FLUID COOLED CONTACT MASK

Abstract: A mask (17) for use in forming features in a surface (12) by high energy pulses of laser radiation is disclosed which comprises a base plate (19) which is located adjacent said surface and has apertures (20) in it through which locations of said surface are exposed to the laser radiation. Channel means (21) formed in a cover (23) of the mask lie in adjoining regions of the mask exposed to the radiation and during ablation of said surface having cooling fluid circulated through there. The mask is described in use for making nozzles (25) in the nozzle plate (13) of an ink jet printhead.

Description: Fluid Cooled Contact Mask This invention relates to masks for use in forming features in a surface by high energy pulses of laser radiation and in particular to a method of forming nozzles in an ink jet printhead having parallel Ink channels with which said nozzles respectively communicate. The use of excimer laser for patterned ablation of surfaces is well known in the literature and the application of laser ablation to the formation of nozzles for an ink jet printhead is described in EP-A-0309146 the content of which is incorporated herein by reference. In this reference the preferred method of nozzle manufacture is to place a contact mask having apertures corresponding to nozzle locations in contact with a nozzle plate attached to the printhead. Exposure to successive pulses of UV light of high intensity causes the nozzles to be ablated. Rocking of the mask and printhead during the pulses enables the nozzles to be undercut so that the nozzle inlets are greater in area than the nozzle outlets. Typical incident energy -2 of the UV light pulses is 0.3-Ucm . Practical tests indicate that a contact mask tends to heat up during exposure to light energy density of this magnitude, which may result in the thermal expansion of the mask. It also causes the mask to become dished due to thermal stress cycling in the mask surface and the mask becomes progressively cracked and damaged, limiting its useful life. One recognised method of avoiding the problems resulting from a high energy density of radiation incident on the contact mask is to employ a projection mask at an expanded part in the path of the incident optical beam, i.e. at a location of the beam where the energy density is less than that at the ablating locations. In the present application, however, a contact mask in contact with the printhead is to be preferred. Contact against the face of the mask, e.g. by locating dowels or by optical alignment, locates the printhead relative to the mask and reduces manufacturing tolerances, particularly in a process incorporating rocking. It is an object of this invention to provide a mask for use in forming features in a surface by high energy pulses of radiation which is of extended useful life. A further object is to provide an .improved method of forming nozzles in an ink jet printhead. The present invention consists in a mask for use in forming features on a surface by laser ablation comprising a baseplate which is located adjacent said surface and is formed with apertures through which respective locations of said surface are exposed to high energy radiation pulses of said laser to form said features, characterised in that channel means are provided adjoining regions of the mask exposed to said high energy pulses through which, during ablation of said surface, fluid is caused to flow to cool the mask. Suitably, said channels means comprise enclosed channels adapted for connection to means for circulating cooling fluid therethrough. Advantageously, a heat exchanger is provided through which fluid heated in the channels is passed for heat extraction therefrom prior to recirculation. Preferably, said mask on the surface thereof on which said laser radiation is incident is formed with a mirror surface to reflect said radiation. In one form the mirror surface is a coating of aluminium. In another form, the mirror surface is a dielectric coating which is of thickness wavelength matched to the wavelength of the incident radiation. The surface of the mask may be such as flatwise to engage the surface in which features are to be ablated. Alternatively, the mask may be formed around the apertures therein with pads which contact the surface in which the features are to be formed respectively around those features. The invention further consists in the method of forming nozzles in an ink jet printhead having parallel ink channels with which said nozzles respectively communicate, characterised by bonding a polymer nozzle plate to corresponding ends of said ink channels, applying a contact mask to said nozzle plate, said mask being formed with apertures at the spacing of said nozzles circulating cooling fluid through channels formed in said mask and exposing said mask to high energy pulses of laser radiation at least in the regions of the mask including said apertures thereby to ablate said nozzles. The invention will now be described, by way of example, with reference to the accompanying, somewhat diagrammatic drawings, in which:- FIGURE 1 is a side elevation partly in section of equipment used for laser ablation of features in a surface, in particular nozzles in a nozzle plate of an ink jet printhead, which includes a mask according to this invention; - - FIGURES 2a and 2b are respectively a sectional side elevation and a sectional plan view of the mask of Figure 1, the side elevation of Figure 2a being taken on the line Ila-IIa of Figure 2b and the plan view of Figure 2b being taken on the line Ilb-IIb of Figure 2a; and FIGURE 3 is a fragmentary sectional view illustrating details of the mask of the earlier figures. In the drawings, like parts are accorded the same references. Referring to Figure 1 an excimer laser 10 affords a high energy optical beam 11 employed for forming features in a surface 12 which, in this case, is a surface of a nozzle plate 13 of an ink jet printhead 1 to which the plate 13 is bonded at corresponding