Patent Publication Number: US-8994136-B2

Title: Digital silicon photomultiplier detector cells

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Korean Patent Application No. 10-2013-0009452, filed on Jan. 28, 2013, in the Korean Intellectual Property Office (KIPO), the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Some example embodiments may relate generally to silicon photomultiplier detector cells. Some example embodiments may relate to silicon photomultiplier detector cells that use digital signals. 
     2. Description of Related Art 
     Examples of techniques of imaging the inside of a human body include positron emission tomography (PET), magnetic resonance imaging (MRI), and X-ray computed tomography (CT). PET is a nuclear medicine imaging method in which an emitted positron is detected using radiopharmaceuticals to show physiological, pathological images of a human body. In PET, an analogue of glucose called F-18-FDG, which is a radioactive isotope, is injected into a body, and radiation produced due to a reaction between the analogue of glucose and a cancer existing in the body is detected after the lapse of a certain period of time (for example, several tens of minutes) to obtain information about the position of the cancer. A detector that detects a radiation produced due to a reaction between an analogue of glucose and a cancer in a body is referred to as a silicon photomultiplier detector or a gamma ray detector. Apparatuses for detecting radiation or gamma rays may be applied not only to radiation detectors for single photon emission CT (SPECT), CT, and the like, but also to various fields such as astronomy. 
     PET apparatuses are medical imaging apparatuses that inject radiopharmaceuticals for emitting positrons into a living body via an intravenous injection or intake, measure gamma rays produced due to positron annihilation by using a circular ring-shaped detector that surrounds the living body, and calculate the distribution of a positron-emitting nuclide within the body via a computer to thereby produce and show an image of the distribution. A photomultiplier tube (PMT) has been used as the detector. 
     PET has recently been incorporated with MRI, which provides anatomical information. In this case, a PMT causes distortion of an electrical signal under a strong magnetic field of MRI. Thus, there is a demand for a detector that is strong against the strong magnetic field of MRI, and a silicon photomultiplier (SiPM) detector has been proposed. 
     The SiPM detector may be roughly broken down into a scintillator, a pixel, and readout electronics. The scintillator transforms high-energy gamma rays of 511 keV into low-energy photons having a wavelength of 400 nm to 450 nm. Since a gamma ray is a very large energy photon, it is not absorbed by silicon but is mostly transmitted by the silicon. Thus, the scintillator transforms the gamma ray to have a wavelength band that can be absorbed by silicon. The pixel transforms an optical signal into an electrical signal by absorbing the photon obtained from the scintillator. 
     The pixel may be an analog type or a digital type. Each analog type pixel includes several thousands of microcells, each of which includes a single avalanche photodiode (APD) and a resistor. A signal of a microcell is turned on or off depending on whether a photon is incident upon the microcell. Thus, the intensity of a pixel signal is determined by summing all of the electrical signals generated by the microcells. The pixel signal is transformed into a digital signal for use in calculations of energy and time by the readout electronics. On the other hand, in a digital type pixel, some of application specific integrated circuits (ASICs) on a printed circuit board (PCB) of analog type pixels are implemented within a microcell. Thus, digital type pixels provide improved time resolution and improved energy resolution compared to analog type pixels. However, the time resolution and the energy resolution of SiPM still need to be further increased. 
     SUMMARY 
     Some example embodiments may provide silicon photomultiplier detector cells capable of increasing photo detection efficiency. 
     Some example embodiments may provide methods of fabricating silicon photomultiplier detector cells capable of increasing photo detection efficiency. 
     In some example embodiments, a silicon photomultiplier detector cell may include a photodiode region and a readout circuit region formed on a same substrate. The photodiode region may comprise: a first semiconductor layer exposed on a surface of the silicon photomultiplier detector cell and doped with first type impurities; a second semiconductor layer doped with second type impurities; and/or a first epitaxial layer between the first semiconductor layer and the second semiconductor layer. The first epitaxial layer may contact the first semiconductor layer and the second semiconductor layer. The first epitaxial layer may be doped with the first type impurities at a concentration lower than a concentration of the first type impurities of the first semiconductor layer. 
     In some example embodiments, a concentration of the second type impurities of the second semiconductor layer may be greater than the concentration of the first type impurities of the first semiconductor layer. 
