ECS Journal of Solid State Science and Technology, 3 (6) M21-M25 (2014) M212162-8769/2014/3(6)/M21/5/$31.00 © The Electrochemical Society

Hydrogen-Free Deposition of Nanocrystalline Diamond byChannel-Spark Electron Beam AblationOmar Alshekhli and Redhouane Hendaz

Laurentian University, School of Engineering, Sudbury, Ontario P3E 2C6, Canada

We report on the preparation of hydrogen-free nanocrystalline diamond films by pulsed electron beam ablation (channel-spark)from a single target on silicon and stainless steel substrates and under different process conditions. The films have been grownfrom highly ordered pyrolytic graphite at room temperature in an argon atmosphere under a pressure of 0.6 Pa. This study aimsat elucidating the influence of the accelerating voltage (13, 14.5, and 16 kV) and electron beam pulse repetition rate (5 and 8Hz) on film morphology, grain size, carbon-carbon sp3 content, and crystalline fraction. The films have been characterized usingvisible-reflectance spectroscopy, visible-Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy(AFM). The correlations between measurement data and film properties are examined and discussed.© 2014 The Electrochemical Society. [DOI: 10.1149/2.004406jss] All rights reserved.

Manuscript submitted February 18, 2014; revised manuscript received April 7, 2014. Published April 17, 2014.

Diamond has a set of technologically attractive properties suchas its extraordinarily high hardness and thermal conductivity, lowfriction coefficient, high stiffness, excellent electrical resistivity, highenergy-bandgap, and excellent chemical stability. The preparation ofartificial diamond has been an ongoing effort for over 100 years.One early and tangible effort is due to Hershey, in late 1920s, whoclaimed to have made diamond from the rapid freezing of excess car-bon dissolved in molten iron.1 Many attempts have ensued, whichare based on high-pressure and high-temperature conditions and cul-minated in the commercial synthesis of diamond from graphite byASEA in Sweden2 and General Electric.3 In the early 1960s, diamondfilms were produced by the pyrolysis of methane over a diamondsubstrate.4 Further successful attempts, throughout the 1970s–1980s,have been reported at producing films of diamond via chemical vapordeposition (CVD) from carbonaceous gas mixtures and hydrogen (orother halogens), e.g., see5,6 for a comprehensive account. The growthof diamond films continues to attract intensive worldwide attentionby the research community, as high-quality films exhibit most of theunsurpassed properties of natural diamond and they are much morecost effective.

Nanocrystalline diamond (NCD) is a nano-composite consistingof nanometer-sized diamond crystallites, which are surrounded byan amorphous carbon matrix. The diamond crystallites are made upof sp3 hybridized carbon bonded atoms, while the amorphous car-bon phase consists of both sp3 and sp2 hybridized carbon atoms (andeventually hydrogen, in case of deposition in a hydrogen-rich at-mosphere). The interfacial region of these two phases consists of aless-distinguishable phase or the grain boundaries. The latter and theamorphous carbon matrix could be considered as one phase due to thesmall volumes, i.e., < 10%, of the amorphous carbon regions.7,8 NCDfilms, with their smooth surface and high sp3 C-C bond content, arepromising candidates in a broad range of applications, such as suchas in optics, tribology, biomedicine, and catalysis.9,10 In addition toCVD methods, the growth of films of diamond has been attemptedvia physical vapor deposition (PVD) routes. These include pulsedlaser ablation,11,12 ion beam evaporation,13 magnetron sputtering,14

and, very recently, pulsed electron beam ablation (PEBA).15,16 PEBAadvantages over other PVD methods include the highly energeticelectron beam, non-thermal equilibrium ablation of virtually all kindsof materials, highly energetic plasma plume, process scalability, lowcapital/operating cost, and small equipment footprint.17,18

This work is part of our systematic study on the portents of processconditions on the composition, growth, and properties of NCD films.In a recent study, we have looked into the effect of substrate on similaraspects of the films.15 The present work has been carried out to assessthe effects of electron beam pulse repetition rate and acceleratingvoltage on the quality and properties of NCD films.

