Abstract:
A device that detects single optical and radiation events and that provides improved blue detection efficiency and lower dark currents than prior silicon SSPM devices. The sensing element of the devices is a photodiode that may be used to provide single photon detection through the process of generating a self-sustained avalanche. The type of diode is called a Geiger photodiode or signal photon-counting avalanche diode. A CMOS photodiode can be fabricated using a “buried” doping layer for the P-N junction, where the high doping concentration and P-N junction is deep beneath the surface, and the doping concentration at the surface of the diode may be low. The use of a buried layer with a high doping concentration compared to the near surface layer of the primary P-N junction allows for the electric field of the depletion region to extend up near the surface of the diode. With a low doping concentration through the bulk of the diode, the induced bulk defects are limited, which may reduce the dark current. The resulting structure provides a diode with improved quantum efficiency and dark current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/656,089, filed Oct. 19, 2012, which claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/549,958, filed Oct. 21, 2011, which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     Described herein is a photodiode having a buried P-N junction. The photodiode may be used in a solid-state photomultiplier that can detect optical or radiation events. 
     2. Discussion of the Related Art 
     Photomultiplier tubes (PMTs) are a standard technology for detecting small light pulses. The photomultiplier tube is a vacuum tube technology that uses a photocathode, dynodes, and an anode. PMTs provide excellent performance characteristics in that they have a large gain (˜10 6 ) and have a good quantum efficiency over the spectral range determined by the photocathode material. PMTs are limited in that they are bulky, require high voltage, and are susceptible to large magnetic fields and helium, for example. 
     Silicon avalanche photodiodes (APDs) are an alternative to PMTs in low-light applications that can have a compact size, high quantum efficiency, insensitivity to magnetic fields and the possibility of mass-production using planar processing. Very large area APDs (40 cm 2 ) as well as monolithic APD arrays (28×28 elements, 0.8-mm pixels) have been fabricated at Radiation Monitoring Devices (RMD), of Watertown, Mass., using a planar process. APD arrays are also being manufactured by other vendors such as Perkin-Elmer, of Waltham, Mass., and Hamamatsu, of Hamamatsu, Japan. 
     The APDs do, however, have some performance limitations. In particular, the gain of most APD designs is not very high. The APDs manufactured by Hamamatsu and Perkin Elmer with reach-through designs exhibit gains in the range of 50-100. The gain of deep-diffused APDs manufactured by RMD is higher (˜1000 or more). These gain values are lower than those for PMTs (with gain ˜10 6 ), and additional circuitry may be needed, such as a relatively low-noise preamplifier, to achieve a high signal-to-noise ratio though the use of APDs. Another disadvantage is that the temporal response of most APD designs is also not as fast as that of high-end PMTs. While APDs are promising devices and are playing an important role in low-light applications, alternative silicon based photodetectors (such as the proposed solid-state photomultipliers) are also promising as they provide a gain comparable to PMTs, very high timing resolution and compact size. 
     The Geiger photodiode (GPD), also termed a single photon counting diode, is based on the avalanche photodiode with a different mode of operation. The response of the GPD is binary for an incident photon with a large gain (˜10 6 ) proportional to the diode junction capacitance and the bias above breakdown. The signal response of each GPD is independent of the number of incident photons, and the probability to generate a self-sustained avalanche is dependent on the bias above breakdown. Once the avalanche is induced either thermally or optically, the diode discharges, creating a sizable current. If a ballast resistor is used in series with the GPD, the self-sustained avalanche can be quenched as the voltage drop across the resistor reduces the bias across the diode to the breakdown voltage. The diode will then recharge and wait for the next avalanche event. The table below provides a brief history of the development of the Geiger photodiodes, and the integration of these diodes for fabrication of solid-state photomultipliers. 
     
       
         
               
               
             
           
               
                   
               
               
                 Year 
                 Development 
               
               
                   
               
             
             
               
                 1973 
                 McIntyre predicts Geiger-mode operation for APDs operated 
               
               
                   
                 above their reverse bias breakdown voltage. 
               
