Abstract:
A method and apparatus for reducing cross-talk between pixels in a semiconductor based image sensor. The apparatus includes neighboring pixels separated by a homojunction barrier to reduce cross-talk, or the diffusion of electrons from one pixel to another. The homojunction barrier being deep enough in relation to the other pixel structures to ensure that cross-pixel electron diffusion is minimized.

Description:
REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/666,080, filed on Mar. 28, 2005, which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a semiconductor based image sensor and, more particularly, to a semiconductor pixel structure for detecting electromagnetic radiation. 
     BACKGROUND 
     Semiconductor based sensors and devices for detecting electromagnetic radiation have been implemented in a semiconductor substrate in CMOS or MOS technology. In these sensors, the regions adapted for collecting charge carriers being generated by the radiation in the semiconductor substrate are formed of a p-n or a n-p junction photodiode with a substrate being of a n type conductivity or p type conductivity respectively. Such junctions are called collection junctions. Of the image sensors implemented in a complementary metal-oxide-semiconductor CMOS or MOS technology, image sensors with passive pixels and image sensors with active pixels are distinguished. The difference between these two types of pixel structures is that an active pixel amplifies the charge that is collect on its photosensitive element. A passive pixel does not perform signal amplification and requires a charge sensitive amplifier that is not integrated in the pixel. 
     One prior semiconductor based image sensor is illustrated in  FIG. 1 . In the semiconductor based image sensor of  FIG. 1 , the photodiode is formed by an n-p collection junction with the substrate being of p type conductivity. The photodiode that collects the charge carriers being generated by the radiation is shown on the right and the diode structure associated with the unrelated (to the detection) readout circuitry is shown on the left of the figure. If the diode structure for the non-related readout circuitry is placed in the neighborhood of the collection junction of the detector photodiode, part of the charges that otherwise would have reached the collection junction will be collected by junctions or components of the un-related readout circuitry. The charge carriers generated by light falling on the regions of the detector that are used for readout circuitry, therefore, are mainly collected by the junctions of this readout circuitry. The area taken by the readout circuitry in the pixels, therefore, is lost for collecting the radiation, which is essentially the reason for the low “fill factor” or low sensitivity of active pixel based sensors. 
     One semiconductor based image sensor, as described in U.S. Pat. No. 6,225,670 and illustrated in  FIG. 2 , provides a solution to the above describe problem with the image sensor illustrated in  FIG. 1 . The semiconductor based detector illustrated in  FIG. 2  has a small, but effective, barrier well between the radiation sensitive volume in the semiconductor substrate and the regions and junctions with unrelated readout circuitry, and also has no or a lower barrier between the radiation sensitive volume in the semiconductor and the photodiode collection junction. The collection junction collects all photoelectrons that are generated in the epitaxial layer beneath the surface of the whole pixel. This is possible because the electrons will see a small but sufficient electrostatic barrier towards the active pixel circuitry and towards the substrate. The only direction in which no, or a low, barrier is present is the collection junction. Virtually all electrons will diffuse towards this junction. Such is pixel structure is also called “well-pixel” because in practice the collection junction in such pixel is implemented as a so-called n-well implantation. 
     However, the well pixel structure of  FIG. 2  may have some cross-talk associated with it. For most applications, the ideal pixel can be considered as a square of Silicon, packed in array of nothing but such squares. The sensitive area is the complete square. The sensitivity is high and constant within the square and zero outside the square. That is, light impinging inside the pixel&#39;s boundary should contribute to the pixel&#39;s signal, and light impinging outside the boundary should not—it should contribute to another pixel&#39;s signal. Reality is less ideal. The optical information entering in a neighboring pixel&#39;s signal is called “optical cross-talk.” Optical cross-talk is expressed in % signal lost to the neighbor. One makes sometimes distinction between left/right/up/down neighbors, and even 2 nd , 3 rd  neighbors. Optical cross-talk is typically also wavelength dependent. Short wavelengths typically suffer less from optical cross-talk than longer wavelengths. Optical cross-talk can be directly derived from the “effective pixel shape” (EPS). EPS can be understood as the pixel response as a function of an infinitesimal light spot that travels over the pixel (and beyond) in X and Y direction. The EPS for an ideal pixel is a square. The EPS for an ideal and for a real pixel and corresponding optical cross-talk are illustrated in  FIG. 3 . 
