INKJET PRINTHEADS

Abstract: Where material is to be removed from an element (4) of an inkjet printhead having a surface (6) intended to form part of the external surface of the printhead, at least part of the material being removed by ablation using a high energy beam directed at said surface, a protective layer (10) is applied to be in sealing engagement with the surface. The layer is believed to protect the surface from high energy free radicals liberated during the ablation process.

Description: INKJET PRINTHEADS The present invention relates to inkjet printheads, and in particular to the production of inkjet nozzles and other instances where removal of material from elements of inkjet printheads is required. Such material can usefully be removed, at least in part, by ablation. The process of ablation - also known as photoablative decomposition - is known: see, for example, R. Srinivasan, J. Vac. Sci. Technol. B, Vol 4, page 923 (1983). Ablation in the context of an inkjet printhead is known from EP-A-0 367 541. In particular, Figures 23-28 of this document disclose the manufacture by ablation of nozzles in an inkjet printer nozzle plate prior to assembly of the nozzle plate on the printhead body and before the application of any non-wetting coating to the nozzle plate. Such non-wetting coatings, which are well known in the art, have a low surface energy, are ink repellent and thus help to prevent the build-up of ink around the nozzle opening. The nozzle plate of EP-A-0 367 541 has in one arrangement an adhesive layer which is used to bond the nozzle plate to the printhead body. This adhesive layer is protected prior to bonding by a backing layer of polyether or Mylar . During the nozzle formation process, both the adhesive layer and backing tape are removed from those areas where the nozzles are formed. A similar adhesive layer and backing tape arrangement is known from W093/22141. EP-A-0 367 541 further discloses an alternative arrangement in which no adhesive layer is employed. The nozzle plate is carried on a backing layer and the face of the nozzle plate which is to be bonded to the print head body is covered by a protective layer. Ablation takes place through this protective layer which is removed prior to the bonding process. Ablation in the context of an inkjet printhead is also known from EP-A-0 576 007. According to this document, nozzles are formed in a nozzle plate after application of a non-wetting coating by ablation of the nozzle plate material using a laser beam which impinges on the opposite surface of the nozzle plate to that on which the non-wetting coating is located. A problem associated with this method is shown in Figure 4a of the document: where the non-wetting coating layer has a thickness less than 20nm, a portion of the layer at the periphery of the nozzle is blown away in the course of ablation, leaving an area where no non-wetting layer exists. As a solution to this problem, the document proposes using a thicker non- wetting coating, ablating it with the aid of a covering layer so as to achieve a satisfactory nozzle shape. EP-A-0 309 146, W092/16822 and W093/15911 - belonging to the same applicant as the present invention - also relate to ablation in the context of inkjet printheads. All three documents disclose methods of removing material from an element of an inkjet printhead - in this case a nozzle plate - using a laser beam which impinges on a surface forming or intended to form part of the external surface of the printhead - in this case a surface having a non-wetting coating. Since, in these examples, the laser beam does not ablate the non-wetting coating from the rear, the above- mentioned problem with the non-wetting coating being blown away does not occur. However, when employing such a method, the originators of the present invention have encountered problems with damage to that surface which is intended to form part of the external surface of the printhead. In the particular case where the surface is the non-wetting coating located around the nozzle exit opening of a printhead, the damage has impaired the performance of the nozzle. Whilst not wishing to be bound by this theory, the originators of the present invention believe that the damage is caused by high energy free radicals and other highly reactive chemical species, liberated during the ablation process, diffusing back onto that surface intended to form part of the external surface of the printhead. Even when a contact mask is located adjacent the surface as per the above-mentioned EP-A-0 309 146 and WO 92/16822, the mask lying flush against the surface or having contact pads which surround the area to be ablated, damage still occurs. This would suggest that the aforementioned diffusion is stjll taking place. The present invention allows the damage referred to above to be avoided. According to the present invention, there is provided a method of removing material from an element of an inkjet printhead, said element including an ablatable material and having a surface intended to form part of the external surface of the printhead, the method being characterised by the steps of: a) applying a protective layer to be in sealing engagement with said surface; b) directing a high energy beam at said protective layer from that side of