Mechanism of diamond formation on substrates abraded with a mixture of diamond and metal powders

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Diamond and Related Materials 5 (1996) 286-291

D IAMOND AND RELATED MATERIALS

Mechanism of diamond formation on substrates abraded with a mixture of diamond and metal powders Y. Chakk, R. Brener, A. Hoffman Chemistry Department and Solid State Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel

Abstract In this work we report a study of CVD diamond formation on silicon substrates abraded with diamond, metal, and a mixture of diamond and metal powders. It was found that the deposited diamond particles density (DPD) obtained after abrasion with diamond powder can be enhanced by a few orders of magnitude by abrasion with a mixed metal/diamond slurry, whereas no enhancement was observed by use for surface abrasion a metal slurry alone. The residual diamond slurry density (RDSD) left on the substrates by abrasion with diamond slurry was measured from AFM images. It was observed that DPD followed by abrasion with pure diamond slurry does not exceed 10% of RDSD, whereas the presence of some metal residues alongside with diamond debris, may increase this value almost to 100%. The enhancement in DPD was in the order: virgin~(Cu, Fe or Ti)< Di < (Cu + Di)< (Fe + Di)< (Ti + Di). These effects are explained qualitatively as follows. It is suggested that metal residues influence the rates of CVD diamond growth through facilitation of conversion of non-sp3-bonded carbon species to the sp3-0ne above the growing surface. This enhancement in sp3-bonded carbon surface concentration at the initial stages of deposition (before a stable substrate is formed) prevents the smallest diamond residues from being completely etched by atomic hydrogen.

Keywords: CVD; Diamond; Surface treatment; Atomic force microscopy

1. Introduction Diamond formation on non-diamond substrates under CVD conditions has been a subject of considerable interest in the last years. In order to grow high quality CVD diamond films for applications in various fields it is important to understand and control the processes involved in their formation. M a n y materials were tried as substrates and several substrate treatments were found to be effective to enhance the density of deposited diamond particles. The methods commonly used are: abrasion with diamond or other hard powders [ 1 - 3 ] , biasing during the initial stages of deposition [4,5] and chemical modification of the near surface region by thin metal [ 6 ] or carbon (DLC) films [7], or by ion implantation [8,9]. Other methods resulting in CVD diamond formation enhancement are: surface damaging with free falling hard particles prior to deposition [ 10], increasing the concentration of C H 4 in CVD reactor during the initial stages of deposition [ 11]. Besides, diamond nucleation enhancement was observed on Pd/A1/O3-wiped Si substrates [ 12]. Different models have been suggested to explain the mechanism of CVD diamond formation on pretreated nondiamond substrates including the creation of highly 0925-9635/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0925-9635(95)00425-4

reactive surface damage sites [13], the presence of residual diamond particles [14], carbide formation [4] and the presence of stable carbonaceous precursors [15,16]. Recently, we have reported a novel method to enhance CVD diamond nucleation density on nondiamond substrates which is based on surface abrasion prior to deposition using a mixture of diamond with metal powders [17]. It has been found that diamond nucleation density obtained by abrasion with a pure diamond slurry can be enhanced by a few orders of magnitude by abrasion with a mixed metal/diamond slurry. In this work we present additional experiments clarifying this phenomena as well as a mechanism which may explain this effect.

2. Experimental details The abrasion pretreatment experiments were performed by sonification of Si (100) substrates with a mixture of diamond ( 1 - 3 g m ) powder with Ti (30-35 gm), Fe (1-3 gm), or Cu (10 gm) metal powders. Each mixed slurry consisted of 0.2 g of diamond powder

