Patent Publication Number: US-2013249035-A1

Title: Silicon photomultiplier and radiation detector

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and hereby claims priority to International Application No. PCT/EP2011/064364 filed on Aug. 22, 2011 and European Application Nos. 10009502.5 filed on Sep. 13, 2010 and 10193436.2 filed on Dec. 2, 2010, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     The invention relates to a silicon photomultiplier with a silicon chip comprising an array of microcells, wherein the microcells form photon-sensitive active areas, each surrounded by photon-insensitive inactive areas. The invention further relates to a radiation detector with a scintillator and a silicon photomultiplier. 
     Silicon photomultipliers (SiPMs) are solid-state photo sensors that have been developed for very low-level light sensing applications. They include of an array of microcells, which are each operated in so-called Geiger mode. These devices have been commercially available for a few years now, and they are well suited for applications in medical imaging, such as Positron Emission Tomography (PET), Single Photon Emission Computed Tomography (SPECT) and Computed Tomography (CT). They generally have reasonably high detection efficiencies, high signal-to-noise ratios due to their internal gain and short pulse widths of a few ns. Promising results on the use of SiPMs for PET have been published in recent years, e.g. in S. Seifert et al., “Ultra Precise Timing with SiPM-Based TOF PET Scintillation Detectors”, 2009, IEEE Nuclear Science Symposium Conference Record; D. Henseler et al., “SIPM Performance in PET Applications: An Experimental and Theoretical Analysis”, 2009, IEEE Nuclear Science Symposium Conference Record and T. Frach et al., “The Digital Silicon Multiplier—Principle of Operation and Intrinsic Detector Performance”, 2009, IEEE Nuclear Science Symposium Conference Record. 
     The most common PET scintillators Lutetiumoxyorthosilicate (LSO) and Lutetiumyttriumoxoorthosilicate (LYSO) emit light in the blue part of the visible spectrum, with a peak emission around 420 nm. The photon detection efficiency (PDE) of SiPMs is normally between 10% and 50% in this wavelength range. The higher-efficient devices compare favourably with classical photomultiplier tubes (PMTs), which have quantum efficiencies of around 25%. But other silicon based devices like regular APDs or blue-enhanced pin diodes (positive intrinsic negative diodes) easily reach quantum efficiencies as high as 80-85%. The PDE of a SiPM is lower mainly because of the limited active area fill factor and also due to a limited avalanche probability. 
     The area fill factor in an analogue SiPM is limited by the space taken up by gaps between the microcells and also by the space for a quenching resistor, typically made of polysilicon material. In a digital SiPM, space is taken up by the cell gaps and a set of transistors (see T. Frach et al., “The Digital Silicon Multiplier—Principle of Operation and Intrinsic Detector Performance”, 2009, IEEE Nuclear Science Symposium Conference Record). 
     The loss in PDE due to the inactive regions matters especially for PET, because the coincidence time resolution of a scintillation event is a strong function of the number of detected photons n. In a first approximation, the coincidence resolving time is proportional to 1/sqrt(n). The photon yield also has a strong impact on the energy resolution of the pulse height spectrum and on the spatial resolution of an event, which is relevant for both PET and SPECT applications. A high light yield is also important for many other applications like Computed Tomography, astrophysics and high energy physics experiments. 
     In most commercially available SiPMs, the width of the cell gaps are between 5 and 15 μm, and the quenching resistor takes up another few 10 to a few 100 μm2 per cell. The gaps are required to electrically isolate the active regions from each other, but also to accommodate the contact lines for bias and/or signal contacts and in some cases other features like optical isolation trenches or guard structures. A high PDE can generally be achieved if a relatively large cell pitch is used, because then the area fractions for the gaps and quench resistors are lower (see table below, which shows known device parameters for 3×3 mm2 SiPMs with different cell sizes). 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 cell size 
                 25 × 25 μm2 
                 50 × 50 μm2 
                 100 × 100 μm2 
               
               
                   
               
             
            
               
                 active area fill 
                 30.8% 
                 61.5% 
                 78.5% 
               
