Influence of Gas Species on Electrical Characteristics of High-Power Pulsed Sputtering

Thin film deposition is an important process for various applications of the material. Nitride films, such as titanium nitride (TiN) and chromium nitride (CrN), have been used in various components because of their good wear, fatigue, and corrosion resistance. Direct current magnetron sputtering (dcMS) is one of the methods of nitride film deposition. However, the target power is limited to prevent the transition from glow discharge to arc discharge due to heating. Therefore, the ionization rate of sputtered particles is low, and it is difficult to control the energy of ions incident on the substrate by bias voltage. High-power impulse magnetron sputtering (HiPIMS) is one of the promising methods of thin film deposition owing to its high power density and high ionization rate of sputtered metals. However, target utilization efficiency is low, and the size of system is large. A high-power pulsed sputtering (HPPS) Penning discharge generated by parallel electric and magnetic fields at the ionization region was developed to overcome disadvantages of HiPIMS. In the HPPS Penning discharge, the magnetic field is used to reduce electron losses through the electron trapping in the gap between the targets. In the case of depositing nitride film, argon and nitrogen mixed gases are used. Although electrical characteristics of HPPS Penning discharge in argon gas have been reported, there is few reports in nitrogen gas. In this paper, the influence of gas species on electrical characteristics of HPPS Penning discharge.<br/>The HPPS Penning discharge unit is compact in size (60 × 67 ×86 mm³). A pair of rectangular Ti plates with a length of 60 mm, a height of 20 mm and a thickness of 5 mm was used as the sputtering target cathode. Two Ti target plates put on the magnet holders in which permanent magnets set were placed on the opposite side with a gap length of 10 mm. A magnetic field was produced perpendicular to the targets. The strength of the magnetic field in the gap was approximately 0.2 T. A cylindrical vacuum chamber was evacuated to 5 × 10⁻³ Pa, and then the nitrogen (N₂) gas or argon (Ar) gas was fed into the chamber with the gas pressure of 4 Pa. A rectangular pulsed voltage with a pulse width of 600 µs and an amplitude of -1000 V was applied to the target cathode using a high-voltage negative pulse power supply (PEKURUS KJ06-3265). The pulse repetition rate was fixed at 1 Hz. To extract ions from plasma, a disk electrode was used. A stainless steel (SUS304) disk electrode with a diameter of 38.5 mm (Surface area; <i>A</i> = 1.16 × 10^-3 mm²) was placed in the chamber at a distance <i>L</i> away from the HPPS Penning discharge ion source. <i>L</i> was varied from 30 to 90 mm. A negative DC voltage of −100 V was applied to the disk electrode using a power supply. The current flowing into the disk electrode <i>I</i>_D [A] were observed by current transformers (Pearson 110A, Pearson 411). The ion density <i>n</i>_i can be obtained from the following by calculating the current density <i>j</i> from the measured <i>I</i>_D.<br/><i>n</i>_i = <i>j</i>/0.61<i>eu</i>_B<br/>where <i>e</i> is an elementary charge, <i>u</i>_B is Bohm velocity. Bohm velocity was calculated the electron temperature is assumed to be 1 eV.<br/>The ion density at Ar gas atmosphere is higher than that of N₂ gas atmosphere. The ion densities at 30 mm of <i>L</i> are 3.1 × 10¹⁸ and 5.3 × 10¹⁸ m⁻³ at N₂ gas and Ar gas atmosphere, respectively. The ion density decreases with increasing <i>L</i>. The ion density at N₂ gas and Ar gas atmosphere showed similar tendency to those at atmosphere.<br/>This work was partly supported by a Grant-in-Aid for Scientific Research (S: 19H05611)