Patent Publication Number: US-2022238738-A1

Title: Silicon carbide ultraviolet light photodetector and manufacturing process thereof

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a silicon carbide ultraviolet light photodetector and to the manufacturing process thereof. 
     Description of the Related Art 
     As is known, in the field of photon detection, the need is felt to have devices enabling detection of ultraviolet (UV) light with a high sensitivity in the spectral region of 100-400 nm. In particular, detection of very weak and ultra-fast signals outside the range of solar light is desired for various applications, such as flame detection, UV astronomy, execution of chemical and biological analyses, and detection of jet engines and missiles plumes. These applications require devices that are very sensitive and have a high signal-to-noise ratio. 
     For such applications, photomultiplier tubes (PMTs) are normally used, but their large size, their brittleness, and the associated costs render solid-state detectors more attractive. 
     Amongst them, commercially available silicon avalanche photodiodes have a moderate quantum efficiency at the non-visible wavelengths but require costly optical filters to obtain a high rejection ratio of solar photons, since their response extends throughout the visible wavelength range. 
     Gallium-nitride based diode photodetectors have demonstrated a high sensitivity in the region of non-visible light and a good gain, but suffer from the problem of having a high dark current due to the high defect density in this type of semiconductor. Silicon carbide based avalanche diodes have a lower dark-current density, thanks to their low thermal generation, and thus represent an advantageous choice for UV-light photodetectors, also considering the more mature process technology and an excellent intrinsic opacity to visible light. 
     US patent application US20170098730, corresponding to Italian patent application 10201500058764, describes a silicon carbide avalanche photodiode for detecting ultraviolet radiation having a completely planar structure. This avalanche photodiode has an active area and an edge ring obtained by implanting aluminum at different doses and energies. This structure enables minimization of the dead area around the active area, reduction of the breakdown voltage, and improvement of the detection efficiency over the entire UV range. A considerable gain is thus obtained (of the order of 10 2  to 10 5 ) measured in the avalanche multiplication condition. 
     However, this solution may be improved as regards dark current, in particular in some frequencies, presumably due to surface implantation processes and soft breakdown (i.e., not sufficiently rapid breakdown) caused by a considerable injection of leakage current starting from the device periphery prior to breakdown and triggering the desired avalanche process. In practice, it is assumed that the confinement of the electrical field in the active area of the photodetector is not always high, and the electrical field extends also laterally, thus causing breakdown over an extensive area. 
     The above effects negatively affect the operation of the device in the single photon condition under illumination—so called single photon avalanche diode (SPAD) or Geiger mode-avalanche photodiode (GM-APD) operating condition. Similar considerations apply to operation as avalanche photodiodes (APDs), since the latter operate in a way similar to SPADs, except for having a linear operating range below the breakdown voltage and more limited gain. 
     BRIEF SUMMARY 
     One or more embodiments of the present disclosure provide a silicon carbide ultraviolet light photodetector that overcomes the drawbacks of the prior art. 
     According to the present disclosure, a silicon carbide ultraviolet light photodetector and the manufacturing process thereof are provided. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding, some embodiments of the present photodetector are now described, purely as non-limiting example, with reference to the attached drawings, wherein: 
         FIG. 1  is a cross-section of an embodiment of the present photodetector; 
         FIG. 1A  shows the doping profile of the photodetector of  FIG. 1 , taken along central axis A, parallel to Cartesian axis Z; 
         FIGS. 2-8  are schematic cross-sections of the photodetector of  FIG. 1  during successive manufacturing steps; 
         FIGS. 9-11  are cross-sections of other embodiments of the present photodetector; 
         FIGS. 12-14  are schematic cross-sections of the photodetector of  FIG. 11  during successive manufacturing steps; 
         FIG. 15  is a cross-section of yet another embodiment of the present photodetector; 
         FIGS. 16 and 17  are schematic cross-sections of the photodetector of  FIG. 15  during successive manufacturing steps; 
         FIG. 18  is a schematic perspective views of an array of photodetectors of the type illustrated in  FIGS. 1, 9-11, and 15 ; and 
         FIG. 19  shows a block diagram of a system including an array of photodetectors, of the type illustrated in  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an embodiment of a photodetector  1  of a silicon carbide APD type, but the photodetector  1  may operate also in SPAD operating condition. 
