Economical deposition of a large area of high quality diamond film by a high power DC arc plasma jet operating in a gas recycling mode

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Economical deposition of a large area of high quality diamond film by a high power DC arc plasma jet operating in a gas recycling mode
  Ž . Diamond and Related Materials 9 2000 1655  1659 Economical deposition of a large area of high qualitydiamond film by a high power DC arc plasma jet operating ina gas recycling mode F.X. Lu a,  , W.Z. Tang a , G.F. Zhong a , T.B. Huang a , J.M. Liu a , G.H. Li b , T.L. Lo b ,Y.G. Zhang b , Z.L. Sun b , S.M. Du b , Q.Y. He b , S.I. Wang b a Uni    ersity of Science and Technology Beijing, Beijing 100083, PR China b  Academy of Sciences of Hebei Pro   ince, Shijiazhuang 050000, PR China  Abstract In the present paper, details of a semi-closed gas recycling system incorporated with a high power jet, which allows more than90% of the feed gas to be recycled while exhausting and feeding a small amount of fresh gas is disclosed. Recent experimentalresults of diamond deposition by the high power jet system operating in gas recycling mode are presented. The effect of gasrecycling on diamond deposition and the quality of the deposited diamond films was discussed. By proper design of the wholesystem, and optimization of process parameters, thick uniform diamond wafers of    60  100 mm in diameter with a thickness upto 2 mm and transparent diamond wafers of    60 mm in diameter has been deposited successfully. It is demonstrated by thepresent study that gas recycling can be used even for high quality diamond film synthesis. This is of technical and economicalimportance in the development of high power DC arc plasma jet CVD systems for economical fabrication of large area highquality diamond wafers.    2000 Elsevier Science S.A. All rights reserved.  Keywords:  Gas recycling; High power DC arc plasma jet; Diamond films; Quality; Economics 1. Introduction Large scale production of CVD diamond wafers at alow cost is the key issue for industrial applications of diamond films. Of the very many deposition methods,the high power DC arc plasma jet is generally con-   sidered as the most promising technique 1 . However,high electricity and gas consumption may partly com-pensate the advantage of low cost due to the highgrowth rate over large substrate area by high power arc      jet 2 . In our previous publications 3,4 we have dis-closed a new type of magnetic and fluid dynamic con-  Corresponding author. Tel.:   86-106233-3336; fax:   86-106233-3336. Ž .  E-mail address: F.X. Lu . trolled large orifice long discharge tunnel plasma torch which guarantees the large area uniformity by rotatingthe arc root. In the present paper, the details of asemi-closed gas recycling system incorporated in thehigh power system is disclosed. Recently, Partlow et al.reported in detail about the operation of a 100-kW   high power jet in gas recycling mode 5 . In the presentpaper additional information on the effect of gas recy-cling on the quality of diamond films is discussed. 2. Experimental  A 100-kW high power DC arc plasma jet CVDsystem equipped with a specially designed large orificelong discharge tunnel arc plasma torch with a magneticfield and fluid dynamics control was used. The details 0925-9635  00  $ - see front matter    2000 Elsevier Science S.A. All rights reserved. Ž . PII: S 0 9 2 5 - 9 6 3 5 0 0 0 0 3 0 5 - 8  ( ) F.X. Lu et al.   Diamond and Related Materials 9 2000 1655  1659 1656 of this high power DC arc plasma jet CVD system can   be found from our previous publications 3,4 . The Ž . main features of this high power system are: 1 largearea uniformity and high growth rate of diamond film Ž deposition 40  50   m  h over   110 mm substrate area . maximum achievable through arc root rotating by thedesign of the plasma torch and the proper control of  Ž . magnetic field and fluid dynamics; 2 a semi-closed gasrecycling system which allows more than 90% of gasesto be recycled while keeping a small amount of gases Ž . being exhausted and renewed; 3 a complete waterrecycling system is also adopted. Argon was used for