Effect of bias on the structure and properties of TiZrN thin films deposited by unbalanced magnetron sputtering
Introduction
Owing to their excellent mechanical properties, low resistance, golden color, and superior corrosion resistance, titanium nitride (TiN) and zirconium nitride (ZrN) have been widely used in various industries over the past 20 years [1], [2], [3], [4]. Their applications include decorative coatings, diffusion barriers in microelectromechanical systems, and hard coatings on tools. However, the demand for harder and tougher materials driven by technological progress necessitates the development of ternary transition-metal nitride films such as TiZrN, TiSiN, and TiAlN. Compared with TiN and ZrN, ternary TiZrN films not only exhibit a low resistivity and golden color but also feature superior mechanical properties and better corrosion resistance [5], [6].
Previous study [7] indicated that the addition of Zr into TiN or Ti into ZrN could enhance the fracture toughness of hard coatings. This approach of incorporating a third element is effective for improving various properties of thin films. In general, the high hardness of hard coatings usually accompanies high residual stress in films deposited via the physical vapor deposition (PVD) process. However, the presence of such high residual stress in thin films can result in reduced adhesion and film spallation. The residual stress can be reduced while retaining the excellent mechanical properties of films by controlling the substrate bias during deposition [8].
The substrate bias is an important parameter in the PVD process. It governs the total energy and momentum of adatoms while influencing the structure and properties of the resulting thin films. In addition, the substrate bias is easily adjusted during thin film deposition. Therefore, we chose to change the substrate bias as a single-variable experimental parameter. Our objective was to investigate the effect of substrate bias on the structure and properties of deposited TiZrN thin films. The influence of substrate bias on the microstructure, surface morphology, hardness, residual stress, resistivity, and packing factor was investigated.
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Experiments
In this research, ternary TiZrN thin films were deposited using a reactive direct current unbalanced magnetron (UBM) sputtering sputtering system. p-Type Si(100) wafers were chosen as the substrate material; the dimensions of the Si substrates were 3.5 × 3.5 × 0.525 cm3. The sputtering sources were a Zr target (99.9%) and a Ti target (99.995%), each with a diameter of 2 in.
First, before the coating process, the specimens were ultrasonically cleaned sequentially in acetone and methanol for 5 min each.
Results and discussion
According to XPS and RBS analyses, the N/(Ti + Zr) ratio for all samples is fixed at 0.9–1.1, and the Zr/(Ti + Zr) ratios are approximately 0.55 (Table 1). The chemical composition of TiZrN thin films is not sensitive to substrate bias. An increase of the substrate bias not only increases the total energy but also enhances the momentum of the adatoms. The momentum of an adatom is directly proportional to its mobility. Adatoms can move to stable sites as they obtain sufficient momentum, resulting in
Conclusions
The window of processing parameters for TiZrN films is very wide in the substrate bias range from − 40 V to − 120 V. In this range, the TiZrN films exhibit high hardness (33.4–37.8 GPa), low resistivity (30–37 μΩ cm), good roughness (0.5–0.6 nm), and a bright-golden color (brilliances > 80). At low substrate biases of − 40 V and − 45 V, the resulting TiZrN thin films retain both their high hardness (33.4–34.5 GPa) and their low residual compressive stress (2.7–3.7 GPa). The residual stress of TiZrN films
Acknowledgment
This research was supported by the National Science Council of the Republic of China under contracts NSC 103-2221-E-007-101. The RBS analysis was carried out at the Accelerator Laboratory, National Tsing Hua University, Taiwan, R.O.C. The XPS analysis was carried out at the Instrument Center of the Material Science and Engineering Department, National Tsing Hua University, Taiwan, R.O.C. SEM and XRD were performed in the Instrument Center, National Chiao Tung University, Taiwan, R.O.C.
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