Patent Publication Number: US-2007103512-A1

Title: Liquid ejection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-323808 filed on Nov. 8, 2005, the entire contents of which are incorporated herein by reference.  
     BACKGROUND  
      1. Technical Field  
      The present invention relates to a liquid ejection apparatus.  
      2. Related Art  
      Typically, a display such as a liquid crystal display or an electroluminescence display includes a substrate that displays an image. The substrate has an identification code (for example, a two-dimensional code) representing encoded information including the site of production and the product number. The identification code is formed by structures (dots formed by colored thin films or recesses) that reproduce the identification code. The structures are provided in multiple dot formation areas (data cells) in accordance with a prescribed pattern.  
      As a method for forming the identification code, a laser sputtering method and a waterjet method have been described in JP-A-11-77340 and JP-A-2003-127537. In the laser sputtering method, films forming a code pattern are provided through sputtering. The waterjet method involves ejection of water containing abrasive material onto a substrate for marking a code pattern on the substrate.  
      However, to form the code pattern in a predetermined size by the laser sputtering method, the interval between a metal foil and a substrate must be adjusted to several or several tens of micrometers. The corresponding surfaces of the substrate and the metal foil thus must be extremely flat and the interval between the substrate and the metal foil must be adjusted with accuracy of the order of micrometer. Therefore, the laser sputtering method is applicable only to certain types of substrates, making it difficult to form identification codes in a wider range of substrates. In the waterjet method, water or dust or abrasive may splash onto and contaminate a substrate, when forming a code pattern on the substrate.  
      To solve these problems, an inkjet method has been focused on as an alternative method for forming an identification code. In the inkjet method, droplets of liquid containing metal particles are ejected from a nozzle. The droplets are then dried and thus form dots. The inkjet method is applicable to a wider variety of substrates and prevents contamination of the substrates caused by formation of the identification codes.  
      However, when drying droplets on a substrate, the inkjet method may have the following problem caused by the surface condition of the substrate or the surface tension of each droplet. Specifically, after having been received by the surface of the substrate, the droplet may spread on the substrate surface as the time elapses. Therefore, if the time necessary for drying the droplet exceeds a predetermined level (for example, 100 milliseconds), the droplet may spread beyond the corresponding data cell and reaches an adjacent data cell. This may lead to erroneous formation of the code pattern.  
      This problem may be avoided by radiating an energy beam (for example, a laser beam) onto a droplet on the substrate at a predetermined point of time and solidifying the droplet.  
      However, in this case, volatile elements or mist evaporated from the droplet may adhere to an optical component, contaminating the optical path of the energy beam. This destabilizes the radiation amount or the radiating position of the energy beam, varying the shape of the obtained pattern.  
     SUMMARY  
      Accordingly, it is an objective of the present invention to provide a liquid ejection apparatus that stabilizes the optical characteristics of energy beams radiated onto droplets of liquid and thus improves controllability for shaping a pattern formed by the droplets.  
      In accordance with one aspect of the present invention, a liquid ejection apparatus including a liquid ejection section, an energy beam radiating section, and a cleaning section is provided. The liquid ejecting section ejects a droplet onto an object. The energy beam radiating section radiates an energy beam onto the droplet on the object. The energy beam radiating section has an optical component that defines a radiation path of the energy beam. The cleaning section cleans the optical component of the energy beam radiating section.  
      Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
       FIG. 1  is a plan view showing a liquid crystal display having an identification code formed by a liquid ejection apparatus according to a first embodiment of the present invention;  
       FIG. 2  is a perspective view schematically showing the liquid ejection apparatus of the first embodiment;  
       FIG. 3  is a plan view schematically showing the liquid ejection apparatus of the first embodiment;  
       FIG. 4  is a view for explaining a head unit;  
       FIG. 5  is a view for explaining a cleaning mechanism of the first embodiment;  
       FIG. 6  is a view for explaining the cleaning mechanism;  
       FIG. 7  is a block diagram representing the electric circuit of the liquid ejection apparatus;  
       FIG. 8  is a view for explaining a cleaning mechanism according to a second embodiment of the present invention; and  
       FIG. 9  is a view for explaining a cleaning mechanism of a modification. 
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      A liquid crystal display having an identification code formed by a method for forming a pattern according to the present invention will now be described with reference to FIGS.  1  to  5 . In the following, direction X, direction Y, and direction Z will be defined as illustrated in  FIG. 2 . First, a liquid crystal display  1  having an identification code formed by a liquid ejection apparatus of the present invention will be explained.  
      As shown in  FIG. 1 , a liquid crystal display  1  has a rectangular glass substrate (hereinafter, refereed to as a substrate)  2 . A rectangular display portion  3  is formed substantially at the center of a surface  2   a  of the substrate  2 . Liquid crystal molecules are sealed in the display portion  3 . A scanning line driver circuit  4  and a data line driver circuit  5  are provided outside the display portion  3 . In the liquid crystal display  1 , the orientation of the liquid crystal molecules is adjusted in correspondence with a scanning signal generated by the scanning line driver circuit  4  and a data signal produced by the data line driver circuit  5 . In accordance with the orientation of the liquid crystal molecules, area light radiated by an illumination device (not shown) is modulated to display an image on the display portion  3  of the substrate  2 .  
      An identification code  10  indicating the product number or the lot number of the liquid crystal display  1  is formed at the left corner of the surface  2   a  of the substrate  2 . The identification code  10  is formed by a plurality of dots D and provided in a code formation area S in accordance with a prescribed pattern. The code formation area S includes 256 data cells, aligned by 16 lines and 16 rows. Each of the data cells C is defined by virtually dividing the code formation area S, which has a square shape of 1 mm×1 mm, into equally sized sections. The dots D are formed in selected ones of the data cells C, thus forming the identification code  10 .  
