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Effects of applied bias voltage on the properties of a-C:H films
g.ORBN6$
ELSEVIER
Surface and Coatings Technology78 (1996) 31 36
I llNOIO l
Effects of applied bias voltage on the properties of a-C'H films Yin Dachuan, Xu Niankan, Liu Zhengtang, Han Yong, Zheng Xiulin Department of Materials Science and Engineering, Northwestern Polytechnical University, X i'an 710072, Shaanxi, People's Republic of China Received 28 July 1994; accepted in final form 25 October 1994
Abstract The effects of the applied negative bias voltage on the hydrogen content, refractive index, extinction coefficient, internal stress, Vickers microhardness, adhesion and growth rate of a-C : H films deposited from a C2H 2 -}-Ar mixture by a d.c.-r.f, plasma-enhanced chemical vapour deposition process have been investigated. The results showed that the properties of a-C : H films strongly depend on the applied bias voltage. In the investigated bias range (-400 to -1000 V), it was found that, apart from the growth rate and extinction coefficient (which were found to increase with increasing bias), all the other properties studied showed a maximum at around - 800 to - 900 V. Compared with those studies reported previously on self-bias voltage effects, these results indicated a slight difference between the effects of the self-bias voltage and applied bias voltage. The experimental results are discussed. Keywords: a-C:H films; Applied bias; d.c.-r.f. PECVD
1. Introduction Hydrogenated amorphous carbon (a-C: H) films have been widely studied in recent years because of their unique diamond-like mechanical, optical, electrical, acoustical, chemical and thermal properties, and their potential applications, such as protective coatings for IR optics, tooling devices, anti-reflection devices and microelectronic devices. To produce these films, a wide variety of techniques, such as r.f. and/or d.c. plasma-enhanced chemical vapour deposition (PECVD) [ 1 - 4 ] , ion beam deposition (IBD) [5,6] and magnetron sputtering [-7] have been utilized. The properties of the resulting films, which can be varied in a wide range, strongly depend on the deposition techniques and their individual deposition conditions. Among these various deposition techniques, PECVD is the most widely employed process to prepare a - C : H films. In this process, r.f. power is often used as the plasma generator. Extensive studies have been carried out on this widely accepted process to investigate the effects of the deposition conditions on the properties of the resulting films. Generally, the self-bias voltage is considered to be an important factor for the final properties of the films. As reported by some authors [8], increasing the self-bias voltage decreases the hydrogen content, sp3:sp 2 ratio, optical gap and internal stress, but increases the refractive index, extinction coefficient and growth rate. With regard to the microhardness, a 0257-8972/96/$15.00© 1996 ElsevierScienceS.A. All rights reserved SSDI 0257-8972(94)02387-5
maximum can be found in the relationship between the bias voltage and the microhardness, but the actual bias where the maximum appears shows a great diversity among different authors. Apart from the bias generated by the r.f. power, an applied d.c. voltage can also supply a negatively biasing effect. The exact bias voltage can be easily controlled by applying d.c. power to the r.f. PECVD system. With this so-called d.c.-r.f. PECVD technique, high quality a-C : H films might be obtained. Recently, Guo et al. [-9] prepared thin anti-reflection a - C : H films, using this technique on germanium slices and achieved extremely high IR transmittance values (average of 90% and peak of 99% in 3-5 gm band). It is necessary to conduct further investigation of this technique. In this paper, we report on the effects of the applied negative d.c. bias voltage on the properties of a - C : H films obtained by the d.c.-r.f. PECVD technique. Comparisons were made between the effects of the applied and self-bias voltages, and the results showed slightly different effects between the two.
2. Experimental details 2.1. Preparation of a-C : H films Fig. 1 shows schematically a diagram of the deposition system. The vacuum chamber was a stainless bell jar
32
D. Yin et aL/Surface and Coatings Technology 78 (1996) 31-36
was determined by a hydrogen evolution method using a gas chromatograph. The results for the hydrogen content were confirmed by the burning method.
/1
~ 8 ~1 1 18 ~ 1 1 ~ 9
DC 5
RF.13.56MHz Fig. 1. Diagram of the deposition system: 1, bell jar; 2, watching window; 3, to diffusion pump and mechanical pump; 4, positive electrode; 5, d.c. power; 6, r.f. power; 7, gas inlet; 8, negative electrode; 9, substrate.
with a diameter of 300 m m and length of 400 mm. The positive electrode, which was grounded, was used as a gas inlet, through which a C2H 2 q-Ar gas mixture was introduced into the chamber. The substrate was placed on the negative electrode, which was capacitively coupled to an r.f. generator and connected to a d.c. power source. The flow rates of the gases were controlled by mass flow controllers. The deposition conditions are listed in Table 1. Before the deposition process, the silicon substrates were carefully cleaned and dried, then placed into the chamber. They were then etched by Ar + implantation in a d.c. discharge of 1.5 kV at 6 x 10 -2 Torr for 10 min.