zE-mail: rhenda@laurentian.ca

Experimental

In this work, the PEBA method is based on a channel-spark processto generate electrons (pulsed electron beam source: PEBS-20, Neo-cera, USA, 15 keV, 1 kA, 100 ns) from a hollow cathode, wherebymagnetically self-pinched electrons are accelerated through a nar-row insulating tube.19 The pulsed electron beam source consists ofa trigger, a hollow cathode, and a dielectric capillary tube, which ispositioned at a 45◦ angle relatively to the target, as schematicallyshown in Fig. 1. The target is a one-inch diameter highly orderedpyrolytic graphite (HOPG ZYB, Optigraph GmbH, Germany). Argonhas been used as the working gas and kept at a constant pressure of∼0.6 Pa, and all runs have been carried out at room temperature. Theexperimental runs have been carried out at an acceleration voltage of13 kV, 14.5 kV or 16 kV, and a pulse repetition rate of 5 or 8 Hz.The same number of electron beam pulses has been used for all ex-periments. The films have been deposited on two kinds of substrates,viz., stainless steel 304 (S.S.) and silicon (100) (Si). The substrateshave been sonically cleaned in acetone and in methanol at 50◦C justbefore they were transferred to the deposition chamber. The operatingconditions have been judiciously optimized prior to deposition. Thechief point was to impart high pressure and high temperature to theplasma particles upon impact on the substrate.20 Elaboration on modeldetails and resulting calculation data will be published in a separatestudy.

Figure 1. Simplified schematics of the PEBA system.

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M22 ECS Journal of Solid State Science and Technology, 3 (6) M21-M25 (2014)

Figure 2. Visible spectroscopic reflectance of NCD film on silicon at 14.5 kV and 5 Hz. Rugged line: Measured data; smooth line: Calculated data.

The thickness of the deposited films has been measured usingvisible-reflectance spectroscopy (M-Probe Series, Semiconsoft) byfitting measured spectra to calculated ones by means of a modi-fied Marquardt–Levenberg minimization scheme. Optical data, viz.,wavelength, refractive index, extinction coefficient21 correspondingto NCD have been used for fitting the reflectance spectrum. The filmstructure and morphology have been revealed using visible-Ramanspectroscopy, AFM, and SEM. AFM (Bruker Multimode AFM IIID)images have been taken in tapping mode using TESPA (doped Si,333–382 kHz, 25 N/m) probe at a scan rate of 0.5 Hz. Film morphol-ogy has further been analyzed using SEM (Model Quanta 250FEG,10 kV). Raman (Horiba Jobin Yvon/XploRA interfaced with an Olym-pus BX41 microscope: 100× magnification, spot size ∼2 μm) datahave been collected in backscattered mode, at a grating of 1200, andan incident laser radiation wavelength of 532 cm−1. Raman spectra(average of 10, 10 s spectra) have been obtained within the range50–2000 cm−1. It is to be mentioned that XRD has been tried on oursamples under a grazing angle of 2◦. However, the peaks could not beclearly resolved from the background noise as the films are too thinfor efficient diffraction (due to weak X-ray absorption of carbon).

Results and Discussion

Figure 2 shows a typical visible reflectance response of depositedfilms. In all cases, the fit between the measured and calculated datais very good (maximum relative error of 5%). Based on obtaineddata, some general trends can be drawn for samples deposited onboth substrates, as shown in Table I. It seems that relatively thickerfilms are grown on stainless steel compared to silicon substrate. Thismay be due to the presence of carbon in stainless steel, which actsas a nucleation site, and, thus, enhances film growth.22,23 In termsof accelerating voltage, the largest film thickness is obtained at14.5 kV and highest frequency, i.e., 8 Hz, for a given substrate. Atthe frequency of 8 Hz, the thickness of films deposited at 16 kV islarger than the thickness of films obtained at 13 kV. At 5 Hz, theredoes not seem to be a substantial effect of accelerating voltage on filmthickness. At high pulse repetition rate, the time between two pulsesis shorter and the target is cooled down (after the pulse is terminated)

less deeply than at a low repetition rate. Accordingly, target heatingby the next pulse starts at a higher temperature at a high repetitionrate. This effect is likely due to an increase in heat accumulation ratein the target as the repetition rate is increased, resulting in a lowerablation threshold.24,25 The enhancement of ablation rate, which isalso a function of accelerating voltage,18 at high pulse repetition ratewill eventually result in a higher deposition rate and thicker films. Atlow repetition rate, the accelerating voltage does not seem to be ableto compensate for the lower heat accumulation rate in the target.