sustaining the arc discharge.H    Ar and CH   H ratios used were: 1:3  3:1 and 2 4 2 1  10%, respectively. Mo was used as the substrate, whose temperature was kept at 700  1200  C duringdiamond deposition. Chamber pressure was 3  30 kPa.Deposited diamond films were examined by Ramanspectroscopy, scanning electron microscopy and X-raydiffraction. The quality of diamond films were evalu- Ž ated by the ratio of the areas covered by or the . intensities of the characteristic diamond peak and thediffused non-diamond peak obtained through a Loren-   zian curve fitting to the Raman spectra 6 . Opticalemission spectroscopy was used for the purpose of plasma diagnostics. Optical properties of the diamondfilms were measured by an IR spectrometer. 3. Results and discussion  3.1. Principles of the semi-closed gas recycling systemincorporated in the 100 kW high power DC arc plasma jetCVD system  A schematic diagram of the gas recycling systemincorporated in our 100 kW DC arc plasma jet CVDsystem is shown in Fig. 1. A series of vacuum pumpsare used. The recycled gas is fed back by a two-stageroots’ pump. A heat exchanger is installed immediately Ž . below the deposition chamber not shown in Fig. 1 , by which the recycled gas is cooled down to nearly roomtemperature before entering the roots’ pumps. Thetorch is designed to allow the recycled gas to mix withthe replenished methane and entering the torch at aproper location, whilst the replenished H and Ar are 2 introduced through the cathode at atmospheric pres-sure. As the recycling system is operated at low pres-sure, so there is no need for the installation of a highpower compressor and buffer tanks to maintain a stablepositive pressure above the atmospheric pressure. Thisefficiently simplifies the design and the operation of thegas recycling system. During regular gas recycling oper-ations, the exhaust gas is compensated exactly by thenewly supplied gases from gas inlet lines which isactually in equilibrium with the exhaust. Whilst the Fig. 1. Schematic diagram of the semi-closed gas recycling system.Some of the valves and the heat exchanger are not shown. amount of recycled gas is controlled by the difference Ž of pressures at the outlet of the roots’ pumps can be . adjusted by valve A to that in the deposition chamber Ž . can be controlled by valve B . The blow down modeoperation can also be done by closing down valve C while keeping valve A fully open. In normal operations,as much as more than 90% of gases can be recycled.Filters were used to separate solid powders and residueoil mist in the recycling gas stream. Our system is inseveral aspects similar to that recently reported by   Partlow et al. 5 . However, the main difference is thatthe recycled gas stream is actually an integrated part of the fluid dynamics required to maintain the uniformityand symmetry of the arc plasma, and our system is alsomore simpler.  3.2. Effects of gas recycling on diamond deposition by high power DC arc plasma jet When operating in gas recycling mode in a system asshown in Fig. 1, one has to be concerned firstly about Fig. 2. Effect of gas recycling on the growth rate of diamond films Ž substrate temperature, 910  C; CH   H , 1.3%; chamber pressure, 4 2 . 3.6 kPa .  ( ) F.X. Lu et al.   Diamond and Related Materials 9 2000 1655  1659  1657 Ž . Fig. 3. Adverse effect of contamination from vacuum pumps on the quality of diamond films deposited in gas recycling mode: a with oil mist Ž . contamination; and b in a reasonably clean vacuum system. the composition in the plasma over the substrate. Dur-ing diamond deposition, there is no doubt that the feedgas is in equilibrium with the exhaust, that is, the exactamount of gas being exhausted is being compensatedby the new feeding gas. In other words, the totalcarbon input per unit time should be equal to thecarbon being exhausted plus that deposited as diamondin the same time period. However, during the coolingdown process, some of the carbonaceous active radicalsin the hot plasma may undergo certain polymerizationreaction processes to form hydrocarbons other than theinput of methane with a higher molecular weight.Therefore, the actual composition in the gas recyclingline is different in carbonaceous species with that in Ž . the feeding starting gas lines. However, by compar-ison of optical emission spectra from a 10-kw DC arc Ž  jet which can work without gas recycling, i.e. the blow . down mode , no additional emission radicals werefound, except that the intensity ratio of Ar  H peaks were different due to the different Ar  H ratio used 2 Ž  Ar  H    5:1 for a 10 kW jet and Ar  H    3:5 for 2 2 . the 100 kW jet .Secondly, as the amount of recycled gas is controlledby the pressure difference    P   at the outlet of the rootspumps and in the deposition chamber, one has to beconcerned about the influence of recycled gas on thegrowth characteristics of diamond films in the gas recy-cling mode. The relationship between growth rate and Ž the amount of recycled gas depicted as the pressure . difference    P   is shown in Fig. 2, where it can be seenthat a linear relationship exists. This is because when   P   increased, the speed of gas flow in the plasmatorch also increased. This would result in a tempera-ture drop of the plasma jet, and accordingly a decreasein growth rate. The uniformity of diamond film deposi-tion over large substrate size is only related to the Ž uniformity and symmetry of the arc plasma jet the . rotating arc , which can be controlled with ease byproper manipulation of the magnetic field and the   effect of fluid dynamics 7 , to which the recycled gasstream is also an important factor. A much highergrowth rate than that shown in Fig. 2 can be obtained Ž . Ž . Fig. 4. Effect of nitrogen left and oxygen right addition on the Ž quality of diamond films deposited by DC arc jet CH   H , 4%; 4 2 . 940  C,   18 kPa .  ( ) F.X. Lu et al.   Diamond and Related Materials 9 2000 1655  1659 1658  with higher methane concentration. The maximumgrowth rate was shown to exceed 40   m  h over thesubstrate area of    110 mm in diameter. A fairly highgrowth rate over large substrate size achieved in thepresent investigation may not due to the gas recycling, we believe this is because of the special design of the   plasma torch 3,4 , which is quite different in principleto the industrial high power arc-heater torch used by   Partlow et al. 5 .  3.3. Effect of gas recycling on the quality of diamond films by the high power DC arc plasma jet In the present investigation, no apparent effect of gas recycling on the quality of diamond films wasfound. However, serious problems may arise from the vacuum pumps and the leakage to the whole system. Inthe course of development of the present system, seri-ous contamination from oil mist from vacuum pumpshas been observed. Black and bright diamond wafers Ž  with very bad Raman signature had been produced see . Fig. 3 . However, for our improved system with a rea-sonably clean vacuum this kind of serious contamina-tion problem was never met again. In order to verifythe adverse effect of vacuum leakage we have intentio-nally introduced a controlled amount of nitrogen and Ž . oxygen the main components of air into the deposi-tion chamber. In both cases a serious deterioration of the quality of deposited diamond films were observed Ž see Fig. 4, where the quality is depicted by the inten-sity ratio of the non-diamond carbon peak to the dia- . mond characteristic peak . Optical emission spectros-copy showed that with the addition of N , a distinct 2 C-N peak located at   390 nm appeared, at whichintensity increased with increasing N addition. Whilst 2 the C2 peaks were found decreased drastically. Withthe addition of O , a drastic decrease in the C2 peak 2  was also observed, however, the OH and CO emissions were not shown in the spectra range of 0.3  0.8   m aslimited by the optical gratings used for the spectrome-ter.In a vacuum-tight system operating in the gas recy-cling mode, the effects of process parameters on thequality of diamond films were the same as that oper-   ated without gas recycling 8 . By proper design of the whole system, and by optimization of the process Ž . Fig. 5. Large area high quality diamond wafers prepared by DC arc plasma jet: a   110 mm and   60 mm diamond wafers prepared with gas Ž . Ž . Ž . recycling; b   60 mm transparent diamond wafer 0.7 mm thick, unpolished deposited in gas recycling mode; c polished transparent diamond Ž .  