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Magnetically enhanced inductively coupled plasma CVD for a-Si:H fabrication
Pergamon PII: 80042-207X(96)00241-2
Vacuum/volume 48/number l/pages 1 to 611997 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/97 $17.00+.00
Magnetically enhanced inductively coupled plasma CVD for a-Si:H fabrication* M Murata, Y Takeuchi and S Nishida, Nagasaki Research & Development Ltd., Fukahori, Nagasaki, 85 7-03 Japan received
20 September
Center, Mitsubishi Heavy Industries,
7996
Radio-frequency (73.56 MHz) excited SiH, plasma is produced with an electrode of a ladder-shaped antenna with rotating magnetic fields. The negative self-bias potential on the electrode, the SiH emission intensity and the deposition rate of a-Si:H films prepared on glass substrates are examined as a function of RF power for different rotating magnetic fields. It is found that when the rotating magnetic fields are applied to the SiH, plasma, the negative self-bias potential becomes low, while the SiH emission intensity and the deposition rate increase. It is also found that the higher the RF power, the better the uniformity of the a-Si:H film, which is desirable for the fabrication of solar cells and thin film transistors. Furthermore, the effect of rotating magnetic fields are discussed. It is found that drift motion plays an important role in determining increases in the deposition rate of a-Si:H films. Copyright 0 1996 Elsevier Science Ltd
Introduction There has been great interest in plasma CVD for fabrication of amorphous hydrogenerated silicon (a-Si:H) in the field of solar cells and thin film transistors (TFT). In particular, high speed deposition rates and uniformity of large-area thin films are needed in such a case. Usually, two plane-parallel electrodes discharge plasmas have been used so far. As is well known, the fundamental properties of a-Si:H films depend on various deposition parameters such as RF power, self-bias potential on the powered electrode, gas-flow distribution, gas-pressure, electrode spacing, substrate temperature and so on.‘-’ Thus, in specifying the design and operation of the RF plasma CVD systems, knowledge of optimization of the deposition parameters is extremely important. However, there are still many problems to be solved in optimization of techniques for the purpose of mass production of large-area and high-quality a-Si:H films prepared under conditions of high deposition rate. To respond to the needs of mass production, various methods have been proposed. Although the horizontal plasma methods is able to prepare a-Si:H films of large size, the films are not uniform because of the inhomogeneous structure of RF plasma. The scanning plasma method using a modulated magnetic field6 is also able to prepare large-sized a-Si:H films, but the deposition rate is low because of magnetic field intensity varying with time. From the point of view of engineering research, Martins et aL7 have
*This paper is a revised version of that presented to Conference on ‘Advances in Materials & Processing Technology 95’, Dublin, August 1995.
investigated large-area a-Si:H films, but the uniformity of the films is inadequate. Recently, inductively coupled plasmas8-‘* have been used in plasma processing such as CVD and etching because of large area and high electron density at low pressure. Hopwood pointed out that large area plasma can be generated using helical and spiral shaped inductive coupling circuit elements which are either external to or immersed in the discharge. Shirakawa et al.” succeeded in the production and control of a loop antenna inductive type RF plasma at very low pressures with the use of permanent magnet arrays placed on discharge vessel walls. However, there are still many problems to be improved such as process development for industrial applications and plasma characterization of theoretical modeling. We have developed a novel CVD device using an electrode of a ladder-shaped antenna with rotating magnetic fields, in order to get high-speed deposition rates and uniformity of large-area a-Si:H films. In this paper, we present the experimental results on properties of the new type of CVD device and on the deposition rate and uniformity of a-Si:H films deposited under various conditions.
Experimental details The plasma CVD system employed in this work is shown in Figure 1. The system consists of a new type of RF electrodeI and two sets of magnetic coils. A stainless steel rectangular solid vacuum chamber, 600mm wide 600 mm high and 400 mm deep, is electrically grounded. A schematic diagram of this new type of electrode is shown in 1
M Murata eta/: Magnetically
enhanced inductively coupled plasma CVD /Magnetic Field by # 2 Magnetic Coils
,I
Magnetic Field by # 1 Magnetic Coils
Ladder - Shapped Antenna Electrode
Figure 3. Configuration of magnetic coils for plasma confinement. The rotating magnetic field B generated by two sets of magnetic coils is sinusoidally modulated at 0.1 Hz in the range from zero to 60 gauss.
