Design and characterization of 64K pixels chips working in single photon processing mode
Sammanfattning: Progress in CMOS technology and in fine pitch bump bonding has madepossible the development of high granularity single photon counting detectors forX-ray imaging. This thesis studies the design and characterization of three pulseprocessing chips with 65536 square pixels of 55 μm x 55 μm designed in acommercial 0.25 μm 6-metal CMOS technology. The 3 chips share the samearchitecture and dimensions and are named Medipix2, Mpix2MXR20 and Timepix.The Medipix2 chip is a pixel detector readout chip consisting of 256 x 256identical elements, each working in single photon counting mode for positive ornegative input charge signals. The preamplifier feedback provides compensationfor detector leakage current on a pixel by pixel basis. Two identical pulse heightdiscriminators are used to define an energy window. Every event falling inside theenergy window is counted with a 13-bit pseudo-random counter. The counter logic,based in a shift register, also behaves as the input/output register for the pixel. Eachcell also has an 8-bit configuration register which allows masking, test-enablingand 3-bit individual threshold adjust for each discriminator. The chip can beconfigured in serial mode and readout either serially or in parallel. Measurementsshow an electronic noise ~160 e- rms with a gain of ~9 mV/ke-. The thresholdspread after equalization of ~120 e- rms brings the full chip minimum detectablecharge to ~1100 e-. The analog static power consumption is ~8 μW per pixel withVdda=2.2 V.The Mpix2MXR20 is an upgraded version of the Medipix2. The mainchanges in the pixel consist of: an improved tolerance to radiation, improved pixelto pixel threshold uniformity, and a 14-bit counter with overflow control. The chipperiphery includes new threshold DACs with smaller step size, improved linearity,and better temperature dependence.Timepix is an evolution of the Mpix2MXR20 which provides independentlyin each pixel information of arrival time, time-over-threshold or event counting.Timepix uses as a time reference an external clock (Ref_Clk) up to 100 MHz whichis distributed all over the pixel matrix during acquisition mode. The preamplifier isimproved and there is a single discriminator with 4-bit threshold adjustment inorder to reduce the minimum detectable charge limit. Measurements show anelectrical noise ~100 e- rms and a gain of ~16.5 mV/ke-. The threshold spread afterequalization of ~35 e- rms brings the full chip minimum detectable charge either to~650 e- with a naked chip (i.e. gas detectors) or ~750 e- when bump-bonded to adetector. The pixel static power consumption is ~13.5 μW per pixel withVdda=2.2 V and Ref_Clk=80 MHz.This family of chips have been used for a wide variety of applications.During these studies a number of limitations have come to light. Among those arelimited energy resolution and surface area. Future developments, such asMedipix3, will aim to address those limitations by carefully exploitingdevelopments in microelectronics.
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