After the wafers are thinned they are very smooth and reflective. A coating needs to be applied to lessen this effect. An anti-reflection (AR) coating is applied onto the back of the CCD to help the CCD reflect less light and therefore absorb more. The coating is applied to the CCD surface in a vacuum chamber. A vacuum chamber evacuates nearly all atmospheric air (similar to the environment of space, but better.) The picture below shows the difference between a coatend and an uncoated CCD. The top wafer is uncoated and bottom wafer is coated.

     
Image courtesy of University of Arizona Imaging Technology Laboratory

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