Method of manufacture of an electrostatic ink jet printer

[0002] The following Australian provisional patent applications are hereby incorporated by cross-reference. For the purposes of location and identification, US patent applications identified by their U.S. patent application serial numbers (USSN) are listed alongside the Australian applications from which the US patent applications claim the right of priority.

[0003] The present invention relates to the manufacture of ink jet print heads and, in particular, discloses a method of manufacture of an electrostatic ink jet printer.

[0004] Many ink jet printing mechanisms are known. Unfortunately, in mass production techniques, the production of ink jet beads is quite difficult. For example, often, the orifice or nozzle plate is constructed separately from the ink supply and ink ejection mechanism and bonded to the mechanism at a later stage (Hewlett-Packard Journal, Vol. 36 no 5, pp33-37 (1985)). These separate material processing steps required in handling such precision devices often add a substantial expense in manufacturing.

[0005] Additionally, side shooting ink jet technologies (U.S. Pat. No. 4,899,181) are often used but again, this limits the amount of mass production throughput given any particular capital investment.

[0006] Additionally, more esoteric techniques are also often utilised. These can include electroforming of nickel stage (Hewlett-Packard Journal, Vol. 36 no 5, pp33-37 (1985)), electro-discharge machining, laser ablation (U.S. Pat. No. 5,208,604), micro-punching, etc.

[0007] The utilisation of the above techniques is likely to add substantial expense to the mass production of ink jet print heads and therefore add substantially to their final cost.

[0008] It would therefore be desirable if an efficient system for the mass production of ink jet print heads could be developed.

[0009] It is an object of the present invention to provide a method for the creation of an electrostatic ink jet printer.

[0010] In accordance with a first aspect of the present invention, there is provided a method of manufacturing an electrostatic ink jet print head wherein an array of nozzles is formed on a substrate utilising planar monolithic deposition, lithographic and etching processes. Preferably, multiple ink jet heads are formed simultaneously on a single planar substrate such as a silicon wafer.

[0011] The print heads can be formed utilising standard vlsi/ulsi processing and can include integrated drive electronics formed on the same substrate. The drive electronics preferably are of a CMOS type. In the final construction, ink can be ejected from the substrate substantially normal

[0012] In accordance with a further aspect of the present invention, there is provided a method of manufacture of an ink jet print head arrangement including a series of nozzle chambers, the method comprising the steps of: (a) utilizing an initial semiconductor wafer having an electrical circuitry layer formed thereon; (b) forming a bottom electrode layer of conductive material on or in the electrical circuitry layer; (c) depositing and etching a first hydrophobic layer on the electrode layer; (d) depositing and etching a first sacrificial layer of sacrificial material on the first hydrophobic layer; (e) depositing and etching a top electrode layer of conductive material on the first sacrificial layer the top electrode layer including predetermined portions interconnecting with the electrical circuitry layer; (f) depositing and etching a membrane layer on the top electrode layer, (g) depositing and etching a second sacrificial layer on the membrane layer, the second sacrificial layer forming a blank for the nozzle chamber walls; (h) depositing and etching an inert material layer on the second sacrificial layer so as to form the nozzle chamber walls surrounding the nozzle chamber in addition to a nozzle fluid ejection hole interconnecting with the nozzle chamber; (i) etching an ink supply channel interconnecting with the nozzle chamber; (j) etching away the sacrificial layers so as to leave an operational device.

[0013] The top electrode layer and the membrane layer can include a concertina edge so as to allow for movement of the membrane layer. The bottom electrode layer can be formed from a metal plane layer of the circuitry layer.

[0014] The ink supply channel can be formed by etching a channel from the back surface of the wafer. The step (b) preferably can include etching a nozzle rim around the nozzle fluid ejection hole and a series of small holes in at least one wall of the nozzle chamber. The hydrophobic layer can comprise substantially polytetrafluoroethylene.

