Abstract:
A method of manufacturing an ink jet print head includes the steps of bonding a body of piezoelectric material to a base plate and cutting channels into the piezoelectric material to form ink chambers which are actuated by applying voltages to electrodes provided on surfaces of the chambers. The base plate carries IC&#39;s which contain the drive circuitry for actuating the ink chambers. To ensure reliable electrical connection between the chamber electrodes and the IC&#39;s, the electrodes and conducting tracks on the base plate are formed in a single step by depositing a conductive layer over both the PZT body and the base plate. A step of masking or selective removal of material may be performed to achieve the necessary patterning of the electrodes and tracks.

Description:
RELATED APPLICATION DATA 
     This is a continuation of International Application No. PCT/GB99/03799 filed Nov. 15, 1999, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to droplet deposition apparatus, particularly inkjet printheads, components thereof and methods for manufacturing such components. 
     BACKGROUND OF THE INVENTION 
     A particularly useful form of inkjet printer comprises a body of piezoelectric material with ink channels formed, for example, by disc cutting. Electrodes can be plated on the channel-facing surfaces of the piezoelectric material, enabling an electrical field to be applied to the piezoelectric “wall” defined between adjacent channels. With appropriate poling, this wall can be caused to move into or out of the selected ink channel, causing a pressure pulse which ejects an ink droplet through an appropriate channel nozzle. Such a construction is shown, for example, in EP-A-0 364 136. 
     It is a frequent requirement to provide a high density of such ink channels, with precise registration across a relatively large expanse of printhead, perhaps an entire page width. A construction that is useful to this end is disclosed in WO 98/52763. It involves the use of a flat base plate that supports the piezoelectric material as well as integrated circuits performing the necessary processing and control functions. 
     Such a construction has several advantages, particularly with regard to manufacture. The base plate acts as a “backbone” for the printhead, supporting the piezoelectric material and integrated circuits during manufacture. This support function is particularly important during the process of butting together multiple sheets of piezoelectric material to form a contiguous, pagewide array of ink channels. The relatively large size of the base plate also simplifies handling. 
     A problem remains of reliably and efficiently establishing electrical connection between the ink channel electrodes and the corresponding pins of the integrated circuits. If the base plate is of suitable material and suitably finished, conductive tracks can be deposited on it, these tracks connecting in known manner with the IC pins. There remains the difficulty of establishing connections to channel electrodes. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide improved apparatus and methods which address this problem. 
     Accordingly, the present invention consists in one aspect in a method of manufacturing a component of a droplet deposition apparatus, the component comprising a body of piezoelectric material having a plurality of channels each with a channel surface and a base, the body being attached to a surface of the base which is free of substantial discontinuities; the method comprising the steps of attaching the body to said surface of the base; and depositing a layer of conductive material so as to extend continuously over at least one of said channel surfaces and said surface of the base to provide an electrode on each channel surface and a conductive track on said surface of the base which is integrally connected to the electrode. 
     The attachment of the body to a surface of the base and subsequent deposition of a continuous layer of conductive material over said at least one channel surface and the base surface results in an effective and reliable electrical connection between channel wall electrodes and substrate conductive tracks. Those tracks can be used to provide connection with one or more integrated circuits carried on the base, either directly or through other tracks and interconnections. 