       FIG. 4  illustrates the optical cross-talk in a well pixel. Impinging light generates photo-electrons in the p-type epitaxial layer. These diffuse randomly until they reach the depletion layer of the photodiode&#39;s collection junction. When electrons are optically generated near the border between two pixels, the electrons can diffuse either way (i.e., to either one of the collection junctions of two neighboring pixels) as illustrated in  FIG. 4 . In such image sensors, the border of the pixels becomes “fuzzy.” When translating this to image quality, the “fuzzyness” is the optical cross-talk between pixels. For the image created by an image sensor with such pixels the effect is a blurriness or lack of sharpness to the image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which: 
         FIG. 1  illustrates one prior semiconductor based image sensor. 
         FIG. 2  illustrates another, conventional semiconductor based image sensor. 
         FIG. 3  illustrates the derivation of optical cross-talk from an effective pixel shape. 
         FIG. 4  conceptually illustrates optical cross-talk in a well pixel. 
         FIG. 5  illustrates one embodiment of an image sensor implementing the methods and apparatus described herein. 
         FIG. 6A  is a cross sectional view illustrating one embodiment of pixels having a homojunction barrier to reduce optical cross talk. 
         FIG. 6B  is a cross sectional view illustrating another embodiment of pixels having a homojunction barrier formed around a trench. 
         FIG. 6C  is a cross sectional view illustrating yet another embodiment of pixels having a homojunction barrier to reduce optical cross talk. 
         FIG. 7  illustrates an alternative embodiment of a pixel matrix structure to reduce optical cross-talk. 
         FIG. 8  illustrates another embodiment of pixel structures to reduce optical cross-talk. 
     
    
    
     DETAILED DESCRIPTION 
     A pixel having a structure to reduce cross-talk is described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques are not shown in detail or are shown in block diagram form in order to avoid unnecessarily obscuring an understanding of this description. 
     Reference in the description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. Any of the signals provided over various buses described herein may be time multiplexed with other signals and provided over one or more common buses. Additionally, the interconnection between circuit components or blocks may be shown as buses or as single signal lines. Each of the buses may alternatively be one or more single signal lines, and each of the single signal lines may alternatively be buses. 
       FIG. 5  illustrates one embodiment of an image sensor implementing the methods and apparatus described herein. Image sensor  1000  includes an imaging core  1010  and components associated with the operation of the imaging core. The imaging core  1010  includes a pixel matrix  1020  having an array of pixels (e.g., pixel  300 ) and the corresponding driving and sensing circuitry for the pixel matrix  1020 . The driving and sensing circuitry may include: one or more scanning registers  1035 ,  1030  in the X- and Y-direction in the form of shift registers or addressing registers; buffers/line drivers for the long reset and select lines; column amplifiers  1040  that may also contain fixed pattern noise (FPN) cancellation and double sampling circuitry; and analog multiplexer (mux)  1045  coupled to an output bus  1046 . FPN has the effect that there is non-uniformity in the response of the pixels in the array. Correction of this non-uniformity needs some type of calibration, for example, by multiplying or adding/subtracting the pixel&#39;s signals with a correction amount that is pixel dependent. Circuits and methods to cancel FPN may be referred to as correlated double sampling or offset compensation and are known in the art; accordingly, a detailed description is not provided. 
     The pixel matrix  1020  may be arranged in N rows of pixels by N columns of pixels (with N≧1), with each pixel (e.g., pixel  300 ) is composed of at least a photosensitive element and a readout switch (not shown). A pixel matrix is known in the art; accordingly, a more detailed description is not provided. 
     The Y-addressing scan register(s)  1030  addresses all pixels of a row (e.g., row  1022 ) of the pixel matrix  1020  to be read out, whereby all selected switching elements of pixels of the selected row are closed at the same time. Therefore, each of the selected pixels places a signal on a vertical output line (e.g., line  1023 ), where it is amplified in the column amplifiers  1040 . An X-addressing scan register(s)  1035  provides control signals to the analog multiplexer  1045  to place an output signal (amplified charges) of the column amplifiers  1045  onto output bus  1046 . The output bus  1046  may be coupled to a buffer  1048  that provides a buffered, analog output  1049  from the imaging core  1010 . 
     The output  1049  from the imaging core  1010  is coupled to an analog-to-digital converter (ADC)  1050  to convert the analog imaging core output  1049  into the digital domain. The ADC  1050  is coupled to a digital processing device  1060  to process the digital data received from the ADC  1050  (such processing may be referred to as imaging processing or post-processing). The digital processing device  1060  may include one or more general-purpose processing devices such as a microprocessor or central processing unit, a controller, or the like. Alternatively, digital processing device  1060  may include one or more special-purpose processing devices such as a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. Digital processing device  1060  may also include any combination of a general-purpose processing device and a special-purpose processing device. 
