Abstract:
In one aspect of the present invention, a color filter capable of controlling wavelengths of light to be transmitted therethrough may include a pair of interferometric films which are substantially parallel to each other and which transmit the light; and a drive member which drives at least one film of the pair of the interferometric films to change a distance of a gap between the interferometric films.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-250423, filed on Sep. 27, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    A general color image sensor has a structure in which every single pixel has a color filter disposed thereon that allows the pixel to detect only one color, that is, only a wavelength range corresponding to one color in operation of the color image sensor. Specifically, in each of many color image sensors, RGB (red, green and blue) color filters are regularly arrayed so that one color filter corresponds to each pixel. Each pixel with a red color filter detects red light, and green light and blue light are also detected in similar manners. 
         [0003]    Thus, in reproducing an image, the color image sensor reproduces green and blue signal in each pixel with a red color filter by calculating intensities of the green and blue colors through signal processing based on information on the adjacent pixel with a green color filter and on the adjacent pixel with a blue color filter. At the same time, red and blue signal in each green pixel, and red and green signal in each blue pixel are also reproduced by using signal processing in similar manners. 
         [0004]    As described above, a conventional color image sensor processes an image by allotting one color to each pixel. As a result, the color image sensor has poor reproducibility characteristics for an object showing a significant spatial color variation or an object containing similar colors. For example, from such an object, the color image sensor is likely to reproduce a more unclear image as to look out of focus than the original object. In addition, the conventional technique has a problem that, in a color image sensor having a large number of pixels, each pixel inevitably has such a small structure that its element for detecting light, such as a photodiode, cannot receive a light beam with a sufficient intensity for its detecting operation. 
         [0005]    Note that an imaging device with a multilayer interferometric filter has already been known (refer to Japanese Patent Application Publication No. 2005-308871). Meanwhile, a technique of employing interferometric films in a reflective color display device has been also known (refer to Japanese Patent Application Publications No. 2005-77718 and No. 2006-20778). 
       SUMMARY 
       [0006]    Aspects of the invention relate to an improved color filter and color image sensor. 
         [0007]    In one aspect of the present invention, a color filter capable of controlling wavelengths of light to be transmitted therethrough may include a pair of interferometric films which are substantially parallel to each other and which transmit the light; 
         [0008]    and a drive member which drives at least one film of the pair of the interferometric films to change a distance of a gap between the interferometric films. 
         [0009]    In another aspect of the invention, a color image sensor, may include a color filter including at least one pair of interferometric films which are substantially parallel to each other and which transmit light; a plurality of photoelectric converters which are arrayed two-dimensionally to receive the respective light transmitted through the color filter and to output electrical signals; and a drive member which changes the wavelengths of the light transmitted through the corresponding pair of the interferometric films by moving at least one film of the pair of the interferometric films to change a distance of a gap between the interferometric films. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0010]    A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
           [0011]      FIG. 1A  is a schematic cross-sectional view showing the color image sensor of the first embodiment.  FIG. 1B  is a schematic plan view of the color image sensor shown in  FIG. 1A .  FIG. 1C  is a schematic cross-sectional view in which cross-sectional views as viewed from the respective directions of the arrow A-A and the arrow B-B of  FIG. 1B  overlap on each other. 
           [0012]      FIGS. 2A to 2F  are schematic cross-sectional views showing the manufacturing process of the color image sensor. 
           [0013]      FIG. 3A  is a schematic cross-sectional view showing the color image sensor of the second embodiment. 
           [0014]      FIG. 3B  is a schematic plan view of the color image sensor shown in  FIG. 3A .  FIG. 3C  is a schematic plan view showing a part, corresponding to one pixel, of the color image sensor shown in  FIG. 3B . 
           [0015]      FIG. 4A  is a cross-sectional view as viewed from the direction of the arrow IVa-IVa of  FIG. 3C .  FIG. 4B  is a cross-sectional view as viewed from the direction of the arrow IVb-IVb of  FIG. 3C . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Various connections between elements are hereinafter described. It is noted that these connections are illustrated in general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. 