     The photodetector  1  of  FIG. 1  has a central axis A. In particular, the photodetector  1  may have circular symmetry so that the central axis A is an axis of symmetry. Alternatively, the photodetector  1  may have a polygonal shape, for example square, in top plan view. 
     The photodetector  1  may be integrated in a die  2 , together with a plurality of other photodetectors  1  to form an array  500 , such as the one illustrated in  FIG. 18 . 
     The die  2  comprises a silicon carbide (SiC) body  3 , formed by a substrate  4 , of an N ++  type, a first epitaxial layer  6 , of an N −  type, and a second epitaxial layer  8 , of a P +  type, stacked on top of each other. The body  3  has a non-planar top surface  3 A, defined by portions of the first and second epitaxial layers  6 ,  8 , and a planar bottom surface  3 B. In particular, the top surface  3 A has a projecting portion  3 A 1 , for example of a planar circular shape; a sloped lateral portion  3 A 2 , for example having a frustoconical shape; and an edge portion  3 A 3 , here planar, as clarified hereinafter. Moreover, the second surface  3 B is parallel to a plane XY of a Cartesian reference system XYZ, wherein the thickness of the body  3  is measured along the axis Z (hereinafter also referred to as thickness direction). In  FIG. 1 , the first and second epitaxial layers  6 ,  8  form an interface  7 , which is planar and parallel to the plane XY. The substrate  4  and the first epitaxial layer  6  are doped, for example, with nitrogen, and the second epitaxial layer  8  is doped, for example, with boron. A buried region  10 , of an N +  type and doped, for example, with phosphorus, extends within the first epitaxial layer  6 , here adjacent and contiguous to the second epitaxial layer, for a part of the interface  7 . The buried region  10  has a variable doping in the thickness direction (parallel to axis Z) with a concentration of dopant species increasing from the interface  7  up to a peak value and then again decreasing down to the doping level of the first epitaxial layer  4 , as illustrated in  FIG. 1A . For instance, at the peak, the buried region  10  has a doping level of 5·10 18  cm −3 . 
     An edge region  11 , of insulating material, extends over part of the top surface  3 A and delimits laterally, in the second epitaxial layer  8 , an anode region  12 . The edge region  11 , for example of tetraethylorthosilicate (TEOS), comprises an inner annular portion  11 A, extending over the peripheral area of the anode region  12  (the central area whereof is thus exposed to the external environment); a sloped annular portion  11 B, as a continuation of the inner annular portion  11 A, laterally surrounding and contiguous to the anode region  12 ; and an outer portion  11 C, as a continuation of the sloped annular portion  11 B, on top of and contiguous to an edge area of the first epitaxial layer  6 . 
     The inner annular portion  11 A of the edge region  11  may be missing. The sloped annular portion  11 B of the edge region  11  has a thickness increasing from the inner annular portion  11 A up to the outer portion  11 C, surrounds at a distance the buried region  10 , and forms a sloped peripheral surface  11 ′ of the edge region  11  (corresponding to the sloped lateral portion  3 A 2  of the top surface  3 A of the body  3 ). In particular, the distance between the lateral edge of the buried region  10  and the sloped peripheral surface  11 ′ is at least 0.5-1 μm. The outer region  11 C of the edge region  11  has a uniform thickness (between 1 and 3 μm, for example 2 μm) and has a bottom surface  11 ″ (corresponding to the edge portion  3 A 3  of the top surface  3 A of the body  3 ) that is contiguous to the first epitaxial layer  6  and extends at a lower level than the interface  7  (in the thickness direction Z). 
     The sloped lateral portion  3 A 2  of the top surface  3 A of the body  3  and thus the peripheral surface  11 ′ of the edge region  11  are inclined by an angle of at least 45°, up to a maximum of 90°, with respect to the plane of the interface  7  for the reasons clarified below. 
     The edge region  11  delimits, in the body  3 , an active area  14 , in the central area whereof the aforementioned breakdown during detection is to be obtained. 
     A top conductive region  15 , for example of nickel silicide (Ni 2 Si), is arranged on, and in direct contact with, the anode region  12 , to form a front ohmic contact. A front contact region  35 , represented by an electrode  16 , extends on a portion of the top conductive region  15 , for external connection. The top conductive region  15  has, for example, an annular shape and overlies a peripheral portion of the anode region  12 . 
