An integrated analog sensor for automatic alignment

Automatic alignment is a computationally intensive task that slows some manufacturing processes that require it. In order to improve alignment speed, we have designed a sensor that combines the imaging operation with the computation of the alignment error. The sensor provides x and y translation error signals when an image of the alignment mark is focused upon it. Photodiodes detect the image, and integrated amplifiers convert the photocurrents into an alignment error signal that is independent of illumination level. Alignment is accomplished in two steps. Coarse alignment is performed by operating as a four quadrant sensor, and fine positioning is achieved using edge detection. Measurements have demonstrated a minimum repeatability of 53 ppm and a maximum bandwidth of 7.5 kHz.