Patent Publication Number: US-6222180-B1

Title: Electro optical devices with reduced filter thinning on the edge pixel photosites and method of producing same

Description:
THIS APPLICATION IS A DIVISIONAL OF APPLICATION SER. NO. 09/196,394, FILED NOV. 19, 1998 
     Attention is directed to copending application U.S. patent application Ser. No. 09/641292, filed Aug. 18, 2000, entitled, “ELECTRO OPTICAL DEVICES WITH REDUCED FILTER THINNING ON THE EDGE PIXEL PHOTOSITES AND MEHTOD FOR PRODUCING SAME”. The disclosure of this application is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electro optical devices with a reduced filter thinning on the edge or outer pixels and a method for reducing the thinning of filter layers on the pixel photosites dosest to the edge of an electro optical device such as a photosensitive chip, as would be used, for example, in a full-color digital copier or scanner. 
     BACKGROUND OF THE INVENTION 
     Image sensors for scanning document images, such as charge coupled devices (CCDs), typically have a row or linear array of photosites together with suitable supporting circuitry integrated onto a semiconductor chip. Usually, a sensor is used to scan line by line across the width of a document with the document being moved or stepped lengthwise in synchronism therewith. A typical architecture for such a sensor array is given, for example, in U.S. Pat. No. 5,153,421. 
     In a full-pagewidth image scanner, there is provided a linear array of photosensors which extends the full width of an original document, such as eleven inches. When the original document moves past the linear array, each of the photosensors converts reflected light from the original image into electrical signals. The motion of the original image perpendicular to the linear array causes a sequence of signals to be output from each photosensor, which can be converted into digital data. 
     A currently preferred design for creating such a long linear array of photosensors is to provide a set of relatively small semiconductor chips, each semiconductor chip defining thereon a linear array of photosensors along with ancillary circuit devices. These chips are assembled end-to-end to form a single linear array of photosensors as disclosed in U.S. Pat. No. 5,473,513. However, there are also single chip applications in which a single chip having a linear array may be used for sensing images and converting those images into electrical signals to be output from each photosensor. These electrical signals can be converted into digital data. 
     With the gradual introduction of color-capable products into the office equipment market, it has become desirable to provide scanning systems which are capable of converting light from full-color images into separate trains of image signals, each train representing one primary color. In order to obtain the separate signals relating to color separations in a full-color image, one technique is to provide on a semiconductor chip multiple parallel linear arrays of photosensors, each of the parallel arrays being sensitive to one primary color. Typically, this arrangement can be achieved by providing multiple linear arrays of photosensors which are physically identical except for a translucent primary-color overlay over the photosensitive areas, or “photosites,” for that linear array. In other words, the linear array which is supposed to be sensitive to red light only will have a translucent red layer placed on the photosites thereof, and such would be the case for a blue-sensitive array and a green-sensitive array. As the chip is exposed to an original full-color image, only those portions of the image, which correspond to particular primary colors, will reach those photosensors assigned to the primary color. These chips can also be assembled end to end to form a full width array comprising a multiple parallel linear arrays of photosites. 
     The most common substances for providing these translucent filter layers over the photosites is polyimide or acrylic. For example, polyimide is typically applied in liquid form to a batch of photosensor chips while the chips are still in undiced, wafer form. After the polyimide liquid is applied to the wafer, the wafer is centrifuged to provide an even layer of a particular polyimide. In order to obtain the polyimide having the desired primary-color-filtering properties, it is well known to dope the polyimide with either a pigment or dye of the desired color, and these dopants are readily commercially available. When it is desired to place different kinds of color filters on a single chip, a typical technique is to first apply an even layer of polyimide over the entire main surface of the chip (while the chip is still part of the wafer) and then remove the unnecessary parts of the filter by photo-etching or another well known technique. Typically, the entire filter layer placed over the chip is removed except for those areas over the desired set of photosites. Acrylic is applied to the wafer in a similar manner. 