[0015] The steps are preferably also utilized to simultaneously separate the wafer into separate printheads

[0016] Notwithstanding any other forms which may fall within the scope of the present invention, preferred forms of the invention will now be described, by way of example only, with reference to the accompanying drawings, which:

[0017] FIG. 1 is a perspective cross-sectional view of a single ink jet nozzle constructed in accordance with the preferred embodiment;

[0018] FIG. 2 is a close-up perspective cross-sectional view (portion A of FIG. 1), of a single ink jet nozzle constructed in accordance with the preferred embodiment;

[0019] FIG. 3 is an exploded perspective view illustrating the construction of a single ink jet nozzle in accordance with the preferred embodiment;

[0020] FIG. 4 provides a legend of the materials indicated in FIGS. 5 to 15; and

[0021] FIG. 5 to FIG. 15 illustrate sectional views of the manufacturing steps in one form of construction of an ink jet printhead nozzle.

[0022] In the preferred embodiment, an ink jet print head is made up of a plurality of nozzle chambers each having an ink ejection port. Ink is ejected from the ink ejection port through the utilisation of attraction between two parallel plates.

[0023] Turning initially to FIG. 1, there is illustrated a cross-sectional view of a single nozzle arrangement 10 as constructed in accordance with the preferred embodiment. The nozzle arrangement 10 includes a nozzle chamber 11 in which is stored ink to be ejected out of an ink ejection port 12. The nozzle arrangement 10 can be constructed on the top of a silicon wafer utilising micro electro-mechanical systems construction techniques as will become more apparent hereinafter. The top of the nozzle plate also includes a series of regular spaced etchant holes, e.g. 13 which are provided for efficient sacrificial etching of lower layers of the nozzle arrangement 10 during construction. The size of the etchant holes 13 is small enough that surface tension characteristics inhibit ejection from the holes 13 during operation.

[0024] Ink is supplied to the nozzle chamber 11 via an ink supply channel, e.g. 15.

[0025] Turning now to FIG. 2, there is illustrated a cross-sectional view of one side of the nozzle arrangement 10. A nozzle arrangement 10 is constructed on a silicon wafer base 17 on top of which is first constructed a standard CMOS two level metal layer 18 which includes the required drive and control circuitry for each nozzle arrangement. The layer 18, which includes two levels of aluminium, includes one level of aluminum 19 being utilised as a bottom electrode plate. Other portions 20 of this layer can comprise nitride passivation. On top of the layer 19 there is provided a thin polytetrafluoroethylene (PTFB) layer 21.

[0026] Next, an air gap 27 is provided between the top and bottom layers. This is followed by a further PTFE layer 28 which forms part of the top plate 22. The two PTFE layers 21, 28 are provided so as to reduce possible stiction effects between the upper and lower plates. Next, a top aluminum electrode layer 30 is provided followed by a nitride layer (not shown) which provides structural integrity to the top electro plate. The layers 28-30 are fabricated so as to include a corrugated portion 23 which concertinas upon movement of the top plate 22.

[0027] By placing a potential difference across the two aluminum layers 19 and 30, the top plate 22 is attracted to bottom aluminum layer 19 thereby resulting in a movement of the top plate 22 towards the bottom plate 19. This results in energy being stored in the concertinaed spring arrangement 23 in addition to air passing out of the side air holes, e.g. 33 and the ink being sucked into the nozzle chamber as a result of the distortion of the meniscus over the ink ejection port 12 (FIG. 1). Subsequently, the potential across the plates is eliminated thereby causing the concertinaed spring portion 23 to rapidly return the plate 22 to its rest position. The rapid movement of the plate 22 causes the consequential ejection of ink from the nozzle chamber via the ink ejection port 12 (FIG. 1). Additionally, air flows in via air gap 33 underneath the plate 22.

