An extensive study of parameters pertinent to electron beam lithography with hydrogen silsesquioxane as a negative tone electron beam resist is presented. With higher developer concentrations contrast and reproducibility are improved significantly at the expense of lower sensitivity. In a similar way extended delays between the baking and exposure degrade the sensitivity but increase the contrast. In contrast, at higher baking temperatures the sensitivity is improved but the contrast and reproducibility deteriorate. These results are discussed within a microscopic model. Contrast values as high as 10 and good reproducibility have been obtained with a developer concentration of 25% tetramethyl ammonium hydroxide and a baking temperature of 90 degreesC. With these optimal parameters an experimental lithographic pattern of 50 nm lines and spaces could be resolved in 220 nm thick HSQ resist film exposed at 50 keV. (C) 2003 American Vacuum Society.