Abstract:
A photodiode for use in an imager having an improved charge leakage. The photodiode has a doped region that is spaced away from the field isolation to minimize charge leakage. A second embodiment of invention provides a second implant to improve charge leakage to the substrate. The photodiodes according to the invention provide improve charge leakage, improved reactions to dark current and an improved signal to noise ratio. Also disclosed are processes for forming the photodiode.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to an improved diode for use in semiconductor devices The invention also relates generally to an improved photodiode having lower charge leakage to the substrate for use in imaging devices.  
         BACKGROUND OF THE INVENTION  
         [0002]    Diodes find use in many solid-state devices. Diodes which respond to light, known as photodiodes, are widely used in many solid-state image sensors, also known as imagers, which were developed in the late 1960s and early 1970s primarily for video image acquisition, transmission, and display. An imager employing a photosensitive diode absorbs incident radiation of a particular wavelength (such as optical photons, x-rays, or the like) and generates an electrical signal corresponding to the absorbed radiation. Typical devices which use photosensitive diodes include charge coupled devices (CCDs), photodiode arrays, charge injection devices, hybrid focal plan arrays, memory and logic circuits and CMOS imagers.  
           [0003]    In the case of photosensitive devices such as CCD&#39;s or CMOS imagers, the photosensitive region is typically a p-n junction. This junction is reverse biased by applying an electrical potential Vo which reverse biases the p-n junction. The p-n junction is then isolated typically by turning off a “reset transistor” that is used to reverse bias the junction. Under illumination, incident light photons create electron-hole pairs that are separated by the electric field in a depletion region of the p-n junction. This separation by the electric field results in one charge type being stored in the depletion region, thus collapsing the depletion region, and reducing the voltage Vo across the p-n junction. It is this reduction in voltage across the diode p-n junction due to light exposure that is measured.  
           [0004]    To provide context for the invention, an exemplary prior art photodiode is described below with reference to FIG. 1. However, it is to be understood that the invention also has utility in any semiconductor diode device which includes a p-n junction where there is leakage to the substrate. Accordingly, the present invention may also find utility in semiconductor devices where p-n junctions are shielded from light, where p-n junctions are formed in a p-well, where p-n junctions are formed in an n-well, or more complicated junctions such, for example, a p-n-p junction. Also, while FIG. 1 shows a simplified photodiode  15  for use as a pixel of an imager, it should be understood that the single photodiode pixel  15  in practical use will be a part of either a row of pixels or an M×N array of pixels arranged in rows and columns.  
           [0005]    The photodiode  15  of FIG. 1 is shown in part as a cross-sectional view of a semiconductor substrate  10  doped with a p-type material to form p-well  12 . A field oxide region  20 , which serves to surround and isolate the photodiode  15  may be formed by thermal oxidation of the doped substrate  10 , or by chemical vapor deposition of an oxide material as in the STI (shallow trench isolation) process. More highly doped p-type regions  40  are formed under the field oxide region  20  and an n-type implant  30  is formed between the field oxide regions  20 . The field oxide regions  20  may be formed before or after doped regions  30 ,  40 . As shown in FIG. 1, p-type regions  40  and n-type implant  30  are typically doped so as to form a junction of regions  30  and  40  that is aligned or self-aligned to the edge of the field oxide  20 . However, the overlapping of the p-type region  40  and the n-type region  30  results in current leakage from the photodiode to the substrate  10  through the depletion region.  
           [0006]    The resolution of the imaging device is a function of the size and performance of each photodetector. To improve resolution, such as by presenting more image lines per inch of visual display, a greater number of photoconductors are required per unit area of the photodetector array. Imager performance is degraded if individual photodiodes have high charge leakage. It is important that the photodetector array be fabricated to allow the photodiode charge to be accurately read within the allowed data sampling time of the system.  
           [0007]    There is needed, therefore, an improved photodiode for use in an imager apparatus that exhibits decreased charge leakage to the substrate through the depletion region. There is also need for an improved p-n junction diode for other integrated circuit applications, such as DRAM applications, which likewise exhibits reduced charge leakage to the substrate through a depletion region. A method of fabricating diodes, including photodiodes exhibiting these improvements is also needed.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a photodiode having improved leakage characteristics to the substrate and improved dark current characteristics. A photodiode comprises a first conductive region of the photodiode that is spaced away from the edge of a field oxide. The present invention also provides methods for forming the photodiode of the present invention.  
           [0009]    The present invention also relates to a p-n junction diode used in other integrated circuit applications where charge leakage to the substrate through the depletion region may occur, such as where a p-n junction is formed adjacent to field oxide isolation at the memory storage node of a DRAM. In this case too the invention also provides a p-n junction with reduced leakage to substrate and method for fabricating the same by spacing the p-n junction from high leakage areas.  
