Abstract:
A piezoelectric printhead or other droplet deposition apparatus has parallel liquid containing channels defined by a base and displaceable walls, and covered by a cover number. The channels each have at least one nozzle for ejecting droplets. Each nozzle may be disposed in the base, the cover then having two ink supply parts spaced lengthwise of each channel on opposite sides of the nozzle. Alternatively two longitudinally spaced nozzles may be provided in the base of each channel. The cover may have a conductive track corrected to wall-displacing electrodes, the points of connection being outside the channels.

Description:
This is a continuation of International Application No. PCT/GB98/03050 filed Oct. 9, 1998. The priority benefit under 35 U.S.C. §119(e) of provisional application No. 60/073,041 filed Jan. 19, 1998 is claimed. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to droplet deposition apparatus, in particular an inkjet printhead, which comprise a channel communicating with a supply of droplet liquid and an opening for ejection of droplets therefrom, at least one channel side wall being displaceable in response to electrical signals, thereby to effect ejection of droplets from the channel. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 a  is a cross-sectional view of the channels of the prior art inkjet printhead construction according to WO92/22429. Piezoelectric ceramic sheet  12  is poled in its thickness direction  17  and formed in one surface with channels  11  bounded on two sides lying parallel to the channel axis by channel walls  13 . By means of electrodes  23  formed on either side of each wall  13 , an electric field can be applied to the piezoelectric material of the walls, causing them to deflect in shear mode in a direction transverse to the channel axis. Pressure waves are thereby generated in the ink which result in the ejection of an ink droplet. These principles are known in the art, e.g. from EP-A-0 364 136. 
     Channels  11  are closed along one side lying parallel to the channel axis by the surface of a cover  14  having conductive tracks  16  at the same pitch interval as the ink channels formed thereon. Solder bonds  28  are formed between tracks  16  and the channel wall electrodes  23 , thereby securing the cover to the base and creating an electrical connection between the electrodes and the track in a single step. To protect them from later being corroded by the ink, electrodes and tracks are then given a passivant coating. 
     As shown in FIG. 1 b , which is a sectional view taken along the longitudinal axis A of a single channel of the prior art printhead of FIG. 1 a , a nozzle plate  20  having respective ink ejection nozzles  22  is mounted at the front of the sheet  12  whilst an ink manifold  26  is defined at the rear by a manifold structure  21 . Tracks  16  are led to the rear of cover  14  for connection to a drive circuit, typically embodied in a microchip  27  which in turn is driven by signal received via input tracks  18 . 
     In printheads of this ilk, the channel walls and in particular the electrodes formed thereon—are often passivated so as to protect from subsequent corrosion by the ink. Reference is made in this regard to WO95/07820. 
     In the device discussed above, however, such conventional passivation prior to attachment of the cover would inhibit the formation of solder bonds between the electrodes and the tracks. On the other hand, passivation after the cover has been attached can only be applied from the end of the channel, resulting in low quality coating of the electrodes and tracks, especially at the midpoint of the channel remote from the channel ends. 
     SUMMARY OF THE INVENTION 
     The present invention has as an objective a printhead construction that retains the connection advantages associated with the conductive tracks formed on the cover of the prior art construction and yet is amenable to passivation. 
     Accordingly, the present invention consists in one aspect in droplet deposition apparatus comprising at least one channel having means for communicating with a supply of droplet liquid and an opening for ejection of droplets; 
     the channel being bounded on at least one side lying parallel to the channel axis by a channel wall associated with actuator means; the actuator means effecting displacement of the channel wall in response to electrical signals, thereby to effect ejection of droplets from the channel; 
     the channel being bounded on a further side lying parallel to the channel axis by a cover surface, the cover surface having formed thereon at least one conductive track for conveying electrical signals to said actuator means, the point of electrical connection between the track and the actuator means lying outside the channel. 
