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Plasma properties in a planar d.c. magnetron sputtering device
Surface and Coatings Technology, 50 (1992) 111-116
111
Plasma properties in a planar d.c. magnetron sputtering device Hu Bingsen
and Cao Zhou
Lanzhou Institute of Physics, P.O. Box 94, Lanzhou (China)
(Received March 16, 1991; accepted in final form August 19, 1991)
Abstract The plasma properties of a planar d.c. magnetron sputtering device were measured using a Langmuir probe. The spatial distribution of the plasma potential, the electron density, and the electron temperature are given for differing discharge parameters. By analysing the above distribution of properties, it was determined that the spatial distribution characteristics of plasma parameters do not change for different cathode materials. The radial distribution of random electron current in the plasma was also measured with a Langmuir probe, and the result shows that there is a drift current that is tens of times larger than the discharge current. For cathode secondary electrons, the results using measured plasma parameters of discharge current calculations are in close agreement with experimental measurements. The calculated and experimental results indicate that charge transport processes within the discharge space affect not only the discharge characteristics, but also the film properties.
I. Introduction D.c. magnetron sputtering has found wide application in thin film deposition owing. In general, films deposited by d.c. magnetron sputtering appear to have homogeneous thicknesses, high density and good adhesion. Therefore, d.c. magnetron sputtering has been widely used in the deposition of optical and metal films. The basic principles [1] and plasma processing [2, 3] of d.c. magnetron sputtering have been reported in investigations of magnetron sputtering techniques, but in some cases detailed results were not given. A planar d.c. magnetron sputtering device, designed in house, was investigated in this work. The plasma parameters and the random electron current were measured with a Langmuir probe [4], and the transport of the charged particles was studied briefly.
2. Experimental apparatus and methods 2.1. Experimental apparatus
The d.c. planar magnetron sputtering system in which the experiments were carried out is shown schematically in Fig. 1. The vacuum chamber is 400 mm in diameter and 280 mm in length. A negative d.c. voltage is applied to the cathode, the chamber is used as a grounded anode, and another anode which is annular in shape is also used to generate a glow discharge. Three different kinds of gas can be fed into the vacuum chamber
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Fig. 1. Schematic diagram of the experimental apparatus. through the bleed valve. The total pressure is measured by both a thermocouple gauge and an ionization gauge. The Langmuir probe, connected to the vacuum chamber through the vacuum valves, can be moved in two directions. Two different measurement circuits for the probe were used in these experiments, one hand operated and the other computer controlled. A schematic diagram of the planar magnetron used is shown in Fig. 2. The annular soft iron serves as a magnetic pole piece. The spatial distribution of magnetic field lines and the magnetic field intensity can be varied by moving the permanent magnets around or to the middle of the soft iron. The magnetic density, Br (the azimuthal component of the magnetic field), was measured with a Hall detector to be 3 4 0 + 1 0 G at a radius of about 15-20 mm on
Elsevier Sequoia
B. Hu, Z. Cao I Properties of d.c. magnetron sputtering plasma
112
0 ----~
N
'S Ring magnet
L/Probe I"/ f ~
O~ Annularanode
S ~ cot bode
in volume and is often assumed to be homogeneous and quasi-neutral in the absence of the probe. In general, one can assume that the electrons have a maxwellian velocity distribution in low temperature plasmas, and thus the total current flowing to the probe would be given by the following relation [5]:
I=Ise exp(eV/KTe) + Isi
//
Soft i r o n
Cytindricat magnet
Fig. 2. Schematic diagram of the magnetron sputtering source.
(1)
where the electron saturation current is given by
I~ = -esp(noC¢/4 ) and the ion saturation current is
4oo
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the surface cathode. The radial distribution Br and the axial distribution Bz (the axial component) of the magnetic density are shown in Figs. 3(a) and (b) respectively.
