Francis M. Houlihan

Evaluation of cycloolefin-maleic anhydride alternating copolymers as single-layer photoresists for 193-nm photolithography

We have developed a fundamentally new class of photoresist matrix resins for use in 193 and 248 nm lithography based on cycloolefin-maleic anhydride alternating copolymers. When used in three-component formulations with cholate-based dissolution inhibitions (DIs) and conventional photoacid generators, these copolymers afford positive-tone resists with potential sub-0.25 micrometer image fidelity. The resists exhibit high contrast (3 - 5.5) and high sensitivity (3 - 5 mJ/cm2 at 248 nm, depending on exact formulation) with low loadings (ca. 1 wt%) of triphenylsulfonium salt photoacid generators. These formulations are sufficiently transparent to be used at 193 nm without further modification.

Radiation chemistry of polymeric materials: novel chemistry and applications for microlithography

In the last two decades, major advances in fabricating very large scale integration (VLSI) electronic devices have placed increasing demands on microlithography, the technology used to generate todays integrated circuits. In 1970, state-of-the-art devices contained several thousand transistors with minimum features of 10-12 μm. Today, they have several million transistors and minimum features of less than 0.3 μm. Within the next 10-15 years, a new form of lithography will be required that routinely produces features of less than 0.2 μm. Short-wavelength (deep-UV) photolithography and scanning and projection electron-beam and X-ray lithography are the possible alternatives to conventional photolithography. The consensus candidate for the next generation of lithography tools is photolithography using 193 nm light. At this wavelength, the opacity of traditional materials precludes their use, and major research efforts to develop alternative materials are currently underway. Notably, the materials being developed for these short UV wavelengths are demonstrating compatibility with the more advanced electron-beam technologies. Materials properties must be carefully tailored to maximize lithographic performance with minimal sacrifice of other performance attributes, eg adhesion, solubility and RF plasma etching stability.

Preliminary lithographic characteristics of an all-organic chemically amplified resist formulation for single-layer deep-UV lithography

When used in conjunction with a nitrobenzylester photoacid generator, poly(t-butoxycarbonyloxy-styrene-sulfone) deep-UV resist films exhibit high contrast, good resolution and linewidth stability. Use of overcoat materials dramatically reduce the surface inhibition problems, improve the latent image stability (time delay) and enhance the sensitivity by isolating the resist surface from environmental contaminants that react with the photogenerated acid. The photospeed of the all organic CAMP formulation is lower compared to the arsenate based system but can be improved by using more aggressive PEB conditions. Coded, 0.35 micrometers l/s pairs could be resolved in 1 micrometers thick resist films at a dose of 20-30 mJ/cm2. The exposure latitude is approximately equals 25% for 0.5 micrometers features, upon exposure with a GCA prototype deep-UV exposure tool with a NA equals 0.35 and 5x reduction optics. This paper will discuss the resolution, depth-of-focus, exposure latitude and processing characteristics obtained during the evaluation of this chemically amplified resist.

Recent advances in 193 nm single-layer photoresists based on alternating copolymers of cycloolefins

We report on our recent investigations on the formulation and processing of 193 nm single layer photoresists based on alternating copolymers of cycloolefins with maleic anhydride. Resists formulated with cycloolefin copolymers are compatible with 0.262 N tetramethylammonium developers, have excellent adhesion, sensitivity, etch resistance and thermal flow properties. The effect of polymer structure and composition, dissolution inhibitor structure and loading as well as the effect of the photoacid generator on the resist dissolution properties was investigated. Based on the results high contrast formulations were evaluated on a GCA XLS (NA equals 0.53, 4X reduction optics) deep-UV stepper to exhibit 0.27 micrometer L/S pair resolution with excellent photosensitivity. Based on the dissolution properties and a spectroscopic examination of the resist, we have designed materials that show less than 0.17 micrometer L/S pair resolution with 193 nm exposures. In this paper, the formulation methodology is detailed and the most recent results upon both with 248 and 193 nm irradiation are described.

Resist outgassing as a function of differing photoadditives

The effect of different photoadditives in high and low activation energy resist resins on resist outgassing during lithographic exposure was studied by quartz microbalance and gas chromatography/mass spectroscopy techniques. The resist outgassing was analyzed both qualitatively and quantitatively and structure-property relationships were developed between resist outgassing and the molecular structure of photoacid generators and additives. The photoadditives examined include, aryl iodonium perfluoroalkylsulfonates, triarylsulfonium perfluoroakylsulfonates, photogenerators of sulfamic acids, 2-nitrobenzyl PAGs and doxyl derivatives.

193-nm single-layer photoresists based on alternating copolymers of cycloolefins: the use of photogenerators of sulfamic acids

Single layer resists for 193 nm based upon resins derived from alternating copolymers of cycloolefins and maleic anhydride will be discussed. Our past work has examined the effect of polymer structure and composition, dissolution inhibitor structure and loading as well as the effect of the photoacid generator on the resist dissolution properties. In this paper, we will report upon on some of our recent investigations aimed at improving performance by use of a new class of photoreactive additives, photogenerators of aminosulfonic acids. One example of these, bis(t- butylphenyl)iodonium cyclamate, will be shown in our high activation 193 nm single layer resist system as being a useful photodecomposable base additive capable of limiting acid diffusion and alleviating post exposure bake delay effects. Finally, we will describe the utility of these materials in low activation energy (acetal based) resist systems.

