Abstract:
An inkjet print head ( 10 ) comprises one or more laser sources ( 12 ). Each laser source ( 12 ) is actuable to emit laser radiation and each laser source is associated with one or more ink chambers (18). Each ink chamber ( 18 ) includes a nozzle aperture ( 20 ) through which ink is dispensed and is arranged to, in use, communicate with an ink supply. Each chamber has a wall ( 16 ) arranged to contact the ink in the ink chamber, the wall being responsive to laser radiation from an associated laser source to produce an acoustic emission capable of displacing ink from the chamber.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an inkjet print head, a method of fabricating an inkjet print head, a print cartridge including such a print head and a printer arranged to operate such a print cartridge. Particularly, but not exclusively, the invention provides an inkjet print head which includes a laser source for generating acoustic waves which in turn produce a driving pressure for ink expulsion.  
           [0002]    In the present specification, the term acoustic is used to describe a longitudinally propagating pressure wave through any of a solid, liquid or gas. In view of the operating frequency of print heads, this pressure wave may in fact have an ultrasonic frequency.  
         BACKGROUND OF THE INVENTION  
         [0003]    Currently inkjet print head cartridges deliver ink using two basic mechanisms, thermal or piezoelectric. Thermal systems rely on rapid heating to generate bubbles in an ink firing chamber, which expand and expel ink. Piezoelectric systems rely on the flexing of a crystal to generate a pressure wave to drive ink out of an ink firing chamber.  
           [0004]    Thermal systems are limited in resolution (minimum drop size) by the lack of control over the bubble generation process, and in delivery rate by the recovery/cooling cycle and time for ink to refill the void left by the bubble. The ink compositions must also be such as to withstand the thermal cycling of the system.  
           [0005]    Piezoelectric devices are relatively expensive to build and limited in firing rate, typically in the order of MHz, by the response times of the material.  
           [0006]    In both cases it is difficult to further miniaturize the systems. In particular, for piezo-crystals it is difficult to make them smaller whilst increasing their operating frequency and generating sufficient deflection to drive ink ejection.  
           [0007]    At the same time, it is known that laser generated acoustic waves in solids can be produced by two mechanisms. In the thermoelastic regime, which occurs at low laser power densities, laser induced temperature rises produce rapid thermal expansion and transient acoustic waves. Such waves were detected early in the development of laser processing, see ‘Calorimetric and acoustic study of ultraviolet laser ablation of polymers’, G. Gorodetsky et al, Appl. Phys. Lett. Vol 46 (1985) pp 828-830. Laser generation of ultrasound in the thermoelastic regime is a nondestructive process. However, in the ablation regime, which occurs at high peak powers, the recoil forces generated by vaporized material leaving the sample generate strong acoustic waves or shock waves. This regime involves the removal of very thin layers of material, although this layer may be a renewable material such as an oil or liquid coating.  
           [0008]    EP1008451A2, ‘Laser-Initiated Ink-Jet Printing Method and Apparatus’, filed Dec. 11, 1999 describes a laser driven ink jet printing head relying on the laser generation of acoustic waves. In this system, single or possibly multiple scanning laser beams are each focussed to generate respective acoustic waves in a liquid contained in a first chamber located above an ink-firing chamber. The acoustic waves are transmitted from the first chamber to the ink-firing chamber through an intermediate body which lies between the chambers and which is almost transparent to the acoustic waves. When the transmitted acoustic wave enters the ink-firing chamber, it causes a droplet of ink to be ejected from a nozzle lying in register with the focussed laser beam. However, in this system, the intermediate body must have sufficient thickness and strength to protect the ink chamber from the pressure perturbations generated by bubble formation and collapse etc in the first chamber, as well as being able to act as an acoustic window allowing acoustic waves to be transmitted to the firing chamber. These two requirements (acoustic window and protective barrier) limit the type and thickness of material that can be used for the intermediate body. These are major disadvantages to the cost and flexibility of a commercial inkjet printing system.  
         DISCLOSURE OF THE INVENTION  
         [0009]    According to the present invention there is provided an inkjet print head as claimed in claim 1.  