said element to which said protective layer is applied, thereby removing material from said protective layer and said element having said surface, at least part of the material removed from said element being removed by ablation; c) removing said protective layer from said surface. Applying a protective layer to be in sealing engagement with that surface intended to form part of the external surface of the printhead effectively protects the surface, in particular at the periphery of the zone in which material removal takes place.from the high energy free radical ablation products discussed above. According to a first embodiment of the invention, the protective layer may be releasably bonded to said surface, advantageously by the use of an adhesive layer. Such an adhesive layer may be transparent to the high energy beam. Removal of the adhesive layer can then take place by the action of the high-energy radicals generated during the ablation of material lying beneath the adhesive layer, for example the material of the nozzle plate. Preferably, the adhesive layer is itself ablatable. This removes any tendency for the adhesive layer to be lifted away from the surface which it is desired to protect as a result of the pressure generated underneath the adhesive layer in the course of the ablation of the underlying element of the inkjet printhead. Whilst this problem can be mitigated by careful metering of the amount of adhesive applied to reduce the thickness of the adhesive layer to the minimum necessary for the protective layer to remain attached, this requirement for metering can be burdensome and is usefully avoided by the use of ablatable adhesive. Furthermore, the originators of the present invention have discovered that this ablatable property prevents refraction of the high energy beam to a location outside of the zone of material removal and the resultant damage to that exterior surface of the printhead located outside of the zone. The UV absorbing characteristic may be an intrinsic property of the adhesive or may be achieved by the use of a UV absorbing additive. Turning now to the protective layer, this also may be transparent to the high-energy beam but will advantageously comprise ablatable material. This will allow the protective layer to be removed by the high energy beam during the material removal process and yet be of sufficient thickness that it will effectively protect the surface from the effects of high energy radicals as described above. Where the protective layer is not ablatable, removal will be by the action of the high-energy radicals generated during the ablation of material lying beneath the protective layer e.g. material of the nozzle plate. A protective layer and/or adhesive that absorbs UV radiation has the further advantage of slowing the ablation of the nozzle plate following the initial impingement of the high energy beam, thereby avoiding damage at the periphery of the nozzle outlet. Such damage can manifest itself as a rounding of the (ideally sharp) edges of the nozzle outlet. The element of the inkjet printhead may be an integral part of the printhead or a separate entity which is attached to the printhead. In the latter case, material removal from the element may take place before or after attachment of the element to the printhead. The element of an inkjet printhead in question may be of homogeneous construction or may comprise at least two layers. In the former case, the whole element will necessarily be of ablatable material. Although in the latter case only one of the inner layers need be ablatable, if the outer layer is not ablatable it should be sufficiently thin that it will be removed by the high energy beam. Another possibility is for material to be removed from the outer layer only. Removal of the protective layer following ablation may be effected mechanically e.g. by peeling, or chemically e.g. by dissolving the layer using a solvent such as acetone, or by a combination of both. Use of the present invention will be particularly important where that surface intended to form part of the external surface of the printhead has a functionality such as non-wetting action or electrical conductivity, which is susceptible to interference from ablation products. It should be noted that there are constraints to the use of the method according to the present invention with a high-energy beam and masking means located adjacent the surface from which material is to be removed, as known for example from EP-A-0 309 146 and W092/16822 relating to the creation of nozzles in the nozzle plate of an ink jet printhead. In such an application, the separation of the masking means and the surface of the nozzle plate caused by the interposition of a protective layer reduces the angle of taper of the nozzle and/or the nozzle density that can be achieved. These constraints are advantageously avoided by the use of high energy beam that is focused and/or masked by means located remote of the protective layer. In particular, the mask may be located such that it is the image of the mask that is projected onto the protective layer via an optical focusing system. Such an arrangement is known from W093/15911, belonging to the applicant of the present invention and incorporated herein by reference. The present invention