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The optical interferometry is a non-invasive plasma diagnostic method. Etch experiment were carried out in a high density plasma etching tool (ICP). The theoretical model for optical interferometric endpoint detection and prediction in plasma etching are presented. It is observed that the optical properties of oxide layer and polysilicon layer can affect the interferometric signals. The endpoint algorithm and real time plasma parameters diagnostics are discussed, then process control with optical interferometry during the poly-silicon etch is described. The endpoint prediction technique has been applied to poly-silicon gate etching in high density plasma etching tools.