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320×240 GaAs pixel detectors with improved X-ray imaging quality
Nuclear Instruments and Methods in Physics Research A 460 (2001) 67–71
320 240 GaAs pixel detectors with improved X-ray imaging quality R. Irsigler*,a, J. Anderssona, J. Alverbroa, Z. Fakoor-Biniaza, C. Fro¨jdhb, P. Helandera, H. Martijna, D. Meiklec, M. O¨stlunda, V. O’Sheac, K. Smithc a ACREO AB, Electrum 233, SE-16440 Kista, Sweden REGAM Medical Systems Int. AB, c/o Mittho¨gskolan, Holmgatan 10, SE-85170 Sundsvall, Sweden c Department of Physics and Astronomy, Kelvin Building, University Avenue, Glasgow G12 8QQ, UK
b
Abstract We report on gain and offset corrections for GaAs X-ray pixel detectors, which were hybridised to silicon CMOS readout integrated circuits (ROICs). The whole detector array contains 320 240 square-shaped pixels with a pitch of 38 mm. The GaAs pixel detectors are based on semi-insulating and VPE grown substrates. The ROIC operates in the charge integration mode and provides snapshot as well as real time video images. Previously we have reported that the image quality of semi-insulating GaAs pixel detectors suffer from local variations in X-ray sensitivity. We have now developed a method to compensate for the sensitivity variations by applying suitable offset and gain corrections. The improvement in image quality is demonstrated in the measured signal-to-noise ratio of flood exposure images. # 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The hybrid semiconductor pixel detector is one of the digital detector concepts, which is used to replace traditional film in X-ray imaging. Digitisation provides easy storage, transmission and duplication of images without any loss of image quality. Computer capabilities such as faster processors, increased memory and better monitors facilitate image-processing techniques. More importantly, using digital equipment rather than traditional equipment, the required X-ray dose can often be reduced. In comparison with other digital X-ray detector concepts such as scintillator-covered Si-CCDs or active pixel CMOS sensors, hybrid detectors offer *Corresponding author.
the advantage of using detector materials with higher absorption for X-rays. One of the promising materials is GaAs, which has about an order of magnitude higher absorption for X-rays than Si for energies in the range 20–60 keV. Moreover these detectors can be operated at room temperature, the process technology is mature and good quality, large-area substrates are readily available. We have reported previously that the image quality of semi-insulating GaAs pixel detectors suffers from local variations in X-ray sensitivity across the detector area [1]. The observed nonuniform sensitivity is probably caused by local variations in the charge collection efficiency (CCE). In semi-insulating LEC-GaAs, the deep donor arsenic antisite defect EL2 compensates residual impurities, which have mainly a flat energy level. This compensation mechanism is
0169-8141/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 0 9 7 - 4
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R. Irsigler et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 67–71
responsible for the appearance of a semi-insulating behaviour even at unintentional doping concentrations (mainly carbon, acceptor) of the order of 1016 cm3, which would otherwise leave the material conducting [2]. It is also well known that these deep donors act as trapping centres for the charge carriers and limit their lifetime [3]. X-ray-generated electron hole pairs can be trapped and hindered from drifting to the readout electrodes of the detector array. Hence only a fraction of the generated charge is detected and the CCE appears to be less than unity. Local variation of sensitivity across the detector area decreases the signal-to noise-ratio (SNR) and image quality substantially. We were able to compensate the observed sensitivity variations by applying suitable offset and gain corrections. The method is described in the following subsection
Fig. 1. Calculated gain map of a GaAs pixel detector. Dark spots belong to low gain map values and therefore high sensitivity.
2. Gain and offset correction The image correction is done pixel by pixel according to imgcorr ¼ gmapðimgorig imgdark Þ
ð1Þ
where imgorig and imgcorr are the pixel values of the images before and after image correction. The value imgdark represents the dark current of the pixel without X-ray exposure. The value gmap is the corresponding element of a gain map matrix that takes care of the non-uniform sensitivity. It can be calculated in various ways. Flood exposure images at two different exposure doses or at two different integration times (img1 and img2) can be used for the calculations. Each element of the gain map matrix is simply given by: imginv ð2Þ gmap ¼ M where 1 imginv ¼ ð3Þ imgdiff
Fig. 2. Histogram of the gain map of a semi-insulating GaAs pixel detector.
