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Alignment sensor for wafersteppers

An IC comprises layer upon layer of different material containing a pattern. The various kinds of processes are performed in different machines. The challenge is to get the new pattern that is projected onto on the previous layer in a waferstepper in exactly the right spot so that the electronic connections, for instance, match properly.

This is the job of the alignment sensor for which TNO has produced a solution based on electronmicroscopy technology. The alignment sensor requires markings on the chip. Those markings can be affected by certain steps in the process leaving the sensor unable to do its job properly, certainly if the normal methods based on visible light are used. So instead of this, we used a sensor that works like a miniature electronmicroscope. By selecting a different shape and using image analysis, the markings remain easy to detect, even when layers lie across them.

Fewer errors

The sensor is sensitive to the whole marking and not just for their visible surfaces. Damage to the upper layer of the marking thus have no effect on the alignment, which drastically reduces the number of alignment errors. This principle that we have used can also be useful in other systems that have to work in a high vacuum with nanometre precision.


Dr. ir. Diederik Maas

  • Helium ion microscopy
  • HIM
  • HeIM
  • metrology
  • SEM


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