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Unbalanced planar magnetron with continuous control of the operating mode from type I to type II
Vacuum/volume
Pergamon PII: SOO42-207X(96)00186-8
47/number II/pages 1395 to 1397/1996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All riahts reserved 0042-207X/96 $15.00+.00
Unbalanced planar magnetron with continuous control of the operating mode from type I to type II* J Kourtev, S Groudeva-Zotova, I Garnev and V Orlinov, institute of Electronics, Sciences, 72 Tzarigradsko Chaussee, 7784 Sofia, Bulgaria received 20 November
Bulgarian Academy
of
1995
A compact and simple planar magnetron design is described based on the known concept for modification of the magnetic field by a co-axially placed solenoid. The magnetron unit ensures sufficient values of the perpendicular B, and the parallel B, components of the magnetic induction above the target surface allowing for a wide range of variation of the operating mode from type I to the unbalanced type Il. The following experimental data characterizing the newly developed magnetron system are reported: (il radial distribution of B, at different distances above the rarger surface and at different solenoid currents I,,,; (ii) longitudinal distribution of B, along axes parallel to the central one, measured again at different I,,,; (iii) VA-characteristics of rhe magnetron unit (with a graphite target) for different values of I,,, (from 0 to 2.6Al and at working gas pressure p - = 4 Pa Arl. Copyright 0 1996 Elsevier Science Lrd
Introduction Unbalanced type II magnetrons represent a further improvement of the magnetron sputtering method for deposition of thin films with enhanced performance.‘,* The basic advantage of this novel technique consists in the wide possibilities for variation of the ion flux to the substrate surface and hence for controlling the structure and properties of the growing films.* In most cases the construction of such unbalanced magnetron systems is characterized by a much higher magnetic flux of the peripheral magnet pole as compared with that of the internal one. Conventionally such systems are realized using permanent magnets3 and are therefore characterized by a fixed unbalance ratio, a nearly constant geometry of the magnetron plasma and a limited variation of the ion flux to the substrates. A more effective method for controlling the unbalance ratio is the utilization of an external solenoid coil, which is normally situated outside the vacuum recipient4 and coaxially with respect to the magnetic poles of the magnetron. Depending on the current direction through the solenoid coil, the superimposed magnetic field of the solenoid can set the initial magnetron system into either type I or type II unbalanced operating mode. The magnetron system newly developed uses an improved magnetic configuration similar to the one reported in Ref. 5, but with a solenoid coil mounted inside the vacuum chamber coaxially along the magnetron unit.
*Paper based on that presented at the 9th International School on Vacuum, Electron and Ion Technologies (VEIT’95), 14-17 September 1995,Sozopol, Bulgaria, 1995
The preliminary investigations carried out with the aim to set into operation and characterize the magnetron system were performed using a graphite target and working gas Ar. The obtained results show that, by choosing appropriate values for the solenoid current Z,, both operating modes of the magnetron system, i.e. as type I or type II unbalanced magnetron, as well as a smooth transition between these modes could be achieved.
Experimental A schematic layout of the unbalanced planar magnetron system allowing for continuous control of the operating mode from type 1 to type II is shown in Figure 1. The magnetron is mounted inside a cylindrical stainless-steel vacuum vessel. The graphite target 4 with a diameter of 65 mm and a thickness of 6 mm is attached to the water-cooled surface of the magnetron unit. The screen 6 confines the discharge to the front surface of the target only. A rotating shutter 3 is placed between the target surface and the substrate holder 7 with the substrates 8. The vacuum system used is of the conventional type for industrial deposition of thin films, equipped with a rotation pump and an oil-diffusion pump to ensure background pressure within the range of 10m4Pa. The magnetic configuration of the magnetron unit consists of an internal disk magnet pole 5 based on SmCo permanent magnets with an outer diameter of 30mm, an annular external pole 2 made of soft iron with outer diameter of 94 mm, and a coaxial solenoid coil 1 comprising of 1800 windings of Cu-wire, allowing coil currents Z, up to 2.6 A without cooling. The maximum values obtained for the perpendicular B, and parallel B= magnetic field components measured over the central magnetic pole at 0 A 1395
J Kout-tev et al: Unbalanced
planar magnetron
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solenoid current are 80 mT and 52 mT at distances 0 and 1.5 cm above the target surface, respectively. The highest value of the magnetic field, B,, generated by the electromagnet at 2.6 A solenoid current, reaches 48mT. All measurements of the magnetic field components were performed using a small-sized (3 mm) Hall probe. For determining the VA-characteristics of the magnetron unit, working gas Ar was injected into the recipient to achieve the desired pressures in the range l-8 Pa. The VA-curves were measured after preliminary sputter-cleaning of the target surface at the maximum current I = 0.5A supplied by the dc target power supply. Another dc constant current power supply was used for the variation of I, and hence for continuous control of the operating mode. A third power supply was utilized for probe holder biasing to realize ion-assisted mode of deposition.
