Photon counting using amorphous selenium: Achieving hole dispersion limited count-rate using the Frisch grid detector de

1. Photon counting using amorphous selenium: Achieving hole dispersion limited count-rate using the Frisch grid detector design Karim S. Karim, A. Goldan Associate Professor, Silicon Thin film Applied Research (STAR) group, Dept. of Electrical and Computer Engineering University of Waterloo, CANADA CMOSET 2009 Workshop (Vancouver) Thank you Dr. Damon for your introduction and Good morning to the committee and everyone in the audience <first, to talk a little about the title> This research proposes unipolar charge-sensing -> which is a technique we are utilizing to enhance the performance For Evaporated solid-state detectors -> by evaporated, we are trying to emphasis a coupling technique to the readout circuit (as opposed to other detectors like CdZnTe where the crystal must be bump-bonded to the readout) And we are interested in Large Area detectors -> which is a requirement imposed by the application being digital radiographyThank you Dr. Damon for your introduction and Good morning to the committee and everyone in the audience <first, to talk a little about the title> This research proposes unipolar charge-sensing -> which is a technique we are utilizing to enhance the performance For Evaporated solid-state detectors -> by evaporated, we are trying to emphasis a coupling technique to the readout circuit (as opposed to other detectors like CdZnTe where the crystal must be bump-bonded to the readout) And we are interested in Large Area detectors -> which is a requirement imposed by the application being digital radiography

2. STAR Group Now, if we consider digital medical imaging modalities, notice that: -Computed tomography (which is inherently digital) was clinically available in 1970s -Magnetic Resonance Imaging which is also inherently digital became clinically available in 1980s -and ultrasound and nuclear medicine transitioned from analog to digital in 1970s However, even till today, radiography has not made full transition from screen film to digital imaging The reason for that is first of all, analog screen-film is a large-area detector and produces images with high spatial resolution. <FIG1> This figure shows a conventional screen film detector. Inside the cassette we have two intensifying screens which are phosphors And the film in sandwiched in-between. X-rays are absorbed by the phosphor and converted into visible or UV light, and the light emitted by the phosphor exposes the film and creates the latent image So, to replace screen-film, the digital detector must meet challenging requirements which are listed in this table. <talk about the table> <FIG2> Here we have a digital detector. This detector uses an amorphous selenium photoconductor for direct detection and the photoconductor is evaporated over the TFT readout panel. The good news is that Such digital selenium-based detectors are available commercially and the success of these detectors is due to two technological advances: 1- The availability of cost-effective large-area amorphous-silicon TFT readout panels, and 2- successful and reliable coupling of the evaporated amorphous-selenium photoconductors to these large-area readout panelsNow, if we consider digital medical imaging modalities, notice that: -Computed tomography (which is inherently digital) was clinically available in 1970s -Magnetic Resonance Imaging which is also inherently digital became clinically available in 1980s -and ultrasound and nuclear medicine transitioned from analog to digital in 1970s However, even till today, radiography has not made full transition from screen film to digital imaging The reason for that is first of all, analog screen-film is a large-area detector and produces images with high spatial resolution. <FIG1> This figure shows a conventional screen film detector. Inside the cassette we have two intensifying screens which are phosphors And the film in sandwiched in-between. X-rays are absorbed by the phosphor and converted into visible or UV light, and the light emitted by the phosphor exposes the film and creates the latent image So, to replace screen-film, the digital detector must meet challenging requirements which are listed in this table. <talk about the table> <FIG2> Here we have a digital detector. This detector uses an amorphous selenium photoconductor for direct detection and the photoconductor is evaporated over the TFT readout panel. The good news is that Such digital selenium-based detectors are available commercially and the success of these detectors is due to two technological advances: 1- The availability of cost-effective large-area amorphous-silicon TFT readout panels, and 2- successful and reliable coupling of the evaporated amorphous-selenium photoconductors to these large-area readout panels

