Polycrystalline diamond films have been grown on silicon at substrate temperatures down to 300 °C in a 3 Torr CH4–H2–O2microwave plasma. The diamond films exhibit characteristic 1333 cm−1Raman phonon peaks with linewidths of 3–7 cm−1and a small nondiamond carbon response. Low temperature diamond synthesis was enhanced by applying a static magnetic field to increase gas dissociation and ionization, and by pulse modulating the 2.45 GHz input microwave power to provide a large atomic hydrogen supply without accompanying positive ion bombardment over most of the discharge cycle. Diamond quality is improved and growth rates are at least an order of magnitude higher in the pulsed mode, compared to continuous wave operation with the same average input microwave power. ©1995 American Institute of Physics.