Two-photon laser-induced fluorescence monitoring of O atoms in a plasma etching environment

Abstract

Atomic radicals are usually the most important reactants in plasma processing. For example, in dry etching or development of organic photoresists, O2 plasmas are used to generate O atoms which can etch the resist spontaneously. However, concentration measurements of these reactive atoms have been limited largely to indirect and often unverifiable methods such as emission spectroscopy, making process optimization difficult. We demonstrate the applicability of two-photon laser-induced fluorescence to monitor atom concentrations in situ by exciting O atoms at 226 nm and detecting fluorescence at 845 nm. A detection limit of <1013 atoms cm−3 is determined using downstream electron paramagnetic resonance spectroscopy. Noise from background plasma-induced emission is eliminated by firing the laser at a time when the applied voltage crosses zero and the emission is extinguished. From the relative intensities of the O(2p4 3P) fine-structure components, the plasma temperature is found to be thermalized with the ambient walls. This technique should also be applicable to detection of other atoms found in plasma reactors, such as N, Cl, S, and H.