Microwave Plasma Chemical Vapor Deposition of Diamond Films on Silicon From Ethanol and Hydrogen

Plasma Science Technology, Vol5, No. 2(2003)Microwave Plasma Chemical Vapor Deposition of Diamond Filmson Silicon From Ethanol and HydrogenMA Zhi-bin(马志斌), WANG Jian-ha(汪建华), WANG Chuan-xin(土传新)MAN Wcl-dong(满卫东)Department of Material Science and Technology, Wuhan Institute of Chemical Tech-nology, Wuhan 430073, Chinaabstract Diamond films with very smooth surface and good optical quality have been de-posited onto silicon substrate using microwave plasma chemical vapor deposition(MPCVD)froma gas mixture of et hanol and hydrogen at a low substrate temperature of 450C. The effects of theubstrate temperature on the diamond nucleation and the morphology of the diamond flm havebeen investigated and observed with scanning clectron microscopy(SEM). The microstructureand the phase of the film have been characterized using Raman spectroscopy and X-ray difrac-tion(XRD). The diamond nucleation density significantly decreases with the increasing of thesubstrate temperature. There are only sparse nuclei when the substrate temperature is higherthan 800 C although the ethanol concentration in hydrogen is very high. That the characteristicdiamond peak in the Raman spectrum of a diamond film prepared at a low substrate temperature of 150C extends into broadband indicates that the film is of nanophase. No graphite peakappeared in the XRD pattern confirms that the film is mainly composed of SP3 carbon. Thediamond peak in the XRD pattern also broadens due to the nanocrystalline of the film.Keywords: diamond filn, microwave plasma chemical vapor deposition, ethanolPACS:8115G;52501 Introductionis more suitable for preparing high quality diamondfilm used as optical windowsdrogen gas source had been studied in hot filament.In this article, the deposition of diamond filmsCVD method [ 2. Diamond films deposited withfrom a gas mixture of ethanol and hydrogen us-ethanol and hydrogen gas mixture may possess a ing MPCVD has been investigated. The nucleationnanophase structure and a smooth surface and hasmorphology and microstructure of the diamond havepotential applications in optics and wear-protecting been investigated with SEM, Raman spectroscopycoatings, Microwave plasma, compared with hot fila- and XRD, respectively. A nanophase diamond filmment, plasma, has many virtues such as little contam- with good IR transmittance has been prepared at anants in plasma due to no electrode discharge andow 51The project supported by the Chen Guang project of the Wuhan govYH中国煤化工CNMHGMA Zhi-bin et al.: Microwave Plasma Chemical Vapor Deposition of Diamond FilmsTo vacuum chamberdensity was also observed with SEM. The surfaceroughness was measured by a roughness meter. Ra5145nm, and XRd were used to characterize the di-amond phase. The IR transmittance from 4000 cmto 500 cm-I was measured by IR spectrometer3 Results and discussion3.1 Nucleation and morphologyThe nucleation density significantly influences themorphology and the growth rate of diamond filnFig. 1 The schematic diagram of ethanol fow rate con-Scratching the substrate using diamond grits is anffective method to improve the nucleation den-sity (Bw5l. In addition, increasing the carbon sourceconcentration during the nucleation stage was also2 Experimental detailsa practical method to promote the nucleation whenthe gas was a mixture of methane and hydrogen (6Deposition of diamond films on P-type(100)sili-In this research, we found that the substrate term-con wafer was carried out in a 2.45 GHz microwaveperature, similasource ceplasma CVD reactor with a mixture of ethanol andhad a significantly effect on the diamond nucleationhydrogen as the feed gas. The schematic diagram The SEM images of the nucleation situation with dif-of ethanol How rate control is shown in Fig. 1. Be- ferent substrate temperatures are shown in Fig. 2cause the ethanol has a low vapor pressure and is liq- the Fig 2a and Fig. 2b are corresponding to 450Cuid at ambient