2.2. Measurement of properties 2.2.1. Hydrogen content The weight per cent hydrogen content was calculated from the weight data of the films and the hydrogen in the films. The weight gain of the samples through deposition was taken as the weight of the films, which was determined using a microgram balance. The weight of hydrogen (both bonded and unbonded) in the films
2.2.2. Film thickness, refractive index and extinction coefficient The film thickness tf, refractive index n and extinction coefficient k were derived from ellipsometry data (at a wavelength of 0.6328 gm) using a computer calculation program.
2.2.3. Mechanical properties: Vickers microhardness, adhesion and internal stress The Vickers microhardness of the films was measured using a microhardness tester under a load of 10 g. The adhesion between the films and the substrates was evaluated using an indentation method. The indentation method can be applied to find the lowest load - known as the critical load Per - - which can make a crack with a particular length. Since a higher critical load indicates better adhesion, the critical load obtained by this method can be utilized to characterize the adhesion of different samples. In our experiments, the critical load represents that which initiates a detectable crack under a microscopic magnification of 800 ×. The measurement was carried out using a Vickers microhardness tester. Since the tester used discrete loads, the critical load which initiates the crack, i.e. the theoretical P¢r, cannot be accurately determined. In the measurement, two loads were obtained: P+ 1, i.e. the lowest available load which cracks the film, and P - l , i.e. the highest load which does not crack the film. Thus, Por can be calculated by [ 10]
Per -
(P-I +P+I) 2
(1)
The internal stress value was obtained quantatively by a bending beam method from the well-known equation
-- 4Est 26 o -
Table 1 Deposition conditions Substrates Total gas pressure Gas flow rate R.f. power frequency R.f. power Negative self-bias voltage Applied d.c. voltage Distance between the two electrodes Deposition time
(2)
3(1 -- vs)Ftf Silicon slices (10 mm x 20 mm, 0.22 mm thick) with (111 ) polished surface 5 × 10 -2 Torr Ar, 1.5 sccm; C2H2, 1.5 sccm 13.56 MHz 4O W <100 V --400 to -- 1000 V 40 mm 35 min
where cr is the internal stress, E~ is the Young's modulus of the substrate, vs is Poisson's ratio of the substrate, tf and ts are the thicknesses of the film and the substrate, respectively, I is the length of the substrate segment, and c5 is the largest deflection (usually the central deflection) in the segment examined. For Si(111) wafers, Es/(1 -- vs) is 2.29 × 1011 Pa [11]. The deflection 6 and length l of the substrates were determined using a multiple-purpose microscope. The final results were calculated from Eq. (2).
D. Yin et al./Surface and Coatings Technology 78 (1996) 31 36
3. Results and discussions 3.1. Growth rate Fig. 2 shows the variation of the growth rate v as a function of the applied bias. As expected, the growth rate was found to increase with increasing bias. At lower applied biases, the growth rate showed little increase. Zou et al. [ 12] studied the effects of the self-bias voltage on the film growth rate and found that the growth rate increases with increasing bias. Apparently, the applied bias and self-bias exhibited similar effects on the growth rate. More complete ionization at the higher biases might be the reason for the increasing growth rate. 3.2. Hydrogen content The variation of the hydrogen content in the films with different applied biases is shown in Fig. 3. Curve 1 was obtained from the data from the hydrogen evolution method and curve 2 was obtained from the data from the burning method. The hydrogen contents depicted in curves 1 and 2 show a slight difference from each other because of different measurement techniques. However, the variations are similar. It is clear that the hydrogen content increased with increasing bias before about 2913 E 2713 "" 25C ~<~ 2313 21( ,~ 19C 17( 15C 13C
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I
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-400 -500 -600 -700 -800 -900 -1000 Applied bias voltage Vb(V) Fig. 2. Variation of growth rate v as a function of applied bias voltage Vb.
33
- 8 0 0 V , but decreased when the bias exceeded this value. This result was contradictory to many reported results [8,12-15] on the effects of the self-bias voltage. Possibly, this is because of the difference in the actual deposition techniques and the difference in the hydrogen content determination techniques. 3.3. Optical constants: refractive index and extinction coefficient Fig. 4 shows the dependence of refractive index n on the applied bias voltage. As shown in this figure, the refractive index of the films, which ranged from 2.0 to 2.7 at a wavelength of 0.6328 lam, was found to have a maximum at around - 8 0 0 V. Previously reported work showed diversity in the results of the dependence of the refractive index on the self-bias voltage. Serra et al. [ 15] reported a maximum refractive index at about - 500 V, whereas Koidl et al. [13] found a monotonic increase in the refractive index with increasing self-bias voltage. This might be caused by different deposition conditions. For our experimental results, the peak of the refractive index probably represented a structural change from a diamond-like to a graphite-like structure which also is in agreement with the following results on the microhardness, adhesion and internal stress. The variation of the extinction coefficient k (at a wavelength of 0.6328 gm) of the films (ranging from 0.01 to 0.12) with the applied bias is shown in Fig. 5. It can be seen that the extinction coefficient increased almost linearly with increasing applied bias voltage. This result is in good agreement with the work of Serra et al. [15] on the effects of the self-bias voltage. 3.4. Internal stress The variation of the internal stress in the films with varying applied bias is shown in Fig. 6. The internal stress in the films was in a compressive state. It
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2.1 -400 -5f00 -6'00 -400 -800 -9'00 -1600 Applied bias voltage Vb(V) Fig. 3. Variation of hydrogen content as a function of applied bias voltage Vb.