     The present invention also consists in a component for a droplet deposition apparatus comprising a body of piezoelectric material formed with a plurality of channels each channel having a channel surface; and a separate base having a base surface free of substantial discontinuities; wherein the body is attached to said base surface and a layer of conductive material extends continuously over said channel surfaces of and said base surface, thereby defining an electrode on each channel surface and a conductive track connected thereto on the base surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a longitudinal sectional view through a known ink jet printhead; 
         FIG. 2  is a transverse sectional view on line AA of  FIG. 1   FIG. 3  is an exploded view of a page wide printhead array according to the prior art; 
         FIG. 4  is an assembled longitudinal sectional view through the printhead shown in  FIG. 3 ; 
         FIG. 5  is an assembled sectional view, similar to that of  FIG. 4 , of a printhead according to a first embodiment of the invention; 
       FIGS.  6 ( a ) and  6 ( b ) are detail sectional views taken perpendicular and parallel to the channel axis of the device of  FIG. 5 ; 
         FIG. 7  is a detail perspective view of the device of  FIG. 5 ; 
         FIG. 8  is a cross-sectional view through a channel of a printhead according to a second embodiment of the invention; 
         FIGS. 9-11  are a sectional views along the channel of third, fourth and fifth embodiments of the invention respectively; 
         FIGS. 12 and 13  are perspective and detail perspective views respectively of the embodiment of  FIG. 11 ; 
         FIG. 14  is a detail view of the area denoted by reference  Figure 194  in FIG.  6 ( b ); 
         FIG. 15  is a perspective view showing a step in the manufacture of a printhead of the kind shown in  FIG. 11 ; and 
         FIG. 16  is a sectional view illustrating a further modification. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It will be helpful to describe first in some detail, examples of the prior art constructions referred to briefly above. 
     Thus,  FIG. 1  shows a prior art inkjet printhead  1  of the kind disclosed in WO 91/17051 and comprising a sheet  3  of piezoelectric material, for example lead zirconium titanate (PZT), formed in a top surface thereof with an array of open-topped ink channels  7 . As evident from  FIG. 2 , which is a sectional view taken along line AA of  FIG. 1 , successive channels in the array are separated by side walls  13  which comprise piezoelectric material poled in the thickness direction of the sheet  3  (as indicated by arrow P). On opposite channel-facing surfaces  17  are arranged electrodes  15  to which voltages can be applied via connections  34 . As is known, e.g. from EP-A-0 364 136, application of an electric field between the electrodes on either side of a wall results in shear mode deflection of the wall into one of the flanking channels—this is shown exaggerated by dashed lines in FIG.  2 —which in turn generates a pressure pulse in that channel. 
     The channels are closed by a cover  25  in which are formed nozzles  27  each communicating with respective channels at the mid-points thereof. Droplet ejection from the nozzles takes place in response to the aforementioned pressure pulse, as is well known in the art. Supply of droplet fluid into the channels, indicated by arrows S in  FIG. 2 , is via two ducts  33  cut into the bottom face  35  of sheet  3  to a depth such that they communicate with opposite ends respectively of the channels  7 . Such a channel construction may consequently be described a double-ended side-shooter arrangement. A cover plate  37  is bonded to the bottom face  35  to close the ducts. 
       FIGS. 3 and 4  are exploded perspective and sectional views respectively of a printhead employing the double-ended side-shooter concept of  FIGS. 1 and 2  in a “pagewide” configuration. Such a printhead is described in WO 98/52763, incorporated herein by reference. Two rows of channels spaced relatively to one another in the media feed direction are used, with each row extending the width of a page in a direction ‘W’ transverse to a media feed direction P. Features common with the embodiment of  FIGS. 1 and 2  are indicated by the same reference Figures used in  FIGS. 1 and 2 . 
     As shown in  FIG. 4 , which is a sectional view taken perpendicular to the direction W, two piezoelectric sheets  82   a ,  82   b  each having channels (formed in their bottom surface rather than their top as in the previous example) and electrodes as described above are closed (again on their bottom surface rather than their top) by a flat, extended base  86  in which openings  96   a ,  96   b  for droplet ejection are formed. Base  86  is also formed with conductive tracks (not shown) which are electrically connected to respective channel electrodes, e.g. by solder bonds as described in WO 92/22429, and which extend to the edge of the base where respective drive circuitry (integrated circuits  84   a ,  84   b ) for each row of channels is located. 
     Such a construction has several advantages, particularly with regard to manufacture. Firstly, the extended base  86  acts as a “backbone” for the printhead, supporting the piezoelectric sheets  82   a ,  82   b  and integrated circuits  84   a ,  84   b  during manufacture. This support function is particularly important during the process of butting together multiple sheets  3  to form a single, contiguous, pagewide array of channels, as indicated at  82   a  and  82   b  in the perspective view of FIG.  3 . One approach to butting is described in WO 91/17051 and consequently not in any further detail here. The size of the extended cover also simplifies handling. 