2.2. Experimental methods Langmuir probe diagnostics are the oldest and probably the simplest techniques available for measuring the properties of plasmas. A single probe was used in these experiments and the probe theory and application can be found in the literature [4, 5]. The magnetron discharge space, except for the cathode sheath and pre-sheath regions, tends to be large
The remaining symbols used in eqn. (1) are defined as follows: V is the applied voltage between the probe and the ground reference: s, is the probe area; cs is the plasma sound speed; c~ is the electron thermal speed; n o is the plasma density; Te is the electron temperature; K is Boltzmann's constant; e is the electron charge. The electron density and temperature can be determined by a computer calculation of an exponential least-squares fit of eqn. (1). Two kinds of plasma probes with different constructions were used. One was a planar probe, either 3.6 mm or 4.5 mm in diameter. The other was a cylindrical tungsten wire either 0.1 mm or 0.2 mm in diameter. The area of the planar probe is nearly unchanged during film deposition, and therefore it has a longer lifetime, but it does create a disturbance in the plasma. The cylindrical probe does not cause large disturbances in the plasma, but has a shorter lifetime since it is easily shorted by a coating of sputtered material. It was found that the measurements taken using the planar probe agreed quite well with those taken using the cylindrical probe. In order to reduce measurement errors, the probes were precleaned in alcohol and trichloroethanol solvents, pre-treated in an argon plasma under a high negative bias, and then operated in the electron saturation regime to clean contaminants from their surface.
3. Experimental results
3.1. Axial distributions of plasma parameters The axial properties of the discharge are shown in Fig. 4. The radial position of the probe is r = 0 mm, which corresponds to the centre of the discharge glow ring. It has been found that the density distributions of charged particles in this position primarily determine the impact angle of the charged particles on the growing film. The greatest distance between the probe and the cathode surface was 30 mm, the position of the annular anode.
B. Hu, Z. Cao / Properties of d.c. magnetron sputtering plasma 30!
magnetic density was constant, while the pressure of the working gas, either argon or helium, was varied from 0.8 to 3 Pa. Data were not taken in active gas plasmas such as 02 because of the short lifetime of the probe. The axial distributions of plasma potentials, electron temperatures and electron densities are given in Figs. 4(a), (b) and (c) respectively. As illustrated in Fig. 4(a), the plasma potential is nearly constant far away from the annular anode and decreases until the cathode pre-sheath is reached. When the operating gas pressure was decreased, the plasma potentials increased slightly, but the axial distribution was basically unchanged. The electron temperatures noted in Fig. 4(b) exhibit complex distributions, and are increasingly pressure dependent as the cathode is approached. When the operating gas pressure is fixed at 0.8 Pa, the electron temperature increases continuously with decreasing distance until the cathode is reached, but rapidly decreases near the cathode sheath. The highest electron temperatures correspond with the lowest operating pressures, owing to a reduction in collisional losses. The plasma densities show the same pressure dependence as the plasma temperatures. It was found that the electron temperatures are higher and the electron desities are lower than values reported elsewhere [2, 3] , probably owing to different measurement positions and electrode structures.
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The discharge characteristics of copper, aluminium and titanium cathodes 100 mm in diameter were measured with discharge currents of 0.1--0.5 A. Under low discharge currents the probe could enter the pre-sheath region and some of the data taken in this region are shown in Fig. 4(a), this is the region of greatest interest for studying gas ionization. In all measurements, the
The radial distributions of the plasma potentials, electron densities and electron temperatures are presented in Fig. 5. The measurements were taken in the region 5 cm above the cathode. The target radius is 2.5 cm, which is both the outer edge of the discharge glow ring and the inner edge of the anode ring. The data in Fig. 5 were taken using an operating pressure of 3 Pa, a discharge current of 0.1 A and three different cathodes (copper, aluminium, titanium) under the socalled "quasi-static discharge model" conditions. As shown in Fig. 5(a), a trough appears in the plasma potential across the region corresponding to the glow ring, parallel to the cathode surface. At the outer edge of the glow ring, the plasma potential was high, but the highest plasma potential, about 30 V, occurs in the centre of the glow ring. These results indicate that the sheath thickness is inhomogeneous near the cathode surface. Assuming that the sheath voltage is nearly equal to the applied voltage, it is likely that the sheath thickness in the glow ring is larger at the edge of the glow ring than it is in the centre of the glow ring. The distribution of plasma potential does not vary significantly for the three cathode materials tested. There is a peak of 60 eV for the copper cathode in the radial distribution of the electron temperature at
B. Hu, Z. Cao / Properties of d.c. magnetron sputtering plasma
114
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electron temperatures and the highest of these studied occurred for copper. This is because, compared with other cathode materials such as aluminium or titanium, copper has the highest sputtering yield under bombardment of argon ions with the same energy and a decrease in the gas density near the cathode occurs [6]. As for the electron temperature distribution, there is also a peak in the radial distribution of the electron density in the glow ring, again indicating that the main ionization occurs in the glow region. The distribution characteristics of the electron density are unchanged for different cathode materials, but result in different values. For the copper and aluminium cathodes, the electron density values are the largest and smallest respectively, while the values with the titanium cathode lie somewhere in between. This diversity is caused by the gas dynamics [6] in the near-cathode region and differences in the secondary electron generation coefficients of the different cathode materials. Variations of these plasma parameters were measured at different discharge currents (0.1-0.5 A) for the titanium cathode. The results indicate that the electron density varies in proportion to the discharge current, while the electron temperature is nearly constant. This is in agreement with previous work [2]. From analysis of the Langmuir probe curves (not shown) using helium as the operating gas, it was determined that the electron velocity tends to be only partially maxwellian distribution.
