Materials Science and Engineering, B26 (1994) 41-48 41

Growth of diamond on Au (selective) and AI203 surfaces

J. S ingh Applied Research Laboratory, Penn State University, State College, PA 16804-0030 (USA)

M. Vel la ika l

Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695 (USA)

M. Akhtar Johnson Matthey Company, 10080 Willow Creek Road, San Diego, CA 92131 (USA)

(Received December 3, 1993; in revised form February 12, 1994)

Abstract

The hot-filament chemical vapor deposition process was used to deposit diamond films on gold and alumina substrates using different deposition conditions. Diamond nucleation and growth on a gold surface readily occurred at low tempera- tures compared with the alumina substrate. This resulted in selective growth of diamond on the gold surface and pattern- ing of diamond films. Scanning electron microscopy was employed to study the morphology of the films.

1. Introduction

Diamond has many desirable properties for ad- vanced microelectronics and coating applications. Some of these properties include (i) the highest hard- ness, (ii) the highest thermal conductivity, (iii) resist- ance to acidic and basic environments and to radiation, (iv) excellent electrical insulation, (v) small dielectric constant, (vi) large hole mobility and (vii) large band gap (5.5 eV). Because of its unique properties, dia- mond is expected to be a future material for high per- formance electronic devices. In the packaging of microelectronic devices, the dissipation of heat gener- ated during the device operation plays a very important role in determining the density of circuits and the nature of interconnects. The heat generation may increase the oxidation rate of interconnect materials, which in turn increases the interconnect resistance. In electronic industries, a gold coating is applied to insula- tors to extract the heat from the microelectronic devices [1]. The heat extraction from the micro- electronic devices can be enhanced by applying a diamond coating, as the thermal conductivity of diamond is better than that of Au and Cu (5 cal cm- 1 s- 1 °C - 1, 0.7 cal cm- l s- 1 o C - 1 and 0.94 cal cm- t s - t °C-~ respectively).

Diamond films can be grown by various techniques, such as hot-filament chemical vapor deposition (HFCVD) [2, 3], microwave-plasma-assisted chemical

vapor deposition [4], a.c. discharge plasma [5, 6], r.f. plasma [7, 8] and laser-assisted chemical vapor deposi- tion [9]. In this paper, we show that diamond can be deposited selectively on gold compared with alumina substrates (insulator). The fundamental aspects of selective diamond growth and morphology on gold surfaces are discussed.

2. Experimental procedure

The specimens (1 cm x 1 cm x 0.2 cm) used in the present study were polycrystalline alumina substrates patterned with gold lines (width, 150 /zm) (Fig. 1). Specimens were obtained from Johnson Matthey, San Diego, CA. A thin film of nickel (1.0/~m) was sputter coated first on the alumina substrate followed by the gold coating (about 60 A). Diamond was grown on these specimens by the HFCVD process at different temperatures, times and methane(CH4)-to-hydro- gen(H 2) gas ratios. The W filament was positioned approximately 8 mm above the substrate and the fila- ment temperature was about 1950°C as measured with an optical pyrometer. Pre-mixed gas of (1-2)vol.%CH4-H 2 was directed on the specimen by a jet above the filament source. The total gas pressure was maintained at about 20 Torr. The substrate tem- perature T S was varied from 775 to 935 °C during deposition and the deposition time was varied from 3 to8 h.

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42 J. Singh et al. / Growth of diamond on Au andA120~

Fig. 1. Photograph of the Au patterned alumina substrate (1 cm x 1 cm x0.2 cm).

The deposited diamond films were characterized using scanning electron microscopy (JEOL JSM 6400FE) and Auger microscopy. The average grain size and morphology of diamond films were deter- mined from high magnification scanning electron micrographs.

3. Results

Figure 2 is a scanning electron micrograph showing the growth of diamond crystallites on an Au surface at 775 °C. The diamond coverage was low (5%-10%). Diamond growth was not observed on the alumina substrate at this temperature. The diamond crystallites were found to be faceted with an average size of about 2 pm. Secondary diamond growth was also observed on the (100) faceted surface whereas the (111) facets were relatively smooth. The diamond coverage on Au surface was increased from 5%-10% to about 20%-30% as the substrate temperature was increased from 775 to 815 °C and the deposition was carried out for 3 h (Fig. 3(a)). Diamond crystallites were still faceted at this deposition temperature and duration of time. By continuing the diamond growth at 815 °C for 8 h, a continuous diamond thin film was obtained only on the Au surface; the growth was not observed on alumina substrate (i.e. selective diamond growth). Overgrowth of diamond crystallites on Au was also observed as shown by arrows in Fig. 3(b). The growth morphology was found to change from faceted crystal- lites to spherical particles (Fig. 3(c)).

