Abstract:
A method for registering a color filter array to a back illuminated imager is disclosed, comprising the steps of providing at least one color filter array comprising filter elements of at least a first color and a second color; providing at least one back illuminated imager having a front side and a back side and comprising a plurality of pixels proximal to the front side, a first portion of the plurality of pixels being associated with the first color, and a second portion of the plurality of pixels being associated with the second color; illuminating the at least one color filter array and the back side of the back illuminated imager with monochromatic light having a wavelength corresponding to the first color; rotating and translating the at least one color filter array relative to the back illuminated imager; measuring a first response of at least one pixel associated with the second color; and repeating the rotating, translating, and measurement steps until the response is a minimum. The aligned back illuminated imager can then be adhered to the color filter array by means of an adhesive.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional patent application No. 60/846,165 filed Sep. 20, 2006, the disclosure of which is incorporated herein by reference in its entirety. 
     
     FIELD OF THE INVENTION 
       [0002]    The field of the present invention is semiconductor device fabrication and device structure. More specifically, the present invention relates to the alignment and application of color filters to back illuminated imagers. 
       BACKGROUND OF THE INVENTION 
       [0003]    CMOS or CCD image sensors are of interest in a wide variety of sensing and imaging applications in a wide range of fields including consumer, commercial, industrial, and space electronics. CCDs are employed either in front or back illuminated configurations. Front illuminated CCD imagers are more cost effective to manufacture than back illuminated CCD imagers such that front illuminated devices dominate the consumer imaging market. Front-illuminated imagers, however, have significant performance limitations such as low fill factor/low sensitivity (the active region of a pixel is typically very small (low fill factor)). As a result, there is a significant amount of interest in the development of color back illuminated imagers. Color back illuminated imagers contain an array of color filter elements sensitive to a plurality of different colors of light, such as the primary colors red, green, and blue. The filter elements can be arranged in a variety of patterns, the most commonly used being the Bayer pattern to be discussed hereinbelow in connection with the present invention. 
         [0004]    In order to maintain color purity, each filter element needs to be precisely aligned, i.e., registered, to a corresponding pixel (at least the light sensitive portion of a pixel). In a traditional front-illuminated imager, color filter elements are applied in set of three steps using conventional photolithography and alignment tools similar to those used to create the imager itself. One common method for obtaining a registered pattern uses colored photoresists. In the photoresist process, a wafer containing a plurality of front-illuminated imagers is coated with a material (photoresist) that is sensitive to ultraviolet (UV) light. On exposure to light, the photoresist is rendered insoluble in particular solvents (developers).  FIGS. 1A-1D  show the process of producing color back illuminated imagers using photoresists of different colors to produce a desired filter pattern.  FIG. 1A  shows a portion of an array  10  of pixels  12  of a front illuminated imager before the application of photoresist.  FIG. 1B  shows a layer  14  of blue photoresist being applied to the entire array  10 . A subset of the pixels  12  is illuminated with a pattern of UV light to which the layer  14  is sensitive. In  FIG. 1C , solvents are applied to the blue photoresist such that those portions  16  exposed to UV light become insoluable and therefore remain on the wafer while other areas  17  are dissolved away. In  FIG. 1D , the process is repeated for photoresists of the other colors (green and red), which results in the device shown. Note, the filter material (insoluable photoresist) need only cover the actual photo sensitive portion of a pixel. In front illuminated imagers, especially those with small pixels, a relatively large portion of the pixel is devoted to signal and control electrodes. 
         [0005]    For back illuminated imagers, the entire area of a pixel can be used for light gathering. As a result, the entire area of the pixel can be covered with the filter material. The back side of the back-illuminated imager, therefore, can provide a single flat surface that can accept an integrated filter, i.e. a filter containing three sets of primary color filter elements aligned in pattern to match the light sensitive portions of the pixels in the imaging array.  FIG. 2  show an example of a color back illuminated imager  18  having an integrated color filter array  20  positioned on a back side  22  of the imager  18  as is known in the prior art. The back illuminated imager  18  includes an array of pixels  24  each having a light sensitive region  26  located on the front side  28  of the imager  18 . The back side  22  of the imager  18  is completely covered by a plurality of filter elements  30  of one or more colors arranged in a patterns such as the Bayer pattern discussed above. 