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and 0.2 g of metal powder in 20 ml ethyl alcohol. The ultrasonic treatment was carried out for 30 min, followed by a 1 rain rinse in acetone. Diamond depositions were performed in a hot filament CVD reactor, previously described [ 18], with a methane to hydrogen mixture of 1:99 vol.%, gas flow rate of 100 sccm and a deposition pressure of 50 Torr. The filament to sample distance was 8 mm and the filament and substrate temperatures were 2000°C and 850°C, respectively. Atomic force microscopy (AFM) measurements were performed to count the residual diamond slurry density (RDSD) left on the substrate surface by the abrasion process, as well as the deposited diamond particles densities (DPD) after different deposition times. These measurements were performed using a Topometrics T M X 2010 instrument, operating in air at room temperature in the contact mode. Pyramidal Si3N4-tips were used, with a typical radius of curvature of 50 nm and a cantilever spring constant of 0.032 N m - 1. Scanning electron microscopy (SEM) measurements were performed to obtain D P D values on variously treated substrates after different deposition times. The D P D values were obtained from SEM measurements performed at a magnification of x 1000 by counting the number of individual crystallites deposited before coalescence of isolated particles into a continuous film occurred. The presence of carbon and metals residues left on the substrate surface following different pretreatments prior to deposition, were determined from Auger electron spectroscopy (AES) measurements. The crystalline quality of continuous diamond films was monitored by micro Raman measurements. All deposited continuous films were composed of crystalline diamond in addition to a small amount of amorphous carbon.

3. Results Typical SEM micrographs obtained after deposition for 15 min on Si substrates pretreated with diamond and mixed diamond/titanium, diamond/iron, and diamond/copper slurries are shown in Fig. 1. As observed from this figure, ultrasonic abrasion with a mixture of diamond and metal particles results in D P D enhancement relative to that obtained following treatment with the diamond slurry alone. A similar effect was observed when other diamond/metal mixed slurries (metal: Ta, Mo, Nb, W, Ni, or Si) and substrates (SiO2, A1203) were used [17]. Typical A F M images obtained on Si substrates, virgin and abraded with pure diamond slurry, are shown in Figs. 2 (a) and (b), respectively. Fig. 2 (b) shows that the substrate surface is covered by small particles with RDSD slightly exceeding 109 particles per cm 2. The

1 gm Fig. 1. Typical SEM images of silicon substrates abraded with (a) pure diamond and mixed (b) diamond/copper (Cu+Di), (c) diamond/iron (Fe + Di), (d) diamond/titanium (Ti + Di) slurries after 15 min of diamond deposition. nature of these particles was investigated by studying the effect of ion implantation on CVD diamond nucleation, and it is believed that these residues are small diamond particles [ 19]. The density of residues left by abrasion with mixed slurry exceeds this value (not shown). However, we could not differentiate between diamond and metal residues. It is assumed that in this case the density of diamond residues is similar to the case of pure diamond abrasion. Measurements of D P D as a function of deposition time, obtained from SEM images, on substrates abraded with pure diamond, pure metals and mixed diamond/ metal (Ti, Fe, Cu) slurries are shown in Fig. 3. The measurements were terminated upon attainment of a continuous diamond film. The value of RDSD left on the substrate by abrasion with pure diamond slurry, as determined from AFM images, is presented in Fig. 3. The following general features are noticeable: (a) for all the cases studied, D P D does not exceed RDSD; (b) D P D obtained after abrasion with pure metal slurry is comparable with that obtained on a virgin substrate

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Y. Chakk et al./Diamond and Related Materials 5 (1996) 286-291

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Fig. 2. A F M images of silicon substrates, (a) virgin, and (b) abraded with pure diamond slurry. The average value of RDSD left by abrasion process slightly exceeds 109 particles per cm 2.

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particles per cm2); (c) D P D obtained after abrasion with pure diamond slurry increases from a few to about 10% of RDSD; (d) D P D obtained after pretreatment with a mixed slurry increases in the order Cu < Fe < Ti; (e) D P D obtained on substrates abraded with a mixed titanium/diamond slurry is slightly less than the value of RDSD. From these results it is suggested that the effect of D P D enhancement is owing to a combined effect produced by simultaneous abrasion with metal and diamond powders [ 17]. Abrasion with pure metal indicates that it is not sufficient to have metal residues alone on the surface to enhance diamond nucleation, but nucleation sites have to be produced by abrasion with the diamond slurry. This is proved also by our studies on the effect of ion implantation on CVD diamond formation on substrates abraded with mixed metal/diamond slurries showing almost complete suppression of diamond nucleation and growth. To further assess this phenomenon, the effect of metalto-diamond weight ratio in the mixed slurry on CVD diamond formation was studied. The dependence of D P D on metal-to-diamond weight ratio in a mixed slurry, when the metal used is Fe, is shown in Fig. 4. (around