     A passivation layer  18 , for example of silicon nitride (Si 3 N 4 ), extends over the edge region  11  and surrounds the top conductive region  15  at the top and laterally, except in the front contact region  35 . 
     A bottom conductive region  20 , for example of nickel silicide, extends underneath the bottom surface  3 B of the body  3 , in contact with the substrate  4 , and forms a rear ohmic contact. A bottom metallization  21  is arranged underneath the bottom conductive region  20 , in contact with the latter. The bottom metallization  21  may be formed by a multilayer structure including three stacked layers of titanium, nickel, and gold. 
     In practice, the first epitaxial layer  6  has an electrical behavior equivalent to that of an intrinsic layer. The anode region  12 , the buried region  10 , and the first epitaxial layer  6  thus form a PN + NI junction; the first epitaxial layer  6  thus operates as cathode region. The photodetector  1  can consequently work as APD or SPAD, where the PN + NI junction is designed to receive photons and generate the avalanche current, as described in U.S. patent application US20170098730, which is incorporated by reference herein in its entirety. 
     The photodetector  1  of  FIG. 1  may be manufactured as illustrated in  FIGS. 2-9  and described in detail hereinafter. 
     Initially,  FIG. 2 , the first epitaxial layer  6  is epitaxially grown on the substrate  4 , for example of 4H-polytype silicon carbide (4H-SiC) of an N type with a thickness, for example, of 350 and a doping level, for example, of 1·10 19  cm −3 . The first epitaxial layer  4  has a thickness of 8-12 for example, approximately 9.5 and a doping level between 8·10 13  cm −3  and 2·10 14  cm −3 , for example, 1·10 14  cm −3 , and is thus quasi-intrinsic. 
     Then, not illustrated, cleaning is carried out, and alignment marks are formed. To this end, a sacrificial oxide layer is thermally grown, portions of the sacrificial layer and of the first epitaxial layer  6  are selectively etched to form zero-layer marks, and the sacrificial oxide layer is removed in a per se known manner. 
     Next,  FIG. 3 , a hard mask  30  is formed, for example of TEOS oxide with a thickness of 0.8 The hard mask  30 , obtained in a known way by depositing and patterning a TEOS layer using a resist mask (not illustrated), has a window  31  where the buried region  10  is to be formed. Then, the buried region  10  is implanted; in particular, a double implantation is carried out, for example with phosphorus ions, first at an energy of 300 keV and with a dose of 1·10 13  cm −2 , then at an energy of 350 keV and with a dose of 1·10 14  cm −2 , as represented schematically in  FIG. 3  by arrows  32 . Both implantations are performed at a temperature of 500° C. A thin layer  10 ′ is thus formed underneath the window  31 . Alternatively, the double implantation may be carried out through a thin sacrificial oxide layer (not illustrated), with a thickness of, for example, 30 nm, and in this case the first implantation may be carried out at an energy of 450 keV, and the second implantation may be carried out at an energy 500 keV. In this case, after the double implantation, the thin sacrificial oxide layer is removed. 
     Then,  FIG. 4 , the hard mask  30  is removed, and the second epitaxial layer  8  is thermally grown. For instance, 4H-polytype silicon carbide (4H—SiC) is grown in two steps, by chemical vapor deposition (CVD) in an epitaxial reactor. In particular, the first step is carried out at a temperature to activate the phosphorus ions in the thin layer  10 ′, for example at 1650° C. for 30 min in an argon environment to form the buried region  10 . In this way, after the second epitaxial growth (described hereinafter), the buried region  10  has a variable profile, as mentioned above, with dopant concentration peak placed at a distance of 0.2-0.7 μm, for example at 0.4 μm, from the interface  7  (which is still to be formed). The second step, of proper epitaxial growth, is carried out at a temperature higher than 1500° C., for example at 1650° C., for 5 min using hydrogen as carrier and HCl 3 Si and C 2 H 4  as silicon and carbon precursors. Then the second epitaxial layer  8  of P +  type is grown, for a thickness of 0.3-0.7 μm, for example of 0.5 μm, and with a doping dose between 1·10 19  cm −3  and 1·10 20  cm −3 , for example 5·10 19  cm −3 , to obtain the body  3 . 
