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, to sense the presence of color in a document. The methods and systems provide for leaving a first filter layer on a non-sensor area to prepare the device for a substantially uniform application of a second filter layer.

Description:
TECHNICAL FIELD 
     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 
     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 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. 
       FIG. 1  is a plan view of two photosensitive chips  10  fashioned from a single wafer  11 . The chips  10  are of a general design found, for example, in a full-color photosensor scanner of the prior art. The chip  10  includes a number of bonding pads  12 , and one or more linear arrays of photosensors  14 . A typical design of a full-page-width scanner will include a plurality of chips  10 , each chip being approximately one-half to one inch in length, the chips  10  being butted end-to-end to form a collinear array of photosensors  14 , which extends across a page image being scanned. 
     Each chip  10  is a silicon-based integrated circuit chip having on a surface three independently functioning linear arrays of photosensors  14 . The photosensors  14  are disposed in 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 . 
     One method of constructing a full-color photosensitive chip  10  according to the prior art is to first construct a wafer  11  having a relatively large number, such as one hundred or more, semiconductor structures, each structure corresponding to one chip  10 . Two such chips are shown in FIG.  1 . For full-color chips, the wafer  11  is coated with multiple layers of translucent filter material by spin coating. A filter liquid, corresponding to a filter of a particular color, is poured near the center of the wafer  11 , and then the wafer  11  is spun about an axis  17  to spread the liquid. The filter material may then be 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. 
     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. 
     SUMMARY OF THE INVENTION 
     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. Accordingly, described herein is a method of fabricating an electro-optical device suitable for use in an image forming system, the method including the steps of imbedding a sensor in a substrate to form a sensor area and a non-sensor area, and applying a first filter layer on at least a portion of the non-sensor area to at least partially planarize the device. The method further includes the step of applying a second filter layer over at least a portion of the substrate without removing the first filter layer on the non-sensor area. At least one of the first and second filter layers may contain a pigment. The method may further include the step of applying a base layer, which may be translucent, on the substrate before the step of applying a first filter layer. The method may also include the step of mounting the electro-optical device in an image forming system. 
     The method may further include the steps of applying a second filter layer on at least a portion of a second non-sensor area to at least partially planarize the device, and applying a third filter layer over at least a portion of the substrate without removing the second filter layer on the second non-sensor area. The first, second, and third filter layers may correspond to primary colors. 
     Also described herein is a method of making a photosensitive chip for image sensing, the method including the steps of imbedding a photosensor in a substrate of a photosensitive chip, covering a sensor area with a filter layer, the sensor area substantially overlying the photosensor. The method further includes permanently covering a non-sensor area with the filter layer to at least partially planarize a surface of the photosensitive chip, the non-sensor area not substantially overlying the photosensor, and applying a second filter layer over at least a portion of the substrate. 
     A method of applying a filter layer of substantially uniform thickness for an image forming system is also described herein, the method including the steps of providing a wafer containing at least two photosensors, and applying a first filter layer on at least a portion of a non-sensor area of the wafer for applying a second filter layer of substantial uniform thickness over the at least two photosensors. 
     In addition, an electro-optical device suitable for use in an image forming system is provided herein. The device includes a substrate, a sensor embedded in the substrate forming a sensor area and a non-sensor area, a first filter layer on at least a portion of the non-sensor area to at least partially planarize the device, and a second filter layer applied over at least a portion of the substrate without removing the first filter layer on the at least a portion of the non-sensor area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  is a plan view of a conventional photosensitive chip. 
         FIGS. 2A-E  illustrate in cross-section electro-optical devices employed in an image forming system and fabricated according to conventional methods. 
         FIGS. 3A-D  illustrate in cross-section electro-optical devices fabricated according to the teachings of the present invention. 
         FIGS. 4A-D  illustrate in cross-section electro-optical devices fabricated according to the teachings of the present invention. 
         FIG. 5  shows a schematic flow chart diagram illustrating the steps for fabricating an electro-optical device for sensing images according to the teachings of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 2A-2E , a series of cross-sectional views of an electro-optical device is depicted illustrating the maimer in which a photosensitive chip can be manufactured according to conventional methods. Parts of two photosensitive chips  10   a  and  10   b  are shown in cross-section fashioned from the single wafer  11 . In  FIG. 2A , three photosensors  14   a-c  are shown embedded in a substrate  18  in part of the photosensitive chips  10   a  and  10   b . Some surface irregularities are also shown as the surface topography  20 . A clear base layer  22  may be disposed on the top surface of the substrate  18 . 
