Abstract:
A method and apparatus relating to a unique printhead configuration is disclosed. The printhead configuration is based on a coplanar arrangement of two sets of electrodes. The electrodes are electrically separated from each other by a dielectric layer, and together create a matrix of charge generating sites. Such a printhead has a very low internal capacitance and therefore is suitable for high speed and high resolution printing.

Description:
FIELD OF THE INVENTION 
     the invention relates to a printhead suitable for use with image forming systems and more particularly relates to a coplanar arrangement of electrodes within a single dielectric layer of the printhead. 
     BACKGROUND OF THE INVENTION 
     many different printing technologies today utilized in image forming systems create and reproduce images in different ways. A process executed by some of these technologies (e.g.,Electron Beam Imaging) includes a step of charging a surface of an image-receiving member, such as a drum, with a latent charge image. The term image-receiving member include a drum, flat or curved surfaces, or a flexible belt. The image-receiving member can also be a liquid crystal or phosphor screen, or similar display panel in which latent charge image results in a visible image. Typically, an exterior surface of the image-receiving member includes a material, such as a dielectric include glass enamel, flame or plasma sprayed high-density aluminum oxides, and plastics, including polyamide, nylon, and other tough thermoplastic or thermoset resins, among other materials. 
     The image-receiving member, or drum, moves past an image forming device, such as printhead, which produces a stream of accelerated electrons as primary charge carriers. The electrons reach the drum, landing in the form of a latent charge image. The latent charge image then receives a developer material, to develop the image, and the image is then by press or electrostatic transfer applied and fused to a medium, such as a sheet of paper, to form a printed document. 
     The printhead most often includes a film having a multi-electrode structure that defines an array of charge generating sites. Each of the charge generating sites, when the electrodes are actuated, generates and directs toward the drum a stream of charge carriers, e.g., electrons, to form a pointwise accumulation of charge on the drum that constitutes the latent image. A representative printhead generally includes a first collection of drive electrodes, e.g., RF-line electrodes, oriented in a first direction across the printing process direction. A second collection of control electrodes, e.g., finger electrodes, oriented transversely to the drive electrodes, forms cross points or intersections with the drive electrodes constituting an array of charge generating sites at which charges originate. A dielectric layer couples to, and physically and electrically separates and insulates, the RF-line electrodes from the finger electrodes. 
     The printhead can also include a third electrode structure, often identified as a screen electrode. This screen electrode couples to the finger electrodes by an insulating structure, such as a spacer layer. The screen electrodes have a plurality of passages aligned with the charge generating sites, to allow the stream of charge carriers to pass through. The screen electrode can be a single conductive sheet having an aperture aligned over each charge generation site. Polarity of charge carriers passing through the passages, or apertures, depends on the voltage difference applied to the finger and screen electrodes. Polarity of particles accumulated on the drum to create latent image is determined by the voltage difference between the screen electrode and the drum surface. The charged particles of appropriate polarity are inhibited from passing through the aperture, depending upon the sign of their charge, so that the printhead emits either positive or negative charge carriers, depending on its electrode operating potentials. 
     A typical structure of the printhead is a vertical arrangement, wherein the RF-line electrodes, dielectric, finger electrodes, spacer, and screen electrode are gradually laminated each on top of the other. This vertical structure for a thin film printhead leads to a relatively high capacitance of the RF-lines. The large capacitance limits the usable charge generating frequency and consequently the speed of printing. 
     SUMMARY OF THE INVENTION 
     There exists in the art a need for a low capacitance printhead. The printhead of the present invention includes a first plurality of electrodes (e.g., RF-line electrodes), and a second plurality of electrodes (e.g., finger electrodes), arranged in a substantially common plane. A subsequently deposited dielectric layer seals the coplanar first plurality of electrodes and isolates electrical connections to the second plurality of electrodes. 
