GB2035908A

GB2035908A - Electrostatic print head and method of fabrication

Info

Publication number: GB2035908A
Application number: GB7939572A
Authority: GB
Original assignee: Dynamics Research Corp
Current assignee: Dynamics Research Corp
Priority date: 1978-11-20
Filing date: 1979-11-15
Publication date: 1980-06-25
Also published as: GB2035908B; FR2441492B1; DE2946866A1; JPS5571564A; FR2441492A1

Abstract

An electrostatic print head has a planar thin film pattern of spaced metal electrode lines 18 produced by photolithographic and electroforming techniques on one surface of a glass substrate 10 and a spaced array of planar thin film buss lines 22 similarly produced on the opposite surface. Each line 18 is connected to one of the lines 22 e.g. by a plated-through hole 26. The electrode lines and buss lines are formed by vacuum deposition of a metal onto the substrate surfaces, etching into intended patterns, and electroforming to the required thickness and cross-sections. <IMAGE>

Description

SPECIFICATION Electrostatic print head and method of fabrication FIELD OF THE INVENTION This invention relates to electrostatic printing and more particularly to an electrostatic print head and method of fabrication.

BACKGROUND OF THE INVENTION In electrostatic printing and plotting systems, there is provided a print head which includes an array of electrodes which can be brought into close proximity with a writing surface and, when electrically energized, cause a charge pattern on the surface to which a toner can be applied to produce a visible image in the shape of the charge pattern. The print heads are often in the form of one or more rows of electrodes terminating in nibs or styli and retained in a support member, the styli being closely spaced to produce an effectively continuous print on an associated surface. Typically, 200 styli per inch are provided for graphic plotters.

Such print heads have been fabricated by hand-writing or an array of wires, the ends of which form the styli, as shown, for example, in patent 3,693,185. Another known technique of electrostatic print head fabrication is by printed circuit techniques such as shown in patents 3, 267,485 and 3,718,936. Fabrication of a print head by hand-writing is necessarily time-consuming, requires skilled labor and is usually expensive and inefficient for quantity manufacture. Printed circuit fabrication techniques have not been wholly satisfactory in producing high performance print heads heretofore, as the production of high density styli without breaks or short circuits is extremely difficult using printed circuit materials and fabrication processes. Moreover, interconnection of electrode lines to the addressing network has been complicated and unreliable.In addition, the shape of the styli at the writing surface is preferably of square cross-section which is difficult to attain using plating or etching types of printed circuit processes.

SUMMARY OF THE INVENTION Briefly, the present invention provides an electrostatic print head fabricated by use of photolithographic and electroforming techniques on a glass or ceramic substrate to achieve a head construction having a precision, high resolution array of styli of intended cross-section and having an interconnect pattern for simple connection to driving circuits.

In accordance with the invention, a planar thin film pattern of spaced metal electrode lines is formed on one surface of a glass or ceramic substrate, and a spaced array of planar thin film buss lines are formed on the opposite substrate surface. Each of the electrode lines includes an enlarged pad area which is in alignment or registration with a corresponding pad area of a respective one of the buss lines. The registered pad areas are conductively interconnected such that each electrode line is in electrical connection with an associated buss line. The electrode lines are formed by photolithographic techniques by which evaporated metal is deposited on the substrate surface and then electroformed and etched in the intended pattern. The buss lines are similarly formed.The electrode lines thus formed are of high resolution having styli or nib ends which are of precisely configured cross-sectional shape and dimensions. Typically, a pair of planar thin film patterns are employed in a single print head to provide a double row of styli which are in staggered relationship to each other.

In accordance with the novel method for fabricating an electrostatic print head, a substrate, preferably of glass, has holes photoetched or otherwise provided therethrough at predetermined positions. Both surfaces of the substrate have a metal vacuum deposited thereon, the deposited metal extending into the previously formed holes to coat the hole surfaces with the metal for purposes of electrically interconnecting the metal-coated substrate surfaces. The deposited metal is typically chromium. A thin layer of a second metal, typically copper, is then electroplated over the chromium surfaces of the substrate and the metal-coated holes. A photoresist pattern is then formed on one surface of the substrate to provide an intended array of electrode lines and pad areas in alignment with respective pad areas of the electrode array.