Experimental results and discussion
Figure 2 presents the radial distribution of the parallel magnetic field component B= on the target surface for two values of the solenoid coil current Z,,,= 0 A (typical type I operating mode) and I,,, = 2 A (typical type II operating mode). As seen from the figure, there is a great symmetry of both magnetic field maxima (B_: B, = 98%) under the two entirely different operating modes of the magnetron unit.
I
10
20
30
40
50
60
70
80
I-I [mm1 of the perpendicular magnetic field component B, on the height H from the target centre for 4 different values of coil current I, (0, 1.0, 1.5 and 2.OA).
Figure 3. Dependence
7 a Figure 1. Schematic view of the planar magnetron system with continuous control of the operating mode from type I to type II. 1,external solenoid; 2, annular external magnetic pole; 3, rotating shutter; 4, graphite target; 5, SmCo permanent magnets; 6, screen; 7, substrate holder; 8, substrates; E,, dc power supply for the target; E,, dc power supply for the solenoid coil; E,,, dc power supply for the substrate bias.
60~
0
From the variety of magnetic field distributions in an unbalanced planar magnetron system, the longitudinal distribution of the perpendicular component B, at different coil currents 1, is of greatest importance, since this dependence underlines most distinctly the differences between type I and type II operating modes. The basic experimental finding regarding this distribution appears to be the change in sign of the variable B, at a certain distance from the target surface, which is characteristic for unbalanced magnetrons of type II only and could be used as a criterion for the respective working mode. Figure 3 presents the summarized results for the longitudinal distribution of B, above the target centre of the magnetron unit for 4 fixed values of the coil current Z, (O,l, 1.5 and 2A). The curves plotted in Figure 3 show a transition of B, through 0 for all non zero values of I,, which is an evidence for type II operating mode. Only at 1, = 0 A, the quantity B, decreases exponentially towards 0, thus indicating an operating mode of type I. The summarized results for the VA-characteristics at Ar gas pressure p = 4Pa and different coil currents 1, (1.4, 1.7, 2.0, 2.3 and 2.6A) are presented in Figure 4. The characteristics were measured by variation of the magnetron discharge current from 50 to 500mA, as these values could be reproducibly obtained using the available power supply. The experimentally obtained values for flowing potential, U, and ion current density, j, on the surface of the substrate holder within the above ranges of variation of I,,, and p, exhibit an increase ofj by more than an order of magnitude (from 0.044 to 0.07 mA cm-‘, respectively) through continuous transition of the operating mode from type I to type II.
I
*
> 3
-6Oe
5000 -40
-20
0
20
40
R [mm1 Figure 2. Dependence of the parallel magnetic field component B= on the radial distance R from the target centre for 2 different values of coil current I, (0 and 2.0 A). 1396
1 ’ . ’ . ’ . ’ 100 200 300 400
’
500
I IdI Figure 4. VA-characteristics of the unbalanced magnetron system at Ar gas pressure p = 4 Pa for 5 different values of coil current I,,, (1.4, 1.7,2.0, 2.3 and 2.6A).
J Kourtev et a/: Unbalanced
planar magnetron
It is to be noted that at coil currents of 1.4 and 1.7 A, the magnetron units operate in type I working mode, while at coil currents higher than 2 A the magnetrons work in type II mode. The lowest running voltages are observed at Z, = 2 A, which is an indication of operating conditions similar to those of a perfectly balanced magnetron. On the grounds of the experimental results presented in Figures 2-4 it can be concluded that the magnetic configuration of the newly developed magnetron unit is fully capable of continuously controlling the operating mode from type I to type II unbalanced magnetron. More detailed investigations are needed, however, to determine the potential possibilities of this magnetron system for ion-assisted deposition of thin coatings with enhanced properties for industrial applications.
Acknowledgements The financial support provided by the Bulgarian National Scientific Fund under contracts F-316 and TN-582 are gratefully acknowledged. References 1. B Window and N Sawides, J Vat Sci Technol, A4, 196 (1986). 2. J Musil, S Kadlec and W-D Muenz, J Vat Sci Technol, A9, 1171 (1991). 3. G A Clarke, N R Osborne and R R Parsons, J Vat Sci Technol, A9,
1166 (1991). 4. T Takahashi, in Electronics and Communications in Japan, Part 2,
69(4), 57 (1986). 5. I Petrov, F Adibi, J E Greene, W D Sproul and W-D Muenz, J Vat Sci Technol, A10(5), 3283 (1992).
