ICMCTF2013 Session F2-1: High Power Impulse Magnetron Sputtering

The high degree of ionization of the sputtered material during the coating process is one of the main features of HIPIMS (High Power Impulse Magnetron Sputtering). The use of HIPIMS leads to better film quality for hard coatings based on metal nitrides and to more conformal coatings during via fillings with high aspect ratios used in microelectronics. Metal oxides are used in many applications like optical coating for filters or transparent conducting oxides (TCOs) with fields of application in photovoltaics, low emissivity coatings, heat mirrors or panel heaters as well as touch panels and displays. At Fraunhofer IST a TCO and optical films has been developed with HIPIMS technology which were applied on glass substrates and photovoltaic absorbers. We will see that HIPIMS is beneficial for metal oxide coatings therefore an overview on applications in industry and research will be given.

Film growth under intense ion bombardment leads to significant microstructural rearrangement and material’s densification. In this context, compared to the more traditional PVD and PECVD techniques, there has been a lot of progress in generating very dense plasmas in pulsed discharges, especially in the case of High Power Impulse Magnetron Sputtering (HiPIMS). For more than a decade, the latter technique has been intensively studied for its unique capability to obtain thin films from a high flux of highly ionized materials. Now, after so many years of investigation, it is time to conclude if this approach meets our expectations. In the present work, we specifically evaluate its capabilities with respect to the fabrication of optical coatings.

In the first part of this work, we study the deposition of high (H) and low (L) refractive index metal oxides in reactive O₂/Ar gas mixtures under high power impulse conditions using a relatively large range of pulse frequencies, pulse durations and discharge voltage and current levels accessible by using different and complementary power supplies. We show that discharge operation in the transition mode between metallic and poisoned target surfaces can give rise to stable deposition conditions and complete hysteresis suppression. Examples include representative H and L optical films such as Nb₂O₅, Ta₂O₅, and SiO₂ exhibiting low absorption, attractive deposition rate, and systematically low internal stress.

In the second part, we demonstrate fabrication of multilayer optical interference filters using the HiPIMS process. Such filters are then compared with those obtained by more traditional sputtering techniques. Evaluation of the overall coating and process performance allows one to judge on the applicability of HiPIMS for meeting the challenging optical filter requirements.

Epitaxial (001)-oriented Mo/V superlattices have been grown by HiPIMS (High power impulse magnetron sputtering) on single-crystalline MgO(100) substrates at growth temperatures ranging from 30 to 700 °C. Superlattice periods of Mo/V 4/4 ML to 16/16 ML were studied. The as-deposited films were characterized by x-ray reflection and diffraction techniques.

Various types of bcc superlattices grown on MgO(100) substrates have been well studied during the last 20 years using magnetron sputtering methods. The best known superlattices are based on Fe/V and Mo/V repeat structures [1,2] but there are more structures such as Nb/Ta, Mo/Nb and Nb/W. Fe/V superlattices can be grown with good quality at temperature of around 200-300°C while for Mo/V a temperature of 700°C is needed to obtain similar quality [1,3]. The main difference in temperature is due to the lower mobility of the molybdenum atoms at the Mo surface during the growth. This study aims to investigate the effect of the HiPIMS process on reducing the growth temperature of Mo/V superlattices using the high energy ionized Mo, V species in the HiPIMS plasma. HiPIMS sputtering parameters such as voltage and pulse length have been optimized relative to the each growth temperature and the requirement of keeping the growth rate as close to conventional magnetron sputtering as possible.

[1] Birch, J. and Hultman, L. and Sundgren, J.-E. and Radnoczi, G

Strain-induced growth-mode transition of V in epitaxial Mo/V(001) superlattices

Phys. Rev. B 1996, 53 8114--8123

[2] Mattson, J. E. and Fullerton, Eric E. and Sowers, C. H. and Bader, S. D.

Epitaxial growth of body centered cubic transition metal films and superlattices onto MgO (111), (011), and (001) substrates

Journal of Vac Sci Tech A: 1995,13, 2, 276 -281

[3] Isberg P.; Granberg P.; Svedberg E. B.; Hjörvarsson B.; Wäppling R.; Nordblad P.

Structure and magnetic properties of Fe/V (110) superlattices

Preferential sputtering from compound targets is well known: the elements of higher sputtering yields are removed from the target at a greater rate, leading to enrichment of the target surface region with the remaining elements of lower sputtering yields. In high power impulse magnetron sputtering, the situation is a bit more complicated since the sputtered atoms are ionized and participate in the sputtered process (self-sputtering). When using compound targets we therefore deal with multiple ion species that cause sputtering, each having different specific sputtering yields, and ionization and target-return probabilities. The situation will be illustrated with sputtering of lanthanum hexaboride, producing boron-rich plasma.

