Deep-UV photoresists

KrF exposure systems (step and repeat) already exist commercially and are used for 0.25 pm linewidth generation. But most emphasis has been recently put on the 195 nm exposnre systems for 0.18 pm linewidth generation. The next generation of components will reqnire linewidths below 0.12 pm. 

The sensitivity of this terpolymer is 170 times that of PMMA and is capable of sub-micron resolntion. Poly(aromatic snlfones) may also be nsed as positive resists [19]. Their quantum efficiency is low, however. Aromatic moieties ensnre a useful plasma etch resistance. 

The resist exhibits reasonable sensitivity. However, the novolac resin itself has a strong absorption around 310 nm, which is bad for the sensitivity. 

Wilkins and others [21] have described another system. In this case, the resin is a copolymer of methyl methacrylate and methacrylic acid (which is transparent above 260 nm and which is soluble in aqueous alkali) and the dissolution inhibitor is an o-nitrobenzyl carboxylate. This system exhibits a good photosensitivity (100 mj/cm in the 230-300 nm range) with a very high contrast (g 5). 

An evolution toward the lower wavelength is in progress with the use of the 193 nm ArF laser source. Optical absorption requirements demand new resist polymers with no 

---

The development of new classes of cationic photoinitiators has played a critical role in the production of highly sensitive, acid-catalyzed deep-uv photoresists. Sulfonium salts have been widely used in this respect (4). These materials are relatively easy to prepare and structural modifications can be used to produce desired wavelength sensitivity. Triphenylsulfonium salts are particularly well suited for deep-uv application and in addition can be photosensitized for longer wavelength. These salts are quite stable thermally and certain ones such as the hexafluoroantimonate salt are soluble in casting solvents and thus easily incorporated within resist materials. 

One attractive possibility is the use of the a-keto-oxime chromophore. It has a strong absorption at 220 nm whose tail, which extends to 240-250 nm, would improve the absorption characteristics of PMMA. Also, the esters possess a N-O bond which is photochemically labile yet sufficiently thermally stable so as to be compatible with the various processing steps. The solution degradation of a-keto oximino methacrylate esters upon irradiation with light of X365 nm has been reported by Delz-enne (2), and we proceeded to investigate the solid state photodegradation of similar copolymers and their possible utility as deep UV photoresists. 

A mixture of three isomeric cresols is used in a commercially available cresol-formaldehyde Novolak resin. This mixed Novolak resin, Varcum resin (12), provides adequate properties as a host resin for near-UV- and mid-UV-photoresist applications. Gipstein and his co-workers prepared pure cresol-formaldehyde Novolak resin from each isomeric cresol and compared their spectroscopic and resist characteristics (13). Their data on the UV-absorption spectra of each cresol-formaldehyde Novolak resin together with the commercially available Varcum resin are as follows the absorbances of 0.2 jim thick Novolak films at 250 nm are 0.165(Varcum), 0.096(o-cresol), 0.092(m-cresol), and 0.055(p-cresol). The so-called "window" in the UV absorption at around 250 nm is a maximum with the p-cresol-formaldehyde Novolak resin, while the other isomeric cresol and formaldehyde Novolak resins yielded similar UV absorptions at this wavelength. The smallest UV absorption at 254 nm is an advantage for the p-cresol-formaldehyde Novolak when the resin is used for a deep UV photoresist with a suitable photoactive compound (14). 

Grant, N.J. Clecak, RJ. Tweig, and C.G. Willson, Deep UV photoresists I. Meldrum s diazo sensitizer, IEEE Trans. Electron Devices ED-28(11), 1300 (1981). 

Johnson, G.J. Stagman, J.C. Sardella, C.R. Spinner III, F. Liou, P. Trefonas, and C. Meister, The effects of absorptive dye loading and substrate reflectivity on a 0.5 p,m i line photoresist process, Proc. SPIE 1925, 552 563 (1993) W. Conley, R. Akkapeddi, J. Fahey, G. Hefferon, S. Holmes, G. Spinillo, J. Sturtevant, and K. Welsh, Improved reflectivity control of APEX E posi tive tone deep UV photoresist, Proc. SPIE 2195, 461 476 (1994). 

Add Labile Cross-Linked Units A Concept for Improved Positive Deep-UV Photoresists... 

A positive deep-UV photoresist system has been described by Ueno and coworkers that consists of 134 t niethanesulfonyloiq )benzene, bisphenol-A... 

Chemical Amplified Deep-UV Photoresists Based on Acetal-Protected Pofy(vinylphenols)... 

A chemically amplified deep UV photoresist system based on acetal chemistiy is reported. Acetal-protected pol vinylphenols) were prepared either by free radical polymerization of the monomers or chemical modification of poly(vinylphenol). In the presence of an add as a catalyst, the polymers thermally decomposed to aqueous base soluble po vinylphenol) and some small molecules. Therefore, the resists were formulated with the acetal-protected polymers and a photoadd generator such as triphei lsulfonium hex-afluoroantimonate. Positive-tone image could be resolved 1 exposing the resist film in deep UV region, post-baking, and developing in tetramethylam-monium hydroxide solutions. 

In this paper we report a new chemically amplified deep UV photoresist system based on acetal-protected poly(vinylphenoIs) and a photoadd generator. 

Inaki, Y, Moghaddam, M.J., Kanbara, K., and Takemoto, K., Pyrimidine polymers as high resolution, high sensitivity deep-UV photoresists, J. Photopoly. Sci Technol, 1, 28, 1988. 

---