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Morphological evolution of polished single crystal (100) diamond surface exposed to microwave hydrogen plasma
Diamond & Related Materials 18 (2009) 1466–1473
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Morphological evolution of polished single crystal (100) diamond surface exposed to microwave hydrogen plasma A. Gaisinskaya a, R. Edrei a, A. Hoffman a,⁎, Y. Feldheim b a b
Schulich Department of Chemistry, Technion, Haifa 32000, Israel 21 Tuval St. Ramat Gan 52522, Israel
a r t i c l e
i n f o
Article history: Received 11 April 2009 Received in revised form 17 September 2009 Accepted 28 September 2009 Available online 6 October 2009 Keywords: Diamond Hydrogen Plasma Morphology
a b s t r a c t It is shown that exposure of polished single crystal diamond surfaces to microwave hydrogen plasma at optimized conditions may be applied as a very efficient method for the smoothing out of single crystal diamond surfaces. The effect of microwave (MW) hydrogen plasma exposure on the morphology of mechanically polished natural single crystal (100) oriented diamond type 2a surfaces is reported. It is shown that the surface morphology is very sensitive to plasma power and exposure time. Under appropriate plasma exposure conditions the diamond surfaces smooth out as reflected in the decrease in the number and depth of the polishing scratches or lines. However adverse effects on the surface morphology through the formation of pits were found to occur upon long exposure times and high plasma power. A systematic study of the influence of hydrogen microwave plasma power and exposure time on the diamond surface morphology is presented. The morphology of the diamond surfaces at the different stages was monitored with sub-nano-metric resolution by atomic force microscopy and scanning electron spectroscopy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The preparation of morphologically defined diamond surfaces is a very difficult task. The complexity in the preparation of this surface originates in their extreme hardness and metastability. Standard preparation by mechanical polishing, ion irradiation and thermal annealing result in ill-defined diamond surfaces: polishing leads to formation of polishing traces resulting in poor morphology whereas ion irradiation and annealing leads to an amorphous surface with degraded optical properties. Recently, several researches have shown that hydrogen plasma treatment improves the surface flatness and smoothes polishing scratches on diamond surfaces [1–8]. Overall these studies show that exposure of diamond surfaces to hydrogen plasma at appropriate conditions may result in smoothing of these surfaces. These studies provide evidence that surface temperature and hydrogen plasma activation may have an effect on the surface morphology. The different authors mention two main mechanisms as being responsible for the observed morphological effects. These are associated with carbon surface assisted diffusion and etching by atomic hydrogen. Below we briefly summarized the works that reported, to the best of our knowledge, the effect of hydrogen plasma on single crystal diamond surface morphology.
⁎ Corresponding author. Tel.: +972 4 8293747; fax: +972 4 8295703. E-mail address: [email protected] (A. Hoffman). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.09.014
Hayashi et al. studied re-polished synthetic 1b diamond (001) surfaces treated with hydrogen plasma [1]. In that study substrates were positioned onto a molybdenum sample holder and heated inductively in the 750 to 850 °C range, while the duration of the treatment was fixed at 5 min. The hydrogen pressure, flow rate and microwave power were 25 Torr, 398 sccm and 750 W, respectively. Under these conditions the authors reported on the formation of successive terraces with atomic steps running parallel to the (110) directions alongside with square pits. The formation of terraces was rationalized on the base of surface diffusion of carbon whereas the formation of the square pits as the effect of etching by atomic hydrogen. They also reported that the diamond morphology is sensitive to treatment temperature and atomic step bunches is not observed at higher temperatures. Rawles et al. [2,3] have studied the effect of hydrogen plasma on type 1a natural diamond powders and type 2a (100) oriented single crystals in a microwave plasma reactor operated at 100 Torr, flow rates of 50–500 sccm, and power levels in the 2.5–4 kW range. In these studies the substrate temperatures was varied in the 650–850 °C range and the diamond samples were positioned onto a molybdenum sample holder. They observed smoothing and nano step bunching on diamond surfaces. Additionally, they found that the diamond particles remained of the same size, but became smoother and well faceted. Based on these findings they proposed that hydrogen atom-assisted carbon surface diffusion is the dominant mechanism affecting the diamond surface morphology and that etching mechanisms are of secondary importance. They also mention that the degree of faceting was sensitive to plasma power and temperature. Treatment at higher power generated deeper
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troughs and the diamond particle's surface became completely faceted after 3 h of exposure. Rawles et al. [3] showed the negligible change of diamond weight after the treatment and the dependence of initial structure on smoothing and pit formation. Kuttel et al. [4] have studied and compared the effect of electron cyclotron resonance and microwave hydrogen plasma on natural boron-doped type 2b (100) oriented diamond surfaces. They observed no changes after prolonged exposure to electron cyclotron resonance hydrogen plasma treatment. However, microwave hydrogen plasma resulted in a decrease of roughness (RMS) from 2 nm to 0.8 nm. These results seem to show that the energy of the atomic hydrogen may affect the surface processes leading to surface smoothing. Komatsu et al. investigated the effect of radio-frequency hydrogen plasma on the morphology of diamond surfaces and reported that smoothness appeared to be improved except for the existence of shallow etch pits with a few nm in depth. However polishing traces whose depths are less than 2 nm are still present after the treatment of hydrogen radio frequency plasma treatment. Thoms et al. [6] described the preparation of smooth diamond (100) surface consisting of terrace several hundred Å wide separated by 40 Å steps. In that work the diamond samples were exposed to MW hydrogen plasma operated at 600 W, 10 Torr of hydrogen and temperature of ~ 800 °C. Koslowski et al. [7] observed enhancement of surface roughness after hydrogen plasma treatment on chemo-mechanical polished diamond (100). A MW Chemical Vapor Deposition (CVD) reactor was used in that work to expose the substrate to hydrogen plasma operated using high purity hydrogen at 40 mbar and a flow rate of 80 sccm/min. The MW generator was operated in a pulse mode with a duty cycle of 50% and a peak MW power of 500–720 W. The substrate was heated indirectly by the plasma to 850 °C. They suggested that the roughness increase due to
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plasma treatment is associated to strain relaxation and anisotropic etching of defects. In this work we systematically investigated the effect of microwave hydrogen plasma exposure on the morphology of mechanically polished type 2a (100) oriented diamond surfaces. In particular the effects of microwave power and exposure time on the surface morphology were investigated. Microwave powers of 500, 700 and 900 W for an exposure time of 30 min were used in our experiments. For the 700 W microwave plasma power the diamonds were exposed for 15, 30 and 60 min. These plasma conditions and exposure times were selected as characteristic treatment times and plasma powers applied in our system and sufficient in order to induced chemical and physical modifications to the diamond surfaces. The morphology of the diamond surfaces was measured by non contact atomic force microscopy and compared to high resolution scanning electron microscopy. This was necessary for the determination of the true size of nano-metric wide pits. The effect of the plasma treatment on the diamond surface is normalized to the morphology of the surface prior to the plasma exposure. This is important as two polished diamond are never the same in terms of density of polishing scratches and their amplitudes as it will be described below. By this normalization processes the effect of plasma exposure on two different diamond surfaces can be compared and analyzed. Finally the effect of hydrogen plasma on the morphology of oriented single crystal and polycrystalline diamond [8] is compared. 2. Experimental details Two sets of natural single crystal (100) oriented type 2a diamonds were mechanically polished following by boiling in organic acids. According to the gemological examination all stones
Fig. 1. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 700 W for 15 min (C–D). Cross-section (across polishing line), parallel section (along polishing line).