     SUMMARY OF THE INVENTION 
     According to a first embodiment of the present invention, a semiconductor device includes a main surface including a plurality of photosites and bonding pads defined in the main surface, wherein the photosites include inner photosites and outer photosites. A dear layer is deposited over the main surface exclusive of the bonding pads and outer photosites. A first primary color filter layer is deposited over at least first inner photosite and first outer photosite, the first primary color filter transmitting a first primary color. A second primary color filter layer is deposited over at least a second inner photosite and a second outer photosite, wherein the second primary color filter layer transmits a second primary color. A third primary color filter layer is deposited over at least a third inner photosite and a third outer photosite, wherein the third primary color layer transmits a third primary color. The clear layer and filter layers are preferably polyimide or acrylic. 
     According to a second embodiment, a semiconductor chip includes a main surface including a plurality of photosites and bonding pads defined in the main surface, wherein the photosites include inner photosites and outer photosites. A first clear layer is deposited over the main surface exclusive of the bonding pads, and a second clear layer is deposited over the main surface exclusive of the bonding pads and outer photosites. A first primary color filter layer is deposited over at least first inner photosite and first outer photosite. The first primary color filter transmits a primary color. A second primary color filter layer is deposited over at least a second inner photosite and a second outer photosite, wherein the second primary color filter layer transmits a second primary color. A third primary color filter layer is deposited over at least a third inner photosite and a third outer photosite, wherein the third primary color layer transmits a third primary color. The clear layer, the second clear layer and the filter layers are preferably polyimide or acrylic 
     The semiconductor devices of the first embodiment may be placed in a digital copier, which includes a raster input scanner scanning documents to generate digital image signals, the raster input scanner including a plurality of generally rectangular chips, which are assembled end to end on a substrate forming a full width array of multiple parallel linear arrays of photosites. Each chip includes a main surface including bonding pads and the photosites defined in the main surface, wherein the photosites include inner photosites and outer photosites, a clear layer deposited over the main surface exclusive of the bonding pads and outer photosites, and a first primary color filter layer deposited over at least first inner photosite and first outer photosite, the first primary color filter transmitting a primary color. 
     The semiconductor devices of the second embodiment may be placed in a digital copier including a raster input scanner scanning documents to generate digital image signals, the raster input scanner including a plurality of generally rectangular chips, which are assembled end to end on a substrate forming a full width array of multiple parallel linear arrays of photosites. Each chip includes a main surface including bonding pads and the photosites defined in the main surface, wherein the photosites include inner photosites and outer photosites, a clear layer deposited over the main surface exclusive of the bonding pads, a second clear layer deposited over the main surface exclusive of the bonding pads and outer photosites, and a first primary color filter layer deposited over at least first inner photosite and first outer photosite, the first primary color filter transmitting a primary color. 
     A method for fabricating a photosensitive device of the first embodiment comprises providing a semiconductor wafer having a main surface defining chip areas separated by V-grooves, the chip areas defining bonding pads and three rows of photosites, wherein the photosites include inner photosites, outer photosites and bonding pads; depositing a dear layer on the semiconductor wafer, soft baking the semiconductor wafer; exposing selective areas of the semiconductor wafer, etching the clear layer covering the bonding pads and outer photosites from the semiconductor wafer; hard baking the semiconductor wafer; and depositing a first primary color filter layer over at least first inner photosite and first outer photosite, the first primary color filter transmitting a primary color. The method for fabricating a semiconductor device includes dicing the semiconductor wafer to provide semiconductor chips. 
