Patent Publication Number: US-2022238744-A1

Title: Semiconductor body, avalanche photodiode and method for fabricating a semiconductor body

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is the national stage entry of International Patent Application No. PCT/EP2020/065058, filed on May 29, 2020, and published as WO 2020/245061 A1 on Dec. 10, 2020, which claims the benefit of priority of European Patent Application Nos. 19177926.3 filed on Jun. 3, 2019, and 19188664.7 filed on Jul. 26, 2019, all of which are incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure is related to a semiconductor body, an avalanche photodiode and a method for fabricating a semiconductor body. 
     BACKGROUND OF THE INVENTION 
     An avalanche photodiode may be implemented as a single-photon avalanche diode, which may be abbreviated SPAD and also named “Geiger-mode avalanche photodiode”. The avalanche photodiode comprises a semiconductor body and a stack having one or more than one isolating layers and one or more than one metallization layers for electrically contacting different regions of the semiconductor body. 
     The semiconductor body comprises a first region of a first type of conductivity and a shallow region of a second type of conductivity that is arranged at the first region. A voltage is provided to the shallow region and the first region such that a space-charge region is generated between the first region and the shallow region. There is an avalanche multiplication region at the border of the first region to the shallow region. The absorption of a photon in the avalanche multiplication region results in the generation of an electron that is multiplied according to the avalanche multiplication effect. Electrons from a depletion region drift towards the avalanche multiplication region. However, also electrons generated outside of the depletion region can diffuse towards the depletion region. Since this diffusion is a slow process, a timing jitter between a light pulse and the avalanche multiplication is increased. 
     SUMMARY OF THE INVENTION 
     It is an object to provide a semiconductor body, an avalanche photodiode and a method for fabricating a semiconductor body that provide structures with reduced timing jitter. 
     These objects are achieved by the subject-matter of the independent claims. Further developments and embodiments are described in the dependent claims. 
     In an embodiment, the semiconductor body comprises a buried layer of a first type of conductivity, a first region of the first type of conductivity, a shallow region of a second type of conductivity at a first surface of the semiconductor body, a sinker of the first type of conductivity located at the first surface of the semiconductor body and a separating region of the first type of conductivity. The separating region encircles at least one of the sinker and the buried layer. The first region is between the buried layer and the shallow region. 
     Advantageously, the separating region hinders electrons to reach an avalanche multiplication region. The avalanche multiplication region is located at the border of the shallow region. Thus, a timing jitter is reduced by the separating region. 
     The first type of conductivity is opposite to the second type of conductivity. The first type of conductivity may be p-type (the regions and layers of the first type of conductivity are p-doped). The second type of conductivity may be n-type (the regions and layers of the second type of conductivity are n-doped). 
     Alternatively, the first type of conductivity may be n-type (the regions and layers of the first type of conductivity are n-doped). The second type of conductivity may be p-type (the regions and layers of the second type of conductivity are p-doped). 
     The word “encircles” means surrounds or encompasses in a plane of the semiconductor body such as e.g. at the first surface or another plane of the semiconductor body that is parallel to the first surface. The separating region encircles at least one of the sinker and the buried layer in planes of the semiconductor body that are different from the first surface, especially below the first surface. 
     In an embodiment, a doping concentration of the separating region is lower than a doping concentration of the sinker and/or the buried layer. There is a gradient of the doping concentration between the separating region and the sinker and also between the separating region and the buried layer. Advantageously, this gradient results in an electric field that hinders electrons to reach an avalanche multiplication region. 
     In an embodiment, the semiconductor body comprises an enhancement region of the first type of conductivity located between the first region and the shallow region. A doping concentration of the enhancement region may be higher than a doping concentration of the first region. The doping concentration of the enhancement region is lower than a doping concentration of the buried layer. The enhancement region may be named enhancement layer. The enhancement layer adjusts the break down voltage. Generally a lower break down voltage reduces the power consumption. 
     In an embodiment, the semiconductor body comprises an isolation region located at the first surface of the semiconductor body. The isolation region encircles the sinker at the first surface. The isolation region may be realized as a shallow trench isolation. Advantageously, the isolation region separates one avalanche photodiode from an adjacent avalanche photodiode. 
     In an embodiment, the sinker encircles the first region. Advantageously, the first region and the enhancement region are enclosed by the sinker, the buried layer and the shallow region at each border or nearly each border. Thus, the first region and the enhancement region are enclosed by crystal material. Advantageously, disturbances and a not desired generation of charge carries is avoided at the border of the buried layer and the shallow region. A doping concentration of the sinker is higher than the minimum doping concentration of the first region. 
     In an embodiment, the sinker extends from the first surface of the semiconductor body to the buried layer. Advantageously, the buried layer is electrically connected via the sinker by a low-resistance connection. 
     In an embodiment, the sinker has a first doping concentration at the first surface of the semiconductor body. This region may be realized by a sinker contact region of the sinker. The sinker has a second doping concentration in a region between the first surface of the semiconductor body and the buried layer. The second doping concentration is lower than the first doping concentration and lower than a maximum doping concentration of the buried layer. The second doping concentration may be higher than the minimum doping concentration of the first region. 
