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Llopart Cudié, Xavier
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Publications (10 of 12) Show all publications
Llopart Cudié, X. (2007). Design and characterization of 64K pixels chips working in single photon processing mode. (Doctoral dissertation). Sundsvall: Mid Sweden Unv
Open this publication in new window or tab >>Design and characterization of 64K pixels chips working in single photon processing mode
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Progress in CMOS technology and in fine pitch bump bonding has made

possible the development of high granularity single photon counting detectors for

X-ray imaging. This thesis studies the design and characterization of three pulse

processing chips with 65536 square pixels of 55 μm x 55 μm designed in a

commercial 0.25 μm 6-metal CMOS technology. The 3 chips share the same

architecture and dimensions and are named Medipix2, Mpix2MXR20 and Timepix.

The Medipix2 chip is a pixel detector readout chip consisting of 256 x 256

identical elements, each working in single photon counting mode for positive or

negative input charge signals. The preamplifier feedback provides compensation

for detector leakage current on a pixel by pixel basis. Two identical pulse height

discriminators are used to define an energy window. Every event falling inside the

energy 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. Each

cell also has an 8-bit configuration register which allows masking, test-enabling

and 3-bit individual threshold adjust for each discriminator. The chip can be

configured in serial mode and readout either serially or in parallel. Measurements

show an electronic noise ~160 e- rms with a gain of ~9 mV/ke-. The threshold

spread after equalization of ~120 e- rms brings the full chip minimum detectable

charge to ~1100 e-. The analog static power consumption is ~8 μW per pixel with

Vdda=2.2 V.

The Mpix2MXR20 is an upgraded version of the Medipix2. The main

changes in the pixel consist of: an improved tolerance to radiation, improved pixel

to pixel threshold uniformity, and a 14-bit counter with overflow control. The chip

periphery includes new threshold DACs with smaller step size, improved linearity,

and better temperature dependence.

Timepix is an evolution of the Mpix2MXR20 which provides independently

in 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 which

is distributed all over the pixel matrix during acquisition mode. The preamplifier is

improved and there is a single discriminator with 4-bit threshold adjustment in

order to reduce the minimum detectable charge limit. Measurements show an

electrical noise ~100 e- rms and a gain of ~16.5 mV/ke-. The threshold spread after

equalization 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 a

detector. The pixel static power consumption is ~13.5 μW per pixel with

Vdda=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 are

limited energy resolution and surface area. Future developments, such as

Medipix3, will aim to address those limitations by carefully exploiting

developments in microelectronics.

Abstract [sv]

Framstegen inom CMOS-teknologin och tekniken för bump bondning har

möjliggjort utveckling av högupplösande bilddetektorer för detektering av

enskilda röntgenfotoner eller laddade partiklar. Denna avhandling behandlar design

och karakterisering av tre pulsräknande utläsningskretsar med 65536 kvadratiska

bildelement med storleken 55 x 55 um2. De tre kretsarna, benämnda Medipix2,

Mpix2MXR20 och Timepix, delar samma arkitektur och dimensioner.

Medipix2 är en utläsningskrets för avbildning med 256 x 256 identiska

bildelement som räknar enskilda fotoner utgående från positiva eller negativa

laddningspulser. Förförstärkarens återkoppling kompenserar individuellt för

läckströmmen i varje bildelement. Ett energifönster kan definieras med hjälp av två

identiska diskriminatorkretsar. Varje händelse som faller inom energifönstret

räknas i en 13-bitars pseudo-random räknare. Räknaren, utformad som ett

skiftregister, fungerar också som in/utregister för varje bildelement. Kretsen kan

läsas ut antingen seriellt eller parallellt. Det elektroniska bruset har uppmätts till

~160 e- rms vid en förstärkning av ~9 mV/ke-. Spridningen i tröskelspänning efter

justering är ~120 e- rms vilket ger en minsta detekterbar laddningspuls över hela

kretsen på ~1100 e-. Den statiska effektförbrukningen i del analoga delen är

~8 mW per bildelement vid Vdda=2,2 V.

Mpix2MXR20 är en uppdaterad version av Medipix2. De huvudsakliga

förändringarna är: bättre strålningshärdighet, jämnare tröskelvärden och en 14-

bitsräknare med overflow. Periferin innehåller också nya DA-omvandlare med

mindre steg, förbättrad linjäritet och mindre temperaturberoende.

Timepix är en vidareutveckling av Mpix2MXR20 som medger detektering av

ankomsttid, time-over-threshold eller pulsräkning individuellt i varje bildelement.

