Micro electro-mechanical system for ejection of fluids

[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 a micron electro-mechanical system for the ejection of fluids.

[0005] In micro-electro mechanical systems (MEMS) it is often necessary to manipulate a fluid. The manipulation of a fluid can result in a clogging of a MEMS system, especially when the fluid contains contaminating bodies which clog fluid passage ways.

[0006] It is an object of the present invention to an integral structure with a MEMS device to filter out foreign bodies in an ink supply.

[0007] In accordance with a first aspect of the present invention there is provided a micro-electro mechanical system for controlling the flow of a liquid, this system being constructed in a planar form. The effect of impurities in the liquid is substantially removed by providing an integral grill structure in the path of the flow of the liquid. This serves to filter foreign bodies in the liquid. Preferably, the micro-electro mechanical system comprises an ink jet printing system and the grill forms one wall of a nozzle chamber for filtering ink entering the nozzle chamber. Further, the filter comprises substantially nitride.

[0008] 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:

[0009] FIG. 1 is a schematic cross-sectional view of a single ink jet nozzle constructed in accordance with the preferred embodiment.

[0010] FIG. 2 is a schematic cross-sectional view of a single ink jet nozzle constructed in accordance with the preferred embodiment, with the thermal actuator in its activated state.

[0011] FIG. 3 is a schematic diagram of the conductive layer utilized in the thermal actuator of the ink jet nozzle constructed in accordance with the preferred embodiment.

[0012] FIG. 4 is a close up perspective view of portion A of FIG. 3.

[0013] FIG. 5 is a cross-sectional schematic diagram illustrating the construction of a corrugated conductive layer in accordance with the preferred embodiment of the present invention.

[0014] FIG. 6 is a schematic cross-sectional diagram illustrating the development of a resist material through a half-toned mask utilized in the fabrication of a single ink jet nozzle in accordance with the preferred embodiment.

[0015] FIG. 7 is an exploded perspective view illustrating the construction of a single ink jet nozzle in accordance with the preferred embodiment.

[0016] FIG. 8 is a perspective view of a portion of an ink jet printhead having ink jet nozzles in accordance with the preferred embodiment.

[0017] The preferred embodiment of the present invention will be discussed with reference to a fluid system which comprises an ink jet printing device wherein it is required to filter the ink supply so as to ensure the continual operation of the ink jet printer. The present invention should not necessarily be restricted to the field of ink jet printing, as will be readily evident.

[0018] In the preferred embodiment, there is provided an ink jet printer having ink ejection nozzles from which ink is ejected with the ink ejection being actuated by means of a thermal actuator which includes a “corrugated” copper heating element encased in a polytetrafluoroethylene (PTFE) layer.

[0019] Turning now to FIG. 1, there is illustrated a cross-sectional view of a single ink jet nozzle 10 in accordance with the present embodiment. The ink jet nozzle 10 includes an ink ejection port 11 for the ejection of ink from a chamber 12 by the actuation of a thermal paddle actuator 13. The thermal paddle actuator 13 comprises an inner copper heating portion 14 and a planar portion 15 which are encased in an outer PTFE layer 16. The outer PTFE layer 16 has an extremely high coefficient of thermal expansion (approximately 770×10⁻⁶, or around 380 times that of silicon). The PTFE layer 16 is also highly hydrophobic which results in an air bubble 17 being formed under the thermal actuator 13 due to out-gassing etc. The top PTFE layer is treated so as to make it hydrophilic. The heating portion 14 is also formed within the lower portion of the actuator 13.

[0020] The heater 14 is connected at ends 20,21 (see also FIG. 7) to a lower CMOS drive layer 18 containing drive circuitry (not shown). For the purposes of actuation of the actuator 13, a current is passed through the copper heating portion 14 which heats the bottom surface of the actuator 13. Turning now to FIG. 2, the bottom surface of the actuator 13, in contact with air bubble 17 remains heated while any top surface heating is carried away by the exposure of the top surface of the actuator 13 to the ink within the chamber 12. Hence, the bottom PTFE layer expands more rapidly resulting in a general rapid bending upwards of the actuator 13 (as illustrated in FIG. 2) which consequentially causes the ejection of ink from the ink ejection port 11. An air inlet channel 28 is formed between two nitride layers 42, 26 such that air is free to flow in the direction of an arrow 29 along a channel 28 and through holes 25, in accordance with any fluctuating pressure influences. The air flow acts to reduce the vacuum on the back surface of actuator 13 during operation. As a result less energy is required for the movement of the actuator 13.

