Abstract:
Ultra thin back-illuminated photodiode array fabrication methods providing backside contact by diffused regions extending through the array substrate. In accordance with the methods, a matrix is diffused into one surface of a substrate, and at a later stage of the substrate processing, the substrate is reduced in thickness and a similar matrix is diffused into the substrate from the other side, this second diffusion being aligned with the first and contacting the first within the substrate. These two contacting matrices provide good electrical contact to a conductive diffusion on the backside for a low resistance contact to the backside. Various embodiments are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/863,558 filed Jun. 8, 2004 now U.S. Pat. No. 7,112,465, which is a divisional of U.S. patent application Ser. No. 10/606,053, filed Jun. 25, 2003, now U.S. Pat. No. 6,762,473. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor photodiodes, and in particular, to the structures of high performance, back-illuminated photodiode arrays and the methods of fabricating such structures. 
     2. Prior Art 
     Conventional photodiode array structures are based on either front illuminated or back illuminated technologies.  FIG. 1   a  is a simplified cross section of an exemplary prior art front illuminated photodiode array and  FIG. 1   b  is a simplified cross section of an exemplary prior art back illuminated photodiode array. The substrate may be either n-type or p-type material, with opposite conductivity type diffused regions  2  therein. This creates a p-on-n or n-on-p structure, respectively. The anode metal pads  3  for the p-on-n structure (the cathode contacts for the n-on-p structure) are always on the device front surface. The opposite polarity electrode is usually coated (plated) on the chip backside in the case of the front illuminated structure (see metal layer  4 ,  FIG. 1   a ), or is made on the device front surface (see metal pads  4 ,  FIG. 1   b ) using metallized through vias  6 , 7  in the case of the back illuminated structure. (See also U.S. Published Patent Application Ser. Nos. 2003/0209652 A1, 2004/0104351 A1 and 2004/0129992 A1.) The blanket-type implantation  5  of the back surface of the die of the same conductivity type as the substrate improves both the charge collection efficiency and DC/AC electrical performance of the devices. 
     Each of the two approaches—the front illuminated and back illuminated structures—has its own advantages and disadvantages. For example, traditional front illuminated structures like that shown in  FIG. 1   a  allow building high performance photodiodes and photodiode arrays, but impose severe constraints on the metal run width. Those constraints limit a design of the front illuminating photodiode array to the use of either a smaller number of elements, or larger gaps between adjacent elements. Note that the metal runs should be accommodated in between adjacent diffusion areas  2  (see  FIG. 1   a ). 
     Back illuminated structures reported recently by several companies take advantage of solder bump technology to electrically connect elements of the array to an external substrate or PC board using the contacts (bumps) on the front surface of the structure. By utilizing solder bump technology, the metal interconnects, which usually reside on top of the active surface between the adjacent elements openings, may be moved to the substrate or PC board upon which the chip is mounted. Such an approach allows minimizing the gaps between adjacent elements of the array, at the same time allowing a virtually unlimited total number of elements. However, several drawbacks of the previously reported back illuminated structures limit their application:
         1) First, these structures are usually fabricated using relatively thick wafers (&gt;50 μm) and the resistivity of the material has to be high enough (&gt;1000 Ohm-cm) to deplete the entire volume at zero bias, which is required for many applications;   2) Second, the application of a high resistivity material usually diminishes the photodiode performance with respect to the leakage current and shunt resistance;   3) Third, if a high resistivity material is not used, then the time response will be very long (micro seconds or even longer) because the time response would be determined by the diffusion processes rather than drift processes of the totally depleted structures;   4) Fourth, there are little or no structural features that isolate adjacent cells from each other within the entire thickness of the device, which results in relatively high cross-talk, especially at zero bias.       

