Method of manufacture of a permanent magnet electromagnetic 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 US patent application serial numbers (USSN) are listed alongside the Australian applications from which the US patent applications claim the right of priority.

[0003] Not applicable.

[0004] The present invention relates to the manufacture of ink jet print heads and, in particular, discloses a method of manufacture of a Permanent Magnet Electromagnetic Ink Jet Printer.

[0005] Many ink jet printing mechanisms are known. Unfortunately, in mass production techniques, the production of ink jet heads 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, pp 33-37 (1985)). These separate material processing steps required in handling such precision devices often add a substantial expense in manufacturing.

[0006] 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.

[0007] Additionally, more esoteric techniques are also often utilised. These can include electroforming of nickel stage (Hewlett-Packard Journal, Vol. 36 no 5, pp 33-37 (1985)), electro-discharge machining, laser ablation (U.S. Pat. No. 5,208,604), micro-punching, etc.

[0008] 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.

[0009] It would therefore be desirable if an efficient system for the mass production of ink jet print heads could be developed.

[0010] It is an object of the present invention to provide a method for the creation of a permanent magnet electromagnetic ink jet printer.

[0011] In accordance with a first aspect of the present invention, there is provided a method of manufacturing a permanent magnet electromagnetic ink jet print head wherein an array of nozzles are 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.

[0012] 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 to the wafer.

[0013] 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 and a buried epitaxial layer formed thereon; (b) depositing and etching a first inert layer, the etching including etching predetermined vias and a nozzle chamber aperture; (c) forming a first conductive coil layer on the first inert layer around the nozzle aperture, the conductive coil layer including predetermined portions interconnecting with the electrical circuitry layer; (d) utilizing the nozzle aperture to etch a nozzle chamber in the wafer; (e) depositing and etching a sacrificial material layer over the wafer including the nozzle chamber, the etching including etching a mould for a series of magnet suspension posts and a permanent magnet above the nozzle aperture; (f) deposit and etch a magnetic material layer, the magnetic material layer forming a permament magnet above the nozzle aperture; (g) deposit and etching an inert material layer interconnecting the permanent magnet to a series of spring posts in a resilient manner; (h) back etching the wafer substantially to the buried epitaxial layer; (i) etching a nozzle fluid ejection aperture through the buried epitaxial layer; (0) etching away the sacrificial layer.

[0014] The conductive coil layer can be formed by first depositing and etching a sacrificial layer forming a mould for the conductive coil layer. The conductive coil layer can be formed utilizing chemical mechanical planarization and can comprise substantially copper.

[0015] The first inert layer can comprise substantially silicon nitride.

[0016] The steps are preferably also utilized to simultaneously separate the wafer into separate printheads.

[0017] 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 in which:

[0018] FIG. 1 is a perspective cross-sectional view of a single ink jet nozzle constructed in accordance with the preferred embodiment;

[0019] FIG. 2 is an exploded perspective view illustrating the construction of a single ink jet nozzle in accordance with the preferred embodiment;

[0020] FIG. 3 provides a legend of the materials indicated in FIGS. 4 to 18;

[0021] FIG. 4 shows a sectional side view of an initial manufacturing step of an ink jet printhead nozzle showing a silicon wafer with a buried epitaxial layer and an electrical circuitry layer;

[0022] FIG. 5 shows a step of etching a subsequently deposited nitride layer;

[0023] FIG. 6 shows a step of etching of a subsequently deposited glass layer;

[0024] FIG. 7 shows a step of electroplating a conductive layer and subsequent planarizing thereof;

[0025] FIG. 8 shows a step of etching the conductive layer;

[0026] FIG. 9 shows a step of crystallographically etching the exposed silicon;

[0027] FIG. 10 shows a step of depositing a sacrificial layer;

[0028] FIG. 11 shows a step of etching the sacrificial layer;

[0029] FIG. 12 shows a step of depositing a magnetic material;

[0030] FIG. 13 shows a step of depositing a layer of nitride;

[0031] FIG. 14 shows a step of back-etching the wafer;

[0032] FIG. 15 shows a step of back-etching the boron doped silicon layer;

[0033] FIG. 16 shows a step of back-etching through the boron doped silicon layer;

[0034] FIG. 17 shows a step of etching sacrificial material; and

[0035] FIG. 18 shows a step of filing the completed ink jet nozzle with ink.

[0036] Turning initially to FIG. 1, there is illustrated a perspective view in section of a single nozzle 1 constructed in accordance with the techniques of the preferred embodiment.

