Patent Publication Number: US-7708686-B2

Title: Color filter imaging array and method of formation

Description:
This application is a continuation of application Ser. No. 10/142,961, filed on May 13, 2002, now U.S. Pat. No. 6,783,900, issued Aug. 31, 2004, the disclosure of which is incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The present invention relates to color filters for use in a solid-state image sensor and, in particular, to a color filter array with a pattern that samples red color most frequently relative to blue and green colors, and method of formation. 
   BACKGROUND OF THE INVENTION 
   Solid-state image sensors, also known as imagers, were developed in the late 1960s and early 1970s primarily for television image acquisition, transmission, and display. An imager 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. There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices (CIDs), hybrid focal plan arrays, and CMOS imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems and data compression systems for high-definition television, among other uses. 
   These imagers typically consist of an array of pixel cells containing photosensors, where each pixel produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be stored, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image. The photosensors are typically phototransistors, photoconductors or photodiodes. The magnitude of the signal produced by each pixel, therefore, is proportional to the amount of light impinging on the photosensor. 
   To allow the photosensors to capture a color image, the photosensors must be able to separately detect red (R) photons, green (G) photons and blue (B) photons. Accordingly, each pixel must be sensitive only to one color or spectral band. For this, a color filter array (CFA) is typically placed in front of the pixels so that each pixel measures the light of the color of its associated filter. Thus, each pixel of a color image sensor is covered with either a red, green or blue filter, according to a specific pattern. 
     FIG. 1  illustrates one such color filter array pattern, known as the “Bayer” pattern, which is described in more detail in U.S. Pat. No. 3,971,065 (the disclosure of which is incorporated by reference herein). In the Bayer pattern, red, green and blue pixels are arranged so that alternating pixels of red and green are on a first row of an image, and alternating pixels of blue and green are on a next row. Thus, when the image sensor is read out, line by line, the pixel sequence for the first line reads GRGRGR etc., and then the alternate line sequence reads BGBGBG etc. This output is called sequential RGB or sRGB. 
   In the Bayer pattern, sampling rates for all three basic color vectors are adjusted according to the acuity of the human visual system. That is, green color, to which the human eye is most sensitive and responsive, is sampled most frequently, whereas blue color, for which the human vision has least resolution, is sampled the least frequently. This is why in the Bayer pattern, the green-sensitive elements, which serve to detect luminance (the color vector which provides the luminance information) occur at every other array position, while the red-sensitive elements alternate with the blue-sensitive elements. 
   As a result of these attributes, the Bayer pattern has vast applications in imaging objects having a more or less uniform representations of colors across the entire visible spectrum. Thus, sampling the green color at twice the frequency of the other primary colors provides a good representation of the luminance component of a particular object being imaged. Nevertheless, if the object being imaged has a relatively low spectral reflectivity in the green part of the wavelength, the image captured with an imager employing a Bayer color filter pattern can be suboptimal. 
   There is needed, therefore, a color filter array pattern of a CMOS-sensor for sensing objects which do not have a uniform representation of colors across the visible spectrum, for example, elements of the human body non-visible to the naked eye, such as the internal organs of the gastrointestinal tract. A method of fabricating such color filter pattern is also needed. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect, the present invention provides a color filter array pattern for use in a solid-state imager for imaging internal organs comprising red sensitive elements located at every other array position, and alternating blue sensitive and green sensitive elements located at the remaining array positions. This way, red color is sampled most frequently and blue and green colors are sampled least frequently. 
   In another aspect, the invention provides a method of using a color filter array pattern of a solid-state imager for imaging objects which do not have a uniform representation of colors across the visible spectrum, for example, internal organs of the human gastrointestinal tract. By employing the color filter pattern of the present invention in in vivo video camera systems or in a small CCD or CMOS imager capsule camera used in medical procedures, such as gastrointestinal endoscopy for example, the predominantly red color of the organs of human gastrointestinal tract is sampled at twice the frequency of the other two basic colors, blue and green. 
   Also provided are methods for forming the color filter array pattern of the present invention. These and other advantages and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a schematic representation of the Bayer color pattern. 
       FIG. 2  illustrates a schematic representation of a color filter pattern of the present invention. 
       FIG. 3  is an exploded three-dimensional representation of the color filter pattern of  FIG. 2 . 
       FIG. 4  is a side cross-sectional view illustrating the principal elements of a solid-state imager having a color filter array constructed in accordance with the present invention. 