     Since the sole point of electrical connection between the track and the actuator in accordance with the present invention lies outside of the channel and thus out of contact with the ink (with its potentially corrosive effects), passivation of this point is no longer required. The channel itself can therefore be conventionally passivated via the open tops of the channels, thereafter, the cover can be attached and electrical contact established between the conductive tracks on the cover and the actuator means associated with the channel walls. Even in a printhead that—because of the type of ink it is designed to fire—does not require passivation, a point of electrical connection lying outside the channel as per the present invention is less likely to fail in fatigue than the channel-length solder bonds of the prior art device of FIGS. 1 a ,  1   b.    
     A corresponding method according to a first aspect of the invention consists in a method of manufacture of droplet deposition apparatus method of manufacture of droplet deposition apparatus, the method comprising the steps of: 
     forming in a base component at least one open-topped channel and, bounding said channel on at least one side lying parallel to the channel axis, a channel wall associated with actuator means for effecting displacement of the channel wall in response to electrical signals, thereby to effect ejection of droplets from the channel; 
     closing the channel on a further side lying parallel to the channel axis by a cover surface, the cover surface having formed thereon at least one conductive track for conveying electrical signals to said actuator means; and 
     electrically connecting the conductive track and the actuator means at a point lying outside the channel. 
     Advantageously, the step of closing the channel results in the electrical connection of the conductive track and the actuator means, thereby simplifying the manufacturing process. 
     The first aspect of the invention also consists in droplet deposition apparatus comprising: a bottom sheet of piezo-material poled droplet deposition apparatus comprising: 
     a bottom sheet of piezo-material poled in a direction normal to said sheet and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels and defined each by facing side walls and a bottom surface extending between said side walls; 
     a top sheet facing said bottom surfaces of said channels and bonded to said side walls to close said channels at the tops thereof; 
     respective nozzles communicating with said channels for the ejection of droplets of liquid therefrom; 
     connection means for connecting said channels with a source of droplet deposition liquid; 
     wherein each channel is formed with a forward part in which electrodes are provided on opposite sides of at least one of the side walls defining the channel, thereby to form a shear mode actuator for effecting droplet expulsion from the channel; and 
     wherein each channel is formed with a rearward part having an electrically-conductive coating which is in electrical contact with the at least one electrode on the channel-facing sides of the side walls in the forward part; 
     sealing means separating the forward part from the rearward part; and wherein 
     the apparatus further comprises conductive tracks formed on that surface of said top sheet that is bonded to said side walls, the conductive tracks being in electrical contact with the electrically-conductive coating in said rearward part. 
     A corresponding method comprises the steps of forming a bottom sheet with a layer of piezo-material poled in a direction method of manufacture of a droplet deposition apparatus comprising the steps of: 
     forming a bottom sheet with a layer of piezo-material poled in a direction normal to said sheet; 
     forming a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined by facing side walls and a bottom surface extending between said side walls, each channel further having a forward part and a rearward part; 
     forming electrodes on opposite sides of at least one of the side walls defining the forward part of each channel, thereby to form a shear mode actuator for effecting droplet expulsion from the channel; and 
     forming in the rearward part of each channel an electrically-conductive coating in electrical contact with a respective electrode; 
     providing a top sheet having a surface formed with conductive tracks thereon; and 
     bonding that surface of the top sheet having conductive tracks thereon to said side walls so as to close said channels at the tops thereof; 
     establishing electrical contact between said tracks and the respective electrically-conductive conductive coating of each channel; and 
     providing sealing means separating the forward and rearward parts of each channel. 
     A second aspect of the present invention consists in droplet deposition apparatus comprising droplet deposition apparatus comprising: 
     at least one longitudinal, open-topped droplet liquid channel defined by facing longitudinal side walls and a bottom, longitudinal surface extending between the side walls; 
     means for applying an electric field to piezoelectric material in at least one of said walls, thereby to effect displacement of the wall relative to said longitudinal channel so as to eject a droplet from the channel; and 
     a cover closing the open, longitudinal top side of the channel; 
     wherein said bottom longitudinal surface of the channel is formed with an opening for droplet ejection, and; 
     the cover incorporates two ports for supply of droplet liquid, the ports being spaced along the channel on either side of the opening. 