     In some example embodiments, the silicon photomultiplier detector cell may further comprise a second epitaxial layer between the same substrate and the second semiconductor layer. The second epitaxial layer may be doped with the first type impurities at a concentration lower than the concentration of the first type impurities of the first semiconductor layer. 
     In some example embodiments, the concentration of the first type impurities of the second epitaxial layer may be equal to the concentration of the first type impurities of the first epitaxial layer. 
     In some example embodiments, the same substrate may be a silicon layer doped with the first type impurities. 
     In some example embodiments, a trench extending from a surface of the silicon photomultiplier detector cell to the same substrate may be formed between the photodiode region and the readout circuit region. 
     In some example embodiments, the trench may be filled with material configured to reduce or prevent optical crosstalk. 
     In some example embodiments, the trench may be filled with at least one of polysilicon and metal. 
     In some example embodiments, a first insulation film may be formed on a sidewall of the trench. 
     In some example embodiments, the silicon photomultiplier detector cell may further comprise the first type impurities may be p-type impurities, and/or the second type impurities may be n-type impurities. 
     In some example embodiments, the silicon photomultiplier detector cell may further comprise a contact layer between the trench and the first epitaxial layer. The contact layer may contact the second semiconductor layer. 
     In some example embodiments, the contact layer may be doped with the second type impurities at a concentration lower than the concentration of the second type impurities of the second semiconductor layer. 
     In some example embodiments, the silicon photomultiplier detector cell may further comprise a second insulation film between the first semiconductor layer and the contact layer. 
     In some example embodiments, a method of fabricating a silicon photomultiplier detector cell including a photodiode region and a readout circuit region formed on a same substrate may comprise, in order to form the photodiode region: forming a first semiconductor layer doped with first type impurities on a first layer; forming, via epitaxial growth, a second layer doped with the first type impurities on the first semiconductor layer; and/or forming a second semiconductor layer doped with second type impurities on the first layer. 
     In some example embodiments, the method may further comprise forming the second layer doped with the first type impurities on the same substrate. 
     In some example embodiments, the method may further comprise forming a trench that exposes the same substrate, by etching the first layer and the second layer, after forming the second layer. 
     In some example embodiments, the method may further comprise forming a first insulation film on a sidewall of the trench; and/or filling the trench with material configured to reduce or prevent optical crosstalk. 
     In some example embodiments, the material that reduces or prevents optical crosstalk may comprise at least one of polysilicon and metal. 
     In some example embodiments, the material that reduces or prevents optical crosstalk may comprise polysilicon. 
     In some example embodiments, the material that reduces or prevents optical crosstalk may comprise metal. 
     In some example embodiments, the method may further comprise forming a contact layer, doped with the first type impurities, in parts of the second layer that contact the trench. 
     In some example embodiments, the method may further comprise forming an insulation film configured to insulate the first semiconductor layer from the contact layer, before forming the second semiconductor layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects and advantages will become more apparent and more readily appreciated from the following detailed description of example embodiments, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a radiation measuring device according to some example embodiments; 
         FIG. 2  is a cross-sectional view of a radiation detecting module according to some example embodiments; 
         FIG. 3  is a block diagram of a detector cell included in a silicon photomultiplier according to some example embodiments; 
         FIG. 4  is a circuit diagram of a detailed structure of a detector unit included in the detector cell illustrated in  FIG. 3 ; 
         FIG. 5A  is a circuit diagram of a detector unit according to some example embodiments; 
         FIG. 5B  is a timing diagram of voltage versus time for a voltage between both ends of a photodiode included in the detector unit of  FIG. 5A ; 
         FIG. 6  is a cross-sectional view of a silicon photomultiplier detector cell according to some example embodiments; 
         FIGS. 7A through 7H  are cross-sectional views illustrating a method of fabricating the silicon photomultiplier detector cell illustrated in  FIG. 6 ; and 
         FIG. 8  shows a result of an experiment for detecting photo detection efficiency according to the structures of silicon photomultiplier detector cells. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. Embodiments, however, may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope to those skilled in the art. In the drawings, the thicknesses of layers and regions may be exaggerated for clarity. 