     The digital processing device  1060  is coupled to an interface module  1070  that handles the information input/output (I/O) exchange with components external to the image sensor  1000  and takes care of other tasks such as protocols, handshaking, voltage conversions, etc. The interface module  1070  may be coupled to a sequencer  1080 . The sequencer  1080  may be coupled to one or more components in the image sensor  1000  such as the imaging core  1010 , digital processing device  1060 , and ADC  1050 . The sequencer  1080  may be a digital circuit that receives externally generated clock and control signals from the interface module  1070  and generates internal signals to drive circuitry in the imaging core  1010 , ADC  1050 , etc. In one embodiment, the voltage supplies that generate the control signals used to control the various components in the pixel structure of  FIG. 5  discussed below may be generated by drivers illustrated by control drivers block  1015 . 
     It should be noted that the image sensor illustrated in  FIG. 5  is only an exemplary embodiment and an image sensor may have other configurations than that depicted in  FIG. 5 . For example, alternative embodiments of the image sensor  1000  may include one ADC  1050  for every pixel  300 , for every column (i.e., vertical output line  1023 ), or for a subset block of columns. Similarly, one or more other components within the image sensor  1000  may be duplicated and/or reconfigured for parallel or serial performance. For example, a fewer number of column amplifiers  1040  than pixel matrix columns may be used, with column outputs of the pixel matrix multiplexed into the column amplifiers. Similarly, the layout of the individual components within the image sensor  1000  may be modified to adapt to the number and type of components. In another embodiment, some of the operations performed by the image sensor  1000  may be performed in the digital domain instead of the analog domain, and vice versa. 
       FIG. 6A  is a cross sectional view illustrating one embodiment of pixels having a homojunction barrier to reduce optical cross talk. Two neighboring pixels of the pixel matrix  1020  are illustrated in  FIG. 6A : pixel A  601  and pixel B  602 . Pixel A and Pixel B in the embodiment illustrated in  FIG. 6A  are formed using an n-p junction photodiode with a substrate that is of a p type conductivity substrate  640 . The n regions  611  and  612  are collection junctions for pixels A and B, respectively, for collecting charge carriers being generated by radiation in epitaxial layer  630  and/or substrate  640 . The radiation may be of any type of radiation, for example, all forms of light including infra-red and ultraviolet as well as the optical spectrum, high energy electromagnetic rays such as x-rays and nuclear particles. The n regions  611  and  612  form photodiodes with epitaxial layer  630  in pixels A and B, respectively. The n region  628  is a junction that may be part of readout circuitry for operating on signals being generated by the charge carriers collected by the collection region  611 . The fabrication and configuration of a pixel is known in the art; accordingly, a more detailed discussion is not provided. It should be noted that the pixels may include other regions and structures that are not illustrated so as not to obscure an understanding of embodiments of the present invention. 
     In this embodiment, the border region  610  between the photodiodes of pixel A  601  and pixel B  602 , respectively, in pixel matrix includes a homojunction barrier  620  that inhibits electrons that are optically generated (by light  605 ) in one pixel (e.g., pixel B  602 ) from diffusing to a neighbor pixel (e.g., pixel A  601 ). The homojunction barrier  620  may be composed of a deep, heavily doped (denoted by “+”) p+ region. In one embodiment, the homojunction barrier  620  may be approximately 2 times or more as heavily doped (denoted by “++”) with respect to a region (e.g., epitaxial layer  630 ) designated as “p−”. “Deep” as used herein means protruding deeper in the epitaxial layer  630  than other p regions (e.g., p region  650 ) in the pixel  1020 . In one particular embodiment, the homojunction barrier  620  may be at least approximately 2 times deeper (depth  671 ) than the depth  672  of the shallower p region  625 . 
     In one embodiment, the homojunction barrier  620  may be disposed in a shallow p region  625 . “Shallow” as used herein means protruding less into the epitaxial layer  630  less than the n regions (e.g., region  612 ) in a pixel (e.g., pixel B  602 ). In one embodiment, the shallow p region  625  may be a “p-well” implant (for example similar to that described in regards to  FIG. 4  at the border between two pixels). Such a p-well may contain an n-region  628  that is used in the fabrication of nMOSFETS. Alternatively, the shallow p region  625  may be a p+ implant used, for example, as an nMOSFET source-drain, with the deeper p region being formed as a p-well. It should be noted that in an embodiment where the p+ region of the homojunction barrier  620  is has depth  671  of approximately 2 to 4 times deeper than the depth  672  shallow p region  625 , the formation of the homojunction region may be referred to as a tub. In yet another embodiment illustrated in  FIG. 6C , the homojunction barrier  620  may not be formed in a shallow p-region but, rather, directly formed in the p− epitaxial layer  630 . 
