Abstract:
Imaging devices having reduced fixed pattern noise are disclosed. The fixed pattern noise in the imaging devices is reduced by measuring and adjusting the spectral characteristics of the imager device on a pixel by pixel basis. The fixed pattern noise of the pixel cells are changed by modifying the absorption, reflectance, refractive index, shape, and/or micro structure of the material.

Description:
FIELD OF THE INVENTION 
     The invention relates to improved semiconductor imaging devices and, in particular, to CCD and CMOS imagers having reduced fixed pattern noise. 
     BACKGROUND OF THE INVENTION 
     The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), complementary metal oxide semiconductor (CMOS) devices, photodiode arrays, charge injection devices and hybrid focal plane arrays, among others. 
     Solid-state image sensors, also known as imagers, were developed in the late 1960s and early 1970s primarily for television image acquisition, transmission, and display. An imager absorbs incident radiation of a particular wavelength (such as optical photons, x-rays, or the like) and generates an electrical signal corresponding to the absorbed radiation. There are a number of different types of semiconductor-based imagers, including CCDs, photodiode arrays, charge injection devices (CIDs), hybrid focal plane arrays, and CMOS imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems and other image based systems. 
     These imagers typically consist of an array of pixel cells containing photosensors, where each pixel cell produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image. The photosensors are typically photogates, phototransistors, photoconductors or photodiodes, where the conductivity of the photosensor or the charge stored in a diffusion region corresponds to the intensity of light impinging on the photosensor. The magnitude of the signal produced by each pixel cell, therefore, is proportional to the amount of light impinging on the photosensor. 
     Active pixel sensor (APS) imaging devices are described in U.S. Pat. No. 5,471,515. These imaging devices include an array of pixel cells, arranged in rows and columns, that convert light energy into electric signals. Each pixel includes a photodetector and one or more active transistors. The transistors typically provide amplification, read-out control and reset control, in addition to producing the electric signal output from the cell. 
     While CCD technology has a widespread use, CMOS imagers are being increasingly used as low cost imaging devices. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital imager applications. 
     A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including a photoconversion device, for example, a photogate, photoconductor, phototransistor, or a photodiode for accumulating photo-generated charge in a portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output transistor, which receives photogenerated charges from a doped diffusion region and produces an output signal which is periodically read out through a pixel access transistor. The imager may optionally include a transistor for transferring charge from the photoconversion device to the diffusion region or the diffusion region may be directly connected to or part of the photoconversion device. A transistor is also typically provided for resetting the diffusion region to a predetermined charge level before it receives the photoconverted charges. 
     In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo-charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion region. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower output transistor. 
     Each pixel cell receives light focused through one or more micro-lenses. Micro-lenses on a CMOS imager help increase optical efficiency and reduce cross talk between pixel cells. A reduction of the size of the pixel cells allows for a greater number of pixel cells to be arranged in a specific pixel cell array, thereby increasing the resolution of the array. In one process for forming micro-lenses, the radius of each micro-lens is correlated to the size of the pixel cell. 
     The micro-lenses refract incident radiation to the photosensor region, thereby increasing the amount of light reaching the photosensor and thereby increasing the fill factor of the imager. Other uses of micro-lens arrays include intensifying illuminating light on the pixel cells of a non-luminescent display device such as a liquid crystal display device to increase the brightness of the display, display associated with a camera, forming an image to be printed, and as focusing means for coupling a luminescent device or a receptive device to an optical fiber. 
     One source of image sensor noise is fixed pattern noise (FPN). FPN may manifest as a stationary background pattern in the image which is caused by mismatches in device parameters. FPN is the systematic signal difference between individual pixel cells or groups of pixel cells. FPN can have a variety of physical causes, including small local variations above each photosensor, differences in electronic response, and variations in the thin film stack above each photosensor, including variations of the color filter and micro-lens layers. FPN in a image sensor is typically around 1.0 to 1.2%, thus the signal to noise ratio due to FPN is about 40 dB. 
