Abstract:
An image sensor and method is provided to improve the measurement of a dark signal reference while substantially suppressing radiation charges that enter an active area of the image sensor from reaching a shielded dark signal detector. In one implementation, dark signal detector is shielded and separated from the active area to substantially reduce the radiation charges that reach the dark signal detector. In another implementation, the image sensor includes a radiation guard that is disposed between the active area and the shielded detector. When radiation or light is permitted to enter the active area, the guard when adequately biased attracts and collects radiated charges that may otherwise travel beyond the active area to reach the shielded detector and contaminate a measurement for the dark signal reference.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    This invention relates generally to electronic imaging devices, and in particular, to measuring dark signals in electronic imaging devices.  
           [0003]    2. Related Art  
           [0004]    Conventional solid-state image sensor, such as a CMOS image sensor, typically has an array of pixels arranged in an active area. Each pixel in the active area has a radiation detector, such as a photodiode, to sense radiation intensity. During an integration period the radiation intensity is sampled and a charge is read out of the radiation detector as an electrical signal by an associated image processor. CMOS image sensors typically have a dark signal level or voltage offset that occurs primarily from low levels of junction leakage. The exact extent of junction leakage can vary with slight changes in manufacturing conditions causing differences in the expected output signal under conditions of zero illumination (darkness). Changes in operating condition such as integration time or temperature may also cause the value of the dark signal level to vary in the CMOS image sensor.  
           [0005]    Knowledge of the dark signal level, sometimes referred to as “black reference level”, is useful in the reconstruction of images captured by the CMOS image sensors. Conventional image sensors often measure current leakage or the dark signal level during an integration period in order to provide a black reference level. Selected pixels in the solid-state image sensor are covered with an opaque material during fabrication to prevent radiation (i.e. light) from directly striking the photo detector. The application of a simple light shield approach during fabrication suffices to establish a dark reference level during an integration period under some conditions.  
           [0006]    A problem exists with utilizing the simple light shield approach when a bright source of radiation, such as a light bulb, sun or long wavelength radiation, such as red light having a wavelength of 600 to 680 nanometers, illuminates the edge of the active area in the solid-state imager sensor. Radiation from bright sources near the edge of the active area is able to penetrate laterally and relatively deep into the semiconductor substrate of the solid-state image sensor creating charge carriers (i.e. electron-hole pairs). While the electron holes may diffuse to a substrate isolation terminal (i.e. ground), the minority carriers (electrons) often diffuse to neighboring covered or otherwise shielded pixels. The diffusion of the minority carriers results in an increase in the measured dark reference level. This increase in dark reference level (also referred to as cross-talk) results in a false or inaccurate black reference level, which in turn, adversely affects the detected image quality.  
           [0007]    A conventional approach to reducing the cross-talk problem in a solid-state image sensor requires dummy pixels between active area pixels and shielded pixels. The dummy pixels act to isolate the shielded pixels that measure the dark reference level from the diffusion of the minority carriers. A single column of dummy or isolation pixels, however, is generally insufficient to prevent cross-talk from occurring. Multiple columns and rows of isolation pixels are needed to correct for gaps or insufficient depth of the photo-detector in each isolation pixel and may require a relatively significant amount of area in the solid-state imager resulting in larger die sizes. Furthermore, the type and amount of light shield material utilized to cover the dark signal detectors (i.e. colored photoresist material) also increase the cost and complexity of fabrication. Thus, there is a need in the art for measurement of the dark signal reference without significantly increasing the cost, die size and complexity of a solid-state image sensor.  
         SUMMARY  
         [0008]    A number of technical advances are achieved in the art by implementing an approach for suppressing radiation charges from reaching a dark signal sensor. Broadly conceptualized, suppressing radiation charges from reaching the dark signal sensor allows a dark signal to be measured simultaneously with the integration and read out of a radiation detector, to produce and improve black reference level for post processing of an image.  
           [0009]    An example implementation of the radiation sensor utilizes a shielded dark signal detector that is sufficiently separated from the radiation detector to substantially suppress radiation charges from reaching the dark signal detector through the active area of the radiation sensor. In addition, the radiation sensor includes a radiation guard ring that is reversed biased in the same manner as the radiation detector and the dark signal detector so as to interdict radiation charges in the active area from reaching the dark signal detector.  
           [0010]    During the integration period when the radiation detectors in the active area are permitted to sense radiation, the ring is preferably reversed biased with respect to the substrate. During that time a depletion region between the guard ring and substrate develops. The depletion region sufficiently suppresses the free electrons from reaching the shielded detectors that enter and then travel laterally through the active area.  
           [0011]    Other systems, methods features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0012]    The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views.  
         [0013]    [0013]FIG. 1 is a top-level illustrating a solid-state image sensor having an exemplary shielded dark signal detector in accordance with an embodiment of the invention.  
         [0014]    [0014]FIG. 2 is a cross sectional view illustrating solid-state image sensor of FIG. 1 with separation between the exemplary shielded dark signal detector and the active area to suppress cross-talk.  
