Optical emission spectroscopic techniques for monitoring etching

A method of etching, the method comprising:

• striking a plasma discharge;
• flowing a gas mixture comprising a hydrogen-containing gas and helium through the plasma discharge to form plasma effluents;
• flowing the plasma effluents through a plurality of apertures to a silicon layer on a substrate, the silicon layer having a first thickness;
• etching the silicon layer with the plasma effluents;
• measuring the intensity of emission from hydrogen from a reaction of plasma effluents with the silicon layer;
• measuring the intensity of emission from helium in the plasma effluents;
• integrating the intensity of the emission from helium over the duration the plasma effluents are being flowed to the silicon layer to obtain an integrated intensity of helium;
• integrating the intensity of the emission from hydrogen over the duration the plasma effluents are being flowed to the silicon layer to obtain an integrated intensity of hydrogen;
• calculating a parameter from the integrated intensity of hydrogen and the integrated intensity of helium;
• comparing the parameter to a reference value corresponding to a target thickness of silicon etched;
• extinguishing the plasma discharge when the integrated intensity is equal to or greater than the reference value to define a partially etched silicon layer on the substrate having a second thickness less than the first thickness.