Abstract:
A droplet discharge includes: a nozzle discharging a liquid as a droplet; a discharge room having a diaphragm and disposed in a channel of the liquid, the channel communicating with the nozzle, the diaphragm pressurizing the liquid by being displaced and being a part of the discharge room; and a fixed electrode facing the diaphragm and generating electrostatic force with respect to the diaphragm by receiving electric charge so as to displace the diaphragm by bringing the diaphragm into contact with and detaching the diaphragm from the fixed electrode. The fixed electrode includes: a first fixed electrode received the electric charge from an outside; and a second fixed electrode made of a material different from a material of the first fixed electrode and received the electric charge through the first fixed electrode.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a droplet discharge head, a droplet discharge apparatus including the droplet discharge head, and the like. 
         [0003]    2. Related Art 
         [0004]    The micro-electro-mechanical systems (MEMS) technology for forming fine elements or the like by processing, for example, silicon has been rapidly advanced. Among micromachined elements formed using the MEMS technology are droplet discharge heads (inkjet heads) for use in a recording apparatus such as a droplet discharge type printer, micropumps, variable optical filters, electrostatic actuators such as motors, and pressure sensors. 
         [0005]    Droplet discharge type (typified by inkjet used to perform printing by discharging ink) apparatus are used in all fields of printing, whether for consumer use or for industrial use. In a droplet discharge type apparatus, a micromachined element such as a droplet discharge head having multiple nozzles is moved relative to a target so as to discharge a liquid onto a predetermined position of the target. In recent years, droplet discharge type apparatuses are also used when manufacturing color filters for use in a liquid crystal display, display substrates using an organic electroluminescence element or an organic light-emitting diode (OLED), microallays of biomolecules such as deoxyribonucleic acids (DNAs), and the like. 
         [0006]    Among discharge heads for realizing the droplet discharge type is one in which at least one wall, for example, a bottom wall (hereafter will be referred to as a “diaphragm” although it is formed integrally with other walls) of a discharge room for storing a discharge liquid flowing on a channel is previously made deformable and, by deforming the diaphragm to increase the pressure in the discharge room, a droplet is discharged from a nozzle communicating with the discharge room. 
         [0007]    In an electrostatic type droplet discharge head, electrostatic force is generated between a diaphragm as a movable electrode and an individual electrode as a fixed electrode opposed to the diaphragm so that the diaphragm is attracted to the individual electrode. Subsequently, when the electrostatic force is weakened or its generation is stopped, restoring force (elastic force) that attempts to restore the diaphragm to its equilibrium position is exerted more strongly. Thus, the diaphragm returns to its original position. By repeating these operations, the diaphragm is driven so that a droplet is discharged. In this case, if various types of control are performed in the droplet discharge head in order to enhance the image quality and printing speed, it is convenient. Specifically, there are strong demands such as one for changing the amount of a droplet to be discharged (hereafter referred to as a “discharge amount”) onto each landing position or one for discharging droplets stably. For these reasons, an inkjet head has been proposed in which an individual electrode is divided into multiple ones, application of a voltage to each electrode is controlled, electrostatic force is changed according to the number of electrodes to which a voltage is applied so as to change the discharge amount (for example, see JP-A-2000-015801.) 
         [0008]    However, as the density increasingly becomes higher, it is difficult to provide multiple individual electrodes and install wiring for each individual electrode. Also, the cost is increased due to such additional wiring. 
       SUMMARY 
       [0009]    An advantage of the invention is to obtain a droplet discharge head and the like that are allowed to change the discharge amount with a simple structure. 
         [0010]    According to a first aspect of the invention, a droplet discharge head includes: a nozzle discharging a liquid as a droplet; a discharge room having a diaphragm and disposed in a channel of the liquid, the channel communicating with the nozzle, the diaphragm pressurizing the liquid by being displaced and being a part of the discharge room; and a fixed electrode facing the diaphragm and generating electrostatic force with respect to the diaphragm by receiving electric charge so as to displace the diaphragm by bringing the diaphragm into contact with and detaching the diaphragm from the fixed electrode. The fixed electrode includes: a first fixed electrode received the electric charge from an outside; and a second fixed electrode made of a material different from a material of the first fixed electrode and received the electric charge through the first fixed electrode. 
         [0011]    According to the first aspect of the invention, the fixed electrode includes the first fixed electrode and the second fixed electrode that is made of a material different from the material of the first fixed electrode and receives electric charge via the first fixed electrode. Thus, by controlling the supply of electric charge to the fixed electrode, it is determined whether the diaphragm is brought into contact with only the first fixed electrode or it is brought into contact with both the first and second fixed electrodes, and then a discharge operation is performed. As a result, a droplet discharge head is obtained that is able to change the amount of a droplet to be discharged at one time. Such a droplet discharge head is easily manufactured since it does not have a complicated structure such as a stepped one. 
         [0012]    In the droplet discharge head according to the first aspect of the invention, the first and second fixed electrodes are preferably electrically coupled to each other via one or more connectors that serve as an electric charge supply path from the first fixed electrode to the second fixed electrode. 
         [0013]    According to the first aspect of the invention, the first and second fixed electrodes are electrically coupled to each other via one or more connectors. Thus, the electric charge supply path is arbitrarily prescribed. Specifically, by setting the number of connectors, widths thereof, or the like, the amount (time) of supply of electric charge from the first fixed electrode to the second fixed electrode is arbitrarily controlled. 
         [0014]    In the droplet discharge head according to the first aspect of the invention, the second fixed electrode is preferably made of a material having an electrical resistivity higher than an electrical resistivity of a material of the first fixed electrode. 
         [0015]    According to the first aspect of the invention, the second fixed electrode is made of a material having an electrical resistivity higher than an electrical resistivity of a material of the first fixed electrode. Thus, the time taken until electrostatic force required to bring the diaphragm into contact with the second fixed electrode is generated is increased compared with the time taken until electrostatic force required to bring the diaphragm into contact with the first fixed electrode is generated. 