      In the first embodiment, the center of each of the data cells C in which a dot D is formed will be referred to as an “ejection target position P” and the length of each side of each data cell C will be refereed to as a “cell width W”.  
      Each of the dots D is formed in a semispherical shape with an outer diameter equal to the cell width W. To form the dots D, droplets Fb of liquid containing metal particles (for example, nickel particles or manganese particles) as pattern forming material are ejected onto the corresponding cells C (the black cells C 1 ). The droplets Fb in the cells C are then irradiated with laser beams, which dry and bake the droplets Fb (see  FIG. 4 ), forming the dots D. Alternatively, the dots D may be formed simply by drying the droplets Fb through radiation of a laser beam.  
      A liquid ejection apparatus  20  for forming the identification code  10  will hereafter be described. In the following case, a plurality of identification codes  10  will be formed on a mother substrate  2 M, or an object from which a plurality of substrates  2  are cut out.  
      As shown in  FIG. 2 , the liquid ejection apparatus  20  has a substantially parallelepiped base  21 . A substrate stocker  22  that accommodates a plurality of mother substrates  2 M is provided at the right-hand side of the base  21 . The substrate stocker  22  is movable in the direction defined by the height of the substrate stocker  22  (in direction Z and the direction opposite to direction Z). Each of the mother substrates  2 M is retrieved from the substrate stocker  22  and transported to the base  21 . The mother substrate  2 M is later removed from the base  21  and returned to the corresponding one of slots defined in the substrate stocker  22 .  
      A running device  23 , which extends in direction Y, is arranged on an upper surface  21   a  of the base  21  and in the vicinity of the substrate stocker  22 . The running device  23  is operably connected to the output shaft of a running motor MS (see  FIG. 7 ). A transport device  24 , which runs in direction Y and the direction opposite to direction Y, is mounted on the running device  23 . The transport device  24  has a transport arm  24   a,  which draws by suction and holds a backside  2 Mb of the mother substrate  2 M. The transport device  24  is operably connected to the output shaft of a transport motor MT (see  FIG. 7 ). The transport device  24  is a SCARA robot movable in the direction defined by the height of the transport device  24 . The transport device  24  supports the transport arm  24   a  in such a manner as to allow the transport arm  24   a  to extend, contract, or pivot on the X-Y plane.  
      A pair of mounting tables  25 R,  25 L are arranged on the upper surface  21   a  of the base  21  and in the vicinity of opposing sides of the base  21 . One of the mother substrates  2 M is mounted on each of the mounting tables  25 R,  25 L with the surface  2 Ma of the mother substrate  2 M facing upward. Each mounting table  25 R,  25 L has a recess  25   a  having an upper opening. The transport arm  24   a  moves in the recess  25   a  of the corresponding one of the mounting tables  25 R,  25 L in a horizontal direction and the direction defined by the height of the transport arm  24   a.  In this manner, the mother substrate  2 M is transported to and placed on the mounting table  25 R,  25 L.  
      When a drive signal is sent to the running motor MS and the transport motor MT, the running device  23  and the transport device  24  (the transport arm  24   a ) are operated. In this manner, the corresponding one of the mother substrates  2 M is thus retrieved from the substrate stocker  22  and placed on the corresponding one of the mounting tables  25 R,  25 L. The mother substrate  2 M is also returned from the mounting table  25 R,  25 L to the corresponding slot of the substrate stocker  22 . A code areas S is defined on the mother substrates  2 M mounted on the mounting tables  25 R,  25 L. In each of the mother substrates  2 M, the rows of the code areas S are defined as the first row of the code areas S 1 , the second row of the code areas S 2 , the third row of the code areas S 3 , the fourth row of the code areas S 4 , and the fifth row of the code areas S 5  sequentially from the foremost row of the code areas S to the rearmost row of the code areas S (see  FIG. 3 ).  
      A multi-joint robot (hereinafter, referred to as a SCARA robot)  26  is arranged between the two mounting tables  25 R,  25 L on the upper surface  21   a  of the base  21 . The SCARA robot  26  has a main shaft  27  that extends upward (in direction Z) from the upper surface  21   a  of the base  21 . A first arm  28   a  is pivotally connected to an upper end of the main shaft  27 . A second arm  28   b  is pivotally connected to the distal end of the first arm  28   a.  A columnar third arm  28   c  is connected to the distal end of the second arm  28   b  in a manner rotatable about the axis of the third arm  28   c.  A head unit  30  is formed at a lower end of the third arm  28   c.    
      The first arm  28   a,  the second arm  28   b,  and the third arm  28   c  are operably connected to the output shaft of a first motor M 1 , the output shaft of a second motor M 2 , and the output shaft of a third motor M 3 , respectively (see  FIG. 7 ). When a drive signal is provided to any one of the first, second, and third motors M 1 , M 2 , M 3 , the SCARA robot  26  operates the corresponding one of the first, second, and third arms  28   a,    28   b,    28   c  to move the head unit  30  in a predetermined scanning area (the area indicated by the double-dotted chain lines of  FIG. 3 ) on the base  21 .  
      As shown by the corresponding arrow of  FIG. 3 , the SCARA robot  26  first rotates the first, second, and third arms  28   a,    28   b,    28   c  to move the head unit  30  in direction Y along the first row of the code areas S 1 . After scanning the first row of the code areas S, the SCARA robot  26  rotates the third arm  28   c  to rotate the head unit  30  counterclockwise at 180 degrees. The SCARA robot  26  then re-rotates the first, second, and third arms  28   a,    28   b,    28   c  to move the head unit  30  in the direction opposite to direction Y along the second row of the code areas S 2 .  