           [0010]    The present invention relies on laser produced pressure waves to generate a driving pressure for ink expulsion. In this sense it could offer the advantages of piezoelectric devices, in that the mechanism is independent of ink chemistry. However, laser devices can be operated at variable pulse repetition rates of up to GHz, overcoming the firing rate limitations of piezoelectric devices. The laser pulse energy is also infinitely variable, offering fine control over the driving pressure and droplet size.  
           [0011]    Preferably the wall comprises a membrane. The prior art makes no reference to the provision within a print head of a laser cooperating with a membrane or to the permanent physical displacement of ink with the resultant pressure waves.  
           [0012]    The present invention overcomes many of the problems of conventional inkjet mechanisms. In relation to firing rate limitations, it will be seen that in principle the firing rate of the print head can be increased to the pulsing rate of the laser source, currently GHz and increasing in the future.  
           [0013]    By using acoustic emissions to directly expel the ink, rather than heat as in the prior art, the requirement of the ink chemistry to withstand rapid temperature cycling is mitigated, allowing a broader range of ink chemistries to be employed within inkjet print heads.  
           [0014]    Because the laser sources used to generate the driving force for ink ejection are in general highly controllable, the print head provides more control over droplet size and speed.  
           [0015]    Preferably, the laser sources are based on semiconductor technology and therefore readily miniaturized and integrated into electrical systems.  
           [0016]    In principle the size of the laser generated acoustic source is limited by the focussability of the laser (around the wavelength of the laser light). In implementing the present invention, a smaller focussed spot or output beam could be an advantage, as for a given laser pulse energy this increases the energy density and this could generate more driving pressure for ink ejection.  
           [0017]    This miniaturization means higher nozzle densities should be possible, thus increasing print head resolution.  
           [0018]    The present invention differs from EP1008451 in the following respects:  
           [0019]    1. The invention generates acoustic waves, preferably using a thermoelastic mechanism, in a solid material rather than an opto-acoustic effect in a liquid.  
           [0020]    2. The invention generates the acoustic wave directly in a membrane. The superficially similar intermediate body in EP1008451 is an acoustic window and plays no part in the generation of the acoustic wave.  
           [0021]    3. The membrane of the invention need not be made of an acoustically transparent material, if sufficiently thin. The intermediate body in EP100851 must be acoustically transparent and cannot be made arbitrarily thin as it also acts to protect the ink chamber from pressure perturbations associated with bubble generation in the buffer solution.  
           [0022]    4. Since the membrane used in the current invention is a solid material, bubble formation does not result after deposition of the laser energy. Therefore, there is no disruption to further laser pulses, which might limit the firing rate of the system.  
           [0023]    5. The current invention does not require an optical system to focus and distribute the laser beam as required for EP1008451. Neither does it use a buffer liquid. The print head is much simpler with fewer moving parts.  
           [0024]    6. In a preferred embodiment of the invention, the laser sources are integrated into the print head and therefore there is no requirement to synchronize the movement of the laser, laser beam and print head mechanism to allow full page width coverage, again making the print head of the invention much simpler. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0026]    FIGS.  1 ( a ) to  1 ( c ) show in schematic form a firing sequence for a laser generated acoustic wave driven print head nozzle of one embodiment of the invention;  
         [0027]    FIGS.  2 ( a ) and  2 ( b ) show in schematic form two variations a laser generated acoustic wave driven print head nozzle of a second embodiment of the invention;  
         [0028]    [0028]FIG. 3 shows a manufacturing sequence for production of one half of a laser generated acoustic wave print head die of a third embodiment of the invention;  
         [0029]    [0029]FIG. 4 is a cross-sectional view of the sequence of FIG. 3 viewed along the line IV-IV;  
         [0030]    [0030]FIG. 5 is schematic view of a portion of a laser generated acoustic wave driven inkjet print head die produced according to the sequence of FIGS. 3 and 4; and  
         [0031]    [0031]FIG. 6 is a cross-sectional view of the print head die of FIG. 5 viewed along the line VI-VI. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    Referring now to FIG. 1 which shows one nozzle  10  for a laser generated acoustic wave driven inkjet print head according to a first embodiment of the present invention. Within the print head, the nozzle  10  is associated with a laser source  12 , for example a semiconductor laser diode, which is switched by control circuitry (not shown) to selectively emit a laser beam  13  through a focussing lens system  14 . If a semiconductor laser diode is selected as the laser source, then optical components may be directly attached to the output facet of the laser to aid miniaturization, see ‘Microlens is deposited directly onto laser-diode facet’, Newsbreaks, Laser Focus World, December 2000. Alternatively, graded index rods or micro-spheres may also be used as focussing elements.  
