Better Coatings through Modular Electronics
Magnetron sputtering is widely used to deposit thin films on a range of substrates, including glass and ceramics. The technology can be easily scaled to larger dimensions; it offers a high deposition rate (allowing a high level of productivity) and tight control of layer thickness, quality and uniformity; and it can be used to deposit a variety of different materials in thicknesses ranging from a few nanometers to several micrometers.
However, to consistently sustain these processes, all hardware components must be tuned to each other. The electronics driving the sputter process should be adapted to the target material and process conditions to obtain the desired layer properties, and the magnetron hardware should work flawlessly with the electronics to achieve arc-free and long-term
stable plasma conditions.
Recently, a new modular electronics system has been developed to achieve these goals, thereby providing greater efficiency and control in magnetron sputtering applications.
Processes
DC sputtering is typically used to deposit metals or weak insulating materials (such as most nitrides). However, a buildup of insulating material next to the plasma racetrack, and the consequent charging effect of the ionic bombardment, often initiates an electrical breakdown that causes arcing.When depositing more highly insulating materials (such as SiO2) by reactive processes, DC sputtering is susceptible to even more pronounced arcing, as well as to a "disappearing anode" effect. During reactive sputter deposition, the insulating layer is not only formed on the substrate, but also on the chamber walls and anode. As this insulating layer builds, the anode gradually loses its ability to collect electrons, resulting in a shift of the current/voltage characteristic of the system over time. The plasma becomes more diffuse and unstable, causing the coating uniformity on the substrate to deteriorate substantially as inhomogeneities in thickness and composition are introduced.
Switching between two full-sized magnetrons-also called dual-magnetron sputtering-solves the disappearing anode problem. This technology uses an AC signal applied between two targets; one target is used as the anode during the first half of the period, while the other target is the cathode of the system and is thus sputter-cleaned. During the second half of the period, the situation is reversed, thereby ensuring that the anode surface always remains clean. If the switching frequency is high enough, dual-magnetron sputtering can also eliminate arcing on the cathode.
Modular Electronics
A new concept called modular electronics can further simplify magnetron sputtering and ensure optimal flexibility, stability and sputter rates. The system consists of a DC power supply, an arc suppressor, a low-frequency (LF) DC/AC converter and a medium-frequency (MF) DC/AC converter. The system is fully integrated with the other sputter components, including the magnetron, magnetic system, target and gas distribution, to further improve overall control and flexibility.MF sputtering can be performed using an AC generator or a DC power supply combined with a DC/AC converter close to the process. However, long cables are needed to connect the power supply to the cathode, and these cables can emit electromagnetic noise over their entire length, which can interfere with the sensitive electronics of the system. Moreover, a special (dis)connection is required to service each coating zone, and upgrades can be difficult and time consuming.
In a modular electronics system, the DC/AC converter is built into the cathode cover. As a result, the length of the MF cables running to the target can be kept to a minimum, drastically reducing electromagnetic radiation and noise. The modular concept also allows for "drop-in" upgrades, in which planar target sputter coaters can be easily changed to high-end rotatable sputter coaters without requiring drastic modifications to the existing coater hardware.
Another advantage of using modular electronics is increased flexibility. Because the converter is built into the cathode cover, the same system can be used for both DC and AC deposition. Additionally, the system's open architecture and high number of "off-the-shelf" components are designed to optimize reliability.
Solving Common Problems
Arcing during magnetron sputtering is unavoidable; however, it can be controlled and suppressed to minimize damage to the substrate, target and other coater components.
To ensure a stable DC sputtering process, the power supply must react quickly to arcing and store a minimal amount of energy. This is achieved in the modular electronics system by using a switching mode power supply that operates on a high frequency (20-60 kHz) and reacts quickly (within 150 μs) to stop the energy supply to the targets when an arc is detected, thereby minimizing the duration of the arc (see Figure 1). After a brief pause, the process automatically restarts. A small output coil is used to smooth the output current and limit the amount of energy that can be stored, thereby minimizing the energy supplied to the arc. The use of a small output coil also reduces the overall machine size; even large-area coating systems operating with 100 kW (200-300 A) and 150 kW (400 A) DC power supplies can be designed with a small footprint.
For applications that require a medium-frequency converter, such as SiO2, the system can be designed to incorporate a MF DC/AC converter that works in a frequency range between 5 and 40 kHz (see Figure 3). Increasing the frequency simultaneously solves both the arcing and disappearing anode problems in these applications.
Better Control
The flexible modular electronics system can optimize stability, flexibility and sputter rates for metallic and reactive sputtering processes in both DC and AC modes. Because the DC/AC converter is built inside the cathode cover, the length of the connections to the target can be minimized, drastically reducing electromagnetic radiation and noise. Additionally, the electronics allow for drop-in upgrades from planar to rotatable technology with minimal coater hardware adaptation. For companies that use magnetron sputtering, the new modular electronics system can provide maximum sputter efficiency and process control.For more information about modular electronics or magnetron sputtering, contact Bekaert at E3-Laan 75/79, B-9800 Deinze, Belgium; (32) 9-381-6161; fax (32) 9-380-0667; e-mail koen.staelens@bekaert.com; or visit http://www.bekaert.com/bac.