Abstract:
A method and apparatus for directing light to a light sensor and filtering out an infrared component from the directed light. In one embodiment, the apparatus includes an array of light sensors disposed on a substrate, wherein the light sensors are operable to convert light intensity into a voltage signal. The apparatus further includes a cover plate disposed over the light sensors such that the cover plate creates a cavity over the array of sensors. The apparatus further includes filter material disposed between the cover plate and the light sensors in the cavity formed between the light sensors and the cover plate. The filter material is operable to filter the light passing through the cover plate. In particular, in one embodiment, light having wavelengths in the infrared range may be filtered out.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Digital cameras and other imaging devices typically have a light sensing apparatus for capturing and storing images. For example, in one popular design, an array of photodiodes, typically arranged in a charge-coupled device (CCD) or on a complimentary metal-oxide semiconductor (CMOS) microchip, are used for capturing and storing images. Each photodiode and its associated circuitry, (the combination of which is often called an Active Pixel Sensor (APS) or more simply, a pixel), converts the light intensity detected at the photodiode into a voltage signal that can be digitized for storage, reproduction, and manipulation. Both CMOS and CCD chips sense light through similar mechanisms, by taking advantage of the photoelectric effect, which occurs when photons interact with crystallized silicon to promote electrons from the valence band into the conduction band. Thus, the quality of the image that is captured is reflective of how much light and the manner in which the light reaches the light sensor (i.e., the photodiode). That is, parameters such as angle of incidence, light beam manipulation, and light beam filtering are important to control in order to ensure the capture of a high quality image that accurately reflects the true and correct image being captured.  
         [0002]     One such parameter that affects the quality of the image captured is the amount of infrared light that reaches the light sensor. It is well known that visible light (to the human eye) has a wavelength range of 400 to 700 nm. Just beyond the visible light range is the infrared range that is defined by light having a wavelength in the range of 700 to 2500 nm. A subset of the infrared range is the near-visible infrared (NIR) range which is more of a concern to the digital imaging industry. The NIR range is defined by light having a wavelength in the range of 700 to 1200 nm. Particularly, in CMOS arrays, too much NIR light causes the captured image to appear washed out. That is, the contrast between the colors is not as sharp as it appears in real life. As such, it is important to filter out NIR light from visible light when capturing an image with a CMOS device or any other device that utilizes a light sensor to convert the intensity of incident light into a voltage signal.  
         [0003]     In the past, interference or absorption IR filters have been designed and used in image-capturing devices to filter out infrared light from visible light in a number of different applications. Typically, an interference IR filter reflects IR light before it reaches the light-capturing device and an absorption IR filter absorbs the IR light before it reaches the light-capturing device. IR filters are designed to pass visible light having wavelengths below 700 nanometers while blocking infrared light having higher wavelengths extending into the near-infrared region (700 to 1200+ nanometers). Such an IR filter is often utilized to protect infrared-sensitive CMOS arrays typically incorporated in digital-imaging devices from infrared wavelengths. Thus, when an infrared filter is used within the optical path (i.e., the filter arranged such that incident light must pass through the filter in order to reach the light sensor), the negative effects of infrared light are reduced when capturing an image.  
         [0004]     For example,  FIG. 1  is a cutaway view of a conventional CMOS array  100  that is typically used in a conventional image-capturing device. The CMOS array  100  includes an IR filter  135  in the optical path between incident light  190  and each pixel  101 ,  102 , and  103 . The conventional CMOS array  100  includes a plurality of pixels  101 ,  102 , and  103  arranged in columns and rows. The columns and rows are not shown for clarity; however, portions of the adjacent pixels  102  and  103  in the same row are shown in  FIG. 1  to the left and to the right of pixel  101 .  
         [0005]     Each pixel  101  includes a photodiode  105  embedded in a silicon substrate  104  and each photodiode  105  is associated with electronic circuitry (not shown for clarity) contained in adjacent metal layers  110 . Together, the photodiode  105  and its associated electronic circuitry in the metal layers  110  form a collection well  107  whereby incident light  190  may be directed toward the photodiode  105 . In order to concentrate incident photons (from the incident light  190 ) into the collection well  107 , the collection well  107  is capped by a miniature, positive-meniscus lens known as a microlens  120 , or lenticular.  
