ICMCTF2009 Session B1-2: Sputtering Coatings and Technologies

In reactive magnetron sputtering, hysteresis effect often limits the deposition rate and achievable compositions of the coatings. Hysteresis is of particular concern in the deposition of oxides due to the large difference between metal and compound sputtering yields and the high reactivity of oxygen.

In this work we have focused on the understanding of sputtering from different oxides. The overall aim is to avoid hysteresis and increase the deposition rate. High sputtering rate has previously been observed from TiO_x target. This was attributed to the sputter reduction of TiO₂ to lower oxides which have substantially higher sputtering yields. To investigate this effect for other materials, a study of sputtering from metal and oxide targets of Al, Ti, V, Nb, and Ta has been carried out. Sputtering was performed by means of RF magnetron reactive sputtering and x-ray photoelectron spectroscopy (XPS) with ion beam sputtering. The evolution of the surface composition during ion bombardment, as measured by XPS, was related to the corresponding hysteresis behaviour in reactive sputtering. Computer modelling of the sputtering process using the Tridyn code was employed in order to describe the preferential sputtering effects and extract the values of surface binding energy

Preferential sputtering could be described using Tridyn. The results have shown an anomalous behaviour of Ti oxide which differs from all the other materials. Even though the formation of lower oxides and preferential sputtering was observed for other oxides as well, the rate increase was substantially lower. Mixed targets consisting of a metal and its compound have been suggested as an alternative to ceramic targets for high rate stable reactive sputter deposition.

Alumina as top-coat of hard coatings is considered as the best material for oxidation-resistance and heat-resistance layer for cutting tools. Particularly, α-alumina with the corundum structure is considered as the best because of its thermal stability compared with the other crystal structures of alumina. Previously, the authors demonstrated the successful deposition of α-Alumina by a reactive magnetron sputtering at temperature around 750°C in a production scale PVD system, by using oxidization layer of CrN or TiAlN as under layer of Alumina.

In this work, the nucleation and crystal growth behaviors of PVD-Alumina were studies. The films were deposited form the metallic aluminum target in Ar and O₂ mixture at the substrate temperature from 600 to 750°C on various kind of substrates and under layer, including Si wafers, Cemented Carbide, CrN, TiAlN etc. The proper process windows for the successful growth of α crystallin e was found on the temperature as well as the deposition rate. Under the optimized conditions around 750°C, the formation of single phase α-Alumina with good crystallinity was confirmed on CrN under layer and some other substrates. However, even at the identical deposition conditions, only γ phase was found on some of the substrates such as Si wafers. In the paper, the effects of the substrates and under layers on the crystal growth of α phase will be demonstrated, and possible growth mechanism of α-Alumina will be discussed.

Alpha alumina has many chemical and mechanical properties that make it an ideal candidate for cutting tool coatings and bio-medical applications. In previous work, we have deposited low-temperature (480C) alpha-alumina with a chrome template layer, and low-temperature (480C) mixed-phase alumina coatings without a chrome template layer using an AC inverted cylindrical magnetron sputtering system. SEM analysis indicated films that were predominantly alpha-phased, and TEM demonstrated pure alpha phase in some samples.

In this work we report on the systematic deposition of alumina films as a function of power (4-6kW), pressure (2-8mTorr), substrate bias (DC and pulsed), and oxygen partial pressure (35-75%). 39 total runs were performed and several substrates were used (glass, silicon, stainless steel, and a titanium alloy). Analysis of each film included XRD, SEM and AFM. Preliminary results indicate that pure alpha films are favorable at 5-6kW, 50% oxygen partia l pressure, pulsed bias, and 2mTorr. In general, pure aluminum films are observed at lower oxygen partial pressures. At lower powers, little alpha alumina is observed as the energy at the substrate is too low. Indicating noticeable pressure dependence, films deposited at lower pressure (2mTorr) tend to exhibit phases fairly independent of power and oxygen partial pressure. Initial evidence suggests that the deposited films have a weak dependence on substrate bias.

Surface acoustic waves (SAWs) on piezoelectric substrates find widespread applications for high frequency signal processing applications. Recently, SAW induced acoustic streaming for agitation and actuation of smallest volumes of fluids has attracted considerable attention in the field of microfluidics. Apart from single crystal substrates like LiNbO₃, low-cost, high quality alternatives are required for on-chip biosensor applications.

Here, we investigate high performance piezoelectric ZnO films produced by radio frequency magnetron sputtering on different host substrates.

We use Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis (ERDA) to analyze the composition of the ZnO layers. Structural characterization by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrates preferential c-axis orientation and an excellent microstructure of our ZnO thin fil ms.

The influence of process conditions such as pressure and temperature are discussed as well as the applicability of different crystalline and non-crystalline substrates considering the advantages and disadvantages for the envisioned applications. We find high coupling factors (close to single crystals). These allow for efficient mixing as required for any microfluidic application. The effects of post annealing, film thickness, roughness, residual stress and resistivity are studied in terms of these properties.

Various electronics and surface engineering applications require deposition of thin dielectric films having certain functional characteristics. In recent years, remarkable progress has been achieved in developing new techniques for reactive sputtering. In this paper, we discuss design features and application capabilities of the S-Gun magnetron for ac (40 kHz) reactive sputtering.

Due to its dual target arrangement, the ac powered S-Gun is uniquely able to realize reactive sputtering processes free of parasitic arcing and disappearing anode effects thus enabling formation of high quality silicon oxide, aluminum oxide, aluminum nitride and other dielectric films with deposition rates up to 100 nm/min. The process module is equipped with a stress adjustment unit to reduce compressive stress in the films by controllably suppressing the flux of charged particles to the substrate by redistributing the discharge current between the targets and the internal shields of the magnetron. If film has excessive tensile stress, deposition with rf substrate bias allows reducing stress to near zero level.

The key advantages of the deposition technique we developed are the following:

- Reliable, arcless reactive sputtering at ambient and elevated temperatures;

- Precise adjustment and high run to run repeatability of the film thickness and uniformity;

- Formation of well-textured and, if required, crystal oriented films;

- Enhancement of the film adhesion using energy of accelerated neutrals and ion-assisted condensation;

- Effective stress control to produce films with either compressive, or near-zero, or tensile stresses.

In the paper, we also discuss some examples of the S-Gun reactive sputter technologies, particularly, deposition of highly c-axis oriented AlN films with low intrinsic stress at relatively low temperature (below 300°C).