               
                 factor 
               
               
                 photon detection 
                     25% 
                     50% 
                     65% 
               
               
                 efficiency (PDE) 
               
               
                 number of cells 
                 14400 
                 3600 
                 900 
               
               
                 per device 
               
               
                   
               
            
           
         
       
     
     On the other hand, most applications require a minimum number of microcells, because saturation effects will occur if the number of microcells is of the same order as the number of impinging photons. This saturation leads to degradation of the energy resolution. Another disadvantage of very large cells is the high cell capacitance and the resulting broader pulse response, which has a negative effect on the time resolution. 
     Depending on the application, the relevant signal levels and the linearity requirements, there is usually an optimum cell size that gives the best performance level: For studies on PET applications, the 50 μm cell size is most commonly used. It seems to give the best trade-off for the average signal levels for 511 keV radiation, after conversion to visible light by standard PET scintillators. 
     The coating of the inactive parts of the SiPM with a reflective material (e.g. aluminium) has been suggested in P. Barton et al., “Effect of Surface Coating on Light Collection Efficiency and Optical Crosstalk for SSPM- Scintillation Detection”, Nucl. Instr. Meth. Phys. Res. A 610 (2009) 393-396. Based on ray-tracing simulations by these authors, a significant increase in overall light yield was expected in particular for geometries with small fill factors. However, the simulations also predict an increase in the optical crosstalk between cells due to the additional reflector and therefore a further limitation to the useful bias voltage range. The use of such additional reflecting layers on the cell gaps has not yet become established in any commercial SiPMs. But the cell gaps of any existing device will at least be partially reflective, because the metal contact lines will introduce some reflectivity to parts of the inactive regions. 
     SUMMARY 
     One potential object is to specify a silicon photomultiplier with improved detection efficiency. 
     Accordingly, the inventors propose a silicon multiplier with a silicon chip comprises an array of microcells, wherein the microcells form photon-sensitive active areas, each surrounded by photon-insensitive inactive areas. According to the proposal, the silicon multiplier further comprises at least one elevated, three-dimensional light concentrating structure located directly on top of the Silicon chip within an inactive area and configured such that photons that would have hit an inactive area are redirected towards an active area. 
     The key idea is the design of a SiPM with three-dimensional light concentration structures, guiding incident light, e.g. light from a scintillator, to the active parts of the microcells. Thus, the purpose of these structures is to enhance the PDE of the SiPM by redirecting the photons that would have hit an inactive area towards an active area of the SiPM, which leads to a large increase of the detection efficiency (modelled by the total light yield of the SiPM). 
     Compared to a simple reflective coating, the above-described structures have the advantage that they do not lead to an increase in optical cross-talk between the microcells. 
     In contrast to the known approach of covering the inactive gaps with a reflective coating, the three-dimensional structures not only lead to a higher increase of the overall efficiency, but they also result in large gains for the fraction of rays which have accumulated only short optical path length delays on their way to the active sensor part. This is particularly important for PET applications, where the time resolution is determined mainly by the light arriving within the first few 100 ps of a scintillation pulse. 
     For optimum performance in PET applications, special emphasis is placed on keeping the optical path lengths of the photons as short as possible before hitting the active part of the sensor. The reason is that large optical path lengths lead to additional time delays before detection and therefore to a deterioration of the time resolution. To achieve the most precise event timing information, the timing trigger should be derived from the first few photons, which arrive within the first few 100 ps of the emission pulse. These early photons have accumulated very short optical path lengths in the LSO and the optical coupling layers. In other words, the photons that are most relevant for the timing information are the ones that successfully hit the active sensor area without being reflected back into the scintillator a second time. 
     Apart from PET there will also be many other applications in different areas of medical imaging (e.g. SPECT, CT and other X- ray detectors) as well as astrophysics, high-energy physics and other analytics applications, where the increase in light yield (and especially the enhancement of early photons) will be a big advantage. 
