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Twin facing target sputtering system for the deposition of multilayer and alloy films
Pergamon PII: 80042-207X(96)00236-2
Vacuum/volume 48/number l/pages 15 to 19/1997 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/97 $17.00+.00
Twin facing target sputtering system for the deposition of multilayer and alloy films M Swarnalatha and S Mohan, Department
of Instrumentation,
Indian institute
of Science, Bangalore
560 012,
India received 24 July
1996
A Facing Target Sputtering (FTSJ technique with a configuration consisting of two sets of vertically parallel facing targets and a substrate holder mounted perpendicular to the target planes has been designed and fabricated. This system is capable of producing thin multilayer films as well as alloy films. The discharge characteristics of the FTS system when dc power is applied is analysed and the discharge currents were found to be 3-5 times the values obtained with a single target (the conventional dc sputtering). The discharge currents also strongly depend on the Inter Target Distance (ITD) showing a decreasing trend with increasing ITD. The influence of pressure, Substrate To Target Distance (STD) and the ITD on the rate of deposition of copper films is studied. The rate of deposition increases with increase in Argon pressure and decreases with increase in STD and ITD. Copyright 0 1996 Elsevier Science Ltd
Introduction
The Facing Target Sputtering (FTS) technique was first developed to deposit magnetic thin films at high deposition rates and low substrate temperatures,‘-’ when the conventional sputtering techniques failed to serve the same purpose. Although magnetron sputtering is also a high rate and low substrate heating type of technique, ‘H in the case of magnetic material targets it requires the use of magnets with higher field strength in order to overcome the absorption of flux by the magnetic material. The target erosion and nonreproducibility in magnetron sputtering are the other persistent problems undergoing complicated investigations and improvements.9~‘0 Apart from having advantages over the conventional techniques, FTS was also found to result in uniform films over large areas,” with smooth, dense and columnless microstructure,‘2,‘3 properties which are highly desirable in the magnetic coatings industry. This technique was further extented for the deposition of well controlled multilayers’4.‘5 and alloy films’6,‘7 and also to the fields of superconducting thin films,‘* hard coatings” and transparent conducting coatings 2o over large areas. FTS, since its conception in 1980 by Naoe et al.,’ has frequently been used in the magnetron mode and some times in the RF magnetron mode but not in the dc diode mode which is the simplest operational mode with all the advantages of the FTS configuration. By incorporating two identical sputtering targets parallel to each other, changing the anode or substrate position from parallel to perpendicular position with respect to the target plane and applying the negative voltage to both the targets from the same power supply the technique offers at least a three fold
increase in deposition rate, decrease in operating pressures and reduced substrate heating and ion bombardment effects. A well controlled deposition rate can be achieved in this mode rather than the magnetron mode by a proper choice of inter electrode distances and pressure. A number of applications in the magnetic memory devices area require the deposition of very thin multilayers of alternate magnetic and nonmagnetic materials or the deposition of ternary and quaternary alloys. The dc FTS could possibly be ideal for achieving well controlled thicknesses in the case of multilayer deposition and desired stiochiometry in the case of alloy films. To investigate the influence of process parameters on the above mentioned characteristics and to prove its usefulness, a twin facing target sputtering system suitable for the sputtering of multilayers as well as alloy films has been fabricated. The system configuration along with its special features are described in this paper. Although this technique is well established for the deposition of magnetic memory devices some of the basic results on the dependence of Inter Target Distance (ITD) and Substrate to Target Distance (STD) on the plasma characteristics and hence, on the rate of deposition and film thickness uniformity are not well reported parameters. Other than a mention of the optimised ITD and STD values, the process involved is not thoroughly diagonised. An attempt in this direction has been made in the present study and some preliminary studies of discharge characteristics and their dependence on some of the system parameters of the FTS operated in dc diode mode are reported in this work. The Z-V characteristics measured at various pressures and inter target distances and their effect on the discharge current are discussed. The effect of inter target distance, substrate to target 15
M Swarnalafha and S Mohan: Deposition of multilayer and alloy films distance and the operating also reported.
