Novolak resists, acid sensitive

The TBOC based resists are examples of resists which can be aqueous alkaline developed. The PHOST resin used in acid sensitive resists are more transparent in the deep UV region than their novolak counterparts and thus have been the focus of more investigations. Previous attempts to use di-azquinone novolak resist in the deep UV region are hampered by the higher absorbance of the novolak resin and the diazoquinone photoproducts (4). Only thin films less than 500 run thick can be used for the deep UV region (4). 

Figure 18. Diazonaphthoquinone-novolac resist. The novolac (Novolak) matrix resin is prepared by acid catalyzed copolymerization of cresol and formaldehyde. The base insoluble sensitizer, a diazohaphthoquinone, undergoes photolysis to produce a carbene which then undergoes Wolff rearrangement to form a ketene. The ketene adds water which is present in, the film, to form a base soluble, indenecarboxylic acid photoproduct.

Various materials have been examined for use as deep UV resists poly(methyl methacrylate) (PMMA) (1), poly(methyl isopropenyl ketone) (PMIPK) fS.7L and the novolak-Meldrum s acid solution inhibition system (S). Each however has a problem related to sensitivity and/or resolution. While PMMA is insensitive to light of X > 230 nm because of its weak absorption, its high resolution properties make it an attractive starting point for the design of a resist that will perform well in the 230-280 region. The photochemical properties of PMMA could be modified by the incorporation of a small percentage of photolabile groups so as to have both the desired sensitivity and base polymer properties. 

The main components of most commercially available positive photoresists are novolak as a binder and naphthoquinone-diazdde as a light-sensitive component. This light-sensitive compound is not base soluble and acts as a dissolution inhibitor for the novolak, which results in a very low dissolution rate of unexposed resist in aqueous base developer. Upon exposure a reaction is induced to yield indene carboxylic acid via a ketene ... 

Depolymerization occurring in exposed resist areas either causes or increases solubility. Both, X-ray and EB resists operating on the basis of the dissolution inhibitor principle in conjunction with acid amplification, have been commercialized, and sensitivity values S(EB) = 9 pCcm and S(X) = 10-30 mjcm have been obtained with formulations containing a novolak resin, an acetal (AC) or orthoester (OE) as dissolution inhibitor and trichloromethyltriazine or tetrabromo-bisphenol as acid generators (Chart 5.13) [151a,b]. Here, S(EB) and S(X) are related to EB and X-ray radiation, respectively. 

A clever variation of the monomeric dissolution inhibitor concept (3-compo-nent system) makes use of the base-catalyzed opening of the lactone ring in cresolphthalein which becomes possible after catalytic deprotection (17b). In the t-BOC protected inhibitor, the quinomethane system cannot form, and the lacton ring is not hydrolysed. The additional acidic functionality improves the dissolution rate in the e q>osed resist (see Eq. 6). The e-beam sensitivity of a 3-component tystem based on this inhibitor in combination with a novolak matrix and a triphenylsulfonium triflate photoacid generator was reported to be 2-3 fiC/cm (17b). 

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