Patent Publication Number: US-7585059-B2

Title: Ink jet head and production method therefor

Description:
TECHNICAL FIELD 
   The present invention relates to an ink jet head and a production method therefor and, more specifically, to an ink jet head of a top shooter type having active areas provided on opposite sides of nozzle holes thereof for pressurizing ink. 
   BACKGROUND ART 
   Known as a prior art related to the present invention is an ink jet head of a side shooter type, which includes porous members such as of sponge provided as vibration absorbers in junctions between a plurality of ink pressurizing chambers and common ink chambers for prevention of a crosstalk phenomenon which may otherwise occur between the ink pressurizing chambers due to pressure waves after ink ejection (see, for example, Patent Document 1).
     Patent Document 1: Japanese Unexamined Patent Publication No. 2000-43252   

   DISCLOSURE OF THE INVENTION 
   Problems to be Solved by the Invention 
   The construction of a typical prior-art ink jet head of a top shooter type will be described with reference to  FIG. 9 , which is an explanatory diagram schematically illustrating the construction of a prior-art ink jet head unit of the top shooter type. 
   As shown in  FIG. 9 , the prior-art ink jet head unit  100  of the top shooter type primarily includes a piezoelectric board  101  polarized along the thickness thereof, and a nozzle plate  109  bonded onto the piezoelectric board  101 . 
   The piezoelectric board  101  includes a plurality of elongated ink chambers  104  formed by dicing, and shallow channel portions  105  provided on end portions of the respective ink chambers  104 . 
   Electrodes  106  and electrode protection films (not shown) for protecting the electrodes  106  are provided on interior surfaces of the ink chambers  104  and the shallow channel portions  105 , and the electrodes  106  respectively extend to the shallow channel portions  105  to be connected to terminals of an external board  108  arranged at a pitch corresponding to the pitch of the shallow channel portions  105 . 
   In the ink chambers  104 , common ink chambers  107 A,  107 B extending perpendicularly to the ink chambers  104  for communication between adjacent ink chambers  104  are provided on opposite sides of nozzle holes  110  with respect to the lengths of the ink chambers  104 . 
   That is, the common ink chambers  107 A,  107 B extend perpendicularly to the plurality of ink chambers  104 , and ink is supplied into the ink chambers  104  through the common ink chambers  107 A,  107 B and ejected from the nozzle holes  110 . 
   When voltages are applied to the electrodes  106  in the respective ink chambers  104  from the external board  108  according to image data, the inner walls of the ink chambers  104  are deformed inward, whereby ink in the ink chambers  104  is pressurized. As a result, the ink is ejected from the nozzle holes  110 . 
   Areas contributable to the ejection of the ink are herein referred to as active areas A.E 1 , A.E 2 , which are provided on opposite sides of each of the nozzle holes  110  in the ink jet head unit  100  of the top shooter type. 
   Such a piezoelectric-type ink jet head unit  100  is capable of controlling the deformation of the piezoelectric body by controlling the voltages for controlling the pressure of the ink and the amount of droplets of the ink to be ejected, thereby permitting easy gradation printing. 
   Unlike the ink jet head of the side shooter type described in BACKGROUND ART, the ink jet head unit  100  of the top shooter type can be driven at relatively low ejection voltages because the active areas A.E 1 , A.E 2  contributable to the ink ejection are present on the opposite sides of each of the nozzle holes  110 . Therefore, the ink jet head unit of the top shooter type is advantageous in terms of heat generation and power consumption. 
   With the provision of the two active areas A.E 1 , A.E 2 , however, the lengths of the active areas A.E 1 , A.E 2  vary among the ink chambers  104 , if the nozzle plate  109  is not bonded at a predetermined position. 
   This is because the lengths of the active areas A.E 1 , A.E 2  are determined by distances between the nozzle hole  110  and the common ink chambers  107 A,  107 B in the prior-art ink jet head unit  100  of the top shooter type. In other words, the active areas A.E 1 , A.E 2  are determined by the lengths of bonding areas of the nozzle plate  109  bonded to upper face portions of the walls of the ink chambers  104  between the nozzle hole  110  and the common ink chambers  107 A,  107 B in the prior-art ink jet head unit  100  of the top shooter type. 
   Therefore, it is essential to bond the nozzle plate  109  to the piezoelectric board  101  so that the nozzle hole  110  is located at a mid-point between the common ink chambers  107 A and  107 B in each of the ink chambers  104  in the prior-art ink jet head unit  100  of the top shooter type. 
   If the active areas A.E 1 , A.E 2  differ in length in each of the ink chambers  104 , the time required for pressure waves occurring in the ink chamber  104  to reach the nozzle hole  110  differs between the active areas A.E 1  and A.E 2 , resulting in a time lag. This results in variations in ejection characteristics of the ink jet head  100 . 
   Further, if the number of times of the ejection per unit time is increased by increasing a driving frequency for higher speed image formation, residual vibration occurring due to a pressure in the ink chambers  104  may make the subsequent ink ejection unstable. 
   The residual vibration which is the cause of the unstable ink ejection is a phenomenon such that the pressure waves (vibration) occurring after the ink ejection are mostly reflected on interfaces between the ink chambers  104  and the common ink chambers  107 A,  107 B back into the ink chambers  104  without attenuation by a water hammer effect, and aversely influences the characteristics of the subsequent ink ejection. 