[0037] Each nozzle 1 includes a nozzle outlet port 2 for the ejection of ink from a nozzle chamber 4 as a result of activation of an electromagnetic piston 5. The electromagnetic piston 5 is activated via solenoid coil 6 which surrounds the piston 5. Upon a current passing through the solenoid coil 6, the piston 5 experiences a force in the direction as indicated by arrow 13. As a result the piston 5 begins movement towards outlet port 2 and thereby imparts momentum to ink within the nozzle chamber 4. Torsional springs, e.g. 8, act against the movement of piston 5, however they do not fully stop the movement of the piston.

[0038] Upon the completion of the ejection cycle, the current to the coil 6 is turned off. As a result, the torsional springs, e.g. 8, act to return the piston 5 to its rest position as initially shown in FIG. 1. Subsequently, surface tension forces cause the chamber 4 to refill with ink and to return ready for “re-firing”.

[0039] Current to the coil 6 is provided via aluminium connectors (not shown) which interconnect the coil 6 with a semi-conductor drive transistor and logic layer 18.

[0040] Construction

[0041] A liquid ink jet print head 1 has one actuator device associated with each of a multitude of nozzles. It will be evident that the actuator 1 has the following major parts, which are constructed using standard semi-conductor and micromechanical construction techniques:

[0042] 1. Drive circuitry 18.

[0043] 2. The nozzle outlet port 2. The radius of the nozzle outlet port 2 is an important determinant of drop velocity and drop size.

[0044] 3. The magnetic piston 5. This can be a cylinder of a rare earth magnetic material such as neodymium iron boron (NdFeB) or samarium cobalt (SaCo). The pistons 5 are magnetised after a last high temperature step in the fabrication of the print heads, to thereby ensure that the Curie temperature is not exceeded after magnetisation. A typical print head may include many thousands of pistons all of which can be magnetised simultaneously and in the same direction.

[0045] 4. The nozzle chamber 4. The nozzle chamber 4 is slightly wider than the piston 5. The gap between the piston 5 and the nozzle chamber 4 can be as small as is required to ensure that the piston does not contact the nozzle chamber during actuation or return. If the print heads are fabricated using a standard 0.5 mm lithography process, then a 1 mm gap will usually be sufficient. The nozzle chamber 4 should also be deep enough so that air drawn in through the nozzle tip 2 when the plunger returns to its quiescent state does not extend to the piston. If it does, the ingested air bubble may form a cylindrical surface instead of a hemispherical surface. If this happens, the nozzle chamber 4 may not refill properly.

[0046] 5. The solenoid coil 6. This is a spiral coil of copper. A double layer spiral is used to obtain a high field strength with a small device radius. Copper is used for its low resistivity, and high electro-migration resistance.

[0047] 6. Springs 8-11. The springs 8-11 return the piston 5 to its quiescent position after a drop has been ejected. The springs can be fabricated from silicon nitride.

[0048] 7. Passivation layers. All surfaces are coated with passivation layers, which may be silicon nitride (Si₃N₄), diamond like carbon (DLC), or other chemically inert, highly impermeable layer. The passivation layers are especially important for device lifetime, as the active device is immersed in the ink.

[0049] Example method of Fabrication

[0050] The print head is fabricated from two silicon wafers. A first wafer is used to fabricate the print nozzles (the print head wafer) and a second wafer is utilised to fabricate the various ink channels in addition to providing a support means for the first channel (the Ink Channel Wafer). FIG. 2 is an exploded perspective view illustrating the construction of a single ink jet nozzle 1 on a print head wafer. The fabrication process proceeds as follows:

[0051] Start with a single silicon wafer, which has a buried epitaxial layer 21 of silicon which is heavily doped with boron. The boron should be doped to preferably 10²⁰atoms per cm³of boron or more, and be approximately 3 mm thick. A lightly doped silicon epitaxial layer 22 on top of the boron doped layer 21 should be approximately 8mm thick, and be doped in a manner suitable for the active semiconductor device technology chosen. This is the starting point for the print head wafer. The wafer diameter should be the same as that of the ink channel wafer.

