Effect of copper emitted from wafers on etch rates of insulator films in capacitively coupled fluorocarbon plasma

Abstract

This article describes the effects of copper emitted from wafers on the etch rates of two kinds of insulator films and discusses copper detection by optical emission spectroscopy (OES) and by a plasma impedance monitor (PIM). Plasma etching of a silicon nitride (SiN) film on a copper film in a passivation layer process is performed using capacitively coupled CHF3∕CF4∕O2∕Ar plasma at 1400W rf power, 250mTorr pressure, total flow of 330SCCM (SCCM denotes cubic centimeter per minute at STP), and etch time fixed at 30s. Under these conditions, it is confirmed that copper is emitted from a wafer into fluorocarbon plasma during overetching of the SiN film and that it adheres to a process chamber wall. The concentration of emitted copper is measured by atomic absorption spectroscopy (AAS) to be about 3×1010atoms∕cm2, and the maximum concentration adhering to the process chamber wall is expected to be 1.2×1015atoms∕cm2 by considering the collection efficiency of copper in AAS. With the increase in the cumulative etch time of a SiN film on a copper film, the concentration of copper sticking to the process chamber wall increases. With additional copper concentration, the etch rate of a SiN film decreases but that of a silicon dioxide (SiO2) film increases. The copper emission into the fluorocarbon plasma can be detected by observing plasma impedance using PIM and optical intensities using OES. These results indicate an increase in plasma density due to the copper emission. Furthermore, x-ray photoelectron spectroscopy measurements on the etched surfaces of SiN and SiO2 films reveal that the mechanism of the etch rate changes: A thicker polymer film is deposited on the SiN film than on the SiO2 film. This indicates that the change in radical composition of fluorocarbon plasma is caused by the catalytic effect of copper on the process chamber wall. Since the thick polymer film inhibits etching reaction between the SiN film and the etching species, the etch rate of the SiN film decreases. On the other hand, on the SiO2 film, the polymer film is thin and the current flowing into the wafer increases due to the increased plasma density. Since the etching species to the wafer increases, the etch rate of the SiO2 film also increases.