     Such a construction again simplifies the manufacture of known printheads, particularly those of the “top shooter” kind discussed in WO91/17051. FIG. 2 shows a sectional view along the channels of such a prior art printhead, with those features that correspond to FIG. 1 being denoted by corresponding reference numbers. Droplet ejection takes place from a nozzle  22  formed in the channel cover component  60  whilst droplet liquid is supplied to the channel via ports  33  formed in the channel base and which are typically connected in their turn to ink supply conduits (not shown) formed in a base component  35  that is separate from the piezoelectric channeled component  12 . 
     In accordance with the invention, an opening communicating with a droplet ejection orifice is formed in the bottom surface of the channel, thereby allowing the cover component to incorporate ports for supply of ink into the channel. A further, separate base component is consequently no longer required. 
     A third aspect of the invention comprises droplet deposition apparatus comprising: 
     at least one longitudinal, open-topped droplet liquid channel defined by facing longitudinal side walls and a bottom, longitudinal surface extending between the side walls; 
     means for supplying droplet liquid to the channel; 
     means for applying an electric field to piezoelectric material in at least one of said walls, thereby to effect displacement of the wall relative to said longitudinal channel so as to eject a droplet from the channel; and 
     a cover closing the open, longitudinal top side of the channel; 
     wherein the bottom longitudinal surface of the channel is formed with two openings for droplet ejection, the openings being spaced along the channel. 
     Such a construction brings to the arrangement of PCT application no. PCT/GB98/01495 the aforementioned advantage of reduced component count. 
     Corresponding method claims are also comprised in the present invention, and other aspects are as set out in other independent claims. 
     Further advantageous embodiments of the invention are set out in the description, drawings and dependent claims. 
     The disclosure of all claims is deemed incorporated here as consistory clauses, unless already set out above. 
     The invention will now be described by way of example by reference to the following diagrams, of which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 3 is a sectional view taken along the channel axis of a printhead according to a first embodiment of a first aspect of the present invention; 
     FIGS. 4 a  and  4   b  show detail of the rear part of the printhead of FIG. 3 before and after attachment of the cover respectively; 
     FIG. 5 is a sectional view taken along the channel axis of a printhead according to a second embodiment of a first aspect of the present invention; 
     FIG. 6 is a sectional view taken along the channel axis of a printhead incorporating both first and second aspects of the present invention; 
     FIG. 7 is a sectional view taken along the channel axis of a printhead according to a second embodiment of a second aspect of the present invention; 
     FIG. 8 is a detail perspective view of the end of the piezoelectric body of the printhead of FIG.  7 . 
     FIGS. 9 and 10 are sectional and detail sectional views respectively of an alternative embodiment of the printhead shown in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 illustrates a printhead according to a first embodiment of the first aspect of the present invention, with those features that are common to FIG.  3  and the prior art printhead of FIGS. 1 and 2 being designated by common reference numerals. 
     As in the prior art device, a piezoelectric ceramic body  12  poled in the thickness direction is formed with channels  11  separated by channel walls  13 . As known from EP-A-0 364 136 referred to above, electrodes  23  are formed along each wall  13  in the ink-containing channel  11  as well as extending along a rearward groove  100  to the rear face  130  of the body. In addition, there is provided a cover  14 , a surface  15  of which closes the open side of each of the channels  11 , a nozzle plate  20  with nozzles  22  for droplet ejection and a manifold for supply of ink into the channel in the form of a transverse cut in the body  12 . Surface  15  of cover  14  has tracks  16  formed thereon (suitable processes are well know) which in turn are connected to microchip  27  (which is illustrated figuratively in FIG.  3  and not to scale) which in turn receives input signals from input tracks  18 . 