SiH4 Gas
Figure 1. Schematic of the experimental set up
Figure 2. The electrode consists of a stainless steel ladder-shaped antenna, 422 mm long and 422 mm wide (17 rods of 6mm in diameter, spacing 20 mm). The heater is made of a stainless steel rectangular plate, 600mm long 500mm wide and 50mm thick. The two sets of magnetic coils are shown in Figure 3, which indicates that the sets of magnetic coils 800 mm in diameter are placed at the left, right, top and bottom of the chamber, spaced 1100 mm apart, respectively. In order to produce magnetic fields, two sinusoidal varying currents generated with a phase-variable
_ Gas Mixing
Box
Ladder - Shapped ‘Antenna Electrode
oscillator and two power amplifiers were fed to two sets of magnetic coils. In addition, phase control between a magnetic field B, and a magnetic field B2 was performed by adjustment of phase between two coil currents, so that a rotating magnetic field B becomes the summation of vectors 8, and B,. Pure SiH, gases were introduced to the chamber. The experiments were carried out at a pressure of 50 mTorr and a gas-flow rate of 50 seem. The RF (13.56 MHz) power was applied to two loading points at a corner of the ladder-shaped antenna electrode via a conventional matching fox and was varied between 20 and 100 W. Magnetic fields, modulated at 0.1 Hz, were applied to the chamber parallel to the grounded electrode, that is the magnetic fields are perpendicular to the RF electric field. Here, the RF voltage applied to the electrode was monitored by using a calibrated voltage probe. Also, the RF current of the electrode was measured by a current probe (Pearson Electronics, Model 2878, 70 MHz bandwidth). The self-bias potential Vh on the electrode is estimated from the relationship V, = (V, + V,)/2, where V, and V, are the maximum and minimum values of the observed potential forms, respectively. Corning # 7059 glass, 300 mm x 300 mm, was used as substrate. The substrates were deposited under the conditions shown in Table 1. In addition, the thickness of the s-Si:H films was measured with a spectrophotometer. Optical emissions from the SiH, plasma were measured using a spectroscopic method, as usual.
Table 1. Experimental
2
of ladder-shaped
antenna
electrode.
used
Gas material
100% SiH4
Gas pressure Gas flow rate Distance between substrate RF power Rotating magnetic field Substrate
50 mm Torr 0 seem 40 mm 2&200 w 0 or 60 gauss Corning # 7059 (300mmx300mmx1.1mm) 200 “C
Substrate Figure 2. Configuration
conditions
temperature
and electrode
M Murata et al: Magnetically
enhanced inductively coupled plasma CVD
Results and discussion
It is well known technique in plasma physics and plasma processing’“18 to employ magnetic fields to confine electrons in glow discharge plasma, and we have used this for plasma confinement. Figure 4 shows the spatial distribution of rotating magnetic fields B in the vicinity of the substrate placed on the electrode. This figure shows that the spatial distribution is a parabolic shape and the magnetic field intensity at the centre of the substrate is a minimum. Thus, electrons in the SiH4 plasma moving from the electrode to the heater are confined by the magnetic fields, and electrons in the SiHl plasma moving in the axial direction of the magnetic coils are confined by the magnetic mirror effect. In addition, the electric field E due to RF power and magnetic field B produce an E x B drift motion. The potential and current waveforms measured on the electrode for RF power of 100 W without rotating magnetic field and with rotating magnetic field are shown in Figures S(a) and (b), respectively. Although the potential and current waveforms are sinusoidal in the case of with rotating magnetic field, the potential in the case of without rotating magnetic field is distorted waveform. Such anharmonic waveforms have already been observed in other studies’ with regard to capacitively coupled discharges. A distortion of the waveform may be due to electron attachment to SiH, molecules or powders as pointed out by Biihm and Perrin.’ It was found that when the rotating magnetic field B is applied to BH, plasma in the chamber, the applied peak-to-peak potential V,, on the ladder-shaped-electrodes increases. Figure 6 shows the applied peak-to-peak potential V,, on the powered electrode as a function of RF power with and without rotating magnetic field. The potential Vpp increases with an increase in the RF power. The slope of VP, against the RF power in the presence of the rotating magnetic field is larger than that in the absence of a rotating magnetic field because electrons in the direction of the electrode from the heater are confined by the magnetic field. In addition, it was observed by the V and I probes shown in Figure 1 that the phase of the potential precedes the current fed to the electrode as shown in Figure 7. In this case, the SiH, plasma is regarded as a kind of inductively coupled plasma source.’
Time
(ns)
Time
(ns)
@I
Figure 5. Potential and current RF power of 100 W.
(0) (0)
waveforms
measured
on the electrode
for
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHz
150
o--.--l 0
20
40
Figure 6.
80
60
RF Power
100
120
(W)
RF potential measured on the electrode as a function of RF
power.
