Abstract:
An inkjet printhead containing at least one ink channel communicating with a nozzle and defined between two opposed side walls; a membrane interconnecting the side walls; and actuating elements arranged adjacent to the membrane for deflecting the same, wherein the side walls and the membrane are formed by a one-piece member.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an inkjet printhead comprising at least one ink channel connected to a nozzle and defined between two opposing side walls; a membrane interconnecting the side walls; and actuating means arranged adjacent to the membrane for deflecting the membrane. 
     A conventional printhead of the type, which is disclosed for example in EP-A-0 819 524, comprises a plurality of ink channels which are arranged side-by-side so that the associated nozzles form a linear nozzle array. The ink channels and the nozzles are formed by grooves cut into the surface of a substrate which may, for example, be made of silicon. The membranes for the various ink channels are formed by a continuous thin sheet which is overlaid on the substrate so that it covers the open top sides of the grooves. This sheet has to be firmly bonded to the regions of the substrate defining the side walls of the ink channels. The actuators are formed by piezo-electric fingers which are supported by a common backing plate and have their respective free ends bonded to the top surfaces of the membranes, so that, when a voltage is applied to one of the piezo-electric fingers, the corresponding membrane is flexed into the volume of the associated ink channel which is filled liquid ink, so that an ink droplet is expelled from the nozzle. 
     U.S. Pat. No. 4,657,631 discloses another type of printhead in which the ink channels are formed by groove-like structures in a metal layer which is formed on the flat surface of the substrate. Thus, the bottom wall of each ink channel is formed by a portion of the substrate, and rigid side walls and a rigid top wall are formed by the metal layer. The actuators are disposed inside of each ink channel and are disposed on the surface of the substrate, so that they are directly exposed to the ink liquid without a membrane intervening between the actuator and the ink. This type of printhead can be manufactured by forming a photo-sensitive layer on the surface of the substrate and by exposing and developing this layer, thereby forming a pattern of ridges which have a shape complementary to that of the ink channels. A metal layer is then formed on the surface of the substrate by sputtering and subsequent electronic plating, until the ridges are buried in the metal layer, the photo-sensitive material is then removed so that the ink channels are formed in the metal layer. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an inkjet printhead which can be manufactured more easily and with a higher production yield. According to the present invention, the side walls and the membrane are formed by a one-piece member. This construction has the advantage that, on the one hand, the membrane can be made very thin so that it has a higher flexibility, and, on the other hand, the member forming the membrane and the side walls of the ink channels can, as a whole, have a comparatively high structural strength. This greatly facilitates the manufacturing process and makes it possible to achieve a high production yield. 
     Advantageously, the one-piece member which forms the membranes and the side walls of the plurality of ink channels is a metal foil which is obtained in a growth process such as electroforming. This has the advantage that the membrane can be made very thin and the thickness thereof can be controlled with high accuracy. Further, since the foil can be formed directly on the surface on the substrate, no assembly process or separate bonding step is required for forming a complete channel plate, including the substrate, and the metal foil which forms the membrane and the side walls of the ink channels. 
     By forming the metal foil on the surface of a photoresist which has appropriately been shaped by means of photolithographic techniques, it is possible to obtain a three-dimensional structure of the foil which forms not only the membrane and the side walls of the ink channels but also the nozzles, with appropriate sizes and shapes. The cross-section of the foil in a plane normal to the axis of the ink channels has a meandering-like shape, with a space formed between the portions of the foil which define the side walls of two adjacent ink channels. This construction greatly reduces the amount of mechanical or acoustic coupling between the adjacent ink channels, so that cross-talk among the various channels of the printhead is reduced significantly. In addition, this cross-sectional shape of the foil has the effect that the thermal expansion of the channel plate is controlled only by the material of the substrate, e.g. silicon, which is particularly useful when the printhead is used for hot-melt ink and, accordingly, operates at high temperatures. Since the nozzles are formed directly by the three-dimensionally structured foil, no mechanical finishing of the nozzle front of the printhead is necessary. 
     By providing a thin metal layer on the surface of the substrate on which the foil is applied in a later manufacturing step, it is possible to obtain a design in which the ink channels and, more important, also the nozzles are completely surrounded by only one type of material, i.e. metal, so that the directionality of the droplet-formation process will not be influenced by differences in the adhesiveness of the walls defining the nozzles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of THE essential parts of an inkjet printhead according to the present invention; 
     FIG. 2 is a perspective bottom view of the printhead with parts of a substrate being removed for illustration purposes; 
     FIG. 3 is a cross-sectional view of a member defining the ink channels of the printhead according to a modified example; and 
     FIGS. 4-9 illustrate a sequence of steps for manufacturing the printhead according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As is shown in FIG. 1, the inkjet printhead comprises a channel plate  10  which has a substrate  12  made of silicon, and a three-dimensionally structured metal foil  14  formed on the flat top surface of the substrate  12 . The foil  14  defines a plurality of parallel ink channels  16  which are arranged side-by-side and each converge to a nozzle  18 , so that the nozzles form a linear array along the front side of the substrate  12 . 
