Download Factors in Arc Parameter Selection on Large

Factors in Arc Parameter Selection on Large Scale Deposition Processes
D. Carter and H. Walde, Advanced Energy Industries Inc, Fort Collins, CO
Abstract
Arcing in magnetron sputtering can be highly disruptive
and damaging to a deposition process. Arc management
technology continues to evolve in an effort to reduce the
impact of sputtering arcs but the utility of each feature and
the optimization of settings is not always straight forward
for a given application. Further complicating the issue is the
fact that arc behaviors are influenced by a number of process
related factors, one of the most important being the type and
quality of the target material being sputtered. It is becoming
recognized that arc parameters effective for one material may
not be ideal for another. In this study we evaluate some of the
material dependencies of arc rate, arc persistence and arc energy and investigate how various arc management parameters
impact these behaviors on a large scale deposition system.
Using high, medium and low melting point materials we also
include target condition as an additional metric and identify
considerations for selecting arc management parameters for
a given process with a given set of objectives.
Introduction
Sputtering arcs are a common source of process defects in
virtually any magnetron sputtering application. Most commonly caused by defects on or within the target [1-3] these
events result in a rapid concentration of energy into a very
small area on the target or other surfaces within the system
[4]. Left unchecked sputtering arcs can cause severe damage
since the power being delivered to the process can be concentrated in its entirety to a spot only a few microns in size
[4]. For this reason, modern power systems are designed to
detect arcs, quickly interrupt power and extinguish the event
thereby minimizing the likelihood of damage. Ultimately, the
impact of these events depends on a number of factors, some
of the more critical being the rate at which they occur and
the manner in which the power system detects and responds
to eliminate them.
The importance of arc rates lies in the fact that each event
results in a collapse of the plasma, and a concentration of
extremely high power density to the arc site. Aside from
reverse-voltage pulsing [5], all arc handling techniques
are fundamentally “reactive” in nature and thus represent
a response to an event that has already occurred. This reality means that even for the fastest, most sophisticated arc
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handling technology, there is a brief moment, before the arc
response is affected, when an extremely high power density
is being delivered to the arc spot. Each event carries the risk
of damage to the target surface as well as particle generation
and associated defects. A high arc rate means higher risk of
damage and increased defect generation. At extremely high
arc rates, process stability becomes a factor because the aggregate time associated with the events, generator response
and then recovery to normal operation, can accumulate into
a significant fraction of the total process time.
Arc energy can also impact process quality since the energy released to an arc influences the likelihood of particle
generation or target damage. Arc energy is determined by a
number of factors, most related to the power delivery system
including the power generator and transmission cabling. It has
been shown that material can also influence arc energy but
in general to a lesser degree compared to the contributions
associated with power delivery [6].
Arc persistence is a behavior not commonly discussed but
important especially in defect generation and target damage. A
persistent arc is one that re-generates or re-ignites immediately
after an attempt is made to extinguish an individual event.
Re-generating, or persistent arcs result in additional process
disruption but more importantly in additional energy being
deposited into an already hot spot on the target. These events
can be attributed to ineffective arc management [6] and can
be particularly damaging since energy can accumulate at a
single site with multiple occurrences until the arc is finally
extinguished.
A holistic view of arcing incorporating arc rates, their persistence and arc energy is important when developing an
effective arc management strategy since each of these ultimately contribute to the eventual process stability and overall
product quality. The aim of this study was to collect arc rate,
persistence and energy data on a sampling of materials commonly used in the large area industry to illustrate the impact
arc handling parameters can have on these behaviors. We
discuss some of the important factors for optimizing these
parameters in hopes of providing guidance for minimizing
the impact of arc activity and maximizing the capabilities of
the arc management features available in modern era power
delivery systems.
© 2011 Society of Vacuum Coaters 505/856-7188
54th Annual Technical Conference Proceedings, Chicago, IL April 16–21, 2011 ISSN 0737-5921
Experimental
Data presented here were generated on a large (48”x36”x30”),
open-volume, rectangular chamber, equipped with a rectangular (12”x44”), planar magnetron. Loaded on the magnetron
was one of three different targets, metallic aluminum, ceramic
aluminum-doped zinc oxide (AZO) or metallic zinc. The
chamber was turbo-molecular pumped to a base of 5x10-6
Torr prior to all data collection runs. All tests were performed
using argon at 1 mTorr as the sputtering gas.
Arc data were gathered using two primary methods. Arc
timing, transients and energy measurements were made from
individual captures of current and voltage using a Tektronix
DPO4050 digital oscilloscope. Arc rates, and differentiation
between primary arcs and re-generating, or persisting arcs,
were made using on-board diagnostics on an Advanced Energy® Ascent™ power supply used as the power system for
all reported testing.
