CCD-world: Reset gate threshold shift

Tue Apr 25 17:15:34 CLT 2000

The following was posted to CCD-world:

**********************************jj
Rodger,

This sounds like a similar problem experienced when burning in flight WF/PC 
II and Cassini CCDs. Here the reset threshold dramatically changed during 
burn-in. In fact some CCDs were impossible to reset. Several formal reports 
were written on the phenomenon experienced. We concluded that the reset gate 
insulator had charged during burn-in (i.e., experienced a very large 
flat-band shift). The problem was resolved by lowering the burn-in voltages 
which were consideralby higher than nominal. I need to look at the reports 
again to remember the details and physics involved. CCD manufacturers run 
through similar tests to make sure the CCD is reliable for long term use. 
There are always surprises when tests like this are performed. 

It would be interesting to generate a few transfer curves to see if there 
really is a flatband shift involved. Do you think it would worth pulling a 
CCD for this purpose? 

Jim Janesick
*******************************jj

In a message dated 4/24/00 3:25:26 PM EST, roger at ctios1.ctio.noao.edu writes:

<< 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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