Harda‐C:H films have been deposited through electron cyclotron resonance (ECR) microwave plasma decomposition of CH4diluted with H2gas. It has been found that hard diamondlike films could only be produced under a rf‐induced negative self‐bias of the substrate stage. Raman spectra indicate the deposition of two distinct film types: one film type exhibiting well‐defined bands at 1360 and 1580 cm−1(the graphitic D and G bands) and another displaying a broad Raman peak centered at approximately 1500 cm−1. The optical gaps of these films are from 1.0 to 1.6 eV, respectively, with resistivities >1012&OHgr; cm. Variation of the mirror magnetic‐field profile of the ECR system was examined, demonstrating the manipulation of film morphology through the extraction of different ion energies.