Abstract:
An optical filter structure for an imager which has customized sub-wavelength elements used to maintain the filter characteristics accordingly across a photo-conversion device to optimize the structure for the angle of incidence as it changes across the imager photo-conversion device. In particular, the layout (e.g., grating period among other parameters) of the sub-wavelength elements used in the structure design are customized to change with the angle of incidence of the optics used to project the image. The sub-wavelength elements are typically built by high resolution lithographic processes such as optical or imprint lithography.

Description:
FIELD OF THE INVENTION 
     Embodiments of the invention relate generally to solid state imaging devices and more particularly to a method and apparatus that optically improve filter characteristics in a solid state image sensor. 
     BACKGROUND OF THE INVENTION 
     Imaging devices, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) sensors, and others, have commonly been used in photo-imaging applications. A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including a photosensor, for example, a photogate, photoconductor or a photodiode for accumulating photo-generated charge in the specified portion of the substrate. Each pixel cell has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some imager circuits, each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference. 
     In a CMOS imager, the active elements of a pixel cell perform the functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and amplification of signals representing pixel reset level and pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor. 
     CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,140,630, 6,177,333, 6,204,524, 6,310,366, 6,326,652, and 6,333,205, assigned to Micron Technology, Inc., the contents of which are incorporated herein by reference. 
     CMOS, CCD and other solid state imagers may use an optical filter to transmit image light to a solid state sensor having an array of pixels. Optical filters employing grating elements can be used as a global light wavelength filter over the top of an imager to filter light across an entire array, for example, as an infra-red (IR) light block. In addition, optical filters having grating elements are now being fabricated as nano-structure-based optics or nano-optics, which are a class of optical devices that allow optical devices to be realized that are thin, offer high performance and are highly reliable. See Kostal et al.,  MEMS Meets Nano - Optics , F IBER  O PTIC  T ECHNOLOGY , November 2005, pp. 8-13; see also  NanoOpto Introduces New Nano - optic Bandpass Filter Designed for High Volume, High Performance Digital Imaging Applications , Press Release, NanoOpto Corporation, Jan. 31, 2005. 
     One drawback with an optical filter is that as the angle of incidence of incoming light gets sharper, i.e., begins angling away from the light path which is normal to the grating, the filter fails to block undesired wavelengths and/or fails to allow the desired light wavelengths to pass through. As a result, the optical filter is less efficient in its filtering characteristics, which can result in image color shading and artifact formation. 
     Accordingly, there is a need and desire for an apparatus and method for improving an optical filter employing grating elements for use in imaging devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a typical optical filter. 
         FIG. 2  is another illustration of a typical optical filter. 
         FIG. 3  is an illustration of an optical filter in accordance with an embodiment described herein. 
         FIG. 4  is another illustration of an optical filter in accordance with an embodiment described herein. 
         FIG. 5  shows a cross sectional view of an imager module constructed in accordance with an embodiment described herein. 
         FIG. 6  is an illustration of an optical filter in accordance with another embodiment described herein. 
         FIG. 7  shows a CMOS image sensor that can be implemented in conjunction with the optical filter constructed in accordance with an embodiment described herein. 
         FIG. 8  shows a processor system incorporating at least one imager and an optical filter constructed in accordance with an embodiment described herein. 
     
    
    
