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Choosing the Best Power Supply for Your PVD Process
The Q2 2008 edition of Flat Panel Focus® offers practical guidelines for power supply selection. It expands upon our Q1 2007 issue of Sputter Spotlight®, providing advice for particular target materials and film compositions.
Power supply selection is both an art and a science. The process of determining the ideal unit for your application must take a number of factors into consideration, including target material, process chemistry, and cathode design. It also must account for your particular performance criteria for sputter rate, film characteristics, and other factors.
That said, for certain processes and/or materials, the options are fairly limited, making power supply selection relatively straightforward. For example, for quartz SiO2, RF is the only viable option. Likewise, AC power supplies were developed specifically to enable dual-cathode processes and therefore are always the ideal choice for that application. Alternating current enables the anode and cathode to trade roles, which cleans the anode surface every half cycle. This periodic cleaning prevents the disappearing anode problem, in which the electrically insulative film eventually coats the substrate and other chamber components, blocking current flow and eventually extinguishing the plasma.
For certain processes and/or materials, the options are fairly limited, making power supply selection relatively straightforward. However, for certain materials, such as SnO2 and SiO2, there is a wide range of possibilities.
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However, for certain materials, such as SnO2 and SiO2, there is a wide range of possibilities in terms of process power selection. For SiO2, those options include pulsed DC, AC, and RF. Your decision among these depends upon your particular priorities. Generally, pulsed DC will give you the highest rate when depositing SiO2, but it also may produce a less stable process than other power methods. It also subjects you to the disappearing anode problem. If you to deposit SiO2 with AC, you trade a bit of sputtering rate, but you eliminate the disappearing anode problem. Finally, RF is both immune to the disappearing anode problem and produces the highest film quality of all of the options. The main drawback is that RF provides the lowest sputter rate. However, RF still may be the best choice if film quality is absolutely critical to your application.
In general, as target material conductivity decreases, the frequency of the process power type recommended for it increases. That is, processes using very insulative targets tend to have the most success with high-frequency power, such as RF, while highly conductive targets are most often paired with DC and pulsed DC.
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As previously mentioned, your choice of power supply must take into account your particular preferences for sputter rate, film characteristics, and other factors. It also must factor in cathode design (rotatable/planar, single/ dual), process chemistry, and target material. In general, as target material conductivity decreases, the frequency of the process power type recommended for it increases. That is, processes using very insulative targets tend to have the most success with high-frequency power, such as RF, while highly conductive targets are most often paired with DC and pulsed DC.
Please note that the conductivity of the deposited film material, as well as of the target material, must be taken into consideration when choosing a power supply. Typically, processes running electrically conductive targets are not subject to the disappearing anode problem. However, in certain reactive applications, although your target material is electrically conductive, the film produced may be electrically insulative, which poses the possibility that your anode may become coated with insulative material. In this case, a dual-cathode process using an AC power supply may be your best option, if avoiding the disappearing anode problem is critical to the success of your manufacturing operation.
For further assistance with selecting the right power supply for your process, please contact us.
Ask the FPD Experts!
Are you struggling to squeeze more profit out of your FPD process?
Bruce Fries and Ken Nauman answer some of your difficult questions. Submit your question or comment to FPDapplications@aei.com. To contact Ken Nauman directly, please call +1.970.214.6280 or e-mail Ken.Nauman@aei.com.
- You talked about CEX in the Q4 2007 edition of FP Focus. What exactly is it for?
- Why is it that some AC power supplies feature CEX and others do not?
- My power supply features CEX. How do I set it up properly?
- My power supply won’t reach set point. Why is this happening, and what can I do about it?
- You talked about CEX in the Q4 2007 edition of FP Focus. What exactly is it for?