       FIG. 5  illustrates a schematic cross-sectional view of a CMOS imager pixel cell having a color filter array constructed in accordance with the present invention. 
       FIG. 6  is a representative diagram of the CMOS imager pixel cell of  FIG. 5 . 
       FIG. 7  illustrates a cross-sectional view of a semiconductor wafer undergoing the process of forming a color pattern layer according to an embodiment of the present invention. 
       FIG. 8  illustrates the semiconductor wafer of  FIG. 7  at a stage of processing subsequent to that shown in  FIG. 7 . 
       FIG. 9  illustrates the semiconductor wafer of  FIG. 7  at a stage of processing subsequent to that shown in  FIG. 8 . 
       FIG. 10  is an illustration of an imaging system having an imager with a color filter pattern according to 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 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. 
   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 or above 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” refers to a picture element unit cell containing a photosensor and transistors for converting electromagnetic radiation to an electrical signal. For purposes of illustration, a representative CMOS imager pixel is illustrated in the figures and description herein. However, this is just one example of the type of imagers and pixel cells thereof with which the invention may be used. The following detailed description is, therefore, not to be taken in a limiting sense, but rather as an exemplary illustration of the invention. 
   Referring now to the drawings, where like elements are designated by like reference numerals, an image sampling array pattern (color filter pattern)  10  of the present invention is illustrated in  FIGS. 2-3 . Three sets of sensor patterns  11 ,  13  and  15  ( FIG. 3 ), each corresponding to a basic color vector, are interlaid to form the image sampling array pattern  10  ( FIGS. 2-3 ). The sensor pattern  11  is formed of red-sensitive elements (also called luminance elements) which are denoted by an “R” and are arranged at every other element position of the sampling array. Since the sensor pattern  11  has the highest number of color sensitive elements, the sensor pattern  11  is called the luminance pattern. As illustrated in  FIGS. 2-3 , the red luminance elements of sensor pattern  11  occur at half the element positions of the array and are uniformly distributed over the entire sampling array  10 . Thus, luminance detail is sampled by the red elements which form the largest population of elements. 
   Sensor pattern  13  has green elements denoted “G” which alternate with the red luminance elements of the sensor pattern  11  in alternate rows. Similarly, sensor pattern  15  has blue elements denoted “B” which alternate with the red luminance elements of the sensor pattern  11  in alternate rows. This way, sensor patterns  13  and  15  form a symmetrical and uniform arrangement in two orthogonal positions, horizontal and vertical, as shown in  FIG. 2 . When an image sensor is read out, line by line, the pixel sequence reads RGRGRG etc., and then the alternate line sequence reads BRBRBR etc. 
   In the arrangement of  FIGS. 2-3 , the red elements form half of the element population, while the blue and green elements form the other half of the element population. Thus, the blue sensitive elements form one fourth of the element population, while the green sensitive elements also form one fourth of the element population. As a result of the twice greater population of the red elements relative to the blue and green ones, red detail is sampled at a twice higher rate than blue detail or green detail. As a result of the red luminance pattern, sampling of an image devoid of all three basic colors, for example, of an image predominant in red and red hues, is symmetrical and uniform in both the horizontal and vertical direction. Thus, the color sampling array pattern  10  is preferably employed for sampling all three basic color vectors according to the primary color of the internal human body, that tends to be in the red spectrum. Sampling the red color at twice the frequency of the other two primary colors provides a good representation of the luminance component of a particular internal body part, organ, tissue or element being imaged. 
   A solid-state imager  20  comprising a color filter layer  100  having color filter pattern  10  of the present invention is schematically illustrated in  FIGS. 4-6 . The imager  20  comprises color filter layer  100  formed over a pixel array  26  as part of the same substrate  30 , which may be any of the types of substrate described above. The pixel array  26  comprises a plurality of pixel sensor cells  28  formed in and over the substrate, and is covered by a protective layer  24  that acts as a passivation and planarization layer for the imager  20 . Protective layer  24  may be a layer of BPSG, PSG, BSG, silicon dioxide, silicon nitride, polyimide, or other well-known light transmissive insulator. 
   The color filter layer  100  having color filter pattern  10  described above is formed over the passivation layer  24 . The color filter layer  100  comprises an array of red sensitive elements located at every other array position, and alternating blue sensitive and green sensitive elements located at the remaining array positions, as described in detail above with reference to the color imaging array pattern  10 . This way, the color filter layer  100  samples red color most frequently and blue and green colors least frequently. 
   As also depicted in  FIGS. 4-6 , a microlens array  22  is formed so that microlens  70  are formed above each pixel cell  28 . The microlens array  22  is formed such that the focal point of the array is centered over the photosensitive elements in each pixel cell  28 . The device also includes a spacer layer  25  under the mircolens array  22  and over the color filter layer  100 . The thickness of spacer layer  25  is adjusted such that the photosensitive element is at a focal point for the light traveling through lenses  70  of microlens array  22 . 
   As shown in  FIGS. 5-6 , each pixel sensor cell  28  contains a photosensor  34 , which may be a photodiode, photogate, or the like. A photogate photosensor  34  is depicted in  FIGS. 5-6 . An applied control signal PG is applied to the photogate  34  so that when incident radiation  101  in the form of photons passes color filter layer  100  and strikes the photosensor  34 , the photo-generated electrons accumulate in the doped region  36  under the photosensor  34 . A transfer transistor  38  is located next to the photosensor  34 , and has source and drain regions  36 ,  40  and a gate stack  42  controlled by a transfer signal TX. The drain region  40  is also called a floating diffusion region or a floating diffusion node, and it passes charge received from the photosensor  34  to output transistors  44 ,  46  and then to readout circuitry  48 . A reset transistor  50  comprised of doped regions  40 ,  52  and gate stack  54  is controlled by a reset signal RST which operates to reset the floating diffusion region  40  to a predetermined initial voltage just prior to signal readout. Details of the formation and function of the above-described elements of a pixel sensor cell may be found, for example, in U.S. Pat. No. 6,376,868 and U.S. Pat. No. 6,333,205, the disclosures of which are incorporated by reference herein. 
   As illustrated in  FIG. 5 , the gate stacks  42 ,  54  of the pixel cell  28  for the transfer  38  and reset  50  transistors include a silicon dioxide or silicon nitride insulator  56  on the substrate  30 , which in this example is a p-type substrate, a conductive layer  58  of doped polysilicon, tungsten, or other suitable material over the insulating layer  56 , and an insulating cap layer  60  of, for example, silicon dioxide, silicon nitride, or ONO (oxide-nitride-oxide). A silicide layer  59  may be used between the polysilicon layer  58  and the cap  60 , if desired. Insulating sidewalls  62  are also formed on the sides of the gate stacks  42 ,  54 . These sidewalls may be formed of, for example, silicon dioxide, silicon nitride, or ONO. A field oxide layer  64  around the pixel cell  28  serves to isolate it from other pixel cells in the array. A second gate oxide layer  57  may be grown on the silicon substrate and the photogate semi-transparent conductor  66  is patterned from this layer. In the case that the photosensor is a photodiode, no second gate oxide layer  57  and no photogate semi-transparent conductor  66  is required. Furthermore, transfer transistor  38  is optional, in which case the diffusion regions  36  and  40  are connected together. 
   The color filter layer  100  of the embodiment described above is manufactured through a process described as follows, and illustrated in  FIGS. 7-9 . Referring now to  FIG. 7 , a substrate  30 , which may be any of the types of substrates described above, having a pixel array  26 , peripheral circuits, contacts and wiring formed thereon by well-known methods, is provided. A protective layer  24  of BPSG, BSG, PSG, silicon dioxide, silicon nitride or the like is formed over the pixel array  26  to passivate it and to provide a planarized surface. 
   A color filter layer  100  is formed over the passivation layer  24 , as also shown in  FIG. 7 . The color filter layer  100  may be formed of a color resist or acrylic material which is used as a light transmitting material. For example, color filter layer  100  may be formed of a plurality of color filter layers, each of the plurality of color filter layers consisting of red filter regions (not shown), green filter regions (not shown) and blue filter regions (not shown), which are formed, for example, from resist or acrylic material of the respective color-filtering qualities. As such, red sensitive resist material, blue sensitive resist material and green sensitive resist material may be employed to form the red, blue and green sensitive elements of each of the plurality of color filter layers that form color filter layer  100 . These red, blue and green elements are disposed side by side, and according to the above-described color filter pattern  10 , so that the red sensitive elements are located at every other array position, with alternating blue sensitive and green sensitive elements located at the remaining array positions. Other embodiments may employ other colored materials, such as paint or dye, as known in the art. The color filter layer  100  may be formed over the passivation layer  24  by conventional deposition or spin-on methods, for example. 
