Managing Arcs for Optimum Deposition Performance
D. Carter and H. Walde, Advanced Energy Industries Inc., Fort Collins, CO
ABSTRACT
Over the years a broad range of arc detection and power
supply response technology has found its way into the power
delivery systems driving thin film sputtering applications.
While different applications carry different propensities for
arcing, it is generally accepted that arcs will occur, at some
level, through the course of a typical deposition process.
The seemingly simple goal of detection and management
to extinguish arcs and recover the plasma to a working state
is in fact quite complex given the sensitivity of sub-micron
films to arc induced particles and the heavy arcing common
on many popular target materials. Effective management of
arcs requires an understanding of both the nature of, and the
contributors to, arcs that are occurring. Minimizing their impact
requires an appropriate response that takes into account the
characteristics of the arcs and also manages the elements that
influence or interact with these characteristics. This study looks
into the factors that influence key arc characteristics including
arc rate, arc energy and arc persistence in large area sputtering
applications. An understanding of these characteristics and
the influence of modern arc response techniques is shown to
offer key insight for methods extending beyond arc handling
and into an approach better characterized as arc management.
Ultimately, improved arc management provides the user better means for controlling the factors that affect arc formation
and persistence. This, in the end, allows for more purposeful
response to the arcs that occur and better control over the
impact arcs have on the deposition being performed.
INTRODUCTION
Sputtering arcs are most commonly the result of target defects
[1-3] that may be inclusions in the bulk or surface imperfections from a variety of sources. Magnetron arcing can produce
high levels of macro particles [4, 5] that eject from the target
and form “killer” defects on the work piece. Equally disruptive is the associated collapse of the uniform discharge. Left
unchecked, arcs can cause damage to the target, substrate,
power supply and other chamber components. Arcs are most
typically resolved by interrupting power for a fraction of a
second and thus suspending the flow of current to the spot.
But this momentary interruption of the process can be disruptive to the deposition and can lead to rate, uniformity or other
quality issues in the thin film.
256
Sputtering power supplies and their incorporated arc response
technology have been driven to reduce the impact of these
events. Over the years detection times have decreased from
milliseconds two decades ago to less than a microsecond today.
Similarly, the energy released to an arc has been reduced by
orders of magnitude from many joules per event down to less
than a millijoule per kilowatt in some cases today.
To minimize the impact of arcs the general trend is toward
faster detection and faster response limiting energy released
and minimizing the duration of plasma disruption. The recognized exception is when higher arc energy is beneficial
for removing residues that can form on a target [6]. Modern
power supplies often offer a number of arc handling parameters that allow adjustment to the arc detection and response
criteria. But optimization requires a sound understanding of
the contributors to arc behaviors and also the impact they
may have on a particular process.
This paper reviews some of the key characteristics of sputtering
arcs, important contributors to these characteristics and the
implications they have on process control and stability. Since
arcs can impact processes differently the ability to tailor the
response is becoming increasingly important and customizable
arc management capabilities are now an important requirement for overall optimization of power delivery.
EXPERIMENTAL
Arcing data were gathered on a large scale planar cathode
with a 12”x44” target area. Multiple target materials were
used to illustrate the effects of arc parameter settings on arc
rates, arc persistence and arc energy. Arc rates were measured
at individual power settings as the power was stair-stepped
in 1 to 2 kW increments from low to high power and then
back to low again. At each power setting a settling time of 2
to 5 minutes was used for data collection and to ensure stable
operation for the condition. This ramp up/ramp down method
was used to minimize thermal stress on ceramic targets and
was also adopted on metal targets for consistency. Tests were
conducted across a range of powers from a few kilowatts up
to 50 kW; power densities ranged from approximately 1 to 15
watt/cm2. Only the metal targets were operated at the highest
power settings. All tests were carried out after pumping the
system to 5x10-6 Torr or less and back filling with argon to
sputtering pressures between 1 mT and 4 mT.
© 2010 Society of Vacuum Coaters 505/856-7188
53rd Annual Technical Conference Proceedings, Orlando, FL April 17–22, 2010 ISSN 0737-5921
150
30
125
25
Arcs per second
Arcs per second
35
20
15
10
5
4 mTorr
3 mTorr
2 mTorr
1 mTorr
100
75
50
25
0
New
Aluminum
Conditioned
Aluminum
Zinc
0
AZO
2000
4000
Material
6000
8000
Power (watts)
10000
a)
b)
Figure 1: Arc rates for a) different materials sputtered at a 20 kW and b) for ceramic AZO at different powers and pressures.