     A method for fabricating a photosensitive chip of the second embodiment comprises providing a semiconductor wafer having a main surface defining chip areas separated by V-grooves, the chip areas defining bonding pads and three rows of photosites, wherein the photosites include inner photosites, outer photosites and bonding pads; depositing a first clear layer on the semiconductor wafer; soft baking the semiconductor wafer; exposing selective areas of the semiconductor wafer, etching the first clear layer covering the bonding pads from the semiconductor wafer; hard baking the semiconductor wafer; depositing a second clear layer on the semiconductor wafer; exposing selective areas of the semiconductor wafer; etching the second clear layer covering the bonding pads and outer photosites from the semiconductor wafer; hard baking the semiconductor wafer; and depositing a first primary color filter layer over at least first inner photosite and first outer photosite, the first primary color filter transmitting a primary color. The method for fabricating a semiconductor device further includes dicing the semiconductor wafer to provide semiconductor chips. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing two chips relevant to the present invention; 
     FIG. 2 is a perspective view showing a semiconductor wafer relevant to the present invention; 
     FIG. 3 is a cross sectional view along the line  3 — 3  in the direction of the arrows in FIG. 1, showing polyimide or acrylic layers deposited on a section of the semiconductor wafer in accordance with the prior art; 
     FIG. 4 is a partial cross sectional view along the line  4 — 4  in the direction of the arrows in FIG. 1, showing polyimide or acrylic layers deposited on a section of the semiconductor wafer in accordance with the prior art; 
     FIG. 5 is a partial cross-sectional view along the line  4 — 4  in the direction of the arrows in FIG. 1, showing a section of the semiconductor wafer before the acrylic or polyimide layers are deposited in accordance with embodiments of the present invention; 
     FIG. 6 shows a clear layer deposited on the section of the semiconductor wafer of FIG. 5 in accordance with the embodiments of the present invention; 
     FIG. 7 shows outer photosites etched out of the section of the semiconductor wafer shown in FIG. 6 in accordance with a first embodiment of the present invention; 
     FIG. 8 shows a filter layer deposited on the semiconductor wafer of FIG. 7 in accordance with the first embodiment of the present invention; 
     FIG. 9 shows a second dear layer deposited on the dear layer of FIG. 6 in accordance with a second embodiment of the present invention; 
     FIG. 10 shows the second clear layer of the outer photosites etched out of the section of the semiconductor wafer shown in FIG. 9 in accordance with the second embodiment of the present invention; 
     FIG. 11 shows a filter layer deposited on the semiconductor wafer of FIG. 10 in accordance with the second embodiment of the present invention; and 
     FIG. 12 is a partial schematic elevational view of an example of a digital copier, which can employ the photosensitive chips produced from the first and second embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     FIG. 1 is a perspective view showing two photosensitive chips  10  relevant to the claimed invention. The chips  10  are generally made of a semiconductor substrate, as is known in the art, in which circuitry and other elements are formed, such as by photolithographic etching. A few of the most relevant structures are a linear array of pixel photosites  12 , each of which forms the photosensitive surface of photosensor circuitry within each chip  10 , and a set of bonding pads  14 . The pixel photosites  12  are typically arranged in a linear array along one main dimension of each chip  10 , with each pixel photosite  12  along the array corresponding to one pixel in the image signal. As will be described in detail below, the pixel photosite  12  includes photosites  12 B,  12 G and  12 R for sensing the three primary colors (blue, green and red) corresponding to the pixel. 
     The bonding pads  14  are distinct surfaces on the main surface of the chips  10 , and are intended to accept wire bonds attached thereto. The bonding pads  14  thus serve as the electronic interface between the chips  10  and any external circuitry. The circuitry for obtaining signals related to light directed to the pixel photosites  12 , and for unloading image data from the chips  10  is generally indicated as  16 . The circuitry  16  is generally deposited between the linear array of pixel photosites  12  and a linear array of bonding pads  14 . 
     Chips  10  are typically formed in batches on semiconductor wafers, which are subsequently cleaved, or “diced,” to create individual chips. Typically, the semiconductor wafers are made of silicon. As is known in the art, photolithographically etched V-grooves  18  define precisely the intended boundaries of a particular chip  10  for dicing. Thus, all of the pixel photosites  12 , bonding pads  14  and circuitry  16  for relatively large number of chips  10  are etched simultaneously onto a single semiconductor wafer  20  as shown in FIG.  2 . The region between the V-grooves  18  is called the tab region. The pixel photosite  12  adjacent to each V-groove is referred to as an outer pixel photosite  12   o . Each outer pixel photosite  12   o  consists of three outer photosites  12 B,  12 G, and  12 R. The other pixel photosites  12  are referred to as inner pixel photosites  12   I  and each inner pixel photosite  12   I  consists of three inner photosites  12 B,  12 G, and  12 R. The inner photosites  12 B and outer photosites  128  form a first row of photosites. The inner photosites  12 G and the outer photosites  12 G form a second row of photosites. The inner photosites  12 R and the outer photosites  12 R form a third row of photosites. 