   Particularly, where the driving frequency is increased, the subsequent ink ejection occurs before the attenuation of the residual pressure waves, leading to significant variations in ink ejection speed. In a certain case, this may result in extremely unstable ink ejection or inability of ink ejection. 
   Not only the top shooter type but also the side shooter type suffer from the problem of the unstable ejection due to the residual vibration. The ink jet head described in BACKGROUND ART copes with the crosstalk problem and other problems attributable to the residual vibration by providing the vibration absorbers such as of sponge in the junctions between the common ink chambers and the ink pressurizing chambers to attenuate the pressure waves when the pressure waves pass through the vibration absorbers. 
   However, the provision of the vibration absorbers for coping with the residual vibration complicates the construction of the ink jet head, and reduces the productivity, thereby increasing production costs. 
   In view of the foregoing, the present invention is directed to an ink jet head which prevents deterioration of ejection characteristics attributable to bonding offset of the nozzle plate and suppresses the residual vibration occurring due to the pressure waves without complication of the construction thereof, thereby ensuring stable ejection characteristics even in high speed driving. 
   Means for Solving the Problems 
   According to the present invention, there is provided an ink jet head, which comprises: a piezoelectric board having a plurality of elongated channels arranged parallel to each other and isolated from each other by channel walls; and a nozzle plate having a plurality of nozzle holes provided in association with the channels, and bonded onto the piezoelectric board with the nozzle holes located at generally longitudinally middle positions of the respective channels; wherein the nozzle plate has trench-like recesses each having a predetermined width and a predetermined depth and extending perpendicularly to the channels as being spaced equidistantly from the nozzle holes longitudinally of the channels; gaps for communication between adjacent channels are defined by the recesses between a surface of the nozzle plate and upper face portions of the channel walls located equidistantly from the nozzle holes; and active areas contributable to ink ejection are defined in each of the channels on opposite sides of the corresponding nozzle hole along the channel. 
   Effects of the Invention 
   According to the present invention, the trench-like recesses each having the predetermined width and the predetermined depth are formed in the nozzle plate as extending perpendicularly to the channels and spaced equidistantly from the nozzle holes longitudinally of the channels, and the gaps for the communication between the adjacent channels are defined by the recesses between the surface of the nozzle plate and the upper face portions of the channel walls located equidistantly from the nozzle holes. Further, the active areas contributable to the ink ejection are defined in each of the channels on the opposite sides of the corresponding nozzle hole along the channel. Therefore, the lengths of the active areas as measured longitudinally of the channel are inevitably equal to each other in the channel, thereby preventing the deterioration of the ejection characteristics which may otherwise occur due to bonding offset of the nozzle plate. 
   According to the present invention, the lengths of the active areas are determined not by the accuracy of the bonding between the piezoelectric board and the nozzle plate but by the positional accuracy of the recesses formed in the nozzle plate. Even if the piezoelectric board and the nozzle plate are not bonded to each other with desired bonding accuracy in a production process, the lengths of the active areas are inevitably equal to each other as long as the positional accuracy of the recesses formed in the nozzle plate falls within a predetermined range. This allows for stable production of a high quality ink jet head. 
   Further, pressure waves generated by pressurizing ink in the respective channels are mostly attenuated when passing through the gaps which communicate with the adjacent channels. Thus, residual vibration is suppressed, so that the ejection characteristics are stably maintained even in high speed driving. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an ink jet head unit including an ink jet head according to an embodiment of the present invention. 
       FIG. 2  is a sectional view taken along a line A-A in  FIG. 1 . 
       FIG. 3  is a process diagram showing a production process for an ink jet head main body. 
       FIG. 4  is a process diagram showing a process for producing the ink jet head unit by incorporating the ink jet head main body. 
       FIG. 5  is a process diagram showing the process for producing the ink jet head unit by incorporating the ink jet head main body. 
       FIG. 6  is a process diagram showing a preparation process for a nozzle plate. 
       FIG. 7  is an explanatory diagram for explaining how pressure waves occurring after ink ejection are absorbed or attenuated when passing through gaps defined by recesses of the nozzle plate. 
       FIG. 8  is an explanatory diagram illustrating a partial section taken along a line B-B in  FIG. 7  on a greater scale for explaining how the pressure waves occurring after the ink ejection are absorbed or attenuated when passing through the gaps defined by the recesses of the nozzle plate. 
       FIG. 9  is an explanatory diagram illustrating the construction of a prior-art ink jet head unit. 
   