     In an embodiment, the sinker is fabricated at least partially using a well proximity effect, shortened WPE, during fabrication of the buried layer. The sinker described in the paragraph above can be fabricated using the WPE. Advantageously, the number of ion implantation steps can be kept low. The sinker fabricated by WPE is different from a sinker fabricated using a mask, an ion implantation step and an annealing step. The difference can be detected by determining doping concentrations of a cross section through the sinker. 
     In an embodiment, the separating region encircles the sinker. 
     In an embodiment, the semiconductor body comprises a deep trench. The deep trench includes at least one isolating layer. Advantageously, the deep trench provides a good isolation between adjacent avalanche photodiodes. 
     In an embodiment, the deep trench encircles the separating region. Advantageously, the separating region is located between the sinker and the deep trench. Thus, electrons generated at the interface between the isolating layer of the deep trench and the crystal can be hindered from drifting towards the avalanche multiplication region. 
     In an embodiment, the separating region encircles the buried layer. The buried layer may be named buried region. 
     In an embodiment, the separating region is located between the sinker and the buried layer. 
     The sinker may be fabricated using WPE. 
     In an embodiment, the semiconductor body comprises a guard ring of the second type of conductivity located at the first surface of the semiconductor body and encircling the shallow region. A doping concentration of the guard ring is lower than a doping concentration of the shallow region. 
     In an embodiment, the semiconductor body comprises a shield ring of the first type of conductivity. The shield ring is arranged between the guard ring and the buried layer. A doping concentration of the shield ring is lower than the doping concentration of the buried layer and may be higher than a minimum doping concentration of the first region. The shield ring may be fabricated by the WPE. 
     In an embodiment, the semiconductor body comprise a substrate. The substrate may be at a second surface of the semiconductor body. The buried layer is arranged between the substrate and the first region. The substrate may have the first type of conductivity. A doping concentration of the substrate is lower than the doping concentration of the buried layer. 
     The doping concentration of the first region may be equal or approximately equal to the doping concentration of the substrate at least at some areas of the first region. 
     The doping concentration of the separating region may be equal or approximately equal to the doping concentration of the substrate at least at some areas of the separating region. 
     In an embodiment, an avalanche photodiode comprises the semiconductor body and a stack of at least one isolating layer and at least one metallization layer. The stack is located at the first surface of the semiconductor body. The avalanche photodiode may be implemented as single-photon avalanche photodiode, abbreviated SPAD. The avalanche photodiode may be back side illuminated (in this case, the light goes through the buried layer to the first region) or front side illuminated (in this case, the light goes through the stack to the shallow region). 
     A device may comprises an array of avalanche photodiodes. The device comprises the semiconductor body and the stack. The device may include an array of pixels. Each pixel comprises one avalanche photodiode. 
     A time-of-flight arrangement may comprise the avalanche photodiode. A low jitter is advantageous in this application. An avalanche photodiode such as a SPAD or an array of avalanche photodiodes such as SPADs may be applied e.g. for time of flight measurement, a four quadrant SPAD detector for adaptive optics, quantum key distribution, single molecule spectroscopy, single molecule conformational dynamics, DNA analysis by capillary electrophoresis, DNA fragment separation in a microchip, a matrix detector for analysis of protein microarray, fluorescence correlation spectroscopy, fluorescence lifetime spectroscopy and/or fluorescence lifetime imaging. 
     In an embodiment, a method for fabricating a semiconductor body comprises
         fabricating a buried layer of a first type of conductivity,   fabricating a shallow region of a second type of conductivity at a first surface of the semiconductor body, and   fabricating a sinker of the first type of conductivity located at the first surface of the semiconductor body.       

     A separating region of the first type of conductivity encircles at least one of the sinker and the buried layer. A first region has a first type of conductivity and is between the buried layer and the shallow region. Advantageously, the separating region reduces the timing jitter. 
     The above list of steps may not specify the chronological order. 
     In an embodiment, the sinker is fabricated at least partially using a well proximity effect during fabrication of the buried layer. Thus, the lower part of the sinker and the buried layer are fabricated at the same time, e.g. using one mask and one ion implantation step or using one mask and two ion implantation steps or using one mask and more than one ion implantation step. Optionally, the lower part of the sinker and the buried layer are fabricated by two high energy implantation steps with e.g. 2000 keV and 1500 keV implantation energy. 
     With this disclosure, the following issues for a back side illuminated single-photon avalanche photodiode, shortened BSI 
     SPAD, with deep trench isolation may be reduced/improved:
         Reduced jitter tail.   No increase of the dark count rate (shortened DCR) due to deep trench isolation.   Reduced process complexity.       

     In the avalanche photodiode, an electric field is introduced which prevents electrons from the deep trench to reach the avalanche multiplication region. The structure which achieves this effect is a split guard ring. An electric field is introduced which prevents electrons from reaching the avalanche multiplication region, when electrons are generated outside of the active area. The method for fabrication saves one mask and corresponding implants by using the well proximity effect. 
