Abstract:
A photodiode structure having an illuminated front-side surface and a back-side surface includes a front-side doped layer having a first conductivity type, a back-side doped layer having the first conductivity type, a front-side active cell region made sensitive to light by the action of at least one plug region formed in the front-side doped layer having a second conductivity type, and a front-side inactive cell region substantially insensitive to light, wherein the first and second conductivity types are opposite conductivity types.

Description:
RELATED CASE INFORMATION 
       [0001]    This application is related to application Ser. No. 13/218,308, entitled “Wafer Structure for Electronic Integrated Circuit Manufacturing,” filed Aug. 25, 2011, and application Ser. No. 13/218,273, entitled “Wafer Structure for Electronic Integrated Circuit Manufacturing,” filed Aug. 25, 2011, and application ser. No. 13/218,335, entitled “Wafer Structure for Electronic Integrated Circuit Manufacturing,” filed Aug. 25, 2011, and application Ser. No. 13/218,345, entitled “Wafer Structure for Electronic Integrated Circuit Manufacturing,” filed Aug. 25, 2011, and application Ser. No. 13/218,352, entitled “Wafer Structure for Electronic Integrated Circuit Manufacturing” filed Aug. 25, 2011, and application Ser. No. 13/218,292, entitled “Wafer Structure for Electronic Integrated Circuit Manufacturing,” filed Aug. 25, 2011, all of which are herein incorporated by reference as if set forth in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to photodiodes and, more particularly, to a structure and method to improve the photodiode response in front-illuminated, back-side contacted, bonded-wafer, Through Silicon Via (TSV) photodiodes. 
       BACKGROUND OF THE INVENTION 
       [0003]    An example of the current state-of-the-art in front-illuminated, back-side contacted, TSV photodiodes is illustrated in U.S. Pat. No. 7,741,141. In this and other patents referenced therein, a photodiode structure is formed consisting of a first doping concentration proximate to a front-side surface, and a second doping concentration proximate to a back-side surface, the front-side doping type being opposite to the backside doping type, with an insulating (intrinsic) region separating the front-surface doping region from the back-surface doping region. The structure formed is either a p-i-n (p-type or anode, insulating, n-type or cathode) or n-i-p (n-type or cathode, insulating, p-type or anode) diode structure. This photodiode structure is often used as part of an X-ray detector comprised of a scintillation material (such as Cadmium Tungstate or Cesium Iodide) attached to the photodiode such that visible light generated in the scintillation crystal by X-rays absorbed therein is subsequently absorbed in the photodiode, generating an electrical current which may be detected and quantized by various electronic means. However, an optical draw-back of this type of structure is the fact that any light absorbed in the non-depleted portion of the front-side doping region (whether anode or cathode) cannot contribute to the desired photo-current, since the electron-hole pairs generated recombine quickly before reaching the depleted region of the photodiode. Such a non-depleted region is called the dead-layer. 
         [0004]    A typical prior photodiode structure  10  is shown in cross section in  FIG. 1 . As previously described an n ++  type cathode layer  18  is shown, as well as an n −  i-layer (collection layer)  16 , and a patterned p +  type anode (dead layer)  14 , separated by patterned isolation layers  12 .  FIG. 1  depicts the anode  14  being struck by photons  19 . A plan view of the photodiode  20  is shown in  FIG. 2 . The plan view simply shows four p +  anodes  24  separated by an isolation region  22 . 
         [0005]      FIG. 3  is a graph  30  that illustrates the percentage  32  of the incident light signal lost as a function of the depth of the front-side dead-layer region, using the absorption coefficient of silicon at 490 nm wavelength, a wavelength approximately near the peak emission of Cadmium Tungstate. As can be seen from  FIG. 3 , a dead layer of only 0.2 microns depth can cause a loss of signal of up to 20% at 490 nm. While the loss due to the dead layer decreases for longer wavelengths, it can still be significant (&gt;10%). 
