A 4096 pixel Photon Counting Chip (PCC) has been developed and tested. It is aimed primarily at medical imaging although it can be used for other applications involving particle counting. The readout chip consists of a matrix of 64 x 64 identical square pixels, whose side measures 170 mm and is bump-bonded to a similar matrix of GaAs or Si pixel diodes covering a sensitive area of 1.18 cm . The electronics in each cell comprises a preamplifier, a discriminator with variable threshold and a 3-bit threshold tune as well as 15-bit counter. Each pixel can be individually addressed for electrical test or masked during acquisition. A shutter allows for switching between the counting and the readout modes and the use of a static logic in the counter enables long data taking periods. Electrical tests of the chip have shown a maximum counting rate of up to 2 MHz in each pixel. The minimum reachable threshold is 1400 e with a variation of 350 e rms that can be reduced to 80 e rms after tuning with the 3-bit adjustment. Electical noise at the input is 170 e rms. Several read-out chips have been bump-bonded to 200 mm thick GaAs detectors. Tests with g-rays and b sources have been carried out. A number of objects have been imaged and 260 mm thick aluminium foil which represents a contrast to the surrounding aire of only 1.9% has been correctly imaged

Bisogni, M G

Campbell, M

Conti, M

Delogu, P

Fantacci, M E

Heijne, Erik H M

Maestro, P

Magistrati, G

Marzulli, V M

Meddeler, G

Mikulec, B

Pernigotti, E

Rosso, V

Schwarz, C

Snoeys, W

Stumbo, S

Watt, J

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN–EP/98–16221 September 1998PERFORMANCE OF A 4096 PIXEL PHOTON COUNTING CHIPM. G. Bisogni1), M. Campbell2), M. Conti3), P. Delogu1), M. E. Fantacci1), E. H. M. Heijne2),P. Maestro1), G. Magistrati4), V. M. Marzulli1), G. Meddeler5), B. Mikulec2),E. Pernigotti1), V. Rosso1), C. Schwarz6), W. Snoeys2), S.Stumbo1), J. Watt5)AbstractA 4096 pixel Photon Counting Chip (PCC) has been developed and tested. It is aimedprimarily at medical imaging although it can be used for other applications involving particlecounting. The readout chip consists of a matrix of 64 x 64 identical square pixels, whose sidemeasures 170 Pm and is bump-bonded to a similar matrix of GaAs or Si pixel diodescovering a sensitive area of 1.18cm2. The electronics in each cell comprises a preamplifier, adiscriminator with variable threshold and a 3-bit threshold tune as well as a 15-bit counter.Each pixel can be individually addressed for electrical test or masked during acquisition. Ashutter allows for switching between the counting and the readout modes and the use of astatic logic in the counter enables long data taking periods. Electrical tests of the chip haveshown a maximum counting rate of up to 2 MHz in each pixel. The minimum reachablethreshold is 1400 e- with a variation of 350 e- rms that can be reduced to 80 e- rms after tuningwith the 3-bit adjustment. Electrical noise at the input is 170 e- rms. Several read-out chipshave been bump-bonded to 200 Pm  thick GaAs detectors. Tests with J-rays and E- sourceshave been carried out. A number of objects have been imaged and 260 Pm thick aluminiumfoil which represents a contrast to the surrounding air of only 1.9 % has been correctlyimaged.(Presented at the 43rd SPIE International Symposium on Optical Science, Engineering, andInstrumentation, July 19-24, 1998, San Diego, California, USA;SPIE Proceedings 3445-31)–––––––––––––––––––––––––1) University and INFN of Pisa, Italy.2) CERN, Geneva, Switzerland.3) University and INFN of Napoli, Italy.4) LABEN S.p.A., Milano, Italy.5) University of Glasgow, Scotland.6) University of Freiburg, Germany.Performance of a 4096 Pixel Photon Counting ChipM. G. Bisognia, M. Campbellb, M. Contic, P. Delogua, M. E. Fantaccia, E. H. M. Heijneb,  P. Maestroa,G. Magistratid, V. M. Marzullia, G. Meddelere, B. Mikulecb, E. Pernigottia*, V. Rossoa, C. Schwarzf,W. Snoeysb, S. Stumboa, J. WatteaUniversity and INFN of Pisa, ItalybCERN, Geneva, SwitzerlandcUniversity and INFN of Napoli, ItalydLABEN S.p.A., Milano, ItalyeUniversity of Glasgow, ScotlandfUniversity of Freiburg, GermanyABSTRACTA 4096 pixel Photon Counting Chip  (PCC) has been developed and tested. It is aimed primarily at medical imaging although itcan be used for other applications involving particle counting. The readout chip consists of a matrix of 64 x 64 identical squarepixels, whose side measures 170 Pm and is bump-bonded to a similar matrix of GaAs or Si pixel diodes covering a sensitivearea of 1.18 cm2. The electronics in each cell comprises a preamplifier, a discriminator with variable threshold and a 3-bitthreshold tune as well as a 15-bit counter. Each pixel can be individually addressed for electrical test or masked duringacquisition. A shutter allows for switching between the counting and readout modes and the use of static logic in the counterenables long data taking periods. Electrical tests of the chip have shown a maximum counting rate of up to 2 MHz in each pixel.The minimum reachable threshold is 1 400 e- with a variation of 350 e- rms that can be reduced to 80 e- rms after tuning with the3-bit adjustment. Electrical noise at the input is 170 e- rms. Several read-out chips have been bump bonded to 200 Pm thickGaAs pixel detectors. Tests with J-ray and E- sources have been carried out. A number of objects have been imaged and a260 Pm thick aluminium foil which represents a contrast to the surrounding air of only 1.9 % has been correctly imaged.Keywords: MEDIPIX, medical imaging, GaAs, pixel detector, x-rays, mammography, photon counting chip, low contrast,ASIC, VLSI.1. INTRODUCTIONIn medical x-ray imaging it is desirable to produce precise 2-dimensional information while limiting the dose to the patient.Another requirement is the possibility to image objects which are very similar in density to the surrounding tissue thuspresenting a low contrast. One way to address these issues is to couple a highly segmented semiconductor detector to a matrix ofreadout electronics. Each small detection element is connected to electronics which counts the number of single x-ray photonsseen by the detector1. It is necessary that the detector material chosen presents a high cross section to the impinging photons andthat the electronics be sufficiently sensitive to detect the small signals generated. Also the problem of interconnect between the2-dimensional detector array and the associated electronics has to be addressed. Hybrid pixel detector technology presents aninteresting solution to this problem.Hybrid pixel detectors were first used successfully in the WA97 experiment at CERN2 following the developments in theRD19 collaboration3. Since that time many particle physics experiments are developing sub-detectors based on this newtechnology. Hybrid pixel detectors consist of one or several ASICs, each containing a 2-dimensional matrix of readoutelectronics, which are bump-bonded to an equally segmented semiconductor detector. Normally the readout requirements for                                                       * Further author information:E.Pernigotti: E-mail: Elena.Pernigotti@cern.ch; Telephone: (+41) 22.767.9146; Fax: (+41) 22.767.3394HEP are rather different from imaging and, as a consequence of this, a new chip, the single Photon Counting Chip (PCC)4, hasbeen realised. In this chip a modified version of the front-end electronics of the LHC1/Omega3 chip5 has been used and eachpixel has been equipped with a 15-bit counter. The new chip has been bump-bonded to a detector made of GaAs as this materialhas a high detection efficiency for x-rays in the diagnostic energy range (10-100 keV)6. A review of the design of the single pixel and of the overall chip is given in section 2. In section 3 we summarise theresults of electrical tests. Some images have been produced using both E- sources and J-rays. Measurements of objects with aknown contrast have also been performed. These are presented in section 4.2. DESIGN OF THE READOUT CHIPPCC is a 64 x 64 matrix of identical square pixels whose side measures  Pm. The chip is designed in the SACMOS1technology of FASELEC (Zurich). The total area of the chip is about 1.7 cm2 with a sensitive area of 1.18 cm2. Each cellcontains a full readout chain and can be bump-bonded to an identically segmented semiconductor detector.Input PreampLatchedComparatorPulseshaperThresholdAdjust(3 bits)TestInputCfbTest(1 bit)CtestRstAnalogResetMask(1 bit)ShutterDataClkClkout1001SelSelMuxMuxShiftRegTo lowerpixelFrom upperpixelFigure 1: Block diagram of the pixel cell.The block diagram of the pixel cell is shown in Figure 1. Each pixel consists of a charge sensitive preamplifier with leakagecurrent compensation followed by a latched comparator with an externally controllable threshold which can be locally adjustedvia 3 fully static flip-flops. The preamplifier can receive signals from the detector via the bump-bond or test signals from anexternal pulse generator via a test capacitance for electrical characterisation. There are 2 additional static flip-flops to masknoisy pixels and to enable electrical testing. These flip-flops along with the threshold adjust flip-flops are organised as a shiftregister. The output of the preamplifier is connected to a short delay line producing a pulse which is used to reset the comparatorand to act as a clock for the counter when the shutter is low. This counter is a shift register whose 14th and 15th bits are fed backvia an exclusive-or gate to the input. This is in fact a pseudo-random counter7. When the shutter is high, the feedback loop of thecounter is broken and the 15 bits are connected in series to the upper and lower pixels of the same column in order to form a15 x 64-bit shift register. The 64 columns of the chip are connected in groups of four and multiplexed onto the 16-bitinput/output bus. The same bus is also used to load the 5 x 64-bit configuration register. Both loading and readout operationscan be performed at a frequency of 10 MHz. A photograph of the chip is shown in Figure 2.Figure 2: Photograph of the single Photon Counting Chip.3. ELECTRICAL MEASUREMENTSA complete characterisation of the electrical performance of the chip has been carried out prior to bump-bonding. As alreadymentioned, electrical tests are possible thanks to a capacitance which is connected to the input of the readout chain and isenabled by a configuration bit. The input capacitance is identical to the one used for LHC1/Omega3, for which calibrationmeasurements gave a value of 14.9 fF. An accurate calibration of the PCC input capacitance has not been carried out, butpreliminary measurements with J-rays sources are in reasonable agreement with that value. The electrical test results aresummarised in Table 1.Minimum Threshold 1 400 e-Threshold linearity 1 400 e- to 7 000 e-Threshold variation (no adjust) 350 e-Threshold variation (adjust ) 80 e-Noise 170 e-Maximum input signal > 80 000 e-Maximum counting rate 2 MHzTable 1: Summary of electrical measurements of the PCC chip.These measurements have been repeated on several chips and show good reproducibility. Some wafers have been probetested and Known Good Die have been selected for bump bonding. The criteria followed for selecting chips were based onfunctionality, minimum threshold and threshold dispersion considerations. Chips with less than 2500 e- as mean minimumthreshold, less than 400 e- before adjustment and more than 4000 working pixels met these criteria. These chips have beenselected for bump-bonding to the detector.4. RADIOACTIVE SOURCE MEASUREMENTSPrior to making images with the chip/detector assemblies the operating point of the electronics has been optimised, the thresholdlocally adjusted and then reduced to the minimum achievable value. The minimum mean threshold obtained with the detectorconnected is 2 000 e- with a variation of 125 e- rms(see Figure 3).EntriesMeanRMS           3984  21.53  1.324Constant   1467.Mean   21.63Sigma  0.9984threshold [ mV ]Number of pixels020040060080010001200140010 12.5 15 17.5 20 22.5 25 27.5 30 32.5Figure 3: Distribution of thresholds after adjustment.The functional behaviour of the assembly as well as the bump-bonding uniformity was verified using a 90Sr source. Thissource produces E- particles with a cut-off energy of 2.32 MeV resulting from the decay of the 90Y daughter isotope. Only a fewpixels did not respond in this test.Figure 4 shows the image of a 6 mm long steel screw placed on the back of the detector and irradiated with a 109Cd sourcewhich has gamma photon emissions at 22.1 keV (~ 85 %) and 25 keV (~ 17%). The spatial resolution of the device isdemonstrated by the possibility to observe the 500 Pm thread of the screw. Moreover, this indicates that the mean threshold ofthe device is below the 22.1 keV line. A second image was taken of a 500 Pm thick Tungsten wire bent in the form on an ‘m’(see Figure 5). Once again the uniformity of response