WO1996006740A1 - Electrostatic printhead utilizing multiplexed control electrodes and integrated drive circuits - Google Patents

Abstract

An electrostatic printhead (20) comprising an addressable array of control electrodes (15) with integrated electronic decoder and drive circuits (11) is described for controlling the direct deposition of charged particles (2) on an information medium (8). Incorporating the decoder and drive circuits (11) directly on the printhead (20) substantially reduces the number of electrical connections, thereby reducing size and complexity while increasing reliability. Multiplexing the electrode array provides a substantial reduction in the number of electronic components necessary to address every control electrode (15), achieving further reduction in cost and complexity. A high precision, electrostatic printhead (20) fabricated from crystalline silicon facilitates the incorporation of integrated electronic components.

Description

ELECTROSTATIC PRINTHEAD UTILIZING MULTIPLEXED CONTROL ELECTRODES AND INTEGRATED DRIVE CIRCUITS

Field of the Invention The invention relates to electrographical printing devices, and in particular to an improved direct electrostatic printhead utilizing an integrated, multiplexed matrix of control electrodes and drive circuits.

Background of the Invention Of the various electrostatic printing techniques, the most widely recognized is that of xerography wherein latent electrostatic images initially formed on a charge retentive surface, such as a photoconductive roller, are developed by suitable toner to render the images visible. The images' are subsequently transferred to paper or other such information medium. In spite of its widespread use, the process of visible image formation on the intermediate receptor contributes substantially to the cost and complexity of printing devices.

Another type of electrostatic printing, known in the art as direct electrostatic printing, deposits pigmented particles, such as toner, directly on an information carrier to form a visible image. In general, this method of printing uses electrostatic fields controlled by addressable electrodes for allowing passage of pigment particles through selected apertures in a printhead structure. A separate electrostatic field is provided to attract the pigment particles to an imaging substrate in image configuration. Many of the methods used in field imaging, (i.e. creating an electric field pattern in the print zone), such as particle charging, particle transport, and particle fusing are similar to those used in "laser printers". However, the novel feature of direct printing is its simplicity of simultaneous field imaging and particle transport to produce a visible image directly on the information carrier.

U.S. Patent No. 3,689,935 granted to Pressman discloses a method to produce text and pictures with pigment particles on an information carrier directly from computer generated signals, without the need for those signals to be intermediately converted to another form of energy such as light energy, as is required in electrographic printers like laser printers.

Pressman discloses an electrostatic line printer incorporating a multilayered particle modulator or printhead comprising a layer of insulating material, a continuous layer of conducting material on one side of the insulating layer and a segmented layer of conducting material on the other side of the insulating layer. At least one row of apertures is formed through the multilayered particle modulator. Each segment of the segmented layer of the conductive material is formed around a portion of an aperture and is insulated from every other segment of the segmented conductive layer. Selected potentials are applied to each of the segments of the segmented conductive layer while a fixed potential is applied to the continuous conductive layer. An overall applied field projects charged particles from a particle source through the row of apertures. The density of the particle stream is modulated according to the pattern of potentials applied to the segments of the segmented conductive layer. The modulated stream of charged particles impinge upon a print-receiving medium interposed in the modulated particle stream and translated relative to the particle modulator to provide line-by-line scan printing. A drawback to the Pressman device is that the particle source must be an airborne stream of charged particles. That stream of airborne particles is of low particle density, resulting in very poor contrast on the print-receiving medium. In addition, it is very difficult to effectively control the airborne particle stream. U.S. Patent No. 5,036,341 granted to Larson discloses a solution where control of many adjacent wire pairs is more effective in attracting toner particles from a magnetic toner carrier, increasing the density of toner particles deposited on the print-receiving medium and providing a more effective means of controlling particle transport. The Larson '341 patent discloses a method which begins with a stream of electronic signals defining the image information. A uniform electric field is created between a high potential on the back electrode and a low (0 volt) potential on the developer sleeve. That uniform field pattern is modified by potentials on addressable wires in a two-dimensional wire mesh array placed in the print zone. The wire mesh array consists of parallel control wires, each of which is connected to an individual voltage source, across the width of the paper surface. The multiple wire electrodes, called print electrodes, are aligned in adjacent pairs parallel to the motion of paper; the orthogonal wires called transverse electrodes are aligned perpendicular to the paper motion. All wires are initially at a V_w (white) potential, preventing all toner transport from the developer sleeve. As image locations on the paper surface pass beneath wire intersections, adjacent transverse and print wire pairs are set to a V_b (black) potential to produce an electrostatic field drawing the toner particles from the developer sleeve. The toner particles are pulled through the apertures being formed in the square region between four crossed wires (i.e. two adjacent rows and two adjacent columns), and deposited on a paper surface in the desired visible image pattern. The toner particle image is then made permanent by heat and pressure fusing the toner particles to the surface of the paper.