               
                 1985 
                 McIntyre experimentally validates operation of Geiger-mode 
               
               
                   
                 APDs. The electric field in the device enables the operation of 
               
               
                   
                 APD devices above their reverse bias breakdown voltage. 
               
               
                 1993 
                 RMD reports on the fabrication of Geiger-mode APDs (GPDs). 
               
               
                   
                 Reducing the size of the APD decreases the amplitude of the 
               
               
                   
                 thermal signal, producing lower DCR in GPD devices. 
               
               
                 1996 
                 Cova et al. reports on the single photon avalanche 
               
               
                   
                 diode response function in relation to fabricated 
               
               
                   
                 devices in a silicon process. 
               
               
                 2001 
                 Buzhan et al. report on the fabrication of an SiPM using the 
               
               
                   
                 MRS process. The integration of resistors in the MRS process 
               
               
                   
                 enables the fabrication of large arrays with two readout contacts. 
               
               
                 2004 
                 RMD reports on the migration GPD pixels to a commercial  
               
               
                   
                 CMOS process. The use of commercial CMOS process 
               
               
                   
                 facilitates the integration of circuit components, and the 
               
               
                   
                 use of an available multi-user service substantially 
               
               
                   
                 reduces the development cost and time. 
               
               
                   
               
             
          
         
       
     
     The solid-state photomultiplier (SSPM) is built from an array of Geiger photodiodes (GPDs). If the GPDs are placed in an array and read out in parallel, the result provides a two-terminal photodetector, where the signal is proportional to the number of GPDs triggered and the incident light intensity. Since the 1970s, solid-state (silicon) photomultipliers have been in development, and by 2001, fabrication of MRS (metal-resistor semiconductor) SSPMs was successful. CMOS (complementary metal-oxide semiconductor) devices were soon to follow. A number of instruments for the medical, scientific, and defense fields are being developed using SSPMs. Broad device characteristic studies have been made discussing the response of SSPMs for detection efficiency and noise terms (cross talk, after pulsing, and dark counts). 
     When operating the GPD pixel, the detection efficiency (DE) is proportional to the probability of creating a Geiger avalanche, or Geiger probability. The Geiger probability is related to the electric field within a GPD, which is typically very high. The probability is roughly linearly related to the bias above the breakdown voltage until a 100% Geiger probability is reached. The detection efficiency refers to the overall product of the photon absorption, charge collection, and generation of a self-sustained avalanche, i.e., the ratio of detected photons to incident photons. 
     The three typical noise sources attributed to SSPMs arise from dark counts, cross talk, and after pulsing. Noise from dark counts, or thermally induced Geiger events, is analogous to dark current in conventional solid-state detectors. The contribution of noise from dark counts is expected to scale with the active area of the SSPM detector. The second significant noise term arises from statistical fluctuations in the optical cross talk between pixels in the SSPM. The Geiger discharge from the GPD pixels in the SSPM produces hot carrier emission of photons from the diode, which may cause cross talk between pixels. The third source of noise in the SSPM arises from the statistical fluctuations associated with after pulsing. The avalanche process creates an ensemble of charge across the diode junction, some of which will populate traps in the silicon. As these traps are liberated, the pixel may avalanche again, yielding another pulse (after pulse). Since there is a probability associated with the occurrence of an after pulse, these pulses add to the parallel noise of the device when integrating the signal. 
     As the proportional response is provided by the number of pixels triggered in the SSPM, there is an upper bound in the SSPM response, since a GPD can trigger once for integration times on the scale of the GPD recharge time. A nonlinear response function developed for SSPMs can be used to understand the effects on the energy resolution as the number of triggered pixels approaches the number of available pixels. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a cross section of a photodiode structure having a buried layer, according to some embodiments. 
         FIG. 2  shows the top view of the photodiode structure with the metal and oxide layers removed. 
         FIG. 3  shows a plot of the measured response with respect to the dark noise of the photodiode structure of  FIG. 1  compared to that of a conventional SSPM GPD pixel. 
         FIG. 4  shows plots of the efficiency of the photodiode of  FIG. 1  versus wavelength compared to a conventional SSPM GPD pixel. 
         FIG. 5  shows a plot of the current versus voltage (IV) characteristic of the photodiode of  FIG. 1  under reverse bias. 
         FIG. 6 a    and  FIG. 6 b    show examples of readout arrangements for solid-state photomultipliers. 
         FIG. 7  shows a diagram of SSPM operation for low-light pulses. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein is a photodiode having a buried semiconductor layer of a high doping concentration, according to some embodiments. The photodiode may be operated as a Geiger photodiode, such that it is reverse biased above its breakdown voltage. An array of such photodiodes can be used to form a solid state photomultiplier (SSPM) that detects electromagnetic radiation. A radiation detector can be produced that includes a scintillator material and a SSPM. The SSPM can detect light produced by the scintillator material in response to incident radiation, including ionizing and non-ionizing radiation. The photodiode described herein may be used for photodetection or for detection of any phenomenon that may generate an electron-hole pair in the diode that may cause the diode to undergo a self-sustained avalanche. 
       FIG. 1  shows a cross section of a photodiode structure  1  having a buried layer, according to some embodiments. Also shown in  FIG. 1  is a material  22  that may produce electromagnetic radiation in response to incident radiation. The electromagnetic radiation produced by material  22  may be detected using an array of photodiodes such as the photodiode illustrated in  FIG. 1 . In some embodiments, the source material  22  is considered a light source, a source of ionizing radiation, or a scintillation material. 
     Various scintillator materials are available which convert high energy incoming radiation, including particles or photons, into light having a wavelength within or near the visible range. The scintillator material may be suitable for detection of neutrons and/or X-rays, or any other suitable type of radiation, including heavy-ions, mesons, gamma rays, beta particles and alpha particles, for example. The scintillator material may have a crystalline structure or an amorphous structure. In some embodiments, the scintillator material may be packaged together with the semiconductor chip to produce an integrated radiation detector. Any suitable scintillator material may be used, examples of which include Cs 2 LiYCl 6 , Cs 2 LiYCl 6 :Ce 3+ , Cs 2 LiYBr 6 , Cs 2 LiYBr 6 :Ce, Cs 2 LiYX 6 :Ce 3+  or CsRbLiYX 6 :Ce 3+  where X is Br or Il, NaI(Ti), Cs 2 NaLaBr 6 , Cs 2 NaGdI 6 , Cs 2 NaLaI 6 , Cs 2 NaLuI 6 , Cs 2 LiLaF 6 , Cs 2 LiLaCl 6 , Cs 2 LiLaBr 6 , Cs 2 LiLaI 6 , Li glass, LiI doped with Eu, LiF+ZnS(Ag), Diphenylanthracene, Polyvinyl toluene, NaI, CsI, BGO, LSO, LYSO, PbWO 4 , SrI, or Stilbene. 
