CCD-world: Red-sensitive CCDs with skinny pixels

[Don Groom]

The following was posted to CCD-world:

Dear Scott Sutherland,

> I have an upcoming project and I am looking 
> for a linear CCD with 5-10 um pixels, a 25-30 mm
> width, and a pixel height of at least 200 um.

What do you mean by pixel height? a (5-10) x 200 um^2 pixel, or a
depletion depth of 200 um?

> I need decent red and NIR sensitivity over 
> the region 800-1050 nm (at least 5% at 1000 nm,
> preferably closer to 10).

You can have considerably better IR response with 200 um depletion--above
50% at 1000 um, for example.  See our SPIE 3649, 80-90 (1999) paper on the
QE of CCDs of various thicknesses. It's also on our web page ccd.lbl.gov

> Back thinned, back illuminated would be even nicer.

There are a lot of tradeoffs. To take advantage of small pixels, you need
a small PSF contribution from lateral diffusion in the CCD. With a 200 um
thick CCD depleted all the way to the back and with a high bias voltage,
diffusion itself will contribute 6--8 um rms. If there is a field-free
region, then it will probably dominate the PSF, contributing an rms width
exactly equal to the field-free thickness. Back-illuminated thin CCDs
usually show fringing in the red.

For red and NIR I don't think it matters much if it is back illuminated,
except that the IR coating can then be optimized. But if it is thinned to
the usual 20 um the NIR reaches the numbers you target, and I know of no
case in which the ff region does not contribute enough to the PSF that
there is no point in 5 or even 10 um pixels. As far as I know the new SITe
UV-B optimized CCDs beat the PSF problem (10 um thick) but lose on the QE.
The Lincoln Labs high-resistivity chips (40 um sensitive region)
significantly extend the red response, but I would expect a diffusion PSF
contribution large enough that there is no point your small pixel width
(Barry Burke can correct me.) Our developmental CCDs have a 300 um
depleted region and no ff region, but also have a diffusion contribution
of 6--8 um to the PSF, depending on the bias voltage.  (See our ESO ODT
99 paper, also on the web page. Steve Holland has recently significantly
extended this analysis, to be published.)

--Don

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Don Groom deg at lbl.gov (Particle Data Group, Supernova Cosmology Project)
http://ccd.lbl.gov   Voice: 510/486-6788 FAX: 510/486-4799
Analog: 50-6008//Berkeley Lab//Berkeley, CA 94720

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