Chapter 15
Chemically Amplified X-ray Resists 1-4
1-3
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J. W. Taylor , C. Babcock , and M. Sullivan 1
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Center for X-ray Lithography, Materials Science Program, Engineering Research Center in Plasma-Aided Manufacturing, and Department of Chemistry, University of Wisconsin at Madison, 3731 Schneider Drive, Stoughton, WI 53589-5397
Downloaded by MONASH UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: April 13, 1993 | doi: 10.1021/bk-1993-0527.ch015
4
Conventional photoresists employed to create microcircuits generally lack sufficient sensitivity to meet the desired manufacturing throughput when X-rays in the region of 0.8 to 1.0 nm are used for exposure. Chemical amplification of the exposure event does provide the sensitivity. Exposures of submicron features with these chemically-amplifiedresists and subsequent development in aqueous base solution demonstrate that several of the commercial negative-tone and positive-tone formulations are capable of high resolution provided the variables which produce the chemistry are understood and controlled. Examples of these variables and the limits of necessary control are demonstrated for two basic commercial formulations, Shipley (negative) XP-90104C and Hoechst (positive) AZ-PF. The variables examined include: preprocessing for adhesion, spin conditions for thickness, pre-baking conditions, exposure, post-bake, and development time, temperature, and normality. Each step has a chemical connection, and these connections are discussed. Finally, the etch selectivity of two chemically-amplified resists is examined, and the resists show similar selectivity to the conventional novolac-based photoresists but are capable of producing features at least to 0.30 microns. The results of a three-wavelength ellipsometer approach to the measurement of surface roughness and resist swelling is also described. The negative tone chemically-amplified resist XP-90104C is shown to swell to less than 1% in aqueous base developer at the temperatures normally used for processing.
The initial requirement for the polymer formulation used as a resist by the semiconductor industry is the resolution of the image which is produced by the exposure agent. When processing throughput becomes a dominant concern, however, the sensitivity of the resist is a factor which must be optimized. Several additional factors, including the flow characteristics (which define the uniformity of the thin film produced by spinning), the thermal stability, the sensitivity to environmental conditions, the resistance to plasma etching, and the stability of the image are also critical to the performance of the resist
0097-6156/93/0527-0224$06.25/0 © 1993 American Chemical Society
In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
Downloaded by MONASH UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: April 13, 1993 | doi: 10.1021/bk-1993-0527.ch015
15.
TAYLOR ET A L
Chemically Amplified X-ray Resists
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Traditionally, the polymer chemist will rely on increasing the quantum efficiency of the exposure agent to achieve a sensitivity increase or will utilize functional groups in die base polymer system to produce a lower activation energy for reaction to yield products which react differently with the developing agent This provides contrast between the exposed and unexposed resist There is some variety in the differentiation approach depending on the method of development With solution development, it is the differential solubility which provides the discrimination between the exposed and unexposed polymer. In the dry or plasma development, it is the differential reactions between the exposed and unexposed resist-coated surface to the radicals and ions striking the resist surface. Once the unwanted resist areas are removed, it is the differential reactions to the etching gas and plasma with the resist and the exposed wafer surface which provide the discrimination. In this latter situation, the resist may also be utilized as a mask for protecting areas where plasma etching is not desired. Polymer matrix selection becomes important in this situation to provide the necessary etch selectivity and protect the covered areas. What then can be done when the traditional lines of chemical synthesis fail to produce the sensitivity that a photoresist system requires when X-ray photons are the exposure agent? One answer has been the development of a new strategy for increasing the effect of the photon absorption, or electron absorption in the case of ebeam exposure. In this new strategy, primarily pioneered for photoresists by research at IBM, AT&T, and in Germany (7-S),the photon event generates an acid, and the acid subsequendy acts as a catalyst to promote the reaction producing the required differentiation. Because the catalyst is produced by one photo event but can participate in several cycles of chemical reactions, the new resists are called "chemically amplified". In the X-ray region these chemically-amplified resists are of critical importance because the absorption cross section is a function of wavelength and decreases substantially as the wavelength decreases. Short wavelength(0.8-1.4 nm) X-rays are the choice for synchrotron-based X-ray lithography to provide the lowest contribution from diffraction and thereby produce the highest image resolution. In like manner, the point source X-ray effort is primarily centered at the Fe emission line at 1.4 nm(9,70). At these short wavelengths, however, the sensitivity of most traditional resists is too poor to be used given the throughput requirement for the production of microcircuit devices by X-ray lithography. For example, as can be seen in Table I. where the results with synchrotron exposures are listed, polymethylmethacrylate - PMMA) - one of the highest resolution resists for X-ray lithography - has a sensitivity of 1500-2000 mJ/cm ; the desired sensitivity for manufacturing is 100 mJ/cm or better(2 ). The modification of PMMA and related acrylic polymers(77)by halogenation, substitution, and copolymerization with other monomers has resulted in improvements in sensitivity to below 100 mJ/cm . However, the resolution of these modified resists is worse than that of PMMA, and they can not be developed in an aqueous developer. Aqueous base development is desired for photoresists because it tends to give better resolution and presents much fewer solvent disposal problems. The novolac/diazonaphthoquinone dissolution-rate inhibition resists, such as those used for UV lithography in most wafer fabrication Unes, can be developed with aqueous solution. These resists also have greater process stability against plasma etching and heat, but they do not meet the sensitivity target value of 100 mJ/cm . For example as shown in Table I, an experimental high-sensitivity resist of this type (Olin-Hunt HEBR 242) tested at the University of Wisconsin Center for X-ray Lithography (CXrL) had a sensitivity of 460 mJ/cm . Other novolac/ diazonaphthoquinone resists are even less sensitive than this, but the matrix does provide etching selectivity. 2
2
2
2
2
In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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IRRADIATION OF POLYMERIC MATERIALS
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TABLE I. SUMMARY OF MEASURED X-RAY RESIST PROPERTIES RESIST (tone)
Developer
PMMA (pos) HEBR-242 (pos)
Resolution
Sensitivity (mJ/cm )
Contrast
MIBK/IPA
1500
3.5