-4100 -5t00 -6})0 -'700 -8t00 -900 -1000 Applied bias voltage Vb(V) Fig. 4. Variation of refractive index n (at wavelength of 0.6328 gm) as a function of applied bias voltage Vb.
34
D. Yin et al./Surface and Coatings Technology 78 (1996) 31-36
0.1
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0.01 f f / / ~ -'~00 -500 -~00 -700 -8100 -9100 -1000 Applied bias voltage Vb(V) Fig. 5. Variation of extinction coefficient k(at wavelength of 0.6328 gm) as a function of applied bias voltage Vb.
2 -13 o
t~
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-400 --500 -600 -700 -800 --900 --1000 Applied bias voltage V b ( V ) Fig. 6. Variation of internal stress a as a function of applied bias voltage lib.
had relatively low values, ranging from - 4 x l0 s to 1.5 x 109 Pa. From Fig. 6, the variation of the internal stress can be seen to be similar to those of the hydrogen content and refractive index, i.e. a maximum can be found at around - 8 0 0 V, and decreasing or increasing the applied bias would result in a rapid decrease in the compressive stress. As discussed above, the peak at about - 800 or - 9 0 0 V might be a result of a change from a diamond-like to a graphite-like structure. According to McKenzie et al. [16], the refractive index of a - C : H films is closely related to the internal stress. They pointed out a clear increase in the refractive index as the compressive stress increases. Our experimental results confirmed their conclusion. This can be seen from Figs. 4 and 6. Jiang et al. [ 17] studied the effects of the applied bias voltage on the internal stress, and obtained a different result compared with the effect of the applied bias. Their results showed a decrease in the internal stress with increasing bias, and no turning point was reported. -
Fig. 7. Variation of Vickers microhardness as a function of applied bias voltage Vb.
3.5. Vickers microhardness
As shown in Fig. 7, the Vickers microhardness of the films increased with increasing applied bias, until it reached a peak at about - 9 0 0 V. Compared with the effect of the self-bias voltage reported by Koidl et al. [ 13], it can be concluded that the applied bias voltage showed a similar effect to that of the self-bias voltage on the film hardness. Since it is observed in the experiment that the microhardness increased with increasing hydrogen content and decreased with decreasing hydrogen content - - though they do not vary synchronously we suggest that the increasing hydrogen content will stabilize more sp 3 bonds, leading to a more diamond-like structure, which might result in an increase in the microhardness. With the increase of the applied bias voltage, an increasing sputtering effect on the weakly bonded atoms and an increasing condensing effect of the high energy impingement would also contribute to the increase in the microhardness. When the bias exceeds - 8 0 0 V or - 9 0 0 V, the spZ:sp 3 ratio might increase slowly, leading to a more graphite-like structure, which might result in a decrease in the microhardness. The microhardness was also varied with the internal stress. As the compressive stress increases, the packing density of the films might increase, causing an increase in the microhardness if the structure remained diamond-like. 3.6. Adhesion
As shown in Fig. 8, with increasing applied bias, the critical load Per increased and passed through a maximum at about - 9 0 0 V . Here, we suggest that high energy ion bombardment causes better adhesion when the internal stress is not high enough to show an apparent degradation effect on the adhesion. As the applied bias voltage increases, the ion implantation energy increases. Higher energy ions will effectively etch the deposited surface, and weakly bonded radicals and
D. Yin et al./Surface and Coatings Technology 78 (1996) 31 36
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-400 -500 -600 -700 -800 -900 -1000 Applied bias voltage Vb(V) Fig. 8. Variation of critical load Per as a function of applied bias voltage Vb.