temperature, the ethanol was trans-with growth time 0.5 h and 800C with growth timeported by hydrogen carrier gas into the reactor. The6h, respectively. A moderate ethanol concentrationthanol Aow rate was controlled through the valves of 10 vol% was used in the deposition, the nucleationand calculated from the difference in the How ratesdensity dccreases from2×103cm-2to2×10f4cmbetween the hydrogen and the mixture gas measuredwhen the substrate temperature is raised fron 450Cby mass How meter. The silicon substrate was firstto 800C. From the Fig. 2b, it is obviously seen thatscratched with a 0.5 um diamond powder and then the nuclei of diamond are very sparse and the di-thoroughly cleaned with acetone before loading intoamond growth is spherical. Although the size of athe CVD reactor, The scratching process provides single diamond cluster is about 40 um in Fig 2b,enough defects on the silicon surface and is helpful there is no continuous film formation and the dia-for diamond nucleation enhancementTo investigate the effects of substrate tempera-In addition there is no chance to form new nucleiture on the nucleation and the morphology, the sub- at the blank between the clusters. The effects oftrate temperature at the nucleation stage was var-the substrate temperature on the nucleation may beied from 400 C to 900C and the density of the nu- suggested that the concentration of the H atom orlei was determined by counting the number of sites-oh radical near the substrate surface may increasedetected in the SEM observation. 'The morphology中国煤化工 aperature and theamed films prepared with different nuclemYHCNMHGanced. The criti-Plasma Science &z'l'echnology, Vol5, No. 2(2003)(b)100mFig. 2 The SEM images of the nucleation situation with different substrate temperatures(a)450°C;(b)800°CFig-3 'The SEM surface morphologies of three samples deposited with different, substrate temperatures(a)450°C;(b)600°C;(c)700°Cal ethanol concentration for a reasonable nucleation most important factor being responsible for the filntensity of 1 x 108 cm-2 may be reduced to 5 vol% morphology in this researchwhen the substrate temperaturc is lowered to 400CFig 3 shows the SEM surface morphology of threehile the nucleationsamples deposited with identical ethanol concentrathe substrate temperature is about 900 C even if the tion of 10 vol. and gas pressure of 6.0 kPa atfeed gas is only ethanoldifferent substrate temperatures of (a)450.C,(bThe morphology of diamond film is dependent on 600 C and (c)700C, respectively. The film shows athe nucleation density and deposition parameters cauliflower growth at substrate temperature 700Csuch as substrate temperature, carbon source conThe size of the spherical structure and the surfacecentration and gas pressure. Among the above depo. roughYHt中国煤化工th thesition parameters, the substrate tenperature is theer a moderateCNMHG1737.s:,MA Zhi-bin et al.: Microwave Plasma Chernical Vapor Deposition of Diamond Films20k56110U(a)10mFig 4 The SEM images of diamond cluster deposited at a substrate temperature of 800C with different ethanolconcentrations: (a )20 vol %;(b)10 vol%ethanol concentration of 10 vol%. This result may a random morphology with increasing ethanol conbe mainly attributed to the high nucleation densitycentration. The deposition of good crystallinityat the nucleatendary observed in Fig 4b. The(100) diamond planes arenucleation during the growth process when the sub- stacked in the form of stairs on the growth surfacstrate temperature is low, whereas increasing the The crystalline facets disappear in Fig 4a duc to thesubstrate temperature will decrease the nucleation increase in the cthanol concentration. Experimentaldensity. A very smooth specimen with a roughness results show that the main effect of the ethanol con-Ra being 0.2 um, shown in Fig. 3a, was obtainecentration on diamond growth is on the microstruc-with a high nucleation density of 2 x 10cm". This ture of the diamond grains and not on the nucleationdiamond film possesses good IR transmittance higherdensityFig. 