ICMCTF2006 Session H3: High Power Impulse Magnetron Sputtering (HIPIMS)

Ionized physical vapour deposition processes are of great interest for surface modification because the flexibility of the thin film deposition process can be increased by ionizing the metallic vapour.

Recently, High-Power Pulsed Magnetron (HPPM) discharges have been implemented to achieve high degree of ionization (≥50%). Nevertheless, the technique has to face a major problem: the deposition rate is minimized compared to conventional DC magnetron discharges. To overcome this drawback, a High-Power Pulsed Magnetron generator has been designed@footnote 1@ allowing to "calibrate" the pulse length with the deposition conditions. Pulse length is shortened in order to minimize the self-sputtering effect responsible of the decrease of the deposition rate ; the ionization degree of the metallic vapour being still significant.

This way, titanium oxide thin films have been deposited on glass and steel substrates using a 450 x 150 mm rectangular titanium target in argon-oxygen atmosphere. The average power delivered to the plasma is ranging between 1.5 and 2kW and peak current and voltages obtained are respectively reaching 200A and 900V.

Films are characterized using Scanning Electron Microscopy, X-Ray Diffraction and Optical Transmission Spectroscopy. One of the major findings is the presence of a pure rutile phase on steel substrates (even without substrate bias) and the significant increase of the refractive index of the films deposited on glass compared to thin films deposited via conventional DC pulsed magnetron sputtering.

super 1@ M. Ganciu, M. Hecq, S. Konstantinidis, J.P. Dauchot, M. Touzeau, L. de Poucques, J. Bretagne, Eur. Patent Appl. n°4447072.2 / 22.03.2004.

High power pulsed magnetron sputtering (HPPMS), which applies a very large power pulse to the target for a short period of time, results in a very high degree of ionization of the sputtered species. Typical peak power densities are on the order of 1,000 to 3,000 W per square centimeter with pulse durations of 100-150 microseconds. HPPMS is very much like the cathodic are process where there is a very high degree of ionization of the evaporant, but there are few if no droplets with HPPMS. The potential of HPPMS is to use these ionized species to improve the structure and properties of the deposited film. Alami et al.¹ have shown that the ionized species follow field lines to a biased substrate producing a dense film in side wall features, and Petrov et al.² have shown that for films grown with high ion-to- neutral ratios for the arriving species at the substrate a low bias voltages can be used to give fully dense films with low stress. HPPMS should allow the deposition of thick films. The major disadvantage of the HPPMS process is that its deposition rate is only 25-30% of the rate for an equivalent amount of power used during conventional DC sputtering. A model by Christie³ has been developed to explain this loss of rate for the HPPMS process, and the model provides insights that hopefully will bring a solution to this loss of rate issue. In this talk, the advantages and disadvantages for HPPMS as it is practiced today will be reviewed, and the needs to make this technology a commercial reality will be discussed.

¹ J. Alami, P. O. Ã.. Persson, D. Music, J. T. Gudmundsson, J. Bohlmark, and U. Helmersson, J. Vac. Sci. Technol. A, 23 (2005) 278.

² I. Petrov, P. B. Barna, L. Hultman, and J. E. Greene, Microstructural evolution during film growth, J. Vac. Sci. Technol. A, 21 (2003) S117.

³ D. J. Christie, J. Vac. Sci. Technol. A, 23 (2005) 330.

An excellent adhesion of hard coatings to steel substrates is paramount in practically all application areas. Conventional methods utilise Ar glow etching or arc discharge pretreatments that have the disadvantage of producing weak interfaces or adding droplets respectively.

A new tool for interface engineering is high power impulse magnetron sputtering (HIPIMS). HIPIMS is based on conventional sputtering with extremely high peak power densities reaching 3 kWcm^-2 at current densities of >2 Acm^-2. HIPIMS of Cr and Nb was used to prepare interfaces on 304 stainless steel and M2 high speed steel (HSS). During the pretreatment, the substrates were biased to -600 V and -1000 V in the environment of a HIPIMS plasma. The bombarding flux density reached peak values of >100 mAcm^-2 and consisted of highly ionised metal plasma containing a high proportion of Cr¹⁺ and Nb¹⁺ but also doubly charged metal ions Cr²⁺ and Nb²⁺ as observed by optical emission spectroscopy. The adhesion was evaluated for a coating consisting of a 0.5 µm thick CrN base layer and a 2.5 µm thick nanoscale multilayer stack of CrN/NbN with bilayer thickness of 3.4 nm with typical hardness of HK_0.025 = 3100 and residual stress of -1.8 GPa. In the case of Cr HIPIMS pretreatment, the adhesion values on M2 HSS reached scratch test critical load values of L_C = 70 N thus comparing well to L_C = 51 N for interfaces pretreated by arc discharge plasmas. Cross-sectional transmission electron microscopy studies revealed a clean interface and large areas of epitaxial growth. The orientation of the coating grains was observed to change together with the orientation of the grains in the polycrystalline steel substrate. Due to the droplet-free pretreatment by HIPIMS, large scale defects in the coating are minimised and the protection capabilities of the coating against wear and corrosion are enhanced.