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A. Gaisinskaya et al. / Diamond & Related Materials 18 (2009) 1466–1473
show a very good (4–5) grade (http://www.gemappraisals.com/pdf/ DiamondGradingScales.pdf). The diamonds were measured by atomic force microscope (AFM) (Nanoscope IV, Veeco) before and after the plasma treatment under atmospheric conditions. The microscope was operated in tapping mode with silicon cantilevers NSC14 (MikroMasch) with tip radius less than 10 nm, and force constant of 5.0 N/m. The AFM data were flattened by second order planarization. HR-SEM analysis was carried out using a Digital Scanning Microscope (DSM LEO-982) and Zeiss Ultra Plus Microscope at primary beam energies of 5 keV and 1 keV. Prior the exposures to MW-H plasma the diamonds were cleaned with analytical acetone followed by 96% ethanol for 20 min each in an ultrasonic bath; this procedure was done four times. Hydrogen plasma treatment was carried out using a microwave ASTEX system operated at a pressure of 50 Torr and power of 500, 700 and 900 W. The maximum hydrogen flow rate was 500 sccm and was automatically controlled to 42% of maximum flow rate (i.e. 210 sccm). The hydrogen plasma exposure was performed by positioning the diamond 5–10 mm below the plasma sphere onto a molybdenum plate. The plasma was ignited using a MW power of 350 W, and a hydrogen pressure of 0.5 Torr. Then the power level (500, 700 and 900 W), exposure time (15 min, 30 min and 60 min) and pressure (50 Torr) were determined according to the experimental plan. The diamond's temperature was measured by a thermo couple connected to the molybdenum plate. The sample temperature increases exponentially with exposure time and MW power during the first 5–7 min from the moment the plasma sphere was ignited and then the temperature stabilized. We estimate that the sample temperature during the plasma exposure was about 470 °C, 550 °C
and 600 °C for power level of 500, 700 and 900 W of plasma power, correspondently. The treatment was stopped automatically by reducing the power level of MW system. Three diamonds were exposed to plasma powers of 500, 700 and 900 W for 30 min and other two diamonds were exposed for 15 and 60 min at 700 W. In other words we investigated two sets of experiments on diamond surface morphology: as a function of plasma time and as a function of power level of MW-H plasma. Polycrystalline diamond film with submicron grain size was grown on 1.0 × 1.0 cm2 Si substrate in a hot filament (HF)-CVD system as described in our previous publication [9]. Deposition parameters were CH4/H2 gas mixture of 1:99 and gas pressure of 50 Torr. The substrate temperature was of 750 °C and deposition time of 30 min. This HFCVD diamond film was exposed to hydrogen microwave plasma at 700 W for 30 min. During plasma exposure the substrate temperature was about 550 °C. After the plasma exposure process the diamond surface was examined by AFM in amplitude mode. Background experiments carried out on silicon surfaces showed that no carbon deposition occurred during the hydrogen plasma exposure that could possible affect the reported effects. 3. Results 3.1. Morphology of the mechanically polished single crystal diamond surfaces The morphology of the as polished and acid clean single crystal diamond surfaces are shown in Figs. 1A–5A. From these figures the morphology of the diamond surfaces are characterized by clear
Fig. 2. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 700 W for 30 min (C–D). Cross-section (across polishing line) parallel section (along polishing line) and pits (pits line).
A. Gaisinskaya et al. / Diamond & Related Materials 18 (2009) 1466–1473
polishing scratches of varying density, amplitude and other irregularities which can be examined by AFM as described below. These were analyzed in detail for each stone. Generally, the diamond surfaces have 10–12 scratches/μm2 (see Tables 1 and 2). There are two types of scratches: shallow scratches (0.3–1.0 nm in height) and broad scratches (1.5–2.5 nm in height), each broad scratch includes a number of shallow scratches. Fig. 3A demonstrates pits of 1.0 ± 0.20 nm deep and of 22 ± 4.1 nm width which are visible on the edges of the shallow scratches. These values (scratch density and size) were determined based a statistical analysis of AFM pictures carried out on different spots of the diamond surfaces and are representative of average values (see Tables 1, 2 and 3). The scratch's shapes strongly depend on polishing procedure and conditions and are unique for each diamond. The diamond surface can include residues of diamond grains (Fig. 2A), spot shapes (Fig. 1A), small pits (Fig. 3A), or broad scratches (Fig. 4A). Due to the fact that each diamond has different initial topography, the plasma effect analysis must take into account the initial topography.
3.2. Hydrogen exposure as a function of time at constant microwave power level Three different diamonds were exposure for 15, 30 and 60 min to the hydrogen plasma at 700 W. The prolonged hydrogen plasma exposure has a strong effect on the (100) diamond surface morphology as it is demonstrated by AFM measurements at Figs. 1–3.
