Patterning of x-ray masks using the negative-acting resist P(SI-CMS)

The copolymer of trimethylsilylmethyl methacrylate with chloromethylstyrene [P(SI-CMS)] is a negative electron-beam and deep-UV resist which can withstand erosion in O2-containing plasma environments [J. R. Maldonado, J. Electron. Mater. 19, 6699 (1990)]. Manipulation of its composition and molecular weight allows control of the etch resistance and radiation sensitivity properties. Methods have been developed to provide reproducible synthesis of P(SI-CMS) with molecular weight and composition tailored to specific lithographic demands. For x-ray mask patterning, the copolymer having a 90:10 mole ratio of SI:CMS [P (SI90-CMS10)] and M(w)BAR between 30 000 and 41 000 g/mol has been found to provide an optimal combination of resist sensitivity, dry-etching resistance, and pattern resolution. P(SI90-CMS10) is used to image the Cr-W-Cr metallization layer of a monolithic x-ray mask structure, by subtractive etching techniques [G. K. Celler et al., Appl. Phys. Lett. 59, 3105 ( 1 99 1 ); G. K. Cellar et al., J. Vac. Sci. Technol. B 10, 3186 (1992); C. W. Jurgensen et al., J. Vac. Sci. Technol. B 9, 3280 (1991)]. The resist pattern is transferred into chromium with a O2-Cl2 plasma, and then into tungsten with a fluorine-based plasma. Resolution of 0.15 mum (line-space) in the resist has been achieved with 50 keV electron-beam exposure, and 0.20 mum patterns (line-space) have been successfully transferred into the tungsten of the x-ray mask.