[0028] The ink jet nozzles of the preferred embodiment can be formed from utilisation of semiconductor fabrication and MEMS techniques. Turning to FIG. 3, there is illustrated an exploded perspective view of the various layers in the final construction of a nozzle arrangement 10. At the lowest layer is the silicon wafer 17 upon which all other processing steps take place. On top of the silicon layer 17 is the CMOS circuitry layer 18 which primarily comprises glass. On top of this layer is a nitride passivation layer 20 which is primarily utilised to passivate and protect the lower glass layer from any sacrificial process that may be utilised in the building up of subsequent layers. Next there is provided the aluminum layer 19 which, in the alternative, can form part of the lower CMOS glass layer 18. This layer 19 forms the bottom plate. Next, two PFTE layers 26, 28 are provided between which is laid down a sacrificial layer, such as glass, which is subsequently etched away so as to release the plate 22 (FIG. 2). On top of the PTFE layer 28 is laid down the aluminum layer 30 and a subsequent thicker nitride layer (not shown) which provides structural support to the top electrode stopping it from sagging or deforming. After this comes the top nitride nozzle chamber layer 35 which forms the rest of the nozzle chamber and ink supply channel. The layer 35 can be formed from the depositing and etching of a sacrificial layer and then depositing the nitride layer, etching the nozzle and etchant holes utilising an appropriate mask before etching away the sacrificial material.

[0029] Obviously, print heads can be formed from large arrays of nozzle arrangements 10 on a single wafer which is subsequently diced into separate print heads. Ink supply can be either from the side of the wafer or through the wafer utilising deep anisotropic etching systems such as high density low pressure plasma etching systems available from surface technology systems. Further, the corrugated portion 23 can be formed through the utilisation of a half tone mask process.

[0030] One form of detailed manufacturing process which can be used to fabricate monolithic ink jet print heads operating in accordance with the principles taught by the present embodiment can proceed utilizing the following steps:

[0031] 1. Using a double sided polished wafer 40, complete a 0.5 micron, one poly, 2 metal CMOS process 42. This step is shown in FIG. 5. For clarity, these diagrams may not be to scale, and may not represent a cross section though any single plane of the nozzle. FIG. 4 is a key to representations of various materials in these manufacturing diagrams, and those of other cross referenced ink jet configurations. FIG. 4 is a key to representations of various materials in these manufacturing diagrams, and those of other cross referenced ink jet configurations.

[0032] 2. Etch the passivation layers 46 to expose 44 the bottom electrode, formed of second level metal. This etch is performed using Mask 1. This step is shown in FIG. 6.

[0033] 3. Deposit 50 nm of PTFE or other highly hydrophobic material.

[0034] 4. Deposit 0.5 microns of sacrificial material, e.g. polyimide 48.

[0035] 5. Deposit 0.5 microns of (sacrificial) photosensitive polyimide.

[0036] 6. Expose and develop the photosensitive polyimide using Mask 2. This mask is a gray-scale mask which defines the concertina edge 50 of the upper electrode. The result of the etch is a series of triangular ridges at the circumference of the electrode. This concertina edge is used to convert tensile stress into bend strain, and thereby allow the upper electrode to move when a voltage is applied across the electrodes. This step is shown in FIG. 7.

[0037] 7. Etch the polyimide and passivation layers using Mask 3, which exposes the contacts for the upper electrode which are formed in second level metal.

[0038] 8. Deposit 0.1 microns of tantalum 52, forming the upper electrode.

[0039] 9. Deposit 0.5 microns of silicon nitride (Si3N4), which forms the movable membrane of the upper electrode.

[0040] 10. Etch the nitride and tantalum using Mask 4. This mask defines the upper electrode, as well as the contacts to the upper electrode. This step is shown in FIG. 8.

[0041] 11. Deposit 12 microns of (sacrificial) photosensitive polyimide 54.

[0042] 12. Expose and develop the photosensitive polyimide using Mask 5. A proximity aligner can be used to obtain a large depth of focus, as the line-width for this step is greater than 2 microns, and can be 5 microns or more. This mask defines the nozzle chamber walls. This step is shown in FIG. 9.

