An analysis of noise on optical emission Spectroscopy measurements

Optical Emission Spectroscopy (OES) is a non-intrusive plasma diagnostic technique that can be used to measure the chemical changes in a plasma that is increasingly being considered for monitoring and control of plasma etch processes. In the practice of collecting OES data for plasma etching, it is inevitable that noise is included in the measurements. The existence of noise can destroy signals or at least make the identification and interpretation of signal patterns unreliable, hence appropriate filtering and pre-processing of the data is needed prior to application of automated feature extraction and analysis techniques. In the absence of a priori knowledge of the noise characteristics and system bandwidth the selection of an appropriate noise suppression filter bandwidth is a challenging problem. This paper explores the characteristics of the noise inherent in OES measurements and proposes a systematic method for establishing a suitable noise filter bandwidth based on the auto-correlation and cross-correlation analyses of the filtered signals and their residuals.