This invention relates to solid-state image sensors. In particular, the invention relates to a system and method for diffracting radiation in a solid-state image sensor so as to increase the intensity of the incident radiation.

2. Related Art.

Solid-state image sensors have broad applications in many areas including commercial, consumer, industrial, medical, defense and scientific fields. Solid-state image sensors convert a received image such as from an object into a signal indicative of the received image. Examples of solid-state image sensors include charge coupled devices (CCD), photodiode arrays, charge injection devices (CID), hybrid focal plane arrays and complementary metal oxide semiconductor (CMOS) imaging devices.

Solid-state image sensors are fabricated from semiconductor materials (such as silicon or gallium arsenide) and include imaging arrays of light detecting (i. e., photosensitive) elements (also known as photodetectors) interconnected to generate analog signals representative of an image illuminating the device. These imaging arrays are typically formed from rows and columns of photodetectors (such as photodiodes, photoconductors, photocapacitors or photogates), each of which generate photo-charges. The photo-charges are the result of photons striking the surface of the semiconductor material of the photodetector, which generate free

charge carriers (electron-hole pairs) in an amount linearly proportional to the incident photon radiation.

Each photodetector in the imaging array receives a portion of the light reflected from the object received at the solid-state image sensor. Each portion is <BR> <BR> called a picture element or"pixel. "Each individual pixel provides an output signal corresponding to the radiation intensity falling upon its detecting area (also known as the photosensitive or detector area) defined by the physical dimensions of the photodetector. The photo-charges from each pixel are converted to a signal (charge signal) or an electrical potential representative of the energy level reflected from a respective portion of the object. The resulting signal or potential is read and processed by video processing circuitry to create an electrical representation of the image.

The detecting area of each pixel is typically smaller than the actual physical pixel dimensions because of manufacturing process constraints, the presence of other circuitry in the pixel area (such as the active elements in CMOS imager arrays) in addition to the photodetector and the proximity of adjacent pixels. The percentage ratio of the detector area to the pixel area is typically referred to as the optical"fill factor." Typically, microlenses (also known as microlenticular arrays or lenslet arrays) <BR> <BR> increase the effective optical fill factor of a pixel by increasing (i. e. , focusing) the amount of radiation that is incident upon the detecting area. The microlens covers an area larger than the detecting area, such that the radiation, which would normally fall outside the detecting area, is refracted by the microlens to the detecting area of the

pixel. Microlenses are typically placed over every pixel in the pixel array, thus increasing the radiation intensity (i. e. , increasing the fill factor) that is incident on every pixel.

As the pixel and microlens sizes decrease, the microlens becomes less effective. Typically, microlenses under 5, um, the optical performance of the microlens degrades and radiation incident upon the microlens is not refracted properly onto the detecting area of the pixel. Thus, a significant amount of the radiation intensity incident upon the pixel is not directed to the detecting area of the pixel.

Therefore, there is a need for a system and method that focuses radiation onto the detecting area of a pixel for smaller pixel sizes such that a sufficient intensity of radiation falls incident upon the detecting area of the pixel so that the radiation is detected by the pixel.

SUMMARY f A number of technical advances are achieved in the art, by implementation of an arrangement of diffraction elements to diffract radiation onto the pixels in a solid- state image sensor. The resulting diffraction pattern has a high intensity area at which a majority of the radiation intensity of the entire diffraction pattern is concentrated. In one approach this high intensity area may be positioned to fall incident upon the detecting area of the pixel. The diffraction element may utilize either an aperture or a lens within an opaque element.

This invention includes a method for detecting radiation in an array of pixels.

The method involves receiving radiation upon a diffraction element positioned over a first pixel and then diffracting that radiation onto the first pixel such that the

diffraction pattern is formed incident upon the first pixel. Among other aspects, the radius of an aperture of the diffraction element is determined in relation to a detecting area of the first pixel and perhaps the distance from the pixel to the diffraction element.

Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.

FIG. 1 is a top plan view of a solid-state image sensor with a plurality of pixels arranged in an array; FIG 2 is a block diagram of one of the individual pixels from FIG. 1; FIG. 3 is a perspective view of one example of the invention using a diffraction element having an aperture ; FIG. 4 is a perspective view of another example of the invention using a diffraction element having a lens ; FIG. 5 is a top view of one example of a Fraunhofer diffraction pattern created by either a circular aperture or lens ;

FIG. 6 is an intensity profile of the Fraunhofer diffraction pattern from FIG. 5; FIG. 7 is a perspective view of another example of a diffraction element; FIG. 8 is a top plan view of one example of a Fraunhofer diffraction pattern created by a strip lens or aperture; and FIG. 9 is a front elevational cross sectional view of an example implementation of this invention where neighboring pixels are optically separated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The solid-state image sensor of this invention includes a diffraction element that focuses the radiation onto the pixels and preferably upon their detecting areas.