atoms will be sputtered away, resulting in a stronger bonding structure in the films, which means better adhesion and a higher Vickers microhardness. If the bias exceeds a particular value ( - 8 0 0 to - 9 0 0 V in our experiment), the structure of the films might become more graphite-like, resulting in a decrease in the adhesion and Vickers hardness. As shown above, it can be seen that the applied d.c. bias, similarly to the self-bias voltage, produced significant effects on the properties of the films. However, these effects are not completely the same as those of the self-bias voltage. As is generally known, the energy of incident ions essentially determines the film properties, so that it is worthwhile to consider the ion energy in the deposition process in both cases. During deposition, the energy of the incident ions is largely determined by the bias voltage and the pressure (which affects the mean free path of the ions) [ 18 ]. There follows some discussion about the energy of incident ions in different biasing methods. In the presence of r.f. power (which starts the glow discharge and remains in the deposition process), regardless of whether the total bias is applied by adding a d.c. source or self-developed by increasing the r.f. power, the cathode potential is a periodic function of the time, because of the modulation effect of the r.f. potential cycles. The frequency of the potential is the frequency of the r.f. power. However, there is a difference in the cathode potential in the two cases. At the same average potential level, the actual potential in the two cases varies in different ranges. In the case of the applied bias, the range of the potential changes little after applying the negative d.c. bias, whereas the range increases significantly in the case of the self-bias voltage while increasing the r.f. power; the maximum potential of the substrate will reach almost two times the value of the average potential [19]. This makes the ions and electrons behave differently. If the existence of inelastic collision is allowed for, the energy of the incident ions in the case of the self-bias voltage will vary in a wide range (from 0 to nearly 2 eVb), while
35
it varies in a smaller range in the case of the applied bias (from 0 to a little more than 1 eVb). The plasma and sheath regions were observed and it was found that the introduction of the applied d.c. bias significantly enlarged the sheath region and eliminated the negative self-bias effects of the r.f. power. (This can be proved by direct measurement of the substrate potential, and measurement of the applied d.c. voltage, which can be carried out separately.) This observation also suggested a difference in the energy conditions in the two cases. The different energy conditions of the incident ions will no doubt affect the deposition process differently. This might be a reason for the slightly different variations of the film properties in the two cases. However, the detailed mechanism remains unknown.
4. Conclusions It can be concluded that the properties of a-C : H films prepared by the d.c.r.f. P E C V D technique strongly depend on the applied negative bias voltage. It is clear that the applied bias voltage affects a large number of factors which decide the final properties of the films. Comparisons between the effects of the applied bias voltage and the self-bias voltage showed a slight difference between the effects. Mostly peaks can be found in the properties when an applied bias voltage is employed, whereas the properties usually show monotonic increases or decreases when self-bias voltage is used in the bias range studied ( - 4 0 0 to - 1 0 0 0 V). The correlated peaks of the properties as a function of applied bias in our experiments might be an indication of a change from a diamond-like to a graphite-like structure in the films.
Acknowledgments We would like to thank Ms. Sheng Meifen and Ms. Li Xiuyi for their great help in the experiments for measurement of the hydrogen content.
References [-1] R.S. Yalamanchi and G.K.M. Thutupall, Thin Solid Films, 164 (1988) 103. [2] L.H. Chou, J. Appl. Phys,, 72 (1992) 2027. L3] Y. Catherine, Mater. Sci. Forum, 52 53 (1989) 175. [-4] S. Jayshree, P. Raghunath, B. Suryadevaraand D. Mosses, Thin Solid Films, 212 (1992) 251. [-5] R.L.C. Wu, Surf. Coat. Technol., 51 (1992) 258. [-6] G.F. Ivanovsky, V.V. Sleptsov, V.M. Elinson, A.M. Baranov, A.A. Kuzin and P.E. Kondrashov, Surf. Coat. Technol., 48 (1991) 189.
36
D. Yin et al./Surface and Coatings Technology 78 (1996) 31-36
[7] M. Rubin, C.B. Hopper and N.-H. Cho, J. Mater. Res., 5 (1990) 2538. [8] J. Robertson, Surf. Coat. Technol., 50 (1992) 185. [9] LJ. Guo, G.F. Zhang, Z.T. Liu, N.K. Xu and X.L. Zheng, J. Northwestern Polytech. Univ., 10 (1992) 565. [10] P.X. Li, Biao Mian Gong Cheng, Shanghai Jiaotong University Press, Shanghai, 1989, p. 60. [11] W.A. Brantley, J. Appl. Phys., 44 (1973) 534. [12] J.W. Zou, K. Schmidt, K. Reichlt and B. Dishler, J. Appl. Phys., 67 (1990) 487. [13] P. Koidl, C. Wild, B. Dishler, J. Wagner and M. Ramsteiner, Mater. Sci. Forum, 52 (1990) 41.
[14] M.A. Tamor, W.C. Vassell and K.R. Carduner, Appl. Phys. Lett., 58 (1991) 592. [15] C. Serra, E. Pascual, F. Maass and J. Esteve, Surf. Coat. Technol., 47 (1991) 89. [16] D.R. McKenzie, D.A. Muller, E. Kravtchinskaia, D. Segal and D.J.H. Cockayne, Thin Solid Films, 206 (1991) 198. [17] X. Jiang, K. Reichelt and B. Stritzer, J. Appl. Phys., 66 (1989) 5805. [18] W. Moiler, Appl. Phys. A,56 (1993) 527. [19] J.L. Vossen and W. Kern, Thin Film Processes, Academic Press, New York, 1978, p. 56.