     Each of the plurality of electrodes has a different arrangement, according to a further aspect of the present invention. One such arrangement includes electrodes with an elongate section having electrode peninsulas extending outwardly therefrom. The other plurality of electrodes includes individual electrodes surrounding each of the extending electrode peninsulas. A single dielectric layer then seals the substantially coplanar electrode layers. 
     The single dielectric layer, according to one aspect of the present invention, can be made of two or more layers, each layer being formed of a different material. 
     A method of making a printhead according to the teachings of the present invention includes applying a metal coating to a substrate material to form electrodes. An etching process forms two sets of substantially coplanar electrodes from the substrate coating material. A dielectric composition then covers each of the two sets of electrode patterns. The dielectric composition is then selectively perforated and a second metal layer is deposited to form electrode interconnections. 
    
    
     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, wherein: 
     FIG. 1 is a diagrammatic illustration of an image forming system suitable for use with the printhead of the present invention; 
     FIG. 2 is a diagrammatic cross-sectional view of a collection of charge generating sites in a printhead; 
     FIG. 3 is a schematic illustration of an electrode configuration of the printhead of the present invention; 
     FIG. 4 is a schematic illustration of an arrangement of electrode configurations according to the teachings of the present invention; 
     FIG. 5 is a partial cross-sectional view of the electrode configuration of FIG. 3; 
     FIGS. 6A-6F illustrate the steps for manufacturing the electrode configuration of the present invention; and 
     FIG. 7 is a schematic illustration of an alternate electrode configuration of the printhead of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally relates to a printhead mounted within an image forming system. A characteristic of the printhead is that there exist two or more distinct coplanar electrode layers, and a dielectric layer, within the printhead. Typically, the printhead includes a set of RF-line electrodes and a set of finger electrodes manufactured from a single metallic layer, and subsequently sealed by a common, relatively thin, dielectric layer. This geometry provides for a printhead with a relatively lower overall capacitance suitable for high resolution and fast printing. 
     FIGS. 1 through 7 illustrate the coplanar thin film printhead according to the teachings of the present invention. Although the present invention will be described with reference to the example embodiments illustrated in the figures, it should be understood that many alternative forms can embody present invention. One of ordinary skill in the art will additionally appreciate different ways to alter the parameters of the embodiments disclosed, such as the size, shape, or type of elements or materials, in a manner still in keeping with the spirit and scope of the present invention. 
     The image forming system is illustrated solely for the purpose of providing a general structure in which the present invention can reside. It is wholly anticipated that other systems or charge transfer apparati can be utilized in combination with different embodiments of the present invention. 
     FIG. 1 schematically illustrates an image forming system  10  of the electron beam imaging (EBI) variety, having an image-receiving member, such as a drum  12  that mounts for rotation about an axis  13 . The drum  12  incorporates an electrically conductive core  14 . coated with a dielectric layer  16 . The dielectric layer  16  receives a charge image from a printhead  18 . A controller  20  drives the printhead. I as desired. Charge generating sites within the printhead  18  generate a charged image and transfer the image to the dielectric layer  16  on the outer surface of the drum  12  as the drum  12  rotates in the direction of the arrow shown. The drum  12  continues to rotate and the entire drum surface including the charged portion of the dielectric layer  16  comes into contact with toner particles  23 ; supplied from a hopper  24  through a feeder  26 . The toner particles  23  electrostatically adhere to the charged image on the dielectric layer  16  to form a toner image. The rotating. drum  12  then carries the toner image towards a nip formed with a pressure roller  28 . The pressure roller  2 g has an outer layer  30  positioned in the path of a receptor, such as a paper sheet  32 . The paper sheet  32  enters between a pair of feed rollers  34 . The pressure in the nip is sufficient to cause the toner particles  23  to transfer to the paper sheet  32 , permanently affixing the toner particles  23  thereto. The paper sheet  32  continues through and exits between a pair of output rollers  36 . After passing through the nip between the drum  12  and the pressure roller  28 , a scraper blade assembly  38  removes any toner particles  23  that may remain on the dielectric layer  16 . A charge eraser  39  positioned between the scrapper blade assembly  38  and the printhead  18  removes any residual charge remaining on the dielectric layer  16  surface. 