     There is needed, therefore, imaging devices for sensing objects which have reduced fixed pattern noise. A reduction of FPN of about 10 to 20% would improve the image quality that can be sensed by the eye. A 1-2% change in optical transmission, either for the full operating range of wavelengths or a portion thereof would result in an appreciable difference in the quality of the image. A method of reducing the fixed pattern noise of the imaging device and methods for fabricating the devices having reduced fixed pattern noise are also needed. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides imaging devices having reduced fixed pattern noise. The fixed pattern noise in the imaging devices is reduced by measuring and adjusting the spectral characteristics of the imager device on a pixel by pixel basis. The invention relates to changing the absorption, reflectance, refractive index, shape, and/or micro structure of the material. In particular, this invention is applicable for any micro-electronic or micro-optical device that requires low noise such as, for example, CCD imagers and CMOS imagers. 
     The present invention provides a method for reducing fixed pattern noise in a solid state imager having a pixel cell array, wherein the fixed pattern noise is reduced by first measuring the fixed pattern noise and then adjusting the spectral characteristics of the imager device on a pixel by pixel basis. The spectral characteristics that may be modified include the absorption, reflectance, refractive index, shape, and/or micro structure of the material on a pixel by pixel basis. In one embodiment of the invention a trim process is used to locally induce physical or chemical changes to the pixel cell. Examples of techniques used to trim the pixel cell include, for example, deposition of a thin film and subsequent beam induced localized ablation/etch of the thin film surface by ion beam or UV laser ablation; beam induced localized deposition; thermally induced change in surface micro structure; beam induced chemical surface change; and direct implantation of absorbing species into the surface layer. Also provided are methods for forming the imaging devices of the present invention having reduced fixed pattern noise. 
     Additional advantages and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side cross-sectional view illustrating the principal elements of a solid-state imager having a trim layer constructed under a color filter array in accordance with an exemplary embodiment of the present invention. 
         FIG. 2  is a side cross-sectional view illustrating the principal elements of a solid-state imager having a trim layer constructed above a color filter array yet below a micro-lens in accordance with an exemplary embodiment of the present invention. 
         FIG. 3  is a side cross-sectional view illustrating the principal elements of a solid-state imager having a trim layer constructed above a micro-lens in accordance with an exemplary embodiment of the present invention. 
         FIG. 4  is a side cross-sectional view illustrating the principal elements of a solid-state imager having a trim layer constructed above a micro-lens in accordance with an exemplary embodiment of the present invention. 
         FIG. 5  is a side cross-sectional view illustrating the principal elements of a solid-state imager having a trim layer constructed above a micro-lens in accordance with an exemplary embodiment of the present invention. 
         FIG. 6  illustrates a schematic cross-sectional view of a CMOS imager pixel cell having a color filter array constructed in accordance with an exemplary embodiment of the present invention. 
         FIG. 7  is a representative diagram of the CMOS imager pixel cell of  FIG. 6 . 
         FIG. 8  illustrates a cross-sectional view of a semiconductor wafer undergoing the process of forming a color pattern layer according to an exemplary embodiment of the present invention. 
         FIG. 9  illustrates the semiconductor wafer of  FIG. 8  at a stage of processing subsequent to that shown in  FIG. 8 . 
         FIG. 10  illustrates the semiconductor wafer of  FIG. 8  at a stage of processing subsequent to that shown in  FIG. 9 . 
         FIG. 11  is a schematic of the process for determining FPN reduction on a pixel by pixel basis. 
         FIG. 12  shows an imager constructed in accordance with an embodiment of the invention. 
         FIG. 13  is an illustration of an imaging system having an imager with reduced fixed pattern noise according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and show by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, 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 the spirit and scope of the present invention. The progression of processing steps described is exemplary of embodiments of the invention; however, the sequence of steps is not limited to that set forth herein and may be changed as is known in the art, with the exception of steps necessarily occurring in a certain order. 
     The terms “wafer” and “substrate” are to be understood as including silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “wafer” and “substrate” in the following description, previous process steps may have been utilized to form regions or junctions in the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, or gallium arsenide. 
     The term “pixel” or “pixel cell” refers to a picture element unit cell containing a photo-conversion device for converting electromagnetic radiation to an electrical signal. Typically, the fabrication of all pixel cells in an image sensor will proceed concurrently in a similar fashion. 
