BACKGROUND OF THE INVENTION 
     The present invention relates to an ink jet nozzle head comprising means defining a nozzle and an ink channel connected to said nozzle, a diaphragm covering at least a portion of said ink channel, actuator means capable of deflecting the diaphragm, and bump means arranged for concentrating the force of the actuator means that is applied to the diaphragm. The present invention also relates to methods of manufacturing the nozzle head and, in particular, the diaphragm thereof. 
     A nozzle head of the present type is used for the generation of ink droplets in an ink jet printer. When a signal is applied to the actuator means, this causes the actuator means to deflect the diaphragm so that the volume of liquid ink contained in the ink channel is pressurized and deflected, and an ink droplet is expelled from the nozzle. 
     EP-A-0 718 102 discloses a nozzle head of this type in which the actuator means are formed by an elastic plate which is arranged in parallel with the diaphragm and spaced apart therefrom by a predetermined distance. An electrode is provided on a surface of the elastic plate for locally heating the same so that thermal stresses are created which cause the elastic plate to buckle. A connecting member or bump is disposed substantially in the central part of the elastic plate and mechanically connects the same to the diaphragm so that, when the plate buckles, the diaphragm is deflected accordingly. 
     In another known type of ink jet nozzle head which is disclosed, for example, in EP-A-0 402 172, the actuator means are formed by a piezoelectric finger which engages the diaphragm with its end face. A plurality of ink channels and nozzles respectively associated therewith are formed in the surface of a channel plate which is covered by the diaphragm. A separate piezoelectric actuator is provided for each of the ink channels. Thus, a multiple-nozzle head is formed in which the nozzles are arranged in a linear array and can be operated independently, so that a higher printing speed and/or image resolution can be achieved. 
     In order to increase the image resolution, it is desirable to make the pitch between the nozzles as small as possible. This means, however, that the associated ink channels must be comparatively narrow. When the width of the ink channels is decreased, the thickness of the diaphragm must be decreased as well in order to assure a sufficient flexibility of the diaphragm. In a practical print head in which the width of each ink channel is in the order of 300 μm, the thickness of the diaphragm which may for example be formed by a thin glass plate should ideally be smaller than 30 μm. However, such a thin diaphragm is difficult to manufacture and to handle. 
     In applicant&#39;s co-pending European patent application No. 96 202 043 it has been proposed to provide bumps on the surface of the diaphragm so that each piezoelectric actuator is connected with the diaphragm only in the limited area of a single bump which is considerably narrower than the piezoelectric finger. This increases the distance over which the diaphragm is allowed to flex, so that the flexibility of the diaphragm will be sufficient even when the thickness thereof is made larger. 
     The bumps can be formed for example by appropriately etching the surface of a glass plate which originally has a uniform thickness. 
     The problem encountered in this approach is that the surface area in which the piezoelectric finger is in contact with the diaphragm becomes very small. It should be observed in this context that the actuator should be firmly bonded to the diaphragm in order for the actuator to be capable of performing not only compression strokes but also suction strokes in which ink liquid is drawn into the ink channel. The smaller the contact area becomes the more difficult it is to firmly bond the actuator to the bump. 
     In addition, if the height of the bump is comparatively small, e.g. in the order of only 1 μm, the cavities formed on either side of the bump are likely to become clogged with adhesive, with the result that the force of the actuator is no longer concentrated on the area of the bump but is partly transmitted also via the adhesive on either side thereof, so that the effect of the bump is diminished or completely eliminated. Similar problems are encountered when the shallow grooves between the bumps are soiled with dust particles or the like or when the surface of the piezoelectric actuator itself is rather rough, due to the granular structure of the piezoelectric material. 
     In order to avoid problems of this kind, the height of the bumps should be increased to at least about 5 μm. This, however, makes the etching process time consuming and expensive. In addition, when an amorphous material such as glass is used for the diaphragm, the etching process is isotropic and, hence, the formation of sufficiently high but narrow bumps is difficult to control. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an ink jet nozzle head which, in spite of a small width of the ink channel, can be manufactured easily and reliably and with reproducible characteristics. 