     The method for fabricating a semiconductor body and an avalanche diode may be implemented e.g. by the semiconductor body and the avalanche diode according to one of the embodiments defined above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following description of figures of embodiments may further illustrate and explain aspects of the semiconductor body, the avalanche diode and the method for fabricating the semiconductor body and the avalanche diode. Regions, areas and layers with the same structure and the same effect, respectively, appear with equivalent reference symbols. In so far as regions, areas and layers correspond to one another in terms of their function in different figures, the description thereof is not repeated for each of the following figures. 
         FIGS. 1A to 1C  show an example of a cross-section of an avalanche photodiode; 
         FIGS. 2A to 2D  show further examples of cross-sections of an avalanche photodiode; 
         FIGS. 3A to 3D  show examples of doping concentrations of the avalanche photodiodes of  FIGS. 1A, 1B and 2A to 2D ; 
         FIGS. 4A to 4D  show examples of electrical fields of the avalanche photodiodes of  FIGS. 1A, 1B and 2A to 2D ; 
         FIGS. 5A and 5B  show an example of a doping concentration of a cross-section of an avalanche photodiode; and 
         FIGS. 6A to 6D, 7A to 7D, 8A to 8D, 9A to 9D, 10A to 10D and 11A to 11D  show examples of layouts of masks for fabricating avalanche photodiodes. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  shows an example of a cross-section of an avalanche photodiode  10 . The avalanche photodiode  10  comprises a semiconductor body  11  and a stack  12 . The semiconductor body  11  comprises a first and a second surface  13 ,  14 . The stack  12  is arranged at the first surface  13  of the semiconductor body  11 . The stack  12  comprises a metallization layer  15 , an isolating layer  16  and contacts  19 ,  21 . Different parts  18 ,  20  of the metallization layer  15  are connected via several contacts  19 ,  21  to regions of the semiconductor body  11 . The stack  12  may comprise at least a further isolating layer and at least a further metallization layer, not shown. The metallization layer  15  may be named first metallization layer, shortened M 1 . 
     The semiconductor body  11  comprises a buried layer  25  of a first type of conductivity, a first region  26  of the first type of conductivity and a shallow region  27  of a second type of conductivity. The first type of conductivity is opposite to the second type of conductivity. The first type of conductivity may be p-type conductivity so that the second type of conductivity is n-type conductivity, as indicated in the figures as an example. Alternatively, the types of conductivity may be reversed such that the first type of conductivity is n-type conductivity and the second type of conductivity is p-type conductivity. The buried layer  25  is realized as a deep implanted buried layer. 
     The stack  12  is on top of the first surface  13  of the semiconductor body  11 . Thus, the stack  12  is on top of the shallow region  27 . The shallow region  27  is arranged at the first surface  13  of the semiconductor body  11 . Thus, the shallow region  27  is on top of the first region  26 . The first region  26  is on top of the buried layer  25 . A part  18  of the metallization layer  15  is coupled by at least one contact  19  to the shallow region  27 . 
     The semiconductor body  11  comprises an enhancement region  28  having the first type of conductivity. The enhancement region  28  is arranged between the first region  26  and the shallow region  27 . In a top view shown in  FIG. 6A , the shallow region  27  has the shape of a closed area such as, for example, an octagon, a quadrat, a rectangle or a circle. Correspondingly, the enhancement region  28  also has the shape of a closed area. The enhancement region  28  is fabricated by an implantation suitable to form the first type of conductivity. The enhancement region  28  may e.g. be fabricated by a p-enhancement implant or a p enhancement implant. 
     The semiconductor body  11  comprises a guard ring  29  of the second type of conductivity. The guard ring  29  may be realized as a well, e.g. an n-well. The guard ring  29  is arranged at the first surface  13  of the semiconductor body  11 . The guard ring  29  encircles the shallow region  27 . Thus, in the top view on the first surface  13  of the semiconductor body  11 , the guard ring  29  encircles the shallow region  27 . In the top view on the first surface  13  of the semiconductor body  11 , the guard ring  29  is located at each border of the area of the shallow region  27  and thus surrounds or encloses the shallow region  27 . 
     The semiconductor body  11  comprises a sinker  35 . The sinker  35  is at the first surface  13  of the semiconductor body  11 . The sinker  35  may be realized as a well, e.g. as a p well. The sinker  35  may have a sinker contact region  36  at the first surface  13  of the semiconductor body  11 . The sinker  35  is of the first type of conductivity. The sinker contact region  36  is a highly doped region of the first type of conductivity. The sinker  35  may extend up to the buried layer  25 . A second part  20  of the metallization layer  15  may be coupled by a contact  21  to the sinker  35  and more especially to the sinker contact region  36 . Thus, the second part  20  of the metallization layer  15  is electrically connected by the contact  21  and the sinker  35  to the buried layer  25 . 
     The first region  26  has a part between the guard ring  29  and the buried layer  25 . The first region  26  has a further part between the guard ring  29  and the sinker  35 . 