   DESCRIPTION OF THE REFERENCE NUMERALS 
   
       
         1 : Piezoelectric board 
         2 : Ink jet head 
         4 : Ink chambers 
         5 : Shallow channel portions 
         6 : Electrodes 
         7 A, 7 B: Common ink chambers 
         10 : Ink jet head main body 
         20 : Base 
         20   a : Head accommodating recess 
         21 A, 21 B: Ink supply pipes 
         22 A, 22 B: Flexible wiring boards 
         23 : Nozzle holes 
         24 A, 24 B: Recesses 
         25 : Nozzle plate 
         25   a : Base film 
         25   b : Water repellent film 
         26 : Ink jet head unit 
         27 : Protection tape 
         30 , 31 : Dicing blades 
         40 : Nozzle hole formation mask 
         40   a , 50   a , 50   b : Openings 
         50 : Recess formation mask 
       A.E 1 ,A.E 2 : Active areas 
       D: Depth of recess 
       L: Length of recess as measured longitudinally of ink chamber 
     
  
   BEST MODE FOR CARRYING OUT THE INVENTION 
   An ink jet head according to the present invention includes: a piezoelectric board having a plurality of elongated channels arranged parallel to each other and isolated from each other by channel walls; and a nozzle plate having a plurality of nozzle holes provided in association with the channels, and bonded onto the piezoelectric board with the nozzle holes located at generally longitudinally middle positions of the respective channels; wherein the nozzle plate has trench-like recesses each having a predetermined width and a predetermined depth and extending perpendicularly to the channels as being spaced equidistantly from the nozzle holes longitudinally of the channels; gaps for communication between adjacent channels are defined by the recesses between a surface of the nozzle plate and upper face portions of the channel walls located equidistantly from the nozzle holes; and active areas contributable to ink ejection are defined in each of the channels on opposite sides of the corresponding nozzle hole along the channel. 
   In the present invention, the piezoelectric board includes two piezoelectric plates having different polarization directions and bonded to each other. 
   The channels may be elongated grooves which are arranged parallel to each other as being isolated from each other by the channel walls and each have a predetermined width and a predetermined depth. Electrodes which respectively generate electric fields perpendicularly to the polarization directions to deform the channel walls in shearing directions when voltages are applied to interior surfaces of the respective channels according to image data are provided on interior surfaces of the respective channels. 
   In the inventive ink jet head, the recesses preferably each have a depth of not greater than 20 μm. 
   With this arrangement, the gaps defined between the upper face portions of the channel walls and the surface of the nozzle plate by the recesses each have a very small height, so that the majority of pressure waves can be more effectively attenuated when passing through the very small gaps. Therefore, residual vibration due to the pressure waves can be more effectively prevented, which may otherwise influence the subsequent ink ejection. Thus, an image can be formed as having a higher quality and a higher fidelity to the image data at a higher speed. 
   Even if the depth is greater than 20 μm, it is possible to provide a pressure wave attenuation effect. However, the maximum attenuation effect can be provided when the recesses each have a depth of not greater than 20 μm as will be described in the following embodiment. 
   In the inventive ink jet head, distances between the recesses and the nozzle holes are preferably determined based on lengths of active areas which are required for providing desired ejection characteristics. 
   The lengths of the active areas contributable to the ink ejection should be optimally determined in consideration of various factors such as the shapes of the ink chambers, the type of the ink and the driving frequency, and are one of the most important factors in designing the ink jet head. 
   In the present invention, as described above, the recesses are located equidistantly from the nozzle holes, and the lengths of the active areas are defined by the distances between the recesses and the nozzle holes. Therefore, even if the nozzle plate and the piezoelectric board are bonded to each other in offset relation in a production process, the active areas can be located on opposite sides of the nozzle holes as each having an optimum length for ensuring the desired ejection characteristics by providing the recesses in optimum positions for ensuring the desired ejection characteristics. 
   Thus, the ink jet head can be stably produced as having a higher quality and a higher performance. 
   In the inventive ink jet head, the recesses of the nozzle plate are preferably formed by an excimer laser process. 
   This is because it is a common practice to employ the excimer laser process for the formation of the nozzle holes in the nozzle plate. 
   Where the nozzle holes are formed by the excimer laser process with the use of a nozzle hole formation mask in a nozzle hole processing step and then the recesses are formed by the excimer laser process with the use of a recess formation mask, the production of the inventive ink jet head can be achieved simply by newly preparing the recess formation mask. Thus, an increase in production costs can be minimized. 
   Where a pulse-controlled excimer laser is employed, the recesses are formed as having the desired depth with a higher level of reproducibility by controlling the number of pulses. 