The method used for arc energy calculation is given below.
Reported energies are averages across a sampling of arcs for
a given setting. Arcs on aluminum were relatively infrequent
and thus the minimum sample size for energy calculations
on aluminum was five arcs. Arcs on AZO were much more
common, sample sizes for reported AZO arc energies were
ten arcs minimum.
Figure 2 shows the impact of shutdown time and recovery
ramp on arc rates for AZO sputtering. Arc rates increase for
shutdown times less than 20 µsec. The increase is more rapid
when the rise in voltage is not regulated. When a ramped
voltage recovery is used, the increase in arc rate is much
less pronounced.
80
70
It is well known that different materials can arc at different rates. Some materials may experience high arc rates
throughout the life of the target, others only during cleaning
or conditioning runs. Ceramic targets often arc at much higher
rates than metals. Figure 1a shows rates measured on a small
sampling of materials. Under similar sputtering conditions a
clean aluminum target arcs at relatively low rates (<< 1 arc
per second) compared to a ceramic target, in this case yielding
a rate of over 30 arcs per second. Sputtering conditions also
affected measured arc rates. Figure 1b shows rates measured
on ceramic AZO sputtered at four different pressures and
increasing powers.
60
The selection of arc handling parameters can strongly influence
the arc rates measured for a given process. Compound AZO
targets are known to have generally high arc rates compared
to metallic materials but the method of arc response can either
increase or decrease rates that are actually experienced.
Effective arc handling involves an interruption of power to
the target allowing the arc to cool and die out. The duration of
the shutdown time is important; it needs to be long enough to
ensure the arc is out but short enough to prevent unnecessary
off time. The manner in which the plasma is re-established
can also influence rates. If power is reapplied too quickly
after the shut down, additional arcs may occur.
Arcs per second
ARC RATES
Unregulated
ramp
50
Regulated
ramp
40
30
20
10
0
0
20
40
60
80
Shut down time (µsec)
100
120
Figure 2: Influence of arc shut down time on arc rates for AZO
sputtered at 20 kW.
ARC PERSISTENCE
Inadequate arc suppression can lead to persistent arcing.
Persistent arcs historically have been termed “hard arcs”
implying that they are different from what has been called
“micro arcs”. In fact persistent arcs are often no different from
micro arcs, other than they may regenerate or persist beyond
the first few response attempts. This can be due to arcs forming
in regions more prone for re-ignition but can also be due to
the arc response. When arcs regenerate the result can be an
increase in total count and energy released into the spot and
higher risk of particles and damage.
257
Total
Arcs per second
Arcs per second
50
45
40
35
30
25
20
15
10
5
0
Primary
Persistent
0
5000
10000
15000
Power (watts)
50
45
40
35
30
25
20
15
10
5
0
Total
Primary
Persistent
20000
0
5000
10000
15000
Power (watts)
20000
a)
b)
Figure 3: Arc rates for AZO sputtered at increasing power levels; a) with 10 µsec arc suppression and b) with 100 µsec arc suppression.
Figure 3 gives two examples of AZO being sputtered at increasing power levels. In Figure 3a, a short (10 µsec) shutdown
time was used. The short off-time shows a high incidence of
persistent arcs. Between 30% and 50% of all arcs persisted
after the initial suppression. Figure 3b shows the result of a
100 µsec off time on the same process sequence. Suppression in both cases used a regulated ramp and the results were
consistent with those in Figure 2 suggesting the increased
rates at short off-time result from arc regeneration after the
initial response. It follows that total rates are reduced when the
primary response shut down is adequate to prevent persistent
arc sites from forming.
It is interesting to note that the number of primary arcs is
generally the same in both cases of Figure 3. This suggests
that the duration of the primary suppression has little effect on
the number of primary arcs that occur. Previous studies have
shown that the strong driving force behind most sputtering
arcs is differential surface charging [3, 4, 7, 8]. Therefore
to address the formation of primary arcs surface charging
must be disrupted. Reverse-voltage pulsing has been proven
for suppressing primary arcs in both metallic and reactive
processes [9, 10] through the ability to scrub surface charge.
Reverse-voltage pulsing was tested here to determine its
utility for ceramic AZO.
Figure 4 shows that AZO falls consistently in line with other
materials and by adding reverse-voltage pulsing a significant
reduction in primary arcs is possible. Pulsed-DC differs from
258
other methods of arc handling as it represents a truly pro-active
method for preventing arcs. All other techniques are essentially
“reactive” in responding to arcs only after they occur.
