Pulsed magnetron sputtering uses pulsed power supply with rectangular wave voltage instead of conventional DC power supply for magnetron sputtering deposition. Pulsed magnetron sputtering technology can effectively suppress the arc generation and then eliminate the resulting coating defects. At the same time, it can increase the sputtering deposition rate and reduce the deposition temperature.
Pulses can be divided into bidirectional pulses and unidirectional pulses (as shown in Figure 1). The bidirectional pulse has two phases in one cycle: positive voltage and negative voltage. In the negative voltage segment, the power supply works for the sputtering of the target. And in the positive voltage segment, electrons are introduced to neutralize the positive charge that accumulates on the target surface, and makes the surface clean to expose metal surface. The pulse voltage applied to the target is same as that of general magnetron sputtering (400-500V). Pulsed magnetron sputtering usually uses square wave pulse waveform. Abnormal arc discharge can be effectively eliminated in the middle frequency band (20-200 kHz), and the target discharge time is controlled to ensure that the target is not poisoned and no arcing occurs, then disconnect the target voltage or even make the target positively charged. Because the velocity of the electrons in the plasma is much higher than the velocity of ion, the transformed the positive voltage of target generally requires only 10% to 20% of the negative bias, and the arc discharge can be prevented. It is considered that the pulse width (ratio of positive and negative voltages) plays a key role, the pulse width of 1:1 has the best suppression effect. The positive voltage has no significant effect on whether arc discharge occurs, but it greatly affects the deposition rate. If positive voltage increases from 10% to 20% (ratio to the negative voltage), the deposition rate will be increased by 50%.
Work principle of pulsed magnetron sputtering
Fig. 1 (a) Unidirectional Pulse (b) Bidirectional Pulse
Bidirectional pulses are more often used for dual-target closed unbalanced magnetron sputtering systems as shown in Figure 2, two magnetron targets are connected to the same pulse power supply in the system, similar to the MF twin target, two targets alternately works as the cathode and anode, when the cathode target are sputtered, the anode target completes the surface cleaning, so periodically changing the polarity of the magnetron target produces a “self-cleaning” effect.
The main parameters of pulsed magnetron sputtering include sputtering voltage, pulse frequency and duty ratio. Since the electrons in the plasma have higher motilities compared with ions, the positive voltage value needs only 10% to 20% of the negative voltage value to effectively neutralize the positive charges that accumulated on the target surface. The pulse frequency is usually in the middle frequency range. The lower frequency limit is determined by the critical value that is lower than breakdown field strength that ensures the formation of field strength by the accumulated charge on the target surface. And the upper limit is determined by the deposition rate. Generally, lower frequency is suggested under the premise of ensuring stable discharge. For the duty cycle, under the premise of the charge that accumulated on the target surface can be completely neutralized in the positive voltage phase during sputtering, duty cycle need be to increased as much as possible to achieve the maximum power supply efficiency.
Fig. 2 Schematic diagram of twin target bidirectional pulsed magnetron sputtering
There is a new development, applying pulsed bias to the substrate. Pulsed bias can greatly increase the ion beam current on the substrate. In magnetron sputtering, the DC negative bias is generally applied to -100 V, and the substrate ion beam is saturated. Increasing the negative bias does not increase the substrate ion beam current. This saturation current is generally considered to be the ion beam current. And electrons cannot access the surface of the substrate. But the use of pulsed bias can overcome this shortage, studies have shown that the pulse bias can not only improve the saturation current of the substrate, but also increase the saturation current as the negative bias voltage increases. When the pulse frequency increases, this effect is more significant. Substrate pulsed negative bias provides a new method to effectively control the current density of the substrate. This effect can be applied to optimize the film structure and adhesion, and shorten the time of sputtering cleaning and substrate heating.