- Email: [email protected]
Flat erosion magnetron sputtering with a moving unbalanced magnet
ARTICLE IN PRESS
Vacuum 80 (2006) 662–666 www.elsevier.com/locate/vacuum
Flat erosion magnetron sputtering with a moving unbalanced magnet Takayuki Iseki Technology Development Department, JVC, Victor Company of Japan Ltd., Shinmei-cho 58-7, Yokosuka, Kanagawa 239-8550, Japan
Abstract A flat erosion-sputtering system has been developed using a rotating unbalanced and asymmetrical magnet. Target utilization and uniformity of erosion on the 5-in. target were greatly improved over the usual magnetron sputtering system by using a novel magnetron cathode to generate a wider, rotating magnetic flux. The novel magnetron cathode used the yoke magnet in which the stronger magnet set shifted towards the center and the weaker towards the periphery. This yoke magnet was attached to slant toward the center of rotation and rotated during sputtering. The estimated target utilization went up to 80% when the target was finally used up, and the uniformity of sputtered film was within 5% over an area 80 mm in diameter. r 2005 Elsevier Ltd. All rights reserved. Keywords: Magnetron sputtering; Flat erosion; Unbalanced magnet; Target utilization
1. Introduction
2. Simulation
Sputtering technology, especially a planar magnetron sputtering, is widely used in many kinds of industrial manufacturing to deposit thin films because of the high rate and wide area deposition. However, the target utilization is only about 20–40% for the conventional planar magnetron sputtering in general [1] due to the narrow and deep groove erosion on the target. A common practice is to rotate the permanent magnet in the circular cathode to expand the erosion area and increase the target utilization. In contrast with the usual system, this increases the target utilization to about 40–60%, in general [2]. However, there still remain some non-eroded areas, especially at the center and outside the target. This paper discusses the way of increasing target utilization still further, and obtaining flat erosion over the entire area of the target, found by investigating the rotation of the permanent magnet with respect to the circular target.
The usual magnetron sputtering system has a permanent magnet that is ‘‘balanced’’. The magnetic flux from one yoke pole is symmetrical to that from the other. In contrast, an ‘‘unbalanced’’ magnetron is simply a design where the magnetic flux pattern at the yoke poles is strongly asymmetrical [3]. The usual configuration of an unbalanced magnet in a sputtering system is a yoke magnet with a stronger outer and weaker inner magnet. Fig. 1 shows the simulated magnetic flux of a balanced and an unbalanced magnetron for a 3-in. target. We assumed the target to be a non-magnetic material. In a balanced magnet made of iron–neodymium–boron (Fe–Nd–B) magnet for yoke poles, the maximum flux density points, shown by the dotted line in Fig. 1(a), line up almost perpendicular to the target surface. That is why the target is sputtered mainly on the narrow erosion area and target utilization is very small. On the other hand, in an unbalanced magnet made of center Fe–Nd–B and outer magnetized Fe, the opposite of the usual configuration of unbalanced magnetron, the maximum flux density points, shown by the dotted line in Fig. 1(b), are inclined towards outside. Therefore, if the target is used continuously, the erosion point on the target surface will shift slightly
Tel.: +81 46 836 9205; fax: +81 46 836 3208.
E-mail address: [email protected]. 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.019
ARTICLE IN PRESS T. Iseki / Vacuum 80 (2006) 662–666
towards inside and target utilization will increase by the shifting of the erosion point with continuous use. In this unbalanced magnetron design, the erosion point changes with target thickness. Therefore, it is reasonable to
663
speculate that the erosion area will expand significantly if the distance between the surface of the target and the magnet is changed. For example, it will be possible to build the mechanism to drive the yoke magnet up and down in the cathode body. However, a planar rotary magnetron sputtering system is widely used and this simple design is comparable to the up and down magnet mechanism.
3. Rotary magnet arrangement design
Fig. 1. Simulation result of magnetic flux: (a) a balanced magnet, (b) an unbalanced magnet.