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Cross lines represent scratches height, width and density whereas parallel lines represent scratches irregularity during the exposure. As can be seen from Figs. 1–3 the hydrogen plasma treatment reduces the scratches height and density. The shallow scratches gradually disappear and the broad scratches decrease in height. The measured dependence of the scratch's height reduction versus microwave exposure time for broad and shallow scratches, at a plasma intensity of 700 W, is presented in Fig. 6A. The difference between initial and final scratch height increases with exposure time. The shallow scratches smooth out faster than the broad scratches at the beginning, but as the process proceeded, the shallow scratches smooth out slower than the broad scratches, as can be seen from the slope in Fig. 6A. The difference of initial and final heights for diamond surface after 30 min of exposure is 0.48 ± 0.17 nm for broad scratches and 0.55 ± 0.11 nm for shallow scratches. As can be seen from Figs. 1C–5C, following the plasma treatment, the shallow scratches disappeared. The difference between the amount of initial and final scratches increases with plasma exposure time as shown in Fig. 6A. Hydrogen plasma cleans the initial polished defects as Figs. 1 and 2 demonstrate. After 30 min of 700 W microwave hydrogen plasma exposure square pits appear (Fig. 2C). Their shape is confirmed with HR-SEM technique as shown in Fig. 7. Their size and amount increase with exposure time, in case of treatment after 30 min, 10 ± 0.35 pits/μm2 appear with average depth of 2.4 ± 0.25 nm and width of 48 ± 5.3 nm (Tables 1 and 3). Careful examination of Fig. 3A demonstrates that this initial sample has already very small pits on its surface before plasma
Fig. 3. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 700 W for 60 min (C–D). Cross-section (across polishing line), parallel section (along polishing line) and pits (pits line).
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Fig. 4. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 500 W for 30 min (C–D). Cross-section (across polishing line), parallel section (along polishing line) and pits (pits line).
treatment. However, as presented in Tables 1 and 3, the number of pits after 60 min of hydrogen plasma exposure increases by 13 ± 0.71 pits/μm2, the average depth increase by 1.0 ± 0.60 nm and the average width increased by 69 ± 7.2 nm.
3.3. Hydrogen exposure as a function of powers level at constant time Our results clearly show that surface morphology is strongly affected by plasma power. The higher power level of the microwave plasma smoothes polishing scratches more productively and causes to their disappearance (Figs. 2, 4 and 5). However the high power level could cause to square pit appearance and an increasing of pit size and pit density (Fig. 8B, Tables 2 and 3). At first, we investigated the diamond surface after hydrogen plasma exposure at a power level of 500 W (Fig. 4). Its initial surface was badly defined with 1.3 ± 0.18 broad scratches/μm2, as shown in Fig. 4A and Table 2. Nonetheless, a convincing influence on the surface topography resulting from exposure to the plasma can be seen (Fig. 4C). However, we do not include it in our quantitative analysis. We focus on two examples: after 700 W of plasma exposure (Fig. 2) and after 900 W of plasma exposure (Fig. 5). The reduced height after 700 W of the plasma exposure is 0.48 ± 0.17 nm for broad scratches and 0.55 ± 0.11 nm for shallow scratches. The reduced scratches density is 7.8 ± 0.0056 scratches/μm2 (Table 2). The square pits present at density of 10 ± 0.35 pits/μm2 of 2.4 ± 0.25 nm in height and of 48 ± 5.3 nm in width (Table 3). Exposure to 900 W of MW-H plasma reduces the height by 0.93 ± 0.14 nm of broad scratches and by 0.63 ± 0.17 nm of shallow scratches. The reduced density of scratches is 8.3 ± 0.071 scratches/
μm2. The pits density is 13 ± 0.88 pits/μm2 of 5.7 ± 0.42 nm in height and 110 ± 2.7 nm in width as can be obtained from Tables 2 and 3. From our results, an increase in plasma power results in a decrease in the number of polishing scratches being left on the surface and if one does not pay attention to the pitting effect, the diamond surface becomes much smoother with increasing power level. We decided not to analyze our results by RMS or by another roughness measurement, contrary to other researches, because the pits cause incorrect results in roughness that could be interpreted as a smoothness effect. The AFM analysis shows that the average width and density of pits on the diamond surface after exposure to the 700 W plasma for 30 min is 48 ± 5.3 nm and 10 ± 0.35 pits/μm2, respectively. HR-SEM analysis presents the same shaped pits (Fig. 7). However, from our HR-SEM analysis the pits density is 7.5 ± 0.71 pits/μm2 and the width is 32 ± 0.31 nm. This difference in width between the measurements carried out by AFM and HR-SEM may be caused by the limitation of the SEM technique to discover the shallow slope of the pits as can seen in Fig. 5E. 3.4. Comparison with chemical vapor deposited diamond films The most popular industrial technique for diamond growth is CVD. CVD produces thin diamond films which don't have polishing scratches but display a surface morphology which comprises disoriented microscopic crystallites. Fig. 9 shows the changes in amplitude morphology of HF-CVD diamond film following exposure to MW hydrogen plasma at 700 W for 30 min, 500 sccm at 50 Torr. As can be seen the exposure to MW-H plasma results in only minor chemical changes and severe surface morphology modification. The MW-H plasma etched very effectively the nondiamond carbon phase
A. Gaisinskaya et al. / Diamond & Related Materials 18 (2009) 1466–1473
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Fig. 5. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 900 W for 30 min (C–D). Cross-section (across polishing line), parallel section (along polishing line) and pits (pits line), (E) pit's cross-section.
