Utilizing venting in a MEMS liquid pumping system

[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 serial numbers (U.S. Ser. No.) 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 venting of an ink jet nozzle.

[0005] Recently, micro-electro mechanical systems (MEMS) have become increasingly popular. One use of such a system is in the ejection of ink in an ink jet printing device. It is important in such devices to have efficient operation.

[0006] It is an object of the present invention to provide an efficient operation of an ink jet nozzle when constructed on a MEMS scale.

[0007] In accordance with a first aspect of the present invention there is provided a micro-mechanical nozzle having thermal actuator which includes a hydrophobic surface surrounded by other hydrophobic surfaces. A method of reducing the operation energy requirements of said actuator comprises the step of providing an air inlet in fluid communication with said hydrophobic surface. The air inlet allow the flow of air into and out of a volume adjacent said hydrophobic surface. Further, the air inlet is surrounded by hydrophobic material and includes a plurality of small spaced apart holes.

[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 incorporating a venting arrangement constructed in accordance with the preferred embodiment.

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

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

[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 the conductive layer of FIG. 3.

[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 the conductive layer of FIG. 3.

[0015] FIG. 7 is an exploded perspective view of the ink jet nozzle of FIG. 1.

[0016] FIG. 8 is a perspective view of a section of an ink jet printhead incorporating a plurality of the ink jet nozzles of FIG. 1.

[0017] The preferred embodiment of the present invention will be discussed with reference to an ink jet nozzle wherein it is required to provide energy efficient operation of an ink jet printing device incorporating a plurality of the ink jet nozzles. The present invention should however, not be restricted to the field of ink jet printing devices.

[0018] In the preferred embodiment, the ink jet nozzle from which ink is ejected is actuated by means of a thermal actuator which includes a “corrugated” copper heating element encased in a polytetrafluoroethylene (PTFE) layer. Those of ordinary skill in the field will appreciate that PTFE is inherently hydrophobic.

[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 means of actuation of a thermal paddle actuator 13. The thermal paddle actuator 13 comprises an inner copper heating portion 14 and paddle 15 which are encased in a PTFE layer 16. The 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 actuator 13 due to out-gassing etc. A top surface of the PTFE layer 16 is treated so as to make it hydrophilic. The heating portion 14 is also formed within a lower portion of the actuator 13.

[0020] The heating portion 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 heating portion 14 which heats the lower portion of the actuator 13. Turning now to FIG. 2, a bottom surface of the actuator 13, in contact with an 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 lower portion of the PTFE layer expands more rapidly resulting in a general 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 a nitride layer 42, and a PTFE layer 26 such that air is free to flow in the direction of an arrow 29 along a channel 28 and through holes 25, to accommodate any fluctuating pressure influences plurality of posts 27 are positioned between the layers 42, 26. As can be seen in FIGS. 1 and 2, air introduced into the ink jet nozzle 10 is in fluid communication with the air bubble 17. Thus the air flow acts to reduce a vacuum acting on the bottom surface of the 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 results in a return of the actuator 13 to its rest position.

[0022] The 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 includes the panel 15, which can be constructed from the same material as the heating portion as 14, i.e. copper, and which contains a series of holes 23. The holes 23 are provided for interconnecting PTFE both above and below the panel 15 so as to resist any movement of the PTFE layers past the panel 15 and thereby reduce any opportunities for the delamination of the PTFE and the copper.

[0023] Turning to FIG. 4, there is illustrated a close up view of a portion of the actuator 13 of FIG. 1 illustrating corrugations 22 of the heating portion 14. The corrugations 22 of the heater 14 allow for a rapid heating of the lower portion. 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 is therefore heated by means of thermal conduction from the resistive element. 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 a corrugated resistive element, the lower portion of the actuator 13 is more rapidly heated. Therefore, the utilization of a corrugated resistive element results in a more rapid heating of the lower portion and therefore a rapid actuation of the actuator 13. Further the corrugations 22 also assist in resisting any delamination of the copper and PTFE.

[0024] Turning now to FIG. 5, the corrugations 22 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 layer 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 copper heating portion 14 is then etched in a serpentine or concertina form. Subsequently, a second PTFE layer 53 is deposited on top of the heating portion 14 so as to form a 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 ink can be supplied via a 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 the channel 38 through side grill portions 40 (see also FIG. 7) into the chamber 12. Importantly, the grill portions 40 which can comprise silicon nitride or similar insulating material act to remove foreign bodies from the ink flow. The grill 40 also helps 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 provided 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, stops ink from being ejected from these holes, whereas, the larger size 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 nozzle 10 can be formed in layers starting with a silicon wafer device 41 having a CMOS layer 18 on top thereof as required. The CMOS layer 18 provides the various drive circuitry for driving the copper heating portion 14.

[0028] On top of the CMOS layer 18 a nitride layer 42 is deposited, providing protection for lower layers from corrosion or etching. Next, a PTFE layer 26 is constructed having the aforementioned holes 25, and posts 27. 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 proceeds by laying down a sacrificial layer on top of layer 26 in which the air bubble underneath 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 and a second PTFE layer 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 the side walls and the grill 40. Subsequently, the top nitride layer 48 is deposited again utilizing the appropriate mask having the holes 19 as required. Subsequently, the various sacrificial layers can be etched away so as to release the structure of the thermal actuator 13.

[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, e.g. 91. The configuration 90 can be utilized in a three color process 1600 dpi print-head utilizing 3 sets of 2 rows of nozzle chambers 92,93, which are interconnected to one ink supply channel, e.g. 94, for each set. The 3 supply channels 94, 95, 96 are interconnected to cyan coloured, magenta coloured and yellow coloured ink reservoirs respectively.

[0032] It will 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 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.

[0036] 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.

[0037] 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:

[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 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.

[0045] 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.

[0046] 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.

[0047] 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 printhead 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 match the docket numbers 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 printheads 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 print technology 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.