Chemically amplified negative resist films of Shipley SAL 605 at 1.0 mu m thickness show a statistically optimized sensitivity that is twice that observed with 0.5 mu m films when exposed to x rays from an electron storage ring. Because of the large "depth of focus" of proximity x-ray Lithography, the differences between thick and thin films are due to resist chemical and physical effects and processing conditions, not to the exposure process. Using identical processing conditions, 1.0-mu m-thick films showed 34% larger linewidths than 0.5 mu m films for isolated Lines from one mask with a target linewidth of 0.215 mu m. Gel permeation chromatography results indicated that there was no dependence of resin or crosslinker concentration on thickness of the applied him. The photogenerated acid and residual solvent were quantified prior to postexposure bake, and neither was able to explain the apparent sensitivity dependence on thickness. The work. suggests that some of the differences arises in the resist application process because of the ability of the thicker film to retain small molecular weight species which facilitate the linewidth production. This implies that thinner films should show better resolution purely because of chemical and physical factors in the resist film.