Characteristics of photosensitive low-k methylsilsesquioxane (MSQ) were investigated by use of electron-beam lithography. Photosensitive low-k MSQ makes it possible to realize via and trench patterns for Cu damascene technology in the ultra-large-scale-integrated (ULSI) circuit multilevel interconnect integration without dryetching processes. In this paper the dependences of exposure dose, humidification treatment and development method on critical dimension were investigated. It is found that longer humidification treatment resulted in the lower critical exposure dose, while the feature sizes were enlarged. The feature sizes had a linear correlation with exposure dose. Then reduction of the critical exposure dose minimizes the feature sizes. The development with ultrasonic wave was developed to reduce the critical exposure dose for 100 nm line and space pattern with the aspect ratio 3.3.