Paper PS1-TuM3
Impact of Self-Absorption on Emission Spectral Lines for Non-Equilibrium Plasma Source

Tuesday, October 30, 2012, 8:40 am, Room 24

Precise optical measuring techniques of spectral lines are necessary for low pressure plasma semiconductor manufacturing analysis. Problems which add to the inaccuracy of the optical measuring techniques include spectral line broadening mechanisms such as self-absorption.

Self-absorption has been widely neglected or its importance overlooked for plasma diagnostics.

In this study several different techniques are used to examine the impact of self-absorption on emission spectral lines from a reactive ion etch (RIE) plasma chamber, measured using a high resolution optical spectrometer. The experiments are performed in an Oxford Instruments plasma lab 100 RIE chamber which operates at a standard single frequency (GEC) 13.56 MHz with maximum RF power of 600 W. The OES spectrums were taken with a Horiba Jobin Yvon Czerny-Turner design Auto MicroHR spectrometer with focal length of 140 mm and spectral resolution of 0.25 nm at 400 nm wavelength.

One technique which is used is to check line intensity ratios within multiplets which abide by so-called LS-coupling rules. The time or spatial fluctuation in the observed intensity of the strongest or metastable line within the multiplet, in respect to the weakest spectral line in the same multiplet shows that self-absorption is present. Another technique which is examined is to change the optical path length by measuring the plasma from different points or different viewports on the plasma chamber. If the increase in signal intensity changes corresponding to optical path length, there is no self-absorption.

A Design of Experiments (DOE) has been used to cover the multidimensional external parameter space e.g. power, pressure, flow rate and chemistry. The results from the techniques are compared to each other for accuracy and the impact of self-absorption is then quantified. In this work the influence of self-absorption of neutral Argon and Oxygen spectral lines for a range of parameters in an RIE plasma chamber has been studied. Almost all the measured spectral lines have been affected, up to some level, by self-absorption. Some spectral lines shapes are changed by self-absorption up to 60%. One of the most widely used actinometry spectral lines for plasma diagnostics, Argon 750 nm, has its intensity affected by self-absorption by up to 40%.

Correction of self-absorption is a necessary step for OES based plasma diagnostics. In the case of Actinometry calculation correction of self-absorption could change the final result up to 20%.