     Detail of the rear part of the printhead prior to attachment of the cover is shown in FIG. 4 a : a passivation layer  140  (not shown in FIG. 3 but indicated by dashed hatching in FIG. 4 a ) is applied over the entirety of the electrodes  23  (indicated by solid hatching in both FIGS. 3 and 4 a ) in the channel and part way along the rearward groove  100 . In contrast to the prior art construction, passivation is carried out before attachment of the cover and advantageously according to the method described in WO95/07820. 
     A mechanical bond between body and surface  15  of cover  14  is achieved by means of adhesive layer  160 , applied to the end surfaces of the walls  13  in the region of the channels  11  prior to assembly of cover and body and preferably in accordance with the method discussed in WO95/04658. FIG. 4 b  illustrates the assembled printhead, with the adhesive bond being indicated at  220 . Such a bond may indeed be tougher and have a longer fatigue life than the corresponding solder bond of the prior art construction described above. 
     Electrical connection between the conductive tracks  16  on the cover and that part of the electrode  23  in the rearward groove  100  is achieved by a protrusion  170  of a malleable, deformable, conductive material such as solder affixed to the end  180  of track  16 . On assembly of the cover to the body, as illustrated in FIG. 4 b , protrusion  170  comes into contact with electrode  23  and is deformed, thereby providing an effective electrical contact  200  between electrode  23  and track  16 . 
     A bead  190  of a sealing paste or high viscosity glue is also applied so as to form on assembly an ink seal  210  between the end of the ink channel  11  and the electrical contact  200 . Such a seal protects the electrical contact from later corrosion by ink. Preferably, the seal is positioned so as to straddle the free end  150  of the passivation layer  140 , thereby preventing the seepage of ink under the passivation layer from where it might otherwise attack the electrode material  23 . 
     FIG. 5 illustrates a second embodiment of the first aspect of the present invention. A ceramic piezoelectric body  290  is, as in the previous embodiment, poled in the thickness direction and formed with channels  11  separated by channel walls  13  which in turn have an electrode  23  formed on each side. Ink ejection, however, takes place from a centrally located nozzle  320  formed either directly in the cover  350  or, as shown, in a nozzle plate  330  communicating with the channel via an aperture  340  formed in the cover. Body  290  is additionally formed with two manifolds  310  for supply of ink from both ends of the channel, as indicated by the arrows  300 . A further structure (not shown) will supply the manifolds with ink from a reservoir. 
     Such a “double-ended” printhead configuration is disclosed in WO91/17051 and has advantages in terms of a lower operating voltage over the “single-ended” configuration described above. Furthermore, the configuration of base  290  is suited to manufacture by moulding—a technique that is potentially more attractive from the point of view of manufacturability than conventional sawing techniques described in the aforementioned EP-A-0 364 136. 
     The connection of the channel electrode  23  to conductive tracks  370  formed on that surface of cover  350  facing body  290  is as already described with regard to FIGS. 3,  4   a  and  4   b , however, and is located in groove  360  formed at one side of the body  290 . Similarly, in the region of the channel itself (the channel walls of which are passivated prior to assembly) and at that end  380  of the body not occupied by an electrical connection, cover  350  is attached to the piezoelectric ceramic body by a conventional adhesive bond (not shown). 
     In order to minimise the distance traveled by the ink from the channel proper  11  to the outlet of the nozzle  320 —thereby reducing pressure losses and consequent reductions in droplet ejection velocity—the nozzle  320  may be formed in the cover  350  itself. Advantageously the nozzle is formed by laser ablation as described, for example, in WO93/15911, and to this end the cover may be made of an easily ablatable material, suitably a polymer such as polyimide, polycarbonate, polyester or polyetheretherketone, typically of 50 μm thickness. 