is also directed to a protective layer for use in the method of removing material discussed above, the protective layer comprising an ablatable material having an adhesive coating, the adhesive coating being ablatable. Use of such a protective layer solves the problems of metering the adhesive and refraction of the high energy beam, as discussed above. The ablatable property may be an intrinsic property of the adhesive or may be achieved by the addition of a UV absorbing additive. Conveniently, the layer of ablatable material may comprise polyester or polyimide. The invention will now be described by way of the example of a nozzle plate of an inkjet printhead, the nozzle plate having a surface provided with a non-wetting coating which in turn forms part of the external surface of the completed printhead. Reference is made to the accompanying figures - which are diagrammatic and not to scale - in which Figure 1 shows the nozzle plate having a non-wetting coating; Figure 2 shows the nozzle plate having a non-wetting coating and protective layer according to a first embodiment of the present invention; Figure 3 shows the nozzle plate/ non-wetting coating/ protective layer arrangement of Figure 2 after material removal; Figure 4 shows the nozzle plate and non-wetting coating of Figure 3 after removal of the protective layer; Figure 5 shows the nozzle plate and non-wetting coating of Figure 1 having a protective layer according to a second embodiment of the present invention; Figure 6 shows a variation of a plate for use in the arrangement of Figure 5. In Figure 1 there is shown a nozzle plate (4) having a non-wetting coating (6) which forms part of the external surface of the printhead. The nozzle plate and non-wetting coating may be of the types disclosed, for example, in EP-A-367 438 belonging to the present applicant and incorporated in the present application by reference. The nozzle plate is attached to a body (2) of an inkjet printhead having ink ejection channels (12). It is noted that correct positioning of each of the nozzles relative to its respective ink ejection channel is conveniently facilitated by attaching the nozzle plate to the printhead body prior to formation of the nozzles. The nozzle plate may comprise any ablatable material - for example plastics material such as polyimide, polycarbonate, polyester, polyetheretherketone, acrylics or non-vitreous inorganic material such as metal, in particular aluminium, or ceramics. Where a plastics material is used, this may include additives of the kind normally employed with plastics, including organic and/or inorganic fillers. The non-wetting coating may be of the kind comprising fluorocarbon functional silane and may be formed by the method described in the above- mentioned EP-A-0 367 438, whereby an adherent first layer comprising cured siloxane is formed on the surface of the nozzle plate, followed by a second layer derived from at least one fluorosilane having a silicon atom to which is attached at least one hydrolysable group and at least one fluorine- containing organic group which donates non-wetting properties to the layer. Figure 2 shows the nozzle plate and non-wetting coating of Figure 1 when provided with a protective layer (10) as per a first embodiment of the present invention. This layer is preferably polyester film, for example the polyethylene terephthlate (PET) film sold by ICI under the trade name Melinex (trade mark), or polyimide (PI) film, for example that sold by UBE under the name Upiiex (trade mark). Both these materials are ablatable and have a high absorbance of ultra-violet radiation in the region of 248 nm wavelength. The protective layer is attached to the non-wetting coating by a layer (8) of pressure sensitive adhesive. The adhesive is preferably applied first to the protective layer (rather than the non-wetting surface) by any suitable means e.g. a wire-wound meter bar, the adhesive-coated protective layer then being applied to the non-wetting coating using pressure. The adhesive is preferably ablatable - this property being either intrinsic or obtained by means of an ultra-violet radiation absorbing additive. It is noted that application of the protective layer need not take place immediately prior to the material removal process. In the case of a nozzle plate, for example, a protective layer may be applied to a sheet of nozzle plate material to which a non-wetting coating has been applied. Such a sheet is subsequently cut up to form individual nozzle plates, each of which is attached to a printhead. Only then would the step of material removal - in this case nozzle formation - take place. Material removal is effected by directing a high energy beam - such as an excimer laser beam - at that side of the nozzle plate (4) on which the non-wetting coating (6) is located such that material is removed from the nozzle plate by ablation. Such methods of material removal are known in the art - see, for example, those documents mentioned at the beginning of the description. As mentioned in W092/16822 in particular, the wavelengths of laser light chosen are typically 193, 248 or 308 nm, corresponding to photon emission at the excimer line of argon fluoride (ArF), krypton fluoride (KrF) or xenon fluoride (XeCI) respectively. Furthermore, in the present