         [0015]    [0015]FIG. 3 is a top-level view illustrating a solid-state image sensor having an exemplary guard ring, in accordance with the invention.  
         [0016]    [0016]FIG. 4 illustrates a cross sectional view of solid-state imaging sensor of FIG. 3 having the exemplary guard ring.  
         [0017]    [0017]FIG. 5 is a flow diagram illustrating an example process for measuring a dark signal while suppressing radiation charges from reaching a dark signal detector.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    In FIG. 1, a top-level view of an exemplar implementation of a solid-state image or radiation sensor  100  having an exemplary shielded dark signal detector  114  and  115  is illustrated. The solid-state image sensor  100  includes an active area  102  having a plurality of cells (only two cells  104  and  106  of a plurality of cells are show in FIG. 1). Each of the cells  104  and  106  has a respective radiation detector  108  and  110  located on the periphery of the active area defined by an outer edge  112 . The radiation detector  108  and  110  receives charges and converts the charges into a corresponding electrical signal for utilization by an image processor.  
         [0019]    The image sensor  100  includes dark signal detectors  114  and  115 . The dark signal detectors  114  and  115  are disposed beneath a shield  116  to prevent radiation charges from directly reaching the dark signal detectors  114  and  115 . The interior edge  118  of the shield  116  extends along an outer edge  112  of the active area  102 . The shield  116  preferably surrounds the perimeter of active area  102  and is fabricated from metal (commonly referred to as top-metal) that is also utilized for signal routing in a CMOS embodiment. In other embodiments, the shield  116  is fabricated from an oblique material, such as glass, ceramic, plastic, or epoxy that block radiation charges from reaching the dark signal detectors  114  and  115 . In the current implementation, the dark signal detectors  114  and  115  are spaced apart from radiation detector  104  in order to prevent radiation charges that are not absorbed by the radiation detector  104  from reaching the dark signal detectors  114  and  115 . Thus, an advantage over the known approaches is achieved by eliminating the operation of dummy columns or rows interposed between the dark signal detectors and the lighted array as power consumption may be reduced.  
         [0020]    Each dark signal detector  114  and  115  in a row of dark signal detectors is read simultaneously via column circuit elements  122 . The dark signal detectors  114  and  115  are arranged in columns outside of the active area  102 . The dark signal detectors  114  and  115  are separately selected and read via an associated column circuit element  122 . In an alternate implementation, the column circuit elements  122  are expanded to handle additional column read-out capability, and thus reduce the fill factor of the radiation sensor  100 . Column-circuit elements  122  are utilized to read a selected portion or row of radiation detectors  108  and  110  in active area  102 . A detailed description of the column circuit elements  122 , however, is not necessary to understand the present invention.  
         [0021]    In FIG. 2, a cross sectional view of solid-state image sensor  100  of FIG. 1 with separation between the exemplary shielded dark signal detector  114  and the active area  102  are illustrated. The active area  102  with column circuit element  206  and the dark signal detector  114  with column circuit element  208  are formed within substrate  202  of solid-state image sensor  100 . The dark signal detector  114  has a sufficient separation  120  from the active area  102  to reduce the interactions (i.e., radiation charges reaching the dark signal detector  114 ) that occur as compared with little or no separation. The separation between the dark signal detector  114  and the active area  102  is preferably at least 50 angstroms and the dark signal detector  114  is located approximately 200 angstroms from the active area  102  to limit cross-talk interactions to 5-15% of the cross-talk interaction that occur with little or no separation. Where column circuit element  204  (or other circuitry of imaging sensor  100 ) is located between the shield  116  and the radiation detector  108 , the column circuit element  204  acts as an extension  210  of the shield  116 . Further, the shield  116  prevents direct radiation from entering the active area  102 . In this instance, the outer edge of the active area  102  is where the edge of the radiation detector  108  meets the column circuit element  204  and the column circuit element  204  represents the extension  210  of the shield  116 .  
         [0022]    Radiation penetrates into the substrate  202  of the solid-state image sensor  100  creating electron hole pairs (only minority carrier shown) even when the radiation detector  108  is biased to absorb and store the radiation charge carriers. The minority carrier  212  received by the image sensor  100  may diffuse back to be collected by the radiation detector  108 . Other minority carriers, such as  214 ,  216  and  218  that are generated deeper in the substrate  202  of the solid-state image sensor  100  are able to diffuse to neighboring detector  110  and dark signal detector  114 .  
         [0023]    Interactions with the dark signal detector  114  are limited, because of the separation  120  of the dark signal detector  114  from the radiation detector  108 . The minority carriers  214  and  216  are prevented from reaching dark signal detector  114  to contaminate the black reference level measurement of the dark signal. The separation of the dark signal detectors from the active area  102  greatly improves the quality of the dark reference level measurement under most imaging conditions. However, if extremely bright light radiation sources are near the edge of the lighted array a significant number of photoelectrons may still diffuse to the reference row. Operation of the image sensor with a precise dark reference under these extreme conditions requires that the distance  120  be increased to about 200 microns. This distance can be reduced substantially by the use of a radiation guard ring as discussed in FIGS. 3 and 4.  