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The radial distributions of the random electron current in the plasma are shown in Fig. 6. The highest random electron current density appears in the glow ring and is tens of times higher than the discharge current density, in the strip ranging from r = 1.5 cm to r=2.5 cm. This indicates that there is an E × B drift current [7] which is tens of times higher than the discharge current. In addition, from the radial distribution of the random electron current, it can be de-
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B. Hu, Z. Cao / Properties of d.c. magnetron sputtering plasma
termined that most of the random current occurs in the glow ring; outside the glow ring it is nearly equal to the discharge current. The E X B drift current near the anode results in a higher magnetic field density, so it is easy to maintain the discharge; this also helps explain why the ignition voltage is always higher than the voltage at which the plasma is extinguished in a d.c. planar magnetron.
4. Discussion
4.1. Plasma distribution characteristics The thickness and the sheath distribution in a d.c. planar magnetron discharge can be estimated from the distribution of the plasma potential under low discharge currents. Using an estimated cathode surface and discharge current, the thickness of the sheath can be calculated theoretically. For example, for a discharge current of 0.3 A and a discharge voltage of 350 V, the calculated thickness of the sheath is about 5 mm in the main discharge region from r = 1 cm to r = 2 cm. This agrees well with the measured results and indicates that Child's law can be applied in the glow ring. From these experiments and the results shown in Fig. 4(a), it can be seen that the glow expands out from the glow ring, but that the distribution of the cathode sheath is inhomogeneous, being much thicker in the glow ring, than that in the centre region. This effect is illustrated more specifically in Fig. 5(a). It is noted that compared with the Larmor frequency of the electron, the magnitude of the magnetic field near the cathode changes slowly when using a permanent magnet in the d.c. planar magnetron. Based on this situation, the motion of charged particles near the cathode region obeys the law of conservation of magnetic moment tz = mV2 /2B = constant where m is the mass of the charged particle, B is the magnetic density, and F is the velocity of the charged particles in the vertical direction. When charged particles move in a magnetic field, the number of magnetic field lines contained by the traces of the particles is constant. That is, when the magnetic field lines are compressed (the magnetic density is increased), the distance between charged particles is less, and so the density of the charged particles is directly proportional to the magnetic field density (i.e. n J B = constant, where no is the density of charged particles). In these experiments, the magnetic field density distributions in the radial and axial directions near the cathode surface were measured, and in comparing Fig. 3 with Figs. 4 and 5, the radial distribution characteristics of the electron density are shown to be very similar to the
115
radial distribution of the magnetic density at a fixed height in front of the cathode surface. The axial distribution of the magnetic field density is also very similar to the axial distribution of the magnetic field density at a fixed radial position. Thus, both experiment and theory indicate that the density distribution of the charged particles is determined by the magnetic field distributions in the discharge space near the cathode region. Moreover, it can be concluded that, in order to extend the utilities of a target of the size of the etch area on the cathode surface, the magnetic field distributions near the cathode need to be changed. 