Knowledge Center
Knowledge Center


The ability of a lithography scanner to align and print various layers accurately on top of each other.


Overlay and alignment function takes place in the lithography scanner. In simple terms, overlay is accomplished by adjusting both the wafer stage position and the reticle stage position using alignment marks on the wafer and the reticle. This is repeated perhaps 100 times to expose one mask layer on one wafer.
In the fab, chipmakers may have three basic and independent control loops for the overlay function-automated process control (APC); CPE; and a scanner baseline control loop, according to GlobalFoundries and KLA-Tencor.

Basically, APC is the central system or controller that determines the operation of the tools and other functions in a fab. One APC technique, called R2R, enables the modification of the processing parameters between the tools.

CPE, which is a separate control loop from APC, improves the overlay within the entire fab flow. This static CPE loop is implemented once per device and layer, according to GlobalFoundries and KLA-Tencor. Then, the scanner loop corrects the mechanical and optical drifts within the scanner.

The three control loops-APC, CPE, and the scanner loop-independently transmit data back and forth into the scanner, metrology tools and other gear. This methodology, however, is inefficient and time consuming, and new methodologies are being developed to streamline the process.