Photoresist diazoquinone positive

The addition of specialized small molecules to a polymer coating is the functional basis for most photoresists. Conventional positive-working photoresists function owing to the difference in solubility caused by the imagewise exposure of a small molecule naphthalene diazoquinone sulfonate ester (NDS). The presence of this small molecule dramatically inhibits the dissolution of the novolac binder while its photodecomposition accelerates the binder dissolution in aqueous base. 

Positive photoresists, by contrast, are based on water-soluble novolak resins with naphthalene diazoquinone sulfonate (NDS) as the photosensi-tiser. On photolysis the NDS causes a rearrangement in the polymer to yield nitrogen gas plus an indene carboxylic acid. This latter functional group considerably increases the solubility of the polymer, hence solubilising those areas of the polymer that had been exposed to light. 

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. 

The latest addition to this list of dry developing resist materials is a contribution from IBM s San Jose Research Laboratory (66-67) that evolved from efforts to design positive-tone resist materials that incorporate chemical amplification. These efforts were stimulated by the fact that the quantum yield of typical diazoquinones of the sort used in the formulation of positive photoresists is 0.2 to 0.3 thus, three or four photons are required to transform a single molecule of sensitizer. This places a fundamental limit on the photo-sensitivity of such systems. 

The commercial positive photoresists are based on the photoinduced Wolf rearrangement of a diazoquinone to a indene carboxylic acld ... 

A positive resist system can be of either two types. The classical diazoquinone system represents a photochemical rearrangement reaction which is the basis of commercial photoresists. Scissloning or degradation of a polymer chain by light or electrons Is a later example of solubility induced change. We will examine this change in detail. 

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). 

The classical example of radiation induced polarity change is the photodecomposition of diazoqui none into an ionizable compound, the indene carboxylic acid. This reaction is the basis of one of the important positive photoresists discussed in Chapter 7. Attaching diazoquinone units to a polymer backbone, for example, via acrylic side groups, makes the unexposed polymer soluble in organic sol vents and the exposed polymer soluble in dilute aqueous basic solutions. 

Positive photoresist chemistry. A. Photolysis of a diazoquinone creates a much more polar environment allowing aqueous base to dissolve a Bakelite-type polymer. B. Photogenerated acid and an example of a polymer whose solubility would change dramatically after add is generated locally. C. A single component positive photoresist. 

For conventional positive diazoquinone/novolac resists (Section 7.2.3), the exposure bleaches the photoactive compound (NDS) and therefore the problem of reflection becomes more severe. Three approaches to solving this problem all involve putting absorbing dyes in various layers of a multilayer configuration. Dyes may be placed directly in the photoresist layer, in the planarizing layer, or in a separate antireflection coating (ARC). The initial work with dyes in photoresists was in the ARC format, where a thin layer of dyed polymer is coated underneath the photoresist. The dyed layer may be directly on the substrate or on top of a planarizing layer. 

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