pressure
on the rate of deposition
is
Experimental The complete Twin Facing Target Sputtering (TFTS) system consisted of a vacuum pumping assembly, a control panel for the pumping system, a vacuum chamber designed to accommodate two sets of facing targets and a substrate holder perpendicular to the plane of the targets, the sputtering targets and the power supply for the targets were built in house. The pumping module consists of a conventional 4.5 in diffstack pump with a pumping speed of 230 1SC’ backed by a direct drive rotary pump along with the necessary valves, gaugues and vacuum lines. The ultimate vacuum that has been achieved at the inlet port of the pumping module with the liquid Nz trap is 2 x 1O-6 mbar in 60min, measured using a Pirani-Penning gauge combination. The design of the vacuum chamber for the FTS system involved the identification and careful consideration of several parameters such as: the target size, location of four targets (two sets of parallel targets) in a suitably designed chamber with respect to size and volume, isolation of the twin facing targets from one another to avoid contamination, linear displacement of the targets to have control on inter target distance, location of substrate holder with respect to targets, and rotation of substrate holder and its linear motion to adjust distance between the substrate and targets. Based on these points and other practical limitations, a number of design options were considered and a horizontally mounted cylindrical chamber was designed and fabricated to the required specifications. The schematic of the chamber design along with dimensions is shown in Figure 1. The main features of the system are as follows: The cylindrical chamber is mounted horizontally with the pumping port to the chamber being welded to the body of the cylinder, to form the bottom base of the chamber. This port is mounted on to the mouth of the pumping module with the help of a collar which accomodates the gauge heads and the air admittance valve. The chamber is closed on either side by a front plate and back plate which are mounted on hinges to enable them to be opened and closed.
PUHPING
FORT
P”“PlWG
po(IT
Figure 1. Chamber design drawing: side and front view. 16
The front plate has a 114mm view port with glass window for viewing. The substrate holder is a rotating disc fitted to the back plate through a Wilson seal shaft and is perpendicular to the plane of the targets. The target ports are 76mm in diameter and located around the circumference of the cylindrical chamber, parallel to each other, one set on the top and one set at the bottom. 85 mm square copper targets were fabricated and mounted in parallel1 in the vacuum chamber as shown in Figure 2. The substrate holder was a circular disc of 100 mm diameter, with clamps fixed to the top half of the disc to mount the substrates. Distance between the targets can be varied from 50 to 150 mm. Distance between substrates and the axis going through the centre of targets can be varied from 75 to 125 mm. Top set of targets are isolated from the bottom set with a shield mounted in between the two to isolate their plasma and avoid deposition of material from one set of targets onto the other. Multilayers can be deposited by rotating the substrate holder and sequentially sputtering from the top and bottom sets of targets. Alloys can be deposited by rotating the substrate holder and simultaneously sputtering from both sets of targets. A 4 KW unregulated variable dc power supply capable of operating both in the dc mode and dc magnetron mode has been designed and fabricated. Sputtering and discharge studies were carried out in argon gas pressures in the lo-‘-IO-* mbar range. Copper films were deposited under different conditions of pressure, ITD and STD. Film thicknesses were measured by Talysurf thickness profilometer and rate of deposition calculated from the thickness and sputtering time. Results and discussion The typical discharge characteristics in the FTS configuration, i.e. the cathode current Zdvs the applied voltage V, to the targets, measured at various pressures and at an inter target distance of 95 mm is shown in Figure 3. The same figure also includes the discharge characteristics for the single target configuration, i.e. power applied to only one of the targets in the same FTS system. This is similar to normal dc diode sputtering except that the anode is perpendicular to the target instead of being parallel which does not make a difference to the discharge characteristics. As is well known for dc diode sputtering, even in this case for the single target configuration the operating pressures are high while
‘igure 2. Interior view of the chamber with the facing targets, Iolder and shield which isolates the top and bottom targets.
substrate
M Swarnalatha
and S Mohanr Deposition
of multilayer and alloy films Pr (mbar)
I
85 x 85 mm. The plasma confinement and enhancement in ionization as a function of inter target distance is shown in Figure 4, where discharge current as a function of ITD at 0.1 mbar pressure and at different applied voltages is plotted. From Figure 4 the following observations can be made: l
l
Applied voltage (volts) Figure 3. I-V characteristics of DC FTS for an ITD = 95 mm and of dc diode sputtering.