     It will be understood that when an element is referred to as being “on,” “connected to,” “electrically connected to,” or “coupled to” to another component, it may be directly on, connected to, electrically connected to, or coupled to the other component or intervening components may be present. In contrast, when a component is referred to as being “directly on,” “directly connected to,” “directly electrically connected to,” or “directly coupled to” another component, there are no intervening components present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, and/or section from another element, component, region, layer, and/or section. For example, a first element, component, region, layer, and/or section could be termed a second element, component, region, layer, and/or section without departing from the teachings of example embodiments. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like may be used herein for ease of description to describe the relationship of one component and/or feature to another component and/or feature, or other component(s) and/or feature(s), as illustrated in the drawings. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Example embodiments may be described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized example embodiments (and intermediate structures). As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will typically have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature, their shapes are not intended to illustrate the actual shape of a region of a device, and their shapes are not intended to limit the scope of the example embodiments. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Reference will now be made to example embodiments, which are illustrated in the accompanying drawings, wherein like reference numerals may refer to like components throughout. 
       FIG. 1  is a perspective view of a radiation measuring device  1  according to some example embodiments. 
     Referring to  FIG. 1 , the radiation measuring device  1  may include a plurality of radiation measuring modules  2 , an imaging processing unit  3 , and an imaging zone  4 . 
     The imaging zone  4  is a zone that accommodates an object, and is provided to measure an image of the object. To fix the object, a supporter  5  may be included in the imaging zone  4 . The radiation measuring modules  2  may receive a radiation from the object and convert the radiation into a detection signal. The radiation measuring modules  2  may include a plurality of radiation detecting modules. Such a radiation detecting module will be described later in detail. The imaging processing unit  3  may produce an image of the object, based on the detect signal produced by the radiation measuring modules  2 . 
       FIG. 2  is a cross-sectional view of a radiation detecting module  20  according to some example embodiments. 
     Referring to  FIG. 2 , the radiation detecting module  20  may include a scintillator  21 , an optical pipe  22 , pixel elements  10 ,  11 ,  12 ,  13 ,  14 ,  15 , and  16 , and a semiconductor chip  24 . 
     The scintillator  21  may receive radiation (for example, a gamma ray) and produce a photon. The optical pipe  22  may be disposed between the scintillator  21  and the semiconductor chip  24 , and may deliver the photon produced by the scintillator  21  to the semiconductor chip  24 . The direction in which the photon is incident upon the semiconductor chip  24  does not limit the scope of the present invention. For example, the photon may be incident upon the semiconductor chip  24  via an upper surface of the scintillator  21  or via a bottom surface of the semiconductor chip  24 . In the latter case, the semiconductor chip  24  may be formed of a material capable of transmitting the photon. 
     The semiconductor chip  24  may include the pixel elements  10 ,  11 ,  12 ,  13 ,  14 ,  15 , and  16 , which are arranged in an array shape, in order to receive the photon via the optical pipe  22  and convert the photon into a corresponding electrical signal. Each of the pixel elements  10 ,  11 ,  12 ,  13 ,  14 ,  15 , and  16  may include detector cells  100 , which will be described later. 
     For example, when each pixel element includes 16 detector cells  100  and pixel elements are formed in a 7×7 array shape in the semiconductor chip  24 , a total of 7×7×16 (=784) detector cells  100  may be formed in the semiconductor chip  24 . Each detector cell  100  may be generally expressed as a microcell. If each microcell (for example, each detector cell  100 ) includes 2 photodiodes, 7×7×16×2 (=1568) photodiodes may be implemented in the single semiconductor chip  24 . 
       FIG. 3  is a block diagram of a detector cell  100  included in a silicon photomultiplier according to some example embodiments. 
     Referring to  FIG. 3 , the detector cell  100  may include a scintillator  110 , a detector unit  130 , and a readout unit  150 . 
     The silicon photomultiplier according to some example embodiments includes a pixel element including at least 500 microcells, each of which has a size of about 20 micrometers. Each microcell independently detects and amplifies photons. When a photon enters each microcell and forms a hole pair together with an electron, an electrical signal corresponding to the hole pair is amplified by an electric field within the silicon photomultiplier, and a signal having a certain magnitude is produced and output. In this case, an output signal of the silicon photomultiplier may be a signal corresponding to a sum of the signals of all of the microcells. 
