- Email: [email protected]
Generation of a highly uniform and dense plasma by distributing hollow cathodes on the electrode surface
SURfACE
&COATIN6S
ELSEVIER
Surface and Coatings Technology 73 (1995) 1-4
UCHNOLDGY
Generation of a highly uniform and dense plasma by distributing hollow cathodes on the electrode surface M. Sugawara a, T. Asami b • Department of Electrical Engineering. H achinohe Institute of Technology. H achinohe. Aomori-ken, 031, Japan b Aiwa Co. Ltd.• Daitoku, Ikenohata, Tokyo, 110. Japan
Received 10 August 1994
Abstract A highly uniform and dense plasma was successfully produced in a parallel-plate reactor with a uniformity of ±2%. The approach is based on modifying the distribution of the current density over the flat electrode surface by incorporating hollow cathodes in order to compensate for the non-uniform part of the current distribution. Experiments were carried out in a vacuum chamber of outer diameter 300 mm with two parallel electrodes (200 mm in diameter, AI) separated by 35 mm, one of which is a hollow-cathode type. The working gas pressure ranged from 0.01 to 0.2 Torr (l Torr = 133.3 Pa) of Ar. The radial ion current distribution was measured at distances of 10, 20 and 30 mm from the surface of the hollow cathode using a negatively biased double probe. Keywords: r.f. discharge; Hollow cathode discharge; Glow discharge
1. Introduction In recent years, there has been growing interest in the subject of plasma-surface interactions, such as plasma etching, plasma deposition and surface modification of materials, resulting from the bombardment of exposed surfaces by active ionic and neutral species. In a d.c. glow discharge, in principle, the abnormal glow regime ensures the uniformity of the current distri• bution over the electrode surface because the whole surface is covered with the glow. However, the uniformity in the radial direction is often degraded by the presence of so-called edge effects and the flow pattern of the operating gas. In an r.f. glow discharge, in addition to these factors, a.c. current conduction through the insulat• ing surface which often surrounds the periphery of the electrode or chamber wall affects the uniformity. The aim of this paper is to demonstrate techniques capable of improving the uniformity while at the same time producing a high-density plasma. A d.c. hollow• cathode discharge, when operated at low pressures, exhibits a considerable increase in the discharge current, the sustaining voltage remaining constant. This hollow• cathode effect has been observed to give a current increase of up to 900-fold [1-4]. A further current enhancement will take place if the two parallel plates are replaced by a hollow cylinder.
I
0257-8972/95/$09.5001995 Elsevier Science SA All rights reserved SSDI 0257 -8972 (94 )02362-X
Therefore, the d.-powered hollow-cathode discharge is a discharge source capable of producing high-density plasmas. When the hollow cathodes are distributed over the flat plate, the current-density distribution can be controlled by the distribution of the hollow cathQdes. 2. Experimental Experiments were carried out in a cylindrical chamber of outer diameter 300 mm with two parallel plates (200 mm in diameter, AI) separated by 35 mm, one of which is a hollow-cathode type. The chamber is shown schematically in Fig. 1. The chamber is constructed of type 304 stamless steel. The d. power (13.56 MHz) is supplied through a matching unit to the powere? electrode while the bottom electrode is grounded, electn• cally connected to the chamber. Probe access is through a port located approximately midway between the electrodes. Pressure is maintained constant using a but• terfly valve and a gas-flow controller and is measured by a Baratron gauge. The double probe construction is shown in Fig. 2. Two identical probes forming a double probe are used. Each probe consists of a nickel wire insulated by an aluminium porcelain sheath running the full length of the probe and terminated with tungsten wire (OJ mm diameter, 2 mm length). The two probes are sleeved in a stainless-steel tube (15 mm diameter). The two probes
2
M. Sugawara. T. AsamifSurface and Coatings Technology 73 (1995) 1-4
R.F. Power Supply
i
Probe Driving Circuit
Double Probe
Grounded Electrode
Fig. 1. Schematic diagram of the discharge chamber.
reF Flange(34mrn $) BNC-Connector
Stainless Tube(15mm $) Tenon
Tungsten WIfe(O 3mm~) AJummwn
~~I~~~porce~~ ~ NIckel wire
50mm
Fig. 2. Construction of the double probe. Probe tips (tungsten) are 0.3 mm diameter and 2 mm long, separated by 1 mm.