     According to one embodiment the inactive areas of the SiPM are formed by cell gaps, which electrically isolate the microcells from each other, and by a frame area surrounding the array of microcells. Accordingly, the at least one light concentrating structure is located in the area of the cell gaps and/or the frame area. 
     The cell gaps can also accommodate contact lines for bias and/or signal contacts and in some cases other features like optical isolation trenches or guard structures. 
     In order to achieve an increase of the detection efficiency that is as large as possible a preferred embodiment provides at least one light concentrating structure, which completely covers the cell gaps and/or the frame area. 
     According to an embodiment, the at least one light concentrating structure is mounted on the Silicon chip and covered by an encapsulation material. Such an arrangement can be manufactured relatively simple, e.g. by a lithography process with subsequent isotropic etching. 
     According to another embodiment, the at least one light concentrating structure is a structured part of an encapsulation material, which is directly located on top of the Silicon chip. Such an arrangement can also be manufactured relatively simple, e.g. by a hot embossing process. 
     In a preferred embodiment, the at least one light concentrating structure has a basically triangular-like cross section. However, the light concentration structures could also have other geometric shapes, which can have higher collection efficiencies than the simple triangular structure. The most important example of an alternative geometry is a so-called compound parabolic concentrator (CPC), where the lateral surfaces, which serve as reflective walls or index boundaries, form sections of a parabola. The gradient of the CPC side wall at the light&#39;s entry aperture is very steep, ideally the walls are parallel to the detector normal at the top of the structure. 
     Both designs are easy to manufacture and result in large gains for the fraction of rays which have accumulated only short optical path length delays on their way to the active sensor part, which is particularly important for all applications, where the time resolution is determined mainly by the light arriving within the first few 100 ps of a light pulse. 
     In order to achieve a good reflectivity of the light concentrating structure the structure can comprise a reflective coating, in particular a metal coating. Alternatively, the at least one light concentrating structure can be made of a solid reflective material. In those implementations, where the light concentration structure is made of solid material, the material can be non-reflective at the bottom side and would therefore not enhance the reflection of IR light emitted within the SiPM. 
     In a further embodiment the at least one light concentrating structure is configured as a hollow three-dimensional structure, so that total internal reflection (TIR) can occur inside the encapsulation material, which covers the structure, at the borders of the structure, i.e. at the lateral surfaces of the hollow structure. At incidence angles that are steeper than the limiting TIR angle, the light can enter the hollow structure and travel through them, to eventually hit a neighbouring microcell. 
     The hollow structure can be filled-up with air or another material with a low refractive index so that total internal reflection can occur. 
     In a preferred embodiment the base of the hollow structure is coated with a reflective layer, so the light impinging there has another chance of being directed to the active region. Favourable, the reflective coating could be applied on top of an absorbing layer, so that the reflectivity from the silicon side is not increased. 
     The inventors further propose a radiation detector with a scintillator and the proposed silicon photomultiplier coupled to the scintillator by at least one coupling material. 
     In order to increase the light yield, a refractive index of the coupling material preferably lies in the range of a refractive index of the scintillator. 
     According to one embodiment, the coupling material can be built by an encapsulation material containing the at least one light concentrating structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  schematically shows a silicon photomultiplier (SiPM) as known from the related art in a top view; 
         FIG. 2  schematically shows a cross section through a section of a SiPM according to a first embodiment of the inventors&#39; proposal; 
         FIG. 3  schematically shows a cross section through a section of a SiPM according to a second embodiment of the inventors&#39; proposal; 
         FIG. 4  schematically shows a cross section through a section of a SiPM according to a third embodiment of the inventors&#39; proposal; and 
