Method of manufacture of a planar thermoelastic bend actuator ink jet printer

[0002] The following Australian provisional patent applications are hereby incorporated by cross-reference. For the purposes of location and identification, U.S. patent applications, identified by their U.S. patent application Ser. Nos. (USSN) are listed alongside the Australian applications from which the U.S. 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 Planar Thermoelastic Bend Actuator 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 planar thermoelastic bend actuator ink jet printer.

[0011] In accordance with a first aspect of the present invention, there is provided a method of manufacturing a planar thermoelastic bend actuator 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 substrate.

[0013] 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:

[0014] FIG. 1 is cross-sectional view, partly in section, of a single ink jet nozzle constructed in accordance with an embodiment of the present invention;

[0015] FIG. 2 is an exploded perspective view illustrating the construction of a single ink jet nozzle in accordance with an embodiment of the present invention;

[0016] FIG. 3 provides a legend of the materials indicated in FIGS. 4 to 19;

[0017] FIG. 4 shows a sectional side view of an initial manufacturing step of an ink jet printhead nozzle showing a silicon wafer layer with an electrical circuitry layer;

[0018] FIG. 5 shows a step of etching the electrical circuitry layer;

[0019] FIG. 6 shows a step of etching the silicon wafer layer;

[0020] FIG. 7 shows a step of depositing an ion diffusion barrier layer;

[0021] FIG. 8 shows a step of depositing a sacrificial material layer;

[0022] FIG. 9 shows a step of etching a stiffener material layer;

[0023] FIG. 10 shows a step of etching the ion diffusion barrier layer;

[0024] FIG. 11 shows a step of depositing a first bend actuator layer;

[0025] FIG. 12 shows a step of etching a previously deposited thermal blanket layer;

[0026] FIG. 13 shows a step of etching a previously deposited second bend actuator layer;

[0027] FIG. 14 shows a step of etching a previously deposited further thermal blanket layer;

[0028] FIG. 15 shows a step of mounting the printhead on a glass blank and back etching the silicon water layer;

[0029] FIG. 16 shows a step of etching a doped layer to form a nozzle rim;

[0030] FIG. 17 shows a step of further etching the doped layer to form a nozzle opeing;

[0031] FIG. 18 shows a step of etching the sacrificial material layer; and

[0032] FIG. 19 shows a step of filling the completed ink jet nozzle with ink.

[0033] In the preferred embodiment, there is provided an ink jet printer having nozzle chambers. Each nozzle chamber includes a thermoelastic bend actuator that utilises a planar resistive material in the construction of the bend actuator. The bend actuator is activated when it is required to eject ink from a chamber.

[0034] Turning now to FIG. 1, there is illustrated a cross-sectional view, partly in section of a nozzle arrangement 10 as constructed in accordance with the preferred embodiment. The nozzle arrangement 10 can be formed as part of an array of nozzles fabricated on a semi-conductor wafer utilising techniques known in the production of micro-electro-mechanical systems (MEMS). For a general introduction to a micro-electric 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. The nozzle arrangement 10 includes a boron doped silicon wafer layer 12 which can be constructed by back etching a silicon wafer 18 which has a buried boron doped epitaxial layer. The boron doped layer can be further etched so as to define a nozzle hole 13 and rim 14.

[0035] The nozzle arrangement 10 includes a nozzle chamber 16 which can be constructed by utilisation of an anisotropic crystallographic etch of the silicon portions 18 of the wafer.

[0036] On top of the silicon portions 18 is included a glass layer 20 which can comprise CMOS drive circuitry including a two level metal layer (not shown) so as to provide control and drive circuitry for the thermal actuator. On top of the CMOS glass layer 20 is provided a nitride layer 21 which includes side portions 22 which act to passivate lower layers from etching that is utilised in construction of the nozzle arrangement 10. The nozzle arrangement 10 includes a paddle actuator 24 which is constructed on a nitride base 25 which acts to form a rigid paddle for the overall actuator 24. Next, an aluminium layer 27 is provided with the aluminium layer 27 being interconnected by vias 28 with the lower CMOS circuitry so as to form a first portion of a circuit. The aluminium layer 27 is interconnected at a point 30 to an Indium Tin Oxide (ITO) layer 29 which provides for resistive heating on demand. The ITO layer 29 includes a number of etch holes 31 for allowing the etching away of a lower level sacrificial layer which is formed between the layers 27, 29. The ITO layer is further connected to the lower glass CMOS circuitry layer by via 32. On top of the ITO layer 29 is optionally provided a polytetrafluoroethylene layer (not shown) which provides for insulation and further rapid expansion of the top layer 29 upon heating as a result of passing a current through the bottom layer 27 and ITO layer 29.

