CCD soft X-ray imaging spectrometer for the ASCA satellite

We describe the development of a charge-coupled device (CCD) array for use as a soft X-ray (0.4-12 keV) imaging spectrometer for the ASCA (formerly Astro-D) satellite. The CCDs are 420×420-pixel frame-transfer devices designed to be closely abutted to other chips on three sides of the imaging array. The imagers are made on 6500-Ω·cm p-type float-zone silicon for depletion depths of about 50 μm under typical CCD bias conditions. The read noise of the CCD is typically 3-4 e^- rms at data rates of 50 KHz resulting in an energy resolution E/δE≈50 at 5.9 keV. The complete focal-plane sensor consists of a 2×2 array of these devices mounted on a common substrate. Radiation damage from energetic protons is mitigated by the use of a narrow potential trough along the center of the CCD channel to confine the small X-ray event charge to a reduced volume and thereby minimize trapping effects. Charged-particle events from the non-X-ray space background are minimized by using a junction on the back of the chip to deplete most of the neutral bulk and draw background charge away from the CCD. Wafer-level device screening at low temperatures and the focal-plane packaging methods are also described