     As shown in  FIG. 1 , photodiode structure  1  may be formed in an epitaxy  2  of semiconductor material. Epitaxy  2  may be formed on a low-resistance substrate  3 . The epitaxy  2  or substrate  3  may be formed of silicon or any other suitable semiconductor material. As discussed below, regions of various doping concentrations are formed in the epitaxy  2 . 
     Photodiode structure  1  may include a region  4  of a first conductivity type (e.g., p-type) and a buried region  6  of a second conductivity type (e.g., n-type) forming the p and n terminals, respectively, of a P-N junction diode. Region  4  may be formed at the upper surface of the epitaxy  2  and extending down to buried region  6 . Buried region  6  contacts region  4 , thereby forming a buried P-N junction  8  at the interface between buried region  6  and region  4 . As shown in  FIG. 1 , buried region  6  may be formed underneath region  4 , i.e., farther from the upper surface of the epitaxy  2 . As an example, the upper surface of buried region  6  may be 0.4-20 μm from the upper surface of the semiconductor region  2 . However, the buried layer  6  may be spaced apart from the upper surface of semiconductor region  2  by any suitable distance, as the techniques described herein are not limited to specific dimensions. 
     The buried region  6  may have a doping concentration higher than that of region  4 . For example, in some embodiments, the buried region  6  may have a doping concentration of from 10 18  to 10 19  cm −3  or greater. By doping buried region  6  to a higher doping concentration than that of the first region  4 , the depletion region of the photodiode may extend farther into the first region  4  from the P-N junction  8  than it extends into the buried region  6 . The depletion region thus extends primarily in an upward direction from the buried P-N junction  8  toward the surface of epitaxy  2 . 
     Side wall breakdown of the photodiode can be prevented by forming a guard ring encompassing region  10  and region  12 , where region  10  is of the same conductivity type as region  4 . Region  12  is of the same conductivity type as the buried region  6 , but a different type than the epitaxy  2  and region  10 . In some embodiments, region  10  may include a diffusion or implant that surrounds region  4 . However, the guard ring formed from regions  10  and  12  may be formed in any suitable shape. For example, the outer edge of guard region  10  may have a rectangular shape (e.g., a square shape) as shown in  FIG. 2 . The drawing in  FIG. 2  shows the regions of the diode structure that are included in the epitaxy  2 , as the upper regions of oxide and metal are not drawn for clarity. In some embodiments, the region  10  may extend from the upper surface of the substrate  2  down to or near the buried layer  6 . The region  12  extends from the surface down to the buried region  6 , making an electrical connection to the buried region  6 . The width of region  10  can be 0.1 μm or more, and the width of region  12  can be 0.1 μm or more. In some embodiments, region  12  may include multiple structures within region  12 , having a doping concentration gradient that increases in concentration from the buried region  6  to the surface of epitaxy  2 . 
     One or more electrical contacts  14  may be formed on the surface of region  12 . Contact(s)  14  may be formed on a region  16  of high doping concentration to form an ohmic contact to region  12 . Region  16  may have a width of no more than 50% of region  12  with a depth of less than 1 μm. Contacts  14  can be connected to through oxide vias to metal layers on top of possible oxides  24  or oxide  25 . Similarly, one or more electrical contacts  18  may be formed on the surface of the intersection of regions  4  and  10 . Contact(s)  18  may be formed on a region  20  of high doping concentration which may be no less than 10 18  cm −3  to form an ohmic contact to region  4  and  10 . The width of region  20  may be greater than 0.2 μm, and region  20  is considered to be wide enough to provide sufficient conduction for both regions  4  and  10 . Metal contacts  14 ,  29 , and  18  may be no larger than the implant contact regions  16  and  20 . In some cases, the width of the metal contact to the implant region is identical. 
     One possible embodiment of the photodiode may have a thin region  29 , which is less than 0.1 μm in depth from the surface of epitaxy  2  and located within region  4 . The region  29  has an electrical conductivity similar to region  4  and may be doped identical or higher than region  4 . 
     Within some embodiments, an electrical connection of the diode is in series with a quenching resistor  28 . This quenching resistor  28  is electrically connected at one end with a possible metal to polysilicon connection  19 . The length and width of the resistor indicates the corresponding resistance. A resistance greater than 1 kΩ may be used for the quenching resistor  28 . The trace shape for the quenching resistor  28  may be of any form (e.g., circular, zig-zag, or straight). The resistor may contact either side of the p-n junction. Through oxide vias  29 ,  14 , and  27  are potentially used to isolate the read out trace for the anode and cathode of the diode. Techniques for readout are not confined to isolating the anode and cathode connections on separate metal layers within possible oxides  25  and  26 . Any method for electrical connection is possible, as long as the diodes within the array are in parallel and the anode and cathode for all diodes in the array are electrically isolated. A final oxide  26  is used to protect the metal traces. The number of metal layers and thickness are only limited by the foundry process rules (i.e., more than 2 metal layers can be fabricated and used). In some cases, the width of the polysilicon resistors, metal traces, and connections are no less than 0.1 μm. 