[0021] The actuator 13 can be deactivated by turning off the current to the heating portion 14. This will result in a return of the actuator 13 to its rest position.

[0022] The thermal actuator 13 includes a number of significant features. In FIG. 3 there is illustrated a schematic diagram of the conductive layer of the thermal actuator 13. The conductive layer defines the planar portion 15, which can be constructed from the same material as the heating portion 14 copper, and which defines a series of holes 23. The holes 23 are provided for interconnecting layers of PTFE both above and below the planar portion 15 so as to resist any movement of the PTFE layers relative to the planar portion 15 and thereby reducing any opportunities for the delamination of the PTFE and copper layers.

[0023] Turning to FIG. 4, there is illustrated a close up view of the heating portion 14 illustrating corrugations 22 of the heating portion 14 within the PTFE. The corrugations 22 of the heater 14 allow for a rapid heating of portions of a bottom layer surrounding the corrugations 22. Any resistive heater which is based upon applying a current to heat an object will result in a rapid, substantially uniform elevation in temperature of the outer surface of the current carrying conductor. The surrounding PTFE volume is therefore heated by means of thermal conduction from the heating portion 14. This thermal conduction is known to proceed, to a first approximation, at a substantially linear rate with respect to distance from a resistive element. By utilizing the corrugations, the bottom surface of the actuator 13 is more rapidly heated as, on average, a greater volume of the bottom PTFE surface is closer to a portion of the resistive element. Therefore a rapid actuation of the actuator 13 results. Further, the corrugation 22 also assist in resisting any delamination of the copper and PTFE layer.

[0024] Turning now to FIG. 5, the corrugated resistive element can be formed by depositing a resist layer 50 on top of the first PTFE layer 51. The resist layer 50 is exposed utilizing a mask 52 having a half-tone pattern delineating the corrugations. After development the resist 50 contains the corrugation pattern. The resist layer 50 and the PTFE layer 51 are then etched utilizing an etchant that erodes the resist layer 50 at substantially the same rate as the PTFE layer 51. This transfers the corrugated pattern into the PTFE layer 51. Turning to FIG. 6, on top of the corrugated PTFE layer 51 is deposited the heating portion 14 which takes on a corrugated form in accordance with its under layer. The heating portion layer 14 is then etched in a serpentine or concertina form. Subsequently, a further PTFE layer 53 is deposited on top of the heating portion 14 so as to form the top layer of the thermal actuator 13. Finally, the second PTFE layer 52 is planarized to form the top surface of the thermal actuator 13 (FIG. 1).

[0025] Returning again now to FIG. 1, it is noted that an ink supply can be supplied through a throughway for channel 38 which can be constructed by means of deep anisotropic silicon trench etching such as that available from STS Limited (“Advanced Silicon Etching Using High Density Plasmas” by J. K. Bhardwaj, H. Ashraf, page 224 of Volume 2639 of the SPIE Proceedings in Micro Machining and Micro Fabrication Process Technology). The ink supply flows from channel 38 through spaced apart apertures 40 (see also FIG. 7) into chamber 12. The apertures 40 are defined between poles 241, forming a filter 341 (see FIG. 7). Importantly, the poles 241 which can comprise silicon nitride or similar insulating material act to remove foreign bodies from the ink flow. The poles 241 also help to pinch the PTFE actuator 13 to a base CMOS layer 18, the pinching providing an important assistance for the thermal actuator 13 so as to ensure a substantially decreased likelihood of the thermal actuator layer 13 separating from a base CMOS layer 18.

[0026] A series of sacrificial etchant holes 19 are defined in a top wall 48 of the chamber 12 to allow sacrificial etchant to enter the chamber 12 during fabrication so as to increase the rate of etching. The small size of the holes 19, does not affect the operation of the device 10 substantially as the surface tension across the holes 19 inhibit ink from being ejected from these holes, whereas the larger sized port 11 allows for the ejection of ink.

