CCD Roughness and Flatness

[Zoran Ninkov]

Posted to CCD-world:
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Recently we have been interested in the surface roughness of back
thinned CCDs.  I couldn't find such measurements elsewhere so we decided
to make them ourselves. The measurements were made using instrumentation
(and staff) in the Center for Optical Manufacturing located in the
Laboratory for Laser Energetics (LLE) in Rochester.

We had available two SITe 512 x 512 back thinned CCDs (24 micron square
pixels).  One was as engineering device (device 1) and the other was a
good device (device 2) intended for use in a proton spectrometer being
built for the LLE..

The areal roughness measurements were made on a Zygo New View 100 using
both a 5x Michelson (field of view 1.41 x 1.06 mm) and a 20X Mirau
(field of view = 0.35 x 0.26 mm) objective.  Data was gathered at 4
random spots on each chip.  The results are summarized below ;

                                    peak-valley        rms roughness
                                    (angstrom)            (angstrom)

device 1
(using 5x objective)            2300                    120
(using 20x)                         2000                    120

device 2
(using 5x)                           1500                    100
(using 20x)                         1250                      75


Subsequently Form Talysurf profile measurements were also carried out on
a Rank Taylor Hobson instrument.  Two measurements were performed on
each chip.  Each ran edge to edge, perpendicular to each other, and
through the approximate center of the chip.

Device 1 was equally and uniformily convex about the center of the chip
in both directions with an approximate 8 micron variation from edge to
center on both sides of center.

On Device 2 the scan in one direction also showed the convex shape about
the center of the chip with a 5.6 micron change between center and
edge.  The scan in the perpendicular direction on this chip however
showed a major defect.  The profile rose normally from the edge until
2.75 mm into the scan (the profile rose 3.5 micron at that point). At
that location a vertical jump of 5.5 micron occurred. Peak height
occurred at approximately 4 mm from the edge.  The balance of the
profile was the familiar second half of the convex shape seen on other
scans.  The edge to peak variation on this chip was 9.1 micron.


If anyone has any comments on how typical such numbers are on other
devices I would appreciate hearing from you.  In addition does anyone
have any idea on how such meaurements would vary between the different
back thinning vendors ?

Thanks ..

Zoran

--
Dr. Zoran Ninkov
Center for Imaging Science
Rochester Institute of Technology
54 Lomb Memorial Drive
Rochester   NY 14623-5604

tel : 716 - 475 7195
fax : 716 - 475 5988
e-mail : ninkov at cis.rit.edu


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