     Summarizing, such parameters as the leakage current, shunt resistance, cross-talk, spectral sensitivity, and temporal response are of main concern for the prior art of back illuminated structures. Additionally, the handling of thin wafers (&lt;70 μm thickness) in the wafer fabrication process is a matter of great concern by itself, and would become increasingly important with the further decrease of the wafer thickness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The main ideas of the invention are demonstrated by the accompanying drawings, in which: 
         FIGS. 1   a  and  1   b  are schematic cross sections of typical, conventional prior art structures for the front illuminated photodiode arrays and back illuminated photodiode arrays, respectively. 
         FIG. 2  is a schematic cross section of an ultra thin, back illuminating photodiode array in accordance with U.S. Pat. No. 6,762,473. 
         FIG. 3  is another example of a schematic cross section of an ultra thin, back illuminated photodiode array in accordance with U.S. Pat. No. 6,762,473 fabricated in accordance with the present invention method using diffusions  8  and  9  applied from both the front side and the backside of the substrate. 
         FIGS. 4   a  through  4   e  illustrate an exemplary sequence for a method for fabricating electrodes of a thin wafer photodiode array structure in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The objectives of the present invention include:
         1) To provide multiple element, backside illuminated 2D-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 silicon multi-element, 2-D photodiode arrays having greatly improved characteristics over prior art arrays, making it useful in such applications as CT scanner applications, etc. 
     Another object is to provide a method of fabricating Si devices on ultra thin wafers, which method can be suitable for fabrication of flip-chip, multi-element, 2-dimensional arrays of silicon photodiodes. 
     Another object is to provide a method of handling ultra thin wafers during wafer fabrication, which method should secure ultra thin wafers against accidental breakage, etc. 
     These and other objects of the present invention will become apparent from the following disclosure. In this disclosure, first preferred embodiments of finished diode arrays will be described, and then the preferred method of fabricating the arrays will be described. 
       FIG. 2  is a simplified cross-sectional view of a semiconductor ultra-thin chip photodiode array in accordance with U.S. Pat. No. 6,762,473. The structure is built using either n-type or p-type bulk silicon  1 . For brevity, the anode in the case of p-on-n structure or the cathode in the case of n-on-p structure will be referred to as “the first electrode”, while the cathode in the case of p-on-n structure and the anode in the case of n-on-p structure will be referred to as “the second electrode”. 
     The material resistivity, thickness of the wafer/die, dopants concentrations and doses, and diffusion conditions are preferably chosen to satisfy the following requirements:
         a) The active area (the first electrode) diffusion  2  extends sufficiently close to the back surface of the finished die that the rest of the volume between the diffusion edge and the die back surface, the space indicated as “d” in the  FIG. 2 , becomes mostly or completely depleted at a zero volt bias;   b) The uniform, “blanket” type implantation  5  of the backside of the wafer with the implant of the same polarity as the one of the majority carriers of the wafer bulk  1  (first conductivity type) provides excellent majority carrier conductivity across the wafer backside;   c) In the method disclosed to fabricate the ultra thin, back illuminated photodiode array in accordance with U.S. Pat. No. 6,762,473, the second electrode, diffusion  8  (first conductivity type), is applied from the front surface of the wafer using the implantation and drive protocols that allow for the diffusion to reach the wafer backside, overlapping thereby with the blanket implantation  5  and providing good electrical contacts between the second electrode network across the entire wafer.       

     Another possibility is to use the structure of ultra thin, back illuminated photodiode array in accordance with U.S. Pat. No. 6,762,473, but fabricated using a method in which the second electrode is applied from both the front side and the backside of the wafer. This is illustrated in  FIG. 3  as diffusions  8  and  9 . 
     Such a structure may be fabricated starting with a thicker substrate (for example 300 μm) for structural stiffness and integrity during the processing, using several masking steps. An exemplary processing method may be described as follows: 
     First, as shown in  FIG. 4   a,  the second electrode  8  implantation/diffusion is applied, followed by a drive. This stage forms a small difference in diffusion depths as seen in  FIG. 4   b  for the first electrodes  2  and second electrodes  8  as formed. 
