June 2012 - Yevgen: Just to remind: we decided, that the problem can be due to:

  1. Residuals of polyimide, as centres of defects formation in SixNy layer: We inspected the surface of clean wafer and the wafer with polyimide by fluorescence microscopy, and did not find any critical dust or residuals. The surface both wafers looks the same.
  2. Small size of protected area (IZM-2 - only pixels area was covered by SixNy): New mask is ready, and last test with Medipix wafer made with this new mask.
  3. Polyimide outgassing, and as a result - different composition and properties of SixNy Several test-samples (dummy Si / aluminum / polyimide / SixNy) prepared, and I sent them to Harry for the resistivity measurement (should be next week).
  4. Unexpected problem with equipment: I checked the logbook in Twente, and have found, that both electrodes was replaced, just after my last deposition (IZM-2), i.e. the machine was in unstable condition. As a confirmation, please see in the attachment the FIB image of Medipix covered by SixNy with polyimide mask (MediPix_PI_SixNy.tif; photo 1 on right). The cross-section is not precisely polished to see details (defects growing), it takes a lot of time, and unfortunately the machine is fully booked, but we will continue next week. The thickness of silicon nitride is the same with one for old sparkproof chip from 3x3 square (TimePix_3x3_H_E07.tif, photo 2 below).
Also, we have found the explanation of bad stripping of polyimide for the last run with Medipix (please see unstripped_PI_SixNy.tif (photo 3) and unstripped_PI_SixNy1.tif (photo 4)). H1 - thickness of SixNy, H2 - thickness of underlying polyimide after the complete deposition process. Theoretically, the thickness of photoresist for the lift-off process should be twice bigger than the thickness of deposited material. So, we are working with double coatings of polyimide now.


  1. On the old and spark proof Timepix chip (TimePix_3x3_H_E07.tif), the trenches in the SiN layer that Martin Fransen observed at the boundaries of the pixel pad (mail 24-4-2012) at the IZM-2 batch do not seem to be present. But for the test with the Medipix chip (MediPix_PI_SixNy.tif) this is less clear. One might even notice a vague indication of a trench. Do you have better pictures already or do you have other indication that the trenches are not present at the Medipix chip?
  2. On the other hand, both pictures on the polyamide deposition (unstripped_PI_SixNy.tif and unstripped_PI_SixNy1.tif) both do not show any trench at the pixel pad, so that looks fine. But it looks like there is a clear trench in the SiN layer in the middle of the pictures between the pixel matrix and the SiN covering the polyamide. Is this due to thermal properties of the polyamide or is something else going on?

The surface of cross-section for the Medipix chip (MediPix_PI_SixNy.tif) has not polished, so, to see the real situation with defects I am going to continue FIBs at Wednesday. Hope, everything will be clear. About unstripped polyimide and defects there - I am working now to improve the PI stripping, but anyway, any present defects close by the PI are not a problem, because it is very far from the pixels area.

-- NielsVanBakel - 2012-06-11

-- NielsVanBakel - 2012-06-11

Topic revision: r1 - 2012-06-11 - NielsVanBakel
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