Patent Publication Number: US-11380812-B2

Title: Methods of making semiconductor radiation detector

Description:
TECHNICAL FIELD 
     The disclosure herein relates to radiation detectors, particularly relates to methods of making semiconductor radiation detectors. 
     BACKGROUND 
     A radiation detector is a device that measures a property of a radiation. Examples of the property may include a spatial distribution of the intensity, phase, and polarization of the radiation. The radiation may be one that has interacted with a subject. For example, the radiation measured by the radiation detector may be a radiation that has penetrated or reflected from the subject. The radiation may be an electromagnetic radiation such as infrared light, visible light, ultraviolet light, X-ray or y-ray. The radiation may be of other types such as α-rays and β-rays. 
     One type of radiation detectors is based on interaction between the radiation and a semiconductor. For example, a radiation detector of this type may have a semiconductor layer that absorbs the radiation and generate charge carriers (e.g., electrons and holes) and circuitry for detecting the charge carriers. 
     SUMMARY 
     Disclosed herein is a method of making an apparatus suitable for detecting radiation, the method may comprises: obtaining a plurality of semiconductor single crystal chunks each having a first surface and a second surface, the second surface being opposite to the first surface; bonding the plurality of semiconductor single crystal chunks by respective first surfaces to a first semiconductor wafer, the plurality of semiconductor single crystal chunks forming a radiation absorption layer; forming a plurality of electrodes on respective second surfaces of each of the plurality of semiconductor single crystal chunks; depositing pillars on each of the plurality of semiconductor single crystal chunks; and bonding the plurality of semiconductor single crystal chunks to a second semiconductor wafer by the pillars. 
     According to an embodiment, the plurality of semiconductor single crystal chunks are cadmium zinc telluride (CdZnTe) chunks. 
     According to an embodiment, the plurality of semiconductor single crystal chunks are bonded to the first semiconductor wafer by glue or plastic molding. 
     According to an embodiment, the first semiconductor wafer is conductive and serve as a common electrode for the plurality of semiconductor single crystals chunks. 
     According to an embodiment, the plurality of electrodes on the plurality of semiconductor single crystal chunks are formed by semiconductor wafer processes. 
     According to an embodiment, the pillars are conductive pillar bumps. 
     According to an embodiment, the pillars are deposited using semiconductor wafer processes. 
     According to an embodiment, the method may further comprises polishing the second surfaces of the plurality of semiconductor single crystal chunks so that the plurality of semiconductor single crystal chunks are of the same thickness. 
     According to an embodiment, the first semiconductor wafer forms a common electrode for the plurality of semiconductor single crystal chunks. 
     According to an embodiment, the plurality of semiconductor single crystal chunks form resistors between the common electrode at the first surfaces and the plurality of electrodes on the second surfaces. 
     According to an embodiment, the radiation absorption layer is configured to detect one of electromagnetic radiation including ultraviolet (UV), X-ray, gamma ray. 
     According to an embodiment, the radiation absorption layer is configured to detect one of particle radiation including alpha particles, beta particles and neutron particles. 
     According to an embodiment, bonding of the plurality of semiconductor single crystal chunks to the second semiconductor wafer is performed by wafer level room temperature bonding. 
     According to an embodiment, the second semiconductor wafer comprises an electronics layer for processing signals generated in the radiation absorption layer. 
     According to an embodiment, the electronics layer comprises an electronics system connected to one of the plurality of electrodes of the plurality of semiconductor single crystal chunks, the electronics system comprises: a first voltage comparator configured to compare a voltage of at least one of the electrodes to a first threshold; a second voltage comparator configured to compare the voltage to a second threshold; a counter configured to register a number of radiation photons or particles reaching the radiation absorption layer; a controller; wherein the controller is configured to start a time delay from a time at which the first voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the first threshold; wherein the controller is configured to activate the second voltage comparator during the time delay; wherein the controller is configured to cause the number registered by the counter to increase by one, if the second voltage comparator determines that an absolute value of the voltage equals or exceeds an absolute value of the second threshold. 
     According to an embodiment, the electronics system further comprises a capacitor module electrically connected to the electrode, and the capacitor module is configured to collect charge carriers from the electrode. 
     According to an embodiment, the controller is configured to activate the second voltage comparator at a beginning or expiration of the time delay. 
     According to an embodiment, the electronics system further comprises a voltmeter, wherein the controller is configured to cause the voltmeter to measure the voltage upon expiration of the time delay. 
     According to an embodiment, the controller is configured to determine a radiation particle energy based on a value of the voltage measured upon expiration of the time delay. 
     According to an embodiment, the controller is configured to connect the electrode to an electrical ground. 
     According to an embodiment, a rate of change of the voltage is substantially zero at expiration of the time delay. 
     According to an embodiment, a rate of change of the voltage is substantially non-zero at expiration of the time delay. 
     Disclosed herein is an apparatus for detecting radiation. The apparatus may comprise a radiation absorption layer that may comprises a first semiconductor wafer, a second semiconductor wafer and a plurality of semiconductor single crystal chunks. Each of the plurality of semiconductor single crystal chunks may have a first surface and a second surface with the second surface being opposite to the first surface. The plurality of semiconductor single crystal chunks may have different sizes and gaps in between, and may be bonded by respective first surfaces to the first semiconductor wafer. A plurality of electrodes may be formed on respective second surfaces of each of the plurality of semiconductor single crystal chunks and the plurality of semiconductor single crystal chunks may be bonded to the second semiconductor wafer by pillars. 