4.2. Discharge, ion and random electron currents From the experimental results we know that the random electron current densities are tens of times higher than the discharge current densities; that is, the E X B drift electron current is much higher near the cathode, and the primary transport mechanism of electrons is the E X B drift in the near-cathode region. However, the relationship between the discharge current and the E XB drift current must be investigated further. Using the experimental value of the plasma density (5X 10 9 cm -3) and an electron temperature (60 eV) at the edge of the cathode sheath (assuming that the discharge region is coincident with the range calculated by Child's law when the discharge current is 0.1 A), the ion current at the edge of the sheath can be computed using the ion acoustic velocity and is equal to 0.08 A. If the effects of the secondary electrons are considered, the actual total ion current is at least 0.09 A; this agrees well with the practical discharge current of 0.1 A. If the number of ions per volume per second formed in the discharge space is known, and one accepts several basic assumptions [8], the ion current attained at the cathode surface can be calculated for the experimentally estimated discharge region: I = 4g'(1+ y)ne =ngcr(2~rMeKTe)-°~U i exp( - Ui/KTe)
(2)
where ~ is a geometric parameter, ne is the electron density, ng is the density of the operating gas, Te is the electron temperature, 3' is the emitting coefficient of the secondary electrons, o- is the microcollision crosssection, and Ui is the ionization potential of the working gas. Using eqn. (2), along with the measured values of the electron density and electron temperature, the discharge current values can be estimated. The experimental data are as follows when the discharge current is 0.2 A: n e = 5 × 109 cm -3, Ze=20 eV, ng=1015 cm -3, t r = 2 X l 0 -16 cm 2, Ui=15.7 eV, the ionization rate is 1.86 x 1017 S- 1 cm- 3, and the thickness of the ionization region is 7 ram. The discharge current calculated by eqn. (2) is 0.2 A. This shows that to some degree, eqn. (2) can be used to determine the
116
B. Hu, Z. Cao / Properties of d,c. magnetron sputtering plasma
relationship between the discharge current, the electron t e m p e r a t u r e and the electron density in a d.c. planar magnetron.
5. Conclusions The plasma properties of a planar d.c. magnetron were measured with a Langmuir probe. The spatial distribution characteristics were analysed in a preliminary manner. T h e electron temperatures and densities were higher at lower pressures, while the plasma potential was constant. Higher discharge currents resulted in a proportional increase in the electron density, but almost no change in the electron temperature was observed. These trends are in good agreement with previous work [2]. T h e spatial distribution of plasma p a r a m e t e r s was unchanged for different cathode materials, but different values of plasma p a r a m e t e r s resulted. For the copper cathode, the electron density was 8 x 1 0 9 c m - 3 and the electron t e m p e r a t u r e was as high as 60 eV, indicating that the electron velocity distributions near the cathode sheath partially deviated from maxwellian velocity distributions. In the glow ring, the E × B drift current was tens of times higher than the discharge current; thus, the distribution of the electron density was very similar
to that of the magnetic density near the cathode. This result is also in good agreement with theory. If the effects of cathode target secondary electrons are considered, and several assumptions are made, the values of discharge current calculated using measured plasma p a r a m e t e r s are also in good agreement with experimental measurements. This implies that the charge transport processes in the discharge space not only affect the discharge characteristics, but also the film properties.
References 1 J. A. Thornton and A. Penfold, in J. Vossen and W. Kern (eds.), Thin Film Processes, Academic Press, New York, 1978. 2 S. M. Rossnagel and H. R. Kaufman, J. Vac. Sci. Technol. A, 4 (1986) 1822. 3 H. Fujita, S, Yagura, H. Ueno and Nagano, J. Phys. D, 19 (1986) 1699. 4 F. F. Chen, in R. H. Huddestone and S. L. Lednard (eds.), Plasma Diagnostic Techniques, Academic Press, New York, 1965. 5 B. Lipschultz, I. Hutchinson, B. Labombard and A. Wan, J. Vac. Sci. TechnoL A, 4(3) (1986) 1810. 6 S. M. Rossnagel and H. R. Kaufman, J. Vac. Sci. Technol. A, 6 (1988) 19. 7 S. M. Rossnagel and H. R. Kaufman, J. Vac. Sci. Technol. A, 5 (1987) 88. 8 S. M. Rossnagel and H. R. Kaufman, J. Vac. Sci. Technol. A, 6 (1988) 223.