         [0016]    In the droplet discharge head according to the first aspect of the invention, indium tin oxide (ITO) is preferably used as a material of the first fixed electrode and titanium is preferably used as a material of the second fixed electrode. 
         [0017]    According to the first aspect of the invention, the droplet discharge head has a long life and discharges droplets favorably since this combination of materials is the best one in terms of the difference between the electrical resistivities, adhesiveness in a case where a substrate serving as a base is made of glass, or the like. Also, titanium is resistant to an etchant (etching solution) necessary when etching ITO in the manufacturing process. Therefore, by previously forming the second individual electrode using titanium, the first and second fixed electrodes are easily formed on a substrate. 
         [0018]    In the droplet discharge head according to the first aspect of the invention, chrome, platinum, or gold is preferably used as a material of the second fixed electrode instead of the titanium. 
         [0019]    According to the first aspect of the invention, if chrome, platinum, or gold is used as the material of the second fixed electrode, a droplet discharge head that is good in electrical resistivity is obtained. 
         [0020]    In the droplet discharge head according to the first aspect of the invention, the first fixed electrode is preferably disposed in a central part in a short side direction of the discharge room and the second fixed electrode is preferably disposed on both sides of the first fixed electrode, and the first fixed electrode and the second fixed electrode are preferably provided side by side along the short side direction of the discharge room. 
         [0021]    According to the first aspect of the invention, the fixed electrode is provided along the short side direction of the discharge room. Therefore, the diaphragm is brought into contact with at least the first fixed electrode along the channel for a liquid, as has been done conventionally, so that a pressure necessary to discharge a droplet is applied. Also, since the second fixed electrode is provided on both sides of the first fixed electrode, the diaphragm is brought into contact with the fixed electrode with good balance. 
         [0022]    In the droplet discharge head according to the first aspect of the invention, one or more fixed electrodes made of a material different from materials of the first and second fixed materials are preferably further provided outside the second fixed electrode in the short side direction of the discharge room. 
         [0023]    According to the first aspect of the invention, one or more fixed electrodes are further provided outside the second fixed electrode. Therefore, the amount of a droplet to be discharged at one time is changed in three or more levels. 
         [0024]    In the droplet discharge head according to the first aspect of the invention, the first fixed electrode and the second fixed electrode are preferably provided side by side along a long side direction of the discharge room. 
         [0025]    According to the first aspect of the invention, the first fixed electrode and the second fixed electrode are side by side along the long side direction of the discharge room. Thus, a droplet discharge head is obtained that changes the amount of a droplet to be discharged at one time at multiple levels even if these fixed electrodes are provided side by side in the long side direction. 
         [0026]    According to a second aspect of the invention, a droplet discharge apparatus includes the above-described droplet discharge head. 
         [0027]    According to the second aspect of the invention, the droplet discharge apparatus includes the above-described droplet discharge head. Therefore, the structure of the droplet discharge head becomes simple and the amount of a droplet to be discharged at one time is changed by simply controlling the voltage application time. As a result, image quality is enhanced, for example, when images are printed. 
         [0028]    According to a third aspect of the invention, a discharge control method for a liquid discharge head including: a nozzle discharging a liquid as a droplet; a discharge room having a diaphragm and disposed in a channel of the liquid, the channel communicating with the nozzle, the diaphragm pressurizing the liquid by being displaced and being a part of the discharge room; and a fixed electrode facing the diaphragm and generating electrostatic force with respect to the diaphragm by receiving electric charge so as to displace the diaphragm by bringing the diaphragm into contact with and detaching the diaphragm from the fixed electrode, the fixed electrode including a first fixed electrode received the electric charge from an outside and a second fixed electrode that is made of a material different from a material of the first fixed electrode and received the electric charge through the first fixed electrode, the method includes controlling a time during which a voltage is applied by supplying electric charge to the fixed electrode, so that an area of contact of the diaphragm with the fixed electrode is changed. 
         [0029]    According to the third aspect of the invention, by simply controlling the time during which a voltage is applied by supplying electrical charge to the fixed electrode, it is determined whether the diaphragm is brought into contact with only the first fixed electrode or it is brought into contact with both the first and second fixed electrodes, and then a discharge operation is performed. That is, the discharge amount of a droplet is changed by performing simple control. 
         [0030]    In the discharge control method for a droplet discharge head according to the third aspect of the invention, a time during which a voltage is applied between the diaphragm and the fixed electrode is preferably set according to respective time constants of the first and second fixed electrodes related to accumulation of electricity. 
         [0031]    According to the third aspect of the invention, the time during which a voltage is applied between the diaphragm and the fixed electrode is set according to the time constants related to accumulation of electricity. This allows efficient design. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0033]      FIG. 1  is an exploded view of a droplet discharge head according to a first embodiment of the invention. 
           [0034]      FIG. 2  is a sectional view of the droplet discharge head according to the first embodiment. 
           [0035]      FIG. 3  is a diagram showing a configuration centering on a drive control circuit  40 . 
           [0036]      FIG. 4  is a partial enlarged view of a recess  11  and an individual electrode  12 . 
           [0037]      FIGS. 5A to 5C  are diagrams showing relations between the voltage application time and the contact of a diaphragm  22 . 
           [0038]      FIGS. 6A and 6B  are diagrams showing example relations between the width of the individual electrode  12  and time constants. 
           [0039]      FIGS. 7A to 7J  are drawings showing a process of manufacturing an electrode substrate  10 . 
           [0040]      FIGS. 8A to 7G  are drawings showing a process of manufacturing the droplet discharge head. 
           [0041]      FIG. 9  is a partial enlarged view of the recess  11  and individual electrode  12 . 
           [0042]      FIG. 10  shows a table showing a typical electrical resistivity of each metal. 