     According to an embodiment, the plurality of semiconductor single crystal chunks are cadmium zinc telluride (CdZnTe) chunks. 
     Disclosed herein is a system comprising the apparatus disclosed herein and a radiation source, wherein the system is configured to perform radiography on human chest or abdomen. 
     Disclosed herein is a system comprising the apparatus disclosed herein and a radiation source, wherein the system is configured to perform radiography on human mouth. 
     Disclosed herein is a cargo scanning or non-intrusive inspection (NII) system, comprising the apparatus disclosed herein and a radiation source, wherein the cargo scanning or non-intrusive inspection (NII) system is configured to form an image using backscattered radiation. 
     Disclosed herein is a cargo scanning or non-intrusive inspection (NII) system, comprising the apparatus disclosed herein and a radiation source, wherein the cargo scanning or non-intrusive inspection (NII) system is configured to form an image using radiation transmitted through an object inspected. 
     Disclosed herein is a full-body scanner system comprising the apparatus disclosed herein and a radiation source. 
     Disclosed herein is a computed tomography (CT) system comprising the apparatus disclosed herein and a radiation source. 
     Disclosed herein is an electron microscope comprising the apparatus disclosed herein, an electron source and an electronic optical system. 
     Disclosed herein is a system comprising the apparatus disclosed herein, wherein the system is an X-ray telescope, or an X-ray microscopy, or wherein the system is configured to perform mammography, industrial defect detection, microradiography, casting inspection, weld inspection, or digital subtraction angiography. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
         FIG. 1A  schematically shows a radiation detector, according to an embodiment. 
         FIG. 1B  shows a radiation detector, according an embodiment. 
         FIG. 2  shows an exemplary top view of a portion of the detector in  FIG. 1A , according to an embodiment. 
         FIG. 3A  and  FIG. 3B  each show a component diagram of an electronics system of the detector in  FIG. 1A  and  FIG. 1B , according to an embodiment. 
         FIG. 4  schematically shows a temporal change of the electric current flowing through an electrode (upper curve) or an electrical contact of a resistor of a radiation absorption layer exposed to radiation, the electric current caused by charge carriers generated by a radiation particle incident on the radiation absorption layer, and a corresponding temporal change of the voltage of the electrode (lower curve), according to an embodiment. 
         FIG. 5  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current), and a corresponding temporal change of the voltage of the electrode (lower curve), in the electronics system operating in the way shown in  FIG. 4 , according to an embodiment. 
         FIG. 6  schematically shows a temporal change of the electric current flowing through an electrode (upper curve) of the radiation absorption layer exposed to radiation, the electric current caused by charge carriers generated by a radiation particle incident on the radiation absorption layer, and a corresponding temporal change of the voltage of the electrode (lower curve), when the electronics system operates to detect incident radiation particles at a higher rate, according to an embodiment. 
         FIG. 7  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current), and a corresponding temporal change of the voltage of the electrode (lower curve), in the electronics system operating in the way shown in  FIG. 6 , according to an embodiment. 
         FIG. 8  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a series of radiation particles incident on the radiation absorption layer, and a corresponding temporal change of the voltage of the electrode, in the electronics system operating in the way shown in  FIG. 6  with RST expires before t e , according to an embodiment. 
         FIG. 9A  schematically shows a bottom view of a semiconductor wafer with a plurality of semiconductor single crystal chunks bonded thereon, according to an embodiment. 
         FIG. 9B  schematically shows a cross-sectional view of the semiconductor wafer and the plurality of semiconductor single crystal chunks of  FIG. 9A , according to an embodiment. 
         FIG. 10A  schematically shows a cross-sectional view of a semiconductor wafer and a plurality of semiconductor single crystal chunks bonded thereon, according to an embodiment. 
         FIG. 10B  schematically shows electrodes on the plurality of semiconductor single crystal chunks of  FIG. 10A , according to an embodiment. 
         FIG. 10C  schematically shows pillars deposited on the plurality of semiconductor single crystal chunks of  FIG. 10B , according to an embodiment. 
         FIG. 10D  schematically shows the plurality of semiconductor single crystal chunks of  FIG. 10C  being bonded to a semiconductor wafer, according to an embodiment. 
         FIG. 11  shows a flow chart of a process of making a semiconductor detector, according to an embodiment. 
         FIG. 12  schematically shows a system comprising the radiation detector described herein, suitable for medical imaging such as chest radiation radiography, abdominal radiation radiography, etc., according to an embodiment 
         FIG. 13  schematically shows a system comprising the semiconductor radiation detector described herein suitable for dental radiography, according to an embodiment. 
         FIG. 14  schematically shows a cargo scanning or non-intrusive inspection (NII) system comprising the radiation detector described herein, according to an embodiment. 
         FIG. 15  schematically shows another cargo scanning or non-intrusive inspection (NII) system comprising the radiation detector described herein, according to an embodiment. 
         FIG. 16  schematically shows a full-body scanner system comprising the radiation detector described herein, according to an embodiment. 
         FIG. 17  schematically shows a computed tomography (CT) system comprising the radiation detector described herein, according to an embodiment. 