      Afterwards, the SCARA robot  26  moves the head unit  30  in direction Y or the direction opposite to direction Y along the third, fourth, and fifth rows of the code areas S 3 , S 4 , S 5  in this order. In this manner, while changing the orientation and the movement direction of the head unit  30  (the scanning direction J), the SCARA robot  26  moves the head unit  30  along the path defined by connecting the code areas S together.  
      Referring to  FIG. 4 , a box-like liquid tank  31 , which retains liquid F, is provided in the head unit  30 . The liquid F is drawn from the liquid tank  31  to a liquid ejection head  32 , which forms a liquid ejecting section.  
      The liquid ejection head  32  (hereinafter, referred to simply as the “ejection head 32”) is provided in a lower portion of the head unit  30 . A nozzle plate  33  is formed on a lower side of the ejection head  32 . A plurality of nozzles N are defined in a lower surface (a nozzle surface)  33   a  of the nozzle plate  33 . Each of the nozzles N is a circular bore extending in a normal direction of the mother substrate  2 M. The nozzles N are aligned in a direction perpendicular to the scanning direction J of the head unit  30 . The pitch of the nozzles N is equal to the cell width W. In the following description, a position immediately below each nozzle N on the surface  2 Ma of the mother substrate  2 M will be referred to as a droplet receiving position PF.  
      Referring to  FIG. 4 , cavities  34  are defined in the liquid tank  31  and communicate with the corresponding nozzles N. An oscillation plate  35  is provided on each of the cavities  34  and oscillates in the direction defined by the height of the oscillation plate  35 . Through oscillation of the corresponding oscillation plate  35 , the volume of each cavity  34  increases and decreases. A plurality of piezoelectric elements PZ are formed on the upper surfaces of the oscillation plates  35  in correspondence with the nozzles N. Each of the piezoelectric elements PZ contracts and extends in response to a drive signal (drive voltage COM 1 : see  FIG. 7 ). This oscillates the corresponding one of the oscillation plates  35  in the direction defined by the height of the oscillation plate  35 . The liquid F is thus supplied from the liquid tank  31  to the associated one of the nozzles N, ejecting a droplet Fb from the nozzle N. More specifically, when the droplet receiving positions PF coincide with the ejection target positions P of the corresponding code areas S through operation of the SCARA robot  26 , the drive voltage COM 1  is supplied to the piezoelectric elements PZ to eject the droplets Fb from the associated nozzles N. After having reached the corresponding one of the ejection target positions P, each of the droplets Fb spreads on the surface  2 Ma of the mother substrate  2 M and develops until the outer diameter of the droplet Fb becomes equal to the size (the cell width W) at which the droplet Fb should be dried.  
      In the first embodiment, the time from when the droplets Fb are ejected from the nozzles N to when the outer diameter of each droplet Fb becomes equal to the cell width W will be referred to as “radiation standby time”. The head unit  30  moves by the distance corresponding to the cell width W in the radiation standby time.  
      A laser head  37 , or an energy beam radiating section, is formed in an upper portion of the head unit  30 . A plurality of semiconductor lasers LD are provided in the laser head  37  in correspondence with the nozzles N. The semiconductor lasers LD are aligned in the direction in which the nozzles N are aligned. Each of the semiconductor lasers LD radiates a laser beam B in response to a drive signal (drive voltage COM 2 : see  FIG. 7 ). The laser beam B has an energy in the wavelength range corresponding to the absorption wavelength of each droplet Fb.  
      A plurality of reflective mirrors M, each of which forms an optical system, extend from a lower end of the laser head  37  in correspondence with the semiconductor lasers LD. Each of the reflective mirrors M is located immediately below the corresponding one of the semiconductor lasers LD. The reflective mirrors M are aligned in the direction in which the nozzles N are aligned. The surface of each reflective mirror M opposed to the corresponding semiconductor laser LD is a reflective surface Ma (an optical surface). The reflective surface Ma totally reflects the laser beam B radiated by the semiconductor laser LD, thus leading the laser beam B to the corresponding radiating position PT on the substrate  2 .  
      In the first embodiment, the droplet receiving positions PF and the radiating positions PT are defined on the movement path of the head unit  30 . The distance between each droplet receiving position PF and the corresponding radiating position PT is equal to the distance covered by movement of the head unit  30  in the radiation standby time (the cell width W)  
      When the radiating positions PT coincide with the corresponding ejection target positions P through operation of the SCARA robot  26 , the drive voltage COM 2  is supplied to the semiconductor lasers LD, causing the semiconductor lasers LD to radiate the laser beams B. Each laser beam B is totally reflected by the reflective surface Ma of the corresponding reflective mirror M and then radiated onto the droplet Fb at the corresponding radiating position PT. The laser beam B evaporates solvent and dispersion medium from the droplet Fb and bakes the metal particles in the droplet Fb. As a result, a semispherical dot D having an outer diameter equal to the cell width W is formed at the ejection target position P on the substrate  2 .  
      The elements evaporated from the droplets Fb float upward from the mother substrate  2 M and adhere to the ejection head  32  and the reflective mirrors M of the head unit  30 , forming adhered matter G. The adhered matter G contaminates the reflective surfaces Ma of the reflective mirrors M and the nozzle surface  33   a  of the ejection head  32 .  