     Referring now to  FIG. 2B , a first filter layer  24  is disposed on top of the clear base layer  22 . The filter layer  24  may contain, for example, acrylic or polyimide and, in addition to filtering light, may act as a photoresist. The filter layer  24  may be applied using the method of spin coating, where a filter liquid is applied near the center of the wafer  11 , and the wafer is then spun about an axis  17  to spread the filter liquid, and thereby form a first filter layer  24 . Referring to  FIG. 2C , the first filter layer  24  is then patterned by removing the layer  24  from selected regions. In particular, the first filter layer  24  is allowed to remain only in an area substantially on top of the second photosensor  14   b.    
     Referring to  FIG. 2D , a second filter layer  26  is disposed over the substrate  18 . The layer  26  may be applied by spin coating. Referring to  FIG. 2E , the second filter layer  26  is then patterned by removing the layer  26  from selected regions. In particular, the second filter layer  26  is allowed to remain only in an area substantially on top of the third photosensor  14   c.    
     In the foregoing method of fabricating a full-color photosensitive chip  10 , a problem arises when applying successive translucent filter layers. In particular, the process of applying a filter coat to the chip causes the coat to be thicker on some photosensors than on others. For example, in the spin coating technique described above, a liquid filter material is poured near the axis  17 , and the wafer  11  is then spun to spread the liquid on the surface of the substrate  18 , and thus form the second filter layer  26 . However, this method results in a thinner application of the filter layer  26  over the outermost photosensors (i.e., photosensor  14   a  in chip  10   a , and photosensor  14   c  in chip  10   b ) because the first filter layer  24  acts as a kind of dam that causes a backup of liquid before the dam, and a dearth of liquid after the dam. The result is that the second filter layer  26  is thicker over the photosensor  14   c  of chip  10   a , than over the photosensor  14   c  of chip  10   b.    
     Sensors placed next to each other in the linear imaging array are formed from different parts of the wafer  11 . Therefore, any two photosensors in the array sensing the same color of light may have above them filter layers of different thicknesses, resulting in different intensities of light passing through the disparate filter layers to the photosensors below. Such variations result in diminished image reproduction quality. 
     Referring to  FIGS. 3A-3D , a series of diagrams is depicted illustrating the manner in which an electro-optical device, such as a photosensitive chip  10 , for sensing images in an image forming system, can be manufactured according to the teachings of the present invention. 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. 
     Referring to  FIG. 3A , two electro-optical devices  28   a  and  28   b  are shown in cross-section fashioned from the single wafer  11 . Three photosensors  14   a-c  are embedded in a substrate  18  in each of the electro-optical devices  28   a  and  28   b . Some surface irregularities are also shown as the surface topography  20 . The surface topography  20  may arise, for example, from circuit elements, such as electrical leads, placed on the electro-optical devices  28   a  and  28   b . A translucent base layer  22  may be disposed on the top surface of the substrate  18 . 
     The photosensors  14   a-c  include a device adapted to output a signal indicative of the frequency or intensity of light impinging thereon. Various photosensors, such as charge coupled devices, and complimentary metal oxide semiconductor sensors, are known in the art that can be used in the photosensitive chip  10 . 
     The translucent base layer  22  can be used for smoothing the surface irregularities that form the surface topography  20 . In other embodiments, the application of this translucent base layer  22  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 electro-optical device to promote or enhance the transfer or flow of a liquid material, such as the filter material, across the surface of the electro-optical device without creating significant layer thickness irregularities as measured across the surface of the device. 
     Referring now to  FIG. 3B , two electro-optical devices  30   a  and  30   b  are shown in cross-section fashioned from the single wafer  11 . A first filter layer  32  is disposed on top of a portion of the translucent base layer  22 . In particular, the first filter layer  32  covers a sensor area  34 . The term sensor area refers to an area of an electro-optical device, such as device  30   a  or  30   b , substantially overlying a photosensor, such as photosensor  14   b . In addition, the first filter layer  32  covers a non-sensor area  36 . The term non-sensor area refers to an area of an electro-optical device, such as device  30   a  or  30   b , not substantially overlying a photosensor. The layer  32  is left on the non-sensor area  36  as a second filter layer  42  ( FIG. 3C ) is applied, as described below. In one embodiment, the first filter layer  32  lying above the non-sensor area  36  may be removed after the second filter layer  32  has been applied. In another embodiment, the first filter layer  32  is left permanently on the non-sensor area  36 , and not removed even after the application of subsequent filter layers. 