of the matrix can be appreciated.0501001502002503003504004505000100200300400500600Figure 4:  A 6 mm screw placed on the back of the detectorand illuminated with 109Cd.Figure 5:  A 500 Pm thick Tungsten wire placed on theback of the detector and illuminated with 109Cd.Another series of images were taken using an 241Am source which has a peak emission at 60 keV. In each case one imagewas taken with the object in place and this was subtracted form the image taken without the object. This attenuates the effects ofdetector non-uniformity. Figure 6 shows the image of a steel object shaped like a cog, but with thickness varying between300 Pm and 500 Pm and an inside diameter of 3 mm. The device was placed about 0.5 cm above the detector with the sourcearound 1 cm away.010002000300040005000600070008000Figure 6: A steel object 0.5 cm from the back of the detector and illuminated with 241Am. Background subtracted.Several measurements have been carried out in order to get information about the minimum detectable contrast, where thecontrast, c, is defined as:nnnc' where n is the number of photons in the background and n’ is the number of photon inside the target image. Aluminium strips ofvariable thickness and a width of around 1 mm have been imaged. The strips were placed ~ 1cm above the detector and thesource was ~ 8 cm away inside a Plexiglas collimator . Table 2 shows the aluminium strip thickness, the theoretical contrastsand the measured contrasts with errors. From 3000 Pm to 1000 Pm there is a systematic underestimate of the contrast of theobjects. This was associated with the photon scattering from the sides of the collimator. For the 540 Pm and the 260 Pm objects(see Figure 7) the collimator was removed and the measured results are in agreement with the theoretical values.Al thickness (Pm) Theoretical contrast Measured contrast3000 20.29 %            17.12 %  r  1.67 %2000 14.03 % 11.92 %  r  0.98 %1000 7.28 %  6.45 %  r  0.56 %540 4.00 %  3.87 %  r  0.48 %260 1.94 %  1.85 %  r  0.24 %Table 2: Theoretical and measured contrasts for Aluminium strips of varying thickness0200400600800100012001400Figure 7: Images of 2 strips of Aluminium. On the left the strip is 540 Pm thick representing a contrast of 4 % and on the rightthe strip is 260 Pm thick representing a contrast of 1.9 %. Background subtracted.5. CONCLUSIONSA single Photon Counting Chip comprising 64 x 64 identical elements has been developed, characterised and bump-bonded to aGaAs detector. A minimum threshold of 2 000 e- with a variation of 125 e- rms after adjustment has been measured electricallysubsequent to bump-bonding. Objects have been imaged using radioactive sources including 109Cd with emissions at 22.1 keVand 25 keV. An object presenting a contrast of only 1.9 % has been successfully imaged.5. ACKNOWLEDGEMENTSThe readout chip was developed as part of the Medipix project, a common development between CERN, University of Freiburg,University of Glasgow and INFN-Pisa. We gratefully acknowledge the contribution of G. Humpston of GEC-Marconi MaterialsTechnology Limited, Caswell, Towcester, Northants., England for the bump-bonding.6. REFERENCES1. E. Pernigotti, “Comparison between integrating readout systems and single photon counting systems for digitalmammography”, to be published on Physica Medica, 1998.2. F. Antinori et al., “First results from the 1994 Lead Beam run of WA97”, Nucl. Phys. A590, pp 139c-146c, 1995.3. E. H. M. Heijne et al., “Development of hybrid and monolithic silicon micropattern detectors”, CERN DRDC/90-81.4. M. Campbell et al., “Readout for a 64 x 64 Matrix with 15-it Single Photon Counting” to be published in IEEE Trans. Nucl.Sci., 1998.5. E. H. M. Heijne et al., “LHC1: a semiconductor pixel detector readout chip with internal, tunable delay providing a binarypattern of selected events”, Nucl. Instr. and Meth. A383, p.52, 1996.6. S. R. Amendolia et al., “A project for digital mammography based on a GaAs pixel detector and on a self triggering singlephoton counting acquisition system”, Physica Medica, Vol. XIII n.4, pp. 157-165, Oct.-Dec. 1997.7. P. Horowitz and W. Hill, “The Art of Electronics” Cambridge University Press, p.438, 1980

Performance of 4096 pixel photon counting chip

Performance of 4096 pixel photon counting chip

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