      To solve this problem, as shown in  FIGS. 2 and 3 , a maintenance mechanism  38  for the ejection head  32  and a cleaning mechanism  40  (a cleaning section) that cleans the head unit  30  are provided on the upper surface  21   a  of the base  21 . The maintenance mechanism  38  has a suction pump  38   a  and a wiping sheet  38   b.  To stabilize ejection of the droplets Fb, the maintenance mechanism  38  draws and drains the liquid F with high viscosity from the ejection head  32  by the suction pump  38   a.  Further, the wiping sheet  38   b  wipes the liquid F off from the ejection head  32 .  
      The cleaning mechanism  40  has a box-like cleaning bath  41  with an upper opening, or a cleaning liquid supply section. The cleaning bath  41  is operably connected to a lift motor ML (see  FIG. 7 ). The cleaning bath  41  is raised and lowered with respect to the upper surface  21   a  of the base  21 . Through actuation of the lift motor ML, the cleaning bath  41  moves between the standby position of  FIG. 5  and the cleaning position of  FIG. 6 .  
      As shown in  FIG. 5 , an inlet pipe  41   a  (an inlet section) is formed in an upper portion of the cleaning bath  41 . The cleaning bath  41  is connected to a cleaning liquid supply section  42  through the inlet pipe  41   a.  The cleaning liquid supply section  42  has a supply tank  42   a  and a supply pump  42   b.  The supply tank  42   a  retains cleaning liquid Fc that washes the adhered matter G off from the nozzle surface  33   a  and the reflective surfaces Ma. The supply pump  42   b  pressurizes the cleaning liquid Fc and supplies the cleaning liquid Fc to the cleaning bath  41 . The cleaning liquid Fc and the liquid F exhibit mutual solubility.  
      An outlet pipe  41   b  (an outlet section) is formed in a lower portion of the cleaning bath  41  at the side opposed to the inlet pipe  41   a.  The cleaning bath  41  is connected to a cleaning liquid discharge section  43  through the outlet pipe  41   b.  The cleaning liquid discharge section  43  has a liquid waste tank  43   a  and a discharge pump  43   b.  The liquid waste tank  43   a  retains the used cleaning liquid Fc. The discharge pump  43   b  drains the cleaning liquid Fc from the cleaning bath  41  into the liquid waste tank  43   a.    
      When a drive signal is input to the supply pump  42   b  and the discharge pump  43   b,  a predetermined amount of the cleaning liquid Fc is supplied from the cleaning liquid supply section  42  to the cleaning bath  41  through the inlet pipe  41   a  and the same amount of the cleaning liquid Fc is drained to the liquid waste tank  43   a  through the outlet pipe  41   b.  In other words, the cleaning liquid supply section  42  and the cleaning liquid discharge section  43  operate to move the cleaning liquid Fc in the cleaning bath  41  from the inlet pipe  41   a  to the outlet pipe  41   b,  causing replacement of the cleaning liquid Fc in the cleaning bath  41 . In this state, the liquid level Fs of the cleaning liquid Fc in the cleaning bath  41  is maintained constant.  
      Through operation of the SCARA robot  26 , the head unit  30  is arranged immediately above the cleaning mechanism  40  (the cleaning bath  41 ) with the laser head  37  (the reflective mirrors M) located at a side of the ejection head  32  that faces the inlet pipe  41   a.    
      In the first embodiment, when the cleaning bath  41  is located at the standby position of  FIG. 5 , the ejection head  32  and the reflective mirrors M are raised from the cleaning liquid Fc. When the cleaning bath  41  is located at the cleaning position of  FIG. 6 , the nozzle surface  33   a  of the ejection head  32  and the reflective surfaces Ma of the reflective mirrors M are immersed in the cleaning liquid Fc.  
      A supersonic oscillator  44 , or an oscillating section, is secured to the cleaning bath  41 . The supersonic oscillator  44  causes supersonic oscillation of the cleaning liquid Fc in the cleaning bath  41 .  
      When the cleaning bath  41  is arranged at the standby position, the SCARA robot  26  operates to move the head unit  30  to a position immediately above the cleaning mechanism  40  (the cleaning bath  41 ). In this manner, referring to  FIG. 5 , the laser head  37  is (the reflective mirrors M are) arranged at a side of the ejection head  32  that faces the inlet pipe  41   a.  In this state, the lift motor ML is actuated to move the cleaning bath  41  to the cleaning position. As a result, as illustrated in  FIG. 6 , the nozzle surface  33   a  of the ejection head  32  and the reflective surfaces Ma of the reflective mirrors M are immersed in the cleaning liquid Fc in the cleaning bath  41 .  
      In this state, the supply pump  42   b,  the discharge pump  43   b,  and the supersonic oscillator  44  are activated to cause supersonic oscillation of the cleaning liquid Fc in the cleaning bath  41 , thus washing the adhered matter G off the nozzle surface  33   a  and the reflective surfaces Ma. Further, since the adhered matter G in a dissolved state flows from the vicinity of the inlet pipe  41   a  toward the outlet pipe  41   b,  the adhered matter G is drained into the liquid waste tank  43   a  through the outlet pipe  41   b.    
      In this manner, by removing the adhered matter G from the nozzle surface  33   a  and the reflective surfaces Ma, ejection of the droplets Fb by the ejection head  32  and radiation of the laser beams B by the laser head  37  are stabilized. Further, while the cleaning liquid Fc flows from the vicinity of the inlet pipe  41   a  to the outlet pipe  41   b,  the reflective mirrors M (the reflective surfaces Ma) are maintained constantly upstream from the nozzle surface  33   a  (the nozzles N). This prevents the liquid F dissolved from the nozzles N into the cleaning liquid Fc from adhering to the reflective surfaces Ma of the reflective mirrors M.  
