The dependence of roughness of poly (methylmethacrylate) resist features on developer composition and development time, using methyl isobutyl ketone:propanol (MIBK:IPA) developer with and without ultrasonic assistance, is investigated using atomic force microscopy. It is found that the roughness decreases with increasing proportion of MIBK in the developer and that for a weak developer (1:4 MIBK:IPA at 25 degreesC), the roughness increases with development time. Ultrasonically assisted development is also found to reduce the roughness when using a weak developer, but has no significant effect with a strong developer (1:1 MIBK:IPA), or when using 3:7 water:IPA. We propose that the origin of the roughness is due to phase separation during development in weak solvents and for long development times. (C) 2004 American Vacuum Society.