Fluorinated dissolution inhibitors for 157-nm lithography

Fluorinated dissolution inhibitors (DIs) for 157 nm lithography were designed and synthesized as part of an ongoing study on the structure/property relationships of photoresist additives. The problem of volatilization of small DI candidates was observed from matrices such as poly(methyl methacrylate) (PMMA) and poly(hexafluorohydroxy-isopropyl styrene) (PHFHIPS) during post-apply bake cycles using Fourier Transform Infrared Spectroscopy (FT-IR). To avoid this problem, low volatility fluorinated inhibitors were designed and synthesized. Three fluorinated DIs, perfluorosuberic acid bis-(2,2,2,-trifluoro-1-phenyl-1-trifluoromethyl-ethyl) ester (PFSE1), perfluorosuberic acid bis-[1-(4-trifluoromethyl-phenyl)-ethyl] ester (PFSE2) and a fluorinated phenylmethanediol diester (FPMD1), largely remained in a PHFHIPS film during the post-apply bake. The dissolution behavior of the two fluorinated diesters was studied and found to slow down the dissolution rate of PHFHIPS with inhibition factors of 1.9 and 1.6, respectively. The absorbance of PHFHIPS films containing 10 wt% of the diester inhibitors is 3.6 AU/micron compared with an absorbance of 3.3 AU/micron for the polymer itself. The absorbance of 10% FPMD1 in PHFHIPS was measured as 3.5 AU/micron compared with an absorbance of 3.4 AU/micron for the polymer itself. Thus, the non-volatility and transparency of the fluorinated inhibitors at 157 nm as well as their ability to reduce the development rate of fluorinated polymers make them suitable for use in a 157 nm resist system.

New polymers for 193-nm single-layer resists based on substituted cycloolefins/maleic anhydride resins

A series of new polymers for 193 nm single layer resist based on maleic anhydride/cycloolefin systems with minimum amount of acrylate units were synthesized. In order to minimize the acrylate content, the cycloolefin moiety of the polymers was functionalized with side groups designed to either promotes adhesion to silicon substrate and/or impart the imaging functionality. All polymers were prepared by free-radical polymerization in moderate to high yields and were characterized by variety of techniques. The initial lithographic evaluation of the new resists was carried out. It was found that acrylates can be successfully replaced with appropriately substituted cycloolefins to provide good resolution. The etch resistance of the new materials generally improves with increase in cycloolefin content. The Onishi and Kunz type plots will be discussed.

Synthesis, characterization, and lithography of α-substituted 2-nitrobenzyl arylsulfonate photo-acid generators with improved resistance to post-exposure bake

A new series of (alpha) -substituted-2-nitrobenzyl arylsulfonate photo-acid generators (PAG) was synthesized. A study of the thermal stability of the PAGs upon varying the (alpha) - substituent was done. The thermal stability was evaluated because there is a correlation between the PAG thermal stability and the post-exposure bake (PEB) temperature tolerance of resists formulated with 2-nitrobenzyl arylsulfonate PAGs and t-BOC polymers. The best thermal stabilities were obtained by having a bulky electron withdrawing group situated at both the (omicron) - and (alpha) -positions of the 2-nitrobenzyl chromophore. This substitution pattern enhances the thermal stability by suppressing the nucleophilic displacement of the sulfonate group by the 2-nitro group oxygen. Increasing the electron withdrawing ability of the (alpha) -substituent decreased the quantum yield for the photogeneration of acid from the 2-nitrobenzyl chromophore. However, one of these (alpha) -substituents, (alpha) -alkoxycarbonyl, was found to give PAGs with optimum thermal stability and quantum yield. These chromophores were used to synthesize PAGs based on strong arylsulfonic acids. The PAGs protected with these new chromophores allow for a higher PEB tolerance, in poly(4-(t-butyoxycarbonyloxy)styrene-sulfone) based resists, than was possible with the 2-(trifluoromethyl)-6-nitrobenzyl chromophore. It was possible to resolve small features (0.35 micrometers ) with a PEB at 135 degree(s)C with PAGs based on this new chromophore. In contrast, formulations based on a 2- (trifluoromethyl)-6-nitrobenzyl PAG of the same acid do not have this resolution at 135 degree(s)C because of poor thermal stability.

Polymers for microlithographic applications: new directions and challenges

Advances in the fabrication technologies associated with electronic devices have placed increasing demands on microlithography, the technology used to generate todays integrated circuits. Within the next few years, a new form of lithography will be required that routinely produces features of less than 0.1 μm. As the exposing wavelength of light decreases to facilitate higher resolution imaging, the opacity of traditional materials precludes their use; and major research efforts to develop alternate materials are underway. As a current example, lithography tools utilizing 193 nm light are now being introduced into the manufacturing environment. Through understanding of materials structure and its relationship to device process requirements and performance, a new class of cyclo-olefin based polymers was designed for these applications. In particular, alicyclic monomers such as norbornene are readily copolymerized with maleic anhydride and substituted acrylates to afford a wide range of alternative matrices that exhibit transparency at the exposing wavelength and aqueous base solubility. Materials properties must be carefully tailored to maximize lithographic performance with minimal sacrifice of other performance attributes. Further reduction in exposing wavelength to 157 nm introduces new challenges in polymer materials design. Efforts to address those challenges will be discussed.