At a higher deposition temperature of 850°C, the density and size of diamond nuclei on Au surface increased dramatically with an average crystallite size of about 5 pm (Fig. 4). Secondary diamond growth was

Fig. 2. Scanning electron micrographs showing the growth of faceted diamond crystallites on the Au substrate by the HFCVD process ( T~ = 775 °C; 1.5 vol.% CH4; deposition time, 3 h).

also observed on (100) surfaces. In addition, diamond growth on alumina was observed (Fig. 5). Diamond crystallites were of a faceted nature on Au and alumina substrates. On comparison of the growth morphology of diamond on Au and alumina surfaces, cubo-octa- hedron diamond crysallites with (100) square faces were dominant on the Au surfaces. The (111) faceted diamond growth morphology was dominant on the alumina surface with an average size of about 6/~m.

The diamond growth morphology on Au surface changed from cubo-octahedron to octahedron on increasing the substrate temperature from 850 to 915 °C (Figs. 4 and 6(b)). In addition, diamond cover- age on the Au surface appeared to be same but it dra- matically increased on the alumina surface (Figs. 5(a) and 6(a)). Octahedron diamond growth morphology

J. Singh et al. / Growth of diamond on Au and Al203 43

Fig. 4. Scanning electron micrographs showing the growth of faceted diamond crystallites on Au surface by HFCVD process (Ts=850 :C, 1.5 vol.% CH4; deposition time, 4 h): (a) low magnification, showing high density of diamond growth; (b) cubo-octahedron diamond crystallites with secondary growth on cube faces.

Fig. 3. Scanning electron micrographs showing the growth of faceted diamond crystallites on Au surface by HFCVD process at T s = 815 °C, with 1.5 voi.% CH 4 for deposition times of(a) 3 h and (b), (c) 8 h.

was found on Au and alumina surfaces at 915 °C. Cauliflower diamond morphology was observed on Au and alumina surfaces at a higher substrate temperature of 935 °C for 4 h (Fig. 7). The average size of diamond particles was about 7/~m.

As the CH 4 in the gas mixture was reduced to 1.0%, diamond coverage on Au surface was found to be about 20% and diamond growth was not observed on alumina surface at 850 °C (Fig. 8). Th e diamond crystallites were of a faceted nature with an average size of about 3 pm.

As the CH4 in the CH4-H2 gas mixture was increased to 2.0 vol.%, diamond coverage on the Au and alumina surfaces was also increased (about 50%) at 835 °C for 2 h (Fig. 9). Th e average sizes of diamond on Au and alumina surfaces were found to be about 3 p m and 4 /~m respectively. Faceted growth morpho- logy was observed on the Au surface (Fig. 9(b)) whereas a non-faceted or spherical growth morphology was observed on the alumina surfaces (Fig. 9(c)).

44 J. Singh et al. / Growth of diamond on Au and AI_,O¢

lOs

Fig. 5. Scanning electron micrographs showing diamond growth on Au and alumina substrate by the HFCVD process (1~,=850 °C; 1.5 vol.% CH4; deposition time, 4 h): (a) low magnification, showing diamond growth on the Au and alumina surfaces; (b) high magnification showing diamond growth on the alumina surface.

Auger surface analysis was conducted between the diamond crystallite on the Au surface of the sample after diamond deposition (Fig. 4). The Auger spectrum revealed mainly an Au peak and carbon peak (Fig. 10). Small Ni peaks were observed probably because of interdiffusion of Ni and Au.

4. Discussion

4.1. Growth morphology Kobashi et al. [10] have reported that the morphol-

ogy of diamond crystals synthesized using a micro- wave-enhanced plasma is a sensitive function of the concentration of methane in the methane-hydrogen gas mixture. They showed that the surface morpholo- gies of diamond crystals changed from (111) orienta- tion (octahedron) at low methane concentrations to

(100) orientation (cubic) as the methane concentration was increased.

Ravi et al. [11] have also observed different growth morphologies of diamond particles grown by the combustion flame techniques as a function of increas- ing hydrocarbon-to-hydrogen (or oxygen) ratio. They proposed that the morphology of diamond changed from octahedron--, cubo-octahedron ~ cube--, spheri- cal, followed by a phase transformation to amorphous carbon and graphite.