the cathode currents are observed to be low for voltages up to IOOOV. As such at the applied voltages used in this study the desired sputtering yields to deposit a film cannot be achieved even at pressures as high as 0.1 mbar with the single target. One has to go to much higher voltages to obtain sputtering yields that will result in film deposition at the anode. Comparatively, the cathode currents are much higher under similar conditions of pressure and voltage for the FTS configuration when the voltage is applied to both the targets connected to the same power supply. The current densities calculated from the measured values of discharge currents (Figure 3), at 0.1 mbar pressure and 1OOOV applied voltage were 0.42mA/cm2 for single target and 1.68mA/cmZ for the FTS configuration. Hence, a four fold increase rather than a two fold increase in current densities is observed in FTS at an ITD of 95 mm. It can also be observed from the figure that realisation of high discharge currents at lower pressures enables FTS to achieve higher deposition rates at low pressures leading to deposition of contamination free films. The discharge characteristics in a FTS system is well known to show a strong dependence of discharge current on the pressure, target size, applied magnetic field and applied power.14 The plasma diagnostics of the dc magnetron and RF magnetron FTS is well reported.’ In all these studies a magnetic field has been used to help in the confinement of the plasma and hence, discharge currents of orders much higher than the currents that can be achieved with conventional magnetrons have been reported at low pressures. The present study however deals with dc FTS where effective plasma confinement even in the absence of any magnetic field is observed. The discharge currents in this case have been found to be much higher than the discharge currents of normal dc diode sputtering but lower than those of magnetron sputtering. Although the target area available for sputtering is twice that in the case of FTS, the current density is more than twice that of the single target and this is due to the confinement of the electrons between the two facing targets which increases the ionization in the space between the targets. The effective plasma confinement depends largely on the size of the targets and the distance between the two targets. The size of the targets is fixed in this case to
l
There is a decrease in discharge current with an increase in ITD. This decrease varies from a maximum of 320 mA at an ITD of 60 mm to a minimum of 185 mA at an ITD of 135 mm resulting in a current density variation from 2.28mA/cm2 to 1.2 mA/cm* at 1000 V. Hence, depending on the ITD the current density enhancement in this case is 5.4 times to 2.85 times that of the single target or normal dc diode sputtering, under similar conditions of applied voltage and pressure. It is also observed that the discharge current which is decreasing with increasing ITD follows the same trend for all the applied voltages. A larger increase in discharge current is however observed at voltages between the 50&6OOV and 600700V range, with the effect being more prominent for ITD’s of 95, 105 and 115 mm. The secondary electron emission for copper is maximum at 600eV which falls in the 600-700V applied voltage range due to which there is sudden increase in the ionization and hence, an increase in discharge current. A possible explanation for the effect being more prominent at ITD’s of 95, 105, and 115 mm, is attempted through Figure 5 which is a schematic representation of the plasma regions for various ITDs, and the following analysis:
In dc FTS the cathode dark space of the two facing targets opposes the electron charge from each other causing a lengthening of the electron path due to reflection. This results in a greater number of gas collisions and hence, enhanced ionization. Consequently a co-operative effect from both the targets will cause an overlap of the negative glow region and forms a central plasma zone between the targets. Depending on the inter target distance (ITD) the central negative glow region is either enhanced or extinguished. Based on the negative glow region and the target size the ITDs are classified into three different ranges to explain the variations of discharge current in the 600-700V range in Figure 4. ITD is equal to or less than d, the target dimension: In this range there is an overlap in the plasma region that is extending from both the targets as shown in Figure 5(a). The co-operative effect from the facing targets results in enhanced ionization at lower voltages (below 600 V) and hence, an increase in currents is observed at 500V itself. At 600V when secondary electron emission is at a maximum, a further increase is noticed. ITDs of 95, 105 and 115 mm, which are greater than d: The ITDs are not close enough for an overlap of the plasma to occur at lower voltages (i.e. < 600 V). When there is maximum secondary electron emission at 600V there is an enhanced ionization due to which an overlap of the plasma region occurs, which was otherwise separated as shown in Figure 5(b) at lower voltages. The overlap in plasma may be caused due to an increase in density as well as energy of the plasma particles. Hence, the effects of both, the secondary electron emission as well as the co-operative effect that causes plasma overlap put together at about the same voltage range causes a sudden and sharper increase in the discharge currents in this ITD range. At even larger ITDs, i.e. 125 and 135 mm, the targets are too far apart, Figure 5(c), and hence, behave as isolated targets. The voltage at which maximum secondary electron emission 17
M Swarnalatha
and S Mohan: Deposition
01 50
of multilayer
I
,I,
and alloy films
1
60
70 Inter
,I
,I
I
III
80 90 100 110 target distance, ITD (mm)
,a,,
120
130
1 0
Figure 4. Discharge current vs inter target distance for various applied voltages occurs is also not sufficient to cause an enhancement in ionization and overlap of the plasma region. Hence, a large increase in currents is not observed in this ITD range.
