Patent Publication Number: US-7588307-B2

Title: Piezolelectric inkjet printhead having temperature sensor and method of making the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a piezoelectric inkjet printhead. More particularly, the present invention relates to a piezoelectric inkjet printhead having a temperature sensor for sensing the temperature of ink in an ink channel, and a method of making the same. 
     2. Description of the Related Art 
     In general, an inkjet printhead is a device that prints an image of a predetermined color by ejecting fine ink droplets onto a desired position of a recording medium. Inkjet printheads may be roughly classified into two types of printheads, based on the method of ink ejection. One of the two types of printheads is a thermally-driven type inkjet printhead, which generates a bubble in ink using a heat source and ejects ink using the force of expansion of the bubble. The other type is a piezoelectric inkjet printhead, which operates through the shape transformation of a piezoelectric element and ejects ink using pressure applied to the ink by the transformation of the piezoelectric element. 
       FIGS. 1 and 2  illustrate partial plan and sectional views, respectively, of a conventional piezoelectric inkjet printhead. Referring to  FIGS. 1 and 2 , the printhead may include a channel forming plate having a manifold  12  and a plurality of pressure chambers  16 , which may be coupled to each other by a plurality of restrictors  14 . The printhead may also include a plurality of nozzles  18 . 
     An ink channel may include the manifold  12 , a restrictor  14 , a pressure chamber  16  and a nozzle  18 . In detail, the manifold  12  may serve as a passage supplying ink flowing from an ink storage region (not shown) to each of a plurality of pressure chambers  16 , and the plurality of restrictors  14  may serve as passages connecting the manifold  12  with the plurality of pressure chambers  16 . The plurality of pressure chambers  16 , which fill with ink to be ejected, may be arranged on one side or both sides of the manifold  12 . 
     A plurality of piezoelectric actuators  40  may be provided on the channel forming plate  10 . As an individual piezoelectric actuator  40  is driven, it causes a corresponding pressure chamber  16  to change its volume, thereby creating a pressure change for ejecting ink, or for inducing the inflow of ink to the pressure chamber  16  from the manifold  12 . A portion of the channel forming plate  10  that constitutes an upper wall, or ceiling, of the pressure chamber  16  may serve as a vibrating plate  20 , which is vibrated by driving the piezoelectric actuator  40 . The channel forming plate  10  may be manufactured by processing a plurality of thin plates, e.g., silicon wafers, metal plates, synthetic resin plates, etc., to form the features making up the ink channels, and then stacking these plates. 
     Each piezoelectric actuator  40  may include a lower electrode  41 , a piezoelectric element  42 , and an upper electrode  43  sequentially stacked on the channel forming plate  10 . A lower electrode insulation layer  31  may be formed between the lower electrode  41  and the channel forming plate  10 . The lower electrode  41  may be formed on an entire surface of the lower electrode insulation layer  31  to serve as a common electrode for multiple piezoelectric actuators  40 . The piezoelectric element  42  may be formed on the lower electrode  41  such that the piezoelectric element  42  is positioned above the corresponding pressure chamber  16 . The upper electrode  43  may be formed on the corresponding piezoelectric element  42  to serve as a drive electrode for applying a voltage across the piezoelectric element  42 . 
     To apply a drive voltage to the piezoelectric actuator  40  having the above-described structure, the upper electrode  43  may be connected to a flexible printed circuit (FPC)  50  for voltage supply. The FPC  50  may include a plurality of drive signal lines  51 , where individual drive signal lines  51  are bonded to individual upper electrodes  43 . 
     In operation, when the vibrating plate  20  is transformed by driving the piezoelectric actuator  40 , the volume of the pressure chamber  16  reduces, which generates a pressure change in the pressure chamber  16  so that ink contained in the pressure chamber  16  is ejected to the outside. Subsequently, when the vibrating plate  20  is restored to an original shape by driving of the piezoelectric actuator  40 , the volume of the pressure chamber  16  increases, which generates a pressure change, i.e., a negative pressure change, in the pressure chamber  16 , so that ink flows from the manifold  12  into the pressure chamber  16  through the restrictor  14 . 
