Abstract:
Back-illuminated, thin photodiode arrays with trench isolation. The trenches are formed on one or both sides of a substrate, and after doping the sides of the trenches, are filled to provide electrical isolation between adjacent photodiodes. Various embodiments of the photodiode arrays and methods of forming such arrays are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/964,331 filed Aug. 10, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to structures of the back-illuminated, pin photodiode arrays manufactured on thin wafers and methods of manufacturing the same. 
     2. Prior Art 
     The structures described herein can be considered an alternative to the conventional structures described previously in U.S. Pat. Nos. 6,762,473 and 7,112,465, as well as in the number of patents from other inventors. See for example U.S. Pat. Nos. 6,933,489, 6,426,991 and 6,707,046. 
     Pin photodiode arrays for imaging applications are 2D arrays combining multiple pixels separated from each other in one of various different ways. A typical die layout (front view) for the 2D backlit pin photodiode array with isolation diffusion between pixels on the front side is shown in  FIG. 1 . 
     In conventional backlit arrays, isolation structures are not usually made on the backside of the array. The example of the vertical structure of such conventional backlit pin photodiode array is shown in  FIG. 2 . The substrate  1  is a first conductivity type. The active pixel diffusion  100  is of the second conductivity type (polarity is different from that of the substrate). Diffusions  100  form diodes (either p-on-n or n-on p polarity) with p-n junctions in the substrate. The diffusion  101  between pixels is of the same conductivity type as the substrate (the first conductivity type). The diffusions  100  and  101  are usually shallow. The shallow blanket diffusion  5  is of the first conductivity type. Oxide layer  11  is a thermal oxide or other passivation layer. The anode pads  20  and cathode pads  21  complete the structure. 
       FIG. 3  shows an example of the structure for backlit pin photodiode arrays with isolation diffusion applied from both sides of the wafer (see also U.S. Pat. Nos. 6,762,473 and 7,112,465). In addition to the structure shown in  FIG. 3 , an additional feature—the isolation diffusion  102  from the backside is applied. The diffusion  102  is of the first conductivity type and is aligned with the diffusion  101  and other structural features on the front side. The diffusions  101  and  102  may or may not meet in the bulk of the wafer. The diffusion  100  may be shallow or as deep as the diffusion  101 . 
     The structure shown in  FIG. 3  is characterized with superior performance parameters, such as leakage current, crosstalk between active pixels of the array, response time, and others. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The main ideas of the invention are demonstrated by the accompanying drawings, in which: 
         FIG. 1  shows the view from the first (front) surface of a conventional (prior art) pin photodiode array with isolation diffusion applied on the first surface of the substrate. 
         FIG. 2  is the cross section of a typical backlit pin photodiode array (prior art). 
         FIG. 3  is an example of a prior art backlit pin photodiode array with isolation diffusion applied from the both surface of the substrate. 
         FIG. 4  shows the cross section of an exemplary structure with front side trenches and backside isolation diffusion in accordance with the present invention. 
         FIGS. 5   a  through  5   c  illustrate an exemplary fabrication sequence for the structure shown in  FIG. 4 . 
         FIG. 6  shows the cross sectional view of an alternative structure with trenches formed from both surfaces of the substrate. 
         FIG. 7  shows the cross section of a structure of an embodiment of the present invention having the trenches on the first surface of the substrate and no isolation diffusion on the second surface of the substrate. 
         FIG. 8  is the cross section of an exemplary structure having the isolation trenches on the second surface of the substrate and isolation diffusion (no trenches) on the first surface of the substrate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The objectives of the present invention include: 
     1) To provide multiple element, backside illuminated pin photodiode arrays with a superior performance of all elements; 
     2) To provide fabrication methods for the backside-illuminated photodiode array on an ultra thin wafer. 
     It is therefore an object of this invention to provide a structure for 2-D pin photodiode arrays having superior performance characteristics, making it useful in such applications as CT scanner applications, etc. 