     Next,  FIG. 5 , a TEOS dielectric layer (not illustrated), is deposited, for example, by plasma-enhanced CVD (PECVD), and the dielectric layer (not illustrated) is selectively etched to form a hard mask for the subsequent etching of the body  3 . Then, portions of the second epitaxial layer  8  are selectively etched and removed, throughout the thickness of the layer, to define the anode region  12 , and surface portions of the first epitaxial layer  6  are also selectively etched and removed. For instance, a dry etch is carried out to obtain the structure of  FIG. 5 . In this step, the shape of the top surface  3 A of the body  3  is defined, with the portions  3 A 1 - 3 A 3 . 
     Next,  FIG. 6 , the top surface  3 A of the body  3  is protected by a protective layer (not illustrated), and the bottom conductive region  20  is formed, for example by back nickel sputtering, with a thickness of 200 nm. After removing the protective layer (not illustrated), rapid thermal annealing (RTA) is carried out, for example at 1000° C., for 60 s, in a nitrogen environment. 
     Then,  FIG. 7 , a field-oxide layer is deposited, intended to form the edge region  11 . For instance, TEOS is deposited by CVD, with a thickness of 1-3 μm, for example 2 μm, and a wet etch of the field-oxide layer is carried out to remove it on the anode region  12  and form the edge region  11 . 
     Thereafter,  FIG. 8 , the bottom conductive region  20  is protected, for example using a resist layer (not illustrated), and a conductive material layer, intended to form the top conductive region  15 , is deposited. For instance, a nickel layer is deposited, via sputtering, for a thickness of 100 nm. The conductive material layer is then defined, for example by masked wet etching, and, after removing the mask, a rapid thermal annealing (RTA) is carried out, for example at 750° C. for 60 s, in a nitrogen environment, to form the top conductive region  15 . 
     Finally, the front and rear electrodes are formed, to obtain the structure of  FIG. 1 . For instance, a metallic multilayer, comprising titanium (with a thickness of 80 nm) and an alloy of aluminum, silicon, and copper (AlSiCu, with a thickness of 3 μm) is deposited on the front surface, by sputtering. Then, the multilayer is patterned by wet etching to form the front contact  35 , which extends over and in direct contact with the top conductive region  15 . Next, the passivation layer  18 , for example of silicon nitride with a thickness of 200 nm, is deposited on the front side and is selectively removed, for example by dry etching, so as to free the central area of the anode region  12  and the front contact  35 . 
     Before or after forming the front contact  35 , the bottom metallization  21  is formed. To this end, on the bottom conductive region  20  a metallic multilayer, for example, formed by a titanium layer with a thickness of 0.1 μm, a nickel layer, with a thickness of 0.4 μm, and a layer of gold, with a thickness of 0.05 μm, is deposited by sputtering. 
     In the photodetector  1 , the buried region  10  represents an enriched region within the first epitaxial layer  6 , which, as mentioned, is practically intrinsic, and thus provides a better confinement of the electrical field in the active area  14  of the photodetector  1 . 
     The presence of the edge region  11  enables further confinement of the electrical field and increase of the breakdown voltage of the edge area  11 , without affecting the breakdown of the central area of the photodetector  1  (active area  14 ). In fact, when, due to a biasing of the photodetector  1  at a higher voltage than breakdown voltage and generation of a photogenerated primary charge carrier an avalanche current is activated, it is desired that the avalanche current is confined in the central area of the photodetector  1 , i.e., that breakdown does not affect the peripheral area. The presence of the edge region  11  thus enables setting of the breakdown voltage of the photodetector  1  at an appropriate value (for example, 80-90 V), optimized with respect to the desired detection behavior, preventing breakdown in the peripheral area. In particular, an inclination of the peripheral surface  11 ′ of the edge region  11  higher than 45° enables a particularly effective confinement to be obtained. 
     With the described process, and in particular by forming the second epitaxial layer  8  by thermal growth, with annealing and activating the dopants in the buried region  10 , it is possible to obtain a defectiveness reduction in the active area  14  of the photodetector  1 , and thus an effective dark current reduction. 
     Thereby, the photodetector has a very low dark current, a high fill factor, and a very low breakdown voltage in the central active area. The photodetector  1  can thus be conveniently used in high-density photodetector arrays. 