In the present study, individual faceted diamond crystals were formed at the beginning of diamond growth. Two types of face were observed in the dia- mond crystallites, i.e. smooth surfaces and rough surfaces. The triangular rough portions were octahe- dral ( 111 ) faces while the smooth square portions were cubic (100) faces. The diamond crystallites with octahedral faces (111) and cubic (100) faces, forming a cubo-octahedron, were found to be common (Fig. 4). At a low deposition temperature (below 850 °C), cubo- octahedral diamond was found to be dominant (Fig. 4), while octahedron diamond morphology was found to be dominant in the temperature ranging from 850 to 915 °C (Fig. 6). At a higher deposition temperature (935 °C), the diamond crystallites were spherical like a cauliflower with multiple growth and twins (Fig. 7). Thus, as the substrate temperature T~ increased, the morphology of diamond crystallites also changed. The sequence of the growth morphology appeared to be cubo-octahedral --" octahedron--- spherical as a function of increasing temperature. At about 925 °C, there was also a transition in the morphology of diamond growth, i.e. from faceted to non-faceted surface (Fig. 11).

The diamond growth morphology sequence was found to be different in all three processes, i.e. HFCVD, microwave chemical vapor deposition [10] and combustion flame techniques [11]. Growth morphology was also reported to depend upon the nature of hydrocarbon species arriving at the diamond surface [ 12].

Figure 12 represents the variation in the crystal shape from octahedron to cubo-octahedron to cube by the growth ratio of (111) faces to (100) faces in their normal directions. For the exact cubo-octahedron shape, the growth ratio will be 1.1547 and will vary in the range 0.577-1.732 depending upon shape modifi- cations. In the present investigation, assuming that the increase in the size of diamond crystal edges is propor- tional to the growth rate, then the ratio A / B is approxi- mately equal to 1.1, which is very close to the value of 1.1547 for exact cubo-octahedron shape, where A is the average length of cubic (100) planes and B is the average length of octahedron (111) planes, as illu- strated in Fig. 4.

J. Singh et al. / Growth of diamond on Au and A l 2 0 ~ 45

03

Fig. 6. Scanning electron micrographs showing the growth of faceted diamond crystallites on the Au and alumina substrate by HFCVD process (T~= 915 °C; 1.5 vol.% CH4: (a) low magnification, showing diamond growth on the Au and alumina surfaces; (b) higher magnification, showing diamond crystallite on the Au surface; (c) higher magnification, showing diamond crystailites on the alumina surface.

Fig. 7. Scanning electron micrograph showing cauliflower growth morphology of diamond crystallites on alumina surface ( T~ = 935 °C; 1.5 vol.% CH4; deposition time, 4 h).

Fig. 8. Scanning electron micrographs showing the growth of faceted diamond crystallites on the Au surface by the HFCVD process ( T s = 800 °C; 1.0 vol.% CH4; deposition time, 4 h).

46 J. Singh et al. / Growth of diamond on Au and AI,_ 01

. . . . . . 1 0 0 0

~i iii .........

500

" 0

-500

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Au C

0 500 1000 1500 2000 Kinetic energy (eV)

Fig. 10. Auger surface analysis of Au surface after diamond growth of the sample (Fig. 4) showing Au and C peaks.

2.5

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1.0

700 1000

~ S p h e r i c a l

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• I I I I

I

I

I I ~'/ I I 750 800 850 900 950

Temperature (°C) --*

Fig. 11. A plot showing the diamond growth morphology change as a function of gas composition and substrate temperature 7",.

Fig. 9. Scanning electron micrographs showing the growth of faceted diamond crystallites on the Au and alumina surface by HFCVD process (T~ = 835 °C; 2.0 vol.% CH4; deposition time, 2 h): (a) low magnification, showing the diamond growth on both Au and alumina surfaces; (b) faceted diamond growth on Au surface; (c) higher magnification, showing diamond crystallites on the alumina surface.

The growth morphology of diamond also depends upon the surface diffusion of hydrocarbon species. Frenklach and Spear [12] have discussed the rela- tionship of atomic structure of (111), (100) and (110) diamond surfaces with growth behavior of these sur- faces and nature of hydrocarbon species arriving at the surfaces. In the early stages of diamond growth, the growing crystal surface is flat and it must grow by either a ledge or a kink mechanism [12]. The most favorable sites for deposition of active hydrocarbon radicals to form diamond are steps, especially kinks or

J. Singh et aL / Growth

ledges. Thus, quick filling of these sites lead to the sharp-edged cubo-octahedral growth morphology [12, 13].