     Another advantage arises from the fact that the surface of the base on which the conductive tracks are required to be formed is flat, i.e. it is free of any substantial discontinues. As such, it allows many of the manufacturing steps to be carried out using proven techniques used elsewhere in the electronics industry, e.g. photolithographic patterning for the conductive tracks and “flip chip” for the integrated circuits. Photolithographic patterning in particular is unsuitable where a surface undergoes rapid changes in angle due to problems associated with the spinning method typically used to apply photolithographic films. Flat substrates also have advantages from the point of view of ease of processing, measuring, accuracy and availability. 
     A prime consideration when choosing the material for the base is, therefore, whether it can easily be manufactured into a form where it has a surface free of substantial discontinuities. A second requirement is for the material to have thermal expansion characteristics to the piezoelectric material used elsewhere in the printhead. A final requirement is that the material be sufficiently robust to withstand the various manufacturing processes. Aluminium nitride, alumina, INVAR or special glass AF45 are all suitable candidate materials. 
     The droplet ejection openings  96   a ,  96   b  may themselves be formed with a taper, as per the embodiment of  FIG. 1 , or the tapered shape may be formed in a nozzle plate  98  mounted over the opening. Such a nozzle plate may comprise any of the readily-ablatable materials such as polyimide, polycarbonate and polyester that are conventionally used for this purpose. Furthermore, nozzle manufacture can take place independently of the state of completeness of the rest of the printhead: the nozzle may be formed by ablation from the rear prior to assembly of the active body  82   a  onto the base or substrate  86  or from the front once the active body is in place. Both techniques are known in the art. The former method has the advantage that the nozzle plate can be replaced or the entire assembly rejected at an early stage in assembly, minimising the value of rejected components. The latter method facilitates the registration of the nozzles with the channels of the body when assembled on the substrate. 
     Following the mounting of piezoelectric sheets  82   a ,  82   b  and drive chips  84   a ,  84   b  onto the substrate  86  and suitable testing as described, for example, in EP-A-0 376 606—a body  80  can be attached. This too has several functions, the most important of which is to define, in cooperation with the base or substrate  86 , manifold chambers  90 , 88  and  92  between and to either side of the two channel rows  82   a ,  82   b  respectively. Body  80  is further formed with respective conduits as indicated at  90 ′,  88 ′ and  92 ′ through which ink is supplied from the outside of the printhead to each chamber. It will be evident that this results in a particularly compact construction in which ink can be circulated from common manifold  90 , through the channels in each of the bodies (for example to remove trapped dirt or air bubbles) and out through chambers  88  and  92 . Body  80  also provides surfaces for attachment of means for locating the completed printhead in a printer and defines further chambers  94   a ,  94   b , sealed from ink-containing chambers  88 , 90 , 92  and in which integrated circuits  84   a ,  84   b  can be located. 
     Turning now to an example of the present invention, reference is made to FIG.  5 . This is a sectional view similar to that of  FIG. 4 , illustrating a printhead in accordance with the present invention. Wherever features are common with the embodiments of  FIGS. 1-4 , the same reference figures as used in  FIGS. 1-4  have been used. 
     As with the previous embodiments, the printhead of  FIG. 5  comprises a “pagewide” base plate or substrate  86  on which two rows of integrated circuits  84  are mounted. In-between lies a row of channels  82  formed in the substrate  84 , each channel of which communicates with two spaced nozzles  96   a ,  96   b  for droplet ejection and with manifolds  88 ,  92  and  90  arranged to either side and between nozzles  96   a ,  96   b  respectively for ink supply and circulation. 