     Even though, as mentioned, the structure of  FIG. 1  operates in a reliable way to maintain the avalanche multiplication in the active area of the photodetector  1  at a voltage of approximately 80 V, when the photodetector has to work at breakdown voltages of between 80 V and 120 V, it is possible to integrate concentric edge rings surrounding the active area  14 . 
       FIG. 9  thus shows a photodetector  100  identical to the photodetector  1  of  FIG. 1 , except for the shape of the edge region, here designated by  111 , so that same elements are designated by the same numbers. 
     In detail, the edge region  111  forms a series of edge rings  140  projecting towards the inside of the body  3 . The edge rings  140 , coaxial to each other, to the anode region  12 , and to the buried region  10 , extend throughout the thickness of the second epitaxial layer, here designated by  108 , as far as into the surface portion of the first epitaxial layer  6 . 
     The edge rings  140 , in the illustrated example two, are formed by appropriately patterning the hard mask used while etching the second epitaxial layer  108  (while performing the etching referred to above with reference to  FIG. 5  for the photodetector  1  of  FIG. 1 ). In practice, in this step, in the body  103 , annular etchings or depressions are obtained, complementary shaped to the edge rings  140 , by removing selective portions of the second epitaxial layer  108  and underlying surface portions of the first epitaxial layer  6 . 
     The edge rings  140  have a quadrangular shape, here trapezoidal, with minor base facing downwards. In this case, the edge rings have lateral walls  140 C inclined by at least 45° with respect to the central axis A (and to the plane XY). Preferably, the inclination is higher than 45°, up to almost 90° (compatibly with the technology), and in this case the edge rings have a quasi-rectangular shape. The edge rings  140  may moreover have a minor base, on their underside  140 A (the side parallel to axis X in  FIG. 9 , in contact with the first epitaxial layer  6 ), with a width of at least 0.2 μm, and may be spaced at a distance (periphery portion  140 B parallel to axis X in  FIG. 9  and in contact with the second epitaxial layer  108 ) of at least 0.2 μm. Preferably, the width of the minor base  140 A is 0.8 μm and the periphery portion  140 B is 2 μm. However, the edge rings  140  may be arranged with constant or variable spacing; in the latter case, the periphery portion  140 B between adjacent edge rings  140  is different. 
     The edge rings  140  have the function of increasing the breakdown voltage of the edge area, which depends upon the geometry and surface electrical charge in the dielectric layer that forms the edge region  111 , thus ensuring that avalanche breakdown of the photodetector occurs in the active area  14  of the photodetector  100 , where the buried region  10  is present. 
     The edge region  111  comprises at least two edge rings; however, simulations made by the present applicant have illustrated that the number of rings, their width, and their spacing are not critical. In particular, it has been shown that the illustrated structure, with two edge rings  140 , represents an optimal compromise between the electrical characteristics and the performance of the photodetector  100  and its dimensions. 
       FIG. 10  shows a different embodiment, wherein the photodetector  200  has a field distribution structure, so called field plate. In detail, in  FIG. 10 , where parts in common with  FIG. 1  are designated by the same reference numbers, the photodetector  200  has an edge region  211  patterned so as to have an inner annular portion  211 A, extending over the periphery of the anode region  12 ; a annular portion  211 B, arranged externally and as a continuation of the inner annular portion  211 A, adjacent to the peripheral surface  12 A of the anode region  12 ; an annular portion  211 D with variable thickness, arranged externally and as a continuation of the sloped annular portion  211 B and having an increasing thickness from the latter; and an outer portion  211 C, of constant thickness, equal to the outer portion  11 C of the edge region  11  of the photodetector  1  of  FIG. 1  (for example, 2 μm). 
     For instance, the sloped annular portion  211 B may be arranged at an angle of approximately 30° with respect to a horizontal plane parallel to the rear surface  3 B of the body  3  (plane XY of the Cartesian reference system XYZ), and the top surface of the annular portion  211 D with variable thickness may be arranged at an angle of approximately 7° with respect to the same horizontal plane parallel to the rear surface  3 B of the body  3 . 
     In this way, the sloped annular portion  211 B and the variable thickness annular portion  211 D delimit a recessed area  241  having an annular shape. The top conductive region  215  here has an ohmic-contact portion  215 A, extending over the peripheral surface of the anode region  12  and similar to the conductive region  15  of  FIG. 1 , and a field-plate portion  215 B, arranged as a continuation of, and peripherally with respect to, the ohmic-contact portion  215 A and extending over the recessed area  241 . 
