Method of fabricating printhead using metal film for protecting hydrophobic ink ejection face

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 nozzle plate having relatively hydrophilic nozzle surface, the nozzle surface at least partially defining the ink ejection face of the printhead; (b) defining a plurality of nozzle openings in the nozzle plate; (c) depositing a hydrophobic polymeric layer onto the nozzle surface; (d) depositing a protective metal film onto the polymeric layer; (e) subjecting the printhead to an oxidizing plasma; and (f) removing the protective metal film, thereby providing a printhead having a relatively hydrophobic ink ejection face. Step (b) may be performed immediately after any of steps (a), (c) or (d).

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 ink jet 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 ink jet printing appears to date back to at least 1929 wherein U.S. Pat. No. 1,941,001 by Hansell discloses a simple form of continuous stream electro-static ink jet printing.

U.S. Pat. No. 3,596,275 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 U.S. Pat. No. 3,373,437 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 U.S. Pat. No. 3,946,398 (1970) which utilizes a diaphragm mode of operation, by Zolten in U.S. Pat. No. 3,683,212 (1970) which discloses a squeeze mode of operation of a piezoelectric crystal, Stemme in U.S. Pat. No. 3,747,120 (1972) discloses a bend mode of piezoelectric operation, Howkins in U.S. Pat. No. 4,459,601 discloses a piezoelectric push mode actuation of the ink jet stream and Fischbeck in U.S. Pat. No. 4,584,590 which discloses a shear mode type of piezoelectric transducer element.

Recently, thermal inkjet 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 U.S. Pat. No. 4,490,728. 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 oxidizing plasma, such as 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 nozzle plate having relatively hydrophilic nozzle surface, said nozzle surface at least partially defining the ink ejection face of the printhead;

(b) defining a plurality of nozzle openings in at least said nozzle plate;

(c) depositing a hydrophobic polymeric layer onto the nozzle surface;

(d) depositing a protective metal film onto at least said polymeric layer;

(e) subjecting said printhead to an oxidizing plasma; and

(f) removing said protective metal film,

thereby providing a printhead having a relatively hydrophobic ink ejection face, wherein step (b) is performed immediately after any of steps (a), (c) or (d).

Optionally, step (c) comprises the sub-steps of:(c)(i) depositing the hydrophobic polymeric layer onto the nozzle surface; and(c)(ii) photopatterning said polymeric layer so as to define a plurality of nozzle openings in said polymeric layer.

Optionally, photopatterning comprises UV-curing at least some of said polymeric material.

Optionally, step (d) comprises the sub-steps of:(d)(i) depositing a protective metal film onto at least said polymeric layer; and(d)(ii) defining a plurality of film openings in said metal film, said film openings being aligned with said nozzle openings.

Optionally, sub-step (d)(ii) comprises the further sub-steps of:(d)(ii)(1) depositing a mask on said protective metal film;(d)(ii)(2) patterning said mask so as to unmask said metal film in a plurality of film opening regions; and(d)(ii)(3) etching said unmasked nozzle opening regions to define said plurality of film openings.

Optionally, step (b) is performed immediately after step (c), and step (b) comprises: defining a plurality of nozzle openings in said nozzle plate and in said polymeric layer.

Optionally, said protective metal film is comprised of a metal selected from the group comprising: titanium and aluminium.

Optionally, said protective metal film has a thickness in the range of 10 nm to 1000 nm.

Optionally, step (f) is performed by wet or dry etching.

Optionally, step (f) is performed by a wet rinse using peroxide or acid.

Optionally, all plasma oxidizing steps are performed prior to removing said protective metal film in step (f).

Optionally, all backside MEMS processing steps are performed prior to removing said protective metal film in step (f).

Optionally, said backside MEMS processing steps include defining ink supply channels from a backside of said wafer, said backside being an opposite face to said ink ejection face.

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 ashing said photoresist scaffold prior to removing said protective metal film.

Optionally, oxidizing plasma is an oxygen ashing plasma.

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

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

Optionally, said hydrophobic polymeric layer is comprised of a polymeric material 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).

