Poly , chain scission positive resists

Another class of "chain scission" positive resists is the poly(olefin sulfones). These polymers are alternating copolymers of an olefin and sulfur dioxide. The relatively weak C-S bond is readily cleaved upon irradiation and several sensitive resists have been developed based on this chemistry (49,50). One of these materials, poly(butene-l sulfone) (PBS) has been made commercially available for mask making. PBS exhibits an e-beam sensitivity of 1.6 pC cm-2 at 20 kV and 0.25 pm resolution. 

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 second olefin copolymer with a more promising structure is that with isobutylene (2-methylpropene-l). Poly isobutylene itself is a chain-scissioning polymer which has been studied often. It is not much used as a positive resist despite its G(s) of 1.5 to 5 (21) because its Tg is so low, about -60°C. 

Another interesting, DUV-sensitive, planarizing layer for the exposure-PCM scheme is poly(dimethyl glutarimide) (PMGI) (structure 3.7). Exposure of PMGI to DUV light or electron beam radiation results in main-chain scission therefore, this resist is positive working. 

Another group of positive UV resists operating on the principle of radiation-induced main chain scission utilizes the efficient photochemistry of polymeric ketones, exemplified hy poly(methyl isopropenyl ketone) PMIPK, to effect image discrimination. Scheme 7.21 shows the photolysis of this resist. ... 

Greater resolution can be obtained nsing deep UV radiation, but now other resists must be used as the conventional ones are optically opaque in this region. Poly(methyl methacrylate) can be nsed as a positive resist, as can several of its derivatives. In each case, the carbonyl gronps absorb at 215 run, and this leads to chain scission and degradation. 

A classical example of a chain scission resist is poly(methyl methacrylate) (PMMA) (see Scheme 1). The high molecular weight PMMA resist imdergoes chain scission induced by e-beam radiation. The low molecular weight fractions in the exposed areas are soluble in the developer, resulting in positive-tone features. 

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