Patent Grant
8480209
This invention relates to inkjet nozzle assemblies and methods of fabricating inkjet nozzle assemblies. It has been developed primarily to reduce electrical losses in supplying power to inkjet actuators.
The following applications have been filed by the Applicant with this application: U.S. Pat. Nos. 7,819,503, 7,901,046, 7,605,009, 7,568,787
The disclosures of these co-pending applications are incorporated herein by reference. The above applications have been identified by their filing docket number, which will be substituted with the corresponding application number, once assigned.
The following patents or patent applications filed by the applicant or assignee of the present invention are hereby incorporated by cross-reference.
The present Applicant has described previously a plethora of MEMS inkjet nozzles using thermal bend actuation. Thermal bend actuation generally means bend movement generated by thermal expansion of one material, having a current passing therethough, relative to another material. The resulting bend movement may be used to eject ink from a nozzle opening, optionally via movement of a paddle or vane, which creates a pressure wave in a nozzle chamber.
Some representative types of thermal bend inkjet nozzles are exemplified in the patents and patent applications listed in the cross reference section above, the contents of which are incorporated herein by reference.
The Applicant's U.S. Pat. No. 6,416,167 describes an inkjet nozzle having a paddle positioned in a nozzle chamber and a thermal bend actuator positioned externally of the nozzle chamber. The actuator takes the form of a lower active beam of conductive material (e.g. titanium nitride) fused to an upper passive beam of non-conductive material (e.g. silicon dioxide). The actuator is connected to the paddle via an arm received through a slot in the wall of the nozzle chamber. Upon passing a current through the lower active beam, the actuator bends upwards and, consequently, the paddle moves towards a nozzle opening defined in a roof of the nozzle chamber, thereby ejecting a droplet of ink. An advantage of this design is its simplicity of construction. A drawback of this design is that both faces of the paddle work against the relatively viscous ink inside the nozzle chamber.
The Applicant's U.S. Pat. No. 6,260,953 describes an inkjet nozzle in which the actuator forms a moving roof portion of the nozzle chamber. The actuator takes the form of a serpentine core of conductive material encased by a polymeric material. Upon actuation, the actuator bends towards a floor of the nozzle chamber, increasing the pressure within the chamber and forcing a droplet of ink from a nozzle opening defined in the roof of the chamber. The nozzle opening is defined in a non-moving portion of the roof. An advantage of this design is that only one face of the moving roof portion has to work against the relatively viscous ink inside the nozzle chamber. A drawback of this design is that construction of the actuator from a serpentine conductive element encased by polymeric material is difficult to achieve in a MEMS fabrication process.
The Applicant's U.S. Pat. No. 6,623,101 describes an inkjet nozzle comprising a nozzle chamber with a moveable roof portion having a nozzle opening defined therein. The moveable roof portion is connected via an arm to a thermal bend actuator positioned externally of the nozzle chamber. The actuator takes the form of an upper active beam spaced apart from a lower passive beam. By spacing the active and passive beams apart, thermal bend efficiency is maximized since the passive beam cannot act as heat sink for the active beam. Upon passing a current through the active upper beam, the moveable roof portion, having the nozzle opening defined therein, is caused to rotate towards a floor of the nozzle chamber, thereby ejecting through the nozzle opening. Since the nozzle opening moves with the roof portion, drop flight direction may be controlled by suitable modification of the shape of the nozzle rim. An advantage of this design is that only one face of the moving roof portion has to work against the relatively viscous ink inside the nozzle chamber. A further advantage is the minimal thermal losses achieved by spacing apart the active and passive beam members. A drawback of this design is the loss of structural rigidity in spacing apart the active and passive beam members.
In all designs of MEMS inkjet nozzles, there is a need to minimize electrical losses. It is particularly important to minimize electrical losses in cases where the design of the nozzle dictates a disadvantageous configuration from the standpoint of electrical losses. For example, a relatively long distance between an actuator and a CMOS electrode supplying current to the actuator can exacerbate electrical losses. Furthermore, bent or tortuous current paths exacerbate electrical losses.
