Low-stress W/Cr films for SCALPEL (R) mask scattering layers

I describe the development of low-stress W/Cr bilayer films, for use as SCA LPEL(R) mask scattering layers. These films are produced by de magnetron sp uttering in argon, and consist of 25-50-nm-thick W layers deposited onto 5- 10-nm-thick Cr layers. X-ray reflectance analysis is used to measure the th icknesses of the individual W and Cr layers with subangstrom precision; sur face and interface roughnesses, film densities, and also the thickness of t he tungsten-oxide overlayer which forms after exposure to air are determine d by this technique as well. Film stress, which is measured using the wafer curvature technique, is controlled by adjusting the deposition conditions such that the Cr layers are in tension while the W layers are in compressio n (and thus have high density and low surface roughness), so that the net s tress in the bilayer is balanced near zero. I present data that illustrates how the net stress in these films varies with argon pressure, background p ressure (i.e., partial pressure of residual gases present in the vacuum cha mber), and Cr layer thickness. I also show how the stress depends on the co mposition of the substrate: i.e., stresses measured in films deposited onto Si wafers are systematically higher (by several hundred MPa) than the stre sses measured for the same films deposited onto silicon-nitride-coated Si w afers. I discuss the implication of these results with regard to the produc tion of high-quality SCALPEL(R) mask blanks for sub 0.12 mu m lithography. (C) 1999 American Vacuum Society. [S0734-211X(99)07604-0].