In a further aspect the present invention provides a printhead obtained or obtainable by a method comprising the steps of:(a) providing a partially-fabricated printhead comprising a plurality of nozzle chambers and a nozzle plate having relatively hydrophilic nozzle surface, said nozzle surface at least partially defining the ink ejection face of the printhead;(b) defining a plurality of nozzle openings in at least said nozzle plate;(c) depositing a hydrophobic polymeric layer onto the nozzle surface;(d) depositing a protective metal film onto at least said polymeric layer;(e) subjecting said printhead to an oxidizing plasma; and(f) removing said protective metal film,
thereby providing a printhead having a relatively hydrophobic ink ejection face, wherein step (b) is performed immediately after any of steps (a), (c) or (d).

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.

Referring toFIG. 1, there is shown a part of printhead comprising a plurality of nozzle assemblies.FIGS. 2 and 3show one of these nozzle assemblies in side-section and cutaway perspective views.

Each nozzle assembly comprises a nozzle chamber24formed by MEMS fabrication techniques on a silicon wafer substrate2. The nozzle chamber24is defined by a roof21and sidewalls22which extend from the roof21to the silicon substrate2. As shown inFIG. 1, each roof is defined by part of a nozzle surface56, which spans across an ejection face of the printhead. The nozzle surface56and sidewalls22are formed of the same material, which is deposited by PECVD over a sacrificial scaffold of photoresist during MEMS fabrication. Typically, the nozzle surface56and sidewalls22are 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 chambers24by capillary action. However, the exterior (ink ejection) surface of the nozzle surface56is also hydrophilic, which causes any flooded ink on the surface to spread.

Returning to the details of the nozzle chamber24, it will be seen that a nozzle opening26is defined in a roof of each nozzle chamber24. Each nozzle opening26is generally elliptical and has an associated nozzle rim25. The nozzle rim25assists with drop directionality during printing as well as reducing, at least to some extent, ink flooding from the nozzle opening26. The actuator for ejecting ink from the nozzle chamber24is a heater element29positioned beneath the nozzle opening26and suspended across a pit8. Current is supplied to the heater element29via electrodes9connected to drive circuitry in underlying CMOS layers5of the substrate2. When a current is passed through the heater element29, it rapidly superheats surrounding ink to form a gas bubble, which forces ink through the nozzle opening. By suspending the heater element29, it is completely immersed in ink when the nozzle chamber24is primed. This improves printhead efficiency, because less heat dissipates into the underlying substrate2and more input energy is used to generate a bubble.

As seen most clearly inFIG. 1, the nozzles are arranged in rows and an ink supply channel27extending longitudinally along the row supplies ink to each nozzle in the row. The ink supply channel27delivers ink to an ink inlet passage15for each nozzle, which supplies ink from the side of the nozzle opening26via an ink conduit23in the nozzle chamber24.

The MEMS fabrication process for manufacturing such printheads was described in detail in our previously filed U.S. application Ser. No. 11/246,684 filed on Oct. 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.

FIGS. 4 and 5show a partially-fabricated printhead comprising a nozzle chamber24encapsulating sacrificial photoresist10(“SAC1”) and16(“SAC2”). The SAC1photoresist10was used as a scaffold for deposition of heater material to form the suspended heater element29. The SAC2photoresist16was used as a scaffold for deposition of the sidewalls22and roof21(which defines part of the nozzle surface56).

In the prior art process, and referring toFIGS. 6 to 8, the next stage of MEMS fabrication defines the elliptical nozzle rim25in the roof21by etching away 2 microns of roof material20. This etch is defined using a layer of photoresist (not shown) exposed by the dark tone rim mask shown inFIG. 6. The elliptical rim25comprises two coaxial rim lips25aand25b, positioned over their respective thermal actuator29.

Referring toFIGS. 9 to 11, the next stage defines an elliptical nozzle aperture26in the roof21by etching all the way through the remaining roof material, which is bounded by the rim25. This etch is defined using a layer of photoresist (not shown) exposed by the dark tone roof mask shown inFIG. 9. The elliptical nozzle aperture26is positioned over the thermal actuator29, as shown inFIG. 11.

With all the MEMS nozzle features now fully formed, the next stage removes the SAC1and SAC2photoresist layers10and16by O2plasma ashing (FIGS. 12 and 13).FIGS. 14 and 15show the entire thickness (150 microns) of the silicon wafer2after ashing the SAC1and SAC2photoresist layers10and16.