This invention may be utilized in association with various types of solid-state image sensors (also referred to as"solid-state imagers,""imagers,"or"image sensors' ;), including but not limited to complementary metal oxide semiconductor (CMOS) imaging devices, charge couples devices (CCD), charge injection devices (CID), and metal oxide semiconductor (MOS) devices. As an example, the implementations of the invention are discussed in association with CMOS image sensors. However, it is appreciated by those skilled in the art that implementation may also utilize CCD, CID or MOS solid-state image sensors.

FIG. 1 shows a number of individual pixels arranged in an array 100 in order to detect radiation falling incident over a given area. Although each pixel in the array in FIG. 1 is shown as being square-shaped, each individual pixel may be fabricated with different shapes such as circular, rectangular, oval, pentagonal, or hexagonal.

The pixels may utilize various types of photodetectors such as photodiodes, photoconductors, photocapacitors, or photogates. These photodetectors generate a charge or electrical potential (generically referred to as signals) dependent upon the wavelength of the incident radiation, the intensity of the incident radiation and the device characteristics of the photodetectors. Thus, the signals generated by the photodetectors are generally indicative of the intensity of the radiation that falls incident upon the detector.

In one approach, each individual pixel may measure only one portion of the radiation spectrum that falls incident upon the pixel and converts it into a measurable signal indicative of the intensity of the incident radiation. For instance, the pixel array 100 includes, among a plurality of other pixels, a first pixel 110, a second pixel 120, and third pixel 130. Each of these first, second and third pixels may be utilized to detect a different portion of the radiation spectrum. The portions of the radiation spectrum detected by the image sensor may include visible light range, infrared range, ultraviolet range, microwave range, x-ray range, and gamma radiation range. So, where the solid-state image sensor is designed to receive visible light, the first pixel 110 may be optimized to detect red light, the second pixel 120 may be optimized to detect green light, and the third pixel 130 may be optimized to detect blue light.

FIG. 2 is a simplified block diagram of first pixel 110 from the sensor array in FIG. 1. The first pixel 110 includes a detecting area 200 and a non-detecting area 202.

The detecting area 200 is photosensitive including a photodetector such as a photodiode, a photoconductor, a photocapacitor, or a photogate. The non-detecting area 202 typically includes circuitry for converting a charge generated by the

detecting area 200 into a measurable signal and for transferring that signal to video processing circuitry. The non-detecting area 202 is not photosensitive and radiation falling on the non-detecting area 202 of the pixel 110 is typically not detected. The relative sizes and configurations of the detecting area and the non-detecting area may be altered in various solid-state image sensor designs as is well known in the art.

FIG. 3 is a perspective view of the first pixel I 10, a first filter 300 and a first diffraction element 302. Filters, such as first filter 300 are selected and typically positioned over an associated pixel, such as first pixel 110 to pass through the portion of the radiation spectrum sensed by its associated pixel while substantially obstructing other portions of the radiation spectrum. For example, if the first pixel 110 is intended to detect the intensity of blue light, first filter 300 is chosen to allow blue light to substantially pass through. As a result, the detecting area of the first pixel 110 will detect the intensity of blue light. Of course, other filters such as red filters, green filters and other filters for the invisible spectrum are known to those of ordinary skill in the art.

The first diffraction element 302 is positioned over the first pixel 110. In the example shown in FIG. 3, the first diffraction element 302 is also positioned over the first filter 300, such that radiation is first diffracted by the diffraction element 302 and then filtered. The first diffraction element 302 may be alternatively placed below the first filter 300. As would be understood by those of ordinary skill in the art having this specification before them, this invention does not require the utilization of any filter. The first diffraction element 302 diffracts the radiation incident upon the diffraction element 302 onto the first pixel 110. The diffracted radiation forms a

diffraction pattern on the pixel, which is detected by the first pixel 110 and converted into an electrical signal.