         FIG. 18  schematically shows an electron microscope comprising the radiation detector described herein, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  schematically shows a cross-sectional view of a radiation detector  100 , according to an embodiment. The detector  100  may include a radiation absorption layer  110  and an electronics layer  120  (e.g., an ASIC) for processing or analyzing electrical signals incident radiation generates in the radiation absorption layer  110 . In an embodiment, the detector  100  does not comprise a scintillator. The radiation absorption layer  110  may include a semiconductor material such as, silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof. The semiconductor may have a high mass attenuation coefficient for the radiation energy of interest. In some embodiments, the radiation may be an electromagnetic radiation such as infrared light, visible light, ultraviolet light, X-ray or y-ray, and the radiation particles may be photons. In some other embodiments, the radiation may be charged particles such as α and β particles or non-charged particles such as neutrons. In some portions of the description, X-ray is used as an example for various types of radiation described herein. 
     As shown in a detailed cross-sectional view of the detector  100  in  FIG. 1B , according to an embodiment, the radiation absorption layer  110  may include a resistor of a semiconductor material such as, silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof, but does not include a diode. 
     When radiation particle hits the radiation absorption layer  110  including a resistor, it may be absorbed and generate one or more charge carriers by a number of mechanisms. A radiation particle may generate  10  to  100000  charge carriers. The charge carriers may drift to the electrical contacts  119 A and  119 B under an electric field. The field may be an external electric field. The electrical contact  119 B includes discrete portions. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single radiation particle are not substantially shared by two different discrete portions of the electrical contact  119 B (“not substantially shared” here means less than 5%, less than 2% or less than 1% of these charge carriers flow to a different one of the discrete portions than the rest of the charge carriers). In an embodiment, the charge carriers generated by a single radiation particle can be shared by two different discrete portions of the electrical contact  119 B. Charge carriers generated by a radiation particle incident around the footprint of one of these discrete portions of the electrical contact  119 B are not substantially shared with another of these discrete portions of the electrical contact  119 B. The area around a discrete portion of the electrical contact  119 B in which substantially all (more than 95%, more than 98% or more than 99% of) charge carriers generated by a radiation particle incident therein flow to the discrete portion of the electrical contact  119 B is called a pixel associated with the discrete portion of the electrical contact  119 B. Namely, less than 5%, less than 2% or less than 1% of these charge carriers flow beyond the pixel associated with the one discrete portion of the electrical contact  119 B. By measuring the drift current flowing into each of the discrete portion of the electrical contact  119 B, or the rate of change of the voltage of each of the discrete portions of the electrical contact  119 B, the number of radiation particles absorbed (which relates to the incident radiation intensity) and/or the energies thereof in the pixels associated with the discrete portions of the electrical contact  119 B may be determined. Thus, the spatial distribution (e.g., an image) of incident radiation intensity may be determined by individually measuring the drift current into each one of an array of discrete portions of the electrical contact  119 B or measuring the rate of change of the voltage of each one of an array of discrete portions of the electrical contact  119 B. The pixels may be organized in any suitable array, such as, a square array, a triangular array and a honeycomb array. The pixels may have any suitable shape, such as, circular, triangular, square, rectangular, and hexangular. The pixels may be individually addressable. 
     The electronics layer  120  may include an electronics system  121  suitable for processing or interpreting signals generated by radiation particles incident on the radiation absorption layer  110 . The electronics system  121  may include an analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuitry such as a microprocessors, and memory. The electronics system  121  may include components shared by the pixels or components dedicated to a single pixel. For example, the electronics system  121  may include an amplifier dedicated to each pixel and a microprocessor shared among all the pixels. 
       FIG. 2  schematically shows that the detector  100  may have an array of pixels  150 . The array may be a rectangular array, a honeycomb array, a hexagonal array or any other suitable array. Each pixel  150  may be configured to detect a radiation particle incident thereon, measure the energy of the radiation particle, or both. For example, each pixel  150  may be configured to count numbers of radiation particles incident thereon whose energy falls in a plurality of bins, within a period of time. All the pixels  150  may be configured to count the numbers of radiation particles incident thereon within a plurality of bins of energy within the same period of time. Each pixel  150  may have its own analog-to-digital converter (ADC) configured to digitize an analog signal representing the energy of an incident radiation particle into a digital signal. The ADC may have a resolution of 10 bits or higher. Each pixel  150  may be configured to measure its dark current, such as before or concurrently with each radiation particle incident thereon. Each pixel  150  may be configured to deduct the contribution of the dark current from the energy of the radiation particle photon incident thereon. The pixels  150  may be configured to operate in parallel. For example, when one pixel  150  measures an incident radiation particle, another pixel  150  may be waiting for a radiation particle to arrive. The pixels  150  may be but do not have to be individually addressable. 
       FIG. 3A  and  FIG. 3B  each show a component diagram of the electronics system  121 , according to an embodiment. The electronics system  121  may include a first voltage comparator  301 , a second voltage comparator  302 , a counter  320 , a switch  305 , a voltmeter  306  and a controller  310 . 
