One of the most significant quantum efficiency losses of back illuminated CCDs is reflection off their back surface.  The thinning  process creates a mirror-like finish with an extremely high specular reflectivity (see right, a 2kx4k thinned and packaged CCD with a picture of the Whirlpool Galaxy reflected in its surface).  This reflection loss approaches 60% in the UV.  The application of a thin film antireflection (AR) coating directly onto the CCD back surface can therefore significantly increase QE.

•  QE Models

•  Measured QE Curves

•  QE vs. Angle Plot

The Imaging Technology Laboratory has pioneered the development of high efficiency AR coatings for back illuminated CCDs.  We have found several suitable materials for silicon AR coatings, including hafnium oxide (HfO₂) and magnesium fluoride (MgF₂).  We routinely produce single layer HfO₂ and double-layer HfO₂-MgF₂ AR coating for our devices.  These coatings provide nearly 100% QE at selected wavelengths with very high QE throughout the entire near-UV to near-IR spectral region.  The picture at right shows our primary vacuum chamber.

For permanent backside charging, we apply a Chemisorption Coating (developed at ITL) to produce a net negative charge on the detector back surface.  There is no backside damage using this process which would cause QE-temperature instabilities and/or reduce the maximum QE obtained.  We routinely achieve flat field uniformity of better than 5% (measured at 400 nm) with Chemisorption Charged devices.

To ensure that our coatings meet thickness specifications, we have a multi-wavelength ellipsometer in-house for quality assurance.