Shape memory alloy ink jet printing mechanism

[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 US patent application serial numbers (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 ink jet printing and in particular discloses a shape memory alloy ink jet printer.

[0005] The present invention further relates to the field of drop on demand ink jet printing.

[0006] Many different types of printing have been invented, a large number of which are presently in use. The known forms of print have a variety of methods for marking the print media with a relevant marking media. Commonly used forms of printing include offset printing, laser printing and copying devices, dot matrix type impact printers, thermal paper printers, film recorders, thermal wax printers, dye sublimation printers and ink jet printers both of the drop on demand and continuous flow type. Each type of printer has its own advantages and problems when considering cost, speed, quality, reliability, simplicity of construction and operation etc.

[0007] In recent years, the field of ink jet printing, wherein each individual pixel of ink is derived from one or more ink nozzles has become increasingly popular primarily due to its inexpensive and versatile nature.

[0008] Many different techniques on ink jet printing have been invented. For a survey of the field, reference is made to an article by J Moore, “Non-Impact Printing: Introduction and Historical Perspective”, Output Hard Copy Devices, Editors R. Dubeck and S Sherr, pages 207-220 (1988).

[0009] Ink Jet printers themselves come in many different types. The utilisation of a continuous stream ink in ink jet printing appears to date back to at least 1929 wherein U.S. Pat. No. 1,941,001 by Hansell discloses a simple form of continuous stream electro-static ink jet printing.

[0010] U.S. Pat. No. 3,596,275 by Sweet also discloses a process of a continuous ink jet printing including the step wherein the ink jet stream is modulated by a high frequency electrostatic field so as to cause drop separation. This technique is still utilized by several manufacturers including Elmjet and Scitex (see also U.S. Pat. No. 3,373,437 by, Sweet et al)

[0011] Piezoelectric ink jet printers are also one form of commonly utilized ink jet printing device. Piezoelectric systems are disclosed by Kyser et. al. in U.S. Pat. No. 3,946,398 (1970) which utilizes a diaphragm mode of operation, by Zolten in U.S. Pat. No. 3,683,212 (1970) which discloses a squeeze mode of operation of a piezoelectric crystal, Stemme in U.S. Pat. No. 3,747,120 (1972) discloses a bend mode of piezoelectric operation, Howkins in U.S. Pat. No. 4,459,601 discloses a piezoelectric push mode actuation of the ink jet stream and Fischbeck in U.S. Pat. No. 4,584,590 which discloses a shear mode type of piezoelectric transducer element.

[0012] Recently, thermal ink jet printing has become an extremely popular form of ink jet printing. The ink jet printing techniques include those disclosed by Endo et al in GB 2007162 (1979) and Vaught et al in U.S. Pat. No. 4,490,728. Both the aforementioned references disclosed ink jet printing techniques rely upon the activation of an electrothermal actuator which results in the creation of a bubble in a constricted space, such as a nozzle, which thereby causes the ejection of ink from an aperture connected to the confined space onto a relevant print media. Printing devices utilizing the electro-thermal actuator are manufactured by manufacturers such as Canon and Hewlett Packard.

[0013] As can be seen from the foregoing, many different types of printing technologies are available. Ideally, a printing technology should have a number of desirable attributes. These include inexpensive construction and operation, high speed operation, safe and continuous long term operation etc. Each technology may have its own advantages and disadvantages in the areas of cost, speed, quality, reliability, power usage, simplicity of construction operation, durability and consumables.

[0014] It is an object of the present invention to provide for a new form of ink jet printing device that utilizes a shape memory alloy in its activation method.