[0027] Turning now to FIG. 7, there is illustrated an exploded perspective view of a single nozzle 10. The nozzles 10 can be formed in layers starting with a silicon wafer substrate 41 having a CMOS layer 18 on top thereof as required. The CMOS layer 18 provides the various drive circuitry for driving the actuator 13.

[0028] On top of the CMOS layer 18 a nitride layer 42 is deposited, providing primarily protection for lower layers from corrosion or etching. Next a PTFE layer 26 is constructed having the aforementioned holes 25 (see FIG. 1), and posts 27 (see FIG. 1). The structure of the PTFE layer 26 can be formed by first laying down a sacrificial glass layer (not shown) onto which the PTFE layer 26 is deposited. The PTFE layer 26 includes various features, for example, a lower ridge portion 30 in addition to vias for subsequent material layers.

[0029] In construction of the actuator 13 (FIG. 1), the process of creating a first PTFE layer 60 proceeds by laying down a sacrificial layer on top of the layer 26 in which the air bubble 17, underneath the actuator 13 (FIG. 1), subsequently forms. On top of this is formed a first PTFE layer utilizing the relevant mask. Preferably, the PTFE layer includes vias for the subsequent copper interconnections. Next, a copper layer 43 is deposited on top of the first PTFE layer 60 and a subsequent PTFE layer 61 is deposited on top of the copper layer 43, in each case utilizing the required mask.

[0030] The nitride layer 46 can be formed by the utilization of a sacrificial glass layer which is masked and etched as required to form side walls and the filter 341. Subsequently, the top nitride layer 48 is deposited again utilizing the appropriate mask having considerable holes 19 as required. Subsequently, the various sacrificial layers can be etched away so as to release the structure of the thermal actuator.

[0031] In FIG. 8 there is illustrated a section of an ink jet printhead configuration 90 utilizing ink jet nozzles constructed in accordance with the preferred embodiment 91. The configuration 90 can be utilized in a three color process, 16 Oodpi printhead utilizing three sets of two rows of nozzle chambers 92,93, which are interconnected to one ink supply channel 94, for each set. The three supply channels 94, 95, 96 are interconnected to cyan colored, magenta colored and yellow colored ink reservoirs respectively. Ink is supplied through respective apertures 200, 202, 204 formed in the wafer substrate and extending through the wafer substrate 206 to the back surface thereof.

[0032] 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.

[0033] Ink Jet Technologies

[0034] 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.

[0035] The most significant problem with thermal inkjet 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 inkjet applications. This leads to an efficiency of around 0.02%, from electricity input to drop momentum (and increased surface area) out.

[0036] The most significant problem with piezoelectric inkjet 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.

[0037] Ideally, the inkjet 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 inkjet technologies have been created. The target features include:

[0038] low power (less than 10 Watts)

[0039] high resolution capability (1,600 dpi or more)

[0040] photographic quality output

[0041] low manufacturing cost

[0042] small size (pagewidth times minimum cross section)

[0043] high speed (<2 seconds per page).

[0044] All of these features can be met or exceeded by the inkjet systems described below with differing levels of difficulty. Forty-five different inkjet 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 below.

[0045] The inkjet 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

[0046] 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 inkjet 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.

[0047] 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.

[0048] Tables of Drop-on-Demand Ink Jets

[0049] 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.

[0050] The following tables form the axes of an eleven dimensional table of ink jet types.

[0051] Actuator mechanism (18 types)

[0052] Basic operation mode (7 types)

[0053] Auxiliary mechanism (8 types)

[0054] Actuator amplification or modification method (17 types)

[0055] Actuator motion (19 types)

[0056] Nozzle refill method (4 types)

[0057] Method of restricting back-flow through inlet (10 types)

[0058] Nozzle clearing method (9 types)

[0059] Nozzle plate construction (9 types)

[0060] Drop ejection direction (5 types)

[0061] Ink type (7 types)

[0062] 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 in the table under the heading Cross References to Related Applications.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] The information associated with the aforementioned 11 dimensional matrix are set out in the following tables.