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First experimental tests with a CdTe photon counting pixel detector hybridized with a Medipix2 readout chip
IFAE Barcelona .
Mid Sweden University, Faculty of Science, Technology and Media, Department of Information Technology and Media.ORCID iD: 0000-0002-8325-5177
CEA France.
CERN.
Show others and affiliations
2004 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 51, no 5, 2379-2385 p.Article 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. Vol. 51, no 5, 2379-2385 p.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:miun:diva-8259DOI: 10.1109/TNS.2004.832324ISI: 000224490900079Scopus ID: 2-s2.0-84905114239OAI: oai:DiVA.org:miun-8259DiVA: diva2:133871
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: 2015-03-11Bibliographically approved
In thesis
1. Design and characterization of 64K pixels chips working in single photon processing mode
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. 139 p.
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

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