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The Smithsonian Astrophysical Observatory (SAO) in collaboration with Stanford Research Institute (SRI) has been developing monolithic CMOS detectors for use as astronomical soft X-ray imaging spectrometers since 2008. The long term goal of this collaboration is to produce X-ray Active Pixel Sensor (APS) detectors with Fano limited performance over the 0.1-10keV band for “Facility Class” missions such as Lynx. Since CMOS x-ray imagers consume very little power; are inherently ”radiation hard”; have high levels of integration, and are capable of very high read rates they are ideal for “Small Satellite” missions as well. SAO/SRI CMOS imagers are presently being proposed for several, more immediate X-ray “Small Sat” real and concept missions. CMOS device fabrication provides the most rapid path forward towards advances in virtually all types of integrated circuits. The same techniques and infrastructure that has produced tremendous capabilities in microprocessors, RAM and FPGAs are now being applied to CMOS based imaging detectors CMOS imaging detectors have found their way into high end consumer cameras and in various (non X-ray) astronomical missions, e.g. the flight imaging detectors for the SoloHi mission, the WISPR imager on the Parker Solar Probe. SAO is presently investigating three different devices that each embody technology that would be highly desirable in an x-ray imaging spectrometer; these are: back thinned high sensitivity NMOS PPD devices; NMOS devices with stitchable reticles; and monolithic PMOS devices that collect photo-holes instead of photo electrons. The back-thinned, high sensitivity NMOS PPD devices, known as Big Minimal IIIs (BMIII) were specifically funded and designed for soft x-ray single photon counting. They embody a 1k by 1k array of 6 Transistor (6T) 16µm PPD pixels. Each pixel has a 135 μV/e sense node. The stitchable reticle devices, known as “Mk by Nk”, can be made seamlessly in any format which fits on a silicon wafer. They consist of an array of 6 Transistor (6T) 10μm PPD pixels (for these test devices M=N=1) with a sense node of 90 μV/e. A stichable reticle CMOS with a choice of format size would be ideal for large focal plane or for a narrow rectangular grating readout. The third device category is a small 256 by 256 16μm pixel PMOS device which collects holes instead of electrons with a 60μV/h+ sense node. SAO/SRI conventional NMOS CMOS devices known as the Big Minimal III (BigMinIII) have recently demonstrated the ability to detect and resolve X-rays with energies below 200eV. Even with this very good sub-1keV response, an NMOS astronomical instrument would still be fundamentally limited by charge collection, read and Random Telegraph Signal (RTS) noise particularly for soft, faint, extended sources and surveys. We have just for the first time performed very preliminary X-ray tests with monolithic FI PMOS devices.
We present details of our new camera design and preliminary device performance with particular emphasis on those aspects of interest to single photon counting X-ray astronomy.
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