     The first voltage comparator  301  is configured to compare the voltage of an electrode or electrical contact of a resistor  300  to a first threshold. The resistor  300  may be formed by semiconductor material in the absorption layer  110 . Alternatively, the first voltage comparator  301  is configured to compare the voltage of an electrical contact (e.g., a discrete portion of electrical contact  119 B) to a first threshold. The first voltage comparator  301  may be configured to monitor the voltage directly, or calculate the voltage by integrating an electric current flowing through the resistor or electrical contact over a period of time. The first voltage comparator  301  may be controllably activated or deactivated by the controller  310 . The first voltage comparator  301  may be a continuous comparator. Namely, the first voltage comparator  301  may be configured to be activated continuously, and monitor the voltage continuously. The first voltage comparator  301  configured as a continuous comparator reduces the chance that the system  121  misses signals generated by an incident radiation particle. The first voltage comparator  301  configured as a continuous comparator is especially suitable when the incident radiation intensity is relatively high. The first voltage comparator  301  may be a clocked comparator, which has the benefit of lower power consumption. The first voltage comparator  301  configured as a clocked comparator may cause the system  121  to miss signals generated by some incident radiation particles. When the incident radiation intensity is low, the chance of missing an incident radiation particle is low because the time interval between two successive photons is relatively long. Therefore, the first voltage comparator  301  configured as a clocked comparator is especially suitable when the incident radiation intensity is relatively low. The first threshold may be 5-10%, 10%-20%, 20-30%, 30-40% or 40-50% of the maximum voltage one incident radiation particle may generate in the resistor. The maximum voltage may depend on the energy of the incident radiation particle (e.g., the wavelength of the incident X-ray), the material of the radiation absorption layer  110 , and other factors. For example, the first threshold may be 50 mV, 100 mV, 150 mV, or 200 mV. 
     The second voltage comparator  302  is configured to compare the voltage to a second threshold. The second voltage comparator  302  may be configured to monitor the voltage directly, or calculate the voltage by integrating an electric current flowing through the resistor or the electrical contact over a period of time. The second voltage comparator  302  may be a continuous comparator. The second voltage comparator  302  may be controllably activate or deactivated by the controller  310 . When the second voltage comparator  302  is deactivated, the power consumption of the second voltage comparator  302  may be less than 1%, less than 5%, less than 10% or less than 20% of the power consumption when the second voltage comparator  302  is activated. The absolute value of the second threshold is greater than the absolute value of the first threshold. As used herein, the term “absolute value” or “modulus” |x| of a real number x is the non-negative value of x without regard to its sign. Namely, 
                  x        =     {             x   ,       if   ⁢           ⁢   x     ≥   0                   -   x     ,       if   ⁢           ⁢   x     ≤   0             .             
The second threshold may be 200%-300% of the first threshold. The second threshold may be at least 50% of the maximum voltage one incident radiation particle may generate in the resistor. For example, the second threshold may be 100 mV, 150 mV, 200 mV, 250 mV or 300 mV. The second voltage comparator  302  and the first voltage comparator  301  may be the same component. Namely, the system  121  may have one voltage comparator that can compare a voltage with two different thresholds at different times.
 
     The first voltage comparator  301  or the second voltage comparator  302  may include one or more op-amps or any other suitable circuitry. The first voltage comparator  301  or the second voltage comparator  302  may have a high speed to allow the system  121  to operate under a high flux of incident radiation. However, having a high speed is often at the cost of power consumption. 
     The counter  320  is configured to register a number of radiation particles reaching the resistor. The counter  320  may be a software component (e.g., a number stored in a computer memory) or a hardware component (e.g., a 4017 IC and a 7490 IC). 
     The controller  310  may be a hardware component such as a microcontroller and a microprocessor. The controller  310  is configured to start a time delay from a time at which the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold (e.g., the absolute value of the voltage increases from below the absolute value of the first threshold to a value equal to or above the absolute value of the first threshold). The absolute value is used here because the voltage may be negative or positive, depending on which side of the resistor&#39;s electrical contact is used. The controller  310  may be configured to keep deactivated the second voltage comparator  302 , the counter  320  and any other circuits the operation of the first voltage comparator  301  does not require, before the time at which the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold. The time delay may expire before or after the voltage becomes stable, i.e., the rate of change of the voltage is substantially zero. The phase “the rate of change of the voltage is substantially zero” means that temporal change of the voltage is less than 0.1%/ns. The phase “the rate of change of the voltage is substantially non-zero” means that temporal change of the voltage is at least 0.1%/ns. 
     The controller  310  may be configured to activate the second voltage comparator during (including the beginning and the expiration) the time delay. In an embodiment, the controller  310  is configured to activate the second voltage comparator at the beginning of the time delay. The term “activate” means causing the component to enter an operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by providing power, etc.). The term “deactivate” means causing the component to enter a non-operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by cut off power, etc.). The operational state may have higher power consumption (e.g., 10 times higher, 100 times higher, 1000 times higher) than the non-operational state. The controller  310  itself may be deactivated until the output of the first voltage comparator  301  activates the controller  310  when the absolute value of the voltage equals or exceeds the absolute value of the first threshold. 
     The controller  310  may be configured to cause the number registered by the counter  320  to increase by one, if, during the time delay, the second voltage comparator  302  determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold. 
