A method of depositing and etching dielectric layers having low dielectric constants and etch rates that vary by at least 3:1 for formation of horizontal interconnects. The amount of carbon or hydrogen in the dielectric layer is varied by changes in deposition conditions to provide low k dielectric layers that can replace etch stop layers or conventional dielectric layers in damascene applications. A dual damascene structure having two or more dielectric layers with dielectric constants lower than about 4 can be deposited in a single reactor and then etched to form vertical and horizontal interconnects by varying the concentration of a carbon:oxygen gas such as carbon monoxide. The etch gases for forming vertical interconnects preferably comprises CO and a fluorocarbon, and CO is preferably excluded from etch gases for forming horizontal interconnects.