Abstract:
A printhead for an inkjet printer is disclosed. The printhead has an elongate wafer substrate and an array of nozzles fabricated on the elongate wafer substrate. The array of nozzles is arranged in a first row and a second row extending parallel to each other and the longitudinal extent of the wafer substrate. Each nozzle has an ejection aperture and an actuator for ejecting printing fluid through the ejection aperture. Each of the actuators has a pair of electrodes spaced apart from each other in a direction transverse to a long edge of the wafer substrate such that one of electrode of each of the electrode pairs is proximate the long edge of the wafer substrate and the other electrode of each of the electrode pairs is remote from the long edge. The printhead also has drive circuitry formed on one surface of the wafer substrate for supplying current to the electrodes of the actuators. The drive circuitry is supplied with power and data along the long edge of the wafer substrate. At least one of the electrodes remote from the long edge of the first row share a current path which conducts current from the drive circuitry to the electrodes with at least one of the electrodes proximate the long edge of the second row, the first row being nearer to the long edge than the second row.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a Continuation Application of U.S. application Ser. No. 11/829,957 filed on Jul. 30, 2007, which is a Continuation in Part Application of U.S. application Ser. No 11/246,687 filed Oct. 11, 2005 the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of printing. In particular, the invention concerns an inkjet printhead for high resolution printing. 
       CROSS REFERENCE TO OTHER RELATED APPLICATIONS 
       [0003]    The following patents or patent applications filed by the applicant or assignee of the present invention are hereby incorporated by cross-reference. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
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                 7,270,405 
                 7,303,268 
                 7,470,007 
                 7,399,072 
               
               
                 7,393,076 
                 7,681,967 
                 7,588,301 
                 7,249,833 
                 7,547,098 
               
               
                 7,703,886 
                 7,524,016 
                 7,490,927 
                 7,331,661 
                 7,524,043 
               
               
                 7,300,140 
                 7,357,492 
                 7,357,493 
                 7,566,106 
                 7,380,902 
               
               
                 7,284,816 
                 7,284,845 
                 7,255,430 
                 7,390,080 
                 7,328,984 
               
               
                 7,350,913 
                 7,322,671 
                 7,380,910 
                 7,431,424 
                 7,470,006 
               
               
                 7,585,054 
                 7,347,534 
                 7,441,865 
                 7,469,989 
                 7,367,650 
               
               
                 11/778,567 
                 7,469,990 
                 7,441,882 
                 7,556,364 
                 7,357,496 
               
               
                 7,467,863 
                 7,431,440 
                 7,431,443 
                 7,527,353 
                 7,524,023 
               
               
                 7,513,603 
                 7,467,852 
                 7,465,045 
                 11/688,863 
                 11/688,864 
               
               
                 7,475,976 
                 7,364,265 
                 11/688,867 
                 11/688,868 
                 11/688,869 
               
               
                 11/688,871 
                 11/688,872 
                 7,654,640 
                 11/741,766 
                 7,645,034 
               
               
                 7,637,602 
                 7,645,033 
                 7,661,803 
                 11/495,819 
                 11/677,049 
               
               
                 11/677,050 
                 7,658,482 
                 7,306,320 
                 7,111,935 
                 7,562,971 
               
               
                 10/760,219 
                 7,604,322 
                 7,261,482 
                 10/760,220 
                 7,002,664 
               
               
                 10/760,252 
                 7,088,420 
                 11/446,233 
                 7,470,014 
                 7,470,020 
               
               
                 7,540,601 
                 7,654,761 
                 7,377,635 
                 7,686,446 
                 7,237,888 
               
               
                 7,168,654 
                 7,201,272 
                 6,991,098 
                 7,217,051 
                 6,944,970 
               
               
                 10/760,215 
                 7,108,434 
                 7,210,407 
                 7,186,042 
                 10/760,266 
               
               
                 6,920,704 
                 7,217,049 
                 7,607,756 
                 10/760,260 
                 7,147,102 
               
               
                 7,287,828 
                 7,249,838 
                 10/760,241 
                 7,431,446 
                 7,611,237 
               
               
                 7,261,477 
                 7,225,739 
                 7,712,886 
                 7,665,836 
                 7,419,053 
               