14
Total arcs per second
Persistent arcs can occur because arc parameters are improperly
set in an attempt to minimize off-time. If arc suppression is
released too quickly (as shown in Figure 2) the spot may not
adequately cool, and the arc can be re-established. Similarly,
if voltage is ramped too rapidly in an attempt to recover the
plasma quickly, the arc breakdown can recur.
12
D.C.
10
8
Pulsed
6
4
2
0
2000
4000
6000
8000
Power (watts)
10000
Figure 4: Arc counts for AZO sputtered at increasing power levels,
a) with DC power and b) with reverse-voltage pulsed DC @ 20 kHz
pulse rate (4 µsec reverse).
ARC ENERGY
Arc energy has become a key performance measure for modern power supplies, many claiming the capability to handle
arcs with very low energy release (~ 1 millijoule per kilowatt
power). Numerous factors contribute to calculated arc energies, however. While some of the contributors are associated
with the sputtering power supply, many are independent of the
generator and its arc detection and response capabilities.
In order to investigate some of the contributors to arc energy,
it is important to establish the calculation method and associated arc-start and arc-end times. Defining these conditions
allows for consistent calculation of energies and meaningful
comparisons between contributing factors. Figure 5 shows a
typical AZO arc captured with oscilloscope current and voltage traces. Defined herein is the time when the arc is formed
(t1) and the time when the arc is extinguished (t2). Selection
of these times can be crucial to the arc energy calculation
(time integral of I*V over the duration). For this study t1
is chosen as the time when the sputtering voltage drops by
20% nominal value and t2 is the time when current falls to
less than 0.5 amp.
The selection of the start and end times defining an arc can
strongly influence the calculated arc energy. As an example, it
may seem intuitive to place t1 at the arc detect level chosen in
the power supply. This threshold, however, typically represents
a relatively low voltage (often between 100V and 200V). Since
current rises most rapidly at the start of the arc and voltage
similarly falls, the placement of t1 can have a large impact
on the calculated arc energy. For the arc shown in Figure 5,
the energy calculated using t1 = 20% of nominal V, results in
arc energy = 2.27 mJ/kW. If t1 is placed at 200V (the arc trip
chosen for the power supply) the calculated energy is only
0.79 mJ/kW. It is arguable that an arc actually begins prior
to a fall in voltage of 20%. For the purpose of this exercise,
20% is a convenient threshold because it provides margin
beyond the typical noise in voltage measurement and thus
minimizes the likelihood of a false calculation. Decreasing
the threshold to 10% increases energy only slightly (8%) to
2.44 mJ/kW. Most important is consistency in the calculation
so comparisons between materials and other contributors
can be made.
Placement of the arc-end time, or t2 tends to be much less
critical since both current and voltage are low towards the
end of the arc and the integrated I*V per incremental time has
minimal impact. To illustrate, using the example of Figure 5,
moving from 0.5 amps to 2 amps results in well less than 1%
change in calculated energy (2.27 mJ/kW versus 2.26 mJ/kW).
Similarly, lowering the current threshold to 0.2 amps increases
the calculated energy by only 0.001 mJ/kW. Again consistency
is most important for comparative evaluation.
Over its duration, energy is deposited into an arc. Figure 6
shows the energy released into the Figure 5 arc from t1 to t2.
The I*V product is highest during the early stages of the arc
and as a result most of the energy is delivered into the spot
during the first few microseconds. Figure 6b shows the percent
of total energy released by time. For this arc, over 50 percent
of the total energy is released during the first microsecond; and
85 percent during the first two microseconds. It is important
to note that the arc trip threshold for this example was 200
volts. This trip condition was satisfied at 1.6 µsec into the
arc (assuming t1 at 20% V fall); at this point, over 65% of the
total energy had already been released into the arc.
While the great majority of energy is absorbed in the first 6
to 8 microseconds, it is evident that current is still flowing up
to this time and the arc is still not fully out. This observation
is important to the arc regeneration discussion above. If arc
suppression time is too short, and voltage is allowed to rise
too quickly, a secondary arc can be established, most likely in
the same spot on the target. In the example shown in Figure 2,
the critical off time was between 10 and 20 microseconds. For
these conditions, Figure 5b shows that very few arcs survive
past ~ 12 µsec. From this there appears the requirement for
avoiding regeneration: shutdown time must be adequate to
allow current to fall to zero (and possibly slightly longer) fully
quenching the arc, before voltage is driven back up.