     The avalanche photodiode  10  may have a cross-section that is symmetrical to a middle axis  37 . Thus, the guard ring  29  seen at the right half of the cross-section is also present at the left half of the cross-section. Correspondingly, the sinker  35  and the second part  20  of the metallization layer  15  are not only shown in the right half of the cross-section, but also in the left half of the cross-section. Thus, the same reference numerals are used in the left halve and in the right halve of the cross-section. Therefore, in the further  FIGS. 1B to 4D , only the “right” half of the avalanche photodiode  10  is shown. 
     Additionally, the semiconductor body  11  comprises an isolation region  38 . The isolation region  38  is located at the first surface  13  of the semiconductor body  11 . The isolation region  38  may be implemented as a shallow trench isolation. The isolation region  38  encircles the sinker  35 . More specifically, the isolation region  38  encircles the sinker contact region  36 . Thus, the isolation region  38  surrounds or encompasses the sinker  35  in a top view on the first surface  13  of the semiconductor body  11 . 
     Furthermore, the semiconductor body  11  comprises a deep trench  39 . The deep trench  39  encircles the sinker  35 . The deep trench  39  may extend from the first surface  13  of the semiconductor body or from the isolation region  38 . On the other side the deep trench  39  extends at least to the buried layer  25 . 
     In one example, the deep trench  39  is manufactured from the back side. Thus, the deep trench  39  is fabricated from the second surface  14 . In case of the deep trench  39  fully isolating each SPAD  10 , the deep trench  39  stops in the isolation region  38 , also named STI. 
     Alternatively, the deep trench  39  may end before reaching the isolation region  38 . Optionally, the isolation region  38  may be omitted. 
     In another example, the deep trench  39  is manufactured from the front side. Thus, the deep trench  39  is fabricated from the first surface  13 . 
     The semiconductor body  11  may comprise a substrate  40 . The substrate  40  is at the second surface  14  of the semiconductor body  11 . The buried layer  25  is located between the substrate  40  and the first region  26 . The buried layer  25  is arranged on top of the substrate  40 . The deep trench  39  may extend up to the substrate  40  and/or up to the second surface  14 . The substrate  40  may be a silicon substrate. The semiconductor body  11  may be made out of silicon. 
     In  FIG. 1A , one pixel realizing one avalanche photodiode  10  is illustrated. A device may comprise an array of avalanche photodiodes  10 , wherein each of the avalanche photodiodes  10  is realized as the example elucidated in  FIG. 1A . In other words, the device may comprise an array of pixels, wherein each pixel comprises one avalanche photodiode  10 . The avalanche photodiode  10  as shown in  FIG. 1A  has an extension DP in the cross-section. The extension DP may be called “device pitch”. In  FIG. 1A , the cross-section of the avalanche photodiode  10  is shown between a first border  48  which may be called “right border” and a second border  49  which may be called “left border”. 
     The border between two avalanche photodiodes  10  of the device is arranged in the isolation region  38  and in the deep trench  39 . The deep trench  39  is typically shared between two neighboring avalanche photodiodes  10 ,  10 ′ (as shown e.g. in  FIGS. 6A to 6D ). Thus, there is a further sinker at the right side of the deep trench  39  and the isolation region  38  shown in the right half of the cross-section and there is also an additional sinker on the left side of the deep trench  39  and the isolation region  38  in the left half of the cross-section comprised by neighboring avalanche diodes. 
       FIG. 1B  shows the example of the avalanche photodiode  10  as shown in  FIG. 1A  with some further explanations. An avalanche multiplication region  50  is formed by parts of the shallow region  27  and parts of the enhancement region  28 . As a photon generates an electron in the avalanche multiplication region  50 , the avalanche multiplication effect is triggered resulting in a high current between the shallow region  27  and the buried layer  25 . A depletion region  51  is located near the avalanche multiplication region  50 . The depletion region  51  is formed by parts of the enhancement region  28  and of the first region  26 . Electrons in the depletion region  51  are transported toward the avalanche multiplication region  50 . During operation, the avalanche multiplication region  50  and the depletion region  51  are inside a space charge region or space charge zone between the shallow region  27  and the guard ring  29  on one side and the enhancement region  28  and the first region  26  on the other side. 
     In further regions of the first region  26 , as indicated by an arrow A, electrons may be generated. Electrons generated outside of the depletion region  51  can diffuse towards the depletion region  51 . Since this is a slow process, the timing jitter is increased, resulting e.g. in a long tail in the jitter (shown in  FIG. 1C ). Since the fill factor of a SPAD (fill factor=(SPAD active area)/(total area)) is typically 30-60%, a significant part of carriers is generated outside of the depletion region  51 . 
     As indicated by an arrow B, an electric field is generated by the doping gradient between the sinker  35  and the first region  26 . The doping gradient decreases along the arrow B. Therefore, a positive electric field is introduced which assists electrons to reach the avalanche multiplication region  50 . Along an arrow C, the doping is approximately constant. But for an area optimized device this constant doping region will be very small or non-existent. Thus, electrons generated at the interface of the deep trench  39  to silicon will increase the dark count rate of the SPAD  10 . 
