Prof. Peter Seitz
CSEM/EPFL
February 22, 2011 | 12:00 to 01:00 | 540 Cory Hall, DOP Center Conference Room
Host: Kris Pister
Since the discovery of X-rays in 1895, radiography imagery has only been black-and-white. The reason for this can be found in any textbook on X-rays: the linear attenuation coefficient of a pure chemical element factorizes into a universal product of a power of the atomic number Z times a power of the X-ray photon energy E. Our careful analysis has shown that the simple textbook equation with constant exponents is incorrect, and that there is a small dependency of the exponents of Z and E: in particular, the exponent of the energy term depends monotonously on Z, allowing us to distinguish between different elements in a sample by simple transmission measurement, albeit demanding good spectral resolution. This possibility to create "color" radiographs and CT imagery requires large arrays of miniaturized, low-noise X-ray detectors and spectrometers. Thorough noise analysis of conventional spectrometer circuitry shows the way to a novel ultra-low-noise X-ray spectrometer pixel that can be realized with commercially available CMOS technologies. Our X-ray pixel covers an area of 20x30 microns, has a fill factor of 56% and a record noise performance of only 12 electrons r.m.s. at room temperature. Since this noise is dominated by Johnson noise originating in the "continuous reset" resistor, a switched reset scheme could bring down the noise to about 5 electrons r.m.s. These insights into the various origins of noise in X-ray pixels can also be used for the design of a novel type of low-noise CMOS image sensor for electronic imaging in the visible and NIR spectral range. By using an in-pixel single-transistor voltage amplifier with a gain of about 10, optimum noise-shaping in the columns of the image sensor and correlated double sampling, sub-electron noise becomes possible. Employing UMC's commercially available 0.18 micron CMOS technology, we have realized a 256x256 array of 11x11 micron pixels for which we measured a record readout noise of 0.86 electrons r.m.s. at room temperature and at a speed of 60 full frames per second.
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