Carbon 41 (2003) 1847–1850

E rosion resistance of polycrystalline diamond films to atomicoxygen

*Jingqi Li , Qing Zhang, S.F. Yoon, J. Ahn, Qiang Zhou, Sigen Wang,Dajiang Yang, Qiang Wang

Microelectronics Center, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798,Singapore

Received 17 February 2003; accepted 28 April 2003

Abstract

Polycrystalline diamond films deposited using hot filament chemical vapor deposition (CVD) technique have beeninvestigated in atomic oxygen simulated as low earth orbit environment to examine their erosion resistance properties. After

16 2exposure to the atomic oxygen beam with a flux of 2.6310 atoms/cm s, the diamond films only show a small mass loss.226 226 3The reaction efficiency is estimated to be between 6.35310 and 8.28310 cm /atom. Oxidation mechanism is

investigated through the reaction temperature influence on the reaction rate. We suggest that atomic oxygen reacts withdiamond surface and forms ether (C–O–C) and carbonyl (.C=O) configurations besides eroding the surface. 2003 Elsevier Science Ltd. All rights reserved.

Keywords: A. Diamond; B. Oxidation; C. Surface oxygen complex; D. Reactivity

1 . Introduction properties of CVD diamond is of great practical impor-tance. In our previous paper[7], the reaction of diamond

Polycrystalline diamond has excellent mechanical, ther- film to atomic oxygen for 3 h was studied. In this paper,mal and optical properties, which are well applied to the reactions are investigated as a function of erosion time.industry, for example, tools, heat sink, electronic field The influence of reaction temperature and oxygen con-emission devices, etc. Its applications in space have also centration on the reaction efficiency is discussed. Ourattracted much attention, for example, space high energy results show that diamond film is highly resistive to atomicparticle detectors, UV detectors[1], and IR windows[2,3]. oxygen erosion.However, to examine whether polycrystalline diamond canwithstand atomic oxygen erosion in a low earth orbit(LEO) environment is a key issue of study. Solid-particle 2 . Experimental conditionserosion resistance of diamond coatings has been investi-

¨gated by Grogler et al.[4] and Heinrich et al.[5]. Using a CVD diamond films used in this study were depositedmultiple-impact jet apparatus, Coad et al.[6] studied the on Si(100) substrates with a diameter of 21 mm usingdamage threshold of both natural and synthetic diamonds hot-filament CVD technique. Prior to deposition, theby high-velocity rain drops. The atmosphere in LEO is substrates were polished with 1-mm diamond paste. CH4predominantly composed of highly active oxygen atoms, and H were used as the reaction gas source. The2

which can cause serious erosion of the surface of materials temperature of Ta filament, ratio of CH /H , total gas4 2

on spacecraft. Unfortunately, the erosion resistance proper- flow, substrate temperature, and deposition pressure, werety of CVD diamond to atomic oxygen has not been well 22008C, 1%, 200 sccm, 7508C, 37 Torr, respectively. Allstudied so far. Therefore, to study atomic oxygen erosion five samples labeled 1–5 were prepared under the same

conditions as mentioned above.Reaction of atomic oxygen with diamond films was*Corresponding author. Tel.:165-679-058-55; fax:165-679-

processed in an atomic oxygen simulation system, where333-18.E-mail address: p144703764@ntu.edu.sg(J. Li). pure oxygen was ionized by microwave energy. The

0008-6223/03/$ – see front matter 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0008-6223(03)00176-3

1848 J. Li et al. / Carbon 41 (2003) 1847–1850

T able 1Parameters of diamond sample erosion with atomic oxygen

Sample Oxygen Reaction Atomic Reaction Erosion Atomic Mass Reactionno. gas flow pressure oxygen area time oxygen flux loss efficiency

2 16 226(sccm) (Pa) energy (cm ) (h) (310 (mg) (3102 3(eV) atoms/cm s) cm /atom)

1 2.7 0.2 5 2.46 1.5 2.6 0.08 6.632 2.7 0.2 5 2.46 2 2.6 0.13 8.073 2.7 0.2 5 2.46 3 2.6 0.20 8.284 2.7 0.2 5 2.46 6 2.6 0.38 7.875 2.7 0.2 5 2.46 9 2.6 0.46 6.35

oxygen ions were then accelerated to gain enough energy detectable impurities except for a very small amount of Oand neutralized into energetic atomic oxygen. The oxygen on the as-grown diamond surface. Atomic ratio of O to Cgas flow rate and the pressure in the reaction chamber wereobtained from fitting the XPS spectra (inFig. 1) is only

212.7 sccm and 2310 Pa, respectively. The atomic oxygen 0.87%. After atomic oxygen erosion, the intensity of O1s16was at an average energy of 5 eV and its flux was 2.6310 core-level spectra increases apparently. The oxygen con-

2atoms/cm s. Five diamond samples were exposed to this centrations of these five samples are shown inFig. 2. It isatomic oxygen flux for different time periods which are found that the oxygen concentration on the diamond filmshown inTable 1.

