CCD-world: GROUND

[MYPIXEL at aol.com]

The following was posted to CCD-world:

*********************jj
Roger .. . . .

Another long shot. . . .  do you have a ground to the CCD? Cold CCDs can work 
without a ground as it floats around. 

We've had trouble with SITe ribbon cables before . . . can you do a very 
careful continuity check on the reset (and ground) lines? You will need to 
remove the bond pad protector. . .  assuming you have the same packaging 
arrangement.

We can do a flat-band test if it comes to that. .  . the manufacturer would 
like to know if this is indeed the problem (and you'll get a CCD replacement).

Jim
************************jj






In a message dated 5/5/00 1:17:25 AM Pacific Daylight Time, 
roger at ctios1.ctio.noao.edu writes:

<< The following was posted to CCD-world:
 
 Folks,
 
 This is the status of my Reset Gate threhold shift story.  This is not
 a very exciting story, until $100K worth of CCDs are down the tubes and
 it isn't clear whether you are the problem or the solution!
 
 Something I hadn't mentined is that one of the CCDs in 8K Mosaic shows
 similar symptoms: the reset FET on the B amplifier wont turn on.  There
 are some differences too : VOS is at 17V instead of 21V;  feedthrough
 pulses are 5 times normal;  VOS responds to the low level on SW when SW
 is increased by more than a few volts, probably because the output node
 has drifted(?) so low.  No voltage within the controler's (+-15V) range
 will turn on this reset FET.
 
 Over the long weekend, Tom Wolfe (from NOAO-Tucson) and I disassembled
 both Mosaic and the spectrograph camera and moved each of the
 malfunctioning CCDs in turn to a test dewar.  This dewar and CCD mount
 has extensively used with CCDs of the same type during device
 acceptance testing.  A separate controller and software setup were also
 used, which had been verified immediately prior to our tests.
 
 --> The symptoms were unchanged for both CCDs, so I must conclude that
 the problem is genuinely within the CCDs, or their integral flat cable
 & connector.
 
 We were fortunate to have purchased a spare 2Kx4K.  After first
 checking that that it worked fine in the test dewar, we loaded it into
 the Mosaic focal plane, which can now be operated in 16 channel mode.
 I installed the CCD with one working channel (removed from Mosaic) in
 in the spectrograph camera.
 
 So, I've been quite busy in the clean room and playing with cryogenics,
 and haven't had an opportunity to investigate the RG threshold shift
 any further.  As it now stands, I have two hypotheses to test: 
 
  - a purely capacitive connection to RG across an open circuit (bad via?),
 
  - or a genuine offset in the threshold due to injection of electrons 
    (how?) into the gate oxide, as suggested by Jim Janesic, and (in the 
    following message) by Dick Bredthauer...
 
 > You do not have an ESD
 > problem, but a hot carrier injection problem.  Many years ago,
 > approximately 1985 we saw the same problem.  We had a buried channel
 > CCD which was to be used for HgCdTe readout at 77K.  It had an oxide
 > nitride gate dielectric.  At normal operating voltages the device would
 > slowly drift until it didn't work at all.   We discovered that
 > electrons were being injected into the oxide nitride interface and
 > shifting the operating threshholds, (ie more positive).  The low
 > temperatures kept them trapped.  The injection occurs at high field
 > strength regions of the device.  By warming the device up to room
 > temperature or higher (50C) and applying a negative voltage (-10v-
 > -30V) to the offending gate you should be able force the electrons out
 > of their interface traps.
 
 If I am driving RG through a parasitic capacitance bridging an open
 via, then the solution is just to maintain the increased RG swing.  I
 may also need to ensure that RG is low during idle periods, so that
 leakage through the high impedance doesn't slowly charge up RG causing
 an additional threshold shift.
 
 I'll let you know when/if I discover anything more about the phenomenon.
 
 
 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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