     The first filter layer  32  may be applied using the method of spin coating, where a filter liquid is applied near the center of the wafer  11 , and the wafer then spun about an axis  17  to spread the filter liquid, and thereby form a first filter layer  32 . The first filter layer  32  is then patterned by removing, or etching away the layer  32  from selected regions. In particular, the first filter layer  32  is removed from the sensor areas  38 , areas above the first photosensor  14   a  and third photosensor  14   c . The result is the electro-optical device  30   a  or  30   b.    
     Covering the sensor area  34  that overlies the second photosensor  14   b  with a first filter layer  32  preferentially allows light having a wavelength within a first range to reach the photosensor  14   b . For example, the first filter layer  32  may be pigmented or dyed so that the only light that reaches the photosensor  14   b  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. In this manner, the photosensor  14   b  can be made sensitive to a first range of frequencies to help in producing color images in image forming systems. 
     Leaving the first filter layer  32  on the non-sensor area  36  as a second filter layer  42  ( FIG. 3C ) is applied, as described below, functions to at least partially planarize the electro-optical devices  30   a  and  30   b . In addition to partially planarizing the surface, leaving the first filter layer over non-sensing areas provides for a more structurally uniform surface that is presented to the second filter layer. The second filter can flow in the same fashion across the entire wafer. Even though the surface is not completely smooth, the differences between chips are reduced as the second filter behaves in a similar fashion on every chip. Partially planarizing the devices  30   a  and  30   b  in this manner improves the uniformity of the second filter layer  42  in a subsequent application. 
     The term planarize denotes adding layers in such a fashion that surface topography is evened out so that the surface approaches a flat, planar surface. The degree of planarization is indicative of how much the surface has been smoothed after the treatment. For example, if there are 2 micron step heights before treatment and treatment A leaves step heights of 1 micron and treatment B leaves step heights of 0.2 microns, it can be said that treatment A has partially planarized the surface and treatment B has almost completely planarized the surface. The amount of surface topography left after the treatment describes the degree of planarization. A conformal layer coats the surface and follows the contours. It does not reduce the topography by much and can be said to have a low degree of planarization. Leaving the filter behind over the non sensor areas does partially planarize the surface, in addition to leaving behind a more periodic, uniform topography. 
     The wafer  11  may eventually be cut up to produce individual chips  10 , which may then be butted together to form a linear array as in FIG.  1 . It is advantageous when photosensors on different chips sensitive to the same color have a filter layer thereon of uniform thickness. When the filter layers over such photosensors have a substantially uniform thickness, a condition that obtains when the principles of the present invention are utilized, the production quality of color documents is increased. 
     Referring now to  FIG. 3C , two electro-optical devices  40   a  and  40   b  are shown in cross-section fashioned from the single wafer  11 . The second filter layer  42  covers a sensor area  44 . The term sensor area refers to an area of an electro-optical device, such as device  40   a  or  40   b , substantially overlying a photosensor, such as photosensor  14   c . In addition, the second filter layer  42  covers a non-sensor area  46 . The term non-sensor area refers to an area of an electro-optical device, such as device  40   a  or  40   b , not substantially overlying a photosensor. The layer  42  is left on the non-sensor area  46  as a third filter layer  52  ( FIG. 3D ) is applied, as described below. In one embodiment, the second filter layer  42  lying above the non-sensor area  46  may be removed after the third filter layer  52  has been applied. In another embodiment, the third filter layer  52  is left permanently on the non-sensor area  46 , and not removed even after the application of subsequent filter layers. 
     The second filter layer  42  may be applied using the method of spin coating, where a filter liquid is applied near the center of the wafer  11 , and the wafer then spun about an axis  17  to spread the filter liquid, and thereby form a second filter layer  42 . The second filter layer  42  is then patterned by removing, or etching away the layer  42  from selected regions. In particular, the second filter layer  42  is removed from the sensor areas  48 , areas above the first photosensor  14   a  and second photosensor  14   b . The result is the electro-optical device  40   a  or  40   b.    
     Covering the sensor area  44  that overlies the third photosensor  14   c  with a second filter layer  42  preferentially allows light having a wavelength within a second range to reach the photosensor  14   c . For example, the second filter layer  42  may be pigmented or dyed so that the only light that reaches the photosensor  14   c  is light having a wavelength within a small range of frequencies near the frequency of a second primary color, such as red, green, or blue. In this manner, the photosensor  14   c  can be made sensitive to a second range of frequencies to help in producing color images in image forming systems. 