     The stiffness of a cover plate formed of such an easily ablatable material may be increased by application of a coating of stiffer material to the inner and outer surfaces of the ablatable cover plate. Particularly suitable for this purpose is silicon nitride: it can also be used as a passivant coating in the process of the aforementioned WO95/07820, is deposited as a smooth coating suitable for the subsequent application of a non-wetting coating, and will not short out electrodes of adjacent channels due to its non-conducting properties. Two layers of such a material placed either side of the polyimide cover and each having a thickness of around 5% of that of the cover (2.5 μm in the case of a 50 μm thick cover) will typically increase bending stiffness by a factor of 5-10 (based on standard compound beam theory and assuming a value of Young&#39;s Modulus for the stiffening material approximately 100 times greater than that of the polymer and good adhesion between the stiff and polymer materials). Such a thin layer has no significant effect on the ease with which the cover plate can be ablated to form a nozzle, particularly if the material of the layer itself is to some degree ablatable. 
     Expressed in broad terms, the cover plate for an inkjet printer comprises a layer of a first, easily ablatable, material having further layers bonded on opposite sides thereof, the further layers each being of a material having a stiffness at least an order of magnitude greater than that of the first material and being of a thickness at least an order of magnitude less than that of the first layer. 
     Referring now to FIG. 6, there is shown a printhead incorporating both first and second aspects of the present invention. Piezoelectric ceramic body  400  is formed with channels  11 , channel-separating walls  13  and electrodes  23  which are supplied with actuating signals via conductive tracks  410  connected to drive circuitry (not shown). Unlike previous embodiments, however, droplet ejection takes place from a nozzle  420  communicating with an opening  430  formed in the body  400  at the closed, bottom surface  440  of the channel  11 —this is in contrast to FIG. 5 where the nozzle  320  is located in a cover  350  closing the open, top side of the channel  11 . 
     Moulding is again the preferred method of manufacture of the channelled body  400 , and the arrangement of FIGS. 4 a  and  4   b  is again employed for electrical connection between the electrodes  23  and conductive tracks  410 . Communication hole  430  may also be formed during the moulding process or may be formed subsequently, e.g. by means of a laser. Cover  450  no longer incorporates a nozzle but is instead formed with ink inlet ports  460 . Such an arrangement has a lower component count than embodiments discussed earlier and has consequential manufacturing advantages. Alternatively, ink supply ports could be formed in the channelled component, e.g. at the channel ends. 
     The printhead of FIG. 7 also employs a cover component  500  having ink inlet ports  520 ,  522  and  524  located at either end and in the middle of a channel  11  formed in a piezoelectric body  530 . Channel walls are separated by a gap  540  into two sections  550 , 560  supplied by ports  520 , 522  and  522 , 524  respectively, with each section being independently actuable by means of respective electrodes  570 ,  580  driven by drive circuits (not shown) via conductive tracks  650 , 660 . For each section there is provided a respective nozzle  610 , 620  formed in a nozzle plate  615  and communicating with a section of the channel  11  via communication holes  630 , 640  formed in the bottom surface of the channel at points located midway between the respective inlet ports for that section. 
     Such a configuration is described in co-pending UK patent application no. 9710530.8 and results in a printhead having two parallel rows of independently actuable printing elements that is compact and which has a reduced actuating voltage per unit droplet ejection velocity due to the “double-ended” ink supply to each channel section. 
     Unlike earlier embodiments, the conductive tracks  650 ,  660  that electrically connect the channel electrodes to the drive chips are formed on the piezoelectric body itself, advantageously in the same step in which the electrodes  570 ,  580  are deposited on the channel walls. Such an arrangement is known from EP-A-0 397 441 and consequently will not be described in further detail here. Connection between track  650 ,  660  and drive chip  590 ,  600  may be achieved by any conventional method, including wire bonding or gold ball connection. 
     Piezoelectric body  530  may be moulded: in addition to having clear manufacturing advantages, such a process permits the end of the channel  11  to be formed as illustrated in FIG. 8, namely with a smooth, continuous transition  700  from the top surface  720  of the body to both the channel wall  730  and the bottom, longitudinal surface  710  of the channel. This in turn avoids discontinuities in the subsequently-deposited electrode material and the associated heating effects which might have a deleterious effect on the operational life of the printhead as a whole. 