Referring toFIGS. 16 to 18, once frontside MEMS processing of the wafer is completed, ink supply channels27are etched from the backside of the wafer to meet with the ink inlets15using a standard anisotropic DRIE. This backside etch is defined using a layer of photoresist (not shown) exposed by the dark tone mask shown inFIG. 16. The ink supply channel27makes a fluidic connection between the backside of the wafer and the ink inlets15.

Finally, and referring toFIGS. 2 and 3, the wafer is thinned to about 135 microns by backside etching.FIG. 1shows 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 channel27extending along its length and supplying ink to a plurality of ink inlets15in each row. The ink inlets, in turn, supply ink to the ink conduit23for 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 surface56being 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 surface56has a hydrophobic polymer deposited thereon immediately after the nozzle opening etch (i.e. at the stage represented inFIGS. 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 O2or an H2ashing 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 O2or H2ashing. 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 O2plasma, 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 toFIG. 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 SAC1and SAC2ashing (as shown inFIGS. 12 and 13), at this stage a thin layer (ca 1 micron) of hydrophobic polymeric material100is spun onto the nozzle surface56, as shown inFIGS. 19 and 20.

After deposition, this layer of polymeric material is photopatterned so as to remove the material deposited within the nozzle openings26. Photopatterning may comprise exposure of the polymeric layer100to UV light, except for those regions within the nozzle openings26. Accordingly, as shown inFIGS. 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 withFIGS. 12 to 18. Significantly, the hydrophobic polymer100is not removed by the O2ashing steps used to remove the photoresist scaffold10and16.

Hydrophobic Polymer Coating Prior to Nozzle Etch with Polymer Used as Etch Mask

As an alternative process, the hydrophobic polymer layer100is deposited immediately after the stage represented byFIGS. 7 and 8. Accordingly, the hydrophobic polymer is spun onto the nozzle surface after the rim25is defined by the rim etch, but before the nozzle opening26is defined by the nozzle etch.

Referring toFIGS. 23 and 24, there is shown a nozzle assembly after deposition of the hydrophobic polymer100. The polymer100is then photopatterned so as to remove the material bounded by the rim25in the nozzle opening region, as shown inFIGS. 25 and 26. Hence, the hydrophobic polymeric material100can now act as an etch mask for etching the nozzle opening26.

The nozzle opening26is defined by etching through the roof structure21, which is typically performed using a gas chemistry comprising O2and a fluorinated hydrocarbon (e.g. CF4or 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 roof21can 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 opening26.

Once the roof21is etched to define the nozzle opening, the nozzle assembly24is as shown inFIGS. 21 and 22. Accordingly, subsequent MEMS processing steps can proceed analogously to the steps described in connection withFIGS. 12 to 18. Significantly, the hydrophobic polymer100is not removed by the O2ashing steps used to remove the photoresist scaffold10and16.

Hydrophobic Polymer Coating Prior to Nozzle Etch with Additional Photoresist Mask

FIGS. 25 and 26illustrate how the hydrophobic polymer100may be used as an etch mask for a nozzle opening etch. Typically, different etch rates between the polymer100and the roof21, 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 polymer100shown inFIG. 24, which enables conventional downstream MEMS processing. Having photopatterned this top layer of resist, the hydrophobic polymer100and the roof21may 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/O2first etches through the hydrophobic polymer100and then through the roof21.

Subsequent O2ashing may be used to remove just the top layer of photoresist (to obtain the nozzle assembly shown inFIGS. 10 and 11), or prolonged O2ashing may be used to remove both the top layer of photoresist and the sacrificial photoresist layers10and16(to obtain the nozzle assembly shown inFIGS. 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 O2and H2ashing, the present inventors have provided a viable means for providing a hydrophobic nozzle surface in an inkjet printhead fabrication process.

Metal Film for Protecting Hydrophobic Polymer Layer

We have described hereinabove three alternative modifications of a printhead fabrication process which result in the ink ejection face of a printhead being defined by a hydrophobic polymer layer.

As already described above, the modification relies on the resistance of certain polymeric materials to standard ashing conditions using, for example, an oxygen plasma. This characteristic of certain polymers allows final ashing steps to be performed without removing the hydrophobic coating on the nozzle plate. However, there remains the possibility of such materials being imperfectly resistant to ashing, particularly aggressive ashing conditions that are typical of final-stage MEMS processing of printheads. Furthermore, there is the possibility that some hydrophobic polymers do not fully recover their hydrophobicity after ashing, which is undesirable given that the purpose of modifying the printhead fabrication process is to maximize the hydrophobicity of the ink ejection face.

It would therefore be desirable to provide an improved process, whereby hydrophobic polymers that are imperfectly resistant to ashing may still be used to hydrophobize an ink ejection face of a printhead. This would expand the range of materials available for use in hydrophobizing printheads. It would further be desirable to maximize the hydrophobicity of the ink ejection face without relying on hydrophobic materials recovering their hydrophobicity post-ashing.

In an improved hydrophobizing modification, the hydrophobic polymeric layer is protected with a thin metal film e.g. titanium or aluminium. The thin metal film protects the hydrophobic layer from late-stage oxygen ashing conditions, and is removed in a final post-ashing step, typically using a peroxide or acid rinse e.g. H2O2or HF rinse. An advantage of this process is that the polymer used for hydrophobizing the ink ejection face is not exposed to aggressive ashing conditions and retains its hydrophobic characteristics throughout the MEMS processing steps.

It will be appreciated that the metal film may be used to protect the hydrophobic polymer layer in any of the three alternatives described above for hydrophobizing the printhead. By way of example, the process outlined in connection withFIGS. 19 to 22will now be described with a protective metal film modification.

Referring then toFIGS. 19 to 22, printhead fabrication proceeds exactly as detailed in these drawings. In other words, a thin layer (ca 1 micron) of hydrophobic polymeric material100is spun onto the nozzle surface56, as shown inFIGS. 19 and 20. After deposition, this layer of polymeric material is photopatterned so as to remove the material deposited within the nozzle openings26. Photopatterning may comprise exposure of the polymeric layer100to UV light, except for those regions within the nozzle openings26. Accordingly, as shown inFIGS. 21 and 22, the printhead now has a hydrophobic nozzle surface with no hydrophobic material positioned within the nozzle openings26.

Turning toFIG. 36, the next stage comprises deposition of a thin film (ca 100 nm) of metal110onto the polymeric layer100. After deposition, the metal may be removed from within the nozzle opening26by standard metal etch techniques. For example, a conventional photoresist layer (not shown) may be exposed and developed, as appropriate, and used as an etch mask for etching the metal film110. Any suitable etch may be used, such as RIE using a chlorine-based gas chemistry.

FIG. 37shows the partially-fabricated printhead after etching the metal film110. It will be seen that the hydrophobic polymer layer100is completely encapsulated by the metal film110and therefore protected from any aggressive late-stage ashing.

Subsequent MEMS processing steps can proceed analogously to the steps described in connection withFIGS. 12 to 18. Significantly, the hydrophobic polymer100is not removed by the O2ashing steps used to remove the photoresist scaffold10and16, because it is protected by the metal film110.

After O2ashing, the metal film is removed by a brief H2O2or HF rinse, thereby revealing the hydrophobic polymer layer100in the completed printhead.

FIGS. 10 to 13show frontside ashing of the wafer to remove all photoresist from within the nozzle chambers. In this case, it is of course necessary to define openings in the protective metal layer110so that the oxygen plasma can access the photoresist.

FIG. 38exemplifies an alternative sequence of MEMS processing steps, which makes use of backside ashing and avoids defining openings in the protective metal layer110. The wafer shown inFIG. 36is subjected to backside MEMS processing so as to define ink supply channels27from the backside of the wafer. The resultant wafer is shown inFIG. 38. Once ink supply channels27are defined from the backside, then backside ashing can be performed to remove all frontside photoresist, including the scaffolds10and16. The hydrophobic polymer layer100still enjoys protection from the ashing plasma. With the photoresist removed, the protective metal film110can simply be rinsed off with H2O2or HF to provide the wafer shown inFIG. 17, except with a hydrophobic polymer layer covering the nozzle plate.

Of course, it will be appreciated that metal film protection of the polymer layer100may be performed prior to the nozzle opening etch. In this scenario, the metal film110, the polymer layer100and the nozzle roof may be etched in simultaneous or sequential etching steps, using a top conventional photoresist layer as a common mask for each etch. Regardless, the polymer layer100still benefits from protection by the metal film110in subsequent ashing steps.

