Layer detection for high aspect ratio etch control

A system for controlling an etch process applied to a multi-layered structure, the system comprising:

• an etch layer detector configured to
• calculate 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, and
• identify 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;
• and
• an etch process controller configured to
• determine that the crossed layer boundary corresponds to a preselected layer boundary of the multi-layered structure, and
• apply 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.