Preparation and characterization of wide area, high quality diamond film using magnetoactive plasma chemical vapour deposition

Surface and Coatings Technology, 43/44 (1990) 1U-21

PREPARATION AND CHARACTERIZATION OF WIDE AREA, HIGH QUALITY DIAMOND FILM USING MAGNETOACTIVE PLASMA CHEMICAL VAPOUR DEPOSITION AKIO HIRAKI, HIROSHI KAWARADA, JIN WEI and JUN-ICHI SUZUKI* Faculty of Engineering, Osaka University, Suita, Osaka 565 (Japan)

Abstract A magnetomicrowave plasma was used for the low pressure deposition of diamond. The important point in the plasma deposition system is to set the electron cyclotron resonance (ECR) condition (875 G in the case of a 2.45 GHz microwave) at the deposition area. The high density plasma (above I >< 10k’ cm 3) necessary for high quality diamond formation was obtained by effective microwave absorption near the magnetic field, satisfying the ECR condition. The plasma is uniform at the discharge area (160 mm in diameter) and uniform diamond films of a high quality are obtained. From an investigation of diamond formation in the range 50—10 2 Torr in the same deposition system, it is obvious that the lower pressure reduces the formation temperature of diamond to 500 C and that the effective species for diamond formation are low energy radicals.

1. Introduction Chemical vapour deposition (CVD) is one of the most suitable techniques for producing thin diamond films [1—3].So far high quality diamond films have been produced near atmospheric pressure (above 30 Torr) [3—8].In plasma-assisted CVD (plasma CVD) methods, the temperature of the plasma is very high. This plasma effectively produces diamond at a high rate [7, 8]. However, control of the plasma at high pressure is limited to small areas. Moreover, the properties of the plasma itself have not been clarified because of the large number of gas phase collisions. Low pressure plasma (around 0.1 Torr), which is at a low temperature, has a number of advantages such as large area, uniform deposition, low temperature deposition etc. In addition, measurement of the plasma parameters, such as electron temperature and plasma density, and the control of the potential difference between the plasma and the substrate can be carried out. *0~leave

from Shimadzu Co., Kyoto 615, Japan.

0257-8972/90/$3.50

Elsevier Sequoia/Printed in The Netherlands

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However, CVD of diamond films using low pressure plasma CVD has not been successful because a high density of atomic hydrogen cannot be formed by this method. The presence of dense atomic hydrogen is required for the formation of diamond from the vapour phase. The main role of the hydrogen atoms is believed to be the selective removal of amorphous carbon (a-C) and the graphite phase (sp2 bonding). As a result only the diamond phase (sp3 bonding) remains [2]. Since the gas density is high, the atomic hydrogen density is expected to be high. Even at a lower pressure (less than 1 Torr), if the plasma density is sufficiently high to form atomic hydrogen at a higher rate than that in a normal low pressure plasma, conditions are suitable for diamond formation. We have developed a magnetoactive microwave plasma CVD system without sacrificing the plasma density at lower pressure [9—13].The important point of the system is to set the electron cyclotron resonance (ECR) condition at the deposition area [9]. The most efficient microwave absorption is expected at the ECR condition. Low pressure, low temperature deposition has been realized [10-15] and plasma diagnosis has been carried out during diamond deposition [11, 14, 15].

2. Magnetomicrowave plasma CVD system The magnetomicrowave plasma CVD system [9] is shown in Fig. 1. It is composed of a microwave generator, a waveguide, Helmholtz-type magnetic

(a) ~ub~ra~

ECR (0.875 kG)

300 )b)

200

100

g 0

Distance (mm)

Fig. 1. (a) Schematic diagram of the magnetomicrowave plasma-assisted CVD system. (b) Corresponding magnetic field distribution in (a) along the axis of the round waveguide.

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coils, a round waveguide (discharge area) with TE11 mode and a reaction chamber. The inner diameter of the round waveguide is 160 mm. Figure 1(b) shows the corresponding profile of the magnetic field used in the deposition. The magnetic field satisfying the ECR condition, where the electron gyration frequency is equal to that of the microwave (2.45 GHz), is 875 G, shown as a circle in Fig. 1(b). The magnetic field distribution is intended to carry out the deposition in a high density plasma. At low pressure, a microwave can propagate in a plasma where the density is higher than the critical density of the microwaves (7.46 x 1010 cm ~ in the case of a 2.45 GHz microwave) and is effectively absorbed around the ECR condition. In other words, the input microwave is not reflected by the high density plasma and can contribute to increase the plasma density above the critical density [16]. This type of plasma formation at 10- ~—iO ~Torr has been thoroughly investigated [161 and applied to ion sources [17]. At 0.1 Torr, the discharge area is uniform and Fig. 2 shows an extended discharge outside the outlet (100 mm in diameter) of the round waveguide for the main discharge region. At this pressure, the discharge is controlled and enhanced by the magnetic field. One of the features can be seen in the flare of the discharge, which is the trace of the confined electron motion by the divergent magnetic field. Thus the plasma obtained is magnetoactive at 0.1 Torr.

