To investigate causes and cures for resist profile edge roughness in ion projection lithography a model for exposure and development of chemically amplified resist exposed with He+ or H+ ions is described. The APEX-E resist system was chosen as a paradigm system. Predominant factors increasing line edge roughness are image blur and low exposure dose. Both effects result in formation of ion clusters in nominally unexposed regions close to feature edges. These clusters can lead to statistically distributed development paths and consequently to line edge roughness;