example, the material of the protective layer (10) is also ablated away (Figure 3). Generally, the non-wetting coating is sufficiently thin - no greater than 1 micron for example - that it may be removed without necessarily being made of an ablatable material. Material may be removed from the nozzle plate to create not only nozzles of the types mentioned in the aforementioned documents but also solvent wettable areas of the type disclosed in EP-A-0 389 217 or nozzles having a formation provided within the bore serving to control the nozzle ink meniscus as per W093/15911 - both of these documents belonging to the present applicant and being incorporated herein by reference. Material removal may take place with the nozzle plate remote from the rest of the printhead or attached to the printhead. The latter method avoids problems with both registration of the nozzles in the nozzle plate with the channels of the inkjet printhead and alignment of the nozzle axes in the plane of the channels. On completion of material removal, the protective layer is removed from the surface of the non-wetting coating (6) to leave the arrangement shown in Figure 4. Any ablation products that might otherwise have diffused back towards the printhead and have been deposited upon the protection layer, are also removed in this step. There is left a nozzle plate having well- formed nozzle bores (14) and outlets (16) having sharp edges (18) and surrounded by an undamaged non-wetting coating (6). According to a second embodiment of the present invention, the protective layer may be of such a constitution - for example a liquid or a gel - that it establishes by itself a sealing engagement with the surface to be protected. The liquid/gel layer according to the second embodiment may simply be applied to the surface to be protected by any conventional means e.g. roll coating. Alternatively, Figure 5 shows an arrangement of this second embodiment where the liquid/gel layer (20) is held between the surface (6) of the printhead and a plate (26) having an aperture (28) through which can pass the hight energy beam. That part of the liquid/gel lying in the path (22) of the beam is removed whilst the liquid/gel lying outside the path of the beam remains and protects the surface of the nozzle plate and in particular that surface lying at the periphery of the nozzle outlet (24). The viscosity of the liquid/gel layer is selected so that the layer remains in place around the periphery of the nozzle aperture, during the ablation process. The non-wetting action of a coated nozzle plate will be taken into consideration, in selecting this viscosity, together with the period of time required for the ablation process. The liquid/gel is preferably ablatable, thereby avoiding the problems with non-ablatable layers discussed earlier. Conveniently the liquid/gel is of sufficiently high viscosity that it remains in location once applied - either to the surface of the printhead or between the surface of the printhead and the plate. A suitable liquid is a high molecular weight liquid such as polypropylene glycol. The plate may hold the liquid/gel in position or may itself be held in position by the liquid/gel layer. In both of cases, the plate offers an additional degree of protection to the surface of the printhead from ablation products. The plate may be of such construction as to be reusable many times or to be discarded after ablation of the nozzle along with the liquid/gel. It may advantageously comprise polyester film, for example the polyethylene terephthlate (PET) film sold by ICI under the trade name Melinex (trade mark). The aperture in the plate can be formed prior to ablation of the nozzle using the same high energy beam as used for nozzle formation, an aperture larger than the outlet aperture of the nozzle being obtained by displacing the plate along the axis of the tapered beam such that a larger section of this tapered beam ablates a larger aperture. The plate can either be in-situ on the surface of the printhead or remote from the printhead during this process. The liquid/gel may be inserted between the plate and the printhead surface in any suitable fashion, for example roll coating of the plate or the printhead surface prior to assembly, or by pumping the liquid between the plate and the printhead surface. An design of plate (26) particularly suited to this latter process is shown in Figure 6: in addition to an aperture (28), the plate has a chamber (30) formed beneath the aperture in which the liquid layer can form in the particularly critical area around the nozzle. Liquid may be fed to the chamber (as indicated by arrows 34) via channels (32) formed in the plate. As has already been pointed out, the present invention is also applicable to the case where the element to be ablated is an integral part of the printhead e.g. where nozzles are created directly in the printhead as per EP-A-0 595 654. It will be clear that both the method of removing material and the protective layer arrangement described in the present document are not restricted to the manufacture of nozzles or nozzle plates but are applicable to any element of an inkjet printhead which includes an ablatable material and which has a surface intended to form part of the external surface of the printhead.