Usually, the actuator material in inkjet nozzles is selected from a material which fulfils a number of criteria. In the case of mechanical thermal bend-actuated nozzles, these criteria include electrical conductivity, coefficient of thermal expansion, Young's modulus etc. In the case of thermal bubble-forming inkjet nozzles, these criteria include electrical conductivity, resistance to oxidation, resistance to cracking etc. Hence, it will be appreciated that the choice of actuator material is usually a compromise of various properties, and that the actuator material may not necessarily have optimal electrical conductivity. In cases where the actuator material itself has sub-optimal electrical conductivity, it is particularly important to minimize electrical losses elsewhere in the nozzle assembly.
Finally, any improvements in nozzle design should be compatible with standard MEMS fabrication processes. For example, some materials are incompatible with MEMS processing since they lead to contamination of the fab.
From the foregoing, it will appreciated that there is a need to improve on the design and fabrication of inkjet nozzles, so as to minimize electrical losses and to provide more efficient drop ejection in the resultant printhead. There is a particular need to improve on the design and fabrication of mechanical thermal bend-actuated inkjet nozzles, where electrical losses may be exacerbated due to inherent aspects of the nozzle design.
In a first aspect the present invention provides a method of forming an electrical connection between an electrode and an actuator in an inkjet nozzle assembly, said method comprising the steps of:
(a) providing a substrate having a layer of drive circuitry, said drive circuitry including the electrode for connection to the actuator;
(b) forming a wall of insulating material over said electrode;
(c) defining a via in at least said wall, said via revealing said electrode;
(d) filling said via with a conductive material using electroless plating to provide a connector post;
(e) forming at least part of the actuator over said connector post, thereby providing electrical connection between the actuator and the electrode.
In a another aspect there is provided a method comprising the further step of:
In another aspect present invention provides a method further comprising the step of:
depositing one or more second metal pads onto said passive beam material prior to deposition of said active beam material, said second metal pad being positioned to facilitate current flow in bend regions of said beam element.
In a second aspect the present invention provides a printhead integrated circuit comprising a substrate having a plurality of inkjet nozzles assemblies formed on a surface of said substrate, said substrate having drive circuitry for supplying power to said nozzle assemblies, each nozzle assembly comprising:
a nozzle chamber for containing ink, said nozzle chamber having a nozzle opening defined therein;
an actuator for ejecting ink through said nozzle opening;
a pair of electrodes positioned at said surface of said substrate, said electrodes being electrically connected to said drive circuitry; and
a pair of connector posts, each connector post electrically connecting a respective electrode to said actuator,
wherein each connector post extends linearly from a respective electrode to said actuator.
a nozzle chamber for containing ink, said nozzle chamber having a nozzle opening defined therein;
an actuator for ejecting ink through said nozzle opening;
a pair of electrodes positioned at said surface of said substrate, said electrodes being electrically connected to said drive circuitry; and
a pair of connector posts, each connector post electrically connecting a respective electrode to said actuator,
wherein each connector post extends linearly from a respective electrode to said actuator.
As shown more clearly in
The moving portion 409 is defined by a thermal bend actuator 410 having a planar upper active beam 411 and a planar lower passive beam 412. The active beam 411 is connected to a pair of electrode contacts 416 (positive and ground). The electrodes 416 connect with drive circuitry in the CMOS layers.
When it is required to eject a droplet of ink from the nozzle chamber 401, a current flows through the active beam 411 between the two contacts 416. The active beam 411 is rapidly heated by the current and expands relative to the passive beam 412, thereby causing the actuator 410 (which defines the moving portion 409 of the roof 404) to bend downwards towards the substrate 403. This movement of the actuator 410 causes ejection of ink from the nozzle opening 408 by a rapid increase of pressure inside the nozzle chamber 401. When current stops flowing, the moving portion 409 of the roof 404 is allowed to return to its quiescent position, which sucks ink from the inlet 406 into the nozzle chamber 401, in readiness for the next ejection.
In the nozzle design shown in
However, with the actuator 410 defining, at least partially, the roof 404 of the chamber 401, there is inevitably a relatively long distance between the active beam 411 and the electrodes 416 to which the active beam is connected. Furthermore, the current path between the electrode 416 and the active beam 411 is tortuous with a number of turns in the relatively thin layer of beam material. The combination of a relatively large distance between electrode 416 and actuator 410, a tortuous current path, and the thinness of the beam material results in appreciable electrical losses.