calculated from a flood exposure image and a dark current image. Dark spots belong to areas of high sensitivity. Fig. 2 shows the corresponding histogram of the calculated gain map. The distribution of the gain map values has a mean value around 1.0 with a sigma of 0.16.
and imgdiff ¼ img1 img2
ð4Þ
The scalar value M is the mean value of the matrix imginv. Fig. 1 shows a gain map that was
3. Images The result of applying the image correction method described above is demonstrated in Fig. 3
R. Irsigler et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 67–71
and 4. Fig. 3 shows a snapshot image of a halogen light bulb obtained using a 150 mm thick, semiinsulating GaAs pixel detector at a detector bias voltage of 60 V. The X-ray tube was operated at 60 kVp. The image is only of poor quality without sharp contrast. The same image after applying the image correction of Eq. (1) is shown in Fig. 4. The image has a much better contrast and appears less noisy. The improvement in image quality is also observed in the SNR of flood exposure images.
Fig. 3. X-ray image of a halogen light bulb using a hybridized GaAs pixel detector. Image quality is poor due to local variations in the sensitivity of the detector.
Fig. 4. X-ray image of a halogen light bulb after image correction.
69
4. Signal-to-noise ratio As the fluctuations in the detection of X-ray photons are described by Poisson statistics, the SNR of a quantum-limited detector should have a square root dependence on the incident exposure. In order to measure the SNR as a function of exposure dose (and dose rate), flood exposure images were acquired and corrected in the manner described above. For each data point, 5 flood exposure images and one dark current image were stored on the hard disk of the readout system. The average of three flood exposures and the dark current image were used to generate the gain and offset maps. The remaining two flood exposures were averaged, gain and offset compensated and finally used for the evaluation of the SNR. The SNR was calculated from the mean signal height and standard deviation of the whole array. The X-ray tube setup was calibrated by using a Si photodiode based diagnostic dosimeter1. The measured exposure dose rate was in good agreement with the specification of the supplier of the X-ray tube2 (4.75 mGy/s at a distance of 25 cm and at an anode voltage of 70 kV). Taking into account the integration time of the ROIC, the total exposure dose to the detector can be easily calculated. Fig. 5 shows the mean signal as a function of dose, acquired at a fixed distance of 44 cm from the tube. The exposure dose was varied by changing the integration time between 6.6 and 38 ms. The mean signal increases linearly with the dose. At a bias voltage of 60 V the detector shows a sensitivity of 67 digital units (DU) mGy. For low exposure doses, the noise floor of the hybridised sensor saturates at approximately 75 DU for an integration time of 31.5 ms. The correction process that removes fixed pattern noise associated with local dark current variations and sensitivity non-uniformities among the pixels improves the SNR from an initial value of 8 to a value of 38 after the correction. As shown in Fig. 6, the SNR increases linearly with the square root of the exposure dose. However 1 2
Wellho¨fer WD10 with H/DN-2X detector. Planmeca Prostyle Intra.
70
R. Irsigler et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 67–71
Fig. 5. Measured signal height in digital units (DU) vs. exposure dose.
Fig. 7. SNR drops for high exposure dose rates because of time-dependent sensitivity fluctuations, which cannot be compensated by the static gain map.
varied within a range of 28–54 cm which corresponds to an exposure dose rate of 3.1 and 1.0 mGy/s, respectively. Fig. 7 shows an increase of the SNR from 31 at 54 cm to a value of 39 at a distance of 36 cm. For smaller distances (high dose rate) the SNR drops very sharply. At high dose rates timedependent fluctuations in the X-ray response become visible and, as a consequence, the correction method becomes less effective because only static sensitivity variations are compensated.
Fig. 6. SNR increases with the square root of the dose but is lower than the photonic noise limited SNR.
the values are lower than expected if the noise in the X-ray sensor is dominated by statistical fluctuations of the integrated charge. This indicates that the noise is not dominated by the quantum statistics but has some additional sources in the detector or read-out system. Although the measured SNR is already quite high further improvements can be expected by lowering the system noise. In order to investigate the influence of the exposure dose rate on the SNR, the distance between the detector and the X-ray tube was
5. VPE GaAs detectors GaAs pixel detectors based on vapour-phase epitaxial (VPE) material were fabricated and thinned down to a total thickness of 170 mm. The 50 mm thick epi layer was grown on a highly doped, n-type substrate. C–V measurements on test diodes showed an unintentional background doping concentration below 1.5 1014 cm3. An abrupt breakdown occurred at around 40 V bias. The VPE GaAs detectors show no time dependent fluctuation in X-ray response even at high radiation dose (small distance to the tube) as previously observed for the semi-insulating pixel detectors. Moreover, the local dark current
R. Irsigler et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 67–71
Fig. 8. X-ray image of a 4.7 mF capacitor after gain and offset correction by using a VPE GaAs detector. Exposure data: 60 kVp, 31.5 ms integration time, 38 V bias voltage.
variations are weaker and the gain map is more homogeneous (standard deviation below 0.1). Fig. 8 shows an X-ray image of a 4.7 mF capacitor obtained using a VPE GaAs pixel detector. Different layers of the metal foil inside the housing are clearly visible.
6. Conclusions Liquid-Encapsulated (LEC) substrate GaAs pixel detectors show local variations in dark
71
current and X-ray sensitivity across the active area. As long as these local variations are not time dependent, significant improvements in image quality and SNR can be achieved by applying suitable gain and offset corrections. After image correction, the SNR approaches values around 40 at a X-ray exposure dose of 50 mGy. Semi-insulating GaAs pixel detectors show time dependent fluctuations in X-ray sensitivity at high exposure dose rates. The compensation method becomes less effective in this case and the SNR drops. The investigated VPE GaAs pixel detectors have better uniformity in the dark current and gain map distribution. Even at high exposure dose rates no time dependent fluctuations were observed.
References [1] R. Irsigler, J. Andersson, J. Alverbro, J. Borglind, C. Fro¨jdh, P. Helander, S. Manolopoulos, V. O’Shea, K. Smith, Nucl. Instr. and Meth. A 434 (1999) 24. [2] M.J. Howes, D.V. Morgan, Gallium Arsenide, Wiley, New York, 1985. [3] M. Rogalla, Th. Eich, N. Evans, R. Geppert, R. Go¨ppert, R. Irsigler, J. Ludwig, K. Runge, Th. Schmid, D. Marder, Nucl. Instr. and Meth. A 395 (1997) 49 EQ.
320 240 GaAs pixel detectors with improved X-ray imaging quality R. Irsigler*,a, J. Anderssona, J. Alverbroa, Z. Fakoor-Biniaza, C. Fro¨jdhb, P. Helandera, H. Martijna, D. Meiklec, M. O¨stlunda, V. O’Sheac, K. Smithc a ACREO AB, Electrum 233, SE-16440 Kista, Sweden REGAM Medical Systems Int. AB, c/o Mittho¨gskolan, Holmgatan 10, SE-85170 Sundsvall, Sweden c Department of Physics and Astronomy, Kelvin Building, University Avenue, Glasgow G12 8QQ, UK
b
Abstract We report on gain and offset corrections for GaAs X-ray pixel detectors, which were hybridised to silicon CMOS readout integrated circuits (ROICs). The whole detector array contains 320 240 square-shaped pixels with a pitch of 38 mm. The GaAs pixel detectors are based on semi-insulating and VPE grown substrates. The ROIC operates in the charge integration mode and provides snapshot as well as real time video images. Previously we have reported that the image quality of semi-insulating GaAs pixel detectors suffer from local variations in X-ray sensitivity. We have now developed a method to compensate for the sensitivity variations by applying suitable offset and gain corrections. The improvement in image quality is demonstrated in the measured signal-to-noise ratio of flood exposure images. # 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The hybrid semiconductor pixel detector is one of the digital detector concepts, which is used to replace traditional film in X-ray imaging. Digitisation provides easy storage, transmission and duplication of images without any loss of image quality. Computer capabilities such as faster processors, increased memory and better monitors facilitate image-processing techniques. More importantly, using digital equipment rather than traditional equipment, the required X-ray dose can often be reduced. In comparison with other digital X-ray detector concepts such as scintillator-covered Si-CCDs or active pixel CMOS sensors, hybrid detectors offer *Corresponding author.