     The controller  310  may be configured to cause the voltmeter  306  to measure the voltage upon expiration of the time delay. The controller  310  may be configured to connect the electrode to an electrical ground, so as to reset the voltage and discharge any charge carriers accumulated on the electrode. In an embodiment, the electrode is connected to an electrical ground after the expiration of the time delay. In an embodiment, the electrode is connected to an electrical ground for a finite reset time period. The controller  310  may connect the electrode to the electrical ground by controlling the switch  305 . The switch may be a transistor such as a field-effect transistor (FET). 
     In an embodiment, the system  121  has no analog filter network (e.g., a RC network). In an embodiment, the system  121  has no analog circuitry. 
     The voltmeter  306  may feed the voltage it measures to the controller  310  as an analog or digital signal. 
     The system  121  may include a capacitor module  309  electrically connected to the electrode or electrical contact of the resistor  300 , wherein the capacitor module is configured to collect charge carriers from the electrode. The capacitor module can include a capacitor in the feedback path of an amplifier. The amplifier configured as such is called a capacitive transimpedance amplifier (CTIA). CTIA has high dynamic range by keeping the amplifier from saturating and improves the signal-to-noise ratio by limiting the bandwidth in the signal path. Charge carriers from the electrode accumulate on the capacitor over a period of time (“integration period”) (e.g., as shown in  FIG. 4 , between t 0  to t 1 , or t 1 -t 2 ). After the integration period has expired, the capacitor voltage is sampled and then reset by a reset switch. The capacitor module can include a capacitor directly connected to the electrode. 
       FIG. 4  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a radiation particle incident on the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve). The voltage may be an integral of the electric current with respect to time. At time to, the radiation particle hits the resistor, charge carriers start being generated in the resistor, electric current starts to flow through the electrode of the resistor, and the absolute value of the voltage of the electrode or electrical contact starts to increase. At time t 1 , the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold V 1 , and the controller  310  starts the time delay TD 1  and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 1 . If the controller  310  is deactivated before t 1 , the controller  310  is activated at t 1 . During TD 1 , the controller  310  activates the second voltage comparator  302 . The term “during” a time delay as used here means the beginning and the expiration (i.e., the end) and any time in between. For example, the controller  310  may activate the second voltage comparator  302  at the expiration of TD 1 . If during TD 1 , the second voltage comparator  302  determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold at time t 2 , the controller  310  causes the number registered by the counter  320  to increase by one. At time t e , all charge carriers generated by the radiation particle drift out of the radiation absorption layer  110 . At time t s , the time delay TD 1  expires. In the example of  FIG. 4 , time t s  is after time t e ; namely TD 1  expires after all charge carriers generated by the radiation particle drift out of the radiation absorption layer  110 . The rate of change of the voltage is thus substantially zero at t s . The controller  310  may be configured to deactivate the second voltage comparator  302  at expiration of TD 1  or at t 2 , or any time in between. 
     The controller  310  may be configured to cause the voltmeter  306  to measure the voltage upon expiration of the time delay TD 1 . In an embodiment, the controller  310  causes the voltmeter  306  to measure the voltage after the rate of change of the voltage becomes substantially zero after the expiration of the time delay TD 1 . The voltage at this moment is proportional to the amount of charge carriers generated by a radiation particle, which relates to the energy of the X-ray photon. The controller  310  may be configured to determine the energy of the radiation particle based on voltage the voltmeter  306  measures. One way to determine the energy is by binning the voltage. The counter  320  may have a sub-counter for each bin. When the controller  310  determines that the energy of the radiation particle falls in a bin, the controller  310  may cause the number registered in the sub-counter for that bin to increase by one. Therefore, the system  121  may be able to detect a radiation image and may be able to resolve energies of each radiation particle. 
     After TD 1  expires, the controller  310  connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode to flow to the ground and reset the voltage. After RST, the system  121  is ready to detect another incident radiation particle. Implicitly, the rate of incident radiation particles the system  121  can handle in the example of  FIG. 4  is limited by 1/(TD 1 +RST). If the first voltage comparator  301  has been deactivated, the controller  310  can activate it at any time before RST expires. If the controller  310  has been deactivated, it may be activated before RST expires. 
       FIG. 5  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current, background radiation, scattered radiation, fluorescent X-rays, shared charges from adjacent pixels), and a corresponding temporal change of the voltage of the electrode (lower curve), in the system  121  operating in the way shown in  FIG. 4 . At time to, the noise begins. If the noise is not large enough to cause the absolute value of the voltage to exceed the absolute value of V 1 , the controller  310  does not activate the second voltage comparator  302 . If the noise is large enough to cause the absolute value of the voltage to exceed the absolute value of V 1  at time t 1  as determined by the first voltage comparator  301 , the controller  310  starts the time delay TD 1  and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 1 . During TD 1  (e.g., at expiration of TD 1 ), the controller  310  activates the second voltage comparator  302 . The noise is very unlikely large enough to cause the absolute value of the voltage to exceed the absolute value of V 2  during TD 1 . Therefore, the controller  310  does not cause the number registered by the counter  320  to increase. At time t e , the noise ends. At time t s  the time delay TD 1  expires. The controller  310  may be configured to deactivate the second voltage comparator  302  at expiration of TD 1 . The controller  310  may be configured not to cause the voltmeter  306  to measure the voltage if the absolute value of the voltage does not exceed the absolute value of V 2  during TD 1 . After TD 1  expires, the controller  310  connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode as a result of the noise to flow to the ground and reset the voltage. Therefore, the system  121  may be very effective in noise rejection. 
