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The design and performance of planar magnetron sputtering cathodes
Vacuum/volume Printed in Great
37/numbers Britain
3/4/pages
363 to 366/l
0042-207X/87S3.00 + .OO Pergamon Journals Ltd
987
The design and performance sputtering cathodes
of planar magnetron
A G Spencer, C A Bishop and R P Howson, Department of Physics, Loughborough Technology, Loughborough, Leics LEll 3TU, UK
University of
The influence of the magnetic field strength and distribution on the operating characteristics of a planar magnetron has been investigated with permanent and electro-magnet units. This electro-magnetron will give variable magnetic fields of O-30 mT at the centre of the race track. Increasing the field up to around 25 mT (250 Gauss) gives sharp improvements in the operating characteristics and increasing the field beyond this results in only marginal improvements. The configuration of the magnetic field by pole pieces and the influence of this on the operating parameters (in particular target utilization and film impurities) has been investigated with permanent magnet systems. It is shown that suitable magnetic fields parallel to the target surface can give uniform target erosion at low operating pressures with efficient line of sight and high energy transfer of material to the substrate. The design of permanent magnet units with commercially available magnets of different materials is included.
Magnetron requirements
The phenomena of sputtering was first noticed and utilized in ordinary glow discharges where a potential is applied between two plates in a chamber at a pressure of around 20 Pa (150 mtorr). This creates a plasma between the plates and the applied potential accelerates ions from the plasma into the cathode. These accelerated ions sputter the cathode material into the chamber and thin films can be deposited by placing a substrate in this flux of sputtered material. The deposition rate is however, slow (I 1 nm s-l) and this makes the process uneconomic for most industrial applications. If the cathode is modified so that a magnetic field exists parallel to the cathode surface, then the plasma electrons follow a convoluted path which allows their confinement and greatly increases the possible plasma densities and sputtering rate. This is the basis of the planar magnetron sputtering cathode. Planar magnetrons’ typically operate at pressures around 1 Pa (7.5 mtorr) and can achieve deposition rates greater than 10 nm s-i kW_‘, which, with tens of kW of input power, makes their industrial use more feasible. As 50% or more of the input power is dumped into the target as heat’, continuous operation at high power requires target cooling. For commercial applications the requirements of a good magnetron design are high deposition rate, uniform deposition, good target utilization and often low heating of the substrate. The deposition rate and uniformity are determined not only by the magnetron design but also by parameters such as the operating pressure, target-to-substrate distance and their relative orientations. For large area deposition such as glass or roll coating, then magnetrons can provide high uniformity. Arcing is a problem which increases with increasing power, and some form of arc suppression is usually necessary for high power operation’. We have found from experience that the main limitation on the
deposition rate is the temperature of the target surface. Because the target must be cooled from behind, the thermal impedance of the target limits the maximum cooling rate, and when there is a maximum permittable surface temperature, this limits the input power. Power supplies permitting, the power and the deposition rate can usually be increased until the temperature of the target starts to damage the target, its cooling system or the substrate. Target utilization is of prime importance in an industrial application, as this determines the proportion of the target that can be deposited as films and so effects the period between target replacements and the resulting down time. A big advantage of magnetron sputtering over other deposition processes is that it is a cold process that can be used to coat thermally sensitive substrates. Where this is important, some care must be taken to ensure that fast electrons from the confined plasma do not escape to heat the substrate.