     In contrast to the printhead embodiments discussed above, the piezoelectric material for the channel wails is incorporated in a layer  100  made up of two strips  110   a ,  110   b . As in the embodiment of  FIG. 4 , these strips will be butted together in the page width direction W, each strip extending approximately 5-10 cm (this being the typical dimension of the wafer in which form such material is generally supplied). Prior to channel formation, each strip is bonded to the continuous planar surface  120  of the substrate  86 , following which channels are sawn or otherwise formed so as to extend through both strip and substrate. A cross-section through a channel, its associated actuator walls and nozzle is shown in FIG.  6 . Such an actuator wall construction is known, e.g. from EP-A-0 505 065 and consequently will not be discussed in any greater detail. Similarly, appropriate techniques for removing both the glue bonds between adjacent butted strips of piezoelectric material and the glue relief channels used in the bond between each piezoelectric strip and the substrate are known from US 5,193,256 and WO 95/04658 respectively. 
     In accordance with the present invention, a continuous layer of conductive material is then applied over the channel walls and substrate. Not only does this form electrodes  190  for application of electric fields to the piezoelectric walls  13 —as illustrated in FIG.  6 ( a )—and conductive tracks  192  on substrate  86  for supply of voltages to those electrodes as shown in FIG.  6 ( b )—it also forms an electrical connection between these two elements as shown at  194 . 
     Appropriate electrode materials and deposition methods are well-known in the art. Copper, Nickel and Gold, used alone or in combination and deposited advantageously by electroless processes utilising palladium catalyst will provide the necessary integrity, adhesion to the piezoelectric material, resistance to corrosion and basis for subsequent passivation e.g. using Silicon Nitride as known in the art. 
     As is generally known, e.g. from the aforementioned EP-A-0 364 136, the electrodes on opposite sides of each actuator wall  13  must be electrically isolated from one another in order that an electric field may be established between them and hence across the piezoelectric material of the actuator wall. This is shown in both the prior art arrangement of FIG.  2  and the embodiment of the present invention shown in FIG.  6 ( a ). The corresponding conductive tracks connecting each electrode with a respective voltage source must be similarly isolated. 
     In the present invention, such isolation may be achieved at the time of deposition for example by masking those areas—such as the tops of the channel walls—where conductive material is not required. Suitable masking techniques, including patterned screens and photolithographically patterned masking materials are well-known in the art, e.g. from WO 98/17477 and EP-A-0 397 441, and will not be described in any further detail. 
     Alternatively, isolation may be achieved after deposition by removing conductive material from those areas where it is not required. Localised vaporisation of material by laser beam, as known e.g. from JP-A-09 010 983, has proved most suitable for achieving the high accuracy required, although other conventional removal methods—inter alia sand blasting, etching, electropolishing and wire erosion may also be suitable.  FIG. 7  illustrates material removal, in this case over a narrow band running along the top of the wall, although several passes of the laser beam (or a single pass of a wider laser beam) can be used to remove material from the entire top surface of the wall so as to maximise the wall top area available for bonding with the cover member  130 . 
     In addition to removing conductive material from the top surface  13 ′ of each piezoelectric actuator wall  13  so as to separate the electrodes  190 ′,  190 ″, on either side of each wall, conductive material must also be removed from the surface of the substrate  86  in such a way as to define respective conductive tracks  192 ′,  192 ″ for each electrode  190 ′  190 ″. At the transition between piezoelectric material  100  and substrate  86 , the end surface of the piezoelectric material  100  is angled or chamfered as shown at  195 . As is known, this has the advantage over a perpendicular cut (of the kind indicated by a dashed line at  197 ) of allowing the vapourising laser beam—shown figuratively by arrow  196 —to impinge on and thereby remove the conductive material without requiring angling of the beam. Preferably, the chamfer  195  is formed by milling after the piezoelectric layer  100  has been attached to the substrate  86  but before the formation of the channel walls which, being typically 300 μm thick and formed of ceramic and glass, are vulnerable to damage. A chamfer angle of 45 degrees has been found to be suitable. 
     It will also be appreciated that the electrodes and conductive tracks associated with the active portions  140   a  need to be isolated from those associated with  140   b  in order that the rows of nozzles might be operated independently. Although this too may be achieved by a laser “cut” along the surface of the substrate  86  extending between the two piezoelectric strips, it is more simply achieved by the use of a physical mask during the electrode deposition process or by the use of electric discharge machining. 