         [0024]    In FIG. 3, a solid-state image sensor  300  having a radiation guard ring  324  is shown. The radiation guard ring  324  surrounds the active area  302  of the solid-state image sensor  300 . The image sensor  300  also includes a dark signal detector  314  and  315 . A portion of the guard ring  306  traverses between cells  302  and  306  of the active area  302 , and the dark signal detector  314 . As depicted, dark signal detectors  314  and  315  are disposed beneath a shield  316  to prevent direct radiation from affecting the measurement of the dark signal level by the dark signal detectors  314  and  315 . In addition, the radiation guard ring  324  prevents the majority of radiation charges from reaching the dark signal detector  314 . Each cell  304  and  306  has a respective radiation detector  308  and  310  for receiving a charge and converting the charge to a corresponding electrical signal for an image processor (not shown). The radiation detectors  308  and  310  are located on the periphery of the outer edge  312  of active area  302 . The interior edge  318  of the shield  316  extends along an outer edge  312  of the active area  302 . The dark signal detector  314  and  315  is disposed beneath the shield  316  and behind the radiation guard ring  324 . The shield  316  and radiation guard ring  324  reduces the number of radiation charges moving laterally towards the dark signal detector  314  and  315  from reaching the dark signal detector  314  and  315 , enabling an accurate black reference level measurement.  
         [0025]    In FIG. 4, a cross sectional view of the solid-state image sensor  300  of FIG. 3 having the exemplary guard ring  324  is illustrated. A signal detector  310  with column circuit element  406 , signal detector  308  with column circuit element  404 , dark signal detector  314  with column circuit element  408  and guard ring  324  are formed in substrate  402 . The dark signal detector  314  and the radiation guard ring  324  have a first conductivity type (e.g., n-type doping) while the substrate has second conductivity type (e.g., p-type doping). The radiation guard ring  324  preferably has a depth approximately equal to or greater than the depth of the dark signal detector  314 . In order to further attract radiation charges traveling near the outer edge  312  of the active area  302 , the radiation guard ring  324  preferably has a doping level that is approximately equal to or greater than the doping level of the dark signal detector  314 . The radiation guard ring has a preferred depth in the substrate equal to or greater than 1.5 microns. In alternate embodiments, the depth of the radiation guard ring  324  may be as shallow as 0.5 microns.  
         [0026]    The radiation guard ring  324  attracts minority carriers  414 ,  416  and  418  when reversed biased. The attraction of the minority carriers  414 ,  416  and  418  to the radiation guard ring  324 , preventing them from reaching the dark signal detector  314 . The separation distance between the active area  302  and the dark signal detector  314  can be substantially reduced to less than 50 microns when a guard ring of about 10 microns width is present. This enables effective measurement of dark reference levels under stringent operational conditions with only a minimal increase in the total die area needed for the image sensor.  
         [0027]    Because the dark reference level measured by the radiation detector  308  and the dark signal detector  314  are principally attributable to similar defects occurring in the formation of the detectors in the substrate  402 , an accurate measurement of the dark signal by the dark signal detector  314  should occur with the radiation detector  308  and the dark signal detector  314  being substantially similar. Thus, the same defects or artifacts that are present in one detector as a result of fabrication are likely to be present in the other detector such that the dark signal measurement obtained by reading the dark signal detector  314  is sufficiently identical to the dark signal component from the radiation detector  308 .  
         [0028]    In FIG. 5, an example process  500  is performed by the solid-state image sensor  300  to measure the dark reference level associated with the radiation detectors while suppressing radiation charges from directly or indirectly reaching the dark signal detector is illustrated. Initially, the radiation detector  308  or a portion of radiation detectors to be read (e.g., a row of radiation detectors) is reset to a known potential (e.g., zero volts) ( 510 ). The dark signal detector may be reset at the same time as the radiation detector  308  in the active area  302 . A biasing potential is applied ( 520 ) to the radiation ring  324  to form a depletion region  420  at a junction between radiation guard ring  324  and the substrate  402 . Radiation charges, that would otherwise reach the dark signal detector  314 , collect in the depletion region  420  formed with the radiation guard ring  324 .  
         [0029]    Next, the integration period commences as the portion of radiation detectors to be read is enabled to receive radiation ( 530 ). For example, when the dark signal detector  314  is formed in the same row as the radiation detector  308 , the detectors  308 ,  314  can share a common reset and enable. Radiation penetrating into the active area  302  (e.g., radiation that penetrates through the radiation detector  308 ) form minority carriers is attracted ( 540 ) to the radiation guard ring  324  before they reach the dark signal detector  314 . Following a predetermined integration period ( 550 ) for sensing radiation, the radiation detectors  308  and the dark signal detector  314  may simultaneously be read out ( 560 ). This method advantageously permits the dark signal detector to be read at the same temporal condition (e.g., same substrate temperature) as the radiation detector  308  to further facilitate an accurate dark reference level measurement for post processing of an image signal.  
         [0030]    While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention.