Optical Etch‐Rate Monitoring: Computer Simulation of Reflectance

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Abstract
We report on a numerical simulation of the signal from a laser etch‐rate monitor which utilizes changes in the optical reflectance of a layered structure while the top layer is being etched. The simulation can be applied to structures having several layers. For a strongly absorbing layer such as aluminum , the reflectance is constant until the layer has been etched completely. For a moderately absorbing layer such as a transition‐metal silicide ( between 1.5 and 2.5), light can be transmitted through the film and can be reflected from the underlying layers when the silicide has been etched down to a few hundred angstroms thick. The only required parameters are the wavelength of the light, and, for each layer, the indexes of refraction and absorption, the initial thickness, and the etch rate. It can also be applied to cases where portions of the film area are being etched at different rates, such as when part of the film is covered with photoresist. The simulation is applied to several etch processes, including silicide‐polysilicon gate and metallization etches. Good agreement is found between experimental and calculated data.

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