t1: Arc begins




Current
t2: Arc ends

Voltage












a)
b)
Figure 5: Typical AZO arcs a) current and voltage traces showing time at initiation (t1) chosen at a 20% drop in nominal sputtering voltage;
and arc-out (t2) when current is less than 0.5 amp and b) distribution of arc duration for AZO arcs measured between 15 and 20 kW power
(measurements made using low inductance transmission cabling).
259
1
2.5
0.8
2
% Energy
Energy (mJ/kW)
3
1.5
1
0.6
0.4
0.5
0.2
0
0
0.00
2.00
4.00 6.00
Time (µsec)
8.00
10.00
0.00
2.00
4.00 6.00
Time (µsec)
8.00
10.00
a)
b)
Figure 6: Arc energy; a) actual energy released into arc over time and b) percent of total energy by time showing >50% of energy is released
in the 1st microsecond and > 85% after the first two microseconds (measurements made using low inductance transmission cabling).
It is clear that fast arc detection is important for minimizing
arc energy. If energy is a concern, setting a higher voltage
trip threshold and perhaps combining both voltage and current thresholds can be used to deliver faster detection and in
turn faster response. Ultimately, how effective arc settings
can be for reducing arc energy depends on other variables in
the system. Two important contributors to arc energy aside
from the power supply are the transmission cable and the
target itself.
These results illustrate the importance of cable impedance
and its potential impact on arc energy. Stored energy will be
minimized when cable impedance matches load impedance.
Therefore cables should be carefully chosen based on the range
of load impedances anticipated for a given system. Matching
cable impedance to load impedance offers the most robust
solution for minimizing stored cable energy, the approach also
addresses the issue of the arc energy curve being front-end
loaded because it does not rely on a generator response.
Transmission cables are known to contribute to arc energy.
The extent of the contribution depends on the energy stored
in the cable’s capacitance and inductance. Figure 7 shows a
comparison of arc energies captured at 10 kW during an extended AZO run using a) low inductance (2.2 µH) transmission
cables and b) high inductance cables (18.4 µH). Arc energies
measured using the high inductance cables increased by nearly
2x on average. And in all cases, a large fraction of the energy
is always released within the first two microseconds, before
generator response is applied.
The previous data reveal an additional contributor to arc
energy, the sputtered material itself. Since most arcs form
on the target at surface imperfections, defects or residues,
it is not surprising that the type of material being sputtered
contributes to the energy absorbed in an arc. Table 1 gives
example arc energies for AZO and aluminum sputtered at
20 kW using the low-L cable. Here and throughout all of
the testing, aluminum demonstrated significantly lower arc
energies compared to AZO.
Similar cable comparisons were made while sputtering aluminum. Energies measured using the low inductance cable at
50 kW averaged 1.24 mJ/kW while energies using the high
inductance cable measured an average of 5.79 mJ/kW. In
addition to increasing released energy, high cable inductance
increased arc duration as well. In our testing arc duration at
50 kW on aluminum was approximately 10 µsec using low-L
cabling and greater than 50 µsec using high-L cabling. This
has implications on the proper selection of shutdown time
based on the above discussion.
260
Table 1 summarizes these and additional contributors to arc
energy namely output capacitance of the power supply and
arc response delay time, a feature available in some supplies
allowing for a delay (usually a few microseconds) prior to
arc response. In practice only delay time is conveniently
adjustable for affecting arc energy. With the exception of arc
response delay, all of the results in Table 1 were collected
with a common set of arc response parameters.




































a)
b)
Figure 7. Distribution of arc energies on AZO sputtered at 10 kW using a) low inductance transmission cable and b) high inductance
cable.
Table 1: Factors influencing arc energy – arc detect and shut down
parameters held constant (except delay as noted).
Arc Energy
(mJ/kW)
Material (@ 20 kW)
Aluminum
AZO
1.67
2.38
Cable inductance (Al @ 50kW)
Low (~ 2.2 µH)
High (~18 µH)
1.24
5.79
Generator output C (AZO @ 10kW)
136 nF
204 nF
2.36
3.47
Arc response delay (AZO @ 10 kW)
0 µsec
5 µsec
2.36
7.21
DISCUSSION AND CONCLUSIONS
Arc rates, arc persistence and arc energies are all influenced
by a number of factors in a large scale sputtering process.
Arc suppression parameters used for detecting and quenching
arcs can influence all of these, but the target type, history and
transmission cabling can influence these behaviors as well.