           [0010]    Additional advantages and features of tie present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a photodiode according to the prior art.  
         [0012]    [0012]FIG. 2 is a cross-sectional view of a semiconductor wafer undergoing the process of a first embodiment of the invention.  
         [0013]    [0013]FIG. 3 illustrates the wafer at a processing step subsequent to that shown in FIG. 2.  
         [0014]    [0014]FIG. 4 is a cross-sectional view of a semiconductor wafer according to the first embodiment of the present invention undergoing a second embodiment of the process to form the photodiode of the first embodiment of the present invention.  
         [0015]    [0015]FIG. 5 illustrates the wafer at a processing step subsequent to that shown in FIG. 4.  
         [0016]    [0016]FIG. 6 illustrates the wafer at a processing step subsequent to that shown in FIG. 5.  
         [0017]    [0017]FIG. 7 is a cross-sectional view of a semiconductor wafer undergoing a first process according to a second embodiment of the invention.  
         [0018]    [0018]FIG. 8 illustrates the wafer at a processing step subsequent to that shown in FIG. 7.  
         [0019]    [0019]FIG. 9 is a cross-sectional view of a semiconductor wafer undergoing a second process according to a second embodiment of the invention.  
         [0020]    [0020]FIG. 10 illustrates a comparison of the current leakage between the photodiodes of the present invention and the photodiodes according to the prior art.  
         [0021]    [0021]FIG. 11 illustrates a processor based system which includes a photodiode based imager device  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]    In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention.  
         [0023]    The terms “wafer” and “substrate” are to be understood as including silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “wafer” or “substrate” in the following description, previous process steps may have been utilized to form regions or junctions in the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, or gallium arsenide.  
         [0024]    The term “pixel” refers to a picture element unit cell containing a photodiode and apparatus for converting electromagnetic radiation to an electrical signal. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.  
         [0025]    The structure of the photodiode  115  of the first embodiment is shown in more detail in FIG. 2. The photodiode  115  may be formed in a substrate  100  doped to a first conductivity type, which for exemplary purposes is treated as a p-well  112  formed in substrate  100 . A field oxide region  120  is formed around the photodiode  115  as is shown in FIG. 2. The field oxide region  120  may be formed by thermal oxidation of the doped substrate  100 , or by chemical vapor deposition of an oxide material according to the STI process. While the invention is described with reference to field oxide regions  120  formed by the Local Oxidation of Silicon (LOCOS) process, it should be understood that the field oxide regions may be formed using the shallow trench isolation (STI) process.  
         [0026]    Still with reference to FIG. 2, a first mask and resist (not shown) is applied to the substrate  100  to form p-type doped regions  140  and n-type region  130  in p-well  112 . The mask and resist are applied to the photodiode  115  to form the n-type region  130  such that n-type region  130  is spaced away from field oxide regions  120  to reduce the electric field at the edges of the defined p-n junction. It should be understood that any suitable doping process may be used, such as ion implantation, or chemical vapor deposition. The n-type region  130  is preferably doped with arsenic, antimony or phosphorous at a dopant concentration level of from about 1×10 11  ions/cm 2  to about 1×10 16  ions/cm 2 . Preferably the dopant is phosphorous which provides a less abrupt junction between the doped region and the substrate, less leakage and little damage to the underlying substrate.  
         [0027]    The p-type regions  140  are shown in the figure as being formed under the field oxide regions  120 . It should be understood that the p-type regions  140  would be formed underneath a STI or LOCOS if these forms of isolation are to be used. It should also be understood that the p-type regions  140  may be formed spaced away from the edge of the field oxide region  120 , as shown, or p-type regions  140  may be formed such that the edge of the p-type region  140  is in contact with the edge of the field oxide region  120 . Any suitable doping process known in the art may be used to form the p-type regions under the field oxide regions  120 , such as, for example ion implantation. The p-type region  140  is preferably doped with boron at a dopant concentration level of from about 1×10 11  ions/cm 2  to about 1×10 14  ions/cm 2 .  
         [0028]    Reference is made to FIG. 3. A second n-type region  110  is formed within the boundaries of first n-type region  130 . A second resist and mask (not shown) are applied to the photodiode and the substrate is doped to form second n-type region  110 . As set forth above, any suitable doping process may be used, such as ion implantation. The second n-type region  110  is doped with arsenic, antimony or phosphorous at a dopant concentration level of from about 1×10 12  ions/cm 2  to about 1×10 16  ions/cm 2 . Preferably the dopant is phosphorous for the reasons set forth above. The resist and mask are removed, leaving a structure that appears as shown in FIG. 3.  