     The sinker  35 , which may be implemented as a p-well, is typically formed with multiple implants with different energies in order to create a good connection with the deep implanted buried layer  25 . The sinker  35  may be fabricated using one mask or two masks. 
     Alternatively, the guard ring  29  may be omitted. 
     Alternatively, the enhancement region  28  may be omitted. 
     In  FIG. 1B  and also in  FIGS. 2A to 2D , the cross-section of one avalanche photodiode  10  is shown between the symmetry axis  37  and the first border  48  of the avalanche photodiode  10 . 
       FIG. 1C  shows an example of two histograms which may be measured with similar or with other avalanche photodiodes as shown in this disclosure. The counts in every histogram bin are shown as a function of a time t. This time t corresponds to the time from a photon hitting the SPAD until the electrical response of the SPAD is measured and includes a constant offset due to the measurement setup. For easier comparison between different measurements, the constant offset is eliminated by shifting the histogram in such a way that the peak position is shifted to the point of time t=0. Thus, a light pulse is applied a little bit earlier that means at a point of time t&lt;0. The number of counts divided by the number of total counts is shown on one axis. The light source is in such a way (i.e. very weak, e.g. by attenuation) that the avalanche photodiodes obtain only a 1% trigger probability. 
     The results achieved with a first avalanche photodiode are marked with AA and the results achieved with a second avalanche photodiode are marked with BB. After the point of time t=0, the counts decrease up to a level which corresponds to the dark count rate, abbreviated as DCR. The width of the shown distribution of the counts is called a jitter. The jitter is typically specified as Full-width-at-half-maximum (abbreviated FWHM) or the full width at 10% of the maximum. The jitter is lower for the measurements marked with BB in comparison to the measurements marked with AA. The measurements marked with AA also result in a higher DCR in comparison to the measurements marked with BB. 
     In the measurements marked with AA, slow carriers (transport to the depletion region  51  by diffusion or weak electric fields only) cause a tail in the jitter characteristics. In the measurements marked with BB, slow carriers cannot reach the avalanche multiplication region  50 . 
     A large jitter tail reduces the signal-to-noise ratio for time-of-flight applications. A time-of-flight arrangement may comprise the avalanche photodiode  10  or an array of avalanche photodiodes  10 . If the avalanche photodiode  10  or an array of avalanche photodiodes  10  are used for time-of-flight measurements, a low jitter is advantageous. 
       FIG. 2A  shows an example of a cross-section of an avalanche photodiode  10 . This implementation is based on the avalanche photodiode shown in  FIGS. 1A and 1B . The semiconductor body  11  comprises a separating region  60 . The separating region  60  encircles the sinker  35 . Thus, the separating region  60  encompasses or surrounds the sinker  35 . At the first surface  13  of the semiconductor body  11 , the isolation region  38  encircles the sinker  35 . Thus, more precisely, the separating region  60  and the isolation region  38  together encircle the sinker  35 . The separating region  60  encircles the sinker  35  in depths between the isolation region  38  and the buried layer  25 . The separating region  60  is located between the deep trench  39  and the sinker  35 . Thus, the deep trench  39  is separated from the sinker  35  by the separating region  60 . The buried layer  25  extends from the deep trench  39  at the first border  48  of the avalanche diode  10  to the second border  49  of the avalanche diode  10 . 
     The separating region  60  is of the first type of conductivity. A minimum doping concentration of the separating region  60  is lower than a maximum doping concentration of the sinker  35 . Thus, there is a doping gradient along an arrow E. At an area marked D, there is a positive field resulting in a movement of electrons towards the deep trench  39 . The doping gradient increases along the arrow E. Therefore, a positive electric field is introduced which hinders electrons to reach the avalanche multiplication region  50 . Thus, electrons generated at the interface of the deep trench  39  to silicon will not increase the DCR of the SPAD  10 . 
     The deep trench  39  is realized by an isolating material. Thus, there is an interface between the semiconductor crystal realizing the sinker  35  and other parts and the isolating layer of the deep trench  39 . This interface may cause the generation of electrons. By the electric field the electrons are driven to the deep trench  39  and do not reach the avalanche multiplication region  50 . 
     The transition of the sinker  35  to the first region  26  causes a drift of electrons from this region towards the depletion region  51 . 
     The avalanche photodiode  10  comprises a spacing between the sinker  35  (named p-well) and the deep trench  39  realized by the separating region  60 . The avalanche photodiode  10  includes a split guard ring. In the example of  FIGS. 1A and 1B , the sinker  35  is shared between two neighboring cells or pixels (i.e. one implant mask opening). In the split guard ring, there are two separate openings in the mask. The split guard ring is formed e.g. by the sinker  35  of one pixel, the separating region  60  of this pixel, a separating region of the neighboring pixel and the sinker of the neighboring pixel. The two separating regions are one structure on the mask. Advantageously, the portion of the DCR that originates from the deep trench  39  is reduced. 