3. Digital Medical Imaging 3 Now, if we consider digital medical imaging modalities, notice that: -Computed tomography (which is inherently digital) was clinically available in 1970s -Magnetic Resonance Imaging which is also inherently digital became clinically available in 1980s -and ultrasound and nuclear medicine transitioned from analog to digital in 1970s However, even till today, radiography has not made full transition from screen film to digital imaging The reason for that is first of all, analog screen-film is a large-area detector and produces images with high spatial resolution. <FIG1> This figure shows a conventional screen film detector. Inside the cassette we have two intensifying screens which are phosphors And the film in sandwiched in-between. X-rays are absorbed by the phosphor and converted into visible or UV light, and the light emitted by the phosphor exposes the film and creates the latent image So, to replace screen-film, the digital detector must meet challenging requirements which are listed in this table. <talk about the table> <FIG2> Here we have a digital detector. This detector uses an amorphous selenium photoconductor for direct detection and the photoconductor is evaporated over the TFT readout panel. The good news is that Such digital selenium-based detectors are available commercially and the success of these detectors is due to two technological advances: 1- The availability of cost-effective large-area amorphous-silicon TFT readout panels, and 2- successful and reliable coupling of the evaporated amorphous-selenium photoconductors to these large-area readout panelsNow, if we consider digital medical imaging modalities, notice that: -Computed tomography (which is inherently digital) was clinically available in 1970s -Magnetic Resonance Imaging which is also inherently digital became clinically available in 1980s -and ultrasound and nuclear medicine transitioned from analog to digital in 1970s However, even till today, radiography has not made full transition from screen film to digital imaging The reason for that is first of all, analog screen-film is a large-area detector and produces images with high spatial resolution. <FIG1> This figure shows a conventional screen film detector. Inside the cassette we have two intensifying screens which are phosphors And the film in sandwiched in-between. X-rays are absorbed by the phosphor and converted into visible or UV light, and the light emitted by the phosphor exposes the film and creates the latent image So, to replace screen-film, the digital detector must meet challenging requirements which are listed in this table. <talk about the table> <FIG2> Here we have a digital detector. This detector uses an amorphous selenium photoconductor for direct detection and the photoconductor is evaporated over the TFT readout panel. The good news is that Such digital selenium-based detectors are available commercially and the success of these detectors is due to two technological advances: 1- The availability of cost-effective large-area amorphous-silicon TFT readout panels, and 2- successful and reliable coupling of the evaporated amorphous-selenium photoconductors to these large-area readout panels

4. Energy Integrating vs. Photon Counting 4

5. Making a case for photon counting: Dose Efficiency Imager’s “Dose Efficiency” considers the following factors: Quality of the x-ray spectrum Quantum efficiency Energy absorption efficiency Scatter rejection efficiency Conversion efficiency (of absorbed photons to image signal) Detector noise(i.e., swank noise, film granularity noise, leakage shot noise, variations in the conversion gain) Readout noise 5 The overall performance of an x-ray imaging system is characterized by its dose efficiency And dose efficiency considers the following factors: <Read and elaborate on the factors> This figure shows the dose efficiency of a commercial mammography detector which is: Energy integrating meaning that the detector has a pre-defined time interval called the “integration time” during which x-rays are absorbed and the charge induced is integrated on the readout pixel. The imager is Area-imaging in that the detector is large-area and it’s not scanned to produce the image The detector uses a Smit-Rontgen antiscatter grid The detective quantum efficiency is 35% And the x-ray spectrum is 30kVp with a Molybdenum target For such a system, you see that the dose efficiency for an average patient breast size of 4.5cm is around 10%, which is very low. Now the question is: <> The overall performance of an x-ray imaging system is characterized by its dose efficiency And dose efficiency considers the following factors: <Read and elaborate on the factors> This figure shows the dose efficiency of a commercial mammography detector which is: Energy integrating meaning that the detector has a pre-defined time interval called the “integration time” during which x-rays are absorbed and the charge induced is integrated on the readout pixel. The imager is Area-imaging in that the detector is large-area and it’s not scanned to produce the image The detector uses a Smit-Rontgen antiscatter grid The detective quantum efficiency is 35% And the x-ray spectrum is 30kVp with a Molybdenum target For such a system, you see that the dose efficiency for an average patient breast size of 4.5cm is around 10%, which is very low. Now the question is: <>

