Power supply with flux-controlled transformer

Details Power supply with flux-controlled transformer Power supply with flux-controlled transformer

2001-11-13

2003-03-11

Sterrett, Jeffrey (Department: 2838)

Electric power conversion systems

Current conversion

Including automatic or integral protection means

C363S095000

Reexamination Certificate

active

06532161

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the control of the delivery of power in a variety of applications, and more particularly to methods and systems to deliver and control pulsed power as may be applicable to plasma thin film processing and other applications.
2. Brief Description of the Prior Art
Thin film layer systems are commonly deposited on glass or polymer substrates with a large area coater. These layer systems are often an implementation of an optical filter. These filters use a combination of interference effects and the transmission and reflection spectra of the discrete metal and dielectric layers to arrive at their composite spectral transmission and absorption characteristics. The metallic layers, commonly titanium, silver, nichrome, aluminum or stainless steel, are typically sputtered with a magnetron operating in the metallic mode in an argon gas environment and powered by a DC supply as described in the book by R. J. Hill, S. J. Nadel, “Coated glass applications and markets”, BOC (1999), p.55-86
A process known as reactive sputtering, where a metal or semiconductor is sputtered in the presence of a reactive gas, typically oxygen or nitrogen, deposits the dielectric layers. The gas then combines with the conductive sputtered material to form a dielectric. This dielectric tends to be deposited on the sputtering target and anode as well as the work piece. The result is an insulating coating on both the target and the anode, which will eventually degrade and perhaps even shut down the process when it is powered by a DC supply. This degradation is primarily due to an effect referred to as a “disappearing anode”. The anode will disappear because it is eventually coated with an insulator, the same reactively formed dielectric compound deposited on the work piece.
One solution to this problem is to use a dual magnetron sputtering arrangement, as shown in FIG.
1
. In this approach the pair of magnetrons A and B is driven by an AC supply that is electrically isolated from the plasma chamber
4
creating plasma
6
. Therefore, they alternate roles between cathode and anode. So, after a brief time acting as an anode, and receiving a tiny deposition of dielectric, the magnetron will act as a cathode, sputtering conductive material as well as the little bit of dielectric that was deposited during the time it acted as an anode. As a result, a clean anode is always available to complete the current path.
Some industrial applications require delivery of power in a pulsed format where the power delivered in opposite polarities must be regulated independently. The pulsed dual magnetron sputtering arrangement used for depositing thin film coatings is such an example. The two magnetrons labeled “A” and “B” are situated in the plasma chamber, in a low-pressure gas environment. The power supply is connected to the magnetrons through gas-tight insulating feed-throughs. Loads of this type can have different voltages for each polarity. The voltages also vary dynamically with time and non-linearly with current. One example of a current versus voltage (I-V) curve for a dual magnetron arrangement is shown in FIG.
2
. The current may vary non-linearly with voltage and I-V characteristics for positive and negative voltages may not be symmetrical. An example of the pulsed voltage and current waveforms as a function of time is shown in FIG.
3
. The power supply can be set to have different amplitudes and pulse widths for positive and negative polarities, which can provide the capability of regulating the power delivered to each magnetron independently. The positive current can be smaller in proportion to the positive voltage than the negative current in proportion to the negative voltage. This is because the I-V characteristics for positive and negative voltages may not be symmetrical, as shown in FIG.
3
. In practical applications these loads can vary in steady state operating voltage, and therefore large signal impedance, depending on physical configuration, magnetic field strength, process gas composition and pressure, target composition, and steady state current.
This technique was reported in G. Este, W. D. Westwood, J. Vac. Sci. Technology, A 6 (3), (May/June 1988), p. 1845-1848. Now, commonly referred to as dual magnetron sputtering (DMS) or dual cathode sputtering. This configuration is widely used in the deposition of low-emissivity (“low-e”) coatings on architectural glass in large in-line coaters. Other significant applications include mirrors, flat panel displays, and anti-reflection (AR) coated glass. DMS is also employed in roll coaters for depositing coatings on plastic films for stick-on glare reduction filters as well as oxygen barriers for plastic food packaging films. An additional benefit of dual magnetron sputtering is the denser and smoother films it produces, due to a much higher flux of energetic ions at the substrate. Silver layers deposited on these smoother films have lower sheet resistance, and therefore lower emissivity, with the same optical transmission characteristics as taught in H. Schilling, et al., ‘New layer system for architectural glass based on dual twin-magnetron sputtered TiO2’, 41st Annual Technical Conf. Proc., SVC (1998), p. 165-173.
Considerable effort has been expended to develop power supplies whose topologies are optimized for driving these dual magnetron systems. Both square-wave pulsed supplies and sinusoidal (or, resonant) power supplies have been used. High power sinusoidal AC supplies are commercially available. Descriptions of these power supplies have appeared in the technical literature and have been presented at conferences within the past few years as disclosed in T. Rettich, P. Wiedemuth, Journal of Non-Crystalline Solids 218 (1997) p. 50-53; T. Rettich, P. Wiedemuth, ‘New application of medium frequency sputtering for large area coating’, 41st Annual Technical Conf. Proc., SVC (1998), p. 182-186; and G. Wallace, Thin Solid Films 351 (1999) p. 21-26. The AC power supplies on the market today control and measure the total power delivered to the two magnetrons. Measurement and control of the power, current and voltage for individual magnetrons is not yet available in sinusoidal AC supplies.
Pulsed supplies inherently offer more flexibility in control of the process. They provide the capability of independently regulating the power delivered to each magnetron. This has some advantages for existing processes, and enables the implementation of new processes. First, independent regulation of power to each magnetron can force each magnetron to receive the same power. Consequently, the racetracks erode at the same rate. When a resonant supply is used, an impedance difference between the two magnetrons can result in faster erosion of one target, which unnecessarily reduces the time between preventative maintenance cycles. Second, it is possible to intentionally operate the two magnetrons at different powers. If one target develops a tendency to arc, its power can be reduced to the point where it arcs at an acceptable rate, and, sometimes, the power to the other magnetron can be increased to compensate and maintain the same deposition rate from the pair. Third, independent regulation enables the creation of controlled mixtures of materials in the film when dissimilar materials are used for the magnetron targets. This would allow the creation of films with customized or graded indexes of refraction. For example, SiO2 can be deposited with a refractive index of about 1.5 and TiO2 can be deposited with a refractive index of about 2.4. If a dual magnetron sputtering arrangement is configured with one Si target and one Ti target, the ratio of Ti to Si can be controlled by controlling the power to each of the magnetrons. Therefore, in principle, it is possible to “dial” the refractive index anywhere between 1.5 and 2.4.
Pulsed power supplies used in dual magnetron sputtering need to be rated at 120 to 200 kW with the ability to regulate on voltage, current, or power are disclosed in P.

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