     The region  30  within the top oxide layers  24 ,  25 , and  26  indicates a region for optical conditioning of the photodiode in some embodiments. The region may be removed by an etching process, and the silicon surface may be treated to improve the optical quantum efficiency through the use of anti-reflective coatings or surface texturing. The area outside of region  30  may or may not include a metal fill at any of the metal layers above epitaxy  2 . 
     Although  FIG. 1  shows a photodiode structure  1  in which buried region  6  is an n-type region and region  4  is a p-type region, a photodiode may be formed in which these regions have the opposite conductivity type. For example, buried region  6  may be formed as a p-type region of a high doping concentration and region  4  may be formed as an n-type region, and the conductivity type of the other regions in substrate  2  may be reversed, as well. The doping concentration of region  4  may be different than region  10 . In some embodiments, region  10  has a concentration of less than or equal to that of region  4 . The differential charge distribution between the junction of regions  10  and  12  is lower than regions  4  and  6 , providing a higher reverse bias breakdown voltage for the junction form from region  10  and  12  compared to the junction form from region  4  and  6 . In some embodiments, region  4  may have any suitable doping concentration or concentration gradient such that the concentrations are not less than that of region  10 . 
     A photodiode structure as shown in  FIG. 1  was fabricated using a 0.18 μm high voltage CMOS process. The resulting photodiode shows a significant improvement in the performance characteristics compared to a conventional CMOS-based GPD design. 
       FIG. 3  shows the count rate of the photodiode structure of  FIG. 1  compared to a conventional SSPM GPD when exposed to the same light conditions. The count rate for the photodiode structure of  FIG. 1  (Buried pn Junction) increases faster than the count rate for a conventional SSPM GPD as a function of the dark count rate. This is a strong indication that the detection efficiency of the photodiode structure of  FIG. 1  has better signal to noise performance than the conventional SSPM GPD. 
       FIG. 4  shows a plot of the detection efficiency at approximately 45% Geiger probability, P g , versus wavelength for the photodiode of  FIG. 1  (Buried pn Junction) along with the detection efficiency from a conventional SSPM GPD. As shown in  FIG. 4 , the photodiode of  FIG. 1  has excellent response to blue light, which enables the photodiode to operate effectively with state-of-the art scintillator materials. 
       FIG. 5  shows a plot of the measured current versus voltage for the photodiode of  FIG. 1 , which shows Geiger behavior when the diode is placed in series with a quenching (ballast) resistor. The solid curve  31  shows the IV characteristics for a photodiode of  FIG. 1 . For comparison, dashed curve  32  shows a parabolic IV relationship. 
     There is a region of operation that spans the diode breakdown voltage and the point where the quenching circuit (resistor) fails. The voltage above breakdown is termed the excess bias or V x . The expected response should follow a parabolic dependence and a deviation  33  above that dependence is related to excess charge generated in the diode. The excess charge has noise associated with it, and degrades the performance. 
     As discussed above, the Geiger photodiode of  FIG. 1  may be a component of a solid-state photomultiplier having an array of such Geiger photodiodes formed on a semiconductor substrate. As shown in  FIG. 6 a    and  FIG. 6 b   , the SSPM can include a plurality of photodiodes in parallel, with each photodiode being in series with an active quenching circuit ( FIG. 6 a   ) or a passive quenching circuit (e.g., a quenching resistor, as shown in  FIG. 6 b   ). The distance of the diodes from  FIG. 1  indicated by epitaxy  2  may come in direct contact with each other or be space at larger distances. The quenching circuit may be designed to sense the current draw from the diode and stop the self-sustained avalanche. The quenching circuits may also be formed on the same semiconductor substrate as the photodiode. A signal processing circuit may also be formed on the substrate to analyze the signals produced by the photodiode array. The array of photodiodes may be read out in parallel. The signal processing circuit may count the number of photodiodes triggered into avalanche by the reception of incident photons. The number of photodiodes triggered is proportional to light yield from the energy deposited from incident radiation striking the scintillator material  22 , or for any pulse light source, the number of triggered diodes is proportional to the incident light intensity on the SSPM. 
       FIG. 7  shows a diagram of the operation of the SSPM for low-light levels. The upper images illustrate arrays of Geiger photodiodes, and triggered diodes are highlighted. The bottom plot shows data from pulsed LED light on a small silicon SSPM. The term p.e. stands for photoelectrons. The number of diodes triggered is proportional to the signal from the SSPM indicated from the arrows to the fine structure in the bottom plot. The overall width of the distribution is associated with the statistics of the average number of photons detected over all light pulses. 
     This invention is not limited in its application to the details of construction and the arrangement of components set forth in the foregoing description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.