Abstract:
An imaging system has an imager comprising a plurality of jots. A readout circuit is in electrical communication with the imager. The readout circuit can be configured to facilitate the formation of an image by defining neighborhoods of the jots, wherein a local density of exposed jots within a neighborhood is used to generate a digital value for a pixel of the image.

Description:
PRIORITY CLAIM 
     This patent application claims the benefit of the priority date of U.S. provisional patent application Ser. No. 60/685,216, filed on May 27, 2005 and entitled SUB-DIFFRACTION-LIMIT (SDL) PIXELS. The entire contents of this provisional patent application are hereby expressly incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to solid state imaging. The present invention relates more particularly to the use of sub-diffraction-limit (SDL) pixels for imaging, such as to emulate the contemporary silver halide emulsion film process. 
     BACKGROUND 
     The relentless drive to reduce feature size in microelectronics has continued now for several decades and feature sizes have shrunk in accordance with the prediction of Gordon Moore in 1975. While the repeal of Moore&#39;s Law has been anticipated for some time, we can still confidently predict that feature sizes will continue to shrink in the near future. 
     Using the shrinking-feature-size argument, it became clear in the early 1990&#39;s that it would be possible to put more than 3 or 4 charge coupled device (CCD) electrodes in a single pixel. Thus, the complimentary metal oxide (CMOS) active pixel sensor concept was born. The effective transistor count in most CMOS image sensors has hovered in the 3-4 transistor range, and if anything, is being reduced as pixel size is shrunk by shared readout techniques. 
     The underlying reason for pixel shrinkage is to keep sensor and optics costs as low as possible as pixel resolution grows. Camera size has recently become highly important in the rapidly expanding camera-phone marketplace. Concomitant with the miniaturization of camera components such as sensors and optics is the miniaturization of optical system components such as actuators for auto-focus and zoom in megapixel camera phones. 
     We are now in an interesting phase in the development of image sensors—for both CCDs and CMOS active pixel sensors. The physical dimensions of the pixel are becoming smaller than the diffraction limit of light at the wavelengths of interest. A perfect lens can only focus a point of light to a diffraction-limited spot, known as an Airy disk and the Airy disk is surrounded by higher order diffraction rings. The Airy disk diameter D A  is given by the equation:
 
 D   A =2.44 λF# 
 
Where λ is the wavelength and F# is the F-number of the optical system.
 
     For example, at 550 nm, and F-number of 2.8, the Airy disk diameter is 3.7 μm. Yet, pixel sizes in megapixel image sensors are at this size and smaller today. We refer to pixel sizes smaller than the 550 nm Airy disk diameter as sub-diffraction-limit (SDL) pixels. 
     Today it is readily possible to build a 6-T SRAM cell in less than 0.7 μm 2  using 65 nm CMOS technology. Smaller device are being prototyped. However, significant issues exist for a 0.25 μm 2  pixel, even though it might be tempting to make a 2 megapixel sensor with 1 mm diagonal using such small pixels. For example, the resolution of the sensor would be well beyond the diffraction limit. In fact, over 40 pixels would fit inside the Airy disk and well over a billion pixels (one gigapixel) can fit on a single chip. Although it is possible to construct such an imager, contemporary imagers do not take advantage of this possibility. Further, contemporary practice does not contemplate the best ways to implement such a device. 
     In view of the foregoing, it is beneficial to provide implementations and practical applications for such SDL pixels. More particularly, it is desirable to provide a method for providing a digital film sensor that emulates, at least to some degree, contemporary silver halide film. 
     BRIEF SUMMARY 
     Systems and methods are disclosed herein to provide a digital film sensor (DFS), such as a gigapixel DFS. In this manner, the contemporary silver halide process can, at least to some degree, be emulated. 
     For example, in accordance with an embodiment of the present invention, an imaging system can comprise an imager having a plurality of sub-diffraction-limit pixels, referred to herein as jots. The imaging system can also comprise a readout circuit that is in electrical communication with the imager. The readout circuit can be configured to form an image by defining neighborhoods of the jots. A local density of exposed jots within a neighborhood can be used to generate a digital value for a single pixel of the image. That is, the digital value can depend upon how many jots within a neighborhood have been exposed (registered a hit by a photon). 
     A neighborhood can comprise either a single jot or a plurality of jots. Every neighborhood does not necessarily comprise the same number jots. A neighborhood can comprise any desired number of jots. For example, some neighborhoods can comprise one jot, other neighborhoods can comprise two jots, yet other neighborhoods can comprise three jots, and yet other neighborhoods can comprise more than three jots. 
