Metal (Al) layer alignment issue

[Girish]

Hi Mahnaz
  I am doing a backend process and am trying to align an aluminum layer to
holes in oxide beneath the aluminum layer (standard contact hole and
metallization process) with the ASML. Resist is 1um Shipley positive and
I use 60mJ/cm^2 energy, which defines the metal patterns well.

- Misalignment: Every die is misaligned and by a different amount both
  in the x-y direction. There are a few devices in the dead centre which
  seem to be aligned and from the centre the misalignment begins to spiral
  out. The alignment of the reticle layer has been checked before by
  aligning to the contact layer beneath, but without metal. So it is not a
  mask or machine offset problem.

_ Rejection of wafers: Sometimes the wafer gets rejected and sometimes it
  exposes it but always with the same misalignment mentioned before.

I am told that the ASML reticles have a layer which just opens up the
area around the marks. I could use that to remove the metal and try
aligning through the passivation layer beneath it.

Can you give any other ideas? Any help/suggestion will be appreciated.

Thanks
Girish