Abstract:
Methods and systems for making an electro-optical device suitable for use in an image forming system are described. The device includes photosensors, which are covered by filter layers that sense the presence of colors in, for example, a color document. The methods and systems involve the use of an inter-filter layer that serves to smooth a top surface of the electro-optical device between successive applications of the filter layers.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to an electro-optical device, and specifically relates to the fabrication of an electro-optical device employed in an image forming system.  
         BACKGROUND OF THE INVENTION  
         [0002]    As copying and scanning of color documents becomes more prevalent, there has arisen a need for a solid-state electro-optical device suitable for sensing images, such as a silicon chip having an array of photosensors. For a photosensor to be sensitive to a specific primary color, a translucent filter layer, such as a polyimide or acrylic layer that has been dyed or pigmented to the specific primary color, may be applied on the surface of the chip. If a single photosensitive chip is intended to have multiple linear arrays of photosensors, each linear array being sensitive to one particular primary color, particular polyimide filter layers are applied to specific linear arrays, thereby creating a full-color photosensitive chip.  
           [0003]    [0003]FIG. 1 is a plan view of a single photosensitive chip  10  of a general design found, for example, in a full-color photosensor scanner of the prior art. A typical design of a full-page-width scanner includes a plurality of chips  10  arranged to form an effective collinear array of photosensors, which extends across a page image being scanned. Each chip  10  is a silicon-based integrated circuit chip having defined in a main surface thereof at least three independently-functioning linear arrays of photosensors, each photosensor being here indicated as  14 . The photosensors are disposed in at least three parallel rows that extend across a main dimension of the chip  10 , these individual rows being shown as  16   a ,  16   b , and  16   c . Each individual row of photosensors on the chip  10  can be made sensitive to a particular color by applying to the particular rows  16   a ,  16   b , and  16   c  a spectrally translucent filter layer that covers only the photosensors in a particular row. For example, the three rows of photosensors can be filtered with a different primary color, such as red, green, and blue. Generally, each individual photosensor  14  is adapted to output a charge or voltage signal indicative of the intensity of light of a certain type impinging thereon. Various structures, such as transfer circuits, or charge-coupled devices, are known in the art for processing signal output by the various photosensors  14 .  
           [0004]    One method of constructing a full-color photosensitive chip  10  is to first create a wafer having a relatively large number, such as one hundred or more, semiconductor structures, each structure corresponding to one chip  10 . Filter layers may then be applied to the structures on the wafer. The filter layers may be applied as an even layer of translucent liquid to the entire wafer. This layer can then be etched away with, for example, a laser except in those areas on the chip structure where the filter is desired to be placed. Lithography can be used, where a photosensitive polymer containing a colorant is exposed to ultraviolet radiation through a mask and then patterned in a developer solution. For full-color chips, multiple layers of translucent filter material are applied to the wafer by spin coating, and then etched away as needed, to yield the three primary-color-filtered linear arrays of photosensors  14 , as known to those of ordinary skill in the art. Only after the filter layers are applied as desired is the wafer “diced,” or sawed into individual chips.  
           [0005]    In the foregoing method of fabricating a full-color photosensitive chip  10 , a problem may arise when applying successive translucent filter layers. In particular, the process of applying a filter coat to the chip may cause the coat to be thicker on some photosensors than on others. Different thicknesses of the filter coat result in different intensities of light passing through the filter material to a particular photosensor. Such variations may result in diminished reproduction quality. For photosensors of a particular type on a single chip, it is desirable that the filter coat be of uniform thickness. In addition, when applying a filter coat, it is desirable to leave a smooth surface on the chip on which to apply the next filter coat. If the surface is not smooth, color reproduction quality can suffer.  
           [0006]    One method of smoothing a first filter coat before applying a second filter coat involves grinding and/or polishing. Specifically, after the first filter coat is applied, another layer, such as a polyimide layer, is applied on top. Next, the polyimide layer is ground and/or polished down to the level of the first filter coat. Finally, the second filter coat is applied on the ground surface. One drawback of this technique, however, is that grinding and/or polishing the polyimide layer can be a time-consuming and inefficient process to smooth a surface of a photosensitive chip, resulting in waste and increased  20  production time.  
