Comprehensive CD characterization of low-k trench etch for 65nm nodes are performed through a specially designed mask with global pattern density (GPD) in the range from 25% to 60%. Unlike traditional means, through this mask we systematically demonstrate global pattern density effects on etch behaviors in correlation with CD uniformity, CD proximity, and CD linearity without local etch loading effect contributed from nearby environment [1-3] and position dependent effect contributed from resist developing or aberrations of the wafer-imaging lens [4]. From our study, CD proximity is the most sensitive item. Wider trench shows larger CD variation as compared with narrow trench when global environment vary. Moreover, we find that low pressure etch conditions in a small chamber volume etcher exhibits less CD variation of global pattern density effect. On the other hand, pressure in a large chamber volume etcher provides better tuning capability in the adjustment of CD variation. The results suggest that residence time might be an influential factor for the GPD dependent CD control.