Silicon CCD Optimized for NIR Wavelengths

CCDs processed on typical substrates exhibit low NIR QE. Because the relatively thin epitaxial layer allows a high percentage of long wavelength photons to pass through and the reflection loss of back surface is high. A thick epitaxial layer allows the longer wavelength photons to bo absorbed into the epitaxial layer where the resultant electrons generated will be collected in the potential wells. The drawback of processing with this method is a resultant degradation of carrier diffusion MTF. Increasing the depleted region under each gate, which can be realized by using high resistivity substrates,can enhance MTF. A new kind of CCD fabricated on high resistivity silicon at Lick Observatory has superior red performance beyond 800 nm wavelength. The application of thin film AR coating directly on to the CCD back surface can significantly reduce reflection loss from UV to NIR and greatly decrease interference fringing on back illuminated CCDs.