     The scintillator  110  receives radiation to produce a photon. In other words, a radioactive isotope (for example, F-18-FDG) injected into a patient or a to-be-scanned target undergoes a radioactive decay event. In the radioactive decay event, a positron is produced. The positron produced in the radioactive decay event interacts with a neighboring electron to cause an electron-positron annihilation event. In the electron-positron annihilation event, two oppositely-directed radiations each having a 511 keV energy are produced. The two radiations move at the speed of light. When the two radiations collide with the scintillator  110 , light (or a photon) is generated. In this case, the light generated by the scintillator  110  may be visible light. In more detail, the light generated by the scintillator  110  may be visible light having a wavelength of about 400 nm to about 450 nm. In this case, the material of the scintillator  110  may be lutetium oxyorthosilicate (LSO), lutetium yttrium oxyorthosilicate (LYSO), mixed lutetium silicate (MLS), lutetium gadolinium oxyorthosilicate (LGSO), lanthanum bromide (LaBr), or a combination thereof. However, the material of the scintillator  110  is not limited thereto, and any other scintillator material may be used. 
     The detector unit  130  receives the light (or photon) from the scintillator  110  to generate a detect signal. The detect signal may be a digitized electrical signal. For example, when the detector unit  130  receives no light, the detect signal has a first digital value. On the other hand, when the detector unit  130  receives light, the detect signal has a second digital value. The detector unit  130  may include a photodiode. The photodiode receives light to generate an electrical signal. The electrical signal is an analog signal, and may be converted into a digital signal after undergoing passive/active quenching and active resetting. 
       FIG. 4  is a circuit diagram of a detailed structure of the detector unit  130 . 
     Referring to  FIG. 4 , the detector unit  130  includes a photodiode  133 , a passive quenching unit  131 , an active quenching unit  135 , and an active reset unit  137 . 
     The photodiode  133  may produce a current based on the photon generated by the scintillator  110 . The detector unit  130  may include a plurality of photodiodes  133 , each of which may be included in each of a plurality of microcells. For example, the photodiode  133  may be an avalanche photodiode. A supply voltage VDD, which is slightly lower than an operational voltage of the photodiode  133  (for example, a breakdown voltage when the photodiode  133  is an avalanche photodiode), may be applied. In this case, when the photodiode  133  receives a photon, the operational voltage of the photodiode  133  becomes less than or equal to the supply voltage VDD. Accordingly, a current flows in the photodiode  133 . 
     When the current flows in the photodiode  133 , a voltage difference is generated between both ends of the passive quenching unit  131 . The passive quenching unit  131  may be, for example, a resistor and an equivalent circuit. Due to a voltage drop caused by the passive quenching unit  131 , the magnitude of a voltage applied to the photodiode  133  is reduced. For example, if the passive quenching unit  131  includes a resistor and an equivalent circuit, the larger the magnitude of the current flowing in the photodiode  133  is, the larger the voltage drop in the passive quenching unit  131 . Accordingly, as the current flowing in the photodiode  133  increases, the voltage applied to the photodiode  133  decreases, because the voltage between both ends of the passive quenching unit  131  increases although the supply voltage VDD is constant. Consequently, while the current flowing in the photodiode  133  is increasing, and at some point a voltage that is lower than the operating voltage of the photodiode  133  is applied on the photodiode  133 . Therefore, the flow of the current generated when the photodiode  133  receives the photons is stopped by the passive quenching unit  131 . 
     The active quenching unit  135  shortens the time taken for the passive quenching unit  131  to prevent a current to flow again. In other words, when the passive quenching unit  131  maintains a voltage between both ends of the photodiode  133  as the operational voltage of the photodiode  133 , the active quenching unit  135  shortens the time taken for the voltage between both ends of the photodiode  133  to be changed from the supply voltage VDD to the operational voltage of the photodiode  133 . Accordingly, the time taken to recover the photodiode  133  may be reduced, and the resolution of the detector unit  130  may be increased. 
     When the reception of the light (or photon) is suspended, the voltage between both ends of the photodiode  133  is restored to the supply voltage VDD. The active reset unit  137  shortens the time taken to perform this restoration. The shortening of the restoration period of time may enable next light (or photon) to be detected when being received. In other words, when the restoration period of time increases, the current light is not distinguished from next light (or photon), and thus the two receptions of light may be recognized as a one-time reception. Therefore, energy resolution decreases. Accordingly, the inclusion of the active reset unit  137  may shorten the restoration period of time and increase energy resolution. 