and to a much lesser extent diamond phase. The morphology was, however, modified to a granular surface with apparent preferential etching of triangular diamond facets. The effect of MW hydrogen
plasma on the surface morphology of diamond films has been recently reported by our group (Shpilman et al. [8]). Hence it can be seen in Fig. 9 that the hydrogen plasma exposed the diamond carbon phase
Table 1 Influence of MW plasma on number of scratches, pits, heights of broad and shallow scratches as a function of different time exposure. 700 W Time [min]
Density of scratches [scratches/μm2]
Density of pits [pits/μm2]
Pristine
Plasma
Delta
Pristine
Plasma
Delta
Pristine
Plasma
Delta
Pristine
Plasma
Delta
15 30 60
11.3 11.8 11.3
4.3 4.0 2.5
7.0 7.8 8.8
0.0 0.0 33.5
0.0 10.0 46.5
0.0 10.0 13.0
1.4 2.2 2.4
1.0 1.8 1.8
0.4 0.5 0.7
0.5 1.0 1.0
0.2 0.4 0.3
0.2 0.6 0.6
Height of broad scratches [nm]
Height of shallow scratches [nm]
Table 2 Influence of MW plasma on number of scratches, pits, heights of broad and shallow scratches as a function of different power exposure. 30 min Power [W]
500.0 700.0 900.0
Density of scratches [scratches/μm2]
Density of pits [pits/μm2]
Height of broad scratches [nm]
Height of shallow scratches [nm]
Pristine
Plasma
Delta
Pristine
Plasma
Delta
Pristine
Plasma
Delta
Pristine
Plasma
Delta
1.3 11.8 9.8
1.3 4.0 1.5
0.0 7.8 8.3
0.0 0.0 0.0
7.8 10.0 12.8
7.8 10.0 12.8
1.5 2.2 1.7
1.7 1.8 0.8
0.3 0.5 0.9
0.3 1.0 0.6
0.3 0.4 0.0
0.1 0.6 0.6
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Table 3 Influence of MW plasma on width and depth of pits as a function of different time and power exposure. 700 W Time [min]
15 30 60
Depth of pits [nm]
Width of pits [nm]
Pristine
Plasma
Delta
Pristine
0.0 0.0 1.0
0.0 2.4 2.0
0.0 2.4 1.0
0.0 0.0 22.0
Pristine
Plasma 0.0 47.8 91.3
Delta 0.0 47.8 69.3
30 min Power [W]
500 700 900
Depth of pits [nm]
Width of pits [nm]
Pristine
Plasma
Delta
0.0 0.0 0.0
5.5 2.4 5.7
5.5 2.4 5.7
0.0 0.0 0.0
Plasma
Delta
65.0 47.8 110.0
65.0 47.8 110.0
Fig. 8. (A) Scratches height reduction versus plasma exposure power for 30 min for shallow scratches (squares) and for broad scratches (rhombi); (B) number of pits (squares) and number of scratches (rhombi) reduction versus plasma exposure for 30 min.
producing smooth rectangular facets. It is most important to note that there are no pitting defects on the polycrystalline surface following hydrogen plasma exposure. 4. Discussion
Fig. 6. (A) Scratches height reduction versus plasma exposure time at 700 W for shallow scratches (squares) and for broad scratches (rhombi); (B) number of pits (squares) and number of scratches (rhombi) reduction versus plasma time at 700 W.
Mechanical polishing of diamond surfaces consist of repeated microcleavage leaving a rough but crystalline surface without any plastic deformation. Generally, the morphology of polished diamond is characterized by polishing scratches consisting of broad patterns which contain a variety of shallow patterns. The scratches themselves have irregularities. Hydrogen microwave plasma treatment can produce a remarkable degree of smoothing of the scratches and create regular surface morphology through disappearance of shallow scratches. However, as found in our studies, plasma exposure for prolonged times or high plasma intensities may cause damage to the
Fig. 7. HR-SEM image at ×100,000 of magnification of single crystal diamond surface after MW hydrogenation (A) at 900 W for 30 min by digital Scanning Microscope Leo-982 at 5 kV of energy high tension and (B) at 700 W for 30 min by Zeiss Ultra Plus Microscope at 1.00 kV of energy high tension.
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Fig. 9. AFM amplitude images, size 2 × 2 μm2, scale of 300 mV, of HF-CVD diamond films before (A) and after exposure to MW hydrogen plasma (B).
diamond surface mainly through the formation of pits. The optimal conditions, based on present experiments and single crystal diamonds used in this study, are 15 min exposure at a power of 700 W and temperature of 550 °C. Our studies show that the pits appeared on the single crystal diamond surface only after prolonged exposure to the hydrogen plasma or on badly defined single crystal diamond surfaces. On the other hand no pits were observed to appear on the CVD diamond surface upon exposure to hydrogen plasma. It was pointed out previously that pits are located predominantly on edges of polishing scratches (Fig. 3). We believe that exposure of the diamond surfaces to atomic hydrogen at high temperature most likely preferentially reacts with surface defects that are located on cross junctions of crystallographic planes which are produced from micro cleavage during mechanical polishing. As the plasma power is increased so does the concentration of active hydrogen and the temperature raises; the prolonged exposure causes prolonged reactions. This makes hydrogen to be more aggressive to the diamond surface and causes the surface to be more susceptible for etching at high energy places, thus resulting in the creation of pits. Considering that we operated the MW hydrogen plasma at relatively low surface temperatures and did not observe weight loss, our results support the possibility that smoothing phenomena is primarily due to the surface carbon diffusion at the diamond surface. In summary, we can unequivocally show that hydrogen plasma flattens the single crystal diamond surface and there are marked influences of MW hydrogen exposure time and power on the level of
diamond surface smoothing. In our experimental system the optimal condition to smooth out single crystal and oriented type 2a (100) diamond surfaces are a plasma power 700 W and 15 min of exposure at 550 °C. Higher power and/or longer exposures times results in the degradation of the diamond surfaces through the formation of nanometric size pits. It is possible that these conditions may differ for other experimental systems. Acknowledgements The authors are very grateful for material support from Word Gemological Insititute and Professor Yeshaya Yarnitsky for his advice. References [1] K. Hayashi, S. Yamanaka, H. Watanabe, T. Sekiguchi, H. Okushi, K. Kajimura, Appl. Surf. Sci. 125 (1998) 120. [2] R.E. Rawles, S.F. Komarov, R. Cat, W.G. Morris, J.B. Hudson, M.P. D'Evelyn, Diamond Relat. Mater. 6 (1997) 791. [3] R.E. Rawles, R. Cat, W.G. Morris, M.P. D'Evelyn, Mater. Res. Soc. Symp. Proc. 416 (1996) 299. [4] O.M. Kuttel, L. Diederich, E. Schaller, O. Carnal, L. Schalapbach, Surf. Sci. 337 (1998) L812. [5] S. Kamtsu, K. Okada, S.B. Chou, T. Aizawa, H. Shigetani, J. Tanaka, Y. Sato, J. Vac. Sci. Technol. A 16 (2) (1998). [6] B.D. Thoms, M.S. Owens, J.E. Butler, Appl.Phys. Lett. 65 (23) (1994). [7] B. Koslowski, S. Strobel, M.J. Wening, R. Martschat, P. Ziemann, Diamond Relat. Mater. 7 (1998) 322. [8] Z. Shpilman, I. Gouzman, E. Grossman, R. Akhvlediani, A. Hoffman, Appl. Phys. 3 (2008). [9] R. Akhvlediani, I. Lior, S. Michelson, A. Hoffman, Diamond Relat. Mater. 11 (2002) 545.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Morphological evolution of polished single crystal (100) diamond surface exposed to microwave hydrogen plasma A. Gaisinskaya a, R. Edrei a, A. Hoffman a,⁎, Y. Feldheim b a b
Schulich Department of Chemistry, Technion, Haifa 32000, Israel 21 Tuval St. Ramat Gan 52522, Israel
a r t i c l e
i n f o
Article history: Received 11 April 2009 Received in revised form 17 September 2009 Accepted 28 September 2009 Available online 6 October 2009 Keywords: Diamond Hydrogen Plasma Morphology