           [0043]      FIG. 11  is a partial enlarged view of the recess  11  and individual electrode  12 . 
           [0044]      FIG. 12  is an outline view of a droplet discharge apparatus using the droplet discharge head. 
           [0045]      FIG. 13  is a drawing showing one example of the main components of the droplet discharge apparatus. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Embodiment 
       [0046]      FIG. 1  is an exploded view of a droplet discharge head according to a first embodiment of the invention and shows a part of the droplet discharge head (the droplet discharge head actually has nozzles more than ones shown in  FIG. 1 ). In this embodiment, a “face eject” type droplet discharge head will be described as a typical example of a device in which an electrostatically-driven actuator is used. Note that in order for the components to be seen easily, the size relations among the components in the following drawings including  FIG. 1  may differ from actual ones. An upper side of each drawing will be referred to as “upper” and a lower side thereof as “lower.” Since the direction in which nozzles are arranged is a direction of a short side of a rectangular discharge room  21  (a diaphragm  22  and an individual electrode  12 ), this direction will be referred to as a “short side direction,” and a direction orthogonal to the short side direction as a “long side direction.” 
         [0047]    As shown in  FIG. 1 , the droplet discharge head according to this embodiment is formed by laminating three substrates, an electrode substrate  10 , a cavity substrate  10  and a nozzle substrate  30 , in ascending order. In this embodiment, the electrode substrate  10  and cavity substrate  20  are anodically bonded to each other. The cavity substrate  20  and nozzle substrate  30  are bonded together using an epoxy adhesive or the like. 
         [0048]    The electrode substrate  10  is mainly made of, for example, a borosilicate and heat-resistant hard glass with a thickness of approximately 1 mm. While a glass substrate is used as the electrode substrate  10  in this embodiment, a monocrystal silicon may be used, for example. On a surface of the electrode substrate  10 , multiple recesses  11  having a depth of, for example, approximately 0.3 μm are formed in alignment with recesses that will become discharge rooms  21  of the cavity substrate  20  to be described later. Individual electrodes  12  as fixed electrodes are provided on the respective inner surfaces of (in particular, on the bottoms) of the recesses  11  in a manner that the individual electrodes are opposed to the discharge rooms  21  (diaphragms  22 ) of the cavity substrate  20 . Here, each individual electrode  12  according to this embodiment includes a first individual electrode  12 A and a second individual electrode  12 B, each of which is made of a different material, and a connector  12 C (see  FIG. 4 ). The first individual electrode  12 A is provided in the center of a rectangular portion inside each recess  11  in the short side direction, and the second individual electrodes  12 B are provided on both sides of the first individual electrode  12 A. The first individual electrode  12 A and second individual electrode  12 B are coupled at multiple locations via the connector  12 C. A lead  13  and a terminal  14  both for electrically coupling an each individual electrode  12  and an external electric charge supply are provided in each recess  11  in a manner that the lead and terminal are integral with the first individual electrode  12 A (these components will collectively be referred to as an “individual electrode  12 ” unless there is a need for referring to these components separately). The individual electrode  12  will be described in detail later. 
         [0049]    Here, a gap in which the diaphragm  22  becomes deformed (displaced) and that has a given size is formed between the diaphragm  22  and individual electrode  12  inside the recess  11 . The size of the gap formed between the diaphragm  22  (insulating film  23 ) and individual  12  will be referred to as a “gap length.” Also, the electrode substrate  10  has a through hole serving as a liquid supply inlet  15  for taking in a liquid provided from an external tank (not shown). 
         [0050]    The cavity substrate  20  is mainly made of, for example, a silicon monocrystal substrate (hereafter referred to as a “silicon substrate”), a surface of which has a ( 110 ) orientation. The cavity substrate  20  has recesses (whose bottom wall is the diaphragm  22  serving as a movable electrode) serving as the discharge rooms  21  for temporarily storing a liquid to be discharged, and a recess serving as a reservoir  24 . Also, an insulating film  23  that is intended to electrically insulate the cavity substrate  20  from the individual electrode  12  and made of a TEOS film (here, an oxide silicon (SiO 2 ) film formed using tetraethyl orthosilicate tetraethoxysilane (ethyl silicate) as a raw material gas) is formed in a thickness of 0.1 μm on the undersurface (surface opposed to the electrode substrate  10 ) of the cavity substrate  20 . Al 2 O 3  (aluminum (alumina) oxide) or the like may be used instead of the TEOS film in order to form the insulating film  23 . In the following description, it will be assumed that the diaphragm  11  and insulating film  23  are integral with each other, unless otherwise mentioned. Also, the cavity substrate  20  has a recess serving as the reservoir (common liquid room)  24  for providing a liquid to each discharge room  21 . Further, the cavity substrate  20  has a common electrode terminal  27  serving as a terminal used when electrical charge is supplied to the cavity substrate  20  (diaphragm  22 ) from an external power supply (not shown). 
         [0051]    The nozzle substrate  30  is also mainly made of, for example, a silicon substrate. The nozzle substrate  30  has multiple nozzles  31 . Each nozzle  31  discharges a liquid pressurized due to displacement of the diaphragm  22 , as a droplet to outside. Also, the nozzle substrate  30  has an orifice  32  serving as a channel for causing the discharge room  21  and reservoir  24  to communicate with each other, and a diaphragm  33  for absorbing a pressure applied in a direction of the reservoir  24  due to deformation of the diaphragm  22 . 
         [0052]      FIG. 2  is a sectional view of the droplet discharge head in the long side direction. In  FIG. 2 , the discharge room  21  stores a liquid to be discharged from the nozzle  31 . By deforming the diaphragm  22  that is a bottom wall of the discharge room  21 , the pressure in the discharge room  21  is increased so that a droplet is discharged from the nozzle  31 . In this embodiment, the diaphragm  22  is formed by forming, on a silicon substrate, a high-concentration boron dope layer that can serve as an electrode and is favorably used in a wet-etching step. Also, in order to prevent a foreign object, moisture (water vapor), or the like from entering the gap, a sealing material  25  for shielding the gap from an external air and sealing the gap is provided on an electrode inlet  26 . 