     In an operation of the detector unit  130 , the photodiode  133  enables a current to flow via the light (photon) received from the scintillator  110 . At this time, the voltage between both ends of the photodiode  133  is adjusted to the level of the operational voltage of the photodiode  133  by the passive quenching unit  131 , and the time taken for the voltage of the photodiode  133  to be changed is reduced by the active quenching unit  135 . 
     Referring back to  FIG. 3 , the readout unit  150  receives the detect signal to generate an output signal OUT. The output signal OUT is not stored in a buffer or a special memory, and the output signal OUT is generated in response to the detect signal. The output signal OUT is transmitted to an external circuit (for example, a radiation calculator) of the silicon photomultiplier detector cell  100 . 
     An operation of the silicon photomultiplier detector cell  100  according to some example embodiments will now be described. A radioactive isotope injected into a patient or a to-be-scanned target generates gamma rays via a radioactive decay events. The scintillator  110  receives the gamma rays to produce the light (or photons). The detector unit  130  receives the light (or photons) from the scintillator  110  to generate the detect signal. The readout unit  150  receives the detect signal from the detector unit  130  and generates an output signal OUT having a format receivable by an external circuit, without storing or counting the detect signal separately. The output signal OUT is transmitted to the external circuit. 
     Accordingly, the silicon photomultiplier according to some example embodiments receives the detect signal and generates the output signal OUT at the same time, and transmits the output signal OUT to the external circuit, without storing the detect signal in the detector cell  100  (or a microcell). Accordingly, a space occupied by internal memory or a buffer is not necessary, leading to a reduction in the size of the silicon photomultiplier and execution of integration. The silicon photomultiplier according to some example embodiments may increase a fill factor. The fill factor represents a percentage of the area of a detecting portion in a detector cell. For example, the fill factor may be increased to at least 65%. 
     Since the energy resolution of the detector cell  100  does not rely on the capacity of internal memory or a buffer but relies on the capacity of the memory of the external circuit, a high energy, resolution detector cell may be obtained. In detail, a Full Width at Half Maximum (FWHM), representing energy resolution, of the silicon photomultiplier according to some example embodiments may be reduced to 10% or less. 
     An energy dynamic range may be represented as a product of the number of detector cells  100  (or microcells) and the capacity of memory capable of storing the detect signal generated by each of the detector cells  100 . The energy resolution of the detector cell  100  does not rely on the capacity of internal memory, but relies on the capacity of external memory. The capacity of the internal memory is limited by the size of the detector cell  100 , whereas the capacity of the external memory may be relatively large. Therefore, the energy dynamic range may increase in proportion to the capacity of the external memory. 
       FIG. 5A  is a circuit diagram of a detector unit  130   a  according to some example embodiments, and  FIG. 5B  is a timing diagram of voltage versus time for a voltage between both ends of the photodiode  133  included in the detector unit  130   a  of  FIG. 5A . 
     Referring to  FIG. 5A , the active quenching unit  135  may include a switch transistor SW 2 . The active reset unit  137  may include a switch transistor SW 1 . The passive quenching unit  131  may include a capacitor CAP_ 1 .  FIG. 5A  also shows first reference voltage VP 1 , second reference voltage VP 2 , third reference voltage VN, and fourth reference voltage −Vop. 
     Referring to  FIGS. 5A and 5B , when light (or photon) is received by the photodiode  133  at a point of time t1, current flows in a node A, and a voltage between both terminals of the photodiode  133  decreases. In other words, the voltage at the node A decreases, and a low voltage is applied to the node A. In other words, passive quenching occurs. Accordingly, when a switch transistor SW 3  is turned on, the supply voltage VDD is applied to a node B, and thus the node B is high and the switch transistor SW 2  and a switch transistor SW 4  are turned on. Therefore, the voltage of the node A drastically decreases between a point of time t2 and a point of time t3. In other words, active quenching occurs. When the switch transistor SW 4  is turned on, the voltage of a node C decreases, and thus the node C is in a low state. Thus, the switch transistor SW 1  is turned on. Therefore, the voltage of the node A stays constant between the point of time t3 and a point of time t4. Since the node C is in a low state, a voltage of a node D is high. However, the time taken for the voltage of the node D to become high since the node C is in a low state is adjustable by a switch transistor SW 6  and a capacitor CAP_ 2 . Accordingly, the node C is in a low state, a switch transistor SW 5  is turned on after a lapse of a certain period of time, and thus the voltage of the node B becomes low. Therefore, the switch transistor SW 2  is turned off, and the voltage of the node A returns to the supply voltage VDD between the point of time t4 and a point of time t5. In other words, active resetting occurs. 
