CCD-world: Reset gate threshold shift

[Tom Droege]

The following was posted to CCD-world:

Roger,

Thank you for sharing your problems.  Sorry that I don't have an answer.
As an old time electronicer, I find CCD work particularly frustrating since
it is hard to measure the thing that needs measuring.  Hearing about
problems that others are having and their approach helps to formulate a
plan when one has a problem.  Thank you.  Now I have this little problem
with ice on the CCD ...

Tom Droege

At 03:07 PM 4/24/00 -0400, you wrote:
>The following was posted to CCD-world:
>
>Dear CCD World,
>
>If you have ever had a new CCD system which gave you a lot of trouble
>you will be able to relate to the expereunce I am having now...
>
>The CCD in question was tested by my colleagues at NOAO-Tucson.  It was
>in fact the best of the batch procured for the second 8Kx8K NOAO Mosaic
>(Cerro Tololo's) and was thus set aside for installation on a cold
>finger in the camera of a fiber fed spectrograph.  After the usual
>teething troubles with a new system, most of which were related to the
>unusual detector mount, the CCD worked fine.  Being under pressure
>check out the new spectrograph optics, we took it to Cerro Tololo
>immediately, keeping it cold and under vacuum while in transit (not
>normally a problem).   It has given me trouble ever since.
>
>I have since determined that the reset transistor switching threshold
>has increased to a quite remarkable 14.3 V.  I've carefully measured
>RG_threshold on both outputs of nine identical CCDs, 18 n total, and
>find that it lies typically between 4 V and 7 V.  I do this by setting
>RG_high to ensure that the Reset FET is safely turned on, then plot
>RG-feedthrough amplitude as a function of RG_low, then extrapolate
>slightly to determine where RG feedthrough is zero, ie the point at
>which the FET doesn't turn on any more.
>
>I haven't yet found any way of manipulating the CCD inputs to modify
>the RG threshold.  We have considerable experience with these devices
>having 16 of them in two 8x8K Mosiacs operating with similar voltages,
>waveforms and the same design controller, so this is quite a surprise.
>
>Note the gate threshold is higher than the drain voltage instead of
>being much lower!  The other bias voltages are:
>
>	Reset Drain   = 13.5 V
>	Last Gate     = -4.0 V
>	Output Drain  = 25.0 V
>	Output Source = ~20 V
>
>The clocks must be ok since the CCD correctly images everything from
>cosmic rays to flat fields, with normal bias levels and normal well
>capacity, provided that RG is switched between 8 and 14.8 V instead of
>0 and 12 V. 
>
>What could have happened?  Static discharge is rarely a problem in our
>very humid environment.  Nonetheless, the CCD has always been handled
>on properly grounded antistatic mats (bench and floor) while using a
>wrist strap with a continuity alarm.  Remember too that I obtained
>normal images with normal voltages immediately prior to taking the CCD
>to Cerro Tololo and it was kept under vacuum and cold the whoel time
>until it ceased to operate.
>
>I wondered if the substrate contact had been lost.  Had a bond wire or
>CCD contact failed?   Inspection under a steroe microscope shows all
>bond wires are in contact with their pads.  Capacitances from the
>clocks to the substrate look normal (measured with CCD unpowered).  I
>also note that the RG threshold of the other amplifier is normal which
>seems to rule out the loss of substrate contact.
>
>-----
>
>As if I didn't have enough problems, amplifier A ceased to operate at
>the same time as the RG threshold shifted on amp B.  Its symptoms
>however are completely different.  My hpothesis is that Last Gate is
>floating.
>
>All the clocks are working since it images through amp B with a
>suitably high RG pulse, and I can verify correct clock reversal when
>the amplifier selection is changed.  I see perfectly normal video
>levels and feedthrough pulses, however no video shifts are seen when LG
>is taken too high (as happens on the other amp).  Amp A is vaguely
>sensitive to light: a vertical overscan can be seen in a flat feild
>with more lines than there are in the image area. The vertical edge
>response is much pooreer than normal (decays over 5-10 lines instead of
>1-2), and the structure along a line is a smooth curve.  This behavior
>can be induced on the other amp by taking LG extremely negative (-8V)
>while raising the low level on SW from -5V to -3V so that the charge
>can't get past the barrier presented by LG until the serial register is
>overfilled.  I hypothesize that a floating LG would behave similarly...
>I haven't actually floated a good one to test this! 
>
>So I replaced the CCD with a mechanical sample on which I had soldered
>the suspect LG pin to its neighbour.  I then performed a continuity
>test with the system pumped and cold.  If LG is open, it as to be in the
>CCD or integral pig-tail cable.  Bummer!
>
>Although I have not yet found anything wrong with the controller or
>dewar, my next step will be to run this CCD in a separate dewar and
>controller.
>
>There is a third phenomenon that appeared at the same time: the noise
>increased from about 4 e- to 20 e-.  I have some doubts that this is
>related since it contains significant harmonic content, though the
>concidental appearance at the same time as the other two problems is
>very suggestive.
>
>I would appreciate any advice or educated guesses as to what might have
>happened, or further tests that I could perform.  And if you have
>nothing to offer, you can at least feel less alone when you get the CCD
>blues.
>
>
>Roger Smith
>
>Senior Electronics Engineer   /   Manager - Array Controller Projects
>Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
>Internet: rsmith at noao.edu	  Coordinates: 29.54 South, 71.16 West
>Phone:    56 (51) 205200	  Bilingual receptionist (08:30-21:00)
>Fax:      56 (51) 205342/205212   Autoforward via US: 1 (520) 318-8259
>Nat. Optical Astronomy Observatories,PO Box 26732,Tucson AZ 85726-6732
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