The recorded Raman spectra of deposited films are composed ofthe typical features of NCD. These features can be clearly distin-guished after the deconvolution (using a mixed Gaussian-Lorentzianfunction, in this case) and extraction of the baseline of the band.Figure 3 depicts the Raman signal for NCD films deposited on sili-con at 14.5 kV and 5 Hz. It is worth mentioning that originally theRaman spectra were generated from 50 cm−1 to 3500 cm−1. How-ever, only the spectral range (1000–1800) cm−1, where the activepeaks can be found, is shown in Fig. 3. The resulting fitted curveis essentially indistinguishable from the overall measured signal.The deconvoluted spectrum consists of many signals, which includethe diamond peak at 1332 cm−1, nanocrystalline diamond peak at1150 cm−1, the so-called G (1536–1565 cm−1) and D (1343–1378cm−1) bands arising from disordered and/or amorphous carbon, andthe t-PA (trans-polyacetylene) signals, which comprise the band(1450–1463 cm−1) and the peak at 1250 cm−1.26 A visible Ramanpeak, assigned G′, may appear within the range 1520–1600 cm−1 dueto defects.27–29

The fraction of sp3 carbon bonded atoms in the nanocomposite iscalculated using Equation 1.30

sp3

sp3 + sp2% =

[1 − IG

75 × (I1332 + I1150) + IG

]× 100 [1]

IG: Intensity of G band peak within the range 1536–1565 cm−1

I1150: Intensity of NCD band peak at 1150 cm−1

I1332: Intensity of diamond band peak at 1332 cm−1

It is to be mentioned that sp3 C-C is a weaker visible Ramanscatterer relatively to sp2 C-C by a factor of 50–230.31 In Eq. 1, a

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ECS Journal of Solid State Science and Technology, 3 (6) M21-M25 (2014) M23

Table I. Results of Raman spectra deconvolution and film properties for all runs. F: Frequency; V: voltage; T: Thickness.

F. (Hz) V. (kV) Sample IG IG’ I1332 ID I1150 I1250 I1480 sp3% (I1150+I1332)/�I I1332/I1150 T. (nm)

8 14.5 S.S. 815 890 910 403 204 455 2188 98.0 94.6 4.5 2158 14.5 Si 390 198 310 150 80 185 829 98.0 94.3 3.9 1678 16.0 S.S. 1233 121 450 437 202 374 478 97.3 94.9 2.2 1168 16.0 Si 599 1460 786 379 353 563 1807 97.6 94.7 2.2 1158 13.0 S.S. 9069 NA 2445 5314 1159 3320 1173 96.7 93.5 2.1 568 13.0 Si 3413 NA 925 1010 465 1177 893 96.8 94.1 2.0 405 14.5 S.S. 662 1993 663 355 390 701 1340 96.7 93.1 1.7 585 14.5 Si 558 1152 447 434 262 602 1233 96.9 93.0 1.7 405 16.0 S.S. 1253 540 432 527 264 734 1678 96.7 91.7 1.6 555 16.0 Si 1681 NA 345 987 210 816 1534 96.1 89.2 1.6 375 13.0 S.S. 7569 NA 1443 4591 1152 3083 6185 96.3 90.1 1.3 465 13.0 Si 1268 2573 651 1271 478 1046 1609 95.7 89.0 1.4 37

conservative factor value of 75 is used. The calculated values of thesp3/(sp2 + sp3) ratio are listed in Table I.

The fraction of sp3 carbon bonded atoms in the films on either sub-strates does not seem to be dramatically affected by the acceleratingvoltage at either repetition rate, but it is higher at the pulse repetitionrate of 8 Hz (in the range of ∼ 96.7%–98.0%) relatively to 5 Hz (inthe range of ∼95.7%–96.9%).

The nanocrystalline percentage in the films deposited on either sub-strate has been inferred by calculating the ratio ((I1332 + I1150)/�(I)),32

where the sum is carried over the entire spectrum. The percentagevalue is highest at a voltage of 14.5 kV for both repetition rates andis overall slightly larger at the higher repetition rate, as shown inTable I. It is worth mentioning that the magnitudes of nanocrystallinepercentage in the films deposited at 16 kV and 13 kV appear to be verysimilar. The crystal size, estimated as I1332/I1150,33 is largest for thefilms obtained at 14.5 kV and 8 Hz on either substrate. For all otherfilms, the crystal size is practically the same irrespective of the processconditions, i.e., accelerating voltage and pulse repetition rate. The ef-fect of the repetition rate on sp3 content, nanocrystalline percentage,and grain size may be explained in terms of the ability to sustainhighly energetic diamond nuclei on the substrate. At a high repetitionrate, the short time interval between successive pulses would meanthat active diamond nuclei are quickly replenished and preserve muchof their high energy (part of it is released as the pulse dies out). Theenergy of the particles impinging on the substrate depends on acceler-

Figure 3. Raman spectrum (and its deconvolution) of film deposited on Si.Other conditions are indicated in the inset.

ating voltage as well, but the latter does not seem to make a dramaticdifference in diamond growth at low repetition rate.

