| The material of high quality diamond films has a great potential for application in each field of modern technology. As the first choice for depositing high quality diamond films, the microwave plasma chemical vapor deposition (MPCVD) technology has been gradually mature in recent years. However, compared to the United States, Europe, Japan and other countries, our country is still in a backward state in the preparation of high quality diamond films, which seriously limits the application of high quality diamond films in high-tech fields of our country. In order to promote the development of MPCVD technology and the application of high quality diamond films in high-tech fields of our country, this paper based on the design of high power MPCVD deposition devices, carried out the deposition technology research of high quality diamond films and the study of mutual interaction between dielectric performance and quality of diamond films.By simulating the distributions of plasma, electric field and gas temperature in resonant chamber, this paper proposed a novel dome-shaped cavity type MPCVD reactor. It has the following advantages:(1) the quartz window was kept far away from the plasma region, so that etching of the window would not become a problem,(2) there were two adjustment mechanisms, making it easier to optimize the plasma distribution,(3) excellent vacuum performance was possessed, the measurement of static pressure rise showed that leakage rate was only about3.7*10-6Pa·m3·s-1,(4) the whole reactor was water-cooled. Experiments showed that the novel dome-shaped MPCVD reactor combining so many excellent capacities was suitable for the preparation of high quality diamond films under high power microwave inputting.Based on the successful establishment of the dome-shaped MPCVD reactor, series of experiments were undertaken to investigate the best process parameters for depositing high quality diamond films. Specifically, deposition temperature, methane concentration and gas flow rate are three important process parameters, then their effects on the growth of diamond films, such as surface morphology, growth rate, quality and texture, were studied. In addition, this paper also studied how the vacuum performance of reactor influenced the preparation of high quality diamond films. Final results shows that diamond films grow best when the deposition temperature near1030℃, methane concentration near3.5%, and gas flow rate greater than100sccm. Moreover, it is concluded that perfect vacuum performance would greatly reduce the growth rate, however, it is the key for one MPCVD reactor to deposit high quality diamond films.With the dome-shaped MPCVD reactor running at9.2kW high microwave power, a high quality free-standing diamond film was successfully deposited under the optimum conditions, and the growth rate was about1.9μm/h. Test results showed that the polished diamond film was colorless and fully transparent, and possessed excellent characteristics (FWHM of Raman peak of only2.0cm-1, a cutoff wavelength of223nm, a nitrogen content lower than1ppm and transmission higher than70%in the6.5-25μm range). It is clear that the deposited diamond film was indeed of quite high quality. At the same time, above results prove that the novel dome-shaped MPCVD reactor has the capability for depositing high quality diamond films under the condition of high power and long time running.Finally, a split-cylinder resonator for dielectric properties test of low loss films was established in this paper. By this split-cylinder resonator, the microwave dielectric properties of diamond films deposited by different methods and possessing various qualities were tested in the Ka wave band. For these four diamond films (1#,2#,3#and4#), Raman studies showed that their values of full width at half maxima increased in sequence, corresponding to the deterioration of their qualities. With the quality decreasing, the dielectric constants of four samples increased subsequently, however, the microwave loss tangents of only2#,3#and4#three samples increased in turn, and for1#sample possessing the best quality, its loss tangent value was a little bigger than that of2#sample. It was found that surface holes and internal black defects of1#sample should be the reason why its loss tangent value was larger than2#. By this dielectric properties test, it could be seen that the diamond films deposited by MPCVD method had smaller microwave loss, and there was intimate relationship between the microwave loss and microstructure of diamond film. In future, systemic research should be carried out about the formation mechanism and impact factors of black defects, moreover, the influences of rooftop twin crystal on the black defect and dielectric property are also worth looking in to. |
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