     Laser machining can also be used in a subsequent step to form the ink ejection holes  96   a ,  96   b  in the base of each channel, as is known in the art. Such holes may directly serve as ink ejection nozzles. Alternatively, there may be bonded to the lower surface of the substrate  86  a separate plate (not shown) having nozzles that communicate with the holes  96   a ,  96   b  and which are of a higher quality that might otherwise be possible with nozzles formed directly in the ceramic or glass base of the channel. Appropriate techniques are well-known, particularly from WO 93/15911 which discloses a technique for the formation of nozzles in situ, after attachment of the nozzle plate, thereby simplifying registration of each nozzle with its respective channel. 
     The conductive tracks  192 ′,  192 ″ defined by laser may extend all the way from the transition area  195  to the integrated circuits  84  located at either side of the substrate. Alternatively, the laser track definition process may be restricted to an area directly adjacent the piezoelectric material and a different—e.g. photolithographic—process used to define further conductive tracks that connect the laser-defined tracks with the integrated circuits  84 . 
     Having established tile electrical connections, it remains only to adhesively bond (e.g. using an offset method) a cover member  130  to the surface of substrate  86 . This cover fulfils several functions: firstly, it closes each channel along those portions  140   a ,  140   b  where the walls incorporate piezoelectric material in order that actuation of the material and the resulting deflection of the walls might generate a pressure pulse in the channel portions and cause ejection of a droplet through a respective opening. Secondly, the cover and substrate define between them ducts  150   a ,  150   b  and  150   c  which extend along either side of each row of active channel portions  140   a ,  140   b  and through which ink is supplied. The cover is also formed with ports  88 ,  90 ,  92  which connect ducts  150   a ,  150   b  and  150   c  with respective parts of an ink system. In addition to replenishing the ink that has been ejected, such a system may also circulate ink through the channels (as indicated by arrows  112 ) for heat, dirt and bubble removing purposes as is known in the art. A final function of the cover is to seal the ink-containing part of the printhead from the outside world and particularly the electronics  84 . This has been found to be satisfactorily achieved by the adhesive bond between the substrate  86  and cover rib  132 , although additional measures such as glue fillets could be employed. Alternatively, cover rib may be replaced by an appropriately shaped gasket member. 
     Broadly expressed, the printhead of  FIG. 5  includes a first layer having a continuous planar surface; a second layer of piezoelectric material bonded to said continuous planar surface; at least one channel that extends through the bonded first and second layers; the second layer having first and second portions spaced along the length of the channel; and a third layer that serves to close on all sides lying parallel to the axis of the channel portions of the channel defined by said first and second portions of said second layer. 
     It will be appreciated that restricting the use of piezoelectric material to those “active” portions of the channel where it is required to displace the channel walls is an efficient way, of utilising what is a relatively expensive material. The capacitance associated with the piezoelectric material is also minimised, reducing the load on—and thus the cost of—the driving circuitry. 
     Whereas the printhead of  FIGS. 5 and 6  employs actuator walls of the “cantilever” type in which only part of the wall distorts in response to the application of an actuating electric field, the actuator walls of the printhead of  FIGS. 8 and 9  actively distort over their entire height into a chevron shape. As is well-known and illustrated in  FIG. 8 , such a “chevron” actuator has upper and lower wall parts  250 , 260  poled in opposite directions (as indicated by arrows) and electrodes  190 ′,  190 ″ on opposite surfaces for applying a unidirectional electric field over the entire height of the wall. The approximate distorted shape of the wail when subjected to electric fields is shown exaggerated in dashed lines  270  on the right-hand side of FIG.  8 . 
     Various methods of manufacturing such “chevron” actuator walls are known in the art, e.g. from EP-A-0 277 703, EP-A-0 326 973 and WO 92/09436. For the printhead of  FIGS. 9 and 10 , two sheets of piezoelectric material are first arranged such that their directions of polarisation face one another. The sheets are then laminated together, cut into strips and finally bonded to an inactive substrate  86 , as already explained with regard to FIG.  5 . 