3 . Results and discussion

Diamond film surfaces were examined using scanningelectron microscopy (SEM)[7]. The as-grown surfaceswere of a mixture of (111) and (100) facets, but dominatedby (111) ones. Some twin crystalline grains were alsofound. The diamond surface morphology, with an averagegrain size of about 4mm, did not show significant variationbefore and after atomic oxygen erosion. The as-growndiamond film was also analyzed by Raman spectroscopy

21[7] in which only diamond peak at 1332 cm wasobserved, suggesting no detectable impurities in the film.

Atomic bonding configuration of the diamond films wasanalyzed using X-ray photoelectron spectroscopy (XPS),which was conducted using a PHI-5702 electron spec- Fig. 2. Oxygen concentrations of samples after erosion versustrometer with a pass energy of 29.35 eV and Al Ka-line reaction time.excitation source. It is found that there are no other

Fig. 1. O1s and C1s core-level XPS spectra of as-grown diamond film.

J. Li et al. / Carbon 41 (2003) 1847–1850 1849

surface is increased linearly with the erosion time. Interest-ingly, the C1s spectrum shows small changes in bothposition and shape. The symmetric peak at 284.1 eV forthe as-grown diamond is shifted to 284.81 eV after theoxidation. The peak becomes more complicated andasymmetric. A shift of10.71 eV could be due to the effectof adsorbed oxygen on the diamond surface[8,9]. Fig. 3shows the fitted C1s peak of Sample 3 after erosion. Itdemonstrates that two fitted peaks possess binding energiesof 286.10 eV and 287.32 eV greater than the main C–Cpeak of diamond by11.29 and12.51 eV, respectively.Judging from the peak positions, we suggest that C–O–C(ether) and C=O (carbonyl) are formed in the erosion,which is in accordance with the results of Goeting et al.[9]and Wilson et al.[10]. Theoretic calculation[11–16] andexperimental observation[17–19] confirm that the twostructures are quite stable during the surface reconstruction Fig. 4. Reaction efficiency of diamond films as a function ofat oxygen environment. reaction time.

Atomic oxygen reaction efficiency is defined as materialvolume loss per oxygen atom as follows:

diamond and therefore were eroded relatively quickly andDm made a rapid mass loss in the first 3 h. The second factor is]]R5 (1)rFSt caused by two compensatory influences; i.e., reaction

temperature and oxygen concentration on the eroding film3wherer53.5 g/cm is the density of diamond film,Dm surface. On the one hand, the erosion temperatures caused

the mass loss of the diamond films after atomic oxygen by atomic oxygen flux were different for the five samples.erosion,F the atomic oxygen flux,S the reaction area and Fig. 5 shows the temperature variation of Sample 4 as at the erosion time. The last four parameters and calculation function of erosion time up to 6 h erosion. The temperaturevalues of all the five samples are listed inTable 1. It is increment was 418C in the first 3 h, while it was onlyimportant to note that the reaction efficiency increases with 6.48C in the following 3 h. Since the reaction rate oferosion time in the early reaction stage, as shown inFig. 4, atomic oxygen with diamond increases with the reactionand then decreases after reaching the maximum value of temperature, the large mass loss rate in the first 3 h

226 38.28310 cm /atom for about 3 h erosion. This phe- probably resulted from the large increase in the tempera-nomenon can be interpreted through two factors. First, a ture. On the other hand, newly formed C–O–C (ether) andvery small amount of impurities such as graphite and C=O (carbonyl) configurations on diamond surface mayamorphous carbon, which usually grow during the prevent the diamond from further atomic oxygen erosion,diamond growth have lower erosion resistance than leading to a reduction of the reaction efficiency. The higher

the concentration of the ether and carbonyl clusters, thegreater the preventing effect. FromFig. 2, the cluster

concentration of Sample 4 and Sample 5 after erosion is

Fig. 3. Deconvolution of C1s peak of the diamond surface afteratomic oxygen erosion. Fig. 5. Variation of reaction temperature versus reaction time.

1850 J. Li et al. / Carbon 41 (2003) 1847–1850

impact resistance of a range of IR-transparent materials.much larger than that of Samples 1, 2 and 3. Thus, theWear 1995;186–187:375–83.preventing effect on Samples 4 and 5 should be more

[3] L i J, Zhang J, Guo W, Lei Q, Sun Y, Gao X, He D. Opticalsignificant than Samples 1, 2 and 3. From the twobrazing technique for bonding diamond films to zinc sulfide.observations, we suggest that, due to rapid increase in theDiamond Relat Mater 2002;11(3–6):753–6.reaction temperature and small coverage of ether and