     Alternatively, channels may be formed in the piezoelectric component by sawing using a disc cutter—as described e.g. in EP-A-0 309 148—and illustrated in the sectional and detail sectional views of FIGS. 9 and 10. It follows that the depth of the channel  11  will run out more gradually at each end, as shown at  800 , and that the piezoelectric channel wall defined between adjacent sawn channels  11  will run continuously between the two active sections  550 , 560 . However, a break  810  in the electrodes on the channel walls at a location between the two sections ensures that each the wall in active section can be actuated independently by signals supplied via electrical input  820 . Such a break may be achieved e.g. by masking during deposition of the metal plating or by removal of the plating by a laser. 
     Connection between the electrodes on the channel walls and the electrical input  820 , whilst not shown in detail, may be achieved by any of the known techniques including wire bond between tracks formed in shallow “run-out” grooves formed in the area  900  rearward of the channel  11  (described in the aforementioned EP-A-0 364 136) or conductive adhesive (e.g. anisotropic conductive adhesive) between conductive tracks formed in area  900  on the surface of the piezoelectric sheet itself and (described in EP-A-0 397 441). 
     As in the embodiment of FIG. 7, each channel  11  is closed along its two active sections  550 ,  560  by appropriate lengths  820 ,  830  of a cover component  500  which is also formed with ports  520 ,  522 ,  540  that allow ink to be supplied to each channel active section and, optionally, allow ink to be circulated through each channel section for cleaning purposes, s is generally known. Ports may be positioned so as to define the edge of an active section, as in the case of port  522 , in which case manufacture is simplified. In the example shown, the width of cover port  522  and the cover closing lengths  820 ,  830  are of the same order of magnitude, typically 2 mm. 
     Ink ejection from each active section is again via openings that communicate the channel with the opposite surface of the piezoelectric component (sheet  860 ) to that in which the channel is formed. In the present embodiment, these openings take the form of slots  840 , 850  which extend some distance—typically 200 μm—in the longitudinal direction of the channel so as to allow some leeway in the placing of the respective nozzles  870 , 880  in nozzle plate  890 . Offsetting of nozzles is generally necessary whenever simultaneous droplet ejection from adjacent channels is not possible e.g. in “shared wall” printheads of the kind illustrated, is generally known e.g. from EP-A-0 376, and will not therefore be discussed in any greater detail. 
     Printheads according to the present invention may also be made in a modular format as described in the aforementioned WO91/17051, each module being formed in opposite end surfaces thereof with respective channel parts so that, upon butting together of modules, further channels are formed between respective pairs of butted modules. In such arrangements, the respective channel parts may include at least part of a slot formed in the channel base and of sufficient length that, even if a pair of butted modules and their respective slot parts are not perfectly aligned, there remains an overlap between the two slot halves sufficient to accommodate a nozzle. 
     As in the previous embodiment, nozzles  870 , 880  are formed in a nozzle plate  890  which, as illustrated, may extend over the substantially the entire length of piezoelectric sheet  860  so as to provide a suitably large area for engagement e.g. of a capping and/or wiping mechanism. 
     It should be understood that this invention has been described by way of examples only and that a wide variety of modifications can be made without departing from the scope of the invention. Features shown in the context of the first aspect of the invention may be equally applicable to the second aspect and vice versa. 
     The piezoelectric channel walls, for example, can be polarised in opposite directions normal to the plane of the channel axes as known, for example, from EP-A-0 277 703. Alternatively, polarisation of the channel walls can be parallel to the plane of the channel axes with electrodes formed in the channel walls themselves as known, for example, from EP-A-0 528 647. 
     Nor is every channel in a printhead required to be capable of droplet ejection: active channels capable of droplet ejection may be alternated in the printhead with inactive—so-called “dummy” channels—as described, for example, in the aforementioned EP-A-0 277 703.