6. Scatter, Noise, & Conversion Efficiency 6

7. Memory Artifacts & Anatomical Noise 7 This figure shows memory artifacts in a Selenium detector. Figure a is the radiograph of an X-shaped lead object. You can see the presence of the X-shaped image in the following radiographs taken from a chest phantom with 1 minute intervals. Of course, the memory artifact is less prominent in the last radiograph because of the recovery of the Selenium receptor (basically memory artifacts relax with time). Note that photon counting systems are not susceptible to memory artifacts. Basically, photon counting systems are immune to low-frequency effects, such as memory artifacts.This figure shows memory artifacts in a Selenium detector. Figure a is the radiograph of an X-shaped lead object. You can see the presence of the X-shaped image in the following radiographs taken from a chest phantom with 1 minute intervals. Of course, the memory artifact is less prominent in the last radiograph because of the recovery of the Selenium receptor (basically memory artifacts relax with time). Note that photon counting systems are not susceptible to memory artifacts. Basically, photon counting systems are immune to low-frequency effects, such as memory artifacts.

8. Photon Counting w/ a-Se 8 Here you see a 70keV spectrum obtained from a 150um thick a-Se at 7V/um. I would like to identify three challenges that we face when operating a-Se in pulse mode. As I mentioned before, the conversion gain is low. How do we improve that? We need to operated the detector at a higher bias, as shown in this figure. 2) The spectrum width is very wide. We can’t definitively say what’s causing such a wide spectrum, even with a fano-factor of 1 But we can speculate that it’s due to variations in the geminate and columnar recombination processes which are Still not fully understood. But we are not trying to do spectroscopy with a-Se, so a single energy bin would be sufficient. 3) The biggest challenge, however, is the low count rate of a-Se. For example, in our measurements, to guarantee… The solution that we propose is: unipolar charge sensing using the Frisch grid detector design Here you see a 70keV spectrum obtained from a 150um thick a-Se at 7V/um. I would like to identify three challenges that we face when operating a-Se in pulse mode. As I mentioned before, the conversion gain is low. How do we improve that? We need to operated the detector at a higher bias, as shown in this figure. 2) The spectrum width is very wide. We can’t definitively say what’s causing such a wide spectrum, even with a fano-factor of 1 But we can speculate that it’s due to variations in the geminate and columnar recombination processes which are Still not fully understood. But we are not trying to do spectroscopy with a-Se, so a single energy bin would be sufficient. 3) The biggest challenge, however, is the low count rate of a-Se. For example, in our measurements, to guarantee… The solution that we propose is: unipolar charge sensing using the Frisch grid detector design

9. Inequality in Charge Transport The material of large-area photoconductors is either amorphous or polycrystalline Suffer from poor charge transport due to traps in the bulk For example, the effective carrier mobilities in a-Se is: 0.003 cm2/V.s for e- 0.14 cm2/V.s for h+ 9

10. Improvements Addressing the problem of carrier transport Utilizing readout techniques: Pulse-shape discrimination Pulse-risetime compensation/correction Modifying the detector structure: P-type-Intrinsic-N-type (P-I-N) contact structure Unipolar Charge-Sensing 10

11. Shockley-Ramo Theorem 11 Shockley-Ramo Theory (1938-1939): Charge induction on any electrode by a single electron in a vacuum tube is due to “electron’s motion”! Qi = qVW VW = Weighting potential Theory is valid for carrier motion inside semiconductor detectors in the presence of space charge. G. Cavalleri et al, Nucl. Instr. and Meth. 92, 137-140 (1971).

12. Unipolar Charge-Sensing A method first proposed by O. Frisch in 1944 to solve the problem of slow drift and trapping effect of positive ions in conventional gas detectors How? By providing an electrostatic shield near the collecting electrode. 12

13. Kinestatic Imagers 13

14. Unipolar Solid-State Area Imagers Considering large-area evaporated photoconductors (such as amorphous selenium), this research proposes unipolar charge-sensing using the Frisch grid detector design to: Enable photon counting operation by improving the photon count-rate Results can be applied to also improve charge collection efficiency, implications of which in energy-integrating mode are: Improved x-ray sensitivity and detective quantum efficiency (DQE) Improved spatial resolution Reduced memory effects 14

15. 15 Photon Counting ModeImproved count-rate To see how much of an improvement in count-rate we can obtain, let’s consider the following hypothetical cases: <Explain the plots> The question that we should ask is: Is it really possible to improve the count rate by more 4 orders of magnitude? And to answer this question, we first need to know how close, practically, we can place the Frisch grids to the collecting pixel electrode. To see how much of an improvement in count-rate we can obtain, let’s consider the following hypothetical cases: <Explain the plots> The question that we should ask is: Is it really possible to improve the count rate by more 4 orders of magnitude? And to answer this question, we first need to know how close, practically, we can place the Frisch grids to the collecting pixel electrode.