Electron motions
Although plasma wave processes are active in a magnetron, much of the behaviour of magnetrons can be understood from the motions of isolated electrons in the various fields present3. The electric field in front of the magnetron is greatly altered by the presence of the conducting plasma so that the bulk of the applied potential is dropped across a small region close to the cathode surface. This region corresponds to the cathode dark space in a glow discharge, but in a magnetron the dark space is small (visually less than about 1 mm thick). Moving electrons in a magnetic field will follow a curved path and if the radius of this curvature is larger than the dark space, then a simple approximation to the electron motion can be used. We can consider first the velocity gained by the electron as it accelerates across the dark 363
A G Spencer et al: Design and performance of planar magnetron sputtering cathodes Drift
/
velocity
Target
/
vd
yrface/
B
,,,/’
pb;entioc p across dark space
Figure 1. An approximation to the electron path where B is parallel to the target.
space and then the circular path followed by an electron with constant speed in a magnetic field (Figure 1). Initially, the magnetic field is assumed to be parallel to the target surface and the E field across the dark space normal to the target surface. Accelerating across the applied potential P the electron will gain a velocity u
u=SQR
(2.q.Pjm)
where 4 = electron charge and m = electron rest mass. The interaction of an electron of velocity u with the B field will produce a circular path of radius r r-
m.v q . B’
Using typical magnetron operating parameters of P = 500 V and B=0.05 T (500 Gauss) gives the electron path radius as r= 1.5 mm, i.e., larger than or comparable to the cathode dark space. Combining the radius of curvature and the electron velocity gives an estimate of the drift velocity ud as
V$. 7-c
Using the same ‘typical’ values gives vd N 2 x 1O-6 m s- ‘. To prevent this large drift velocity removing electrons from the discharge, the drift path must form a loop. Any loop will do and common shapes are circular or two parallel sides with semicircular ends, but some more tortuous paths have been used4. The magnetic field is generally provided from a simple magnetic circuit behind the target and the field from this arrangement will be dome-shaped (Figure 2). Only at the centre of the target is the magnetic field parallel to the target and so normal to the electron velocity. Away from the central region the field is at some angle TV to the target (Figure 3). As they accelerate across the dark space
---_
Figure 2. A typical magnetron with hidden poles and the resulting domed magnetic field. 364
Flectron drift Into paper
/
Target /
/
Figure 3. The effect of an inclined B field on the electron motion.
the electrons will therefore gain a component of velocity u’=v sin (a) parallel to the B field. This velocity will carry the electron along the magnetic field line, across the dome and down on the other side, where it may be mirrored by the converging B field or re-enter the dark space. So the drift velocity ud is approximately u/n and the initial velocity u across the dome is u sin (a). Quite a small value of alpha (~20”) makes u,, and u‘ comparable and then the electron movement will not be along the drift direction but diagonally across the dome field3. Electrons created by ion impacts on the target where B is parallel to the target surface will have a small velocity v ’ across the dome. Electrons created at or reflected from a point on the target where ozis not zero will move across the dome through the region where B is parallel to the target so the highest electron density will be in this region. The creation rate of ions will be highest here and, if the ions do not move large distances from their point of creation, the maximum sputtering of the target will be beneath this region. In our magnetrons when a dome-shaped field was used, roughly triangular erosion profiles have been obtained with the maximum erosion where B is parallel to the target (Figure 4(a) ) and similar results have been obtained elsewhere’. This suggests we may simply relate the magnetic field distribution and subsequent electron motions to the target erosion profile. Our experiments with various magnetic pole designs support this. Pole design and erosion profiles As mentioned above, we found that an ordinary dome-shaped field produced a roughly triangular erosion profile. The first magnetron constructed by us used a magnetic circuit entirely behind the target surface and this magnetron design, the magnetic field and the resulting erosion profile is shown in Figure 4(a). This triangular erosion profile is far from ideal as the target is eroded right through when only 50% of the material under the plasma has been sputtered. To produce a more uniform erosion profile the magnetic field distribution was modified by placing the poles of the magnetic circuit in front of the target surface. This arrangement is shown in Figure 4(b) and a flatter erosion profile has resulted. The centre pole is a little beneath the outer pole in this design and so the magnetic field lines slope upwards slightly from the centre of the magnetron. The electron velocities induced by this sloping field will also be outwards and this is reflected in the maximum erosion of the target at the outer edge. The angles of the field lines to the target are less than in Figure 4(a) and the erosion profile is correspondingly flatter. In the magnetron design of Figure 4(c), the poles are 5 mm above the target and so the magnetic field at the target surface is curved in the opposite sense to the original dome field of Figure 4(a). The electron motions resulting from this will be outwards
A G Spencer et a/: Design and performance of planar magnetron sputtering cathodes
B field
1
/I/
j-L\
\ I
/,/~,E$+~aigei (a)
Cooled
backing
120 mT
-
/ plate
IO mm H
(b)
Figure 4. Cross-sections of three magnetrons fields and erosion profiles.