         SUMMARY OF THE INVENTION  
         [0007]    For the foregoing reasons, there exists in the art a need for systems and methods for fabricating an electro-optical device for sensing images in an image forming system. The methods can include smoothing a surface of a photosensitive chip by applying a smoothing or inter-filter layer. The inter-filter layer can be applied on a portion of a filter layer, and then left thereon, thereby smoothing the surface to ready it for the application of another filter layer.  
           [0008]    In particular a method of fabricating an electro-optical device suitable for use in an image forming system is presented herein including applying a first filter layer above a substrate, and then applying an inter-filter layer over at least the first filter layer. The method also includes applying a second filter layer over at least a portion of the inter-filter layer without removing the inter-filter layer.  
           [0009]    In accordance with the teachings of the present invention, a method of fabricating an electro-optical device, such as a linear array chip, is also presented herein that includes providing a substrate that functions as a foundation for the application of other layers. Next, a first photosensor and a second photosensor can be, if desired, inserted into the substrate of the electro-optical device. The photosensors can be embedded into the substrate to avoid creating topographical artifacts. A clear base layer is then optionally applied on the substrate. An area of the base layer that overlies the first photosensor is subsequently covered with a patterned first filter coat or layer. If the base layer is omitted, an area of the substrate that overlies the first photosensor is covered with a patterned first filter layer. The first filter layer preferentially allows light having a wavelength within a first range to reach the first photosensor. For this purpose, the first filter layer may contain a dye or pigment. Next, an inter-filter layer is applied on the patterned first filter layer and on an area of the base layer not covered by the patterned first filter layer, thereby smoothing a top surface of the electro-optical device. If the base layer is omitted, an inter-filter layer is applied on the patterned first filter layer and on an area of the substrate not covered by the patterned first filter layer. The inter-filter layer may be translucent, or colorless and may be composed of polyimide or acrylic. In another embodiment, the inter-filter layer can be colored and act as a filter itself. For example, a colored inter-filter layer can be used for cyan, magenta, and yellow filters. Cyan can be laid down over a first photosensor and then yellow can be utilized as an inter-filter layer, the combination yielding green. Subsequently, without removing the inter-filter layer, an area of the inter-filter layer that overlies the second photosensor is covered with a patterned second filter layer. The second filter layer preferentially allows light having a wavelength within a second range to reach the second photosensor. A second inter-filter layer may further be applied on the patterned second filter layer and on an area of the inter-filter layer not covered by the patterned second filter layer, thereby smoothing a second top surface of the electro-optical device. Additional filter layers may be applied in the above manner as needed.  
           [0010]    The present invention also provides for an electro-optical device for image sensing having a substrate, and first and second photosensors disposed within the substrate. The device also includes an optional base layer disposed on the substrate. A patterned first filter layer is disposed on an area of the base layer that overlies the first photosensor. If the base layer is omitted, a patterned first filter layer is disposed on an area of the substrate that overlies the first photosensor. The first filter layer preferentially allows light having a wavelength within a first range to reach the first photosensor. The device further includes an inter-filter layer that is disposed permanently on the patterned first filter layer and on an area of the base layer not covered by the patterned first filter layer. If the base layer is omitted, the device further includes an inter-filter layer disposed permanently on the patterned first filter layer and on an area of the substrate not covered by the patterned first filter layer. The inter-filter layer thereby smoothes a top surface of the electro-optical device. The electro-optical device also includes a patterned second filter layer disposed over the second photosensor on the inter-filter layer. The second filter layer preferentially allows light having a wavelength within a second range to reach the second photosensor.  
           [0011]    The electro-optical device may further include a second inter-filter layer disposed on the patterned second filter layer and on an area of the inter-filter layer not covered by the patterned second filter layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The aforementioned features and advantages, and other features and aspects of the present invention, will become better understood with regard to the following description and accompanying drawings.  
         [0013]    [0013]FIG. 1 is a plan view of a conventional photosensitive chip.  