111
Plasma properties in a planar d.c. magnetron sputtering device Hu Bingsen
and Cao Zhou
Lanzhou Institute of Physics, P.O. Box 94, Lanzhou (China)
(Received March 16, 1991; accepted in final form August 19, 1991)
Abstract The plasma properties of a planar d.c. magnetron sputtering device were measured using a Langmuir probe. The spatial distribution of the plasma potential, the electron density, and the electron temperature are given for differing discharge parameters. By analysing the above distribution of properties, it was determined that the spatial distribution characteristics of plasma parameters do not change for different cathode materials. The radial distribution of random electron current in the plasma was also measured with a Langmuir probe, and the result shows that there is a drift current that is tens of times larger than the discharge current. For cathode secondary electrons, the results using measured plasma parameters of discharge current calculations are in close agreement with experimental measurements. The calculated and experimental results indicate that charge transport processes within the discharge space affect not only the discharge characteristics, but also the film properties.
I. Introduction D.c. magnetron sputtering has found wide application in thin film deposition owing. In general, films deposited by d.c. magnetron sputtering appear to have homogeneous thicknesses, high density and good adhesion. Therefore, d.c. magnetron sputtering has been widely used in the deposition of optical and metal films. The basic principles [1] and plasma processing [2, 3] of d.c. magnetron sputtering have been reported in investigations of magnetron sputtering techniques, but in some cases detailed results were not given. A planar d.c. magnetron sputtering device, designed in house, was investigated in this work. The plasma parameters and the random electron current were measured with a Langmuir probe [4], and the transport of the charged particles was studied briefly.
2. Experimental apparatus and methods 2.1. Experimental apparatus
The d.c. planar magnetron sputtering system in which the experiments were carried out is shown schematically in Fig. 1. The vacuum chamber is 400 mm in diameter and 280 mm in length. A negative d.c. voltage is applied to the cathode, the chamber is used as a grounded anode, and another anode which is annular in shape is also used to generate a glow discharge. Three different kinds of gas can be fed into the vacuum chamber
[.'l
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Fig. 1. Schematic diagram of the experimental apparatus. through the bleed valve. The total pressure is measured by both a thermocouple gauge and an ionization gauge. The Langmuir probe, connected to the vacuum chamber through the vacuum valves, can be moved in two directions. Two different measurement circuits for the probe were used in these experiments, one hand operated and the other computer controlled. A schematic diagram of the planar magnetron used is shown in Fig. 2. The annular soft iron serves as a magnetic pole piece. The spatial distribution of magnetic field lines and the magnetic field intensity can be varied by moving the permanent magnets around or to the middle of the soft iron. The magnetic density, Br (the azimuthal component of the magnetic field), was measured with a Hall detector to be 3 4 0 + 1 0 G at a radius of about 15-20 mm on
Elsevier Sequoia
B. Hu, Z. Cao I Properties of d.c. magnetron sputtering plasma
112
0 ----~
N
'S Ring magnet
L/Probe I"/ f ~
O~ Annularanode
S ~ cot bode
in volume and is often assumed to be homogeneous and quasi-neutral in the absence of the probe. In general, one can assume that the electrons have a maxwellian velocity distribution in low temperature plasmas, and thus the total current flowing to the probe would be given by the following relation [5]:
I=Ise exp(eV/KTe) + Isi
//
Soft i r o n
Cytindricat magnet
Fig. 2. Schematic diagram of the magnetron sputtering source.
(1)
where the electron saturation current is given by
I~ = -esp(noC¢/4 ) and the ion saturation current is
4oo
(a)
Isi = 0.61npC~Soe
3OO
v
200
100
. 4
I
!
i
i
I
6
10
18
20
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30
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0
5
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15
20
H (ram) Fig. 3. (a) The radial distribution of the magnetic field density, and (b) the axial distribution of the magnetic density.
the surface cathode. The radial distribution Br and the axial distribution Bz (the axial component) of the magnetic density are shown in Figs. 3(a) and (b) respectively.
2.2. Experimental methods Langmuir probe diagnostics are the oldest and probably the simplest techniques available for measuring the properties of plasmas. A single probe was used in these experiments and the probe theory and application can be found in the literature [4, 5]. The magnetron discharge space, except for the cathode sheath and pre-sheath regions, tends to be large
The remaining symbols used in eqn. (1) are defined as follows: V is the applied voltage between the probe and the ground reference: s, is the probe area; cs is the plasma sound speed; c~ is the electron thermal speed; n o is the plasma density; Te is the electron temperature; K is Boltzmann's constant; e is the electron charge. The electron density and temperature can be determined by a computer calculation of an exponential least-squares fit of eqn. (1). Two kinds of plasma probes with different constructions were used. One was a planar probe, either 3.6 mm or 4.5 mm in diameter. The other was a cylindrical tungsten wire either 0.1 mm or 0.2 mm in diameter. The area of the planar probe is nearly unchanged during film deposition, and therefore it has a longer lifetime, but it does create a disturbance in the plasma. The cylindrical probe does not cause large disturbances in the plasma, but has a shorter lifetime since it is easily shorted by a coating of sputtered material. It was found that the measurements taken using the planar probe agreed quite well with those taken using the cylindrical probe. In order to reduce measurement errors, the probes were precleaned in alcohol and trichloroethanol solvents, pre-treated in an argon plasma under a high negative bias, and then operated in the electron saturation regime to clean contaminants from their surface.
