WO2008109910A1

WO2008109910A1 - Method of fabricating printhead having hydrophobic ink ejection face

Info

Publication number: WO2008109910A1
Application number: PCT/AU2007/000303
Authority: WO
Inventors: Gregory John Mcavoy; Kia Silverbrook; Emma Rose Kerr; Misty Bagnat; Vincent Patrick Lawlor
Original assignee: Silverbrook Research Pty Ltd
Priority date: 2007-03-12
Filing date: 2007-03-12
Publication date: 2008-09-18
Also published as: TW200836932A; CA2675856A1; CN101610909B; CN101610909A; TWI419794B; TW200836930A; EP2121330A1; SG176493A1; EP2121330A4; TWI403420B; JP5205396B2; CA2675856C; JP2010520080A

Abstract

A method of fabricating a printhead having a hydrophobic ink ejection face is provided. The method comprises the steps of: (a) providing a partially-fabricated printhead comprising a plurality of nozzle chambers and a relatively hydrophilic nozzle surface, the nozzle surface at least partially defining the ink ejection face; (b) depositing a layer of relatively hydrophobic polymeric material onto the nozzle surface, the polymeric material being resistant to removal by ashing; and (c) defining a plurality of nozzle openings in the nozzle surface, thereby providing a printhead having a relatively hydrophobic ink ejection face. Steps (b) and (c) may be performed in any order.

Description

METHOD OF FABRICATING PRINTHEAD HAVING HYDROPHOBIC INK EJECTION

FACE

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 reliability 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 inkjet 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 inkjet printing including the step wherein the inkjet 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 inkjet printers are also one form of commonly utilized inkjet 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 inkjet 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 electro-thermal 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.

However, whilst hydrophobic front faces and hydrophilic ink chambers are desirable, there is a major problem in fabricating such printheads by MEMS techniques. The final stage of MEMS printhead fabrication is typically ashing of photoresist using an oxygen plasma. However, organic, hydrophobic materials deposited onto the front face are typically removed by the ashing process to leave a hydrophilic surface. Moreover, a problem with post-ashing vapour deposition of hydrophobic materials is that the hydrophobic material will be deposited inside nozzle chambers as well as on the front face of the printhead. The nozzle chamber walls become hydrophobized, which is highly undesirable in terms of generating a positive ink pressure biased towards the nozzle chambers. This is a conundrum, which creates significant demands on printhead fabrication.

Accordingly, it would be desirable to provide a printhead fabrication process, in which the resultant printhead has improved surface characteristics, without comprising the surface characteristics of nozzle chambers. It would further be desirable to provide a printhead fabrication process, in which the resultant printhead has a hydrophobic front face in combination with hydrophilic nozzle chambers.

Summary of the Invention

In a first aspect the present invention provides a method of fabricating a printhead having a hydrophobic ink ejection face, the method comprising the steps of:

(a) providing a partially- fabricated printhead comprising a plurality of nozzle chambers and a relatively hydrophilic nozzle surface, said nozzle surface at least partially defining the ink ejection face; (b) depositing a layer of relatively hydrophobic polymeric material onto the nozzle surface, said polymeric material being resistant to removal by ashing; and

(c) defining a plurality of nozzle openings in said nozzle surface, thereby providing a printhead having a relatively hydrophobic ink ejection face, wherein steps (b) and (c) are performed in any order.

Optionally, step (c) is performed prior to step (b), and the method comprises the further step of defining a corresponding plurality of aligned nozzle openings in said deposited polymeric material.

Optionally, said corresponding plurality of aligned nozzle openings are defined by photopatterning said polymeric material. Optionally, step (c) is performed after step (b), and said polymeric material is used as a mask for etching said nozzle surface.

Optionally, said polymeric material is photopatterned to define a plurality of nozzle opening regions prior to etching said nozzle surface.

Optionally, (c) is performed after step (b), and step (c) comprises the steps of: depositing a mask on said polymeric material; patterning said mask so as to unmask said polymeric material in a plurality of nozzle opening regions; etching said unmasked polymeric material and said underlying nozzle surface to define the plurality of nozzle openings; and removing said mask.

Optionally, said mask is photoresist, and said photoresist is removed by ashing.

Optionally, a same gas chemistry is used to etch said polymeric material and said nozzle surface.

Optionally, said gas chemistry comprises O₂ and a fluorine-containing compound.

Optionally, in said partially- fabricated printhead, a roof of each nozzle chamber is supported by a sacrificial photoresist scaffold, said method further comprising the step of removing said photoresist scaffold by ashing.

Optionally, a roof of each nozzle chamber is defined at least partially by said nozzle surface.

Optionally, said nozzle surface is spaced apart from a substrate, such that sidewalls of each nozzle chamber extend between said nozzle surface and said substrate.

Optionally, a roof and sidewalls of each nozzle chamber are comprised of a ceramic material depositable by CVD.

Optionally, said roof and sidewalls are comprised of a material selected from the group comprising: silicon oxide, silicon nitride and silicon oxynitride. Optionally, said hydrophobic polymeric material forms a passivating surface oxide in an O₂ plasma.

Optionally, said hydrophobic polymeric material recovers its hydrophobicity after being subjected to an O₂ plasma.

Optionally, said polymeric material is selected from the group comprising: polymerized siloxanes and fluorinated polyolefins.

Optionally, said polymeric material is selected from the group comprising: polydimethylsiloxane (PDMS) and perfluorinated polyethylene (PFPE).

Optionally, at least some of said polymeric material is UV-cured after deposition.

In a further aspect the present invention provides a printhead obtained or obtainable by the method of the present invention.

In a second aspect the present invention provides a printhead having an ink ejection face, wherein at least part of the ink ejection face is coated with a hydrophobic polymeric material selected from the group comprising: polymerized siloxanes and fluorinated polyolefins.

Optionally, said polymeric material is resistant to removal by ashing.

Optionally, said polymeric material forms a passivating surface oxide in an oxygen plasma.

Optionally, said polymeric material recovers its hydrophobicity after being subjected to an oxygen plasma.

Optionally, the polymeric material is selected from the group comprising: polydimethylsiloxane (PDMS) and perfluorinated polyethylene (PFPE).

In a further aspect the present invention provides a printhead comprising 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, Optionally, a nozzle surface, having the hydrophobic polymer coated thereon, at least partially defines the ink ejection face.

Optionally, each roof defines at least part of the nozzle surface of the printhead, each roof having a hydrophobic outside surface relative to the inside surfaces of each nozzle chamber by virtue of said hydrophobic coating.

Optionally, at least part of the ink ejection face has a contact angle of more than 90° and the inside surfaces of the nozzle chambers have a contact angle of less than 90°.

Optionally, each nozzle chamber comprises a roof and sidewalls comprised of a ceramic material.

Optionally, the ceramic material is selected from the group comprising: silicon nitride, silicon oxide and silicon oxynitride.

Optionally, said roof is spaced apart from a substrate, such that sidewalls of each nozzle chamber extend between said nozzle surface and said substrate.

Optionally, the ink ejection face is hydrophobic relative to ink supply channels in the printhead.

Optionally, said actuator is a heater element configured for heating ink in said chamber so as to form a gas bubble, thereby forcing a droplet of ink through said nozzle opening.

Optionally, said heater element is suspended in said nozzle chamber.

Optionally, said 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, said thermal bend actuator defines at least part of a roof of each nozzle chamber, whereby actuation of said actuator moves said actuator towards a floor of said nozzle chamber.

Optionally, said nozzle opening is defined in said actuator or in a static portion of said roof. Optionally, said hydrophobic polymeric material defines a mechanical seal between said actuator and a static portion of said roof, thereby minimizing ink leakage during actuation

Optionally, said hydrophobic polymeric material has a Young's modulus of less than 1000 MPa.

In a third aspect the present invention provides a nozzle assembly for an inkjet printhead, said nozzle assembly comprising: a nozzle chamber having a roof, said roof having a moving portion moveable relative to a static portion and a nozzle opening defined in said roof, such that movement of said moving portion relative to said static portion causes ejection of ink through the nozzle opening; an actuator for moving said moving portion relative to said static portion; and a mechanical seal interconnecting said moving portion and said static portion, wherein said mechanical seal comprises a polymeric material selected from the group comprising: polymerized siloxanes and fluorinated polyolefins.

Optionally, said nozzle opening is defined in said moving portion.

Optionally, said nozzle opening is defined in said static portion.

Optionally, said first and second elements are cantilever beams.

Optionally, said thermal bend actuator defines at least part of the moving portion of said roof, whereby actuation of said actuator moves said actuator towards a floor of said nozzle chamber.

Optionally, the polymeric material has a Young's modulus of less than 1000 MPa.

Optionally, said polymeric material is hydrophobic and is resistant to removal by ashing. Optionally, said polymeric material recovers its hydrophobicity after being subjected to an O₂ plasma.

Optionally, the polymeric material is coated on the whole of said roof, such that an ink ejection face of said printhead is hydrophobic.

Optionally, each roof forms at least part of a nozzle surface of the printhead, each roof having a hydrophobic outside surface relative to the inside surfaces of each nozzle chamber by virtue of said polymeric coating.

Optionally, said polymeric coating has a contact angle of more than 90° and the inside surfaces of the nozzle chambers have a contact angle of less than 90°.

Optionally, said polymeric has a contact angle of more than 110°.

Optionally, inside surfaces of said nozzle chamber have a contact angle of less than 70°.

Optionally, said nozzle chamber comprises sidewalls extending between said roof and a substrate, such that said roof is spaced apart from said substrate.

Optionally, said roof and said sidewalls are comprised of a ceramic material depositable by CVD.

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 10 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 photopatterning of the polymeric coating; Figure 22 is a perspective of the nozzle assembly shown in Figure 21 ;

Figure 23 shows the nozzle assembly of Figure 7 after deposition of a hydrophobic polymeric coating;

Figure 24 is a perspective of the nozzle assembly shown in Figure 23;

Figure 25 shows the nozzle assembly of Figure 23 after photopatterning of the polymeric coating;

Figure 26 is a perspective of the nozzle assembly shown in Figure 25;

Figure 27 is a side sectional view of an inkjet nozzle assembly comprising a roof having a moving portion defined by a thermal bend actuator;

Figure 28 is a cutaway perspective view of the nozzle assembly shown in Figure 27; Figure 29 is a perspective view of the nozzle assembly shown in Figure 27;

Figure 30 is a cutaway perspective view of an array of the nozzle assemblies shown in Figure 27;

Figure 31 is a side sectional view of an alternative inkjet nozzle assembly comprising a roof having a moving portion defined by a thermal bend actuator; Figure 32 is a cutaway perspective view of the nozzle assembly shown in Figure 31 ;

Figure 33 is a perspective view of the nozzle assembly shown in Figure 31; Figure 34 shows the nozzle assembly of Figure 27 with a polymeric coating on the roof forming a mechanical seal between a moving roof portion and a static roof portion; and

Figure 35 shows the nozzle assembly of Figure 31 with a polymeric coating on the roof forming a mechanical seal between a moving roof portion and a static roof portion.

Description of Optional Embodiments

The present invention may be used with any type of printhead. The present Applicant has previously described a plethora of inkjet printheads. It is not necessary to describe all such printheads here for an understanding of the present invention. However, the present invention will now be described in connection with a thermal bubble- forming inkjet printhead and a mechanical thermal bend actuated inkjet printhead. Advantages of the present invention will be readily apparent from the discussion that follows.

Thermal Bubble-Forming InkJet Printhead

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 contents of which is herein incorporated by reference. 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 ("SACl") and 16 ("SAC2"). The SACl 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 SACl 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 SACl 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.

Nozzle Etch Followed by Hydrophobic Polymer Coating

As an alternative to the process described above, the nozzle surface 56 has a hydrophobic polymer deposited thereon immediately after the nozzle opening etch (i.e. at the stage represented in Figures 10 and 11). Since the photoresist scaffold layers must be subsequently removed, the polymeric material should be resistant to the ashing process. Preferably, the polymeric material should be resistant to removal by an O₂ or an H₂ ashing plasma. The Applicant has identified a family of polymeric materials which meet the above-mentioned requirements of being hydrophobic whilst at the same time being resistant to O₂ or H₂ ashing. These materials are typically polymerized siloxanes or fluorinated polyolefins. More specifically, polydimethylsiloxane (PDMS) and perfluorinated polyethylene (PFPE) have both been shown to be particularly advantageous. Such materials form a passivating surface oxide in an O₂ plasma, and subsequently recover their hydrophobicity relatively quickly. A further advantage of these materials is that they have excellent adhesion to ceramics, such as silicon dioxide and silicon nitride. A further advantage of these materials is that they are photopatternable, which makes them particularly suitable for use in a MEMS process. For example, PDMS is curable with UV light, whereby unexposed regions of PDMS can be removed relatively easily.

Referring to Figure 10, there is shown a nozzle assembly of a partially- fabricated printhead after the rim and nozzle etches described earlier. However, instead of proceeding with SACl and SAC2 ashing (as shown in Figures 12 and 13), at this stage a thin layer (ca 1 micron) of hydrophobic polymeric material 100 is spun onto the nozzle surface 56, as shown in Figures 19 and 20.

After deposition, this layer of polymeric material is photopatterned so as to remove the material deposited within the nozzle openings 26. Photopatterning may comprise exposure of the polymeric layer 100 to UV light, except for those regions within the nozzle openings 26. Accordingly, as shown in Figures 21 and 22, the printhead now has a hydrophobic nozzle surface, and subsequent MEMS processing steps can proceed analogously to the steps described in connection with Figures 12 to 18. Significantly, the hydrophobic polymer 100 is not removed by the O₂ ashing steps used to remove the photoresist scaffold 10 and 16.

Hydrophobic Polymer Coating Prior to Nozzle Etch With Polymer Used as Etch Mask As an alternative process, the hydrophobic polymer layer 100 is deposited immediately after the stage represented by Figures 7 and 8. Accordingly, the hydrophobic polymer is spun onto the nozzle surface after the rim 25 is defined by the rim etch, but before the nozzle opening 26 is defined by the nozzle etch.

Referring to Figures 23 and 24, there is shown a nozzle assembly after deposition of the hydrophobic polymer 100. The polymer 100 is then photopatterned so as to remove the material bounded by the rim 25 in the nozzle opening region, as shown in Figures 25 and 26. Hence, the hydrophobic polymeric material 100 can now act as an etch mask for etching the nozzle opening 26. The nozzle opening 26 is defined by etching through the roof structure 21, which is typically performed using a gas chemistry comprising O₂ and a fluorinated hydrocarbon (e.g. CF₄ or C₄F₈). Hydrophobic polymers, such as PDMS and PFPE, are normally etched under the same conditions. However, since materials such as silicon nitride etch much more rapidly, the roof 21 can be etched selectively using either PDMS or PFPE as an etch mask. By way of comparison, with a gas ratio of 3:1 (CF₄:O₂), silicon nitride etches at about 240 microns per hour, whereas PDMS etches at about 20 microns per hour. Hence, it will be appreciated that etch selectivity using a PDMS mask is achievable when defining the nozzle opening 26.

Once the roof 21 is etched to define the nozzle opening, the nozzle assembly 24 is as shown in Figures 21 and 22. Accordingly, subsequent MEMS processing steps can proceed analogously to the steps described in connection with Figures 12 to 18. Significantly, the hydrophobic polymer 100 is not removed by the O₂ ashing steps used to remove the photoresist scaffold 10 and 16.

Hydrophobic Polymer Coating Prior to Nozzle Etch With Additional Photoresist Mask

Figures 25 and 26 illustrate how the hydrophobic polymer 100 may be used as an etch mask for a nozzle opening etch. Typically, different etch rates between the polymer 100 and the roof 21, as discussed above, provides sufficient etch selectivity. However, as a further alternative and particularly to accommodate situations where there is insufficient etch selectivity, a layer of photoresist (not shown) may be deposited over the hydrophobic polymer 100 shown in Figure 24, which enables conventional downstream MEMS processing. Having photopatterned this top layer of resist, the hydrophobic polymer 100 and the roof 21 may be etched in one step using the same gas chemistry, with the top layer of a photoresist being used as a standard etch mask. A gas chemistry of, for example, CF₄/O₂ first etches through the hydrophobic polymer 100 and then through the roof 21. Subsequent O₂ ashing may be used to remove just the top layer of photoresist (to obtain the nozzle assembly shown in Figures 10 and 11), or prolonged O₂ ashing may be used to remove both the top layer of photoresist and the sacrificial photoresist layers 10 and 16 (to obtain the nozzle assembly shown in Figures 12 and 13). The skilled person will be able to envisage other alternative sequences of MEMS processing steps, in addition to the three alternatives discussed herein. However, it will be appreciated that in identifying hydrophobic polymers capable of withstanding O₂ and H₂ ashing, the present inventors have provided a viable means for providing a hydrophobic nozzle surface in an inkjet printhead fabrication process.

Thermal Bend Actuator Printhead

Having discussed ways in which a nozzle surface of a printhead may be hydrophobized, it will be appreciated that any type of printhead may be hydrophobized in an analogous manner. However, the present invention realizes particular advantages in connection with the Applicant's previously described printhead comprising thermal bend actuator nozzle assemblies. Accordingly, a discussion of how the present invention may be used in such printheads now follows.

In a thermal bend actuated printhead, a nozzle assembly may comprise a nozzle chamber having a roof portion which moves relative to a floor portion of the chamber. The moveable roof portion is typically actuated to move towards the floor portion by means of a bi-layered thermal bend actuator. Such an actuator may be positioned externally of the nozzle chamber or it may define the moving part of the roof structure.

A moving roof is advantageous, because it lowers the drop ejection energy by only having one face of the moving structure doing work against the viscous ink. However, a problem with such moving roof structures is that it is necessary to seal the ink inside the nozzle chamber during actuation. Typically, the nozzle chamber relies on a fluidic seal, which forms a seal using the surface tension of the ink. However, such seals are imperfect and it would be desirable to form a mechanical seal which avoids relying on surface tension as a means for containing the ink. Such a mechanical seal would need to be sufficiently flexible to accommodate the bending motion of the roof. A typical nozzle assembly 400 having a moving roof structure was described in our previously filed US Application No. 11/607,976 filed on December 4, 2006 (the contents of which is herein incorporated by reference) and is shown here in Figures 27 to 30. The nozzle assembly 400 comprises a nozzle chamber 401 formed on a passivated CMOS layer 402 of a silicon substrate 403. The nozzle chamber is defined by a roof 404 and sidewalls 405 extending from the roof to the passivated CMOS layer 402. Ink is supplied to the nozzle chamber 401 by means of an ink inlet 406 in fluid communication with an ink supply channel 407 receiving ink from a backside of the silicon substrate. Ink is ejected from the nozzle chamber 401 by means of a nozzle opening 408 defined in the roof 404. The nozzle opening 408 is offset from the ink inlet 406.

As shown more clearly in Figure 28, the roof 404 has a moving portion 409, which defines a substantial part of the total area of the roof. Typically, the moving portion 409 defines at least 50% of the total area of the roof 404. In the embodiment shown in Figures 27 to 30, the nozzle opening 408 and nozzle rim 415 are defined in the moving portion 409, such that the nozzle opening and nozzle rim move with the moving portion.

The nozzle assembly 400 is characterized in that 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. Hence, the actuator 410 typically defines at least 50% of the total area of the roof 404.

Correspondingly, the upper active beam 411 typically defines at least 50% of the total area of the roof 404.

As shown in Figures 27 and 28, at least part of the upper active beam 411 is spaced apart from the lower passive beam 412 for maximizing thermal insulation of the two beams. More specifically, a layer of Ti is used as a bridging layer 413 between the upper active beam 411 comprised of TiN and the lower passive beam 412 comprised of SiO₂. The bridging layer 413 allows a gap 414 to be defined in the actuator 410 between the active and passive beams. This gap

414 improves the overall efficiency of the actuator 410 by minimizing thermal transfer from the active beam 411 to the passive beam 412. However, it will of course be appreciated that the active beam 411 may, alternatively, be fused or bonded directly to the passive beam 412 for improved structural rigidity. Such design modifications would be well within the ambit of the skilled person.

The active beam 411 is connected to a pair of contacts 416 (positive and ground) via the Ti bridging layer. The contacts 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. Since the gap 460 between the moving portion 409 and a static portion 461 is so small, surface tension can generally be relied up to seal this gap when the moving portion is actuated to move towards the substrate 403.

The 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. Turning to Figure 12, it will be readily appreciated that the nozzle assembly may be replicated into an array of nozzle assemblies to define a printhead or printhead integrated circuit. A printhead integrated circuit comprises a silicon substrate, an array of nozzle assemblies (typically arranged in rows) formed on the substrate, and drive circuitry for the nozzle assemblies. A plurality of printhead integrated circuits may be abutted or linked to form a pagewidth inkjet printhead, as described in, for example, Applicant's earlier US Application Nos. 10/854,491 filed on May 27, 2004 and 1 1/014,732 filed on December 20, 2004, the contents of which are herein incorporated by reference.

An alternative nozzle assembly 500 shown in Figures 31 to 33 is similar to the nozzle assembly 400 insofar as a thermal bend actuator 510, having an upper active beam 51 1 and a lower passive beam 512, defines a moving portion of a roof 504 of the nozzle chamber 501.

However, in contrast with the nozzle assembly 400, the nozzle opening 508 and rim 515 are not defined by the moving portion of the roof 504. Rather, the nozzle opening 508 and rim 515 are defined in a fixed or static portion 561 of the roof 504 such that the actuator 510 moves independently of the nozzle opening and rim during droplet ejection. An advantage of this arrangement is that it provides more facile control of drop flight direction. Again, the small dimensions of the gap 560, between the moving portion 509 and the static portion 561, is relied up to create a fluidic seal during actuation by using the surface tension of the ink.

The nozzle assemblies 400 and 500, and corresponding printheads, may be constructed using suitable MEMS processes in an analogous manner to those described above. In all cases the roof of the nozzle chamber (moving or otherwise) is formed by deposition of a roof material onto a suitable sacrificial photoresist scaffold.

Referring now to Figure 34, it will be seen that the nozzle assembly 400 previously shown in Figure 27 now has an additional layer of hydrophobic polymer 101 (as described in detail above) coated on the roof, including both the moving 409 and static portions 461 of the roof. Importantly, the hydrophobic polymer 101 seals the gap 460 shown in Figure 27. It is an advantage of polymers such as PDMS and PFPE that they have extremely low stiffness. Typically, these materials have a Young's modulus of less than 1000 MPa and typically of the order of about 500 MPa. This characteristic is advantageous, because it enables them to form a mechanical seal in thermal bend actuator nozzles of the type described herein - the polymer stretches elastically during actuation, without significantly impeding the movement of the actuator. Indeed, an elastic seal assists in the bend actuator returning to its quiescent position, which is when drop ejection occurs. Moreover, with no gap between a moving roof portion 409 and a static roof portion 461, ink is fully sealed inside the nozzle chamber 401 and cannot escape, other than via the nozzle opening 408, during actuation. Figure 35 shows the nozzle assembly 500 with a hydrophobic polymer coating 101. By analogy with the nozzle assembly 400, it will be appreciated that by sealing the gap 560 with the polymer 101, a mechanical seal 562 is formed which provides excellent mechanical sealing of ink in the nozzle chamber 501.