Chamber
Substrate
In order to exploit this widening phenomenon by means of sputtering cathode with a rotary magnet mechanism, I have devised some types of magnet arrangement shown in Fig. 2. Type A has a rotary unbalanced magnet that is tilted. Tilting this magnet makes the distance between the target and the magnet surface different on the left and right sides of the cathode, as can be seen in Fig. 2(type A). Therefore, when this magnet rotates, the distance between the target and the magnet changes continuously, so the erosion area will move continuously and expand the sputtered area. Type B has a rotary unbalanced magnet with the center magnet shifted. The distance between the center and side yoke poles is different on the left and right sides of the cathode, as can be seen in Fig. 2(type B). Therefore, the maximum flux density point on the target face will also be different on the right and left, and the erosion area will move continuously and expand the sputtered area as the magnet rotates.
Chamber
Substrate
Erosion (moving)
Erosion (moving) Target
(Plasma) Fe
FeNdB
FeNdB Fe
Fe cathode Rotation axis
Target
(Plasma)
Yoke magnet cathode (Tilted arrangement) Rotation axis (Rotation)
(Type A)
Fe Yoke magnet (Center magnet shifted) (Rotation)
(Type B)
Erosion (moving) Target
(Plasma) Fe
FeNdB
Fe cathode Rotation axis
(Type A+B)
Yoke magnet (Tilted and center magnet shifted arrangement) (Rotation)
Fig. 2. Structures of rotary magnet arrangement: type A: tilted, type B: center magnet shifted, type A+B: tilted and center magnet shifted.
ARTICLE IN PRESS T. Iseki / Vacuum 80 (2006) 662–666
664
And more, a new type of rotary magnet has been devised with a configuration that is a mix of type A and type B arrangements. In a word, this sputtering system has a center-pole-shifted unbalanced magnet with a tilted arrangement and a target rotating mechanism as illustrated in Fig. 2(type A+B). 4. Experimental Experimental magnetron sputtering system has a 5-in. circular cathode with rf (13.56 MHz) power generation. This time, by changing only the design and rotation mechanism of the yoke magnet, experiments could investigate both the usual magnetron sputter and the effect rotation had on the sputter. The yoke magnet comprised a center pole magnet and an outer pole with a base. The center pole was a Fe–Nd–B magnet and the outer pole and 7 6 5 4 3 2 1
5. Result and discussion Fig. 3 shows a cross-sectional profile of a target that was sputtered in systems of type A, type B and type A+B, compared with sputtering results from the usual fixed
(Fixed magnetron)
Initial (a)
7 6 5 4 3 2 1 Target thickness (mm)
base were Fe, magnetized by the center magnet to make an unbalanced magnet. Only in the case of the usual rotary magnetron, a balanced magnet was used for an experiment. An aluminum plate 5-in. in diameter and 5 mm thick was used for the target. The distance between target and substrate was set at 100 mm. Sputtering gas pressure and rf power were 0.5 Pa and 300 W, respectively, throughout all experiments. Magnet rotation speed was 8 rpm and tilted angle of magnet was 101 in type A and type A+B. After 140 h of sputtering, target profiles were measured and the surface states of the targets were observed.
Experimental Calculated
(Usual rotary magnetron)
(b)
7 6 5 4 3 2 1
(type A)
(c)
7 6 5 4 3 2 1
(type B)
(d)
7 6 5 4 3 2 1
(type A+B)
(e) 0
10
20
30
40
50
60
70
Target position from center (mm) Fig. 3. Cross-sectional profiles of sputtered target: (a) usual fixed magnet, (b) usual rotary magnet, (c) type A rotary magnet, (d) type B rotary magnet, (e) type A+B rotary magnet.