         [0053]      FIG. 3  is a diagram showing a configuration centering on a drive control circuit  40 . A controller and the like that control the contact (hold) and detachment of the diaphragm  22  and perform control for discharging a droplet from the droplet discharge head will be described with reference to  FIG. 3 . The drive control circuit  40  includes a head controller  41  configured centering on a CPU  42   a.  The CPU  42   a  of the head controller  41  receives a signal including printing data from an external device  50  such as a computer via a bus  51 . 
         [0054]    The head controller  41  includes a ROM  43   a,  a RAM  43   b,  and a character generator  43   c,  and is coupled to the CPU  42   a  via an internal bus  42   b.  The CPU  42   a  perform a process according to a control program stored in the ROM  43   a  to generate a discharge control signal corresponding to printing data. At that time, the CPU  42   a  uses a memory area in the RAM  43   b  as a work area. Also, when printing characters or the like, the CPU  42   a  performs a process according to character data or the like stored in the character generator  43   c.  A discharge control signal generated by the CPU  42   a  is transmitted to a logic gate array  45  via an internal bus  42   b.  The logic gate array  45  generates a SEG signal concerning supply of electric charge to each individual electrode  12  provided for each nozzle  31 , as will be described later, according to the discharge control signal. Also, a COM generation circuit  46   a  generates a COM signal concerning supply of electric charge to the cavity substrate  20  (diaphragm  22 ), as will be described later. A drive pulse generation circuit  46   b  generates a signal for synchronization. These signals are transmitted to a driver IC  48  via a connector  47 . 
         [0055]    The driver IC  48  is electrically coupled to the terminal  14  and the common electrode terminal  27  directly or via wiring  49  such as a flexible print circuit or a wire. If the number of terminals of the driver IC  48  is smaller than that of the nozzles  31 , multiple driver ICs  48  may be provided. The driver IC  48  is a means for, upon receiving power from the power supply circuit  52 , applying a voltage (drive voltage) between the diaphragm  22  and individual electrode (that is, making a potential difference therebetween)  12  by actually supplying (charging) electric charge to the cavity substrate  20  (diaphragm  22 ) and/or individual electrode  12 , holding the supplied electric charge, and discharging (hereafter referred to as “output”) the cavity substrate and/or individual electrode, according to the above-described signals. By repeating such output, a voltage to be applied by output produced by the drive IC  48  comes to have a waveform of a pulse (actually, the voltage comes to have a waveform of a trapezoid since none of the rise time and fall time is zero; however, such output will be referred to as a “pulse” for convenience). 
         [0056]    By applying a voltage by supplying electric charge, electrostatic force is generated between the diaphragm  12  and individual electrode  12 . Thus, the diaphragm  22  is attracted to the individual electrode  12  so that it is deformed and brought into contact with the individual electrode  12 . For this reason, the removal volume (volume of the discharge room  21 ) is increased. Conversely, if the potential difference between the diaphragm  22  and individual electrode  12  is eliminated or reduced by discharging the diaphragm  22  and individual electrode  12 , generation of the electrostatic force is stopped or reduced. If the restoring force of the diaphragm  22  becomes larger than the force by which the diaphragm  22  is attracted, the diaphragm  22  attempts to return to its original position, thereby detaching itself from the individual electrode  12 . A pressure (hereafter referred to as a “restoring pressure”) caused by this restoring force is applied to a liquid. Thus, the liquid is pushed out of the nozzle  31  so that a droplet is discharged. This droplet lands on, for example, recording paper that is a recording target. Thus, recording such as printing is performed. 
         [0057]      FIG. 4  is a partial enlarged view of the recess  11  and individual electrode  12 . As described above, the first individual electrode  12 A and second individual electrode  12 B, each of which is made of a different material, are formed inside each recess  11 . In this embodiment, indium tin oxide (ITO) that is formed by doping indium oxide with tin oxide and is transparent in a visible light area is used as the material of the first individual electrode  12 A (lead  13  and terminal  14 ). On the other hand, titanium (Ti) is used as the materials of the second individual electrode  12 B and connector  12 C. Although it varies with conditions such as the temperature, the electrical resistivity of titanium is typically 5.5×10 −5  (Ω·cm) and higher than that of ITO. The widths of the first individual electrode  12 A and second individual electrode  12 B are not specified herein. However, since a larger width makes the electrical resistivity lower, electric charge more rapidly extends across the first individual electrode  12 A if the first individual electrode  12 A has a larger width. This is preferable in terms of discharging a droplet with a faster response. Consideration must be given to the balance between the width of contact of the diaphragm  22  with the first individual electrode  12 A and that of the diaphragm  22  with the second individual electrodes  12 B. 
         [0058]    In this embodiment, three connectors  12 C are provided. The first individual electrode  12 A and second individual electrode  12 B are electrically coupled to each other via the connectors  12 C. Electrical charge from the driver IC  48  is supplied to the first individual electrode  12 A via the terminal  14  and lead  14 , and extends across the first individual electrode  12 A. If the electric charge continues to be supplied from the driver IC  48 , it is also supplied to the second individual electrode  12 B via the first individual electrode  12 A and the connectors  12 C. Since supply of the electric charge to the second individual electrode  12 B is performed via the first individual electrode  12 A and connectors  12 C and since the electrical resistivity of the second individual electrode  12 B is higher than that of the first individual electrode  12 A, it takes time until electric charge that causes electrostatic force that brings the diaphragm  22  into contact with the second individual electrode  12 B extends across the second individual electrode  12 B. 