     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 they 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 embodiments, 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 them. 
       FIG. 1  illustrates a typical optical filter  120  employing grating elements  150 . A complete filter will also include integrating fill material  180  and index matching, or anti-reflection, coatings  190  on either side of the elements of grating structure  150 , as shown in  FIG. 2 . 
     Referring to  FIGS. 1 and 2 , a typical optical filter used as a global filter  120  for a solid state image sensor  110  comprises grating elements  150  that are constructed above an image sensor  110 , which may represent an entire sensor array in the case of a global filter. It should be appreciated that an actual image sensor will comprise several fabricated layers between a photosensor level and an upper micro-lens level. For example, film layers  112 ,  114  and  116  can be formed. A global filter  120  may be provided above the micro-lens level. In a typical optical filter, the distance between the outside edges of the grating elements  150  that filter the incoming light, i.e., the grating period, is equal across the entire filter above the image sensor  110 . For example, for devices designed as a global filter for operation in the near IR range, with undesired wavelengths from roughly 600 nm to 3 μm, the distance between the outside edge of one structural grating element to the outside edge of the next structural grating element (i.e., the grating period) is on the order of several hundred nanometers or less. In other words, the grating period between the grating elements is sufficiently less than the wavelength of the light that the optical filter intends to block that these wavelengths of light are reflected and prevented from passing through the filter. 
     However, in the typical optical filter employed as global filter  120 , the characteristics of the light being passed are affected by the angle of incidence of the light entering the filter. For example, as the angle of incidence gets sharper, i.e., begins angling away from a path which is normal to the filter  120  and image sensor  110 , the global filter  120  fails to block undesired wavelengths of light and thus, allows light at these wavelengths to pass through, causing the characteristics of the light reaching the photosensors in image sensor  110  to be different than expected. Due to the angle of incidence, the grating period becomes skewed, making the grating elements of the global filter no longer entirely effective in blocking undesired light from reaching image sensor  110 . 
       FIGS. 3 and 4  show embodiments of the invention which address this problem. The embodiments provide a method and apparatus of inversely changing the affected light characteristics by altering the uniform grating period between grating elements of the optical filter. The typical optical filter, shown in  FIGS. 1 and 2 , will change its transmitted wavelength spectrum as the incident angle changes, while optical filter  220  having a variable (non-uniform) grating period, as shown in  FIGS. 3 and 4 , will have less of such a change. Optical filter  220  will improve the properties of a combined layer stack and grating structure as shown in  FIGS. 1 and 2 . It should be appreciated that the embodiments, for example, can be implemented and applied in the construction of any type of global color filter, that is, a filter designed to be a light block and/or a pass band filter across an entire array. 
     In one embodiment of the invention, the grating period of the optical filter  220  non-uniformly changes, i.e., increases, across the surface of the optical filter  220  and thus over an image sensor  210  containing an array of pixels. The grating elements are formed at non-uniform grating periods across the optical filter and as a result, the angular shift in transmitted light can be partly compensated for. The grating period of the optical filter  220  is based on the wavelength of light desired to be passed through the optical filter  220 . The grating period corresponds to the distance from the outside edge of one grating element to the outside edge of the next grating element along the surface of the optical filter  220 . It should be appreciated that other grating parameters can be modified as well to influence the transmission wavelengths such as the grating material and the shape, thickness and duty cycle of the grating structures. 
     Similar to  FIGS. 1 and 2 , between optical filter  220  and image sensor  210 , film layers  212 ,  214  and  216  can be formed. These additional film layers  212 ,  214 ,  216  further define and/or filter the reflected or transmitted wavelengths of the optical filter. Although  FIGS. 3 and 4  illustrate one optical filter and a specific number of film layers, the specific number of film layers and optical filters can be varied, and their arrangement (e.g., two gratings sandwiching film layers, a grating buried in a film stack, etc.) can be adjusted to meet design objectives. 
     The grating period of optical filter  220  is a certain distance A in the center of the optical filter  220  corresponding to a first desired wavelength to be passed through the optical filter  220 . Then, the grating period increases progressively as the grating period approaches the edge of the optical filter. In other words, the grating period will become greater, i.e., the distance between the outside edges of the elements will become farther, as the grating elements  250  reach an edge of the optical filter  220 . For example, the resulting grating elements  250  of the optical filter  220  along the outer-most grating elements, on the edges, will have a greater spaced grating period, for example grating period C. The grating elements  250  will progress to the desired wavelength grating period at the center-most grating elements, for example grating period A. In between the outer-most grating period C and the inner-most grating period A, the grating periods, e.g., grating period B, will be less than C, but greater than A. It should be appreciated that although the grating period is described in relation to three grating periods A, B, C, the embodiments should not be limited to only three possible grating periods, but that the grating periods are an unlimited gradual progression from the outer-most grating periods to the inner-most grating periods, e.g., grating periods C to A. 
     Widening the outside grating periods of the optical filter compensates for the change in light characteristics exhibited by the angling of incident light. Thus, the embodiments allow a global filter, e.g., IR block, to have a reduced amount of the undesired change of cutoff wavelength of the transmitted light, e.g., IR light, passing through the optical filter  220  and reaching the respective image sensor  210 . The resonant wavelength of an optical filter is predominantly determined by the grating period. 
     Similar to the typical optical filter, the optical filter of the embodiments include integrating fill material  280  and index matching, or anti-reflection, coatings  290  on either side of the grating element  250 . It should be appreciated that the materials used in constructing the optical filter grating elements can include silicon nitride (SiNx), silicon dioxide (SiO 2 ), aluminum (Al), gold (Au) and metals such as silver (Ag) can be used in various combinations. The same materials can likewise be used to form the coatings on either side of the grating elements. It should also be appreciated that the use of several material and structure combinations can yield the same optical function, while allowing optical filters to be readily designed with reliability, and environmental and usage objectives in mind. Therefore, the mentioned list and their resulting combinations should not be considered an exhaustive list. 
     The general design steps required to create a single layer optical filter include forming the filter having grating elements formed at non-uniform grating periods across the filter with the grating period becoming wider the farther they are from the center of the filter. The grating period may change continuously from the center of the filter to the edge of the filter, or may change in stepped increments. Materials can be selected which have minimal absorption in the wavelength range of interest and selecting a substrate that is optically transparent and has a coefficient of thermal expansion that effectively matches that of the optical filter  220  across the range of operating and storage temperatures. For example, when IR light is the desired light to be blocked by a global filter constructed in accordance with optical filter  220 , the filter is formed of aluminum with grating elements having a grating period of approximately 650 nm at the center of the filter and approximately 720 nm at the edge of the filter. 
     In an embodiment of the invention, the change in grating periods can be calculated as follows:
 