     Finally, while the invention is described with reference to a CMOS imager, it should be appreciated that the invention may be applied in any micro-electronic or micro-optical device that requires low noise for optimized performance. Other suitable micro-optical devices include CCDs and displays. 
     Referring now to the drawings, where like elements are designated by like reference numerals. A solid-state imager  20  is schematically illustrated in  FIG. 1 . The imager  20  comprises a trim layer  80  and a color filter layer  100  formed over a pixel cell array  26  as part of the same substrate  30 , which may be any of the types of substrate described above. The pixel cell array  26  comprises a plurality of pixel sensor cells  28  formed in and over the substrate, and is covered by a protective layer  24  that acts as a passivation and planarization layer for the imager  20 . Protective layer  24  may be a layer of BPSG, PSG, BSG, silicon dioxide, silicon nitride, polyimide, or other well-known light transmissive insulator. 
     The trim layer  80  may be formed of any material with suitable optical properties that can be inserted into the light path to modify the (angular) spectral intensity of an imager pixel cell. The trim layer  80  may be formed of any material that has the desired spectral transmission characteristics that can be adjusted by localized photon or particle beams. The materials forming the trim layer  80  are stable under normal storage and operating conditions of the device after trimming has been done. When the trim layer  80  is formed under the micro-lens  70 , the trim layer  80  may be a thin film that is deposited onto the wafer or a thin metal layer that is formed over the protective layer  24  by conventional methods. These conventional methods for forming the trim layer  80  include, for example, sputtering or evaporative metal deposition. The metal may be any metal whose spectral characteristics can be adjusted by localized photon or particle beams and which is stable under normal storage and operating conditions of the device after spectral trimming has been performed. Examples of suitable metals for the trim layer  80  include, for example, tungsten or aluminum. Moreover, the trim layer  80  may be formed from amorphous carbon which may be deposited by conventional methods as understood by the person having ordinary skill in the art. The trim layer  80  may also be formed of other inorganic films such as, for example, metal oxides. As understood by those having ordinary skill in the art, materials that show slow transmission changes under light exposure or thermal stress (i.e., materials that undergo gradual irreversible chemical or physical phase changes under the influence of visible light and/or low or high temperatures). The trim layer  80  is formed such that the trim layer  80  does not significantly inhibit light from reaching the pixel sensor cells  28 . The trim layer  80  preferably has a thickness of from about 1 angstrom to about 250 angstroms, preferably from about 5 to about 100 angstroms, more preferably from about 10 to about 75 angstroms, most preferably from about 15 to about 50 angstroms. 
     The physical and/or chemical characteristics of the trim layer  80  may be changed after the imager is formed to modify the spectral response of the imager  20 . As discussed in more detail below, examples of techniques used to trim the pixel cell include, for example, deposition of a thin film and subsequent beam induced localized ablation/etch of the thin film surface by ion beam or UV laser ablation; beam induced localized deposition; thermally induced change in surface micro structure; beam induced chemical surface change; and direct implantation of absorbing species into the surface layer. For example, the trim layer  80  may be modified by using a laser, particle beams, ion beams, localized heating or infrared exposure. It should be understood that different lasers can change spectral components differently so that the FPN can be reduced on a pixel by pixel basis. Thus, each trim layer  80  may address a separate wavelength range by changing its spectral absorbance in response to localized heating or particle beam exposure, depending on the desired reduction in FPN needed for that specific pixel cell. 
     Reference is now made to  FIG. 2  which schematically illustrates a second embodiment of the solid-state imager  20  of the present invention. The illustrated embodiment comprises a trim layer  80  formed over the color filter layer  100  and spacer layer  25 , the color filter layer  100  being formed over a pixel cell array  26  as part of the same substrate  30 , which may be any of the types of substrate described above. The trim layer  80  may be formed of any of the materials discussed above with reference to  FIG. 1 . 