         [0033]    Alternatively, in this embodiment, it will be seen that one laser source may be associated with more than one nozzle, with the focussing system being movable to shift the focus of the beam from one nozzle to another.  
         [0034]    In any case, for any given nozzle, the beam is focussed on a membrane  16  which acts to close one side of an ink chamber  18  associated with the nozzle. A nozzle aperture  20  is defined in the ink chamber, which is also in fluid communication with an ink supply (not shown). FIG. 1( a ) shows the system at rest, where the ink is contained within the chamber. In FIG. 1( b ) a laser pulse is emitted and the laser beam focussed down onto the membrane  16 . Either through the thermoelastic mechanism, or by ablation, a strong acoustic wave is generated in the thin membrane  16 . It will be seen that a pressure pulse generated in the ablation regime, especially a shock wave, is likely to be stronger than that generated in the thermoelastic regime. The pressure pulse generated by thermal expansion and/or momentum transfer, propagates through the membrane and is transmitted into the ink, generating a pressure pulse in the ink chamber which in turn causes the ejection of an ink droplet  22 , FIG. 1( c ). The sequence shown in FIG. 1 is then repeated up to the maximum pulse rate of the laser, although the maximum firing rate is determined by the recovery time of the system to the pressure pulse.  
         [0035]    In an alternative embodiment of the invention, FIG. 2, the laser source  12 , is placed in very close proximity to the membrane  16 . It is therefore possible to eliminate the need for focussing optics of the first embodiment entirely.  
         [0036]    [0036]FIG. 2 also shows alternative ink chamber shapes for concentrating laser generated acoustic waves in the ink. In FIG. 2( a ), the chamber  18  is defined by, for example, wet etching the layer of material which, once etched, forms the walls of the chamber. This provides a funnel type chamber narrowing towards the nozzle aperture  20 . In FIG. 2( b ) a further layer  24  is deposited over the chamber walls. In this case, the nozzle aperture defined in this layer is anisotropically etched to provide relative parallel side walls. These may prove less prone to wear than the acute edged walls of FIG. 2( a ). In either case, these ink chamber shapes are intended to reflect and/or confine the laser generated acoustic wave, increasing the pressure generated in the ink chamber.  
         [0037]    For either of the embodiments of FIGS. 1 and 2, the selection of the optimum membrane material and its thickness can be determined experimentally. However, some properties are clear. Material with a relatively high absorption coefficient and low thermal diffusivity will aid the conversion of laser to thermal/acoustic energy. The material should have a high ablation threshold at the selected laser wavelength to prevent excess erosion of the membrane. Finally a sandwich of materials, each with different properties may amplify the desired response to the laser pulse.  
         [0038]    More particularly, polyimide (Kapton®) is an example of a suitable membrane material. Firstly, it is known that polyimide finds application as a radiating film for audio loudspeakers, which suggests that it may have a suitable lifetime for use in a print head. Furthermore, this material has been extensively studied with UV lasers and the ablation threshold found to be in the range 0.1-0.3 J/cm 2 , see Gorodetsky et al. At these energy densities both acoustic emission and deflection have been detected in other polymers such as polymethylmethacrylate (Acrylic), see ‘Nanometer-Nanosecond Oscillatory Expansion and Contraction Behaviour of Polymer films Induced by 248 nm Excimer Laser Excitation’, T. Masubuchi et al., ChemPhysChem Vol 3 (2000) pp 137-139 (see appendix A), which is incorporated herein by reference, and these too may be useful as membrane materials.  
