High Power Impulse Magnetron Sputtering (HiPIMS) is a newly developed PVD technology for adhesion enhancement pretreatment and deposition of high density, high performance thin films. It is characterised by a very high pulse power density (up to 3 kWcm-2) at very low duty cycles <5%.
The enhanced power density input in the discharge promotes the ionisation of sputtered material from the target and yields an increased instantaneous plasma density. The use of ionised sputtered target material from the plasma phase is an advantage to a number of applications such as substrate pre-treatment to provide crystalline coating-substrate interface with high adhesion and filling of trenches of semiconductor devices.
The typical characteristics of HiPIMS plasma are:
Peak power ~ kW/cm2
Average power ~ W/cm2
Frequency ~ 10-1000 Hz
Pulse width ~ 10-500 μs
Typical discharge voltage and current waveforms are shown in the picture on the right.
It can be seen that the applied voltage to the magnetron source lasts for very short period of time (~50μs). However, the amount of transferred energy is enough to sustain the plasma until the next pulse.
The introduction of pulsing dramatically changes the energetics of the plasma discharge. The plasma density rises to ~1x1019m-3 compared to ~1x1016m-3 during conventional sputtering. Such increase in plasma density results in increased probability of ionising collisions, where sputtered material is ionised alongside process gas atoms. Some publications indicate that the fraction of ionised sputtered material reaches as high as 90% compared to conventional magnetron sputtering.
The OES spectrum of Ti plasma generated by HiPIMS clearly shows this trend. (The image is reproduced from J. Bohlmark et al., J. Vac. Sci. Technol. A 23, 18 (2005))
| Parameter || HiPIMS || DC Sputtering |
| Peak Power Density || 103 Wcm-2 || 1 Wcm-2 |
| Average Power Density || 1 Wcm-2 || 1 Wcm-2 |
| Current Density || 1-103 Acm-2 || 0.01-0.1 Acm-2 |
| Discharge Voltage || 500-1000 V || 500 V |
| Process Gas Pressure || 10-3-10-2 Torr || 10-3-10-2 Torr |
| Magnetic Field Strength || 0.010-0.100 T || 0.010-0.100 T |
| Electron Density || 1018-1019 m-3 || 1016 m-3 |
| Electron Temperature || 1-5 eV || 1-7 eV |
| Degree of Metal Ionisation || 30-100 % || <5 % |
| Ion energy |
(average for metal ions)
| 20 eV || 5 eV |
In addition to increased number of ions, HiPIMS plasma generates ions with energies in the range of 50 eV to 100 eV (compared to only about 2 eV to 10 eV in conventional sputtering). These high energy ions transfer their energy to the coating as it grows, creating very dense coatings. Coatings deposited using HiPIMS do not have the porosity and columnar features typically found in coatings deposited using conventional sputtering techniques.
Interruption of columnar growth and re-nucleation
The control of ion energy allows the operator to control the growing films over their phase composition, microstructure, as well as mechanical and optical properties. For example, HiPIMS coatings tend to be under compressive stress. The compressive film stress level is controllable by changing the peak power density, since the ion energy scales with peak power.