ELSEVIER
Surface and Coatings Technology78 (1996) 31 36
I llNOIO l
Effects of applied bias voltage on the properties of a-C'H films Yin Dachuan, Xu Niankan, Liu Zhengtang, Han Yong, Zheng Xiulin Department of Materials Science and Engineering, Northwestern Polytechnical University, X i'an 710072, Shaanxi, People's Republic of China Received 28 July 1994; accepted in final form 25 October 1994
Abstract The effects of the applied negative bias voltage on the hydrogen content, refractive index, extinction coefficient, internal stress, Vickers microhardness, adhesion and growth rate of a-C : H films deposited from a C2H 2 -}-Ar mixture by a d.c.-r.f, plasma-enhanced chemical vapour deposition process have been investigated. The results showed that the properties of a-C : H films strongly depend on the applied bias voltage. In the investigated bias range (-400 to -1000 V), it was found that, apart from the growth rate and extinction coefficient (which were found to increase with increasing bias), all the other properties studied showed a maximum at around - 800 to - 900 V. Compared with those studies reported previously on self-bias voltage effects, these results indicated a slight difference between the effects of the self-bias voltage and applied bias voltage. The experimental results are discussed. Keywords: a-C:H films; Applied bias; d.c.-r.f. PECVD
1. Introduction Hydrogenated amorphous carbon (a-C: H) films have been widely studied in recent years because of their unique diamond-like mechanical, optical, electrical, acoustical, chemical and thermal properties, and their potential applications, such as protective coatings for IR optics, tooling devices, anti-reflection devices and microelectronic devices. To produce these films, a wide variety of techniques, such as r.f. and/or d.c. plasma-enhanced chemical vapour deposition (PECVD) [ 1 - 4 ] , ion beam deposition (IBD) [5,6] and magnetron sputtering [-7] have been utilized. The properties of the resulting films, which can be varied in a wide range, strongly depend on the deposition techniques and their individual deposition conditions. Among these various deposition techniques, PECVD is the most widely employed process to prepare a - C : H films. In this process, r.f. power is often used as the plasma generator. Extensive studies have been carried out on this widely accepted process to investigate the effects of the deposition conditions on the properties of the resulting films. Generally, the self-bias voltage is considered to be an important factor for the final properties of the films. As reported by some authors [8], increasing the self-bias voltage decreases the hydrogen content, sp3:sp 2 ratio, optical gap and internal stress, but increases the refractive index, extinction coefficient and growth rate. With regard to the microhardness, a 0257-8972/96/$15.00© 1996 ElsevierScienceS.A. All rights reserved SSDI 0257-8972(94)02387-5
maximum can be found in the relationship between the bias voltage and the microhardness, but the actual bias where the maximum appears shows a great diversity among different authors. Apart from the bias generated by the r.f. power, an applied d.c. voltage can also supply a negatively biasing effect. The exact bias voltage can be easily controlled by applying d.c. power to the r.f. PECVD system. With this so-called d.c.-r.f. PECVD technique, high quality a-C : H films might be obtained. Recently, Guo et al. [-9] prepared thin anti-reflection a - C : H films, using this technique on germanium slices and achieved extremely high IR transmittance values (average of 90% and peak of 99% in 3-5 gm band). It is necessary to conduct further investigation of this technique. In this paper, we report on the effects of the applied negative d.c. bias voltage on the properties of a - C : H films obtained by the d.c.-r.f. PECVD technique. Comparisons were made between the effects of the applied and self-bias voltages, and the results showed slightly different effects between the two.
2. Experimental details 2.1. Preparation of a-C : H films Fig. 1 shows schematically a diagram of the deposition system. The vacuum chamber was a stainless bell jar
32
D. Yin et aL/Surface and Coatings Technology 78 (1996) 31-36
was determined by a hydrogen evolution method using a gas chromatograph. The results for the hydrogen content were confirmed by the burning method.
/1
~ 8 ~1 1 18 ~ 1 1 ~ 9
DC 5
RF.13.56MHz Fig. 1. Diagram of the deposition system: 1, bell jar; 2, watching window; 3, to diffusion pump and mechanical pump; 4, positive electrode; 5, d.c. power; 6, r.f. power; 7, gas inlet; 8, negative electrode; 9, substrate.
with a diameter of 300 m m and length of 400 mm. The positive electrode, which was grounded, was used as a gas inlet, through which a C2H 2 q-Ar gas mixture was introduced into the chamber. The substrate was placed on the negative electrode, which was capacitively coupled to an r.f. generator and connected to a d.c. power source. The flow rates of the gases were controlled by mass flow controllers. The deposition conditions are listed in Table 1. Before the deposition process, the silicon substrates were carefully cleaned and dried, then placed into the chamber. They were then etched by Ar + implantation in a d.c. discharge of 1.5 kV at 6 x 10 -2 Torr for 10 min.
