ACTIVE PLANAR AUTOFOCUS

摘要

A system for inspecting a constant layer depth relative to a particular device layer. The system has an image sensor with a fixed focal plane. A focus sensor senses the surface topography of the substrate and outputs a focus data stream. A stage moves the substrate in an XY plane, and a motor moves the substrate in a Z dimension. A controller operates the system in one of a setup mode and an inspection mode. In the setup mode the controller controls XY movement of the substrate so as to scan a first portion of the substrate. The controller receives the focus data stream, concurrently receives XY data, and stores correlated XYZ data for the substrate. In the inspection mode the controller controls XY movement of the substrate so as to scan a second portion of the substrate. The controller receives the focus data stream, concurrently receives XY data, and subtracts the stored Z data from the focus data stream to produce a virtual data stream. The controller feeds the virtual data stream plus an offset to the motor for moving the substrate up and down during the inspection, thereby holding the focal plane at a desired Z distance, regardless of the surface topography of the substrate.