     Second, as shown in  FIG. 4   b,  the first electrode  2  implantation/diffusion is applied followed by a drive. The future back surface of the wafer after backside grinding and polishing is shown schematically with the dashed line  10 . 
     The array is then reduced in thickness by grinding the backside of the array, preferably to provide a substrate thickness of under approximately 100 μm, and more preferably to approximately 50 or 30 μm. The final thickness achieved, of course, is preferably selected in accordance with the resistivity of the substrate and the depth of the first electrode  2  diffusion so that the diffusion  2  is spaced away from the backside of the substrate an amount that approximately equals the depletion depth for the substrate material at zero bias. 
     Third, the second electrode  9  is applied (implantation/diffusion) on the backside of the thin wafer, followed by a drive, as illustrated in  FIG. 4   c.  At this point, there might (but not necessarily) be a small gap between the two electrodes of the same polarity,  8  and  9 , as shown in  FIG. 4   c.  In other words, the diffusion  8  might not necessarily contact the diffusion  9 , and an additional drive might still be necessary to provide good electrical contact between the front and backside of the thin wafer through the diffusion regions  8  and  9 . However, there may be an option when the two electrodes  9  and  8  contact each other without an additional drive, creating a good conducting path between the front surface and the backside of the wafer as illustrated in  FIG. 4   d.    
     Fourth, as shown in  FIG. 4   d , the second electrode  8  receives an additional enhancement  12  followed by a drive to ensure superior electrical contacts and to activate dopants. By the end of this stage, the diffusion profiles of regions  2 , 8 , and  9  almost reach their final configuration (see the solid lines and hatched areas in  FIG. 4   d ). As an alternative, this additional enhancement may be done before reducing the thickness of the array. In that regard, the processing order set out in the appended claims is intended to be representative of one possible order, though is not to be considered a limitation of the claims. 
     Then a blanket implant of the first conductivity type is made to the backside of the wafer, which implant improves both the charge collection efficiency and DC/AC electrical performance of the photodiode arrays. Activation of the implant does not significantly alter the first and second electrode diffusions. Alternatively, a diffusion for the backside could be used if desired. 
     The photodiode arrays exhibit very low cross talk because of the excellent isolation of each pixel. Also, because of the small depletion volume, the arrays exhibit low noise and low temperature sensitivity. When used in X-ray systems, they exhibit low radiation damage, and have thermal characteristics similar to scintillators to which they will be mounted. The technique of using deep diffusions from both sides of a thin substrate for making electrical contact to the backside of the substrate may, of course be used in other semiconductor devices. Also while the deep diffusion in the preferred embodiment is of the same conductivity type as the substrate, this is not a limitation of the invention, as the deep diffusions may be of the opposite conductivity type, if desired. 
     For many applications, the photodiode response time is a critical parameter. Using a p-type starting material can minimize the response time for the Si arrays. This is because the transit time for electrons as minority carriers is less than ⅓ of that for the holes. Hence, the structure shown in  FIG. 4   e  can be built using p-type Si as a starting material to ensure fast response time. 
     With a p-type starting material, regions  8 ,  9  and  5  normally will be p-type regions of higher conductivity than the starting material, with regions  2  being n-type regions. However with an n-type starting material, regions  8 ,  9  and  5  normally will be n-type regions of higher conductivity than the starting material, with regions  2  being p-type regions. However for some applications, it may be desirable to reverse the conductivity type of the starting material, so that with an n-type starting material, regions  8 ,  9  and  5  will be p-type regions, with regions  2  being n-type regions of higher conductivity than the starting material, and with a p-type starting material, regions  8 ,  9  and  5  will be n-type regions, with regions  2  being p-type regions of higher conductivity than the starting material. 
     While preferred exemplary embodiments of the present invention have been disclosed herein, such disclosure is only for purposes of understanding the exemplary embodiments and not by way of limitation of the invention. It will be obvious to those skilled in the art that various changes in fabrication process and structure of the photodiode arrays may be made without departing from the spirit and scope of the invention, as set out in the full scope of the following claims.