       FIG. 2B  shows a further example of the avalanche diode  10  which is based on the examples shown in  FIGS. 1A, 1B and 2A . The separation region  60  encircles the buried layer  25 . Thus, the separating region  60  encompasses or surrounds the buried layer  25  in a plane that is formed by the buried layer  25  and is parallel to the first surface  13 . The buried layer  25  is separated by the separating region  60  from the deep trench  39 . As shown in the cross-section of  FIG. 2B , the buried layer  25  does not reach the deep trench  39 . 
     The sinker  35  is realized using the well proximity effect, abbreviated WPE. The buried layer  25  is fabricated using a mask that is realized such that a photoresist covers an area near the first and the second border  48 ,  49 . For example, the photoresist may cover the area of the isolation region  38 . This mask has the effect that a buried layer  25  is realized wherein the buried layer  25  has a gap (which is formed by the separating region  60 ) to the deep trench  39 . The separation region  60  between the sinker  35  and the deep trench  39  as well as between the buried layer  25  and the deep trench  39  is a result of this photoresist. The photoresist has the effect that ions are scattered from the photoresist into the semiconductor body  11  such that the doping concentration of the first type of conductivity is increased in the area between the first surface  13  and the buried layer  25  such that the sinker  35  is fabricated. The sinker  35  is fabricated by doping atoms during implantation of the buried layer  25  using the WPE effect. 
     In the right side of  FIG. 2B , a simulation of the doping concentration is shown. At the first surface  13  of the semiconductor body  11 , the sinker contact region  36  is fabricated using a separate fabrication step with a mask and an ion implantation step for realizing the sinker contact region  36 . The doping between the sinker contact region  36  and the buried layer  25  is realized using WPE. As shown in the simulation, the doping of the sinker  35  is of the first type of conductivity and has a lower concentration in comparison to the maximum doping concentration of the sinker contact region  36  and also in comparison to the maximum doping concentration of the buried layer  25 . The buried layer  25  may be called “deep implanted buried layer”. 
     In an area indicated by an arrow F in the left and in the right figure, a doping is caused by the well proximity effect, abbreviated WPE, due to the implantation of the deep implant buried layer  25 . Thus, the sinker  35  is realized (however, with a lower doping concentration than the sinker  35  in  FIGS. 1A, 1B and 2A ). 
     A doping gradient increases along an arrow G. Therefore, a positive electric field is introduced which hinders electrons to reach the avalanche multiplication region  50 . Thus electrons generated at the interface of the deep trench  39  to the silicon will not increase the DCR of the SPAD  10 . 
     As shown in  FIG. 2B , the “p-well” is eliminated, more precisely no mask for fabrication the p-well is required. Advantageously, process complexity is reduced (one mask process step and several implantation process steps can be skipped). The portion of the DCR originating from the deep trench  39  is reduced. 
       FIG. 2C  shows a further example of the avalanche diode  10  which is based on the above-shown examples. The separating region  60 ′ encircles the buried layer  25 . The separating region  60 ′ surrounds or encompasses the buried layer  25 . The separating region  60 ′ is located between the buried layer  25  and the sinker  35 . Thus, the separating region  60 ′ separates the buried layer  25  from the sinker  35 . The doping of the separating region  60 ′ is fabricated using the WPE effect. A doping concentration in the separating region  60 ′ may be slightly higher than the doping concentration of the substrate  40 . The doping concentration of the separating region  60 ′ may be slightly higher than a minimum doping concentration of the first region  26 . Thus, there is a slightly conducting bridge between the buried layer  25  and the sinker  35 . The doping concentration of the separating region  60 ′ is lower than a maximum doping concentration of the buried layer  25  and also lower than a maximum doping concentration of the sinker  35 . The maximum doping concentration of the sinker  35  may be obtained in the sinker contact region  36 . 
     The semiconductor body  11  comprises a shield ring  62  that is arranged between the guard ring  29  and the buried layer  25 . The shield ring  62  is of the first type of conductivity. The shield ring  62  encircles the first region  26 . The doping of the shield ring  62  is fabricated using the WPE effect. A doping concentration of the shield ring  62  is lower than a maximum doping concentration of the buried layer  25 . The doping concentration of the shield ring  62  may be also lower than a maximum doping concentration of the enhancement region  28 . The doping concentration of the shield ring  62  is higher than the doping concentration of the substrate  40 . The doping concentration of the shield ring  62  may be similar to the doping concentration of the separating region  60 , as shown in  FIG. 3D . 
     According to  FIG. 2C , the sinker  35  extends to the deep trench  39 , such as also shown in  FIGS. 1A and 1B . 
     In an alternative embodiment, the guard ring  29  is omitted. In this case, the shield ring  62  is arranged between the first surface  13  of the semiconductor body  11  and the buried layer  25  or between the shallow region  27  and the buried layer  25 . 
