REVIEW OF SCIENTIFIC INSTRUMENTS 80, 104704 共2009兲

Double circular erosion patterns on dielectric target in magnetron
sputtering
Yoshifumi Suzaki, Hayato Miyagawa, and Seiki Ejima
Department of Advanced Materials Science, Kagawa University, 2217-20 Hayashi-cho, Takamatsu,
Kagawa 761-0396, Japan

共Received 25 April 2009; accepted 24 August 2009; published online 13 October 2009兲
In rf magnetron sputtering, a circular erosion pattern forms on the surface of a circular metal
conductor target with permanent magnets on its back. In this case, the theory behind the erosion
pattern has been established. However, in the case of a dielectric target, a double circular erosion
pattern is formed. So far, this pattern has been phenomenologically recognized by experimenters;
however, it has not yet been investigated. In this study, we performed a magnetron sputteringexperiment with a SiO2 dielectric target, and confirmed the formation of a double circular erosion
pattern. The dimensions of the double circular erosion pattern varied depending on the insulation
resistance or the thickness of the SiO2 target. Furthermore, we found that the dimensions of a double
circular erosion pattern changed by making a gap between the SiO2 target and guard ring. Based on
the experimental results, we have proposed a qualitative model to explain the formation mechanismof double circular erosion patterns. © 2009 American Institute of Physics. 关doi:10.1063/1.3227236兴
I. INTRODUCTION

The magnetron sputtering method is generally used forthe fabrication of films. Film thickness distribution changes
in relation to the distance between the target and the substrate; thus, fabrication of a uniform film continues to be a
subject of investigation. Moreover, because of the permanent
magnets on the backside of the target, the erosion of thetarget by sputtering is not uniform, and the depth of theerosion is different along the magnetic field.1,2 This makes it
difficult to use the target effectively. In order to improve theutilization rate of the target, many recent attempts have beenmade.3,4
In rf magnetron sputtering, a circular erosion pattern isformed on the surface of the circular metal conductor target
with permanent magnets behind the target, as shown in Fig.
1. The theory behind the formation of this erosion pattern has
been established.5,6 However, in the case of a dielectric target, a double circular erosion pattern is formed. So far, thispattern has been phenomenologically recognized by experimenters; however, it has not yet been investigated. The dimensions of the erosion patterns may vary in relation to thethickness of the dielectric target, so that insulation resistanceis different.
In this study, we perform a magnetron sputtering experiment with a SiO2 dielectric target, and confirm the formationof a double circular erosion pattern. Based on the experimental results, we propose a qualitative model to explain theformation mechanism of double circular erosion patterns fordesigning the sputtering system with high utilization rate.

mm. A schematic diagram of the placement of the target isshown in Fig. 2. Using a target consisting of a combinationof SiO2 targets and Cu targets we measured how the double
circular erosion pattern changes. Patterns of the combinations of the targets are shown in Table I. Here, S indicates the
SiO2 target, C indicates the Cu target, and G indicates thegap shown in Fig. 2, where there is a distance of 5 mm
between the metal electrode and the guard ring. A combination of targets 1 mm thick was set for measuring differencesin erosion. Moreover, the change in erosion when there is a
gap between the guard ring and the surface of the target
was measured. The purity, electrical conductivity, anddielectric constant of the SiO2 target were 99.99%, less than
10−18 共⍀ m兲−1 and 3.8.
We used a noncontact three dimension measuring system
共Ryokosha Corp., NH-3N兲 for the measurement of the target
erosion pattern. A measurement of three directions along thediameter via the center of the target was performed. A profileand view of erosion on the metal conductor 共Cu兲 target,
which was performed for comparison, are shown in Figs.

II. CONDITION OF THE TARGET

In our experiment, the thickness of the target waschanged by combining SiO2 and Cu targets 1 mm thick. A
double circular erosion pattern was measured by changing
the thickness of the target. The diameter of the target is 700034-6748/2009/80共10兲/104704/6/$25.00

FIG. 1. Schematic design of the target. Because of the permanent magnets
on the backside of the target, the erosion of the targets by sputtering is not
uniform, and the depth of the erosion is different along the magnetic field.