Timepix utnyttjar en extern klocka (Ref_Clk) med frekvenser upp till 100 MHz som

distribueras över hela bildmatrisen. Förförstärkaren är förbättrad och en enkel

diskriminator med 4 bitars tröskeljustering används för att minimera lägsta

detekterbara laddningspuls. Mätningar visar ett elektroniskt brus på ~100 e- rms

och förstärkningen 16,5 mV/ke-. Med en tröskelspridning på 35 e- rms blir minsta

detekterbara laddning för den nakna kretsen (t.ex. i en gasfylld detektor) ~650 eoch

för en bondad detektor ~750 e-. Den statiska effektförbrukningen är ~13,5 mV

per bildelement vid Vdda=2,2 V och Ref_Clk= 80 MHz.

Den här kretsfamiljen har använts i ett antal olika applikationer. Under dessa

studier har ett antal begränsningar konstaterats. Bland dessa märks begränsad

energiupplösning och begränsad detektorarea. Framtida utvecklingsprojekt, t.ex.

Medipix3, kommer att inriktas på att avhjälpa dessa begränsningar genom att

utnyttja den senaste utvecklingen på mikroelektronikområdet.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden Unv, 2007. p. 139
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 27
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-45 (URN)978-91-85317-56-1 (ISBN)
Public defence
2007-05-25, M 102, Mittuniversitetet Sundsvall, Sundsvall, 10:15 (English)
Supervisors
Available from: 2007-12-05 Created: 2007-12-05 Last updated: 2011-04-07Bibliographically approved
McMullan, G., Cattermole, D. M., Chen, S., Henderson, R., Llopart Cudié, X., Summerfield, C., . . . Faruqi, A. R. (2007). Electron imaging with medipix2 hybrid pixel detector. Ultramicroscopy, 107(4-5), 401-413
Open this publication in new window or tab >>Electron imaging with medipix2 hybrid pixel detector
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2007 (English)In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 107, no 4-5, p. 401-413Article in journal (Refereed) Published
Abstract [en]

The electron imaging performance of Medipix2 is described. Medipix2 is a hybrid pixel detector composed of two layers. It has a sensor layer and a layer of readout electronics, in which each 55 μm×55 μm pixel has upper and lower energy discrimination and MHz rate counting. The sensor layer consists of a 300 μm slab of pixellated monolithic silicon and this is bonded to the readout chip. Experimental measurement of the detective quantum efficiency, DQE(0) at 120 keV shows that it can reach 85% independent of electron exposure, since the detector has zero noise, and the DQE(Nyquist) can reach 35% of that expected for a perfect detector (4/π2). Experimental measurement of the modulation transfer function (MTF) at Nyquist resolution for 120 keV electrons using a 60 keV lower energy threshold, yields a value that is 50% of that expected for a perfect detector (2/π). Finally, Monte Carlo simulations of electron tracks and energy deposited in adjacent pixels have been performed and used to calculate expected values for the MTF and DQE as a function of the threshold energy. The good agreement between theory and experiment allows suggestions for further improvements to be made with confidence. The present detector is already very useful for experiments that require a high DQE at very low doses.

Keywords
Hybrid pixel detectors; Imaging detectors; Electron microscopy
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9380 (URN)10.1016/j.ultramic.2006.10.005 (DOI)
Available from: 2009-07-13 Created: 2009-07-13 Last updated: 2017-12-13Bibliographically approved
Llopart Cudié, X., Ballabriga, R., Campbell, M., Tlustos, L. & Wong, W. (2007). Timepix, a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 581(1-2), 485-494
Open this publication in new window or tab >>Timepix, a 65k programmable pixel readout chip for arrival time, energy and/or photon counting measurements
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2007 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 581, no 1-2, p. 485-494Article in journal (Refereed) Published
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9383 (URN)10.1016/j.nima.2007.08.079 (DOI)
Available from: 2009-07-13 Created: 2009-07-13 Last updated: 2017-12-13Bibliographically approved
Tlustos, L., Ballabriga, R., Campbell, M., Heijne, E. H., Kincade, K., Llopart Cudié, X. & Stejskal, P. (2006). Imaging properties of the Medipix2 system exploiting single and dual enery thresholds. IEEE Transactions on Nuclear Science, 53, 1323-1328
Open this publication in new window or tab >>Imaging properties of the Medipix2 system exploiting single and dual enery thresholds
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2006 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 53, p. 1323-1328Article in journal (Refereed) Published
Abstract [en]