      Further, in the first embodiment, a non-illustrated air supply device blasts dry air onto the nozzle surface  33   a  and the reflective mirrors M after the nozzle surface  33   a  and the reflective mirrors M have been raised from the cleaning liquid Fc. The cleaning liquid Fc is thus removed from the nozzle surface  33   a  and the reflective mirrors M.  
      The electric circuit of the liquid ejection apparatus  20  will be explained in the following, with reference to  FIG. 7 .  
      As illustrated in  FIG. 7 , a controller  51  has a CPU, a RAM, and a ROM. In accordance with various types of data and different control programs stored in the ROM, the controller  51  operates the running device  23 , the transport device  24 , and the SCARA robot  26  while activating the ejection head  32 , the laser head  37 , and the cleaning mechanism  40 .  
      An input device  52  having a start switch and a stop switch is connected to the controller  51 . Through manipulation of the switches by the operator, an image of the identification code  10  is input to the controller  51  as a prescribed form of imaging data Ia. In accordance with the imaging data Ia, the controller  51  generates bit map data BMD and the drive voltages COM 1 , COM 2 .  
      The bit map data BMD is data that indicates whether to turn on or off the piezoelectric elements PZ in accordance with the value of each bit (0 or 1). That is, the bit map data BMD instructs whether or not to eject the droplets Fb onto the data cells C defined on a two-dimensional imaging plane (the surface  2 Ma of the corresponding mother substrate  2 M).  
      A driver circuit  53  of the running device  23  is connected to the controller  51 . The running motor MS and a rotation detector MSE are connected to the driver circuit  53 . The rotation detector MSE outputs a prescribed signal when rotation of the running motor MS is detected. The controller  51  sends a drive signal to the driver circuit  53  to drive the running motor MS. In response to the drive signal of the controller  51 , the driver circuit  53  rotates the running motor MS in a forward direction or a reverse direction. The controller  51  also computes the movement direction and the movement amount of the transport device  24  in correspondence with the detection signal of the rotation detector MSE.  
      A driver circuit  54  of the transport device  24  is connected to the controller  51 . The driver circuit  54  has the transport motor MT and a rotation detector MTE. The rotation detector MTE outputs a prescribed signal when rotation of the transport motor MT is detected. In response to a drive signal of the controller  51 , the driver circuit  54  rotates the transport motor MT in a forward direction or a reverse direction. Further, in correspondence with the detection signal of the rotation detector MTE, the driver circuit  54  calculates the movement direction and the movement amount of the transport arm  24   a.    
      A driver circuit  55  of the SCARA robot  26  is connected to the controller  51 . The first motor M 1 , the second motor M 2 , and the third motor M 3  are connected to the driver circuit  55 . In response to a drive signal of the controller  51 , the driver circuit  55  rotates the first, second, and third motors M 1 , M 2 , M 3  in a forward direction or a reverse direction. Rotation detectors M 1 E, M 2 E, M 3 E are connected to the driver circuit  55 . In correspondence with detection signals of the rotation detectors M 1 E, M 2 E, M 3 E, the driver circuit  55  computes the movement direction and the movement amount of the head unit  30 .  
      The controller  51  moves the head unit  30  in the scanning direction J through the driver circuit  55  and provides different types of control signals to the corresponding driver circuits in correspondence with the computation results obtained by the driver circuit  55 .  
      More specifically, in correspondence with the timing at which the droplet receiving positions PF, which move together with the head unit  30 , coincide with the corresponding ejection target positions P on the mother substrate  2 M, the controller  51  outputs an ejection timing signal LP 1  to the driver circuit  56 . Also, in correspondence with the timing at which the head unit  30  reaches the position immediately above the cleaning bath  41 , the controller  51  sends a cleaning start signal SP to the driver circuit  58 .  
      A driver circuit  56  is connected to the controller  51 . The controller  51  sends the ejection timing signal LP 1  to the driver circuit  56 . Further, the controller  51  provides the drive voltage COM 1  to the driver circuit  56  synchronously with a prescribed reference clock signal. The controller  51  also generates ejection control signals SI from the bit map data BMD synchronously with a prescribed reference clock signal. The ejection control signals SI are serially transferred to the driver circuit  56  of the ejection head  32 . The driver circuit  56  sequentially converts the serial ejection control signals SI to parallel signals in correspondence with the piezoelectric elements PZ.  
      When receiving the ejection timing signal LP 1  from the controller  51 , the driver circuit  56  supplies the drive voltage COM 1  to the piezoelectric elements PZ that are selected in accordance with the ejection control signals SI. In other words, the controller  51  operates to eject the droplets Fb from the nozzles N corresponding to the bit map data BMD when the receiving positions PF coincide with the corresponding ejection target positions P. The controller  51  operates the driver circuit  56  to send the parallel ejection control signals SI, which have been converted from the serial forms, to a driver circuit  57  of the laser head  37 .  
      The driver circuit  57  is connected to the controller  51 . The controller  51  supplies the drive voltage COM 2  synchronized with a prescribed reference clock signal to the driver circuit  57 .  
      After the radiation standby time has elapsed since reception of the ejection control signals SI from the driver circuit  56 , the driver circuit  57  supplies the drive voltage COM 2  to the semiconductor lasers LD corresponding to the ejection control signals SI. Specifically, the controller  51  operates in such a manner that the head unit  30  moves (the reflective mirrors M move) to cover the distance corresponding to the radiation standby time and then radiates the laser beams B onto the droplets Fb at the ejection target positions P when the radiating positions PT coincide with the ejection target positions P.  