     According to the present invention, the diaphragm has at least two layers, and the bump means forms a spacer which is disposed between these two layers. This solution has the advantage that the diaphragm has a smooth and continuous outer surface and accordingly provides a sufficiently large bonding surface for firmly bonding the diaphragm to the actuator. Nevertheless, the force of the actuator is concentrated by the bump so that the inner layer of the diaphragm facing the ink channel is allowed to flex over the whole length between the bump and the lateral edge of the ink channel. Moreover, the cavities formed on either side of the bump are completely enclosed between the two layers of the diaphragm. This eliminates the risk that these cavities can become clogged with adhesive or other foreign matter. As a result the height of the bumps or, conversely, the depth of the grooves between them, can be reduced considerably, which drastically increases the production efficiency. 
     When the diaphragm is deflected, the two layers thereof behave substantially like two separate diaphragms, because they are spaced apart by the bumps and, accordingly, there is no friction between the two layers. Thus, the stiffness of the diaphragm as a whole is approximately equal to only twice the stiffness of a single layer. In comparison, the stiffness of a diaphragm consisting only of a single layer with twice this thickness would be four times as large, because the bending stiffness of a plate is approximately equal to the third power of the thickness. Thus, the thickness of the two layers of the diaphragm according to the present invention can be allowed to be so large that it will impose no serious manufacturing problems. 
     Once a channel plate has been prepared and the ink channels have been formed in the surface thereof, a glass plate which is to form the inner layer of the diaphragm and the bumps is superposed on the channel plate so as to cover the open faces of the ink channels. If the channel plate itself is made of glass, the superimposed glass plate may be compression bonded to the channel plate, so that an integral member is formed. Selected portions of the outer surface of the superposed glass plate are then etched away so that only the bumps are left in the form of elongate ridges extending along the center lines of the ink channels. Since the glass plate is already fixedly connected to the channel plate in this state, the bumps can be positioned precisely relative to the ink channels. Then, a second glass plate forming the outer layer of the diaphragm is superimposed on the bumps and is fixed thereto by compression bonding. 
     Since the diaphragm is thus integrally connected with the channel plate, it can be handled easily and safely in the subsequent mounting steps, and, in particular, there is no risk that the gaps between the bumps can be terminated. 
     The process described above can be performed with comparatively large wafers which are then diced to form a plurality of integrated channel plate/diaphragm units for a plurality of nozzle heads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a portion of a nozzle head with parts broken away for illustration purposes; 
     FIG. 2 is an enlarged cross-sectional view of the nozzle head according to FIG. 1 as viewed in the direction of arrow  11  in FIG. 1; 
     FIG. 3 is a cross-sectional view corresponding to FIG. 2, but showing an active state of the nozzle head; and 
     FIGS. 4 to  7  are cross-sectional views illustrating steps of a manufacturing process for the nozzle head. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As is shown in FIG. 1, an ink jet nozzle head  10  comprises a channel plate  12  which has a front face  14  formed with a linear array of equidistant nozzles  16 . A plurality of ink channels  18  are formed in the top surface of the channel plate  12 . These ink channels are arranged in parallel to one another and are connected to corresponding nozzles  16 . 
     A diaphragm  20  is bonded to the top surface of the channel plate  12  so as to cover the open faces of the ink channels  18  and the nozzles  16 . 
     An actuator member  22  is superimposed on the diaphragm  20  and forms a plurality of piezoelectric actuators  24  which are configured as parallel, downwardly extending fingers, the lower end faces of which being bonded to the diaphragm  20 . Each actuator  24  is opposed to one of the ink channels  18 . 
     A backing plate  26  is overlaid on the top side of the actuator member  22  and is bonded thereto for absorbing reaction forces caused by the expansion and retraction strokes of the individual actuators  24 . 
     As is shown in FIG. 2, the diaphragm  20  is an integral, layered structure with a lower layer or inner layer  28  facing the ink channels  18  and a top layer or outer layer  30  connected to the actuators  24 . The two layers  28  and  30  are interconnected and at the same time held in a spaced-apart relationship by a plurality of elongated bumps  32  which are centered on the ink channels  18 . The width of the actuators  24  is only slightly smaller than the width of the ink channels  18 , whereas the width of the bumps  32  is considerably smaller than that of the actuators. 