       FIG. 6  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a radiation particle incident on the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve), when the system  121  operates to detect incident radiation particles at a rate higher than 1/(TD 1 +RST). The voltage may be an integral of the electric current with respect to time. At time to, the radiation particle hits the resistor, charge carriers start being generated in the resistor, electric current starts to flow through the electrical contact of resistor, and the absolute value of the voltage of the electrode or the electrical contact starts to increase. At time t 1 , the first voltage comparator  301  determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold V 1 , and the controller  310  starts a time delay TD 2  shorter than TD 1 , and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 2 . If the controller  310  is deactivated before t 1 , the controller  310  is activated at t 1 . During TD 2  (e.g., at expiration of TD 2 ), the controller  310  activates the second voltage comparator  302 . If during TD 2 , the second voltage comparator  302  determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold at time t 2 , the controller  310  causes the number registered by the counter  320  to increase by one. At time t e , all charge carriers generated by the radiation particle drift out of the radiation absorption layer  110 . At time t h , the time delay TD 2  expires. In the example of  FIG. 6 , time t h  is before time t e ; namely TD 2  expires before all charge carriers generated by the radiation particle drift out of the radiation absorption layer  110 . The rate of change of the voltage is thus substantially non-zero at t h . The controller  310  may be configured to deactivate the second voltage comparator  302  at expiration of TD 2  or at t 2 , or any time in between. 
     The controller  310  may be configured to extrapolate the voltage at t e  from the voltage as a function of time during TD 2  and use the extrapolated voltage to determine the energy of the radiation particle. 
     After TD 2  expires, the controller  310  connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode to flow to the ground and reset the voltage. In an embodiment, RST expires before t e . The rate of change of the voltage after RST may be substantially non-zero because all charge carriers generated by the radiation particle have not drifted out of the radiation absorption layer  110  upon expiration of RST before t e . The rate of change of the voltage becomes substantially zero after t e  and the voltage stabilized to a residue voltage VR after t e . In an embodiment, RST expires at or after t e , and the rate of change of the voltage after RST may be substantially zero because all charge carriers generated by the radiation particle drift out of the radiation absorption layer  110  at t e . After RST, the system  121  is ready to detect another incident radiation particle. If the first voltage comparator  301  has been deactivated, the controller  310  can activate it at any time before RST expires. If the controller  310  has been deactivated, it may be activated before RST expires. 
       FIG. 7  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by noise (e.g., dark current, background radiation, scattered radiation, fluorescent X-rays, shared charges from adjacent pixels), and a corresponding temporal change of the voltage of the electrode (lower curve), in the system  121  operating in the way shown in  FIG. 6 . At time to, the noise begins. If the noise is not large enough to cause the absolute value of the voltage to exceed the absolute value of V 1 , the controller  310  does not activate the second voltage comparator  302 . If the noise is large enough to cause the absolute value of the voltage to exceed the absolute value of V 1  at time t 1  as determined by the first voltage comparator  301 , the controller  310  starts the time delay TD 2  and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 2 . During TD 2  (e.g., at expiration of TD 2 ), the controller  310  activates the second voltage comparator  302 . The noise is very unlikely large enough to cause the absolute value of the voltage to exceed the absolute value of V 2  during TD 2 . Therefore, the controller  310  does not cause the number registered by the counter  320  to increase. At time t e , the noise ends. At time t h , the time delay TD 2  expires. The controller  310  may be configured to deactivate the second voltage comparator  302  at expiration of TD 2 . After TD 2  expires, the controller  310  connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode as a result of the noise to flow to the ground and reset the voltage. Therefore, the system  121  may be very effective in noise rejection. 
       FIG. 8  schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a series of radiation particles incident on the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve), in the system  121  operating in the way shown in  FIG. 6  with RST expires before t e . The voltage curve caused by charge carriers generated by each incident radiation particle is offset by the residue voltage before that radiation particle. The absolute value of the residue voltage successively increases with each incident photon. When the absolute value of the residue voltage exceeds V 1  (see the dotted rectangle in  FIG. 8 ), the controller starts the time delay TD 2  and the controller  310  may deactivate the first voltage comparator  301  at the beginning of TD 2 . If no other radiation particle incidence on the resistor during TD 2 , the controller connects the electrode to the electrical ground during the reset time period RST at the end of TD 2 , thereby resetting the residue voltage. The residue voltage thus does not cause an increase of the number registered by the counter  320 . 
       FIG. 9A  schematically shows a bottom view of a semiconductor wafer  902  with a plurality of semiconductor single crystal chunks  904  bonded thereon. The semiconductor single crystal chunks  904  may be cut from one or more manufactured semiconductor single crystals. They may be arranged on the semiconductor wafer  902  and fixed thereon. The arrangement need not have a high precision. The bonding may be by glue, plastic molding, or other known mechanism or techniques to be developed. Although not shown in  FIG. 9A , there may be gaps between neighboring single crystal chunks  904  on the semiconductor wafer  902  and the widths of the gaps may not be the same. This bonding of semiconductor single crystals to a wafer may be referred to as wafer reconstruction. In one embodiment, the semiconductor single crystal chunks may be CdZnTe chunks that are suitable to make radiation detectors. For example, the CdZnTe chunks may have size of less than 1 cm, around 1 cm or larger than 1 cm. 