1397
Pergamon PII: SOO42-207X(96)00186-8
47/number II/pages 1395 to 1397/1996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All riahts reserved 0042-207X/96 $15.00+.00
Unbalanced planar magnetron with continuous control of the operating mode from type I to type II* J Kourtev, S Groudeva-Zotova, I Garnev and V Orlinov, institute of Electronics, Sciences, 72 Tzarigradsko Chaussee, 7784 Sofia, Bulgaria received 20 November
Bulgarian Academy
of
1995
A compact and simple planar magnetron design is described based on the known concept for modification of the magnetic field by a co-axially placed solenoid. The magnetron unit ensures sufficient values of the perpendicular B, and the parallel B, components of the magnetic induction above the target surface allowing for a wide range of variation of the operating mode from type I to the unbalanced type Il. The following experimental data characterizing the newly developed magnetron system are reported: (il radial distribution of B, at different distances above the rarger surface and at different solenoid currents I,,,; (ii) longitudinal distribution of B, along axes parallel to the central one, measured again at different I,,,; (iii) VA-characteristics of rhe magnetron unit (with a graphite target) for different values of I,,, (from 0 to 2.6Al and at working gas pressure p - = 4 Pa Arl. Copyright 0 1996 Elsevier Science Lrd
Introduction Unbalanced type II magnetrons represent a further improvement of the magnetron sputtering method for deposition of thin films with enhanced performance.‘,* The basic advantage of this novel technique consists in the wide possibilities for variation of the ion flux to the substrate surface and hence for controlling the structure and properties of the growing films.* In most cases the construction of such unbalanced magnetron systems is characterized by a much higher magnetic flux of the peripheral magnet pole as compared with that of the internal one. Conventionally such systems are realized using permanent magnets3 and are therefore characterized by a fixed unbalance ratio, a nearly constant geometry of the magnetron plasma and a limited variation of the ion flux to the substrates. A more effective method for controlling the unbalance ratio is the utilization of an external solenoid coil, which is normally situated outside the vacuum recipient4 and coaxially with respect to the magnetic poles of the magnetron. Depending on the current direction through the solenoid coil, the superimposed magnetic field of the solenoid can set the initial magnetron system into either type I or type II unbalanced operating mode. The magnetron system newly developed uses an improved magnetic configuration similar to the one reported in Ref. 5, but with a solenoid coil mounted inside the vacuum chamber coaxially along the magnetron unit.
*Paper based on that presented at the 9th International School on Vacuum, Electron and Ion Technologies (VEIT’95), 14-17 September 1995,Sozopol, Bulgaria, 1995
The preliminary investigations carried out with the aim to set into operation and characterize the magnetron system were performed using a graphite target and working gas Ar. The obtained results show that, by choosing appropriate values for the solenoid current Z,, both operating modes of the magnetron system, i.e. as type I or type II unbalanced magnetron, as well as a smooth transition between these modes could be achieved.
Experimental A schematic layout of the unbalanced planar magnetron system allowing for continuous control of the operating mode from type 1 to type II is shown in Figure 1. The magnetron is mounted inside a cylindrical stainless-steel vacuum vessel. The graphite target 4 with a diameter of 65 mm and a thickness of 6 mm is attached to the water-cooled surface of the magnetron unit. The screen 6 confines the discharge to the front surface of the target only. A rotating shutter 3 is placed between the target surface and the substrate holder 7 with the substrates 8. The vacuum system used is of the conventional type for industrial deposition of thin films, equipped with a rotation pump and an oil-diffusion pump to ensure background pressure within the range of 10m4Pa. The magnetic configuration of the magnetron unit consists of an internal disk magnet pole 5 based on SmCo permanent magnets with an outer diameter of 30mm, an annular external pole 2 made of soft iron with outer diameter of 94 mm, and a coaxial solenoid coil 1 comprising of 1800 windings of Cu-wire, allowing coil currents Z, up to 2.6 A without cooling. The maximum values obtained for the perpendicular B, and parallel B= magnetic field components measured over the central magnetic pole at 0 A 1395
J Kout-tev et al: Unbalanced
planar magnetron
I
+zf
/
I
1
/’
: ,
1
23
4
’
5
wb
“if&
L-lT
677 “’
T
solenoid current are 80 mT and 52 mT at distances 0 and 1.5 cm above the target surface, respectively. The highest value of the magnetic field, B,, generated by the electromagnet at 2.6 A solenoid current, reaches 48mT. All measurements of the magnetic field components were performed using a small-sized (3 mm) Hall probe. For determining the VA-characteristics of the magnetron unit, working gas Ar was injected into the recipient to achieve the desired pressures in the range l-8 Pa. The VA-curves were measured after preliminary sputter-cleaning of the target surface at the maximum current I = 0.5A supplied by the dc target power supply. Another dc constant current power supply was used for the variation of I, and hence for continuous control of the operating mode. A third power supply was utilized for probe holder biasing to realize ion-assisted mode of deposition.