The size and coverage of diamond were also dependent upon the substrates and substrate tempera- ture ranging from 775 to 915 °C. The density of diamond nuclei on the gold surface increased as a func- tion of increasing substrate temperature, i.e. from about 20% at 775 °C to about 90% at 850 °C and about 100% at 915 °C (Figs. 13 and 14), whereas the diamond growth was not observed on an alumina substrate up to a temperature of about 815 °C and then it is increased as a function of increasing substrate temperature (Fig. 14).

4.2. Activation energy The activation energy Q was measured by plotting

the natural logarithm of diamond crystal size as a func- tion of inverse substrate temperature (Fig. 15). In the present study, the activation energies were estimated to be about 20 kcal mol- 1 for the growth of diamond on Au and alumina surfaces. The present Q values are very close to the value reported by Chen et al. [14] for the growth of diamond on copper substrate. Spitsyn et al. [15] reported the activation energy of diamond growth on diamond via pyrolysis of methane to be about 60 kcal mol- ~ and it dropped to 25 kcal mol- when atomic hydrogen was introduced into the reaction chamber. Thus atomic hydrogen plays an important role for the growth of diamond.

Derjaguin et al. [16] reported that the growth rate of diamond initially increased with increasing deposition

Cube

0 .7 0 .8 0 .6

C u b o - o c t a h e d r a

1.0 1.15 1.3

Octahedron

1.5

Fig. 12. Cubo-octahedral diamond shapes revealing the ratio of growth rates for the (100) and (111) axes. The growth ratio of the (100) value to the (111) value is given for each shape.

of diamond on Au and Al20¢

1Or

47

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9

8

7

6

5

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• Gas Ratio CH 4 (1o5) : H 2 (100)

• Au Surface • Pressure = 20 Torr • AI203 Surface

/ / Selective Growth of Diamond on Gold (i.e. No Diamond I Formation on AI203) ] •

"

S'I 1 I z I I I

700 800 900 1000

Temperature (°C) Fig. 13. A plot of the average diamond crystallite size as a function of temperature for Au and alumina.

100

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.90

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700 800 900 1000

Temperature (°C) Fig. 14. A plot of the percentage of surface coverage with diamond particles as a function of temperature for Au and alumina.

48 J. Singh et al. / Growth of diamond on Au and AL O ~

A 1 o

m

0

-1 | !

8.00e-4 9 .000-4 1 .00e-3 1 .10e-3 l I T

Fig. 15. A plot showing the natural logarithm of the diamond crystallite size on Au as a function of inverse temperature.

5. Summary

Diamond was found to selectively grow on the gold surface at a relatively lower temperature (800 °C) than on the alumina surface. Negligible or no diamond growth was observed on the alumina surface up to 800 °C. The density of diamond growth on gold and alumina surfaces increased parabolically as a function of substrate temperature. The growth morphology of diamond was found to be dependent on the substrate temperature and composition of the CH4-H 2 gas mixture. This small value of activation energy (20 kcal mo1-1) was attributed to the surface diffusion pheno- mena for diamond growth.

temperature and then decreased, showing a maximum growth of diamond at about 1000 °C. The main factor determining these trends is the competition between the formation of diamond and nondiamond phases. Frenklach and Spear [12] proposed a growth mechan- ism for diamond, which consisted of two steps: surface activation by the removal of hydrogen atoms followed by the addition of one or two acetylene molecules. The hydrogen removal step is a thermally activated process. The growth rate of diamond also depends upon the specific energy of the crystallographic planes [13]. Under certain conditions, a higher specific energy plane generally has a higher growth rate [13]. The quickly growing planes gradually decrease their areas, while slowly growing planes increase their areas. It is interesting to note in the present investigation that the diamond growth was dominated by cubo-octahedron at a low deposition temperature. This means that all possible (111) crystallographic planes of diamond were taking an active role in the initial growth of diamond leading to a cubo-octahedron (Fig. 4). At a relatively higher deposition temperature, it appeared that only selective (111) crystallographic planes were taking an active role in the growth of diamond, leading to octa- hedron shapes (Fig. 6). At still higher deposition temperatures, the number of active ( 111 ) crystallogra- phic planes reduced to minimum (close to zero) and the shape of diamond crystals was spherical (Fig. 7). Thus the surface energy, surface diffusion of species and temperature were the rate-contr011ing steps for the growth of diamond. The high resolution electron mic- roscopy of diamond is under way to provide further insight into the growth mechanisms of diamond.

Acknowledgment

The work was performed at North Carolina State University, Raleigh, NC, USA.

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