(a)
-----
(b) -----
IT0 =75mm j -ITD:85mm
ITD=95mm;-
ITD=105mm;
, (C)
------ITD=
Figure 5. ITDs. 18
Schematic
-
--ITD=llSmm
I
125mm
j -
representation
IT0
= 135mm
of the plasma regions for various
To summarize, ITDs close to the target dimensions cause effective electron confinement in the space between the targets to occur. This results in maximum ionization and higher current densities, even at lower voltages. At larger ITDs the number of electrons scattering away from the space between the targets is more and hence, plasma confinement is minimum and the targets behave more as isolated or individual targets. Further work is in progress to establish an emphirical relationship between the target dimensions, ITD and the current densities as a function of applied voltage and pressure. Copper films were sputtered from the facing targets onto substrates placed perpendicular to the targets under various conditions of pressure, inter target distance and substrate to target distance (STD). Films were deposited at various pressures ranging from 0.1 to 0.02 mbar at an ITD of 95 mm. Films were also deposited at different STDs of 75, 85, 95, and 105mm keeping the pressure constant at 0.05mbar and repeated for an ITD of 80 mm as well as 95 mm. The thicknesses of all the films were measured by forming a sharp step using the Talysurf thickness profilometer. Figure 6 shows the increase in deposition rate with an increase in argon pressure. Figure 7 shows the influence of substrate to target distance on the rate of deposition. The rate of deposition decreases with increase in STD as is common in any sputtering technique. However this effect is more pronounced at an ITD of 80mm showing a steep decrease in rate of deposition with increase in STD. This may be due to the fact that the plasma is more dense at 80 mm ITD which could cause more collisions of the sputtered particles in the plasma region leading to reduced energy and more scattering of the sputtered particles. Hence, the reduced energy and scattering could result in a lower percentage of sputtered particles reaching the substrate, when ITD is small and STD is larger.
M Swarnalafha
and S Mohan:
Deposition of multilayer
and alloy films
Conclusions A twin facing target sputtering system for the deposition of multilayer thin films and alloy films has been fabricated. The discharge characteristics of the dc FTS in the absence of a magnetic field has been studied and compared with normal dc diode sputtering. In this case the current densities were 3-5 times higher than the current densities of dc diode sputtering. The discharge current as a function of ITD showed a decrease in I, with increasing ITD. A sharp increase in discharge current at applied voltages where secondary electron emission is maximum has been observed. This sharp increase in current also depends on the ITD showing a more prominent increase at ITDs close to the target dimensions than for larger ITDs. An explanation for this behaviour based on effective plasma confinement and secondary electron emission is given. The rate of deposition showed an increase with increase in pressure and decrease in STD.
I
Acknowledgements 0 .lO
0.08 Pressure
(mbar
1
Figure 6. Rate of deposition vs pressure.
bd
ITD = 95mm
M
ITD-8Qmm
References
Pr = 0.05 mbar
‘~
80 Substrate
90 to target
100 distance,
This work was done under the financial support of the Indian Institute of Science, Bangalore. The authors wish to thank J. Raghunath for his help and suggestions during the design and fabrication stage of the work. Helpful discussions and cooperation of colleagues of vacuum and thin films laboratory are also gratefully acknowledged.
110
STD (mm)
Figure 7. Rate of deposition vs substrate to target distance.
The thickness uniformity of the sputtered films in the FTS technique varies largely with ITD and STD. Optimization of ITD and STD values to achieve thickness unformity over large areas is also critical and this work will be consequently published as another study. A smaller ITD and STD chosen to give high deposition rates may not result in the desired thickness uniformity in the films. Therefore an optimization of system parameters to achieve good thickness uniformity as well as high deposition rates would make this technique more advantageous compared to the other sputtering techniques.