In this study, nanocrystalline TiO₂ films were reactively sputtered onto glass and steel substrates in a balanced magnetron sputtering system using the new deep oscillation magnetron sputtering (DOMS) and conventional pulsed dc magnetron sputtering (PDCMS) techniques. No external substrate bias or heating were used for the depositions. For the DOMS TiO₂ depositions, different peak target discharge currents (powers) were used. The crystalline phase and microstructure of the TiO₂ coatings were characterized and compared. With DOMS, a virtually arc free high power pulsed magnetron sputtering process for TiO₂ has been observed. The TiO₂ films deposited by PDCMS exhibited only the anatase phase whereas the TiO₂ coatings deposited with DOMS showed different crystalline phases depending on the peak discharge current. For the TiO₂ coatings deposited with DOMS at a relatively low target peak current of about 50 A or less, only the anatase phase was observed. With an increase in the peak target current for the DOMS coatings, an increase in the amount of the rutile phase in the coatings was observed along with an increase in the density and a decrease in the grain size of the TiO₂ coatings. At high peak target currents of 200 A or greater, only the rutile phase was produced. The mechanical and optical properties of the anatase and rutile TiO₂ coatings will also be discussed.

The High Power Impulse Magnetron Sputtering System (HiPIMS) equipped with 2” in diameter target has been investigated by means of time-resolved mass- and energy-resolved analyser (plasma monitor) from Hiden Ltd., gridded retarding field energy analyser (RFEA) from Impedans Ltd. and so called modified Katsumata probe. All the methods allow to determine ion velocity distribution functions (IVDF) in forward direction to substrate as a function of retarding electric field. However, except plasma monitor latter methods are not able to resolve the mass of particles. The newly developed modified Katsumata probe uses a static magnetic field created by Sm-Co permanent magnets to intercept the most of plasma electron and pull them away back to the plasma bulk. Furthermore, the plasma monitor and the modified Katsumata probe are characterized in very small angular acceptance in comparison with the gridded RFEA. The high power impulse magnetron sputtering system was equipped with pure metallic targets (titanium or iron). As working gas a mixture of Ar and O₂ was used. The working gas pressure ranging between 0.5 Pa to 5 Pa. All the diagnostic instruments were placed at position of substrate. All the measurements were carried out under the same conditions as the thin oxide films TiO₂ and Fe₂O₃ were deposited. A comparative study of all the aforementioned methods has been carried out.

Results clearly demonstrate an influence of angular resolution on measured IVDF. Unlike gridded RFEA the modified Katsumata probe was able to distinguish different groups of ions coming on the substrate in different times of plasma pulse. The gridded RFEA has angular acceptance more than 70° and ions reaching its input orifice originate from different direction unlike the modified Katsumata probe which accepts ions only from cone of a small solid angle. The temporally resolved investigation with plasma monitor revealed that sputtered particles reach a substrate later on. All the plasma diagnostic methods revealed significantly enhanced energy tail in ion velocity distributions measured in HiPIMS in contrast to dc magnetron or mid-frequency pulsed-dc magnetron. An influence of working gas pressure on velocity distributions of argon, metallic and reactive gas ions specifically on presence of high-energy tail is discussed. The plasma monitor proved that under certain plasma conditions appearance of double ionized sputtered and working gas particles can be observed.

Recently an inhomogenity of the HiPIMS discharge has been reported (1,2). In our previous papers we demonstrated and explained the influence of the power and pressure on the instabilities. Furthermore we investigated transition form stochastic to periodic behaviour. In this contribution an explanation of the shape and mechanism of the instabilities is presented, based on the experimental observations. The experimental results of a 4 camera setup, photomultiplier tube data correlated with the biased flat probe are presented. The theory combines a particle approach together with a global approach in demonstrating that both violation of the β limit and modified Rayleigh–Taylor instability could be responsible for the shape of the instability. Further optical emission spectroscopy experiments provide an understanding on the correlation between instabilities and the deposition conditions.

1. Ehiasarian AP, Hecimovic A, de los Arcos T, New R, Schulz-von der Gathen V, Boeke M, et al., Appl. Phys. Lett. 2012 Mar 12;100(11).

2. Anders A., Applied Physics Letters. 2012 May 31;100(22):224104–224104–5.