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Depositions were performed for values of iron-todiamond weight ratios: 0 (pure diamond case), 1:10, 1:2 and 1:1. As can be seen from Fig. 4, D P D is sensitive to very small quantities of metal in the mixed slurry, which is indicated by its rapid increase for the metal-todiamond weight ratio value of 0.1 as compared with the pure diamond slurry case. Besides, although D P D increases slowly between 1:2 and 1:1 weight ratios, the difference in average sizes of deposited crystallites, as

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Y. Chakk et al./Diamond and Related Materials 5 (1996) 286-291

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Fig. 5. Typical AFM images of silicon substrates abraded with (a) pure diamond and mixed diamond/iron slurries when Fe-to-diamond weight ratios are (b) 1:10, (c) 1:2 and (d) 1:1, after 30 min of diamond deposition.

obtained from AFM images (see Fig. 5), is large, which is shown in the same figure. These effects may be related to the quantities of metal left on the substrate by abrasion. AES examinations of the abraded surface show that increasing the iron-todiamond weight ratio in mixed slurry increases the quantity of Fe left on the substrate. A mechanism to rationalise the effect of substrate surface abrasion by mixed diamond/metal slurry on DPD enhancement is suggested in the following section.

4. Mechanism of D P D enhancement

It is well established both, experimentally and theoretically [20-24], that during the CVD diamond formation process, the main active species in the gas environment near the substrate surface are hydrogen, methyl radicals (CH3), methane ( C H 4 ) and acetylene (CzHz), with small amounts of ethylene (C2H4) and ethane (CzH6). However, it is generally believed that methyl radicals (CH3) are the main carbon source for CVD diamond formation [24-26]. Enhancement in CH3 concentration above the growing surface may be achieved by changing

the deposition parameters (e.g., filament temperature, filament-to-substrate distance, initial gas-phase composition, etc.) [22,25,27]. These changes affect the steady state gas concentrations above the growing surface. Thus, diamond formation may be facilitated by increasing the CH 3 concentration on the growing surface by choosing optimal deposition conditions [28,29]. It is well established that under typical diamond CVD conditions the diamond growth rate from CH 3 radicals is faster than its etching rate by atomic hydrogen [30], otherwise diamond could not grow. This is true at steady state conditions. However, at the initial stages of deposition, the concentration of surface carbon species available for growth at steady state conditions is depleted until the formation of a stable carbide substrate is achieved. It is suggested that this depletion of carbon active species at the initial stages of deposition results in annihilation of the smallest diamond residues. This may explain the large difference between the value of DPD followed by abrasion with pure diamond slurry and RDSD left by abrasion. Based on these considerations, a mechanism of CVD diamond formation on substrates abraded with mixed slurry is suggested. As it was concluded before, differ-