[4] G rogler T, Zeiler E, Franz A, Plewa O, Jilbert GH, Field JE.carbonyl clusters in the first 3 h, the erosion rate increases Erosion resistance of CVD diamond-coated titanium alloysignificantly. However, the reaction temperature ap- for aerospace applications. Surf Coat Technolproaches its saturated value after 3 h erosion. The prevent- 1999;112:129–32.ing effect of created ether and carbonyl clusters dominates [5] H einrich G, Grogler T, Rosiwal SM, Singer RF. CVDthe process, reducing the reaction efficiency thereafter. diamond coated titanium alloys for biomedical and aerospace

applications. Surf Coat Technol 1997;94–95:514–20.It should be pointed out that the peak value of the[6] C oad EJ, Pickles CSJ, Jilbert GH, Field JE. Aerospacereaction efficiency inFig. 4 is a factor of 4 larger than that

erosion of diamond and diamond coatings. Diamond Relatof natural diamond. The explanation to the finding can beMater 1996;5(6–8):640–3.2-fold. First, CVD diamond surface is much rougher than

[7] L i J, Zhang Q, Yong SF, Ahn J, Zhou Q, Wang S et al.that of natural diamond. Thus the effective reaction area ofReaction of diamond thin films with atomic oxygen simu-

diamond film should be larger than the measured area lated as low-earth-orbit environment. J Appl Phys2value of 2.46 cm . Second, CVD diamond usually has a 2002;92(10):6275–7.very small amount of non-diamond component at the [8] S ugino T, Itagaki T, Shirafuji J. Formation of pn junctionsdiamond grain boundaries, such as graphites or amorphous by bonding of GaAs layer onto diamond. Diamond Relatcarbons, which could be easily eroded by atomic oxygen. Mater 1996;5(6–8):714–7.

[9] G oeting CH, Marken F, Sosa AG-, Compton RG, Foord JS.Although the reaction efficiency for diamond films isElectrochemically induced surface modifications of boron-larger than that of natural diamond, it is much smaller than

225 doped diamond electrodes: an X-ray photoelectron spec-that of common space protection material, say, 2.23103 troscopy study. Diamond Relat Mater 2000;9(3–6):390–6.cm /atom for Teflon. Therefore, CVD polycrystalline

[10] W ilson JIB, Walton JS, Beamson G. Analysis of chemicaldiamond film is a very prospective material for passivationvapor deposited diamond films by X-ray photoelectron

application in LEO environment. spectroscopy. J Electron Spectr Relat Phenom2001;121:183–201.

[11] F renklach M, Huang D, Tomas RE, Rudder RA, Markunas4 . Conclusion RJ. Activation energy and mechanism of CO deposorption

from (100) diamond surface. Appl Phys Lett1993;63(22):3090–2.Upon exposing CVD polycrystalline diamond films to

[12] W hitten JL, Cremaschi P, Tomas RE, Rudder RA, Markunasatomic oxygen simulated for low-earth-orbit environment,RJ. Effects of oxygen on surface reconstruction of carbon.two different reactions occur. On the one hand, oxygenAppl Surf Sci 1994;75(1–4):45–50.atoms react with diamond surface and erode away the

[13] S kokov S, Weiner B, Frenklash M. Molecular-dynamicsdiamond films. On the other hand, oxygen atoms also reactstudy of oxygenated (100) diamond surfaces. Phys Rev B

with surface carbon atoms to reconstruct them into rela- 1994;49(16):11374–82.tively stable structures, such as ether (C–O–C), carbonyl [14] Z heng XM, Smith PV. The stable configurations for oxygen(.C=O) clusters or both, resulting in a reduction of the chemisorption on the diamond (100) and (111) surfaces. Surerosion rate. We find that after exposure to an atomic Sci 1992;262(1–2):219–34.

16 2oxygen beam with a flux of 2.6310 atoms/cm s, the [15] S kokov S, Weiner B, Frenklash M. Theoretical study ofoxygenated (100) diamond surfaces in the presence ofdiamond films show very small mass loss with reaction

226 226 3 hydrogen. Phys Rev B 1997;55(3):1895–902.efficiencies between 6.35310 and 8.28310 cm /¨[16] L arsson K, Bjorkman H, Hjort K. Role of water and oxygenatom. Such a small reaction efficiency suggests that CVD

in wet and dry oxidation of diamond. J Appl Physdiamond is highly erosion-resistive to atomic oxygen and it2001;90(2):1026–34.is very good passivation material for space application.

[17] P ehrsson PE, Mercer TW. Oxidation of the hydrogenateddiamond (100) surface. Surf Sci 2000;460(1–3):49–66.

[18] d e Theije FK, Reedijk MF, Arsic J, van Enckevort WJP,R eferences Vlieg E. Atomic structure of diamondh111j surfaces etched

in oxygen water vapor. Phys Rev B 2001;64:085403-1-7.[1] H ouchedez J-F, Bergonzo P, Castex M-C, Dhez P, Hainaut [19] H ossain MZ, Kubo T, Aruga T, Takagi N, Tsuno T, Fujimori

O, Sacchi M et al. Diamond UV detectors for future solar N, Nishijima M. Chemisorbed states of atomic oxygen andphysics missions. Diamond Relat Mater 2001;10(3–7):673– its replacement by atomic hydrogen on the diamond. Surf Sci80. 1999;436(1–3):63–71.

[2] S eward CR, Coad EJ, Pickles CSJ, Field JE. The liquid