     The separation between the layers  28  and  30  is exaggerated in the drawing. In practice, the distance between these layers and hence the height of cavities  34  enclosed by the layers  28 ,  30  and the bumps  32  may amount to only 1 or 2 μm. 
     FIG. 3 illustrates how the diaphragm  20  is deflected when one of the actuators  24  performs a compression stroke. The length of the stroke of the actuator is also exaggerated in the drawing and amounts to less than 0.1 μm in a practical embodiment. It will be observed that the outer layer  30  is allowed to flex over a distance L 1  which corresponds to the width of the gap between two adjacent actuators  24 , whereas the inner layer  28  is allowed to flex over a length L 2  which corresponds to the distance between the bump  32  and the edge of the ink channel  18 . Both lengths L 1  and L 2  are considerably larger than the distance D between the edge of the actuator  24  and the edge of the ink channel  18 . Thus, even when the thickness of each of the layers  28  and  30  is 30 μm or larger, the stiffness of the diaphragm  20  as a whole is small enough for efficiently transferring the mechanical energy of the actuator  24  to the ink volume in the channel  18 . 
     The cavities  34  between the layers  28  and  30  are slightly compressed when the diaphragm is deflected, but as the stroke length is small in comparison to the height of the cavities. Thus the layers  28  and  30  will never contact each other. 
     A manufacturing process for the nozzle head described above will now be explained with reference to FIGS. 4 to  7 . 
     FIG. 4 shows a portion of a wafer  36  from which a number of channel plates  12  are to be formed. The ink channels  18  and the nozzles  16  are formed in a top surface of the wafer  36 . Then, as is shown in FIG. 5, a glass plate  38  is disposed on the top surface of the wafer  36  and is bonded thereto. If the substrate  36  is also made of glass, the bonding may be achieved without adhesive, by compression bonding or thermocompression bonding. The thickness of the glass plate  38  corresponds to that of the lower layer  28  of the diaphragm plus the height of the bumps  32 . 
     As is illustrated in FIG. 6, a pattern of parallel grooves  40  is formed in the top surface of the glass plate  38 , for example by means of conventional etching techniques. The material left between the grooves  40  forms the bumps  32 . Since the ink channels  18  are visible through the transparent glass plate  38 , the masking for the etching process can be applied appropriately in order to center the bumps  32  on the ink channels. 
     Finally, as is illustrated in FIG. 7, another thin glass plate which is to form the outer layer  30  is superimposed on the bumps  32  and is bonded thereto by compression bonding or thermocompression bonding, thereby forming the integral structure of the diaphragm  20 . 
     The wafer  36  with the diaphragm  20  formed thereon is then diced to form a plurality of integral channel plate/diaphragm units for a plurality of nozzle heads  10 . The lower end faces of the actuators  24  of the actuator member  22  are bonded to the top surface of the diaphragm  22  by means of an adhesive. 
     Since the layered structure of the diaphragm  20  comprising the two layers  28  and  30  and the bumps  32  disposed therebetween has a comparatively high inherent strength, the diaphragm  20  can be handled relatively safely as a separate member, so that it is also possible to manufacture the diaphragm separately and then bond it to the channel plate  12  by means of an adhesive or another suitable bonding technique. The diaphragm  20  can also be made from other materials such as a metal or even a plastic. 
     It is clear that to maintain the cleanliness of the cavities  34  the diaphragm  20  is closed on all sides. 
     While only specific embodiments of the present invention have been described above, it will occur to a person skilled in the art that the invention can be modified in various ways without departing from the spirit and scope of the invention as defined in the appended claims.
An ink jet nozzle head including a substrate defining a nozzle and an ink channel connected to the nozzle, a diaphragm covering at least a portion of the ink channel, an actuator capable of deflecting the diaphragm, and a bump arranged for concentrating the force of the actuator that is applied to the diaphragm, wherein the diaphragm has at least two layers, and the bump forms a spacer for separating these two layers.