Diazonaphthoquinone/novolak resists

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.

Dammel, Diazonaphthoquinone based Resists, p. 58, SPIE Press, Bellingham, WA (1993). 206t E Yeh, H.Y. Shi, and A. Reiser, A percolation view of novolak dissolution and inhibition, Proc. SPIE 1672, 204 (1992) Percolation view of novolak dissolution and dissolution inhibition, Macromolecules 25, 5345 5352 (1992). 

Figure 1. Dissolution rates of a composite resist made of a diazonaphthoquinone sensitizer and o-chloro-m-cresol-formaldehyde Novolak resin after 5 /cm2 electron beam exposures. Note this kind of an induction period appeared only in the high-molecular-weight fraction resin.

Novolak diazonaphthoquinone positive-tone resists, the most important imaging system of semiconductor production today1510,1511 is an archetypal example of the industrial applications of photochemistry. Novolak is a phenol formaldehyde polymer (Bakelite) that dissolves in aqueous hydroxide, but the addition of a small amount of the diazonaphthoquinone 585 dramatically decreases the solubility. When irradiated, 585 undergoes the photo-Wolff rearrangement (see also Scheme 6.171), leading to ring contraction and subsequently to carboxylic acid formation (Scheme 6.284). Such a photochemically altered site is readily soluble and can be removed with a basic developer solution. 

Material. The resist material photoactive compound (PAC), a diazonaphthoquinone derivative in a novolak resin matrix, was used as a control and also reformulated to obtain PAC/pyrene. Pyrene was obtained from Aldrich and was added to the PAC in this study in the quantity of 2.5% of the total solids content of the resist. The resists were mixed on a roller overnight and then filtered through a 0.8pm silver membrane filter. 

Most of the ICs were obtained makinguseofaresistformulation discovered by Suss, namely, diazonaphthoquinone as PAC and novolak resin (Chart 12.1) as polymer matrix [12]. The resolution achieved with this resist formulation was smaller than 500 nm [13]. Novolak is obtained through a polycondensation reaction between formaldehyde and cresols [4,14]. The novolak resin is photochemically inert at 436 and 365 nm, and is easily soluble in basic developers due to its phenolic OH groups, but upon addition of naphthoquinone the dissolution rate decreases dramatically [15,16]. 

Resists are comprised of several components. The classic composition of negative photoresist is an azide sensitized rubber such as a polyisoprene polymer base with a bis-aryldiazide sensitizer, and a carrier solvent that makes the resist a liquid, allowing it to be spun on the wafer as a thin layer (sometimes additives are included such as dyes to control the light rays). A typical positive photoresist would be aphenol-formaldahyde Novolak resin structure and a diazonaphthoquinone sensitizer in a carrier solvent. 

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