     A printhead configuration known to those skilled in the art is most common in EBI printing technologies. The printhead includes a first electrode layer having a plurality of electrodes, called RF-line electrodes, sealed and electrically isolated from a second electrode layer by at least one dielectric layer. The second electrode layer also comprises a plurality of electrodes, known as finger electrodes, which cross the plurality of RF electrodes creating a matrix of plasma generating sites from where the charge, used for imaging, is emitted. 
     The illustration of FIG. 2 generally illustrates a portion of the conventional printhead configuration. The printhead includes a first electrode layer, e.g., the RF-line electrode layer  52 , covered and sealed by a dielectric layer  54 . On the opposite side of the dielectric layer  54  is a second electrode layer, e.g., the finger electrode layer  56 . The printhead can also include a spacer layer  53  supporting a screen electrode  55 . The screen electrode  55  aids in the proper alignment of the emitted charge carriers as is understood by one of ordinary skill in the art. The printhead is oriented with respect to a drum  57 , similar to the drum  12  of FIG. 1, for generating and transferring charge from a charge generating site  65  to the drum  12  to form the latent image. 
     FIG. 3 illustrates a possible electrode configuration  58  according to the teachings of the present invention. A finger electrode  60  extends in a single plane having a generally U-shape configuration, which surrounds the RF electrode  66 . The finger electrode  60  includes an extension that couples to a contact  62  in approximately the same plane. The RF electrode  66  in connection with the RF-line  64 , form the RF-line electrode, which generally extends across the electrode configuration  58  to contribute to the formation of a sufficient number of charge generating sites. 
     Those of ordinary skill in the art will readily recognize that the RF-electrodes  66  are shown as peninsulas extending from the RF-lines  64 , but the RF-electrodes  66  can have any suitable shape, as long as they are in mutual proximity with the finger electrodes  60  in a manner sufficient for charge generation to occur. RF-line  64  is in substantially the same plane as the finger electrode  60 , and can have various different forms as well, such as curves, waves, zigzags, peaks and valleys, or other suitable patterns or shapes. The finger electrode  60  can also exist in other forms or shapes, again, as long as there is sufficient proximity to the RF-electrode  66  to result in the desired charge generating capabilities. 
     The illustrated coplanar electrode configuration is partially covered by a dielectric layer  61 . The dielectric layer  61  serves to electrically separate and insulate the coplanar finger electrode  60  from the coplanar RF-electrode  66 , as well as the RF-line  64 , and the contact  62 . While the RF-electrodes  66  and RF-lines  64  are fully sealed by the dielectric layer  61 , the finger electrodes are only partially covered to allow for an electrical contact with generated charged particles and for finger electrode interconnections. A cutaway illustration of the electrode configuration  58  is further illustrated in FIG.  5 . 
     The dielectric layer, or dielectric composition, as disclosed herein includes a number of different structures and materials. The dielectric, for example, can be a single layer of a single material type, or can include a plurality of layers of either the same or different dielectric materials. A plethora of compositions can form the dielectric. Some possible materials include silicon dioxide, aluminum oxide, magnesium oxide, silicon nitride, and boron nitride. 
     FIG. 4 illustrates a section of a thin film element  68  of the printhead  18  according to the teachings of the present invention. As illustrated, the series of RF-lines  64  extend in parallel fashion relative to one another across the film segment  68 . Each RF-line  64  interconnects a set of periodically placed coplanar RF-electrodes  66  surrounded by generally U-shaped finger electrodes. The coplanar electrodes (RF-electrodes  66 , finger electrodes  60 , as well as RF-lines  64 ) are covered by a dielectric layer (not shown) provided with openings in registration with finger electrodes  60  and contacts  62 . The finger electrodes  60  are connected into fingers by metal strips deposited across the dielectric openings (not shown). 
