CCD-world: CCD cleaning

[Roy Tucker]

The following was posted to CCD-world:

At 10:33 AM 6/24/99 +0100, you wrote:
>The following was posted to CCD-world:

>I'd be interested to hear about the bad results from using a single-hair
>brush.  I had been led to believe it was a good idea.

     For what it's worth, this was my first attempt to try the single-hair
brush technique and I'm not inclined to try it again.

     Several years ago I attempted to clean a frontside Sony imager in a
Pulnix camera. This was a glassless device that was to be used in an
interferometer. I trimmed all but one hair from a small camel's hair art
brush and worked under a binocular microscope. I delicately brushed away
the contamination but found when the camera was powered up that there were
dead pixels in scratch-like patterns, not a pretty sight.

     According to the Va-Tran literature, their snow gun produces a fine
spray of tiny dry ice particles. It is the direct impact of these particles
that transfers momentum to dislodge contaminating dust particles and is
more effective than aerodynamic forces in a gas stream. The dry ice
particles cannot cause physical damage themselves since, at the point of
contact with the surface against which they are impacting, the pressure is
sufficiently great to cause a transient liquification of the CO2.

     Three years ago, I sent them several dead CCDs that were thoroughly
contaminated with dust (I let them sit on my desk for a couple of weeks
before the test) to see how effective the method was. The devices were
carefully examined upon their return. Indeed, the devices were clean and
the bond wires were apparently unaffected.

     My primary concern is possible electrostatic discharge damage from
this high-speed spray of particles. I'm not sure how to model this. What
would be the maximum number of electrons that one might expect to ride on a
dry ice particle and how would that energy be concentrated during the
impact with a CCD surface? Let's assume that a dry ice particle will impact
a one picofarad structure on the surface of a CCD. To charge this structure
to one volt would require dumping over 6 million electrons into it. I may
be mistaken but I don't think that amount of charge could ride on a single
dry ice particle. I would expect that during cleaning attempts the imager
would be operating and providing a display so that one may recognize when
the offending particles had been dislodged. Wouldn't the fact that the
device is in operation and the gate structures are being actively driven
provide some protection from ESD from the CO2 snow spray? Alternatively,
the device could be cleaned while all device pins were shorted together.

     All comments will be appreciated. Thank you.

					Best regards,
					  - Roy Tucker
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