ICMCTF2011 Session F2-1: High Power Impulse Magnetron Sputtering

A main parts of a typical sputtering apparatus are a power supply, a sputter target sitting inside a vacuum chamber, a substrate powered by a bias power supply. Understanding of each of the consisting parts limitations and capabilities is of high importance to the coating developer, which is even more the case when HPPMS technology is used. HPPMS and HIPIMS are acronyms of an emerging sputtering technology that has gained substantial interest among academics and industrials alike. HPPMS (HIPIMS) which stands for high power (im)pulsed magnetron sputtering is a physical vapor deposition technique in which the power is applied to the target in low-duty-cycle pulses (< 10%) and low frequency (< 10 kHz). As a result, pulsed target power densities of several kW cm^{− 2} are obtained. The HPPMS mode of operation results in generation of a large number of charged particles in the plasma, including electrons, ions, and multi-fold charged ions. The dynamics of such plasma have been especially studied with regards to the high ionization degree of the sputtered atoms, their charge, and their transport route as they are expanding away from the sputter-target. A number of studies have utilized the abnormal plasma dynamics observed in the HPPMS plasma in order to grow dense and smooth coatings on flat and complex-shaped substrates. They found that HPPMS provides new and parameters to control the deposition process, tailor the properties and optimize the performance of elemental and compound films.

In the present review paper, an attempt is made at demonstrating the main features of HPPMS by trying to find the red-line through the more-than-250 publications on the subject. In order to do this, the power supply types in the market are reviewed. This is followed by a short description of the type of magnetrons needed for the HPPMS operation. Finally, examples of coatings deposited by HPPMS are given and the benefits of using this technique are highlighted.

We present a two-zone non-stationary model of a high-power pulsed magnetron sputtering discharge. The model splits the magnetron discharge into two zones, namely the high density plasma ring above the target racetrack including a target sheath and the bulk plasma region between the plasma ring and the substrate. By solving the particle and energy conservation equations for these two zones, the model makes it possible to evaluate time evolutions of the averaged process gas and target material neutral and ion densities, as well as the fluxes of these particles to the target and substrate during a pulse period. Consequently, the target and substrate current density waveforms, together with the effective electron temperature, can be calculated. In addition, the important deposition characteristics, such as the deposition rate, the fraction of target material ions in the total ion flux to the substrate and the ionized fraction of target material atoms in the flux to the substrate can be evaluated. The geometric input parameters of the model are the vacuum chamber dimensions, the target-to-substrate distance, the target and substrate diameters and the plasma ring size (defined roughly by the geometry of the magnetic field). The main process input parameters are the process gas pressure, the magnetic field strength, the target voltage waveform during a pulse period and the repetition frequency of the pulses. Furthermore, additional material parameters, such as the sputtering yields, the secondary electron emission coefficients, the ionization and excitation cross-sections for the process gas and target material must be provided. The effects of various process parameters on the discharge and deposition characteristics were examined. The model predictions were compared with the experimental results obtained for two different high-power pulsed magnetron systems.^1,2 It was shown that the model provides a good qualitative picture of the complicated processes determining the sputtering and deposition mechanisms in the high-power pulsed magnetron discharges investigated.

²A. Anders, J. Andersson and A. Ehiasarian: High power impulse magnetron sputtering: Current-voltage-time characteristics indicate the onset of sustained self-sputtering, J. Appl. Phys. 102, 113303 (2007); Erratum: J. Appl. Phys. 103, 039901 (2008).

Sideway photos of the HIPIMS plasma have been recorded using Charge Coupled Device (CCD) camera with gate width of 5 μs. For each photo Abel inversion has been performed to compute the radial emission profile of the plasma. Bandpass interference filters were used to isolate desired wavelength in order to observe lines of Ti⁰, Ti¹⁺, Ar⁰ and Ar¹⁺ particles in HIPIMS plasma discharge with Titanium (Ti) target in Ar atmosphere. Result is the temporal evolution of radial emission profile of both metal and gas atoms and singly charged ions.

Besides conventional unipolar high power impulse sputtering (HIPIMS) the bipolar mode is offering additional degrees of freedom, especially in case of reactive sputtering. For oxides improvement of process stability and coating quality is reported using bipolar mode instead of unipolar.

In this paper we investigated the influence of the used pulse arrangement on the resuting voltage-current response at the target, as well as the plasma and the film properties. For basic investigation pure titanium without additional oxygen was investigated. The pulse on-time for positive and negative pulse were kept constant. The off-time between the positive and negative pulse was varied. From a symmetric arrangement of defined pulse and pause for positive and negative pulse the off-time separating positive and negative pulse was shortened to a minimum pause of 20μs. From nearly no influence the peak current was modified and got unsymmetric when bringing positive and negative pulse close together. The remaining ions form the positive pulse led to faster ignition of the negative pulse. The correlated OES spectra, deposition rates, and film structure is investigated.

Tantalum thin films exhibit two crystalline phases, α (body-centered-cubic) and β (metastable tetragonal). α is the phase commonly found in bulk tantalum, which exhibits good ductility, high melting temperature, and low resistivity. β, however, has high resistivity, high hardness, yet is very brittle. Thus, α is typically preferred for wear and corrosion resistant applications. In this study, modulated pulse power (MPP) magnetron sputtering was used to deposit thin and thick tantalum coatings to determine the effect of several deposition parameters, such as target power (1-3 kW), working pressure (3 – 10 mTorr), substrate-to-target distance (60 – 165 mm), and negative substrate bias (0 – -150 V) on the phase control without substrate heating and post annealing process. The phase, microstructure, and mechanical properties of the coatings were characterized using X-ray diffraction, scanning electron microscopy, micro scratch test and nanoindentation.

In the present work, we propose to stabilize the HiPIMS discharge by controlling the target magnetic field using metal spacers with different thicknesses in between the magnetron surface and the target. We demonstrate a straightforward discharge optimization and enhanced reproducibility, while using various metal target materials (such as Nb, Ta or Ti, and even semiconducting Si), and different levels of target erosion. We show the effect of such an approach on the magnetic field in front of the target, and on its consequences on the deposition rate, the coating properties, and the overall process optimization.