     One consequence of the entire actuator wall height being defined by piezoelectric material is that there is no need to saw wall-defining grooves into the inactive substrate  86 . There remains, of course, the need for the length of the nozzles  96   a ,  96   b  to be kept to a minimum so as to minimise losses that would otherwise reduce the droplet ejection velocity. To this end, the substrate can be reduced in thickness either locally by means of a trench  300  as shown in FIG.  9  and formed advantageously by sawing, grinding or moulding—or overall per FIG.  10 . Both arrangements need to provide free passage for a disc cutter (shown diagrammatically in dashed lines at  320 ) used to form the channels in the piezoelectric strips. 
     Following channel formation and in accordance with the present invention, conductive material is then deposited and electrodes/conductive tracks defined. In the examples shown, piezoelectric strips  110   a  and  110   b  are chamfered to facilitate laser patterning, as described above. Nozzle holes  96   a ,  96   b  are also formed at two points along each channel. 
     Finally a cover member  130  is bonded to the tops of the channel walls so as to create the closed, “active” channel lengths necessary for droplet ejection. In the printhead of  FIG. 9 , the cover member need only comprise a simple planar member formed with ink supply ports  88 ,  90 ,  92  since gaps  150   a ,  150   b ,  150   c  necessary for distributing the ink along the row of channels are defined between the lower surface  340  of that cover member  130  and the surface  345  of the trench  300 . Sealing of the channels is achieved at  330  by the adhesive bond (not shown) between the lower surface  340  of the cover  130  and the upper surface of the substrate. Broadly expressed, the printhead of this third invention embodiment includes a first layer of inactive material; a second layer of piezoelectric material comprising first and second portions formed with channels and bonded to the first layer in a spaced relationship; a third layer that serves to close the channels on all sides lying parallel to their axes; and outlets formed in the first layer for ink ejection from said channels in said portions of the second layer. 
     In the embodiment of  FIG. 10 , the simplicity of substrate  86  formed without trench  300  is offset by the need to form a trench-like structure  350  (defined, for example, by a projecting rib  360 ) in the cover  130  so as to define ink supply ducts  150   a ,  150   b ,  150   c.    
     Turning to the embodiment of  FIG. 11 , this also employs the combination of a simple substrate  86  and a more-complex cover  130 , in this case a composite structure made up of a spacer member  410  and a planar cover member  420 . Unlike previous embodiments, however, it is the substrate  86  rather than the cover that is formed with ink supply ports  88 ,  90 ,  92  and the cover  130  rather than the substrate that is formed with holes  96  for droplet ejection. In the example shown, these holes communicate with nozzles formed in a nozzle plate  430  attached to the planar cover member  420 . 
       FIG. 12  is a cut-away perspective view of the printhead of  FIG. 11  seen from the cover side. The strips  110   a ,  110   b  of “chevron”-poled piezoelectric laminate have been bonded to substrate  86 , and subsequently cut to form channels. A continuous layer of conductive material has then been deposited over the strips and parts of the substrate and electrodes and conductive tracks defined thereon in accordance with the present invention. As explained with regard to  FIGS. 5 and 6 , the strips are chamfered on either side (at  195 ) to aid laser patterning in this transition area. 
       FIG. 13  is an enlarged view with spacer member  410  removed to show the conductive tracks  192  in more detail. Although not shown for reasons of clarity, it will be appreciated that these, like channels  7 , extend across the entire width of the printhead. In the area of the substrate adjacent each strip (indicated by arrow  500  with regard to strip  110   b ) the tracks are continuous with the electrodes (not shown) on the facing walls of each channel, having been deposited in the same manufacturing step. This provides an effective electrical contact in accordance with the present invention. 
     However, elsewhere on the substrate—as indicated at  510 —more conventional techniques, for example photolithographic, can be used to define not only tracks  192  leading from the channel electrodes to the integrated circuits  84  but also further tracks  520  for conveying power, data and other signals to the integrated circuits. Such techniques may be more cost effective, particularly where the conductive tracks are diverted around ink supply ports  92  and which would otherwise require complex positional control of a laser. They are preferably formed on the alumina substrate in advance of the ink supply ports  88 ,  90 ,  92  being drilled (e.g. by laser) and of the piezoelectric strips  110   a ,  110   b  being attached, chamfered and sawn. Following deposition of conductive material in the immediate area of the strips, a laser can then be used to ensure that each track is connected only with its respective channel electrode and no other. 