     When the temperature of ink changes, the viscosity of the ink may also change. If the viscosity of the ink increases, the flow resistance of the ink may also increase, and thus the volume and ejection speed of an ink droplet ejected through the nozzle  18  may be reduced. Therefore, overall ink ejection performance may be reduced and satisfactory printing quality may not be obtained. Accordingly, it may be desirable to provide appropriate compensation for increased ink viscosity by raising the temperature of the ink through heating, or by raising the driving voltage applied to the piezoelectric actuator  40 . 
     To manage this compensation, it may be desirable to accurately sense the temperature of the ink inside the inkjet printhead. However, it may not be straightforward to directly install a temperature sensor for sensing the temperature of ink in the inkjet printhead. 
     SUMMARY OF THE INVENTION 
     The present invention is therefore directed to a piezoelectric inkjet printhead having a temperature sensor and a method of making the same, which substantially overcome one or more of the problems due to the limitations and disadvantages of the related art. 
     It is therefore a feature of an embodiment of the present invention to provide an inkjet printhead having a temperature sensor directly attached thereto. 
     It is therefore another feature of an embodiment of the present invention to provide a method of making a piezoelectric inkjet printhead, wherein temperature sensor mounting elements may be formed at the same time, and of the same materials, as elements of piezoelectric actuators. 
     At least one of the above and other features and advantages of the present invention may be realized by providing a piezoelectric inkjet printhead having a channel forming plate including an ink channel having a pressure chamber coupled to a nozzle, a piezoelectric actuator including a lower electrode on the channel forming plate, a piezoelectric element on the lower electrode, and an upper electrode on the piezoelectric element, the piezoelectric actuator corresponding to the pressure chamber, an insulation element on the lower electrode and spaced apart from the piezoelectric element, a first electrode on the insulation element, and a temperature sensor on the first electrode. 
     The temperature sensor may be a thermistor. The insulation element and the piezoelectric element may be formed of a first material. The first material may be lead zirconate titanate (PZT). The insulation element and the piezoelectric element may be coplanar. The insulation element may have an elongated rectangular shape and may be disposed adjacent to and in parallel with the piezoelectric element. 
     The first electrode and the upper electrode may be coplanar. The first electrode and the upper electrode may be formed of a second material. The second material may be Ag—Pd. The piezoelectric inkjet printhead may further include a second electrode disposed adjacent to the first electrode, wherein the first and second electrodes are attached to electrodes of the temperature sensor. The first and second electrodes may each have an elongated rectangular shape and may be disposed with long sides thereof opposing each other and in parallel to each other. The insulation element may have the first and second electrodes disposed thereon. 
     The piezoelectric inkjet printhead may further include a plurality of piezoelectric actuators, wherein the plurality of piezoelectric actuators and the first and second electrodes are disposed parallel to each other and in a same column. The piezoelectric inkjet printhead may further include a set of signal lines provided on a flexible printed circuit, wherein a first subset of the signal lines is coupled to the first electrode and a second subset of the signal lines is coupled to the upper electrode. 
     At least one of the above and other features and advantages of the present invention may also be realized by providing a method of forming an inkjet printhead having a piezoelectric actuator and a temperature sensor, including forming a lower electrode of the piezoelectric actuator on a channel forming plate, forming an insulation element on a portion of the lower electrode, forming a first electrode on the insulation element, and attaching a temperature sensor on the first electrode. 
     The temperature sensor may be a thermistor. The method may further include forming a second electrode on the insulation element in parallel with the first electrode, wherein the temperature sensor is attached to the first and second electrodes. Attaching the temperature sensor on the first electrode may include mounting the channel forming plate in a heating block, disposing a solder material between the temperature sensor and the first electrode, placing the temperature sensor on the first electrode, and heating the heating block to melt the solder. 
     The insulation element and a piezoelectric element of the piezoelectric actuator may be formed of a first material layer. The insulation element may be formed from the first material layer simultaneously with the piezoelectric element. The first electrode and an upper electrode of the piezoelectric actuator may be formed of a second material layer. The first electrode may be formed simultaneously with the upper electrode. 