   In the inventive ink jet head, the nozzle plate may be composed of a polymer material. 
   In this arrangement, the polymer material is preferably one of a polyimide film and polyether sulfone. 
   This is because the polymer material such as the polyimide film or polyether sulfone absorbs the excimer laser to be decomposed into molecules or atoms with its molecular bonds cut and evaporate. Thus, the nozzle holes and the recesses can be each formed as having a shape strictly conforming to the pattern of the mask and, therefore, the polymer material is advantageous for forming the nozzle holes and the recesses with a higher level of accuracy. 
   As described above, the formation of the recesses can be achieved simply by preparing the recess formation mask in addition to the nozzle hole formation mask, thereby minimizing the increase in production costs. 
   According to another aspect of the present invention, there is provided a production method for the inventive ink jet head, comprising the steps of: forming a plurality of channels in a piezoelectric board; forming nozzle holes in a nozzle plate; forming trench-like recesses having a predetermined width and a predetermined depth in the nozzle plate, the recesses extending perpendicularly to the channels as being spaced equidistantly from the nozzle holes of the nozzle plate longitudinally of the channels; and bonding the nozzle plate to the piezoelectric board with the nozzle holes being located at generally longitudinally middle positions of the respective channels. 
   The present invention will hereinafter be described in detail by way of embodiment thereof illustrated in the drawings. 
   EMBODIMENT 
   An ink jet head and a production method therefor according to embodiment of the present invention will be described with reference to  FIGS. 1 to 8 . 
     FIG. 1  is a perspective view of an ink jet head unit including an ink jet head according to an embodiment of the present invention, and  FIG. 2  is a sectional view taken along a line A-A in  FIG. 1 .  FIG. 3  is a process diagram showing a production process for an ink jet head main body, and  FIGS. 4 and 5  are process diagrams showing a process for producing the ink jet head unit by incorporating the ink jet head main body.  FIG. 6  is a process diagram showing a preparation process for a nozzle plate.  FIGS. 7 and 8  are explanatory diagrams for explaining how pressure waves occurring after ink ejection are absorbed or attenuated when passing through gaps defined by recesses of the nozzle plate and, particularly,  FIG. 8  illustrates a partial section taken along a line B-B in  FIG. 7  on a greater scale. 
   As shown in  FIGS. 1 and 2 , an ink jet head  2  incorporated in an ink jet head unit  26  primarily includes a piezoelectric board  1  as an ink jet head main body  10  having a plurality of elongated ink chambers (channels)  4  arranged parallel to each other and isolated from each other by ink chamber walls (channel walls)  4   a , and a nozzle plate  25  having a plurality of nozzle holes  23  provided in association with the ink chambers  4  and bonded onto the piezoelectric board  1  with the nozzle holes  23  being located at generally longitudinally middle positions of the respective ink chambers  4 . 
   The nozzle plate  25  has trench-like recesses  24 A,  24 B extending perpendicularly to the ink chambers  4  as being spaced equidistantly from the nozzle holes  23  longitudinally of the ink chambers  4  and each having a predetermined width and a predetermined depth. Gaps for communication between adjacent ink chambers  4  are defined by the recesses  24 A,  24 B between a surface of the nozzle plate  25  and upper face portions of the ink chamber walls  4   a  located equidistantly from the nozzle holes  23 , and active areas A.E 1 , A.E 2  contributable to ink ejection are defined in each of the ink chambers  4  on opposite sides of the corresponding nozzle hole  23  along the ink chamber  4 . 
   What should be herein noted is that the lengths of the active areas A.E 1 , A.E 2  disposed on the opposite sides of the nozzle hole  23  are inevitably equal to each other as measured longitudinally of the ink chamber  4  even if the nozzle plate  25  is bonded to the ink jet head main body  10  in offset relation, and that the small gaps are defined by the recesses  24 A,  24 B between the nozzle plate  25  and the upper face portions of the ink chamber walls  4   a , because the recesses  24 A,  24 B are spaced equidistantly from the nozzle holes  23 . 
   Since the lengths of the active areas A.E 1 , A.E 2  are inevitably equal to each other, the times required for pressure waves generated in the respective active areas A.E 1 , A.E 2  by pressurization of ink in the ink chambers  4  to reach the nozzle holes  23  are equal to each other, thereby preventing the deterioration of the ejection characteristics. 
   The pressure waves generated in the ink chambers  4  are mostly absorbed or attenuated by the small gaps defined by the recesses  24 A,  24 B when passing through the gaps, thereby suppressing the deterioration of the ejection characteristics occurring due to residual vibration. Thus, the ejection characteristics are stably maintained even in high speed driving. 
   A production process for the ink jet head main body will be described in detail with reference to  FIG. 3 . 
   As shown in  FIG. 3(   a ), a dicing blade  30  is moved up and down with respect to a piezoelectric board  1  as indicated by arrows to form a plurality of ink chambers  4  and shallow channel portions  5 . 