[0015] In accordance with a first aspect of the present invention there is provided a method of ejecting ink from a chamber comprising the steps of: a) providing a cantilevered beam actuator incorporating a shape memory alloy; and b) transforming said shape memory alloy from its martensitic phase to its austenitic phase or vice versa to cause the ink to eject from said chamber. Further, the actuator comprises a conductive shape memory alloy panel in a quiescent state and which transfers to an ink ejection state upon heating thereby causing said ink ejection from the chamber. Preferably, the heating occurs by means of passing a current through the shape memory alloy. The chamber is formed from a crystallographic etch of a silicon wafer so as to have one surface of the chamber substantially formed by the actuator. Advantageously, the actuator is formed from a conductive shape memory alloy arranged in a serpentine form and is attached to one wall of the chamber opposite a nozzle port from which ink is ejected. Further, the nozzle port is formed by the back etching of a silicon wafer to the epitaxial layer and etching a nozzle port hole in the epitaxial layer. The crystallographic etch includes providing side wall slots of non-etched layers of a processed silicon wafer so as to the extend the dimensions of the chamber as a result of the crystallographic etch process. Preferably, the shape memory alloy comprises nickel titanium alloy.

[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 an exploded perspective view of a single ink jet nozzle as constructed in accordance with the preferred embodiment;

[0018] FIG. 2 is a top cross sectional view of a single ink jet nozzle in its quiescent state taken along line A—A in FIG. 1;

[0019] FIG. 3 is a top cross sectional view of a single ink jet nozzle in its actuated state taken along line A—A in FIG. 1;

[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, shape memory materials are utilized to construct an actuator suitable for injecting ink from the nozzle of an ink chamber.

[0023] Turning to. FIG. 1, there is illustrated an exploded perspective view 10 of a single ink jet nozzle as constructed in accordance with the preferred embodiment. The ink jet nozzle 10 is constructed from a silicon wafer base utilizing back etching of the wafer to a boron doped epitaxial layer. Hence, the ink jet nozzle 10 comprises a lower layer 11 which is constructed from boron doped silicon. The boron doped silicon layer is also utilized a crystallographic etch stop layer. The next layer comprises the silicon layer 12 that includes a crystallographic pit 13 having side walls etch at the usual angle of 54.74. The layer 12 also includes the various required circuitry and transistors for example, CMOS layer (not shown). After this, a 0.5 micron thick thermal silicon oxide layer 15 is grown on top of the silicon wafer 12.

[0024] After this, comes various layers which can comprise a two level metal CMOS process layers which provide the metal interconnect for the CMOS transistors formed within the layer 12. The various metal pathways etc. are not shown in FIG. 1 but for two metal interconnects 18, 19 which provide interconnection between a shape memory alloy layer 20 and the CMOS metal layers 16. The shape memory metal, layer is next and is shaped in the form of a serpentine coil to be heated by end interconnect/via portions 21,23. A top nitride layer 22 is provided for overall passivation and protection of lower layers in addition to providing a means of inducing tensile stress to curl upwards the shape memory alloy layer 20 in its quiescent state.

[0025] The preferred embodiment relies upon the thermal transition of a shape memory alloy 20 (SMA) from its martensitic phase to its austenitic phase. The basis of a shape memory effect is a martensitic transformation which creates a polydemane phase upon cooling. This polydemane phase accommodates finite reversible mechanical deformations without significant changes in the mechanical self energy of the system. Hence, upon re-transformation to the austenitic state the system returns to its former macroscopic state to displaying the well known mechanical memory. The thermal transition is achieved by passing an electrical current through the SMA. The actuator layer 20 is suspended at the entrance to a nozzle chamber connected via leads 18, 19 to the lower layers.

[0026] In FIG. 2, there is shown a cross-section of a single nozzle 10 when in its quiescent state, the section basically being taken through the line A—A of FIG. 1. The actuator 30 is bent away from the nozzle when in its quiescent state. In FIG. 3, there is shown a corresponding cross-section for a single nozzle 10 when in an actuated state. When energized, the actuator 30 straightens, with the corresponding result that the ink is pushed out of the nozzle. The process of energizing the actuator 30 requires supplying enough energy to raise the SMA above its transition temperature, and to provide the latent heat of transformation to the SMA 20.