with the measured
magnetic
than 0.5 at%. The long overhanging poles of the design in Figure 4(c) should be avoided as the magnetic field tends to leave the front surface of such a pole, and then the magnetic field in front of the pole is far from normal to the surface. The material sputtered from the unshielded poles amounted up to 40 at% of the material forming the film. Sputtering of the poles and of other surfaces can be prevented by earth shielding 5-6 Allowable impurity levels vary with the application and if the highest possible purity is required, then the initial hidden pole design of Figure 4(a) is to be favoured, and target utilization must be sacrificed unless a moving magnet or target system is used’. Magnetic field strength The design in Figure 4(c) is based on an electro-magnet and with a magneto-motance (current x turns) of 16 kA, a magnetic field strength of 30 mT (300 Gauss) can be produced in the centre of the pole gap. The operation of the magnetron was studied for magnetic fields between 0 and 30 mT. The operating potential, current, pressure and magnetic field strength are all inter-related in a complex fashion. In order to study the effect of the field strength, the magnetron current and pressure were held constant and the operating potential was plotted as a function of the field strength (Figure 5). The current density was estimated by dividing the total current by the area of the target and as this current density is increased, stronger magnetic fields are required for stable operation. Once the minimum stable magnetic field is exceeded, the operating potential drops rapidly and above field strengths of about 25 mT the curves are fairly flat. A magnetic field of 25 mT (250 Gauss) is, therefore, a-reasonable minimum figure to aim for in a magnetron design and this is comparable with figures given elsewheres. Higher magnetic field strengths will allow higher current densities and the maximum field of 30 mT allows stable operation of our electro-magnetron up to current densities of 420 Am _ 2. Magnetic materials
the centre of the dome and so the electron density will no longer peak where B is parallel to the target. The erosion profile again agrees with this argument because it now shows maximum erosion at the edges of the target. The erosion profile is not symmetrical because the magnetrons are circular (around the centre line of Figure 4) and the field therefore converges radially across the target. Placing these magnetic poles in front of the target means that they will be exposed to the plasma. In our designs the pole pieces are at the target potential, which means the poles can sputter and contaminate the film. To minimize the sputtering of the exposed poles, the magnetic field in front of the poles should be normal to the pole surface. This produces the maximum removal rate of electrons from the vicinity of the poles and, therefore, reduces ionization and sputtering. A field normal to the pole surface can be easily obtained by shaping the pole rather than altering the field. The design of Figure 4(b) originally had an inner pole with a square cross-section which produced films containing 2 at% of Fe (electron microprobe and Auger analyses) with the bulk of this impurity, therefore, being introduced by sputtering of the inner pole. This pole was, therefore, modified to the design of Figure 4(b) where the pole surface is more normal to the field lines above it. The Fe content of the deposited films then decreased to less
from
The type of magnetic material chosen strongly influences the magnetron design. In all cases the pole gap must be smaller than, or comparable to, any other gap across which the magnetic flux might leak. With the relatively low coercivity AlNiCo-type materials the magnets must be longer than the pole gap to prevent their demagnetization. The designs of Figure 4(a) and 4(b) use cylindrical AlNiCo magnets 100 mm long and the resulting
552 I
I
I
IO
20
I 30
Mognetlc field strength at centre of pole gop (mT)
Figure 5. The magnetron field strength.