Furthermore, in Larson '341 , one voltage source can affect a plurality of apertures by multiplexing the electrode array, thereby reducing the number of drive circuits needed for the printhead. For example, in a device with M rows and N columns, the number of electronic drive circuits is reduced from M*N to M + N. However, a drawback in the device of the Larson '341 patent is that during operation of the control electrode matrix, the individual wires can be sensitive to opening or closing of adjacent apertures. This cross-coupling results in undesired printing due to the thin wire border between apertures. International Patent Application PCT/SE90/00398, also by Larson, discloses a method to substantially reduce cross-coupling defects in the wire lattice electrode matrix by using an array of looped wires to enclose the apertures for the passage of pigment particles. Using two wires connected as a loop for each dot position results in more effective control of the adjacent wire pairs since only one electronic drive circuit is connected to each electrode loop across the linear array. The looped wire electrodes, arranged in a two-dimensional matrix of rows and columns, can be constructed as a woven wire mesh or a laminated structure using printed circuit fabrication methods. Circuit fabrication methods are preferred for reasons of accuracy, repeatability, and automated assembly. The woven wire mesh alternates the row and column electrode distance within each aperture so that the electrode matrix, as a whole, behaves as if all of the electrodes are substantially at a uniform distance from the particle carrier. However, a two-layer control electrode circuit does not perform well because the layer closest to the particle carrier dominates in controlling the opening and closing of apertures. The control electric fields acting between the control electrode matrix and the particle carrier are very sensitive to the distance between the control electrode matrix surface and the particle carrier surface. If the rows and columns are at different distances, as with layered circuit boards, their ability to accurately control the electric fields is greatly reduced. A single layer control electrode matrix would be more effective in controlling the apertures. U.S. Patent No. 5,121 ,144 granted to Larson shows a control electrode matrix on a single insulating layer with one circular electrode surrounding each passage to eliminate the cross-coupling. The electrodes are arranged in rows and columns on a single surface-insulating substrate with a single electronic drive needed for each electrode. The ring electrode design requires a single electronic driver for each dot position and is effective in eliminating cross-coupling and increasing maximum print speed.

In spite of these advantages, the design tends to increase the size, complexity and manufacturing costs of the device by an undesirable amount because of the large number of electronic drivers required, along with their associated cables and connectors. Consequently, there is a need in the electrographic printing art for a direct electrostatic printhead employing an addressable array of preferred control electrodes, and yet compact, robust and inexpensive.

Summary of the Invention The present invention reduces the size, cost and complexity of electrostatic printers and the like, and furthermore makes advantageous use of materials and integrated circuit fabrication techniques to make an improved electrostatic printhead.

In accordance with one aspect of the present invention, an electrostatic printhead for controlling the direct deposition of charged particles on an information medium comprises an electrode matrix formed on an insulating substrate having a plurality of apertures disposed through it. The electrode matrix is arranged for electrostatic control of charged particle passage through the apertures by application of at least one electrostatic potential to at least one control electrode. At least one electronic decoder circuit and at least one control potential source is attached directly to the substrate and electrically connected to the electrode matrix for the addressable application of control potentials.

A significant reduction in size and complexity is achieved when the electronic circuits comprise, at least in part, integrated circuit dies which are attached directly to the substrate and electrically connected to the control electrode matrix. A further improvement in print operation may be achieved by applying a conductive film over a portion of the control electrode surface, which reduces charge accumulation. A preferred form for the electrode matrix comprises multiplexing the electrode array to reduce the number of decoder and drive circuits. In particular, a passive multiplexing network, involving for example, resistive or capacitive components, will allow addressable access to an MxN array of electrodes with only M + N galvanic connections.