a b s t r a c t It is shown that exposure of polished single crystal diamond surfaces to microwave hydrogen plasma at optimized conditions may be applied as a very efficient method for the smoothing out of single crystal diamond surfaces. The effect of microwave (MW) hydrogen plasma exposure on the morphology of mechanically polished natural single crystal (100) oriented diamond type 2a surfaces is reported. It is shown that the surface morphology is very sensitive to plasma power and exposure time. Under appropriate plasma exposure conditions the diamond surfaces smooth out as reflected in the decrease in the number and depth of the polishing scratches or lines. However adverse effects on the surface morphology through the formation of pits were found to occur upon long exposure times and high plasma power. A systematic study of the influence of hydrogen microwave plasma power and exposure time on the diamond surface morphology is presented. The morphology of the diamond surfaces at the different stages was monitored with sub-nano-metric resolution by atomic force microscopy and scanning electron spectroscopy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The preparation of morphologically defined diamond surfaces is a very difficult task. The complexity in the preparation of this surface originates in their extreme hardness and metastability. Standard preparation by mechanical polishing, ion irradiation and thermal annealing result in ill-defined diamond surfaces: polishing leads to formation of polishing traces resulting in poor morphology whereas ion irradiation and annealing leads to an amorphous surface with degraded optical properties. Recently, several researches have shown that hydrogen plasma treatment improves the surface flatness and smoothes polishing scratches on diamond surfaces [1–8]. Overall these studies show that exposure of diamond surfaces to hydrogen plasma at appropriate conditions may result in smoothing of these surfaces. These studies provide evidence that surface temperature and hydrogen plasma activation may have an effect on the surface morphology. The different authors mention two main mechanisms as being responsible for the observed morphological effects. These are associated with carbon surface assisted diffusion and etching by atomic hydrogen. Below we briefly summarized the works that reported, to the best of our knowledge, the effect of hydrogen plasma on single crystal diamond surface morphology.
⁎ Corresponding author. Tel.: +972 4 8293747; fax: +972 4 8295703. E-mail address: [email protected] (A. Hoffman). 0925-9635/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2009.09.014
Hayashi et al. studied re-polished synthetic 1b diamond (001) surfaces treated with hydrogen plasma [1]. In that study substrates were positioned onto a molybdenum sample holder and heated inductively in the 750 to 850 °C range, while the duration of the treatment was fixed at 5 min. The hydrogen pressure, flow rate and microwave power were 25 Torr, 398 sccm and 750 W, respectively. Under these conditions the authors reported on the formation of successive terraces with atomic steps running parallel to the (110) directions alongside with square pits. The formation of terraces was rationalized on the base of surface diffusion of carbon whereas the formation of the square pits as the effect of etching by atomic hydrogen. They also reported that the diamond morphology is sensitive to treatment temperature and atomic step bunches is not observed at higher temperatures. Rawles et al. [2,3] have studied the effect of hydrogen plasma on type 1a natural diamond powders and type 2a (100) oriented single crystals in a microwave plasma reactor operated at 100 Torr, flow rates of 50–500 sccm, and power levels in the 2.5–4 kW range. In these studies the substrate temperatures was varied in the 650–850 °C range and the diamond samples were positioned onto a molybdenum sample holder. They observed smoothing and nano step bunching on diamond surfaces. Additionally, they found that the diamond particles remained of the same size, but became smoother and well faceted. Based on these findings they proposed that hydrogen atom-assisted carbon surface diffusion is the dominant mechanism affecting the diamond surface morphology and that etching mechanisms are of secondary importance. They also mention that the degree of faceting was sensitive to plasma power and temperature. Treatment at higher power generated deeper
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troughs and the diamond particle's surface became completely faceted after 3 h of exposure. Rawles et al. [3] showed the negligible change of diamond weight after the treatment and the dependence of initial structure on smoothing and pit formation. Kuttel et al. [4] have studied and compared the effect of electron cyclotron resonance and microwave hydrogen plasma on natural boron-doped type 2b (100) oriented diamond surfaces. They observed no changes after prolonged exposure to electron cyclotron resonance hydrogen plasma treatment. However, microwave hydrogen plasma resulted in a decrease of roughness (RMS) from 2 nm to 0.8 nm. These results seem to show that the energy of the atomic hydrogen may affect the surface processes leading to surface smoothing. Komatsu et al. investigated the effect of radio-frequency hydrogen plasma on the morphology of diamond surfaces and reported that smoothness appeared to be improved except for the existence of shallow etch pits with a few nm in depth. However polishing traces whose depths are less than 2 nm are still present after the treatment of hydrogen radio frequency plasma treatment. Thoms et al. [6] described the preparation of smooth diamond (100) surface consisting of terrace several hundred Å wide separated by 40 Å steps. In that work the diamond samples were exposed to MW hydrogen plasma operated at 600 W, 10 Torr of hydrogen and temperature of ~ 800 °C. Koslowski et al. [7] observed enhancement of surface roughness after hydrogen plasma treatment on chemo-mechanical polished diamond (100). A MW Chemical Vapor Deposition (CVD) reactor was used in that work to expose the substrate to hydrogen plasma operated using high purity hydrogen at 40 mbar and a flow rate of 80 sccm/min. The MW generator was operated in a pulse mode with a duty cycle of 50% and a peak MW power of 500–720 W. The substrate was heated indirectly by the plasma to 850 °C. They suggested that the roughness increase due to
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plasma treatment is associated to strain relaxation and anisotropic etching of defects. In this work we systematically investigated the effect of microwave hydrogen plasma exposure on the morphology of mechanically polished type 2a (100) oriented diamond surfaces. In particular the effects of microwave power and exposure time on the surface morphology were investigated. Microwave powers of 500, 700 and 900 W for an exposure time of 30 min were used in our experiments. For the 700 W microwave plasma power the diamonds were exposed for 15, 30 and 60 min. These plasma conditions and exposure times were selected as characteristic treatment times and plasma powers applied in our system and sufficient in order to induced chemical and physical modifications to the diamond surfaces. The morphology of the diamond surfaces was measured by non contact atomic force microscopy and compared to high resolution scanning electron microscopy. This was necessary for the determination of the true size of nano-metric wide pits. The effect of the plasma treatment on the diamond surface is normalized to the morphology of the surface prior to the plasma exposure. This is important as two polished diamond are never the same in terms of density of polishing scratches and their amplitudes as it will be described below. By this normalization processes the effect of plasma exposure on two different diamond surfaces can be compared and analyzed. Finally the effect of hydrogen plasma on the morphology of oriented single crystal and polycrystalline diamond [8] is compared. 2. Experimental details Two sets of natural single crystal (100) oriented type 2a diamonds were mechanically polished following by boiling in organic acids. According to the gemological examination all stones
Fig. 1. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 700 W for 15 min (C–D). Cross-section (across polishing line), parallel section (along polishing line).