     The method may further include bonding a flexible printed circuit to the printhead, the flexible printed circuit including a first signal line coupled to the first electrode and a second signal line coupled to an upper electrode of the piezoelectric actuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIGS. 1 and 2  illustrate partial plan and sectional views, respectively, of a conventional piezoelectric inkjet printhead; 
         FIG. 3  illustrates a plan view of a piezoelectric inkjet printhead having a temperature sensor according to an embodiment of the present invention; 
         FIG. 4  is a sectional view taken along line A-A′ of  FIG. 3 ; and 
         FIGS. 5A-5E  illustrate partial sectional views, taken along line B-B′ of  FIG. 3 , of stages in a method of making an inkjet printhead according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Korean Patent Application No. 10-2005-0008003, filed on Jan. 28, 2005, in the Korean Intellectual Property Office, and entitled: “Piezoelectric Inkjet Printhead Having Temperature Sensor And Method of Attaching Temperature Sensor to Inkjet Printhead,” is incorporated by reference herein in its entirety. 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the figures, the dimensions of layers and regions are exaggerated for clarity of illustration. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being “under” another layer, it can be directly under, and one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. Like reference numerals refer to like elements throughout. 
     According to the present invention, a temperature sensor may be directly attached to an inkjet printhead. Thus, it may be possible to more accurately sense the temperature of ink contained in the printhead, thereby enabling active and appropriate compensation depending on the temperature of the ink so that printing quality may be improved. 
     The temperature sensor may be a thermistor, such that temperature sensor calibration for individual printheads is not required. Temperature sensor mounting elements may be formed at the same time, and of the same materials, as elements of piezoelectric actuators. The temperature sensor may be mounted on the inkjet printhead using a soldering process. 
       FIG. 3  illustrates a plan view of a piezoelectric inkjet printhead having a temperature sensor according to an embodiment of the present invention, and  FIG. 4  is a sectional view taken along line A-A′ of  FIG. 3 . Referring to  FIGS. 3 and 4 , a piezoelectric inkjet printhead according to the present invention may include a channel forming plate  100  having a plurality of ink channels formed therein, a piezoelectric actuator  140  for providing the driving force required for ejecting ink, and a temperature sensor  165  for sensing the temperature of ink contained in the printhead. 
     The ink channels may include a plurality of pressure chambers  104 , which fill with ink to be ejected and which generate pressure changes for ejecting ink. The ink channels may also include an ink inlet  101 , through which ink from an ink storage region (not shown) flows, a manifold  102 , which is a common channel supplying the ink from the ink inlet  101  to the pressure chambers  104 , a plurality of restrictors  103 , which are individual channels supplying ink from the manifold  102  to each of the pressure chambers  104 , and a plurality of nozzles  106 , for ejecting ink from the pressure chambers  104 . A damper  105  may be provided between each of the plurality of pressure chambers  104  and the corresponding nozzles  106 , in order to concentrate energy on the nozzles  106  and to buffer sudden pressure changes. 
     The channel forming plate  100  may include three channel plates  110 ,  120  and  130 . Each of the three channel plates  110 ,  120  and  130  may be formed of, e.g., a silicon substrate. The three channel plates  110 ,  120  and  130  may be individually formed, then sequentially stacked and bonded. Where the three channel plates  110 ,  120  and  130  are silicon substrates, mutual bonding of the three channel plates  110 ,  120  and  130  may be performed by, e.g., silicon direct bonding (SDB). 
     In detail, the plurality of pressure chambers  104  may be formed at a predetermined depth in a lower surface of a first channel plate  110 , and the ink inlet  101  may be formed to vertically pass through the first channel plate  110 . A vibrating plate  111 , to be transformed by driving the piezoelectric actuator  140 , may be formed at the upper portion of each pressure chamber  104  in the first channel plate  110 . Each of the pressure chambers  104  may have an elongated rectangular shape, with a long dimension oriented in the direction of ink flow. The pressure chambers  104  may be arranged in two columns, with one column disposed along each side of the manifold  102 , or may be arranged in one column on one side of the manifold  102 . 
     A second channel plate  120  may be bonded to the lower surface of the first channel plate  110 . The manifold  102  may be formed in the second channel plate  120 . One end of the manifold  102  may be connected to the ink inlet  101 . Referring to  FIG. 4 , the manifold  102  may be formed to a predetermined depth from the upper surface of the second channel plate  120 . Alternatively, the manifold  102  may be formed to vertically pass through the second channel plate  120  (not shown). Restrictors  103 , which are individual channels connecting the manifold  102  to one end of each of the pressure chambers  104 , may be formed in the second channel plate  120 . The restrictor  103  may be formed to a predetermined depth from the upper surface of the second channel plate  120 , as illustrated in  FIG. 4 . Alternatively, the restrictor  103  may be formed to vertically pass through the second channel plate  120  (not shown). Dampers  105 , connecting each of the pressure chambers  104  to each of the nozzles  106 , may be formed in the second channel plate  120  and may be aligned with the other end of each of the pressure chambers  104 , opposite the restrictors  103 . The dampers  105  may vertically pass through the second channel plate  120 . 