         [0006]    Thus, what is desired is an alternative photodiode structure that minimizes the dead layer so that the photodiode response can be maximized. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    A photodiode structure having an illuminated front-side surface and a back-side surface comprise a front-side doped layer having a first conductivity type; a back-side doped layer having the first conductivity type; a front-side active cell region made sensitive to light by the action of at least one plug region formed in the front-side doped layer having a second conductivity type; and a front-side inactive cell region substantially insensitive to light, wherein the first and second conductivity types are opposite conductivity types. The photodiode includes a through-via traversing the at least one plug region or the inactive cell region. In an embodiment of the invention, the first conductivity comprises an n-type conductivity, and the second conductivity comprises a p-type conductivity. The inactive cell region comprises a pixel isolation region that can be formed using a silicon trench, a heavily doped region of the first conductivity type, or other deep trenches filled with non-conductive materials including oxide or a combination of oxide and intrinsic, polycrystalline semiconductor. In an embodiment of the invention, the back-side doped layer comprises a cathode layer. The photodiode structure of the present invention can be fabricated in silicon, GaAs, or other semiconductor materials. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The following detailed description of the preferred embodiments of the present invention will be better understood when read in conjunction with the following drawings. The embodiments shown in the drawings illustrate the preferred embodiments of the present invention; however, the invention is not limited to the precise arrangements and instrumentalities shown. Drawings are not to scale. 
           [0009]    In the drawings: 
           [0010]      FIG. 1  is a cross-sectional view of a photodiode according to the prior art including a dead layer; 
           [0011]      FIG. 2  is a plan view of the prior art photodiode shown in  FIG. 1 ; 
           [0012]      FIG. 3  is a graph of the lost signal as a function of dead layer depth in microns associated with the photodiode of  FIG. 1 ; 
           [0013]      FIG. 4  is a cross-sectional view of a photodiode according to the present invention; 
           [0014]      FIG. 5  is a detailed cross-sectional view of the photodiode according to the present invention; 
           [0015]      FIG. 6  is a front-side plan view of the photodiode with the front-side dielectric and metal interconnection layers omitted; 
           [0016]      FIGS. 7 through 9  are detailed front-side plan views of alternative embodiments of the photodiode of the present invention; 
           [0017]      FIG. 10  is a detailed front-side plan view of an embodiment of the photodiode of the present invention showing the front-side metal interconnections; 
           [0018]      FIG. 11  is a process flow diagram showing cross-sectional view of the photodiode of the present invention at various points during the fabrication process; 
           [0019]      FIG. 12  is a back-side plan view of the photodiode showing the back-side metal interconnections; and 
           [0020]      FIG. 13  is a diagram of an X-ray imaging system incorporating the photodiode of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0021]    The photodiode  40  of the present invention circumvents the problem of dead-layer absorption by locating the front-side doping region in the septum between active photodiode pixels as shown in the cross-sectional view of  FIG. 4 . Using an example where the front-side plug regions or patterned layer  44  is p-type (shown as p +  in  FIG. 4 ), and is formed in the front-side doped layer  46 , which is n-type (shown as n −  in  FIG. 4 ). The back-side doped region  48  of the photodiode  40  is also n-type (shown as n ++  in  FIG. 4 ). In the embodiment shown in  FIG. 4 , the front-side plug region  44  acts as the anode of a p-i-n photodiode. In previous art, the anode would extend across the entire photodiode Desired Active Region, such that the electric field associated with the depletion region of the photodiode is essentially perpendicular to the front-side surface. In the preferred embodiment of the present invention, the front-side plug region  44  is located in the septum between pixels, such region including an isolation region  42  constructed such that light impinging upon this region does not contribute appreciable electrical signal to the pixels bordering such region. 
         [0022]    The location of the front-side plug region  44  in the septum between adjacent pixels causes the electric field associated with the depletion region of the photodiode to be essentially parallel to the front-side surface. At some depth below the front-side surface of the photodiode, the electric field lines will curve until they become essentially perpendicular to the front-side surface and back-side surface. Thus, using the example where the front-side plug region  44  is the anode, the electrons of the electron-hole pairs generated by the absorption of light in the photodiode  40  will move along curved electric field lines and be collected approximately laterally by the anode comprised of the front-side plug region  44 , while the holes of the electron-hole pairs generated by the absorption of light in the photodiode  40  will move along curved electric field lines and be collected approximately vertically by the cathode comprised of the back-side doping region  48 . 