     Each ink channel  16  has a pair of opposed side walls  20  which are interconnected by a membrane  22 , so that the membrane  22  forms a top wall of the ink channel opposite to the substrate  12 . The side walls  20  and membranes  22  of all the ink channels  16  are integrally formed by a one-piece member, i.e. by the metal foil  14 . 
     The printed further comprises a plurality of piezo-electric actuators  24  only one of which is shown in FIG.  1 . The actuators  24  are disposed on each of the membranes  22 , so that, when they are electrically energized, they perform expansion stroke, and the membrane  22  flexes into the interior of the ink channel. Thus, when the ink channel is filled with liquid ink, an ink droplet will be expelled from the nozzle  18 . 
     The foil  14  also forms a front wall  26  in which the nozzles  18  are defined. Each nozzle is connected to the associated ink channel  16  by a funnel portion  28  which is also an integrated part of the foil  14 . Further, the foil  14  forms crater-like reinforcement members  30  which are partially cut by the front wall  26  and are provided between the nozzles  18  for improving the structural strength of the front wall  26 . 
     As can be seen more clearly in FIGS. 2 and 3, the foil  14  has a meander-like cross-sectional shape in a plane normal to the longitudinal direction of the ink channels  16 , with V-shaped concave spaces  32  formed between the side walls  20  of each pair of adjacent ink channels. Thus, when an individual actuator  24  is energized and an acoustic pressure wave is generated in the ink liquid contained in the associated ink channel  16 , the space  32  will prevent this pressure wave from being transmitted to the neighboring ink channels, so that cross-talk among the various channels is avoided. In addition, due to its meander-like cross-section, the foil  14  can behave like an expansion bellow and can adapt itself to thermal expansions of the substrate  12  without causing a distortion of the channel plate  10  as a whole. On the other hand, when exposed to vertical pressure, the foil  14  has a comparatively high strength and is surprisingly robust, even when the thickness of the foil is only in the order of several μm. 
     In a typical embodiment, the thickness of the foil in the portion forming the membranes  22  is between 3 and 30 μm, preferably 15 μm, with an accuracy of ±1 μm. This assures a high flexibility of the membranes  22  so that the mechanical energy of the actuators  24  is readily transmitted to the ink liquid. The high level of accuracy in the membrane thickness assures a uniform performance of all of the ink channels. 
     FIG. 3 illustrates a modified embodiment in which an outwardly projecting ridge or bump  34  is formed along the longitudinal center line of each membrane  22  which allows for a high amount of deflexion of the membrane with little strain on the foil material, even when the width of the actuator  24  is comparatively large. 
     A process of manufacturing the channel plate  10  will now be described in conjunction with FIGS. 4-9. 
     At first, as is shown in FIG. 4, a thin layer  36  of metal, e.g. of nickel or a nickel alloy is formed on the flat top surface of the substrate  12 . The layer  36  covers the whole surface of the substrate with the exception of a portion  38  directly adjacent to the nozzle side of substrate and a hole  40  near the rear end of each ink channel. Then, a three-dimensionally structured photoresist  42  is applied on the substrate and on the layer  36  by means of photolithographic techniques. The shape of the photoresist  42  is complementary to the relief of the bottom side of the foil  14  shown in FIG.  2 . 
     In the next step, the foil  14  is formed on the surface of the layer  36  and on the surface of the photoresist  42  by means of a nickel-electroforming process. The result is shown in FIG.  6 . Then, as is shown in FIG. 7, an ink feed channel  44  is formed in the substrate  12  by etching, laser drilling, powder blasting or the like. 
     In order to form the nozzle face  46  of the channel plate FIG. 8, a dicing cut is performed in the substrate  12 , the photoresist  42 , and the foil  14 . The plane of this dicing cut is slightly offset from the front wall  26  of the foil  14 , so that a small horizontal flange  48  is formed along the upper edge of the front wall  26 . 
     Finally, the photoresist  42  is removed, so that the ink channel  16  and the nozzle  18  are formed, as is shown in FIG.  9 . The ink channel communicates with the ink feed channel  44  through the hole  40  formed in the layer  36 . As is also shown in FIG. 9, the front edge of the layer  36  is located in the same longitudinal position as the upper and outer edge of the wall of the nozzle  18 , so that the mouth of the nozzle  18  lies in a plane normal to plane of the substrate  12  and is slightly recessed from the main part of the front wall  26 . As a result, the droplets will be expelled in a direction strictly aligned with the longitudinal direction of the ink channel, and no finishing needs to be applied to the nozzle face. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.