Imaging chemistry

After image transfer, the patterned resist must be readily and completely removable without substrate damage. The pattern often can be stripped from the substrate with a mild organic solvent. Proprietary stripper formulations or plasma oxidation treatments are utilized when the imaging chemistry or image transfer process has iasolubilized the pattern. 

Coordination chemistry can play a central role in diffusion transfer imaging chemistries, and reactions of coordination compounds control the diffusion of image-providing compounds in several image transfer systems. Of course, silver halide diffusion transfer has already been discussed in which a silver halide solvent enables the diffusion of undeveloped silver halide to a receiver, where it is developed into an image. 

Several other imaging systems based on Com/developer amplification are known, including systems with various diffusion transfer imaging chemistries. The principles are the same as those outlined here. Basically, most imaging chemistries useful with silver halide systems can be transformed into a low-silver or (sometimes) non-silver system with a suitable developer/Com complex redox combination for amplification on imagewise catalytic nuclei. 

Wolf reaction) and o-nitrophenylaldehyde photochemistry, but they do not involve electron transfer processes, and so are excluded from this chapter. Similarly, radiation or electron-beam initiated imaging chemistries are excluded. 

Browne, M.W., Mirror Image Chemistry Yielding New Products The NewYork Times Aug. 13,... 

When the minimum dimensions of a resist pattern are <100 nm, their dimensional tolerances approach the scale of the molecular components of the film. At this level, LER, random fluctuation in the width of a resist feature, may limit the advancement of lithography. Possible contribution to LER include polymer molecular weight, molecular weight distribution, molecular structure of resist components, inhomogeneity in component distribution within the film, statistical effects influencing film dissolution, intrinsic properties of the imaging, chemistry, image contrasts of irradiation, etc. 

Nucleophilic aromatic substitution reactions follow the well-established two-step addition-elimination mechanism via a Meisenheimer intermediate (Fig. 8.3). Indeed, reaction of fluoride ion with trifluoro- -triazine, gives the corresponding perfluorocarbanion system that has been directly observed by NMR spectroscopy, supporting this mechanistic rationale. This reactivity has been termed mirror-image chemistry, which contrasts the very well-known chemistry of... 

Reactions of perfluorinated alkenes, such as hexafluoropropene, with fluoride ion give perfluoroalkylcarbanions which can act as nucleophiles in S Ar reactions with perfluoroheteroaromatic systems (Fig. 8.13). These reactions are another example of mirror-image chemistry and reflect well-known Friedel-Crafts reactions of hydrocarbon systems that proceed by reaction of the corresponding electrophile and carbocationic intermediates. Poly substitution processes are possible and, indeed, all five fluorine atoms may be replaced upon reaction with an excess of tetrafluoroethylene and fluoride ion. ... 

FIGURE 8.13 Mirror-image chemistry, negative Friedel-Crafts reactions. 

Contamination by airborne basic substances such as amines and N-methylpyrrolidone (NMP) has been shown to be responsible for the formation of the surface insoluble layer. Because of the catalytic nature of the imaging chemistries, a trace amount on the order of 10 ppb of airborne basic substances absorbed in the resist film interfaces with desired acid-catalyzed reactions, and also certain polymers and groups can function as internal contamination. Activated carbon filtration of the enclosing atmosphere can alleviate the contamination problem. Application of a protective... 

Positive-Tone Imaging Chemistry. The ester, carbonate, and ketal acidol-ysis reactions that form the basis of most positive tone CA resists are thought to proceed by specific acid catalysis (28). In this mechanism (type AalI), illustrated here for the hydrolysis of tert-butyl acetate (29), the first step is a rapid equilibrium where the proton is transferred between the photogenerated acid and the ester ... 

Negative-Tone Imaging Chemistry. In many negative-tone CA resists cross-linking is employed to effect insolubilization of the exposed pattern. In the... 

Fig. 7. Representative imaging chemistry employed in negative-tone CA resists (top) epoxy polymers requiring organic solvent development (center) PHOST-based cross-linking systems requiring aqueous development and (bottom) monomeric cross-linking agents used in PHOST matrix polymers.

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