In the illustration of FIG. 3, the diffraction element 302 has an aperture 304 within an opaque portion 306. Typically, when radiation passes through an aperture, the image formed by the radiation has a diameter approximately equal to the diameter of the aperture. However, as the diameter of the aperture is decreased, the radiation is diffracted and the diameter of the diffracted image is larger than the diameter of the aperture. The diameter of the aperture 304 is preferably on the order of several micrometers such that the radiation that passes through the aperture 304 is diffracted, forming a diffraction pattern incident upon the first pixel 110.

Generally, the resulting diffraction pattern has a high intensity area at which a majority of the radiation intensity of the entire diffraction pattern is concentrated.

Preferably, the aperture of the diffraction element 302 is positioned such that the high intensity area created by radiation being diffracted thereby falls incident upon the detecting area of the first pixel 110. Furthermore, the aperture 304 may also be situated within the diffraction element 302 such that the center of the aperture 304 is concentric with the center of the detecting area of the first pixel in order to increase the portion of the high intensity area that is incident upon the detecting area.

In another approach shown in FIG. 4, the diffraction element 400 includes a lens 402 situated within the opaque portion 404 of the diffraction element 400. As in the other approach, the diameter of the lens 402 may also be on the order of several micrometers in order to diffract radiation incident upon the diffraction element 400 onto the first pixel 110, thus forming a diffraction pattern incident upon the first pixel

110. Similarly, the diffraction element 400 is also positioned over a filter 406, such that radiation is first diffracted by the diffraction element 400 and then filtered by the filter 406. Also, as in the other approach, the lens 402 is also preferably situated within the diffraction element 400 such that the lens 402 is concentric with the center of the detecting area of the first pixel HO. It would be understood by one skilled in the art having the this specification before them that the type of lens or aperture utilized in this invention may be changed as long as the radiation incident upon the diffraction element is diffracted onto the pixel.

Generally, the solid-state image sensor is utilized in conjunction with a camera or other such image recording device. As such, radiation typically passes through a front camera lens (not shown) prior to reaching the diffraction element and pixel array 100, FIG. 1. Due to the presence of the camera lens, the radiation incident on the diffraction element is generally parallel. As the radiation is parallel, the diffraction pattern caused by the diffraction element (either 304, FIG. 3, or 402, FIG. 4) is generally a Fraunhofer's diffraction pattern, such as that shown in FIG. 5. However, if the radiation falling incident upon the diffraction element (either 304, FIG. 3, or 402, FIG. 4) is not parallel, the diffraction image created would be a Fresnel's diffraction pattern. It is appreciated by those skilled in the art that Fraunhofer and Fresnel diffraction patterns are similar in shape and differ only in the locations of the minimum intensities of the radiation within the diffraction pattern. Thus, this invention will function acceptably regardless of the type of radiation incident upon the diffracting element.

FIG. 5 illustrates a Frauhofer's diffraction pattern 500 formed by a substantially circular aperture (or lens). The Fraunhofer's diffraction pattern appears as a series of concentric radiation circles 502, 504, 506, and 508 with areas of minimum intensity 510,512, 514, 516 between each concentric circle, wherein the center circle, referred to in the art as the Airy Spot 512, has the highest radiation intensity : The radius of the Airy Spot 512 may be calculated by the following formula: 0. 61L p af where"p"is the radius of the Airy Spot 502, "a"is the diameter of the lens or aperture within the diffraction element,"f'is the distance from the diffraction element to the pixel, and"L"is the wavelength of radiation incident upon the diffraction element (either 302, FIG. 3, or 400, FIG. 4).

For example, a Fraunhoffer's diffraction pattern created by green light (550 nm wavelength) incident upon a diffraction element having a 2 micrometer diameter aperture located at a distance of 8 micrometers between the diffraction element and the pixel would theoretically result in a diffraction pattern with an Airy Spot 502, FIG. 5, having a radius of 0.67. Similarly, a Fraunhofer's diffraction pattern created by red light (650 nm wavelength) incident upon the same diffraction element would result an Airy Spot 510 with a radius of 0.79. A Fraunhofer's diffraction pattern created by blue light (450 nm wavelength) incident upon the same diffraction element (either 302, FIG. 3, or 400, FIG. 4) would result in an Airy Spot 502, FIG. 5, with a radius of 0.55.