         FIG. 5  schematically shows a cross section through a section of a radiation detector according to an embodiment of the inventors&#39; proposal. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  schematically shows a silicon photomultiplier  1  (SiPM) as it is known from the related art. The SiPM  1  comprises an array of microcells  2 , which are built on a silicon chip  3 . These microcells  2  form photon-sensitive active areas  2 ′. The active areas  2 ′ are each surrounded by photon-insensitive inactive areas  4 ′, which are formed by cell gaps  5 , which electrically isolate the microcells  2  from each other, and by a frame area  6  surrounding the array of microcells  2 . 
       FIG. 2  schematically shows a cross section through a section of a SiPM  20  according to a first embodiment of the inventors&#39; proposal. Microcells  21  are built on a silicon chip  22  and form photon-sensitive active areas  23  of the SiPM  20 . The active areas  23  and the microcells  21  respectively are each surrounded by photon-insensitive inactive areas  24 , which are formed by cell gaps  25  and by a not shown frame area surrounding the array of microcells  21 . The cell gaps  25  electrically isolate the microcells  21  from each other and can also accommodate contact lines for bias and/or signal contacts and in some cases other features like poly-Silicon or metal quench resistors, transistors, optical isolation trenches or guard structures. 
     A three-dimensional light concentrating structure  27  is located directly on top of the silicon chip  22  within an inactive area  24 - 1  formed by the cell gaps  25 . The light concentrating structure  27  is configured such that photons that would have hit the inactive area  24 - 1  are redirected towards an active area  23 . Thereby, the light concentrating structure  27  completely covers the cell gaps  25 . Therefore, in the shown typical SiPM configuration, where the microcells  21  form a two-dimensional array with the cell gaps  25  forming a two-dimensional rectangular grid, the light concentrating structure  27  is also a two-dimensional, interlaced grid.  FIG. 2  shows a first embodiment, wherein the light concentrating structure  27  has a basically triangular-like cross section. The reflectivity of the structure&#39;s surface can for example be achieved by a reflective coating, e.g. in the form of a metallization layer. In this case the refractive index of the structure&#39;s material itself does not influence the optical properties much. Instead, the material can be chosen to allow optimal three-dimensional patterning and to provide a smooth seed layer for a high-reflectivity metal coating. 
     Alternative to a reflective coating, the light concentrating structure  27  itself can be made of a solid reflective material. 
     The light concentrating structure  27  is covered by an encapsulation material  28 , which is directly located on top of the silicon chip  22 . This can be achieved by integrating the light concentrating structure  27  between the silicon chip  22  and the encapsulation material  28 , e.g. by a lithography process with subsequent isotropic etching. Alternatively, the light concentrating structure  27  can be a structured part of the encapsulation material  28  itself, which is directly located on top of the silicon chip  22 . Such structuring can, for instance, be made by a hot embossing process. 
       FIG. 2  shows a sample ray  29  of schematically indicated incident light  30 , which comes, for instance, from a not shown scintillator. The ray  29  is hitting the light concentrating structure  27 , which leads to a redirection of the ray  29  to the active area  23  of a microcell  21 . Without the light concentrating structure  27  this sample ray  29  would have hit an inactive area  24  of a cell gap  25 . 
       FIG. 3  shows an alternative configuration of an SiPM  20 ′, which differs from the first embodiment in that a light concentrating structure  27 ′ is configured as a hollow three-dimensional structure, so that total internal reflection (TIR) can occur inside the encapsulation material  28  at the borders of the light concentrating structure  27 ′. At incidence angles that are steeper than a limiting TIR angle, the light can enter the light concentrating structure  27 ′ and travel through it, to eventually hit a neighbouring microcell  21 . Favourable, the base of the hollow structure is coated with a reflective layer, so the light impinging there has another chance of being directed to an active area  23 . The use of other, low-index materials instead of air would produce a similar effect. 
       FIG. 3  shows two sample rays  31  and  32  of schematically indicated incident light  30 ′. These rays  31  and  32  are hitting the light concentrating structure  27 ′, which leads to a redirection of the rays  31 ,  32  to active areas  23  of microcells  21 . Without the light concentrating structure  27  sample ray  31  would have hit an inactive area  24  of a cell gap  25 . 