It will be appreciated by ordinary workers in this field that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A method of fabricating a printhead having a hydrophobic ink ejection face, the method comprising the steps of: (a) providing a partially- fabricated printhead comprising a plurality of nozzle chambers and a relatively hydrophilic nozzle surface, said nozzle surface at least partially defining the ink ejection face;

(b) depositing a layer of relatively hydrophobic polymeric material onto the nozzle surface, said polymeric material being resistant to removal by ashing; and (c) defining a plurality of nozzle openings in said nozzle surface, thereby providing a printhead having a relatively hydrophobic ink ejection face, wherein steps (b) and (c) are performed in any order.

2. The method of claim 1 , wherein step (c) is performed prior to step (b), and the method comprises the further step of defining a corresponding plurality of aligned nozzle openings in said deposited polymeric material.

3. The method of claim 2, wherein said corresponding plurality of aligned nozzle openings are defined by photopatterning said polymeric material.

4. The method of claim 1 , wherein step (c) is performed after step (b), and said polymeric material is used as a mask for etching said nozzle surface.

5. The method of claim 4, wherein said polymeric material is photopatterned to define a plurality of nozzle opening regions prior to etching said nozzle surface.

6. The method of claim 1, wherein (c) is performed after step (b), and step (c) comprises the steps of: depositing a mask on said polymeric material; patterning said mask so as to unmask said polymeric material in a plurality of nozzle opening regions; etching said unmasked polymeric material and said underlying nozzle surface to define the plurality of nozzle openings; and removing said mask.

7. The method of claim 6, wherein said mask is photoresist, and said photoresist is removed by ashing.

8. The method of claim 6, wherein a same gas chemistry is used to etch said polymeric material and said nozzle surface.

9. The method of claim 8, wherein said gas chemistry comprises O₂ and a fluorine-containing compound.

10. The method of claim 1 , wherein, in said partially- fabricated printhead, a roof of each nozzle chamber is supported by a sacrificial photoresist scaffold, said method further comprising the step of removing said photoresist scaffold by ashing.

11. The method of claim 1 , wherein a roof of each nozzle chamber is defined at least partially by said nozzle surface.

12. The method of claim 11, wherein said nozzle surface is spaced apart from a substrate, such that sidewalls of each nozzle chamber extend between said nozzle surface and said substrate.

13. The method of claim 1 , wherein a roof and sidewalls of each nozzle chamber are comprised of a ceramic material depositable by CVD.

14. The method of claim 13, wherein said roof and sidewalls are comprised of a material selected from the group comprising: silicon oxide, silicon nitride and silicon oxynitride.

15. The method of claim 1, wherein said hydrophobic polymeric material forms a passivating surface oxide in an O₂ plasma.

16. The method of claim 15, wherein said hydrophobic polymeric material recovers its hydrophobicity after being subjected to an O₂ plasma.

17. The method of claim 1, wherein said polymeric material is selected from the group comprising: polymerized siloxanes and fluorinated polyolefins.

18. The method of claim 17, wherein said polymeric material is selected from the group comprising: polydimethylsiloxane (PDMS) and perfluorinated polyethylene (PFPE).

19. The method of claim 1, wherein at least some of said polymeric material is UV-cured after deposition.

20. A printhead obtained or obtainable by the method according to claim 1.