Figure 4. Spatial distribution
of rotating
magnetic
fields.
It was also found that when the rotating magnetic field is applied to a SM, plasma in the chamber, the reduction of the phase difference between V(t) and Z(t) indicate that the discharge becomes more resistive than that without the rotating magnetic fields. It was also found that the negative self-bias potential Vb on the electrodes is a few volts in this inductively coupled plasma 3
M Murata eta/: Magnetically
(0)
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHz
(0)
- 20b
enhanced inductively coupled plasma CVD
I
I
I
I
I
I
20
40
60
80
100
120
RF Power Figure 7. Phase difference between potential electrode as a function of RF power.
(WI and current
measured
on the
source using an electrode of a ladder-shaped antenna. Moreover when the rotating magnetic field is applied to a SiH, plasma in the chamber, the negative self-bias potential Vh on the electrodes decreases. We measured the negative self-bias potential as a function of RF power of different rotating magnetic field intensities. Figure 8 shows the dependence of the self-bias potential V,, on RF power. In the absence of the rotating magnetic field, when the RF power is increased, the bias potential Vh increases. This means that the rotating magnetic field applied to the chamber effectively suppresses the ion bombardment. Thus, it is expected that the plasma CVD system, using the rotating magnetic field, offers high-quality a-Si:H films. In contrast, in a plasma CVD system without a rotating magnetic field, the powered electrode is bombarded by positive ions accelerated by the high self-bias potential and is eroded.19 Thus, the substrates will
10 (0) (0)
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHz
be deposited by eroded material due to an ion bombardment effect, which is undesirable. Optical emission spectroscopy is widely used as a simple diagnostic tool for SiH4 plasma. Figures 9(a) and (b) show a typical example of the optical emission spectrum from the SiH, plasma without and with the rotating magnetic field, respectively. Strong emission lines of Si* (at 288.1 nm), SiH* (at 412.7nm) and Hcc (at 656.3 nm) were observed. Si* and SiH* emission intensities of Figure 9(b) are larger than those of Figure 9(a). Figure 10 shows SiH* emission intensity as a function of RF power with and without the rotating magnetic field. The emission intensity of SiH* is proportional to the RF power, with the same tendency as that reported by Tanaka and Matsuda.’ In the presence of the rotating magnetic field, a rapid rise and saturation of the SiH* emission intensity with the RF power are observed. This suggests that the deposition rate of a-Si:H films with the rotating magnetic field is higher than that without the rotating magnetic field, so that a-Si: H deposition rate is considered to be proportional to SiH* emission intensity.’ The saturation of SiH* emission intensity suggests a saturation of the a-Si: H deposition rate due to the deficiency of SiH, molecules in the plasma.iA Figure 11 shows spatial distributions of the deposition rate of a-Si:H films as a function of RF power with and without the rotating magnetic field. In this experiment, in order to avoid powder formation of SiH, plasma which is caused by the rapid reaction of radicals with parent molecules,‘~z” the gas pressure was kept at 50 mTorr. In addition, Tanaka and Matsuda ’ pointed out that the gas pressure in SiH, plasma necessary for highquality a-Si:H deposition is typically below 50 m Torr. Comparison of the spatial distribution for RF power of 100 W clearly demonstrates that the uniformity of the deposition rate is determined by the RF power and the rotating magnetic field. In
0a8’ 0.8
0.4 I
SiH*
O.11 300
400
Wave
500
600
Length
(nm)
400
500
600
Wave
Length
(nm)
700
800
700
8
5 0.8
z
SiH*
a
s
I
0.6 -
0.4 -
-5
Si* 0.2
-10'
0
20
40
60 RF Power
Figure 8. Self-bias power. 4
potential
80
100
1
0
300
(W)
V, on the electrode
as a function
of RF
Figure 9. Optical power of 100 W.
emission
spectrum
from
100% SiHl plasma
for RF
M Murara et al: Magnetically
enhanced inductively coupled plasma CVD 7-
(0) (0)
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHz
6-
(0) (a)
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHi
5-
4/ 3-
2-
l-
OO 50 Figure 10. SiH* emission
150
100 RF intensity
Power
as a function
Figure 12. Deposition of RF power.
the presence of the rotating magnetic field, the uniformity of the deposition rate is within +2% at 1OOW. In the absence of the rotating magnetic field, the uniformity of the deposition rate distribution is within + 15% at 100 W. Moreover the deposition rate and the uniformity of a-Si:H at RF power of 200 W in the presence of the rotating magnetic field is 5.67&S and within +4%, respectively. Figure 12 shows the deposition rate of a-Si:H films as a function of RF power with and without the rotating magnetic field. When the RF power is increased, the deposition rate increases in both cases. However, in the presence of the rotating magnetic
I
I
I
100
150
200
RF Power
200
(WI
I
50
magnetic
rate as a function
J
(WI
of RF power for different rotating
fields.
