The heart of CCD optimization is thinning.  We have designed and constructed a suite of linear agitation thinning machines which move the CCD in an acid bath (at right).  Uniformity to better than one micron is obtained with this method.  Etching is done in an acid mixture selective to p+ silicon.  Thinning is accomplished on a die basis.  We use a wax border to protect the front-side device circuitry and substrate traces from being attacked by the acid.  We have successfully thinned devices as large as a 4kx4k 15-micron pixel CCD.

We use a second acid etch to remove any remaining p+ silicon and to remove stains which sometimes form on the surface (see left).  This etch is  non-selective and can therefore be used to thin into the epitaxial layer to tailor device thickness, if necessary.  Resolution can also be improved by ultra-thinning the device to eliminate any field-free region in the CCD. We make use of this fact to ultra-thin devices to be used in applications requiring the highest possible spatial resolution.