       FIG. 4  shows an alternative configuration of an SiPM  20 ″, which differs from the first and second embodiments in that a light concentrating structure  27 ″ has another geometric shape, which has a higher collection efficiency than a structure with a simple triangular-like cross-section with straight-lined lateral surfaces. The shown alternative geometry is a so-called compound parabolic concentrator (CPC), where the lateral surfaces (reflective walls or index boundaries) of the light concentrating structure  27 ″ form sections of a parabola. The gradient of the CPC lateral surfaces at the light&#39;s entry aperture is very steep, ideally the walls are parallel to the detector normal at the top of the structure. Similar to the geometry shown in  FIGS. 2 and 3 , two-dimensional interlacing grids of structures with a CPC cross section would be formed. 
     In the shown example, the light concentrating structure  27 ″ are formed by hollow compartments, so that the light is guided by total internal reflection inside the encapsulation material  28 . Alternatively, the light concentrating structure  27 ″ can be filled with a low-index material, or the walls of the structure could be covered with a reflective coating (either as a coating on the wall of a hollow compartment or as a coating on a solid structure). As a further option, the CPC structures could also be made of solid, reflective material, in particular metal. 
       FIG. 4  shows a sample ray  40  of schematically indicated incident light  30 ″. The sample ray  40  hits at a limiting collection angle. All rays with steeper incidence than this angle will be collected by the CPC structure. At larger angles with the surface normal, a fraction of the rays can still be collected, depending on the exact position where they enter the concentrator structure. 
     In both implementations (hollow or filled structures), at least photons with steep incidence angle have a high chance of being (re-)directed to an active area  23 . 
       FIG. 5  schematically shows a radiation detector  60  with a scintillator  61  configured, for instance, as an LSO or an LYSO and a SiPM  62 . By way of example, a light concentrating structure  63  of the SiPM  62  is configured like the light concentrating structure  27  of  FIG. 2 . The light concentrating structure  63  is covered by an encapsulation material  64  and the SiPM  61  is coupled to the scintillator  61  by an additional coupling material  65 . 
     Light extraction is made more difficult by a change of the refractive index from the scintillator, e.g. LSO with a refractive index of 1.82) to another refractive index of the coupling material. The light yield can generally be increased by matching the refractive index of the encapsulation material  64  and/or the coupling material  65  to that of the scintillator  61 , so that the refractive index of the additional coupling material  65  and/or encapsulation material  64  lies in the range of the refractive index of the scintillator  61 . 
     It has been found that the matching of the index or generally the increase of the refractive index of these two layers  64  and  65  is particularly advantageous in the presence of light concentration structures. The reason is that for a given incidence angle distribution in the scintillator, a lower-index encapsulation material  64  will lead to a broader angle distribution and a higher-index encapsulation material  64  will lead to a narrower angle distribution on the SiPM  61 . If the angle distribution at the SiPM  61  is narrow and the rays enter at steep incidence angles, then a larger part of the distribution will be within the acceptance range of the light concentrating structure  63 , and the light concentration is more effective. Therefore it is preferable to have a high-index material as the encapsulation material  64 . The refractive index of the optical coupling material  65  matters for the light extraction of the scintillator, but the index of the encapsulation material  64  is even more relevant for controlling the fraction of rays within the acceptance angle of the light concentrating structure  63 . By way of example, for an LSO scintillator a preferred range for the refractive index of the additional optical coupling material  65  is between 1.5 and 1.82, preferably 1.82, and for the encapsulation material  64  between 1.7 and 2. 
       FIG. 5  shows a sample ray  50  of schematically indicated incident light  30 ″′. The sample ray  50  has an angle within the acceptance range of the light concentrating structure  63  after it enters the optically dense encapsulation material  64 . But inside the less dense optical coupling material  65 , the ray had a larger angle that may have been rejected by the light concentrating structure  63 . 
     Alternative to the arrangement shown in  FIG. 6 , the SiPM  61  can be directly coupled to the scintillator  61  by the encapsulation material  64 . In such an embodiment the encapsulation material  64  itself serves as a coupling material. 
     Alternatively to the shown embodiments, the light concentrating structure can cover the cell gaps only partially and/or can cover at least part of the frame area of the SIPM as well. Furthermore the proposed SiPM can comprise more than one light concentrating structures, which together cover the inactive areas of the SiPM completely or partially. 
     The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).