      A driver circuit  58  of the cleaning mechanism  40  is connected to the controller  51 . The controller  51  sends the cleaning start signal SP and a cleaning stop signal TP to the driver circuit  58 . The lift motor ML is connected to the driver circuit  58 . In response to the cleaning start signal SP or the cleaning stop signal TP of the controller  51 , the driver circuit  58  rotates the lift motor ML in a forward direction or a reverse direction to raise or lower the cleaning bath  41 . A rotation detector MLE is connected to the driver circuit  58  and outputs a prescribed signal when rotation of the lift motor ML is detected. The driver circuit  58  calculates the movement direction and the movement amount of the cleaning bath  41  in correspondence with the detection signal of the rotation detector MLE.  
      When receiving the cleaning start signal SP from the controller  51 , the driver circuit  58  rotates the lift motor ML in the forward direction to move the cleaning bath  41  to the cleaning position. Further, the driver circuit  58  determines whether the cleaning bath  41  has reached the cleaning position in correspondence with the detection signal of the rotation detector MLE. When the cleaning bath  41  reaches the cleaning position, the driver circuit  58  activates the supersonic oscillator  44  to cause supersonic oscillation of the cleaning liquid Fc in the cleaning bath  41 . The driver circuit  58  also operates the supply pump  42   b  and the discharge pump  43   b  to start introduction of the cleaning liquid Fc into the cleaning bath  41  and drainage of the cleaning liquid Fc from the cleaning bath  41 .  
      When receiving the cleaning stop signal TP from the controller  51 , the driver circuit  58  rotates the lift motor ML in the reverse direction so as to return the cleaning bath  41  to the standby position. The driver circuit  58  determines whether the cleaning bath  41  has reached the standby position in correspondence with the detection signal of the rotation detector MLE. When the cleaning bath  41  reaches the standby position, the driver circuit  58  deactivates the supply pump  42   b,  the discharge pump  43   b,  and the supersonic oscillator  44 .  
      A method for forming the identification code  10  by the liquid ejection apparatus  20  will hereafter be explained.  
      First, the input device  52  is manipulated by the operator to provide the imaging data Ia to the controller  51 . The controller  51  then drives the running device  23  and the transport device  24  through the driver circuit  53  and the driver circuit  54 , respectively, to transport the corresponding mother substrate  2 M from the substrate stocker  22  and place the mother substrate  2 M on the mounting table  25 R (the mounting table  25 L).  
      Further, the controller  51  generates the bit map data BMD from the imaging data Ia and produces the drive voltages COM 1 , COM 2 . The controller  51  then operates the SCARA robot  26  through the driver circuit  55 , starting movement of the head unit  30 . In correspondence with the computation results of the driver circuit  55 , the controller  51  determines whether the droplet receiving positions PF coincide with the rearmost ones of the data cells C in the first rows of the code areas S 1  in direction Y (the ejection target positions P).  
      Meanwhile, the controller  51  sends the ejection control signals SI and the drive voltage COM 1  to the driver circuit  56  and the drive voltage COM 2  to the driver circuit  57 .  
      When the droplet receiving positions PF coincide with the data cells C rearmost in the first row of the code areas S 1  in direction Y (the ejection target positions P), the controller  51  outputs the ejection timing signal LP 1  to the driver circuit  56  and supplies the drive voltage COM 1  to the piezoelectric elements PZ selected in accordance with the ejection control signals SI. The droplets Fb are thus simultaneously ejected from the selected ones of the nozzles N. After having reached the corresponding ejection target position P, each droplet Fb spreads on the surface  2   a  of the substrate  2 . By the time the radiation standby time elapses since starting of ejection of the droplets Fb, the outer diameter of each droplet Fb becomes equal to the cell width W.  
      Further, the controller  51  sends the parallel ejection control signals SI, which have been converted from the serial forms, to the driver circuit  57  through the driver circuit  56 . After the radiation standby time has elapsed since starting of ejection, the controller  51  supplies the drive voltage COM 2  to the semiconductor lasers LD selected in accordance with the ejection control signals SI. As a result, the selected semiconductor lasers LD simultaneously radiate the laser beams B.  
      The laser beams B are then totally reflected by the reflective surfaces Ma of the reflective mirrors M and radiated onto the droplets Fb at the radiating positions PT (the ejection target positions P). This evaporates the solvent or the dispersion medium from the droplets Fb and bakes the metal particles of the droplets Fb. As a result, the dots D having an outer diameter equal to the cell width W are provided on the surface  2 Ma of the mother substrate  2 M.  
      Afterwards, the controller  51  continuously moves the head unit  30  along the scanning path in the same manner as has been described. Each time the droplet receiving positions PF coincide with the ejection target positions P, the controller  51  operates to eject the droplets Fb from the selected nozzles N. Further, the laser beams B are radiated onto the droplets Fb when the outer diameter of each droplet Fb becomes equal to the cell width W. After having formed all of the dots D, the controller  51  operates the running device  23  and the transport device  24  to return the mother substrate  2 M from the mounting table  25 R (the mounting table  25 L) to the substrate stocker  22 .  
      During formation of the dots D, the adhered matter G deposits on the reflective surfaces Ma of the reflective mirrors M and the nozzle surface  33   a  of the ejection head  32 . This gradually degrades the optical characteristics of the laser beams B radiated by the laser head  37  and ejection performance of the droplets Fb by the ejection head  32 .  