ARTICLE IN PRESS T. Iseki / Vacuum 80 (2006) 662–666
Fig. 4. Photographs of target surface: (a) usual fixed magnet, (b) usual rotary magnet, (c) type A rotary magnet, (d) type B rotary magnet, (e) type A+B rotary magnet.
magnetron and a rotary magnetron. In these figures, solid lines were experimental cross-sections and short dash lines were estimated cross-sections if target was finally used with the assumption of constant sputtering ratios in all surface. Fig. 4 shows photographs of the target surface. With sputtering by the usual fixed magnetron (a), the erosion area was concentrated very narrowly as a deep groove on a circle of about 66 mm diameter. With rotary magnetron sputtering (b), flatness of the target was slightly improved, and erosion was expanded over an area between 50 and 90 mm in diameter. With sputtering from the type A magnetron (c), the erosion area was expanded into a circle of about 80 mm in diameter, with a groove not as deep as that from the usual fixed magnetron. It gave better target utilization than the usual fixed magnetron, but it did not seem to be better than the usual rotary magnetron.
665
With the type B magnetron (d), the sputtered target’s profile was similar to that produced by the usual rotary magnetron. Erosion was expanded wide and broad across a 40–90 mm diameter area, making this appear to be a somewhat better target utilization than the usual rotary magnetron. In this cathode structure, center magnet was widely shifted from center so that the magnetic flux area was also widely expanded and moved by rotation. From this calculated profile, target utilizations were estimated about 70%. In these cases, the erosion profiles were circular or doughnut shaped, so the center and outer part of the target area were not sputtered as much. On the other hand, with the type A+B magnetron (e), target utilization was remarkably improved, and the expanded erosion area was mostly flat as seen in the target profile and photograph. The center and outer part of the target were also sputtered, producing better uniformity of surface profile than with other sputtering systems. In this magnet arrangement, as can be seen in Fig. 2(type A+B), the magnetic flux between the center and right side yokes would be more concentrated, and the maximum flux density point would be shifted more to the right towards outside of the target. Also, the maximum flux density point between the center and left side yokes would be shifted more towards inside of the target. By rotating this yoke magnet continuously, the expectation was that the erosion area would be broadened and expanded from the center to the outer part of the target. From this calculated profile, target utilizations were estimated about 80% over. Fig. 5 shows the thickness uniformity of sputtered films produced by the newly developed magnetron (c) compared with the usual fixed magnetron (a) and the usual rotary magnetron (b). With the newly developed magnetronsputtering system, thickness uniformity was 5% over an 80 mm diameter area, much better than with the usual systems. However, sputtering rate was lower than usual magnetron sputtering because magnetic flux density at the target surface was lower with this magnet arrangement. In fact, the sputtering rate was 23 nm/min and about 20% lower than the usual fixed magnetron, so it was not as good as the usual. It will be possible to restore the sputtering rate by using stronger or larger permanent magnets. 6. Conclusions A flat erosion sputtering system has been developed using a rotary unbalanced magnet. Both target utilization and the uniformity of erosion of a 5-in. target were greatly improved by using a cathode magnet with the center magnet shifted and tilted in an arrangement with a rotating axis. As this yoke magnet rotates, the magnetic flux moves to cover most of the target face, achieving mostly full and flat erosion of the cathode. Target utilization was estimated at up to 80% if the target is used to completion, and thin film uniformity can be within 5% across an 80 mm diameter area. These characteristics of sputtering were
ARTICLE IN PRESS T. Iseki / Vacuum 80 (2006) 662–666
666
120 100 (20%)
80 60 40
Film thickness uniformity (%)
20
(Usual fixed magnet) (a)
120 100 (18%)
80 60 40 20
(Usual rotary magnet) (b)
120 100 (5%)
80 60 40 (Type A+B)
20 0
(c) -80
-60
-40
-20 0 20 Substrate position (mm)
40
60
80
Fig. 5. Thickness uniformity of sputtered films: (a) usual fixed magnet, (b) usual rotary magnet, (c) type A+B rotary magnet.
greatly improved compared with conventional magnetron sputtering systems, which use fixed or rotary magnets. Acknowledgement The author would like to thank Mr. Toshiaki Yamamoto, retired from JVC, for providing the simulation data.
References [1] Van Vorous T. Solid State Technol 1976;19:62. [2] Hosokawa N, Tsukada T, Misumi T. J Vac Sci Technol 1977;14:143. [3] Kelly PJ, Arnell RD. J Vac Sci Technol 1998;16(5):2858–69.