ends of parallel channels 15 which extend in the printhead in a plane normal to that of the drawing. An example of this process is described in EP-A-030916 referred to earlier. In this process the surface 12 is ablated by exposure to pulses of high energy UV light generated by the laser 10. The wavelengths of 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 chloride (Xe Cl). The pulse period generated by such lasers is typically 10~30ns, delivered at frequencies of up to 200Hz or higher. The energy density of the pulses may be concentrated, by means of a suitable lens 16, to a level depending on the ablation threshold of the surface 12. "typically where the surface is a polymer suitable for the nozzle plate for an ink jet printhead, the threshold energy _2 density for ablation is 0.1-0.2Jcm . In a process for ablating the surface at a suitably high rate, an energy density in the _2 range 0.3-1Jem will be selected : but for the ablation of surfaces having a higher threshold energy density a higher exposure energy -2 density up to 10Jem may be employed. In known art a projection mask disposed in the region of the lens 16 is used, but where small precise features are to be ablated, or rocking of the surface 12 is employed it is convenient to use a contact mask 17 including apertures 20 made in a base plate 19 of the mask which is located precisely relatively to the surface 12, e.g. by dowels or optical means (not shown). The mask 17 is exposed to the full energy density of the incident light pulse. A problem with the contact mask is that it may absorb energy during the period of exposure to light pulses and progressively heat up during the ablation process. As a result the mask may expand by thermal expansion, which limits the accuracy of manufacture of the nozzles. Further, it has a tendency to become dished, due to thermal stress cycling and its surface becomes cracked and damaged so that the mask has a limited life. These difficulties would generally be avoided with a projection mask where the energy density can be lower, and the rate of heating correspondingly less. Another problem with a contact mask arises when it is used to form nozzles of an ink jet printhead when heating of the mask gives rise to thermal degradation of the non-wetting coating formed on the outer face of the polymer nozzle plate. This coating is the subject of EP-A-0367438 and is formed on the polymer sheet from which the nozzle plate is made. The coating as well as being of low surface energy, i.e. non-wetting, is rub resistant and tolerant up to 180°C which is attained during its manufacture. There is however evidence that when an uncooled mask overheats during nozzle ablation, the coating degrades as a result of which ink in the nozzles, instead of being confined to the nozzle, then spreads over the outer surface of the nozzle plate. For this reason, therefore, cooling of the mask is desirable. The degree of heating of a contact mask depends upon the optical absorption or reflection coefficient of the mask at the wavelength of the incident light energy. For example, if the mask is formed of silicon, whose absorption coefficient is about OΛ, and of thickness -2 lOOμm, under incident energy of 0.5Jem , the mask will heat about 10 C per pulse. When the mask is metallised with Aluminium, which has an absorption coefficient of about 0.1 (i.e. approximates to a mirror of 90£ reflection efficiency) the temperature rise is still about 2.5 C per pulse. Thus at a typical pulse rate of 200Hz the contact mask will be found to rise in temperature at about 500 C per -2 second absorbing heat at a rate of lOWcm . In an ablation process requiring several thousand pulses, it has hitherto been practical only to ablate at lower frequencies i.e. l-2Hz, so that the mask cools between pulses and is limited to an acceptable peak temperature. It will thus be seen that only an extremely high quality mirror coating having an absorption coefficient less than 0.001 will be suitable for a passive contact mask - one in which nothing is done about absorption of energy - without overheating in limited periods at highest laser pulse rates. A high quality mirror coating although it absorbs less heat, also loses less heat by infra-red radiation. One does not accordingly want to rely on heat conduction into the printhead to cool the mask, therefore cooling by other means is desirable to keep the mask temperature within reasonable range. To prevent overheating, the contact mask in Figure 1 incorporates fluid channels 21. The cooling fluid which is caused to flow through the channels 21 by means of a pump, may be gas but in view of the limited space available for the channels is, preferably, a liquid such as water including inhibitants to limit oxidation or solubility of the channel walls or a hydrocarbon solvent. The channels 21 are formed in a cover 23 of the mask which is bonded to the base plate 19• Both the cover and base plate may be made of metal or silicon or a high temperature polymer bonded or glued together. Advantageously, the bond is a low vapour pressure bond such as a diffusion or solder bond. In the mask illustrated, the apertures 20 comprise a line of apertures at the spacing of channel nozzles 25 which are ablated into the nozzle plate 13 by the beam 11 and respectively communicate with the channels 15 of the printhead. As shown in Figure 3. apertures 27 in the cover which overlie the apertures 20 in the base plate 19 may have a larger diameter than the apertures 20 to facilitate ablation of the nozzles 25 by relative rocking between the incident light beam 11 and the printhead 14 without shading or occluding the exit of the nozzles 25. The cooling channels 21 formed in the cover include deflectors 29 which impart sinous flow to the cooling liquid to ensure optimum heat absorption therein. The channels 21 are placed so as to cool, as much as practical, the area of the mask exposed to the incident light pulse on the mask. The surface of the cover 23 may be coated by a mirror surface (for