MEMS metrology Part 1: Building Blocks
In MEMS manufacturing, the device structures predominantly utilize the “bulk” of the substrate to provide structural rigidity, as well as a platform for the electronics and sensing elements of the MEMS device. In many cases, lithography layers that require alignment are on different Z planes separated by hundreds of microns. This lithography requirement is beyond the scope of any Logic or DRAM process flow. The technical considerations here are many, but from a lithographer’s perspective, the main concerns are alignment target acquisition and accurate quantification of overlay.
Typically, for dark field alignment systems, the target acquisition issue is solved by offsetting the wafer in Z in order to capture the target/signal. Once captured, the wafer then needs to move back to its original Z position to expose the layer in the focal plane of the photoresist film. Having accomplished this, the printed layer needs to be measured with respect to the previous layer to determine the overlay performance of the alignment. This is where things get tricky – a result is normally obtained using “box in box” overlay structures; but of course this assumes that both boxes are in the focal plane of the measurement system, which they are not, and this makes measurement by conventional methods impossible.
ASML's alignment system is uniquely suited for MEMS applications
ASML’s unique phase grating alignment system provides the first real “building block” to solve this problem, as it enables the lithographer to capture a target hundreds of microns away in Z, even when the focus system is leveling on the top surface of the wafer. Moreover, once alignment is established, measurement of target locations at any Z plane is possible. Of course, the alignment laser beam angle needs to be calibrated, but this a systematic offset that once calculated can be removed from the overlay calculation. With these basic building blocks both of the lithographer’s key issues are resolved. Furthermore, the ASML stage is capable of achieving system specification overlay with only 2-point alignment. This has tremendous benefit to MEMS processing as alignment can take place beyond the device array, preventing resist striations generated by “welling” resist in deep RIE etched recesses. Finally, with a 2point alignment approach, protection and recovery of targets is simple compared to a field by field or EGA scheme.
These concepts, and a real case study, are reported in the following SPIE paper: Three-Dimensional Metrology in MEMS Applications - The 27th Annual Symposium on Microlithography, 3 March - 8 March 2002, Santa Clara, CA, U.S.A.
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