[0052] Next, fabricate the drive transistors and data distribution circuitry required for each nozzle according to the process chosen, in a standard CMOS layer 18 up until oxide over the first level metal. On top of the CMOS layer 18 is deposited a silicon nitride passivation layer 25. Next, a silicon oxide layer 27 is deposited. The silicon oxide layer 27 is etched utilising a mask for the copper coil layer. Subsequently, the copper layer 30 is deposited through the mask for the copper coil. The layers 27, 25 also include vias for the interconnection of the copper coil layer 30 to the underlying CMOS layer 18 (not shown). Next, the nozzle chamber 4 (FIG. 1) is etched. Subsequently, a sacrificial material is deposited to entirely fill the etched volume (not shown). On top of the sacrificial material a silicon nitride layer 31 is deposited, including site portions 32. Next, the magnetic material layer 33 is deposited utilising the magnetic piston mask. This layer also includes the posts, e.g. 34.

[0053] A final silicon nitride layer 35 is then deposited onto an additional sacrificial layer (not shown) deposited to cover the bare portions of nitride layer 31 to the height of the magnetic material layer 33, utilising a mask for the magnetic piston and the torsional springs, e.g. 8 to 11. The torsional springs, e.g. 8 to 11, and the magnetic piston 5 (see FIG. 1) are liberated by etching the aforementioned sacrificial material.

[0054] For a general introduction to a micro-electro mechanical system (MEMS) reference is made to standard proceedings in this field including the proceedings of the SPIE (International Society for Optical Engineering), volumes 2642 and 2882 which contain the proceedings for recent advances and conferences in this field.

[0055] 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:

[0056] 1. Using a double sided polished wafer 51 deposit 3 microns of epitaxial silicon heavily doped with boron 21.

[0057] 2. Deposit 10 microns of epitaxial silicon 22, either p-type or n-type, depending upon the CMOS process used.

[0058] 3. Complete a 0.5 micron, one poly, 2 metal CMOS process 18. The metal layers 18 are copper instead of aluminum, due to high current densities and subsequent high temperature processing. This step is shown in FIG. 4. For clarity, these diagrams may not be to scale, and may not represent a cross section though any single plane of the nozzle. FIG. 3 is a key to representations of various materials in these manufacturing diagrams, and those of other cross referenced ink jet configurations.

[0059] 4. Deposit 0.5 microns of low stress PECVD silicon nitride (Si3N4) 52. The nitride 52 acts as a dielectric, and etch stop, a copper diffusion barrier, and an ion diffusion barrier. As the speed of operation of the print head is low, the high dielectric constant of silicon nitride is not important, so the nitride layer 52 can be thick compared to sub-micron CMOS back-end processes.

[0060] 5. Etch the nitride layer 52 using Mask 1. This mask defines the contact vias 53 from the solenoid coil to the second-level metal contacts, as well as the nozzle chamber. This step is shown in FIG. 5.

[0061] 6. Deposit 4 microns of PECVD glass 54.

[0062] 7. Etch the glass 54 down to nitride 52 or second level metal 8 using Mask 2. This mask defines the solenoid. This step is shown in FIG. 6.

[0063] 8. Deposit a thin barrier layer of Ta or TaN (not shown).

[0064] 9. Deposit a seed layer (not shown) of copper. Copper is used for its low resistivity (which results in higher efficiency) and its high electromigration resistance, which increases reliability at high current densities.

[0065] 10. Electroplate 4 microns of copper 55.

[0066] 11. Planarize using CMP. Steps 4 to 11 represent a copper dual damascene process, with a 4:1 copper aspect ratio (4 microns high, 1 micron wide). This step is shown in FIG. 7.

[0067] 12. Etch down to silicon using Mask 3. This mask defines the nozzle cavity. This step is shown in FIG. 8.

[0068] 13. Crystallographically etch the exposed silicon using KOH. This etch stops on (111) crystallographic planes 56, and on the boron doped silicon buried layer 21. This step is shown in FIG. 9.

[0069] 14. Deposit 0.5 microns of low stress PECVD silicon nitride 57.

[0070] 15. Open the bond pads using Mask 4.

[0071] 16. Wafer probe. All electrical connections are complete at this point, bond pads are accessible, and the chips are not yet separated.

[0072] 17. Deposit a thick sacrificial layer 58 (e.g. low stress glass), filling the nozzle cavity. Planarize the sacrificial layer 58 to a depth of 5 microns over the nitride 57 surface. This step is shown in FIG. 10.

[0073] 18. Etch the sacrificial layer 58 to a depth of 6 microns using Mask 5. This mask defines the permanent magnet 33 plus the magnet support posts 34. This step is shown in FIG. 11.