     The edge region  211  may be formed using an appropriate photolithographic process, including masking resist reflow. 
     The field-plate portion  215 B, of metal such as nickel silicide, thus forms an electrical field redistribution layer causing the structure of the photodetector  200  to be even stronger to edge breakdown. In particular, as evident to the person skilled in the art, the inclination of the sloped annular portion  211 B and of the variable thickness annular portion  211 D, as well as the length of the field-plate portion  215 B, may be calibrated based on the variability of the manufacturing process steps which are used to define the photodetector  200 . 
       FIG. 11  shows a different embodiment of the present photodetector wherein the buried region no longer faces the interface  7  between the first and second epitaxial layers  6 ,  8 , but is arranged at a distance from this interface. 
     In detail,  FIG. 11  shows a photodetector  300  comprising, in addition to the first and second epitaxial layers  6 ,  8 , an epitaxial buffer layer  345 , extending between the first and second epitaxial layers  6 ,  8  and overlying the buried region  310 . The epitaxial buffer layer  345  thus forms an interface  7 ′ with the second epitaxial layer  8 , is of an N type, and has an intermediate doping level between the first epitaxial layer  6  and the buried region  310 . The epitaxial buffer layer  345  moreover has a thickness of 0.2-0.4 μm. In addition, in this case, the depth of the buried region  310  where a peak concentration occurs is between 0.3 and 0.7 μm, for example at approximately 0.4 μm from the interface  7 ′. 
     The photodetector  300  of  FIG. 11  has an edge region  311  with an edge ring  140 ; however, it may not have edge rings  140  and be shaped as in  FIG. 1 , or have a number of edge rings  140 , as in  FIG. 9 . 
     The photodetector  300  of  FIG. 11  may be manufactured as illustrated in  FIGS. 12-14 . 
     In detail, as shown in  FIG. 12  and as described with reference to  FIG. 2 , the first epitaxial layer  4  is epitaxially grown on the substrate  4 , for example 4H-polytype silicon carbide (4H—SiC) of an N type with a thickness of, for example, 350 μm and a doping level of, for example, 1·10 19  cm −3 . The first epitaxial layer  4  is doped with nitrogen ions, and has a thickness of approximately 9.5 μm and a doping level of, for example, 1·10 14  cm −3 . 
     After cleaning and forming alignment marks, as described above, selective surface implantation is carried out, at a low energy, of dopant ions of an N type, for example phosphorus, using the hard mask  30 ,  FIG. 13 . For instance, implantation may be carried out at an energy of 80 keV and with a dose of 1·10 14  cm −2 , to form the thin layer  310 ′, which has a peak concentration at approximately 0.1 μm from the surface of the first epitaxial layer  6 . 
     After removing the hard mask  30 ,  FIG. 14 , thermal annealing and a three-step epitaxial growth using a CVD process are carried out in an epitaxial reactor. In detail, first phosphorus ions of the thin layer  310 ′ are activated at a temperature of, for example, 1650° C. for 30 s in argon; then, the epitaxial buffer layer  345  is grown at a temperature of 1650° C. for 30 min using hydrogen as gas carrier and HCl 3 Si and C 2 H 4  as silicon and carbon precursors, with doping nitrogen N. The epitaxial buffer layer  345  of a 4H—SiC type with a doping level of 1·10 16  cm −3  and a thickness of 0.2-0.4 μm is thus obtained, as mentioned above. Then, a further epitaxial growth is carried out at 1650° C. for 3 min using hydrogen as carrier and HCl 3 Si and C 2 H 4  as silicon and carbon precursors, with doping aluminum Al. The second epitaxial layer  8  of a P +  type is thus obtained for a thickness of, for example, 0.2 μm and with a doping dose comprised between 1·10 19  cm −3  and 1·10 20  cm −3 , for example, 5·10 19  cm −3 . 
     The process proceeds with the steps already described with reference to  FIGS. 5-8 , until the final structure of  FIG. 11 . 
     In this embodiment, the low-energy implantation of the buried layer  310 , followed by the double epitaxial growth (first of an N type and then of a P type) reduces damage in the active area  14 . Moreover, the electrical field is almost completely confined in the buffer layer  345  and within the active area  14 , thus reducing the risk of breakdown in the peripheral area. 
