Lithography, wafer alignment

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In a lithography process I have a hard mask which is opaque. I have to make a wafer alignment, however I use optical techniques to align my wafer (lasers etc.) . How can I align my wafer through an opaque layer?
 

phyzguy

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In a lithography process I have a hard mask which is opaque. I have to make a wafer alignment, however I use optical techniques to align my wafer (lasers etc.) . How can I align my wafer through an opaque layer?
The mask must not be opaque everywhere, otherwise what is the point? Aren't there clear areas that you can see through? Typically one includes alignment mark regions that are clear through which you can see the underlying wafer.
 
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https://en.wikipedia.org/wiki/Photolithography said:
Photolithography, also termed optical lithography or UV lithography, is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate. A series of chemical treatments then either engraves the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photo resist. For example, in complex integrated circuits, a modern CMOS wafer will go through the photolithographic cycle up to 50 times.
 

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