Air-gap fabrication using a sacrificial polymeric thin film synthesized via initiated chemical vapor deposition

Thin films of a sacrificial polymer based on poly(cyclohexyl methacrylate) (PCHMA) were synthesized using initiated chemical vapor deposition (iCVD). iCVD is able to make cross-linked PCHMA in situ on a substrate in one step without using any solvents. Although cross-linked, the polymer was found to decompose cleanly, leaving behind a maximum of 0.3% of residue by thickness upon completion of thermal annealing. Cross-linking renders the polymer stable in practically all solvents, so photoresist used for patterning can be removed by dissolution instead of ashing. The high etch rate (0.35 mu m/min) in oxygen reactive-ion etching in addition to the stability in solvents eliminates the need of a hard mask during etching. This represents an improvement over previously reported spin-on sacrificial materials. Infrared spectroscopy and solubility tests confirm the identity of the iCVD polymer and its stability in solvents. The onset of thermal decomposition is 270 degrees C, allowing the polymer to survive high-temperature processing steps. Fabrication using conventional lithographic, etching, and deposition techniques resulted in void structures having feature sizes of a minimum of 1.5 mu m. This work represents a novel approach for air-gap fabrication using a sacrificial polymer, combining the rationale behind cross-linking and the technique required for thin-film deposition.