     FIG. 2 shows a typical semiconductor wafer  20 , in isolation, wherein a relatively large number of chips  10  are created in the wafer  20  prior to dicing thereof. Each chip  10  has a distinct chip area within the main surface of the wafer  20 . The phrase “chip area” refers to a defined area within the main surface of the wafer  20  which is intended to comprise a discrete chip  10  after the dicing step, when individual chips  10  are separated from the rest of the wafer  20 . 
     FIG. 3 is a cross sectional view along line  3 — 3  in the direction of the arrows in FIG.  1 . On the main surface of chip  10  there is provided an inner pixel photosite  12 , with three separate inner photosites  12 B,  12 G and  12 R, each sensitive to one primary color. As shown in FIG. 3, within each inner pixel photosite  12   I  is deposited a photosite  12 G, sensitive to green light, a photosite  12 R sensitive to red light, and a photosite  12 B, sensitive to blue light. The three photosites  12 B,  12 G and  12 R are on the whole identical as circuit elements except that the surface of each photosite  12 B,  12 G and  12 R is superimposed thereon by a distinct primary-color filter layer  30 . The blue filter layer, green filter layer and red filter layer are indicated by reference numerals  30 B,  30 G, and  30 R. 
     As is known in the art, such filter layers preferably comprise a polyimide or acrylic which has been doped with a commercially available dye or pigment blended to yield a primary color filter. As is further known in the art, it is common to provide filter layers such as  30 B,  30 G and  30 R, by first placing a polyimide or acrylic in liquid form over the entire main surface of the chip  10 , and then removing the polyimide or acrylic by photolithography in all areas of the chip  10  except where the filter area is desired. To ensure a uniform coating of these materials, the semiconductor wafer  20  is partially planarized by using clear layer  40 , which is preferably a dear polyimide or clear acrylic layer. This clear layer  40  acts to smooth the topography of the semiconductor wafer  20  and partially fill the V-grooves  18  as shown in FIG.  4 . Since the clear layer  40  only partially planarizes the semiconductor wafer  20 , the V-grooves  18  still allow some of the filter material to be channeled away from the outer pixel photosite  12   o , causing thinning of the filter material over the outer pixel photosite  12   o  as shown in FIG.  4 . The outer pixel photosites  12   o  on the semiconductor wafer  20  have substantially thinner filter layers  30  than the inner pixel photosites  12   I  due to the topography of the semiconductor wafer  20  as explained above. The two embodiments of the present invention enhance the image sensing capability of the photosensitive chips  10  by increasing the thickness of the filter layers  30  of the outer pixel photosites  12   o . 
     FIG. 5 is a cross-sectional view along the line  4 — 4  in the direction of the arrows in FIG. 1, showing a section of the semiconductor wafer  20  before the acrylic or polyimide layers are deposited in accordance with the first embodiment of the present invention. A clear layer  40 , which is preferably polyimide or acrylic, is deposited on the semiconductor wafer  20  to smooth the topography of the semiconductor wafer  20  as in the prior art. The coated semiconductor wafer  20  is soft baked (partially baked), and certain areas of the semiconductor wafer  20  are selected for exposure to ultraviolet light using a mask. The dear layer  40  is etched out of the bonding pads  14 . An etched out bonding pad  14  is shown in prior art FIG.  3 . According to the first embodiment of the present invention, the dear layer  40  is also etched out of the outer pixel photosites  12   o  as shown in FIG.  7 . Preferably, the clear layer  40  is etched out using a well-known solvent. The semiconductor wafer  20  is then hard baked. A filter layer  30  is deposited on the semiconductor wafer  20  as shown in FIG.  8 . By etching out the outer pixel photosites  12   o , the outer pixel photosites  12   o  now have a deeper well to collect additional filter material so that there is a thicker filter layer  30  in the outer pixel photosites  12   o  as indicated in FIG.  8 . This enhances the image sensing capability of the photosensitive chips  10 . 