     A neighborhood can comprise a plurality of jots from a single exposure (a single frame). Alternatively, a neighborhood can comprise a plurality of jots from a plurality of exposures. The number of jots used to define a neighborhood is variable. A jot may belong to one neighborhood for one exposure and to a different neighborhood for another exposure. 
     The number of jots used to define a neighborhood can be variable according to a region growing process. The number of jots used to define a neighborhood can be variable according to either a spatial or a temporal region growing process. The number of jots used to define a neighborhood can be variable according to both a spatial and a temporal region growing process. 
     The region growing process can be a process whereby the size of the neighborhood is determined dynamically. That is, various sizes of neighborhoods are tried and the size providing the best results is used to form an image. For example, the size of each neighborhood can be increased until a resolution maximum is defined and the neighborhood size that provides the resolution maximum can be used to form the image. An image can be divided into any desired number of sub-images and the region growing process can be performed independently for each sub-image. 
     The neighborhoods can have different sizes as a result of this region growing process. For example, neighborhoods comprised of single jots may provide maximum resolution in one portion of an image, while neighborhoods comprised of four jots may provide maximum resolution in another part of an image. 
     Each jot can be sensitive to a single photon of light. Alternatively, each jot can require more than one photon to register a hit. The size of the jots within an imager does not have to be uniform. Each jot can be read out as a logical “1” or a logical “0”. 
     The imager can comprise color filters covering a plurality of the jots so as to facilitate color imaging therewith. Different colored filters can cover individual jots or groups of jots in the fashion of the Bayer filters that are used in contemporary color imaging sensors. 
     A digital processor can be configured to facilitate the generation of the image. Optionally, the digital processor can be integrated on the same chip as the jots. The processor can be configured to use an algorithm to form an image from a jot pattern. The algorithm can be dynamically varied so as to trade spatial resolution for light sensitivity. Thus, when less light is available the number of jots in a neighborhood can be increased. The processor can be configured to use a single or a plurality of readouts of the jots to form a single image. 
     A jot may comprise an integrating silicon photodetector, a high gain amplifier, a reset circuit, and a selection switch for reading out the jot. The imager can be made using a CMOS-compatible process. Each of the jots can have a total area less than 1 square micron. 
     A method for digital imaging can comprise setting a grain size digitally. The grain size can be set so as to be the smallest grain size that provides a picture having acceptable quality as measured using a parameter other than grain size. For example, the parameter can be intensity resolution. 
     The method can comprise digitally developing an image. The method can comprise using a plurality of jots in a manner that provides a desired balance between intensity resolution and spatial resolution. The method can comprise setting a grain size of an imager so as to provide a desired effective International Standards Organization (ISO) speed/resolution. The method can comprise selecting a grain size after exposure so as to enhance image quality. The method can comprise performing a region growing image processing function. The method can comprise determining a jot count of a grain based upon a light level. The method can comprise reading a plurality of jots more than once per exposure. Different jots can be read during each reading thereof. 
     The method can comprise adding exposures such that grain construct is both spatial and temporal. The method can comprise using different mapping on consecutive readouts so as to dither grain position. The method can comprise varying a grain size during a plurality of readouts of an exposure. The method can comprise mapping light density of exposed jots to define intensity. Light density of exposed jots with overlapping neighborhoods can be mapped to define intensity. Light density of exposed jots with non-overlapping neighborhoods can be mapped to define intensity. The method can comprise mapping jots to define a pixel image. 
     This invention will be more fully understood in conjunction with the following detailed description taken together with the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the relationship between the size of an exemplary Airy disk and exemplary SDL pixels; 
         FIG. 2  is an exemplary chart showing the density of exposed grains versus exposure for film and digital sensors; 
         FIG. 3  is a semi-schematic diagram showing an exemplary array of jots according to an embodiment of the present invention, wherein a plurality of the jots have registered photon hits; 
         FIG. 4  is a semi-schematic diagram showing the array of  FIG. 3 , wherein the jots have been digitally developed using 4×4 neighborhoods according to an embodiment of the present invention; 
         FIG. 5  is a semi-schematic diagram showing the array of  FIG. 3 , wherein the jots have been digitally developed using 3×3 neighborhoods according to an embodiment of the present invention; and 
         FIG. 6  is a semi-schematic diagram showing an imaging system comprising an SDL imager and a readout circuit/processor according to an embodiment of the present invention. 