         [0029]    It is desired that the second n-type region  110  is doped to a higher dopant concentration than the first n-type region  130  such that the charge collected in the photodiode  115  is isolated in the second n-type region  110 . By isolating charge collected by the photodiode  115  in the second n-type region  110 , charge leakage to the substrate is reduced. For example, if the first n-type region  130  is doped at a dopant concentration of 1×10 12  ions/cm 2  the second n-type region  110  would be doped at a doping concentration of about 1×10 13  ions/cm 2  or greater. However, it should be understood that in the present invention the specific level of doping concentration is not critical, provided that the second n-type region  110  is doped at a higher concentration than the first n-type region  130 . For example, it should be Understood that a first n-type region  130  that is doped n− and a second n-type region  110  doped n+, where n+ is simply defined to be a concentration greater than n− is within the scope of the present invention.  
         [0030]    For the photodiode  115  of the first embodiment, the processing is essentially complete at this stage, and conventional processing methods may then be used to form contacts and wiring to connect the photodiode  115  to an imager apparatus. For example, the entire surface of the photodiode may then be covered with a passivation layer of, e.g., silicon dioxide or BPSG, which is CMP planarized and etched to provide contact holes, which are then metallized to provide contacts to the photodiode  115 .  
         [0031]    An alternative second embodiment of the process is illustrated by FIGS. 4 through 6. As shown in FIG. 4, the process also begins with a substrate doped to a first conductivity type, which for exemplary purposes is treated as a p-type substrate. A field oxide region  120  is formed around the photodiode  115  as is shown in FIG. 2. The field oxide region  120  may be formed as described above.  
         [0032]    A first mask and resist (not shown) is applied to the substrate  100  and the substrate  100  is doped to form p-type doped regions  140  beneath field oxide regions  120  as illustrated in FIG. 4. As noted above with respect to FIG. 2, any suitable doping process may be used, such as ion implantation. The photodiode region  115  is shown in p-well  112 .  
         [0033]    Reference is now made to FIG. 5. A second mask and resist (not shown) are applied to the photodiode  115  to form the first n-type region  130  such that n-type region  130  is spaced away from the field oxide regions  120  to facilitate reduction of the electric field across the p-n photodiode junction at the photodiode edge. Any suitable doping process may be used to form n-type region  130 , such as ion implantation.  
         [0034]    A second n-type region  110  is formed within the boundaries of the first n-type region  130 . A third resist and mask (not shown) is applied to the photodiode  115  and the substrate is doped to form second n-type region  110 . As set forth above, any suitable doping process may be used, such as ion implantation. Again, as was true of FIG. 3, it is important that the second n-type region  110  is doped to a higher dopant concentration than the first n-type region  130  such that the charge collected in the photodiode  115  is isolated in the second n-type region  110 . The resist and mask are removed, leaving a structure as illustrated in FIG. 6.  
         [0035]    For the photodiode  115  of the first embodiment, the processing is essentially complete at this stage, and conventional processing methods may then be used to form contacts and wiring to connect the photodiode  115  to an imager apparatus as described in more detail above with reference to FIG. 3.  
         [0036]    A third embodiment of the present invention is illustrated by FIGS.  7 - 9 . Reference is now made to FIG. 7. The photodiode  215  may be formed in a substrate  200  doped to a first conductivity type to form p-well  212 . A field oxide region  220  is formed around the photodiode  215  as shown in FIG. 7. The field oxide region  220  may be formed by thermal oxidation of the doped substrate  200 , or by chemical vapor deposition of an oxide material. While the invention is described with reference to field oxide regions  220  formed by the LOCOS process, it should be understood that the field oxide regions may be formed using the STI process.  
         [0037]    Still with reference to FIG. 7, a first mask and resist (not shown) is applied to the substrate  200  and the substrate is doped to form p-type doped regions  240  under field oxide region  220 . It should be understood that the p-type regions  240  would be formed underneath a STI or LOCOS formed isolation. It should also be understood that the p-type regions  240  may be formed spaced away from the edge of the field oxide region  220 , as shown, or p-type regions  240  may be formed such that the edge of the p-type region  240  is in contact with the edge of the field oxide region  220 . Any suitable doping process may be used, such as, for example, ion implantation. The p-type region  240  is preferably formed by doping the substrate  200  with boron at a dopant concentration level of from about 1×10 11  ions/cm 2  to about 1×10 14  ions/cm 2 .  
         [0038]    Reference is now made to FIG. 8. A second mask and resist (not shown) are applied to the photodiode  215  to form the n-type region  230  such that n-type region  230  is spaced away from the field oxide regions  220  to facilitate the reduction of the electric field across the p-n junction of the photodiode at the photodiode edge. Any suitable doping process may be used, such as ion implantation. The n-type region  230  may be doped with arsenic, antimony or phosphorous at a dopant concentration level of from about 1×10 11  ions/cm 2  to about 1×10 16  ions/cm 2 . Preferably the dopant is phosphorous as phosphorous gives less abrupt junctions, less leakage and causes little damage to the underlying substrate.  