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 assembly400having a moving roof structure was described in our previously filed U.S. application Ser. No. 11/607,976 filed on Dec. 4, 2006 (the contents of which is herein incorporated by reference) and is shown here inFIGS. 27 to 30. The nozzle assembly400comprises a nozzle chamber401formed on a passivated CMOS layer402of a silicon substrate403. The nozzle chamber is defined by a roof404and sidewalls405extending from the roof to the passivated CMOS layer402. Ink is supplied to the nozzle chamber401by means of an ink inlet406in fluid communication with an ink supply channel407receiving ink from a backside of the silicon substrate. Ink is ejected from the nozzle chamber401by means of a nozzle opening408defined in the roof404. The nozzle opening408is offset from the ink inlet406.

As shown more clearly inFIG. 28, the roof404has a moving portion409, which defines a substantial part of the total area of the roof. Typically, the moving portion409defines at least 50% of the total area of the roof404. In the embodiment shown inFIGS. 27 to 30, the nozzle opening408and nozzle rim415are defined in the moving portion409, such that the nozzle opening and nozzle rim move with the moving portion.

The nozzle assembly400is characterized in that the moving portion409is defined by a thermal bend actuator410having a planar upper active beam411and a planar lower passive beam412. Hence, the actuator410typically defines at least 50% of the total area of the roof404. Correspondingly, the upper active beam411typically defines at least 50% of the total area of the roof404.

As shown inFIGS. 27 and 28, at least part of the upper active beam411is spaced apart from the lower passive beam412for maximizing thermal insulation of the two beams. More specifically, a layer of Ti is used as a bridging layer413between the upper active beam411comprised of TiN and the lower passive beam412comprised of SiO2. The bridging layer413allows a gap414to be defined in the actuator410between the active and passive beams. This gap414improves the overall efficiency of the actuator410by minimizing thermal transfer from the active beam411to the passive beam412.

However, it will of course be appreciated that the active beam411may, alternatively, be fused or bonded directly to the passive beam412for improved structural rigidity. Such design modifications would be well within the ambit of the skilled person.

The active beam411is connected to a pair of contacts416(positive and ground) via the Ti bridging layer. The contacts416connect with drive circuitry in the CMOS layers.

When it is required to eject a droplet of ink from the nozzle chamber401, a current flows through the active beam411between the two contacts416. The active beam411is rapidly heated by the current and expands relative to the passive beam412, thereby causing the actuator410(which defines the moving portion409of the roof404) to bend downwards towards the substrate403. Since the gap460between the moving portion409and a static portion461is so small, surface tension can generally be relied up to seal this gap when the moving portion is actuated to move towards the substrate403.

The movement of the actuator410causes ejection of ink from the nozzle opening408by a rapid increase of pressure inside the nozzle chamber401. When current stops flowing, the moving portion409of the roof404is allowed to return to its quiescent position, which sucks ink from the inlet406into the nozzle chamber401, in readiness for the next ejection.

Turning toFIG. 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 U.S. application Ser. No. 10/854,491 filed on May 27, 2004 and Ser. No. 11/014,732 filed on Dec. 20, 2004, the contents of which are herein incorporated by reference.

An alternative nozzle assembly500shown inFIGS. 31 to 33is similar to the nozzle assembly400insofar as a thermal bend actuator510, having an upper active beam511and a lower passive beam512, defines a moving portion of a roof504of the nozzle chamber501.

However, in contrast with the nozzle assembly400, the nozzle opening508and rim515are not defined by the moving portion of the roof504. Rather, the nozzle opening508and rim515are defined in a fixed or static portion561of the roof504such that the actuator510moves 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 gap560, between the moving portion509and the static portion561, is relied up to create a fluidic seal during actuation by using the surface tension of the ink.

The nozzle assemblies400and500, 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 toFIG. 34, it will be seen that the nozzle assembly400previously shown inFIG. 27now has an additional layer of hydrophobic polymer101(as described in detail above) coated on the roof, including both the moving409and static portions461of the roof. Importantly, the hydrophobic polymer101seals the gap460shown inFIG. 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 portion409and a static roof portion461, ink is fully sealed inside the nozzle chamber401and cannot escape, other than via the nozzle opening408, during actuation.

FIG. 35shows the nozzle assembly500with a hydrophobic polymer coating101. By analogy with the nozzle assembly400, it will be appreciated that by sealing the gap560with the polymer101, a mechanical seal562is formed which provides excellent mechanical sealing of ink in the nozzle chamber501.

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.