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

The present invention relates to semiconductor devices with a reduced filter thinning of outer photosites and a method for reducing the thinning of filter layers of the outer photosites. 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. The semiconductor device further includes 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.

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 closest 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-page-width 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 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
 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 clear 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.

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 12B, 12G and 12R 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.sub.o. Each outer pixel
 photosite 12.sub.o consists of three outer photosites 12B, 12G, and 12R.
 The other pixel photosites 12 are referred to as inner pixel photosites
 12.sub.i and each inner pixel photosite 12.sub.i consists of three inner
 photosites 12B, 12G, and 12R. The inner photosites 12B and outer
 photosites 12B form a first row of photosites. The inner photosites 12G
 and the outer photosites 12G form a second row of photosites. The inner
 photosites 12R and the outer photosites 12R 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 12B, 12G
 and 12R, each sensitive to one primary color. As shown in FIG. 3, within
 each inner pixel photosite 12.sub.i is deposited a photosite 12G,
 sensitive to green light, a photosite 12R sensitive to red light, and a
 photosite 12B, sensitive to blue light. The three photosites 12B, 12G and
 12R are on the whole identical as circuit elements except that the surface
 of each photosite 12B, 12G and 12R 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 30B, 30G, and
 30R.
 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 30B, 30G and 30R,
 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 clear 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.sub.o, causing thinning of the filter material
 over the outer pixel photosite 12.sub.o as shown in FIG. 4. The outer
 pixel photosites 12.sub.o on the semiconductor wafer 20 have substantially
 thinner filter layers 30 than the inner pixel photosites 12.sub.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.sub.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 clear 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 clear
 layer 40 is also etched out of the outer pixel photosites 12.sub.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.sub.o, the outer
 pixel photosites 12.sub.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.sub.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 is 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 clear 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 clear 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.sub.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.sub.o, the outer pixel photosites
 12.sub.o now have a deeper well than the inner pixel photosites 12.sub.i,
 to collect additional filter material so that there is a thicker filter
 layer 30 in the outer pixel photosites 12.sub.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 turn, 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 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.