3. Formation of diamond at low pressure The substrates used in the experiments were Si(100) wafers. These wafers were polished with diamond particles in alcohol using ultrasonic vibrations to make high density nucleation sites for diamond. No remaining diamond particles could be observed after cleaning in organic solvents. The substrates were loaded into the deposition system and it was evacuated to b.c Torr. Hydrocarbon (CH4) [2,3] and/or carbon oxides (CO [18] and C02) diluted with H2 were 3then into the system. Thetototal mm introduced Microwaves (2.45deposition GHz) were applied the flow rate was 100 cm round waveguide where a magnetic field distribution was present as shown in Fig. 1(b). The input microwave power was between 400 and 1300 W. The substrate temperatures were between 500 and 900 ~C. The pressures during the deposition were varied in the range 10 ~—50Torr. Low pressure diamond deposition can be effectively produced using carbon monoxide as reaction gas. Table 1 shows the experimental conditions for a CO—H 2 mixture and the corresponding products [12]. At 0.1 Torr, the formation temperature is reduced to 600 ~C. The graphite phase appears above 850 C where high quality diamond forms at a pressure of 1—SO Torr. This is one of the low pressure effects. A lower pressure leads to a lower temperature of deposition of diamond films. In order to form the diamond phase below 1 Torr, a positive d.c. bias is added to the substrate [10—13].As the effect of substrate potential increases ~.

13

—~

a .a

E E 5)

a a a

a

a

~

0

‘-~0

‘O 0

14 TABLE 1 Experimental conditions and products of diamond formation [12] Pressure (Torr)

CO (%) in H

T0~b(CC)

Bias

Product

3- 20

800—900

0

D {11i}, {100}

0.5--i

5

800—900

+

D 111}, {100}

0.1

35

900 750 750 600

+ + +

G(D) D {100} SiC D{111~

700

+

D

10-50

0.01

2

5

—

D, diamond; G, graphite; T,~b,substrate temperature. Braces indicate a dominent facet plane.

at 0.1 Torr, SiC forms at low negative d.c. voltage (—10 V) and the silicon substrates are sputtered at high negative d.c. voltage (—60 V) [12, 13]. From these results it is speculated that fast ions or neutrals bombard the substrate under negative bias. However, under positive bias only electrons or slow neutrals can reach the substrate. This situation is suitable for diamond formation at low pressure. Figures 3(a) and 3(b) show a scanning electron micrograph and a reflection electron diffraction pattern of the diamond film formed at 0.1 Torr and 650 ~C using a mixture of 5% CO and H2. The grain sizes are 200—500 nm and almost all of the planes are of {111} orientation. All the observed Debye—Scherrer rings are identified as those of diamond. The temperature for the formation of diamond films depends on the reaction gases. Figures 4(a)—4(d) show scanning electron micrographs of films formed at 580 CC during a period of 4 h using 5%CH4—H2, 1O%CO2—H2, 5%CO—H2 and S%CH4—10%CO2—H2 respectively. For the CO—H2 mixture, diamond is obtained as islands (Fig 4(c)), but not as a complete film. For the CH4—H2 and C02—H2 mixtures, the diamond phase is not obtained (Figs. 4(a) and 4(b)). However, with the CH4—C02—H2 mixture, high quality diamond films of a sufficient thickness with {111} facets are obtained as shown in Fig. 4(d). A crystallinity similar to that obtained with the CO—H2 mixture at 650 ~C (Fig. 3) is observed. Figure 5 shows the Raman spectrum of the film presented in Fig. 4(d). There is a sharp peak at 1332 cm~which is equivalent to that of natural diamond. The broad band near 1500 cm’ is small. Thus high quality diamond is obtained at a temperature of 580 ~C. By decreasing the temperature to 500 °C,high quality, uniform diamond films can be obtained using the CH4—C02--H2 mixture. Thus diamond can be formed on aluminium [19], the melting point of which is 660 ~‘C.On increasing the temperature above 650 °C, the crystallinity deteriorates [15]. Thus the optimum temperature for the deposition of high quality diamond is lower than for the other reaction gases. The presence of oxygen has some effect on the low temperature deposition. The role of oxygen is still unknown.