Vacuum 80 (2006) 662–666 www.elsevier.com/locate/vacuum
Flat erosion magnetron sputtering with a moving unbalanced magnet Takayuki Iseki Technology Development Department, JVC, Victor Company of Japan Ltd., Shinmei-cho 58-7, Yokosuka, Kanagawa 239-8550, Japan
Abstract A flat erosion-sputtering system has been developed using a rotating unbalanced and asymmetrical magnet. Target utilization and uniformity of erosion on the 5-in. target were greatly improved over the usual magnetron sputtering system by using a novel magnetron cathode to generate a wider, rotating magnetic flux. The novel magnetron cathode used the yoke magnet in which the stronger magnet set shifted towards the center and the weaker towards the periphery. This yoke magnet was attached to slant toward the center of rotation and rotated during sputtering. The estimated target utilization went up to 80% when the target was finally used up, and the uniformity of sputtered film was within 5% over an area 80 mm in diameter. r 2005 Elsevier Ltd. All rights reserved. Keywords: Magnetron sputtering; Flat erosion; Unbalanced magnet; Target utilization
1. Introduction
2. Simulation
Sputtering technology, especially a planar magnetron sputtering, is widely used in many kinds of industrial manufacturing to deposit thin films because of the high rate and wide area deposition. However, the target utilization is only about 20–40% for the conventional planar magnetron sputtering in general [1] due to the narrow and deep groove erosion on the target. A common practice is to rotate the permanent magnet in the circular cathode to expand the erosion area and increase the target utilization. In contrast with the usual system, this increases the target utilization to about 40–60%, in general [2]. However, there still remain some non-eroded areas, especially at the center and outside the target. This paper discusses the way of increasing target utilization still further, and obtaining flat erosion over the entire area of the target, found by investigating the rotation of the permanent magnet with respect to the circular target.
The usual magnetron sputtering system has a permanent magnet that is ‘‘balanced’’. The magnetic flux from one yoke pole is symmetrical to that from the other. In contrast, an ‘‘unbalanced’’ magnetron is simply a design where the magnetic flux pattern at the yoke poles is strongly asymmetrical [3]. The usual configuration of an unbalanced magnet in a sputtering system is a yoke magnet with a stronger outer and weaker inner magnet. Fig. 1 shows the simulated magnetic flux of a balanced and an unbalanced magnetron for a 3-in. target. We assumed the target to be a non-magnetic material. In a balanced magnet made of iron–neodymium–boron (Fe–Nd–B) magnet for yoke poles, the maximum flux density points, shown by the dotted line in Fig. 1(a), line up almost perpendicular to the target surface. That is why the target is sputtered mainly on the narrow erosion area and target utilization is very small. On the other hand, in an unbalanced magnet made of center Fe–Nd–B and outer magnetized Fe, the opposite of the usual configuration of unbalanced magnetron, the maximum flux density points, shown by the dotted line in Fig. 1(b), are inclined towards outside. Therefore, if the target is used continuously, the erosion point on the target surface will shift slightly
Tel.: +81 46 836 9205; fax: +81 46 836 3208.
E-mail address: [email protected]. 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.019
ARTICLE IN PRESS T. Iseki / Vacuum 80 (2006) 662–666
towards inside and target utilization will increase by the shifting of the erosion point with continuous use. In this unbalanced magnetron design, the erosion point changes with target thickness. Therefore, it is reasonable to
663
speculate that the erosion area will expand significantly if the distance between the surface of the target and the magnet is changed. For example, it will be possible to build the mechanism to drive the yoke magnet up and down in the cathode body. However, a planar rotary magnetron sputtering system is widely used and this simple design is comparable to the up and down magnet mechanism.
3. Rotary magnet arrangement design
Fig. 1. Simulation result of magnetic flux: (a) a balanced magnet, (b) an unbalanced magnet.
