Abstract:
The invention, in various exemplary embodiments, incorporates multiple image sensor arrays, with separate respective color filters, on the same imager die. One exemplary embodiment is an image sensor comprising a plurality of arrays of pixel cells at a surface of a substrate, wherein each pixel cell comprises a photo-conversion device. The arrays are configured to commonly capture an image. An image processor circuit is connected to said plurality of arrays and configured to combine the captured images, captured by the plurality of arrays, and output a color image.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of semiconductor devices and more particularly to multi-array image sensor devices. 
     BACKGROUND OF THE INVENTION 
     The semiconductor industry currently produces different types of semiconductor-based image sensors that use micro-lenses, such as charge coupled devices (CCDs), CMOS active pixel sensors (APS), photodiode arrays, charge injection devices and hybrid focal plane arrays, among others. These image sensors use micro-lenses to focus electromagnetic radiation onto photo-conversion devices, e.g., photodiodes. Also, these image sensors can use color filters to pass particular wavelengths of electromagnetic radiation for sensing by the photo-conversion devices, such that the photo-conversion devices typically are associated with a particular color. 
     Micro-lenses help increase optical efficiency and reduce crosstalk between pixel cells of an image sensor.  FIGS. 1A and 1B  show a top view and a simplified cross section of a portion of a conventional color image sensor using a Bayer color filter patterned array  100  (described below). The array  100  includes pixel cells  10 , each being formed on a substrate  1 . Each pixel cell  10  includes a photo-conversion device  12   r ,  12   g ,  12   b , for example, a photodiode, and a charge collecting well  13   r ,  13   g ,  13   b . The illustrated array  100  has micro-lenses  20  that collect and focus light on the photo-conversion devices  12   r ,  12   g ,  12   b , which in turn convert the focused light into electrons that are stored in the respective charge collecting wells  13   r ,  13   g ,  13   b.    
     The array  100  can also include or be covered by a color filter array  30 . The color filter array  30  includes color filters  31   r ,  31   g ,  31   b , each disposed over a pixel cell  10 . Each of the filters  31   r ,  31   g ,  31   b  allows only particular wavelengths of light to pass through to a respective photo-conversion device. Typically, the color filter array is arranged in a repeating Bayer pattern that includes two green color filters  31   g  for every red color filter  31   r  and blue color filter  31   b , arranged as shown in  FIG. 1A . 
     Between the color filter array  30  and the pixel cells  10  is an interlayer dielectric (ILD) region  3 . The ILD region  3  typically includes multiple layers of interlayer dielectrics and conductors that form connections between devices of the pixel cells  10  and from the pixel cells  10  to circuitry  150  peripheral to the array  100 . A dielectric layer  5  is typically provided between the color filter array  30  and microlenses  20 . 
     One major disadvantage of the Bayer pattern color filter, and of other color filter patterns that use alternating RGB filters over a single array, is that crosstalk among the pixels can effectively reduce color reconstruction capabilities. Crosstalk can occur in two ways. Optical crosstalk occurs from several sources, on being when light enters the microlens at a wide angle and is not properly focused on the correct pixel. An example of angular optical crosstalk is shown in  FIG. 1B . Most of the filtered red light  15  reaches the correct photo-conversion device  12   r , but some of the filtered red light  16  intended for red photo-conversion device  12   r  is misdirected to adjacent green and blue pixels. 
     Electrical crosstalk can also occur in the array through a blooming effect. Blooming occurs when the intensity of a light source is so intense that the charge collecting well  13   r ,  13   g  of the pixel cell  10  cannot store any more electrons and provides extra electrons  17  into the substrate and adjacent charge collecting wells. Where a particular color, e.g., red, is particularly intense, this blooming effect can artificially increase the response of adjacent green and blue pixels. 