     Reference is now made to  FIG. 3  which schematically illustrates a third embodiment of the solid-state imager  20  of the present invention which comprises a trim layer  80  formed over the micro-lens  70 . The micro-lens  70  is formed over the color filter layer  100  and spacer layer  25 , the color filter layer  100  being formed over a pixel cell array  26  as part of the same substrate  30 , which may be any of the types of substrate described above. The trim layer  80  may be formed of any of the materials discussed above with reference to  FIG. 1 . However, since the trim layer  80  is formed after the micro-lens  70 , the selection of the trim layer  80  is limited due to thermal constraints of the color filter layer  100  and micro-lens  70  during the deposition of the trim layer  80  because the spectral transmission of the color filter layer  100  and the micro-lens  70  may be affected by the processing temperature of the addition of the trim layer  80 . Thus, when the trim layer  80  is formed above the micro-lens  70 , the trim layer is preferably selected from materials such as thin silicon films and thin metal films that are deposited onto the micro-lens  70 . Alternatively, the trim layer  80  can be selectively removed in selected areas of variable density, allowing a direction dependent transmission adjustment. 
     Reference is now made to  FIG. 4  which schematically illustrates a fourth embodiment of the solid-state imager  20  of the present invention which comprises a color filter layer  105  and spacer layer  25  formed over a pixel cell array  26  as part of the same substrate  30 , which may be any of the types of substrate described above. In this embodiment, the color filter layer  105  is modified to reduce the FPN of the imager  20 . For example, the color filter layer  105  can be formed of a color resist or acrylic material which is used as a light transmitting material. The color filter layer  105  may be modified by the addition of additional chemical adjuvants, such as, for example, monomers or other additives, that can be “bleached” to reduce their spectral absorption upon exposure to (UV) light. Thus, by destroying or changing the molecular structure of the color filter layer  105  or by causing absorption of the UV light inside the color filter layer  105 , the spectral characteristics of the color filter layer  105  can be selectively modified. 
     Reference is now made to  FIG. 5  which schematically illustrates a fifth embodiment of the solid-state imager  20  of the present invention which comprises a trimmable micro-lens  75  formed above the spacer layer  25  and color filter layer  100 . The color filter layer  100  is formed over a pixel cell array  26  as part of the same substrate  30 , which may be any of the types of substrate described above. In this embodiment, the trimmable micro-lens  75  may itself be modified to reduce the FPN of the imager  20 . The trimmable micro-lens  75  may be modified by the addition of additional chemical adjuvants, such as, for example, additive monomers or other additives, that can be “bleached” to reduce their spectral absorption upon exposure to (UV) light. Thus, by destroying or changing the molecular structure of the trimmable micro-lens  75  or by causing absorption of the UV light inside the trimmable micro-lens  75 , the spectral characteristics of the trimmable micro-lens  75  can be selectively modified. 
     Additionally, since the shape of the micro-lens  75  is the result of a thermal reflow process, rastering can create individual (and even anisotropic) reflow conditions for each micro-lens  75 . While not wishing to be bound by theory, it is believed that the lower scan speed and higher temperature would produce a different reflow result and thus a different spectral response in the imager  20  by individually shaping the lens  75 . 
     Reference is now made to  FIGS. 6-10 .  FIG. 6  shows an expanded view of the solid-state imager discussed above. The pixel array  26  shown in  FIGS. 1-5  comprises a plurality of pixel sensor cells  28  formed in and over the substrate, and is covered by a protective layer  24  that acts as a passivation and planarization layer for the imager  20 . Protective layer  24  may be a layer of BPSG, PSG, BSG, silicon dioxide, silicon nitride, polyimide, or other well-known light transmissive insulator. 
     The color filter layer  100  is formed over the passivation layer  24 . The color filter layer  100  comprises an array of red, blue and green sensitive elements which may be arranged in a pattern understood by the person having ordinary skill in the art as exemplified by U.S. Pat. Nos. 6,783,900 and 3,971,065 which are herein incorporated by reference. 
     As also depicted in the figures, a micro-lens array  22  is formed so that micro-lens  70  are formed above each pixel cell. The micro-lens array  22  is formed such that the focal point of the array is centered over the photosensitive elements in each pixel cell. The device also includes a spacer layer  25  under the microlens array  22  and over the color filter layer  100 . The thickness of spacer layer  25  is adjusted such that the photosensitive element is at a focal point for the light traveling through lenses  70  of micro-lens array  22 . 