     Another object is to provide a method of fabricating Si devices on thin wafers, which method can be suitable for fabrication of flip-chip, multi-element, 2-dimensional pin photodiode arrays. 
     These and other objects of the present invention will become apparent from the following disclosure. In this disclosure, first preferred embodiments of the arrays structures will be described, and then the preferred method of fabricating the arrays will be described. 
     The embodiments of the present invention are shown in and described with respect to  FIGS. 4 through 8 . 
       FIG. 4  shows a first example of a structure in accordance with the present invention. 
       FIGS. 5   a  through  5   c  show the steps of and exemplary fabrication method of the structure of  FIG. 4 . 
     The structure in  FIG. 4  includes (electrical) isolation trenches  10  on the front side (first surface) and isolation diffusion  4  made from the backside (second surface) of the substrate  1 . The pin photodiode array of the structure shown in  FIG. 4  is made using semiconductor substrate  1  of a first conductivity type. The basic steps of the fabrication method for the structure of  FIG. 4  are disclosed herein with  FIGS. 5   a  through  5   c.    
     As shown in  FIG. 5   a , deep trenches (defining regions  10 ) are created in the first surface of the substrate between pixels (active elements) of the array. The trenches form a matrix enclosing (surrounding) each pixel of the array. The trenches may be created using dry etch, wet etch, or other techniques known in the industry, preferably by a substantially directional etch. The trenches may, for example, propagate from the first surface of the substrate into the substrate bulk to a depth ranging from a few microns to more than 100 um. The maximum depth of the trenches depends on the target thickness of the array and requirements of the mechanical integrity of the wafer. The walls and bottoms of trenches are then doped with the dopant  2  of the first conductivity type (same as the substrate) with a concentration heavier than that of the substrate. The trenches are then backfilled with silicon oxide, poly, or other isolation material  10  as shown in  FIG. 5   a . Chemical-mechanical polishing (CMP) may be applied to improve the surface planarity. The diffusions  3  of the second conductivity type with a concentration greater than that of the substrate is applied in the middle of each pixel. This diffusion creates p/n junctions with the semiconductor substrate. The shape and size of the diffusion  3  may vary, and depend on the required performance parameters, such as active pixel size and gaps between neighboring pixels. Layer  11  shown in  FIG. 5   a  is thermal oxide or other passivation material. 
     The next step is shown in  FIG. 5   b , where the wafer is lapped from the backside (second surface of the substrate) to the thickness of approximately 300 um or less, and preferably less than 200 um. This may be followed by polishing and CMP. Temporary wafer bonding to the first surface of the substrate may be necessary before the wafer thinning. After polishing, diffusion  4  of the first conductivity type in a concentration heavier than that of the substrate is made from the second surface of the substrate. Next, a deep thermal drive is applied to bring diffusions  2  and  4  closer to each other and to bring diffusion  3  closer to the second surface of the substrate, as shown in  FIG. 5   c . The distance between the p/n junction (bottom edge of the diffusion  3 ) and the second surface of the substrate is preferably made approximately 200 um or smaller. The diffusions  2  and  4  may touch, but this is not a requirement of the invention. The spacing between the diffusions  2  and  4  may be, by way of example, as small as a few microns and as large as several tens of microns. The final spacing between these two diffusions depends on the required array performance parameters (crosstalk, etc.). The anti-reflection coating oxide layer  12  may be created during this process step. 
     The result of the final exemplary steps of the array fabrication is shown in  FIG. 4 . A shallow blanket diffusion  5  of the same conductivity type as the substrate is made on the second surface of the substrate. This blanket diffusion may be made through the oxide layer  12  by ion implantation, or alternatively may be formed before the oxide coating is created. The surface enhancement of the diffusions  2  and  3  may be required, which step is well known in the industry. Then the contacts are opened on the first surface of the substrate to deposit and pattern metal to form the contact pads  20  and  21 . In the final structure, the pads  20  have good electrical contact to the diffusion  3  and the pads  21  have good electrical contact to the diffusion  2 . 