         [0059]    Here, if the first and second individual electrodes  12 A and  12 B are in contact with each other in a larger area, an electric charge supply path from the first individual electrode  12 A to the second individual electrode  12 B is widen. Thus, no difference may be made between the time taken until electric charge extends across the second individual electrode  12 B and the time taken until electric charge extends across the first individual electrode  12 A. For this reason, the connectors  12 C are provided to limit the path through which electric charge is supplied from the first individual electrode  12 A to second individual electrode  12 B. Thus, a difference is made between the above-described times. However, if the contact area is reduced too much, it takes too much time until electric charge extends across the second individual electrode  12 B. This may deteriorate responsiveness. Therefore, if an attempt is made to set a difference between the time taken until electric charge extends across the first individual electrode  12 A and the time taken until electric charge extends across the second individual electrode  12 B, the widths of the connectors  12 C, the number thereof, or the like are adjusted. For example, the time difference is preferably approximately 2 μs. Here, since electric charge does not instantly extend across the first individual electrode  12 A but it is supplied from a region close to the lead  13  and terminal  14  toward a region distant therefrom as described above, the time difference may vary slightly depending on the locations at which the connectors  12 C are provided. 
         [0060]      FIGS. 5A to 5C  are diagrams showing a relation between the time during which a voltage is applied and the contact of the diaphragm  22 .  FIG. 5A  shows a pulse waveform of a voltage to be applied by the driver IC  48 . Here, the voltage of a COM signal to the common electrode terminal  27  is defined as GND, and the voltage of a SEG signal to be applied to control supply of electric charge to each of the individual electrodes  12  is defined as V. In this embodiment, as shown in  FIG. 5A , the time (time during which a voltage is applied) during which electric charge is supplied is changed by the driver IC  48 . Also, with regard to the time taken until electrostatic force that brings the diaphragm  22  into contact with the first individual electrode  12 A and second individual electrode  12 B is obtained, a difference is made between the first individual electrode  12 A and second individual electrode  12 B. This difference is made by forming the first and second individual electrodes  12 A and  12 B using different materials so that the respective individual electrodes have different electrical resistivities. Thus, whether the diaphragm  22  is brought into contact with only the first individual electrode  12 A ( FIG. 5B ) or it is brought into contact with the entire individual electrode  12  (first individual electrode  12 A, second individual electrode  12 B, and connectors  12 C) ( FIG. 5C ) is determined by adjusting the time during which electric charge is supplied. By changing the area (hereafter referred to as a “contact area”) in which the diaphragm  11  is in contact with the individual electrode  12  so as to change the removal volume (volume of the discharge room  21 ), the amount of a droplet to be discharged from the nozzle  31  is changed. 
         [0061]    The driver IC  48  starts to apply a voltage at time ta. If the applied voltage is maintained at V during ≢t 1 , only the first individual electrode  12 A accumulates electric charge that causes electrostatic force required to bring the diaphragm  22  into contact with the first individual electrode  12 A (the second individual electrode  12 B does not accumulate electric charge required to bring the diaphragm  22  into contact with the second individual electrode  12 B). By discharging the individual electrode  12  after the diaphragm  22  has been brought into contact with the first individual electrode  12 A, the diaphragm  22  is detached from the first individual electrode  12 A. A droplet is discharged from the nozzle  31  by a restoring pressure caused at this time. 
         [0062]    On the other hand, if the applied voltage is maintained at V during Δt 2 , the second individual electrodes  12 B as well as the first individual electrode  12 A accumulates electric charge that causes electrostatic force required to bring the diaphragm  22  into contact with the second individual electrodes  12 B. By discharging the individual electrode  12  after the diaphragm  22  has been brought into contact with both the first and second individual electrodes  12 A and  12 B in this manner, the diaphragm  22  is detached from the individual electrode  12 . A droplet is discharged from the nozzle  31  by a restoring pressure caused at this time. Since the diaphragm  22  has also been brought in contact with the second individual electrode  12 B, the removal volume is increased. As a result, the amount of a droplet to be discharged from the nozzle  31  is increased compared with that in a case where the applied voltage is maintained at V. 
         [0063]      FIGS. 6A and 6B  are diagrams showing an example relation between the widths of the first and second individual electrodes  12 A and  12 B and the time constants thereof. Specifically,  FIGS. 6A and 6B  show the ratio between the widths of the first individual electrode  12 A made of ITO (referred to as an “ITO part” in  FIGS. 6A and 6B ) and second individual electrodes  12 B made of titanium (referred to as a “Ti part” in  FIGS. 6A and 6B ), a time constant τ 1 (s) of the first individual electrode  12 A (ITO part), a time constant τ 2 (s) of the second individual electrode  12 B (Ti part), and a time constant τ 3  (considered as the sum of the time constant τ 1 (s) and time constant τ 2 (s)) up to the second individual electrode  12 B via the first individual electrode  12 A.  FIG. 6A  shows a case where the first and second individual electrode  12 A and  12 B are electrically coupled to each other at one connector  12 C.  FIG. 6B  shows a case where these electrodes are electrically coupled to each other at three connectors  12 C. Note that the first and second individual electrode  12 A and  12 B have identical lengths. 
         [0064]    Here, the time constant τ denotes a value showing a primary frequency response in a linear system expressed by the following Formula 1 and typically denotes the time taken until approximately 63.2% of a final value is reached. 
         [0000]        e ( t )= E (1−exp(− t /τ))   Formula 1 
         [0065]    where e is a voltage between the diaphragm  11  and individual electrode  12 , E is a voltage V applied by the driver IC  48 , t is a time, and τ is a time constant. 