I. Naoe, M., Yamanaka, S. and Hoshi, Y.. fEEE Trans. Mug., 1980, 16, 646. 2. Matsuoka, M., Hoshi, Y. and Naoe, M., J. Appl. Phys., 1986, 60. 2096. 3. Kume, M., Kishimoto, D., Nakatsuka, Y. and Abe, Y., IEEE Trans. Msg., 1986, 22, 1161. 4. Hoshi, Y. and Naoe, M., IEEE Trans. Ma.9.. 1991, 27,487O. 5. Chapin, J.S., Res. Develop., 1974, 25, 37. 6. Thornton, J.A., J. Vat. Sci. Technol., 1978, 15, 171. I. Escrivao, M.L., Moutinho, A.M.C. and Maneira, J.P., J. Vat. Sci. Technol.. 1994, 12, 723. 8. Petrov, I., lvanov, I., Orlinov, V. and Sundgren, J.E., J. VU. Sci. Technol.. 1993, 11, 2733. 9. Ivanov, I., Kazansky, P., Hultman, L.. Petrov, 1. and Sundgren, J.E., J. Vat. Sci. Technol., 1994, 12, 314. 10. Musil, J., Rusnak, K., Jezek, V. and Vicek. J., Vucuum, 1995, 46, 341. 11. Qihua Fan, J. Vat. Sci. Technol.. 1992, AlO, 3371. 12. Ito, H. and Naoe, M., IEEE Trans. Mug., 1990, 26, 181. 13. Kawanabe, T., Park, J. and Naoe, M., Mater. Sci. Engng, 1991,134, 1305. 14. Song, K. and Naoe, M., Journal of Mugnetics and Magneric Mnrerials, 1992, 104, 1855. 15. Fukizawa, A. and Naoe, M., IEEE Trans. Mug., 1987,23, 140. 16. Jiang, E.Y ., Wang, Z.J., Sun, C.Q., Lu, Q., Zhang, X.X. and Liu, Y.G., Journal qf Magneticsand Magnek Muterials, 1989.81, 25. 17. Hirata, T. and Naoe, M., J. Appl. Phys., 1990,67, 5047. 18. Terada, N., Ihara, H., Jo, M., Hirabayashi. M., Kimura, Y., Matsutani, K., Hirata, K., Ohno, E., Sugise, R. and Kawashima, F., Jap. J. Appl. Phys., 1988, 27, L639. 19. Akiyama, S., Nakagawa, S. and Naoe, M.. Mater. Sci. Engng, 1991, 134, 1309. 20. Akiyama, S., Nakagawa, S. and Naoe, M., IEEE Trans. Mug., 1991, 27, 5094.
19
Vacuum/volume 48/number l/pages 15 to 19/1997 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x/97 $17.00+.00
Twin facing target sputtering system for the deposition of multilayer and alloy films M Swarnalatha and S Mohan, Department
of Instrumentation,
Indian institute
of Science, Bangalore
560 012,
India received 24 July
1996
A Facing Target Sputtering (FTSJ technique with a configuration consisting of two sets of vertically parallel facing targets and a substrate holder mounted perpendicular to the target planes has been designed and fabricated. This system is capable of producing thin multilayer films as well as alloy films. The discharge characteristics of the FTS system when dc power is applied is analysed and the discharge currents were found to be 3-5 times the values obtained with a single target (the conventional dc sputtering). The discharge currents also strongly depend on the Inter Target Distance (ITD) showing a decreasing trend with increasing ITD. The influence of pressure, Substrate To Target Distance (STD) and the ITD on the rate of deposition of copper films is studied. The rate of deposition increases with increase in Argon pressure and decreases with increase in STD and ITD. Copyright 0 1996 Elsevier Science Ltd
Introduction
The Facing Target Sputtering (FTS) technique was first developed to deposit magnetic thin films at high deposition rates and low substrate temperatures,‘-’ when the conventional sputtering techniques failed to serve the same purpose. Although magnetron sputtering is also a high rate and low substrate heating type of technique, ‘H in the case of magnetic material targets it requires the use of magnets with higher field strength in order to overcome the absorption of flux by the magnetic material. The target erosion and nonreproducibility in magnetron sputtering are the other persistent problems undergoing complicated investigations and improvements.9~‘0 Apart from having advantages over the conventional techniques, FTS was also found to result in uniform films over large areas,” with smooth, dense and columnless microstructure,‘2,‘3 properties which are highly desirable in the magnetic coatings industry. This technique was further extented for the deposition of well controlled multilayers’4.‘5 and alloy films’6,‘7 and also to the fields of superconducting thin films,‘* hard coatings” and transparent conducting coatings 2o over large areas. FTS, since its conception in 1980 by Naoe et al.,’ has frequently been used in the magnetron mode and some times in the RF magnetron mode but not in the dc diode mode which is the simplest operational mode with all the advantages of the FTS configuration. By incorporating two identical sputtering targets parallel to each other, changing the anode or substrate position from parallel to perpendicular position with respect to the target plane and applying the negative voltage to both the targets from the same power supply the technique offers at least a three fold
increase in deposition rate, decrease in operating pressures and reduced substrate heating and ion bombardment effects. A well controlled deposition rate can be achieved in this mode rather than the magnetron mode by a proper choice of inter electrode distances and pressure. A number of applications in the magnetic memory devices area require the deposition of very thin multilayers of alternate magnetic and nonmagnetic materials or the deposition of ternary and quaternary alloys. The dc FTS could possibly be ideal for achieving well controlled thicknesses in the case of multilayer deposition and desired stiochiometry in the case of alloy films. To investigate the influence of process parameters on the above mentioned characteristics and to prove its usefulness, a twin facing target sputtering system suitable for the sputtering of multilayers as well as alloy films has been fabricated. The system configuration along with its special features are described in this paper. Although this technique is well established for the deposition of magnetic memory devices some of the basic results on the dependence of Inter Target Distance (ITD) and Substrate to Target Distance (STD) on the plasma characteristics and hence, on the rate of deposition and film thickness uniformity are not well reported parameters. Other than a mention of the optimised ITD and STD values, the process involved is not thoroughly diagonised. An attempt in this direction has been made in the present study and some preliminary studies of discharge characteristics and their dependence on some of the system parameters of the FTS operated in dc diode mode are reported in this work. The Z-V characteristics measured at various pressures and inter target distances and their effect on the discharge current are discussed. The effect of inter target distance, substrate to target 15
M Swarnalafha and S Mohan: Deposition of multilayer and alloy films distance and the operating also reported.