A further reduction in size may be achieved by fabricating at least a portion of the electronic circuits directly on the insulating substrate as integrated circuits. A preferred embodiment for such integration would entail the use of a crystalline silicon wafer for the printhead substrate, upon which is fabricated at least a portion of the electronic circuits. Additionally, the formation of precision apertures through a (OOD-oriented silicon wafer may be accomplished by means of an anisotropic etchant.

In accordance with another aspect of the present invention, a printhead for electrostatic printing comprises a solid membrane defining at least one electrically insulating surface and having a plurality of apertures disposed therethrough. A plurality of addressable electrodes is disposed on the insulating surface, each substantially adjacent to an aperture on the surface, and each galvanically isolated from the others. At least one electronic decoder and drive circuit is attached to the membrane and is electrically connected to the addressable electrodes. In operation, the addressable electrodes of the printhead are preferably located near a donor of charged particles such that the charged particles undergo an electrostatic interaction with said addressable electrodes.

In a preferred embodiment, the membrane may comprise an insulating, flexible material, such as polyimide foil, upon which conductive electrode material is laminated or otherwise deposited. Alternatively, the membrane may comprise a rigid, insulating material such as alumina, upon which the electrode material is deposited, thereby achieving greater reliability. Furthermore, a rigid substrate material offers the possibility of fabricating integrated electronic circuits directly on the printhead surface. In still another preferred embodiment, a crystalline silicon wafer may be used for the substrate of a high precision printhead, utilizing fabrication techniques in the integrated circuit arts. ln accordance with a method of the present invention, electrical control signals are supplied to an electrostatic printhead by transmitting encoded electrical control signals through an electrical connector to an electronic decoder. The encoded control signals are decoded and distributed over the electrostatic printhead. Brief Description of the Drawings

Figure 1a is a schematic perspective of one embodiment of the invention. Figure 1 b is an enlarged view of the control electrode matrix with surrounding means in Figure 1a.

Figure 2 is a sectional view through the print zone of Figure 1. Figure 3 is a plan view of the central portion of one embodiment of a preferred electrostatic printhead.

Figure 4 is a sectional view through the print zone of a preferred electrostatic printhead. Figure 5 is a plan view of the central portion one embodiment of a preferred electrostatic printhead. Figure 6a is a plan view of a square aperture etched in a rigid silicon substrate.

Figure 6b is a sectional view through the etched square aperture of Figure 6a.

Description of the Preferred Embodiments

As shown in figures 1a and 1 b, an electrostatic printer using a preferred embodiment of the invention comprises a container 1 for pigment particles 2, and which presents a mounting surface for the printhead 20. A particle carrier such as a developing roller 4 is disposed within container 1 , and may enclose a multiple magnetic core 5 for attracting magnetic pigment particles 2 toward development roller 4. Alternative means may as well be employed for attaching toner particles 2 to development roller 4, such as electrostatic attraction. The printhead 20 is comprised of a control electrode matrix 3 with electrode leads 7 and electronic drive circuits 1 1. In operation, the printhead 20 is positioned relative to the development roller

4 and back electrode 9 such that control electrode matrix 3 is maintained in addressable potentiostatic cooperation with back electrode 9 and a segment of development roller 4 passing thereby.

A printhead substrate 6 supports control electrode matrix 3, which is comprised of an array of individual control electrodes 15, each preferably circumscribing an aperture 10 passing therethrough. The control electrodes 15 and apertures 10 should be arranged such that the geometric projection of their location on a given print line should provide continuous and uniform coverage. They may, for example, be arranged in a skewed row and column configuration as described in Larson '144. The present invention, however, is not limited to a specific arrangement of apertures and control electrodes, nor to the profile of the aperture itself. An information medium 8, usually paper, is preferably positioned adjacent to back electrode 9 for printing thereon and is advanced in a preset direction, defined here by arrow 22. A voltage source (not shown) connected to back electrode 9 attracts charged pigment particles 2 from developing roller 4, through apertures 10 in control electrode matrix 3, depositing pigment particles 2 on information medium 8. Coded control voltage signals from a remote source (not shown) are supplied to electronic drive circuit 1 1 through external connector 16, and are subsequently decoded and distributed to drive the individual control electrodes 15 (Figure 1 b). The application of control potentials to an electrode 15 creates electric fields that partially open or close apertures 10 to passage of toner particles 2. A visible image pattern is produced on information carrier 8 corresponding to the pattern of the control voltage signals.

Referring to figure 2, a section through the print zone of figure 1 shows that control electrode matrix 3 is comprised of substrate 6 upon which is laminated or deposited a conductive layer making up etched electrodes 15 and electrode leads 7. Control electrode matrix 3 is shown with one control electrode in the print condition where pigment particles 2 pass through one aperture 10a to information medium 8. Although it is preferred to utilize an electrode matrix with apertures, where toner particles 2 pass through apertures 10a to deposit on information medium 8, it is not necessarily critical to the inventive aspects of the present embodiment. For example, the information medium 8 may pass between control electrode matrix 3 and development roller 4. In this case, electric fields generated by control electrode matrix 3 permit or restrict toner 2 transport from development roller 4 onto information medium 8, without passage through aperture 10.

A preferred embodiment of an electrostatic printhead 20 is shown in figure 3, which displays a portion of a control electrode matrix 3 with attached electronic decoder and drive circuits 1 1. Substrate 6 may be a thin, flexible insulating foil, such as polyimide about 25 μm thick, upon which is laminated or otherwise deposited a conductive layer, such as copper. The conductive layer is etched or otherwise formed to define conductive leads 7 and electrodes 15 of control electrode matrix 3. An insulating layer 12, shown in figures 2 and 4, is applied over the circuit traces 7. Surface conductivity treatment 14 is applied over insulating layer 12, to both sides of the control electrode matrix 3 prior to attachment of the electronic dies. The conductive coatings 14 function to avoid charge buildup on the substrate during operation of the printer. Apertures 10 for passage of pigment particles though substrate 6 are typically about 150 μm in diameter, and may be formed by means of focused laser energy. Electronic decoder and drive circuit dies 1 1 are attached to the substrate preferably by means of adhesive bonding. Conductive traces 7 may be connected to the appropriate terminals of the die 1 1 by conductive wires 13, attached by wire bonding. A central feature of the invention is integration of electronic decoder and drive circuits 1 1 with the control electrode matrix 3. In doing so, the numerous electrical connections 7 between the decoder and drive circuits 1 1 and the electrode matrix 3 may be fabricated using integrated circuit methods, thereby eliminating numerous bulky and potentially unreliable connectors. Moreover, the entire matrix of individual control electrodes 15 is preferably multiplexed into an MxN array, thereby requiring only M + N separate control signals to address any of the electrodes 15 of the electrode array 3. The control electrode array 3 may be multiplexed by "passive" component means, where passive herein characterizes a component which itself cannot function as a source of power. Such means may, for example, be a resistor- capacitor grid, where each node comprises a parallel resistor-capacitor combination with respective component values R_m and C„. For separate control voltage pulses V_m, V_n applied through each of a resistor and capacitor, the network functions to substantially add the voltages, V_m + V_n, at a node for characteristic times within about R^C,,. An example of such an RC multiplexing network is disclosed in United States Patent 4,353,080, by D.A. Cross, and is herein incorporated by reference. The integrated electronic decoder circuit 1 1 may be a serial- to-parallel converter, which accepts logic-level inputs and has a voltage output suitable for driving control electrode matrix 3. The commercially available Supertex, Inc. model # HV31 may be used for the present embodiment.

The aforementioned features not only reduce the size, cost and complexity of direct electrostatic printheads, but increases their robustness and reliability by substantially reducing the size and number of connections to the printhead. Thus, in the preferred embodiment, connectors between the decoder and drive circuit 1 1 and the electrode array 3 are substantially eliminated, while external connectors 16 to the printhead 20 itself will provide encoded electrical signals over far fewer electrical contacts. The invention further facilitates printhead 20 maintenance and reliability by providing simultaneous or single-step replacement of both the control electrode array 3 and the decoder/drive circuits.

Another preferred embodiment of the printhead 20 makes use of a rigid ceramic material, such as alumina, for substrate 6, as shown in figure 4. Conductive traces 7 are etched from a solid layer of conductive material deposited, for example, by well known vacuum evaporation techniques. In like fashion to that shown in figures 2 and 4, an insulating layer 12 is applied over the circuit traces 7. Furthermore, as shown previously in figure 3, surface conductivity treatment 14 is applied to both sides of the control electrode matrix 3 prior to circuit attachment. The conductive coatings 14 function to avoid charge buildup on the substrate during operation of the printer. Apertures 10 for passage of pigment particles therethrough preferably have a diameter at least equal to the material thickness and may be formed by focused laser energy. Electronic circuit dies 1 1 are attached to the substrate preferably using adhesive bonding. Conductive traces 7 are attached to the appropriate terminals of the die with conductive wires 13 which may be secured by wire bonding.

In accordance with well-known surface mount die bonding and interconnect technology, electronic circuit dies 1 1 way be mounted in a packaging medium, such as a ceramic substrate or housing having interconnects between the package and die 11. The packages are subsequently mounted to the substrate 6 by such processes as ball-grid array bounding or flip- clip solder bounding.

Still another preferred form of the printhead 20 makes use of crystalline silicon for substrate 6, as shown in figure 5. Conductive traces 7 are deposited on the silicon substrate by well known integrated circuit fabrication techniques. An insulating layer 12 is applied over the circuit traces as shown previously in Figure 4. Surface conductivity treatment 14 is applied to both sides of the control electrode matrix 3 over the insulating layer 12. The conductive coatings function to avoid charge buildup on the substrate during operation of the printer. Apertures 10 for passage of pigment particles are preferably drilled through the substrate 6 by laser energy. Additionally, in the present embodiment electronic decoder and drive circuits 1 1 may be formed directly into the substrate, as for example in the form of monolithic or thin film devices, thereby eliminating the separate assembly steps of die placement, adhesive bonding and wire bonding. Such processes would be well known to those skilled in the art of integrated circuit fabrication. As shown in figures 6a and 6b, a preferred method for creating apertures 10 in a silicon substrate 6 comprises an anisotropic etching process, such as disclosed in "The Fabrication of High Precision Nozzles by the Anisotropic Etching of (100) Silicon" (Bassous, E, Baran, E.F., J. Electroche . Soc.: Sol. St. Sci. Techno!., vol. 125, no. 8, 1978, pp. 1321-1327) and incorporated herein by reference. An array of substantially square apertures 10 may be fabricated by use of an anisotropic etchant acting on a single crystal (001 )-oriented silicon wafer. For selected etchants, the etch rate depends strongly on crystal lographic direction, being preferably much larger in the <001 > direction than < 1 1 1 > direction. For such conditions, the equilibrium shape of etch holes takes the basic form of a truncated pyramidal cavity bounded by the top and bottom surface planes of the substrate as shown in Figure 6b. The size and shape of the cavities further depends on the geometry of the exposure mask, the duration of etching, and the wafer thickness. In perfect single crystal silicon, the angle between (001) and (1 1 1) planes is 57.54 degrees, defining a preferred equilibrium contour.

The foregoing description should be taken as illustrative and not as limiting. While the present invention has been disclosed herein with reference to preferred embodiments, it should be understood that the invention, itself, is not limited by the particular methods and devices described therewith. It is also possible to apply the invention to other printing methods that -9- use an aperture control electrode to influence the flow of charged particles to an information medium.

Claims

CLAIMS:

1. In an electrographic image recording apparatus, an electrostatic printhead for controlling direct deposition of particles on an information medium, comprising: an electrode matrix formed on an insulating substrate, said substrate having a plurality of electrodes arranged for potentiostatic control of particle deposition on an an information medium; and an electronic decoder circuit attached directly to said substrate and electrically connected to said electrode matrix for the addressable application of said control potentials.

2. The electrostatic printhead of claim 1 , wherein said substrate has a plurality of apertures disposed therethrough; and said electrode matrix being arranged for potentiostatic control of particle passage through said apertures.

3. The electrostatic printhead of Claim 1 , wherein at least a portion of said electronic decoder circuit comprises an integrated circuit.

4. The electrostatic printhead of Claim 1, wherein a conductive film is deposited over at least a portion of the surfaces of said control electrode matrix, said conductive film remaining galvanically isolated from said control electrode matrix.

5. The electrostatic printhead of Claim 1 , wherein said electronic decoder includes a multiplexing network.

6. The electrostatic printhead of Claim 1 , wherein at least a portion of said electronic decoder circuit comprises an integrated circuit, said integrated circuits fabricated directly on said substrate of said control electrode matrix.

7. The electrostatic printhead of claim 1 , wherein said substrate comprises a crystalline silicon wafer.

8. The electrostatic printhead of Claim 1, wherein said substrate comprises (001)- oriented silicon wafer.

9. The electrostatic printhead of claim 2, wherein said substrate comprises (001)- oriented silicon wafer and said apertures anisotropically etched therethrough.

10. A printhead for electrostatic printing, comprising: a solid membrane defining at least one insulating surface, said membrane having a plurality of apertures disposed therethrough; a plurality of addressable electrodes, each disposed on said insulating surface and substantially adjacent to an aperture on said surface, each of the addressable electrodes being galvanically isolated from the others; and at least one electronic decoder circuit attached to said membrane, said electronic circuit being electrically connected to said electrodes.

1 1. The printhead of claim 10, further comprising a donor of particles, wherein said membrane having addressable electrodes is positioned near said donor of particles, said particles being in potentiostatic cooperation with said electrodes.

12. The printhead of claim 10, wherein said membrane comprises a flexible, electrically insulating foil, upon which said electrodes are disposed.

13. The printhead of claim 10, wherein at least a portion of said electronic decoder comprises an integrated circuit.

14. The printhead of claim 10, wherein a conductive film is applied over at least a portion of the surfaces of said addressable electrodes, said conductive film being galvanically isolated from said addressable electrodes.

15. The printhead of claim 10, wherein said electronic decoder includes a multiplexing network, at least a portion of which comprises passive components.

16. The printhead of claim 10, wherein said membrane comprises a rigid, insulating material.

17. The printhead of claim 10, wherein said membrane comprises a crystalline silicon wafer.

18. The printhead of claim 10, wherein at least a portion of said electronic decoder comprises an integrated circuit fabricated directly on said substrate.

19. The printhead of claim 10, wherein said substrate comprises a (001 )-oriented silicon wafer and said apertures anisotropically etched therethrough.

20. The printhead of claim 10, further comprising a source of electrical control signals and an electrical connector, wherein said source of electrical control signals is electrically connected to said electronic decoder through said electrical connector.

21. A method of supplying electrical control signals to the control electrodes of an electrostatic printhead comprising: transmitting encoded electrical control signals through an electrical connector to an electronic decoder; decoding said encoded electrical control signals in said electronic decoder to generate electrode drive signals; and transmitting said electrode drive signals to said control electrodes over non-detachable transmission lines.

22. A printhead for electrostatic printing, comprising: a solid membrane defining an insulating surface, said membrane having a plurality of apertures disposed therethrough; a plurality of addressable electrodes disposed on said insulating surface, each electrode having a portion thereof disposed substantially adjacent to an aperture on said surface, said plurality of addressable electrodes arranged in a multiplexed network; and an electronic decoder and drive circuit disposed on said membrane, said electronic circuit being electrically connected to said multiplexed network of electrodes.

23. The printhead of claim 22, wherein said multiplexed network of electrodes comprises passive multiplexing components.

24. An electrostatic printhead comprising: a semiconductor wafer having a substantially insulating surface region and an array of apertures disposed therethrough; a plurality of electrodes formed on said surface region, each electrode having a portion thereof substantially adjacent to an aperture; and an electronic decoder and drive circuit formed on said semiconductor wafer and electrically connected to the plurality of electrodes.

25. The electrostatic printhead of Claim 24, wherein the semiconductor wafer comprises (OOD-oriented silicon.

26. The electrostatic printhead of Claim 24, wherein the array of apertures are anisotropically etched therethrough.

27. The electrostatic printhead of Claim 24, wherein at least a portion of the electronic decoder and drive circuit comprise a monolithic integrated circuit fabricated on said semiconductor wafer.

28. A method for transmitting drive signals to the electrodes of an electrostatic printhead comprising: multiplexing control signals; encoding the multiplexed control signals; transmitting the encoded multiplexed control signals to the electrostatic printhead; decoding the multiplexed control signals; converting the multiplexed control signals to multiplexed drive signals; demultiplexing the multiplexed drive signals; and transmitting the drive signals to the electrodes.