       FIG. 2D  shows a further example of the avalanche photodiode  10  which is based on the above-shown examples, especially of the examples shown in  FIGS. 2B and 2C . A first part  60 ′ of the separating region separates the buried layer  25  from the sinker  35 . A second part  60  of the separating region separates the sinker  35  from the deep trench  39 . Thus, the embodiments shown in  FIGS. 2A and 2C  are combined in the example shown in  FIG. 2D . 
     As discussed above, a doping gradient increases along the arrow E. Therefore, a positive electric field is introduced which hinders electrons generated at the interface of the deep trench  39  to silicon to reach the avalanche multiplication region  50 . 
     Additionally, a doping gradient increases along arrows H. Therefore, a positive electric field is introduced which hinders electrons to reach the avalanche multiplication region  50 . Thus, electrons generated outside of this barrier do not contribute to the jitter tail. The barrier is realized by the transition of the separating region  60 ′ to the buried layer  25  and optionally also by the transition of the shield ring  62  to a region between the shield ring  62  and the sinker  35 . 
     The avalanche photodiode  10  realizes a p-well to deep trench spacing and a p-barrier by WPE. Advantageously, a reduced DCR originates from the deep trench  39 . The jitter is reduced because carrier diffusion towards the avalanche multiplication region  50  is suppressed by electric fields (indicated by the arrows E and H). 
       FIGS. 3A to 3D  show examples of doping concentrations of the above-described examples of the avalanche photodiode  10 . Moreover,  FIGS. 4A to 4D  show examples of electric fields during operation for the different examples of the avalanche photodiode  10 .  FIGS. 3A and 4A  correspond to the example of the avalanche photodiode  10  shown in  FIGS. 1A and 1B ;  FIGS. 3B and 4B  correspond to the example shown in  FIG. 2A ;  FIGS. 3C and 4C  correspond to the example shown in  FIG. 2B ; and  FIGS. 3D and 4D  correspond to the example shown in  FIG. 2D . The simulations shown in  FIGS. 3A to 3D and 4A to 4D  are performed by using a simulation tool. 
     In  FIG. 4B , arrow I indicates an area with a negative electric field along most of the deep trench  39 . In  FIGS. 4C and 4D , arrow J indicates an area with a negative electric field all along the deep trench  39 . Moreover, in  FIG. 4D , arrow K indicates an area with a negative electric field. No carriers generated right from here can reach the avalanche multiplication region  50 . 
       FIGS. 5A and 5B  show details of characteristics of the avalanche photodiode  10  fabricated using WPE, as shown in  FIGS. 2B and 3C . In  FIG. 5A , details of the doping concentration of the sinker  35  are shown. The sinker  35  realized using WPE (for example as shown in  FIGS. 2B, 3C and 5A ) differs from the sinker  35  as shown in  FIGS. 1A, 1B, 2A, 3A and 3B . A doping concentration of the sinker  35  as shown in  FIGS. 1A, 1B, 2A  typically has a high value from the first surface  13  of the semiconductor body  11  up to the buried layer  25 . For example, the doping concentration inside the sinker  35  may be approximately constant from the first surface  13  to the buried layer  35 . Contrary to that, as shown in  FIGS. 5A and 3C , the minimum of the doping concentration of the sinker  35  is lower than the maximum of the doping concentration of the buried layer  25  and also of sinker contact region  36 . 
     Moreover, some regions of the sinker  35  are not directly below the sinker contact region  38 . 
     The doping concentration of the sinker  35 , created by the WPE, has certain properties related to the WPE: The doping concentration of the sinker  35  is always lower than the highest doping concentration of the buried layer  25 . For example, the highest doping concentration of the buried layer  25  is five times larger than the doping concentration of the sinker  35  (typically the ratio is 10 times to 20 times). The width of the sinker  35  is larger closer to the first surface  13  (this is indicated by the two arrows L and M). The sinker  35  may have a wasp waist shape. The sinker  35  has a decreasing lateral dimension with increasing distance from the first surface  13 . 
       FIG. 5B  shows an example of the doping concentration on a line  65  shown in  FIG. 5A . The doping concentration has a first peak approximately near the first surface  13  of the semiconductor body  11  and decreases with increasing distance from the first surface  13 . The doping concentration rises again to a second peak which is achieved by the doping concentration of the buried layer  25 . Thus, the second peak shown in  FIG. 5B  is a result of the buried layer  25  and not part of the sinker  35 . In one example, the doping concentration is only the boron doping concentration. Thus, one can discriminate between a fabrication process with WPE and without WPE by a measurement of doping concentrations in a cross section of the sinker  35  and the buried layer  25 . 
     The following  FIGS. 6A to 11D  show examples of layouts of masks for the fabrication of the avalanche diode  10 . The example layouts mostly show octagonal structures. 
     Alternatively, the layouts may also be quadratic, circular or rectangular. In most cases, the layers and regions of the avalanche diode  10  in a top view are identical or approximately identical with the layout of the masks for fabricating these layers and regions. Differences may result e.g. from a diffusion process, overexposure or underexposure of photoresist etc.. In case of the buried layer  25  and the sinker  35 , differences may result from the WPE, the large distance from the photoresist to the buried layer  25 , a diffusion process and a high ion implantation dose and energy. 