     According to a different manufacturing process, the buried region may be arranged at a distance from the second epitaxial layer  8  and embedded within the first epitaxial layer  6 , using a high-energy buried implantation, as described with reference to  FIGS. 15-17 . 
     In detail,  FIG. 15  shows a photodetector  400  wherein the first and second epitaxial layers  6 ,  8  are adjacent and contiguous to each other and form the interface  7 , as in  FIG. 1 . The buried region, here designated by  410 , extends at a short distance from the interface  7  so that the maximum concentration area is arranged, also in this case, approximately at a distance of 0.2-0.7 μm from the interface  7 . In practice, an extremely thin buffer portion (designated by  450 ) of the first epitaxial layer  6  is arranged between the buried region  410  and the interface  7  and operates similarly to the buffer region  345  of  FIG. 11 . 
     The photodetector  400  may be manufactured as illustrated in  FIGS. 16 and 17 . 
     In detail,  FIG. 16 , initially the first epitaxial layer  6  is grown on the substrate  4  as described with reference to  FIG. 2 . The thickness, material, and doping level of the substrate  4  and of the first epitaxial layer  6  may be the same as those described with reference to  FIG. 2 . Then, a P +  type epitaxial growth is carried out on the first epitaxial layer  6 , for a thickness of less than 0.2 μm, for example, 0.1 μm, and with a doping dose of, for example, 5·10 19  cm −3 . Boron may be used as dopant ion species. The structure of  FIG. 16  is thus obtained. 
     Next, in a not illustrated way, cleaning is carried out, and the alignment marks are formed, as described above for the embodiment of  FIGS. 2-8 . 
     Then,  FIG. 17 , a hard mask  430  is formed, for example of TEOS oxide with a thickness of 0.8 μm. The hard mask  430 , obtained in a known way by depositing a TEOS layer, for example by PECVD, and patterning using a resist mask (not illustrated), has a window  431  where it is desired to form the buried region  410 . The buried region  410  is implanted using the mask  430 , for example with phosphorus ions, with an energy to depth confine the dopant atoms, at a distance from the top surface of the first epitaxial layer  6 . For instance, a double implantation is carried out, first at an energy of 420 keV with a dose of 1·10 13  cm −2 , then at an energy of 490 keV with a dose of 1·10 14  cm −2 , both at a temperature of 500° C., as represented schematically in  FIG. 17  by the arrows  432 . The buried region  410  is thus formed underneath the window  431 . 
     Then the steps already described with reference to 5-8 follow, until the final structure of  FIG. 15 . 
     Possibly, as clear to the person skilled in the art, additional annealings may be carried out to reduce the defectiveness caused by the high-energy implantation of the buried region  410  in the active area  14 . 
     With the solution of  FIGS. 15 to 17 , a thin buffer layer  450 , of a quasi-intrinsic N type is formed between the buried region  410  and the anode region  12  and contributes to confining the electrical field, albeit maintaining the breakdown voltage below 100 V with reverse biasing. 
     According to another alternative, the deep implantation of the buried region  410  is performed before carrying out the second epitaxial growth of a P +  type to form the second epitaxial layer. 
       FIG. 18  shows an array  500  of photodetectors  1 ,  100 ,  200 ,  300 ,  400 , integrated in a single die  2 . The array  500  may comprise any number of same photodetectors  1 ,  100 ,  200 ,  300 ,  400 . In use, the photodetectors  1 ,  100 ,  200 ,  300 ,  400  of the array  500  are arranged facing an external light source  560  adapted to emit ultraviolet radiation. 
     The array  500  of photodetectors  1 ,  100 ,  200 ,  300 ,  400  may be used, as illustrated in  FIG. 19 , in a generic system  600 , wherein the array  500  is coupled to a microcontroller  561 , in turn coupled to a computer  562  controlling a display  563 . The microcontroller  561  processes the output signal of the array  500  and supplies a processed signal to the computer  562 ; this may thus analyze the processed signal and display the associated information on the display  563 . 
     Finally, it is clear that modifications and variations may be made to the photodetector and to the manufacturing process thereof, described and illustrated herein, without thereby departing from the scope of the present disclosure. For instance, the various embodiments described can be combined to provide further solutions. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.