         [0039]    For the photodiode  215  of the second embodiment, the processing is essentially complete at this stage, and conventional processing methods may then be used to form contacts and wiring to connect the photodiode  215  to an imager apparatus. For example, the entire surface of the photodiode may then be covered with a passivation layer of, e.g., silicon dioxide or BPSG, which is CMP planarized and etched to provide contact holes, which are then metallized to provide contacts to the photodiode  215 .  
         [0040]    A second process for preparing the photodiode of the third embodiment of the present invention is described with reference to FIG. 9. As shown in FIG. 9, this process also begins with a substrate  200  doped to a first conductivity type, which for exemplary purposes is treated as a p-well  212 . A field oxide region  220  is formed around the photodiode  215 . The field oxide region  220  may be formed as described above.  
         [0041]    Still with reference to FIG. 9, a mask and resist (not shown) is applied to the substrate  200  and the substrate is doped to form p-type doped regions  240  beneath field oxide regions  220  as well as n-type region  230 , provided that n-type region  230  is spaced away from the field oxide regions  220  to facilitate the reduction of the electric field across the p-n photodiode at the photodiode edge. As set forth above with respect to FIGS.  7 - 8 , any suitable doping process may be used to dope regions  230 ,  240 , such as, for example, ion implantation. The resist and mask are then removed, leaving the structure illustrated in FIG. 9.  
         [0042]    For the photodiode  215  of the second embodiment, the processing is essentially complete at this stage, and conventional processing methods may then be used to form contacts and wiring to connect the photodiode  215  to an imager apparatus as described in more detail above.  
         [0043]    The spacing of the n-type region  130 ,  230  away from the field oxide region  120 ,  220  is dependant upon several factors. The spacing of the n-type region  130 ,  230  is dependant upon the dopant concentration and the bias voltage. Preferably the spacing of the n-type region  130 ,  230  from the field oxide region  120 ,  220  should be about the magnitude of one depletion region. It should be understood that the width of the depletion region will vary depending upon the generation of the process design as well as the bias voltage, the dopant concentration, thermal processing and junction depth. For example, in a 0.25 μm process using a 5V bias voltage across the photodiode, the space between the n-type region  130 ,  230  and the field oxide region  120 ,  220  is from about 0.05 μm to about 1.0 μm, preferably from about 0.1 μm to about 0.8 μm, most preferably about 0.2 to about 0.7 μm.  
         [0044]    Reference is now made to FIG. 10. The figure shows the reverse biased leakage from the photodiode versus the voltage applied to the photodiode on a log/log scale. Curve A represents a photodiode where the n-type region was set back from the field oxide region approximately 0.6 μm while curve B represents a photodiode where the n-type region and the field oxide region were self aligned as described in reference to FIG. 1 above. As can be seen from FIG. 10, as the voltage increases above 1V, the reverse biased leakage from the photodiode processed according to the prior art (curve B) increases asymptotically. In contrast, the reverse biased leakage of the photodiode fabricated according to the present invention (curve A) does not increase as rapidly as the prior art photodiode.  
         [0045]    A typical processor based system which includes a photodiode based imager device according to the present invention is illustrated generally at  300  in FIG. 11. A processor based system is exemplary of a system having digital circuits which could include CMOS imager devices, CCD imager devices or the like. Without being limiting, such a system could include a computer system, camera system, scanner, machine vision system, vehicle navigation system, video telephone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system and data compression system for high-definition television, all of which can utilize the present invention.  
         [0046]    A processor system, such as a computer system, for example generally comprises a central processing unit (CPU)  344  that communicates with an input/output (I/O) device  346  over a bus  352 . The photodiode containing imager  342  also communicates with the system over bus  352 . The computer system  300  also includes random access memory (RAM)  348 , and, in the case of a computer system may include peripheral devices such as a floppy disk drive  354  and a compact disk (CD) ROM drive  356  which also communicate with CPU  344  over the bus  352 . Photodiode containing imager  342  is preferably constructed as an integrated circuit which includes pixels containing photodiodes optimized for low leakage, as previously described with respect to FIGS. 2 through 9. The imager  342  may be combined with a processor, such as a CPU, digital signal processor or microprocessor, in a single integrated circuit, or may reside on a separate integrated circuit.  
         [0047]    It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. For example, although the operation of the present invention has been described with respect to a photodiode formed by n-type implants into a p-well, the present invention applies equally well to a photodiode formed by a p-type implant into an n-well or even more complicated photodiodes such a p-n-p diodes, as well as to diodes in general which have p-n junction leakage to a substrate. Similarly, the processes described above are examples of the many processes that could be used. The above description and drawings illustrate preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention which comes within the spirit and scope of the following claims should be considered part of the present invention.