         [0006]    If the integrated filter  20  is produced by semiconductor manufacturing processes similar to those used for front-illuminated imager color filters, the resulting color back illuminated imager  18  may have poor registration of the color filter elements  30  to the to the light sensitive regions  26  of the pixels  24 . Poor registration may result because the pixels  24  on the front side  28  of the imager  18  are not directly visible to manufacturing equipment used to locate the positions of the color filter elements  30  on the back side  22 . 
         [0007]    The alignment of a color filter pattern on the back side  22  of the back illuminated imager  18  in two dimensions now becomes very important, since any shift of the pattern will result in degradation of color fidelity. Further, because such a back-illuminated color filter  20  would be constructed on the back side  24  of the imager  18  using photoresists and employing photolithography, the manufacturing of the integrated filter  20  is subject to the same solvent and etching material limitations as is found with front-illuminated imager color filters. Also, because the application of photoresists is repeated three times—one for each color—the likelihood of defects in one or more of the colors is greatly increased. Defects in a color filter pattern produces an imager  18  that does not function properly, even though the underlying imager functions properly electrically. The production of defect-free color mask patterns on the back side  22  of the imager  18  is made even more difficult when the thickness of the semiconductor material is 4 to 10 μm or less, leading to warpage and other distortions of the light sensitive regions  26  of the pixels  24 . 
         [0008]    Accordingly, what would be desirable, but has not yet been provided, is a method for aligning and affixing a monolithic integrated color filter array to the back side of a back-illuminated imager in which the manufacturing process of the color filter array is independent of the manufacturing process of the imager. 
       SUMMARY OF THE INVENTION 
       [0009]    Disclosed is an apparatus and method for registering a color filter array to a back illuminated imager, comprising the steps of providing at least one color filter array comprising filter elements of at least a first color and a second color; providing at least one back illuminated imager having a front side and a back side and comprising a plurality of pixels proximal to the front side, a first portion of the plurality of pixels being associated with the first color, and a second portion of the plurality of pixels being associated with the second color; illuminating the at least one color filter array and the back side of the back illuminated imager with monochromatic light having a wavelength corresponding to the first color; rotating and translating the at least one color filter array relative to the back illuminated imager; measuring a first response of at least one pixel associated with the second color; and repeating the rotating, translating, and measurement steps until the response is a minimum. The color filter array can also comprise elements of a third color, wherein a third response of at least one pixel associated with the third color is measured repeatedly while rotating and translating the color filter array relative to the back illuminated imager until the response for both colors that do not correspond to the illuminating source are minimized. This process can be repeated by substituting light of second color and then a third color for the illuminating source and then finding a best fit translation and rotation vector based on the three sets of measurements. The aligned back illuminated imager can then be adhered to the color filter array by means of an adhesive. 
         [0010]    A device can be constructed from the at least one back illuminated imager and the at least one color filter array, comprising a transparent substrate; at least one color filter array comprising a plurality of filter elements of at least a first color and a second color substantially overlying said transparent substrate; an adhesive layer substantially overlying the at least one color filter array; and at least one back illuminated imager having a front side and a back side and comprising a plurality of pixels proximal to the front side, a first portion of the plurality of pixels being associated with the first color, and a second portion of the plurality of pixels being associated with the second color, wherein the at least one back illuminated imager is oriented to the at least one color filter array based on rotating and translating the at least one color filter array relative to the at least one back illuminated imager so as to minimize a response of at least one pixel associated with the second color to illuminated with monochromatic light corresponding to the first color. The at least one back illuminated imager can be a charge coupled device (CCD), a CMOS based back illuminated imager, or a plurality of back illuminated imagers arranged on a wafer. The elements of the at least one color filter array can be arranged in a Bayer pattern comprising three primary colors. The filter elements of the at least one color filter array can be made with organic dyes. Alternatively, the filter elements can include multiple thin layers of inorganic materials acting as interference filters. 
