Patent Publication Number: US-7905577-B2

Title: Printhead substrate having electrothermal transducers arranged at high density, printhead, and printing apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printhead substrate including electrothermal transducers (heaters) to generate thermal energy for discharging ink. The present invention also relates to a printhead having the printhead substrate assembled therein, and to a printing apparatus configured to print an image by using the printhead. 
     2. Description of the Related Art 
     For example, Japanese Patent Laid-Open No. 2002-374163 describes an ink jet printhead in which electrothermal transducers (heaters), a driver, and a logic circuit are formed on a silicon substrate. In accordance with image data, the logic circuit drives the electrothermal transducers through the driver. An arrangement of power supply wirings (lines) for the electrothermal transducers in a substrate (printhead substrate) is described in, e.g., U.S. Pat. No. 6,409,315. 
     Resolution of a color ink jet printer utilizing the thermal ink jet technique has been increased year by year. Also, in an ink jet printhead used to print an image with high quality, a density at which a plurality of discharge ports (nozzles) are arrayed to discharge ink droplets has been increased from 600 dpi (dots/inch) to 900 dpi and further to 1200 dpi per color of ink. A practical arrangement of such a printhead is described in, e.g., U.S. Pat. No. 6,474,790. 
     Further, the size of an ink droplet discharged from the printhead has been reduced to lessen graininess in a halftone area of a gray-scale image and an intermediate-tone or highlight area of a photographic color image. In a printhead to discharge color ink, for example, there is a tendency to reduce the droplet size year by year from an ink discharge amount of about 15 pl several years ago to 5 pl and further to 2 pl. Such a printhead is adaptable for a user&#39;s needs for higher-quality printing when the user wants to print a graphic or photographic color image with higher quality. However, when the user wants to print a relatively rough image, e.g., a color graph in business forms, which does not require high resolution, a contradiction to the demand for higher-speed printing is resulted because the number of scans necessary for printing is increased due to the reduction of the droplet size. 
     With the view of solving such a problem, a printhead has been proposed which has both features of higher-quality printing realized with the reduction in size of the discharged ink droplet and a higher-speed printing using the ink droplet of a larger size. For example, U.S. Pat. No. 5,754,201 proposes a method of arranging a plurality of electrothermal transducers in one nozzle and modulating an ink discharge amount. As another example, U.S. Pat. No. 6,137,502 proposes a method of arranging, in one printhead substrate, a plurality of discharge ports (nozzles) which have different ink discharge amounts from each other. 
     In addition, a method of transferring image data having an increased data quantity to a printhead and driving the printhead in a more compact way is proposed as a time-division driving method in U.S. Pat. No. 6,966,629, for example. With the proposed time-division driving method, a plurality of adjacent electrothermal transducers (heaters) are set as one group and the electrothermal transducers (heaters) in the group are driven successively on the time basis such that plural one of the electrothermal transducers in each group are not driven at the same time. 
     In recent ink jet printers (ink jet printing apparatuses), as described above, the droplet size of the discharged ink has been reduced to obtain an image with higher quality. On the other hand, a higher printing speed has also been demanded. When the same image is simply formed, the same ink amount is required. For example, when the amount of the ink droplet is reduced to ½ just by decreasing the droplet size of the discharged ink, the printing speed is also reduced to ½. In that case, the reduction of the printing speed can be avoided by discharging the ink in the same amount toward a printing medium. To realize it, however, the number of heaters (electrothermal transducers) arranged per unit length of a nozzle array has to be doubled. If the number of heaters is doubled while the pitch between two adjacent heaters is kept the same, the size of a printhead substrate in which the heaters are arranged is increased by two folds or more. The so-called serial scanning printing apparatus, in particular, suffers from drawbacks of increasing the size of the printhead which is moved in the apparatus at a high speed, making production more difficult, increasing the size of the printing apparatus, and generating larger vibration and noise. To avoid those drawbacks, it is necessary to reduce the heater pitch and to array the heaters at a higher density. 
     On the other hand, from the viewpoint of stably discharging the ink droplets, a stable voltage has to be applied to the heaters. If all the heaters are driven at the same time, large current flows at a time and a large voltage drop is caused due to the line resistance. As one example of proposals to avoid such a large voltage drop, a time-division driving method is employed which limits the number of heaters driven at the same time and drives the grouped heaters successively at different timings. 
     Because the reduction in size of the ink droplet is contradictory to obtaining a higher printing speed, it is important to increase the size of the ink droplet so as to realize a higher printing speed in addition to the reduction in size of the discharged ink droplet. In one example of the so-called side-shooter ink jet printhead having discharge ports on the side opposed to the electrothermal transducers (heaters) as described later, the discharge ports and the heaters are arranged corresponding to small ink droplets and large ink droplets, respectively. With the printhead thus constructed, by selectively driving the heaters for the small ink droplets and the heaters for the large ink droplets, the small ink droplets and the large ink droplets can be selectively discharged and higher quality of a printed image and a higher printing speed can be both realized. However, because the number of arranged heaters is increased, it is necessary to reduce the heater pitch and to array the heaters at a higher density. 
     Thus, in any of the cases of arraying the discharge ports at a higher density to reduce the size of the ink droplet for realizing higher quality of the printed image and the case of selectively using the small ink droplets and the large ink droplets to realize higher image quality and a higher printing speed, the heaters have to be arrayed at a higher density. 
     Such a requirement can be realized by employing heaters each having a small area to reduce the size of the ink droplet and by arraying the heaters at a higher density corresponding to the printing density. However, when the heaters are arranged in the form of a row at a higher density, rules for a line (wiring) width and a line-to-line distance are decided based on the restriction on the current density in lines associated with the heaters and the rate determined in a production process using photolithography. Accordingly, it is difficult to ensure a heater area for realizing the desired ink discharge amount. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a printhead substrate, a printhead, and a printing apparatus, which can ensure an area for arrangement of each of electrothermal transducers and can array the electrothermal transducers at a higher density while realizing higher quality of a printed image and a higher printing speed. 
     According to a first aspect of the present invention, a printhead substrate includes an electrothermal transducer array constituted by a plurality of electrothermal transducers which are arranged along an ink supply port adapted to supply ink, the transducers being configured to generate thermal energy to discharge the ink, and a circuit configured to perform time-division driving for a plurality of blocks obtained by dividing the plurality of electrothermal transducers in the electrothermal transducer array. In units of a group constituted by plural electrothermal transducers belonging to each of the different blocks, one-end sides of the plural electrothermal transducer in the group are connected to a common wiring which is arranged to extend along the ink supply port and is separated from common wirings for other groups. The other-end sides of the plural electrothermal transducer in the group are connected respectively to an individual wiring. Plural lines connected to the common wiring are each arranged in an adjacent or sandwiched relation to the electrothermal transducers in the group. 
     According to a second aspect of the present invention, a printhead includes the printhead substrate and a discharge port forming member forming ink discharge ports corresponding to the electrothermal transducers. 
     According to a third aspect of the present invention, in a printing apparatus configured to print an image on a printing medium by using the printhead and by discharging ink through the ink discharge ports, the printing apparatus includes a mechanism arranged to relatively move the printhead and the printing medium. 
     According to the present invention, the plural electrothermal transducers belonging to each of the blocks having different driving timings are constituted as one group and are connected to the common wiring in units of one group. Plural lines are connected to the common wiring such that the plural lines are extended to return with respect to the electrothermal transducers and are each arranged to extend in an adjacent or sandwiched relation to the electrothermal transducers. With such an arrangement, the width of each of the plural lines can be reduced. As a result, higher quality of a printed image and a higher printing speed can be realized by arraying the heaters at a higher density while ensuring a sufficient area for arrangement of each of the electrothermal transducers. Further, the electrothermal transducers can be arrayed at the higher density by employing the known production process. In addition, downsizing of both the printhead substrate and the printhead can be resulted. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of a heater portion of a printhead substrate according to a first exemplary embodiment of the present invention. 
         FIG. 2  is an electric circuit diagram corresponding to heaters and lines (wirings) in  FIG. 1 . 
         FIG. 3  is a schematic plan view of a heater portion of a printhead substrate according to a second exemplary embodiment of the present invention. 
         FIG. 4  is an electric circuit diagram corresponding to heaters and lines in  FIG. 3 . 
         FIG. 5  is a schematic plan view of a heater portion of a printhead substrate as one Comparative Example. 
         FIG. 6  is an electric circuit diagram corresponding to heaters and lines in  FIG. 5 . 
         FIG. 7  is a schematic plan view of a heater portion of a printhead substrate as another Comparative Example. 
         FIG. 8  is an explanatory circuit diagram illustrating one example of a heater driving circuit which can be formed on the printhead substrate according to the present invention. 
         FIG. 9  is a timing chart illustrating the operation of the heater driving circuit in  FIG. 8 . 
         FIG. 10  is a perspective view, partly cut away, illustrating one example of a printhead using the printhead substrate according to the present invention. 
         FIG. 11  is an explanatory view illustrating one example of a printing apparatus using the printhead in  FIG. 10 . 
         FIG. 12  is a diagram illustrating a time-division driving method. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below with reference to the drawings. 
     First Exemplary Embodiment 
       FIGS. 1 and 2  are explanatory views of a heater portion of a printhead substrate according to a first exemplary embodiment of the present invention. 
     Prior to describing the construction of the heater portion of the printhead substrate, which is the feature of the present invention, a description is first given of exemplary constructions of a driving circuit formed in the printhead substrate, signals used to drive the driving circuit, a printhead, and a printing apparatus using the printhead. 
     Exemplary Construction of Driving Circuit 
     The printhead substrate includes, as described in detail later, electrothermal transducers (also called “heaters” hereinafter) and a driving circuit arranged to drive the heaters. The heaters and the driving circuit are formed on the same printhead substrate by employing the semiconductor process technique. 
       FIG. 8  is an explanatory circuit diagram illustrating one example of the driving circuit formed on the printhead substrate. 
     In  FIG. 8 , the driving circuit includes heaters  1101  which generate thermal energy, and transistors  1102  which perform switching to selectively supply desired currents for driving of the heaters  1101 . An AND gate  1201  serves as a heater selection circuit outputting a heater driving signal which turns on/off each of the transistors  1102 . When the transistor  1102  is turned on, a current is allowed to flow from a power supply wiring (line)  1105  to the corresponding heater  1101  and further to a ground wiring (GND line)  1110  through the transistor  1102 , whereby the heater  1101  is driven. 
     A shift register (S/R)  1104  temporarily stores data including image data (DATA). Based on the stored data, whether a current is supplied to each heater  1101 , i.e., whether the heater  1101  is turned on/off to discharge ink from a nozzle of the later-described printhead, is determined. A transfer clock input terminal  1107  receives a transfer clock (CLK) applied to the shift register  1104 . A data input terminal  1106  serially receives the image data (DATA) used to turn on/off the heater  1101  and block control data decoded by a decoder  1203 . A latch circuit  1103  holds the image data and the block control data, and a latch signal input terminal  1108  inputs a latch timing signal (LT) to the latch circuit  1103 . The power supply wiring (line)  1105  applies a predetermined voltage to the heater  1101  for supply of the current. The ground wiring (GND line)  1110  grounds the heater  1101  through the transistor  1102 . A heat-enable signal input terminal  1202  receives a heat-enable signal which specifies the timing of driving the heater  1101 . 
     In the first exemplary embodiment, of the data stored in the shift register  1104 , one bit of the image data is input in common to the AND gates  1201  corresponding to four adjacent heaters. In other words, circuit elements are divided into groups in units of those four adjacent heaters, transistors, and corresponding four AND gates. 
     The block selection signal output from the decoder  1203  enables the so-called time-division driving to be performed such that the timings of driving the heaters are shifted from each other on the time basis. Further, plural ones of the heaters in one group cannot be driven at the same time. In addition, as described in detail later, lines on the VH (high voltage) side of the heaters are common in one group. 
     A description is next given of a signal for turning on the heater driving signal from the AND gate  1201  (i.e., a signal enabling the heater to be driven) and of a turning-on condition. The present invention employs the time-division driving method to drive the heaters. The term “time-division driving method” means a method of setting a time-basis zone (block) and driving the heaters successively per block, instead of driving all the heaters in a heater row (array) at the same time, thereby reducing the number of heaters driven at the same time. 
     First, the data is input to and stored in the shift register  1104 , which temporarily stores the data, through the data input terminal  1106  in sync with the transfer clock (CLK)  1107 . The data stored in the shift register  1104  is taken out to the latch circuit  1103  at the timing of the latch signal (LT)  1108 . 
     One part of the data taken out of the shift register  1104  provides an image data signal, and the other part provides the block selection signal generated through the decoder  1203 . The AND gate  1201  is turned on to drive the heater on condition that the image data signal, the time-division signal, and the heat-enable signal are all produced at the same time. 
     Example of Driving Signal 
       FIG. 9  is a timing chart of various signals used to drive the printhead driving circuit in  FIG. 8 . The operation of the printhead driving circuit in  FIG. 8  will be described with reference to  FIG. 9 . 
     The transfer clocks (CLK) corresponding to the number of bits of the data stored in the shift register  1104  are input to the transfer clock input terminal  1107 . In this example, it is assumed that the data transfer to the shift register  1104  is performed in sync with the timing of a rise of the transfer clock (CLK). The image data and the block control data both used to turn on/off each heater  1101  are input through the data input terminal  1106 . Because 6-bit data is transferred herein, pulses of the transfer clocks (CLK) for 6 bits are input and temporarily stored in the shift register  1104 . Further, the latch signal (LT) is applied to the latch signal input terminal  1108  to hold the data (DATA) in the latch circuit  1103 . 
     With reference to  FIG. 12 , a description is next given of only a portion of the circuit in  FIG. 8 , which is related to the time-division driving method used in the first exemplary embodiment of the present invention. 
     When the time-division driving method is performed with time division at N(2 n ), the heater array is divided into groups (blocks) each of which includes a number N(2 n ) of heaters and which exist in number m. The data input to the shift register is provided by two kinds of data, i.e., the block control data specifying which one of the time-division blocks is selected, and the image data corresponding to the relevant time-division block. 
     Because the block control data is decoded by the decoder  1202  (not shown in  FIG. 12 ), the input block control data has n bits in the case of the time division at N(2 n ). Therefore, the data input to the shift register is provided by the block control data of n bits and the image data of m bits. This means that the shift register and the latch circuit each having (n+m) bits are required. In the first exemplary embodiment, the shift register  1104  receives two bits of B 1  and B 2  as the block control data which is decoded to drive the heaters with the time division into four blocks. More specifically, in the first exemplary embodiment, four adjacent heaters constitute one group and each heater in one group corresponds to each of the different time-division blocks. Hence, plural heaters in one group are not driven at the same time. In the present invention, as described later, lines (wirings) are connected together in units of one group including the plural heaters which are not driven at the same time. 
     When the time-division driving is performed, all the heaters (four time-division blocks×4 bits=16) are driven by inputting the data of (n+m) bits (6 bits in the first exemplary embodiment) successively N(2 n ) times (four times in the first exemplary embodiment), thus inputting the heater driving signals in a one-to-one relation to all the heaters. The switching transistor  1102  serves as a switch which allows the current to selectively flow into the heater in the above-described circuit arrangement. 
     The switching transistor  1102  is turned on for a time corresponding to the heat-enable signal. For the time during which the switching transistor  1102  is turned on, the current from the power supply line  1105  flows into the GND line  1110  through the heater  1101  and the switching transistor  1102 . With the supply of the current, the heater  1101  generates heat necessary for discharging ink and the ink is discharged from the nozzle of the printhead in accordance with the image data. 
     Exemplary Construction of Printhead 
       FIG. 10  is an explanatory view illustrating the exemplary construction of the printhead using the above-described printhead substrate. More specifically,  FIG. 10  is a perspective view, partly cut away, illustrating the nozzle portion of the printhead. The printhead in the first exemplary embodiment is the so-called side-shooter ink jet printhead. In the side-shooter ink jet printhead, ink supplied to a position above the heater  1101  is bubbled upon heating of the heater  1101 , and bubble generating energy generated at that occasion is utilized to discharge the ink through a discharge port (nozzle)  1132  formed at a position corresponding to the heater  1101 . 
     The printhead substrate  1000  can be formed of, e.g., a Si wafer (substrate) having a thickness of 0.5-1 mm. On the surface of the printhead substrate  1000 , the heaters  1101  and the heater driving circuit, described above with reference to  FIG. 8 , can be formed by employing the semiconductor process technique. The discharge ports  1132  through which is discharged the ink can be formed by photolithography using a discharge port forming member (resin material)  1131 , along with ink passage walls positioned corresponding to the heaters  1101  on the printhead substrate  1000 . 
     An ink supply port  1121  through which is supplied the ink can be formed by anisotropic etching utilizing the crystal orientation of the Si wafer of the printhead substrate  1000 . More specifically, the ink supply port  1121  is formed as a through hole shaped into an elongate groove and having slopes which are inclined such that the through hole is gradually narrowed from the rear surface toward the front surface of the printhead substrate  1000 . 
     In the printhead substrate  1000  according to the first exemplary embodiment, two heater arrays are formed one on each of both sides of the ink supply port  1121 , and the discharge ports  1132  are formed in an opposed relation to the heaters  1101  in the two heater arrays. Stated another way, the heaters (electrothermal transducers)  1101  are arrayed along the lengthwise direction of the ink supply port  1121 . The discharge ports  1132  in each of two discharge port arrays are formed at the same pitch P, and the discharge ports  1132  in one discharge port array are shifted by pitches P 1  and P 2  from respective adjacent discharge ports  1132  in the other discharge port array in the direction in which the discharge ports  1132  are arrayed. The pitches P, P 1  and P 2  can be optionally set, for example, such that the pitches P 1  and P 2  are each ½ of the pitch P. The heaters  1101  are arranged corresponding to those pitches of the discharge ports  1132 . Note that the discharge port array can be formed in number other than two, i.e., one or three or more. 
     A channel member for introducing the ink is connected to the ink supply port  1121  and a container containing the ink (i.e., an ink tank) is combined with the channel member, whereby a printhead cartridge including the printhead and the ink tank can be constructed. A color image can be printed by constructing a printhead cartridge in combination of containers (ink tanks) containing respective inks of plural colors and the printhead substrates corresponding to the respective inks of plural colors. 
     Exemplary Construction of Printing Apparatus 
       FIG. 11  is an explanatory view illustrating the exemplary construction of a printing apparatus to which is mountable the printhead using the above-described printhead substrate. 
     In the printing apparatus according to the first exemplary embodiment, a printhead cartridge H 1000  is mounted to a carriage  102  in a replaceable manner. The carriage  102  has an electric connector which can be electrically connected to an external signal input terminal of the printhead cartridge H 1000 . The printhead driving signal, etc. are transmitted from the electric connector of the carriage  102  through the external signal input terminal of the printhead cartridge H 1000 . 
     The printing apparatus according to the first exemplary embodiment is the so-called serial scanning printing apparatus. The carriage  102  is supported so as to be able to reciprocally move in the direction of main scanning, indicated by an arrow X, along a guide shaft  103  mounted to a main body of the printing apparatus. The carriage  102  is coupled to a timing belt  107  looped between a motor pulley  105  and a driven pulley  106 . When the motor pulley  105  is driven to rotate by a main scanning motor  104 , the carriage  102  is moved in the direction of main scanning under control. A printing medium  108  is supported on a platen (not shown) with its rear surface rested on the platen such that a flat print surface is formed in a printing region. In the illustrated example, the discharge port array is arranged to extend in a direction perpendicular to the direction of main scanning. 
     When an image is printed on the printing medium  108 , print scanning by the printhead cartridge H 1000  and an operation of conveying the printing medium  108  are repeated. More specifically, in the print scanning, the printhead cartridge H 1000  is moved in the direction of main scanning together with the carriage  102  while the ink is discharged through the discharge ports. In the conveying operation, the printing medium  108  is conveyed by a predetermined distance in the direction of sub-scanning. 
     Construction of Heater Portion of Printhead Substrate 
     A description is next given of the construction of the heater portion of the printhead substrate which is the feature of the present invention. 
       FIG. 1  is a schematic enlarged plan view of a portion including the heaters  1101  of the printhead substrate  1000  according to the first exemplary embodiment of the present invention, and  FIG. 2  is an explanatory diagram of an electric circuit connected to the heaters  1101  in  FIG. 1 . The printhead substrate  1000  is formed by using the multilayer wiring technique. On the printhead substrate  1000 , wirings (lines made of aluminum, copper, gold, or an alloy containing aluminum, copper or gold) interconnecting the components have a multilayered structure in which a wiring layer is sandwiched between insulating layers. The lines of upper- and lower-side wiring layers are connected to each other via through holes (openings of the insulating layer) formed at desired positions on the printhead substrate  1000 , thereby forming a circuit. 
     The heaters  1101  are each formed of a resistance (made of, e.g., a tantalum alloy) and, as indicated by rough hatching in  FIG. 1 , lines  513  and  514 , and individual wiring  515  are formed at opposite ends of the resistance to supply heating energy. The individual wiring (first lines)  515  are each formed in an area below the heater  1101  as viewed on the drawing sheet of  FIG. 1 , (i.e., in an area on one side of the heater  1101  in a direction crossing the direction in which the heaters  1101  are arrayed) to extend downward from there as viewed on the drawing sheet of  FIG. 1 , for connection to the switching transistor  1102 . The lines (second lines)  513  are each formed to return from an area above the heater  1101  as viewed on the drawing sheet of  FIG. 1 , (i.e., in an area on the other side of the heater  1101  in the direction crossing the direction in which the heaters  1101  are arrayed), to extend downward from there as viewed on the drawing sheet of  FIG. 1 , and to further extend in an adjacent or sandwiched relation to the heaters  1101  in the same group (block). The line  513  is also called a return wiring hereinafter. The lines (third lines)  514  are each formed between the heaters  1101  and the ink supply port  1121  to interconnect the plural lines  513  (four in the first exemplary embodiment) in units of group (i.e., in the same group). The line  514  is also called an interconnecting wiring hereinafter. Thus, the plural lines (return wirings)  513  each arranged in an adjacent or sandwiched relation to the heaters  1101  are connected to one interconnecting wiring  514 . 
     In the printhead substrate  1000  of the multilayered structure, the resistances constituting the heaters  1101  are formed in the same one layer by using the multilayer wiring technique, and the lines  513 ,  514  and  515  are also formed in the same one layer as the resistances. Generally, the resistances and the lines  513 ,  514  and  515  are covered with the same one insulating layer (protective film layer) formed thereon. On that occasion, an optimum film thickness of the insulating layer (protective film layer) is determined in consideration of reliability of an insulating film and the efficiency of utilization of the heat generated by the heaters  1101 . 
     The film thickness of the underlying wiring layer (i.e., the layer in which the lines  513 ,  514  and  515  are formed) is also determined in view of satisfactory coverage depending on the optimum film thickness of the insulating layer. In the general production method using photolithography, the film thickness of a wiring layer is limited to be not larger than the film thickness of an insulating layer. In the first exemplary embodiment, the film thickness of the insulating layer is about 3000 A (angstroms) and the film thickness of the wiring layer is about 2000 A. Further, in the first exemplary embodiment, the shape (W×L) of each heater  1101  is set to 15×15 μm to obtain an ink discharge amount Vd, and the heaters  1101  are arrayed at 1200 dpi (dots/inch) to realize high-quality and high-speed printing. Note that 1200 dpi corresponds to 21 μm in terms of the array pitch P of the heaters  1101 . 
     The lines  513 ,  514  and  515  are formed in conformity with arrangement rules depending on production conditions (generally called “wiring rules”). The arrangement rules are based on the production method using photolithography. The first exemplary embodiment employs the arrangement rules of setting a minimum line width of the wiring to 2 μm and a minimum space width to 2 μm. Therefore, the return wiring  513  has a line width  501  of 2 μm and a space width  502  of 2 μm. A ratio of a heater width W to the pitch P of the heaters  1101  (i.e., W/P) is 71.4%. Such a ratio is preferably not less than 70% from the viewpoint of realizing a high-density array of the heaters. 
     The ink supply port  1121  is formed, as described above, to supply the ink toward the discharge ports  1132  (see  FIG. 10 ) from the rear side of the printhead substrate  1000 . The heaters  1101  are arranged at a high density so as to form a heater array along the ink supply port  1121 . In the first exemplary embodiment, four adjacent heaters  1101  constitute one group and the power supply lines  1105  and  1110  are formed in units of the group. As shown in  FIG. 2 , the power supply lines  1105  and  1110  connected respectively to a connection terminal VH 1  and a ground terminal GNDH 1  are formed for the heater group constituted by four heaters  1101  located on the right side as viewed on the drawing sheet of  FIG. 2 . Similarly, the power supply lines  1105  and  1110  connected respectively to a connection terminal VH 2  and a ground terminal GNDH 2  are formed for the heater group constituted by four heaters  1101  located on the left side as viewed on the drawing sheet of  FIG. 2 . While the power supply line and the ground line are each formed as a single line in  FIG. 8  described above,  FIG. 2  illustrates the case where the power supply line and the ground line are formed for each heater group. 
     Thus, the four heaters  1101  in one heater group are connected in parallel to the common power supply line  1105  and the common ground line  1110 . As described above, the heaters  1101  in one heater group belong to the time-division blocks in which the heaters are driven at different timings, and the transistors  1102  corresponding to the four heaters  1101  drive those four heaters  1101  successively with a time shift therebetween without driving them at the same time. Such a driving method is called the time-division driving method. The first exemplary embodiment employs a 4-division driving method. 
       FIGS. 1 and 2  show only eight heaters  1101  corresponding to eight discharge ports (nozzles)  1132 . When a nozzle array including thirty-two discharge ports  1132  is formed, for example, thirty-two heaters  1101  are arrayed and the 4-division driving is performed by dividing the thirty-two heaters into 8 groups in units of four heaters. Correspondingly, the power supply lines  1105  and  1110  are each formed in number eight. 
     In the first exemplary embodiment, the four return wirings  513  in each heater group are interconnected at a position between the heater array and the ink supply port  1121  by the interconnecting wiring (common wiring)  514  which is extended substantially parallel to the lengthwise direction of the ink supply port  1121 . The interconnection of the return wirings  513  enables a reduction of the line width  501  of each of the return wirings  513  positioned adjacent to the heaters  1101 . In other words, the line width  501  of one return wiring  513  can be reduced in comparison with the case where the return wirings  513  are not interconnected by the interconnecting wiring  514 . As a result, the width W of each heater  1101  can be increased while keeping the same pitch P. 
     In the first exemplary embodiment, the number of the heaters  1101  is the same as that of the return wirings  513 , and the number of the return wirings  513  in each heater group is four. The return wirings  513  are each arranged in an adjacent or sandwiched relation to the heaters  1101  as shown in  FIG. 1 , and the density of a current flowing through one return wiring  513  is set to be held from exceeding a predetermined value. Further, the above-described arrangement of the return wirings  513  is effective in not only avoiding an uneven distribution of the current density among the return wirings  513  in each heater group, but also preventing the occurrence of a line resistance difference and a current difference depending on positions of the heaters  1101 . 
     Particularly, in the first exemplary embodiment, the heaters  1101  not driven at the same time based on the time-division driving method constitute one group, the interconnecting wiring  514  is formed corresponding to one group, and the plural lines (return wirings)  513  each arranged in an adjacent or sandwiched relation to the heaters  1101  are connected to the interconnecting wiring  514 . Therefore, the width of each of the return wirings in one group can be further reduced. 
