Hi, everyone

I am having run-in/run-out problem when I try to align the photo-mask
to the the wafer. The second layer pattern on the mask seems to be
slightly scaled (larger ) compare with the first layer pattern on the
wafer, which give a 1um alignment offset over a 1cm square wafer. The
two layers mask pattern are made on the same mask plate, and the
environment temperature remains the same for the two exposures. I am
wondering what could be the sources of this error? I baked the wafer
at 110C after first exposure, because I was using a AZ5214 image
reversal process. Could this baking have any effect to scale the
pattern on the wafer? It seems to me although the wafer expands during
baking, it will shrink to the exact original size after it is fully
cooled, unless the above PR layer have a relative shift with respect
to the wafer during this process.

I hope to reduce the alignment error below 0.5um or even smaller. One
suggestion is to use quartz plate (instead of soda lime I am using
now) to improve the accuracy of the mask. I am wondering will quartz
material help in this case?

Any comment would be appreciated!

Thanks,

Wei

--
Wei Tang
Department of Materials Science and Engineering, UCLA
Cell: 310-357-0158
Website: http://tangweipku.googlepages.com