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In order to produce an accurate model of CCD quantum efficiency it is very important to know precisely the optical constants of the materials comprising the gate structure of the device. Widely used Henke data are quite inaccurate at energies close to the absorption edges. We undertook an effort to measure the transmission of the corresponding films in order to to fill in this gap.
The CCD gate structure contains three different materials: silicon dioxide, silicon nitride (Si3N4), and polycrystalline silicon, which is heavily doped with phosphorus. All the X-ray absorption edges of these materials, namely, silicon L (100 eV), nitrogen K (400 eV), oxygen K (532 eV), and silicon K (1840 eV) are within the range of interest for ACIS.
In the course of technology development for manufacturing backside CCD, Lincoln Lab implemented a technique for etching away the thick silicon substrate, leaving only a thin layer of material at the surface of the wafer. This technology turned out to be exceptionally well suited for making thin films supported by a rigid frame, allowing them to be handled easily. Usually, at low energies the transmission of a material is derived from the results of the measurement of total electron yield. With the Lincoln Lab technology it is possible to make extremely thin free standing films and measure transmission directly. We used synchrotron radiation to measure the transmission of these materials as a function of energy. There is no single beamline which can cover such a wide range of energies, and for this reason measurements were performed at three different synchrotrons.