Metachrome coating

[Paddy Oates]

Tim and Alistair,	
	Can you pass this to Alistair if you're his Go Between, Tim, Thanx.

	Hi, We have used EEV CCDs here at RGO for a number of years and 
these have been coated with both a polymer (laser) dye and laterly, by 
vapor deposition, metachrome. None of the devices I have had experience 
with has had this problem, though I do know the polymer coatings were supposed 
to lose their response over a number of years. 
	Also I measured the Metachrome coatings done for us by EEV and always 
found them to be aroun 50% lower in the UV than the Polymer coatings. Best 
Polymer coating I have seen had around 18% in the UV, best metachrome around 
12%...
	
	We have however had cause to try and remove a Metachrome coating. One 
of our large devices for an echelle spectrograph got contaminated with vacuum 
grease - this was in the days when we were using it as a means of ensuring 
good thermal contact with the cold block. Unfortunately the EEV 05-50 we were 
using was a 2-edge buttable and the silicon along the 2 long edges came right 
up to the package edge (hence the difficulty we had in getting a good clamp on 
it to enure good thermal contact). During operation the grease crept up the 
sides and gott onto the coating. EEV volunteered to try and clean it. They 
said getting the grease off the silicon was easier than getting the metachrome 
off the silicon. The coating permits the grease to 'soak through' so it wasnt 
'just' on the coating! 
	The attempt they made got the grease and the coating off but resulted 
in a dead chip. The O/P amplifiers didnt work anymore. We got a perfectly 
reasonable looking device back - you couldnt tell the difference before the 
grease contamination and after  the cleaning BUT....

	Polymer coatings are easier to get off - I have done that myself 
using warm iso-propyl alcohol. The trouble is getting a clean enough solution 
to ensure you dont deposit bits of grot/hair/cheese sandwiches... onto the 
chip when it dries! I did achieve a rather acceptable result with the chip 
still working... I didnt get it re-coated it was just a try and see operation.
I am not sure whether this will work for Metachrome BUT I guess whatever you 
do it would be wise to have a fall-back option (er i.e. another chip!) if the 
process does the dirty on the FETs or makes it in some other way non-operable. 
We managed to negotiate another device out of EEV for another reason so we 
werent left high and dry...
	If you dont get anywhere I could ring EEV and ask them what they use 
to do this sort of thing....

Cheers
Paddy



On Fri, 26 Apr 1996 11:14:55 -1000 (HST) Tim Abbott wrote:

> From: Tim Abbott <tmca at cfht.hawaii.edu>
> Date: Fri, 26 Apr 1996 11:14:55 -1000 (HST)
> Subject: Metachrome coating
> To: ccd-world at cfht.hawaii.edu
> 
> 
> Forwarded from Alistair Walker, CTIO:
> 
> > Subject: Metachrome coating
> > 
> > 
> > 
> > 
> > At CTIO we have a Tektronix 2048 CCD which was made several
> > years ago as part of the development for the HST STIS instrument.
> > This CCD is front-illuminated with 21 micron pixels, and four
> > years ago was coated with "metachrome" by Thomson, to give
> > QE of 15-20% below 4700A.  We have been using this CCD at the
> > Schmidt telescope for the past 12 months, where it gives over
> > a one degree field.
> > 
> > A sudden occurence two weeks ago was the appearance of what
> > looked like a long hair-like "crack" in the surface, this 
> > meanders for many mm.  If the CCD is viewed with the naked eye
> > the surface color of the coating is slightly different in the 
> > regions near the defect.  When the latter is viewed in
> > oblique illumination under a microscope, it is found to be 
> > raised above the surroundings (ie is a hill) and there is no
> > actual break in the coating.  
> > 
> > Our intrpretation is that the coating has partly de-laminated
> > and the visible defect is the junction between the attached
> > and the non-attached areas.  I would be interested in knowing
> > whether anyone has had similar experience with these coatings
> > becoming detached from large CCDs and, if it is necessary to
> > remove the coating, what solvent works best.  We have been using
> > Thomson 1024 CCDs with metachrome coatings for seven years with 
> > no problems, but of course the Tek device is very much larger.
> > 
> > Alistair Walker
> > 
> >      
> >    
> > 
> 
> 
> -- 
> Timothy M. C. Abbott                
http://www.cfht.hawaii.edu/~tmca/tmca.html

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