Chapter 5
New Directions in the Design of Chemically Amplified Resists E. Reichmanis, M. E. Galvin, Κ. E. Uhrich, P. Mirau, and S. A. Heffner
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AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974
The design of a robust, manufacturable, deep-UV resist requires a fundamental understanding of the interactions between the varied components of such a material at a molecular level. Two factors that have a critical impact on the ability of a given resin to be used in a chemically amplified resist formulation are polymer/additive miscibility and inter- and intra-chain hydrogen bonding interactions. Experiments with trimethylsilyloxystyrene containing materials suggest that efficient chemically amplified resist systems should consist of a polar PAG and polar matrix in order to maximize the interaction between the photogenerated acid and the polymer. Similarly, intra- and inter molecular interactions between polymer chains may be important in determining the solubility characteristics of a matrix. For instance, the substitution pattern on a given chain will be shown to perturb hydrogen bonding interactions and concomitantly, dissolution characteristics. It will be demonstrated how issues such as those described above will affect resist performance and the groundwork will be laid for the design of resist chemistry through understanding and manipulation of polymer structure, molecular properties and synthetic methods.
The progress that has been made in the fabrication of microelectronic devices in general, and the lithographic technology used to generate the high-resolution circuit elements that are characteristic of those devices in particular, can only be classified as remarkable. Even more remarkable is that the technology of choice for fabricating those devices remains photolithography. Thus, conventional photolithography will be able to print features as small as 0.35 μηι and will remain the dominant lithographic
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Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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REICHMANIS ET AL.
Design of Chemically Amplified Resists
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technology well into this decade (1). However, the ultimate resolution of a printing technique is governed, at the extreme, by the wavelength of light (or radiation) used to form the image (2). It is this physical limitation that is now driving the industry to explore and develop new lithographic technologies. Each of the alternatives, be it 248 or 193 nm photolithography, X-ray or electron-beam lithography, will require not only the development of a manufacturing worthy tool, but the design and development of a manufacturable resist material and process (3,4). Upon examination of each of the alternative lithographic technologies, it appears unlikely that a material which undergoes only one radiation induced chemical event per absorbed unit radiation dose will provide sufficient sensitivity to ensure adequate throughput. A new class of resists that achieves differential solubility from acid catalyzed chemical reactions was discovered by Ito, Willson and Frechet (5-8) and independently presented by Crivello (9) who utilized the fact that arylonium salts efficiently produce strong acids upon irradiation (10-12). These resists are nominally classified as chemically amplified resists and are compatible with current lithographic process technology. Chemically amplified resists generally exhibit high contrast, good process latitude, excellent thermal stability and good dry-etching resistance. The process sequence for these chemically amplified materials is similar to that for conventional positive resists, although the post-exposure bake (FEB) assumes a different role. For these resists, the exposure and PEB steps play an equally important role in effecting differential solubility between the exposed and unexposed regions of a resist film. Presented here will be selected design isssues that must be addressed in order to effectively build a manufacturable, production worthy chemically amplified resist material. DESIGN ISSUES Lithographic resists must be carefully designed to meet the specific requirements of a given lithographic technology. Although these requirements vary according to the radiation source and device processing sequence, the following resist properties are common to all lithographic technologies: sensitivity, contrast, resolution, optical density (for UV resists), etching resistance, purity and manufacturability. These properties can be achieved by careful manipulation of polymer structure, molecular properties and synthetic methods. The materials issues that must be considered in designing resists with the appropriate properties are given below (13). The polymer resin must i.
exhibit solubility in solvents that allow the coating of uniform, defect free, thin films, or be amenable to vapor-deposition to achieve the same result, ii. be sufficiently thermally stable to withstand the temperatures and conditions used with standard processes, iii. exhibit no flow during pattern transfer of the resist image into the device substrate,
Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS
iv. possess a reactive functionality that will facilitate pattern differentiation after irradiation, and; v. for UV exposure, have absorption characteristics that will permit uniform imaging through the thickness of a resist film. Using the development of a deep-UV resist as an example, the performance criteria for a production worthy deep-UV resist are given in Table 1 (4,13).
Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch005
Table 1: Deep-UV resist performance criteria PARAMETER
CRITERIA
Sensitivity Contrast Resolution Optical Density
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