   Round profiles present between the ink chambers  4  and the shallow channel portions  5  conform to the outer shape of the dicing blade  30 . 
   The piezoelectric board  1  has a thickness of 1.02 mm, and is prepared by bonding two piezoelectric plates having different polarization directions and respectively having thicknesses of 0.22 mm and 0.8 mm. 
   The shallow channel portions  5  may be formed only on one edge of the piezoelectric board  1 , but are formed on opposite edges of the piezoelectric board  1  in this embodiment. 
   The shallow channel portions  5  later function as external connection terminals with electrodes  6  (see  FIG. 2 ) formed on surfaces thereof in the subsequent step. Where the shallow channel portions  5  are provided on the opposite edges of the piezoelectric board  1  as in this embodiment, a connection pitch is doubled as compared with a case in which the shallow channel portions  5  are provided only on one edge of the piezoelectric board  1 . Therefore, the pitch of the ink chambers  4  as measured in a widthwise direction is not limited by a connection pitch limit of the external connection terminals. 
   Further, there is no need to reduce the connection pitch of the external connection terminals to the connection pitch limit, advantageously allowing for highly reliable connection with external boards. 
   The ink chambers  4  each have a depth of about 250 μm and a width of 80 μm, and are arranged at a pitch of 169.3 μm. This permits a nozzle density of 150 DPI. 
   On the other hand, the shallow channel portions  5  each have a depth of 25 μm and a width of 80 μm which equals to the width of the ink chamber  4 . 
   The shallow channel portions  5  which later serve as the external connection terminals may be subjected to a blade process a plurality of times so as to be broadened. Thus, the reliability of the connection to the external boards is further improved. In general, the shallow channel portions  5  each having a width of 80 μm ensures sufficiently reliable connection. 
   Even if the pitch of the ink chambers  4  laid out according to this embodiment is reduced to less than 169.3 μm to increase the nozzle density, the reliable connection to the external boards can be ensured by providing the shallow channel portions  5  on the opposite edges of the piezoelectric board  1  for the sufficient connection pitch, and broadening the shallow channel portions  5  as described above. 
   After the ink chambers  4  and the shallow channel portions  5  are formed in the piezoelectric board  1  as described above, electrodes  6  (see  FIG. 2 ) are formed on interior surfaces of the ink chambers  4  and interior surfaces of the shallow channel portions  5  by an evaporation method, a sputtering method or a plating method. 
   In the formation of the electrodes  6 , an electrode material is also applied on surface portions of the piezoelectric board  1  other than the ink chambers  4  and the shallow channel portions  5 . In this state, short-circuit may occur between the adjacent ink chambers  4 . Therefore, the surface portions of the piezoelectric board  1  are ground by a thickness of 20 μm by means of a dicing machine for removal of the unwanted electrode material present on the surface portions of the piezoelectric board  1  after the formation of the electrodes  6 . 
   As a result, the thickness of the piezoelectric board  1  is reduced from 1.02 mm to 1.0 mm, and the electrodes  6  are present only on the interior surfaces of the ink chambers  4  and the shallow channel portions  5 . Thus, the shallow channel portions  5  serve as the external connection terminals for connection to the external boards. 
   A permissible variation in parallelism between the rear and front surfaces of the piezoelectric board  1  resulting from the grinding process is 1 μm at the maximum. 
   In turn, common ink chambers  7 A,  7 B are formed by means of a wider dicing blade  31  as shown in  FIG. 3(   b ). Thus, the common ink chambers  7 A,  7 B are provided inward of the shallow channel portions  5  as extending perpendicularly to the ink chambers  4 . 
   The common ink chambers  7 A,  7 B each have a smaller depth than the ink chambers  4 , so that portions of the electrodes  6  present on the interior surfaces of the ink chambers  4  are respectively electrically connected to portions of the electrodes  6  present on the interior surfaces of the shallow channel portions  5  (see  FIG. 2 ). 
   The common ink chambers  7 A,  7 B serve to supply ink to all the ink chambers  4 . Since the common ink chambers desirably each have a lower flow path resistance to the ink, the common ink chambers  7 A,  7 B each have the greatest possible width. 
   The process described above is performed on a wafer, so that a plurality of ink jet head main bodies  10  are formed in a piezoelectric board  1  in the wafer state. 
   The wafer is diced by a dicing machine to provide the ink jet head main bodies  10 . 
   Next, a process for producing an ink jet head unit  26  shown in  FIGS. 1 and 2  by incorporating the ink jet head main body  10  produced by the above mentioned process will be described in detail with reference to  FIGS. 4 and 5 . 
   As shown in  FIG. 4(   a ), a base  20  is first prepared. The preparation of the base  20  is achieved by counter-boring a 3-mm thick plate of aluminum, stainless steel or a ceramic material to a depth of 0.95 mm, drilling opposite end portions of a bottom of a head accommodating recess  20   a  formed by the counter-boring, and connecting ink supply pipes  21 A,  21 B to the resulting plate. 