     It would, therefore, be advantageous to have alternative color filter arrangements for use in an image sensor to provide more accurate color data and which mitigates against optical and electrical crosstalk. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention, in various exemplary embodiments, incorporates multiple image sensor arrays, having separate respective color filters, on the same imager die. One exemplary embodiment is an image sensor comprising a plurality of arrays of pixel cells at a surface of a substrate, wherein each pixel cell comprises a photo-conversion device. Each array is configured to capture the same image by an optical system which provides the same image to each array. An image processor circuit is connected to the plurality of arrays and configured to combine images captured by the respective arrays, and produce an output image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages and features of the invention will become more apparent from the detailed description of exemplary embodiments provided below with reference to the accompanying drawings in which: 
         FIG. 1A  is a top plan view of a portion of a conventional Bayer pattern color image sensor; 
         FIG. 1B  is a cross sectional view of a portion of a conventional color image sensor; 
         FIG. 2  is a top plan view of a 2×2 array image sensor according to an embodiment of the present invention; 
         FIG. 3  is a top plan view of a 3×1 array image sensor according to another embodiment of the present invention; 
         FIG. 4  is a cross sectional view of a portion of the array according to an embodiment of the present invention; 
         FIG. 5A  is a graph showing the relationship between parallax shift and the center-to-center distance between arrays according to an embodiment of the present invention; 
         FIG. 5B  is a graph showing the relationship between parallax shift and the distance of an object from the lenses of the array according to an embodiment of the present invention; 
         FIG. 6  is a cross sectional view of a portion of the array according to another embodiment of the present invention; 
         FIG. 7  is a cross sectional view of a portion of the array according to another embodiment of the present invention; 
         FIG. 8A  is a top plan view of an imager employing a pixel array according to an embodiment of the present invention; 
         FIG. 8B  is a top plan view of an imager employing a pixel array according to another embodiment of the present invention; and 
         FIG. 9  is a top plan view of an image system employing an imager pixel array according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and illustrate specific embodiments in which the invention may be practiced. In the drawings, like reference numerals describe substantially similar components throughout the several views. 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. 
     The terms “wafer” and “substrate” are to be understood as including silicon, 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. 
     The term “pixel” or “pixel cell” refers to a picture element unit cell containing a photo-conversion device for converting electromagnetic radiation to an electrical signal. Typically, the fabrication of all pixel cells in an image sensor will proceed concurrently in a similar fashion. 
       FIG. 2  is a top plan view of a multi-array image sensor  200  according to an embodiment of the present invention. In the illustrated embodiment, image sensor  200  is a 2×2 array sensor. Image sensor  200  includes a substrate  201  on which four pixel arrays  213   r ,  213   b ,  213   g ,  213   g ′ and associated support circuitry  250  for each array  213   r ,  213   b ,  213   g ,  213   g ′ are fabricated. The illustrated embodiment contains two green pixel arrays  213   g ,  213   g ′, one red pixel array  213   r ′ and one blue pixel array  213   b . Opaque walls  260  separate the individual arrays. Four imaging filters  231   r ,  231   b ,  231   g ,  231   g ′ are respectively arranged above the pixel arrays  213   r ,  213   b ,  213   g ,  213   g ′ in a Bayer pattern. Red filter  231   r  is arranged above red pixel array  213   r , blue filter  231   b  is arranged above blue pixel array  213   b  and green filters  231   g ,  231   g ′ are arranged above respective green pixel arrays  213   g ,  213   g ′. Four imaging lenses  220  are arranged above the imaging filters  231   r ,  231   b ,  231   g ,  231   g′.    
     Including a multi-array color image sensor on a single die allows the reduction of color crosstalk artifacts, especially for compact camera modules with pixel sizes less than 6 microns by 6 microns. Multiple imaging arrays  213   r ,  213   b ,  213   g ,  213   g ′ arranged on a single die and containing separate color filters  231   r ,  231   b ,  231   g ,  231   g ′ achieves superior color performance while reducing the focal length of the imaging lens system. This arrangement can significantly reduce color crosstalk; moreover, an imaging lens with a shorter focal length can minimize parallax effects and allow a camera module employing image sensor  200  to be more compact. 
     The multi-array layout has several advantages over conventional systems. In addition to providing a lens with a shorter focal length, the color filters can be more easily embedded in the imaging lens itself rather than requiring individual color filters for each pixel, providing better flexibility to tune and optimize the color filters for better color image reconstruction and maximum photon throughput. As discussed above, color crosstalk can be significantly reduced because the different color filter arrays are completely isolated. 
     In addition, color aliasing will be reduced and demosaicing will be unnecessary, as compared to an array using a Bayer pattern filter, because of the single pixel accuracy of each color filtered array. Using separate color arrays also allows more flexibility in areas such as exposure time control and separate pixel parameter optimization such as, e.g., spectral response, conversion gain, etc. 