A layer of nickel is electroplated onto all exposed copper surfaces of the substrate.

Preferably, a layer of gold is then electroplated onto the connector areas of the buss lines by which the head is connected to a signal source to enhance the conductivity of the connector areas. The styli ends of the electrode lines are then electroformed to an intended cross-sectional shape and size. The photoresist material is removed by use of an appropriate solvent, and with the nickel plating serving as a resist material, the copper and chromium are selectively etched away to produce the resulting pattern of the electrode lines and pad area on one substrate surface, and buss lines and aligned pad areas on the opposite substrate surface.

The structure thus formed is then conformal coated with a plastic material and assembled into a completed head. If desired, the styli faces can be ground and finished to intended surface finish. The styli are disposed along an edge of the substrate to provide an array of styli in a position to contact a writing surface when the print head is in use. One end of the buss line array includes the connector areas adapted for connection to an electrical source operative to selectively apply energizing signals to the buss lines and thence to the associated electrode lines for producing an intended electrostatic charge pattern via the styli on the writing surface. The charge pattern is rendered visible by subsequent application of a toner to the pattern in a manner wellknown in the art.

DESCRIPTION OF THE DRA WINGS The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: Figure 1 is a cutaway pictorial view of a portion of an electrostatic head according to the invention; Figure 2 is a cutaway elevation view of the writing end of a print head according to the invention; Figure 3 is a plan view of the electrode lines and pad areas on one substrate surface; Figure 4 is a plan view of the buss lines and pad areas on the opposite substrate surface; Figure 5 is a sectional elevation view of the substrate containing the electrode lines and buss lines on respective opposite surfaces; Figure 6 is a cutaway pictorial view of an alternative embodiment of the invention; and Figure 7 is a cutaway pictorial view of a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVEN TION An electrostatic print head according to the invention is shown in preferred embodiment in Figs. 1 and 2 and includes first and second glass substrates 10 and 1 2 each having on one surface thereof a respective array of electrode lines 14 and 1 6 and terminating in a respective pattern of styii 1 8 and 20. Each substrate includes a spaced array of buss lines 22 and 24, respectively, on an opposite surface and selectively inter-connected to the electrode lines by respective plated-through holes 26 and 28. The substrates 10 and 12 are spaced from one another by an interposed layer 30 of plastic or other electrically insulative material.Plastic layers 32 and 34 are provided over the respective buss arrays, and outer glass plates 36 and 38 are disposed over the laminated structure along with glass spacers 40 and 42 provided along the writing face of the print head. This structure can be housed within any convenient head assembly for mounting in an electrostatic printer.

It will be appreciated that a print head can be constructed to the principles of the invention employing one or more arrays of electrodes. For many purposes, a single array of electrodes is fully adequate. For other purposes, the illustrated staggered arrangement of two electrode arrays is preferable, while for yet other purposes, more than two electrode arrays can be provided. A description follows of the fabrication of the electrode lines and buss lines on glass substrate 10 of the embodiment of Fig. 1. It is understood that substrate 1 2 is fabricated in like manner.

Referring to Fig. 3, there is shown a glass substrate 10 having formed on one surface thereof a parallel spaced array of metal electrode lines 14. The lines of the array all terminate at one end at one edge of the substrate to provide styli 18, and at the other end terminate in a staggered manner at respective enlarged pad areas 44. On the opposite surface of substrate 10, shown in Fig. 4, a spaced parallel array of buss lines 22 is formed extending in a direction transverse to the direction along which the electrode lines 14 extend. Each of the buss lines 22 includes an enlarged pad area 46 in alignment with the pad area 44 of a respective electrode line 14, and a plated-through connection 26 (Fig.