     A third channel plate  130  may be bonded to the lower surface of the second channel plate  120 . The plurality of nozzles  106  may be formed in the third channel plate  130 . The nozzles  106  may vertically penetrate the third channel plate  130 . 
     The plurality of piezoelectric actuators  140  may be formed on the first channel plate  110  so as to provide each of the corresponding pressure chambers  104  with a driving force for ejecting ink. Each piezoelectric actuator  140  may include a lower electrode  141 , serving as a common electrode for multiple piezoelectric actuators  140 . Each piezoelectric actuator  140  may also include a piezoelectric element  142 , which is transformed when a driving voltage is applied thereto, and an upper electrode  143  serving as a drive electrode. Thus, the piezoelectric actuator  140  may have a structure in which the lower electrode  141 , the piezoelectric element  142  and the upper electrode  143  are sequentially stacked. 
     A lower electrode insulation layer  112  may be formed between the lower electrode  141  and the first channel plate  110 . The lower electrode insulation layer  112  may be formed of, e.g., a silicon oxide layer. The lower electrode  141  may be formed on an entire surface of the lower electrode insulation layer  112 . The lower electrode  141  may be formed of one conductive metal material layer, or may be formed of two thin metal layers such as, e.g., Ti and Pt. Each piezoelectric element  142  may be formed on the lower electrode  141  and arranged above the corresponding pressure chamber  104 . Thus, multiple piezoelectric elements  142  may be formed on the lower electrode  141 , such that each of the multiple piezoelectric elements  142  is adjacent to, but separated from, a neighboring piezoelectric element  142 , and is coplanar therewith. The piezoelectric elements  142  may be formed from a single layer of material, e.g., a piezoelectric material such as a PZT ceramic material. 
     A plurality of upper electrodes  143  may be formed on the piezoelectric elements  142 , with each upper electrode  143  corresponding to one piezoelectric element  142 . The upper electrodes  143  may serve as drive electrodes for applying a driving voltage to the piezoelectric elements  142 . Each piezoelectric element  142  may be transformed when the driving voltage is applied thereto, such that deformation of the piezoelectric element  142  warps a vibration plate  111  on each of the pressure chambers  104 . To apply the drive voltage to the piezoelectric actuator  140  having the above construction, a drive signal line  151  may be provided, e.g., on a flexible printed circuit  150  (FPC), and bonded to the upper electrode  143 . 
     A temperature sensor  165  for detecting the temperature of ink in the printhead may be provided on the channel forming plate  100 . The temperature sensor  165  may be, e.g., a thermistor. The thermistor may be, e.g., an integrated circuit (IC) chip, which may be separately manufactured and then assembled onto the printhead. 
     Common forms of temperature sensors include resistance temperature detector sensors (RTDs) and thermistors. The RTD uses a temperature sensor, e.g., a metal such as Pt, whose resistance changes significantly with temperature. The thermistor has a similar resistance response to temperature change, but is typically a semiconductor device, e.g., one obtained by mixing and sintering oxides of Mn, Ni, Cu, Co, Cr, Fe, etc. The thermistor is widely used as a temperature sensor, and may be manufactured in various types and used in various ways. For example, the thermistor may be a thermistor chip obtained by forming electrodes on both sides of the thermistor and manufacturing the thermistor in the form of an integrated circuit chip. 
     In manufacturing inkjet printheads, tens or hundreds of printheads may be manufactured at one time. If RTDs are used as temperature sensors for the printheads, deviations in dimensions of the RTDs may occur, e.g., variations in thickness, width, or length. Accordingly, calibration of each RTD may be required for each of the printheads after the manufacturing of the printheads. 
