The initial stages of thermal oxidation for Zr–Cu–Al–Ni amorphous metal thin films were investigated using X-ray photoelectron spectroscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The asdeposited films had oxygen incorporated during sputter deposition, which helped to stabilize the amorphous phase. After annealing in air at 300 °C for short times (5 min) this oxygen was found to segregate to the surface or buried interface. Annealing at 300 °C for longer times leads to significant composition variation in both vertical and lateral directions, and formation of a surface oxide layer that consists primarily of Zr and Al oxides. Surface oxide formation was initially limited by back-diffusion of Cu and Ni (b30 min), and then by outward diffusion of Zr (N30 min). The oxidation properties are largely consistent with previous observations of Zr–Cu–Al–Ni metallic glasses, however some discrepancies were observed which could be explained by the unique sample geometry of the amorphous metal thin films.
Bulk amorphous metals (i.e., bulk metallic glasses, BMGs) have been extensively investigated over the last 50 years . In particular, Zr–Cubased BMGs such as Zr–Cu–Al–Ni (ZCAN) have attracted significant interest due to their good glass forming ability, high mechanical strength and corrosion resistance [1,2]. More recently, amorphous metal thin films (AMTFs) have been shown to have potential as a replacement for polycrystalline thin films for a wide range of applications . ZCAN is among the most thoroughly investigated AMTFs for applications such as nanoscale patterning [4,5], mechanical coatings , nanolaminates [7,8] and metal-insulator-metal devices [9,10].
Understanding the thermal oxidation of these materials is critical for the aforementioned and other potential applications. For example, the deposition of dielectric layers for metal-insulator-metal (MIM) devices often requires temperatures N 300 °C , and many other applications require similar thermal processes. The oxidation of AMTFs may either improve or degrade the materials properties. For example, while oxidation may be unfavorable for microelectronic applications where low resistivity and a well-defined interface are required, it may be desirable as a passivation layer for biomedical devices, which is an emerging application for both bulk  and thin film  amorphous metals. Although significant research has been conducted to better understand the oxidation of ZCAN as a BMG, these studies are largely limited to long annealing times at high temperatures which allows significant diffusion of all metal species. Understanding the initial oxidation (and associated composition variation) of AMTFs such as ZCAN  has received considerably less attention and is expected to be of interest to the scientific community at large.
In addition to thermal oxidation, the role of small quantities of oxygen incorporated during the material synthesis has been studied for many amorphous metals, including ZCAN in both thin film  and bulk [2,15] forms, and incorporation of oxygen has been shown to have significant effects on both structural and physical material properties . Thus, the thermal stability of oxygen in these films has implications related to the processing requirements for many applications.
In this study the thermal oxidation of amorphous oxygen-containing ZCAN thin films was investigated using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and energydispersive X-ray spectroscopy (EDS). Our results provide insight into the initial stages of thermal oxidation for ZCAN thin films during a 300 °C anneal in air.
2. Experimental details
ZCAN thin films (≈50 nm thick) were deposited onto thermally oxidized Si wafers (SiO2 thickness ≈ 140 nm) and SiN windows (for topdown TEM analysis). DC magnetron sputtering was used to deposit the ZCAN films using a 3-in. Zr40Cu35Al15Ni10 target (Kamis Incorporated), a power of 60 W, a pressure of 3 mTorr, an Ar flow rate of 20 sccm, and a target to substrate distance of 4 in. The as-deposited samples were thermally oxidized in air using a tube furnace. The samples were inserted into a tube furnace held at the target temperature. After oxidation at the desired conditions (5, 15, 30, 60 and 120 min at 300 °C and 60 min at 400 °C) the samples were removed and allowed to cool to room temperature.
An FEI 80–200 kV Titan Scanning/Transmission Electron Microscope (S/TEM) operating at 200 kV with ChemiSTEM energy dispersive X-ray spectroscopy (EDS) was used for TEM analysis. Samples were prepared by depositing the films directly onto TEM grids with ≈20 nm thick SiN windows, or fabricated as thin film lift outs using a focused ion beam (FIB) in an FEI 3D DualBeam Scanning Electron Microscope. EDS line scans were collected with a step size between 1 and 2 nm and the data was averaged over 5 points to reduce noise.
XPS measurements were performed with a PHI Quantera Scanning ESCA system using monochromatic Al Kα radiation with a 100 μm spot size. The data were acquired with a 45° emission angle and an electron analyzer pass energy of 140 eV for the sputter depth profile data. The energy scale of the spectrometer is calibrated to Au 4f7/2 at 84.0 eV and Cu 2p3/2 at 932.7 eV. The sputter depth profiles were acquired by alternately sputtering the sample and then acquiring high resolution data at each sputter cycle. A monoatomic 2 kV Ar+ ion beam rastered over a 2 × 2 mm2 area was used for ion milling. XPS sputter times were adjusted to approximate depths by correlating to TEM cross-sectional images of films annealed at 300 °C for 60 min, where approximate sputter rates were determined separately for the metal film and the surface oxide. Quantification calculations were made using PHI MultiPak which incorporates established sensitivity factors corrected for the transmission function of the analyzer and the reported values should be regarded as semi-quantitative. Chemical state resolved depth profile analysis was performed by fitting the Zr 3d spectra into both metallic (Zrmetal) and oxide (Zr-oxide) components at each sputter cycle. The quantification of the sputter depth profiles does not take into account effects of preferential sputtering.
3. Results and discussion
Plan-view TEM analysis of ZCAN films as-deposited and after a 300 °C, 60 min anneal is shown in Fig. 1. A high-resolution bright field (HRTEM) image of the as-deposited film is shown in Fig. 1a. The salt-and-pepper pattern and the lack of long range order suggest that the films are amorphous, consistent with previous studies . A high-angle annular dark field (HAADF) image of the as-deposited film is shown in Fig. 1b, and reveals high electron density (light contrast) regions on the order of 10 nm surrounded by low electron density (dark contrast) regions. The shortrange inhomogeneities observed in this low resolution HAADF image may be due to slight porosity in the film structure or segregation of low electron density elements (e.g., oxygen) to the boundaries of metal clusters. EDS analysis of this as-deposited film (data not shown) revealed a uniform 2-dimensional composition of all metal species within the spatial resolution of the technique. Fig. 1c shows a HAADF image of the ZCAN film after annealing to 300 °C for 60 min in air. These annealing conditions resulted in reduction in the clustering seen in the as-deposited sample and the development of significant long-range inhomogeneities. These inhomogeneities are seen as light and dark regions ranging from ≈10 to 100 nm that are dispersed throughout the ZCAN film. To gain insight into possible compositional variation between the dark and light regions, in Fig. 1d we show image contrast EDS data along the dashed line shown in Fig. 1c. The line scan suggests that the dark regions in the 300 °C annealed film represent oxygen-rich regions as indicated by peaks in O concentration near line distances of 40, 135, 280 and 390 nm. These regions also have a significant reduction in Cu content and a slight reduction in Ni content, suggesting these regions are composed primarily of Zr- and Al-oxides. The EDS line scan also includes a region of light contrast centered near the line distance of 300 nm. No significant change in composition is observed through this regime suggesting the lighter contrast is likely due to an increase in film thickness. Thus, thickness variation is introduced during the 300 °C anneal. It should be noted that the Cu concentration measured by EDS for this analysis was artificially increased via scattering effects from Cu present in the TEM specimen holder, however this does not affect the trends observed in the line scan.