Diamondlike carbon (DLC) films were deposited using unbalanced planar circular magnetron sputtering of a graphite target with argon as the support gas. The properties of the DLC films were studied as functions of the applied negative substrate bias voltage, 50-200 V, and argon gas pressure, 2-6 mTorr. The hydrogen content was measured by elastic recoil spectroscopy and impurity content was measured by Rutherford backscattering spectroscopy; the physical thickness was measured by mechanical profilometry. For the first time films with densities as high at 3 g/cm(3) have been made by this technique. The hydrogen content in the films is 12-16 at. % and 20-32 at. % for an argon gas pressure of 2 and 6 mTorr, respectively. As the bias voltage increases. the hydrogen content decreases. The transmission electron diffraction of the films indicated that they are amorphous. The bonding structure, optical property, and optical band gap were determined by Fourier transform infrared (FTIR) spectroscopy, and ultraviolet-visible spectroscopy. FTIR spectra from DLC films deposited at 6 mTorr display the C-H stretching absorptions centered around 2900 cm(-1) with a bonding configuration of sp(3)/sp(2)-C:H at 2920, 2860, and 3012 cm(-1), as well as the deformation peaks of sp(3)C-CH3 at 1450 and 1370 cm(-1). The decreasing ratio of sp(3)CH(2) to sp(3)CH(1) in the C-H stretching region indicates a decrease in hydrogen content with bias voltage.