       FIG. 6  shows another alternative structure of the backlit pin photodiode array with trench isolation. In this version, additional trenches are formed, and a second conductivity type diffusion  3  is made inside the trenches, preferably at their bottom as shown. A highly conductive path  6  (more conductive than the substrate) in each such trench that connects diffusion  3  with the first surface of the substrate is made, either with a high concentration diffusion of the second conductivity type applied to the wall of the trench, or with doped (the same conductivity type) poly, or by another method known in the industry. These trenches are then also backfilled with isolation material  15 . The depth of the trenches  15  and the depth of the p/n junction (the bottom edge of the diffusion  3 ) are made as deep as is necessary to bring the bottom edge of the p/n junction within the approximately 200 um or less from the second surface of the substrate  1 . 
     The structure of  FIG. 6  also has trenches made from the second surface (backside) of the substrate. These trenches are aligned with the trenches on the first surface of the substrate. A first conductivity type diffusion  7  with a concentration heavier than that of the substrate is made inside the trenches on the second surface of the substrate. The trenches are then backfilled with isolation material  16 , which could be the same as the material  10  used to backfill the trenches of the first surface of the substrate. The final drive is made with the purpose of minimizing the gaps between diffusions  2  and  7  (or to cause them to contact) and to bring the p/n junction between regions  3  and  1  closer than approximately 200 um from the second surface of the substrate. The blanket shallow diffusion  5  on the second surface of the substrate is applied when all drives are completed, which is similar to the case of the structure shown in  FIG. 4 . The metal pads  20  and  21  complete the structure. Note that wafer thinning may not be required for this structure. 
       FIG. 7  shows still other embodiment of the array with isolating trenches. The structure of  FIG. 7  is similar to the structure shown in  FIG. 4 , but no isolation diffusion is applied from the second surface of the substrate. Instead, the wafer thinning from the backside intercepts the isolation diffusion applied to the bottom of the trench on the first surface of the substrate. The structure of  FIG. 7  is completed with the blanket doping deposition  5  on the second surface of the substrate and optional antireflection (AR) coating. 
       FIG. 8  shows still another version of an array with isolating trenches. The structure of  FIG. 8  is similar to the structure shown in  FIG. 6 , but no trenches are made on the first surface of the substrate. Instead, the isolating diffusion  2  from the first surface of the substrate is driven as close as possible to the diffusion  7  applied inside trenches on the second surface of the substrate. Isolating diffusion  2  encloses (surrounds) each active pixel of the array. However, these two diffusions (diffusions  2  and  7 ) may, but do not necessarily meet. All diffusions ( 2 ,  3 , and  7 ) may be driven simultaneously, providing the same depth for the isolation diffusion  2  on the first surface of the substrate and diffusion  7  on the second surface of the substrate. The thickness of the wafer can be, by way of example, from approximately 10 um up to approximately 300 um. The p/n junction depth on  FIG. 8  (the depth of the diffusion  3  from the first surface of the substrate) is less than 300 um for a 300 um thick substrate, and may be close to the depth of the isolation diffusion  2  (this may be valid for the other alternative structures shown in  FIGS. 4 through 7  as well). The p/n junction depth may be as deep as several tens of microns. Alternatively, diffusion  3  may be very shallow (from a few microns to several microns deep) and may be significantly shallower than the depth of the isolation diffusion  2 . 
     As in all other versions of the structure, a blanket, shallow diffusion  5  is applied to the second surface of the substrate after all drives are completed. 
     Note that either n- or p-type substrate may be used for the embodiments and objectives throughout this disclosure. 
     The current invention presents alternative structures and methods of their preparation, which allow building pin photodiode arrays with superior performance parameters. 
     While certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.