     As shown in  FIGS. 6-7 , each pixel sensor cell contains a photosensor  34 , which may be a photodiode, photogate, or the like. A photogate photosensor  34  is depicted in  FIGS. 6-7 . An applied control signal PG is applied to the photogate  34  so that when incident radiation  101  in the form of photons passes color filter layer  100  and strikes the photosensor  34 , the photo-generated electrons accumulate in the doped region  36  under the photosensor  34 . A transfer transistor  42  is located next to the photosensor  34 , and has source and drain regions  36 ,  40  and a gate stack  42  controlled by a transfer signal TX. The drain region  40  is also called a floating diffusion region, and it passes charge received from the photosensor  34  to output transistors  44 ,  46  and then to readout circuitry  48 . A reset transistor  50  comprised of doped regions  40 ,  52  and gate stack  54  is controlled by a reset signal RST which operates to reset the floating diffusion region  40  to a predetermined initial voltage just prior to signal readout. Details of the formation and function of the above-described elements of a pixel sensor cell may be found, for example, in U.S. Pat. Nos. 6,376,868 and 6,333,205, the disclosures of which are incorporated by reference herein. 
     As illustrated in  FIG. 6 , the gate stacks  42 ,  54  for the transfer  42  and reset  54  transistors include a silicon dioxide or silicon nitride insulator  56  on the substrate  30 , which in this example is a p-type substrate, a conductive layer  58  of doped polysilicon, tungsten, or other suitable material over the insulating layer  56 , and an insulating cap layer  60  of, for example, silicon dioxide, silicon nitride, or ONO (oxide-nitride-oxide). A silicide layer  59  may be used between the polysilicon layer  58  and the cap  60 , if desired. Insulating sidewalls  62  are also formed on the sides of the gate stacks  42 ,  54 . These sidewalls  62  may be formed of, for example, silicon dioxide, silicon nitride, or ONO. A field oxide layer  64  around the pixel sensor cell  28  serves to isolate it from other pixel cells in the array. A second gate oxide layer  57  may be grown on the silicon substrate and the photogate semi-transparent conductor  66  is patterned from this layer. In the case that the photosensor is a photodiode, no second gate oxide layer  57  and no photogate semi-transparent conductor  66  is required. Furthermore, transfer transistor  42  is optional, in which case the diffusion regions  36  and  40  are connected together. 
     The image devices  20  described above with reference to  FIGS. 1-3  are manufactured through a process described as follows, and illustrated in  FIGS. 8-10 . 
     Referring now to  FIG. 8 , a substrate  30 , which may be any of the types of substrates described above, having a pixel cell array  26 , peripheral circuits, contacts and wiring formed thereon by well-known methods, is provided. A protective layer  24  of BPSG, BSG, PSG, silicon dioxide, silicon nitride or the like is formed over the pixel cell array  26  to passivate it and to provide a planarized surface. 
     A trim layer  80  is formed over passivation layer  24  as shown in  FIG. 8 . The trim layer  80  may be formed of any material with suitable optical properties that can be inserted into the light path to modify the (angular) spectral intensity of an imager pixel cell. The trim layer  80  may be formed of any material that has the desired spectral transmission characteristics that can be adjusted by localized photon or particle beams. The materials forming the trim layer  80  are stable under normal storage and operating conditions of the device after trimming has been done. The trim layer  80  may be a thin film that is deposited onto wafer, such as, for example, a thin metal layer that is formed over the protective layer  24  by conventional methods. The conventional methods for forming the trim layer  80  include, for example, sputtering or evaporative metal deposition. Examples of suitable metals for the trim layer  80  include, for example, tungsten or aluminum. Moreover, the trim layer  80  may be formed from amorphous carbon which may be deposited by conventional methods as understood by the person having ordinary skill in the art. The trim layer  80  may also be formed of other inorganic films such as, for example, metal oxides. As understood by those having ordinary skill in the art, materials that show slow transmission changes under light exposure or thermal stress can be used to fabricate the trim layer  80 . 