     Leaving the second filter layer  42  on the non-sensor area  46  as a third filter layer  52  ( FIG. 3D ) is applied, as described below, functions to at least partially planarize the electro-optical devices  40   a  and  40   b . Partially planarizing the devices  40   a  and  40   b  in this manner improves the uniformity of the third filter layer  52  in a subsequent application. 
     Referring now to  FIG. 3D , two electro-optical devices  50   a  and  50   b  are shown in cross-section fashioned from the single wafer  11 . The third filter layer  52  covers a sensor area  54 . In addition, the third filter layer  52  covers a non-sensor area  56 . In one embodiment, the third filter layer  52  lying above the non-sensor area  56  may be removed, or left behind permanently. 
     The third filter layer  52  may be applied using the method of spin coating, where a filter liquid is applied near the center of the wafer  11 , and the wafer then spun about an axis  17  to spread the filter liquid, and thereby form a third filter layer  52 . The third filter layer  52  is then patterned by removing, or etching away the layer  52  from selected regions. In particular, the third filter layer  52  is removed from the sensor areas  58 , areas above the second photosensor  14   b  and third photosensor  14   c . The result is the electro-optical device  50   a  or  50   b.    
     Covering the sensor area  54  that overlies the first photosensor  14   a  with a third filter layer  52  preferentially allows light having a wavelength within a third range to reach the photosensor  14   a . For example, the third filter layer  52  may be pigmented or dyed so that the only light that reaches the photosensor  14   a  is light having a wavelength within a small range of frequencies near the frequency of a third primary color, such as red, green, or blue. In this manner, the photosensor  14   a  can be made sensitive to a third range of frequencies to help in producing color images in image forming systems. 
     Those of ordinary skill will readily recognize that any number of additional layers can be formed on the substrate. Leaving filter layers on non-sensor areas as described above will likewise improve the uniformity of these layers. 
     The principles of the present invention can also be applied to other primary colors, such as cyan, magenta, and yellow (CMY). Referring to  FIG. 4A-D , a series of diagrams is depicted illustrating the manner in which an electro-optical device, such as a photosensitive chip  10 , for sensing images in an image forming system, can be manufactured according to the present invention using the three primary colors CMY. 
     Referring to  FIG. 4A , a base layer  22  is disposed on a substrate  18  as in FIG.  3 A. In  FIG. 4B , a cyan filter layer  60  overlies the photosensors  14   a  and  14   b . The cyan filter layer  60  is etched away from above the third photosensor  14   c . The cyan filter layer is placed over the non-sensor areas  62  to at least partially planarize the electro-optical device, thereby allowing a more uniform application of subsequent layers. Referring to  FIG. 4C , a yellow filter layer  64  is placed over the photosensors  14   b  and  14   c . Likewise, the yellow filter layer  64  is placed over the non-sensor areas  66  to at least partially planarize the electro-optical device. Referring to  FIG. 4D , a magenta filter layer  68  is placed over the photosensors  14   a , and  14   c . Likewise, the magenta filter layer  68  is placed over the non-sensor areas  70 . The cyan and yellow, magenta and yellow, and magenta and cyan filter layer combinations produce sensitivity to green, red, and blue colors, respectively. The electro-optical device thus obtained can be used in an image forming system according to the present invention. 
     Referring to  FIG. 5 , a flow chart is shown illustrating the steps of fabricating an electro-optical device for image sensing according to the teachings of the present invention. In step  72 , a substrate  18  of the electro-optical device is provided, which functions as a foundation on which additional layers are applied. The substrate  18  may be part of a wafer  11  on which several photosensitive chips  10  are manufactured. In step  74 , any suitable number of photosensors, such as first, second, and third photosensors  14   a-c , are imbedded in the substrate  18 , which results in the formation of sensor areas and non-sensor areas. The wafer  11  may be diced up into chips  10  containing such triples of photosensors  14 . In optional step  76 , a base layer  22  is applied on the substrate  20  by, for example, adding a liquid base layer material and spinning the wafer  11  to spread the material on the surface thereof. In step  78 , a first filter layer  32  is applied on at least a portion of the non-sensor area to at least partially planarize the device. Subsequently, in step  80  a second filter layer  42  is applied over at least a portion of the substrate  18  without removing the first filter layer  32  on the at least a portion of the non-sensor area. 
     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.