Low noise, high resolution and high dose efficiency are the common requirements for most X-ray imaging applications. Especially in medical applications the dose efficiency is a necessity for detector systems. We present the imaging performance of the Medipix2 readout chip bump bonded to a 300 mu m thick Si detector as a function of the detection threshold, a free parameter not available in conventional integrating imaging systems. Spatial resolution has been measured using the modulation transfer function (MTF) and it varies between 8.2 Ip/mm and 11.0 Ip/mm at 70%. An associated measurement of noise power spectrum (NPS) permits us to derive the detective quantum efficiency (DQE) which can be as a high as 25.5 % for a broadband incoming spectrum. The influence of charge diffusion in the sensor together with threshold variation in the readout chip is discussed. Although the Medipix2 system is used in photon counting mode with a single threshold in energy, the system is also capable of counting within a given energy window of down to ~1.4 keV. First measurements and images using this feature reveal capabilities that allow to identify fluorescence and other sources of disturbance.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9379 (URN)
Available from: 2009-07-13 Created: 2009-07-13 Last updated: 2017-12-13Bibliographically approved
Ballabriga, R., Campbell, M., Heijne, E. H., Llopart Cudié, X. & Tlustos, L. (2006). The Medipix3 prototype, a pixel readout chip working in single photon counting mode with improved spectrometric performance. In: procedings of Nuclear Science Symposium IEEE 2006, Ocotber 2006, San Diego USA.
Open this publication in new window or tab >>The Medipix3 prototype, a pixel readout chip working in single photon counting mode with improved spectrometric performance
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2006 (English)In: procedings of Nuclear Science Symposium IEEE 2006, Ocotber 2006, San Diego USA, 2006Conference paper, Published paper (Refereed)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9384 (URN)
Note
Published on CD-romAvailable from: 2009-07-13 Created: 2009-07-13 Last updated: 2011-04-07Bibliographically approved
Campbell, M., Chefdeville, M., Colas, P., Colijn, A. P., Fornaini, A., Giomataris, Y., . . . Visschers, J. L. (2005). Detection of single electrons by means of a Micromegas-covered Medipix2 pixel CMOS readout circuit. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 540(2-3), 295-304
Open this publication in new window or tab >>Detection of single electrons by means of a Micromegas-covered Medipix2 pixel CMOS readout circuit
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2005 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 540, no 2-3, p. 295-304Article in journal (Refereed) Published
Abstract [en]

A small drift chamber was read out by means of a MediPix2 readout chip as a direct anode. A Micromegas foil was placed above the chip, and electron multiplication occurred in the gap. With a He/isobutane 80/20 mixture, gas multiplication factors up to tens of thousands were achieved, resulting in an efficiency for detecting single electrons of better than 90%. We recorded many frames containing 2D images with tracks from cosmic muons. Along these tracks, electron clusters were observed, as well as δ-rays.

Keywords
Micromegas; MediPix2; TPC; Single electron; Pixel; Pixel segmented anode
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9382 (URN)10.1016/j.nima.2004.11.036 (DOI)000228031400009 ()
Available from: 2009-07-13 Created: 2009-07-13 Last updated: 2017-12-13Bibliographically approved
Chmeissani, M., Fröjdh, C., Gal, O., Llopart, X., Ludwig, J., Ponchut, C., . . . Tlustos, L. (2004). First experimental tests with a CdTe photon counting pixel detector hybridized with a Medipix2 readout chip. Paper presented at IEEE Nuclear Science Symposium/Room-Temperature Semiconductor Detector Workshop/Symposium on Nuclear Power Systems (2003 NSS/RTSD/SNPS), Nov 15, 2003, Rome Italy. IEEE Transactions on Nuclear Science, 51(5), 2379-2385
Open this publication in new window or tab >>First experimental tests with a CdTe photon counting pixel detector hybridized with a Medipix2 readout chip
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2004 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 51, no 5, p. 2379-2385Article in journal (Refereed) Published
Abstract [en]

We present preliminary tests of hybrid pixel detectors consisting of the Medipix2 readout chip bump-bonded to a 1-mm-thick CdTe pixel detector. This room temperature imaging system for single photon counting has been developed within the Medipix2 European Collaboration for various imaging applications with X-rays and gamma rays, including dental radiography, mammography, synchrotron radiation, nuclear medicine, and radiation monitoring in nuclear facilities. The Medipix2 + CdTe hybrid detector features 256 × 256 square pixels, a pitch of 55 μm, a sensitive area of 14×14 mm2. We analyzed the quality of the detector and bump-bonding and the response to nuclear radiation of the first CdTe hybrids. The CdTe pixel detectors, with Pt ohmic contacts, showed an ohmic response when negatively biased up to less than 60 V (electrons collection mode). Tests were also performed in holes collection mode, where a nonresistive behavior was observed above +15 V. We performed a series of imaging tests at low voltage bias with gamma radioactive sources and with an X-ray tube. Under uniform irradiation, we observed for all detectors the presence of numerous, stable structures in the form of small circles of about 200 μm diameter, with the central pixels showing a reduced counting efficiency with respect to the periphery (in electrons counting regime). Also long filament structures have been observed. Further investigations will reveal whether they are due to an intrinsic detector response (e.g., due to Te inclusions) or to the bump-bonding process.