         [0039]    It is clear that to provide a practical laser driven print head, the laser source itself should be miniaturized preferably to have dimensions of the order of approx. 100 μm. Furthermore, the laser would need to be selected for maximum peak power and for the desired pulsing rate for application in a print head, regardless of the operating wavelength. As mentioned more generally above, one potential laser source is a pulsed semiconductor laser diode chip.  
         [0040]    There are numerous pulsed laser diodes on the market and the characteristics of the two are given below, by way of example:  
                                                                                                 Energy           Wavelength   Peak Power   Pulse Length   Emitting Area   Density       Make &amp; Model   (nm)   (W)   (nsec)   (μmxμm)   (J/cm2)                                Laser Diode Inc.   905   4   50   3000 × 80   0.083       CYD 60       Hamamatsu   860   23   100   300 × 1   0.766       L5758                          
 
         [0041]    If the lasers are placed sufficiently close to the target, especially as in the case of the embodiment of FIG. 2, it can be assumed that the achievable energy density on the target is the same as the above values. If polyimide were used as a membrane material, with an ablation threshold in the range 0.1-0.3 J/cm2, the above two lasers would be capable of generating acoustic emission from the membrane. As such, these figures show that sufficient laser energy density is available at the output facet of the laser diode to generate acoustic waves.  
         [0042]    However, currently individual laser diode chips are packaged with circuitry etc to form packages of 5-10 mm size and these would probably be too large for practical use.  
         [0043]    A more ideal semiconductor laser type for the print-head application would be a vertical cavity surface-emitting laser (VCSEL), such as those manufactured by EMCORE, New Jersey, see “Optical Devices,” which is incorporated herein by reference and which appears as appendix B to this application. These are produced as semiconductor diode arrays, currently to a maximum of 12 on a 3.2×0.4 mm die and as such have dimensions of the order required to produce a practical print head.  
         [0044]    In a third embodiment of the invention, FIGS.  3  to  6 , such laser diode structures  30  are incorporated within an integrated print head chip fabricated within a die of an array of such print heads on a silicon wafer substrate  32 . This provides a print head containing the drive circuitry, identification circuitry etc. necessary to form a high-resolution print head with a closely spaced 2-dimensional array of nozzles. This integration is analogous to the manner in which existing thermal inkjet print heads including resistive heaters are produced. As in this case, the head is then incorporated into a print cartridge in a conventional manner with connections to the print cartridge circuitry being made through wire bonds. The print cartridge circuitry is then in turn connected to printer control circuitry, which in accordance with an image mask selectively fires individual lasers  30  to deposit ink onto a print medium.  
         [0045]    Referring now to FIGS. 3 and 4, which show a manufacturing sequence running in the direction of the arrow A for producing the print head of the third embodiment. Only the top half of the print head is shown, with the remainder being a mirror image of the top half centered about a common ink feed slot  34  cut through the wafer  32 . (In a colour print head this two row array of nozzles with a common ink feed slot would be reproduced for each colour to be dispensed by the print head.)  
         [0046]    The process begins by fabricating a plurality of Vertical Cavity Surface Emitting Laser (VCSEL) diodes  30  directly on the silicon wafer substrate  32 . In the present embodiment, each VCSEL diode is 30×30 μm in size indicated by the numeral d.  
         [0047]    A polymer barrier layer  36  defined by conventional photolithographic and etch steps is then deposited around the VCSEL diodes to planarize the substrate. As explained below, the layer  36  is preferably deeper than the VCSELs to define a cavity in the region of the VCSEL. Furthermore, the layer  36  need not actually contact the sides of the VCSEL so providing for heat dissipation.  