     
    
    
     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , the relationship between the size of an exemplary Airy disk  11 , e.g., an Airy disk for light having a wavelength of 550 nm, and exemplary sub-diffraction-limit (SDL) pixels  12  is shown graphically. Airy disk  11  has a diameter of 3.7 μm. SDL pixels  12  are square and are 0.5 μm on a side. Of course, the actual size of an Airy disk depends upon the wavelength of light being used to form the Airy disk and the size of the SDL pixels can be larger or smaller than 0.5 μm. As can be seen the, Airy disk  11  is substantially larger than each individual SDL pixel  12  and a plurality of SDL pixels  12  can thus fit within Airy disk  11 . 
     According to an embodiment of the present invention, sub-diffraction-limit (SDL) pixels can be used in a new solid-state imaging paradigm. More particularly, SDL pixels can be used in a digital imaging emulation of the well known silver halide emulsion film process. The SDL pixels can be used in a binary mode to create a gigapixel digital film sensor (DFS). 
     According to an embodiment of the present invention, oversampling of the SDL pixels can be performed. For example, the optical resolution of an image can be highly oversampled. For SDL pixels, such oversampling can mitigate color aliasing problems, such as those that occur due to the use of color filter arrays. Further, a diffraction effect can be used to eliminate the need for anti-aliasing optical filters. For deep-SDL pixels (those SDL pixels having a diameter substantially less than one micron), improved resolution of the optical image can be achieved using digital signal processing. 
     According to an embodiment of the present invention, SDL pixels are used in the emulation of film. In film, silver halide (AgX) crystals form grains in the sub-micron to the several micron size range. A single photon striking the grain can result in the liberation of a single silver atom. This grain is effectively tagged as exposed and constitutes a latent image. In the subsequent wet chemical development process, the one silver atom results in a runaway feedback process that chemically liberates all the silver atoms in the exposed grain. This liberation of silver atoms leaves an opaque spot in the film, where the silver halide has been converted to metallic silver. Unexposed grains are washed away. The image intensity is thus proportional to a local density of silver grains. 
     Referring now to  FIG. 2 , a chart shows the density of exposed grains versus the log exposure thereof for a typical silver halide emulsion film. As indicted by the chart, the probability that any particular grain is exposed under illumination grows linearly at first, but only eventually approaches unity. This process gives rise to film&#39;s particular D-log H contrast curve, where D is density and H is light exposure. The smaller the grain size, the lower the probability that the grain will be struck by a photon in a given exposure, and the slower the film speed will be since more light is required to ensure a high probability that all grains are struck by photons. However, the spatial resolution of the image is determined by grain size, with smaller grain sizes and slower film having higher image resolution. 
     In a developed film image, the grains are binary-like since they are either exposed or not exposed. The local image intensity is determined by the density of exposed grains, or in digital parlance, by the local spatial density of logical 1&#39;s. 
     According to an embodiment of the present invention, the concept of binary-like development of images in silver halide emulsion film is emulated to provide a digital-film sensor (DFS). For example, an embodiment of the present invention can comprise an array of deep-SDL pixels. With sufficiently high conversion gain and sufficiently low readout noise, the presence of a single photoelectron can be determined. 
     In practice, several photoelectrons can contribute to pushing the output signal above some threshold. However, either single photon or multiple photon sensitivity can be used. From the discussion above, it is evident that a pixel that only needs to detect a single photoelectron has much lower performance requirements for full-well capacity and dynamic range than an analog pixel in a conventional image sensor. 
     According to one or more embodiments of the present invention, the implementation of a jot can be accomplished is any of several ways. A brute force approach can be to make a conventional active pixel with very high conversion gain (low capacitance). Other approaches include using avalanche or impact ionization effects to achieve in-pixel gain, as well as the possible application of quantum dots and other nanoelectronics devices to define the jots. Stacked structures are also possible, especially since performance requirements are reduced. Of course, it is generally desirable to minimize dark current. 
     At the start of the exposure period, the jot can be reset to a logical ‘0’. If the jot is subsequently hit by a photon during an exposure, then the jot is set to a logical ‘1’, either immediately or upon readout. This can be accomplished in a fashion analogous to that performed with memory chips that have been used as image sensors. Due to the single-bit nature of the analog-to-digital conversion resolution, high row-readout rates can be achieved, thus facilitating scanning of a gigapixel sensor having approximately 50,000 rows in milliseconds and thereby enabling multiple readouts per exposure or frame. 
     The read out binary image can be digitally developed to provide a conventional image having somewhat arbitrary pixel resolution. Such development can be accomplished using a two step process. According to this two step process, image intensity resolution can be traded for spatial resolution. 