     In contrast, the thermistor may be separately manufactured and provided in the form of a chip. Thus, it may have relatively uniform characteristics, obviating the need for calibration of individual printheads. Accordingly, a thermistor may be used as the temperature sensor  165  for measuring the ink temperature. Where a thermistor  165   a  is used as the temperature sensor  165 , it may include two electrodes  165   b  formed on two sides thereof and may be provided as a premade chip. The thermistor  165   a  may be directly attached to the inkjet printhead. 
     In detail, an insulation element  162  may be formed on the lower electrode  141  on the channel forming plate  100 . The insulation element  162  may insulate the lower electrode  141  from an electrode  163  coupled to the temperature sensor  165 . The insulation element  162  may be disposed adjacent to, but spaced apart from, a piezoelectric element  142  of a piezoelectric actuator  140 . The insulation element  162  may be shaped similarly to the piezoelectric element  142  and may be arranged in parallel to the piezoelectric element  142 . The insulation element  162  may be formed on the lower electrode  141  together with the piezoelectric element  142 . The insulation element  162  and the piezoelectric element  142  may be formed from a same material layer, e.g., a PZT layer. The insulation element  162  and the piezoelectric element  142  may be formed simultaneously from the same material layer, as described below. 
     The electrode  163  may be formed on the insulation element  162 . Two electrodes  163  may be formed in parallel to each other on the insulation element  162  so as to correspond to two electrodes  165   b  of the temperature sensor  165 . The electrode  163  and the upper electrode  143  of the piezoelectric actuator  140  may be formed of a same material layer. The electrode  163  and the upper electrode  143  may be simultaneously formed from the same material layer, as described below. 
     The temperature sensor  165  may be attached on the electrode  163 . For example, two electrodes  165   b  of a thermistor  165   a  may be attached on the two electrodes  163  for temperature sensing, respectively. The electrodes  165   b  may be attached on the two electrodes  163  using, e.g., solder  164 , as described below. 
     Signal lines  152  for temperature sensing may be bonded to each of the electrodes  163 . Referring to  FIG. 3 , a set of signal lines  152  may be provided on a FPC  150  together with a set of drive signal lines  151 , which are coupled to the upper electrodes  143  of the piezoelectric actuators  140 . 
     A method of attaching a temperature sensor to an inkjet printhead according to the present invention will now be described with reference to  FIGS. 5A-5E , which illustrate partial sectional views, taken along line B-B′ of  FIG. 3 , of stages in a method of making an inkjet printhead according to the present invention. 
     Referring to  FIG. 5A , a lower electrode  141  of a piezoelectric actuator  140  may be formed on a channel forming plate  100 . As described above, the channel forming plate  100  may have a structure including a first channel plate  110 , a second channel plate  120 , and a third channel plate  130 , which may be sequentially stacked and bonded. Each of the first through third channel plates  110 ,  120  and  130  may be formed of a silicon substrate. An ink channel is formed in the channel forming plate  100  and may include an ink inlet  101 , a manifold  102 , a plurality of restrictors  103 , a plurality of pressure chambers  104 , a plurality of dampers  105 , and a plurality of nozzles  106 . Note, however, that this structure is merely exemplary, and is described in detail merely to provide a full and complete understanding of the present invention. 
     An insulation layer  112  may be formed between the lower electrode  141  and the channel forming plate  100 . The insulation layer  112  may be, e.g., a silicon oxide layer. The lower electrode  141  may be formed on an entire surface of the insulation layer  112 . The lower electrode  141  may be formed of one conductive metal material layer, or may be formed of two thin metal layers such as, e.g., Ti and Pt. 
     After forming the lower electrode  141  on the channel forming plate  100  as described above, an insulation element  162  is formed on a partial portion of the lower electrode  141 . The insulation element  162  may be formed of the same material layer, e.g., a PZT layer, as the piezoelectric element  142  of a piezoelectric actuator  140 . Thus, the insulation element  162  and the piezoelectric element  142  may be the same material. The insulation element  162  may be simultaneously formed together with the piezoelectric element  142 , i.e., a separate process is not required to form the insulation element  162 . 
     In detail, the insulation element  162  and the piezoelectric element  142  may be formed by coating a piezoelectric material layer, e.g., PZT in paste form, to a predetermined thickness on the lower electrode  141  by, e.g., screen printing and drying/sintering the coated piezoelectric material. The piezoelectric material layer may be patterned, e.g., by the screen printing, so that the piezoelectric element  142  is formed above a pressure chamber  104 , and the insulation element  162  is formed adjacent to and in parallel with the piezoelectric element  142 . 