         [0023]    Electrical connection of the front-side plug regions  44  may be made by a conductive through via  49 A or  49 B (isolated with oxide isolation and described in further detail below), as described in previous art, but an electrical connection may also be made by a bond pad and metal wire formed on the front-side surface. Via  49 A is shown traversing p +  plug region  44 , and an alternative via  49 B is shown traversing the isolation region  42 . Either via can be used in conjunction with the present invention. 
         [0024]    In previous art, the pixel isolation region  42  was comprised of a deep silicon trench. In the preferred embodiment of the present invention, this isolation method is certainly possible; however, the pixel isolation region  42  can be alternatively comprised of a doping region of opposite type to the front-side plug region  44 . In the example where the front-side plug region  44  is p-type, the pixel isolation region  42  may be an n-type doping region. The formation of the front-side plug region  44 , the back-side doped region  48 , and the pixel isolation region  42  may be made using well-known methods of doping in semiconductor technology such as but not limited to ion-implantation, epitaxial growth, wafer bonding, or solid source diffusion, any of which such methods may be followed by one or more thermal annealing steps to both diffuse and/or activate such doping. The isolation region  42  is shown on the left side of  FIG. 4  as extending to the upper surface of the back-side doped region  48 . However, on the right side of  FIG. 4  an alternative embodiment is shown. The isolation region  42  can extend only partially through the front-side doped region  46 , or may extend well into the back-side doped region  48  if desired for a particular application. 
         [0025]    Referring now to  FIG. 5 , a more detailed cross-sectional view  500  of the photodiode of the present invention is shown, and in particular, revealing details related to the through-via  549 . As before, photodiode  500  includes plug regions  544 , front-side doped region  546 , and back-side doped region  548 . However, additional details are shown in  FIG. 5 . At the top surface of the photodiode a patterned metal layer  502  couples the via  549  to the plug region  544 . Note that metal layer  502  can be extended to cover both plug regions  544 . Since plug region  544  is essentially a dead-layer, it does not contribute appreciably to the generation of a photocurrent, and therefore may be hidden from the incident photons, i.e. entirely excluded from the desired active pixel region  546  that is not occluded by metal region  502 . The metal via  549  thus makes contact with a front-side metal region  502 , as well as a back-side metal region  506 . The via  549  is completely electrically isolated from the doped regions due to oxide layer  504 . The plug region is thus electrically contacted through top-side metal layer  502 , via  549 , and through the back-side metallization comprising metal layers  506 ,  512 A,  514 A,  516 A, oxide layer  510 , and metal ball  518 A. In a preferred embodiment of the invention, metal layers  502  and  506  are conventional aluminum or other known metals, metal layer  512 A is nickel, metal layer  514 A is copper, and metal layer  516 A is gold. The metal ball  518 A is conventionally formed of solder, gold or other known conductive materials. The back-side electrical connection (cathode) to the photodiode is made through the metallization stack made up of metal layers  508 ,  512 B,  514 B,  516 B, and metal ball  518 B, which is made of similar materials to the metallization of the plug region (anode). 
         [0026]    Referring now to  FIG. 6 , the front side of the photodiode  600  is shown stripped of any oxide or dielectric layers, and of any front-side metallization. Contacts are also not shown in  FIG. 6 . Thus, a simple plan view remains showing only the isolation region  602 , the p +  anode region  604 , and the active n −  regions  606 . Four pixels are shown in  FIG. 6 , each having a crossed anode pattern. 
         [0027]    Referring now to  FIG. 7  a detailed plan view of a single pixel is shown. As before, the isolation region  602 , the anode region  604 , and the active regions  606  are shown, wherein the anode comprises a cross pattern. However, in  FIG. 7 , the top-side via contacts for accessing the anode (plug regions) are shown. Contacts  608 A and  608 B are shown in the plug region  604 , and alternative contact  608 C is shown in the isolation region  602 . 
