Lateral diffusion in small-pixel CCDs

[roger smith x294]

Posted to CCD-world:
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> From richard at ucolick.org Thu Jan  8 21:02 CDT 1998
> 
> I'm curious why you said in your response to John Geary "Presumably 8um
> pixels binned by two will eliminate the resolution loss in the blue
> that we suffer with our current 15 um Lorals." If these are standard
> thickness thinned CCDs you may not gain much if the loss is due to
> lateral diffusion. Lateral diffusion through the undepleted part of the
> silicon will be the same. With the 8um pixels you'll be able to sample
> the diffusion better, but not reduce it.


Richard,

Yes indeed.  I realized after sending my message to John that I hadn't
thought this through.  Without thinking about how, I was assuming that
someone building an 8 um CCD would be working on reducing the lateral
diffusion to that scale.  I should have asked whether they will invoke
process changes (resistivity/doping) or be thinned more to narrow the
field free region in order to obtain spatial resolution commensurate
with the smaller pixels.

Our impression, formed only from looking at line widths on our
spectrographs, is that the 15 um Loral CCDs give poorer resolution than
we expect over **most of the visible spectrum**.  We attribute this
resoltion loss to lateral diffusion.  The observation that it affects
more than the blue wavelengths is consistent with the discussion last
year on CCD World which highlighted the fact that even red photons (650
nm) have an absorbtion length (3 um or so) that deposits most of their
charge near the backside and thus on the far side of the field free
region.

I think this raises an interesting question:  if one builds a small
pixel CCD, does one have to make it thinner to get comparable MTF.  If
so, red response will be sacrificed and fringing will set in at shorter
wavelengths and be a stronger effect.  It would seem that eliminating
the field free region by deepening the depletion region is preferable
both for 8 um and conventional pixel sizes.  As someone involved in a
project using high resitivity silicon perhaps you could comment.

Roger

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