      In the first embodiment, after the mother substrate  2 M is received in the substrate stocker  22 , the controller  51  first operates the SCARA robot  26  through the driver circuit  55  to move the head unit  30  to the position immediately above the cleaning bath  41 . The controller  51  then sends the cleaning start signal SP to the driver circuit  58  so as to move the cleaning bath  41  to the cleaning position. At this position, the nozzle surface  33   a  of the ejection head  32  and the reflective surfaces Ma of the reflective mirrors M are immersed in the cleaning liquid Fc. In this state, the controller  51  actuates the supersonic oscillator  44  through the driver circuit  58 , causing supersonic oscillation of the cleaning liquid Fc in the cleaning bath  41 . Further, the controller  51  operates the supply pump  42   b  and the discharge pump  43   b  to start introduction of the cleaning liquid Fc into the cleaning bath  41  and drainage of the cleaning liquid Fc from the cleaning bath  41 .  
      As a result, the adhered matter G deposited on the nozzle surface  33   a  and the reflective surfaces Ma is dissolved into the cleaning liquid Fc. The adhered matter G is thus discharged from the cleaning bath  41  into the liquid waste tank  43   a  together with the cleaning liquid Fc. Since the nozzle surface  33   a  and the reflective surfaces Ma are cleaned in this manner, the initial states of the ejection performance of the ejection head  32  and the optical characteristics of the laser head  37  (the laser beams B) are restored.  
      After having cleaned the nozzle surface  33   a  and the reflective surfaces Ma for a predetermined time, the controller  51  outputs the cleaning stop signal TP to the driver circuit  58 . The cleaning bath  41  is thus returned to the standby position and the supply pump  42   b,  the discharge pump  43   b,  and the supersonic oscillator  44  are deactivated. Subsequently, the controller  51  operates the non-illustrated air supply device to blast the dry air onto the nozzle surface  33   a  and the reflective mirrors M, thus drying and removing the cleaning liquid Fc.  
      The first embodiment has the following advantages.  
      (1) Since the adhered matter G is removed also from the nozzle surface  33   a,  the droplets Fb are ejected stably by the ejection head  32 . This further enhances the controllability for shaping the dots D.  
      (2) The cleaning liquid supply section  42  and the cleaning liquid discharge section  43  are connected to the cleaning bath  41 . The cleaning liquid supply section  42  introduces the cleaning liquid Fc into the cleaning bath  41  and the cleaning liquid discharge section  43  discharges the cleaning liquid Fc from the cleaning bath  41 . Therefore, the adhered matter G on the reflective surfaces Ma and the nozzle surface  33   a  is dissolved into the cleaning liquid Fc and then drained from the cleaning bath  41  together with the cleaning liquid Fc. This maintains the cleaning performance of the cleaning mechanism  40 . The optical characteristics of the laser beams B are thus maintained stable for a relatively long time.  
      (3) The reflective mirrors M are immersed in the cleaning liquid Fc in the vicinity of the inlet pipe  41   a  and the ejection head  32  is immersed in the cleaning liquid Fc in the vicinity of the outlet pipe  41   b.  Therefore, the reflective mirrors M (the reflective surfaces Ma) are maintained upstream from the nozzle surface  33   a  (the nozzles N). This prevents the liquid F dissolved from the nozzles N into the cleaning liquid Fc from adhering to the reflective surfaces Ma.  
      (4) The cleaning bath  41  has the supersonic oscillator  44  that causes supersonic oscillation of the cleaning liquid Fc. The reflective mirrors M and the nozzle surface  33   a  are thus further effectively cleaned.  
      A second embodiment of the present invention will now be described with reference to  FIG. 8 . Same or like reference numerals are given to parts of the second embodiment that are the same as or like corresponding parts of the first embodiment and detailed description thereof will be omitted. In the following description of the second embodiment, the configuration of a maintenance mechanism  38  of the ejection head  32  will be explained in detail.  
      As shown in  FIG. 8 , a plate-like mirror securing section  45  is supported by a lower end of the laser head  37  in a manner movable in the direction defined by the height of the mirror securing section  45 . The reflective mirrors M of the first embodiment are pivotally secured to a lower end of the mirror securing section  45 . In the second embodiment, the mirror securing section  45  moves downward from the state in which the laser beams B are radiated (the state indicated by the double-dotted chain lines of  FIG. 8 ). The reflective surfaces M 1  of the reflective mirrors M pivot counterclockwise about the axis C 1  (in the direction indicated by the corresponding arrow of  FIG. 8 ). The reflective surfaces Ma of the reflective mirrors M thus face downward and are located substantially on the same plane as the nozzle surface  33   a.    
      Hereinafter, the position of each reflective mirror M at which the laser beams B are radiated onto the radiating positions PT (see  FIG. 4 ) will be referred to as the mirror reflecting position. The position of the reflective mirror M at which the reflective mirror M is located on the same plane as the nozzle surface  33   a  will be referred to as the mirror cleaning position.  
      The maintenance mechanism  38  has a drive roller  46   a  that rotates counterclockwise and a driven roller  46   b.  A wiping sheet  38   b,  or a wiping member forming the cleaning liquid supply section, is wound around the outer circumference of the driven roller  46   b.  When the drive roller  46   a  rotates, the wiping sheet  38   b  is continuously reeled off the driven roller  46   b  and wound around the outer circumference of the drive roller  46   a.    
      When the head unit  30  is arranged immediately above the maintenance mechanism  38 , the laser head  37  is (the reflective mirrors M are) located upstream from the ejection head  32 , or the position closer to the driven roller  46   b.    
      More specifically, when the head unit  30  is sent to the position immediately above the maintenance mechanism  38  through operation of the SCARA robot  26 , the laser head  37  (the reflective mirrors M) of the head unit  30  and the ejection head  32  (the nozzle surface  33   a ) are arranged between the drive roller  46   a  and the driven roller  46   b.  In this state, the reflective mirrors M are located upstream from the ejection head  32 .  