   Then, the ink jet head main body  10  is placed in the head accommodating recess  20   a  of the base  20  as shown in  FIG. 4(   b ), and an adhesive is injected into a gap defined between the head accommodating recess  20   a  and the ink jet head main body  10 , whereby the ink jet head main body  10  is bonded to the head accommodating recess  20   a  and the gap between the ink jet head main body and the head accommodating recess is sealed. 
   Since the depth of the head accommodating recess  20   a  is 0.95 mm as described above, the 1.0-mm thick ink jet head main body  10  projects by 50 μm from the surface of the base  20 . 
   Subsequently, flexible wiring boards  22 A,  22 B as the external boards are connected via an anisotropically electrically conductive resin to the shallow channel portions  5  as the external connection terminals each projecting by 45 μm from the surface of the base  20  as shown in  FIG. 5(   c ). 
   Thus, voltages can be applied to the respective ink chambers  4  of the ink jet head main body  10  based on image data to externally drive the ink jet head. 
   In addition to the aforesaid connection method using the anisotropically electrically conductive resin, exemplary methods for the connection to the external boards include a method in which leads of the external boards are connected directly to the external connection terminals of the ink jet head main body  10 , and a method in which the leads of the external boards are connected to the external connection terminals of the ink jet head main body  10  by wire bonding. 
   Connector portions of the flexible wiring boards  22 A,  22 B to be connected to the shallow channel portions  5  each have a thickness of about 50 μm. Therefore, the surfaces of the flexible wiring boards  22 A,  22 B project by about 50 μm from the surface of the ink jet head main body  10  when the connection is established in the aforesaid manner. 
   Even in such a state, no particular problem occurs. If the surfaces of the flexible wiring boards  22 A,  22 B should be flush with the surface of the ink jet head main body  10  with the flexible wiring boards  22 A,  22 B connected to the ink jet head main body  10 , only opposite edge portions of the ink jet head main body  10  to be overlapped with the flexible wiring boards  22 A,  22 B may be counter-bored to a depth of 50 μm. 
   In this case, the shallow channel portions  5  as the external connection terminals should each have a depth of 5 μm. Therefore, the shallow channel portions  5  should be initially formed as having a depth of 75 μm, rather than a depth of 25 μm as described above, in the production process for the ink jet head main body  10 . 
   Next, an electrode protection film (not shown) having a thickness of about 10 μm is formed for protecting the electrodes  6  formed on the interior surfaces of the ink chambers  4  of the ink jet head main body  10 . 
   In the formation of the electrode protection film, an electrode protection film material adheres everywhere on the surface of the ink jet head main body  10 , the flexible wiring boards  22 A,  22 B, the base  20  and the ink supply pipes  21 A,  21 B. Therefore, the flexible wiring boards  22 A,  22 B which need not be covered with the electrode protection film is masked with a masking tape for prevention of the adhesion of the electrode protection film material prior to the formation of the electrode protection film. 
   In turn, a nozzle plate  25  formed with nozzle holes  23  and recesses  24 A,  24 B is bonded onto the surface of the ink jet head main body  10 . 
   The nozzle plate  25  has a greater outer size than the ink jet head main body  10  (see  FIG. 5(   c )), and is bonded so as to cover the head accommodating recess  20   a  of the base  20  (see  FIG. 4(   b )). 
   After the nozzle plate  25  is bonded to the ink jet head main body  10 , an adhesive is injected into gaps between the nozzle plate  25  and the base  20  and between the nozzle plate  25  and the flexible wiring boards  22 A,  22 B to externally seal the ink jet head main body  10 . 
   Thus, the ink jet head unit  26  shown in  FIGS. 1 and 2  is produced. 
   A preparation method for the aforesaid nozzle plate  25  will be described in detail with reference to  FIG. 6  { FIGS. 6(   a ) to  6 ( c )}. 
   As shown in  FIG. 6(   a ), a water repellent film  25   b  which is repellent to the ink is formed on an ink ejection side of a base film  25   a  of a polymer material such as a polyimide film or polyether sulfone, and a protection tape  27  is applied onto the water repellent film for protection of the water repellent film. 
   Then, the resulting film is irradiated with a pulse-controlled excimer laser with the use of a nozzle hole formation mask  40  having openings  40   a  located at positions corresponding to the positions of the nozzle holes  23  (see  FIG. 1 ) as shown in  FIG. 6(   b ). 
   At this time, the excimer laser passes through the base film  25   a  and the water repellent film  25   b  to reach parts of the protection tape  27 , whereby the polymer material of the base film  25   a  is partly decomposed into molecules or atoms with its molecular bonds cut to evaporate. Thus, the nozzle holes  23  are formed as extending through the base film  25   a  and the water repellent film  25   b  at the positions of the openings  40   a  of the nozzle hole formation mask  40 . 
   Subsequently, the resulting film is irradiated with a pulse-controlled excimer laser with the use of a recess formation mask  50  having openings  50   a ,  50   b  located at positions corresponding to the positions of the recesses  24 A,  24 B as shown in  FIG. 6(   c ). 