5) is provided through the substrate to electrically interconnect each buss line with a respective electrode line.

The electrode lines and buss lines are formed by thin film metal deposition, etching and electroforming techniques to provide line patterns of very high definition, each line being of uniform composition with no breaks or cracks to impair electrical conductivity.

Very high resolution is also achieved by use of the novel process. Typically, line densities of at least 200 lines per inch can be provided After formation of the electrode lines and buss lines on the respective surfaces of the glass substrate, the writing ends of the electrode lines are plated to an intended thickness and cross-section to form the writing styli 18 and 20. Typically, the electrode lines are plates to a square cross-section, although other crosssectional configurations can also be provided.

The substrate having the electrode and buss patterns is fabricated in the following manner.

The glass substrate 10 has holes photoetched or otherwise provided therethrough at predetermined positions at which aligned pad areas 44 and 46 will be formed. A metal is vacuum deposited onto both surfaces of the substrate and onto the walls of the previously formed holes through the substrate. The metal is typically chromium which provides good adherence to the glass surfaces. A thin layer of copper is then electroplated over the chromium substrate surfaces and holes. A photoresist pattern is formed and processed by wellknown techniques on one surface of the substrate to produce the array of electrode lines 14 and pad areas 44. A photoresist pattern is also provided on the opposite substrate surface to produce the pattern of buss lines 22 and pad areas 46. The exposed copper sur faces.defined by means of the photoresist patterns are electroplated with a layer of nickel. The connector areas of the buss lines 22 can be gold-plated to provide high conductivity electrical contacts.

The ends of the electrode lines 14 providing the styli 1 8 are then masked and electroplated with nickel to an intended cross-sectional shape and size. The photoresist material is then removed by an appropriate solvent, and the copper and chromium, which were previously covered by the resist patterns, are selectively etched away to leave the nickelplated patterns of electrode lines on one substrate surface and buss lines on the opposite substrate surface.

To assemble the thus formed structure into a print head as in Fig. 1, a conformal coating of plastic material is provided over both substrate surfaces to produce a plate containing the electrode and buss lines which is then assembled into the laminated head structure.

The styli can be ground to an intended surface finish if desired. The connector areas of the substrate can be constructed for connection to external driving circuitry after complete assembly of the head, or can be constructed for connection to driving circuitry or interconnecting wiring during the final assembly of the head.

An alternative embodiment is shown in Fig.

6 wherein the styli 50 extend beyond the surface of the head, rather then being substantially flush with the head surface as in the embodiments described above. The shape and extent of protrusion of the styli can be provided by plating the styli to the intended final shape or by selective etching of the surrounding portions of the substrate. The protruding styli can be of a different metal from the metal employed in forming the electrode lines to provide intended electrical and mechanical properties.

A further embodiment of the invention is shown in Fig. 7 and includes a glass or ceramic substrate 60 having formed on one surface thereof an array of electrodes having ends formed as styli 64 of intended crosssectional shape, as described above. A similar array of styli 66 and associated electrode lines is formed on the opposite surface of substrate 60, in intended relationship to the array 64.

First and second electrically insulative substrates 68 and 60 are disposed as shown over the respective electrodes on the opposite surfaces of substrate 60. Each substrate 68 and 70 contains an array of buss lines 72 and 74, respectively, which extend along an axis transverse to the axis along which the electrode lines extend. The buss lines 72 and 74 are each connected such as via a plated-through hole 76 to associated electrode lines. Connection between the buss lines and the electrode lines can be made by reflow soldering, conductive plastic, welded or compression bonding or other known electrical connection techniques.

It will be appreciated that the invention can be modified and alternatively implemented without departing from the spirit of the invention. Accordingly, it is not intended to limit the invention by what has been particularly shown and described, except as indicated in the appended claims.

Claims

1. An electrostatic print head comprising: a glass or ceramic substrate; an array of electrode lines of electrically conductive material on one surface of said substrate extending along a first direction; an array of buss lines of electrically conductive material on the opposite surface of said substrate and extending along a second direction transverse to said first direction; each of said electrode lines and buss lines including an interconnecting pad area of electrically conductive material, the pad area of each electrode line being in registration with a pad area of a respective buss line; a conductive interconnection between each pair of aligned pad areas thereby to connect each electrode line with a respective buss line; one end of said array of electrode lines being arranged along an edge of the substrate and defining styli adapted to contact a surface on which an electrical charge pattern is produced by the styli for electrostatic printing; and said array of buss lines having an area adapted for electrical connection to a source of driving signals.

2. The electrostatic print head of claim 1 wherein the styli of said electrode lines are of predetermined cross-sectional shape and size.

3. The electrostatic print head of claim 1 further including a second substrate having a like array of electrode lines and buss lines and disposed in association with the first substrate such that the styli of the electrode lines of the second substrate are in predetermined spaced relationship to the styli of the first substrate.

4. The electrostatic print head of claim 1 wherein the styli extend from the edge of the substrate to define a protruding array of styli.

5. The method of fabricating an electrostatic print head comprising the steps of: etching holes through a glass or ceramic substrate at predetermined positions; vacuum depositing a first metal layer on both surfaces of the substrate and onto the surfaces of the holes through the substrate; forming a photoresist pattern on one surface of electroplated metal defining an array of electrode lines extending along a first direction, with the ends of the electrode lines lying along an edge of the substrate and defining writing styli, and a pad area on each electrode line surrounding a respective one of the holes through the substrate; forming a photoresist pattern on the opposite surface of electroplated metal to define an array of buss lines extending along a trans verse direction to the first direction, with a connector area on each of the buss lines and a pad area on each of the buss lines in registration with a pad area of a respective electrode line and surrounding a respective hole through the substrate; plating a layer of third metal on all exposed surfaces of said second metal defined by the photoresist patterns; plating the styli of the electrode lines to intended cross-sectional shape and size; removing the photoresist material from both surfaces of the substrate; and selectively etching the first and second metal to leave the third metal electrode line and buss line patterns on respective substrate surfaces.

6. The method of claim 5 wherein said first metal is chromium, said second metal is copper and said third metal is nickel.

7. The method of claim 5 further including the step of plating a gold coating on the connector areas of the buss lines.

8. The method of fabricating an electrostatic print head comprising the steps of: forming by photoectching and electroforming an array of electrode lines of electrically conductive material on one surface of a glass or ceramic substrate, the ends of the electrode lines lying along an edge of the substrate and defining styli for contacting a writing surface, and a pad area on each electrode line; forming by photoetching and electroforming an array of buss lines of electrically conductive material on the opposite surface of the substrate, and a pad area on each buss line in registration with a pad area of a respective electrode line; etching holes through the substrate at the registered pad areas; forming a conductive connection through the etched holes thereby to interconnect an electrode line with a respective buss line; plating the styli of the electrode lines to an intended cross-sectional shape and size; and providing areas of the buss lines for connection to an electrical source for selectively applying signals to the buss lines and to associated electrode lines.

9. The method of fabricating an electrostatic print head comprising the steps: forming by photoetching and electroforming a first array of electrode lines of electrically conductive material on one surface of a glass or ceramic substrate; forming by photoetching and electroforming a second array of electrode lines of electrically conductive material on the opposite surface of the substrate; the ends of the electrode lines of each array lying along an edge of the substrate and defining styli for contacting a writing surface, the styli of the first array being in predetermined relationship with the styli of the second array; plating the styli of the first and second arrays of electrode lines to an intended cross-sectional shape and size; providing a first array of buss lines on a second substrate which is disposed on said first array of electrode lines; providing a second array of buss lines on a third substrate which is disposed on said second array of electrode lines; and coonnecting the buss lines of said first and second arrays of buss lines to associated electrode lines of said first and second arrays of electrode lines.

10. An electrostatic print head substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of fabricating an electrostatic print head substantially as hereinbefore described with reference to the accompanying drawings.