         [0048]    Following this step a polyimide membrane layer  16 , corresponding to the membrane of the first two embodiments, is deposited and patterned as necessary as it need only extend over the openings in the layer  36  in which the VCSELs are located. As in the second embodiment, this layer covers the field of view of each VCSEL disposed beneath the layer  16  and lies close enough to the laser source so as not to require focusing optics between the laser source and the layer. If the layer  16  is in intimate contact with the VCSEL, then there may be some burning during operation of the device. For this reason, in a preferred embodiment, the layer  16  is spaced slightly from the emitting surface of the VCSEL. This can be achieved using a number of different fabrication techniques. For example, the polyimide could be provided in tape form and rolled over the surface of the wafer after the layer  36  has been formed. As long as the layer  36  is slightly deeper than the VCSELs and air gap will be formed within the VCSEL cavity. The tape layer can then be patterned and etched as required.  
         [0049]    Alternatively, the VCSEL cavity could be filled with a temporary soluble filler such as a wax. The polyimide could be provided in liquid form and spun over the surface of the wafer and cured. The VCSEL cavity would then need to be suitably shaped to allow the filler to be dissolved in the region of the VCSEL. It may also be possible to do without the filler and simply shape the VCSEL cavity to allow the polyimide layer to be etched back from around the VCSEL.  
         [0050]    In any case, once the membrane  16  has been laid down, a further polymer barrier layer  40  is then deposited on the membrane layer and patterned to form the walls of respective ink chambers  18  corresponding to each VCSEL. As in a conventional ink jet print head, each ink chamber  18  is in fluid communication (as indicated by the numeral  39 ) with an ink feed slot which passes through the substrate  32  to supply ink from a reservoir (not shown) within the body of the cartridge through to the nozzles. Again, in a colour print head, it will be seen that different groups of nozzles will communicate with respective ink feed slots for different colours, however, this does not affect the description or implementation of the present invention.  
         [0051]    Finally, a metallic or polymer orifice plate  42  with pre-drilled holes corresponding to the nozzle apertures  18  is applied to the layer  40 . It is this surface through which, in use, ink will be ejected from the print head as a result of the acoustic wave generated by the VCSELs within the print head.  
         [0052]    Referring now to FIGS. 5 and 6 which show a portion comprising  8  chambers of a complete print head fabricated as shown in FIGS. 3 and 4. The ink feed slot while not shown lies behind the orifice plate  42  between the two rows of nozzle apertures  20 . It will also be seen that it is the membrane  16  which is visible within an empty ink chamber.  
         [0053]    Finally, it is acknowledged that FIGS.  3  to  6  are shown for exemplary purposes only. It is clear that further circuitry needs to be incorporated in a commercial print head including but not limited to: conductive traces connecting each VCSEL to power, ground and signal supplies; identification circuitry to enable printer control circuitry to identify and operate the print cartridge including the print head correctly; temperature measurement circuitry etc.  
         [0054]    It will be seen that variations of the above embodiments are possible. For example, there is an opportunity to use the properties of a coherent laser source to provide a pattern of focussed light on the membrane and so form acoustic sources of different patterns. This approach is possible particularly where the laser beam is very strongly absorbed in the membrane and as such the resultant acoustic waves are effectively generated from a surface source i.e. optical absorption depth&lt;&lt;wavelength of acoustic wave, rather than a volume source. The acoustic waves will therefore have a high degree of coherence.  
         [0055]    The most general form of this technique would be to use diffractive optical elements (DOE), to shape the laser beam, see the document entitled ‘Pattern Formation DOEs’ by the Diffractive Optics Group, Heriot-Watt University, a copy of which document appears as appendix C to this application. This document is incorporated herein by reference. This would allow almost any light pattern to be generated on the membrane, offering considerable scope in the acoustic source shapes available. One known technique is to use a series of concentric rings to generate focussed acoustic waves through constructive interference (see ‘Micromachined Acoustic-wave Liquid Ejector’, by X. Zhu et al., a copy of which is appended hereto as appendix D and which is incorporated herein by reference), in effect forming the acoustic equivalent of an optical Fresnel lens. As with conventional optical elements, it has been shown to be possible to integrate DOEs directly onto semiconductor laser structures, see “Optoelectronics” published by Chalmers University of Technology, a copy of which is appended hereto as appendix E and which is incorporated herein by reference.  
         [0056]    An alternative miniature laser source could be a fibre laser, or a higher power laser source fed to a fibre bundle, which is then split to drive multiple firing chambers.