         [0014]    FIGS.  2 A-G illustrate in cross-section the steps for fabricating an electro-optical device for sensing images in an image forming system according to the teachings of the present invention.  
         [0015]    [0015]FIG. 3 is a schematic flow chart diagram illustrating the steps for fabricating an electro-optical device according to the teachings of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    FIGS.  2 A-G illustrate in cross-section an electro-optical device  200  for sensing images in an image forming system. Image forming systems include electrophotographic, electrostatic or electrostatographic, ionographic, and other types of image forming or reproducing systems that are adapted to capture and/or store image data associated with a particular object, such as a document. The system of the present invention is intended to be implemented in a variety of environments, such as in any of the foregoing types of image forming systems, and is not limited to the specific systems described below.  
         [0017]    Referring to FIG. 2A, a photosensitive chip, such as the chip  10  of FIG. 1, can be fabricated by providing a substrate  20 . A number of photosensors  21 - 23  can be disposed within the substrate  20 . Some surface irregularities are also shown as the surface topography  24 . With a purpose of smoothing the surface irregularities that form the surface topography  24 , a clear base layer  25  may be disposed on the top surface of the substrate. In other embodiments, the application of this clear base layer  25  may be omitted. As used herein, the term “smoothing” is intended to include reducing, eliminating, or preventing the formulation of relatively sharp profiles of irregularities or other formed topographical structures present in one or more layers of the chip, so as to promote or enhance the transfer or flow of a fluid material, such as the filter material, across the surface of the chips without creating significant layer thickness irregularities as measured across the surface of the chip.  
         [0018]    Referring now to FIG. 2B, a first filter layer  26  is disposed on top of the clear base layer  25 . In one embodiment, the filter layer  26  may be applied using the technique of spin coating, as known to those of ordinary skill in the art. The filter layer  26  may contain, for example, acrylic, or polyimide and, in addition to filtering light, may act as a photoresist.  
         [0019]    Referring to FIG. 2C, the first filter layer  26  is patterned. In particular, methods known to those of ordinary skill in the art, such as etching, may be used to form a patterned first filter layer  27 . One of the aims of the filter coat patterning is to dispose the first filter layer  26  on an area  266  of the base layer  25  that overlies the first photo sensor  2   1 . If the base layer  25  is omitted, the first filter layer  26  is disposed on an area  201  of the substrate  20  that overlies the first photosensor  2   1 . Covering an area  266  of the base layer  25  that overlies the first photo sensor  21  with a patterned first filter layer  27  preferentially allows light having a wavelength within a first range to reach the first photo sensor  2   1 . For example, the first filter layer  26  may be pigmented or dyed so that the only light that reaches the first photo sensor  21  is light having a wavelength within a small range of frequencies near the frequency of a first primary color, such as red, green, or blue.  
         [0020]    Referring to FIG. 2D, an inter-filter layer  28  is disposed permanently over the patterned first filter layer  27  and on an area  288  of the base layer  25  not covered by the patterned first filter layer  27 . If the base layer is omitted, an inter-filter layer  28  is disposed permanently on the patterned first filter layer  27  and at least on a portion  202  of the substrate  20 , such as on an area  202  of the substrate  20  not covered by the patterned first filter layer  27 . The term “inter-filter layer” as used herein is intended to include any suitable layer compatible with the other chip layers for allowing radiation to pass therethrough and for smoothing the topography (surface) of the chip  10 . The inter-filter layer can be composed of any suitable material sufficient to allow radiation to pass therethrough, such as acrylic, polyimide or other optically transmissive film-forming polymer material. The inter-filter layer  28  acts to smooth the top surface of the assembly shown in FIG. 2C to prepare the surface for the application of a second filter layer. The inter-filter layer  28  is disposed permanently in the sense that it is not necessary to remove the inter-filter layer  28  by grinding and/or polishing to the level of the patterned first filter layer  27  prior to the application of the second filter layer. Instead, in what is an advantage of the present invention, the second filter layer is applied directly on the inter-filter layer  28  without having to remove or grind down the layer  28 . In one embodiment, the inter-filter layer  28  is translucent and clear. In another embodiment, the inter-filter layer  28  may be translucent, but have a slight color. This latter embodiment may be useful in cases where the inter-filter layer is used to modify the incoming wavelengths in a similar fashion as the filters. The inter-filter layer  28  may be composed of any optically transmissive, film-forming polymer, such as acrylic, polyimide, polymethylmethacrylate (PMMA), and/or diazonovolak compounds.  