         [0017]    Embodiments of the present invention will be explained with reference to the drawings as next described, wherein like reference numerals designate identical or corresponding parts throughout the several views. 
       First Embodiment 
       [0018]    Firstly, with reference to  FIGS. 1A to 1C , description will be given of a configuration of a first embodiment of a color image sensor according to the present invention.  FIG. 1A  is a schematic cross-sectional view showing the color image sensor of the first embodiment.  FIG. 1B  is a schematic plan view of the color image sensor shown in  FIG. 1A .  FIG. 1C  is a schematic cross-sectional view in which cross-sectional views as viewed from the respective directions of the arrow A-A and the arrow B-B of  FIG. 1B  overlap on each other. 
         [0019]    In the color image sensor, multiple photodiode regions  5 , each of which serves as a photoelectric converter corresponding to a pixel, are arranged in horizontal and vertical lines on a substrate  6 , and an interlayer insulating film  4  is formed thereon. In the interlayer insulating film  4 , an unillustrated wiring layer is formed. A color filter  20  is disposed on the interlayer insulating film  4 , and multiple microlenses  2  are arranged at positions corresponding to the respective photodiode regions  5 . Though pixels are arranged in  3  horizontal and  3  vertical lines in the example shown in  FIGS. 1A to 1C , the number of pixels may typically be far larger than this. For example, several million pixels may be arranged in a matrix in a plane. 
         [0020]    In this embodiment, an arrangement is employed in which the single color filter  20  covers the entire matrix of the multiple photodiode regions  5 . The color filter  20  has a lower interferometric film  1   a  directly disposed on the interlayer insulating film  4  and an upper interferometric film  1   b  disposed parallel to the lower interferometric film (interference film)  1   a . Each of the lower and upper interferometric films  1   a  and  1   b  is formed of a stable film made, for example, of amorphous silicon. Between the lower and upper interferometric films  1   a  and  1   b , support structures  8  are interposed in a peripheral region outside an optical path, and thus a gap  21  is formed between the lower and upper interferometric films  1   a  and  1   b.    
         [0021]    Lower motion control electrodes  7   a  are attached on the lower surface of the lower interferometric film  1   a , and upper motion control electrodes  7   b  are attached on the upper surface of the upper interferometric film  1   b . The lower and upper motion control electrodes  7   a  and  7   b  are disposed in the peripheral region outside the optical path so as to face each other in the direction of the optical path. The color image sensor is configured such that a potential difference caused between the lower and upper motion control electrodes  7   a  and  7   b  can change the gap distance between the lower and upper interferometric films  1   a  and  1   b . Specifically, if such a potential difference is caused between the two electrodes, an electrostatic force is applied between the two electrodes to deform the support structures  8 , and thereby moves the upper interferometric film  1   b  up and down to change the gap distance. This change in the gap distance between the lower and upper interferometric films  1   a  and  1   b  changes the wavelength of interfered light beams. In this way, the wavelength of the light beams transmitted through this color filter is made variable. 
         [0022]    As shown in  FIGS. 1A and 1C , the light-shielding metal layers  3  are formed in the interlayer insulating film  4  to prevent light beams transmitted through each adjacent microlenses  2  from interfering with one another. 
         [0023]    In addition, a distance meter  40  for measuring a distance between the lower and upper interferometric films  1   a  and  1   b  is also disposed as shown in  FIG. 1C . 
         [0024]    In the color image sensor with the above configuration, light beams incident from the respective microlenses  2  are caused to have a certain limited wavelength by the color filter  20  when the light beams is being transmitted therethrough, and then reach the photodiode regions  5 . Thereafter, the light beams are photoelectrically converted in the respective photodiode regions  5  to be detected as electrical image data signals. 