     
     
       SUMMARY DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1A  is a schematic diagram of a portion of an array pixels of a front illuminated imager before the application of photoresist; 
           [0012]      FIG. 1B  is a schematic diagram showing a layer of blue photoresist being applied to the entire array of  FIG. 1 ; 
           [0013]      FIG. 1C  is a schematic diagram showing solvents being applied to the blue photoresist such that those portions exposed to UV light become insoluable and therefore remain on the wafer while other areas are dissolved away; 
           [0014]      FIG. 1D  is a schematic diagram of a color front illuminated imager in the prior art after photoresists of multiple colors are applied and patterned; 
           [0015]      FIG. 2  is a schematic diagram showing a color back illuminated imager in the prior art having an integrated color filter array positioned on a back side of the imager; 
           [0016]      FIG. 3  is a schematic diagram depicting a monolithic color filter array constructed in accordance with an embodiment of the present invention; 
           [0017]      FIG. 4  is a schematic block diagram showing equipment for aligning and affixing the monolithic color filter array of  FIG. 3  to a back illuminated imager according to an embodiment of the present invention; and 
           [0018]      FIG. 5  depicts a monolithic color filter array having the configuration of a Bayer pattern which can be used in conjunction with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The following embodiments are intended as exemplary, and not limiting. In keeping with common practice, figures are not necessarily drawn to scale. 
         [0020]      FIG. 3  depicts a monolithic color filter array  32  constructed in accordance with an embodiment of the present invention.  FIG. 4  shows a back illuminated imager  34  to which the monolithic color filter array  32  is to be aligned and affixed using test equipment  36 . The manufacture of the color filter array  32  is performed in a separate series of operations. The color filter array  32  includes a transparent substrate  38 . The transparent substrate  38  can be made from a variety of suitable materials, such as glass or quartz. The composition of the substrate  38  should be mechanically compatible with the semi-conductor material of the back illuminated imager  34 . It should also be stable over temperature and time. A plurality of color filter elements  40  of at least one color, preferably three primary colors, (e.g., red-blue-green or magneta-cyan-yellow), and incorporated into the monolithic color filter array  32  substantially overly the transparent substrate  38 . Each of the plurality of color filter elements  40  is sized and shaped to substantially underlay at least the light-sensitive regions  42  of a pixel elements  44  of the back illuminated imager  34 .  FIG. 3  also shows an adhesive layer  46  substantially overlying the plurality of color filter elements  40 . The adhesive layer  46  is provided so that the monolithic color filter array  32  may be adhered to the back surface  48  of the back illuminated imager  34 . This allows for the plurality of color filter elements  32  and the back surface  48  of the back illuminated imager  34  to be in near intimate contact during the alignment process, separated only by a thin liquid layer (the adhesive layer  46 ). This prevents light incident on the color filter elements  40  from spreading beyond the boundaries of the light-sensitive regions  42  of pixel elements  44 , which would result in optical losses and reduce sensitivity. 
         [0021]    Because the color filter array  32  is built separately from the back-illuminated imager  34 , the materials for the color filter elements  40  can be freely chosen without being subject to limitations in processing conditions which may contaminate or destroy the pixel elements  44  in the imager  34 . Producing the monolithic color filter array  32  would not require any compromises to be made in chemicals used, filter materials, process temperature, application methods or other conditions that are imposed if the color filter elements  40  were to be created on an already processed imager array. Further, the entire array of color filter elements  40  can be fully inspected for defects. Separating the manufacture of the color filter array  32  from the manufacture of the imager  34  allows for greater efficiency and higher yield in both the imager  34  and the monolithic color filter array  32 . Furthermore, the color filter array production process can be separately optimized for pattern fidelity. The monolithic color filter array  32  can be independently inspected for conformance to size and placement of each of the color filter elements  40 . Filter arrays that do not meet quality standards can be rejected without having to reject an entire back-illuminated imager, as would be the case for an imager that has integrally manufactured color filter elements. 