Target material and target history largely determine the intrinsic arc rate for a given process. The likelihood of arc related
damage is a function of the cumulative power delivered to all
arc events. Therefore high arc rates pose high risk for product
or system damage. Arc detection and response parameters
influence arc rates by determining how effectively primary
events are suppressed. If shut down time is too short, arcs
can regenerate and persist. In a similar manner, the rise in
voltage after response influences persistence. If voltage rises
too fast (in combination with short shutdown time) arcs will
regenerate and persist for multiple responses. Persistent arcs
lead to higher counts and longer total shutdown time. Properly
optimized arc parameters reduce the occurrence of persistent
arcs and improve overall process stability and quality. Primary
arc rates can be reduced by preventing differential charging
that creates the initial event. Reverse-voltage pulsing is the
proven method for reducing primary arc rates.
Arc energy is strongly dependent on the material being
sputtered and also on other components outside the power
generator. A single set of arc suppression parameters can yield
widely varied arc energies dependent on material, transmission
cable reactance and stored energy elsewhere in the system. Arc
energy can be increased using an arc response delay but the
ability to reduce arc energy is limited since energy is strongly
loaded in the early stages of the arc transient. A significant
portion of arc energy is released typically before an arc detect
threshold can be reached. Therefore arc suppression settings
(other than delay time) can realistically influence only a fractional portion of the arc energy curve and are often second
order to many other contributors in the system.
While many arcs have similar characteristics, no two arcs
are the same. Understanding how material, cabling, and arc
handling settings influence these characteristics is important
for proper optimization of arc suppression. With these multiple
factors all influencing arc rates, persistence and energy, it is
equally important that a power delivery system have adequate
control parameters to influence those behaviors that can be
261
controlled. Improved understanding of these interdependencies
along with evolving generator capabilities gives the user the
best advantage for managing the arcs that occur and minimizing their impact on the process being performed.
5.
C.E. Wickersham, Jr., J.E. Poole, J.S. Fan, L. Zhu, “Video
analysis of inclusion induced macroparticle emission
from aluminum sputtering targets,” J. Vac. Sci. Technol.
A 19(6), 2741 (2001)
ACKNOWLEDGEMENTS
6.
F. Milde, D. Schulze, G. Teschner, P. Wiedemuth, and
T. Rettich, “Experience with High Power DC Supplies
with Fast Arc Suppression in Large Area Coating,” 44th
Annual Technical Conference Proceedings of the Society
of Vacuum Coaters, p. 375, 2001
7.
A. Belkind, A. Freilich, and R. Scholl, “Electrical Dynamics of Pulsed Plasmas,” 41st Annual Technical Conference
Proceedings of the Society of Vacuum Coaters, p. 321,
1998
8.
R.A. Scholl, “Advances in arc handling in reactive and
other difficult processes,” 37th Annual Technical Conference Proceedings of the Society of Vacuum Coaters, p.
312, 1994
9.
P.J. Kelly, P.S. Henderson, R.D. Arnell, G.A. Roche,
and D. Carter, “Reactive pulsed magnetron sputtering
process for alumina films,” J. Vac. Sci. Technol. A 18
(6), Nov/Dec 2000
The authors would like to thank Astrid Borkowski and GfE
GmbH for providing the AZO target materials used in this study
and in addition, Karen Peterson for her invaluable assistance
with configuration and general support on the test system.
REFERENCES
1.
A. Anders, “Physics of arcing, and implications to sputter
deposition”, Thin Solid Films 502 (2006) 22 – 28.
2.
V. Pavate, M. Abburi, S. Chiang, K. Hansen, G. Mori,
M. Narasimhan, S. Ramaswami, J. Nulman, “Correlation
between aluminum alloy sputtering target metallurgical
characteristics, arc initiation, and in-film defect densities”, SPIE Proc., Vol. 3214, pp 42-47
3.
C.E. Wickersham, Jr., J.E. Poole, A. Leybovich, L. Zhu,
“Measurements of the critical inclusion size for arcing
and macroparticle ejection from aluminum sputtering
targets,” J. Vac. Sci. Technol. A 19(6), 2767 (2001)
4.
C.E. Wickersham, Jr., J.E. Poole, J.S. Fan, “Arc generation
from sputtering plasma-dielectric inclusion interactions,”
J. Vac. Sci. Technol. A 20(3), 833 (2002)
262
10. D. Carter, “Arc prevention in magnetron sputtering processes,” 51st Annual Technical Conference Proceedings
of the Society of Vacuum Coaters, p. 380, 2008
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Managing Arcs for Optimum Deposition