     If only one filter layer  30  was to be deposited on semiconductor wafer  20 , then the semiconductor wafer  20  would be soft baked. Then, certain areas of the semiconductor wafer  20  would be selectively exposed to ultraviolet light using a mask and the filter layer  30  would be etched out of the semiconductor wafer  20  except for the pixel photosites  12 . The semiconductor wafer  20  would then be hard baked and diced to provide chips  10 . 
     However, two additional filter layers  30  are preferably added to semiconductor wafer  20 . Therefore, after the first filter layer  30  is deposited on the semiconductor wafer  20 , the semiconductor wafer  20  is soft baked. Certain areas of the semiconductor wafer  20  are selected for exposure to ultraviolet light using a mask. Preferably, the filter layer  30  is etched out of the semiconductor wafer  20  except for one row of photosites in each chip area, which is shown by the partial cross section of chips  10  in FIG.  8 . The two other rows of photosites in each chip area have substantially the same configuration as shown in the partial cross section of chips  10  in FIG. 7. A second filter layer  30  is deposited on the semiconductor wafer  20 , and these two rows now have substantially the same configuration as shown in FIG.  8 . The semiconductor wafer  20  is then soft baked, selectively exposed to ultraviolet light, selectively etched and hard baked so that two rows of photosites in each chip area each have a different filter layer  30 . The two rows having filter layers  30  in each chip area have substantially the same configuration as shown by the partial cross section in FIG.  8 . The last row of photosites, which does not have a filter layer  30 , has substantially the same configuration as shown in the partial cross section in FIG.  7 . 
     A third filter layer  30  is deposited on the semiconductor wafer  20 . The semiconductor wafer  20  is then soft baked, selectively exposed to ultraviolet light, selectively etched and hard baked so that three rows of photosites now have substantially the same configuration as shown in FIG.  8 . However, each filter layer  30  preferably has a different filter material. Preferably, the three filter layers  30  are red, green and blue. 
     In the second embodiment of the present invention, there is provided a semiconductor wafer  20  having a first dear layer  40  as in the prior art. The coated semiconductor wafer  40  is soft baked (partially bake) and exposed to ultraviolet light as in the prior art The first clear layer  40  is etched out of the bonding pads  14 . An etched out bonding pad  14  is shown in prior art FIG.  3 . The semiconductor wafer  20  is then hard baked. A second dear layer  50  is deposited on the semiconductor wafer  20  as shown in FIG.  9 . The twice coated semiconductor wafer  20  is soft baked (partially baked), and exposed to ultraviolet light. The second clear layer  50  is etched out of both the outer pixel photosites  12   o  as shown in FIG.  10  and the bonding pads  14  as shown in prior art FIG.  3 . Preferably, the second clear layer  50  is etched out using a well-known solvent. The semiconductor wafer  20  is then hard baked. A filter layer  30  is deposited on the semiconductor wafer  20  as shown in FIG.  11 . By etching out the outer pixel photosites  12   o , the outer pixel photosites  12   o  now have a deeper well than the inner pixel photosites  12   I  to collect additional filter material so that there is a thicker filter layer  30  in the outer pixel photosites  12   o  as indicated in FIG.  11 . This enhances the image sensing capability of the photosensitive chips  10 . 
     If only one filter layer  30  was to be deposited on semiconductor wafer  20 , then the semiconductor wafer  20  would be soft baked. Then, certain areas of the semiconductor wafer  20  would be selectively exposed to ultraviolet light using a mask and the filter layer  30  would be etched out of the semiconductor wafer  20  except for the pixel photosites  12 . The semiconductor wafer  20  would then be hard baked and diced to provide chips  10 . 
     However, two additional filter layers  30  are preferably added to semiconductor wafer  20 . Therefore, after the first filter layer  30  is deposited on the semiconductor wafer  20 , the semiconductor wafer  20  is soft baked. Certain areas of the semiconductor wafer  20  are selected for exposure to ultraviolet light using a mask Preferably, the filter layer  30  is etched out of the semiconductor wafer  20  except for one row of photosites in each chip area, which is shown by the partial cross section of chips  10  in FIG.  11 . The two other rows of photosites in each chip area have substantially the same configuration as in the partial cross section of chips  10  in FIG. 10. A second filter layer  30  is deposited on the semiconductor wafer  20 , and these two rows now have substantially the same configuration as shown in FIG.  11 . The semiconductor wafer  20  is then soft baked, selectively exposed to ultraviolet light, selectively etched and hard baked so that two rows of photosites in each chip area each have a different filter layer  30 . The two rows having filter layers  30  in each chip area have substantially the same configuration as shown by the partial cross section in FIG.  11 . The last row of photosites, which does not have a filter layer  30 , has substantially the same configuration as shown by the partial cross section in FIG.  10 . 