pressure
on the rate of deposition
is
Experimental The complete Twin Facing Target Sputtering (TFTS) system consisted of a vacuum pumping assembly, a control panel for the pumping system, a vacuum chamber designed to accommodate two sets of facing targets and a substrate holder perpendicular to the plane of the targets, the sputtering targets and the power supply for the targets were built in house. The pumping module consists of a conventional 4.5 in diffstack pump with a pumping speed of 230 1SC’ backed by a direct drive rotary pump along with the necessary valves, gaugues and vacuum lines. The ultimate vacuum that has been achieved at the inlet port of the pumping module with the liquid Nz trap is 2 x 1O-6 mbar in 60min, measured using a Pirani-Penning gauge combination. The design of the vacuum chamber for the FTS system involved the identification and careful consideration of several parameters such as: the target size, location of four targets (two sets of parallel targets) in a suitably designed chamber with respect to size and volume, isolation of the twin facing targets from one another to avoid contamination, linear displacement of the targets to have control on inter target distance, location of substrate holder with respect to targets, and rotation of substrate holder and its linear motion to adjust distance between the substrate and targets. Based on these points and other practical limitations, a number of design options were considered and a horizontally mounted cylindrical chamber was designed and fabricated to the required specifications. The schematic of the chamber design along with dimensions is shown in Figure 1. The main features of the system are as follows: The cylindrical chamber is mounted horizontally with the pumping port to the chamber being welded to the body of the cylinder, to form the bottom base of the chamber. This port is mounted on to the mouth of the pumping module with the help of a collar which accomodates the gauge heads and the air admittance valve. The chamber is closed on either side by a front plate and back plate which are mounted on hinges to enable them to be opened and closed.
PUHPING
FORT
P”“PlWG
po(IT
Figure 1. Chamber design drawing: side and front view. 16
The front plate has a 114mm view port with glass window for viewing. The substrate holder is a rotating disc fitted to the back plate through a Wilson seal shaft and is perpendicular to the plane of the targets. The target ports are 76mm in diameter and located around the circumference of the cylindrical chamber, parallel to each other, one set on the top and one set at the bottom. 85 mm square copper targets were fabricated and mounted in parallel1 in the vacuum chamber as shown in Figure 2. The substrate holder was a circular disc of 100 mm diameter, with clamps fixed to the top half of the disc to mount the substrates. Distance between the targets can be varied from 50 to 150 mm. Distance between substrates and the axis going through the centre of targets can be varied from 75 to 125 mm. Top set of targets are isolated from the bottom set with a shield mounted in between the two to isolate their plasma and avoid deposition of material from one set of targets onto the other. Multilayers can be deposited by rotating the substrate holder and sequentially sputtering from the top and bottom sets of targets. Alloys can be deposited by rotating the substrate holder and simultaneously sputtering from both sets of targets. A 4 KW unregulated variable dc power supply capable of operating both in the dc mode and dc magnetron mode has been designed and fabricated. Sputtering and discharge studies were carried out in argon gas pressures in the lo-‘-IO-* mbar range. Copper films were deposited under different conditions of pressure, ITD and STD. Film thicknesses were measured by Talysurf thickness profilometer and rate of deposition calculated from the thickness and sputtering time. Results and discussion The typical discharge characteristics in the FTS configuration, i.e. the cathode current Zdvs the applied voltage V, to the targets, measured at various pressures and at an inter target distance of 95 mm is shown in Figure 3. The same figure also includes the discharge characteristics for the single target configuration, i.e. power applied to only one of the targets in the same FTS system. This is similar to normal dc diode sputtering except that the anode is perpendicular to the target instead of being parallel which does not make a difference to the discharge characteristics. As is well known for dc diode sputtering, even in this case for the single target configuration the operating pressures are high while
‘igure 2. Interior view of the chamber with the facing targets, Iolder and shield which isolates the top and bottom targets.
