EP2490898B1

EP2490898B1 - Printhead having polysilsesquioxane coating on ink ejection face

Info

Publication number: EP2490898B1
Application number: EP09847455.4A
Authority: EP
Inventors: Gregory John Mcavoy; Ronan Padraig Sean O'reilly; Emma Rose Kerr
Original assignee: Memjet Technology Ltd
Current assignee: Memjet Technology Ltd
Priority date: 2009-07-24
Filing date: 2009-07-24
Publication date: 2016-07-13
Anticipated expiration: 2029-07-24
Also published as: CN102470675A; AU2009350310B2; KR101356333B1; SG176101A1; CN102470675B; WO2011009153A1; AU2009350310A1; JP5608737B2; JP2012529383A; EP2490898A4; CA2760206C; CA2760206A1; EP2490898A1; KR20120037015A

Description

Field of the Invention

The present invention relates to the field of printers and particularly inkjet printheads. It has been developed primarily to improve print quality and printhead maintenance in high resolution printheads.

Background of the Invention

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 inkjet 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.
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.
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).
Ink Jet printers themselves come in many different types. The utilization of a continuous stream of ink in inkjet printing appears to date back to at least 1929 wherein US Patent No. 1941001 by Hansell discloses a simple form of continuous stream electro-static inkjet printing.
US Patent 3596275 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 electro-static field so as to cause drop separation. This technique is still utilized by several manufacturers including Elmjet and Scitex (see also US Patent No. 3373437 by Sweet et al )
Piezoelectric ink jet printers are also one form of commonly utilized ink jet printing device. Piezoelectric systems are disclosed by Kyser et. al. in US Patent No. 3946398 (1970 ) which utilizes a diaphragm mode of operation, by Zolten in US Patent 3683212 (1970 ) which discloses a squeeze mode of operation of a piezoelectric crystal, Stemme in US Patent No. 3747120 (1972 ) discloses a bend mode of piezoelectric operation, Howkins in US Patent No. 4459601 discloses a piezoelectric push mode actuation of the ink jet stream and Fischbeck in US 4584590 which discloses a shear mode type of piezoelectric transducer element.'
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 US Patent 4490728 . Both the aforementioned references disclosed ink jet printing techniques that 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 electrothermal actuator are manufactured by manufacturers such as Canon and Hewlett Packard.
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.
In the construction of any inkjet printing system, there are a considerable number of important factors which must be traded off against one another especially as large scale printheads are constructed, especially those of a pagewidth type. A number of these factors are outlined below.
Firstly, inkjet printheads are normally constructed utilizing micro-electromechanical systems (MEMS) techniques. As such, they tend to rely upon standard integrated circuit construction/fabrication techniques of depositing planar layers on a silicon wafer and etching certain portions of the planar layers. Within silicon circuit fabrication technology, certain techniques are better known than others. For example, the techniques associated with the creation of CMOS circuits are likely to be more readily used than those associated with the creation of exotic circuits including ferroelectrics, gallium arsenide etc. Hence, it is desirable, in any MEMS constructions, to utilize well proven semi-conductor fabrication techniques which do not require any "exotic" processes or materials. Of course, a certain degree of trade off will be undertaken in that if the advantages of using the exotic material far out weighs its disadvantages then it may become desirable to utilize the material anyway. However, if it is possible to achieve the same, or similar, properties using more common materials, the problems of exotic materials can be avoided.
A desirable characteristic of inkjet printheads would be a hydrophobic ink ejection face ("front face" or "nozzle face"), preferably in combination with hydrophilic nozzle chambers and ink supply channels. Hydrophilic nozzle chambers and ink supply channels provide a capillary action and are therefore optimal for priming and for re-supply of ink to nozzle chambers after each drop ejection. A hydrophobic front face minimizes the propensity for ink to flood across the front face of the printhead. With a hydrophobic front face, the aqueous inkjet ink is less likely to flood sideways out of the nozzle openings. Furthermore, any ink which does flood from nozzle openings is less likely to spread across the face and mix on the front face - they will instead form discrete spherical microdroplets which can be managed more easily by suitable maintenance operations.
Hitherto, the present Applicant has described the use of PDMS (polydimethylsiloxane) for coating the front face of a printhead and providing a hydrophobic surface. However, whilst PDMS has excellent hydrophobic properties and can be readily incorporated into a printhead MEMS fabrication process, it has relatively poor wear-resistance and may be scratched or otherwise damaged by a wiper blade used for printhead maintenance (see, for example, US Application No. 12/014,772, filed on January 16, 2008 ; publication number US 2009/179940 . It would therefore be desirable to provide a printhead having a hydrophobic ink ejection face, which can be readily produced by a MEMS fabrication process and which has good wear-resistance.
EP 0665107A2 discloses a highly reliable ink jet recording head excellent in mechanical strength, weatherability, ink resistance, and adhesion to the substrate. For its production, a cationically polymerized curing product of an epoxy resin having a structural unit is used as a resin material which coats an ink flow path pattern formed from a dissoluble resin on the substrate. An ejection Face is coated with polysilses quioxanes.

Summary of the Invention

In view of the above, the present teaching provides, a printhead having polysilsesquioxane coating on ink ejection face in accordance with the claims which follows.
In a first aspect, there is provided a printhead having an ink ejection face, wherein at least part of the ink ejection face is coated with a hydrophobic polymeric material, the polymeric material being comprised of a polysilsesquioxane. Printheads according to the present invention have excellent durability and wear-resistance making them compatible with various printhead maintenance operation involving contact with the ink ejection face (e.g. wiping). Moreover, the polysilsesquioxane can be deposited in a thin layer (0.5 to 2 microns) by a spin-on process, which is readily incorporated into a MEMS printhead fabrication process.
Optionally, the polysilsesquioxane is selected from the group consisting of: poly(alkylsilsesquioxanes) and poly(arylsilsesquioxanes)
Optionally, the polysilsesquioxane is selected from the group consisting of: poly(methylsilsesquioxane) and poly(phenylsilsesquioxane).
Optionally, the polymeric material is deposited and hardbaked onto a nozzle plate of the printhead during MEMS printhead fabrication.
The printhead comprises a plurality of nozzle assemblies formed on a substrate, each nozzle assembly comprising: a nozzle chamber, a nozzle opening defined in a roof of the nozzle chamber and an actuator for ejecting ink through the nozzle opening. The polymeric material is coated on a nozzle plate of the printhead, the nozzle plate being at least partially defined by the roof of each nozzle chamber.
Optionally, each roof has a hydrophobic outside surface relative to the inside surfaces of each nozzle chamber by virtue of the hydrophobic coating.
Optionally, each nozzle chamber comprises a roof and sidewalls comprised of a ceramic material.
Optionally, the ceramic material is selected from the group consisting of: silicon nitride, silicon oxide and silicon oxynitride.
Optionally, the roof is spaced apart from the substrate, such that sidewalls of each nozzle chamber extend between the nozzle plate and the substrate.
The actuator is a thermal bend actuator comprising:

a first active element for connection to drive circuitry; and
a second passive element mechanically cooperating with the first element, such that when a current is passed through the first element, the first element expands relative to the second element, resulting in bending of the actuator.

Optionally, the thermal bend actuator defines at least part of a roof of each nozzle chamber, whereby actuation of the actuator moves said a moving portion of the roof towards a floor of said nozzle chamber.
Optionally, the nozzle opening is defined in said moving portion of the roof.
Optionally, nozzle opening is defined in a stationary portion of the roof.
Optionally, the polymeric material defines a mechanical seal between the moving portion and a stationary portion of the roof, thereby minimizing ink leakage during actuation of the actuator.

Brief Description of the Drawings

Optional embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a partial perspective view of an array of nozzle assemblies of a thermal inkjet printhead;
Figure 2 is a side view of a nozzle assembly unit cell shown in Figure 1;
Figure 3 is a perspective of the nozzle assembly shown in Figure 2;
Figure 4 shows a partially-formed nozzle assembly after deposition of side walls and roof material onto a sacrificial photoresist layer;
Figure 5 is a perspective of the nozzle assembly shown in Figure 4;
Figure 6 is the mask associated with the nozzle rim etch shown in Figure 7;
Figure 7 shows the etch of the roof layer to form the nozzle opening rim;
Figure 8 is a perspective of the nozzle assembly shown in Figure 7;
Figure 9 is the mask associated with the nozzle opening etch shown in Figure 10;
Figure 10 shows the etch of the roof material to form the elliptical nozzle openings;
Figure 11 is a perspective of the nozzle assembly shown in Figure 10;
Figure 12 shows the oxygen plasma ashing of the first and second sacrificial layers;
Figure 13 is a perspective of the nozzle assembly shown in Figure 12;
Figure 14 shows the nozzle assembly after the ashing, as well as the opposing side of the wafer;
Figure 15 is a perspective of the nozzle assembly shown in Figure 14;
Figure 16 is the mask associated with the backside etch shown in Figure 17;
Figure 17 shows the backside etch of the ink supply channel into the wafer;
Figure 18 is a perspective of the nozzle assembly shown in Figure 17;
Figure 19 shows the nozzle assembly of Figure 7 after deposition of a hydrophobic polymeric coating;
Figure 20 is a perspective of the nozzle assembly shown in Figure 19;
Figure 21 shows the nozzle assembly of Figure 19 after deposition of a protective metal film; and
Figure 22 shows the nozzle assembly of Figure 21 after etching through the protective metal film, the polymeric coating and the nozzle roof;
Figure 23 shows the completed nozzle assembly after backside MEMS processing and removal of photoresist;
Figure 24 is a perspective of the nozzle assembly shown in Figure 23;
Figure 25 is a side-sectional view of a partially-fabricated alternative inkjet nozzle assembly after a first sequence of steps in which nozzle chamber sidewalls are formed;
Figure 26 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 25;
Figure 27 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a second sequence of steps in which the nozzle chamber is filled with polyimide;
Figure 28 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 27;
Figure 29 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a third sequence of steps in which connector posts are formed up to a chamber roof;
Figure 30 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 29;
Figure 31 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a fourth sequence of steps in which conductive metal plates are formed;
Figure 32 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 31;
Figure 33 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a fifth sequence of steps in which an active beam member of a thermal bend actuator is formed;
Figure 34 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 33;
Figure 35 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a sixth sequence of steps after coating with a polymeric layer, protecting with a metal layer and etching a nozzle opening;
Figure 36 is a side-sectional view of completed inkjet nozzle assembly, after backside MEMS processing and removal of photoresist; and
Figure 37 is a cutaway perspective view of the inkjet nozzle assembly shown in Figure 36.

Description of Optional Embodiments

Thermal Bubble-Forming Inkjet Printhead ; not part of the invention Referring to Figure 1, there is shown a part of printhead comprising a plurality of nozzle assemblies. Figures 2 and 3 show one of these nozzle assemblies in side-section and cutaway perspective views.
Each nozzle assembly comprises a nozzle chamber 24 formed by MEMS fabrication techniques on a silicon wafer substrate 2. The nozzle chamber 24 is defined by a roof 21 and sidewalls 22 which extend from the roof 21 to the silicon substrate 2. As shown in Figure 1, each roof is defined by part of a nozzle surface 56, which spans across an ejection face of the printhead. The nozzle surface 56 and sidewalls 22 are formed of the same material, which is deposited by PECVD over a sacrificial scaffold of photoresist during MEMS fabrication. Typically, the nozzle surface 56 and sidewalls 22 are formed of a ceramic material, such as silicon dioxide or silicon nitride. These hard materials have excellent properties for printhead robustness, and their inherently hydrophilic nature is advantageous for supplying ink to the nozzle chambers 24 by capillary action. However, the exterior (ink ejection) surface of the nozzle surface 56 is also hydrophilic, which causes any flooded ink on the surface to spread.
Returning to the details of the nozzle chamber 24, it will be seen that a nozzle opening 26 is defined in a roof of each nozzle chamber 24. Each nozzle opening 26 is generally elliptical and has an associated nozzle rim 25. The nozzle rim 25 assists with drop directionality during printing as well as reducing, at least to some extent, ink flooding from the nozzle opening 26. The actuator for ejecting ink from the nozzle chamber 24 is a heater element 29 positioned beneath the nozzle opening 26 and suspended across a pit 8. Current is supplied to the heater element 29 via electrodes 9 connected to drive circuitry in underlying CMOS layers 5 of the substrate 2. When a current is passed through the heater element 29, it rapidly superheats surrounding ink to form a gas bubble, which forces ink through the nozzle opening. By suspending the heater element 29, it is completely immersed in ink when the nozzle chamber 24 is primed. This improves printhead efficiency, because less heat dissipates into the underlying substrate 2 and more input energy is used to generate a bubble.
As seen most clearly in Figure 1, the nozzles are arranged in rows and an ink supply channel 27 extending longitudinally along the row supplies ink to each nozzle in the row. The ink supply channel 27 delivers ink to an ink inlet passage 15 for each nozzle, which supplies ink from the side of the nozzle opening 26 via an ink conduit 23 in the nozzle chamber 24.
The MEMS fabrication process for manufacturing such printheads was described in detail in our previously filed US Application No. 11/246,684 filed on October 11, 2005 . The latter stages of this fabrication process are briefly revisited here for the sake of clarity.
Figures 4 and 5 show a partially-fabricated printhead comprising a nozzle chamber 24 encapsulating sacrificial photoresist 10 ("SAC1") and 16 ("SAC2"). The SAC1 photoresist 10 was used as a scaffold for deposition of heater material to form the suspended heater element 29. The SAC2 photoresist 16 was used as a scaffold for deposition of the sidewalls 22 and roof 21 (which defines part of the nozzle surface 56).
In the prior art process, and referring to Figures 6 to 8, the next stage of MEMS fabrication defines the elliptical nozzle rim 25 in the roof 21 by etching away 2 microns of roof material 20. This etch is defined using a layer of photoresist (not shown) exposed by the dark tone rim mask shown in Figure 6. The elliptical rim 25 comprises two coaxial rim lips 25a and 25b, positioned over their respective thermal actuator 29.
Referring to Figures 9 to 11, the next stage defines an elliptical nozzle aperture 26 in the roof 21 by etching all the way through the remaining roof material, which is bounded by the rim 25. This etch is defined using a layer of photoresist (not shown) exposed by the dark tone roof mask shown in Figure 9. The elliptical nozzle aperture 26 is positioned over the thermal actuator 29, as shown in Figure 11.
With all the MEMS nozzle features now fully formed, the next stage removes the SAC1 and SAC2 photoresist layers 10 and 16 by O₂ plasma ashing (Figures 12 and 13). Figures 14 and 15 show the entire thickness (150 microns) of the silicon wafer 2 after ashing the SAC1 and SAC2 photoresist layers 10 and 16.
Referring to Figures 16 to 18, once frontside MEMS processing of the wafer is completed, ink supply channels 27 are etched from the backside of the wafer to meet with the ink inlets 15 using a standard anisotropic DRIE. This backside etch is defined using a layer of photoresist (not shown) exposed by the dark tone mask shown in Figure 16. The ink supply channel 27 makes a fluidic connection between the backside of the wafer and the ink inlets 15.
Finally, and referring to Figures 2 and 3, the wafer is thinned to about 135 microns by backside etching. Figure 1 shows three adjacent rows of nozzles in a cutaway perspective view of a completed printhead integrated circuit. Each row of nozzles has a respective ink supply channel 27 extending along its length and supplying ink to a plurality of ink inlets 15 in each row. The ink inlets, in turn, supply ink to the ink conduit 23 for each row, with each nozzle chamber receiving ink from a common ink conduit for that row.
As already discussed above, this prior art MEMS fabrication process inevitably leaves a hydrophilic ink ejection face by virtue of the nozzle surface 56 being formed of ceramic materials, such as silicon dioxide, silicon nitride, silicon oxynitride, aluminium nitride etc.
In a preferred process for hydrophobizing the nozzle surface 56 (and as described in US 2009/0139961 , the wafer is coated with a hydrophobic polymer 80 immediately after the nozzle rim etch at the stage exemplified in Figures 7 and 8.
A thin layer (about 1 to 2 microns) of the hydrophobic polymer 100 is spun onto the wafer and hardbaked to provide the partially-fabricated printhead shown in Figures 19 and 20.
Referring now to Figure 21, a protective metal film 90 (ca. 100 nm thickness) is then deposited onto the polymer layer 80. The metal film is typically comprised of titanium or aluminium and protects the hydrophobic polymer 80 from late-stage oxygen ashing conditions. Hence, the polymer layer 80 is not exposed to aggressive ashing conditions and retains its hydrophobic characteristics throughout the MEMS processing steps.
Figure 22 shows the wafer after etching the nozzle opening 26 through the metal film 110, the polymer layer 80 and the nozzle roof 21. This etching step utilizes a conventional patterned photoresist layer (not shown) as a common mask for all nozzle etching steps. In a typical etching sequence, the metal film 90 is first etched, either by standard dry metal-etching (e.g. BCl₃/Cl₂) or wet metal-etching (e.g. H₂O₂ or HF). A second dry etch is then used to etch through the polymer layer 80 and the nozzle roof 21. Typically, the second etch step is a dry etch employing O₂ and a fluorinated etching gas (e.g. SF₆ or CF₄).
Once the nozzle opening 26 is defined as shown in Figure 22, backside MEMS processing steps (e.g. etching ink supply channels, wafer thinning etc) and late-stage ashing of photoresist can proceed in accordance with known protocols, analogous to the steps described above in connection with Figures 14 to 18. Final removal of the metal film 90 using a H₂O₂ or HF rinse yields the completed nozzle assembly shown in Figures 23 and 24, having the hydrophobic polymer layer 80.

Thermal Bend Actuator Printhead

From the foregoing, it will be appreciated that any type of printhead may be hydrophobized in an analogous manner. However, the polymeric coatings are particularly advantageous for use in the Applicant's thermal bend actuator nozzle assemblies, because the polymer layer acts as a mechanical seal between a moving roof portion and a stationary body of the printhead. These advantages are discussed in greater detail in the Applicant's US Publication No. 2008/0225076 ,
Figures 25 to 37 shows a sequence of MEMS fabrication steps for an inkjet nozzle assembly 100 described in our earlier US Publication No. US 2008/0309728 . The completed inkjet nozzle assembly 100 shown in Figures 36 and 37 utilizes thermal bend actuation, whereby a moving portion of a roof bends towards a substrate resulting in ink ejection.
The starting point for MEMS fabrication is a standard CMOS wafer having CMOS drive circuitry formed in an upper portion of a silicon wafer. At the end of the MEMS fabrication process, this wafer is diced into individual printhead integrated circuits (ICs), with each IC comprising drive circuitry and plurality of nozzle assemblies.
As shown in Figures 25 and 26, a substrate 101 has an electrode 102 formed in an upper portion thereof. The electrode 102 is one of a pair of adjacent electrodes (positive and earth) for supplying power to an actuator of the inkjet nozzle 100. The electrodes receive power from CMOS drive circuitry (not shown) in upper layers of the substrate 101.
The other electrode 103 shown in Figures 25 and 26 is for supplying power to an adjacent inkjet nozzle. In general, the drawings shows MEMS fabrication steps for a nozzle assembly, which is one of an array of nozzle assemblies. The following description focuses on fabrication steps for one of these nozzle assemblies. However, it will of course be appreciated that corresponding steps are being performed simultaneously for all nozzle assemblies that are being formed on the wafer. Where an adjacent nozzle assembly is partially shown in the drawings, this can be ignored for the present purposes. Accordingly, the electrode 103 and all features of the adjacent nozzle assembly will not be described in detail herein. Indeed, in the interests of clarity, some MEMS fabrication steps will not be shown on adjacent nozzle assemblies.
In the sequence of steps shown in Figures 25 and 26, an 8 micron layer of silicon dioxide is initially deposited onto the substrate 101. The depth of silicon dioxide defines the depth of a nozzle chamber 105 for the inkjet nozzle. After deposition of the SiO₂ layer, it is etched to define walls 104, which will become sidewalls of the nozzle chamber 105, shown most clearly in Figure 26.
As shown in Figures 27 and 28, the nozzle chamber 105 is then filled with photoresist or polyimide 106, which acts as a sacrificial scaffold for subsequent deposition steps. The polyimide 106 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 104.
In Figures 29 and 30, a roof member 107 of the nozzle chamber 105 is formed as well as highly conductive connector posts 108 extending down to the electrodes 102. Initially, a 1.7 micron layer of SiO₂ is deposited onto the polyimide 106 and wall 104. This layer of SiO₂ defines a roof 107 of the nozzle chamber 105. Next, a pair of vias are formed in the wall 104 down to the electrodes 102 using a standard anisotropic DRIE. This etch exposes the pair of electrodes 102 through respective vias. Next, the vias are filled with a highly conductive metal, such as copper, using electroless plating. The deposited copper posts 108 are subjected to CMP, stopping on the SiO₂ roof member 107 to provide a planar structure. It can be seen that the copper connector posts 108, formed during the electroless copper plating, meet with respective electrodes 102 to provide a linear conductive path up to the roof member 107.
In Figures 31 and 32, metal pads 109 are formed by initially depositing a 0.3 micron layer of aluminium onto the roof member 107 and connector posts 108. 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. The metal pads 109 are positioned over the connector posts 108 and on the roof member 107 in predetermined 'bend regions' of the thermoelastic active beam member.
In Figures 33 and 34, a thermoelastic active beam member 110 is formed over the SiO₂ roof 107. By virtue of being fused to the active beam member 110, part of the SiO₂ roof member 107 functions as a lower passive beam member 116 of a mechanical thermal bend actuator, which is defined by the active beam 110 and the passive beam 116. The thermoelastic active beam member 110 may be comprised of any suitable thermoelastic material, such as titanium nitride, titanium aluminium nitride and aluminium alloys. As explained in the Applicant's earlier US Publication No. 2008/0129793 vanadium-aluminium alloys are a preferred material, because they combine the advantageous properties of high thermal expansion, low density and high Young's modulus.
To form the active beam member 110, 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 110. After completion of the metal etch and as shown in Figures 33 and 34, the active beam member 110 comprises a partial nozzle opening 111 and a beam element 112, which is electrically connected at each end to positive and ground electrodes 102 via the connector posts 108. The planar beam element 112 extends from a top of a first (positive) connector post and bends around 180 degrees to return to a top of a second (ground) connector post.
Still referring to Figures 33 and 34, the metal pads 109 are positioned to facilitate current flow in regions of potentially higher resistance. One metal pad 109 is positioned at a bend region of the beam element 112, and is sandwiched between the active beam member 110 and the passive beam member 116. The other metal pads 109 are positioned between the top of the connector posts 108 and the ends of the beam element 112.
Referring to Figure 35, a hydrophobic polymer layer 80 is deposited onto the wafer and covered with a protective metal layer 90 (e.g. 100 nm aluminum). After suitable masking, the metal layer 90, the polymer layer 80 and the SiO₂ roof member 107 are then etched to define fully a nozzle opening 113 and a moving portion 114 of the roof. The etch is typically a two-stage etch process as described above in connection with Figure 22.
The moving portion 114 comprises a thermal bend actuator 115, which is itself comprised of the active beam member 110 and the underlying passive beam member 116. The nozzle opening 113 is defined in the moving portion 114 of the roof so that the nozzle opening moves with the actuator during actuation. Configurations whereby the nozzle opening 113 is stationary with respect to the moving portion 114, as described in US Publication No. 2008/0129793 , are also possible and within the ambit of the present invention.
A perimeter space or gap 117 around the moving portion 114 of the roof separates the moving portion from a stationary portion 118 of the roof. This gap 117 allows the moving portion 114 to bend into the nozzle chamber 105 and towards the substrate 101 upon actuation of the actuator 115. The hydrophobic polymer layer 80 fills the gap 117 to provide a mechanical seal between the moving portion 114 and stationary portion 118 of the roof 107. The polymer has a sufficiently low Young's modulus to allow the actuator to bend towards the substrate 101, whilst preventing ink from escaping through the gap 117 during actuation.
In the final MEMS processing steps, and as shown in Figures 36 and 37, an ink supply channel 120 is etched through to the nozzle chamber 105 from a backside of the substrate 101. Although the ink supply channel 120 is shown aligned with the nozzle opening 113 in Figures 36 and 37, it could, of course, be positioned offset from the nozzle opening.
Following the ink supply channel etch, the polyimide 106, which filled the nozzle chamber 105, is removed by ashing in an oxidizing plasma and the metal film 90 is removed by an HF or H₂O₂ rinse to provide the nozzle assembly 100.

Polymer Layer Comprising MSQ

The hydrophobic polymer layer 80 has proven to be an important feature of the Applicant's printheads. Not only does it hydrophobize the front face of the printhead, which helps to improve overall print quality, it also assists with printhead maintenance by presenting a planar hydrophobic surface for a printhead maintenance means (e.g. wiper blade) employed to maintain the printhead in an operable condition. Of course, in the case of the thermal bend-actuated printheads 100 described above, the polymer 80 provides the additional function of mechanically sealing the moving part of the nozzle from the body of the printhead.
Hitherto, the Applicant has proposed the use of polydimethylsiloxane (PDMS). This material can be readily incorporated in MEMS fabrication processes, has excellent hydrophobicity and a Young's modulus which allows efficient thermal bend actuation. However, PDMS has relatively poor wear-resistance and can be scratched or otherwise damaged by repeated contact with, for example, a wiper blade.
The Applicant has now found that polysilsesquioxanes provide superior wear-resistance to PDMS whilst still maintaining all the advantages of PDMS. Polysilsesquioxanes belong to the general class of polymers known as polymerized siloxanes or silicones, and have the empirical formula (RSiO_1.5)_n, where R is hydrogen or an organic group and n is an integer representing the length of the polymer chain. The organic group may be C_1-12 alkyl (e.g. methyl), C_1-10 aryl (e.g. phenyl) or C_1-16 arylalkyl (e.g. benzyl). The polymer chain may be of any length known in the art (e.g. n is from 2 to 10,000).
Poyl(alkylsilsesquioxanes) and poly(arylsilsesquioxanes), such as poly(methylsilsesquioxane) and poly(phenylsilsesquioxane) have been shown to have excellent hydrophobicity, durability and wear-resistance when used as the polymer layer 80 in the Applicant's printheads. For example, printheads coated with MSQ or PSQ could be wiped clean without damage, even after ink and paper fibres were baked onto the printhead for 1 hour.
Poly(methylsilsesquioxane) is also known in the art as methylsilsequioxane, MSQ, MSSQ, PMSQ and PMSSQ. Poly(phenylsilsesquioxane) is also known in the art as phenylsilsequioxane, PSQ, PSSQ, PPSQ and PPSSQ. For the sake of brevity, the Applicant shall hereinafter refer to poly(methylsilsesquioxane) as MSQ and refer to poly(phenylsilsesquioxane) as PSQ.
MSQ has a low dielectric constant (k = 2.7) and has been used previously as an insulating material. However, the use of MSQ as a hydrophobic coating for MEMS inkjet printheads was not previously known.
MSQ or PSQ may be incorporated into printheads as the polymer layer 80 by the MEMS fabrication process described above. A MSQ or PSQ solution is spun on to the wafer to a depth of about 0.5 to 5 microns (e.g. 1 micron) and then hardbaked to promote adhesion to the nozzle plate and to provide a durable ink ejection face for the printhead. Hardbaking may include a UV curing step. For example, a typical hardbaking process may comprise the following steps:

1. Contact bake @ 110°C for 2 min immediately after coating
2. Contact bake @ 300°C for 6.5 min
3. UV expose for 130sec (∼1300mJ)
4. Oven cure for 1 hour (starting @ 180°C and ramping up @ ∼4°C/min)

Although the Applicant's hardbaking process described above provides MSQ-coated or PSQ-coated printheads having excellent durability, it will be appreciated that hardbaking may follow any conventional procedure.
MSQ and PSQ each have a Young's modulus of about 3 GPa, which is somewhat higher than that of PDMS. However, the Applicant has found that thermal bend-actuated printheads still operate efficiently when the polymer layer 80 is comprised of MSQ or PSQ, notwithstanding its higher Young's modulus. Moreover, the overall robustness of MSQ and PSQ usually outweighs any downsides arising from their higher Young's moduli. Of course, in thermal bubble-forming printheads where there are no moving parts, the Young's modulus of the polymeric layer 80 is irrelevant to nozzle actuation.
The present inventors consider that the use of MSQ or PSQ represents a significant breakthrough in inkjet printhead technology. Hydrophobizing inkjet printheads, especially those manufactured by a MEMS fabrication process, was seen as a very significant challenge for all industry players. The present Applicant has demonstrated that MSQ or PSQ may be incorporated into a MEMS fabrication process and provides a hydrophobic ink ejection face having excellent durability and wear-resistance. This desirable combination of features had not been achieved previously in the art.