     Each arrangement of finger electrode  60  surrounding each RF-electrode  66  creates a charge generating site for depositing electric charges on charge receiving elements, such as the dielectric drums  12  (as show in FIGS.  1  and  2 ). The finger electrode  60  does not need to surround each RF-electrode  66 , but merely needs to be sufficiently proximal to the RF-electrode  66  to create the charge generating sites. 
     FIG. 5 is a cross-sectional view of the electrodes configuration  58  of FIG. 3. A substrate  70  forms the base of the configuration  58 . The finger electrode  60  and the RF-electrodes  66  lie atop the substrate  70 . The dielectric layer  61  extends across the top of the substrate  70 , the finger electrode  60 , the RF-line  64  (not shown), and the RF-electrode  66  in a predetermined fashion such that the dielectric layer  61  electrically insulates the finger electrode  60  and the RF-electrode  66  from each other. The finger connection is shown in further detail as a metal strip  71  extending from atop the dielectric layer  61  to make electrical contact with the extension of the finger electrode  60  through the finger contact pad  62 . A spacer element  74  supports a screen  77 . The screen contains a screen hole  72 , therethrough. 
     A stepwise illustration of one suitable method for manufacturing of the electrode configuration  58  according to the present invention, is shown in FIGS. 6A-6F. A substrate  70  is first coated with a metal layer  76  suitable for forming both the finger and RF electrode layers, FIG.  6 A. The metal layer  76  is patterned by a standard photo-imaging and etching process to form two sets of coplanar electrodes (finger electrodes  60  and RF-electrodes  66 ), FIG.  6 B. Such photo-imaging and etching processes are well known the art and are not described further herein. A layer of dielectric material  61  covers the finger electrodes  60  the RF-electrodes  66 , and the RF-line  64  (not shown) through use of well known thin film application technologies, such as, e.g., sputtering, evaporation, and the like, FIG. 6C. A dry or wet etching process perforates, or removes portions of, the dielectric layer  61 , to clear areas used for an interconnection of finger electrodes  60  and to enable finger electrode electrical contact  62  with emitted charged particles, FIG.  6 D. The perforations made through the dielectric layer  61  extend to the electrode layer. An additional metal layer  71  is applied to cover the entire structure, FIG. 6E. A further etching process selectively removes portions of the additional metal layer  71  to form the finger electrode interconnections  92 , FIG.  6 F. 
     FIG. 7 illustrates an alternate embodiment of the printhead of the present invention. The illustrated electrode configuration  78  includes a substrate  80  that forms a base layer. A finger electrode  82  and an RF-electrode  84  are arranged on the substrate  80  similarly to that shown in FIG. 5. A dielectric layer  86  fully seals the finger electrode  82 , while the RF-electrode  84  remains exposed. The dielectric layer  86  extends sufficiently beyond an edge of the finger electrode  82  to serve as an electric insulator between the finger electrode  82  and the RF-electrode  84 . A spacer  88  provides support for a screen electrode  90 . 
     The coplanar design of the present invention placing the electrode layers in substantially the same plane along with the dielectric layer, as disclosed herein, has several advantages. The distance required between the finger and RF-electrodes does not rely upon a thickness of a deposited dielectric layer. The proper distance, instead, is achieved by lateral placement of the electrodes within substantially the same plane, rather than in an axial fashion. Coplanar printheads have a reduced capacitance and are suitable for high resolution fast printing. Overall, the cost of manufacture is reduced due to the lesser amount of printhead manufacturing steps and lesser amounts of dielectric material required. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode for carrying out the invention. Details of the structure may vary substantially without departing from the spirit of the invention, and exclusive use of all modifications that come within the scope of the appended claims is reserved. It is intended that the invention be limited only to the extent required by the appended claims and the applicable rules of law.