Polyphthalaldehyde

Films of polyphthalaldehyde, sensitized by cationic photoinitiators, have been imaged at 2-5-mJ/cm in the deep ultraviolet (DUV) (see Section 3.10), at 1 pC/cm (20 kV) electron beam radiation and at an unspecified dose of Al-A x-ray radiation. The ultimate utility of this "self-developing" resist system will depend upon its efficacy as an etch barrier. It seems clear that such materials would not serve as adequate etch masks for... 

Recently, many similar systems have been reported in the literature. Examples include acid-photogenerating compounds in combination with t-BOC-protected maleimide or hydroxyphenyl maleimide copolymers (69, 70), tert-butyl ethers of phenolic resins (71), silylated phenolic resins (72, 73), and polycarbonates that contain acid-labile linkages in their backbone (74, 75). Onium-salt-photogenerated acid has also been used in another two-component system to catalytically depolymerize polyphthalaldehyde (76). 

Other three-component systems based on this chemistry have made use of the following acid-labile dissolution inhibitors polyphthalaldehyde (80), ketals of (J-ketoesters (81), and compounds containing C-O-Si bonds (82). Similar resists have also been used with other radiation sources these will be discussed in subsequent sections. 

Figure 3.40. SEM of positive image self-developed in polyphthalaldehyde by a KrF excimer laser. Reproduced with permission from reference 105. Copyright 1986 Electrochemical Society.)...

Ito and Wilson (12) have extensively investigated another resist system of this type based on polyphthalaldehyde (Equation 24). 

In a typical experiment, polyphthalaldehyde was dissolved in bis-(2-methoxyethyl) ether or cyclohexanone, to which was added the onium salt at 10 wt% to the polymer. Films spin coated on Si wafers were baked at 100°C for 10 minutes and then image-wise exposed. Optical micrographs of the resist images generated upon UV, e-beam, and x-ray radiations are exhibited in Figures 7a, b, and c, respectively. 

The UV sensitivity of the polyphthalaldehyde-onium salt system is dependent on the concentration and structure of the onium salts. At 10 wt% loading the sensitivity to narrow band width 254 nm radiation is 1-7 mJ/cm2 and is insensitive to the structure of the salts. However, at 2 wt% loading of triphenylsulfonium or diphenyliodonium hexafluoroarsenates or -hexyloxybenzenediazonium tetrafluoroborate much higher doses are required to achieve self-development and the sensitivity decreases with the salt structures in the sequence listed. 

Formation of monomeric phthalaldehyde due to acidolysis and depolymerization of polyphthalaldehyde initiated by the photolysis of onium salts is clearly demonstrated by IR spectroscopy (Figure 8). Figure 8c shows vaporization of the aldehyde monomer upon heating. The monomeric phthalaldehyde is crystalline at room temperature but sublimes under vacuum without heating. We have successfully delineated self-developed images in a 2.8 p thick film at a dose of 1.0 pC/cm2 of 20 keV e-beam radiation, which were subsequently transferred to silicon oxide by treatment with either buffered HF or CF4/02 plasma. 

We are continuing to explore the limits of the PBOCST and polyphthalaldehyde resist systems, as well as other materials that incorporate chemical amplification. 

Figure 7. Self-developed images in polyphthalaldehyde (a) 254 nm deep UV, Ph3SAsF6, (b) 20 keV e-beam, PhjAsFg, and (c) Al-Ko x-ray, Ph2IAsF6.

Figure 8. IR spectra of polyphthalaldehyde/Ph2IAsF6 before and after exposure to 254 nm radiation and after postbake.

Synthesis of polyphthalaldehyde, PBOCST, and early contributions to the chemical amplification concept by Professor Jean Frechet of the University of Ottawa are gratefully acknowledged. The authors thank S. MacDonald, R. Kwong and R. Cox for preparation of various diazonium salts, and J. Carothers, D. Mathias, C. Hrusa, and C. Cole for assistance in TGA, GPC, and 13C NMR measurements. The iodonium and sulfonium salts used in the preliminary stages of this work were generously provided by V. Lee. 

Fig. 108 Acid-catalyzed depolymerization of polyphthalaldehyde for self-development...

The very first chemical amplification resist designed for use in bilayer lithography employed Si-containing polyphthalaldehyde as a top layer resin,... 

FIGURE 5.48 Photochemistry of poly(4-t-butoxycarbonyloxy styrene) doped with (a) triphenylsulfonium hexafluoroantimonate, (b) a dry developable polycarbonate, and (c) self-developing polyphthalaldehyde. 

Figure 25 Three-dimensional nanoengraving in polyphthalaldehyde (PPA) with a hot AFM tip with a 10 nm resolution defined by the tip s curvature (tool NanoFrazor). Courtesy ofP Paul and F. Hoizner from SwissLitho Ltd.

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