The microwave film prepared by the microwave method has developed rapidly in the past 30 years with its controllable high quality and achievable large area. The high quality diamond film with a diameter of 2 inches can pass the 2.45GHz microwave plasma chemical vapor deposition (CVD) technology. Meet the needs. However, for some special applications, such as high power microwave and terahertz window materials, diamond films are required to have a large deposition area (more than 4 inches in diameter) and thickness (greater than 1.35 mm), while requiring high optical transparency of the diamond film. Thermal conductivity and low dielectric loss. These characteristics put forward high requirements for the preparation technology of diamond film. At present, only the microwave frequency is lowered to 915MHz, and the preparation requirement of large-diameter diamond film with a diameter of 4-6 inches can be satisfied.

Internationally, Germany's Fraunhofer and Japan's ASTeX/Seki have successively introduced the 915MHz microwave CVD device after successfully developing the 2.45GHz microwave CVD device. Fraunhofer has built a unique 915MHz/60kW ellipsoidal microwave CVD device and demonstrates its ability to produce a 6-inch large-area diamond film. ASTeX/Seki focused on the crystal orientation and morphology control technology of large-area diamond films using a 915MHz/60kW microwave CVD device and applied it to the preparation of large-size single crystals. Element Six of the UK reported its commercial high-power millimeter-wave transmission window with a diameter of 119 mm. Mallik of India reported in detail the uniformity and integrity control issues during the preparation of 100mm large-area self-supporting diamond films.

In fact, from the high power stability of the device to the uniformity and integrity control of the diamond film, with the large increase in deposition area and microwave input power, both technical difficulty and development cost, large-area high quality at 915MHz The preparation of diamond film materials is greatly increased compared to the preparation of diamond films below 2 inches. Although the development of microwave CVD technology at 915MHz has made great breakthroughs for decades, the related process research and technical details are less reported than the research heat and maturity of 2.45GHz microwave CVD technology.

Although the domestic development of 2.45GHz high-power microwave CVD equipment has made great progress in recent years, it is still in the blank of 915MHz microwave CVD technology research. After more than 10 years of unremitting efforts, the Hebei Institute of Laser Research and Hebei Presman Diamond Co., Ltd. and Professor Tang Weizhong of Beijing University of Science and Technology have finally made major breakthroughs in the field of microwave CVD preparation of large-area high-quality diamond film technology, using self-developed The 915MHz/75kW microwave CVD device successfully prepared a double-sided polished high-quality diamond film window material with a diameter of φ127mm and a thickness of 1mm. Its optical transmittance is close to the theoretical value. This achievement breaks the gap in China in this field and is related to the country. The development of high-tech fields provides advanced material support.