[0074] 19. Deposit 6 microns of permanent magnet material 59 such as neodymium iron boron (NdFeB). Planarize. This step is shown in FIG. 12.

[0075] 20. Deposit 0.5 microns of low stress PECVD silicon nitride 60.

[0076] 21. Etch the nitride 60 using Mask 6, which defines the spring 8 to 11. This step is shown in FIG. 13.

[0077] 22. Anneal the permanent magnet material 59 at a temperature which is dependant upon the material.

[0078] 23. Place the wafer in a uniform magnetic field of 2 Tesla (20,000 Gauss) with the field normal to the chip surface. This magnetizes the permanent magnet 59.

[0079] 24. Mount the wafer on a glass blank 61 and back-etch the wafer 51 using KOH, with no mask. This etch thins the wafer 51 and stops at the buried boron doped silicon layer 21. This step is shown in FIG. 14.

[0080] 25. Plasma back-etch the boron doped silicon layer 21 to a depth of 1 micron using Mask 7. This mask defines the nozzle rim 62. This step is shown in FIG. 15.

[0081] 26. Plasma back-etch through the boron doped layer using Mask 8. This mask defines the nozzle 2, and the edge of the chips.

[0082] 27. Plasma back-etch nitride up to the glass sacrificial layer through the holes in the boron doped silicon layer. At this stage, the chips are separate, but are still mounted on the glass blank. This step is shown in FIG. 16.

[0083] 28. Strip the adhesive layer to detach the chips from the glass blank.

[0084] 29. Etch the sacrificial glass layer 58 in buffered HF. This step is shown in FIG. 17.

[0085] 30. Mount the print heads in their packaging, which may be a molded plastic former incorporating ink channels which supply different colors of ink to the appropriate regions of the front surface of the wafer.

[0086] 31. Connect the print heads to their interconnect systems.

[0087] 32. Hydrophobize the front surface of the print heads.

[0088] 33. Fill the completed print heads with ink 63 and test them. A filled nozzle is shown in FIG. 18.

[0089] 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 specific embodiment without departing from the spirit or scope of the invention as broadly described. The present embodiment is, therefore, to be considered in all respects to be illustrative and not restrictive.

[0090] 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, PHOTO CD (PHOTO CD is a registered trade mark of the Eastman Kodak Company) printers, portable printers for PDAs, wallpaper printers, indoor sign printers, billboard printers, fabric printers, camera printers and fault tolerant commercial printer arrays.

[0091] Ink Jet Technologies

[0092] 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.

[0093] 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.

[0094] 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 print head, but is a major impediment to the fabrication of pagewidth print heads with 19,200 nozzles.

[0095] 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:

[0096] low power (less than 10 Watts)

[0097] high resolution capability (1,600 dpi or more)

[0098] photographic quality output

[0099] low manufacturing cost

[0100] small size (pagewidth times minimum cross section)

[0101] high speed (<2 seconds per page).

[0102] 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 list under the heading Cross References to Related Applications.

[0103] 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

[0104] For ease of manufacture using standard process equipment, the print head 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 print head designed is covered in U.S. patent application Ser. No. 09/112,764, which is 0.35 mm wide, giving a chip area of 35 square mm. The print heads each contain 19,200 nozzles plus data and control circuitry.

[0105] Ink is supplied to the back of the print head 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.

[0106] Tables of Drop-on-Demand Ink Jets

[0107] 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.

[0108] The following tables form the axes of an eleven dimensional table of ink jet types.

[0109] Actuator mechanism (18 types)

[0110] Basic operation mode (7 types)

[0111] Auxiliary mechanism (8 types)

[0112] Actuator amplification or modification method (17 types)

[0113] Actuator motion (19 types)

[0114] Nozzle refill method (4 types)

[0115] Method of restricting back-flow through inlet (10 types)

[0116] Nozzle clearing method (9 types)

[0117] Nozzle plate construction (9 types)

[0118] Drop ejection direction (5 types)

[0119] Ink type (7 types)

[0120] 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. Forty-five such inkjet types were filed simultaneously to the present application.

[0121] 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 forty-five examples can be made into ink jet print heads with characteristics superior to any currently available ink jet technology.

[0122] 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 simultaneously filed patent applications by the present applicant are listed by USSN numbers. In some cases, a print technology may be listed more than once in a table, where it shares characteristics with more than one entry.

[0123] 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. The information associated with the aforementioned 11 dimensional matrix are set out in the following tables.