example, an aluminium coating) . This limits the heat absorbed during the pulse period of typically 10-30 ns and thus reduces the peak temperature attained by the surface layer of the cover to typically 1-200°C temperature rise. Without the coating, the layer may reach 500-1000 C or more during the pulse causing the mask cover to deteriorate and distort, as well as increase the rate of heating of the mask. The material of the base plate 19 round the apertures 20 (as illustrated in Figure 3) nay similarly be coated with a dielectric mirror coating. The choice of coating, i.e. metallised coating or dielectric coating, is made primarily to ensure that the life of the cooled mask when exposed to UV laser pulses is adequate for the manufacturing duty specified. For aluminium the threshold energy of ablation is limited by surface segregation of impurities in the _2 deposited metal and will not exceed 0.8 - 1.1 Jem . For higher energy density a dielectric coating is required. In order to ensure that the material round the apertures is effectively cooled between light pulses, if the pulse frequency is f and the thermal diffusivity is k, the distance between the apertures 20 and the cooling channel is preferably, less than -Jk/f. As shown in Figure 3. the heat in the material in the base plate 19 in this region then has time to diffuse towards the cooling channels and to become essentially uniform before the next pulse. The thermal diffusivity k = K/pc where K = the thermal conductivity, P= the density and c the specific heat of the mask material. A contact pad 28 may be placed round each of the apertures 20 of the mask on the side of the base plate facing the nozzle plate 13 which ensures a good contact between the mask and surface 12 of the nozzle plate. Alternatively, the base plate may lie flush against the surface 12. The cooling channels are filled with cooling fluid, preferably liquid, which is circulated through an inlet 22 and an outlet (not shown) formed in the cover suitably at respective ends of the channels, so that the heat is continuously removed during ablation. The fluid is then passed prior to recirculation through a heat exchanger (not shown) , which dissipates the heat keeping the mask at a steady temperature, preferably less than 20-40 C above ambient or similar, when the thermal expansion of the cooled mask 17 is kept within acceptable limits.

Claims: Claims 1. A mask (17) for use in forming features (25) on a surface (12) by laser ablation comprising a baseplate (19) which is located adjacent said surface and is formed with apertures (20) through which respective locations of said surface are exposed to high energy radiation pulses of said laser to form said features, characterised in that channel means (21) are provided adjoining regions of the mask exposed to said high energy pulses through which, during ablation of said surface, fluid is caused to flow to cool the mask. 2. A mask as claimed in Claim 1, characterised in that said channel means comprise enclosed channels (21) adapted for connection to means for circulating cooling fluid therethrough. 3. A mask as claimed in Claim 2, characterised in that said channels include deflectors (29) which impart sinous flow to fluid passing therethrough. 4. A mask as claimed in Claim 2 or Claim 3. characterised in that a heat exchanger is provided through which fluid heated in the channels is passed for heat extraction therefrom prior to recirculation. 5. A mask as claimed in any preceding claim, characterised in that the channel means extend through locations adjacent said mask apertures. 6. A mask as claimed in Claim . characterised in that said channel means are spaced from said mask apertures within a distance fk^where k is the thermal diffusivity of the base plate material of of the mask and f is the frequency of the laser pulses. 7. A mask as claimed in any preceding claim, characterised in that said mask on the surface thereof on which said laser radiation is incident is formed with a mirror surface to reflect said radiation. 8. A mask as claimed in Claim 7. characterised in that said mirror surface comprises a coating of aluminium. 9. A mask as claimed in Claim 7, characterised in that said mirror surface is a dielectric coating which is of thickness wavelength matched to the wavelength of the incident radiation. 10. A mask as claimed in any preceding claim, characterised in that said mask is adapted so that said base plate flatwise contacts said surface in which said features are to be formed. 11. A mask as claimed in any one of Claims 1 to 9 > characterised in that contact pads (28) are provided on the mask surface which faces the surface in which said features are to be formed, said pads extending respectively around said mask apertures. 12. The method of forming nozzles (25) in an ink jet printhead having parallel ink channels (15) with which said nozzles respectively communicate, characterised by bonding a polymer nozzle plate to corresponding ends of said ink channels, applying a contact mask to said nozzle plate, said mask being formed with apertures at the spacing of said nozzles circulating cooling fluid through channels formed in said mask and exposing said mask to high energy pulses of laser radiation at least in the regions of the mask including said apertures thereby to form said nozzles by ablation. 13. The method of Claim 12, characterised by rocking said printhead, nozzle plate and mask relatively to the axis of said radiation thereby to form said nozzles with an undercut. 14. The method claimed in Claim 12 or Claim 13, characterised by circulating water as said cooling fluid. 15. The method claimed in Claim 12 or Claim 13, characterised by circulating hydrocarbon solvent as said cooling fluid.

Inventors: HARVEY ROBERT ALAN, PATON ANTHONY DAVID, SPEAKMAN STUART PHILIP, HARVEY, ROBERT, ALAN, PATON, ANTHONY, DAVID, SPEAKMAN, STUART, PHILIP
Applicants: XAAR LTD, XAAR LIMITED
Application year: 1992
Main classification: G01L25
IPC codes: B23K26, B41J2, G03F1, G03F7