are arranged to lie on an equal potential surface. For bias voltages of 40 V and above applied between the two probes, the probe current is saturated and therefore the variation of the probe current indicates the distribu• tion of the ion current density. The time-average of the ion saturation current using +40 V bias was recorded on an X- Y recorder as a function of the radial position. The experimental circuit is shown in Fig. 3. R F.Power (1356MHz)
The hollow-cathode discharges were sustained in verti• cal grooves which were machined into a thick disc and were formed in a circular shape. The construction of the cathode is shown in Fig. 4. Grooves of equal width and depth are made co-axial with respect to the axis of the disc. Three groove widths (5, 7.4 and 10 mm) were employed. It is generally believed that there is an opti• mum value of the parameter pw, where w is the groove
S-laS (+40V)
gl
--:::l.-
Low Pass
FLiter
..
-e::J
-
lKQ
-.-
y
x-v Recorder
1
x
YII~ Fig. 3. The double probe circuit. Ion saturation currents are recorded on an X-Y recorder in conjunction with the radial position.
3
M. Sugawara, T. Asami/Surface and Coatings Technology 73 (1995) 1-4
40
~O~.~0~6TT~o~r::Ir:------==::::::::-_0 .08Torr
O.lTorr 0.12Torr 0.16Torr 0.2Torr ~o
0 ' - - - - - - - - - 5.....0 -------"":'1"="'"00
Fig. 4. Construction of the hollow-cathode type electrode. Groove width, 5 rom. Uniformly distributed grooves of equal depth. Dimensions in mm.
Center
Distance from the center (mm)
Edge
Fig. 5. Measurements of the radial ion current density. X = 10 mm; rf power, 30 W; width, 5 mm.
width and p the operating pressure. At this point the two separate negative glows are about to coalesce. Therefore, three combinations of width and operating pressure were examined. A typical radial distribution of the ion current across a surface 10 rom from the cathode is shown in Fig. 5.
Without compensalion(X-lOmm)
20
With compensation(X-lOmm)
3. Results and discussion Results taken at w=5 mm and p=O.l3 Torr (1 Torr= 133.3 Pa) of Ar are shown in Fig. 6, where at the bottom a cross-sectional view of the electrode is shown. The position X is measured from the top surface of the electrode. It can be seen that around the periphery of the disc electrode a dense plasma is formed. In order to flatten the radial distribution, the high edge density should be suppressed and at the same time the lower central density should be enhanced. The depression and enhancement of the density can be carried out either by diminishing or enhancing the hollow-cathode effect. Enhancement and depression of the bollow-cathode effect were controlled by modifying the depth of the grooves. At the bottom of the figure, the compensated distribution of the groove depth is shown. The compen• sated grooves produced a uniformity of + 1.7%, as shown in Fig. 7. Similarly, for the case of 7.4 and 10 mm widths, uniformities of + 1.8% and +2.9% were obtained, respectively.
l' ~
15 .........- •
c:
Without compensaliol1(X"20mm)
~
~
~
U
With c[mpensation(X-20mm)
c:
.~
E
10
~
Without compensation(X-JOlllln)
Uniformity; ±l.7"/o ~ __________________-t_______ • ------f------ -----·f------- -- -1With compel1sation(X"'JOl1un)
If.)
..s 5
Distance from the center Fig. 6. Compensated distribution of the radial ion current density. d. power, 30 W; width,S mm.
4
M. Sugawara. T. AsamijSurface and Coatings Technology 73 (1995) 1-4
It was found experimentally that a hollow cathode employing a larger w results in lower uniformity. It is suspected that in lower pressure operation, the rippled density distribution which is produced by the hollow• cathode discharge extends to greater distances from the cathode because of the longer mean free path.
References [1] A. Guntherschultze, Z. Tech. Phys., 11 (1930) 49. [2] S. Caroli (Ed.), Improved Hollow CarhOOe Lamps for Aromic Specrroscopy. Ellis Horwood, Chichester, 1985. [3] A. van Engel, Ionized Gases, Oxford University Press, Oxford, 1965. [4] P.F. Little and A. von EngeL Proc. R. Soc. London So. A, 224 (1954) 209.