3. Experimental results
3.1. Axial distributions of plasma parameters The axial properties of the discharge are shown in Fig. 4. The radial position of the probe is r = 0 mm, which corresponds to the centre of the discharge glow ring. It has been found that the density distributions of charged particles in this position primarily determine the impact angle of the charged particles on the growing film. The greatest distance between the probe and the cathode surface was 30 mm, the position of the annular anode.
B. Hu, Z. Cao / Properties of d.c. magnetron sputtering plasma 30!
magnetic density was constant, while the pressure of the working gas, either argon or helium, was varied from 0.8 to 3 Pa. Data were not taken in active gas plasmas such as 02 because of the short lifetime of the probe. The axial distributions of plasma potentials, electron temperatures and electron densities are given in Figs. 4(a), (b) and (c) respectively. As illustrated in Fig. 4(a), the plasma potential is nearly constant far away from the annular anode and decreases until the cathode pre-sheath is reached. When the operating gas pressure was decreased, the plasma potentials increased slightly, but the axial distribution was basically unchanged. The electron temperatures noted in Fig. 4(b) exhibit complex distributions, and are increasingly pressure dependent as the cathode is approached. When the operating gas pressure is fixed at 0.8 Pa, the electron temperature increases continuously with decreasing distance until the cathode is reached, but rapidly decreases near the cathode sheath. The highest electron temperatures correspond with the lowest operating pressures, owing to a reduction in collisional losses. The plasma densities show the same pressure dependence as the plasma temperatures. It was found that the electron temperatures are higher and the electron desities are lower than values reported elsewhere [2, 3] , probably owing to different measurement positions and electrode structures.
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The discharge characteristics of copper, aluminium and titanium cathodes 100 mm in diameter were measured with discharge currents of 0.1--0.5 A. Under low discharge currents the probe could enter the pre-sheath region and some of the data taken in this region are shown in Fig. 4(a), this is the region of greatest interest for studying gas ionization. In all measurements, the
The radial distributions of the plasma potentials, electron densities and electron temperatures are presented in Fig. 5. The measurements were taken in the region 5 cm above the cathode. The target radius is 2.5 cm, which is both the outer edge of the discharge glow ring and the inner edge of the anode ring. The data in Fig. 5 were taken using an operating pressure of 3 Pa, a discharge current of 0.1 A and three different cathodes (copper, aluminium, titanium) under the socalled "quasi-static discharge model" conditions. As shown in Fig. 5(a), a trough appears in the plasma potential across the region corresponding to the glow ring, parallel to the cathode surface. At the outer edge of the glow ring, the plasma potential was high, but the highest plasma potential, about 30 V, occurs in the centre of the glow ring. These results indicate that the sheath thickness is inhomogeneous near the cathode surface. Assuming that the sheath voltage is nearly equal to the applied voltage, it is likely that the sheath thickness in the glow ring is larger at the edge of the glow ring than it is in the centre of the glow ring. The distribution of plasma potential does not vary significantly for the three cathode materials tested. There is a peak of 60 eV for the copper cathode in the radial distribution of the electron temperature at
B. Hu, Z. Cao / Properties of d.c. magnetron sputtering plasma
114
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electron temperatures and the highest of these studied occurred for copper. This is because, compared with other cathode materials such as aluminium or titanium, copper has the highest sputtering yield under bombardment of argon ions with the same energy and a decrease in the gas density near the cathode occurs [6]. As for the electron temperature distribution, there is also a peak in the radial distribution of the electron density in the glow ring, again indicating that the main ionization occurs in the glow region. The distribution characteristics of the electron density are unchanged for different cathode materials, but result in different values. For the copper and aluminium cathodes, the electron density values are the largest and smallest respectively, while the values with the titanium cathode lie somewhere in between. This diversity is caused by the gas dynamics [6] in the near-cathode region and differences in the secondary electron generation coefficients of the different cathode materials. Variations of these plasma parameters were measured at different discharge currents (0.1-0.5 A) for the titanium cathode. The results indicate that the electron density varies in proportion to the discharge current, while the electron temperature is nearly constant. This is in agreement with previous work [2]. From analysis of the Langmuir probe curves (not shown) using helium as the operating gas, it was determined that the electron velocity tends to be only partially maxwellian distribution.
