| Chemical vapor deposition?CVD?diamond film obtains great prospect in various applications due to its incomparable physicochemical property.Nowadays,the researches on the deposition of high quality CVD diamond are mainly carried out in H2-CH4 gas mixture with microwave plasma CVD?MPCVD?technology which is considered as a preference technique to prepare diamond films with lower impurities.Besides that the improvement of growth rate and quality during deposition process is very important and necessary to satisfy the requirements of commercial process.However,H2 is kind of combustible gas,which will introduce some potential dangers mixed with other combustible gas or fuel gas at high deposition temperature.In order to avoid the risk effectively and ensure the controllability of diamond film growth,the deposition of diamond films were systematic investigated in CO2-CH4 mixtures without adding H2 by Woosinent-R2.0-type MPCVD apparatus.The surface morphology,crystal structure,film quality and growth rate of diamond films were systematic studied at various deposition parameters,such as different gas ratios of CO2/CH4,microwave power,deposition pressure and N2 concentration in CO2/CH4 plasma.Eventually,we obtained some beneficial results to improve the quality and growth rate of diamond films.And we also achieved the controllability of diamond films deposition in the reaction gas without H2.The primary results in this research are listed following:In the condition of various gas ratios of CO2/CH4,the surface morphology,quality and orientation of diamond films were discussed.It was difficult to obtain diamond films on the silicon when CO2/CH4?30%while keeping the CH4 gas flow rate constant at 50 sccm.And the various characteristic diamond films were deposited when CO2/CH4=40%80%.With the increase of CO2/CH4,the surface morphology of films changed from nano-linear to micron and nanometer granule.The diamond film possesses relative high quality with the growth rate of 3.4?m/h when CO2/CH4 is 60%.There was nothing deposited on silicon when CO2/CH4?80%.The influence of microwave power and deposition pressure on growth of diamond film was studied under the condition of CO2/CH4=30/50.The experimental results show that the morphology and quality of diamond films were greatly influenced by microwave power.It should be notable that increasing microwave power can improve the degree of ionization of the reaction gas.The grain was irregular when the microwave power was 0.9 kW.The reason is that the reaction gas is less ionized and the plasma is nonuniform.With the improvement of microwave power,the degree of ionization of the gas and the grain size of diamond film were also obvious increase.The grains were relatively regular shape with?100?plane and the crystallization quality was improved when microwave power increase gradually from1.2 kW to 1.5 kW.The diamond film with?100?orientation,which grain size was 4?m,was obtained when the microwave power was 1.8kW.Pressure has great influence on surface morphology,crystalline quality and orientation.The diamond films preferred to?111?orientated growth with many hole on the surface at low pressure.As the pressure rises,the gas molecules are more dense and the concentrations of carbon are increased,which can be obtained?111?and?100?surface of the diamond.In addition,the increasing C2H2 species and oxygen-containing species promoted the secondary nucleation and etching which gradually makes appeared crystals become incomplete and eventually hinders the further growth of grains.Finally,the effect of nitrogen addition on CO2-CH4 reaction system was investigated.It was found that the effects of the addition of nitrogen into CO2-CH4 is focus on refine the grain size of diamond film and improve the deposition rate.The experimental results showed that the surface of the diamond film was typical of the micron diamond film which grain size was about 2?m in CO2-CH4 reaction gas.After the introduction of nitrogen,the grain size of diamond film with a typical of nanometer decreased greatly.With the increase of nitrogen concentration,the trend of deposition rate of the diamond film was increased firstly and then decreased.When the N2 concentration was 10%,the maximum deposition rate of 4?m/h was obtained.When the nitrogen concentration was 20%,the surface of the diamond film presented a"needle"shape and the deposition rate was low,which is mainly related to the CN group in the reaction system.At the same time,the quality of the diamond film with some ditches on the surface was poor,which is due to the increasing nitric effect of nitrogen on the diamond film. |
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