Copper Oxidation Studied by In Situ Raman Spectroscopy
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Copper Oxidation Studied by In Situ Raman Spectroscopy Robert Schennach1 and Andreas Gupper2 1 Institute of Solid State Physics, Graz University of Technology, Graz, Austria 2 Research Institute for Electron Microscopy, Graz University of Technology, Graz, Austria
ABSTRACT
The growing importance of copper in the semiconductor industry has led to a renewed interest in the properties and growth modes of copper oxides under a variety of conditions. While thermal oxidation of copper has been studied extensively over the last decades, recent surface studies seem to ignore the possible formation of Cu3O2. It has been shown earlier that thermal oxidation of copper leads to multilayer structures, which consist of CuxO, Cu2O, Cu3O2 and CuO, depending on the oxidation conditions. These oxides were analysed ex situ using X-ray Photoelectron Spectroscopy (XPS) combined with depth profiling, Linear Sweep Voltammetry (LSV) and Galvanostatic Reduction (GR). In this work it will be shown that Raman Spectroscopy can be used to follow the formation of the different copper oxides in situ. The experiments were performed using a Raman Microscope with a sample heating extension, which enables in situ copper oxidation in air between room temperature and 300 °C. Raman spectra were acquired in the range between 3000 ∆cm-1 to 150 ∆cm-1. From these spectra one can see that Cu2O is formed between 70 °C and 130 °C, Cu3O2 is formed between 150 °C and 250 °C and CuO starts to form at temperatures higher than 250 °C.
INTRODUCTION
Copper is being increasingly studied today because of its use for electronic interconnect systems where processing is done at temperatures below 200 ˚C. The oxides that form on copper at these temperatures can be both chemically and mechanically unstable and show mechanical instability if Cu2+ forms [1]. Given the enormous importance of copper in today’s technological world, it is surprising that its oxidation at low temperature is not better understood. There remain open questions about the structure of the oxide films that develop at these conditions and their chemical nature. This is so, even with the enormous number of modern techniques (Rutherford Back Scattering (RBS) [2], Ellipsometry [3], Atom Probe [4] and Scanning Tunneling Microscopy (STM) [5], Pulsed Field Desorption Mass Spectrometry [6]) and matured techniques (XPS [1, 7, 8, 9, 10, 11, 12], low angle x-ray diffraction [13], optical spectroscopy [7, 13, 14, 15, 16, 17, 18, 19, 20], angle resolved XPS [21], and factor analysis and artificial neural network XPS depth profiling [63, 22]), that have been applied. There are different temperature regimes where different Cu oxides form [16]: (1) Below 70 °C, where an amorphous oxide CuxO grows; (2) 70–110 °C, where Cu2O grows; (3) 110–200 °C,
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where Cu3O2 grows; (4) 200–270 °C, where Cu3O2 and CuO grow and (5) 270–330 °C, where cupric oxide, CuO, grows. More recently it was shown [15] that near room temperature and atmospheric pressure, a precursor oxide forms that has Cu2O characteristics but c
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