Polymer Layer Containing Nanoparticles

Although, as described above, MSQ and PSQ have significant advantages over PDMS for use as a polymer coating, there may be some instances where PDMS is still the material of choice. For example, in low-powered thermally bend-actuated printheads, the lower Young's modulus of PDMS may be advantageous for minimizing drop ejection energies. It would be desirable to improve, for example, the wear-resistance characteristics of PDMS without comprising its low Young's modulus.
In other scenarios, the polymer coating of a printhead may have properties which do not suit the particular fluid being ejected from the printhead. It should be noted that thermal bend-actuated printheads may eject both aqueous and non-aqueous liquids (e.g. polymers for printing OLEDs), and the ink ejection face of the printhead may have characteristics which complement the inherent properties of the fluid being ejected. These properties may include, for example, the fluid's hydrophilicity, hydrophobicity, viscosity, surface tension and/or boiling point.
Alternatively, the ink ejection face may have characteristics which complement a particular type of printhead maintenance regime employed (e.g. printhead capping/wiping as described in US Application No. 12/014,772 ; or printhead flooding/non-contact maintenance as described in US 7,401,886 ) . For example, wear-resistance is important for printhead maintenance regimes involving contact with the printhead, but less important for non-contact maintenance regimes.
Alternatively, the ink ejection face may have characteristics which complement a particular type of nozzle actuator. For example, fatigue-resistance is important for thermal bend-actuators where the polymeric material seals a moving portion of the nozzle to the body of the printhead. However, fatigue-resistance is less important in non-moving nozzles, such as the thermal bubble-forming nozzles described above.
The ability to 'tune' the characteristics of the ink ejection face without changing fundamentally the MEMS fabrication process would be highly desirable. This 'tuning' may improve, for example, the toughness, wear-resistance, fatigue-resistance and/or the surface energy characteristics of the ink ejection face. As a trivial example, when printing hydrophobic liquids such as polymers, the ink ejection face should preferably be relatively hydrophilic rather than hydrophobic (in contrast with printing aqueous inks).
The availability of silicone polymers incorporating nanoparticles (sometimes known in the art as "fillers") means that the characteristics of the silicone polymer (e.g. PDMS, MSQ, PSQ) may be modified by changing the nanoparticles incorporated therein. The use of different nanoparticles will correspondingly 'tune' the characteristics of the ink ejection face defined by the polymer layer 80.
Of course, the nanoparticles may be of any suitable type, size and shape depending on the particular application. The nanoparticles may comprise inorganic particles, organic particles or a combination of both. Some examples of inorganic nanoparticles are metal oxides, metal carbonates and metal sulfates. More specifically, the inorganic nanoparticles may be, for example, silica (including colloidal silica), zirconium oxide, titanium oxide, aluminium oxide, calcium carbonate, tin oxide, zinc oxide, copper oxide, chromium oxide, calcium oxide, tungsten oxide, iron oxide, cobalt oxide, barium sulfate etc. Some examples of organic nanoparticles are cross-linked silicone resin particles (e.g. PDMS, MSQ, PSQ), cross-linked polyolefin resin particles (e.g. polystyrene, polyethylene, polypropylene), cross-linked acryl resin particles, cross-linked styrene-acryl resin particles, cross-linked polyester particles, polyimide particles, melamine resin particles, carbon nanotubes etc.
As used herein, the term "nanoparticles" refers to particles have an average particle size in the range of 1 to 1000 nm, more usually 1 to 100 nm, and more usually 1 to 50 nm. Average particles sizes of about 20 nm are generally preferred. The particles may be monodisperse or polydisperse.
The nanoparticles may be present in an amount ranging from 1 to 70 wt.%, optionally 5 to 60 wt.%, optionally 10 to 50 wt.%. The amount of nanoparticles present will depend on the requisite characteristics of the polymer film.
The nanoparticles may be incorporated into the polymer by any suitable process, such as the sol-gel process, which is well known to the person skilled in the art. The resulting polymer may be deposited by any suitable process, such as a spin-on process followed by hardbaking.
PDMS polymers incorporating silica nanoparticles are known in the art and such polymers may be used as the polymer layer 80 in the present invention. The silica nanoparticles impart the desirable characteristics of wear-resistance and fatigue-resistance to the PDMS polymer. Depending on the amount of silica particles present, the PDMS may also have a relatively hydrophilic surface which is useful in some applications.

Claims

A printhead comprising a plurality of nozzle assemblies (100) formed on a substrate (101), each nozzle assembly comprising:
a nozzle chamber (105),

a nozzle opening (113) defined in a roof (107) of the nozzle chamber, the roof having a moving portion (114) comprising a thermal bend actuator (115) for ejecting ink through the nozzle opening, said thermal bend actuator comprising:
a first active element (110) for connection to drive circuitry; and

a second passive element (116) mechanically cooperating with the first element, such that when a current is passed through the first element, the first element expands relative to the second element, resulting in bending of the actuator towards a floor of said nozzle chamber,

wherein a hydrophobic polymeric material (80) is coated on a nozzle plate of the printhead, said nozzle plate being at least partially defined by the roof of each nozzle chamber, and characterised in that an ink ejection face is coated with a hydrophobic polymeric material selected from the group consisting of: polysilsesquioxanes.
The printhead of claim 1, wherein said hydrophobic polymeric material is selected from the group consisting of: poly(alkylsilsesquioxanes) and poly(arylsilsesquioxanes)
The printhead of claim 1, wherein said polymeric material is selected from the group consisting of: poly(methylsilsesquioxane) and poly(phenylsilsesquioxane).
The printhead of claim 1, wherein each roof has a hydrophobic outside surface relative to the inside surfaces of each nozzle chamber by virtue of said hydrophobic coating.
The printhead of claim 1, wherein each nozzle chamber comprises a roof and sidewalls comprised of a ceramic material.
The printhead of claim 5, wherein the ceramic material is selected from the group consisting of: silicon nitride, silicon oxide and silicon oxynitride.
The printhead of claim 1, wherein said roof is spaced apart from the substrate, such that sidewalls of each nozzle chamber extend between said nozzle plate and said substrate.
The printhead of claim 1, wherein said nozzle opening is defined in said moving portion of said roof.
The printhead of claim 1, wherein said nozzle opening is defined in a stationary portion of said roof.
The printhead of claim 1, wherein said polymeric material defines a mechanical seal between said moving portion and a stationary portion of said roof, thereby minimizing ink leakage during actuation of said actuator.