     The semiconductor single crystal chunks  904  as shown in  FIG. 9A  are rectangular or square, but in some embodiments, some of the chunks  904  may have various other shapes, such as but not limited to, round, parallelogram, or irregular shapes. In one embodiment, the semiconductor wafer  902  may be conductive. The size of the wafer  902  may be any suitable size, for example, 4 inches, 5 inches, 6 inches, 8 inches, 12 inches, or 18 inches. 
       FIG. 9B  schematically shows a cross-sectional view of the semiconductor wafer  902  and the plurality of semiconductor single crystal chunks  904  according to one embodiment. As shown in  FIG. 9B , each semiconductor single crystal chunks  904  may have a first surface  906  bonded to the semiconductor wafer  902  and a second surface  908  opposite to the first surface  906 . A gap  910  is shown in  FIG. 9B  to illustrate that there may be gaps between neighboring chunks  904 . The semiconductor wafer  902  may be conductive and form a common electrode for the plurality of semiconductor single crystal chunks  904 , while the plurality of semiconductor single crystal chunks  904  may for a radiation absorption layer  110 . That is, the semiconductor wafer  902  may be an embodiment of the electrical contact  119 A shown in  FIG. 1B . In one embodiment, the semiconductor single crystal chunks  904  may have a thickness of around 1 to 2 mm before being polished. As shown in  FIG. 9B , the semiconductor single crystal chunks  904  may have different thicknesses. 
       FIG. 10A  schematically shows a semiconductor wafer  902  with a plurality of semiconductor single crystal chunks  904  bonded thereon, according to an embodiment. The plurality of semiconductor single crystal chunks  904  may have the same thickness. In one embodiment, the same thickness of the plurality of semiconductor single crystal chunks  904  may be obtained by polishing after they have been bonded on the semiconductor wafer  902 . 
       FIG. 10B  schematically shows electrodes  912  on the plurality of semiconductor single crystal chunks  904  of  FIG. 10A , according to an embodiment. Each of the chunks  904  may have a plurality of electrodes  912 . The electrodes  912  may be obtained using semiconductor wafer processes. For example, the electrodes  912  may be generated using known or yet to be developed semiconductor wafer processes. Each chunk  904  may have a plurality of electrodes, for example, hundreds or thousands of electrodes. In one embodiment, each chunk  904  may have about 5000 electrodes. 
       FIG. 10C  schematically shows pillars  914  deposited on the plurality of semiconductor single crystal chunks  904 . In one embodiment, the pillars  914  may be conductive, for example, made of copper. The pillars  914  may be deposited using semiconductor wafer processes. For example, the pillars  914  may be obtained using known or to be developed semiconductor wafer processes. 
       FIG. 10D  schematically shows that the plurality of semiconductor single crystal chunks  904  may be bonded to a second semiconductor wafer  920  by the pillars  914 , according to an embodiment. Compared to  FIG. 10C , the reference numerals  908  for the second surface,  910  for the gap,  912  for the electrodes and  914  for the pillars are omitted on  FIG. 10D  to reduce clutter. The semiconductor wafer  920  may comprise the electronics layer  120  as described herein. In one embodiment, the semiconductor wafer  920  may comprise a plurality of ASIC to read out and process the signals from the plurality of semiconductor single crystal chunks. In one embodiment, the bonding may be a wafer level bonding and performed using room temperature bonding. The final product of the processes  9 A- 9 B and  10 A- 10 D may be used as one radiation detector  100  or may be cut into smaller modules such that each smaller module may be used as a radiation detector  100 . 
       FIG. 11  shows a flow chart of a process  1100  of making a semiconductor radiation detector (e.g., detector  100 ) as described herein. According to one embodiment, the process  1100  may start at block  1102 , in which a plurality of semiconductor single crystal chunks (e.g., chunks  904 ) may be obtained. Each of the plurality of semiconductor single crystal chunk may have a first surface and a second surface, and the second surface may be on an opposite side to the first surface. At block  1104 , the plurality of semiconductor single crystal chunks may be bonded to a first semiconductor wafer (e.g., wafer  902 ) by respective first surfaces. At block  1106 , a plurality of electrodes (e.g., electrodes  912 ) may be formed on respective second surfaces of each of the plurality of semiconductor single crystal chunks. At block  1108 , pillars (e.g., pillars  914 ) may be deposited on each of the plurality of semiconductor single crystal chunks for bonding to a second semiconductor wafer (e.g., wafer  920 ). In one embodiment, the semiconductor single crystal chunks may be CdZnTe chunks. 
       FIGS. 12-18  schematically show various systems each comprising an image sensor  9000 . The image sensor  9000  may be an embodiment of an image sensor comprising one or more semiconductor radiation detectors described herein. It should be noted a radiation detector according to an embodiment may be used to detect one or more types of radiation and X-ray is just one example. For example, the radiation may be an electromagnetic radiation such as infrared light, visible light, ultraviolet light, X-ray or y-ray. The radiation may be of other types such as charged and non-charged particles as described herein. 