substrate
M Swarnalatha
and S Mohanr Deposition
of multilayer and alloy films Pr (mbar)
I
85 x 85 mm. The plasma confinement and enhancement in ionization as a function of inter target distance is shown in Figure 4, where discharge current as a function of ITD at 0.1 mbar pressure and at different applied voltages is plotted. From Figure 4 the following observations can be made: l
l
Applied voltage (volts) Figure 3. I-V characteristics of DC FTS for an ITD = 95 mm and of dc diode sputtering.
the cathode currents are observed to be low for voltages up to IOOOV. As such at the applied voltages used in this study the desired sputtering yields to deposit a film cannot be achieved even at pressures as high as 0.1 mbar with the single target. One has to go to much higher voltages to obtain sputtering yields that will result in film deposition at the anode. Comparatively, the cathode currents are much higher under similar conditions of pressure and voltage for the FTS configuration when the voltage is applied to both the targets connected to the same power supply. The current densities calculated from the measured values of discharge currents (Figure 3), at 0.1 mbar pressure and 1OOOV applied voltage were 0.42mA/cm2 for single target and 1.68mA/cmZ for the FTS configuration. Hence, a four fold increase rather than a two fold increase in current densities is observed in FTS at an ITD of 95 mm. It can also be observed from the figure that realisation of high discharge currents at lower pressures enables FTS to achieve higher deposition rates at low pressures leading to deposition of contamination free films. The discharge characteristics in a FTS system is well known to show a strong dependence of discharge current on the pressure, target size, applied magnetic field and applied power.14 The plasma diagnostics of the dc magnetron and RF magnetron FTS is well reported.’ In all these studies a magnetic field has been used to help in the confinement of the plasma and hence, discharge currents of orders much higher than the currents that can be achieved with conventional magnetrons have been reported at low pressures. The present study however deals with dc FTS where effective plasma confinement even in the absence of any magnetic field is observed. The discharge currents in this case have been found to be much higher than the discharge currents of normal dc diode sputtering but lower than those of magnetron sputtering. Although the target area available for sputtering is twice that in the case of FTS, the current density is more than twice that of the single target and this is due to the confinement of the electrons between the two facing targets which increases the ionization in the space between the targets. The effective plasma confinement depends largely on the size of the targets and the distance between the two targets. The size of the targets is fixed in this case to
l
There is a decrease in discharge current with an increase in ITD. This decrease varies from a maximum of 320 mA at an ITD of 60 mm to a minimum of 185 mA at an ITD of 135 mm resulting in a current density variation from 2.28mA/cm2 to 1.2 mA/cm* at 1000 V. Hence, depending on the ITD the current density enhancement in this case is 5.4 times to 2.85 times that of the single target or normal dc diode sputtering, under similar conditions of applied voltage and pressure. It is also observed that the discharge current which is decreasing with increasing ITD follows the same trend for all the applied voltages. A larger increase in discharge current is however observed at voltages between the 50&6OOV and 600700V range, with the effect being more prominent for ITD’s of 95, 105 and 115 mm. The secondary electron emission for copper is maximum at 600eV which falls in the 600-700V applied voltage range due to which there is sudden increase in the ionization and hence, an increase in discharge current. A possible explanation for the effect being more prominent at ITD’s of 95, 105, and 115 mm, is attempted through Figure 5 which is a schematic representation of the plasma regions for various ITDs, and the following analysis:
In dc FTS the cathode dark space of the two facing targets opposes the electron charge from each other causing a lengthening of the electron path due to reflection. This results in a greater number of gas collisions and hence, enhanced ionization. Consequently a co-operative effect from both the targets will cause an overlap of the negative glow region and forms a central plasma zone between the targets. Depending on the inter target distance (ITD) the central negative glow region is either enhanced or extinguished. Based on the negative glow region and the target size the ITDs are classified into three different ranges to explain the variations of discharge current in the 600-700V range in Figure 4. ITD is equal to or less than d, the target dimension: In this range there is an overlap in the plasma region that is extending from both the targets as shown in Figure 5(a). The co-operative effect from the facing targets results in enhanced ionization at lower voltages (below 600 V) and hence, an increase in currents is observed at 500V itself. At 600V when secondary electron emission is at a maximum, a further increase is noticed. ITDs of 95, 105 and 115 mm, which are greater than d: The ITDs are not close enough for an overlap of the plasma to occur at lower voltages (i.e. < 600 V). When there is maximum secondary electron emission at 600V there is an enhanced ionization due to which an overlap of the plasma region occurs, which was otherwise separated as shown in Figure 5(b) at lower voltages. The overlap in plasma may be caused due to an increase in density as well as energy of the plasma particles. Hence, the effects of both, the secondary electron emission as well as the co-operative effect that causes plasma overlap put together at about the same voltage range causes a sudden and sharper increase in the discharge currents in this ITD range. At even larger ITDs, i.e. 125 and 135 mm, the targets are too far apart, Figure 5(c), and hence, behave as isolated targets. The voltage at which maximum secondary electron emission 17
M Swarnalatha
and S Mohan: Deposition
01 50
of multilayer
I
,I,
and alloy films
1
60
70 Inter
,I
,I
I
III
80 90 100 110 target distance, ITD (mm)
,a,,
120
130
1 0
Figure 4. Discharge current vs inter target distance for various applied voltages occurs is also not sufficient to cause an enhancement in ionization and overlap of the plasma region. Hence, a large increase in currents is not observed in this ITD range.