     As a result, the heaters  1101  can be arrayed at a higher density, and higher-quality and higher-speed printing of an image can be realized with stable discharge of the ink. 
     Comparative Examples 
       FIGS. 5-7  illustrate Comparative Examples for the purpose of comparing with the printhead substrate  1000  according to the present invention. The interconnecting wiring and the plural return wirings  513  connected to the interconnecting wiring are not formed in printhead substrates of Comparative Examples. 
     In  FIGS. 5 and 6 , printing elements each including a discharge port (see  1132  in  FIG. 10 ), a separated channel, and a heater  1101  are arranged at an array density of 600 dpi in the form of a row along an ink supply port  1121 , and the amount of ink discharged through the discharge port is set to Vd. As in the above-described first exemplary embodiment, the heaters  1101  positioned in lower portions of the printing elements are arranged in the form of a row similarly to the printing elements. Roughly hatched areas in  FIG. 5  represent lines  513  and  515  connected to each heater  1101 . 
     In order to arrange the printing elements each having the ink discharge amount of Vd in the form of a row at the array density of 600 dpi, the heaters  1101  are also required to be arranged in the form of a row at the array density of 600 dpi. For realizing such an array density, the pitch P of the heaters  1101  has to be set to 42 μm. In this Comparative Example, electrodes for the heaters  1101  are formed by a method of arranging the electrodes in a perpendicular relation to the direction of the heater row as in the above-described first exemplary embodiment of the present invention, i.e., by a method of arranging the heater electrodes (connecting portions to the lines) to position in the vertical direction, as viewed on the drawing sheet of  FIG. 5 , perpendicularly to the direction in which the heaters  1101  are arrayed. When individual return wirings  513  each have a line width  501  of 8 μm and a space width  502  of 8 μm, the width W of each heater  1101  is 18 μm at maximum in conformity with the wiring rules. Thus, the heaters  1101  each having a square shape of 18×18 μm, for example, are arranged to ensure the ink discharge amount Vd. The ratio of the heater width W to the pitch P (i.e., W/P), is 42.9%. 
       FIG. 6  is an electric circuit diagram representing the arrangement of the heaters  1101  and the lines  513 ,  514  and  515  in  FIG. 5 . 
     It is here assumed that the heaters  1101  (each having the size of, e.g., 18×18 μm) with the same ink discharge amount Vd are arranged at a higher array density of 900 dpi than 600 dpi shown in  FIG. 6 . In such a case, the pitch P of the heaters  1101  is required to be set to 28 μm. When the heaters  1101  are formed by the method arranging the electrodes in a perpendicular relation to the direction of the heater row such that the line width  501  of the return wiring  513  is specified to 8 μm and the space width  502  is specified to 8 μm in conformity with the wiring rules, the width W of the heater  1101  is 4 μm at maximum. It is hence understood that the heaters  1101  cannot be arranged at the pitch P of 28 μm. 
     An obstacle to such a higher-density array of the heaters  1101  is the return wiring  513 . The arrangement of the heaters  1101  (each having the size of, e.g., 18×18 μm) with the ink discharge amount Vd at the pitch P of 28 μm can be realized if the line width  501  of the return wiring  513  is specified to 2 μm and the space width  502  is specified to 2 μm in conformity with the wiring rules.  FIG. 7  is an explanatory view illustrating the case where the return wirings  513  are formed in conformity with those wiring rules. In the case of  FIG. 7 , however, the cross-sectional area of each return wiring  513  is reduced to ¼ of that in the case of  FIG. 5  and the current density is quadrupled if the film thickness of the return wiring  513  is not changed. 
     The same cross-sectional area of the return wiring  513  can be maintained by increasing the film thickness of the return wiring  513  four times so as to compensate for the reduction of the line width. However, because the return wiring  513  is a line connected to the heater electrode, the film thickness of only the return wiring  513  has to be increased without increasing the film thickness of the insulating layer (protective film layer) formed on the return wiring  513 . That requirement straightly deteriorates the line coverage, thus resulting in a risk of reducing reliability of the heater  1101 . Also, because increasing the film thickness of the insulating layer (protective film layer) to ensure the satisfactory line coverage means a larger film thickness of the insulating layer (protective film layer) formed on the heater  1101 , there arises a risk of greatly reducing the efficiency of ink bubbling and impeding stability of the ink discharge. Further, when trying to realize a higher array density of the heaters  1101 , as another example, by a method of using a layer different from the layer in which the heaters  1101  are formed, and forming the heaters  1101  and electrode lines in a three-dimensionally overlapped relation, it is apparent that the production process is more complicated. 