     Shorter focal length lenses may be used because the image area per lens can be 25% of the area of the a typical Bayer pattern sensor array. This shorter focal length translates to an extended depth of field of the imaging module and may alleviate the need for an auto-focus mechanism. 
     In the above embodiments, once each array has captured an image, the image is assigned a color according to the color of the filter above the array. These images can now be combined to form a single RGB color output in which each pixel of the output uses pixel values from a corresponding pixel from each of the arrays. 
     The RGB color output may exhibit some degree of parallax error in at least one linear direction because the arrays are attempting to capture the same image from different positions on the same plane. In general, the shorter focal length eliminates a large portion of the parallax inherent to using separate color arrays on the same die. For situations where parallax error is large enough to interfere with the proper reconstruction of an image, e.g., where an object to be captured is very close to the imager lens, post processing of the captured image (discussed in detail below) may be performed. 
       FIG. 3  is a top plan view of another multi-array image sensor  300  according to another embodiment of the present invention. In the illustrated embodiment, image sensor  300  is a 3×1 array sensor. Image sensor  300  includes a substrate  301  which houses three pixel arrays  313   r ,  313   b ,  313   g  and associated support circuitry  350  for each array. This embodiment contains one green pixel array  313   g , one red pixel array  313   r  and one blue pixel array  313   b . Opaque walls  360  separate the individual arrays. Three imaging filters  331   r ,  331   b ,  331   g , are arranged above each pixel array  313   r ,  313   b ,  313   g . Red filter  331   r  is arranged above pixel array  313   r , blue filter  331   b  is arranged above pixel array  313   b  and green filter  331   g  is arranged above pixel array  313   g . Three imaging lenses  320  are arranged above the respective imaging filters  331   r ,  331   b ,  331   g.    
     As set forth above, once each array has captured an image, the image is assigned a color according to the color of the filter above the array. These images can now be combined to form a single RGB color output. 
     A 3×1 array image sensor has the additional advantage of confining any parallax errors to one linear direction. By arranging the arrays such that parallax only occurs in one linear direction, the amount and type of post-processing necessary to reconstruct the image from the captured image data may be reduced. 
       FIG. 4  is a cross sectional view of a portion of an image sensor  400  according to any embodiment of the present invention (e.g., according to the 2×2 or 3×1 pixel arrays shown in  FIGS. 2 and 3 ). Sensor  400  includes a substrate  401  on which a plurality of pixel arrays  413   r ,  413   g  and associated support circuitry (not shown) for each array are fabricated. The illustrated portion shows one green pixel array  413   g  and one red pixel array  413   r . It should be appreciated that there is at least one blue array and possibly another green array that are not shown in the cross sectional view. Opaque walls  460  separate the individual arrays and imaging lenses  431   r ,  431   g  that are arranged above each respective pixel array  413   r ,  413   g.    
     As an object to be imaged  470  moves closer to the imaging lenses  420 , the individual arrays  413   r ,  413   g  will exhibit an increase in parallax shift between them. The magnitude of the parallax shift between two arrays is approximated by the following formula: 
                     n   ·   w     =     d   =       F   ·   D     O               (   1   )               
where d is the parallax shift distance on the image pixel array, F is the focal length of the lens, D is the center-to-center distance between the two arrays, and O is the distance between the object to be imaged  470  and the imaging lens  420 . Further, w is the center-to-center distance between two pixels and n is the number of pixels to be shifted based on the parallax calculation.
 
     According to formula 1, a decrease in focal length F and/or a decrease in center-to-center distance between the arrays D will result in a decrease in overall parallax shift for a given object distance O. The object distance O can be manually set, or can be detected by a conventional distance detection device, e.g. an infrared sensor or autofocus mechanism (not shown). Once the object distance O has been measured or approximated, the parallax shift calculation can be performed. 
     In the example shown in  FIG. 4 , a portion of object  470  is captured by pixel  475  on sensor array  413   g , but by pixel  476  on sensor array  413   r  instead of corresponding pixel  475 . By selecting one array, e.g.,  413   g , as a reference array, the parallax shift d is calculated from the object distance O, focal length F and center-to-center distance D. The required number of pixels n to shift the output image of  413   r  can then be calculated by dividing the parallax shift d by the center-to-center between pixels w. In this example, when providing an RGB output, the output of pixel  475  for the green array will be used with the output of pixel  476  of the red array. Of course, the RGB output will also include a corresponding pixel from a second green array and the blue array as well, shifted as necessary to correct for parallax shift. 