wafers. Above: samples prepared without gas recycling, below: samples cut from large wafers prepared with gas recycling; and d typical Ramanspectra of high quality transparent diamond films.  ( ) F.X. Lu et al.   Diamond and Related Materials 9 2000 1655  1659  1659 parameters, large area high quality diamond wafers Ž  60  100 mm in diameter, 2 mm maximum thick- . ness and transparent free standing diamond films of  Ž .  60 mm thickness 0.7 mm were successfully de- Ž . posited see Fig. 5 . Optical properties measured fromthe polished transparent samples were close to that of  Ž natural type IIa diamond single crystals   71% trans- . mittivity in the IR spectral region . Thermal conductiv-ity as measured by a laser thermal deflection method was as high as 18 w  cm k. It is also worth mentioning Ž . that the growth rate was fairly high 8  10   m  h inthe preparation of these high quality diamond wafers inthe gas recycling mode. In our case, we believe that it is Ž the characteristics of the plasma torch due to its . special design and the process of optimization thatmade the deposition of high quality large area diamondfilms technically feasible, but not due to the gas recy-   cling. Our plasma torch 3,4 is different to most of thehigh power arc plasma torches concurrently employedfor large area diamond film deposition, e.g. to that used   by Partlow et al. 5 .Thus, we have demonstrated that large area highquality diamond wafers can be produced by high powerDC arc plasma jet operating in gas recycling mode. Apparently this is of technical and economical signifi-cance. The reason why gas recycling did not showpronounced adverse effects on the quality of diamondfilms may be due to the fact that the content anddistribution of active radicals in the plasma only de-pend on the overall C  H ratio and the temperatureand pressure in the plasma, but are independent on thetypes of carbonaceous species in the recycling andfeeding gas lines. 4. Summary We have disclosed a semi-closed gas recycling systemincorporated in the 100 kW DC arc plasma jet CVDsystem which allows as much as 90% of gas to berecycled whilst a small amount of gas is being continu-ously exhausted and newly supplied. The principles of this gas recycling system have been discussed. The volume of the recycled gas is controlled by the pressuredifference at the outlet of the roots’ pumps and in thedeposition chamber. When operating in the recyclingmode, the growth rate of diamond film was inverselyproportional to the pressure difference. The quality of diamond films was not notably affected by gas recy-cling, whilst a minute leakage to the pumps and the whole system may result in serious deterioration to thequality. By proper design and optimization of theprocess parameters, large area high quality diamond wafers were successfully prepared in the gas recyclingmode at a reasonably high growth rate. This is of technical and economical significance in developing thehigh power DC arc plasma jet CVD system for indus-trial applications. References    Ž . 1 J.V. Busch, J.P. Dismukes, Diamond Relat. Mater. 3 1994 295.   2 W.D. Partlow, J. Schreurs, R.M. Young et al., Application of Diamond Film and Related Materials, in: A. Feldman et al, Ž . Eds. , NIST Special Publications 885, USA, 1995, pp. 519.   3 F.X. Lu, G.F. Zhong, L. Wang et al., Proceedings of the Interna-tional Diamond Symposium Seoul, 1996, p. 115.   4 F.X. Lu, G.F. Zhong, J.G. Sun et al., Diamond Relat. Mater. Ž . 7  6 1998 737.   5 I.A. Martorell, W.D. Partlow, R.M. Young, J.J. Schreurs, H.E. Ž . Saunders, Diamond Relat. Mater. 8 1999 29  36.   6 G.F. Zhong, W.Z. Tang, F.Z..Shen, W.X. Yu, Y.M. Tong, F.X.Lu, Proceedings of the Third Pacific Rim Conference on Ad- vanced Materials and Processing, Hawaii, USA, July 15  18,1998, pp. 2941.   7 F.X. Lu, W.Z. Tang, G.F. Zhong et al., to be presented at ADC  FCT’99 conference, Tsukuba, Aug. 30  Sept. 3, Japan,1999.   8 G.F. Zhong, Ph.D thesis, University of Science and TechnologyBeijing, 1998
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