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show a very good (4–5) grade (http://www.gemappraisals.com/pdf/ DiamondGradingScales.pdf). The diamonds were measured by atomic force microscope (AFM) (Nanoscope IV, Veeco) before and after the plasma treatment under atmospheric conditions. The microscope was operated in tapping mode with silicon cantilevers NSC14 (MikroMasch) with tip radius less than 10 nm, and force constant of 5.0 N/m. The AFM data were flattened by second order planarization. HR-SEM analysis was carried out using a Digital Scanning Microscope (DSM LEO-982) and Zeiss Ultra Plus Microscope at primary beam energies of 5 keV and 1 keV. Prior the exposures to MW-H plasma the diamonds were cleaned with analytical acetone followed by 96% ethanol for 20 min each in an ultrasonic bath; this procedure was done four times. Hydrogen plasma treatment was carried out using a microwave ASTEX system operated at a pressure of 50 Torr and power of 500, 700 and 900 W. The maximum hydrogen flow rate was 500 sccm and was automatically controlled to 42% of maximum flow rate (i.e. 210 sccm). The hydrogen plasma exposure was performed by positioning the diamond 5–10 mm below the plasma sphere onto a molybdenum plate. The plasma was ignited using a MW power of 350 W, and a hydrogen pressure of 0.5 Torr. Then the power level (500, 700 and 900 W), exposure time (15 min, 30 min and 60 min) and pressure (50 Torr) were determined according to the experimental plan. The diamond's temperature was measured by a thermo couple connected to the molybdenum plate. The sample temperature increases exponentially with exposure time and MW power during the first 5–7 min from the moment the plasma sphere was ignited and then the temperature stabilized. We estimate that the sample temperature during the plasma exposure was about 470 °C, 550 °C
and 600 °C for power level of 500, 700 and 900 W of plasma power, correspondently. The treatment was stopped automatically by reducing the power level of MW system. Three diamonds were exposed to plasma powers of 500, 700 and 900 W for 30 min and other two diamonds were exposed for 15 and 60 min at 700 W. In other words we investigated two sets of experiments on diamond surface morphology: as a function of plasma time and as a function of power level of MW-H plasma. Polycrystalline diamond film with submicron grain size was grown on 1.0 × 1.0 cm2 Si substrate in a hot filament (HF)-CVD system as described in our previous publication [9]. Deposition parameters were CH4/H2 gas mixture of 1:99 and gas pressure of 50 Torr. The substrate temperature was of 750 °C and deposition time of 30 min. This HFCVD diamond film was exposed to hydrogen microwave plasma at 700 W for 30 min. During plasma exposure the substrate temperature was about 550 °C. After the plasma exposure process the diamond surface was examined by AFM in amplitude mode. Background experiments carried out on silicon surfaces showed that no carbon deposition occurred during the hydrogen plasma exposure that could possible affect the reported effects. 3. Results 3.1. Morphology of the mechanically polished single crystal diamond surfaces The morphology of the as polished and acid clean single crystal diamond surfaces are shown in Figs. 1A–5A. From these figures the morphology of the diamond surfaces are characterized by clear
Fig. 2. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 700 W for 30 min (C–D). Cross-section (across polishing line) parallel section (along polishing line) and pits (pits line).
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polishing scratches of varying density, amplitude and other irregularities which can be examined by AFM as described below. These were analyzed in detail for each stone. Generally, the diamond surfaces have 10–12 scratches/μm2 (see Tables 1 and 2). There are two types of scratches: shallow scratches (0.3–1.0 nm in height) and broad scratches (1.5–2.5 nm in height), each broad scratch includes a number of shallow scratches. Fig. 3A demonstrates pits of 1.0 ± 0.20 nm deep and of 22 ± 4.1 nm width which are visible on the edges of the shallow scratches. These values (scratch density and size) were determined based a statistical analysis of AFM pictures carried out on different spots of the diamond surfaces and are representative of average values (see Tables 1, 2 and 3). The scratch's shapes strongly depend on polishing procedure and conditions and are unique for each diamond. The diamond surface can include residues of diamond grains (Fig. 2A), spot shapes (Fig. 1A), small pits (Fig. 3A), or broad scratches (Fig. 4A). Due to the fact that each diamond has different initial topography, the plasma effect analysis must take into account the initial topography.
3.2. Hydrogen exposure as a function of time at constant microwave power level Three different diamonds were exposure for 15, 30 and 60 min to the hydrogen plasma at 700 W. The prolonged hydrogen plasma exposure has a strong effect on the (100) diamond surface morphology as it is demonstrated by AFM measurements at Figs. 1–3.