80, 104704-1© 2009 American Institute of Physics

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104704-2

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Rev. Sci. Instrum. 80, 104704 共2009兲
TABLE I. A combination of the targets: Cu and SiO2. S indicates the SiO2
target, C indicates the Cu target, and G indicates the gap shown in Fig. 2.

FIG. 2. Schematic diagram of the placement of the target. The diameter of
the target is 70 mm. The thickness of the target was changed by combining
SiO2 targets and Cu targets 1 mm thick.

3共a兲 and 3共b兲, respectively. The condition of the sputtering
was high frequency electricity 共13.56 MHz, 200 W兲, and theaccumulated electric discharge time was 150 h. In Fig. 3共a兲,a typical single circular erosion pattern on the metal conductive target was confirmed. Moreover, clear erosion is shownin Fig. 3共b兲. In a magnetron sputtering system, electrons aretrapped by the magnetic field and high density plasma iscreated in the narrow circle-shaped region near the target
surface. A single circular erosion pattern is formed under thisregion because the sputtering rate is higher than the value
under the other region.
III. RESULTS OF MEASUREMENTS OF THE TARGETEROSION PATTERN

Conditions of sputtering in the dielectric target were thepower of rf supply 共200 W兲, reflection electricity at less than
1%, and the discharge time at 2 h. A typical photo of the
double circular erosion pattern on the surface of the SiO2
target provided by the experiment is shown in Fig. 4. Thiserosion pattern is clearly different from the one in Fig. 3共b兲.
A profile measurement was performed for this pattern. Results of the measurement of the erosion pattern profile of thetarget are as follows. In this paper, we defined a double circular erosion pattern as having two local maximums in acircle-shaped erosion pattern.
The relationship between thickness of SiO2 共S兲 and theerosion pattern, with a constant gap G = 0, is shown in Fig. 5.

Figure 5共a兲 shows erosion depth profiles on the targets. In
S1C4, deep erosion occurred in the circle-shaped area whereplasma density was high. Erosion depth became shallower as
S increased. In S2C3, erosion depth was about half as deepas in S1C4. Moreover, the shape of the erosion was distorted

FIG. 3. 共Color online兲 Erosion on the metal conductor 共Cu兲 target. 共a兲 and 共b兲 show a profile and a photo of the erosion on the surface. The condition of the
sputtering was high frequency electricity 共200 W兲, and the accumulated electric discharge time was 150 h. A typical single circular erosion pattern was
confirmed on the target.

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104704-3

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Rev. Sci. Instrum. 80, 104704 共2009兲

FIG. 4. 共Color online兲 Typical photo of the double circular ring of erosion
on the surface of the SiO2 target. The erosion pattern is clearly different
from the one in Fig. 3共b兲.

but had only one maximum. Its distorted depth profile will be
discussed later. In S3C2, S4C1, and S5, erosion depth profiles had two local maximums in a circle-shaped erosion pattern. The formation of a double circular erosion pattern was
confirmed at S = 3, 4, and 5 mm; a single circular erosion
pattern was present at S = 1 and 2 mm. Figure 5共b兲 shows
relations between S and the radius of the double circularerosion pattern. S increased, and the radius of the inner andouter circular erosion patterns decreased and increased, respectively. The difference of the radius of the two circularerosion patterns increased.
The relationship between thickness of SiO2 共S兲 and theerosion pattern, with a constant gap G = 1 mm, is shown in
Fig. 6. Figures 6共a兲 and 6共b兲 show erosion depth profiles on
the targets and radius of the double circular erosion patterns,
respectively. In Fig. 6共a兲, erosion depth at S = 1 was morethan three times as deep as at the other three profiles. At
S = 2 – 4 mm, the erosion depth decreased linearly with anincrease in S. In Fig. 6共b兲, the formation of a double circular

FIG. 5. 共Color online兲 Relationship between thickness of SiO2 共S兲 and theerosion pattern, with a constant gap G = 0. 共a兲 shows erosion depth profiles
on the targets. Deep erosion occurred in the circle-shaped area where plasma
density was high. 共b兲 shows the relations between S and the radius of the
double circular erosion pattern. The triangular diagrams in these figures are
the same as in Table I.