The typical morphology of the films as revealed by AFM analy-sis, and depicted through their 3D topography in Fig. 4, consists ofcrystallites33,34 with a size in the range ∼10 nm to ∼150 nm, consistentwith the grain size expected for NCD films, viz., a few tens of nanome-ters. The crystallites appear to grow along a preferential direction forall samples. With respect to the grain size criterion, the film depositedat 14.5 kV and 8 Hz exhibits the largest grains. Films obtained at16 kV show slightly larger grains than those of films deposited at13 kV for either repetition rate (Fig. 4). As can be observed in thesame figure, the surface of films prepared at 13 kV (irrespective of therepetition rate) is remarkably smoother, as estimated from Ra (arith-metic mean roughness, shown in Fig. 4), than the surface of filmsdeposited under other conditions. The roughness of the films obtained14.5 kV and 16 kV is practically the same irrespective of the repeti-tion rate, and larger than the roughness of films deposited at 13 kV.The reason behind this finding is not yet clear, however, roughness isgenerally affected by the film crystal size (relatively larger at 14.5 and16 kV) as well as other factors, namely, in this case, the size of nano-particulates landing on the substrate from the gas phase (plasma).Nano-sized particles have been reported to form during film deposi-tion by PEBA, and, their size to increase with accelerating voltage.35

The aforementioned AFM results are overall in good agreement withthose obtained from Raman analysis (Table I) and visible reflectancespectroscopy (Table I).

SEM images (Fig. 5) of NCD films deposited on silicon revealvariations (despite some charging) in morphology as a function ofaccelerating voltage and at 5 Hz (at 8 Hz, the scans are blurred andare not presented here). Typical images are shown in Fig. 5 for filmsdeposited on Si. As can be seen, the crystallite size of films preparedat 13 kV is smallest relatively to other films at 14.5 kV and 16 kV, andis comparable for the latter films. The findings are in good agreementwith AFM (Fig. 4) and Raman data, as reported in Table I. In orderto better appreciate the morphology of the films at 8 Hz, a sample onsilicon has been prepared that is much thicker (∼ 3 times) than the film(at 14.5 kV) in Table I, and was coated with carbon to reduce charging.The corresponding SEM image is depicted in Fig. 6a, which clearlyshows the crystalline nature of the film. Figure 6b shows the presenceof clusters of NCD with larger crystallites surrounded by areas of NCDwith much smaller size crystallites. Such clusters were not clearly vis-ible in the films reported in Table I. The formation of clusters is knownto occur in PEBA under some specific process conditions.35 The arrowin Fig. 6b points to the locus of the SEM image in Fig. 6a. For visualconvenience, Fig. 6a depicts the structure of NCD in the cluster, whichexhibits crystallites of very large size relatively to the size of thosein the remainder of the film. It is worthwhile to mention that nano-indentation tests, performed on the deposited films, have resulted inhardness data (to be published in a separate paper) within the rangereported for NCD films by other authors using different depositionmethods.20

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M24 ECS Journal of Solid State Science and Technology, 3 (6) M21-M25 (2014)

Figure 4. AFM 3D view of film morphology onSi. The process conditions are shown in the inset.

Figure 5. SEM images of NCD films on Si at5 Hz and different accelerating voltage.

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ECS Journal of Solid State Science and Technology, 3 (6) M21-M25 (2014) M25

Figure 6. SEM images of NCD films on Si at 8 Hzand 14.5 kV at different magnifications: a) ×50,000and b) ×1,000.

Conclusions

In conclusion, films of nanocrystalline diamond have been de-posited on silicon and stainless steel substrates by pulsed electronbeam ablation and characterized by visible-reflectance spectroscopy,AFM, SEM, and visible-Raman spectroscopy. Film properties suchas thickness, morphology, grain size, sp3 content, and degree of crys-tallinity have been estimated and discussed as a function of two pro-cess conditions, viz., electron beam accelerating voltage and pulserepetition rate. The findings have shown that overall film propertiesand quality are remarkably affected by the aforementioned processconditions.

Acknowledgments

This research has been supported by the Canada Foundation forInnovation (CFI) and the Natural Sciences and Engineering ResearchCouncil of Canada (NSERC).

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