Extended front-to-back alignment capability for MEMS/MOEMS applications

A novel front-to-back alignment method, which does not require additional alignment sensors, is being developed for optical projection lithography tools. The system is designed such that it can be easily retrofitted to existing systems. By embedding a pair of tiny optics into the wafer stage, the existing Through-The-Lens (TTL) and/or Off-Axis (OA) alignment systems can be utilized, thereby avoiding the added complexity of supplementary alignment electronics and hardware. The front-to-back alignment is accomplished by imaging the alignment marks on the wafer back surface to the front and then using the standard front-side alignment system. To calibrate the front-to-back alignment module and to minimize the alignment errors, new metrology software is introduced. The front-to-back alignment accuracy is specified to be less than or equal to 500 rim. (3 sigma), which provides typically twice the improvement compared to current state-of-the-art implementations. The front-to-back alignment capability is being developed in response to the requirements of the MEMS/MOEMS market.