Pulsed DC Sputtering Technical Notes

Pulsed DC sputtering is commonly used to deposit compound thin films from an electrically conductive sputter target. An effective way to make these compound thin films is to use a reactive sputtering process, where a chemically reactive gas is intentionally inlet into the deposition chamber during sputtering to react with the sputtered material.

Reactive sputtering can be used with a base metal target (e.g., titanium plus oxygen gas) or to control the stoichiometry of the film from a conductive compound/ceramic target (e.g., ITO plus oxygen gas). The reactive gas partial pressure required to form a particular stoichiometric compound on the substrate usually causes the surface of the target to be partially reacted. This produces areas on the target surface with different electrical properties, resulting in varying charge accumulation and arc formation to dissipate the charge difference. These arcs are strong enough to cause local evaporation resulting in the undesirable ejection of macroparticles, unstable process parameters, and possible target damage.

KJLC utilizes our bipolar pulsed DC supply to mitigate arc formation. Instead of applying a constant negative DC voltage to the target, the potential is reversed to a positive voltage (15% of the negative voltage magnitude) for a short duration, many thousands of times a second. This positive pulse draws an electron current from the plasma and neutralizes charge build-up on the target surface, whereas the longer period of applied negative voltage sputters the target.

The duration and frequency of the positive voltage reversal are fully controllable from 1-10µs (1µs resolution) and 2-100 kHz, respectively. The full range of duty cycle allows a user to set the appropriate pulsing parameters to outpace charge build-up creating a stable reactive sputtering process while maximizing the deposition rate.

It is important to note that the DC pulse frequency is high enough to maintain a stable reactive deposition process but is not as high as the 13.56 MHz typically used in Radio Frequency (RF) processes. Pulsed DC provides advantages over RF as it does not require complicated matching networks, offers higher deposition rates, and typically has better arc control.

KJLC has pulsed DC supplies with 1 kW and 2 kW maximum single-output power, which are a good fit for 2" to 4" magnetron sputter cathodes. Figure 1 is our recommended size of power supply for the size of the sputter cathode. Additionally, the power supplies have a suitably large current range for reactive sputtering processes at 2.5 amps and 5 amps, respectively.

When some target materials are exposed to a reactive gas, their emission of secondary electrons increases, and the voltage required to sustain the target plasma reduces. As the voltage decreases, the current must increase to maintain a fixed power. Furthermore, it is sometimes beneficial to choose pulsing parameters that operate the target at the lowest voltage and highest current. Sputtering at lower voltages is especially useful when working with complex oxides, such as ITO, to reduce high energy neutral bombardment on the growing thin film. The high energy neutral bombardment can be detrimental by causing preferential re-sputtering of the film, driving it non-stoichiometric. As for the voltage output of these supplies, both the 1 kW and 2 kW units can output up to 800 V, which is well suited to ignite and sustain a plasma at standard operating pressures.

Nitrogen gas (N₂) is commonly used for making reactively sputtered nitride films, although it is only moderately chemically reactive in its diatomic form. It may be beneficial to increase its reactivity by applying RF bias on the substrate while doing a pulsed DC sputtering process on the sputter cathode. The RF bias creates a plasma to crack the nitrogen into monoatomic form and adds energy to promote the reaction. KJLC has rigorously tested the pulsed DC supplies and verified stable operation while RF bias is applied to the substrate.