Abstract

A high quality freestanding nanocrystalline diamond (NCD) film grown in a microwave plasma CVD system has been studied and applied in extremely thin terahertz traveling wave tube windows. An average fracture strength from the 100~260μm thick freestanding films was found to be ~1814MPa, showing superior mechanical strength. A seven-layered 60μm microcrystalline diamond (MCD)/NCD composite film was fabricated for 850GHz traveling wave tube (TWT) windows. Then window components were assembled using laser cut MCD/NCD disks without lapping or polishing. The tests of the window showed this extremely thin MCD/NCD film meets the requirements for mechanical and vacuum properties of the window, while CST simulated bandwidths were found to be about 44.2GHz and 71.4GHz for the return loss ≤-30dB and -20dB, respectively. The results of this study demonstrate the NCDs grown at the relatively low CH4/H2, and high substrate temperature possess superior mechanical properties at least comparable to ultra-nanocrystalline diamonds (UNCDs) and thus can be employed in fabrication of the extremely thin composite diamond for THz TWT windows. This novel process of MCD/NCD and MCD/ UNCD composite diamond films provides a practical method for the fabrication of extremely thin THz TWT windows.

Keywords: Nanocrystalline diamond (NCD); Mechanical property; Air-tightness; Extremely thindiamond window; Terahertz traveling wave tubes