Salutory lessons

[Paddy Oates]

Mike and all,
			BAKED CHIPS NOT FRIED

	I have just completed a rigorous 2 week blitz on one of our thinned 
Loral CCDs. AS some of you may know I have been having problems getting 
repeatable consistent results with our Thinned devices. The response at room 
temperature always being okay BUT a drastic reduction in response being seen 
when the device was cooled (400nm response was 20nA at RT, 7-8nA at -100C i.e. 
a third of the RT response).
	So, last week I baked the cryostat, NO CCD, using a 7m heating tape 
(5W/sq inch) and retested the chip before flooding, after flooding and when 
cold - NO change. I then put the chip in and repeated the operation - this time 
hey presto, NO change in response when cold PLUS, the room temperature response 
after a few days dropped in the UV, as you would expect at ambient temp but 
under vacuum and was recovered to exactly the same level after UV flooding. I 
have done this 3 times now, warming up each time. I have tested at -120 and am 
just about to do another run at -100. I will then let the cryo warm up and 
repeate it again. I will then do the same thing with the next device. 

	I have been trying to work out what happened to get what I suppose must 
be water vapour onto the chip surface so that it was affected in this way. The 
cryo was new, but had not been baked, kept under vacuum. had not been allowed 
to form water vapour accidentaly inside, the flooding being done in dry air or 
Oxygen. Whatever the mechanism, the baking appears to have got rid of the 
contaminant, by the way the cryo body temp reached about 120C, the chip and 
cold block got (accidentaly at one point) to 150C.!! 

	This has been a gruelling lesson to learn. I want now to ensure 
continued repeatability and show that we can always get back to the response we 
expect when the camera comes up to ambient during periods of 'quiet'.

Thanks for the responses I got to my initial queries aboout this problem - I 
hope this report helps if some of you are still having similar problems.

Cheers for now, 

Paddy

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The Path of Excess Leads to the Tower of Wisdom...

The pleasure of satisfying a savage instinct, undomesticated by the
ego, is uncomparably much more intense than the one of satisfying a tamed
instinct - the reason is becoming the enemy that prevents us from a lot of
possibilities of pleasure

If you believe in the light, its because of obscurity, if you believe
in happiness its because of unhappiness. If you believe in God, then
you have to believe in the devil


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