     Referring now to  FIG. 3 , a representative portion of an exemplary digital film sensor (DFS) can comprise a plurality of jots  32  arranged in an array  31  according to an embodiment of the present invention. Expose jots  33  are indicated as being black. Either one photon or a plurality of photon may be required to expose a jot. 
     Referring now to  FIG. 4 , a neighborhood  41  can be defined herein as being comprised of a group of jots. Each neighborhood of  FIG. 4  is a 4×4 array of jots. Thus, each 4×4 array of  FIG. 4  defines one neighborhood  41 . Alternatively, a neighborhood can comprise any other number, e.g., 2, 3, 5, 20, 100, of jots. Indeed, a neighborhood can even comprise a single jot, if desired. 
     Each neighborhood is at least somewhat analogous to a grain of contemporary silver halide film. The terms neighborhood and grain can thus generally be used interchangeably herein. 
     According to one embodiment of the present invention, if any jot in a grain or neighborhood  41  has been hit by a photon and is a logical ‘1’, the neighborhood is considered exposed and all jots in the neighborhood are set to ‘1’. The digital development process allows the flexibility of setting a grain or neighborhood size during readout to adjust the effective speed, e.g. International Standards Organization (ISO) speed of the DFS. 
     Referring now to  FIG. 5 , according to an embodiment of the present invention a region-growing approach can be used for digital development. Different sizes of neighborhoods can be tried during an exposure. Alternatively, the size of a neighborhood  41  can be selected to optimize image quality after the exposure. 
     Thus, the first step of digital development can be performed as a region-growing image processing function. In any case, the development can be accomplished in a jot area-amplification fashion. This first step of digital development can be used in very high jot-count image sensors under low light conditions and corresponds to large-grain film emulsions for very high film speed. 
     Unlike film where the grain boundaries are fixed during an exposure, it is possible to provide an imaging process where the jots are read out several times during a single exposure. The exposures can be added (logically ‘OR’d) together so that the grain construct is both spatial and temporal. 
     The neighborhood mapping function can be different for each readout. That is, the number and/or location of jots in each neighborhood can be different for each readout. The use of different neighborhood mapping functions for each readout is somewhat analogous to dithering the grain position in a film emulsion during exposure, and perhaps even varying the grain size during the exposure. 
     In the second step of digital development, the grains which form a binary image can be converted to a conventional digital image that contains pixels with intensity values between 0 and 255, for example. In this case, a local density of exposed grains can be mapped into a pixel image. The more exposed grains in a neighborhood, the higher the pixel value. 
     Neighborhoods can overlap or can be distinct. If they overlap, this second step is like a blurring convolution process followed by subsampling. At high magnification, a conventional film image appears to be binary due to the presence or absence of silver grains. But, at the lower magnifications used for digitizing film, the same image appears as a continuous gray tone that can be digitized into an array of pixels. 
     According to an embodiment of the present invention, digital color imaging can be performed in a manner analogous to the procedure used in contemporary color image sensors. Jots can be covered with color filters. Red (R), green (G), and blue (B) jots can be treated separately and later the digitally-developed images combined to form a conventional RGB image. R, G, and B jots need not appear at the same spatial frequency, and since the deep-SDL nature of the jot pitch results in blurring from diffraction effects, color aliasing is not an issue. 
     Like film, we expect such a jot-based DFS to exhibit D-log H exposure characteristics. This is true because the physics and mathematics of jot exposure are nominally very similar to those of film. The dynamic range can be large and the exposure characteristics more appealing for photographic purposes. 
     The DFS imaging may be superior to contemporary imaging techniques. One or more embodiments of the present invention provide for the use of deep-SDL pixels and introduce a paradigm shift with respect to contemporary solid-state image sensors. Pixel sizes can be measured in nanometers, conversion gain becomes extremely large, charge-handling capacity can be minute, and pixel resolution can be increased by orders of magnitude. 
     Referring now to  FIG. 6 , an SDL imaging system is shown. According to an embodiment of the present invention, light from a subject passes through optics  61  and is incident upon SDL imager  62 . SDL imager  62  comprises a plurality of jots that can be organized into neighborhoods so as to emulate, at least to a degree, the effect of grain structure in contemporary silver halide film. 
     Such organization of the jots into neighborhoods can be performed by readout circuit and processor  63 , as discussed in detail above. Information from readout circuit and processor  63  can be provided to a memory for storage, to another processor for further processing (color balance, compression, etc.) and/or to a display. 
     One or more embodiments of the present invention provide applications for SDL pixels. More particularly, one or more embodiments of the present invention provide a method for providing a digital film sensor that emulates, at least to some degree, contemporary silver halide film. 
     Embodiments described above illustrate, but do not limit, the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.