[0037] The back surface of the nozzle arrangement 10 is placed in an ink reservoir so as to allow ink to flow into nozzle chamber 16. When it is desired to eject a drop of ink, a current is passed through the aluminium layer 27 and ITO layer 29. The aluminium layer 27 provides a very low resistance path to the current whereas the ITO layer 29 provides a high resistance path to the current. Each of the layers 27, 29 are passivated by means of coating by a thin nitride layer (not shown) so as to insulate and passivate the layers from the surrounding ink. Upon heating of the ITO layer 29 and optionally PTFE layer, the top of the actuator 24 expands more rapidly than the bottom portions of the actuator 24. This results in a rapid bending of the actuator 24, particularly around the point 35 due to the utilisation of the rigid nitride paddle arrangement 25. This accentuates the downward movement of the actuator 24 which results in the ejection of ink from ink ejection nozzle 13.

[0038] Between the two layers 27, 29 is provided a gap 60 which can be constructed via utilisation of etching of sacrificial layers so as to dissolve away sacrificial material between the two layers. Hence, in operation ink is allowed to enter this area and thereby provides a further cooling of the lower surface of the actuator 24 so as to assist in accentuating the bending. Upon de-activation of the actuator 24, it returns to its quiescent position above the nozzle chamber 16. The nozzle chamber 16 refills due to the surface tension of the ink through the gaps between the actuator 24 and the nozzle chamber 16.

[0039] The PTFE layer has a high coefficient of thermal expansion and therefore further assists in accentuating any bending of the actuator 24. Therefore, in order to eject ink from the nozzle chamber 16, a current is passed through the planar layers 27, 29 resulting in resistive heating of the top layer 29 which further results in a general bending down of the actuator 24 resulting in the ejection of ink.

[0040] The nozzle arrangement 10 is mounted on a second silicon chip wafer which defines an ink reservoir channel to the back of the nozzle arrangement 10 for resupply of ink.

[0041] Turning now to FIG. 2, there is illustrated an exploded perspective view illustrating the various layers of a nozzle arrangement 10. The arrangement 10 can, as noted previously, be constructed from back etching to the boron doped layer. The actuator 24 can further be constructed through the utilisation of a sacrificial layer filling the nozzle chamber 16 and the depositing of the various layers 25, 27, 29 and optional PTFE layer before sacrificially etching the nozzle chamber 16 in addition to the sacrificial material in area 60. To this end, the nitride layer 21 includes side portions 22 which act to passivate the portions of the lower glass layer 20 which would otherwise be attacked as a result of sacrificial etching.

[0042] 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:

[0043] 1. Using a double sided polished wafer deposit 3 microns of epitaxial silicon heavily doped with boron 12.

[0044] 2. Deposit 10 microns of epitaxial silicon 18, either p-type or n-type, depending upon the CMOS process used.

[0045] 3. Complete a 0.5 micron, one poly, 2 metal CMOS process 20. 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.

[0046] 4. Etch the CMOS oxide layers down to silicon 18 or second level metal using Mask 1. This mask defines the nozzle cavity and the bend actuator electrode contact vias 28, 32. This step is shown in FIG. 5.

[0047] 5. Crystallographically etch the exposed silicon 18 using KOH as shown at 40. This etch stops on <111> crystallographic planes 61 , and on the boron doped silicon buried layer 12. This step is shown in FIG. 6.

[0048] 6. Deposit 0.5 microns of low stress PECVD silicon nitride 41 (Si3N4). The nitride 41 acts as an ion diffusion barrier. This step is shown in FIG. 7.

[0049] 7. Deposit a thick sacrificial layer 42 (e.g. low stress glass), filling the nozzle cavity. Planarize the sacrificial layer 42 down to the nitride 41 surface. This step is shown in FIG. 8.

[0050] 8. Deposit 1 micron of tantalum 43. This layer acts as a stiffener for the bend actuator.

[0051] 9. Etch the tantalum 43 using Mask 2. This step is shown in FIG. 9. This mask defines the space around the stiffener section of the bend actuator, and the electrode contact vias.