FIG. 6. 共Color online兲 The relationship between thickness of SiO2 共S兲 andthe erosion pattern, with a constant gap G = 1 mm. 共a兲 and 共b兲 show erosion
depth profiles on the targets and radius of the double circular erosion patterns, respectively.

erosion pattern was confirmed, except in the case of S
= 1 mm. In the case of G = 1 mm, a double circular erosion
pattern was formed at S = 2, 3, and 4 mm, whereas a double
circular erosion pattern was formed at S = 3 and 4 mm, with
G = 0. This difference may depend on the influence of thesize of G.
For S fixed at 1 mm, and G changed from 0 to 4 mm, theerosion profile results are shown in Fig. 7. Figures 7共a兲 and7共b兲 show erosion depth profiles on the targets and radius of

FIG. 7. 共Color online兲 The relationship between the gap 共G兲 and the erosion
pattern, with a constant thickness of SiO2 S = 1 mm. 共a兲 and 共b兲 show erosion depth profiles on the targets and radius of the circular erosion patterns,
respectively. A double circular erosion pattern did not appear whenG = 0 – 3 mm.

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Rev. Sci. Instrum. 80, 104704 共2009兲

FIG. 9. 共Color online兲 Two models of sputtering of the target at different
thicknesses of SiO2; 共a兲 S = 2 mm and 共b兲 5 mm with a constant value of
G = 0. When S = 2 mm in 共a兲, a deep single circular erosion pattern appeared. However, when S = 5 mm in 共b兲, a shallow double circular erosion
pattern appeared instead because the size of the charged domain was larger.

FIG. 8. 共Color online兲 The relationship between the gap 共G兲 and erosion
pattern, with a constant thickness of SiO2 S = 2 mm. 共a兲 and 共b兲 show erosion depth profiles on the targets and radius of the double circular erosion
patterns, respectively. In the case of G = 1 – 3 mm, the formation of a double
circular erosion pattern was confirmed, although not in the case of G = 0.
The radius of the inner circular erosion pattern was constant; therefore, theradius of the outer pattern increased as G increased.

the double circular erosion patterns, respectively. In Fig.
7共a兲, erosion depth is deepest at G = 0. Erosion depth is approximately constant in the case of G = 1 – 3 mm. The erosion depth is extremely shallow in the case of G = 4 mm.
The erosion pattern changes whether there is a gap or not.
The size of G may not have an influence on the erosion
pattern. Because the size of S was much smaller than G, at
G = 4 mm, the reason why the erosion depth became shallow
thought to be because the Ar+ ions may have gone greatly
astray into the guard ring side. In Fig. 7共b兲, the formation of
a double circular erosion pattern was confirmed at G
= 4 mm. Therefore, at G = 0, the target was eroded like aconductive target and a double circular erosion pattern did
not appear. On the other hand, at G = 1 – 3 mm, the target
was eroded irregularly; therefore, a double circular erosion
pattern did not appear. This pattern will be discussed later.
The relationship between the gap 共G兲 and the erosion
pattern, with a constant thickness of SiO2 S = 2 mm is shownin Fig. 8. Figures 8共a兲 and 8共b兲 show erosion depth profiles
on the targets and radius of the double circular erosion patterns, respectively. In Fig. 8共a兲, when G = 0, the target waseroded greatly. In the case of G = 1 – 3 mm, the erosion
depth was smaller than at G = 0, and grew deeper linearlyas the size of G increased. In Fig. 8共b兲, in the case of
G = 1 – 3 mm, the formation of a double circular erosion pattern was confirmed, except in the case of G = 0. The radius of
the inner circular erosion pattern was constant; therefore, theradius of the outer pattern increased as G increased.
IV. DISCUSSION