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(a)

1 jim

(b) Fig. 3. (a) Scanning electron micrograph of a diamond film deposited on a silicon substrate using a 5%CO—H 2 mixture at 0.1 Torr and 650 C. (b) Reflection electron diffraction pattern of the film shown in (a).

4. Relationship between plasma density and diamond formation We can measure the plasma density during diamond formation, because diamond is formed below 0.1 Torr, where a distinct plasma sheath appears. For the precise measurement of plasma parameters, a double probe method was chosen. Two probes with equal surface areas were floated from ground potential and placed perpendicular to the direction of the magnetic field. The

16 -d

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C)

.0

C C)

a

E

—4

~C

a a .0

0

5)

a

a aa

N

CDL) 0)

0

a.

(0

a

H L~

LC~

0

~

17

(a)

1600 (b)

1500 RAMAN

ILIOO

i~oo

1200

SHIFT (cm~)

Fig. 5. (a) Reflection electron diffraction pattern of the film shown in Fig. 4(d). (b) Raman spectrum of the film shown in Fig. 4(d).

electric circuit of the double probe for measuring the plasma density is shown in Fig. 6(a). The ion current was monitored using the double probe which is not affected by the strong applied magnetic field and the secondary electrons. The positions of the probes were varied along the axis of the magnetic field. The input microwave (2.45 GHz) power was 400—1300W. An example of the current—voltage relationship is shown in Fig. 6(b). The electron temperature and plasma density were calculated from the current—voltage relationship.

18

11. C. Source

Probe

~

~

-

EIJJJ

L H::: (a)

—axis

//

x-y Recorder

dVd

-

(b) Fig. 6. (a) Electric circuit of the double probe method for measuring plasma density. (b) Current-voltage relationship in the double probe method.

The plasma density was measured at various levels of input microwave power (400-1300W) [13]. The results are shown in Fig. 7. The atmosphere is pure hydrogen at 0.1 Torr. The probes were placed at the ECR condition (875 G) and were perpendicular to the direction of the axis of the magnetic field. The plasma density increases with increasing input microwave power and is higher than the critical plasma density of the microwave (7.46 x lO~cm3 in the case of 2.45 GHz). The microwaves are effectively absorbed at the higher input microwave power. There is no saturation of the plasma density up to 1300 W.

19

10 p

=

0.1 Tort

xlQ ii

4 8

2.0 E

(1)

~

6

Ne— -

____

ci)

1.5 ~j

Te

1~~

I

-~

0.5~

______ 0

400

600

_____

I

800

1000

Input microwave power

1200

P~~(W)

Fig. 7. Dependence of the plasma density and electron temperature on the input microwave power (hydrogen atmosphere at 0.1 Torr).

The dependence of the deposition rate on the input microwave power was also investigated. In this case, the reaction gas used was the 5%CH4— 10%C02—H2 mixture. The substrates were set at the ECR condition. The normalized deposition rate of the diamond film increases as the input power increases as shown in Fig. 8. The trend is similar to that shown in Fig. 7. On increasing the plasma density, the deposition rate increases and the crystallinity of the diamond films improves. The lower the present plasma magnedensity 3 limit usingofthe for diamond formation is about (2—3) In x 1010 tomicrowave plasma CVD system. thecm high density plasma above 1 x 1011 cm3, high quality diamond films with {111} facets are obtained at 0.1 Torr using the CH 4—C02—H2 mixture as shown in Fig. 4(d). High density plasmas effectively form high quality diamond film.

io7

Z

02

0

~

400 600 800 1000 1200 Input microwave power Pir(W)

Fig. 8. Dependence of the normalized deposition rate on the input microwave power (CH

4 -CO2 H2 mixture at 0.1 Torr). The substrate temperature is maintained at 650 C for all depositions.

5. Conclusions We have applied a magnetomicrowave plasma CVII system for low pressure, low temperature deposition of diamond. The following results have been obtained. (i) A high density (greater than 1 x 1011 cm ~) plasma is obtained at 0.1 Torr using efficient microwave absorption near the ECR condition. This low temperature plasma is very advantageous for low temperature and large area deposition. (ii) Of the reactant gases, the presence of carbon oxides (CO and/or C02) is advantageous in the low pressure deposition of diamond. Using CH4 and CO2 diluted with H2, a complete diamond film is formed at 500 C. Acknowledgments The authors are grateful to Shimadzu Co. for technical cooperation. They also wish to thank Idemitsu Petrochemical Inc. and Semiconductor Energy Laboratory Inc. for collaboration in developing the magnetomicrowave plasma CVD system. This work was supported in part by a Grant-inAid for Developmental Scientific Research (63850008) from the Ministry of Education and Culture of Japan.

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