Hitherto, MEMS fabrication of inkjet nozzles relied primarily on standard PECVD (plasma-enhanced chemical vapor deposition) and mask/etch steps to build up a nozzle structure. The use of PECVD to deposit simultaneously the active beam 411 and a connection to the electrode 416 has advantages from a MEMS fabrication standpoint, but inevitably leads to a thin, tortuous connection which is disadvantageous in terms of current losses. The current losses are exacerbated further when the beam material does not have optimal conductivity. For example, a vanadium-aluminium alloy has excellent thermoelastic properties, but poorer electrical conductivity compared to, for example, aluminium.
A further disadvantage of PECVD is that a via 418 having sloped sidewalls is required for effective deposition onto the sidewalls. Material cannot be deposited onto vertical sidewalls by PECVD due to the directionality of the plasma. There are several problems associated with sloped via sidewalls. Firstly, a photoresist scaffold having sloped sidewalls is required—this is typically achieved using out-of-focus photoresist exposure, which inevitably leads to some loss of accuracy. Secondly, the total footprint area of the nozzle assembly is increased, thereby reducing nozzle packing density—this increase in area is significantly worsened if the height of the nozzle chamber is increased.
One attempt to alleviate the problem of current losses in the nozzle assembly 400 is to introduce a highly conductive intermediate layer 417, such as titanium or aluminium, between the electrode contact 416 and the active beam material 411 (see
A further disadvantage of the nozzle assembly shown in
In light of the above-mentioned problems, the present Applicants have developed a new method for fabricating a mechanical thermal bend inkjet nozzle assembly, which does not rely on PECVD for forming connections from CMOS contacts to the actuator. As will be described in greater detail, the resultant inkjet nozzle assembly has minimal electrical losses and has an additional structural advantage of a planar ink ejection face. Whilst the invention is exemplified with reference to a mechanical thermal bend inkjet nozzle assembly, it will readily appreciated that it may be applied to any type of inkjet nozzle fabricated by MEMS techniques.
As shown in
The other electrode 3 shown in
Turning initially to
After deposition of the SiO2 layer, it is etched to define the wall 4, which will become a sidewall of the nozzle chamber 5, shown most clearly in
In a second sequence of steps the nozzle chamber 5 is filled with photoresist or polyimide 6, which acts as a sacrificial scaffold for subsequent deposition steps. The polyimide 6 is spun onto the wafer using standard techniques, UV cured and/or hardbaked, and then subjected to chemical mechanical planarization (CMP) stopping at the top surface of the SiO2 wall 4.
In a third sequence of steps, a roof member 7 of the nozzle chamber 5 is formed as well as highly conductive connector posts 8 down to the electrodes 2. Initially, a 1.7 micron layer of SiO2 is deposited onto the polyimide 6 and wall 4. This layer of SiO2 defines a roof member 7 of the nozzle chamber 5. Next, a pair of vias are formed in the wall 4 down to the electrodes 2 using a standard anisotropic DRIE. The dark tone mask shown in
Next, the vias are filled with a highly conductive metal, such as copper, using electroless plating. Copper electroless plating methods are well known in the art and may be readily incorporated into a fab. Typically, an electrolyte comprising a copper complex, an aldehyde (e.g. formaldehyde) and a hydroxide base deposits a coating of copper onto exposed surfaces of a substrate. Electroless plating is usually preceded by a very thin coating (e.g. 0.3 microns or less) of a seed metal (e.g. palladium), which catalyses the plating process. Hence, electroless plating of the vias may be preceded by deposition of a suitable catalyst seed layer, such as palladium, by CVD.
In the final step of this third sequence of steps, the deposited copper is subjected to CMP, stopping on the SiO2 roof member 7 to provide a planar structure.
In a fourth sequence of steps, conductive metal pads 9 are formed, which are configured to minimize power losses in any regions of potentially high resistance. These regions are typically at the junction of the connector posts 8 with a thermoelastic element, and at any bends in the thermoelastic element. The thermoelastic element is formed in subsequent steps and the function of the metal pads 9 will be understood more readily once the nozzle assembly is described in its fully formed state.