FIG. 6 is a plot 600 of an intensity profile of the Frauhofer's diffraction pattern that may be calculated as where"JI"is a Bessel function. The radiation intensity of the Airy Spot 502, FIG. 5, includes about 84% of the total intensity of radiation incident upon the diffraction element. The minimum radiation intensities 602, FIG. 6,604, and 606 (same as minimum radiation intensities 510, FIG. 5, 512, and 514) occur periodically at u = 0.61 (PI), 1.22 (PI), 1.83 (PI),... 0.61n (PI). The Airy Spot 606, FIG. 6, occurs at the center of the pattern.

The radius of the aperture (or lens) within the diffraction element (either 302, FIG. 3, or 400, FIG. 4) may be determined with reference to the size of the detecting area of the associated pixel and the distance between the pixel and diffraction element. In view of this information, the diffraction element is preferably designed such that the Airy Spot 606, FIG. 6, falls incident within the detecting area. As such, about 84% of the incident radiation intensity falls within the detecting area of an associated pixel. If the pixel requires less than 84% of the incident radiation to generate a charge, the diffraction element may also be designed such that only a portion of the area of the Airy Spot 606 falls within the detecting area. Of course, the diffraction element may also be designed in order to further allow radiation circles 504, FIG. 5,506, and 508 that are outside the Airy Spot 606, FIG. 6, to also fall

incident within the detecting area, thus further increasing the intensity of radiation incident upon the detecting area.

In the implementation described above, the radius of the aperture (or lens) within the diffraction element is selected in relation to the wavelength of the radiation intended for the pixel associated with the diffraction element. In a typical solid-state image sensor, a plurality of pixels are arranged in an array in order to detect various portions of the radiation spectrum (e. g. blue, green and red for use in the visible spectrum or x-ray, gamma ray, infrared, etc. in the invisible spectrum) over a desirable surface area. In this illustration, a separate diffraction element may be positioned over each pixel. The radius of the lens (or aperture) within each diffraction element is preferably determined in relation to the mean wavelength of red, green, and blue light, such that the radius of the lenses (or apertures) may be uniform throughout each diffraction element associated with the pixel array to simply manufacturing of the diffraction elements, which may be fabricated in a single sheet for placement over the pixel array. Alternatively, the lenses (and apertures) of the diffraction elements may be configured with reference to the average wavelength of red, green, and blue light, the median wavelength, or any other wavelength between the longest (red) and shortest (blue) wavelengths associated with pixels in the pixel array.

In another example implementation shown in FIG. 7, the diffraction element 700 positioned over the first pixel 110 may have an aperture (or lens) 702 shaped like a rectangular strip situated within the opaque portion 704 of the diffraction element 700. Similarly, the diffraction element 700 is also positioned over a filter 706, such that radiation is first diffracted by the diffraction element 700 and then filtered by the

filter 706. Unlike the prior example having a circular aperture/lens, in this example the diffraction element 700 will create a diffraction pattern having a strip shaped high intensity area, or Airy Strip, at its center.

FIG. 8 illustrates an exemplary diffraction pattern 800 that would be formed by the strip aperture or lens. The size of the Airy Strip 802 created by the rectangular aperture 702 diffraction element 700 can be determined by the following formula: 0. 61L bf <BR> <BR> where"p"is the width of the Airy Strip 802, "b"is the width of the lens or aperture within the diffraction element,"f'is the distance between the diffraction element 702 and the first pixel 110, and"L"is the wavelength of the radiation. As in the other example implementation, the size of the aperture 702 is determined by the width of the detecting area and the distance of the diffraction element 702 from the first pixel 110.

It would be understood by one skilled in the art having this specification before them that this invention may be used in association with various pixel designs in association with various lens (or aperture) shapes as long as a sufficient intensity of radiation created by the diffraction pattern falls incident within the detecting area of the pixels of an array. It would also be understood that the diffraction element may also include multiple apertures or lenses for altering the diffraction pattern.

In another implementation of this invention illustrated in FIG. 9, individual pixels within a pixel array 100, FIG. 1, may be optically separated from neighboring pixels. An opaque wall 900 may be placed between the neighboring pixels 110,120,

and 130. Preferably, the opaque wall 900 is located along the border of each pixel and is of a sufficient height from the surface of the pixels such that most of the radiation from the diffraction pattern (formed by the diffraction elements 902,904, and 906 over its associated pixel 110,120 and 130, respectively) is blocked from falling incident upon the surface of a neighboring pixel. The opaque wall 900 may be constructed from silicon or another semiconductor material.

While various implementations of the application have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.