The primary parameters investigated were arc detection limit,
response delay time and shutdown time. The arc detection
criteria employed throughout was a simple voltage threshold ranging from -100V to -500V. Proper selection of this
parameter requires some knowledge of the material being
sputtered, as shown below.
Response delay is defined as the time delay from when the
power system detects the arc (when the detection criterion is
met) to the point when the system switches power off. Delay
was varied from 0 (equating to an actual delay of about ½
µsec) to 50 microseconds. This delay time is alternatively
called arc duration since it is the time we allow the arc to
survive before responding to extinguish it. Generally, short
delay time is preferred, since increasing the delay allows more
energy to be deposited into the arc. This parameter was investigated, first to determine energy versus time characteristics,
and second, to consider possible benefits from increasing arc
energy, namely for preventing the build-up of surface defects,
or nodules, during long campaign runs.
Shutdown time, the amount of time power is switched off
in order to extinguish the arc, was tested between 5 µsec
and 50 µsec. This parameter was shown to be influential in
decreasing arc rates on AZO in previous work [6]. We look
more closely at this behavior here and contrast its effects on
different materials in this study.
Results – Detection thresholds
While an in-depth evaluation of detection criteria was not
the intent of this work, it is difficult to discuss arc parameter
selection without some mention of detect criteria. During data
collection for arc energy and arc rates a few related observations were made that merit mention. First, when dissimilar
materials are involved, a single set of detect criteria is often
not adequate. This becomes apparent when arcs are allowed
to propagate for more than a few microseconds as they did
during our arc response delay testing. During long duration
arcs it becomes evident that aluminum arcs and AZO arcs,
for instance, propagate at very different voltages. Figure 1
illustrates this behavior (shown on arcs allowed to stay lit
for 50 µsec). After the initial voltage drop, aluminum arcs
propagate at approximately 30 to 50 volts while AZO arcs
maintain a voltage around to 80 to 100 volts. This behavior
has implications on the proper selection of detection criteria.
For instance a trip threshold of 50 volts would probably be
adequate for aluminum, but could be very dangerous for
AZO presenting high risk to system components due to the
possibility of arcs going undetected.
Volts
200
~ 40 V
0
-200
-400
Amps
0
-600
50
100
150
a)
Volts
200
200
~ 100 V
0
-200
-400
Amps
0
-600
25
50
75
b)
100
Figure 1: Voltage and current for arcs on a) aluminum and b) AZO.
Arcs were allowed to propagate for extended times to show the higher
arc voltage typical on AZO. Time scale is 10 µsec per division.
One other consideration regarding detection criteria comes
from analyzing the evolution of arcs over time. Although arcs
evolve (voltage decays and current rises) quite rapidly, when
referenced on a microsecond scale, the selection of threshold
criteria can impact detection times rather significantly. Figure
2 shows the impact threshold level has on detection time for
AZO arcs. As voltage falls through the duration of the arc,
we observe that the threshold voltage along the curve can
have the effect of delaying detection time. As an example,
for AZO, setting the arc trip threshold at -150V versus -350V
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100
75
60
-100V
-100
45
-200V
-200
30
-300V
-300
15
-400V
-400
0
-500
-15
-600
-30
-700
-45
-800
Voltage -60
Current
-75
6
8
-900
-4
-2
0
2
4
Time, micro-sec
Cathode Current, amps
Cathode Voltage, volts
0
-6
x 10
Figure 2: Voltage and current for an arc on AZO. Selection of trip
voltage can shift arc response time significantly.
In the previous study mentioned above [6] both primary and
persistent arc rates on AZO were shown to increase with
increasing power for short (10 µsec) arc shutdown (Figure
3). With sufficient shutdown time, persistent rates decreased,
causing total arc counts to fall as well. A shutdown time of
20 µsec was shown to achieve a significant reduction in
persistent arcs but the test plan lacked resolution between
10 and 20 µsec. Here we look more closely at this behavior
and attempt to more accurately resolve the time necessary
for preventing arc persistence.
50
45
Arcs per second
40
35
Total
Primary
25
Persistent
20
15
10
5
0
0
5000
10000
15000
20000
Power (watts)
Figure 3: Primary and persistent arc rates on AZO at increasing
power [6].
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800
700
20 µsec duration
600
30 µsec duration
500
50 µsec duration
40 µsec duration
400
300
200
Arc rates and arc persistence
30
Figure 4 shows 10 kW, AZO arc counts versus shutdown
time with particular attention to shutdown times less than
20 µsec. Trends are compared for arc duration from 20 µsec
to 50 µsec. As expected, longer shutdown time resulted in
lower arc counts. The reduction in arc counts was seen for
all arc duration conditions tested. Above 20 µsec shutdown,
arc counts remain relatively flat. The lowest arc rates were
seen for shorter arc duration. Shorter shutdown and longer
duration both increase arc counts. Arc rates for 0 µsec duration were highly erratic suggesting a conditioning issue, these
data were deemed unreliable and thus omitted from the chart.
Arc counts - arcs/sec
has the effect of delaying arc detection by more than a full
microsecond. This has implications on the speed at which
arcs are detected and potentially on the amount of energy
ultimately released into the arc.
0
20
40
60
80
100
Shutdown time (µsec)
Figure 4: Arc rates on AZO for at increasing arc duration (delay)
and arc response shutdown time.
Figure 5 expands the region for shutdown times less than
20 µsec and plots the fraction of arc counts classified as
persistent. Persistent arcs are defined as those occurring
immediately after the initial arc event and the attempted
response to extinguish the primary arc. When an arc persists
it is likely a case where the initial arc response is inadequate
to eliminate the primary event, or the hot, high emission spot
associated with the arc. This situation is most pronounced at
the shortest shutdown times where virtually all initial arcs
persist through multiple attempted shutdowns. Persistent
arcs throughout, were highest when shutdown times were
shortest. Arc persistence for the 0 µsec duration appeared
consistent with other trials here, so despite the erratic rates
mentioned above, the persistence behavior for this condition
appeared to be reliable.
Arc Energy
90%
80%
70%
0 µsec duration
20 µsec duration
30 µsec duration
40 µsec duration
50 µsec duration
60%
50%
40%
30%
20%
10%
0%
0
5
10
15
20
Shutdown time (µsec)
Figure 5: Persistent arcs on AZO for increasing arc duration (delay)
and arc response shutdown time.
With the finer resolution, we observed that shutdown times
between 8 µsec and 14 µsec were necessary to prevent persistent arcs and the optimal time was dependent on arc duration.
Longer arc duration increased the minimum shutdown time
needed to prevent persistent arcs. For the shortest arc duration, 8 µsec shutdown appeared adequate. When arc duration
was increased to 50 µsec, the minimum shutdown needed
increased to 14 µsec.
This behavior was very material sensitive. Figure 6 compares
arc persistence on AZO to both metallic Zn and reactive ZnO
for arcs with a duration of 20 µsec. Arc counts were too low
on aluminum to be meaningful. Shutdown time needed to
prevent persistent arcs increased on zinc to approximately 25
µsec in metallic mode and to something well over 50 µsec
in reactive mode.
Persistent arcs - % of total
100%
90%
ZnO
80%
Zinc
70%
AZO
60%
50%
Numerous factors, many external to the sputtering chamber,
contribute to arc energy [6]. Within the chamber, the material being sputtered is a factor by affecting arc voltage, arc
current and the rate of fall or rise of the voltage and current
respectively. The time elapsed during the power system’s
response to an arc contributes as well. The power system’s
response time is determined largely by the system being used
(e.g. the capability of its arc detect and response features) and
the selection of arc detection criteria as discussed above. The
goal of this portion of the study was to quantitatively evaluate
the impact of arc response timing on our selected materials,
and also to use available controls to purposefully control arc
energy and study its effect on target condition, namely on
residue buildup, or nodule formation on long duration runs.
Figure 7 is arc energy measured for increasing response
delay times (arc duration) on each of our three materials. As
expected, energy increases directly with increased arc duration. ZnO, AZO and Zn (not shown) all showed similar trends
to one another. Aluminum was notably different from the
others, having arc energies consistently lower by more than
50% for most cases. The increase in energy is nearly linear
with delay time for all materials, AZO had the steepest curve,
with maximum arc energies averaging around 25 mJ/kW for
arc duration of 50 µsec. The maximum for AZO was four
times the maximum measured on aluminum, approximately
26 mJ/kW versus 6.6 mJ/kW respectively, for the 50 µsec
response delay condition.
30
Arc Energy (mJ/kW)
Persistent arcs - % of total
100%
AZO
25
ZnO
Aluminum
20
AZO
15
10
ZnO
Al
5
0
40%
0
30%
10
20
30
40
50
60
Arc response delay (µsec)
20%
Figure 7: Arc energy measured for increasing arc duration on
AZO, ZnO and Al.
10%
0%
0
10
20
30
40
50
60
Shutdown time (µsec)
Figure 6: Persistent arcs on AZO, Zn and ZnO. Shutdown time
needed to prevent arc persistence is material dependent. Arc
duration is 20 µsec.
Higher arc energies are a common concern for target damage
and particle generation, but for some processes, long-term
operating stability may be a priority. Previous studies have
suggested improved long-term stability can be achieved if arc
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energy is sufficient to remove or prevent formation of target
residues [7]. AZO is known to be prone for nodule formation
and so, increased arc energy, as achieved in Figure 7, was
thus investigated as a means to reduce nodule formation on
long AZO runs.
Photographs of nodule growth were taken after extended runs
we completed using either low energy arc response settings
or high energy settings. The effect of increasing arc energy
on nodule formation is shown in Figure 8. Figure 8a shows
nodule build up on the AZO target after five hours of sputtering
at 15 kW (75 kW-hr) using a 0 µsec arc response delay (arc
energy approximately 1.6 mJ/kW). After this test the target
was cleaned of all nodules, conditioned and the test repeated
but with an arc delay of 20 µsec, equating to approximately
9.6 mJ/kW arc energy. Figure 8b is a photograph taken after
75 kW-hr at the increased arc energy.
Discussion
Arc rates are known to be influenced by material type and
sputtering conditions (flow, pressure, power). We show here
that arc decay and then reformation behavior are also influenced by arc response method. The behaviors seen in Figure
4 suggests that when shutdown time is too short, arcs do
not fully decay and/or the spot where the arc originates has
inadequate time to cool making arc re-ignition more likely
once voltage is reapplied. The result is increased rates and
high arc persistence at short arc shutdown time.
The impact of arc duration shown in Figure 5 supports this
view showing that long-duration arcs are more likely to
persist. This suggests that hotter spots, those experiencing
higher energy, require longer cool-down time and it follows
that longer duration arcs show higher persistence for a given
shutdown time.
From these results proper selection of arc shutdown time needs
to factor in the importance of reducing arc rates and preventing arc persistence. Figure 6 illustrates that material clearly
plays a role in these arc decay and regeneration behaviors, and
must also be considered when selecting shutdown parameters.
a)
b)
Figure 8: Nodule formation on AZO after 75 kW-hr operation at
a) 0 arc response delay, 1.6 mJ/kW arc energy and b) 20 µsec arc
response delay, 9.6 mJ/kW arc energy.
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Arc energy as should be expected, increases with arc duration. This too, is material sensitive, and is likely driven by the
predominant defect present in the target material. We found
Zn, ZnO and AZO arc energies to all be very similar, while
aluminum stood out as having significantly lower energies.
Since each of the former materials share the primary metal
constituent, we suggest zinc containing inclusions (oxides)
are the driver for arc energies in each of these materials, while
aluminum oxides are likely the primary arc generating sites
on the Al target. The contrasting dielectric properties of the
different oxide materials are believed to be a strong determinant in the arc energy trends we observe.
High arc energy was shown to decrease nodule formation on
long cycle AZO runs. An exhaustive study was not possible
since our target was fully consumed after only a few trials, but
the impact of energy on nodule growth was quite clear. The
notion of a critical energy required to fully prevent nodule
nucleation is worthy of exploration, but additional testing
would be needed and still more to answer whether less energy could offer proportional benefits with less concern for
particle generation.
Conclusions
References
Arc handling parameters influence not only how arcs are
detected but also key factors important to deposition quality
and process consistency. Arc shutdown time can influence arc
rates, especially at short shutdown time when arc persistence
becomes a dominating contributor to rate. Arc persistence
can be a result of inadequate shutdown. The likelihood of arc
persistence is influenced by target material and by the amount
of energy delivered to the initial arc event. Arc persistence
was found to be very high on AZO when shutdown time was
less than 10 µsec. Persistent arcs were completely eliminated
on AZO when shutdown time was 14 µsec or greater. Longer
shutdown time was required to reduce arc persistence during
Zn and ZnO sputtering. Over 30 % of arcs still persisted on
ZnO using a 50 µsec shutdown.
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ranging from 1.6 mJ/kW to 25.5 mJ/kW for the same range
of arc duration. Increasing arc energy was shown to decrease
nodule growth on AZO targets. A noticeable reduction in
nodule growth was observed by increasing arc energy from
1.6 mJ/kW to 9.6 mJ/kW.
Acknowledgements
The authors wish to thank Astrid Borkowski and GfE GmbH
for providing the AZO target materials used in this study and
also, Karen Peterson for her invaluable assistance with configuration of the test system and data collection throughout
the effort.
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