operating
potential
as a function
of the magnetic
365
A G Spencer
et al: Design and performance of planar magnetron sputtering cathodes
magnetic field strength is about 40 mT at the centre of the pole gap. High coercivity materials like barium ferrite or the more expensive neodinium iron and samarium cobalt are much less susceptible to demagnetization and so can be bought as flat blocks magnetized across their shortest side. These blocks can be built up into magnetrons of virtually any shape and in a design like Figure 2, barium ferrite can easily provide 30 mT at the centre of the pole gap. Conclusions Planar magnetron sputtering cathodes require direct cooling of the target and the thermal impedance of the target is a limiting factor on the cooling rate. This ultimately limits the power which can be put into the magnetron. The erosion profile can be related to the electron motions and, hence, to the magnetic field distribution. On this basis the target utilization can be improved by flattening (so that the field lines are parallel to the target) the magnetic field with the use of magnet poles in front of the target. Sputtering of these exposed poles can be minimized by shaping the poles so that the magnetic field lines are normal to the pole surface. The magnetic field strength required in front of the target is, at minimum, 25 mT. The current density at which the magnetron
366
will operate with stability increases with the magnetic field strength so that fields higher than 25 mT are required for high current densities. In our electromagnetron a field of 30 mT allowed stable operation up to current densities of 420 Am-‘.
Acknowledgements The authors construction work.
wish to thank Mr E M Stenlake for much of the involved and ICI and the SERC for supporting this
References
’ J S Chapin, R & D p 37 (January 1974). ’ R K Waits, J Vat &i Technol, 13, 179 (1978). 3 J A Thornton. J Vat Sci Technol. 15. 171 (1978). 4 S Schiller, U Heisig and K Goedicke, 7th Int Confon Vuc Met, Tokyo, Japan, (November 1982). ’ L Holland, British Patent 610529, accepted 18 October 1948. 6 L Maissel and R Glang, Handbook of Thin Film Technology, Chapter 4, McGraw-Hill, New York (1970). ’ K Abe, S Kobayashi, T Kamel, T Shim& H Tateishi and S Aiuchi, Thin Solid Films, 96, 255 (1982). ’ K Roll, J Vat Sci Technol, A4 14 (1986).
37/numbers Britain
3/4/pages
363 to 366/l
0042-207X/87S3.00 + .OO Pergamon Journals Ltd
987
The design and performance sputtering cathodes
of planar magnetron
A G Spencer, C A Bishop and R P Howson, Department of Physics, Loughborough Technology, Loughborough, Leics LEll 3TU, UK
University of
The influence of the magnetic field strength and distribution on the operating characteristics of a planar magnetron has been investigated with permanent and electro-magnet units. This electro-magnetron will give variable magnetic fields of O-30 mT at the centre of the race track. Increasing the field up to around 25 mT (250 Gauss) gives sharp improvements in the operating characteristics and increasing the field beyond this results in only marginal improvements. The configuration of the magnetic field by pole pieces and the influence of this on the operating parameters (in particular target utilization and film impurities) has been investigated with permanent magnet systems. It is shown that suitable magnetic fields parallel to the target surface can give uniform target erosion at low operating pressures with efficient line of sight and high energy transfer of material to the substrate. The design of permanent magnet units with commercially available magnets of different materials is included.