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R(mm)
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,
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3.3. Random electron currents
~ctrr-3)
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The radial distributions of the random electron current in the plasma are shown in Fig. 6. The highest random electron current density appears in the glow ring and is tens of times higher than the discharge current density, in the strip ranging from r = 1.5 cm to r=2.5 cm. This indicates that there is an E × B drift current [7] which is tens of times higher than the discharge current. In addition, from the radial distribution of the random electron current, it can be de-
CU
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~
,
,
,
,
,
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Fig. 5. Radial distribution of plasma parameters for cathodes of aluminium (O), copper ( + ) and titanium (A): (a) potential, (b) electron temperature and (c) electron density.
t',4 I
E
r = 15 mm. This implies that the most intense area of ionization is located in this region. It can also be seen in Fig. 5(c) that the primary ionization occurs in an annular area between 10 mm and 20 mm in radius; this region will be considered in later calculations and discussion. Different cathode materials result in different
100
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0
6
10
1
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310
R (ram) Fig. 6. Radial distribution of the random electron current.
B. Hu, Z. Cao / Properties of d.c. magnetron sputtering plasma
termined that most of the random current occurs in the glow ring; outside the glow ring it is nearly equal to the discharge current. The E X B drift current near the anode results in a higher magnetic field density, so it is easy to maintain the discharge; this also helps explain why the ignition voltage is always higher than the voltage at which the plasma is extinguished in a d.c. planar magnetron.
4. Discussion
4.1. Plasma distribution characteristics The thickness and the sheath distribution in a d.c. planar magnetron discharge can be estimated from the distribution of the plasma potential under low discharge currents. Using an estimated cathode surface and discharge current, the thickness of the sheath can be calculated theoretically. For example, for a discharge current of 0.3 A and a discharge voltage of 350 V, the calculated thickness of the sheath is about 5 mm in the main discharge region from r = 1 cm to r = 2 cm. This agrees well with the measured results and indicates that Child's law can be applied in the glow ring. From these experiments and the results shown in Fig. 4(a), it can be seen that the glow expands out from the glow ring, but that the distribution of the cathode sheath is inhomogeneous, being much thicker in the glow ring, than that in the centre region. This effect is illustrated more specifically in Fig. 5(a). It is noted that compared with the Larmor frequency of the electron, the magnitude of the magnetic field near the cathode changes slowly when using a permanent magnet in the d.c. planar magnetron. Based on this situation, the motion of charged particles near the cathode region obeys the law of conservation of magnetic moment tz = mV2 /2B = constant where m is the mass of the charged particle, B is the magnetic density, and F is the velocity of the charged particles in the vertical direction. When charged particles move in a magnetic field, the number of magnetic field lines contained by the traces of the particles is constant. That is, when the magnetic field lines are compressed (the magnetic density is increased), the distance between charged particles is less, and so the density of the charged particles is directly proportional to the magnetic field density (i.e. n J B = constant, where no is the density of charged particles). In these experiments, the magnetic field density distributions in the radial and axial directions near the cathode surface were measured, and in comparing Fig. 3 with Figs. 4 and 5, the radial distribution characteristics of the electron density are shown to be very similar to the
115