Chamber
Substrate
In order to exploit this widening phenomenon by means of sputtering cathode with a rotary magnet mechanism, I have devised some types of magnet arrangement shown in Fig. 2. Type A has a rotary unbalanced magnet that is tilted. Tilting this magnet makes the distance between the target and the magnet surface different on the left and right sides of the cathode, as can be seen in Fig. 2(type A). Therefore, when this magnet rotates, the distance between the target and the magnet changes continuously, so the erosion area will move continuously and expand the sputtered area. Type B has a rotary unbalanced magnet with the center magnet shifted. The distance between the center and side yoke poles is different on the left and right sides of the cathode, as can be seen in Fig. 2(type B). Therefore, the maximum flux density point on the target face will also be different on the right and left, and the erosion area will move continuously and expand the sputtered area as the magnet rotates.
Chamber
Substrate
Erosion (moving)
Erosion (moving) Target
(Plasma) Fe
FeNdB
FeNdB Fe
Fe cathode Rotation axis
Target
(Plasma)
Yoke magnet cathode (Tilted arrangement) Rotation axis (Rotation)
(Type A)
Fe Yoke magnet (Center magnet shifted) (Rotation)
(Type B)
Erosion (moving) Target
(Plasma) Fe
FeNdB
Fe cathode Rotation axis
(Type A+B)
Yoke magnet (Tilted and center magnet shifted arrangement) (Rotation)
Fig. 2. Structures of rotary magnet arrangement: type A: tilted, type B: center magnet shifted, type A+B: tilted and center magnet shifted.
ARTICLE IN PRESS T. Iseki / Vacuum 80 (2006) 662–666
664
And more, a new type of rotary magnet has been devised with a configuration that is a mix of type A and type B arrangements. In a word, this sputtering system has a center-pole-shifted unbalanced magnet with a tilted arrangement and a target rotating mechanism as illustrated in Fig. 2(type A+B). 4. Experimental Experimental magnetron sputtering system has a 5-in. circular cathode with rf (13.56 MHz) power generation. This time, by changing only the design and rotation mechanism of the yoke magnet, experiments could investigate both the usual magnetron sputter and the effect rotation had on the sputter. The yoke magnet comprised a center pole magnet and an outer pole with a base. The center pole was a Fe–Nd–B magnet and the outer pole and 7 6 5 4 3 2 1
5. Result and discussion Fig. 3 shows a cross-sectional profile of a target that was sputtered in systems of type A, type B and type A+B, compared with sputtering results from the usual fixed
(Fixed magnetron)
Initial (a)
7 6 5 4 3 2 1 Target thickness (mm)
base were Fe, magnetized by the center magnet to make an unbalanced magnet. Only in the case of the usual rotary magnetron, a balanced magnet was used for an experiment. An aluminum plate 5-in. in diameter and 5 mm thick was used for the target. The distance between target and substrate was set at 100 mm. Sputtering gas pressure and rf power were 0.5 Pa and 300 W, respectively, throughout all experiments. Magnet rotation speed was 8 rpm and tilted angle of magnet was 101 in type A and type A+B. After 140 h of sputtering, target profiles were measured and the surface states of the targets were observed.
Experimental Calculated
(Usual rotary magnetron)
(b)
7 6 5 4 3 2 1
(type A)
(c)
7 6 5 4 3 2 1
(type B)
(d)
7 6 5 4 3 2 1
(type A+B)
(e) 0
10
20
30
40
50
60
70
Target position from center (mm) Fig. 3. Cross-sectional profiles of sputtered target: (a) usual fixed magnet, (b) usual rotary magnet, (c) type A rotary magnet, (d) type B rotary magnet, (e) type A+B rotary magnet.
