APS and Thinning

[MYPIXEL at aol.com]

*******
JJ here

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In a message dated 97-11-05 14:39:35 EST, ninkov at cis.rit.edu writes:


 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.


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One of the benefits of APS is low cost (i.e., CMOS direct). Are you sure you
want to get into thinning? Why not just stick with the expensive CCDs for
high QE. Or are you looking towards mass production  thinning (that would be
exciting for the CCD too).

Watch out when you loose your substrate to thinning. Most of our work these
days at Pixel Vision focuses on achieving high speed for thinned CCDs (30
frame/sec rates). Amazing new things discovered on this front. Guess we will
present a paper at SPIE (San Jose) on this.

*****
 
 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.
 

*****
You are dealing in the devils world. Anything is possible. If surface
roughness makes it easier to grow a good oxide (flash oxide) and accumulate
then you are on to something. I haven't seen any evidence that roughness has
a significant effect on either. But usually one thing leads to another when
doing research in this area. Let us know if you need some pre-thinned CCDs
for your experiments . . .

JJ

*****

 


 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
 
  >>