     A third filter layer  30  is deposited on the semiconductor wafer  20 . The semiconductor wafer  20  is then soft baked, selectively exposed to ultraviolet light, selectively etched and hard baked so that three rows of photosites now have substantially the same configuration as shown in FIG.  11 . However, each filter layer  30  preferably has a different filter material. Preferably, the three filter layers  30  are red, green and blue. 
     FIG. 12 is a partial schematic elevational view of a digital copier, which can utilize the photosensitive chips  10  of the present invention by assembling them in generally the same manner as in U.S. Pat. No. 5,153,421. However, it is understood that the photosensitive chips  10  may be used together in a full width array or independently in a single chip application in any imaging or scanning device. 
     An original document is positioned in a document handler  227  on a raster-input scanner (RIS) indicated generally by reference numeral  228 . The RIS contains document illumination lamps, optics, a mechanical scanning device and a plurality of photosensitive chips  10  as shown in FIG.  1 . The photosensitive chips  10  may include any one of the photosensitive arrays described above. The RIS captures the entire original document and converts it to a series of raster scan lines. This information is transmitted to an electronic subsystem (ESS) which controls a raster output scanner (ROS). 
     The digital copier employs a photoconductive belt  210 . Preferably, the photoconductive belt  210  is made from a photoconductive material coated on a ground layer, which, in tum, is coated on an anti-curl backing layer. Belt  210  moves in the direction of arrow  213  to advance successive portions sequentially through the various processing stations deposited about the path of movement thereof. Belt  210  is entrained about stripping roller  214 , tensioning roller  220  and drive roller  216 . As roller  216  rotates, it advances belt  210  in the direction of arrow  213 . 
     Initially, a portion of the photoconductive surface passes through charging station A At charging station A, a corona generating device indicated generally by the reference numeral  222  charges the photoconductive belt  210  to a relatively high, substantially uniform potential. 
     At an exposure station B, a controller or electronic subsystem (ESS), indicated generally by reference numeral  229 , receives the image signals representing the desired output image and processes these signals to convert them to a continuous tone or grayscale rendition of the image which is transmitted to a modulated output generator, for example the raster output scanner (ROS), indicated generally by reference numeral  230 . Preferably, ESS  229  is a self-contained, dedicated minicomputer. The image signals transmitted to ESS  229  may originate from a RIS  228  as described above or another type of scanner utilizing the photosensitive chips  10 , thereby enabling the digital copier to serve as a remotely located printer for one or more scanners. Alternatively, the printer may serve as a dedicated printer for a high-speed computer or for one or more personal computers. The signals from ESS  229 , corresponding to the continuous tone image desired to be reproduced by the printer, are transmitted to ROS  230 . ROS  230  includes a laser with rotating polygon mirror blocks. The ROS  230  will expose the photoconductive belt  210  to record an electrostatic latent image thereon corresponding to the continuous tone image received from ESS  229 . As an alternative, ROS  230  may employ a photosensitive array of light emitting diodes (LEDs) arranged to illuminate the charged portion of photoconductive belt  210  on a raster-by-raster basis. 
     After the electrostatic latent image has been recorded on photoconductive surface  212 , belt  210  advances the latent image to a development station, C, where toner, in the form of liquid or dry particles, is electrostatically attracted to the latent image using commonly known techniques. The latent image attracts toner particles from the carrier granules forming a toner powder image thereon. As successive electrostatic latent images are developed, toner particles are depleted from the developer material. A toner particle dispenser, indicated generally by the reference numeral  244 , dispenses toner particles into developer housing  246  of developer unit  238 . 