2.2. Measurement of properties 2.2.1. Hydrogen content The weight per cent hydrogen content was calculated from the weight data of the films and the hydrogen in the films. The weight gain of the samples through deposition was taken as the weight of the films, which was determined using a microgram balance. The weight of hydrogen (both bonded and unbonded) in the films
2.2.2. Film thickness, refractive index and extinction coefficient The film thickness tf, refractive index n and extinction coefficient k were derived from ellipsometry data (at a wavelength of 0.6328 gm) using a computer calculation program.
2.2.3. Mechanical properties: Vickers microhardness, adhesion and internal stress The Vickers microhardness of the films was measured using a microhardness tester under a load of 10 g. The adhesion between the films and the substrates was evaluated using an indentation method. The indentation method can be applied to find the lowest load - known as the critical load Per - - which can make a crack with a particular length. Since a higher critical load indicates better adhesion, the critical load obtained by this method can be utilized to characterize the adhesion of different samples. In our experiments, the critical load represents that which initiates a detectable crack under a microscopic magnification of 800 ×. The measurement was carried out using a Vickers microhardness tester. Since the tester used discrete loads, the critical load which initiates the crack, i.e. the theoretical P¢r, cannot be accurately determined. In the measurement, two loads were obtained: P+ 1, i.e. the lowest available load which cracks the film, and P - l , i.e. the highest load which does not crack the film. Thus, Por can be calculated by [ 10]
Per -
(P-I +P+I) 2
(1)
The internal stress value was obtained quantatively by a bending beam method from the well-known equation
-- 4Est 26 o -
Table 1 Deposition conditions Substrates Total gas pressure Gas flow rate R.f. power frequency R.f. power Negative self-bias voltage Applied d.c. voltage Distance between the two electrodes Deposition time
(2)
3(1 -- vs)Ftf Silicon slices (10 mm x 20 mm, 0.22 mm thick) with (111 ) polished surface 5 × 10 -2 Torr Ar, 1.5 sccm; C2H2, 1.5 sccm 13.56 MHz 4O W <100 V --400 to -- 1000 V 40 mm 35 min
where cr is the internal stress, E~ is the Young's modulus of the substrate, vs is Poisson's ratio of the substrate, tf and ts are the thicknesses of the film and the substrate, respectively, I is the length of the substrate segment, and c5 is the largest deflection (usually the central deflection) in the segment examined. For Si(111) wafers, Es/(1 -- vs) is 2.29 × 1011 Pa [11]. The deflection 6 and length l of the substrates were determined using a multiple-purpose microscope. The final results were calculated from Eq. (2).
D. Yin et al./Surface and Coatings Technology 78 (1996) 31 36
3. Results and discussions 3.1. Growth rate Fig. 2 shows the variation of the growth rate v as a function of the applied bias. As expected, the growth rate was found to increase with increasing bias. At lower applied biases, the growth rate showed little increase. Zou et al. [ 12] studied the effects of the self-bias voltage on the film growth rate and found that the growth rate increases with increasing bias. Apparently, the applied bias and self-bias exhibited similar effects on the growth rate. More complete ionization at the higher biases might be the reason for the increasing growth rate. 3.2. Hydrogen content The variation of the hydrogen content in the films with different applied biases is shown in Fig. 3. Curve 1 was obtained from the data from the hydrogen evolution method and curve 2 was obtained from the data from the burning method. The hydrogen contents depicted in curves 1 and 2 show a slight difference from each other because of different measurement techniques. However, the variations are similar. It is clear that the hydrogen content increased with increasing bias before about 2913 E 2713 "" 25C ~<~ 2313 21( ,~ 19C 17( 15C 13C
O
I
1
I
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I
I
-400 -500 -600 -700 -800 -900 -1000 Applied bias voltage Vb(V) Fig. 2. Variation of growth rate v as a function of applied bias voltage Vb.
33
- 8 0 0 V , but decreased when the bias exceeded this value. This result was contradictory to many reported results [8,12-15] on the effects of the self-bias voltage. Possibly, this is because of the difference in the actual deposition techniques and the difference in the hydrogen content determination techniques. 3.3. Optical constants: refractive index and extinction coefficient Fig. 4 shows the dependence of refractive index n on the applied bias voltage. As shown in this figure, the refractive index of the films, which ranged from 2.0 to 2.7 at a wavelength of 0.6328 lam, was found to have a maximum at around - 8 0 0 V. Previously reported work showed diversity in the results of the dependence of the refractive index on the self-bias voltage. Serra et al. [ 15] reported a maximum refractive index at about - 500 V, whereas Koidl et al. [13] found a monotonic increase in the refractive index with increasing self-bias voltage. This might be caused by different deposition conditions. For our experimental results, the peak of the refractive index probably represented a structural change from a diamond-like to a graphite-like structure which also is in agreement with the following results on the microhardness, adhesion and internal stress. The variation of the extinction coefficient k (at a wavelength of 0.6328 gm) of the films (ranging from 0.01 to 0.12) with the applied bias is shown in Fig. 5. It can be seen that the extinction coefficient increased almost linearly with increasing applied bias voltage. This result is in good agreement with the work of Serra et al. [15] on the effects of the self-bias voltage. 3.4. Internal stress The variation of the internal stress in the films with varying applied bias is shown in Fig. 6. The internal stress in the films was in a compressive state. It
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2.2
~Z
2.1 -400 -5f00 -6'00 -400 -800 -9'00 -1600 Applied bias voltage Vb(V) Fig. 3. Variation of hydrogen content as a function of applied bias voltage Vb.