         [0028]    Referring now to  FIG. 8 , a detailed plan view of a single pixel for an alternative embodiment of the present invention is shown. As before, the isolation region  602 , the anode region  604 , and the active regions  606  are shown, wherein the anode comprises a bar pattern, coupled to a peripheral anode region. In  FIG. 8 , the top-side via contacts for accessing the anode (plug regions) are shown. Contacts  608 A and  608 B are shown in the in the plug region  604 , and alternative contact  608 C is shown in the isolation region  602 . 
         [0029]    Referring now to  FIG. 9  a detailed plan view of a single pixel is shown for another alternative of the present invention. As before, the isolation region  602 , the anode region  604 , and the active regions  606  are shown, wherein the anode comprises a segmented bar pattern that is not coupled to the peripheral anode region. In  FIG. 9 , the top-side via contacts for accessing the anode (plug regions) are shown. Contacts  608 A,  608 B,  608 C are shown in each of the segmented plug regions  604 , and alternative contact  608 D is shown in the isolation region  604 . 
         [0030]    In addition to the embodiments of the present invention shown and described above, numerous other configurations of anode regions are possible that do not extend throughout the entire desired active pixel region. 
         [0031]    Referring now to  FIG. 10 , a detailed plan view of the top-side of a pixel is shown, including the top-side metal. As before, the isolation region  602 , the anode region  604 , and the active regions are shown, wherein the anode comprises a cross pattern. Via contacts  608 A in the isolation region, and contacts  608 B and  608 C in the anode region, are also shown. One or more of contacts  608 A,  608 B, and  608 C can be used. In addition, top-side contacts  612  for the anode region are shown, which are electrically coupled to the via contacts through the top-side metal layer  610 . 
         [0032]    Referring now to  FIG. 11 , a series of cross sectional diagrams are shown that illustrate the process of forming a photodiode  1100  according to the present invention. The process flow in  FIG. 11  is highly simplified, and those of skill in the art will realize that many conventional processing steps have been omitted. Also, some of the conventional processing details are also omitted. In step  1100 A, a handle wafer  1104 A and a top wafer  1102 A are bonded together. In step  1100 B, the two wafers are thinned using grinding or other known techniques, to form thinned wafers  1102 B and  1104 B. In step  1100 C, nitride and oxide layers  1108  and  1110  are formed, that form an etch stop for forming the via trench  1106  as shown. In step  1100 D, a liner oxide is formed in via  1112 , and via  1112  is filled with a conducting material. In step  1100 E, the p +  anode regions  1114  are shown. In the embodiment shown in  FIG. 11 , note that the via  1112  traverses the anode region. As previously described, it can also traverse the isolation region. In step  1100 F, the isolation regions  1116  are formed. 
         [0033]    Referring now to  FIG. 12 , the backside  1200  of the photodiode of the present invention is shown. The backside of the isolation region  1202 , the backside of the anode region  1204 , and the back side of the active regions  1206  are shown in dashed lines, because these regions are obscured by thinned wafers  1102 B and  1104 B as shown in  FIG. 11 . The metallized regions  1208 A,  1208 B,  1208 C, and  1208 D connect the vias  1112  to their respective anode contact pads. Note that the anode pads may be routed to locations arbitrarily distant from the via such that the anode contact pad is not immediately adjacent to its active pixel region  1206 . The metallized regions  1210 A and  1210 B connect the backside cathode region  1200  to one or more backside cathode contact pads. Just as with the anode contact pads, the cathode contact pads may be located arbitrarily distant from the region where such metallization  1210 A and  1210 B make contact to the backside cathode region  1200 . 
         [0034]      FIG. 13  shows an X-ray imaging system  1300  incorporating an X-ray detector comprised of an X-ray source  1302  for emitting X-ray photos  1304 , a scintillator material  1306  coupled to a photodiode structure  1308  including a plurality of photodiodes according to the present invention. The X-ray imaging system  1300  can comprise a computed tomography system, a digital radiography system, an X-ray baggage security scanner, or other known X-ray systems. 
         [0035]    It will be apparent to those skilled in the art that various modifications and variations can be made to the invention without departing from the spirit or scope of the invention. For example, numerous geometric features have been shown and described in conjunction with the layout embodiments of the photodiode of the present invention. As will be appreciated by those skilled in the art, all of these geometric features can be changed as required, as well as the placement of the contacts, and the shape of the metal regions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.