Answer: CEX stands for common exciter oscillator and is used to connect multiple power supplies together that are connected to multiple electrodes or cathodes in the same chamber. The use of CEX serves to efficiently contain the plasma and alleviate the potentially debilitating effects of crosstalk between the electrodes or cathodes. This type of crosstalk can lead to target damage, substrate arcing, and substrate damage. It may also lead to eventual power supply damage. In addition, crosstalk may prevent you from reaching the desired power level, which decreases throughput.
Larger substrates, such as those frequently used in FPD and architectural glass manufacturing, and more commonly being used in solar applications, can require a dozen or more cathodes per chamber. In order to fit into a single chamber, the space between cathodes must shrink. When closely-spaced targets are not synchronized using a feature such as the PEII power supply’s CEX, they can be at different potentials, causing them to interfere with one another. This interference is called crosstalk and can lead to the serious process, film, and equipment problems described above.
Please see our Enhanced Plasma Containment for Inline Sputtering Systems application note for instructions on using the PEII low-frequency power supply’s unique CEX capability to create an alternating cathode arrangement that significantly improves process control, film quality, and uptime.
- Why is it that some AC power supplies feature CEX and others do not?
Answer: AC power supplies are either fixed or variable frequency. In order to use CEX to synchronize multiple AC power supplies, the output of all of those power supplies must be at the same frequency. Therefore, it’s fairly straightforward to use CEX to synchronize fixed-frequency AC power supplies, such as AE’s PEII power supply. However, the output of variable-frequency power supplies, such as AE’s Crystal® power supply, depends on the load impedance, and this varies based on process conditions. This means that each power supply in the chamber will likely produce a unique frequency based on the unique impedance. Therefore, by definition, it is impossible to synchronize multiple variable-frequency power supplies with CEX or any other phase synchronization feature.
Please note that the benefits of CEX aren’t limited to processes using fixed-frequency AC power supplies. The Pinnacle® Plus+ pulsed-DC power supply, as well as the Pulsar® and Sparc-le® V DC pulsing accessories, feature CEX to synchronize the output of multiple pulsed-DC units. This delivers the same plasma-containment and crosstalk-alleviation benefits to pulsed-DC power arrangements as described above for AC power supply arrangements. CEX is also available on certain AE RF power supplies.
- My power supply features CEX. How do I set it up properly?
Answer: CEX setup involves simple connections from the CEX/Drive Out port of one unit to the CEX/Drive In port of another, as shown in the illustration below. Please note that it’s also necessary to insert a CEX termination plug in the CEX/Drive Out connector on the last unit.
The following illustration shows the back panels of multiple PEII power supplies that are properly connected in a CEX arrangement. Please also see your power supply’s user manual for further instructions, or contact us and we’d be happy to answer questions about CEX setup for your specific system. Please also see our Enhanced Plasma Containment for Inline Sputtering Systems application note for more information on CEX benefits and setup.
Figure 1. Proper CEX setup for connected PEII power supplies
- My power supply won’t reach set point. Why is this happening, and what can I do about it?
Answer: In an integrated system, below-set-point power delivery triggers an alarm, but in a lab environment, it actually causes thinning of the deposited film. As to the cause of the power-delivery problems you are experiencing, there are a couple of possibilities. First, your power supply simply may not be able to deliver the power you need because of a voltage or current limit that is due to impedance mismatch. The solution is to find a more appropriate power supply for your needs. To minimize this problem, AE power supplies have extremely wide impedance ranges, and some are offered in specific impedance configurations to meet your particular needs, such as the Pinnacle® Plus DC/Pulsed-DC power supply, which is available in a high-Z and low-Z configuration.
Another possibility is that your arc rate is too high. In order to extinguish an arc, the power supply shuts down for a very short period of time and then turns on again. Usually, after it turns back on, its output returns to set point. However, if there are too many arcs, the power supply may be shutting down so often that the amount of actual delivered power is falling below set point. For this possibility, the solution is to check your arc parameters and make sure they are set up appropriately for your process. Also evaluate your process parameters and contact Ken for further assistance.