-4100 -5t00 -6})0 -'700 -8t00 -900 -1000 Applied bias voltage Vb(V) Fig. 4. Variation of refractive index n (at wavelength of 0.6328 gm) as a function of applied bias voltage Vb.
34
D. Yin et al./Surface and Coatings Technology 78 (1996) 31-36
0.1
3500 o
0.1
3000
0.09
2500
~0.07
~ 2000
e~ o
0.05
1500 G,
u~ 0.03
lO00 -400 -100 -6'00 -I00 -8JO0 -900 -1600 Applied bias voltage Vb(V)
0.01 f f / / ~ -'~00 -500 -~00 -700 -8100 -9100 -1000 Applied bias voltage Vb(V) Fig. 5. Variation of extinction coefficient k(at wavelength of 0.6328 gm) as a function of applied bias voltage Vb.
2 -13 o
t~
O
-400 --500 -600 -700 -800 --900 --1000 Applied bias voltage V b ( V ) Fig. 6. Variation of internal stress a as a function of applied bias voltage lib.
had relatively low values, ranging from - 4 x l0 s to 1.5 x 109 Pa. From Fig. 6, the variation of the internal stress can be seen to be similar to those of the hydrogen content and refractive index, i.e. a maximum can be found at around - 8 0 0 V, and decreasing or increasing the applied bias would result in a rapid decrease in the compressive stress. As discussed above, the peak at about - 800 or - 9 0 0 V might be a result of a change from a diamond-like to a graphite-like structure. According to McKenzie et al. [16], the refractive index of a - C : H films is closely related to the internal stress. They pointed out a clear increase in the refractive index as the compressive stress increases. Our experimental results confirmed their conclusion. This can be seen from Figs. 4 and 6. Jiang et al. [ 17] studied the effects of the applied bias voltage on the internal stress, and obtained a different result compared with the effect of the applied bias. Their results showed a decrease in the internal stress with increasing bias, and no turning point was reported. -
Fig. 7. Variation of Vickers microhardness as a function of applied bias voltage Vb.
3.5. Vickers microhardness
As shown in Fig. 7, the Vickers microhardness of the films increased with increasing applied bias, until it reached a peak at about - 9 0 0 V. Compared with the effect of the self-bias voltage reported by Koidl et al. [ 13], it can be concluded that the applied bias voltage showed a similar effect to that of the self-bias voltage on the film hardness. Since it is observed in the experiment that the microhardness increased with increasing hydrogen content and decreased with decreasing hydrogen content - - though they do not vary synchronously we suggest that the increasing hydrogen content will stabilize more sp 3 bonds, leading to a more diamond-like structure, which might result in an increase in the microhardness. With the increase of the applied bias voltage, an increasing sputtering effect on the weakly bonded atoms and an increasing condensing effect of the high energy impingement would also contribute to the increase in the microhardness. When the bias exceeds - 8 0 0 V or - 9 0 0 V, the spZ:sp 3 ratio might increase slowly, leading to a more graphite-like structure, which might result in a decrease in the microhardness. The microhardness was also varied with the internal stress. As the compressive stress increases, the packing density of the films might increase, causing an increase in the microhardness if the structure remained diamond-like. 3.6. Adhesion
As shown in Fig. 8, with increasing applied bias, the critical load Per increased and passed through a maximum at about - 9 0 0 V . Here, we suggest that high energy ion bombardment causes better adhesion when the internal stress is not high enough to show an apparent degradation effect on the adhesion. As the applied bias voltage increases, the ion implantation energy increases. Higher energy ions will effectively etch the deposited surface, and weakly bonded radicals and
D. Yin et al./Surface and Coatings Technology 78 (1996) 31 36
9C
©
"-~7Ca. o
O 5(
i
30I lC I
I
I
I
.....
I
I
I
-400 -500 -600 -700 -800 -900 -1000 Applied bias voltage Vb(V) Fig. 8. Variation of critical load Per as a function of applied bias voltage Vb.