     With continued reference to FIG. 12, after the electrostatic latent image is developed, the toner powder image present on belt  210  advances to transfer station D. A print sheet  248  is advanced to the transfer station, D, by a sheet feeding apparatus,  250 . Preferably, sheet feeding apparatus  250  includes a nudger roll  251  which feeds the uppermost sheet of stack  254  to nip  255  formed by feed roll  252  and retard roll  253 . Feed roll  252  rotates to advance the sheet from stack  254  into vertical transport  256 . Vertical transport  256  directs the advancing sheet  248  of support material into the registration transport  290  and past image transfer station D to receive an image from photoreceptor belt  210  in a timed sequence so that the toner powder image formed thereon contacts the advancing sheet  248  at transfer station D. Transfer station D includes a corona-generating device  258 , which sprays ions onto the backside of sheet  248 . This attracts the toner powder image from photoconductive surface  212  to sheet  248 . The sheet is then detached from the photoreceptor by corona generating device  259  which sprays oppositely charged ions onto the back side of sheet  248  to assist in removing the sheet from the photoreceptor. After transfer, sheet  248  continues to move in the direction of arrow  260  by way of belt transport  262 , which advances sheet  248  to fusing station F. 
     Fusing station F includes a fuser assembly indicated generally by the reference numeral  270  which permanently affixes the transferred toner powder image to the copy sheet Preferably, fuser assembly  270  includes a heated fuser roller  272  and a pressure roller  274  with the powder image on the copy sheet contacting fuser roller  272 . The pressure roller  274  is loaded against the fuser roller  272  to provide the necessary pressure to fix the toner powder image to the copy sheet. The fuser roller  272  is internally heated by a quartz lamp (not shown). Release agent stored in a reservoir (not shown), is pumped to a metering roll (not shown). A trim blade (not shown) trims off the excess release agent. The release agent transfers to a donor roll (not shown) and then to the fuser roll  272 . Or alternatively, release agent is stored in a presoaked web (not shown) and applied to the fuser roll  272  by pressing the web against fuser roll  272  and advancing the web at a slow speed. 
     The sheet then passes through fuser  270  where the image is permanently fixed or fused to the sheet. After passing through fuser  270 , a gate  280  either allows the sheet to move directly via output  284  to a is finisher or stacker, or deflects the sheet into the duplex path  300 , specifically, first into single sheet inverter  282  here. That is, if the sheet is either a simplex sheet, or a completed duplex sheet having both side one and side two images formed thereon, the sheet will be conveyed via gate  280  directly to output  284 . However, if the sheet is being duplexed and is then only printed with a side one image, the gate  280  will be positioned to deflect that sheet into the inverter  282  and into the duplex loop path  300 , where that sheet will be inverted and then fed to acceleration nip  202  and belt transports  310 , for recirculation back through transfer station D and fuser  270  for receiving and permanently fixing the side two image to the backside of that duplex sheet, before it exits via exit path  284 . 
     After the print sheet is separated from photoconductive surface  212  of belt  210 , the residual toner/developer and paper fiber particles adhering to photoconductive surface  212  are removed therefrom at cleaning station E. Cleaning station E includes a rotatably mounted fibrous brush in contact with photoconductive surface  212  to disturb and remove paper fibers and a cleaning blade to remove the nontransferred toner particles. The blade may be configured in either a wiper or doctor position depending on the application. Subsequent to cleaning, a discharge lamp (not shown) floods photoconductive surface  212  with light to dissipate any residual electrostatic charge remaining thereon prior to the charging thereof for the next successive imaging cycle. 
     Controller  229  regulates the various printer functions. The controller  229  is preferably a programmable microprocessor, which controls all of the printer functions hereinbefore described. The controller  229  provides a comparison count of the copy sheets, the number of documents being recirculated, the number of copy sheets selected by the operator, time delays, jam corrections, etc. The control of all of the exemplary systems heretofore described may be accomplished by conventional control switch inputs from the printing machine consoles selected by the operator. Conventional sheet path sensors or switches may be utilized to keep track of the position of the document and the copy sheets. 
     While the invention has been described in detail with reference to specific and preferred embodiments, it will be appreciated that various modifications and variations will be apparent to the artisan. All such modifications and embodiments as may occur to one skilled in the art are intended to be within the scope of the appended claims.