field, higher deposition rates with RF power are observed. This means that the Ex B drift motion in the SiH4 plasma plays an important role in determining the increase of the deposition rate of a-Si:H films. Conclusions
We have developed a new type of plasma CVD device using a ladder-shaped antenna electrode and two sets of magnetic coils. The deposition rate and the uniformity of a-Si:H deposited on a 300mm x 300mm substrate under a gas pressure of 50m Torr, RF power density of 200 W and applied magnetic field intensity of 60 gauss was 5.67 A/S and within + 4%, respectively. The rotating E x B drift motion in the SiH4 plasma is found to be useful for high-speed deposition rates and uniformity of large area a-Si:H films. It is also found that when the rotating magnetic field is applied to the SiH, plasma, the negative self-bias potential becomes low, while the SiH emission intensity and the deposition rate increase. References
: without the Rotating Magnetic Field (0) : with the Rotating Magnetic Field
CO)
GOGauss at O.lHz
8
12i :-‘_ 1 oow
/
1 B
2
2ow
6.
4ow
2ow
I
OO
I
I
I
10
20
30
Diagonal Distance
of Substrate
(cm)
Figure 11. Spatial distribution of the deposition rate of a-SiH films prepared on a 300 mm x 300 mm substrate as a function of RF power for different rotating magnetic fields. Thickness of the a-SiH films are measured with a double monochromator along the diagonal line of the substrate.
K. and Matsuda, A., Mater. Sci. Reports, 1974,2(4), 139. Btihm, C. and Perrin, J., J. Phys. D: App. Phys., 1991, 24, 865. Perrin, J., J. Non-Cryst. Solids, 1991, 137/138, 639. Knight, J.C., .I. App. Phys., 1979,8, 101, Supplement 18-l. Nishimura, K., Nakayama, T., Tsuge, K., Tawada, Y., Hirata, H. and Izumina, M., Design parameters of large-scale horizontal plasma furnace, Proc. of the 1st International Photovoltail, Science Engineering Confirence, Kobe, 1984, pp. 183. Fujiyama, H., Yamashita, T., Takahashi, T., Matsuo, H., Murata. M., Kaneko, S. and Morita, S., Trial production of large-area uniform a-Si:H films by scanning plasma method using a modulated magnetic field, Proc. of the 8th International Symposium on Plasma Chemistry, Vol. 3, 1987, pp. 1448. Martins, R., Ferreira, I., Carvalbo, N. and Guimaraes, L., J. NonCryst. Solids, 1991, 1371138, 757. Hopwood, J., Plasma Source Scient@c Technology, 1992, 1, 109. Hopwood, J., Guarnieri, CR., Whitehair, S.J. and Cuomo, J.J., J. Vat. Sci. Technol., 1993, All(l), 147. Shirakawa, T., Toyoda, H. and Sugai, H., Jup. J. App. Phys., 1990, 29(6), L1015.
1. Tanaka, 2. 3. 4. 5.
7. 8. 9.
10.
5
M Murata et al: Magnetically
enhanced inductively coupled plasma CVD
II. Turner, M.M., Phys. Reo. Let?., 1993, 71(12), 1844. 12. Amorim, J., Macil, H.S. and Sudano, J.P., J. Vat. Sci. Technol., 1991, B9(2), 362. 13. Murata, M., Takeuchi, Y., Sasagawa, E. and Hammamoto, K., Rec. Sci. Instrum., 1996, 67, 1542. 14. Chen. F.F.. Introduction to Plasma Phvsics. Plenum Press. New York, 1974. 15. Chapman, B.N., IBM Technical Disclosure Bulletin, 1979. 21(12), 5006.
16. Hamasaki, T., Hirose, M., Kurata, H., Taniguchi, M. and Osaka, Y., Jap. J. App. Phys., 1980, 20, 28 1, Supplement 20-I. 17. Nguyen, S.V., Chrisman, G., Dobuzinsky, D. and Harmon, D., Solid &ate Technology, 1990, 10, 73. 18. Murata, M., Takeuchi, Y. and Kojyo, D., Surf. Coat. Technol., 1995, 74-75, 195. 19. Horwitz, C.M., J. Vat. Sci. Technol., 1983, Al(l), 60. 20. Watanabe, Y. and Shiratani, M., Proc. oj’Japanese Symposium on Plasma Chemistry, 1992, 5, 85.
