On treatment of ultra-low-k SiCOH in CF₄ plasmas: correlation between the concentration of etching products and etching rate

Abstract

Low-pressure rf plasmas have been applied for etching of ultra-low-k SiCOH wafers using an Oxford Plasmalab System 100. In pure CF₄ plasmas, SiCOH layers have been etched for different power values. Using quantum cascade laser absorption spectroscopy in the mid-infrared spectral range, the correlation of online and in situ measured concentrations of two etching products, CO and SiF₄, with the ex situ determined etching rates has been studied. The concentration of SiF₄ was found to range between 0.6 and 1.4 × 10¹³ molecules cm⁻³. In contrast the concentrations of CO were measured to be only about 50 % of the SiF₄ density with 7 × 10¹² molecules cm⁻³ in maximum. The production rate of SiF₄, determined from the time behavior of its concentration after plasma ignition, was found to be between 1 and 5 × 10¹² cm⁻³ s⁻¹. The etching rates varied between 2 and 7 nm s⁻¹. Both parameters increase nearly linearly with the applied rf power. It was found that for power values of up to 1.1 kW, the etching rate depends nearly linearly on the in situ monitored concentrations of both etching products. Therefore, the concentration of the etching products can be directly used as a measure of the etching rate.