     Both the piezoelectric element  142  and the insulation element  162  may have substantially rectangular shapes, with a major length of each being approximately equal. The major sizes of the respective rectangles may be parallel to each other, i.e., they may be disposed adjacent to but spaced apart from each other. A minor length of the insulation element  162  may be longer than the corresponding minor length of the piezoelectric element  142 . That is, referring to  FIG. 5A , the left-right dimension of the insulation element  162  may be greater than the left-right dimension of the adjacent piezoelectric element  142 . 
     Still referring to  FIG. 5A , one or more electrodes  163  for temperature sensing may be formed on the insulation element  162 . The electrodes  163  may be formed of the same material as that of an upper electrode  143  of an adjacent piezoelectric actuator  140 . The electrodes  163  may be formed simultaneously with the upper electrode  143 , and a separate process is not required to form the electrodes  163 . 
     In detail, the electrodes  163  and upper electrode  143  may be formed by, e.g., coating an electrode material layer such as an Ag—Pd paste to a predetermined thickness on the insulation element  162  and the piezoelectric layer  142 , respectively, using, e.g., a screen printing process, and sintering the same. 
     The electrodes  163  may be substantially rectangular and may be formed in parallel to each other on the insulation layer  162 . The electrodes  163  may have a major length approximately equal to a corresponding major length of an adjacent upper electrode  143 , or may be aligned with ends substantially even with the adjacent upper electrode  143 . That is, referring to  FIG. 3 , the ends of the electrodes  163  and the end of the adjacent upper electrode  143  may be aligned, e.g., near the right edge of the printhead in  FIG. 3 . Thus, a FPC may be easily coupled to both the electrodes  163  and the upper electrodes  143  of the piezoelectric actuators  140 , as will be described below. 
     As described above, a piezoelectric element  142  and an upper electrode  143  may be formed on the channel forming plate  100  simultaneously with an insulation element  162  and an electrode  163 , respectively. Accordingly, the manufacture of the inkjet printhead may be simplified. 
     Referring now to  FIG. 5B , the channel forming plate  100  may be mounted in a heating block  170 . A groove or recess  172  for receiving the channel forming plate  100  may be formed in the upper surface of the heating block  170 . With the channel forming plate  100  mounted in the heating block  170 , a process may be performed to attach the temperature sensor  165  to the electrode(s)  163 . 
     In detail, referring to  FIG. 5C , solder  164  may be disposed on the electrodes  163 . The solder  164  may be formed by, e.g., printing a predetermined solder material on the electrodes  163  using, e.g., a printing mask  180 , or by dispensing a predetermined solder material using a dispenser. The type of solder and methods of application thereof may be of the same kinds typically used for semiconductor manufacturing. 
     Referring to  FIG. 5D , the temperature sensor  165 , e.g., a thermistor, may be positioned on the solder  164 , such that electrodes  165   b  of the temperature sensor  165  are in contact with the solder  164 . The positioning of the thermistor chip  165  may be performed using, e.g., a positioning mask  190 , or by using a pick and place device as is commonly used for semiconductor manufacturing. 
     Referring to  FIG. 5E , the solder  164  may be heated, e.g., to about 200° C., so that a reflow process is performed on the solder  164 . Of course, the heating temperature of the solder  164  may change depending on the type of solder used. Heating of the solder  164  may be indirectly performed by heating the heating block  170 . Alternatively, the heating of the solder  164  may be performed within a heating oven, in which case the heating block  170  illustrated in  FIGS. 5B-5E  need not be used. After the solder  164  is reflowed by heating, the solder  164  is cooled down. The cooling of the solder  164  may be performed by natural cooling. Thus, the above process may be used to attach electrodes  165   b  of the temperature sensor  165  on the electrodes  163 . 
     Referring now to  FIGS. 3 and 4 , signal lines  152  for temperature sensing may be bonded to each of the electrodes  163 . The signal lines  152  may be provided as part of a FPC  150 , and may be provided together with a set of drive signal lines  151  for the piezoelectric actuators  140 . The drive signal lines  151  may be bonded to the upper electrodes  143  of the piezoelectric actuators  140  simultaneously with bonding of the signal lines  152  to the electrodes  163 . 
     Exemplary embodiments of the present invention have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.