&COATIN6S
ELSEVIER
Surface and Coatings Technology 73 (1995) 1-4
UCHNOLDGY
Generation of a highly uniform and dense plasma by distributing hollow cathodes on the electrode surface M. Sugawara a, T. Asami b • Department of Electrical Engineering. H achinohe Institute of Technology. H achinohe. Aomori-ken, 031, Japan b Aiwa Co. Ltd.• Daitoku, Ikenohata, Tokyo, 110. Japan
Received 10 August 1994
Abstract A highly uniform and dense plasma was successfully produced in a parallel-plate reactor with a uniformity of ±2%. The approach is based on modifying the distribution of the current density over the flat electrode surface by incorporating hollow cathodes in order to compensate for the non-uniform part of the current distribution. Experiments were carried out in a vacuum chamber of outer diameter 300 mm with two parallel electrodes (200 mm in diameter, AI) separated by 35 mm, one of which is a hollow-cathode type. The working gas pressure ranged from 0.01 to 0.2 Torr (l Torr = 133.3 Pa) of Ar. The radial ion current distribution was measured at distances of 10, 20 and 30 mm from the surface of the hollow cathode using a negatively biased double probe. Keywords: r.f. discharge; Hollow cathode discharge; Glow discharge
1. Introduction In recent years, there has been growing interest in the subject of plasma-surface interactions, such as plasma etching, plasma deposition and surface modification of materials, resulting from the bombardment of exposed surfaces by active ionic and neutral species. In a d.c. glow discharge, in principle, the abnormal glow regime ensures the uniformity of the current distri• bution over the electrode surface because the whole surface is covered with the glow. However, the uniformity in the radial direction is often degraded by the presence of so-called edge effects and the flow pattern of the operating gas. In an r.f. glow discharge, in addition to these factors, a.c. current conduction through the insulat• ing surface which often surrounds the periphery of the electrode or chamber wall affects the uniformity. The aim of this paper is to demonstrate techniques capable of improving the uniformity while at the same time producing a high-density plasma. A d.c. hollow• cathode discharge, when operated at low pressures, exhibits a considerable increase in the discharge current, the sustaining voltage remaining constant. This hollow• cathode effect has been observed to give a current increase of up to 900-fold [1-4]. A further current enhancement will take place if the two parallel plates are replaced by a hollow cylinder.
I
0257-8972/95/$09.5001995 Elsevier Science SA All rights reserved SSDI 0257 -8972 (94 )02362-X
Therefore, the d.-powered hollow-cathode discharge is a discharge source capable of producing high-density plasmas. When the hollow cathodes are distributed over the flat plate, the current-density distribution can be controlled by the distribution of the hollow cathQdes. 2. Experimental Experiments were carried out in a cylindrical chamber of outer diameter 300 mm with two parallel plates (200 mm in diameter, AI) separated by 35 mm, one of which is a hollow-cathode type. The chamber is shown schematically in Fig. 1. The chamber is constructed of type 304 stamless steel. The d. power (13.56 MHz) is supplied through a matching unit to the powere? electrode while the bottom electrode is grounded, electn• cally connected to the chamber. Probe access is through a port located approximately midway between the electrodes. Pressure is maintained constant using a but• terfly valve and a gas-flow controller and is measured by a Baratron gauge. The double probe construction is shown in Fig. 2. Two identical probes forming a double probe are used. Each probe consists of a nickel wire insulated by an aluminium porcelain sheath running the full length of the probe and terminated with tungsten wire (OJ mm diameter, 2 mm length). The two probes are sleeved in a stainless-steel tube (15 mm diameter). The two probes
2
M. Sugawara. T. AsamifSurface and Coatings Technology 73 (1995) 1-4
R.F. Power Supply
i
Probe Driving Circuit
Double Probe
Grounded Electrode
Fig. 1. Schematic diagram of the discharge chamber.
reF Flange(34mrn $) BNC-Connector
Stainless Tube(15mm $) Tenon
Tungsten WIfe(O 3mm~) AJummwn
~~I~~~porce~~ ~ NIckel wire
50mm
Fig. 2. Construction of the double probe. Probe tips (tungsten) are 0.3 mm diameter and 2 mm long, separated by 1 mm.