       FIG. 5A , for example, is a graph showing the relationship between parallax shift d and the center-to-center distance D between arrays based on a pixel size of 2 microns and an object distance O of 100 mm.  FIG. 5B  is another graph, which shows the relationship between parallax shift d and the distance of an object O from the lenses of the array, based on a pixel size of 2 microns and a center-to-center distance D of 3 mm. 
     As object distance O increases, parallax shift d becomes negligible. For example, for the design example shown in  FIG. 5B , with a pixel size of 2 microns and a center-to-center distance D of 3 mm, parallax shift d becomes negligible at object distances O greater than 1 m. 
     Where an image contains objects having different object distances O, especially when these distances O are both small and large, the image may be corrected for one object or the other, depending on user preferences and processing settings. Again, as focal length F and center-to-center distance D decrease, the parallax shift d will be less pronounced even for objects having small object distances O, allowing for fewer tradeoffs when capturing images having both near and far objects. 
       FIG. 6  is a cross sectional view of a portion of the array according to another embodiment of the present invention (e.g., according to the 2×2 or 3×1 pixel arrays shown in  FIGS. 2 and 3 ). Sensor  600  includes a substrate  601  on which a plurality of pixel arrays  613   r ,  613   g ,  613   b  and associated support circuitry (not shown) for each array are fabricated. The illustrated portion shows one red pixel array  613   r , one green pixel array  613   g  and one blue pixel array  613   b , each having a respective color filter  631   r ,  631   g ,  631   b . For simplicity, the embodiment shown in  FIG. 6  shows pixel arrays  613   r ,  613   g ,  613   b  that are eight pixels wide, but it should be appreciated that the pixel arrays  613   r ,  613   g ,  613   b  may contain as many or as few pixels as desired. It should also be appreciated that there may be additional arrays that are not shown in the cross sectional view. Opaque walls  660  separate the individual arrays and arrays of imaging lenses  620  that are arranged above each respective pixel array  613   r ,  613   g ,  613   b.    
     In this embodiment, rather than fabricating a single lens over each array  613   r ,  613   g ,  613   b  (as in the embodiments shown in  FIGS. 2-4 ), respective arrays of microlenses  620  are fabricated over one or more pixels in each array  613   r ,  613   g ,  613   b . The individual lenses may cover and focus light on any number of pixels; in the specific embodiment shown in  FIG. 6A , each mircolens in each array  620  covers and focuses light on a four pixel section (in a 2×2 pattern) of the pixels arrays  613   r ,  613   g ,  613   b.    
       FIG. 7  is a cross sectional view of a portion of the array according to another embodiment of the present invention (e.g., according to the 2×2 or 3×1 pixel arrays shown in  FIGS. 2 and 3 ). Sensor  700  includes a substrate  701  on which a plurality of pixel arrays  713   r ,  713   g ,  713   b  and associated support circuitry (not shown) for each array are fabricated. The illustrated portion shows one red pixel array  713   r , one green pixel array  713   g  and one blue pixel array  713   b , each having a respective color filter  731   r ,  731   g ,  731   b . For simplicity, the embodiment shown in  FIG. 7  shows pixel arrays  713   r ,  713   g ,  713   b  that are eight pixels wide, but it should be appreciated that the pixel arrays  713   r ,  713   g ,  713   b  may contain as many or as few pixels as desired. It should also be appreciated that there may be additional arrays that are not shown in the cross sectional view. Opaque walls  760  separate the individual arrays and arrays of imaging lenses  720  that are arranged above each respective pixel array  713   r ,  713   g ,  713   b . The embodiment shown in  FIG. 7  additionally contains lens elements  780   r ,  781   r ,  780   g ,  781   g ,  780   b ,  781   b.    
