AbStrACt
Pulsed DC power is widely used for reactive magnetron sputtering, particularly for dielectric compounds. Its introduction was transformational, enabling reactive sputtering of dielectrics that were essentially impossible with straight
DC. This was accomplished by periodically discharging the voltage on dielectric films reactively deposited on the target, actually preventing the occurrence of arcs. The concept of voltage reversal to prevent target arcs during reactive sputtering has been known since at least the 1970s, when it was presented in a patent application. Further development showed that a pulsed implementation could be industrially practical.
Asymmetric pulsing was a key concept, with a reversal large enough to discharge the target surface, but small enough to prevent sputtering of the chamber surfaces and anodes. An important aspect of this development was the use of voltage reversal as an effective way to handle the few arcs that did occur, since it rapidly removed the energy stored in the cable system, independent of its length. Now, with continuously increasing demands for diverse films deposited at high quality with minimum defects, increased control and synchronization of multiple pulsed supplies is required. This paper covers strategies for delivering higher power and more control to meet current and future requirements.
IntroDuCtIon
Since its introduction, pulsed DC arc handling has become an important technique in the arsenal of the sputtering process engineer. It offers several sputtering advantages. A primary advantage is dramatically reduced arcing, especially for reactive sputtering processes (arc prevention), but with process characteristics very similar to the pure DC approach. Further advantages include low arc energy, reduced particle generation and reduced sensitivity to imperfections in magnetron targets and process gases. In normal operation, the sputtering power is delivered in a periodic pulse train in which a positive (reverse) voltage on the order of 5 % to 20 % of the magnitude of the sputtering voltage is applied after every sputtering pulse. This positive voltage tends to discharge voltage buildup on any oxide that is on the target surface, actually http://dx.doi.org/10.14332/svc14.proc.1820
preventing the occurrence of process arcs, which is important for reactive sputtering of some materials. When an arc on the target is detected, the output voltage is immediately reversed to quench the arc, after which the pulsing sequence resumes.
The introduction of asymmetric bipolar pulsed DC in the early 1990s was immediately disruptive to the paradigm for reactive sputtering. Suddenly, it was possible to do reactive sputtering of difficult compounds, such as Al
2
O x
, with a single magnetron and a DC power supply, along with a pulsed DC accessory [1-4].
The concept of voltage reversal for prevention of arcing on a sputtering magnetron was disclosed by Cormia in a patent issued in 1977 [5]. Cormia specifically described the concept of full voltage reversal using a sinusoidal waveform. An enabling refinement to this concept was disclosed by Drummond in a patent issued in 1995, describing the invention of voltage reversal with pulsed DC, with timeframes and reversal magnitudes that enable practical industrial reactive sputtering processes
[1]. It has been used in products developed manufactured by
Advanced Energy ® for two decades to inhibit arc formation and to swiftly quench arcs when they do occur. To suppress arc formation, voltage is reversed on a periodic basis, dissipating energy buildup in insulative materials deposited on the target surface during reactive sputtering. Voltage reversal offered the means to actively suppress the buildup of charge before an arc occurs, in many cases preventing the events altogether.
To quench arcs, voltage reversal temporarily removes energy in response to process conditions that indicate an arc event.
It reduces arc events occurring even on metal targets by an order of magnitude or more. By proactively inhibiting arc formation, quenching arcs that do occur, and minimizing arc energy by dissipating charges stored in output cabling, this technique enables deposition of even the most inherently arcprone materials, such as AZO, Al
2
O
3
, and ZnO. An additional significant benefit of this technique is that it dissipates energy stored in the plasma, the magnetron, and the power supply’s output cabling. This lowers arc energy typically to below 1 mJ/kW, which significantly decreases particulate generation and film damage.
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More recently, ion energy has been correlated to pulse parameters. This correlation has been used to tune processes to influence for example crystal orientation, and tribological performance for very thin MoS
2
films [6].
ArC MAnAgeMent
Arcs can occur when insulative material deposited on or included in the target is charged by ion bombardment to the point where it initiates an electrical breakdown, as shown in
Figure 1. To inhibit arc formation and enable more effective quenching of arcs that may still occur, a periodic reversal in voltage clears charge buildup by showering the cathode with electrons as shown in Figure 2. In this manner, voltage reversal is self-triggered and does not require an arc event in to dissipate charge buildup. This was the first-ever proactive measure in reducing arc incidence, in many cases preventing the events altogether [1-4]. This reduces production of macro-particles which may be included in the deposited film as shown in Figure 1.
Figure 1: Charge buildup forms around insulative regions that lead to arcing and production of macro-particles that are included in the deposited film.
Figure 2: Reverse-voltage pulse modulation for active charge cleaning and arc prevention.
A power supply with voltage reversal capability periodically reverses target voltage to about one-tenth of the operating target voltage, which is generally slightly more positive than the plasma potential. This dissipates energy buildup on insulative regions on the target, in the plasma, on the magnetron, and in the power supply’s output stage and cabling, while maintaining plasma stability. During the reversal, electrons are delivered to the target surface, which discharges ions on any insulating layers. Voltage reversal of a defined magnitude will discharge the current in the cable over a range practical lengths, in essence compensating for cable length. This is the approach taken by Drummond [1] as described earlier.
Voltage reversal length and magnitude are controlled to eliminate ion-cloud disturbances. The reversal is typically held for 1 to 10 μs, which is adjustable depending on process conditions. The electrons actually arrive very quickly, but the positive potential is maintained slightly longer because stray inductances and capacitances can delay the appearance of the voltage on the target surface. The period between reversals is adjustable, typically between 10 to 2,000 μs.
Using the equation t b
=  r characteristics of Al period t b
2
O
3
(E b
 o
E b
/J, and assuming material
= 108 v/m ,  r
= 10), the discharge
of 50 μs suffices to discharge a surface bombarded with a current density of approximately 103 A/m 2 (100 mA/cm 2 ).
The power supply is sufficiently isolated from the target voltage reversal, enabling it to recover quickly and preserve process stability. This is accomplished with appropriate circuit design, typically incorporating an inductive element for short term isolation.
MAnAgIng DePoSItIon ProCeSSeS
Pulsing has been found to enhance the film and/or target properties of some sputtered materials. It is possible to find optimal pulse parameters for deposition of ITO using Pulsed
DC. By pulsing at a certain frequency and duty cycle, a “low voltage” saddle can be located, resulting in reduced nodule formation and improved film properties.
When DC sputtering NiCr in the oxide mode, it is well known that powder builds up on the target surface after some number of days of constant production, as shown in Figure 3. Pulsing greatly reduces the powder buildup and enables a much longer campaigns between maintenance cycles.
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Pulsed DC DC Only
Figure 3: Pulsed DC can effectively reduce nodule formation on NiCr targets, pictured here, comparing Pulsed
DC and straight DC power.
Sputtering a planar Zn target with DC grows the familiar hairs on the target surface resulting in highly frequent Ar cleaning cycles per hour. Sputtering with Pulsed DC reduces the Ar cleaning cycle to once per shift.
With both standard and pulsed-DC power, voltage reversal improves film quality, enabling manufacturers to choose process power based on their priorities for film quality and cost. However, the pairing of pulsed DC and voltage reversal excels where highly arc-prone materials and highly precise film characteristics are required. It enables the manufacture of extremely high-quality films with highly arc-prone materials such as AZO, AL
2
O
3
, and ZNO.
Power SySteM SolutIonS
Voltage reversal enables deposition of highly arc-prone materials, such as AZO, ZnO, Al
2
O
3
, with reduced film defects, and excellent transmission, resistivity, and uniformity. Figure 4 shows practical industrial power solutions for DC and pulsed
DC sputtering manufactured by Advanced Energy ® . These are industrially rated solutions that handle arcs by voltage reversal. The pulsed DC solutions shown in Figure 4 not only suppresses arcs, but also quench them when they do occur, and minimize available arc energy. A key benefit is their ability to dissipate energy stored inductively in output cabling and in the power supply’s output stage, reducing arc energy and resulting particulate formation and film damage.
ConCluSIonS
Pulsed DC power has multiple benefits. Initially developed for the purpose of preventing arcs and reducing arc energy, the approach was found to have additional benefits. Workers found that film characteristics could be tuned by changing pulse parameters, and that process operation could be enhanced.
One example of process enhancement is the reduction of nodule formation in some processes when pulsed DC is used.
ACknowleDgeMentS
The authors would like to acknowledge the diligent and persistent efforts of those who brought this enabling technology to the thin film community.
Ascent ® AMS with Arc Management
System™ Technology Series
Pinnacle ® Plus Series Solvix ® Series
Figure 4: Power solutions which reverse voltage to minimize arc energy during arc handling. The second and third from the left use periodic voltage reversal to also prevent arcs in reactive deposition and metallic sputtering processes.
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reFerenCeS
1. G.N. Drummond, US Patent 5,427,669, Thin film DC plasma processing system, June 27, 1995.
2. G.N. Drummond, R.A. Scholl, US Patent 5,718,813,
“Enhanced reactive DC sputtering system,” 17 February
1998.
3. R.A. Scholl, “A new method of handling arcs and reducing particulates in DC plasma processing,” 36 th Annual
Technical Conference Proceedings of the Society of
Vacuum Coaters, pp. 405, 1993.
4. R.A. Scholl, “Reactive PV deposition of insulators,” 39 th
Annual Technical Conference Proceedings of the Society of Vacuum Coaters , pp. 31, 1996.
5. R.L. Cormia, et al., US Patent 4,046,659, Method for coating a substrate, Sep 6, 1977.
6. C. Muratore, A.A. Voevodin, Control of molybdenum disulfide basal plane orientation during coating growth in pulsed magnetron sputtering discharges, Thin Solid
Films 517 (2009) 5605–5610.
http://dx.doi.org/10.1016/j.tsf.2009.01.190
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