are arranged to lie on an equal potential surface. For bias voltages of 40 V and above applied between the two probes, the probe current is saturated and therefore the variation of the probe current indicates the distribu• tion of the ion current density. The time-average of the ion saturation current using +40 V bias was recorded on an X- Y recorder as a function of the radial position. The experimental circuit is shown in Fig. 3. R F.Power (1356MHz)
The hollow-cathode discharges were sustained in verti• cal grooves which were machined into a thick disc and were formed in a circular shape. The construction of the cathode is shown in Fig. 4. Grooves of equal width and depth are made co-axial with respect to the axis of the disc. Three groove widths (5, 7.4 and 10 mm) were employed. It is generally believed that there is an opti• mum value of the parameter pw, where w is the groove
S-laS (+40V)
gl
--:::l.-
Low Pass
FLiter
..
-e::J
-
lKQ
-.-
y
x-v Recorder
1
x
YII~ Fig. 3. The double probe circuit. Ion saturation currents are recorded on an X-Y recorder in conjunction with the radial position.
3
M. Sugawara, T. Asami/Surface and Coatings Technology 73 (1995) 1-4
40
~O~.~0~6TT~o~r::Ir:------==::::::::-_0 .08Torr
O.lTorr 0.12Torr 0.16Torr 0.2Torr ~o
0 ' - - - - - - - - - 5.....0 -------"":'1"="'"00
Fig. 4. Construction of the hollow-cathode type electrode. Groove width, 5 rom. Uniformly distributed grooves of equal depth. Dimensions in mm.
Center
Distance from the center (mm)
Edge
Fig. 5. Measurements of the radial ion current density. X = 10 mm; rf power, 30 W; width, 5 mm.
width and p the operating pressure. At this point the two separate negative glows are about to coalesce. Therefore, three combinations of width and operating pressure were examined. A typical radial distribution of the ion current across a surface 10 rom from the cathode is shown in Fig. 5.
Without compensalion(X-lOmm)
20
With compensation(X-lOmm)
3. Results and discussion Results taken at w=5 mm and p=O.l3 Torr (1 Torr= 133.3 Pa) of Ar are shown in Fig. 6, where at the bottom a cross-sectional view of the electrode is shown. The position X is measured from the top surface of the electrode. It can be seen that around the periphery of the disc electrode a dense plasma is formed. In order to flatten the radial distribution, the high edge density should be suppressed and at the same time the lower central density should be enhanced. The depression and enhancement of the density can be carried out either by diminishing or enhancing the hollow-cathode effect. Enhancement and depression of the bollow-cathode effect were controlled by modifying the depth of the grooves. At the bottom of the figure, the compensated distribution of the groove depth is shown. The compen• sated grooves produced a uniformity of + 1.7%, as shown in Fig. 7. Similarly, for the case of 7.4 and 10 mm widths, uniformities of + 1.8% and +2.9% were obtained, respectively.
l' ~
15 .........- •
c:
Without compensaliol1(X"20mm)
~
~
~
U
With c[mpensation(X-20mm)
c:
.~
E
10
~
Without compensation(X-JOlllln)
Uniformity; ±l.7"/o ~ __________________-t_______ • ------f------ -----·f------- -- -1With compel1sation(X"'JOl1un)
If.)
..s 5
Distance from the center Fig. 6. Compensated distribution of the radial ion current density. d. power, 30 W; width,S mm.
4
M. Sugawara. T. AsamijSurface and Coatings Technology 73 (1995) 1-4
It was found experimentally that a hollow cathode employing a larger w results in lower uniformity. It is suspected that in lower pressure operation, the rippled density distribution which is produced by the hollow• cathode discharge extends to greater distances from the cathode because of the longer mean free path.
References [1] A. Guntherschultze, Z. Tech. Phys., 11 (1930) 49. [2] S. Caroli (Ed.), Improved Hollow CarhOOe Lamps for Aromic Specrroscopy. Ellis Horwood, Chichester, 1985. [3] A. van Engel, Ionized Gases, Oxford University Press, Oxford, 1965. [4] P.F. Little and A. von EngeL Proc. R. Soc. London So. A, 224 (1954) 209.