Claims: Claims 1. Method of removing material from an element of an inkjet printhead, said element including an ablatable material and having a surface intended to form part of the external surface of the printhead, the method being characterised by the steps of: a) applying a protective layer to be in sealing engagement with said surface; b) directing a high energy beam at said protective layer from that side of said element to which said protective layer is applied, thereby removing material from said protective layer and said element having said surface, at least part of the material removed from said element being removed by ablation; c) removing said protective layer from said surface. 2. Method according to Claim 1 wherein the protective layer is releasably bonded to said surface. 3. Method according to Claim 2 wherein the protective layer is releasably bonded to said surface by an adhesive layer. 4. Method according to Claim 3 wherein the adhesive layer is ablatable. 5. Method according to Claim 4 wherein the adhesive layer includes an ultra-violet-radiation-absorbing additive 6. Method according to Claim 4 wherein the ablatable property is an intrinsic property of the adhesive. 7. Method according to Claim 3 wherein the adhesive layer is transparent to the high energy beam. 8. Method according to any of the previous claims wherein the protective layer comprises ablatable material. 9. Method according to Claim 8 wherein the ablatable material comprises polyester or polyimide. 10. Method according to Claim 1 wherein the protective layer establishes by itself a sealing engagement with said surface. 11. Method according to Claim 10 wherein the protective layer is a liquid or a gel. 12. Method according to Claim 11 wherein said liquid or gel is ablatable. 13. Method according to Claim 12 wherein the liquid is polypropylene glycol. 14. Method according to Claim 11 wherein said protective layer lies between said surface and a member lying adjacent the periphery of a zone of material removal. 15. Method according to Claim 14 wherein said member is a plate lying spaced from said surface and having an aperture through which said high energy beam passes. 16. Method according to Claim 14 wherein said aperture is formed by said high energy beam prior to said removal of material from said element having said surface. 17. Method according to Claim 15 wherein said member includes a chamber located on that side of the plate lying closest said surface, liquid or gel being pumpable into said chamber thereby to establish a protective layer. 18. Method according to any one of the preceding claims, wherein the element comprises a body portion comprising ablatable material and a surface layer of a different material to that of the body portion. 19. Method according to Claim 18 wherein material is removed only from one layer. 20. Method according to any of the previous claims wherein said surface intended to form part of the external surface of the printhead has a functionality susceptible to interference from ablation products. 21. Method according to Claim 20, wherein said functionality comprises non-wetting action or electrical conductivity. 22. Method according to any of the previous claims wherein material removal takes place with said element integral with said inkjet printhead. 23. Method according to any of Claims 1-21 wherein material removal takes place with said element remote from said inkjet printhead. 24. Method according to any of the previous claims wherein said high energy beam is an excimer laser beam. 25. Method according to any of the previous claims wherein the high energy beam is shaped to define the geometry of material removal, by means located remote of the protective layer. 26. Method according to Claim 25 wherein masking means are located remote of the protective layer and are imaged onto the protective layer by means of an optical focusing system. 27. A protective layer arrangement for use in the method according to Claim 1 and comprising a layer of ablatable material having an adhesive coating, the adhesive coating being ablatable. 28. A protective layer arrangement according to Claim 27 wherein the ablatable property is an intrinsic property of the adhesive. 29. A protective layer arrangement according to Claim 28 wherein the adhesive layer includes an ultra-violet-absorbing additive. 30. A protective layer arrangement according to any of Claims 27-28 wherein the layer of ablatable material comprises polyester or polyimide.

Inventors: HARVEY ROBERT ALAN, TEMPLE STEPHEN, HARVEY, ROBERT, ALAN, TEMPLE, STEPHEN
Applicants: HARVEY ROBERT ALAN, TEMPLE STEPHEN, XAAR LTD, HARVEY, ROBERT, ALAN, TEMPLE, STEPHEN, XAAR LIMITED
Application year: 1995
Main classification: B41J2
IPC codes: B23K26, B41J2