     In the detector unit  130   a , the speed at which the node A enters into a low state may increase according to the photo detection efficiency of the photodiode  133 , and a weak photon may be incident without difficulties. A photodiode having improved photo detection efficiency will now be described. 
       FIG. 6  is a cross-sectional view of a silicon photomultiplier detector cell  200  according to some example embodiments. Since the structure of a photodiode having improved photo detection efficiency is focused on in some example embodiments, components other than the photodiode of the silicon photomultiplier detector cell  200  will not be described herein. For convenience of explanation, the structure of the silicon photomultiplier detector cell  200  is divided into a photodiode region D and a readout circuit region R, and only complementary metal-oxide-semiconductor (CMOS) layers  330  and  340  in the readout circuit region R are illustrated. 
     Referring to  FIG. 6 , the silicon photomultiplier detector cell  200  may be divided into the photodiode region D and the readout circuit region R on a substrate  210 . The substrate  210  may be an n-type or p-type silicon substrate. Since the substrate  210  may be a substrate that conforms to the standards of CMOS devices of the readout circuit region R, the readout circuit region R and the photodiode region D may be on the same substrate  210 . Hereinafter, the substrate  210  is set to be a silicon substrate doped with p-type impurities, for convenience of explanation. 
     The silicon photomultiplier detector cell  200  may include a trench  250  formed deeply toward the substrate  210 , so as to separate the photodiode region D from the readout circuit region R. The trench  250  may extend from the surface of the silicon photomultiplier detector cell  200  to the substrate  210 . The trench  250  may be filled with material(s) having a function of electrical and optical shielding between the photodiode region D and the readout circuit region R. For example, the trench  250  may be filled with an insulative material. In detail, a first insulation film  252  enabling electrical insulation may be formed in a region of the trench  250  that contacts the photodiode region D, the readout circuit region R, the substrate  210 , and the outside of the silicon photomultiplier detector cell  200 , and the first insulation film  252  may be filled with a film  254  capable of reducing or preventing optical crosstalk. The film  254  capable of reducing or preventing optical crosstalk may be a polysilicon film or a metal film. The polysilicon film or the metal film may not only reduce or prevent optical crosstalk, but also may play the role of a field plate. 
     The photodiode region D may include a first semiconductor layer  220 , which is exposed to the surface of the silicon photomultiplier detector cell  200  and doped with first type impurities, a second semiconductor layer  230  doped with second type impurities, and a first epitaxial layer  240 , which is disposed between the first and second semiconductor layers  220  and  230  and doped with the first type impurities. The photodiode region D may be formed of silicon, and may be doped with p-type or n-type impurities depending on the type of semiconductor layer. 
     The first semiconductor layer  220  may be a silicon layer doped with p-type impurities. The impurity concentration of the first semiconductor layer  220  may be equal to or greater than that of the substrate  210 . The second semiconductor layer  230  may be a silicon layer doped with n-type impurities. The impurity concentration of the second semiconductor layer  230  may be equal to or greater than that of the first semiconductor layer  220 . The first epitaxial layer  240  may be disposed between the first semiconductor layer  220  and the second semiconductor layer  230  and may be formed on the second semiconductor layer  230  in a straight growth manner. The first epitaxial layer  240  may contain p-type impurities, which are the same type of impurities as the first semiconductor layer  220 . 
     A contact layer  270 , which is disposed between the trench  250  and the first epitaxial layer  240  and contacts the second semiconductor layer  230 , may be further disposed in the photodiode region D. The contact layer  270  may be doped with the same type of impurities as the second semiconductor layer  230 , and the impurity concentration of the contact layer  270  may be less than that of the second semiconductor layer  230 . 
