolak-based mid-ultra violet (MUV) photoresists or poly(4-hydroxystyrene)-based deep UV (DUV) photoresists at wavelengths of 365 and 248 nm, respectively. 

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