Tadao Okimoto

Electrical and Optical Characteristics of High-Power Pulsed Sputtering Glow Discharge

Droplet-free metal-plasma sources are promising for enhanced adhesion of deposited films with a smooth surface. High-power pulsed sputtering (HPPS) plasma is an arc-free discharge and a glow-discharge plasma with instantaneous power consumption of several tens of kilowatts, although the average power is the same as conventional sputtering discharge systems. Sputtered metallic species are significantly ionized. In this paper, two metal-plate cathode targets are positioned in parallel to form a gap. A magnetic field with a strength of 0.3 T is oriented parallel to the electric field, and a Penning discharge is generated at the gap. When a negative pulse voltage is applied to the two targets with the same polarity, a glow plasma is generated at the gap. Gas ions are first produced and accelerated toward the target, where metals are sputtered and simultaneously ionized. The plasma source is compact in size (60 times 60 times 60 mm3), with a gap length of 10 mm. The pulsed voltage is rectangular in shape, with amplitude ranging from -600 to -1500 V and pulse-widths of 30 and 100 mus. The repetition rate of the applied pulse is 625 Hz. Electrical and optical characteristics are investigated to determine fundamental characteristics of the HPPS glow metal plasma, and the plasma density are estimated using an electrode immersed in the HPPS glow plasma. The electrode is set nearby the plasma source. With a current-limiting resistor of 10 Omega and an applied voltage of -1200 V, the peak discharge current is approximately 76 A, which results in a peak instantaneous power consumption of approximately 30 kW at an argon gas pressure of 2 Pa. In this case, the consumed energy per pulse is 0.8 J, which corresponds to an average power of 500 W at a repetition rate of 625 Hz. The peak power density at the target surface is approximately 1.3 kW/cm2. The ion density at the holder-electrode set, positioned near the HPPS plasma source, is estimated to be on the order of 1016 m-3, and it is time dependent. Optical emissions from titanium ions with a charge of +1 and excited titanium atoms are observed. Emissions are also observed from argon atoms and ions. Ions are extracted at the holder electrode. It is found that the titanium-ion spectrum intensity is proportional to the argon-ion spectrum intensity. The waveform of the ion current has a sharp peak at the initial stage followed by a stationary state. Thus, the holder electrode is immersed in the plasma, and a transient ion sheath is formed around the holder electrode. It can be shown that the stationary ion current is proportional to the ion density at the surface of the holder electrode.