     Thus, with Comparative Examples in which the interconnecting wiring  514  used in the above-described first exemplary embodiment of the present invention is not used, a difficulty arises in arraying the heaters  1101  at a higher density. 
     Second Exemplary Embodiment 
       FIG. 3  is a schematic plan view illustrating a heater arrangement in a printhead substrate according to a second exemplary embodiment of the present invention, and  FIG. 4  is an electric circuit diagram corresponding to the arrangement of heaters  1101  and lines  513 ,  514  and  515  in  FIG. 3 . Similar components in  FIGS. 3 and 4  to those in the above-described first exemplary embodiment are denoted by the same characters and a description of those components is not repeated here. 
     In the second exemplary embodiment of the present invention, as in the first exemplary embodiment, resistances constituting the heaters  1101  are formed in the same layer by using the multilayer wiring technique, and the lines  513 ,  514  and  515  are also formed in the same layer as the resistances. Further, as in the first exemplary embodiment, the return wirings  513  in each heater group are interconnected by the interconnecting wiring  514 , which is formed between a heater array (row) and an ink supply port  1121  and which is extended substantially parallel to the lengthwise direction of the ink supply port  1121 . 
     Also, in the second exemplary embodiment, the heaters in each heater group, which are interconnected by the interconnecting wiring, are not driven at the same time. 
     In the above-described first exemplary embodiment, all the heaters  1101  have the same shape. In the second exemplary embodiment, however, the heaters  1101  are formed as two differently-shaped heaters  1101 A and  1101 B having large and small sizes. The heaters  1101 A are large-size heaters each discharging the ink in a relatively large amount Vd 1 , and the heaters  1101 B are small-size heaters each discharging the ink in a relatively small amount Vd 2 . A discharge port  1132  corresponding to each of the large-size heaters  1101 A is formed as a large discharge port having a relatively large opening area, and a discharge port  1132  corresponding to each of the small-size heaters  1101 B is formed as a small discharge port having a relatively small opening area. Further, in the above-described first exemplary embodiment, the heaters  1101  and the return wirings  513  are formed in the same number, i.e., four, in one heater group. In the second exemplary embodiment, however, the number of the return wirings  513  is two in one heater group, as seen from  FIG. 3 . That feature is advantageous in setting a heater width (W 1 +W 2 ) given by the sum of widths W 1  and W 2  of the heaters  1101 A and  1101 B to a larger value while ensuring a certain space  504  between the large-size heater  1101 A and the small-size heater  1101 B. 
     The number of the return wirings  513  to be arranged is set to a value at which the density of a current flowing through one return wiring  513  does not exceed a predetermined value and which is not too small. If the number of the return wirings  513  is too small, there arises a risk that an uneven distribution of the current density may occur in the heater group and a line resistance difference and a current difference may be caused depending on positions of the heaters  1101 A and  1101 B. Thus, the number of the return wirings  513  to be arranged in each heater group is set to a plural number that satisfies the above-described conditions. 
     The large-size heater  1101 A with the ink discharge amount Vd 1  has a square shape (18×18 μm) in a plan view with the width W 1  and the length L 1  being each 18 μm, and the small-size heater  1101 B with the ink discharge amount Vd 2  has a square shape (12×12 μm) in a plan view with the width W 2  and the length L 2  being each 12 μm. A heater pitch P 1  for an area including the return wiring  513  is 23 μm and a heater pitch P 2  for an area including the space  504  is 19 μm. By employing the wiring rules that specify a minimum line width to 2 μm and a minimum space width to 2 μm, the line width  501  of the return wiring  513  is set to 4 μm and the space width  502  is set to 2 μm. Further, the width of the space  504  between the heaters  1101 A and  1101 B is set to 4 μm. Accordingly, a ratio of the sum of the heater widths W 1  and W 2  to the sum of the pitches P 1  and P 2  (i.e., {(W 1 +W 2 )/(P 1 +P 2 )}) is 71.4%. 
     Thus, according to the second exemplary embodiment, the ink can be stably discharged and a high-quality image can be printed at a high speed while realizing a higher array density of the large-size heater  1101 A and the small-size heater  1101 B which correspond to the discharge ports having different ink discharge amounts. 
     Other Exemplary Embodiments 
     The present invention is not limited to only the above-described serial printing apparatus, and it can also be applied to, e.g., the so-called full-line printing apparatus using a long printhead which is extended over an entire printing area of a printing medium in the widthwise direction thereof. Therefore, the printhead substrate of the present invention can also be utilized to constitute a printhead used in the full-line printing apparatus. Any of the serial printing apparatus and the full-line printing apparatus is just required to include a mechanism capable of relatively moving the printhead and the printing medium. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2006-338095 filed Dec. 15, 2006, which is hereby incorporated by reference herein in its entirety.