   At this time, the polymer material of the base film  25   a  is partly decomposed into molecules or atoms with its molecular bonds cut to evaporate as in the formation of the nozzle holes  23 , whereby the recesses  24 A,  24 B are formed as each having a predetermined depth. The formation of the recesses  24 A,  24 B each having the predetermined depth is achieved by controlling the pulse number of the excimer laser to be applied. 
   Variations in the distances between the nozzle holes  23  and the recesses  24 A,  24 B, i.e., variations in the lengths of A.E 1  and A.E 2 , are determined by the positional accuracy of the recess formation mask  50  with respect to the nozzle holes  23  in the laser irradiation. If necessary, correction of the laser processing positions may be made with the use of a dummy nozzle plate. Therefore, the variations can be easily suppressed to less than 5 μm. 
   If the variations in the distances between the nozzle holes  23  and the recesses  24 A,  24 B, i.e., the lengths of A.E 1  and A.E 2 , are less than 5 μm, the variations will not influence the ejection characteristics of the ink jet head unit  26 . 
   Thus, the nozzle plate  25  is prepared. A feature of the aforesaid preparation method for the nozzle plate  25  is that the recesses  24 A,  24 B each having the predetermined depth can be formed at positions spaced equidistantly from the nozzle holes  23  simply by additionally performing the excimer laser process with the use of the recess formation mask  50  in the prior-art nozzle plate production process. 
   The excimer laser process is conventionally employed for the formation of the nozzle holes  23 . Therefore, the preparation of the nozzle plate  25  according to this embodiment can be achieved simply by newly preparing the recess formation mask  50 , so that existing equipment can be used as it is. Accordingly, an increase in production costs can be minimized. 
   Even if the nozzle plate  25  is bonded to the ink jet head main body  10  in offset relation, the times required for pressure waves occurring in the respective active areas A.E 1 , A.E 2  to reach the nozzle holes  23  are equal to each other, thereby preventing deterioration of the ejection characteristics. Further, the pressure waves are mostly attenuated by the small gaps defined by the recesses  24 A,  24 B to suppress residual vibration, whereby the ejection characteristics can be stably maintained even in high speed driving as described above. The nozzle plate  25  which provides these effects can be prepared by the aforesaid simple method with a minimum increase in production costs as described above. 
   As a result, the ink jet head  26  can be stably produced as having a higher quality and a higher performance without an increase in production costs. 
   The pressure wave absorption/attenuation effect observed when the pressure waves pass through the gaps will be explained with reference to  FIGS. 7 and 8 .  FIG. 8  is an enlarged partial sectional view taken along a line B-B in  FIG. 7 . 
   As shown in  FIG. 7 , pressure waves generated in the active areas A.E 1 , A.E 2  within the ink chambers  4  when the interior surfaces of the ink chambers  4  are deformed inward by application of voltages to eject the ink from the nozzle holes  23  travel toward the recesses  24 A,  24 B and the common ink chambers  7 A,  7 B in directions opposite to ink supply paths as indicated by arrows. When the pressure waves reach the recesses  24 A,  24 B, the pressure waves are partly reflected on the interior surfaces of the recesses  24 A,  24 B back into the ink chambers  4 , but the pressure waves mostly pass through the gaps defined by the recesses  24 A,  24 B. 
   As shown in  FIG. 8 , the pressure waves are mostly absorbed by the viscosity of the ink when passing through the small gaps defined by the recess  24 A between the nozzle plate  25  and the ink chamber walls  4   a , or repeatedly reflected on the interior surface of the recess  24 A to be attenuated, and then reach the adjacent ink chambers  4  or the common ink chambers  7 A,  7 B (see  FIG. 7 ). Though the recess  24 B is not shown in  FIG. 8 , the same absorption/attenuation effect occurs in the recess  24 B. 
   Therefore, the pressure waves are not reflected on the interior surfaces of the common ink chambers  107 A,  107 B back into the ink chambers  104  as in the prior-art ink jet head  100  shown in  FIG. 9 , but are mostly absorbed or attenuated when passing through the gaps defined by the recesses  24 A,  24 B, thereby suppressing the residual vibration. 
   The pressure wave absorption/attenuation effect is significantly influenced by the heights of the gaps, i.e., the depths D of the recesses  24 A,  24 B of the nozzle plate  25  (see  FIG. 8 ). Therefore, the depths of the recesses  24 A,  24 B should be optimally determined to provide a preferred absorption/attenuation property. 
   To this end, an experiment was performed with the use of the ink jet head unit  100  having the prior-art structure shown in  FIG. 9  and ink jet heads  26  including recesses  24 A,  24 B having four different depths according to the embodiment of the present invention to evaluate the ejection characteristics of the respective ink jet units and check if residual vibration occurs due to pressure waves. The results are shown in the following Table 1. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Active 
               Lengths L 
               Depths D of 
               Residual 
             