     A color filter layer  100  is formed over the trim layer  80 , as also shown in  FIG. 9 . The color filter layer  100  may be formed of a color resist or acrylic material which is used as a light transmitting material. For example, color filter layer  100  may be formed of a plurality of color filter layers, each of the plurality of color filter layers consisting of red filter regions (not shown), green filter regions (not shown) and blue filter regions (not shown), which are formed, for example, from resist or acrylic material of the respective color-filtering qualities. As such, red sensitive resist material, blue sensitive resist material and green sensitive resist material may be employed to form the red, blue and green sensitive elements of each of the plurality of color filter layers that form color filter layer  100 . These red, blue and green elements may be formed in any pattern know to those skilled in the art. Other embodiments may employ other colored materials, such as paint or dye, as known in the art. The color filter layer  100  may be formed over the trim layer  80  by conventional deposition or spin-on methods, for example. 
     A spacing layer  25  is formed over the color filter layer  100 , as illustrated in  FIG. 9 . Lenses  70  may then be formed, as shown in  FIG. 10 , from a lens forming layer, for example, so that each lens  70  overlies a pixel cell  28 . Alternative constructions in which a lens  70  overlies multiple pixel cells  28  are also encompassed by the present invention. It should also be understood that the preceding examples discuss one embodiment of the present invention. Of course, it should be understood that other embodiments of the invention may be similarly fabricated with the trim layer  80  being located, for example, in the various positions discussed in  FIGS. 1-5 . 
     Reference is now made to  FIG. 11 .  FIG. 11  shows a method  200  for reducing FPN according to the present invention. As illustrated in this figure, the imager device is first built as discussed herein with reference to  FIGS. 1-10  (step  202 ). After the imager device is built, the raw sensor responses are then measured (step  204 ). As understood by the person having ordinary skill in the art, the raw sensor responses are measured by observing the spectral transmission of imager  20 . The FPN per pixel cell is then calculated based on the spectral observations (step  206 ). As set forth above, FPN in a image sensor is typically around 1.0 to 1.2%, thus the signal to noise ratio due to FPN is about 40 dB. A reduction of FPN of from 10 to 20% would improve the image quality that can be sensed by the eye. Likewise, a controlled 1-2% change in optical transmission, either for the full operating range of wavelengths or a portion thereof would suffice to achieve an appreciable difference in the quality of the image. The FPN data is then used as an input to perform pixel by pixel correction of FPN, thus improving the image quality of the device (step  208 ). 
     Examples of techniques used to perform pixel by pixel correction of the imager device in accordance with the acquired FPN data include, for example, deposition of a thin film and subsequent beam induced localized ablation/etch of the thin film surface by ion beam or UV laser ablation; beam induced localized deposition; thermally induced change in surface micro structure; beam induced chemical surface change; and direct implantation of absorbing species into the surface layer. 
     As discussed above with reference to  FIGS. 1-10 , the trim layer  80  may be deposited as a thin film. Based on the FPN data, the trim layer  80  may be selectively removed by beam induced localized ablation/etch of the thin film surface by ion beam or UV laser ablation, as understood by the person having ordinary skill in this art. Different materials, due to their physical or atomic or molecular structure, show certain optical spectral transmission properties. The thickness of the films can be modulated or their chemical composition or microstructure can be changed (e.g. phase change) under the influence of the localized photon or particle beam and thus the optical spectral transmission properties can be modified. Chemical reactions inside the film can be induced by the local heating or photon exposure of the trimming beam. In an alternative embodiment, it is understood that chemicals for the reaction due to the influence of the trimming beam can be present in the film as-deposited, by either being added in a second removable film for the trim layer  80  (not shown) or by adding the chemical in the gas atmosphere above the imager device during the trimming process. The resulting change in chemical composition or phase affects the spectral characteristics of the trim layer  80 , thus enabling the correction of FPN for the specific pixel cell. It should be understood that conventional methods for trimming the layer  80  can be used. Examples include ion beam subtraction, UV laser ablation, changes in pulse energy or pulse duration of the ablation laser. 
     Another exemplary method for reducing the FPN of the imager includes beam induced localized deposition. Exemplary methods include using mask repair technology to selectively add, remove or modify the trim layer  80 . For broad spectral response adjustment, very thin metal films are preferred. For more targeted spectral range adjustments, films with embedded dye molecules or functional molecular groups that change their spectral absorbance upon heating/irradiation can be used. These dyes could be destroyed by photons/heat/particle beams or generated by them, thereby modifying the spectral response of the imager pixel cell. 