[0052] 10. Etch nitride 41 still using Mask 2. This clears the nitride from the electrode contact vias 28, 32. This step is shown in FIG. 10.

[0053] 11. Deposit one micron of gold 44, patterned using Mask 3. This may be deposited in a lift-off process. Gold is used for its corrosion resistance and low Young's modulus. This mask defines the lower conductor of the bend actuator. This step is shown in FIG. 11.

[0054] 12. Deposit 1 micron of thermal blanket 45. This material should be a non-conductive material with a very low Young's modulus and a low thermal conductivity, such as an elastomer or foamed polymer.

[0055] 13. Pattern the thermal blanket 45 using Mask 4. This mask defines the contacts between the upper and lower conductors, and the upper conductor and the drive circuitry. This step is shown in FIG. 12.

[0056] 14. Deposit 1 micron of a material 46 with a very high resistivity (but still conductive), a high Young's modulus, a low heat capacity, and a high coefficient of thermal expansion. A material such as indium tin oxide (ITO) may be used, depending upon the dimensions of the bend actuator.

[0057] 15. Pattern the ITO 46 using Mask 5. This mask defines the upper conductor of the bend actuator. This step is shown in FIG. 13.

[0058] 16. Deposit a further 1 micron of thermal blanket 47.

[0059] 17. Pattern the thermal blanket 47 using Mask 6. This mask defines the bend actuator, and allows ink to flow around the actuator into the nozzle cavity. This step is shown in FIG. 14.

[0060] 18. Mount the wafer on a glass blank 48 and back-etch the wafer using KOH, with no mask. This etch thins the wafer and stops at the buried boron doped silicon layer 12. This step is shown in FIG. 15.

[0061] 19. Plasma back-etch the boron doped silicon layer 12 to a depth of 1 micron using Mask 7. This mask defines the nozzle rim 14. This step is shown in FIG. 16.

[0062] 20. Plasma back-etch through the boron doped layer 12 using Mask 8. This mask defines the nozzle 13, and the edge of the chips.

[0063] 21. Plasma back-etch nitride 41 up to the glass sacrificial layer 42 through the holes in the boron doped silicon layer 12. At this stage, the chips are separate, but are still mounted on the glass blank. This step is shown in FIG. 17.

[0064] 22. Strip the adhesive layer to detach the chips from the glass blank 48.

[0065] 23. Etch the sacrificial glass layer 42 in buffered HF. This step is shown in FIG. 18.

[0066] 24. 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.

[0067] 25. Connect the print heads to their interconnect systems.

[0068] 26. Hydrophobize the front surface of the print heads.

[0069] 27. Fill the completed print heads with ink 50 and test them. A filled nozzle is shown in FIG. 19.

[0070] 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 present embodiment is, therefore, to be considered in all respects to be illustrative and not restrictive.

[0071] The presently disclosed ink jet printing technology is potentially suited to a wide range of printing systems including: colour 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 inbuilt pagewidth printers, portable colour and monochrome printers, colour and monochrome copiers, colour 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 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.

[0072] Ink Jet Technologies

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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:

[0077] low power (less than 10 Watts)

[0078] high resolution capability (1,600 dpi or more)

[0079] photographic quality output

[0080] low manufacturing cost

[0081] small size (pagewidth times minimum cross section)

[0082] high speed (<2 seconds per page).

[0083] 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 above under the heading Cross References to Related Applications.

[0084] 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

[0085] 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 IJ38, 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.

[0086] 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.

[0087] Tables of Drop-on-Demand Ink Jets

[0088] 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.

[0089] The following tables form the axes of an eleven dimensional table of ink jet types.

[0090] Actuator mechanism (18 types)

[0091] Basic operation mode (7 types)

[0092] Auxiliary mechanism (8 types)

[0093] Actuator amplification or modification method (17 types)

[0094] Actuator motion (19 types)

[0095] Nozzle refill method (4 types)

[0096] Method of restricting back-flow through inlet (10 types)

[0097] Nozzle clearing method (9 types)

[0098] Nozzle plate construction (9 types)

[0099] Drop ejection direction (5 types)

[0100] Ink type (7 types)

[0101] 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 which matches the docket numbers in the table under the heading Cross References to Related Applications.

[0102] 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 print heads with characteristics superior to any currently available ink jet technology.

[0103] 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, a printer may be listed more than once in a table, where it shares characteristics with more than one entry.

[0104] 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.

[0105] The information associated with the aforementioned 11 dimensional matrix are set out in the following tables.