Vacuum/volume 48/number l/pages 1 to 611997 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/97 $17.00+.00
Magnetically enhanced inductively coupled plasma CVD for a-Si:H fabrication* M Murata, Y Takeuchi and S Nishida, Nagasaki Research & Development Ltd., Fukahori, Nagasaki, 85 7-03 Japan received
20 September
Center, Mitsubishi Heavy Industries,
7996
Radio-frequency (73.56 MHz) excited SiH, plasma is produced with an electrode of a ladder-shaped antenna with rotating magnetic fields. The negative self-bias potential on the electrode, the SiH emission intensity and the deposition rate of a-Si:H films prepared on glass substrates are examined as a function of RF power for different rotating magnetic fields. It is found that when the rotating magnetic fields are applied to the SiH, plasma, the negative self-bias potential becomes low, while the SiH emission intensity and the deposition rate increase. It is also found that the higher the RF power, the better the uniformity of the a-Si:H film, which is desirable for the fabrication of solar cells and thin film transistors. Furthermore, the effect of rotating magnetic fields are discussed. It is found that drift motion plays an important role in determining increases in the deposition rate of a-Si:H films. Copyright 0 1996 Elsevier Science Ltd
Introduction There has been great interest in plasma CVD for fabrication of amorphous hydrogenerated silicon (a-Si:H) in the field of solar cells and thin film transistors (TFT). In particular, high speed deposition rates and uniformity of large-area thin films are needed in such a case. Usually, two plane-parallel electrodes discharge plasmas have been used so far. As is well known, the fundamental properties of a-Si:H films depend on various deposition parameters such as RF power, self-bias potential on the powered electrode, gas-flow distribution, gas-pressure, electrode spacing, substrate temperature and so on.‘-’ Thus, in specifying the design and operation of the RF plasma CVD systems, knowledge of optimization of the deposition parameters is extremely important. However, there are still many problems to be solved in optimization of techniques for the purpose of mass production of large-area and high-quality a-Si:H films prepared under conditions of high deposition rate. To respond to the needs of mass production, various methods have been proposed. Although the horizontal plasma methods is able to prepare a-Si:H films of large size, the films are not uniform because of the inhomogeneous structure of RF plasma. The scanning plasma method using a modulated magnetic field6 is also able to prepare large-sized a-Si:H films, but the deposition rate is low because of magnetic field intensity varying with time. From the point of view of engineering research, Martins et aL7 have
*This paper is a revised version of that presented to Conference on ‘Advances in Materials & Processing Technology 95’, Dublin, August 1995.
investigated large-area a-Si:H films, but the uniformity of the films is inadequate. Recently, inductively coupled plasmas8-‘* have been used in plasma processing such as CVD and etching because of large area and high electron density at low pressure. Hopwood pointed out that large area plasma can be generated using helical and spiral shaped inductive coupling circuit elements which are either external to or immersed in the discharge. Shirakawa et al.” succeeded in the production and control of a loop antenna inductive type RF plasma at very low pressures with the use of permanent magnet arrays placed on discharge vessel walls. However, there are still many problems to be improved such as process development for industrial applications and plasma characterization of theoretical modeling. We have developed a novel CVD device using an electrode of a ladder-shaped antenna with rotating magnetic fields, in order to get high-speed deposition rates and uniformity of large-area a-Si:H films. In this paper, we present the experimental results on properties of the new type of CVD device and on the deposition rate and uniformity of a-Si:H films deposited under various conditions.
Experimental details The plasma CVD system employed in this work is shown in Figure 1. The system consists of a new type of RF electrodeI and two sets of magnetic coils. A stainless steel rectangular solid vacuum chamber, 600mm wide 600 mm high and 400 mm deep, is electrically grounded. A schematic diagram of this new type of electrode is shown in 1
M Murata eta/: Magnetically
enhanced inductively coupled plasma CVD /Magnetic Field by # 2 Magnetic Coils
,I
Magnetic Field by # 1 Magnetic Coils
Ladder - Shapped Antenna Electrode
Figure 3. Configuration of magnetic coils for plasma confinement. The rotating magnetic field B generated by two sets of magnetic coils is sinusoidally modulated at 0.1 Hz in the range from zero to 60 gauss.