[0027] Obviously, the SMA martensitic phase must be pre-stressed to achieve a different shape from the austenitic phase. For printheads with many thousands of nozzles, it is important to achieve this pre-stressing in a bulk manner. This is achieved by depositing the layer of silicon nitride 22 using Plasma Enhanced Chemical Vapour Deposition (PECVD) at around 300° C. over the SMA layer. The deposition occurs while the SMA is in the austenitic shape. After the printhead cools to room temperature, the substrate under the SMA bend actuator is removed by chemical etching of a sacrificial substance. The silicon nitride layer 22 is under tensile stress, and causes the actuator to curl upwards.

[0028] The weak martensitic phase of the SMA provides little resistance to this curl. When the SMA is heated to its austenitic phase, it returns to the flat shape into which it was annealed during the nitride deposition. The transformation being rapid enough to result in the ejection of ink from the nozzle chamber.

[0029] There is one SMA bend actuator 30 for each nozzle. One end 31 of the SMA bend actuator is mechanically connected to the substrate. The other end is free to move under the stresses inherent in the layers.

[0030] Returning to FIG. 1 the actuator layer is therefore composed of three layers:

[0031] 1. An SiO₂lower layer 15. This layer acts as a stress ‘reference’ for the nitride tensile layer. It also protects the SMA from the crystallographic silicon etch that forms the nozzle chamber. This layer can be formed as part of the standard CMOS process for the active electronics of the printhead.

[0032] 2. A SMA heater layer 20. A SMA such as nickel titanium (NiTi) alloy is deposited and etched into a serpentine form to increase the electrical resistance.

[0033] 3. A silicon nitride top layer 22. This is a thin layer of high stiffness which is deposited using PECVD. The nitride stoichiometry is adjusted to achieve a layer with significant tensile stress at room temperature relative to the SiO₂lower layer. Its purpose is to bend the actuator at the low temperature martensitic phase.

[0034] As noted previously the ink jet nozzle of FIG. 1 can be constructed by utilizing a silicon wafer having a buried boron epitaxial layer. The 0.5 micron thick dioxide layer 15 is then formed having side slots 45 which are utilized in a subsequent crystallographic etch. Next, the various CMOS layers 16 are formed including drive and control circuitry (not shown). The SMA layer 20 is then created on top of layers 15/16 and being interconnected with the drive circuitry. Subsequently, a silicon nitride layer 22 is formed on top. Each of the layers 15, 16, 22 include the various slots eg. 45 which are utilized in a subsequent crystallographic etch. The silicon wafer is subsequently thinned by means of back etching with the etch stop being the boron layer 11. Subsequent boron etching forms the nozzle hole leg. 47 and rim 46 (FIG. 3). Subsequently, the chamber proper is formed by means of a crystallographic etch with the slots 45 defining the extent of the etch within the silicon, oxide layer 12.

[0035] A large array of nozzles can be formed on the same wafer which in turn is attached to an ink chamber for filling the nozzle chambers.

[0036] One form of detailed manufacturing process which can be used to fabricate monolithic ink jet printheads operating in accordance with the principles taught by the present embodiment can proceed utilizing the following steps:

[0037] 1. Using a double sided polished wafer deposit 3 microns of epitaxial silicon heavily doped with boron.

[0038] 2. Deposit 10 microns of epitaxial silicon, either p-type or n-type, depending upon the CMOS process used.

[0039] 3. Complete drive transistors, data distribution, and timing circuits using a 0.5 micron, one poly, 2 metal CMOS process. 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.

[0040] 4. Etch the CMOS oxide layers down to silicon or aluminum using Mask 1. This mask defines the nozzle chamber, and the edges of the printheads chips. This step is shown in FIG. 6.

[0041] 5. Crystallographically etch the exposed silicon using, for example, KOH or EDP (ethylenediamine pyrocatechol). This etch stops on <111> crystallographic planes, and on the boron doped silicon buried layer. This step is shown in FIG. 7.