Magnetron requirements
The phenomena of sputtering was first noticed and utilized in ordinary glow discharges where a potential is applied between two plates in a chamber at a pressure of around 20 Pa (150 mtorr). This creates a plasma between the plates and the applied potential accelerates ions from the plasma into the cathode. These accelerated ions sputter the cathode material into the chamber and thin films can be deposited by placing a substrate in this flux of sputtered material. The deposition rate is however, slow (I 1 nm s-l) and this makes the process uneconomic for most industrial applications. If the cathode is modified so that a magnetic field exists parallel to the cathode surface, then the plasma electrons follow a convoluted path which allows their confinement and greatly increases the possible plasma densities and sputtering rate. This is the basis of the planar magnetron sputtering cathode. Planar magnetrons’ typically operate at pressures around 1 Pa (7.5 mtorr) and can achieve deposition rates greater than 10 nm s-i kW_‘, which, with tens of kW of input power, makes their industrial use more feasible. As 50% or more of the input power is dumped into the target as heat’, continuous operation at high power requires target cooling. For commercial applications the requirements of a good magnetron design are high deposition rate, uniform deposition, good target utilization and often low heating of the substrate. The deposition rate and uniformity are determined not only by the magnetron design but also by parameters such as the operating pressure, target-to-substrate distance and their relative orientations. For large area deposition such as glass or roll coating, then magnetrons can provide high uniformity. Arcing is a problem which increases with increasing power, and some form of arc suppression is usually necessary for high power operation’. We have found from experience that the main limitation on the
deposition rate is the temperature of the target surface. Because the target must be cooled from behind, the thermal impedance of the target limits the maximum cooling rate, and when there is a maximum permittable surface temperature, this limits the input power. Power supplies permitting, the power and the deposition rate can usually be increased until the temperature of the target starts to damage the target, its cooling system or the substrate. Target utilization is of prime importance in an industrial application, as this determines the proportion of the target that can be deposited as films and so effects the period between target replacements and the resulting down time. A big advantage of magnetron sputtering over other deposition processes is that it is a cold process that can be used to coat thermally sensitive substrates. Where this is important, some care must be taken to ensure that fast electrons from the confined plasma do not escape to heat the substrate.
Electron motions
Although plasma wave processes are active in a magnetron, much of the behaviour of magnetrons can be understood from the motions of isolated electrons in the various fields present3. The electric field in front of the magnetron is greatly altered by the presence of the conducting plasma so that the bulk of the applied potential is dropped across a small region close to the cathode surface. This region corresponds to the cathode dark space in a glow discharge, but in a magnetron the dark space is small (visually less than about 1 mm thick). Moving electrons in a magnetic field will follow a curved path and if the radius of this curvature is larger than the dark space, then a simple approximation to the electron motion can be used. We can consider first the velocity gained by the electron as it accelerates across the dark 363
A G Spencer et al: Design and performance of planar magnetron sputtering cathodes Drift
/
velocity
Target
/
vd
yrface/
B
,,,/’
pb;entioc p across dark space
Figure 1. An approximation to the electron path where B is parallel to the target.
space and then the circular path followed by an electron with constant speed in a magnetic field (Figure 1). Initially, the magnetic field is assumed to be parallel to the target surface and the E field across the dark space normal to the target surface. Accelerating across the applied potential P the electron will gain a velocity u
u=SQR
(2.q.Pjm)
where 4 = electron charge and m = electron rest mass. The interaction of an electron of velocity u with the B field will produce a circular path of radius r r-
m.v q . B’
Using typical magnetron operating parameters of P = 500 V and B=0.05 T (500 Gauss) gives the electron path radius as r= 1.5 mm, i.e., larger than or comparable to the cathode dark space. Combining the radius of curvature and the electron velocity gives an estimate of the drift velocity ud as
V$. 7-c
Using the same ‘typical’ values gives vd N 2 x 1O-6 m s- ‘. To prevent this large drift velocity removing electrons from the discharge, the drift path must form a loop. Any loop will do and common shapes are circular or two parallel sides with semicircular ends, but some more tortuous paths have been used4. The magnetic field is generally provided from a simple magnetic circuit behind the target and the field from this arrangement will be dome-shaped (Figure 2). Only at the centre of the target is the magnetic field parallel to the target and so normal to the electron velocity. Away from the central region the field is at some angle TV to the target (Figure 3). As they accelerate across the dark space
---_
Figure 2. A typical magnetron with hidden poles and the resulting domed magnetic field. 364
Flectron drift Into paper
/
Target /
/
Figure 3. The effect of an inclined B field on the electron motion.