ARTICLE IN PRESS T. Iseki / Vacuum 80 (2006) 662–666
Fig. 4. Photographs of target surface: (a) usual fixed magnet, (b) usual rotary magnet, (c) type A rotary magnet, (d) type B rotary magnet, (e) type A+B rotary magnet.
magnetron and a rotary magnetron. In these figures, solid lines were experimental cross-sections and short dash lines were estimated cross-sections if target was finally used with the assumption of constant sputtering ratios in all surface. Fig. 4 shows photographs of the target surface. With sputtering by the usual fixed magnetron (a), the erosion area was concentrated very narrowly as a deep groove on a circle of about 66 mm diameter. With rotary magnetron sputtering (b), flatness of the target was slightly improved, and erosion was expanded over an area between 50 and 90 mm in diameter. With sputtering from the type A magnetron (c), the erosion area was expanded into a circle of about 80 mm in diameter, with a groove not as deep as that from the usual fixed magnetron. It gave better target utilization than the usual fixed magnetron, but it did not seem to be better than the usual rotary magnetron.
665
With the type B magnetron (d), the sputtered target’s profile was similar to that produced by the usual rotary magnetron. Erosion was expanded wide and broad across a 40–90 mm diameter area, making this appear to be a somewhat better target utilization than the usual rotary magnetron. In this cathode structure, center magnet was widely shifted from center so that the magnetic flux area was also widely expanded and moved by rotation. From this calculated profile, target utilizations were estimated about 70%. In these cases, the erosion profiles were circular or doughnut shaped, so the center and outer part of the target area were not sputtered as much. On the other hand, with the type A+B magnetron (e), target utilization was remarkably improved, and the expanded erosion area was mostly flat as seen in the target profile and photograph. The center and outer part of the target were also sputtered, producing better uniformity of surface profile than with other sputtering systems. In this magnet arrangement, as can be seen in Fig. 2(type A+B), the magnetic flux between the center and right side yokes would be more concentrated, and the maximum flux density point would be shifted more to the right towards outside of the target. Also, the maximum flux density point between the center and left side yokes would be shifted more towards inside of the target. By rotating this yoke magnet continuously, the expectation was that the erosion area would be broadened and expanded from the center to the outer part of the target. From this calculated profile, target utilizations were estimated about 80% over. Fig. 5 shows the thickness uniformity of sputtered films produced by the newly developed magnetron (c) compared with the usual fixed magnetron (a) and the usual rotary magnetron (b). With the newly developed magnetronsputtering system, thickness uniformity was 5% over an 80 mm diameter area, much better than with the usual systems. However, sputtering rate was lower than usual magnetron sputtering because magnetic flux density at the target surface was lower with this magnet arrangement. In fact, the sputtering rate was 23 nm/min and about 20% lower than the usual fixed magnetron, so it was not as good as the usual. It will be possible to restore the sputtering rate by using stronger or larger permanent magnets. 6. Conclusions A flat erosion sputtering system has been developed using a rotary unbalanced magnet. Both target utilization and the uniformity of erosion of a 5-in. target were greatly improved by using a cathode magnet with the center magnet shifted and tilted in an arrangement with a rotating axis. As this yoke magnet rotates, the magnetic flux moves to cover most of the target face, achieving mostly full and flat erosion of the cathode. Target utilization was estimated at up to 80% if the target is used to completion, and thin film uniformity can be within 5% across an 80 mm diameter area. These characteristics of sputtering were
ARTICLE IN PRESS T. Iseki / Vacuum 80 (2006) 662–666
666
120 100 (20%)
80 60 40
Film thickness uniformity (%)
20
(Usual fixed magnet) (a)
120 100 (18%)
80 60 40 20
(Usual rotary magnet) (b)
120 100 (5%)
80 60 40 (Type A+B)
20 0
(c) -80
-60
-40
-20 0 20 Substrate position (mm)
40
60
80
Fig. 5. Thickness uniformity of sputtered films: (a) usual fixed magnet, (b) usual rotary magnet, (c) type A+B rotary magnet.
greatly improved compared with conventional magnetron sputtering systems, which use fixed or rotary magnets. Acknowledgement The author would like to thank Mr. Toshiaki Yamamoto, retired from JVC, for providing the simulation data.
References [1] Van Vorous T. Solid State Technol 1976;19:62. [2] Hosokawa N, Tsukada T, Misumi T. J Vac Sci Technol 1977;14:143. [3] Kelly PJ, Arnell RD. J Vac Sci Technol 1998;16(5):2858–69.