      A cleaning liquid supply section  47  is formed between the drive roller  46   a  and the driven roller  46   b.  The cleaning liquid supply section  47  is located upstream from the reflective mirrors M and above the wiping sheet  38   b.  The cleaning liquid supply section  47  sprays the cleaning liquid Fc onto the wiping sheet  38   b,  which is reeled off the driven roller  46   b.    
      A first pressing roller  48  and a second pressing roller  49  are arranged between the drive roller  46   a  and the driven roller  46   b  in such a manner that the wiping sheet  38   b  is held between the reflective mirrors M and the ejection head  32 . The first and second pressing rollers  48 ,  49  rotate counterclockwise through the wiping sheet  38   b.  The wiping sheet  38   b  is pressed upward by the first and second pressing rollers  48 ,  49 . The pressing force generated by the first and second pressing rollers  48 ,  49  constantly acts to cause slidable contact between the wiping sheet  38   b  and the reflective surfaces Ma and the nozzle surface  33   a.    
      In the second embodiment, the SCARA robot  26  is first operated to move the head unit  30  to the position immediately above the maintenance mechanism  38 . The reflective mirrors M are then pivoted to move the reflective mirrors M from the mirror reflecting positions to the mirror cleaning positions. Subsequently, the drive roller  46   a  is activated to rotate and the cleaning liquid supply section  47  is caused to spray the cleaning liquid Fc onto the wiping sheet  38   b.  The wiping sheet  38   b,  which has received the cleaning liquid Fc, thus contacts and slides on the reflective surfaces Ma and the nozzle surface  33   a.  This washes the adhered matter G off the reflective surfaces Ma and the nozzle surface  33   a.  The initial states of the ejection performance of the ejection head  32  and the optical characteristics of the laser head  37  (the laser beams B) are thus restored.  
      The second embodiment has the following advantages.  
      (1) The reflective mirrors M are secured to the lower end of the laser head  37  through the mirror securing section  45 . The position of each reflective mirror M is switched between the mirror reflecting position and the mirror cleaning position. Specifically, when the wiping sheet  38   b  that has received the sprayed cleaning liquid Fc is moved along the nozzle surface  33   a  in a slidable contact manner, the wiping sheet  38   b  slides also on the reflective surface Ma of each reflective mirror M located at the mirror cleaning position.  
      In this manner, the adhered matter G is washed off the reflective surfaces Ma by the wiping sheet  38   b.  The initial states of the optical characteristics of the laser head  37  (the laser beams B) are thus restored. As a result, the optical characteristics of the laser beams B radiated onto the droplets Fb are stabilized and the controllability for shaping the dots D is improved.  
      (2) The wiping sheet  38   b  slides on the nozzle surface  33   a  of the ejection head  32  at a position downstream from the reflective mirrors M. Therefore, even if the liquid F flows from the nozzles N and received by the wiping sheet  38   b,  the liquid F is prevented from re-adhering to the reflective surfaces Ma of the reflective mirrors M.  
      The illustrated embodiments may be modified in the following forms.  
      To immerse only the reflective mirrors M in the cleaning liquid Fc, a partition  61  may be provided, as illustrated in  FIG. 9 , to prevent the ejection head  32  from being exposed to the cleaning liquid Fc. Alternatively, for the same purpose, the cleaning bath  41  may be reduced in size. In this case, dissolution of the liquid F from the nozzles N into the cleaning liquid Fc is suppressed. Contamination of the reflective mirrors M by the liquid F is thus avoided.  
      In the first embodiment, for example, the cleaning liquid Fc may be sprayed onto the reflective mirrors M in the cleaning bath  41 . That is, the reflective surfaces Ma may be cleaned in any suitable manner as long as the cleaning liquid Fc is supplied to the reflective surfaces Ma and the adhered matter G is thus removed.  
      In the first embodiment, the cleaning liquid Fc may be volatile. In this case, the nozzle surface  33   a  and the reflective mirrors M dry naturally after having been cleaned.  
      In the second embodiment, for example, the wiping sheet  38   b  may slide on only the reflective mirrors M. In this case, contamination of the reflective mirrors M by the liquid F from the nozzles N is avoided.  
      In each of the first and second embodiments, a carriage and a movable stage may be provided instead of the SCARA robot  26 . The carriage holds the head unit  30  and moves on the base  21  in a specific direction. The movable stage carries the substrate  2  and moves in a direction perpendicular to the specific direction. That is, any suitable structure may be employed as long as the head unit  30  is movable relative to the corresponding mother substrate  2 M.  
      In each of the first and second embodiments, the droplets Fb may be moved in a desired direction by energy generated by the laser beams B. Alternatively, the laser beams may be radiated onto only the outer ends of the droplets Fb, causing solidification (pinning) only in the surfaces of the droplets Fb. In other words, the present invention is applicable to any suitable method by which a pattern is formed through radiation of the laser beams B onto the droplets Fb.  
      In each of the first and second embodiments, a pattern may be formed by droplets Fb that provide oval dots or linear marks instead of the semispherical dots D.  
      In each of the first and second embodiments, for example, ion beams or plasma light may be employed instead of the laser beams B. That is, any suitable form of energy may be supplied to the droplets Fb on the substrate  2  as long as a pattern is formed by the droplets Fb.  
      In each of the first and second embodiments, various types of thin films, metal wirings, or color filters of the liquid crystal display  1  or a field effect type device (an FED or an SED) may be formed instead of the dots D as the marks forming the identification code  10 . The field effect type device emits light from a fluorescent substance by electrons released by electron release elements. In other words, the droplets Fb may form any suitable marks.  
      In each of the illustrated embodiments, the substrate  2  may be, for example, a silicone substrate, a flexible substrate, or a metal substrate.