     Yet another method for pixel by pixel FPN correction includes thermally induced change in surface micro structure of the device. Under this method, the trim layer  80 , trimmed color filter layer  105  or trimmed micro-lens  75  can be modified by controlled local heating and cooling (thus effecting phase change of the material) and the surface roughness can be modified by structural rearrangement (i.e., phase change) of the surface molecules/atoms of the trim layer  80 , trimmed color filter layer  105  or trimmed micro-lens  75 . This thermally induced change can affect the spectral response of the imager pixel cell. 
     Yet another method for pixel by pixel FPN correction includes beam induced chemical surface change. As discussed above, energy from the particle beam can result in a phase change of the surface of the device. Tools suitable for use in this method are those which generate a localized particle beam, such as, for example, an e-beam writer, ion beam writer or the like that directs the beam at the surface of the substrate. Accelerated particles are ‘shot’ at the target (usually under vacuum) and implanted into the surface layer(s) of the target. They either act directly to modify its spectral transmission or can be induced to react (e.g. thermally) with trim layer  80 , trimmed color filter layer  105  or trimmed micro-lens  75  and form absorbing molecules. This change can be used to reduce FPN of the pixel cell. 
     Another method for pixel by pixel FPN correction includes direct implantation of absorbing species into the rim layer  80 , trimmed color filter layer  105  or trimmed micro-lens  75 . For example, if the entire pixel cell is targeted for a uniform transmission change, a buried film close to the micro-lens focal point may be desirable. If light transmission from a certain angle is meant to be adjusted, trimming a film on top of the micro-lenses with small holes facing in the proper direction may be the method of choice. 
     While the process of forming the trim layer  80 , the trimmed micro-lens  75  and the trimmed color filter layer  105  and selectively modifying these layers have been described with reference to a CMOS imager device, it should be understood that the process may be also used with pixel cells of other types of imagers as well, for example, with a CCD imager. Accordingly, the pixel cell formed as described above may be employed in CCD image sensors as well as CMOS image sensors. The imager devices of the present invention may also be formed as different size megapixel imagers, for example imagers having arrays in the range of about 0.1 megapixels to about 20 megapixels. 
       FIG. 12  illustrates an exemplary imager  700  that may utilize any embodiment of the invention. The Imager  700  has a pixel array  705  comprising pixels constructed as described above with respect to  FIGS. 1-10 , or using other pixel architectures. Row lines are selectively activated by a row driver  710  in response to row address decoder  720 . A column driver  760  and column address decoder  770  are also included in the imager  700 . The imager  700  is operated by the timing and control circuit  750 , which controls the address decoders  720 ,  770 . The control circuit  750  also controls the row and column driver circuitry  710 ,  760 . 
     A sample and hold circuit  761  associated with the column driver  760  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  762  for each pixel and is digitized by analog-to-digital converter  775  (ADC). The analog-to-digital converter  775  supplies the digitized pixel signals to an image processor  780  which forms a digital image. 
     If desired, the imager  20  may be combined with a processor, such as a CPU, digital signal processor or microprocessor. The imager  20  and the microprocessor may be formed in a single integrated circuit. An exemplary processor system  400  using a CMOS imager having a filter array in accordance with the present invention is illustrated in  FIG. 13 . A processor based system is exemplary of a system having digital circuits which could include CMOS or other imager devices. Without being limiting, such a system could include a computer system, camera system, scanner, machine vision system, vehicle navigation system, video telephone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system and other image processing systems. 
     As shown in  FIG. 13 , an exemplary processor system  400 , for example, a camera generally comprises a central processing unit (CPU)  444 , e.g., a microprocessor, that communicates with an input/output (I/O) device  446  over a bus  452 . The imager  20  also communicates with the system over bus  452 . The computer system  400  also includes random access memory (RAM)  448 , and may include peripheral devices such as a floppy disk drive  454 , a compact disk (CD) ROM drive  456  or a flash memory  458  which also communicate with CPU  444  over the bus  452 . The floppy disk  454 , the CD ROM  456  or flash memory  458  stores images captured by imager  20 . The imager  20  is preferably constructed as an integrated circuit, with or without memory storage, as previously described with respect to  FIGS. 1-10 . 
     While the invention has been described in detail in connection with exemplary embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.