       FIGS. 6A, 7A, 8A, 9A, 10A and 11A  show a top view of layouts of masks corresponding to the avalanche photodiode  10  shown in  FIGS. 1A and 1B .  FIGS. 6B, 7B, 8B, 9B, 10B and 11B  show a top view corresponding to the example shown in  FIG. 2A .  FIGS. 6C, 7C, 8C, 9C, 10C and 11C  show top views corresponding to the example shown in  FIG. 2B . Similarly,  FIGS. 6D, 7D, 8D, 9D, 10D and 11D  show top views corresponding to the example shown in  FIG. 2D . Thus, the four avalanche photodiode  10  and the four further avalanche photodiode  10 ′ shown in  FIGS. 6A to 6D  are typically not realized on one wafer or one die; instead the avalanche photodiode  10  and the further avalanche photodiode  10 ′ shown in  FIG. 6A  may be realized on one die, the avalanche photodiode  10  and the further avalanche photodiode  10 ′ shown in  FIG. 6B  may be realized on another die etc.. The same applies to the other  FIGS. 7A to 11D . 
       FIGS. 6A to 6D  show the layout for the masks to realize the shallow region  27 , the deep trench  39  and the sinker  35 . The shallow region  27  has an orthogonal form in the top view in the four examples shown in  FIGS. 6A to 6D . As mentioned above, the shallow region  27  may also be circular, rectangular or quadratic. The deep trench  39  has the form of four stripes on the border of a rectangle or quadrat. The deep trench  39  is located at the border from one pixel to another pixel. Thus, two adjacent pixels or avalanche photodiodes  10 ,  10 ′ have a common deep trench  39 . As shown in  FIGS. 6A, 6B and 6D , an inner border or boundary of the sinker  35  has an orthogonal shape in the top view. As shown in  FIG. 6C , there is no mask for realization of the sinker  35 , since the sinker  35  is realized using the WPE effect during fabrication of the buried layer  25 . 
       FIGS. 7A to 7D  show the layout of the masks that are already elucidated in  FIGS. 6A to 6D  and additionally the layout for the enhancement region  28  and the guard ring  29 . The area of the enhancement region  28  is smaller in comparison to the area of the shallow region  27 . 
       FIGS. 8A to 8D  show the layout for the masks for the buried layer  25  and the guard ring  29  (the guard ring  29  is also shown in  FIGS. 7A to 7D ). As shown in  FIGS. 8A and 8B , the buried layer  25  may be, for example, fabricated over the complete wafer and, thus, without using any mask. Thus, the buried layer  25  extends into the complete area of the avalanche photodiode  10  and also into the complete area of the array of avalanche photodiodes. Only in case that the semiconductor body  11  comprises not only the array of the avalanche diodes  10  but also further parts such as an integrated circuit, a mask is required for the realization of the buried layer  25  to keep the area of the integrated circuit free from the buried layer  25 . 
     The layout of the mask for the buried layer  25  shown in  FIG. 8C  is designed such that the separating region  60  between the sinker  35  and the buried layer  25  on one side and the deep trench  35  on the other side can be realized. As shown in  FIG. 8D , the mask for the buried layer  25  is realized such that a large gap between the buried layer  25  and the deep trench  39  is realized. The use of masks as shown in  FIGS. 8C and 8D  result into the WPE. 
       FIGS. 9A to 9D  show an example of a layout of an inverse mask of the isolation region  38 . Thus, the isolation region  38  is fabricated at areas on the first surface  13  of the semiconductor body  11  which are not covered by the mask shown in  FIGS. 9A to 9D . The isolation region  38  has the form of stripes at the borders of the avalanche photodiodes  10 ,  10 ′. Thus, the avalanche photodiode  10  and the adjacent avalanche photodiode  10 ′ have a common isolation region  38 . 
       FIGS. 10A to 10D  show an example of layouts of the mask for the realization of the sinker contact region  36  and also the inverse mask for the realization of the isolation region  38 . The inner border of the sinker contact region  36  has an orthogonal form. The sinker contact region  36  has the form of four triangles at the four corners of the avalanche photodiode  10 , wherein the triangles are connected by small stripes. The sinker contact region  36  is realized also in case the sinker  35  is realized by WPE (as shown in  FIG. 10C ). 
       FIGS. 11A to 11D  show the examples of the layouts of different masks shown above with the exception of the masks of the isolation region  38  and the sinker contact region  36 . 
     Instead of the word “region” sometimes the words “layer” or “well” could be used. 
     The embodiments shown in  FIGS. 1A to 11D  as stated represent example embodiments of the improved semiconductor body, avalanche photodiode and method for fabricating a semiconductor body, therefore they do not constitute a complete list of all embodiments according to the improved semiconductor body, avalanche photodiode and method for fabricating. Actual semiconductor bodies, avalanche photodiodes and methods for fabricating may vary from the embodiments shown in terms of layers, shape, size and materials, for example.