 g=g (0)/cos(alpha)
 
where g is the local grating constant (g changes across the filter as a function of the angle of incident light), g(o) is a grating constant at the vertical incidence of light (i.e., center of the filter), and alpha is the angle of incidence of the light wavelength (where 0 degrees=vertical). The result will be an increased separation between grating lines (the outside edges of the grating elements) gradually across the filter while keeping the center-most grating period constant. Due to the symmetry requirements on an imager, a circular shape for the grating lines is the preferred embodiment, where the grating period between the circles gradually increase as described above. Circles exceeding the outer-most grating elements could be cut-off resulting in a rectangular grating pattern that matches the image sensor size. It should be appreciated that the optical filter  220  should be aligned with the image sensor  210  so that the optical center of the image sensor  210  lines up with the center of the optical filter  220 .
 
       FIG. 5  shows a cross-sectional view of an imager module constructed in accordance with an embodiment of the invention. The imager module comprises a pixel array implemented in a substrate (e.g., a silicon substrate)  510  and a layer of protective silicon oxide  520 , which may be also serve as a support for metal interconnects. The imager module also includes a color filter array  560  (e.g., a Bayer CFA) to allow only light of a specific wavelength to pass to each pixel within the active pixel array of substrate  510 , a layer of micro-lenses  530  that concentrate the incident light to the sensitive area of the underlying pixel, and a main lens  540  that focuses a light ray  550  from the object onto the micro-lenses  530 . The imager module further includes an optical filter  580  above the micro-lenses  530 . The optical filter  580  is formed between lens  540  and micro-lenses  530 . It should also be appreciated that the optical filter  580  can be implemented as part of the main lens system. For example, the optical filter  580  can be integrated into lens  540 , e.g., a lens wafer or a lens with the optical filter on the surface of the lens with a molded lens on top of the filter, as shown in  FIG. 6 . 
     The embodiments described herein provide an improved optical filter that reduces the change of cutoff wavelength of the optical filter with the angle of light incidence. Additionally, embodiments described herein exhibit a number of advantages including eliminating the shift of the transition wavelength of the filter with incident angle of light; reducing or eliminating the formation of unwanted transmission peaks at wavelengths intended to be blocked; and reducing or eliminating the change in transmission at wavelengths intended to pass through the filter. 
       FIG. 7  illustrates an imager  1100  that may utilize any embodiment described above. The imager  1100  has a pixel array  1105  having pixels working in conjunction with the optical filter (i.e.,  FIG. 3 ) constructed in accordance with an embodiment. Row lines are selectively activated by a row driver  1110  in response to row address decoder  1120 . A column driver  1160  and column address decoder  1170  are also included in the imager  1100 . The imager  1100  is operated by the timing and control circuit  1150 , which controls the address decoders  1120 ,  1170 . 
     A sample and hold circuit  1161  associated with the column driver  1160  reads a pixel reset signal Vrst and a pixel image signal Vsig for selected pixels. A differential signal (Vrst−Vsig) is amplified by differential amplifier  1162  for each pixel and is digitized by analog-to-digital converter  1175  (ADC). The analog-to-digital converter  1175  supplies the digitized pixel signals to an image processor  1180 , which forms a digital image. 
       FIG. 8  shows a system  1200 , a typical processor system modified to include an imaging device  1210  (such as the imaging device  1100  illustrated in  FIG. 7 ) and an optical filter (i.e.,  FIG. 3 ) of an embodiment described above. The processor system  1200  is an example of a system having digital circuits that could include image sensor devices. Without being limiting, such a system could include a computer system, digital camera system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, and other systems employing an imager. 
     System  1200 , for example a digital camera system, generally comprises a central processing unit (CPU)  1220 , such as a microprocessor, that communicates with an input/output (I/O) device  1270  over a bus  1280 . Imaging device  1210  also communicates with the CPU  1220  over the bus  1280 . The processor-based system  1200  also includes random access memory (RAM)  1290 , and can include removable memory  1230 , such as flash memory, which also communicate with the CPU  1220  over the bus  1280 . The imaging device  1210  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. 
     It should be appreciated that there are likely many alternatives for materials that may be suitably employed to provide the optical filter for integrated image sensors including metals, polymers, semiconductors, and dielectrics. If the requirements of the optical filter cannot be met with a single material than a combination of materials can be used. It should also be appreciated that there are several varying grating parameters that can be derived such as e.g., increasing the period of the grating to increase the resonant wavelength; increasing the duty cycle will result in a change in the resonant wavelength and/or a variation in the full width at half maximum; changing the incidence angle to change the resonant wavelength; and varying the index of the grating structure or of the filter to change the resonant wavelength. 
     The processes and devices described above illustrate preferred methods and typical devices of many that could be used and produced. The above description and drawings illustrate embodiments, which achieve the described objects, features, and advantages. However, it is not intended that the embodiments be strictly limited to the above-described and illustrated embodiments. Any modification, though presently unforeseeable, of the embodiments that comes within the following claims should be considered part of the invention.