Experimental results and discussion
Figure 2 presents the radial distribution of the parallel magnetic field component B= on the target surface for two values of the solenoid coil current Z,,,= 0 A (typical type I operating mode) and I,,, = 2 A (typical type II operating mode). As seen from the figure, there is a great symmetry of both magnetic field maxima (B_: B, = 98%) under the two entirely different operating modes of the magnetron unit.
I
10
20
30
40
50
60
70
80
I-I [mm1 of the perpendicular magnetic field component B, on the height H from the target centre for 4 different values of coil current I, (0, 1.0, 1.5 and 2.OA).
Figure 3. Dependence
7 a Figure 1. Schematic view of the planar magnetron system with continuous control of the operating mode from type I to type II. 1,external solenoid; 2, annular external magnetic pole; 3, rotating shutter; 4, graphite target; 5, SmCo permanent magnets; 6, screen; 7, substrate holder; 8, substrates; E,, dc power supply for the target; E,, dc power supply for the solenoid coil; E,,, dc power supply for the substrate bias.
60~
0
From the variety of magnetic field distributions in an unbalanced planar magnetron system, the longitudinal distribution of the perpendicular component B, at different coil currents 1, is of greatest importance, since this dependence underlines most distinctly the differences between type I and type II operating modes. The basic experimental finding regarding this distribution appears to be the change in sign of the variable B, at a certain distance from the target surface, which is characteristic for unbalanced magnetrons of type II only and could be used as a criterion for the respective working mode. Figure 3 presents the summarized results for the longitudinal distribution of B, above the target centre of the magnetron unit for 4 fixed values of the coil current Z, (O,l, 1.5 and 2A). The curves plotted in Figure 3 show a transition of B, through 0 for all non zero values of I,, which is an evidence for type II operating mode. Only at 1, = 0 A, the quantity B, decreases exponentially towards 0, thus indicating an operating mode of type I. The summarized results for the VA-characteristics at Ar gas pressure p = 4Pa and different coil currents 1, (1.4, 1.7, 2.0, 2.3 and 2.6A) are presented in Figure 4. The characteristics were measured by variation of the magnetron discharge current from 50 to 500mA, as these values could be reproducibly obtained using the available power supply. The experimentally obtained values for flowing potential, U, and ion current density, j, on the surface of the substrate holder within the above ranges of variation of I,,, and p, exhibit an increase ofj by more than an order of magnitude (from 0.044 to 0.07 mA cm-‘, respectively) through continuous transition of the operating mode from type I to type II.
I
*
> 3
-6Oe
5000 -40
-20
0
20
40
R [mm1 Figure 2. Dependence of the parallel magnetic field component B= on the radial distance R from the target centre for 2 different values of coil current I, (0 and 2.0 A). 1396
1 ’ . ’ . ’ . ’ 100 200 300 400
’
500
I IdI Figure 4. VA-characteristics of the unbalanced magnetron system at Ar gas pressure p = 4 Pa for 5 different values of coil current I,,, (1.4, 1.7,2.0, 2.3 and 2.6A).
J Kourtev et a/: Unbalanced
planar magnetron
It is to be noted that at coil currents of 1.4 and 1.7 A, the magnetron units operate in type I working mode, while at coil currents higher than 2 A the magnetrons work in type II mode. The lowest running voltages are observed at Z, = 2 A, which is an indication of operating conditions similar to those of a perfectly balanced magnetron. On the grounds of the experimental results presented in Figures 2-4 it can be concluded that the magnetic configuration of the newly developed magnetron unit is fully capable of continuously controlling the operating mode from type I to type II unbalanced magnetron. More detailed investigations are needed, however, to determine the potential possibilities of this magnetron system for ion-assisted deposition of thin coatings with enhanced properties for industrial applications.
Acknowledgements The financial support provided by the Bulgarian National Scientific Fund under contracts F-316 and TN-582 are gratefully acknowledged. References 1. B Window and N Sawides, J Vat Sci Technol, A4, 196 (1986). 2. J Musil, S Kadlec and W-D Muenz, J Vat Sci Technol, A9, 1171 (1991). 3. G A Clarke, N R Osborne and R R Parsons, J Vat Sci Technol, A9,
1166 (1991). 4. T Takahashi, in Electronics and Communications in Japan, Part 2,
69(4), 57 (1986). 5. I Petrov, F Adibi, J E Greene, W D Sproul and W-D Muenz, J Vat Sci Technol, A10(5), 3283 (1992).
1397