         [0066]    In this embodiment, the time taken until approximately 63.2% of electric charge that can be accumulated in the first individual electrode  12 A is accumulated and the time taken until approximately 63.2% of electric charge that can be accumulated in the second individual electrode  12 B is accumulated are denoted as τ 1  and τ 2 , respectively. It is conceivable that it takes a time longer than a time shown as the time constant until electric charge required to generate electrostatic force is supplied to the first individual electrodes  12 A and second individual electrode  12 B and accumulated therein, although it depends on the discharge amount, discharge speed, performance design items, and the like. For example, it is conceivable that when electrostatic force required to make contact is generated at triple the constant, approximately 95% of electric charge that can be accumulated is accumulated. In this case, the times are denoted by Δt 1  and Δt 2  shown in  FIG. 5A . Thus, the time constants of the first and second individual electrodes  12 A and  12 B can be referred to, although the time constants are not used as the voltage application time directly. By setting the voltage application time according to the time constants, efficient design is achieved. 
         [0067]    As described above, according to the first embodiment, the individual electrode  12  includes the first individual electrode  12 A made of ITO and the second individual electrode  12 B that is made of titanium and receives electric charge via the first individual electrode  12 A. Thus, by controlling supply of electric charge from the driver IC  48 , it is determined whether the diaphragm  22  is brought into contact with only the first individual electrode  12 A or it is brought into contact with the both the first and second individual electrode  12 A and  12 B, and then a discharge operation is performed. As a result, the discharge amount of a droplet to be discharged at one time is changed. In this case, since the first and second individual electrode  12 A and  12 B are electrically coupled to each other via one or more (here, three) connectors, the electric charge supply path from the first individual electrode  12 A to the second individual electrode  12 B is arbitrarily set. Also, since the individual electrode  12  is provided along the short side direction of the discharge room  21  (diaphragm  22 ), the diaphragm  22  for applying a pressure required for discharge to a liquid is brought into contact with the individual electrode  12  along the channel of the liquid. 
         [0068]    Also, since the first individual electrode  12 A is made of ITO and the second individual electrode  12 B is made of titanium that is a material having a electrical resistivity higher than ITO, a difference is made between the time taken until electrostatic force required to bring the diaphragm  22  into contact with the first individual electrode  12 A is generated and the time taken until electrostatic force required to bring the diaphragm  22  into contact with the second individual electrode  12 B is generated. In particular, the combination of ITO and titanium is the best one in terms of the difference between the electrical resistivities, adhesiveness in a case where a substrate serving as a base is made of glass, or the like. Thus, the droplet discharge head has a long life and performs favorable discharge. 
         [0069]    Further, the time during which a voltage is applied by supplying electric charge to the individual electrode  12  is simply controlled by the drive IC  48 , whereby the discharge amount is controlled. That is, the amount of a droplet to be discharged at one time is changed by performing simple control. Also, since the voltage application times Δt 1  and Δt 2  are set according to the time constants τ 1  and τ 2 , respectively, efficient design is achieved. 
       Second Embodiment 
       [0070]      FIGS. 7A to 7J  are drawings showing a process of manufacturing the electrode substrate  10 . A method for manufacturing a droplet discharge head according to a second embodiment of the invention will now be described focusing on manufacture of the electrode substrate  10 . The drawings on the right hand side represent a section in the long side direction of a portion in which the first individual electrode  12 A and the like are formed, and the drawings on the left hand side represent a section in the long side direction of a portion in which the second individual electrode  12 B is formed. Actually, multiple electrode substrates  10  are simultaneously formed using a wafer-shaped glass substrate. Then, the electrode substrates and other substrates are bonded together and then cut into individual droplet discharge heads.  FIGS. 7A to 7J  show only a part of the electrode substrate  10  of one droplet discharge head (the same goes for the following drawings). 
         [0071]    Chrome (Cr) or the like is deposited on one surface of a glass substrate  61  with a thickness of approximately 1 mm so as to form a film  62  (hereafter referred to as a “mask film  62 ”) that will serve as a mask (Fig. A). The mask film  62  is formed, for example, by physical vapor deposition (PVD). Among PVD techniques are sputtering, vacuum deposition, and ion-plating. A photoresist  63  is applied to all of a surface of the mask film  62 . Then, photolithography is performed. Specifically, the photoresist photosensitive resin applied to all of the surface of the chrome film is exposed to light using a mask aligner and developed using a developer. As a result, a pattern of the photoresist  63  for forming a portion that will become the recess  11  of the electrode substrate  10  later is formed on the glass substrate  61 . 
         [0072]    After the photoresist pattern is formed, wet-etching is performed using a cerium nitrate ammonium solution so as to eliminate an unnecessary portion of the mask film  62  ( FIG. 7B ). Thus, an etching pattern of a portion of the mask film  62  that will become the recess  11  is formed on the glass substrate  61 . Then, the glass substrate  61  is wet-etched using a ammonium fluoride solution so as to form the recess  11  having a sidewall with a height of approximately 0.3 μm ( FIG. 7C ). Then, the mask film  62  is peeled off. 
         [0073]    Then, titanium is deposited on, for example, all of the surface on which the recess  11  is formed, so as to form a film  64  (hereafter referred to as a “titanium film  64 ”) ( FIG. 7D ). The titanium film  64  is formed, for example, a PVD technique such as sputtering. A photoresist  65  is applied to the titanium film  64  using the above-described photolithography technique, and then patterned. Subsequently, the titanium film  64  is dry-etched using sulfur hexafluoride (SF 6 ) ( FIG. 7E ). Then the photoresist  65  is peeled off to form the second individual electrode  12 B and connector  12 C ( FIG. 7F ). 