     Lens elements  780   r ,  781   r ,  780   g ,  781   g ,  780   b ,  781   b  are optimized to produce the best focal spot resolution and aberration free performance for the wavelength range of each color filter  731   r ,  731   g ,  731   b . For example, since pixel array  713   r  is associated with a single red color filter  731   r , the set of lenses  780   r ,  781   r  can be optimized for the red wavelength range rather than the entire range of visible light. Individual color arrays can also be optimized for varying the dopant implants and epixatial (EPI) layer thickness among different arrays  713   r ,  713   g ,  713   b . For example, by fabricating the EPI layer (not shown) within blue pixel array  713   b  to have a thickness small enough to only respond to blue light wavelengths, blue color filter  731   b  may be omitted. 
     A more detailed single chip CMOS image sensor  800  is illustrated by the block diagram of  FIG. 8A . The image sensor  800  includes a pixel cell array  801  according to an embodiment of the invention. The array  801  comprises a red array  813   r , a green array  813   g  and a blue array  813   b , similar to the embodiment shown in  FIG. 3 . The array  801  can also comprise one or more arrays of the embodiment shown in  FIG. 2 , or any other similar arrangement using multiple color arrays. 
     The rows of pixel cells in array  801  are read out one by one. Accordingly, pixel cells in a row of array  801  are all selected for readout at the same time by a row select line, and each pixel cell in a selected row provides a signal representative of received light to a readout line for its column. In the array  801 , each column also has a select line, and the pixel cells of each column are selectively read out in response to the column select lines. 
     The row lines in the array  801  are selectively activated by a row driver  882  in response to row address decoder  881 . The column select lines are selectively activated by a column driver  884  in response to column address decoder  885 . The array  801  is operated by the timing and control circuit  883 , which controls address decoders  881 ,  885  for selecting the appropriate row and column lines for pixel signal readout. 
     The signals on the column readout lines typically include a pixel reset signal (V rst ) and a pixel image signal (V sig ) for each pixel cell. Both signals are read into a sample and hold circuit (S/H)  886  in response to the column driver  884 . A differential signal (V rst −V sig ) is produced by differential amplifier (AMP)  887  for each pixel cell, and each pixel cell&#39;s differential signal is amplified and digitized by analog-to-digital converter (ADC)  888 . The analog-to-digital converter  888  supplies the digitized pixel signals to an image processor  889 , which performs appropriate image processing, which can include combining the outputs of multiple arrays and performing the parallax adjustment calculation described above, before providing digital signals defining an image output. 
     Another embodiment of a single chip CMOS image sensor  800 ′ is illustrated by the block diagram of  FIG. 8B . The image sensor  800 ′ includes the same elements as image sensor  800  shown in  FIG. 8A , and additionally includes individual row address decoders  881   r ,  881   g ,  881   b  and row drivers  882   r ,  882   g ,  882   b  for each array  813   r ,  813   g ,  813   b , thereby allowing for individual control of red, green, and blue exposure times, and also allowing for white balancing responsive to variations in exposure times. 
     The row lines in the arrays  813   r ,  813   g ,  813   b  are each selectively activated by a respective row driver  882   r ,  882   g ,  882   b  in response to row address decoder  881   r ,  881   g ,  881   b . The column select lines are selectively activated by a column driver  884  in response to column address decoder  885 . The array  801  is operated by the timing and control circuit  883 , which controls address decoders  881   r ,  881   g ,  881   b ,  885  for selecting the appropriate row and column lines for pixel signal readout. 
       FIG. 9  illustrates a processor system  900  including the image sensor  800  of  FIG. 8A . The processor system  900  is exemplary of a system having digital circuits that could include image sensor devices. Without being limiting, such a system could include a computer system, camera system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, and data compression system. 
     The processor system  900 , for example a camera system, generally comprises a central processing unit (CPU)  995 , such as a microprocessor, that communicates with an input/output (I/O) device  991  over a bus  993 . Image sensor  800  also communicates with the CPU  995  over bus  993 . The processor-based system  900  also includes random access memory (RAM)  992 , and can include removable memory  994 , such as flash memory, which also communicate with CPU  995  over the bus  993 . Image sensor  800  may be combined with a processor, such as a CPU, digital signal processor, or microprocessor, with or without memory storage on a single integrated circuit or on a different chip than the processor. The parallax adjustment calculation may be performed by the image sensor  800 , or by the CPU  995 . 
     It is again noted that the above description and drawings are exemplary and illustrate preferred embodiments that 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, including the use of other imager technologies such as CCD arrays, should be considered part of the present invention.