         [0025]    Then by detecting electrical image data signals as described above after the distance between the lower and upper interferometric films  1   a  and  1   b  is changed, the color image sensor can obtain a data set representing a different color. In this way, the color image sensor can obtain data sets representing the respective RGB colors after, for example, three rounds of such data detection. Finally, by overlapping these color data sets, the color image sensor can reproduce a color image. 
         [0026]    As described above, the color image sensor produces an image by imaging, that is, detecting colors of, an object with all the pixels one color by one color till all the colors desired to be detected are obtained and then by overlapping these obtained colors by using signal processing. In general, a microelectromechanical system (MEMS) is supposed to have an operation speed on the order of several ten μsec. Accordingly, the color image sensor can be used in high-speed imaging by enhancing performance of peripheral elements such as an analog-to-digital converter (ADC), photodiodes and the like. 
         [0027]    Note that this embodiment is applicable to both types of a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor. 
         [0028]    In the image sensor of this embodiment, the gap distance between the lower and upper interferometric films  1   a  and  1   b  can be continuously changed by utilizing a potential difference. This enables the image sensor to reproduce not only the RGB colors but basically all the colors. Accordingly, the image sensor of this embodiment is capable of detecting colors other than the RGB colors, and thus has improved color reproducibility characteristics. 
         [0029]    Here, though the gap distance between the lower and upper interferometric films  1   a  and  1   b  may be controlled within a predetermined range every time before a certain color is detected, an alternative operation is also possible. In the alternative operation, the color filter  20  is continuously or periodically moved during an imaging session while the distance meter  40  keeps monitoring the height of the color filter  20 , and the image sensor reads image signals when the color filter  20  has the height providing the color desired to be obtained. This operational mode eliminates the need to precisely control the movement of the color filter  20 , and the precision of the color to be obtained in this operational mode depends on the precision of the height detection instead. Accordingly, employment of this operational mode can simplify the MEMS structure of this image sensor even more. 
         [0030]    Next, with reference to  FIGS. 1D and 2A  to  2 F, description will be given of a manufacturing process of the color image sensor of this first embodiment.  FIG. 1D  is a schematic cross-sectional view showing a finished form of the manufacturing process of the color image sensor of this first embodiment.  FIGS. 2A to 2F  are schematic cross-sectional views showing the manufacturing process of the color image sensor.  FIG. 1D  is basically the same as  FIG. 1C  except that the upper motion control electrodes  7   b  and the support structures  8  in  FIG. 1C  are integrally formed in  FIG. 1D . 
         [0031]    Firstly, as shown in  FIG. 2A , multiple photodiode regions  5 , each of which corresponds to a pixel, are arranged in horizontal and vertical lines on a substrate  6 , and then an interlayer insulating film  4  and light-shielding metal layers  3  are formed thereon. Specifically, the light-shielding metal layers  3  are formed in the interlayer insulating film  4 . Thereafter, a lower interferometric film la is formed on the interlayer insulating film  4 . The steps so far are no different than in conventional techniques. Then, as shown in  FIG. 2B , lower motion control electrodes  7   a  are formed on the lower interferometric film  1   a . Then, as shown in  FIG. 2C , a sacrificial layer  30  is deposited on the lower interferometric film  1   a  and the lower motion control electrodes  7   a  and thereafter patterned. 
         [0032]    Then, as shown in  FIG. 2D , an upper interferometric film  1   b  is deposited on the sacrificial layer  30  and thereafter patterned. Subsequently, as shown in  FIG. 2E , upper motion control electrodes  7   b  and a metal layer including support structures  8  are deposited on the upper interferometric film  1   b  and thereafter patterned. Then, as shown in  FIG. 2F , microlenses  2  are formed on the upper interferometric film  1   b . Lastly, the sacrificial layer  30  is removed off, and, as a result, the form shown in  FIG. 1D  is obtained. 
