Ramiz Raza The previous alignment mark would still have impression on the second layer and by focusing the scope on the edges of the impression formed, you should be able to align you second mask. Hope this helps.

Yes, in most cases if the alignment marks are suitable, you should easily be able to pattern multilayers.

What stack (material and thickness) do you have, and how large or small are the alignment marks?

Jules

Hi Ramiz Raza

Your question is sound simple, but actually it becomes problematic for even experienced researchers, I met it several times even I knew it well ( neglected or forgot the issue during the process development). To answer your question clearly and sufficiently:

1. The first mask must have a proper alignment masks. Asking your experienced colleagues for proper design that suitable for your process, especially for multiple steps process or process involving the use of really thin-films;

2. And this alignment mask will be formed/etched on the wafer after the processing steps like etching or metalization ;

3. In the second resist coating; the resit will cover the previously formed alignment masks on the wafer, and you can focus the optic there to see well the resist coated alignment masks for correct alignment of the second layer.

However, problem appeared if the thickness of the second coated resist is too thick to the etched depth of the previous alignment mask- or the etch depth of the alignment mask is too shallow ( happened often when process ultrathin layer); Sometimes, we must do an extra step of etching just for sufficient deep imprinting the first alignment masks into the wafer.

4. After processed, the first processed alignment mask can be wider or narrower than its original design; while the second mask can be transparency or dark ones. To see and align well, then these both factors must be given sufficient attention for proper design of the second alignment masks. For instance, after the first mask, following with the etching; a dried etching do not widening the alignment mask too much, but if followed by a wet etching, especially if the under etching is happened, then the alignment masks will be created with much larger than its designed ( and expected ) dimension . Therefore, this things must be known in advance for proper design of alignment mask in the second mask.

Generally, mask relates inherently to the process development of the device, which is never perfect. Therefore you really need to have deep advice/cross-check from a really experienced process engineer to obtain a good design; and even that should do a double check before officially mask order ( not only the finance issue, it takes months to re-order the masks).

Hope it helps

Cheers

Hien D Tong

it is normal practice to cover the markers with photoresist in order to perform exposures after the first mask. Visibility of the markers should be preserved by making them properly, i.e. with a sufficient contrast, depending on the substrate and the layer you etched your markers in. For a more specific comment, please provide the substrate and layers you are working with.

regards

A. Bagolini Tong Duy Hien Jules Lloyd Hammond I am using Silicon wafer , SU-8 photoresist and performing 2nd layer but If photoresist is coated over first alignment marks then how will I do 2nd alignment through MJB4 mask aligner for 2nd Mask? . I am not doing Etching.