SiH4 Gas
Figure 1. Schematic of the experimental set up
Figure 2. The electrode consists of a stainless steel ladder-shaped antenna, 422 mm long and 422 mm wide (17 rods of 6mm in diameter, spacing 20 mm). The heater is made of a stainless steel rectangular plate, 600mm long 500mm wide and 50mm thick. The two sets of magnetic coils are shown in Figure 3, which indicates that the sets of magnetic coils 800 mm in diameter are placed at the left, right, top and bottom of the chamber, spaced 1100 mm apart, respectively. In order to produce magnetic fields, two sinusoidal varying currents generated with a phase-variable
_ Gas Mixing
Box
Ladder - Shapped ‘Antenna Electrode
oscillator and two power amplifiers were fed to two sets of magnetic coils. In addition, phase control between a magnetic field B, and a magnetic field B2 was performed by adjustment of phase between two coil currents, so that a rotating magnetic field B becomes the summation of vectors 8, and B,. Pure SiH, gases were introduced to the chamber. The experiments were carried out at a pressure of 50 mTorr and a gas-flow rate of 50 seem. The RF (13.56 MHz) power was applied to two loading points at a corner of the ladder-shaped antenna electrode via a conventional matching fox and was varied between 20 and 100 W. Magnetic fields, modulated at 0.1 Hz, were applied to the chamber parallel to the grounded electrode, that is the magnetic fields are perpendicular to the RF electric field. Here, the RF voltage applied to the electrode was monitored by using a calibrated voltage probe. Also, the RF current of the electrode was measured by a current probe (Pearson Electronics, Model 2878, 70 MHz bandwidth). The self-bias potential Vh on the electrode is estimated from the relationship V, = (V, + V,)/2, where V, and V, are the maximum and minimum values of the observed potential forms, respectively. Corning # 7059 glass, 300 mm x 300 mm, was used as substrate. The substrates were deposited under the conditions shown in Table 1. In addition, the thickness of the s-Si:H films was measured with a spectrophotometer. Optical emissions from the SiH, plasma were measured using a spectroscopic method, as usual.
Table 1. Experimental
2
of ladder-shaped
antenna
electrode.
used
Gas material
100% SiH4
Gas pressure Gas flow rate Distance between substrate RF power Rotating magnetic field Substrate
50 mm Torr 0 seem 40 mm 2&200 w 0 or 60 gauss Corning # 7059 (300mmx300mmx1.1mm) 200 “C
Substrate Figure 2. Configuration
conditions
temperature
and electrode
M Murata et al: Magnetically
enhanced inductively coupled plasma CVD
Results and discussion
It is well known technique in plasma physics and plasma processing’“18 to employ magnetic fields to confine electrons in glow discharge plasma, and we have used this for plasma confinement. Figure 4 shows the spatial distribution of rotating magnetic fields B in the vicinity of the substrate placed on the electrode. This figure shows that the spatial distribution is a parabolic shape and the magnetic field intensity at the centre of the substrate is a minimum. Thus, electrons in the SiH4 plasma moving from the electrode to the heater are confined by the magnetic fields, and electrons in the SiHl plasma moving in the axial direction of the magnetic coils are confined by the magnetic mirror effect. In addition, the electric field E due to RF power and magnetic field B produce an E x B drift motion. The potential and current waveforms measured on the electrode for RF power of 100 W without rotating magnetic field and with rotating magnetic field are shown in Figures S(a) and (b), respectively. Although the potential and current waveforms are sinusoidal in the case of with rotating magnetic field, the potential in the case of without rotating magnetic field is distorted waveform. Such anharmonic waveforms have already been observed in other studies’ with regard to capacitively coupled discharges. A distortion of the waveform may be due to electron attachment to SiH, molecules or powders as pointed out by Biihm and Perrin.’ It was found that when the rotating magnetic field B is applied to BH, plasma in the chamber, the applied peak-to-peak potential V,, on the ladder-shaped-electrodes increases. Figure 6 shows the applied peak-to-peak potential V,, on the powered electrode as a function of RF power with and without rotating magnetic field. The potential Vpp increases with an increase in the RF power. The slope of VP, against the RF power in the presence of the rotating magnetic field is larger than that in the absence of a rotating magnetic field because electrons in the direction of the electrode from the heater are confined by the magnetic field. In addition, it was observed by the V and I probes shown in Figure 1 that the phase of the potential precedes the current fed to the electrode as shown in Figure 7. In this case, the SiH, plasma is regarded as a kind of inductively coupled plasma source.’
Time
(ns)
Time
(ns)
@I
Figure 5. Potential and current RF power of 100 W.
(0) (0)
waveforms
measured
on the electrode
for
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHz
150
o--.--l 0
20
40
Figure 6.
80
60
RF Power
100
120
(W)
RF potential measured on the electrode as a function of RF
power.