         [0074]    Further, ITO is deposited on all of the surface on which the recess  11  is formed, so as to form a film  66  (hereafter referred to as a “ITO film  66 ”) that will become the first individual electrode  12 A, lead  13  and terminal  14  ( FIG. 7G ). While the method for forming the ITO film is not limited to a particular one, the ITO film is formed, for example, by sputtering. A photoresist  67  is applied to the ITO film  66  by photolithography and then patterned. Subsequently, the ITO film  66  is wet-etched using a mixed liquid of hydrochloric acid, nitric acid, and pure water ( FIG. 7H ). Then, the photoresist  67  is peeled off to form the first individual electrode  12 A, lead  13 , and terminal  14  ( FIG. 7I ). Then, the liquid supply inlet  15  is made. Thus, the electrode substrate  10  is manufactured ( FIG. 7J ). Here, taking into account damages due to etching, the second individual electrode  12 B and connector  12 C are formed using titanium, and then the first individual electrode  12 A, lead  13 , and terminal  14  are formed using ITO. Depending on the material of each electrode, etching method, etchant, or the like, the order in which these components are formed is limited thereto. 
         [0075]      FIGS. 8A to 8G  are drawings showing a process of manufacturing the droplet discharge head. The process of manufacturing the droplet discharge head will now be described with reference to  FIGS. 8A to 8G . While multiple droplet discharge heads are simultaneously formed using one wafer,  FIGS. 8A to 8G  show only one droplet discharge head. 
         [0076]    One surface (surface to which the electrode substrate  10  is to be bonded) of a silicon substrate  71  is mirror-polished to form a substrate (that will become the cavity substrate  20 ) with a thickness of, for example, 220 μm ( FIG. 8A ). Subsequently, a surface of the silicon substrate  71  on which a boron dope layer  72  is to be formed is opposed to a diffusion source of a solid that contains B 2 O 3  as a main ingredient. Then, the silicon substrate  71  is put into a vertical furnace so that boron is diffused through the silicon substrate  71 . Thus, the boron dope layer  72  is formed. An insulating film  23  is formed in a thickness of 0.1 μm on the surface on which the boron dope layer  72  is formed, by plasma CVD under the conditions: processing temperature of 360° C., a high-frequency output of 250 W, a pressure of 66.7 Pa (0.5 Torr), gas flow rate (TEOS flow rate) of 100 cm 3 /min (100 sccm), and oxygen flow rate of 1000 cm 3 /min (1000 sccm). 
         [0077]    Then, the silicon substrate  71  and electrode substrate  10  are heated up to 360° C., and then a negative electrode is coupled to the electrode substrate  10  and a positive electrode is coupled to the silicon substrate  71 . Then, these substrates anodically bonded to each other by applying a voltage of 800 V therebetween. The surface of the silicon substrate  71  included in the anodically-bonded substrate (hereafter referred to as a “bonded substrate”) is polished until the thickness of the silicon substrate  71  becomes approximately 60 μm. Subsequently, in order to eliminate an affected layer, the silicon substrate  71  is wet-etched using a potassium hydroxide solution for approximately 10 min. Thus, the thickness of the silicon substrate  71  is made approximately 50 μm ( FIG. 8C ). 
         [0078]    Silicon oxide using TEOS is deposited on the wet-etched surface of the bonded substrate by plasma CVD, so as to form a hard mask (hereafter referred to as a “TEOS hard mask”)  73 . The hard mask is formed in a thickness of 1.5 μm under the conditions: processing temperature of 360° C., a high-frequency output of 700 W, a pressure of 33.7 Pa (0.25 Torr), gas flow rate (TEOS flow rate) of 100 cm 3 /min (100 sccm), and oxygen flow rate of 1000 cm 3 /min (1000 sccm). 
         [0079]    After the TEOS hard mask  73  is formed, resist-patterning is performed to wet-etch portions of the TEOS hard mask  73  that will become the discharge room  21  and electrode inlet  26 . Then, using a hydrofluoric acid solution, these portions are wet-etched until the TEOS hard mask  73  is eliminated. Thus, the TEOS hard mask  73  is patterned and the silicon substrate  71  is exposed. With regard to a portion that will become reservoir  24 , the TEOS hard mask  73  is slightly left to secure the thickness of the bottom of the reservoir  24 . Also, with regard to a portion that will become the electrode inlet  26  which is fragile, the thickness of the resist may be slightly secured to prevent a fracture in a later step. Then, wet-etching is performed and then the resist is peeled off ( FIG. 8D ). 
         [0080]    Subsequently, the bonded substrate is immersed into a potassium hydroxide solution with a concentration of 35 wt/%. Then wet-etching is performed until the thicknesses of portions that will become the discharge room  21  and electrode inlet  26  become approximately 10 μm. Further, the bonded substrate is immersed into a potassium hydroxide solution with a concentration of 3 wt/%, and then wet-etching is continued until the boron dope layer  72  is exposed and it is determined that an etching stop at which the progress of the etching becomes extremely slow has taken effect sufficiently ( FIG. 8E ). By performing etching using the two potassium hydroxide solutions having different concentrations in this way, surface roughness of a portion that will become the diaphragm  22  of the discharge room  21  is suppressed, thereby improving the thickness accuracy. As a result, the discharge performance of the droplet discharge head is stabilized. 
         [0081]    When the wet-etching is complete, the bonded substrate is immersed into a hydrofluoric acid solution, and then the TEOS hard mask  73  on the surface of the silicon substrate  71  is peeled off. Subsequently, in order to eliminate a portion of the boron dope layer  72  that will become the electrode inlet  26 , a silicon mask having an opening in a portion that will become the electrode inlet  26  is attached to the surface of the silicon substrate  71  included in the bonded substrate. For example, RIE dry-etching (anisotropic dry-etching) is performed for 30 min. under the conditions: RF power of 200 W, pressure of 40 Pa (0.3 Torr), and CF 4  flow rate of 30 cm 3 /min (30 sccm). Then, plasma is applied to only a portion that will become the electrode inlet  26 , so as to make an opening. In order to improve the accuracy of alignment between the bonded substrate and mask, the silicon mask is preferably attached to the bonded substrate using pin alignment in which a pin is threaded through the bonded substrate and silicon mask. While the opening is made by anisotropic dry-etching herein, the boron dope layer  72  may be broken by puncturing it with a pin or the like. Then, sealing is performed using the sealing material  25  in order to shield the gap from an outside air ( FIG. 8F ). While the material of the sealing material  25 , sealing method, and the like are not limited to particular ones, the sealing is performed, for example, by applying an epoxy resin to an opening of the electrode inlet  26  or depositing a silicon oxide thereon. 