the electrons will therefore gain a component of velocity u’=v sin (a) parallel to the B field. This velocity will carry the electron along the magnetic field line, across the dome and down on the other side, where it may be mirrored by the converging B field or re-enter the dark space. So the drift velocity ud is approximately u/n and the initial velocity u across the dome is u sin (a). Quite a small value of alpha (~20”) makes u,, and u‘ comparable and then the electron movement will not be along the drift direction but diagonally across the dome field3. Electrons created by ion impacts on the target where B is parallel to the target surface will have a small velocity v ’ across the dome. Electrons created at or reflected from a point on the target where ozis not zero will move across the dome through the region where B is parallel to the target so the highest electron density will be in this region. The creation rate of ions will be highest here and, if the ions do not move large distances from their point of creation, the maximum sputtering of the target will be beneath this region. In our magnetrons when a dome-shaped field was used, roughly triangular erosion profiles have been obtained with the maximum erosion where B is parallel to the target (Figure 4(a) ) and similar results have been obtained elsewhere’. This suggests we may simply relate the magnetic field distribution and subsequent electron motions to the target erosion profile. Our experiments with various magnetic pole designs support this. Pole design and erosion profiles As mentioned above, we found that an ordinary dome-shaped field produced a roughly triangular erosion profile. The first magnetron constructed by us used a magnetic circuit entirely behind the target surface and this magnetron design, the magnetic field and the resulting erosion profile is shown in Figure 4(a). This triangular erosion profile is far from ideal as the target is eroded right through when only 50% of the material under the plasma has been sputtered. To produce a more uniform erosion profile the magnetic field distribution was modified by placing the poles of the magnetic circuit in front of the target surface. This arrangement is shown in Figure 4(b) and a flatter erosion profile has resulted. The centre pole is a little beneath the outer pole in this design and so the magnetic field lines slope upwards slightly from the centre of the magnetron. The electron velocities induced by this sloping field will also be outwards and this is reflected in the maximum erosion of the target at the outer edge. The angles of the field lines to the target are less than in Figure 4(a) and the erosion profile is correspondingly flatter. In the magnetron design of Figure 4(c), the poles are 5 mm above the target and so the magnetic field at the target surface is curved in the opposite sense to the original dome field of Figure 4(a). The electron motions resulting from this will be outwards
A G Spencer et a/: Design and performance of planar magnetron sputtering cathodes
B field
1
/I/
j-L\
\ I
/,/~,E$+~aigei (a)
Cooled
backing
120 mT
-
/ plate
IO mm H
(b)
Figure 4. Cross-sections of three magnetrons fields and erosion profiles.
with the measured
magnetic
than 0.5 at%. The long overhanging poles of the design in Figure 4(c) should be avoided as the magnetic field tends to leave the front surface of such a pole, and then the magnetic field in front of the pole is far from normal to the surface. The material sputtered from the unshielded poles amounted up to 40 at% of the material forming the film. Sputtering of the poles and of other surfaces can be prevented by earth shielding 5-6 Allowable impurity levels vary with the application and if the highest possible purity is required, then the initial hidden pole design of Figure 4(a) is to be favoured, and target utilization must be sacrificed unless a moving magnet or target system is used’. Magnetic field strength The design in Figure 4(c) is based on an electro-magnet and with a magneto-motance (current x turns) of 16 kA, a magnetic field strength of 30 mT (300 Gauss) can be produced in the centre of the pole gap. The operation of the magnetron was studied for magnetic fields between 0 and 30 mT. The operating potential, current, pressure and magnetic field strength are all inter-related in a complex fashion. In order to study the effect of the field strength, the magnetron current and pressure were held constant and the operating potential was plotted as a function of the field strength (Figure 5). The current density was estimated by dividing the total current by the area of the target and as this current density is increased, stronger magnetic fields are required for stable operation. Once the minimum stable magnetic field is exceeded, the operating potential drops rapidly and above field strengths of about 25 mT the curves are fairly flat. A magnetic field of 25 mT (250 Gauss) is, therefore, a-reasonable minimum figure to aim for in a magnetron design and this is comparable with figures given elsewheres. Higher magnetic field strengths will allow higher current densities and the maximum field of 30 mT allows stable operation of our electro-magnetron up to current densities of 420 Am _ 2. Magnetic materials