(a)
-----
(b) -----
IT0 =75mm j -ITD:85mm
ITD=95mm;-
ITD=105mm;
, (C)
------ITD=
Figure 5. ITDs. 18
Schematic
-
--ITD=llSmm
I
125mm
j -
representation
IT0
= 135mm
of the plasma regions for various
To summarize, ITDs close to the target dimensions cause effective electron confinement in the space between the targets to occur. This results in maximum ionization and higher current densities, even at lower voltages. At larger ITDs the number of electrons scattering away from the space between the targets is more and hence, plasma confinement is minimum and the targets behave more as isolated or individual targets. Further work is in progress to establish an emphirical relationship between the target dimensions, ITD and the current densities as a function of applied voltage and pressure. Copper films were sputtered from the facing targets onto substrates placed perpendicular to the targets under various conditions of pressure, inter target distance and substrate to target distance (STD). Films were deposited at various pressures ranging from 0.1 to 0.02 mbar at an ITD of 95 mm. Films were also deposited at different STDs of 75, 85, 95, and 105mm keeping the pressure constant at 0.05mbar and repeated for an ITD of 80 mm as well as 95 mm. The thicknesses of all the films were measured by forming a sharp step using the Talysurf thickness profilometer. Figure 6 shows the increase in deposition rate with an increase in argon pressure. Figure 7 shows the influence of substrate to target distance on the rate of deposition. The rate of deposition decreases with increase in STD as is common in any sputtering technique. However this effect is more pronounced at an ITD of 80mm showing a steep decrease in rate of deposition with increase in STD. This may be due to the fact that the plasma is more dense at 80 mm ITD which could cause more collisions of the sputtered particles in the plasma region leading to reduced energy and more scattering of the sputtered particles. Hence, the reduced energy and scattering could result in a lower percentage of sputtered particles reaching the substrate, when ITD is small and STD is larger.
M Swarnalafha
and S Mohan:
Deposition of multilayer
and alloy films
Conclusions A twin facing target sputtering system for the deposition of multilayer thin films and alloy films has been fabricated. The discharge characteristics of the dc FTS in the absence of a magnetic field has been studied and compared with normal dc diode sputtering. In this case the current densities were 3-5 times higher than the current densities of dc diode sputtering. The discharge current as a function of ITD showed a decrease in I, with increasing ITD. A sharp increase in discharge current at applied voltages where secondary electron emission is maximum has been observed. This sharp increase in current also depends on the ITD showing a more prominent increase at ITDs close to the target dimensions than for larger ITDs. An explanation for this behaviour based on effective plasma confinement and secondary electron emission is given. The rate of deposition showed an increase with increase in pressure and decrease in STD.
I
Acknowledgements 0 .lO
0.08 Pressure
(mbar
1
Figure 6. Rate of deposition vs pressure.
bd
ITD = 95mm
M
ITD-8Qmm
References
Pr = 0.05 mbar
‘~
80 Substrate
90 to target
100 distance,
This work was done under the financial support of the Indian Institute of Science, Bangalore. The authors wish to thank J. Raghunath for his help and suggestions during the design and fabrication stage of the work. Helpful discussions and cooperation of colleagues of vacuum and thin films laboratory are also gratefully acknowledged.
110
STD (mm)
Figure 7. Rate of deposition vs substrate to target distance.
The thickness uniformity of the sputtered films in the FTS technique varies largely with ITD and STD. Optimization of ITD and STD values to achieve thickness unformity over large areas is also critical and this work will be consequently published as another study. A smaller ITD and STD chosen to give high deposition rates may not result in the desired thickness uniformity in the films. Therefore an optimization of system parameters to achieve good thickness uniformity as well as high deposition rates would make this technique more advantageous compared to the other sputtering techniques.
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