Abstract: Controlling an etch process applied to a multi-layered structure, by calculating a spectral derivative of reflectance of an illuminated region of interest of a multi-layered structure during an etch process applied to the multi-layered structure, identifying in the spectral derivative a discontinuity that indicates that an edge of a void formed by the etch process at the region of interest has crossed a layer boundary of the multi-layered structure, determining that the crossed layer boundary corresponds to a preselected layer boundary of the multi-layered structure, and applying a predefined control action to the etch process responsive to determining that the crossed layer boundary corresponds to the preselected layer boundary of the multi -layered structure.