Elsevier

Thin Solid Films

Volume 618, Part A, 1 November 2016, Pages 13-20
Thin Solid Films

Effect of bias on the structure and properties of TiZrN thin films deposited by unbalanced magnetron sputtering

https://doi.org/10.1016/j.tsf.2016.05.021Get rights and content

Highlights

  • The hardness of TiZrN reaches 37.5GPa.

  • TiZrN maintains low residual stress.

  • Resistivity is inversely proportional to the packing density.

Abstract

The objective is to investigate the substrate bias effect on the structure and properties of the TiZrN thin films. The TiZrN thin films were deposited by direct current unbalanced magnetron sputtering system with dual guns (Ti and Zr) targets onto Si (100) substrates at different substrate bias ranging from − 35 V to − 150 V. Experimental results indicated that all the specimens have strong (111) texture in X-ray Diffraction patterns. In this study, we discovered a transition bias of − 35 V, above which a significant improvement of properties was found, including high hardness, excellent brilliance, low resistivity and fine surface morphology. Within the bias range of − 40 to − 120 V, the hardness of TiZrN films is around 35.5 GPa, the resistivity is about 33.5 μΩ-cm, and the brilliance is larger than 80. The roughness is between 0.5 nm and 0.6 nm. The TiZrN films maintain excellent properties through a large range of applying bias, indicating that the process window is considerably wide. However, structure damage and thin film delamination were found when substrate bias reached − 150 V. The Rutherford backscatterng spectrometry result and scanning electron microscope image further support the structure damage at − 150 V. For protective coatings, low residual stress is required to avoid delamination. By adjusting substrate bias, residual stress can be controlled to lower value. In this study, the residual stress of TiZrN films gradually decreases with decreasing the substrate bias ranging from − 65 V to − 35 V. The TiZrN thin films with high hardness, lower residual stress could be obtained simultaneously at low substrate bias of − 40 V and − 45 V, the hardness and residual stress are 33.4–34.5 GPa and − 2.7 to − 3.7 GPa, respectively.

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.

Section snippets

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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