         [0021]    Referring to FIG. 2E, a second filter layer  29  is disposed over the inter-filter layer  28 . In one embodiment, the second filter layer  29  can also be applied using spin coating, as known to those of ordinary skill in the art. The second filter layer  29  may contain, for example, acrylic or polyimide and, in addition to acting as a filter of light, may act as a photoresist.  
         [0022]    As illustrated in FIG. 2F, the second filter layer  29  can then be patterned. In particular, methods known to those of ordinary skill in the art, such as etching, may be used to form a patterned second filter layer  298 . One of the aims of the patterning is to dispose the second filter layer  29  over at least a portion  299  of the inter-filter layer  28 , such as on an area  299  of the inter-filter layer  28  that overlies the second photosensor  22 . Covering an area  299  of the inter-filter layer  28  that overlies the second photosensor  22  with a patterned second filter layer  298  preferentially allows light having a wavelength within a second range to reach the second photosensor  22 . For example, the second filter layer  29  may be pigmented or dyed so that only light having a wavelength within a small range of frequencies near the frequency of a second primary color reaches the second photosensor  22 .  
         [0023]    As illustrated in FIG. 2G, if additional layers are desired, a second inter-filter layer  270  can be disposed over the patterned second filter layer  298  and over an area  271  of the first inter-filter layer  28  not covered by the patterned second filter layer  298 , as described above with reference to FIG. 2D.  
         [0024]    Those of ordinary skill will readily recognize that any number of additional layers and intermediate inter-filter layers can be formed on the substrate. For example, a third filter layer pigmented to pass light corresponding to another primary color can be disposed over photosensor  23 .  
         [0025]    Referring to FIG. 3, a flow chart is shown illustrating the steps of fabricating an electro-optical device  200  for image sensing according to the teachings of the present invention. In step  31 , a substrate  20  of the electro-optical device  200  is provided, which functions as a foundation on which additional layers are applied. In step  32 , any suitable number of photosensors, such as a first photosensor  21  and a second photosensor  22 , are inserted into the substrate  20  of the electro-optical device  200 . In optional step  33 , a base layer  25  is applied on the substrate  20  by, for example, spin coating. In other embodiments of the present invention, the step of applying a base layer  25  may be omitted. In step  34 , an area of the base layer  25  that overlies one or more photosensors, such as the first photosensor  21 , is covered with a patterned first filter layer  27 . The first filter layer  26  preferably allows light having a wavelength within a first range to reach the first photosensor  21 . To cover the base layer  25  over the first photosensor  21  with the patterned first filter layer  27 , the layer  26  may initially be applied on the whole surface of the base layer  25  and subsequently etched to leave the patterned filter layer  27  disposed over the first photosensor  21 . Next, in step  35 , the inter-filter layer  28  is applied on the patterned first filter layer  27  and on an area of the base layer  25  not covered by the patterned first filter layer  27 , thereby smoothing a top surface of the electro-optical device  200 . Subsequently, in step  36 , without removing the inter-filter layer  28 , the inter-filter layer  28  may be covered over the second photosensor  22  with a second filter layer  29 , the second filter layer  29  preferentially allowing light having a wavelength within a second range to reach the second photosensor  22 . As in step  34 , the second filter  29  may first be applied over the whole surface and then etched to leave the patterned second filter layer  298  over the second photosensor  22 .  
         [0026]    This process of smoothing the top surface with an inter-filter layer before applying a filter layer can be repeated as needed. For example, a second inter-filter layer can be added over the second filter layer for the coating of a third filter layer.  
         [0027]    While the present invention has been described with reference to illustrative embodiments thereof, those skilled in the art will appreciate that various changes in form and detail may be made without departing from the intended scope of the present invention as defined in the appended claims.