     A second insulation film  280  may be further disposed on a part of the surface of the silicon photomultiplier detector cell  200  between the first semiconductor layer  220  and the contact layer  270 . The second insulation film  280  may reduce or prevent electrical leakage between the first semiconductor layer  220  and the contact layer  270 . The second insulation film  280  may be formed of silicon nitride or silicon oxide. As illustrated in  FIG. 6 , the first epitaxial layer  240  is defined by the first and second semiconductor layers  220  and  230 , the contact layer  270 , and the second insulation film  280 . 
     The first semiconductor layer  220  may be connected an anode electrode, and the second semiconductor layer  230  may be connected to a cathode electrode via the contact layer  270 . 
     Photo detection efficiency may be determined by a light-incidence area (i.e., a fill factor), quantum efficiency, and an avalanche triggering probability that an avalanche is triggered while an electron or a hole generated from light is being accelerated by an electric field within a depletion region. 
     As illustrated in  FIG. 6 , since the second semiconductor layer  230  is disposed across the photodiode region D, an area where an avalanche occurs and an overlap area between the first and second semiconductor layers  220  and  230  are large. Thus, photo detection efficiency may be increased. Since the first and second semiconductor layers  220  and  230  perform a photodiode function and only the first epitaxial layer  240  is formed therebetween, a thickness of the first epitaxial layer  240 , namely, a depletion width, is large. Thus, electrons produced from light undergo more impact ionizations while moving across the depletion width, and thus more electrons and holes are additionally produced. This leads to a high avalanche triggering probability, thus increasing the photo detection efficiency. 
     A second epitaxial layer  290  may be formed between the substrate  210  and the second semiconductor layer  230 . The second epitaxial layer  290  may be doped with a different type of impurities from the impurity type of the second semiconductor layer  230 . For example, the second epitaxial layer  290  may be doped with p-type impurities. Therefore, a current in the second semiconductor layer  230  may be prevented from leaking into the readout circuit region R or such leakage may be reduced. 
     In addition, since an epitaxial layer  310  is exposed to the outside of the silicon photomultiplier detector cell  200  in the readout circuit region R, a bias voltage may be applied to the substrate  210  via the upper surface of the silicon photomultiplier detector cell  200 . 
     Various circuit devices may be stacked in the readout circuit region R. For convenience of explanation, the two CMOS layers  330  and  340  are illustrated in  FIG. 6 . 
       FIGS. 7A through 7H  are cross-sectional views illustrating a method of fabricating the silicon photomultiplier detector cell  200  of  FIG. 6 . 
     Referring to  FIG. 7A , a first layer  410  is grown on the substrate  210 . The substrate  210  may be a silicon layer doped with p-type impurities. The first layer  410  may also be doped with p-type impurities. The first layer  410  may be formed by epitaxially growing p-type silicon having a low doping concentration via chemical vapor deposition (CVD). The first layer  410  may be commonly used to form a device of a readout circuit region, for example, a CMOS. Thus, the thickness of the first layer  410  may be similar to a thickness required to form a device of the readout circuit region R. Since such an layer epitaxially grown across the substrate  210  may be used as a base member for forming a photodiode and a readout circuit, a photodiode and a signal processing device may be simultaneously formed. 
     As illustrated in  FIG. 7B , the second semiconductor layer  230  is formed in a part of the first layer  410 . To form the second semiconductor layer  230 , silicon doped with p-type impurities may be epitaxially grown by ion implantation or ion diffusion. The second semiconductor layer  230  may be a shallow isolation layer. 
     As illustrated in  FIG. 7C , a second layer  420  may be additionally grown on the first layer  410  such as to cover the second semiconductor layer  230 . The second layer  420  may be doped with p-type impurities, and the impurity concentration of the second layer  420  may be equal to that of the first layer  410 . Thus, the second layer  420  formed on the first layer  410  may not be separated from the first layer  410 . 
     As illustrated in  FIG. 7D , to divide the silicon photomultiplier detector cell  200  into the photodiode region D and the readout circuit region R, the trench  250  is formed by etching the first and second layers  410  and  420  by a desired width (that may or may not be predetermined). The trench  250  may be formed by etching the first and second layers  410  and  420  so that the substrate  210  is exposed. In some cases, a part of the substrate  210  may be etched. Due to the formation of the trench  250 , a part of the first layer  410  between the second semiconductor layer  230  and the substrate  210  becomes the second epitaxial layer  290 . A part of the second layer  420  on the second semiconductor layer  230  may be the first epitaxial layer  240 . 