radial distribution of the magnetic density at a fixed height in front of the cathode surface. The axial distribution of the magnetic field density is also very similar to the axial distribution of the magnetic field density at a fixed radial position. Thus, both experiment and theory indicate that the density distribution of the charged particles is determined by the magnetic field distributions in the discharge space near the cathode region. Moreover, it can be concluded that, in order to extend the utilities of a target of the size of the etch area on the cathode surface, the magnetic field distributions near the cathode need to be changed. 4.2. Discharge, ion and random electron currents From the experimental results we know that the random electron current densities are tens of times higher than the discharge current densities; that is, the E X B drift electron current is much higher near the cathode, and the primary transport mechanism of electrons is the E X B drift in the near-cathode region. However, the relationship between the discharge current and the E XB drift current must be investigated further. Using the experimental value of the plasma density (5X 10 9 cm -3) and an electron temperature (60 eV) at the edge of the cathode sheath (assuming that the discharge region is coincident with the range calculated by Child's law when the discharge current is 0.1 A), the ion current at the edge of the sheath can be computed using the ion acoustic velocity and is equal to 0.08 A. If the effects of the secondary electrons are considered, the actual total ion current is at least 0.09 A; this agrees well with the practical discharge current of 0.1 A. If the number of ions per volume per second formed in the discharge space is known, and one accepts several basic assumptions [8], the ion current attained at the cathode surface can be calculated for the experimentally estimated discharge region: I = 4g'(1+ y)ne =ngcr(2~rMeKTe)-°~U i exp( - Ui/KTe)
(2)
where ~ is a geometric parameter, ne is the electron density, ng is the density of the operating gas, Te is the electron temperature, 3' is the emitting coefficient of the secondary electrons, o- is the microcollision crosssection, and Ui is the ionization potential of the working gas. Using eqn. (2), along with the measured values of the electron density and electron temperature, the discharge current values can be estimated. The experimental data are as follows when the discharge current is 0.2 A: n e = 5 × 109 cm -3, Ze=20 eV, ng=1015 cm -3, t r = 2 X l 0 -16 cm 2, Ui=15.7 eV, the ionization rate is 1.86 x 1017 S- 1 cm- 3, and the thickness of the ionization region is 7 ram. The discharge current calculated by eqn. (2) is 0.2 A. This shows that to some degree, eqn. (2) can be used to determine the
116
B. Hu, Z. Cao / Properties of d,c. magnetron sputtering plasma
relationship between the discharge current, the electron t e m p e r a t u r e and the electron density in a d.c. planar magnetron.
5. Conclusions The plasma properties of a planar d.c. magnetron were measured with a Langmuir probe. The spatial distribution characteristics were analysed in a preliminary manner. T h e electron temperatures and densities were higher at lower pressures, while the plasma potential was constant. Higher discharge currents resulted in a proportional increase in the electron density, but almost no change in the electron temperature was observed. These trends are in good agreement with previous work [2]. T h e spatial distribution of plasma p a r a m e t e r s was unchanged for different cathode materials, but different values of plasma p a r a m e t e r s resulted. For the copper cathode, the electron density was 8 x 1 0 9 c m - 3 and the electron t e m p e r a t u r e was as high as 60 eV, indicating that the electron velocity distributions near the cathode sheath partially deviated from maxwellian velocity distributions. In the glow ring, the E × B drift current was tens of times higher than the discharge current; thus, the distribution of the electron density was very similar
to that of the magnetic density near the cathode. This result is also in good agreement with theory. If the effects of cathode target secondary electrons are considered, and several assumptions are made, the values of discharge current calculated using measured plasma p a r a m e t e r s are also in good agreement with experimental measurements. This implies that the charge transport processes in the discharge space not only affect the discharge characteristics, but also the film properties.
References 1 J. A. Thornton and A. Penfold, in J. Vossen and W. Kern (eds.), Thin Film Processes, Academic Press, New York, 1978. 2 S. M. Rossnagel and H. R. Kaufman, J. Vac. Sci. Technol. A, 4 (1986) 1822. 3 H. Fujita, S, Yagura, H. Ueno and Nagano, J. Phys. D, 19 (1986) 1699. 4 F. F. Chen, in R. H. Huddestone and S. L. Lednard (eds.), Plasma Diagnostic Techniques, Academic Press, New York, 1965. 5 B. Lipschultz, I. Hutchinson, B. Labombard and A. Wan, J. Vac. Sci. TechnoL A, 4(3) (1986) 1810. 6 S. M. Rossnagel and H. R. Kaufman, J. Vac. Sci. Technol. A, 6 (1988) 19. 7 S. M. Rossnagel and H. R. Kaufman, J. Vac. Sci. Technol. A, 5 (1987) 88. 8 S. M. Rossnagel and H. R. Kaufman, J. Vac. Sci. Technol. A, 6 (1988) 223.