       FIG. 12  schematically shows a system comprising the image sensor  9000  described herein. The system may be used for medical imaging such as chest X-ray radiography, abdominal X-ray radiography, etc. The system comprises an X-ray source  1201 . X-ray emitted from the X-ray source  1201  penetrates an object  1202  (e.g., a human body part such as chest, limb, abdomen), is attenuated by different degrees by the internal structures of the object  1202  (e.g., bones, muscle, fat and organs, etc.), and is projected to the image sensor  9000 . The image sensor  9000  forms an image by detecting the intensity distribution of the X-ray. 
       FIG. 13  schematically shows a system comprising the image sensor  9000  described herein. The system may be used for medical imaging such as dental X-ray radiography. The system comprises an X-ray source  1301 . X-ray emitted from the X-ray source  1301  penetrates an object  1302  that is part of a mammal (e.g., human) mouth. The object  1302  may include a maxilla bone, a palate bone, a tooth, the mandible, or the tongue. The X-ray is attenuated by different degrees by the different structures of the object  1302  and is projected to the image sensor  9000 . The image sensor  9000  forms an image by detecting the intensity distribution of the X-ray. Teeth absorb X-ray more than dental caries, infections, periodontal ligament. The dosage of X-ray radiation received by a dental patient is typically small (around 0.150 mSv for a full mouth series). 
       FIG. 14  schematically shows a cargo scanning or non-intrusive inspection (NII) system comprising the image sensor  9000  described herein. The system may be used for inspecting and identifying goods in transportation systems such as shipping containers, vehicles, ships, luggage, etc. The system comprises an X-ray source  1401 . X-ray emitted from the X-ray source  1401  may backscatter from an object  1402  (e.g., shipping containers, vehicles, ships, etc.) and be projected to the image sensor  9000 . Different internal structures of the object  1402  may backscatter X-ray differently. The image sensor  9000  forms an image by detecting the intensity distribution of the backscattered X-ray and/or energies of the backscattered X-ray photons. 
       FIG. 15  schematically shows another cargo scanning or non-intrusive inspection (NII) system comprising the image sensor  9000  described herein. The system may be used for luggage screening at public transportation stations and airports. The system comprises an X-ray source  1501 . X-ray emitted from the X-ray source  1501  may penetrate a piece of luggage  1502 , be differently attenuated by the contents of the luggage, and projected to the image sensor  9000 . The image sensor  9000  forms an image by detecting the intensity distribution of the transmitted X-ray. The system may reveal contents of luggage and identify items forbidden on public transportation, such as firearms, narcotics, edged weapons, flammables. 
       FIG. 16  schematically shows a full-body scanner system comprising the image sensor  9000  described herein. The full-body scanner system may detect objects on a person&#39;s body for security screening purposes, without physically removing clothes or making physical contact. The full-body scanner system may be able to detect non-metal objects. The full-body scanner system comprises an X-ray source  1601 . X-ray emitted from the X-ray source  1601  may backscatter from a human  1602  being screened and objects thereon, and be projected to the image sensor  9000 . The objects and the human body may backscatter X-ray differently. The image sensor  9000  forms an image by detecting the intensity distribution of the backscattered X-ray. The image sensor  9000  and the X-ray source  1601  may be configured to scan the human in a linear or rotational direction. 
       FIG. 17  schematically shows an X-ray computed tomography (X-ray CT) system. The X-ray CT system uses computer-processed X-rays to produce tomographic images (virtual “slices”) of specific areas of a scanned object. The tomographic images may be used for diagnostic and therapeutic purposes in various medical disciplines, or for flaw detection, failure analysis, metrology, assembly analysis and reverse engineering. The X-ray CT system comprises the image sensor  9000  described herein and an X-ray source  1701 . The image sensor  9000  and the X-ray source  1701  may be configured to rotate synchronously along one or more circular or spiral paths. 
       FIG. 18  schematically shows an electron microscope. The electron microscope comprises an electron source  1801  (also called an electron gun) that is configured to emit electrons. The electron source  1801  may have various emission mechanisms such as thermionic, photocathode, cold emission, or plasmas source. The emitted electrons pass through an electronic optical system  1803 , which may be configured to shape, accelerate, or focus the electrons. The electrons then reach a sample  1802  and an image detector may form an image therefrom. The electron microscope may comprise the image sensor  9000  described herein, for performing energy-dispersive X-ray spectroscopy (EDS). EDS is an analytical technique used for the elemental analysis or chemical characterization of a sample. When the electrons incident on a sample, they cause emission of characteristic X-rays from the sample. The incident electrons may excite an electron in an inner shell of an atom in the sample, ejecting it from the shell while creating an electron hole where the electron was. An electron from an outer, higher-energy shell then fills the hole, and the difference in energy between the higher-energy shell and the lower energy shell may be released in the form of an X-ray. The number and energy of the X-rays emitted from the sample can be measured by the image sensor  9000 . 
     The image sensor  9000  described here may have other applications such as in an X-ray telescope, X-ray mammography, industrial X-ray defect detection, X-ray microscopy or microradiography, X-ray casting inspection, X-ray non-destructive testing, X-ray weld inspection, X-ray digital subtraction angiography, etc. It may be suitable to use this image sensor  9000  in place of a photographic plate, a photographic film, a PSP plate, an X-ray image intensifier, a scintillator, or another semiconductor X-ray detector. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.