Photoresist positive tone

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Positive-tone photoresists, 15 161-163 acid-catalyzed chemistry in, 15 169-170 Positive-working dye processes, 19 284... 

Positive-Tone Photoresists. The ester, carbonate, and ketal acidolysis reactions which form the basis of most positive tone CA resists are thought to proceed under specific acid catalysis (62). In this mechanism, illustrated in Figure 22 for the hydrolysis of tert-butyl acetate (type A l) (63), the first step involves a rapid equilibrium where the proton is transferred between the photogenerated acid and the acid-labile protecting group ... 

In a subsequent investigation by the author [1] molecular positive tone photoresist blends consisting of nonpolymeric octakis(dimethylsilyloxy)sil-sesquioxane, (VII), and acid-IabUe bulky substiutents including 2-t-butyl tetracyclo-dodec-3-ene-5-carboxyIate, (VIII), N-(2-tetrahydro-2H-pyran-2-yloxy)-5-norbornene-2,3-dicarboximide, (IX), and norbornene anhydride, (X), were prepared and activated at 248 nm, 193 nm, 157 nm, and 134 nm, respectively. 

Carr [2] prepared positive tone photoresists activated below 200 nm by copolymerizing fluorinated bridged carbocyclic compounds, (XI) and (Xll), with other fluorinated unsaturated bridged carbocyclic monomers. 

Positive tone photoresist resins activated below 200 nm were prepared by Inoue [3] by copolymerizing polycyclic monomers, (Xlll) and (XIV), with t-butyl-trifluoromethyl acrylate, (XV). 

Fig. 2 Typical exposure or characteristical curves, do is the maximum thickness of a resist layer (A) a negative tone photoresist (B) a positive tone photoresist.

Photosensitized degradation of poly(olefin sulfones) similar to the Hg(3P) photosensitized reactions of olefin sulfones make them subject to photodegradation in easily accessible wavelength regions. Almost all poly(olefin sulfones) have been reported only as positive tone electron beam resists (4). As the only exception, poly(5-hexene-2-one sulfone) has been reported as a positive tone photoresist with or without a photosensitizer, benzophenone (5). Because this polymer has a carbonyl chromophore, its photosensitivity is clearly derived from the polymer structure itself. 

During the 1970s and 1980s, the common exposure wavelengths were 436 and 365 nm using the G- and I-lines from an Hg lamp [4]. Both negative- and positive-tone photoresists were employed. 

Figure 17.10 shows the process sequence of the hydrophilic overlayer (HOL) process. First, a chemically amplified or non-chemically amplified positive-tone photoresist comprising hydrophobic polymer and appropriate PAG is coated to a nominal thickness on an appropriate substrate such as a silicon wafer, followed by a soft bake to dry out the nonaqueous solvent. Next, the photoresist film is exposed to radiation of appropriate wavelength to generate photoacid from the PAG. Then the exposed film is again baked (called PEB) at the standard temperature to enhance the diffusion of the photoacid and thermolysis of the acid-labile protecting groups of the polymers. 

Photo-rearrangements in polymers are important because they can lead to pronounced property changes. For example, polymers containing o-nitrobenzyl pendant groups become soluble in aqueous solution since benzyl ester groups are converted into carboxyl groups. Therefore, such polymers are applicable as positive-tone photoresists in lithographic processes [50, 51] (see Section 9.1). 

SCHEME 57.1. Chemical amplification in a positive-tone photoresist. 

Novolak based resists that have not been exposed to UV light are therefore almost insoluble, but the exposed resist is highly soluble. Novolaks are therefore used as positive tone photoresists. Novolak chemistry has proved very effective for producing high resolution photolithographic patterns (Fig. 13.2) and is used extensively industry. An example of a Novolak-based commercial positive resist is S1813 from Rohm Haas, which was developed for the microelectronics integrated circuits industry. 

Figure 4.42. Molecular structures and photoinduced reactions of common photoresists. Shown are (a) the diazonaphthoquinone (DNQ) positive tone photoresist, and (b) the SU-8 epoxy-based negative tone photoresist.

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