The metal pads 9 are formed by initially depositing a 0.3 micron layer of aluminium onto the roof member 7 and connector posts 8. Any highly conductive metal (e.g. aluminium, titanium etc.) may be used and should be deposited with a thickness of about 0.5 microns or less so as not to impact too severely on the overall planarity of the nozzle assembly. Following deposition of the aluminium layer, a standard metal etch (e.g. Cl2/BCl3) is used to define the metal pads 9. The clear tone mask shown in
In a fifth sequence of steps exemplified by
To form the active beam member 10, a 1.5 micron layer of active beam material is initially deposited by standard PECVD. The beam material is then etched using a standard metal etch to define the active beam member 10. The clear tone mask shown in
After completion of the metal etch and as shown in
As is shown most clearly in
In a sixth sequence of steps, exemplified in
As can be seen most clearly in
A perimeter gap 17 around the moving portion 14 of the roof separates the moving portion from a stationary portion 18 of the roof This gap 17 allows the moving portion 14 to bend into the nozzle chamber 5 and towards the substrate 1 upon actuation of the actuator 15.
In a seventh sequence of steps, exemplified in
The use of photopatternable polymers to coat arrays of nozzle assemblies was described extensively in our earlier U.S. application Ser. No. 11/685,084 filed on 12 Mar. 2007 and U.S. application Ser. No. 11/740,925 filed on 27 Apr. 2007 , the contents of which are incorporated herein by reference. Typically, the hydrophobic polymer is polydimethylsiloxane (PDMS) or perfluorinated polyethylene (PFPE). Such polymers are particularly advantageous because they are photopatternable, have high hydrophobicity, and low Young's modulus.
As explained in the above-mentioned U.S. applications, the exact ordering of MEMS fabrication steps, incorporating the hydrophobic polymer, is relatively flexible. For example, it is perfectly feasible to etch the nozzle opening 13 after deposition of the hydrophobic polymer 19, and use the polymer as a mask for the nozzle etch. It will appreciated that variations on the exact ordering of MEMS fabrication steps are well within the ambit of the skilled person, and, moreover, are included within the scope of the present invention.
The hydrophobic polymer layer 19 performs several functions. Firstly, it provides a mechanical seal for the perimeter gap 17 around the moving portion 14 of the roof The low Young's modulus of the polymer (<1000 MPa) means that it does not significantly inhibit bending of the actuator, whilst preventing ink from escaping through the gap 17 during actuation. Secondly, the polymer has a high hydrophobicity, which minimizes the propensity for ink to flood out of the relatively hydrophilic nozzle chambers and onto an ink ejection face 21 of the printhead. Thirdly, the polymer functions as a protective layer, which facilitates printhead maintenance.
In a final, eighth sequence of steps, exemplified in
Following the ink supply channel etch, the polyimide 6, which filled the nozzle chamber 5, is removed by ashing (either frontside ashing or backside ashing) using, for example, an O2 plasma to provide the nozzle assembly 100.
The resultant nozzle assembly 100 shown in
Secondly, the connector posts 8 extend perpendicularly from the surface of the substrate 1, allowing the height of the nozzle chamber 5 to be increased without impacting on the overall footprint area of the nozzle assembly 100. By contrast, the nozzle assembly 400 requires sloped connections between the electrode 416 and the active beam member 411 so that the connections can be formed by PECVD. This slope inevitably impacts on the overall footprint area of the nozzle assembly 400, which is particularly disadvantageous if the height of the nozzle chamber 401 were to be increased (for example, to provide improved drop ejection characteristics). In accordance with the present invention, nozzle assemblies having relatively large volume nozzle chambers can be arranged in rows with a nozzle pitch of, for example, less than 50 microns.
Thirdly, the nozzle assembly 100 has a highly planar ink ejection face 21, in the absence of any pit or via in the region of the electrodes 2. The planarity of the ink ejection face is advantageous for printhead maintenance, because it presents a smooth wipeable surface for any maintenance device. Furthermore, there is no risk of particles becoming trapped permanently in electrode vias or other contoured features of the ink ejection face.
It will, of course, be appreciated that the present invention has been described by way of example only and that modifications of detail may be made within the scope of the invention, which is defined in the accompanying claims.
This application is a continuation of U.S. application Ser. No. 12,972,506 filed Dec. 19, 2010, which is a continuation of U.S. application Ser. No. 11/763,440 filed Jun. 15, 2007 issued U.S. Pat. No. 7,866,795, all of which is herein incorporated by reference.
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| Number | Date | Country | |
|---|---|---|---|
| Parent | 12972506 | Dec 2010 | US |
| Child | 13118463 | US | |
| Parent | 11763440 | Jun 2007 | US |
| Child | 12972506 | US |