         [0006]     One particular kind of pixel  101  is a standard three-transistor pixel which is well known in the art and will not be discussed in detail herein. When a broad wavelength band of visible light  190  is incident on a pixel  101 , a variable number of electrons are released from the semiconductor  104  in proportion to the photon-flux density incident on the surface of a photodiode. In effect, the number of electrons produced is a function of the wavelength and the intensity of light striking the semiconductor  104 . Electrons are collected in a potential well (not shown) until an integration period is complete (as determined by the associated circuitry), and then the collected electrons are converted into a voltage signal. The voltage signal can then passed through an analog-to-digital converter (not shown in  FIG. 1 ), which forms a digital electronic representation of the image, pixel by pixel, captured by the CMOS array  100 .  
         [0007]     The columns and rows of pixels  101  that form the CMOS array  100  are collectively covered by a cover glass  130  or a cover plate to form a shellcase package. The cover glass  130  fits securely on portions of the metal layer  110  over the array of pixels  101  such that a cavity  121  is formed over the microlens  120  of each pixel  101 . In conventional CMOS arrays  100 , this cavity  121  is filled only with air or may be a vacuum. Typically, the shellcase package is manufactured as a unit and then any modifications, such as adding an absorption filter  135 , are accomplished during a separate manufacturing phase. As can be seen in the conventional CMOS array  100  of  FIG. 1 , an IR filter  135  is disposed on top of the cover glass  130 . The IR filter  135  is designed to filter out or absorb light having wavelengths in the infrared range (i.e., greater than about 700 nm). As such, any NIR light within the incident light  190  will be filtered out before reaching the pixel  101  below.  
         [0008]     Conventional IR filters  135  are commonly manufactured from dyed glass and comprise the most widely used types of filters for the attenuation of infrared light in digital-image-capturing devices. The absorption of specific wavelengths, that is, the filter&#39;s spectral performance, is a function of the physical thickness of the conventional IR filter  135  and the amount of dye present in the glass of the filter. Conventional IR filters  135  are made primarily from colored filter glass, and represent the largest class and most widely used type of filters for applications that do not require a precise definition of transmitted wavelengths. These conventional filters are commonly available in the form of glass, plastic-coated glass, acetate. Among the materials used in glass filters are the rare earth transition elements, colloidal dyes (such as selenide), and other molecules having high extinction coefficients that produce reasonably sharp absorption transitions.  
         [0009]     Conventional absorption filters, such as absorption filter  135 , are expensive and bulky and add to the overall depth of the optical path and bulk to the shellcase package. A typical absorption filter  135  is 10 microns thick which adds additional depth to the top of the shellcase package that includes the CMOS array  100 . Further, the inclusion of a typical absorption filter  135  in a digital-image-capturing device requires an additional manufacturing step of affixing the absorption filter to the top of the glass cover  130 . Because this manufacturing step is typically not performed in a clean room during the fabrication of the CMOS array  100 , particulates and/or dust often may become embedded between the cover glass  130  and the absorption filter  135 . Such dust and particulates may greatly affect the performance of the CMOS array  100  in the image-capturing device. Thus, the manufacturing complexity and assembly process adds to the cost and time required to produce a CMOS array  100  suitable for digital image-capturing devices.  
         [0010]     Thus, it would be most beneficial to have a shellcase package with an integrated absorption filter that does not require the additional manufacturing steps associated with or the inherent drawbacks of conventional absorption filters  135  as shown in  FIG. 1 .  
       SUMMARY OF THE INVENTION  
       [0011]     An embodiment of the invention is directed to an apparatus for directing light to a light sensor and filtering out an infrared component from the directed light. The apparatus includes an array of light sensors disposed on a substrate, wherein the light sensors are operable to convert light intensity into a voltage signal. The apparatus further includes a cover plate disposed over the light sensors such that the cover plate creates a cavity over the light sensor array. The cover plate is operable to pass light. That is, the cover plate does not contain a film covering of glass filter that may filter out o portion of any incident light. The apparatus further includes filter material disposed between the cover plate and the light sensors in the cavity formed between the light sensors and the cover plate. The filter material is operable to filter a portion of the light passing through the cover plate. In particular, in one embodiment, light having wavelengths in the infrared range (700-2500 nm) may be filtered out such that only light having wavelengths below about 700 nm is able to pass through the cover plate and the filter material to excite electrons in the light sensor below.  
         [0012]     Having an integrated IR filter that is underneath the cover plate in the cavity formed therein is advantageous because the overall depth of the apparatus is reduced as compared to an apparatus having a filter plate coupled on top of the cover plate. Further, there is a smaller likelihood that particulates or dust may become embedded in the apparatus such that optical performance is degraded because the integrated filter material is injected or otherwise manufactured in concert with the rest of the apparatus. As such, since this manufacturing is typically performed in a clean room and the apparatus is self-contained, i.e., a single shellcase package, the chances of particulates or dust becoming embedded in the apparatus within the optical path is greatly reduced.  