Figure 4. Spatial distribution
of rotating
magnetic
fields.
It was also found that when the rotating magnetic field is applied to a SM, plasma in the chamber, the reduction of the phase difference between V(t) and Z(t) indicate that the discharge becomes more resistive than that without the rotating magnetic fields. It was also found that the negative self-bias potential Vb on the electrodes is a few volts in this inductively coupled plasma 3
M Murata eta/: Magnetically
(0)
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHz
(0)
- 20b
enhanced inductively coupled plasma CVD
I
I
I
I
I
I
20
40
60
80
100
120
RF Power Figure 7. Phase difference between potential electrode as a function of RF power.
(WI and current
measured
on the
source using an electrode of a ladder-shaped antenna. Moreover when the rotating magnetic field is applied to a SiH, plasma in the chamber, the negative self-bias potential Vh on the electrodes decreases. We measured the negative self-bias potential as a function of RF power of different rotating magnetic field intensities. Figure 8 shows the dependence of the self-bias potential V,, on RF power. In the absence of the rotating magnetic field, when the RF power is increased, the bias potential Vh increases. This means that the rotating magnetic field applied to the chamber effectively suppresses the ion bombardment. Thus, it is expected that the plasma CVD system, using the rotating magnetic field, offers high-quality a-Si:H films. In contrast, in a plasma CVD system without a rotating magnetic field, the powered electrode is bombarded by positive ions accelerated by the high self-bias potential and is eroded.19 Thus, the substrates will
10 (0) (0)
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHz
be deposited by eroded material due to an ion bombardment effect, which is undesirable. Optical emission spectroscopy is widely used as a simple diagnostic tool for SiH4 plasma. Figures 9(a) and (b) show a typical example of the optical emission spectrum from the SiH, plasma without and with the rotating magnetic field, respectively. Strong emission lines of Si* (at 288.1 nm), SiH* (at 412.7nm) and Hcc (at 656.3 nm) were observed. Si* and SiH* emission intensities of Figure 9(b) are larger than those of Figure 9(a). Figure 10 shows SiH* emission intensity as a function of RF power with and without the rotating magnetic field. The emission intensity of SiH* is proportional to the RF power, with the same tendency as that reported by Tanaka and Matsuda.’ In the presence of the rotating magnetic field, a rapid rise and saturation of the SiH* emission intensity with the RF power are observed. This suggests that the deposition rate of a-Si:H films with the rotating magnetic field is higher than that without the rotating magnetic field, so that a-Si: H deposition rate is considered to be proportional to SiH* emission intensity.’ The saturation of SiH* emission intensity suggests a saturation of the a-Si: H deposition rate due to the deficiency of SiH, molecules in the plasma.iA Figure 11 shows spatial distributions of the deposition rate of a-Si:H films as a function of RF power with and without the rotating magnetic field. In this experiment, in order to avoid powder formation of SiH, plasma which is caused by the rapid reaction of radicals with parent molecules,‘~z” the gas pressure was kept at 50 mTorr. In addition, Tanaka and Matsuda ’ pointed out that the gas pressure in SiH, plasma necessary for highquality a-Si:H deposition is typically below 50 m Torr. Comparison of the spatial distribution for RF power of 100 W clearly demonstrates that the uniformity of the deposition rate is determined by the RF power and the rotating magnetic field. In
0a8’ 0.8
0.4 I
SiH*
O.11 300
400
Wave
500
600
Length
(nm)
400
500
600
Wave
Length
(nm)
700
800
700
8
5 0.8
z
SiH*
a
s
I
0.6 -
0.4 -
-5
Si* 0.2
-10'
0
20
40
60 RF Power
Figure 8. Self-bias power. 4
potential
80
100
1
0
300
(W)
V, on the electrode
as a function
of RF
Figure 9. Optical power of 100 W.
emission
spectrum
from
100% SiHl plasma
for RF
M Murara et al: Magnetically
enhanced inductively coupled plasma CVD 7-
(0) (0)
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHz
6-
(0) (a)
: without the Rotating Magnetic Field : with the Rotating Magnetic Field GOGauss at O.lHi
5-
4/ 3-
2-
l-
OO 50 Figure 10. SiH* emission
150
100 RF intensity
Power
as a function
Figure 12. Deposition of RF power.