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Cross lines represent scratches height, width and density whereas parallel lines represent scratches irregularity during the exposure. As can be seen from Figs. 1–3 the hydrogen plasma treatment reduces the scratches height and density. The shallow scratches gradually disappear and the broad scratches decrease in height. The measured dependence of the scratch's height reduction versus microwave exposure time for broad and shallow scratches, at a plasma intensity of 700 W, is presented in Fig. 6A. The difference between initial and final scratch height increases with exposure time. The shallow scratches smooth out faster than the broad scratches at the beginning, but as the process proceeded, the shallow scratches smooth out slower than the broad scratches, as can be seen from the slope in Fig. 6A. The difference of initial and final heights for diamond surface after 30 min of exposure is 0.48 ± 0.17 nm for broad scratches and 0.55 ± 0.11 nm for shallow scratches. As can be seen from Figs. 1C–5C, following the plasma treatment, the shallow scratches disappeared. The difference between the amount of initial and final scratches increases with plasma exposure time as shown in Fig. 6A. Hydrogen plasma cleans the initial polished defects as Figs. 1 and 2 demonstrate. After 30 min of 700 W microwave hydrogen plasma exposure square pits appear (Fig. 2C). Their shape is confirmed with HR-SEM technique as shown in Fig. 7. Their size and amount increase with exposure time, in case of treatment after 30 min, 10 ± 0.35 pits/μm2 appear with average depth of 2.4 ± 0.25 nm and width of 48 ± 5.3 nm (Tables 1 and 3). Careful examination of Fig. 3A demonstrates that this initial sample has already very small pits on its surface before plasma
Fig. 3. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 700 W for 60 min (C–D). Cross-section (across polishing line), parallel section (along polishing line) and pits (pits line).
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Fig. 4. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 500 W for 30 min (C–D). Cross-section (across polishing line), parallel section (along polishing line) and pits (pits line).
treatment. However, as presented in Tables 1 and 3, the number of pits after 60 min of hydrogen plasma exposure increases by 13 ± 0.71 pits/μm2, the average depth increase by 1.0 ± 0.60 nm and the average width increased by 69 ± 7.2 nm.
3.3. Hydrogen exposure as a function of powers level at constant time Our results clearly show that surface morphology is strongly affected by plasma power. The higher power level of the microwave plasma smoothes polishing scratches more productively and causes to their disappearance (Figs. 2, 4 and 5). However the high power level could cause to square pit appearance and an increasing of pit size and pit density (Fig. 8B, Tables 2 and 3). At first, we investigated the diamond surface after hydrogen plasma exposure at a power level of 500 W (Fig. 4). Its initial surface was badly defined with 1.3 ± 0.18 broad scratches/μm2, as shown in Fig. 4A and Table 2. Nonetheless, a convincing influence on the surface topography resulting from exposure to the plasma can be seen (Fig. 4C). However, we do not include it in our quantitative analysis. We focus on two examples: after 700 W of plasma exposure (Fig. 2) and after 900 W of plasma exposure (Fig. 5). The reduced height after 700 W of the plasma exposure is 0.48 ± 0.17 nm for broad scratches and 0.55 ± 0.11 nm for shallow scratches. The reduced scratches density is 7.8 ± 0.0056 scratches/μm2 (Table 2). The square pits present at density of 10 ± 0.35 pits/μm2 of 2.4 ± 0.25 nm in height and of 48 ± 5.3 nm in width (Table 3). Exposure to 900 W of MW-H plasma reduces the height by 0.93 ± 0.14 nm of broad scratches and by 0.63 ± 0.17 nm of shallow scratches. The reduced density of scratches is 8.3 ± 0.071 scratches/
μm2. The pits density is 13 ± 0.88 pits/μm2 of 5.7 ± 0.42 nm in height and 110 ± 2.7 nm in width as can be obtained from Tables 2 and 3. From our results, an increase in plasma power results in a decrease in the number of polishing scratches being left on the surface and if one does not pay attention to the pitting effect, the diamond surface becomes much smoother with increasing power level. We decided not to analyze our results by RMS or by another roughness measurement, contrary to other researches, because the pits cause incorrect results in roughness that could be interpreted as a smoothness effect. The AFM analysis shows that the average width and density of pits on the diamond surface after exposure to the 700 W plasma for 30 min is 48 ± 5.3 nm and 10 ± 0.35 pits/μm2, respectively. HR-SEM analysis presents the same shaped pits (Fig. 7). However, from our HR-SEM analysis the pits density is 7.5 ± 0.71 pits/μm2 and the width is 32 ± 0.31 nm. This difference in width between the measurements carried out by AFM and HR-SEM may be caused by the limitation of the SEM technique to discover the shallow slope of the pits as can seen in Fig. 5E. 3.4. Comparison with chemical vapor deposited diamond films The most popular industrial technique for diamond growth is CVD. CVD produces thin diamond films which don't have polishing scratches but display a surface morphology which comprises disoriented microscopic crystallites. Fig. 9 shows the changes in amplitude morphology of HF-CVD diamond film following exposure to MW hydrogen plasma at 700 W for 30 min, 500 sccm at 50 Torr. As can be seen the exposure to MW-H plasma results in only minor chemical changes and severe surface morphology modification. The MW-H plasma etched very effectively the nondiamond carbon phase
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Fig. 5. AFM image and roughness profile of single crystal diamond surface pristine (A–B), and after MW hydrogenation at 900 W for 30 min (C–D). Cross-section (across polishing line), parallel section (along polishing line) and pits (pits line), (E) pit's cross-section.
