Towards a single-photon energy-sensitive pixel readout chip: pixel level ADCs and digital readout circuitry

Unlike conventional CMOS imaging, a single\ud photon imager detects each individual photon impinging on\ud a detector, accumulating the number of photons during a\ud certain time window and not the charge generated by the all\ud the photons hitting the detector during said time window.\ud The latest developments in the semiconductor industry\ud are allowing faster and more complex chips to be designed\ud and manufactured. With these developments in mind we are\ud working towards the next step in single photon X-ray imaging:\ud energy sensitive pixel readout chips. The goal is not only\ud to detect and count individual photons, but also to measure\ud the charge deposited in the detector by each photon, and\ud consequently determine its energy. Basically, we are aiming\ud at a spectrometer-in-a-pixel, or a “color X-ray camera”.\ud The approach we have followed towards this goal is the\ud design of small analog-to-digital-converters at the pixel level,\ud together with a very fast digital readout from the pixels to\ud the periphery of the chip, where the data will be transmitted\ud off-chip.\ud We will present here the design and measurement on prototype\ud chips of two different 4-bit pixel level ADCs. The\ud ADCs are optimized for very small area and low power, with\ud a resolution of 4-bits and a sample rate of 1 Msample/s. The\ud readout architecture is based around current-mode sense\ud amplifiers and asynchronous token-passing between the pixels.\ud This is done in order to achieve event-by-event readout\ud and, consequently, on-line imaging. We need to read eventby-\ud event (photon-by-photon), because we cannot have memory\ud on the pixels due to obvious size constraints. We use\ud current-mode sense amplifiers because they perform very\ud well in similar applications as very fast static-RAM readout

Design of pixel-level ADCs for energy-sensitive hybrid pixel detectors

Single-photon counting hybrid pixel detectors have shown\ud to be a valid alternative to other types of X-ray imaging\ud devices due to their high sensitivity, low noise, linear behavior\ud and wide dynamic range. One important advantage of these\ud devices is the fact that detector and readout electronics are\ud manufactured separately. This allows the use of industrial\ud state-of-the-art CMOS processes to make the readout\ud electronics, combined with a free choice of detector material\ud (high resistivity Silicon, GaAs or other). By measuring not\ud only the number of X-ray photons but also their energies (or\ud wavelengths), the information content of the image increases,\ud given the same X-ray dose. We have studied several\ud possibilities of adding energy sensitivity to the single photon\ud counting capability of hybrid pixel detectors, by means of\ud pixel-level analog-to-digital converters. We show the results of\ud simulating different kinds of analog-to-digital converters in\ud terms of power, area and speed

On evolution of CMOS image sensors

CMOS Image Sensors have become the principal technology in majority of digital cameras. They started replacing the film and Charge Coupled Devices in the last decade with the promise of lower cost, lower power requirement, higher integration and the potential of focal plane processing. However, the principal factor behind their success has been the ability to utilise the shrinkage in CMOS technology to make smaller pixels, and thereby have more resolution without increasing the cost. With the market of image sensors exploding courtesy their inte- gration with communication and computation devices, technology developers improved the CMOS processes to have better optical performance. Nevertheless, the promises of focal plane processing as well as on-chip integration have not been fulfilled. The market is still being pushed by the desire of having higher number of pixels and better image quality, however, differentiation is being difficult for any image sensor manufacturer. In the paper, we will explore potential disruptive growth directions for CMOS Image sensors and ways to achieve the same

Development and Performance of Kyoto's X-ray Astronomical SOI pixel (SOIPIX) sensor

We have been developing monolithic active pixel sensors, known as Kyoto's X-ray SOIPIXs, based on the CMOS SOI (silicon-on-insulator) technology for next-generation X-ray astronomy satellites. The event trigger output function implemented in each pixel offers microsecond time resolution and enables reduction of the non-X-ray background that dominates the high X-ray energy band above 5--10 keV. A fully depleted SOI with a thick depletion layer and back illumination offers wide band coverage of 0.3--40 keV. Here, we report recent progress in the X-ray SOIPIX development. In this study, we achieved an energy resolution of 300~eV (FWHM) at 6~keV and a read-out noise of 33~e- (rms) in the frame readout mode, which allows us to clearly resolve Mn-K$\alpha$ and K$\beta$. Moreover, we produced a fully depleted layer with a thickness of $500~{\rm \mu m}$. The event-driven readout mode has already been successfully demonstrated.Comment: 7pages, 12figures, SPIE Astronomical Telescopes and Instrumentation 2014, Montreal, Quebec, Canada. appears as Proc. SPIE 9147, Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ra

Investigation of the Performance of Photon Counting Arrays Based on Polycrystalline Silicon Thin-Film Transistors

Projection x-ray imaging is commonly employed to visualize internal human anatomy and used to produce diagnostic images. Modern projection imaging is typically performed using an active matrix, flat panel imager that is comprised of a converter layer overlying a pixelated array. The images are formed by converting x-ray photons into electrical signals, and then integrating those signals over a frame time – a method referred to as fluence integration. Recently, imagers employing a second method for creating x-ray images – referred to as photon counting – have been developed and used to perform mammographic imaging (a form of projection imaging). Photon counting involves measuring the energy of each interacting x-ray photon and storing digital counts of the number of photons exceeding one or more energy thresholds. Because the imaging information is stored digitally, photon counting imagers are less susceptible to noise than fluence-integrating imagers – which improves image quality and/or decreases the amount of radiation required to acquire an image. Current photon counting mammographic imagers are based on crystalline silicon and are limited in detection area. In order to produce an image, the array is moved in a scanning motion across the object of interest. A photon counting imager with larger detection area would benefit other projection imaging modalities – such as radiography (which produces, for example, chest x-ray images) or fluoroscopy (which is used for non-invasively inserting stents and other medical devices). However, techniques to increase detection area, such as tiling multiple arrays, result in increased imager complexity or cost. For this reason, our group has been exploring the possibility of creating photon counting arrays using a different semiconductor material, referred to as polycrystalline silicon (poly-Si). This material is fabricated using a thin-film process, which allows the economic manufacture of monolithic, large-area arrays and has favorable material properties for creating complex, high speed circuits. Using poly-Si, a set of prototype arrays have been designed and fabricated. The design of the arrays consists of four components: an amplifier, a comparator, a clock generator, and a counter. Several circuit variations were created for each component, and circuit simulations were performed in order to determine energy resolution and count rate values for each variation of each component. For the amplifier component, all circuit variations were determined to have an energy resolution of ~10% when presented with a 70 keV input x-ray photon (a typical photon energy level used in diagnostic imaging). This energy resolution value is comparable to those reported for photon counting imagers fabricated using crystalline silicon. In addition, while count rate values for the amplifier component were roughly one order of magnitude too low for radiographic and fluoroscopic applications (which require count rates on the order of 1 million counts per second per square millimeter [cps/mm2]), a hypothetical amplifier circuit variation with count rate capabilities suitable for these applications (while preserving the same ~10% energy resolution) was designed. In addition, the count rate values for the various comparator, clock generator, and counter circuit variations ranged from 100 to 3000 kcps/mm2. Finally, due to improvements in the poly-Si fabrication process (driven largely by the display industry), future photon counting arrays employing this material can have pixel pitches as small as 250 um – a size approaching that suitable for radiographic and fluoroscopic imaging.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144006/1/akliang_1.pd