Polycrystalline diamond films previously grown on silicon were polished to an RMS roughness of 15 nm and bonded to the silicon in a dedicated ultrahigh vacuum bonding chamber. Successful bonding under a uniaxial mechanical stress of 32 MPa was observed at temperatures as low as 950°C. Scanning acoustic microscopy indicated complete bonding at fusion temperatures above 1150°C. Cross-sectional transmission electron microscopy later revealed a 30 nm thick intermediate amorphous layer consisting of silicon, carbon and oxygen.