   The red, blue and green filter elements are preferably squares of generally less than  50  microns wide, although other geometrical shapes may be used also, and are placed in registration with the photosensitive elements (for example photodiodes) of the semiconductor layer. 
   Next, a spacing layer  25  is formed over the protective layer  24 , as illustrated in  FIG. 8 . Refractive lenses  70  may then be formed, as shown in  FIG. 9 , from a lens forming layer, for example, so that each lens  70  overlies a pixel cell  28 . Alternative constructions in which a lens  70  overlies multiple pixel cells  28  are also encompassed by the present invention. 
   The color filter layer  100  is essentially complete at this stage, and conventional processing methods may now be performed to package the imager  20 . Pixel arrays having the color filter array pattern of the present invention, and described with reference to  FIGS. 2-9 , may be further processed as known in the art to produce a CMOS imager. 
   The filter array of the present invention may be also used with pixels of other types of imagers as well, for example, with a CCD imager. If desired, the imager  20  may be combined with a processor, such as a CPU, digital signal processor or microprocessor. The imager  20  and the microprocessor may be formed in a single integrated circuit. An exemplary processor system  400  using a CMOS imager having a filter array in accordance with the present invention is illustrated in  FIG. 10 . A processor based system is exemplary of a system having digital circuits which could include CMOS or other imager devices. 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. 
   As shown in  FIG. 10 , an exemplary processor system  400  generally comprises a central processing unit (CPU)  444 , e.g., a microprocessor, that communicates with an input/output (I/O) device  446  over a bus  452 . The imager  20  also communicates with the system over bus  452 . The computer system  400  also includes random access memory (RAM)  448 , and may include peripheral devices such as a floppy disk drive  454 , a compact disk (CD) ROM drive  456  or a flash memory  458  which also communicate with CPU  444  over the bus  452 . The floppy disk  454 , the CD ROM  456  or flash memory  458  stores images captured by imager  20 . The imager  20  is preferably constructed as an integrated circuit, with or without memory storage, which includes a color filter layer  100  having color filter pattern  10  of the present invention, as previously described with respect to  FIGS. 2-9 . 
   Since the color filter array pattern for use in a solid-state imager, as described above, comprises red sensitive elements located at every other array position, and alternating blue sensitive and green sensitive elements located at the remaining array positions, red color is sampled most frequently and blue and green color are sampled least frequently. For this reasons, the color filter array pattern of the present invention may be employed for obtaining images and data measurements from a variety of organ systems, tissues and cells for use in splanchnology (study of viscera), neurology (study of nervous system), osteology (study of bones), syndesmology (study of ligaments and joints) and myology (study of muscles), among others. This way, sampling red color (the primary color of internal body organs, tissues and cells) at twice the frequency of the other two primary colors provides a good representation of the luminance component of the particular internal body organ, tissue or cell being imaged. 
   Accordingly, and in a preferred embodiment of the present invention, the imager  20  is constructed as an integrated circuit with a color filter layer  100  and color filter pattern  10  of the present invention, and further as a part of an in vivo video camera system or an in vivo measurement system, which detects images and analyzes data of various systems of the human body, such as the digestive or muscular systems, for example. In vivo video camera and measurement systems typically include swallowable electronic capsules which collect data from various internal body organs or tissues and further transmit data to a receiver system. These swallowable intestinal capsules may also include a transmission system for transmitting the measured data at various radio frequencies to the receiver system. 
   Other in vivo detecting and measuring systems, to which the imager  20  comprising color filter layer  100  with color filter pattern  10  of the present invention may be attached, are endoscopes, which are typically long tubes that patients swallow to provide images of the upper or lower gastrointestinal tract. The endoscopes may be fiber optic endoscopes or video endoscopes. In video endoscopes, for example, a small electronic camera is placed at the area of interest and stores the images until after the test finishes. 
   More detail on in vivo video cameras and swallowable capsules are provided, for example, in U.S. Pat. No. 5,604,531 to Iddan et al.; U.S. Pat. No. 4,278,077 to Mizumoto; U.S. Pat. No. 5,267,033 to Hoshino; and E. N. Rowland and H. S. Wolff,  The Radio Pill: Telemetering from the Digestive Tract , British Communications and Electronics (August 1960, pp. 598-601), the disclosures of which are incorporated by reference herein. 
   It should again be noted that although the invention has been described with specific reference to imaging circuits having a pixel array, the invention has broader applicability and may be used in any imaging apparatus. Similarly, the process for the fabrication of the color filter layer  100  described above is but one method of many 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.