[0043] 13. Deposit 3 microns of PECVD glass 56. This step is shown in FIG. 10.

[0044] 14. Etch to a depth of 1 micron using Mask 6. This mask defines the nozzle rim 58. This step is shown in FIG. 11.

[0045] 15. Etch down to the sacrificial layer 54 using Mask 7. This mask defines the roof of the nozzle chamber, and the nozzle 60 itself. This step is shown in FIG. 12.

[0046] 16. Back-etch completely through the silicon wafer 46 (with, for example, an ASE Advanced Silicon Etcher from Surface Technology Systems) using Mask 8. This mask defines the ink inlets 62 which are etched through the wafer 40. The wafer 40 is also diced by this etch.

[0047] 17. Back-etch through the CMOS oxide layer through the holes in the wafer 40. This step is shown in FIG. 13.

[0048] 18. Etch the sacrificial polyimide 54. The nozzle chambers 64 are cleared, a gap is formed between the electrodes and the chips are separated by this etch. To avoid stiction, a final rinse using supercooled carbon dioxide can be used. This step is shown in FIG. 14.

[0049] 19. Mount the print heads in their packaging, which may be a molded plastic former incorporating ink channels which supply the appropriate color ink to the ink inlets at the back of the wafer.

[0050] 20. Connect the print heads to their interconnect systems. For a low profile connection with minimum disruption of airflow, TAB may be used. Wire bonding may also be used if the printer is to be operated with sufficient clearance to the paper.

[0051] 21. Hydrophobize the front surface of the print heads.

[0052] 22. Fill the completed print heads with ink 66 and test them. A filled nozzle is shown in FIG. 15.

[0053] The aforementioned half-tone mask process follows fabrication steps as discussed in Australian Provisional Patent entitled Image Creation Method and Apparatus (IJ16) lodged by the present applicant simultaneously herewith, the contents of which are hereby incorporated by cross-reference.

[0054] It would be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the preferred embodiment without departing from the spirit or scope of the invention as broadly described. The preferred embodiment is, therefore, to be considered in all respects to be illustrative and not restrictive.

[0055] The presently disclosed ink jet printing technology is potentially suited to a wide range of printing systems including: color and monochrome office printers, short run digital printers, high speed digital printers, offset press supplemental printers, low cost scanning printers, high speed pagewidth printers, notebook computers with in-built pagewidth printers, portable color and monochrome printers, color and monochrome copiers, color and monochrome facsimile machines, combined printer, facsimile and copying machines, label printers, large format plotters, photograph copiers, printers for digital photographic ‘minilabs’, video printers, PhotoCD printers, portable printers for PDAs, wallpaper printers, indoor sign printers, billboard printers, fabric printers, camera printers and fault tolerant commercial printer arrays.

[0056] Ink Jet Technologies

[0057] The embodiments of the invention use an ink jet printer type device. Of course many different devices could be used. However presently popular ink jet printing technologies are unlikely to be suitable.

[0058] The most significant problem with thermal ink jet is power consumption. This is approximately 100 times that required for high speed, and stems from the energy-inefficient means of drop ejection. This involves the rapid boiling of water to produce a vapor bubble which expels the ink. Water has a very high heat capacity, and must be superheated in thermal ink jet applications. This leads to an efficiency of around 0.02%, from electricity input to drop momentum (and increased surface area) out.

[0059] The most significant problem with piezoelectric ink jet is size and cost. Piezoelectric crystals have a very small deflection at reasonable drive voltages, and therefore require a large area for each nozzle. Also, each piezoelectric actuator must be connected to its drive circuit on a separate substrate. This is not a significant problem at the current limit of around 300 nozzles per printhead, but is a major impediment to the fabrication of pagewidth printheads with 19,200 nozzles.

