CCD Question

[Zoran Ninkov]

Jim Janesick ...

I use both back (SITe) and front (Kodak 4K x 4K) illuminated cooled CCDs
in the lab here at RIT so I am always interested in knowing more about
how they work.  In addition we have designed and fabricated active pixel
sensors (APS - for us this is two MOS capacitors per pixel for
storage-sense + amplifier and two switches) devices using our campus
microelectronics facility.  For a variety of reasons (fill factor) we
have been thinking about ways of fabricating back illuminated APS
devices.

My questions regarding back side surface roughness came up in talking
with people in the Center for Optics Manufacturing in the Laboratory for
Laser Energetics (DOE lab) here in Rochester.  They have been working on
a method for optical finishing called "magnetorheological finishing"
(MRF).  In MRF a magnetically-field-stiffened ribbon of fluid is used to
polish out a workpiece.  A spot of the stiffened fluid is swept across
the surface of interest performing a sub-aperture lap.  The standard
fluid used is the carbonyl ion in water, although cerium oxide oxide is
added for accelerated removal rates.  Determinstic finishing is
accomplished by mounting the part to be finished on a computer numerical
controlled machine.  

The main interest of the group has been in the determinstic
manufacturing of aspheric components.  They quote, as an example, of
starting with plano-convex aspheres (hyperboloids), 47 mm in diamter,
with 140 microns of aspheric departure and a surface roughness of 1
micron rms.   In a two step process requiring about a 140 minutes they
removed material so as to have the form rms error reduced to 0.8 micron
from that required for the desired asphere shape.  The rms surface
roughness measured was 10 Angstrom and no sub-surface damage was
introduced (as judged by HF acid etching identical parts). The group has
also investigated planarizing silicon wafers, evidently with good
initial results.

As an interested user, but non-CCD thinner, I have this maybe simplistic
notion that the smoother the back surface of the CCD is after thinning
the better it would be ?  (i.e. passivation may not be as difficult).
The rougher the surface is the more places there are for "bad" sites and
traps to occur.  The above MRF technique attracted my attention because
of the amazingly small (at least to me, and much less than the micron
you mentioned in your e-mail) surface roughness number, and the control
they have on the final accuracy of the surface shape. Also the technique
in initial tests has worked on silicon.

More information on the MRF method is available in ;

SPIE Critical Reviews of Optical Science and technology volume CR67,
page 251 - Golini et al [1997]
SPIE vol 3134  Jacobs et al [1997], and
(for pretty pictures)  Laser Focus World September 1995 , page 83 -


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