         [0013]     Further, filter material is less expensive than a filter cover plate or a filter film and the filter material is not subject to degradation due to scratching and/or abrasion as is the case with conventional filter cover on a typical conventional image-capture apparatus. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0015]      FIG. 1  is a cutaway view of a conventional CMOS array having an absorption filter in the optical path between incident light and each pixel;  
         [0016]      FIG. 2  is a cutaway view of a CMOS array having an absorption filter material disposed inside a cavity within the shellcase package that lies in the optical path between incident light and each pixel; and  
         [0017]      FIG. 3  is a block diagram of a system that includes a CMOS array of  FIG. 2 , disposed therein. 
     
    
     DETAILED DESCRIPTION  
       [0018]     The following discussion is presented to enable a person skilled in the art to make and use the invention. The general principles described herein may be applied to embodiments and applications other than those detailed above without departing from the spirit and scope of the present invention. The present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed or suggested herein.  
         [0019]      FIG. 2  is a cutaway view of a CMOS array  200  having a filter material  235  in the optical path between cover glass  230  and each photodiode  205 . As is the case with conventional CMOS arrays (such as CMOS array  100  depicted in  FIG. 1 ), the CMOS array  200  includes a plurality of pixels  201 ,  202  and  203  arranged in columns and rows (all pixels are not shown for clarity) disposed in a silicon substrate  204 . Each pixel, such as pixel  201 , includes an associated photodiode  205 , electronic circuitry (also not shown for clarity) contained in adjacent metal layers  210 , and a microlens  220  as described previously. The columns and rows of pixels  201  again form the CMOS array  200  and are collectively covered by the cover glass  230 . The covered CMOS array  200  is referred to as a shellcase package.  
         [0020]     The cover glass  230  fits securely on top of the array of pixels  201  such that a cavity  221  is formed above the pixel array. In this embodiment, the cavity  221  spans the entire array of pixels  201  such that the cover glass  230  is coupled to the contact points with the metal layers on the substrate in a limited number of places such as the outermost edges (not shown) of the pixel CMOS array  200 . In another embodiment, individual cavities (not shown) may be formed to correspond to an associated pixel  201  on a one-for-one basis. Regardless of which embodiment or how the cover glass  230  is coupled with the metal layers  210 , a cavity  221  is formed in some fashion between each pixel  201  and the cover glass  230 .  
         [0021]     In this embodiment, the formed cavity  221  is filled with a filter material. The filter material may be a polymer, (i.e., a gel-like substance) filled with an absorptive dye. The absorptive dye is manufactured such that tiny particulates interact with the incident light  290  which absorbs (i.e., filters) infrared light from the light  290  that passes through the cover glass  230  and the cavity  221  to the photodiode  205 . In one embodiment, the absorptive dye in the polymer is customized such that light having wavelengths above 650 nm is substantially filtered out while light having wavelengths below 650 nm is left substantially unfiltered and able to pass through to the photodiode  205 .  
         [0022]     The quality of filter material used in the manufacture of the polymer is of optical grade and provides uniformity of density and color over the entire volume of the cavity  221 . The filter material filters light through absorption so the spectral performance of the filter material is dependent upon the thickness and optical density of the filter material. Increasing the thickness will produce a corresponding increase in the blocking level of unwanted wavelengths, but also reduces the peak in-band transmission, causing falloff at the ends of absorption bands.  
         [0023]     Using a polymer filter material is cost effective (when compared to the separately fabricated glass or plastic filters, such as absorptive filter  135  in  FIG. 1 ) and optically satisfactory for filtering infrared light. Polymer filter material is commercially available, making this filter material suitable for a wide variety of applications, despite the gentle handling typically required.  
         [0024]     There are several advantages to using polymer filter material in the cavity between the cover glass  230  and the photodiode  205 . These advantages include the relatively low cost and the stability under a wide variety of climates and operating conditions. In addition, the polymer filter material may be constructed with light-absorbing chemical species mixed throughout the filter material, rather than being deposited on the surface of a filter plate as is the case with film or glass filters described above with respect to  FIG. 1 . Thus, polymer filter material is not prone to destruction by minor scratches or abrasions. Further, polymer filter material is not sensitive to the angle of incident light  290  and provides uniform spectral characteristics at virtually any angle of incidence.  
         [0025]     Additionally, typical manufacturing steps include the injecting of the polymer filter material into the cavity while the entire shellcase package is being fabricated in a clean room. Thus, the chance of particulates and/or dust becoming embedded inside the shellcase package or between the cover plate and a conventional filter plate (as is the case in prior art) is greatly reduced.  
