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Proximity aligners were a bridge tool between the
original contact aligners and projection aligners. Yield from
contact aligners was extremely low, mainly because the mask was just
a stencil laying over the photoresist. It would smear and scratch
the resist when in contact and then pull off some of the resist upon
being removed. Proximity aligners would position the mask about 5
microns above the resist—i.e. in proximity. This was quite a feat in
its day, requiring alignment through six degrees of freedom while
holding mask flatness even tighter.
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