16. Food (for thought) Realistically speaking, can the unipolar Frisch grid design improve the count-rate by ~4 orders of magnitude? Can we implement the Frisch grids very close to the collecting pixel electrodes to yield a nearly ideal unipolar charge-sensing operation? Is there a physical phenomenon that limits the count-rate given a nearly-ideal unipolar device? 16

17. Frisch Grid Fabrication 17 Here is a design that we’ve been working on, and the idea is actually very simple: Since the a-Se is evaporated, we can use photolithography on the substrate and mount the Frisch grid on an insulator Here is a design that we’ve been working on, and the idea is actually very simple: Since the a-Se is evaporated, we can use photolithography on the substrate and mount the Frisch grid on an insulator

18. Weighting Potential Distribution 18 So, by lithography, we can mount the Frisch grid very close to the pixel And here we’ve done a simulation of the weighting potential distribution for a grid width of 5um, and 5um grid spacing, And the grids are deposited over 500nm of Silicon Nitride You can see that the weighting potential distribution shows a very sharp rise from the grid to the pixel and Signal risetime is reduced substantially. So this result is very promising But going back to the question that would it be possible to achieve photon count-rate of as high as 10M counts/s Takes us to the second thing we need to consider, now that we can place the grids very close to the pixel, what can fundamentally limit the count-rate given such weighting potential distribution? So that’s why we carried out a series of transient photoconductivity measurements using the time-of-flight techniques So, by lithography, we can mount the Frisch grid very close to the pixel And here we’ve done a simulation of the weighting potential distribution for a grid width of 5um, and 5um grid spacing, And the grids are deposited over 500nm of Silicon Nitride You can see that the weighting potential distribution shows a very sharp rise from the grid to the pixel and Signal risetime is reduced substantially. So this result is very promising But going back to the question that would it be possible to achieve photon count-rate of as high as 10M counts/s Takes us to the second thing we need to consider, now that we can place the grids very close to the pixel, what can fundamentally limit the count-rate given such weighting potential distribution? So that’s why we carried out a series of transient photoconductivity measurements using the time-of-flight techniques

19. Food for thought (again) Realistically speaking, can the unipolar Frisch grid design improve the count-rate by ~4 orders of magnitude? Can we implement the Frisch grids very close to the collecting pixel electrodes to yield a nearly ideal unipolar charge-sensing operation? YES! Thanks to evaporated photoconductors which enable grid construction using photolithography. Is there a physical phenomenon that limits the count-rate given a nearly-ideal unipolar device? 19

20. Time-of-Flight (TOF) TOF transient photoconductivity technique: Directly measure carrier drift mobility and deep trapping time 20

21. TOF Measurements 21 The extended decay in the tail is due to the dispersion of holes in the drifting packet Dispersion is due to: diffusion multiple trapping and release Mutual Coulombic repulsion

22. Food for thought (conclusion) Realistically speaking, can the unipolar Frisch grid design improve the count-rate by ~4 orders of magnitude? Can we implement the Frisch grids very close to the collecting pixel electrodes to yield a nearly ideal unipolar charge-sensing operation? YES! Thanks to evaporated photoconductors which enable grid construction using photolithography. Is there a physical phenomenon that limits the count-rate given a nearly-ideal unipolar device? YES! Carrier dispersion limits the count-rate. However, a-Se is till capable of achieving a count-rate of ~1M [quanta/s] at E=10V/µm. 22

23. Summary Many of the limitations associated with today’s energy-integrating imagers can be alleviated by switching to photon-counting. However, poor carrier transport in large-area evaporated photoconductors has greatly limited their photon count rate. To circumvent the problem of poor carrier transport, this research proposes unipolar charge-sensing using the Frisch grid design. Results show substantial increase in photon count rate using the unipolar Frisch detectors 23

24. 24 Acknowledgements