Positive-acting resists

Fig. 10. Chemistry of dia2onaphthoquiaone—ciesol novolac positive-acting resist. During pattemwise exposure, the DNQ undergoes photolysis that...

However, the important considerations are sensitivity and tone (positive or negative). When covalent bonds are formed, the increase in molecular weight results in negative-acting resists. A molecular weight decrease because of polymeric backbone scissions produces positive-acting resists. 

Positive-acting resists are subject to the same problems as are negative-acting resists, although they may be easier to remove cleanly, after exposure, from areas to be etched or plated. Problems related to plating photoresist-coated boards are reviewed in Chap. 29. 

Materials that exhibit enhanced solubility after exposure to radiation are defined as positive resists. Positive acting materials are particularly attractive for the production of VLSI devices because of their high resolution properties. The chemistry of these systems generally involves either chain-scission or solution-inhibition mechanisms. 

The positive resist materials evolved from discoveries made by the Kalle Corporation in Germany who developed the first positive-acting photoresist based on the use of a novolac matrix resin and a diazoquinone photoactive compound or sensitizer. The original materials were designed to produce photoplates used in the printing industry. These same materials have been adopted by semi-conductor fabrication engineers and continue to function effectively in that more demanding application. 

A modification of these systems involving replacement of poly (4-vinylphenol) with poly(methyl isopropenyl ketone) (PMIPK) was reported by Nakane and co-workers (41). Although PMIPK is a positive-acting singlecomponent resist in the deep UV, it functions as a negative resist when mixed with an aromatic bisazide such as 2,6 di(4-azidobenzylidene)-4-... 

Apart from multi-level layer resist systems, conventional positive-tone resists can be classified into two categories one-component and two-component systems. Classical examples of the former systems are polyfmethyl methacrylate), and poly (butene-1-sulfone) (2,3). Typical examples of the latter system are AZ-type photoresists, which are mixtures of cresol-formaldehyde-Novolak resins and a photoactive compound acting as a dissolution inhibitor... 

Both negative and positive acting, oxygen RIE resistnt e-beam resist systems have been prepared through the incorporation of the trimethylsilylmethyl functionality into standard resist chemistry. Resins containing >10 wt% silicon display an RIE resistance more than 10 times greater than conventional photoresists and allow submicron pattern transfer with minimum linewidth loss. 

The origin of these effects lies in the different radiation susceptibilities of the two copolymer components. PDMS is a negative-acting resist whose sensitivity has been reported to be for a molecular weight of 71,800 (5). PMMA, on the other hand, is positive... 

By general convention, the sensitivity, S, is related to the thickness, d, of the resist layer measured after exposure and development, and is obtained from exposure characteristic curves, as are illustrated in Fig. 9.3. In the case of positively functioning resists, S = corresponds to the exposure light dose required to completely remove the irradiated polymer from the substrate, i.e. the dose at which the normalized thickness of the resist layer is equal to zero, dirr/do=0. In the case of negatively acting resists, the sensitivity is reported as S = or... 

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