         [0022]    Referring now to  FIG. 4 , in a preferred embodiment, a first fixture  52  holds the back illuminated imager  34  having a back surface  48  and a front surface  50 , the front surface  50  being the location for a plurality of light sensitive pixel elements  44  that can be aligned relative to the at least one color filter array  32  held by a second fixture  54 . The imager  34  can be a partially packaged device that is inserted into the first fixture  52 . This is a preferred embodiment since all of the electrical connections to the imager  34  are already established and tested and the package will contain an opening to allow light to fall on the imager  34 . Alternatively, several imagers can be located on a surface of a wafer whose output electrodes are connected to the test equipment  36  by means of a probe card (not shown). Ideally the imagers on the wafer are located at positions close to the edge of the wafer. Likewise the at least one color filter array  32  can be a plurality of color filter arrays to be aligned with the plurality of imagers on the wafer. Individual defect-free color filter arrays can be selected. 
         [0023]    The imager(s)  34  is/are situated such that the back surface(s)  48  can be illuminated using monochromatic light. At least one light source  56  of at least one wavelength is designed to illuminate the color filter array  32 . In some embodiments, the at least one light source  56  can be an array of light sources of the three primary colors described above, corresponding to three primary colors used in the monolithic color filter array  32 . The at least one light source  56  can be one or more red, green and blue light emitting diodes (LEDs) arranged as an LED array  56 . The LED array  56  is constructed such that the diodes of a single color may be selected by the test equipment  36 . Further, the intensity of the illumination from each color can be varied under control of the test equipment  36 . The LED array  56  can be designed to provide uniform, collimated light over the area of a single imager  34 . The illumination source  56  for use with large diameter wafers can comprise multiple LED arrays disposed in the approximate positions of the imagers under test. 
         [0024]    The color filter elements  40  of independently fabricated monolithic color filter arrays  32  can be precisely registered with the light-sensitive regions  42  of a pixel elements  44  in the imager  34 . Registration is performed by observing the electrical signals emanating from the imager  34 . The color filter array  32  to be aligned is interposed between at least one light source  56  and the imager  34 . The color filter array  32  can be translated and rotated with respect to the back surface  48  of the imager. Alternatively, the color filter array  32  can be held stationary and the imager  34  rotated and translated. The color filter array  32  or the imager  34  can be moved such that controlled contact can be established between the back surface  48  of the imager  34  and the adhesive layer  46 . The test equipment  36  controls all motions of the color filter array  32  and imager  34  with respect to each other. 
         [0025]    An imager output block  58  includes equipment for collecting the analog voltage signals  60  representing the output signals of the plurality of light sensitive pixel elements  44  and may contain equipment, such as data acquisition modules or a microcontroller containing one or more analog-to-digital converters for converting these analog voltage signals  60  to digital signals  62 . A test and analysis block  64  contains at least one processor  66  and memory  68  for receiving and processing the digital signals  62 . The at least one processor  66  operates on a program stored in the memory  68  for determining the light output of the plurality of pixel elements  44 , and for determining a set of control signals to be applied to one or both of the fixtures  52 ,  54  for adjusting the relative position of the imager  34  with the color filter array  32 . The second fixture  54  can be configured to be movable relative to the first fixture  54  according to three degrees of freedom of translation and three degrees of freedom of rotation. 
         [0026]    In operation, the color filter array  32  is moved to near intimate contact with the back surface  48  of the imager  34 . The imager  34  is illuminated with light of a single wavelength by the at least one light source  56  corresponding to a color associated with one type of the color filter elements  40 . The analog voltages  60  produced by the plurality of pixel elements  44  is measured and converted to digital signals  62  by the imager output block  58 , which in turn sends the plurality of digital signals  62  to the at least one processor  66  in the test and analysis block  64 . The at least one processor  66  then signals one or both of the fixtures  52 ,  54  to rotate and/or translate its position so as to minimize the measured response (output voltages) from the subset of the pixel elements  44  that do not correspond to the color (wavelength) selected for illumination. Optimizing for a minimum response from the subset of pixel elements associated with the subset of color filter elements that do not correspond to the selected wavelength (color) of illumination also has the effect of maximizing the response of the subset of pixel elements associated with the color filter elements that do correspond to the selected color of illumination. Optionally, the response of the selected color of illumination can also be measured. 
