Nanoscale investigation of long-term native oxidation of Cu films

Abstract

The long-term native oxidation of Cu films has been investigated at the nanoscale by angle resolved X-ray photoelectron spectroscopy, spectroscopic ellipsometry, and high-resolution transmission electron microscopy. Cu films with different microstructures and crystallographic textures were deposited by with or without a negative substrate bias voltage of − 50 V. The oxidation rate of the Cu films deposited at − 50 V is much lower than that of the Cu films deposited at 0 V. The electron microscopy observation showed that the Cu films deposited at 0 V have two oxide layers, namely, an outer CuO and an inner Cu₂O, whereas the oxide layer of the Cu films deposited at − 50 V consists of only Cu₂O layer. The difference of texture between the Cu films deposited at 0 V and − 50 V is found to affect native oxidation behavior. It is also considered that many defects in the Cu films deposited at 0 V provide preferential nucleation sites and subsequent growth, which promotes Cu oxidation. The difference in the oxidation behavior of the two Cu films and high-purity bulk Cu can be mainly explained by the difference in texture and microstructure.

Introduction

In ultra large-scale integrated circuits, the continued demand for a lower resistivity of metal interconnects has led to the choice of copper as an alternative to aluminum, because copper offers a lower bulk resistivity (1.67 µΩcm) than aluminum (2.67 µΩcm) [1], [2]. The reliability of Cu interconnects is an important factor that determines the degradation of the device. However, Cu oxidation is an obstacle in the reliability for widespread applications of Cu technology, because Cu oxidizes at a significant rate even at temperatures lower than 150 °C [3], [4]. Furthermore, a native oxide layer on the Cu surface is not self-protective and grows continuously in air even at room temperature [5]. Hence, it is very important to understand the nanoscale oxidation behavior of Cu films. On the other hand, the oxidation behavior of Cu films must be different from the bulk Cu due to the difference of the microstructure and defects between the bulk Cu and Cu films. Because of its very slow oxidation rate at room temperature, any detailed investigation on a long-term native oxidation of Cu films has not been reported. Furthermore, it is considered that the microstructural defects and the crystallographic texture in the Cu films affect the nanoscale oxidation behavior, however few studies have devoted any attention to the relation between them.

In our previous report [6], the Cu films deposited using a non-mass separated ion beam deposition system were found to have a different microstructure and crystallographic texture, depending on a negative substrate bias voltage. Based on the previous results, we report here a long-term native oxidation at room temperature for Cu films deposited with or without a negative substrate bias voltage and discuss the results in terms of their different texture and microstructure. We have utilized angle resolved X-ray photoelectron spectroscopy (AR-XPS), spectroscopic ellipsometry (SE), and high-resolution transmission electron microscopy (HRTEM). AR-XPS was used to analyze a surface composition in a nanometer scale native oxide layer. SE was utilized to measure the oxide thickness of Cu films depending on the exposure time to air. HRTEM was also used to directly observe a native oxide layer on the two types of Cu films exposed to the air for a long time.