[0042] 6. Deposit 12 microns of sacrificial material. Planarize down to oxide using CMP. The sacrificial material temporarily fills the nozzle cavity. This step is shown in FIG. 8.

[0043] 7. Deposit 0.1 microns of high stress silicon nitride (Si3N4).

[0044] 8. Etch the nitride layer using Mask 2. This mask defines the contact vias from the shape memory heater to the second-level metal contacts.

[0045] 9. Deposit a seed layer.

[0046] 10. Spin on 2 microns of resist, expose with Mask 3, and develop. This mask defines the shape memory wire embedded in the paddle. The resist acts as an electroplating mold. This step is shown in FIG. 9.

[0047] 11. Electroplate 1 micron of Nitinol. Nitinol is a ‘shape memory’ alloy of nickel and titanium, developed at the Naval Ordnance Laboratory in the U.S. (hence Ni—Ti—NOL). A shape memory alloy can be thermally switched between its weak martensitic state and its high stiffness austenic state.

[0048] 12. Strip the resist and etch the exposed seed layer. This step is shown in FIG. 10.

[0049] 13. Wafer probe. All electrical connections are complete at this point, bond pads are accessible, and the chips are not yet separated.

[0050] 14. Deposit 0.1 microns of high stress silicon nitride. High stress nitride is used so that once the sacrificial material is etched, and the paddle is released, the stress in the nitride layer will bend the relatively weak martensitic phase of the shape memory alloy. As the shape memory alloy—in its austenic phase—is flat when it is annealed by the relatively high temperature deposition of this silicon nitride layer, it will return to this flat state when electrothermally heated.

[0051] 15. Mount the wafer on a glass blank and back-etch the wafer using KOH with no mask. This etch thins the wafer and stops at the buried boron doped silicon layer. This step is shown in FIG. 11.

[0052] 16. Plasma back-etch the boron doped silicon layer to a depth of I micron using Mask 4. This mask defines the nozzle rim. This step is shown in FIG. 12.

[0053] 17. Plasma back-etch through the boron doped layer using Mask 5. This mask defines the nozzle, and the edge of the chips. At this stage, the chips are still mounted on the glass blank. This step is shown in FIG. 13.

[0054] 18. Strip the adhesive layer to detach the chips from the glass blank. Etch the sacrificial layer. This process completely separates the chips. This step is shown in FIG. 14.

[0055] 19. Mount the printheads 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.

[0056] 20. Connect the printheads to their interconnect systems.

[0057] 21. Hydrophobize the front surface of the printheads.

[0058] 22. Fill with ink and test the completed printheads. A filled nozzle is shown in FIG. 15.

[0059] 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.

[0060] 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 inbuilt 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 trademark 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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:

[0065] low power (less than 10 Watts)

[0066] high resolution capability (1,600 dpi or more)

[0067] photographic quality output

[0068] low manufacturing cost

[0069] small size (pagewidth times minimum cross section)

[0070] high speed (<2 seconds per page).

[0071] 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.

[0072] 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.

[0073] 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 printhead 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.

[0074] 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 printhead is connected to the camera circuitry by tape automated bonding.

[0075] 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.

[0076] The following tables form the axes of an eleven dimensional table of ink jet types.

[0077] Actuator mechanism (18 types)

[0078] Basic operation mode (7 types)

[0079] Auxiliary mechanism (8 types)

[0080] Actuator amplification or modification method (17 types)

[0081] Actuator motion (19 types)

[0082] Nozzle refill method (4 types)

[0083] Method of restricting back-flow through inlet (10 types)

[0084] Nozzle clearing method (9 types)

[0085] Nozzle plate construction (9 types)

[0086] Drop ejection direction (5 types)

[0087] Ink type (7 types)

[0088] 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 match the docket numbers in the table under the heading Cross Referenced to Related Application.

[0089] 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.

[0090] 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 print technology may be listed more than once in a table, where it shares characteristics with more than one entry.

[0091] 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.

[0092] The information associated with the aforementioned 11 dimensional matrix are set out in the following tables.