             
                 
               areas 
               of recesses 
               recesses 
               vibration 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
               Prior-art 
               1.15 mm 
               — 
               — 
               X 
             
             
               structure 
             
             
               Comparative 
               1.15 mm 
               1.35 mm 
               0.01 mm 
               ⊚ 
             
             
               Example 1 
             
             
               Comparative 
               1.15 mm 
               1.35 mm 
               0.02 mm 
               ◯ 
             
             
               Example 2 
             
             
               Comparative 
               1.15 mm 
               1.35 mm 
               0.03 mm 
               Δ 
             
             
               Example 3 
             
             
               Comparative 
               1.15 mm 
               1.35 mm 
               0.04 mm 
               X 
             
             
               Example 4 
             
             
                 
             
          
         
       
     
   
   As apparent from the results of the experiment shown in Table 1, the influence of the residual vibration tends to diminish, as the heights of the gaps or the depths of the recesses  24 A,  24 B of the nozzle plate  25  are reduced. Further, it was found that, where the recesses  24 A,  24 B each have a depth of 40 μm, a pressure wave component reflected on the interior surfaces of the recesses  24 A,  24 B back into the ink chambers  4  is increased as in the prior-art ink jet head  100  shown in  FIG. 9 , thereby adversely influencing the ink ejection characteristics. 
   Though not apparent from Table 1, it was confirmed that, where the lengths L of the recesses  24 A,  24 B as measured longitudinally of the ink chambers  4  (see  FIG. 7 ) are each set to the greatest possible level, the effect of suppressing the residual vibration occurring due to the pressure waves is enhanced. 
   In the ink jet unit  26  according to this embodiment, the small gaps are formed between the active areas A.E 1 , A.E 2  and the common ink chambers  7 A,  7 B by the provision of the recesses  24 A,  24 B in the nozzle plate  25 . Therefore, the pressure waves generated in the ink chambers  4  can be speedily attenuated when passing through the gaps. 
   As a result, the influence of the residual vibration is minimized, so that the subsequent ink ejection smoothly occurs. Thus, an image having a higher quality and a higher fidelity to image data can be formed at a higher speed.