Development of Advanced Pixelated Si Sensors for the Next Generation of X-ray Observatories

The highest priority recommendation from the Astro2020 Decadal was for NASA to fund technology development for the LuvEx (Large UV/Opt/IR), Lynx (X-ray), and Origins (far IR) flagship mission concepts with the goal of retiring technology risk to make all three feasible in the 2030s-2040s timeframe. This directed technology development funding will start immediately for the LuvEx concept technologies, and begin five years later for the X-ray and far IR instruments. As identified by the Astro2020 Decadal, advances in X-ray technology would be relevant to either the Lynx mission concept and/or any X-ray probe started in this decade to complement the Athena mission. Three sensor technologies; monolithic CMOS, digital CCD, and hybrid CMOS – were identified in the Lynx mission concept study as potentially viable for the High-Definition X-ray Imager (HDXI) focal plane and for the readout of the X-ray Grating Spectrometer (XGS). Individually, each sensor technology already meets some of the Lynx requirements, however no single technology currently meets all of them; further significant development and demonstration will be needed. The primary advantages of monolithic CMOS technology lie in its ability to provide both high read rates and soft X-ray response while demonstrating low read noise and near Fano-limited resolution at low energies. The primary disadvantage of the monolithic CMOS sensor technology is its (present) rather modest (~10 μm) depletion depth, providing smaller QE above ~2 keV than the other technologies. Given that the key science pillars of the Lynx mission concept (that were strongly endorsed by the Astro2020 Decadal) – investigation of high redshift (z>6) black holes and nearby galaxy haloes – emphasize observatory performance at softest (E<1 keV) X-ray energies, it is critical to advance the sensor technology with the best demonstrated low energy performance to meet the full suite of requirements in other areas. We propose a three year Strategic Astrophysics Technologies (SAT) program to make initial, focused improvements to existing monolithic CMOS sensor technology with the ultimate goal of reaching the science requirements for both the HDXI and the XGS readout on the Lynx mission concept. Specifically, we propose to make a custom wafer run duplicating our demonstrated BigMin III (BMIII) sensors with two key improvements. First, we will make the sensors with higher resistivity Si. This will increase the depletion depth to ~20-25 μm. Second, we will utilize an alternate treatment for the backside surface – Molecular Beam Epitaxy (MBE) as opposed to the chemical treatment used in our previous generation of sensors. The chemical treatment previously has been shown to be unstable. The MBE process has shown outstanding results with BI sensors used in optical through ultraviolet wavelengths. The MBE process, similar to that employed for UV response, will effectively eliminate any backside dead layer, further improving CMOS soft X-ray response. We will be re-using existing BMIII fabrication reticles to reduce both cost and risk. Similarly, our present camera electronics and test facilities will be re-used. This program is the first step of our longer term vision to advance the monolithic CMOS sensor technology to meet the Lynx requirements. Additional advancements required in future proposals will be to extend the depletion depth to ~50 μm and increase the read-out rate to 100 frames per second while maintaining noise performance. Subsequent development will be to produce either larger format sensors on a curved package or four-side abuttable devices to tile the Lynx focal plane and extended XGS readout. With the initial funding provided by the SAT program, and subsequently by directed funding, all technology hurdles related to monolithic CMOS can be overcome to meet the Lynx requirements or those of a notional probe concept emphasizing soft X-ray science.

The Strategic Astrophysics Technology program (SAT) supports focused development efforts for key technologies to the point at which they are ready to feed into major missions in the three science themes of the Astrophysics Division: Exoplanet Exploration, Cosmic Origins, and the Physics of the Cosmos. This program is specifically designed to address middle technology readiness level (TRL) "gaps" between levels 3 and 6: the maturation of technologies that have been established as feasible, but which are not yet sufficiently mature to incorporate into flight missions without introducing an unacceptable level of risk. NASA does not require a data management plan for proposals to SAT.

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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