         [0026]     Further yet, because the filter is no longer embodied in a plate or film that is external to the shellcase package, the overall depth of the shellcase package is reduced. Reducing the depth of the shellcase package is advantageous because the shellcase package then has a lower profile that may be able to be fit in shallower digital-image capturing devices.  
         [0027]      FIG. 3  shows a block diagram of a system  300  that includes a CMOS array  200  of  FIG. 2 , disposed therein. The system  300  may be a digital camera, digital camera-phone, or other electronic device utilizing a digital image-capturing apparatus. Such an apparatus may be of any size and number of pixels each containing a respective photodiode.  
         [0028]     The CMOS array  200  is able to integrate a number of processing and control functions, which lie beyond the primary task of photon collection, directly onto a single shellcase package. These features generally include timing logic, exposure control, analog-to-digital conversion, shuttering, white balance, gain adjustment, and initial image processing algorithms. In order to perform all of these functions, the CMOS integrated circuit architecture more closely resembles that of a random-access memory cell rather than a simple photodiode array. One popular CMOS array  200  is built around active pixel sensor (APS) technology in which both the photodiode  205  and a readout amplifier (not shown, although within the metals layers  210  are incorporated into each pixel  201 . This enables the charge accumulated by the photodiode  205  to be converted into an amplified voltage signal inside the pixel  201  and then transferred in sequential rows and columns to the analog signal-processing portion of the chip.  
         [0029]     Thus, each pixel  201  contains, in addition to a photodiode  205 , a triad of transistors that converts accumulated electron charge to a measurable voltage, resets the photodiode, and transfers the voltage to a vertical column bus. The resulting array is an organized checkerboard of metallic readout busses that contain a photodiode  205  and associated signal preparation circuitry at each intersection, i.e., each pixel  201 . The busses apply timing signals to the photodiodes  205  and return readout information back to the analog decoding and processing circuitry housed away from the CMOS array  200 . This design enables signals from each pixel  201  in the array to be read with simple x, y addressing techniques.  
         [0030]     The photodiode  205  is a key element of a digital image sensor. Sensitivity is determined by a combination of the maximum charge that can be accumulated by the photodiode  205 , coupled to the conversion efficiency of incident photons to electrons and the ability of the device to accumulate the charge in a confined region without leakage or spillover. These factors are typically determined by the physical size and aperture of the photodiode  205 , and its spatial and electronic relationship to neighboring elements in the CMOS array  200 . Another factor is the charge-to-voltage conversion ratio, which determines how effectively integrated electron charge is translated into a voltage signal that can be measured and processed.  
         [0031]     Photodiodes are typically organized in an orthogonal grid that may range in size from 128×128 pixels (16 K pixels) to a more common 1280×1024 (over a million pixels). Several of the latest CMOS arrays  200 , such as those designed for high-definition television (HDTV), contain several million pixels organized into very large arrays of over 2000 square pixels. The signals from all of the pixels  201  composing each row and each column of the array must be accurately detected and measured (read out) in order to assemble an image from the photodiode charge accumulation data.  
         [0032]     The system of  FIG. 3  includes a central processing unit (CPU)  315  coupled with a bus  320 . Also coupled with the bus  320  is a memory  325  for storing digital images captured by the CMOS array  200 . The CPU  315  facilitates an image capture by controlling the CMOS array  200  through the bus  325  and, once an image is captured, storing of the image in a digital format in the memory  325 .  
         [0033]     The CMOS array  200  includes several components for facilitating the capture and digitizing of an image as described above with respect to  FIG. 2  and that which is well-known in the art with respect to image capture electronics. Each pixel  201  in the CMOS array  200  is coupled to row control circuitry  350  via connections  351  and to column control circuitry  360  via connections  361  which facilitate the control signals for capturing an image. Further, each pixel  201  in the CMOS array  200  is coupled to Vdd  311  and GROUND  312  (individual connection not shown).  
         [0034]     During a typical image capture procedure, the voltage signal for each pixel  201  is read by the column control circuitry  360  and sent to a multiplexor  370 . The multiplexor  370  combines each voltage signal into a single multiplexed signal which represents the voltage signal captured at each photodiode  205  of each pixel  201 . After an amplification stage  380 , this signal is converted into a digital signal via an analog-to-digital converter  390  before being communicated to the bus  320 . The CPU  315  then facilitates the storage in the memory  325  of the multiplexed digital signal  
         [0035]     By incorporating the filter material which accomplishes the task of filtering out infrared light form any incident light that may reach the CMOS array  200 , the system  300  in  FIG. 3  may be contained in a shallower environment and/or housing.