[0060] Ideally, the ink jet technologies used meet the stringent requirements of in-camera digital color printing and other high quality, high speed, low cost printing applications. To meet the requirements of digital photography, new ink jet technologies have been created. The target features include:

[0061] low power (less than 10 Watts)

[0062] high resolution capability (1,600 dpi or more)

[0063] photographic quality output

[0064] low manufacturing cost

[0065] small size (pagewidth times minimum cross section)

[0066] high speed (<2 seconds per page).

[0067] All of these features can be met or exceeded by the ink jet systems described below with differing levels of difficulty. Forty-five different ink jet technologies have been developed by the Assignee to give a wide range of choices for high volume manufacture. These technologies form part of separate applications assigned to the present Assignee as set out in the table under the heading Cross References to Related Applications.

[0068] The ink jet designs shown here are suitable for a wide range of digital printing systems, from battery powered one-time use digital cameras, through to desktop and network printers, and through to commercial printing systems.

[0069] For ease of manufacture using standard process equipment, the printhead is designed to be a monolithic 0.5 micron CMOS chip with MEMS post processing. For color photographic applications, the print head is 100 mm long, with a width which depends upon the ink jet type. The smallest printhead designed is IJ38, which is 0.35 mm wide, giving a chip area of 35 square mm. The printheads each contain 19,200 nozzles plus data and control circuitry.

[0070] Ink is supplied to the back of the printhead by injection molded plastic ink channels. The molding requires 50 micron features, which can be created using a lithographically micromachined insert in a standard injection molding tool. Ink flows through holes etched through the wafer to the nozzle chambers fabricated on the front surface of the wafer. The print head is connected to the camera circuitry by tape automated bonding.

[0071] Tables of Drop-on-Demand Ink Jets

[0072] Eleven important characteristics of the fundamental operation of individual ink jet nozzles have been identified. These characteristics are largely orthogonal, and so can be elucidated as an eleven dimensional matrix. Most of the eleven axes of this matrix include entries developed by the present assignee.

[0073] The following tables form the axes of an eleven dimensional table of ink jet types.

[0074] Actuator mechanism (18 types)

[0075] Basic operation mode (7 types)

[0076] Auxiliary mechanism (8 types)

[0077] Actuator amplification or modification method (17 types)

[0078] Actuator motion (19 types)

[0079] Nozzle refill method (4 types)

[0080] Method of restricting back-flow through inlet (10 types)

[0081] Nozzle clearing method (9 types)

[0082] Nozzle plate construction (9 types)

[0083] Drop ejection direction (5 types)

[0084] Ink type (7 types)

[0085] The complete eleven dimensional table represented by these axes contains 36.9 billion possible configurations of ink jet nozzle. While not all of the possible combinations result in a viable ink jet technology, many million configurations are viable. It is clearly impractical to elucidate all of the possible configurations. Instead, certain ink jet types have been investigated in detail. These are designated IJ01 to IJ45 above which matches the docket numbers in the tables under the heading Cross References to Related Applications.

[0086] Other ink jet configurations can readily be derived from these forty-five examples by substituting alternative configurations along one or more of the 11 axes. Most of the IJ01 to IJ45 examples can be made into ink jet printheads with characteristics superior to any currently available ink jet technology.

[0087] Where there are prior art examples known to the inventor, one or more of these examples are listed in the examples column of the tables below. The IJ01 to IJ45 series are also listed in the examples column. In some cases, print technology may be listed more than once in a table, where it shares characteristics with more than one entry.

[0088] Suitable applications for the ink jet technologies include: Home printers, Office network printers, Short run digital printers, Commercial print systems, Fabric printers, Pocket printers, Internet WWW printers, Video printers, Medical imaging, Wide format printers, Notebook PC printers, Fax machines, Industrial printing systems, Photocopiers, Photographic minilabs etc.

[0089] The information associated with the aforementioned 11 dimensional matrix are set out in the following tables.