the centre of the dome and so the electron density will no longer peak where B is parallel to the target. The erosion profile again agrees with this argument because it now shows maximum erosion at the edges of the target. The erosion profile is not symmetrical because the magnetrons are circular (around the centre line of Figure 4) and the field therefore converges radially across the target. Placing these magnetic poles in front of the target means that they will be exposed to the plasma. In our designs the pole pieces are at the target potential, which means the poles can sputter and contaminate the film. To minimize the sputtering of the exposed poles, the magnetic field in front of the poles should be normal to the pole surface. This produces the maximum removal rate of electrons from the vicinity of the poles and, therefore, reduces ionization and sputtering. A field normal to the pole surface can be easily obtained by shaping the pole rather than altering the field. The design of Figure 4(b) originally had an inner pole with a square cross-section which produced films containing 2 at% of Fe (electron microprobe and Auger analyses) with the bulk of this impurity, therefore, being introduced by sputtering of the inner pole. This pole was, therefore, modified to the design of Figure 4(b) where the pole surface is more normal to the field lines above it. The Fe content of the deposited films then decreased to less
from
The type of magnetic material chosen strongly influences the magnetron design. In all cases the pole gap must be smaller than, or comparable to, any other gap across which the magnetic flux might leak. With the relatively low coercivity AlNiCo-type materials the magnets must be longer than the pole gap to prevent their demagnetization. The designs of Figure 4(a) and 4(b) use cylindrical AlNiCo magnets 100 mm long and the resulting
552 I
I
I
IO
20
I 30
Mognetlc field strength at centre of pole gop (mT)
Figure 5. The magnetron field strength.
operating
potential
as a function
of the magnetic
365
A G Spencer
et al: Design and performance of planar magnetron sputtering cathodes
magnetic field strength is about 40 mT at the centre of the pole gap. High coercivity materials like barium ferrite or the more expensive neodinium iron and samarium cobalt are much less susceptible to demagnetization and so can be bought as flat blocks magnetized across their shortest side. These blocks can be built up into magnetrons of virtually any shape and in a design like Figure 2, barium ferrite can easily provide 30 mT at the centre of the pole gap. Conclusions Planar magnetron sputtering cathodes require direct cooling of the target and the thermal impedance of the target is a limiting factor on the cooling rate. This ultimately limits the power which can be put into the magnetron. The erosion profile can be related to the electron motions and, hence, to the magnetic field distribution. On this basis the target utilization can be improved by flattening (so that the field lines are parallel to the target) the magnetic field with the use of magnet poles in front of the target. Sputtering of these exposed poles can be minimized by shaping the poles so that the magnetic field lines are normal to the pole surface. The magnetic field strength required in front of the target is, at minimum, 25 mT. The current density at which the magnetron
366
will operate with stability increases with the magnetic field strength so that fields higher than 25 mT are required for high current densities. In our electromagnetron a field of 30 mT allowed stable operation up to current densities of 420 Am-‘.
Acknowledgements The authors construction work.
wish to thank Mr E M Stenlake for much of the involved and ICI and the SERC for supporting this
References
’ J S Chapin, R & D p 37 (January 1974). ’ R K Waits, J Vat &i Technol, 13, 179 (1978). 3 J A Thornton. J Vat Sci Technol. 15. 171 (1978). 4 S Schiller, U Heisig and K Goedicke, 7th Int Confon Vuc Met, Tokyo, Japan, (November 1982). ’ L Holland, British Patent 610529, accepted 18 October 1948. 6 L Maissel and R Glang, Handbook of Thin Film Technology, Chapter 4, McGraw-Hill, New York (1970). ’ K Abe, S Kobayashi, T Kamel, T Shim& H Tateishi and S Aiuchi, Thin Solid Films, 96, 255 (1982). ’ K Roll, J Vat Sci Technol, A4 14 (1986).