4, the size and the density of thethan 62% over the range of 4000 cm-1 N 500 cm-1diamond cluster grown on the same substrate withand may be applied to optical windowsdifferent ethanol concentrations are about cqual. InIncreasing the substrate temperature higher than creasing the ethanol concentration will increase the800C will result in a very low nucleation den-roughness of the surface and decrease the quality ofsity and develop a separated spherical growth. Twothe film, and not be uscful for the nucleation cn-kinds of half ball growth with different fine struc- hancement. Similar effects may be observed abouttures are shown in Pig. 4, in which the specimensthe continuous film growth rate. The film growthwere prepared at an identical substrate tempera-rate is essentially deternined by the substrate temture of 800 C and all with a growth time forperature and has no relationship with the ethanol6h. The ethanol concentrations are respectivelyconcentration. It is worthy to note that the growth(a)20 vol. and(b)10 vol % The sizes of the clus- rate of the balled-cluster shown in Fig 4 is approachters are about 40 um and 38 um, respectively cor-to 4 um/h, being much higher than the filn growthresponding to Fig 4a and Fig 4b. The microstruc. rate of I um/h obtained in this research. This dif-ture of the cluster is cssentially dependent on theference is similar to the result made from CO2-C2H2ethanol concentration. A transition of growth wasgas sy中国煤化工 hat insufficientobserved from a significant density of(100) facets teYHaCNMHGhis differenc1738、疆翻解州持求,Plasma Science Technology, VoL 5, No. 2(2003)tensity. The diamond peak at 1332 cm-I is nearly15481321470covered over by the D-band when the ethanol con-centration is as high as 10 vol %(in Fig 5a). Decreasing the ethanol concentration is helpful for thesuppression of non diamond carbon growth and as a1140esuntration is 6 vol%(in Fig 5b). The band around 1470 cm is promi-nent and attributed to the diamond structure con-16001800taining SP carbon. The bands around 1140 cm-1Raman shift / cmtand 1470cm-I always appear at the same time inthe Raman spectrum when the crystalline size of theFig. 5 The Raman spectra of diamond film depositeddiamond film is less than 25 nml. It is suggestat a substrate temperature 450C with differentthat the film prepared here is of nanophase film. Fur-thanol concentrations in ethanol-hydrogen gas mixtureher detailed research indicates that the size of crys-(a)10 vol%;(b)6vol%talline increases with the decreasing of the ethanolconcentration or with the increasing of the substratetemperature demonstrated from the changes in thedemonstrated through increasinethanol concentration in hydrogen. The difference ofIn the customary Raman measurement using thethe mechanisms between t514.5 nm line of the Ar-ion laser, the scattering effi-in MPCVD using ethanol and hydrogen as feed gasis a very effective means of detecting percentage levels of graphitic carbon in diamond film. In the Ra-3.2 Diamond phase identificationman spectra of Fig. 5, although the intensity of thebroadened band around 1548 cm-L associated withfig. 5 is the Raman spectra of diamond film de-amorphous carbon is higher than diamond peak, itposited at a low substrate temperature of 450C is reasonable to say that the film is mainly made ofwith different ethanol concentrations in hydrogendiamond and contains small amounts of amorphousFig 5a and Fig 5b are corresponding to 10 vol. and carbon. This result can be further confirmed by the6 vol%, respectively. The structure and the phase of XRD patterns of the specimens analyzed herethe films deduced from the Raman spectra possessThe patterns of Fig 6a and Fig 6b are corresponda feature of nanocrystalline diamond film containing ing to different ethanol concentrations as 10 voL.a little amorphous carbon 8). The broadened char- and 6 vol%, respectively. There is no graphite peakacteristic peaks of diamond and graphite are due in the XRD patterns of Fig. 