     An insulation film  252   a  may be formed on a side wall of the trench  250 . The insulation film  252   a  may not only help formation of the contact layer  270 , which will be described later, but also insulate the photodiode region D from the readout circuit region R. The insulation film  252   a  may be also formed on a lower surface of the trench  250 . 
     As illustrated in  FIG. 7E , the contact layer  270  may be formed by implantation. The contact layer  270  may be formed along the insulation film  252   a  while contacting the second semiconductor layer  230 , and may be exposed to the outside. The contact layer  270  may be doped with the same type of impurities as those of the second semiconductor layer  230 , but the impurity concentration of the contact layer  270  may be less than that of the second semiconductor layer  230 . 
     As illustrated in  FIG. 7F , the film  254  capable of reducing or preventing optical crosstalk may be formed by filling the trench  250  with polysilicon, and then an exposed part of the film  254  capable of reducing or preventing optical crosstalk may be covered with an insulation film  252   b . The film  254  capable of reducing or preventing optical crosstalk may reduce or prevent optical crosstalk between the photodiode region D and the readout circuit region R. The trench  250  may be filled with a material other than polysilicon as long as the material is capable of reducing or preventing optical crosstalk. For example, the trench  250  may be filled with metal instead of polysilicon. The polysilicon or metal may not only reduce or prevent optical crosstalk, but also serve as a field plate. The insulation film  252   b  formed on the surface of the film  254  capable of reducing or preventing optical crosstalk may insulate the photodiode region D from the readout circuit region R. The first insulation film  252  including the insulation films  252   a  and  252   b  may be a silicon oxide film or a silicon nitride film. The first insulation film  252  may not only perform an insulation function but also reflect incident light. 
     As illustrated in  FIG. 7G , the second insulation film  280  may be formed in contact with the contact layer  270  in order to electrically separate the first semiconductor layer  220 , which will be formed later, from the contact layer  270 . The second insulation film  280  may be a silicon oxide film or a silicon insulation film. 
     As illustrated in  FIG. 7H , the first semiconductor layer  220  may be formed by doping an exposed part of the second layer  420  with n-type impurities. The impurity type of the first semiconductor layer  220  may be opposite to that of the second semiconductor layer  230 , and the impurity concentration of the first semiconductor layer  220  may be less than that of the second semiconductor layer  230 . A part of the second layer  420  between the first and second semiconductor layers  220  and  230  may be the first epitaxial layer  240 . In the readout circuit region R, necessary layers may be formed. As illustrated in  FIG. 7H , the two CMOS layers  330  and  340  may be formed. 
     In order to check photo detection efficiency, a comparative example having a diode region in which a p-type first semiconductor layer, an n-type second semiconductor layer, an n-type first epitaxial layer, an n-type third semiconductor layer, a p-type second epitaxial layer, a p-type substrate are formed from the surface of the diode region was set, and a silicon photomultiplier detector cell according to some example embodiments including a p-type first semiconductor layer, a p-type first epitaxial layer, an n-type second semiconductor layer, a p-type second epitaxial layer, and a p-type substrate was set.  FIG. 8  shows experimental data obtained by detecting photo detection efficiency depending on the structures of silicon photomultiplier detector cells. Referring to  FIG. 8 , the photo detection efficiency of the silicon photomultiplier detector cell according to some example embodiments is about 1.6 times greater than that of the comparative example. When considering up to a fill factor, the photo detection efficiency of the silicon photomultiplier detector cell according to some example embodiments may be further increased. 
     A silicon photomultiplier detector cell according to some example embodiments may measure the amount of radiation without regard to the capacity of memory because the radiation is counted in real time, thereby increasing energy resolution. 
     A silicon photomultiplier detector cell according to some example embodiments may not only improve energy resolution and time resolution of a detector by increasing photo detection efficiency, but also improve the contrast of tumoral tissue or the like and the accuracy of the position of tumoral tissue or the like. 
     It should be understood that the exemplary embodiments described therein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments.