Full length articleAuger electron spectroscopy and UV–Vis spectroscopy in combination with multivariate curve resolution analysis to determine the Cu2O/CuO ratios in oxide layers on technical copper surfaces
Introduction
Since the strategy used for packaging power modules in the automotive industry changed from the use of aluminum boxes to a direct packaging approach using molded epoxides, the surface of the lead frame, which is mostly based on copper, has become the focus of research. The reliability of electronic control units is directly dependent on the adhesion of the mold compound to the copper surface. In recent years, several studies have been conducted on the influence of copper surface oxide layers on this adhesion [[1], [2], [3], [4], [5]]. In addition, the oxidation of copper has been the focus of extensive research. Consequently, studies on the formation and thickness of native oxide layers [6] [7,8], the kinetics of oxidation at different temperature ranges from 25 to 350 °C [[9], [10], [11], [12]] and up to 1200 °C [[13], [14], [15], [16], [17], [18]] have been conducted. In many of these studies, the influence of oxides on adhesion of packaging material for electronic control units was investigated [1,[19], [20], [21]]. In addition, copper and its oxides are also in the focus for solar cell applications [16,22,23]. In all of these applications, the relative content of the copper oxides is of essential importance because with a change in the respective contents, the optical, chemical and mechanical properties are different.
The standard measurement techniques used for the determination of oxide layer thicknesses are ultra-high vacuum (UHV) techniques such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and time-of-flight secondary-ion mass spectrometry (TOF-SIMS). However, the highly specialized and bulky equipment required for these measurement techniques prevents them from being used in the manufacturing area. Consequently, extensive research has been performed with the aim of finding an alternative measurement system based on optical or electrochemical methods such as interferometry, sequential electrochemical reduction analysis, and infrared reflection techniques [[24], [25], [26], [27], [28]]. UV–Vis reflectance spectroscopy has been used extensively for the characterization, identification, and quantitative analysis of copper oxide film thicknesses.
As well as information on the thickness of the oxide itself, most UHV measurement techniques also generate information about the ratio of the copper oxides Cu2O and CuO in the layer [22,[29], [30], [31], [32], [33], [34]]. This data is difficult to obtain by non-UHV techniques due to the lower sensitivity of the equipment [[35], [36], [37], [38]]. There are several reports in the literature on the use of UV–Vis measurements to investigate different oxides [[35], [36], [37],39]. However, UV–Vis spectroscopy is not a well-established method for measuring oxide ratios because the spectra are very complex, presenting numerous overlapping and broad bands. Thus, interpretation of the spectra is not an easy task, necessitating an alternative approach. To address this issue, a multivariate data evaluation for interpreting the data in real time is needed.
Often, principle component analysis (PCA), is used to explain the maximum variance in data sets. The general purpose of PCA is to emphasize this variance via sequential calculations. The calculation is based not on chemical or physical properties but on the data itself. The model obtained must be interpreted if it can be explained in this context or if there is no true chemical information in the spectra.
MCR is a powerful tool for analyzing multicomponent systems using a bilinear model of pure component contributions. Thus, MCR can be used to elucidate the influence of different pure spectra on their combined spectrum. Originally, MCR was developed for in-line monitoring of wet chemical reactions [[40], [41], [42]]. However, it has been applied in the study of copper [[43], [44], [45]] and its oxides [46].
In the literature, the use of MCR to analyze UV–Vis spectra is well reported [47,48]. However, the use of MCR with photoemission spectroscopy is not widely known, but there are several researchers working on this field [[49], [50], [51]].
In this study, the ratio of Cu2O to CuO in thermally produced oxide layers on copper surfaces are investigated by AES depth profiles and UV–Vis spectroscopy. The data analysis used to derive the Cu2O/CuO ratio is performed using MCR in combination with reference spectra for metallic copper, Cu2O, and CuO. Once the oxide layer thickness is known, the total amounts of the oxides present can be calculated and interrelated.
In order to crosscheck the results of the MCR calculations, the change of the optical band gap is studied. By creating a Tauc plots [52] for all the UV–Vis measurements separately, the optical band gap can be determined for every investigated sample and the reference compounds Cu2O and CuO. In this manner, the changes of the relative contents of the copper oxides can be traced. The Tauc plots are generated according to the literature. [[53], [54], [55]] The derivation of optical band gaps from Tauc plots is often used in literature even with copper substrates [[56], [57], [58], [59]].
Section snippets
Sample preparation
Oxygen-free extra low phosphorus Cu (Cu-PHC) samples (Wieland-Werke AG; Wieland-K14; Cu ≥ 99.95%; P ≈ 0.003%) with dimensions of 21 mm × 49 mm × 3 mm were used. The samples were cleaned with Zestron Vigon A 200 for 6 min, rinsed twice with deionized water for 2 min, and dried under N2 gas. The oxidation was performed on a heating plate (H-22, Gerhardt, Bonn) at 175 °C for 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12 or 15 min in air under atmospheric pressure under a fume hood. The temperature was
Results and discussion
An initial analysis of the oxide films was performed by AES in combination with Ar-ion depth profiling. AES provides detailed information on the development of chemical composition and the chemical state of the copper. Thus, AES depth profiling serves as an established reference method for other spectroscopies, as will be demonstrated below. All samples were measured with the same regular cycles of recording and sputtering. Three prominent Auger spectra taken at characteristic depths are shown
Conclusions
We have demonstrated that MCR analysis provides access to detailed chemical information regarding the chemical states in copper oxide films. Most importantly, we obtained similar results upon applying MCR to both AES and UV–Vis data. In this manner, we were able to develop a novel approach for the investigation of the oxidation states in copper oxide films using non-destructive UV–Vis spectroscopy. In addition, the easy-to-use UV–Vis equipment facilitates these measurements in environments
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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