and to a much lesser extent diamond phase. The morphology was, however, modified to a granular surface with apparent preferential etching of triangular diamond facets. The effect of MW hydrogen
plasma on the surface morphology of diamond films has been recently reported by our group (Shpilman et al. [8]). Hence it can be seen in Fig. 9 that the hydrogen plasma exposed the diamond carbon phase
Table 1 Influence of MW plasma on number of scratches, pits, heights of broad and shallow scratches as a function of different time exposure. 700 W Time [min]
Density of scratches [scratches/μm2]
Density of pits [pits/μm2]
Pristine
Plasma
Delta
Pristine
Plasma
Delta
Pristine
Plasma
Delta
Pristine
Plasma
Delta
15 30 60
11.3 11.8 11.3
4.3 4.0 2.5
7.0 7.8 8.8
0.0 0.0 33.5
0.0 10.0 46.5
0.0 10.0 13.0
1.4 2.2 2.4
1.0 1.8 1.8
0.4 0.5 0.7
0.5 1.0 1.0
0.2 0.4 0.3
0.2 0.6 0.6
Height of broad scratches [nm]
Height of shallow scratches [nm]
Table 2 Influence of MW plasma on number of scratches, pits, heights of broad and shallow scratches as a function of different power exposure. 30 min Power [W]
500.0 700.0 900.0
Density of scratches [scratches/μm2]
Density of pits [pits/μm2]
Height of broad scratches [nm]
Height of shallow scratches [nm]
Pristine
Plasma
Delta
Pristine
Plasma
Delta
Pristine
Plasma
Delta
Pristine
Plasma
Delta
1.3 11.8 9.8
1.3 4.0 1.5
0.0 7.8 8.3
0.0 0.0 0.0
7.8 10.0 12.8
7.8 10.0 12.8
1.5 2.2 1.7
1.7 1.8 0.8
0.3 0.5 0.9
0.3 1.0 0.6
0.3 0.4 0.0
0.1 0.6 0.6
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Table 3 Influence of MW plasma on width and depth of pits as a function of different time and power exposure. 700 W Time [min]
15 30 60
Depth of pits [nm]
Width of pits [nm]
Pristine
Plasma
Delta
Pristine
0.0 0.0 1.0
0.0 2.4 2.0
0.0 2.4 1.0
0.0 0.0 22.0
Pristine
Plasma 0.0 47.8 91.3
Delta 0.0 47.8 69.3
30 min Power [W]
500 700 900
Depth of pits [nm]
Width of pits [nm]
Pristine
Plasma
Delta
0.0 0.0 0.0
5.5 2.4 5.7
5.5 2.4 5.7
0.0 0.0 0.0
Plasma
Delta
65.0 47.8 110.0
65.0 47.8 110.0
Fig. 8. (A) Scratches height reduction versus plasma exposure power for 30 min for shallow scratches (squares) and for broad scratches (rhombi); (B) number of pits (squares) and number of scratches (rhombi) reduction versus plasma exposure for 30 min.
producing smooth rectangular facets. It is most important to note that there are no pitting defects on the polycrystalline surface following hydrogen plasma exposure. 4. Discussion
Fig. 6. (A) Scratches height reduction versus plasma exposure time at 700 W for shallow scratches (squares) and for broad scratches (rhombi); (B) number of pits (squares) and number of scratches (rhombi) reduction versus plasma time at 700 W.
Mechanical polishing of diamond surfaces consist of repeated microcleavage leaving a rough but crystalline surface without any plastic deformation. Generally, the morphology of polished diamond is characterized by polishing scratches consisting of broad patterns which contain a variety of shallow patterns. The scratches themselves have irregularities. Hydrogen microwave plasma treatment can produce a remarkable degree of smoothing of the scratches and create regular surface morphology through disappearance of shallow scratches. However, as found in our studies, plasma exposure for prolonged times or high plasma intensities may cause damage to the
Fig. 7. HR-SEM image at ×100,000 of magnification of single crystal diamond surface after MW hydrogenation (A) at 900 W for 30 min by digital Scanning Microscope Leo-982 at 5 kV of energy high tension and (B) at 700 W for 30 min by Zeiss Ultra Plus Microscope at 1.00 kV of energy high tension.
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Fig. 9. AFM amplitude images, size 2 × 2 μm2, scale of 300 mV, of HF-CVD diamond films before (A) and after exposure to MW hydrogen plasma (B).
diamond surface mainly through the formation of pits. The optimal conditions, based on present experiments and single crystal diamonds used in this study, are 15 min exposure at a power of 700 W and temperature of 550 °C. Our studies show that the pits appeared on the single crystal diamond surface only after prolonged exposure to the hydrogen plasma or on badly defined single crystal diamond surfaces. On the other hand no pits were observed to appear on the CVD diamond surface upon exposure to hydrogen plasma. It was pointed out previously that pits are located predominantly on edges of polishing scratches (Fig. 3). We believe that exposure of the diamond surfaces to atomic hydrogen at high temperature most likely preferentially reacts with surface defects that are located on cross junctions of crystallographic planes which are produced from micro cleavage during mechanical polishing. As the plasma power is increased so does the concentration of active hydrogen and the temperature raises; the prolonged exposure causes prolonged reactions. This makes hydrogen to be more aggressive to the diamond surface and causes the surface to be more susceptible for etching at high energy places, thus resulting in the creation of pits. Considering that we operated the MW hydrogen plasma at relatively low surface temperatures and did not observe weight loss, our results support the possibility that smoothing phenomena is primarily due to the surface carbon diffusion at the diamond surface. In summary, we can unequivocally show that hydrogen plasma flattens the single crystal diamond surface and there are marked influences of MW hydrogen exposure time and power on the level of
diamond surface smoothing. In our experimental system the optimal condition to smooth out single crystal and oriented type 2a (100) diamond surfaces are a plasma power 700 W and 15 min of exposure at 550 °C. Higher power and/or longer exposures times results in the degradation of the diamond surfaces through the formation of nanometric size pits. It is possible that these conditions may differ for other experimental systems. Acknowledgements The authors are very grateful for material support from Word Gemological Insititute and Professor Yeshaya Yarnitsky for his advice. References [1] K. Hayashi, S. Yamanaka, H. Watanabe, T. Sekiguchi, H. Okushi, K. Kajimura, Appl. Surf. Sci. 125 (1998) 120. [2] R.E. Rawles, S.F. Komarov, R. Cat, W.G. Morris, J.B. Hudson, M.P. D'Evelyn, Diamond Relat. Mater. 6 (1997) 791. [3] R.E. Rawles, R. Cat, W.G. Morris, M.P. D'Evelyn, Mater. Res. Soc. Symp. Proc. 416 (1996) 299. [4] O.M. Kuttel, L. Diederich, E. Schaller, O. Carnal, L. Schalapbach, Surf. Sci. 337 (1998) L812. [5] S. Kamtsu, K. Okada, S.B. Chou, T. Aizawa, H. Shigetani, J. Tanaka, Y. Sato, J. Vac. Sci. Technol. A 16 (2) (1998). [6] B.D. Thoms, M.S. Owens, J.E. Butler, Appl.Phys. Lett. 65 (23) (1994). [7] B. Koslowski, S. Strobel, M.J. Wening, R. Martschat, P. Ziemann, Diamond Relat. Mater. 7 (1998) 322. [8] Z. Shpilman, I. Gouzman, E. Grossman, R. Akhvlediani, A. Hoffman, Appl. Phys. 3 (2008). [9] R. Akhvlediani, I. Lior, S. Michelson, A. Hoffman, Diamond Relat. Mater. 11 (2002) 545.