A method includes forming a line feature above a substrate. Carbon-containing spacers are formed on sidewalls of the line feature. A first dielectric layer is formed above the carbon spacers and the line feature. The first dielectric layer is planarized to expose upper ends of the carbon-containing spacers. An ashing process is performed to remove the carbon-containing spacers and define air gaps adjacent the line feature. A cap layer is formed to seal the upper ends of the air gaps.