     Thereafter, both electrodes and tracks will require passivation, e.g. using Silicon Nitride deposited in accordance with WO 95/07820. Not only does this provide protection against corrosion due to the combined effects of electric fields and the ink (it will be appreciated that all conductive material contained within the area  420  defined by the inner profile  430  of spacer member  410  will be exposed to ink), it also prevents the electrodes on the opposite sides of each wall being short circuited by the planar cover member  430 . Both cover and spacer are advantageously made of molybdenum which, in addition to having similar thermal expansion characteristics to the alumina used elsewhere in the printhead, can be easily machined, e.g. by etching, laser cutting or punching, to high accuracy. This is particularly important for the holes for droplet ejection  96  and, to a lesser extent, for the wavy, bubble-trap-avoiding, inner profile  430  of the spacer member  410 . Bubble traps are further avoided by positioning the trough  440  of the wavy profile such that it aligns with or even overlies the edge of the respective ink port  92 . Crest  450  of the wavy profile is similarly dimensioned (to lie a distance—typically 3 mm, approximately 1.5 times the width of each strip  110   a ,  110   b —from the edge of the adjacent strip  110   a ,  110   b  to ensure avoidance of bubble traps without affecting the ink flow into the channels. 
     Spacer member  410  is subsequently secured to the upper surface of substrate  86  by a layer of adhesive. In addition to its primary, securing function, this layer also provides back-up electrical isolation between the conductive tracks on the substrate. Registration features such as notch  440  are used to ensure correct alignment. 
     The last two members to be adhesively attached—either separately or following assembly to one another—are the planar cover member  420  and nozzle plate  430 . Optical means may be employed to ensure correct registration between the nozzles formed in the nozzle plate and the channels themselves. Alternatively, the nozzles can be formed once the nozzle plate is in situ as known, for example, from WO 93/15911. 
     A further feature is illustrated in  FIG. 14 , which is a detail view of the area denoted by reference  Figure 194  in FIG.  6 ( b ). The fillet  550  created when adhesive is squeezed out during creating of the joint between the piezoelectric layer  100  and substrate  86  is advantageously retained when chamfer  195  is formed on the end surface of the layer as described above. This adhesive fillet is subsequently exposed when the assembly is subjected to a pre-plating cleaning step (e.g. plasma etching) and provides a good key for the electrode material  190  in an area that would otherwise be vulnerable to plating faults. 
     A further modification is explained with reference to FIG.  15 . As already explained above, the piezoelectric material for the channel walls is incorporated in a layer  100  made up of two strips  110   a ,  110   b  each butted with other strips in the direction W necessary for a wide array of channels. Depending on whether the actuator is of the “cantilever” or “chevron” type, the piezoelectric layer will be polarised in one or two (opposed) directions and, in the latter case, may be formed from two oppositely-polarised sheets laminated together as shown at  600  and  610  in FIG.  15 . To facilitate relative positioning, strips  110   a ,  110   b  are connected together by a bridge piece  620  that is removed in the chamfering step that takes place once strip  100  and substrate  86  have been bonded together using adhesive. 
     A still further modification is illustrated in FIG.  16 . Here, the integrated circuit  84  is not mounted on the substrate  86  but on an auxiliary substrate  700 , which may be single or multi-layer. The substrate  86  is appropriately bonded to the auxiliary substrate  700  and wire bonds  702  connect the conductive tracks on the substrate  86  with the pins of the integrated circuit. Further wire bonds  704  then interconnect the integrated circuit with pads  708  on the auxiliary substrate  700 . 
     The present invention has been explained with regard to the figures contained herein but is in no way restricted to such embodiments. In particular, the present techniques are applicable to printheads of varying width and resolution, pagewide double-row being merely one of many suitable configurations. Printheads having more than two rows, for example, are easily realised using tracks used in multiple layers as well-known elsewhere in the electronics industry. 
     All documents, particularly patent applications, referred to are incorporated in the present application by reference.