Wafer polishing device with movable window

A wafer polishing device with movable window can be used for in-situ monitoring of a wafer during CMP processing. During most of the CMP operation, the window remains below a polishing surface of a polishing device to protect the window from the deleterious effects of the polishing process. When the window moves into position between the wafer and a measurement sensor, the window is moved closer to the polishing surface. In this position, at least some polishing agent collected in the recess above the window is removed, and an in-situ measurement can be taken with reduced interference from the polishing agent. After the window is positioned away from the wafer and measurement sensor, the window moves farther away from the wafer and polishing surface. With such a movable window, the limitations of current polishing devices are overcome.