High-density plasma etching of carbon-based low-k materials in a integrated circuit

Abstract

A plasma etching process for etching a carbon-based low-k dielectric layer in a multi-layer inter-level dielectric. The low-k dielectric may be divinyl siloxane-benzocyclobutene (BCB), which contains about 4% silicon, the remainder being carbon, hydrogen, and a little oxygen. The BCB etch uses an etching gas of oxygen, a fluorocarbon, and nitrogen and no argon. An N,/O,ratio of between 1:1 and 3:1 produces vertical walls in the BCB. In a dual-damascene structure, the inter-level dielectric includes two BCB layers, each underlaid by a respective stop layer. Photolithography with an organic photoresist needs a hard mask of silicon oxide or nitride over the upper BCB layer. After the BCB etch has cleared all the photoresist, the bias power applied to the cathode supporting the wafer needs to be set to a low value while the separately controlled plasma source power is set reasonably high, thereby reducing faceting of the exposed hard mask. Chamber pressures of no more than 5 milliTorr increase the selectivity of BCB over photoresist. Substrate temperatures of less than 0° C. increase the BCB etch rate. A low fluorocarbon flow increases the etch rate, but a minimum amount of fluorocarbon is required for the silicon component of BCB. In a counterbore dual-damascene etch, the lower stop layer is composed of nitride, and the preferred fluorocarbon is difluoroethane (CH,F,). A silicon-free carbon-based low-k dielectric can be etched under similar chamber conditions with a etching gas of oxygen and nitrogen in about equal amounts but including no fluorocarbon nor argon.