the presence of the rotating magnetic field, the uniformity of the deposition rate is within +2% at 1OOW. In the absence of the rotating magnetic field, the uniformity of the deposition rate distribution is within + 15% at 100 W. Moreover the deposition rate and the uniformity of a-Si:H at RF power of 200 W in the presence of the rotating magnetic field is 5.67&S and within +4%, respectively. Figure 12 shows the deposition rate of a-Si:H films as a function of RF power with and without the rotating magnetic field. When the RF power is increased, the deposition rate increases in both cases. However, in the presence of the rotating magnetic
I
I
I
100
150
200
RF Power
200
(WI
I
50
magnetic
rate as a function
J
(WI
of RF power for different rotating
fields.
field, higher deposition rates with RF power are observed. This means that the Ex B drift motion in the SiH4 plasma plays an important role in determining the increase of the deposition rate of a-Si:H films. Conclusions
We have developed a new type of plasma CVD device using a ladder-shaped antenna electrode and two sets of magnetic coils. The deposition rate and the uniformity of a-Si:H deposited on a 300mm x 300mm substrate under a gas pressure of 50m Torr, RF power density of 200 W and applied magnetic field intensity of 60 gauss was 5.67 A/S and within + 4%, respectively. The rotating E x B drift motion in the SiH4 plasma is found to be useful for high-speed deposition rates and uniformity of large area a-Si:H films. It is also found that when the rotating magnetic field is applied to the SiH, plasma, the negative self-bias potential becomes low, while the SiH emission intensity and the deposition rate increase. References
: without the Rotating Magnetic Field (0) : with the Rotating Magnetic Field
CO)
GOGauss at O.lHz
8
12i :-‘_ 1 oow
/
1 B
2
2ow
6.
4ow
2ow
I
OO
I
I
I
10
20
30
Diagonal Distance
of Substrate
(cm)
Figure 11. Spatial distribution of the deposition rate of a-SiH films prepared on a 300 mm x 300 mm substrate as a function of RF power for different rotating magnetic fields. Thickness of the a-SiH films are measured with a double monochromator along the diagonal line of the substrate.
K. and Matsuda, A., Mater. Sci. Reports, 1974,2(4), 139. Btihm, C. and Perrin, J., J. Phys. D: App. Phys., 1991, 24, 865. Perrin, J., J. Non-Cryst. Solids, 1991, 137/138, 639. Knight, J.C., .I. App. Phys., 1979,8, 101, Supplement 18-l. Nishimura, K., Nakayama, T., Tsuge, K., Tawada, Y., Hirata, H. and Izumina, M., Design parameters of large-scale horizontal plasma furnace, Proc. of the 1st International Photovoltail, Science Engineering Confirence, Kobe, 1984, pp. 183. Fujiyama, H., Yamashita, T., Takahashi, T., Matsuo, H., Murata. M., Kaneko, S. and Morita, S., Trial production of large-area uniform a-Si:H films by scanning plasma method using a modulated magnetic field, Proc. of the 8th International Symposium on Plasma Chemistry, Vol. 3, 1987, pp. 1448. Martins, R., Ferreira, I., Carvalbo, N. and Guimaraes, L., J. NonCryst. Solids, 1991, 1371138, 757. Hopwood, J., Plasma Source Scient@c Technology, 1992, 1, 109. Hopwood, J., Guarnieri, CR., Whitehair, S.J. and Cuomo, J.J., J. Vat. Sci. Technol., 1993, All(l), 147. Shirakawa, T., Toyoda, H. and Sugai, H., Jup. J. App. Phys., 1990, 29(6), L1015.
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II. Turner, M.M., Phys. Reo. Let?., 1993, 71(12), 1844. 12. Amorim, J., Macil, H.S. and Sudano, J.P., J. Vat. Sci. Technol., 1991, B9(2), 362. 13. Murata, M., Takeuchi, Y., Sasagawa, E. and Hammamoto, K., Rec. Sci. Instrum., 1996, 67, 1542. 14. Chen. F.F.. Introduction to Plasma Phvsics. Plenum Press. New York, 1974. 15. Chapman, B.N., IBM Technical Disclosure Bulletin, 1979. 21(12), 5006.
16. Hamasaki, T., Hirose, M., Kurata, H., Taniguchi, M. and Osaka, Y., Jap. J. App. Phys., 1980, 20, 28 1, Supplement 20-I. 17. Nguyen, S.V., Chrisman, G., Dobuzinsky, D. and Harmon, D., Solid &ate Technology, 1990, 10, 73. 18. Murata, M., Takeuchi, Y. and Kojyo, D., Surf. Coat. Technol., 1995, 74-75, 195. 19. Horwitz, C.M., J. Vat. Sci. Technol., 1983, Al(l), 60. 20. Watanabe, Y. and Shiratani, M., Proc. oj’Japanese Symposium on Plasma Chemistry, 1992, 5, 85.