     The difference in doping concentrations between the p− epitaxial layer  630  and the p+ homojunction barrier  620  represents a weak electrostatic barrier and electric field that counteracts the diffusion of electrons from p− towards p+, hence it will inhibit electrons from passing from one pixel (e.g., pixel B  602 ) to another neighboring pixel (e.g., pixel A  601 ). The diffusion of electrons from the area of one pixel to the neighbor pixel is impeded by a p+ region of the homojunction barrier  620  in the p− epitaxial layer  630  disposed between the collection regions  611  and  612 . In an alternative embodiment, an epitaxial layer may not be used and the regions may be disposed directly in another type of charge generation layer, for example, tub regions or substrate. In either configuration, the homojunction barrier  620  may protrude into the substrate. The homojunction barrier  620  may result in a crisper separation of the optical volumes of neighboring pixels by reducing the mixing of signals of neighboring pixels. 
       FIG. 6B  is a cross sectional view illustrating an alternative embodiment of pixels having a homojunction barrier to reduce optical cross talk. In this embodiment, the homojunction barrier  620  is formed around a trench  680 . The formation of a trench is known in the art; accordingly, a detailed description is not provided. 
     Although formation of the homojunction barrier  620  is discussed at times in relation to an implantation operation for ease of explanation, it should be noted that other fabrication techniques may be used to generate the doped region, for example, diffusion and epitaxial growth. Such fabrication techniques are known in the art; accordingly, a detailed discussion is not provided. In addition, the pixels structures have been illustrated and discussed in regards to a using an n-p junction photodiode with a substrate that is of a p type conductivity substrate only for ease of explanation purposes. In an alternative embodiment, the pixels may be formed using a p-n junction photodiode with a substrate that is of a n type conductivity substrate and, correspondingly, an n type homojunction barrier  620 . 
     In alternative embodiments, other structures may be utilized to reduce cross-talk between neighboring pixels, for example, as described below. 
       FIG. 7  illustrates an alternative embodiment of a pixel structure to reduce cross-talk. In this embodiment, reduction of cross-talk may be achieved by a dummy photodiode collection region  710  (e.g., n-implant {that is typically but not necessarily of the same nature as the real photodiode}) between the real photodiode collection regions  720  and  730 . This dummy photodiode may also be additionally covered by a metal light shield  715 . Alternatively, the metal light shield  715  need not be used. Although the structure illustrated in  FIG. 7  may require additional room for the dummy diode plus buffer space, it may provide an effective countermeasure for cross-talk. The photo-charge that attempts to cross the border between tow pixels is collected by the dummy photodiode. 
       FIG. 8  illustrates another embodiment of a pixel structure to reduce cross-talk. In this embodiment, cross-talk may be reduced by embedding a pixel  801  in a deeper tub region  810  than the p-well region  820 . The photosensitive volume is now confined to the p-tub  810 . Each pixel is contained in a separate p-tub. For example, pixel  801  is contained in p-tub  810  and pixel  802  is contained in p-tub  830 . Since electrons cannot diffuse between p-tubs, in the n-type substrate  850 , there may be no resulting cross-talk at all. 
     It should be noted that the semiconductor manufacturing processes of fabricating the various regions and layers described above are known in the art; accordingly, more detailed descriptions are not provided. 
     Embodiments of the present have been illustrated with a photodiode device type and CMOS technology for ease of discussion. In alternative embodiments, other device types (e.g., photogate and phototransistor), device technologies (e.g., charge coupled device (CCD) and buried channel CMOS), and process technologies (e.g., nMOS, buried channel CMOS and BiCMOS) may be used. Furthermore, the image sensors discussed herein may be applicable for use with all types of electromagnetic (EM) radiation (i.e., wavelength ranges) such as, for example, visible, infrared, ultraviolet, gamma, x-ray, microwave, etc. In one particular embodiment, the image sensors and pixel structures discussed herein are used with EM radiation in approximately the 300-1100 nanometer (nm) wavelength range (i.e., visible light to near infrared spectrum). Alternatively, other the image sensors and pixel structures discussed herein may be used with EM radiation in other wavelength ranges. 
     The image sensor and pixel structures discussed herein may be used in various applications including, but not limited to, a digital camera system, for example, for general-purpose photography (e.g., camera phone, still camera, video camera) or special-purpose photography (e.g., in automotive systems, hyperspectral imaging in space borne systems, etc). Alternatively, the image sensor and pixel structures discussed herein may be used in other types of applications, for example, machine and robotic vision, document scanning, microscopy, security, biometry, etc. 
     Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.