6. That the intensityto the micro-grain of diamond film. The represen- of diamond(220) peak is stronger and much highertation of that the band around 1140 cm associ- than that of diamond (111)pcak indicates that theated with nanocrystalline diamond is obvious and the deposited film has a preferential(220)orientation.1332cm-diamond Raman peak is not clear may be The intensity of diamond peak and the peak ratiattributed to the appearance of disordered graphite of (2中国煤化工 e influenced byband(D-band)around 1350 cm-I with higher in- substraCNMHGncentration1739ic;主s瘫凉:磁影活级潘和;MA Zhi-bin et al.: Microwave Plasma Chemical Vapor Dcposition of Diamond Films90850Fig.6 The XRD patterns of specimens deposited atFig7 Infrared transmittance spectra of freestandinga lower substrate temperature as 450.C with differentnanophase diamond films prepared with substrate tem-ethanol concentrations:(a)10 vol %:(b)6vol.perature 450C: gas pressure 6.0 kPa, growth time 14h15 vol %; (c)20 vol % respectivelyThe diamond growth changed from random growthto predominant(220) texture with the decreasedethanol concentration. Combining the Fig 5a andthe substrate temperature will result in the decreasethe Fig 6a obtained frin the IR transmittance, which is mainly attributedconclusion can be obtained that the nanophase dia-to the large scattering from the rough surface de-mond films may be prepared at a substrate temperaposited at high substrate temperature. Increasingture as low as 450 C when the feed gas is a mixturethe ethanol concentration is also harmful to the IRof ethanol and hydrogentransmittance shown in Fig 2b 2c. In this researcthe highest IR transmittance was obtained at a sub-3.3 Infrared transmittancestrate temperature of 450C and a moderate ethanolconcentration of 10 vol%The infrared transmittance of the diamond filmwas measured by IR spectrometer. Free-standing4 Conclusionsnanophase films were prepared by chemical etchingof the silicon substrate on which the films had beenDiamond nucleation density on silicon is sensitivedeposited with different conditions. A transmissionto the substrate temperature when the feed gas is aspectrum of a 13 um thick nanophase film depositedmixture of ethanol and hydrogen. Decreasing thewith ethanol concentration 10 vol% at a substrate substrate temperature is useful for the nucleationtemperature of 450C is shown in Fig. 7a. It shows improvement. A very high nucleation density ofthat a Ch absorption band around 2900 cm- indi2 108 cm-2 and a smooth film surface with a rough-cates the presence of hydrogen in the film. The area ness Ra being 0.2 uin have been obtained with a sub-of the CH absorption band decreases with the in-strate temperature of 450C and an ethanol concencreasing of the substrate temperature. The spectrumtration of 10 vol % The diamond film prepared withshown in Fig. 7a shows a clear interference pattern. ethanol at a low substrate temperature has a goodIn Fig. 7a, the IR transmittance is higher than 62 optic中国煤化工 R spectrometerover the range of 500 cm-a 4000 cm-1.IncreasingCNMHGfied by Rama17404A斯赫Plasma Science Technology, VoL 5, No. 2(2003)spectrum and XRD pattern. In addition, increasing3 Saijo K, Yagi M, Shibuki K, et al. Surf. Coat.substrate temperature will result in the increase inTechnol,1990,43/44:30the surface roughness degree due to the sparse spher4 Yarbroough W A, Messier R, Science, 1990, 247ical growth. Good crystallinity with layer growth has5 Ali N, Ahmed W, Rego C A, et al. J Mater. Resbeen observed through increasing the substrate tem2000.15:593perature or decreasing the ethanol concentration6 Stoner B R, MA G H M, Wolter S D, et al. Phys.Rev.B,1992,45:110677 CHEN Chai-fu, LIN Chen-HONG Tsao-8 Knight D S, white W B J Mater. Res, 1989, 4References38859 Olsen J M, Michael. J Mater. Res, 1996, 111 Yoshikawa M, Katagiri G, Ishida H, et al. Appl.(Manuscript received 29 October 2002)2 Murakawa M, Takeuchi S, Miyawa H, et al. SurfEr-mail address of ma Zhi-binCoat. T'echnoL. 1988mazb@mail. whict. edu. c中国煤化工CNMHG1741