atoms will be sputtered away, resulting in a stronger bonding structure in the films, which means better adhesion and a higher Vickers microhardness. If the bias exceeds a particular value ( - 8 0 0 to - 9 0 0 V in our experiment), the structure of the films might become more graphite-like, resulting in a decrease in the adhesion and Vickers hardness. As shown above, it can be seen that the applied d.c. bias, similarly to the self-bias voltage, produced significant effects on the properties of the films. However, these effects are not completely the same as those of the self-bias voltage. As is generally known, the energy of incident ions essentially determines the film properties, so that it is worthwhile to consider the ion energy in the deposition process in both cases. During deposition, the energy of the incident ions is largely determined by the bias voltage and the pressure (which affects the mean free path of the ions) [ 18 ]. There follows some discussion about the energy of incident ions in different biasing methods. In the presence of r.f. power (which starts the glow discharge and remains in the deposition process), regardless of whether the total bias is applied by adding a d.c. source or self-developed by increasing the r.f. power, the cathode potential is a periodic function of the time, because of the modulation effect of the r.f. potential cycles. The frequency of the potential is the frequency of the r.f. power. However, there is a difference in the cathode potential in the two cases. At the same average potential level, the actual potential in the two cases varies in different ranges. In the case of the applied bias, the range of the potential changes little after applying the negative d.c. bias, whereas the range increases significantly in the case of the self-bias voltage while increasing the r.f. power; the maximum potential of the substrate will reach almost two times the value of the average potential [19]. This makes the ions and electrons behave differently. If the existence of inelastic collision is allowed for, the energy of the incident ions in the case of the self-bias voltage will vary in a wide range (from 0 to nearly 2 eVb), while
35
it varies in a smaller range in the case of the applied bias (from 0 to a little more than 1 eVb). The plasma and sheath regions were observed and it was found that the introduction of the applied d.c. bias significantly enlarged the sheath region and eliminated the negative self-bias effects of the r.f. power. (This can be proved by direct measurement of the substrate potential, and measurement of the applied d.c. voltage, which can be carried out separately.) This observation also suggested a difference in the energy conditions in the two cases. The different energy conditions of the incident ions will no doubt affect the deposition process differently. This might be a reason for the slightly different variations of the film properties in the two cases. However, the detailed mechanism remains unknown.
4. Conclusions It can be concluded that the properties of a-C : H films prepared by the d.c.r.f. P E C V D technique strongly depend on the applied negative bias voltage. It is clear that the applied bias voltage affects a large number of factors which decide the final properties of the films. Comparisons between the effects of the applied bias voltage and the self-bias voltage showed a slight difference between the effects. Mostly peaks can be found in the properties when an applied bias voltage is employed, whereas the properties usually show monotonic increases or decreases when self-bias voltage is used in the bias range studied ( - 4 0 0 to - 1 0 0 0 V). The correlated peaks of the properties as a function of applied bias in our experiments might be an indication of a change from a diamond-like to a graphite-like structure in the films.
Acknowledgments We would like to thank Ms. Sheng Meifen and Ms. Li Xiuyi for their great help in the experiments for measurement of the hydrogen content.
References [-1] R.S. Yalamanchi and G.K.M. Thutupall, Thin Solid Films, 164 (1988) 103. [2] L.H. Chou, J. Appl. Phys,, 72 (1992) 2027. L3] Y. Catherine, Mater. Sci. Forum, 52 53 (1989) 175. [-4] S. Jayshree, P. Raghunath, B. Suryadevaraand D. Mosses, Thin Solid Films, 212 (1992) 251. [-5] R.L.C. Wu, Surf. Coat. Technol., 51 (1992) 258. [-6] G.F. Ivanovsky, V.V. Sleptsov, V.M. Elinson, A.M. Baranov, A.A. Kuzin and P.E. Kondrashov, Surf. Coat. Technol., 48 (1991) 189.
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D. Yin et al./Surface and Coatings Technology 78 (1996) 31-36
[7] M. Rubin, C.B. Hopper and N.-H. Cho, J. Mater. Res., 5 (1990) 2538. [8] J. Robertson, Surf. Coat. Technol., 50 (1992) 185. [9] LJ. Guo, G.F. Zhang, Z.T. Liu, N.K. Xu and X.L. Zheng, J. Northwestern Polytech. Univ., 10 (1992) 565. [10] P.X. Li, Biao Mian Gong Cheng, Shanghai Jiaotong University Press, Shanghai, 1989, p. 60. [11] W.A. Brantley, J. Appl. Phys., 44 (1973) 534. [12] J.W. Zou, K. Schmidt, K. Reichlt and B. Dishler, J. Appl. Phys., 67 (1990) 487. [13] P. Koidl, C. Wild, B. Dishler, J. Wagner and M. Ramsteiner, Mater. Sci. Forum, 52 (1990) 41.
[14] M.A. Tamor, W.C. Vassell and K.R. Carduner, Appl. Phys. Lett., 58 (1991) 592. [15] C. Serra, E. Pascual, F. Maass and J. Esteve, Surf. Coat. Technol., 47 (1991) 89. [16] D.R. McKenzie, D.A. Muller, E. Kravtchinskaia, D. Segal and D.J.H. Cockayne, Thin Solid Films, 206 (1991) 198. [17] X. Jiang, K. Reichelt and B. Stritzer, J. Appl. Phys., 66 (1989) 5805. [18] W. Moiler, Appl. Phys. A,56 (1993) 527. [19] J.L. Vossen and W. Kern, Thin Film Processes, Academic Press, New York, 1978, p. 56.