DESY initiated development of a multi-megapixel monolithic soft X-ray CMOS imager for synchrotron and FEL applications.

DESY initiated development of and is leading an international collaboration to bring to users a multi-megapixel monolithic soft X-ray CMOS imager for synchrotron and FEL applications

With the increased brilliance of state-of-the-art Synchrotron radiation sources and the advent of Free Electron Lasers enabling revolutionary science with EUV to X-ray photons comes an urgent need for suitable photon imaging detectors. Requirements include high frame rates, very large dynamic range, single-photon counting capability with low probability of false positives, and (multi)-megapixels. PERCIVAL (Pixelated Energy Resolving CMOS Imager, Versatile and Large) is currently being developed by a collaboration of RAL/STFC, DESY, Elettra Sincrotrone Trieste, Diamond Light Source, Pohang Accelerator Lab (Rep. Korea), and Synchrotron Soleil to address this need for the soft X-ray regime. PERCIVAL is a monolithic active pixel sensor (MAPS), i.e. based on CMOS technology.

The Percival 2Megapixel imager (Percival 2M, or P2M) is a stitched CMOS sensor with 1484x1408 pixel imaging area. Pixel size is 27x27um2 for a total imaging area of ~4x4cm2.

Post-processing for back-side illuminated (BSI) imaging with ultrathin entrance windows enables high quantum efficiencies in the primary target energy range between 250eV and 1keV.


Photograph of Percival 2Megapixel sensor wafer prior to dicing

One key requirement for X-ray imagers at free electron lasers is the capability to handle – and quantify - large amounts of photons arriving in a pixel within much less than a picosecond. This makes the traditional method of photon counting, traditionally very successful in synchrotron applications, unuseable – integration and subsequent quantification of charge is instead the method of choice – and requires large dynamic range. Simultaneously, many science cases require reliable discrimination between zero and one photon in a given pixel, placing stringent requirements on noise in particular for soft X-ray detection. Percival meets this challenge by dynamically adjusting its gain (3 possible levels) in each pixel. The classical 3T CMOS imager (aka MAPS) pixel is enhanced by two capacitors C0 and C1 and 3 switches (SW0, SW1, AB). During exposure, these 3 additional switches are biased at an intermediate voltage (around 0.7V). Small charge deposits get integrated on the diode alone. As its voltage reaches the switch’s gate voltage, overflow onto the (larger) capacitance C0 becomes possible, and more charge can be integrated at a lower gain. Should C0 also reach the gate voltage of SW1, the largest capacitance C1 is used for a low-gain large-charge-deposit measurement. In the event of charge deposits beyond the system’s maximum capacity of about 3.5Me- per pixel and frame (corresponding to 50000 photons at 250eV), the “antiblooming” switch AB enables controlled draining of excess charge and thus avoids neighbouring pixels being affected.

Percival Pixel Schematic. (JINST, iWoRID 2018 Proceedings)

The Percival 2M sensor is contacted by close to 1000 microscopic wirebonds to a ceramic redistribution board. Power supplies and finely regulated biases necessary to run the sensor, as well as most of the electrical diagnostics, are located on an in-vacuum PowerBoard integrated into the detector head.


Percival Detector Head (JINST)

The system is controlled by an FPGA on the carrier board that forms the interface for control, machine and other auxiliary information (trigger, bunch id, experiment flags ,etc). As a Percival 2M sensor running at its maximum design speed of 300Hz frame rate generates 20 Gbit/s data, a separate data processing card – shared with other developments at DESY, namely AGIPD and LAMBDA – processes this data on an FPGA for output on up to four 10Gb ethernet lines.

The project started with first small prototype TestSensors in 2012.

Percival Test Sensor of the 1st generation (JINST)

 A first Percival 2M wafer underwent processing for BSI imaging to enable soft X-ray imaging at NASA’s Jet Propulsion Laboratory (JPL) in 2019. The sensor’s stitching architecture was developed such that fabricating a larger, 10x10cm2 imaging area 13 Megapixel monolith would be possible from the same set of foundry masks. First laboratory tests of Percival 2M performance used front-side-illuminated sensors (i.e. lacked the post-processing necessary for the backside illumination that enables good response to soft X-rays). Visible light and harder X-rays can be used to assess the sensor’s performance, and were e.g. used to demonstrate pixel noise below 16e-.

Percival 2M first light image using visible light on an FSI sensor (JINST)

Percival 2M pixel noise – averaging at below 16e-. This enables single photon discrimination with an acceptable number of false positives per image in the 2-Megapixel imager even at 250eV (69e- signal generated in the Si sensor).