Using a metal conductive target, a single circular erosion
pattern was formed on the surface of the target after sputter-ing. It was caused because there was a circular high-density
sputtering electric discharge near the surface of the target due
to the permanent magnets on the back of the target. Thisphenomenon has already been observed and described theoretically. In this paper, we confirm that the Cu target waseroded in the shape of a circle 关Fig. 3共b兲兴.
However, the formation of a double circular erosion pattern was confirmed on the surface of the dielectric target
共Fig. 4兲. Because the Ar+ ions that reached the target surface
are not neutralized, the target surface is charged positively,
and, as a result, Ar+ ions avoided a charged domain throughCoulomb repulsion. Moreover, a double circular erosion pattern was formed. The erosion patterns were different when
the states of the insulation resistance varied with the thickness of the dielectric target. We changed the thickness of
SiO2 from 1 to 5 mm and measured the erosion on the surface of the target after sputtering. In Fig. 5共a兲, insulation
resistance increased with the thickness of SiO2, and the erosion depth on the target decreased. A double circular erosion
pattern was not formed on 1 mm thick SiO2, but was formedon over 2 mm thick SiO2. In addition, the distance between
the two circles of erosion increased with the thickness of
SiO2 关Fig. 5共b兲兴. The thickness of the dielectric target increased, thus insulation resistance of the surface of the target
increased; as a result, the target surface became more positively charged. Based on these results, we developed twomodels of sputtering of the target at different thicknesses of
SiO2; 2 and 5 mm, as shown in Fig. 9. With thin SiO2, thecharged domain was narrow because there was little quantityof Ar+ charged on the surface, and, as a result, a distinct
double circular erosion pattern did not appear. However, theerosion depth was deep because of the small quantity of theelectrical charge.
Moreover, in the experiment, when there was a gap between the target and the guard ring as shown in Fig. 2, different double circular erosion patterns with different depths
were formed. Because the guard ring was connected to thecathode, it was thought that Ar+ ions were led to the guard
ring side when the gap was large.
When the gap was fixed 共G = 1 mm兲 and the thickness of
SiO2 was changed, the results are shown in Figs. 6共a兲 and6共b兲. Comparing Fig. 6共a兲 with Fig. 5共a兲, when G = 0 and 1
mm, a double circular erosion pattern was formed when the
thickness of SiO2 was more than 3 and 2 mm, respectively.

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104704-5

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Suzaki, Miyagawa, and Ejima

FIG. 10. 共Color online兲 Schematic diagrams of the single and double circular rings of erosion at different gaps, 共a兲 G = 0 and 共b兲 3 mm with a constantvalue of S = 2 mm. When G = 0 in 共a兲, the double circular erosion pattern
was not formed. However, when G = 3 mm in 共b兲, because the flow of theAr+ ion spreads toward the guard ring side, and the size of the charged
domain became larger, a double circular erosion pattern was formed instead.

These results show that Ar+ ions flow through the guard ringside when a gap is made. Consequently, when the thicknessof SiO2 was constant and the gap was 0–4 mm, a double
circular erosion pattern did not appear clearly. When SiO2
was 1 mm thick, and the gap was increased, the erosion
depth became shallow; in addition, a double circular erosion
pattern tended to form, as shown in Figs. 7共a兲 and 7共b兲. This
result shows that Ar+ ions were not accumulated in the target
neighborhood very much at 1 mm because insulation resistance was low. When the gap was increased, Ar+ ions led to
the guard ring side increased; as a result, a double circularerosion pattern was formed. Furthermore, at 2 mm of SiO2,
when there was not a gap 共G = 0兲, the erosion was deep, but
a double circular erosion pattern did not appear, as shown in
Figs. 8共a兲 and 8共b兲. However, when there was a gap, theerosion depth was shallower than the depth without a gap. In
addition, the radius of the inner circle did not depend on the
gap size. However, the radius of the outer circle increased as
the gap size increased. Figure 10 shows schematic diagramsof the single and double circular rings of erosion at different
gaps, 共a兲 G = 0 and 共b兲 3 mm. When there is not a gap, asshown in Fig. 10共a兲, the spread of the Ar+ ions was narrow,and a charged domain on the surface of the target was small.
Thus, a double circular erosion pattern was not formed, andthe erosion depth was deep. When there was a gap, as shownin Fig. 10共b兲, the flow of the Ar+ ion spreads toward theguard ring side, and the charged domain was wide. A double
circular erosion pattern was thus formed, and the erosion
depth was shallow.
S1C4 showed a single circular erosion pattern in Fig. 5
because insulation resistance of 1 mm thick SiO2 is low. In
S2C3 共in Fig. 5兲 and S1C3G1 共in Fig. 7兲, the erosion depths
were shallower than in S1C4. Moreover, the erosion profiles
were distorted but had only one maximum value. These distortions seem to be caused by surface charge. We will discussthese two combination targets. The simple fitting function for
the erosion shape for metal targets was introduced throughthe concept of a virtual line source in a collisionless plasma
by Kusumoto and Iwata,6 They assumed a two-dimensional
Maxwellian source of temperature T, located at the distanceh from the target surface in a uniform electric field E.
Then the incident flux distribution n共x兲 could be obtained asfollows:

FIG. 11. Fittings of experimental erosion depth data for 共a兲 the Cu target, 共b兲
S2C3 and S1C3G1 using Eq. 共1兲 by Kusumoto and Iwata 共Ref. 6兲. For the
Cu target in 共a兲, the calculated data indicated by the solid line corresponded
closely to experimental data indicated by open circles across the eroded
region. For S2C3 in 共b兲, erosion depth at the center is shallower than thesolid line 关the same line as in 共a兲兴. Moreover in 共b兲, the erosion center
widened at both the inner and outer sides. However in 共c兲, erosion depth atthe outer side is deeper than the solid line. These two tendencies are consistent with the prediction made regarding by the models in Figs. 9 and 10.

ndx ⬇

冑

冊

冉

1
␤ e␤共1−r兲 冑r+
冑r d ␰ ,8␲ r

共1兲

where r, ␰, and ␤ are defined byr = 冑1 + ␰2,

␰=

共x − m兲
h

and

␤=

qEh
,
2kT

共2兲

respectively. m is the position of the erosion center. k and q
denote the Boltzmann constant and elementary electriccharge. Figure 11共a兲 shows the fit of calculated erosion depth
data using Eq. 共1兲 in semilogarithmic plots for a Cu target.
The calculated data indicated by the solid line correspondclosely to the experimental data indicated by open circles
across the eroded region. Figures 11共b兲 and 11共c兲 show experimental data of S2C3 and S3C3G1, respectively. The
solid lines in Fig. 11共b兲 and 11共c兲 are the calculated data for
the Cu target, which is the same as the data in Fig. 11共a兲. For
S2C3 in Fig. 11共b兲, erosion depth at the center is shallower
than the solid line, and the erosion center widened at both theinner and outer sides. This result was caused by an increasein electric charge at the center of erosion. As the electriccharge increases, the profile is expected to change from asingle circular pattern to a distorted pattern, and finally to adouble circular pattern. However, for S1C3G1 in Fig. 11共c兲,
erosion depth at the outer side is deeper than the solid line.
This result was caused by the gap between the guard ring andthe surface of the target. These two tendencies are consistent

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104704-6

Rev. Sci. Instrum. 80, 104704 共2009兲

Suzaki, Miyagawa, and Ejima

with the prediction made regarding by the models in Figs. 9
and 10.
V. CONCLUSION

In this study, we performed a magnetron sputtering experiment on a SiO2 dielectric target and were able to confirm
the formation of a double circular erosion pattern, which
agreed with qualitative theory. Moreover, insulation resistance varied with the thickness of the SiO2 target; thus thedimensions of a double circular erosion pattern also varied.
Furthermore, we confirmed in the experiments that the depthof a double circular erosion pattern changed by making a gap
between the SiO2 target and guard ring. Results are summarized as follows.
共1兲 The erosion depth is shallow when SiO2 is thick or the
gap is large. On the contrary, the erosion depth is deep
when SiO2 is thin or the gap is small.
共2兲 When SiO2 is thick because the insulation resistance ishigh the erosion depth is shallow. Moreover, when the
gap is large because Ar+ ions are led to the guard ringside, and the charged domain of the Ar+ ions becomeslarge, the erosion depth decreases.
共3兲 When the SiO2 is thin or the gap is small, a single circular erosion pattern is formed. Under these conditions,a double circular erosion pattern is formed.

These results suggest that thickness and dimensions of
the gap in an SiO2 target should be balanced to achieve a
deep erosion depth and a double circular erosion pattern.
Thus, the most suitable condition in our equipment was SiO2
at 2 mm, and the gap at 3 mm. The sputtering system withthe maximum utilization rate of the dielectric target can be
designed by using the obtained fundamental results.
In this study, on a dielectric target, the formation of adouble circular erosion pattern was confirmed; this agreed
with qualitative theory. In addition, we will examine double
circular erosion patterns that are caused by making a gap
between a target and guard ring, and will formulate a theory
about these results in the future.
ACKNOWLEDGMENTS

We thank Mr. Ian Willey for his careful proofreading of
our manuscript.
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