the advantage of using detector materials with higher absorption for X-rays. One of the promising materials is GaAs, which has about an order of magnitude higher absorption for X-rays than Si for energies in the range 20–60 keV. Moreover these detectors can be operated at room temperature, the process technology is mature and good quality, large-area substrates are readily available. We have reported previously that the image quality of semi-insulating GaAs pixel detectors suffers from local variations in X-ray sensitivity across the detector area [1]. The observed nonuniform sensitivity is probably caused by local variations in the charge collection efficiency (CCE). In semi-insulating LEC-GaAs, the deep donor arsenic antisite defect EL2 compensates residual impurities, which have mainly a flat energy level. This compensation mechanism is
0169-8141/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 0 9 7 - 4
68
R. Irsigler et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 67–71
responsible for the appearance of a semi-insulating behaviour even at unintentional doping concentrations (mainly carbon, acceptor) of the order of 1016 cm3, which would otherwise leave the material conducting [2]. It is also well known that these deep donors act as trapping centres for the charge carriers and limit their lifetime [3]. X-ray-generated electron hole pairs can be trapped and hindered from drifting to the readout electrodes of the detector array. Hence only a fraction of the generated charge is detected and the CCE appears to be less than unity. Local variation of sensitivity across the detector area decreases the signal-to noise-ratio (SNR) and image quality substantially. We were able to compensate the observed sensitivity variations by applying suitable offset and gain corrections. The method is described in the following subsection
Fig. 1. Calculated gain map of a GaAs pixel detector. Dark spots belong to low gain map values and therefore high sensitivity.
2. Gain and offset correction The image correction is done pixel by pixel according to imgcorr ¼ gmapðimgorig imgdark Þ
ð1Þ
where imgorig and imgcorr are the pixel values of the images before and after image correction. The value imgdark represents the dark current of the pixel without X-ray exposure. The value gmap is the corresponding element of a gain map matrix that takes care of the non-uniform sensitivity. It can be calculated in various ways. Flood exposure images at two different exposure doses or at two different integration times (img1 and img2) can be used for the calculations. Each element of the gain map matrix is simply given by: imginv ð2Þ gmap ¼ M where 1 imginv ¼ ð3Þ imgdiff
Fig. 2. Histogram of the gain map of a semi-insulating GaAs pixel detector.
calculated from a flood exposure image and a dark current image. Dark spots belong to areas of high sensitivity. Fig. 2 shows the corresponding histogram of the calculated gain map. The distribution of the gain map values has a mean value around 1.0 with a sigma of 0.16.
and imgdiff ¼ img1 img2
ð4Þ
The scalar value M is the mean value of the matrix imginv. Fig. 1 shows a gain map that was
3. Images The result of applying the image correction method described above is demonstrated in Fig. 3
R. Irsigler et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 67–71
and 4. Fig. 3 shows a snapshot image of a halogen light bulb obtained using a 150 mm thick, semiinsulating GaAs pixel detector at a detector bias voltage of 60 V. The X-ray tube was operated at 60 kVp. The image is only of poor quality without sharp contrast. The same image after applying the image correction of Eq. (1) is shown in Fig. 4. The image has a much better contrast and appears less noisy. The improvement in image quality is also observed in the SNR of flood exposure images.
Fig. 3. X-ray image of a halogen light bulb using a hybridized GaAs pixel detector. Image quality is poor due to local variations in the sensitivity of the detector.
Fig. 4. X-ray image of a halogen light bulb after image correction.
69
4. Signal-to-noise ratio As the fluctuations in the detection of X-ray photons are described by Poisson statistics, the SNR of a quantum-limited detector should have a square root dependence on the incident exposure. In order to measure the SNR as a function of exposure dose (and dose rate), flood exposure images were acquired and corrected in the manner described above. For each data point, 5 flood exposure images and one dark current image were stored on the hard disk of the readout system. The average of three flood exposures and the dark current image were used to generate the gain and offset maps. The remaining two flood exposures were averaged, gain and offset compensated and finally used for the evaluation of the SNR. The SNR was calculated from the mean signal height and standard deviation of the whole array. The X-ray tube setup was calibrated by using a Si photodiode based diagnostic dosimeter1. The measured exposure dose rate was in good agreement with the specification of the supplier of the X-ray tube2 (4.75 mGy/s at a distance of 25 cm and at an anode voltage of 70 kV). Taking into account the integration time of the ROIC, the total exposure dose to the detector can be easily calculated. Fig. 5 shows the mean signal as a function of dose, acquired at a fixed distance of 44 cm from the tube. The exposure dose was varied by changing the integration time between 6.6 and 38 ms. The mean signal increases linearly with the dose. At a bias voltage of 60 V the detector shows a sensitivity of 67 digital units (DU) mGy. For low exposure doses, the noise floor of the hybridised sensor saturates at approximately 75 DU for an integration time of 31.5 ms. The correction process that removes fixed pattern noise associated with local dark current variations and sensitivity non-uniformities among the pixels improves the SNR from an initial value of 8 to a value of 38 after the correction. As shown in Fig. 6, the SNR increases linearly with the square root of the exposure dose. However 1 2
Wellho¨fer WD10 with H/DN-2X detector. Planmeca Prostyle Intra.