Place, publisher, year, edition, pages
IEEE, 2004
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-8259 (URN)10.1109/TNS.2004.832324 (DOI)000224490900079 ()2-s2.0-84905114239 (Scopus ID)
Conference
IEEE Nuclear Science Symposium/Room-Temperature Semiconductor Detector Workshop/Symposium on Nuclear Power Systems (2003 NSS/RTSD/SNPS), Nov 15, 2003, Rome Italy
Note

IEEE Nuclear Science Symposium/Room-Temperature Semiconductor Detector Workshop/Symposium on Nuclear Power Systems (2003 NSS/RTSD/SNPS), Nov 15, 2003, Rome, Italy

Available from: 2009-01-15 Created: 2009-01-15 Last updated: 2017-12-14Bibliographically approved
Tlustos, L., Campbell, M., Heijne, E. H. & Llopart Cudié, X. (2004). Signal variations in high granularity Si pixel detectors. IEEE Transactions on Nuclear Science, 51, 3006-3012
Open this publication in new window or tab >>Signal variations in high granularity Si pixel detectors
2004 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 51, p. 3006-3012Article in journal (Refereed) Published
Abstract [en]

Fixed pattern noise is one of the limiting factors of image quality and degrades the achievable spatial resolution. In the case of silicon sensors non-uniformities due to doping inhomogeneities can be limited by operating the sensor in strong overdepletion. For high granularity photon counting pixel detectors an additional high frequency interpixel signal variation is an important factor for the achievable signal to noise ratio (SNR). It is common practice to apply flatfield corrections to increase the SNR of the detector system. For the case of direct conversion detectors it can be shown that the Poisson limit can be reached for floodfield irradiation. However when used for imaging with spectral X-ray sources flatfield corrections are less effective. This is partly a consequence of charge sharing between adjacent pixels, which gives rise to an effective energy spectrum seen by the readout, which is different from the spectral content of the incident beam. In this paper we present simulations and measurements of the limited applicability of flatfield corrections for spectral source imaging and investigate the origins of the high frequency interpixel noise component The model, calculations and measurements performed suggest that flatfield correction maps for photon counting detectors with a direct conversion Si sensor can be obtained from electrical characterization of the readout chip alone.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9378 (URN)
Available from: 2009-07-13 Created: 2009-07-13 Last updated: 2017-12-13Bibliographically approved
Colas, P., Colijn, A. P., Fornaini, A., Giomataris, Y., van der Graaf, H., Heijne, E. H., . . . Visschers, J. L. (2004). The readout of a GEM or Micromegas-equipped TPC by means of the Medipix2 CMOS sensor as direct anode. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 535(1-2), 506-510
Open this publication in new window or tab >>The readout of a GEM or Micromegas-equipped TPC by means of the Medipix2 CMOS sensor as direct anode
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2004 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 535, no 1-2, p. 506-510Article in journal (Refereed) Published
Abstract [en]

We have applied the Medipix2 pixel CMOS chip as direct anode readout for a TPC. For the gas amplification two options have been investigated: (i) a three-stage GEM system and (ii) a Micromegas mesh. The structure of the cloud of primary electrons, left after interactions of 55Fe quanta with the gas is visible with unprecedented precision. This proof-of-principle is an essential step in our project to realize a monolithic pixel sensor with integrated Micromegas, to be developed specially for the readout of TPCs, and applicable for drift chambers in general.

Keywords
GEM; Micromegas; Medipix2; TPC; Pixel; Pixel-segmented anode
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9381 (URN)10.1016/j.nima.2004.07.180 (DOI)000225580500092 ()
Available from: 2009-07-13 Created: 2009-07-13 Last updated: 2017-12-13Bibliographically approved
Llopart Cudié, X. (2003). First test measurements of a 64k pixel redout chip working in single photon counting mode. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 509(1-3), 157-163
Open this publication in new window or tab >>First test measurements of a 64k pixel redout chip working in single photon counting mode
2003 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 509, no 1-3, p. 157-163Article in journal (Refereed) Published
Abstract [en]

The Medipix2 chip is a pixel detector readout chip consisting of 256×256 identical elements, each working in single photon counting mode for positive or negative input charge signals. The chip is designed and manufactured in a six-metal 0.25 μm CMOS technology. This paper describes several electrical measurements which have been carried out on the chip prior to detector bump bonding using a dedicated readout system. Threshold linearity and variation has been measured for both electron and hole collection. The noise is 100e RMS and the threshold can be adjusted to 120e RMS for both polarities. The minimum operating threshold is 1000e.

Keywords
Photon counting, Pixel, CMOS, X-rays, Midipix
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:miun:diva-9376 (URN)10.1016/S0168-9002(03)01565-1 (DOI)000185047700026 ()
Available from: 2009-07-13 Created: 2009-07-13 Last updated: 2017-12-13Bibliographically approved
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