         [0027]    In some embodiments, rotation can be optimized first, wherein the at least one light source  56  comprises two LEDs of the same color widely separated for use with pixel elements disposed at extreme positions on a semiconductor wafer. It may be necessary to use at least two LEDs because it may be difficult construct a single LED light source to illuminate a semiconductor wafer that is 8-12″ in diameter. Illuminating pixels that are far apart with a single LED can exaggerate small errors in rotation. Once optimized for rotation, all of the color filter elements  40  are parallel to the pixel elements  44  and have the same center of rotation. Then, translation can be optimized using one of the two LEDs above. If response is optimized for rotation first before translation, then the adjustment for translation is simplified because all of the signal responses from the pixel elements  44  change in the same way uniformly. A person skilled in the art would appreciate that other optimization algorithms could be used, wherein translation optimization can be performed before rotation optimization, or a combination of both could be employed simultaneously. 
         [0028]    In a preferred embodiment, the at least one light source  56  can comprise a plurality of arrays of LEDs of three primary colors so as to minimize errors caused by spread of an applied beam of light. Spreading of the light beam can also be minimized when the color filter array  32  and the imager  34  are in near intimate contact. For increased accuracy, the process outlined above can be carried out sequentially using red, then blue, and then green LEDs. By recording the relative positions of the color filter array  32  and the imager  34  using each of the three colors, a ‘best fit’ set of translation and rotation vectors can be determined. Once the optimized position and orientation of the color filter array  32  is determined, the color filter array  32  can be directly affixed to the rear surface  48  of the imager  34  using a suitable adhesive. 
         [0029]    In a preferred embodiment, the alignment technique of the present invention can be used in conjunction with a monolithic filter array having the Bayer pattern previously discussed. The Bayer pattern is depicted in  FIG. 5 . A Bayer pattern can have RGB pixels in the ratio 1:2:1 organized as shown. In a filter array  68  constructed with colors distributed according to a Bayer pattern, there are twice as many green filter elements  70  as there are of blue  72  or red  74  filter elements. This results in greater proximity of green filter elements  70  to each other compared to the blue  72  or red  74  filter elements. Using the filter array  68  having a color distribution according to a Bayer pattern in which green is represented twice as often as red or blue is a suitable configuration of pixel color distribution because the human eye is more sensitive to green. If the Bayer pattern array  68  is illuminated with red or blue light, ideally one fourth of the pixels can have identical outputs and three-fourths will have no output. A down side to using such a pattern is that the arrangement of green filter elements  70  is symmetrical, so that that amount of movement away from one green pixel can be cancelled by the simultaneous movement by the same amount toward another green pixel, with the result that another green filter element is associated with the same pixel, thereby making it more difficult to achieve proper alignment. In such circumstances, it is best to first align on either red  74  or blue  72  filter elements, which are asymmetrical in a Bayer filter pattern, so that moving away from either a red  74  or blue  72  filter element has a lower probability of moving into an area of the Bayer array associated with another red or blue filter element. Thus, in an embodiment employing a color filter array  68  having a Bayer pattern, it would be preferable to illuminate the filter array with either red or blue light, say, for example, red light, and then adjust the relative position of the filter array  68  and/or the imager to minimize the signal to be detected in green and blue. Best results can be obtained if all of the data from the imaging array  68  is used. In a two megapixel array, for example, there are 1,000,000 green pixels and 500,000 blue and green pixels, respectively. Using blue illumination, for example, the best alignment position can be found by simultaneously minimizing the signals in the 1,500,000 other pixels. The large degree of data redundancy ensures that the best solution can be found. 
         [0030]    The present invention is subject to modifications. For example, although the present invention is independent of the type of color filter array used, filter elements of a monolithic color filter array can be made with organic dyes. The color filter elements can include multiple thin layers of inorganic materials acting as interference filters, such as dichroic filters. 
         [0031]    It is to be understood that the exemplary embodiments are merely illustrative of the invention and that many variations of the above-described embodiments may be devised by one skilled in the art without departing from the scope of the invention. It is therefore intended that all such variations be included within the scope of the following claims and their equivalents.