         [0082]    When the sealing is complete, a mask having an opening in a portion that will become the common electrode terminal  27  is attached to the surface of the silicon substrate  71  included in the bonded substrate. Then, sputtering or the like is performed using, for example, platinum (Pt) as a target so as to form the common electrode terminal  27 . Then, the nozzle substrate  30  previously manufactured in another process is attached to the surface of the cavity substrate  20  included in the bonded substrate using an epoxy adhesive, and bonded thereto ( FIG. 8G ). Dicing is performed along dicing lines so that the bonded substrate is cut into individual droplet discharge heads. Thus, the droplet discharge heads are completed. Further, each droplet discharge head is coupled to the IC driver  48  via the wiring  49 . 
         [0083]    As described above, when manufacturing the electrode substrate  10 , the second individual electrode  12 B is formed using titanium and then the first individual electrode  12 A is formed using ITO. Thus, these electrodes are formed without being damaged by each other. 
       Third Embodiment 
       [0084]      FIG. 9  is a partial enlarged view of the recess  11  and individual electrode  12  according to a third embodiment of the invention. While the first and second individual electrode  12 A and  12 B are provided side-by-side in the short side direction of the rectangular diaphragm  22  in the above-described embodiments, these electrodes may be provided side-by-side, for example, in the long side direction thereof. Also in this case, these electrodes may be electrically coupled to each other via the connector  12 C so that the electric charge supply path is limited. 
       Fourth Embodiment 
       [0085]      FIG. 10  shows a table showing a typical electrical resistivity of each metal. In the above-described embodiments, ITO is used as the material of the first individual electrode  12 A and titanium is used as the material of the second individual electrode  12 B. While it is conceivable that titanium is most favorable in terms of the relations in electrical resistivity and the like with ITO, the material is not limited thereto. For example, adhesiveness to the glass that serves as a base of the electrode substrate  10  must be also considered. For example, chrome (Cr), platinum (Pt), gold (Au) and the like are conceivable as metal materials other than titanium. Further, other alloys, metal oxides such as titanium oxide, and the like may be used. 
         [0086]    Also, the material of the first individual electrode  12 A is not limited to ITO. For example, indium zinc oxide (IZO) and the like may be used as the material thereof. 
         [0087]      FIG. 11  is a partial enlarged view of the recess  11  and individual electrode  12 . In the above-described embodiments, the first individual electrode  12 A is provided in the center of the recess  11  and the second individual electrode  12 B is provided on both sides of the first individual electrode  12 A. However, the invention is not limited thereto and, for example, a second individual electrode  12 B- 2  made of a different material may additionally be provided outside a second individual electrode  12 B- 1 , as shown in  FIG. 11 . Also, in the above-described embodiments, titanium is used as the material of the connector  12 C like the second individual electrode  12 B. However, the invention is not limited thereto and a different material may be used. 
       Fifth Embodiment 
       [0088]    While the three-layered droplet discharge head including the electrode substrate  10 , cavity substrate  20 , and nozzle substrate  30  has been described in the above-described embodiments, the invention is also applicable to a four-layered droplet discharge head including an independent substrate (hereafter referred to as a “reservoir substrate”) as a reservoir. 
       Sixth Embodiment 
       [0089]      FIG. 12  is an outline view of a droplet discharge apparatus using a droplet discharge head manufactured according to the above-described embodiments.  FIG. 13  is a drawing showing one example of the main components of the droplet discharge apparatus. The droplet discharge apparatus shown in  FIGS. 12 and 13  is intended to perform printing using the droplet discharge method (inkjet method). Such a apparatus is called “serial type” apparatus. In  FIG. 13 , the droplet discharge apparatus mainly includes a drum  101  for supporting printing paper  110  as a printing target and a droplet discharge head  102  for discharging ink onto the printing paper  110  to perform recording. Although not shown, the droplet discharge apparatus also includes an ink supply means for providing ink to the droplet discharge head  102 . The printing paper  110  is held by the drum  101  in a manner that it is pressed against the drum  101  by a paper pressure roller  103  provided in parallel to the axis direction of the drum  101 . A lead screw  104  is provided in parallel to the axis direction of the drum  101 , and the droplet discharge head  102  is held by the lead screw  104 . By rotating the lead screw  104 , the droplet discharge head  102  is moved in the axis direction of the drum  101 . 
         [0090]    On the other hand, the drum  101  is rotary-driven by a motor  106  via a belt  105  and the like. The drive control circuit  40  drives the lead screw  104  and motor  106  according to printing data and a control signal. Further, the drive control circuit  40  drives an oscillation drive circuit (not shown) to vibrate the diaphragm  22 , and performs control so that printing is performed on the printing paper  110 . 
         [0091]    While ink is discharged onto the printing paper  110  in this embodiment, the liquid to be discharged from the droplet discharge head is not limited to ink. For example, the following liquids may be discharged from the droplet discharge head provided in a relevant droplet discharge apparatus: a liquid that includes a pigment for a color filter and is to be discharged onto a substrate which will become a color filter; a liquid that includes a compound which will become a light-emitting element and is to be discharged onto a display such as OLED; and a liquid that includes, for example, a conductive metal and is used to install wiring on a substrate. Also, a liquid including a probe such as deoxyribo nucleic acids (DNA), other nucleic acids (e.g., ribo nucleic acids, peptide nucleic acids, etc.), or a protein may be discharged from a dispenser as a droplet discharge head onto a substrate that will become a microarray of biomolecules. Further, the droplet discharge head may be used to discharge a dye for cloth.