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ences in DPD and size distribution of deposited crystallites are owing to the presence of metal particles embedded alongside with diamond into the substrate by abrasion with mixed slurry. These metal residues are expected to be highly reactive resulting in local changes in the composition of surface species involved in diamond growth process. The catalytic activity of metals to hydrogenate acetylene and ethylene has been widely studied [31,32]. It is known that different metals may facilitate the processes of C2H 2 and C2H 4 hydrogenation, resulting in the formation of sp3-bonded carbon species. The interaction of graphite crystals with hydrogen in the presence of small particles of some transition metals at the temperatures of CVD diamond formation was studied [33-35]. It was found that the presence of metals facilitates the process of graphite hydrogenation resulting in methane formation. Besides, it was speculated, that during the diamond growth in the presence of platinum, hydrogen is converted to atomic hydrogen which destroys the nuclei of graphite or of other black forms of carbon as rapidly as such nuclei are formed between the pressures of 1 to 2000 atmospheres [36]. In diamond CVD process such metal-catalysed reactions may reduce non-sp3-bonded carbon fraction above the growing surface, thus inevitably increasing the sp 3one. This enhancement in sp3-bonded carbon fraction in the near surface region may result in both, enhancement in the number of deposited crystallites and their size distribution. This is owing to the fact that sp 3bonded products being transported by surface diffusion to diamond residues contribute to their homoepitaxial growth and prevent the smallest of them from being completely etched by atomic hydrogen at the initial stages of deposition. This results in enhancement of both DPD and average size of deposited particles. Studies on the effect of supersaturation of a CVD reactor by hydrocarbons on nucleation density support this explanation. It was observed that increasing of input concentration of CH 4 from 10 to 90 vol.~/o at the initial stages of deposition increased the density of deposited diamond particles by nearly two orders of magnitude

[12]. The differences in observed DPD followed by abrasion with different metal/diamond mixed slurries may be explained as follows. It is known that different metals display different surface catalytic activity towards hydrogenation of carbon species. The mechanism of catalytic reactions is of a complex nature and consists of several consecutive steps involving diffusion of the reactants to the substrate surface and their adsorption, reactions among adsorbed species, and desorption of the reaction products. If diffusion rate of the reactants to the surface is constant, the first step, which probably determines the catalytic activity of metal, is adsorption of the reactants to its surface.

It is known that various metals show different tendencies to attach adsorbates to their surfaces [37]. The behaviour of some transition metals such as Ti and Fe towards the adsorption of hydrogen, for example, is very similar. However, metals which come directly after the transition metals (e.g., Cu), possesses entirely different adsorptive properties: hydrogen can be chemisorbed on Cu in very small amount. The same is true for hydrocarbon species. For example, heats of chemisorption of C2H 4 on Fe and Cu are 285 and 76 kJ tool -1, respectively [32]. These differences may be expected to affect the rates of hydrogenation of non-sp3-bonded carbon species presented above the growing surface. This may explain the fact that although DPD enhancement is observed after copper/diamond abrasion, as compared with pure diamond case, its effect is less pronounced than that obtained for Fe or Ti. It should be stressed that the above explanation gives only a qualitative picture of the process, and does not take into consideration possible chemical changes which could undergo small metal particles during the ultrasonic treatment and the initial stages of deposition. However, we believe that our explanation would only change in detail, but not in principle. Further studies are necessary to elucidate the basic processes involved in DPD enhancement obtained upon substrate treatment with mixed metal/diamond slurries.

5. Summary and conclusions In summary, the effect of substrate surface abrasion with mixed metal/diamond slurry on CVD diamond formation is studied. It is found that abrasion with mixed slurry can enhance deposited DPD by a few orders of magnitude, as compared with that obtained after the treatment with pure diamond powder. DPD followed by abrasion with pure diamond slurry does not exceed 10% of RDSD left on the substrate by abrasion, whereas the presence of some metal (such as Ti) residues alongside with diamond debris, may increase this value almost to 100%. It is suggested that metal residues left by the treatment with mixed slurry influence the rate of CVD diamond growth through conversion of sp- and sp2-bonded carbon species to the sp3-0ne above the growing surface. This enhancement in sp3-bonded carbon surface concentration at the initial stages of deposition prevents the smallest diamond residues from complete annihilation by atomic hydrogen. The differences in deposited DPD followed by abrasion with various metal/diamond mixed slurries are owing to both, the physical nature of the metal, and its quantity. Further experiments are required to clarify these effects.

I1. Chakk et al./Diamond and Related Materials 5 (1996) 286-291

Acknowledgment W e w o u l d like to a c k n o w l e d g e the I s r a e l i M i n i s t r y of S c i e n c e for f i n a n c i a l s u p p o r t d u r i n g this r e s e a r c h w o r k .

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