Preparation Of Diamond Film Vacuum Windows

Compared with other vacuum window material,CVD diamond not only has excellent mechanical properties,such as high hardness and high elastic modulus,but also its advantageous thermal properties and optical properties including high thermal conductivity,low thermal expansion coefficient,wide spectral transmission range,and etc.It is the combination of excellent performances that make diamond become an excellent vacuum window material.In this paper,free-standing nano-crystalline diamond films were deposited on silicon wafers,using an optimized gas flow rates of an ethanol gas highly diluted with hydrogen in a reactor of Microwave Plasma Chemical Vapor Deposition.The effects of pretreatment of substrate surface on the diamond nucleation density were researched.During the diamond deposition process using C₂H₆O-H₂ microwave plasma,an optical emission spectroscopy was provided to diagnose the internal radicals of the plasma.Furthermore,the effects of parameters such as alcohol concentration,work pressure and temperature of substrate on diamond deposition was investigated.Then,the diamond vacuum window was made by welding the free-standing nano-crystalline diamond films to metal flange.The air leak rate and welding technology of diamond vacuum window was investigated.Experimental results revealed that:（1）Substrate after grinding with powder ultrasonic treatment can improve the diamond nucleation density and the highest nucleation density of 10⁹cm^-2 was obtained.（2）The main radicals in C₂H₆O-H₂ microwave plasma are Hα_、H_β、CH、C₂ and OH、CO.In a horizontal direction,the distribution of radicals is symmetrical.The concentration of the radicals near the substrate surface is higher than that of the top of the plasma in a vertical direction.Carbon-containing radicals are increased with the increasing of the ethanol concentration and operating pressure,however,oxygen-containing radicals are reduced with the increasing the increasing of the ethanol concentration and operatingpressure.Increasing the concentration of ethanol can reduce the electron temperature and the quality of diamond decreased.The size of diamond grain decreases with the increasing of ethanol concentration.Reducing the operating pressure can rise the electron temperature and conductive to the preparation of nano-diamond films.In addition,reducing substrate temperature will result in the decrease of grain size of diamond films and the increase of non diamond phase.（3）The air leakage rate of 10 μm free-standing nano-diamond films is 8.8 × 10^-9 Pa ·m³ / s.Increasing the thickness or decreasing the grain of free-standing nano-diamond films will significantly reduce the air leakage rate.（4）The welding experiment of the invar alloy flange and the free-standing nano-diamond has been performed using microwave plasma.The results show that the invar alloy flange and the free-standing nano-diamond may be firm welded.However the silicon framework is easy to crack due to the large different of the thermal expansion coefficient between silicon and invar alloy.The framework of free-standing nano-diamond is needed for subsequent studies.