Pulsed DC Sputtering is a physical vapor deposition technique with a wide range of applications in the semiconductor, optical and industrial coating industries. Pulsed DC Sputtering is particularly effective for the sputtering of metals and dielectric coating – coatings which are insulating non-conducting materials that can acquire a charge.
It is most often used with Reactive Sputtering where there is a chemical reaction occurring in the plasma between the vaporized target material and ionized gases like Oxygen to form deposition molecules such as silicon oxides. Pulsed DC Sputtering has revolutionized the Reactive Sputtering of “difficult” dielectric materials like Alumina, Titania and Silica with high deposition rates that are impossible with straight DC Sputtering alone.
Sputtering Process
The primary advantages of Pulsed DC Sputtering over conventional DC Sputtering is that it dramatically reduces or eliminates the formation of arcs which occur when the target material used as a coating takes on a charge. This charge when allowed to build discharges in high voltage arcs into the plasma forming droplets causing defects in thin film quality control – as well as potentially damaging the DC power system.
The basic configuration of either a Pulsed or conventional DC Sputtering system is the target material to be used as a coating is placed in a vacuum chamber parallel to the substrate to be coated. The vacuum chamber is evacuated to a base pressure removing H2O, Air, H2, and then backfilled with a high purity inert process gas – usually Argon due to its size and ability to convey kinetic energy upon impact with the target material. For Reactive Sputtering, reactive gases such as Oxygen that will combine with the target material in the plasma to produce oxide molecules are also entered into the vacuum chamber.
A Pulsed DC electrical current typically in the few hundreds of volts range is then applied to the target coating material – which is the cathode or point at which electrons enter the system known as the negative bias – and a positive charge is applied to the substrate to be coated which becomes the anode.
The great challenge of the Reactive Sputtering of dielectric materials such as Alumina, Titania and Silica is preventing arcs when the target material takes on a charge that can seriously degrade or destroy the thin film quality.
With conventional DC Sputtering of dielectrics, an insulating film forms on the surface of the target coating material around the area commonly referred to as the “racetrack” – a circular depression that forms on the surface of the target that has this shape as a result of the circular magnetic field holding the bombarding gas ions close to the target for maximum sputtering. This insulating layer takes on a positive charge as a result of the gas ion collisions which “Poisons” the target – dramatically reducing the sputter rate and ultimately leading to arcing.
Compared to conventional DC Sputtering, arcing can be greatly decreased or even eliminated by pulsing the DC voltage in the 10–350 kHz range with duty cycles in the 50–90% range.
During the “on time” part of the duty cycle, a strong negative pulse of a few hundred volts is applied to the target to initiate sputtering. The gas in the vacuum chamber is ionized as a result of collisions with the target surface sputtering off atoms, and the plasma glow is ignited. Sputtering only takes place while this “on time” negative pulse is being applied to the target.
At the end of this high voltage “on time” part of the duty cycle the voltage is either turned off – or more frequently reversed with a low voltage positive charge to “cleanse” the target of any charge buildup. This is a low voltage short duration cycle reversal, usually around a 20 volt positive reversal that is about one tenth of the “on time” duration and about ten to twenty percent of the on time voltage. During this positive cycle, the target’s surface is discharged or “scrubbed” making it ready for the next negative voltage pulse sputtering the target coating material into the plasma.
Because this buildup of dielectric charge develops over time, problems with maintaining process stability and control that are not evident at the beginning of a run can change dramatically thirty minutes later as the thin film process environment changes. Continuously pulsing the DC circuit also in and of itself is usually not enough to totally eliminate arcs from forming which can be a very expensive, quality control breakdown event that can require a restarting of the run.
Today most state-of-the-art Pulsed DC power sources also contain arc suppression circuitry to detect and extinguish an arc at the moment it forms by detecting changes in the target voltage. When the rapid fall in the target voltage associated with an arc is detected – typically between 50 and 150V – it activates a reverse voltage pulse in an attempt to quench the arc.
An arc which is eliminated with a single reversal of the voltage pulse is known as a “micro-arc”. Those that require more than one reversal of the voltage pulse to suppress are called “hard-arcs” after which time the normal Pulsed DC cycle resumes.
Output cabling of the sputtering coating equipment can also acquire a charge that contributes to arcing. In addition to the basic Pulsed DC power supply arc handling circuitry is designed to detect and dissipate charge buildups, be proactive inhibiting arc formation, extinguishing arcs that do form, and compensate for charge buildups in a system’s cabling to prevent expensive breakdowns in the thin film quality control environment.
There are also two types of Pulsed DC Magnetron Sputtering. Uni-Polar Pulsed and Bi-Polar Pulsed Sputtering where two pulses from adjacent magnetrons set 180 degrees out of phase with each can also greatly reduces the effects of dielectric buildup. The combination of a Pulsed DC power source with a high voltage “on time” cycle followed by a low voltage reversal of current, combined with arc handling circuitry has enable the deposition of even the most difficult to sputter dielectric coating materials such as AZO, ZnO and Al2 O3.
While Pulsed DC Sputtering was initially developed for its arc handling capabilities for sputtering dielectric materials, it has also been found that by changing the pulse parameters other beneficial results can be controlled like the presence of more gas ions in the plasma flux which improves film adhesion, and enables the control of other important thin film deposition properties.
The primary advantages of Pulsed DC Sputtering is that at optimum pulsing frequencies and duty cycles, higher thin film deposition rates can be achieved compared to RF Sputtering without the problems with arcing that results in expensive quality control issues.
Matt Hughes is President of Semicore Equipment Inc, a leading worldwide supplier of sputtering equipment for the electronics, solar energy, optical, medical, military, automotive, and related high tech industries. Please let our helpful support staff answer any questions you have regarding “What is Pulsed DC Sputtering?” and how to implement the best techniques and equipment for your specific Pulsed DC Sputtering Equipment needs by contacting us at sales@semicore.com or by calling 925-373-8201.
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Photo: CC BY-SA 2.0 Gisela Giardino