         [0033]    Note that as the modification of the above procedure, the upper interferometric film  1   b  may be formed after the upper motion control electrodes  7   b  is formed. Still alternatively, the lower motion control electrodes  7   a  may be formed concurrently with the interlayer insulating film  4  including the wiring layers to position under the lower interferometric film  1   a.    
         [0034]    According to this first embodiment, each pixel can detect multiple colors at different timings, respectively. This enables the image sensor to obtain fine image data without increasing the number of pixels therein. 
         [0035]    Moreover, since the color image sensor of this embodiment has a structure in which the distance between the interferometric films can be continuously changed, each pixel therein is capable of detecting any wavelength. This enables the image sensor to detect not only the RGB colors but also the other colors, and thus to have improved color reproducibility characteristics. Furthermore, the capability of each pixel of detecting all the colors necessary to reproduce an image not only eliminates the need for conventionally-required color correction of the pixel but also allows the color image sensor to obtain an increased number of signals and thus to have improved image reproducibility characteristics. In addition, since the interferometric films, the support structures and the like can be implemented with reliable materials, the reliability of the color image sensor is not decreased unlike the problematic case in which a color image sensor with a conventional color filter formed of an organic film has a seriously decreased reliability. 
         [0036]    Moreover, this structure can be implemented using the interferometric films  1   a  and  1   b  each formed of a stable film made of a material such as amorphous silicon, and the support structures  8  can be formed of insulating films. Accordingly, an appropriate selection of materials for these films can provide an even higher reliability with the color image sensor. 
         [0037]    Especially, the MEMS color filter of this embodiment integrally formed for all the pixels has a simple structure since the motion control electrodes  7   a  and  7   b  and the support structures  8  need not be formed in the pixels, and thus can be manufactured by a simple process. 
       Second Embodiment 
       [0038]    Secondly, with reference to  FIGS. 3A to 4B , description will be given of a second embodiment of a color image sensor according to the present invention. Note, however, that the same or similar components as in the first embodiment are denoted by the same reference numerals and the redundant description thereof will be omitted.  FIG. 3A  is a schematic cross-sectional view showing the color image sensor of the second embodiment.  FIG. 3B  is a schematic plan view of the color image sensor shown in  FIG. 3A .  FIG. 3C  is a schematic plan view showing a part, corresponding to one pixel, of the color image sensor shown in  FIG. 3B .  FIG. 4A  is a cross-sectional view as viewed from the direction of the arrow IVa-IVa of  FIG. 3C .  FIG. 4B  is a cross-sectional view as viewed from the direction of the arrow IVb-IVb of  FIG. 3C . 
         [0039]    In this embodiment, the upper interferometric film  1   b  is separated into portions corresponding to the respective pixels. In addition, the MEMS movement units, that is, the lower motion control electrode  7   a , the upper motion control electrodes  7   b  and the support structures  8 , are provided for each pixel so as to individually change the gap distance between the lower and upper interferometric films  1   a  and  1   b  in the pixel. Here, in the pixel, the pair of the lower motion control electrodes  7   a , the pair of the upper motion control electrodes  7   b  and the pair of the support structures  8  are each diagonally disposed with respect to the microlens  2 , so as not to obstruct a gapless arrangement of the microlenses  2 . With this arrangement, the microlenses  2  can be arranged in an array with no gap between one another. 
         [0040]    The MEMS color filter of this embodiment is implemented to have a structure allowing an MEMS operation on the one-pixel basis. Thus, if an object image includes a part showing a significant color variation or a part containing similar colors, the color image sensor of this embodiment can detect, from the part, colors critical for image reproduction on the one-pixel basis after detecting basic colors such as the RGB colors from the entire object image. Here, the color image sensor calculates such colors critical for image reproduction by using signal processing. For example, the color image sensor can minutely detect colors around the target wavelengths from the part containing similar colors and can additionally detect colors only in the part showing a significant color variation. 
         [0041]    Embodiments of the invention have been described with reference to the examples. However, the invention is not limited thereto. 
         [0042]    Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and example embodiments be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following.