70
R. Irsigler et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 67–71
Fig. 5. Measured signal height in digital units (DU) vs. exposure dose.
Fig. 7. SNR drops for high exposure dose rates because of time-dependent sensitivity fluctuations, which cannot be compensated by the static gain map.
varied within a range of 28–54 cm which corresponds to an exposure dose rate of 3.1 and 1.0 mGy/s, respectively. Fig. 7 shows an increase of the SNR from 31 at 54 cm to a value of 39 at a distance of 36 cm. For smaller distances (high dose rate) the SNR drops very sharply. At high dose rates timedependent fluctuations in the X-ray response become visible and, as a consequence, the correction method becomes less effective because only static sensitivity variations are compensated.
Fig. 6. SNR increases with the square root of the dose but is lower than the photonic noise limited SNR.
the values are lower than expected if the noise in the X-ray sensor is dominated by statistical fluctuations of the integrated charge. This indicates that the noise is not dominated by the quantum statistics but has some additional sources in the detector or read-out system. Although the measured SNR is already quite high further improvements can be expected by lowering the system noise. In order to investigate the influence of the exposure dose rate on the SNR, the distance between the detector and the X-ray tube was
5. VPE GaAs detectors GaAs pixel detectors based on vapour-phase epitaxial (VPE) material were fabricated and thinned down to a total thickness of 170 mm. The 50 mm thick epi layer was grown on a highly doped, n-type substrate. C–V measurements on test diodes showed an unintentional background doping concentration below 1.5 1014 cm3. An abrupt breakdown occurred at around 40 V bias. The VPE GaAs detectors show no time dependent fluctuation in X-ray response even at high radiation dose (small distance to the tube) as previously observed for the semi-insulating pixel detectors. Moreover, the local dark current
R. Irsigler et al. / Nuclear Instruments and Methods in Physics Research A 460 (2001) 67–71
Fig. 8. X-ray image of a 4.7 mF capacitor after gain and offset correction by using a VPE GaAs detector. Exposure data: 60 kVp, 31.5 ms integration time, 38 V bias voltage.
variations are weaker and the gain map is more homogeneous (standard deviation below 0.1). Fig. 8 shows an X-ray image of a 4.7 mF capacitor obtained using a VPE GaAs pixel detector. Different layers of the metal foil inside the housing are clearly visible.
6. Conclusions Liquid-Encapsulated (LEC) substrate GaAs pixel detectors show local variations in dark
71
current and X-ray sensitivity across the active area. As long as these local variations are not time dependent, significant improvements in image quality and SNR can be achieved by applying suitable gain and offset corrections. After image correction, the SNR approaches values around 40 at a X-ray exposure dose of 50 mGy. Semi-insulating GaAs pixel detectors show time dependent fluctuations in X-ray sensitivity at high exposure dose rates. The compensation method becomes less effective in this case and the SNR drops. The investigated VPE GaAs pixel detectors have better uniformity in the dark current and gain map distribution. Even at high exposure dose rates no time dependent fluctuations were observed.
References [1] R. Irsigler, J. Andersson, J. Alverbro, J. Borglind, C. Fro¨jdh, P. Helander, S. Manolopoulos, V. O’Shea, K. Smith, Nucl. Instr. and Meth. A 434 (1999) 24. [2] M.J. Howes, D.V. Morgan, Gallium Arsenide, Wiley, New York, 1985. [3] M. Rogalla, Th. Eich, N. Evans, R. Geppert, R. Go¨ppert, R. Irsigler, J. Ludwig, K. Runge, Th. Schmid, D. Marder, Nucl. Instr. and Meth. A 395 (1997) 49 EQ.