               
                 7,191,978 
                 10/962,426 
                 7,524,046 
                 10/962,417 
                 10/962,403 
               
               
                 7,163,287 
                 7,258,415 
                 7,322,677 
                 7,258,424 
                 7,484,841 
               
               
                 7,195,412 
                 7,207,670 
                 7,270,401 
                 7,220,072 
                 7,588,381 
               
               
                 11/544,547 
                 11/585,925 
                 7,578,387 
                 11/706,298 
                 7,575,316 
               
               
                 7,384,206 
                 7,628,557 
                 7,470,074 
                 7,425,063 
                 7,429,104 
               
               
                 7,556,446 
                 7,367,267 
                 11/754,359 
                 11/778,061 
                 11/765,398 
               
               
                 11/778,556 
                 7,399,065 
                 7,695,204 
                 7,322,761 
                 11/223,021 
               
               
                 11/223,020 
                 11/014,730 
                 7,079,292 
               
               
                   
               
             
          
         
       
     
       BACKGROUND OF THE INVENTION 
       [0004]    The quality of a printed image depends largely on the resolution of the printer. Accordingly, there are ongoing efforts to improve the print resolution of printers. The print resolution strictly depends on the spacing of the printer addressable locations on the media substrate, and the drop volume. The spacing between the nozzles on the printhead need not be as small as the spacing between the addressable locations on the media substrate. The nozzle that prints a dot at one addressable location can be spaced any distance away from the nozzle that prints the dot at the adjacent addressable location. Movement of the printhead relative to the media, or vice versa, or both, will allow the printhead to eject drops at every addressable location regardless of the spacing between the nozzles on the printhead. In the extreme case, the same nozzle can print adjacent drops with the appropriate relative movement between the printhead and the media. 
         [0005]    Excess movement of the media with respect to the printhead will reduce print speeds. Multiple passes of a scanning printhead over a single swathe of the media, or multiple passes of the media past the printhead in the case of pagewidth printhead reduces the page per minute print rate. 
         [0006]    Alternatively, the nozzles can be spaced along the media feed path or in the scan direction so that the spacing between addressable locations on the media are smaller than the physical spacing of adjacent nozzles. It will be appreciated that the spacing the nozzles over a large section of the paper path or scan direction is counter to compact design and requires the paper feed to carefully control the media position and the printer control of nozzle firing times must be precise. 
         [0007]    For pagewidth printheads, the large nozzle array emphasizes the problem. Spacing the nozzles over a large section of the paper path requires the nozzle array to have a relatively large area. The nozzle array must, by definition, extend the width of the media. But its dimension in the direction of media feed should be as small as possible. Arrays that extend a relatively long distance in the media feed direction require a complex media feed that maintains precise positioning of the nozzles relative to the media surface across the entire array. Some printer designs use a broad vacuum platen opposite the printhead to get the necessary control of the media. In light of these issues, there is a strong motivation to increase the density of nozzles on the printhead (that is, the number of nozzles per unit area) in order to increase the addressable locations of the printer and therefore the print resolution while keeping the width of the array (in the direction of media feed) small. 
         [0008]    The Applicant has developed a range of pagewidth printheads with very high nozzle densities. The printheads use one or more printhead integrated circuits (ICs) that each have an array of nozzles fabricated on a silicon wafer substrate using semiconductor etching and deposition techniques. Each nozzle is a MEMS (micro-electro-mechanical systems) device with an actuator mounted in a chamber for ejecting ink through a respective nozzle aperture. 
         [0009]    To keep the printzone (i.e. the area encompassed by all the nozzles on the printhead) as narrow as possible, the printhead IC&#39;s on each printhead are mounted end to end in a line transverse to the paper feed direction. This keeps the width of the total nozzle array small to avoid, or at least minimize, the media feed control problems discussed above. However, end to end printhead ICs mean that the power and data to the nozzles must be fed to the side of each IC. 
         [0010]    The drive circuitry for each printhead IC is fabricated on the wafer substrate in the form of several metal layers separated by dielectric material through which vias establish the required inter layer connections. The drive circuitry has a drive FET (field effect transistor) for each actuator. The source of the FET is connected to a power plane (a metal layer connected to the position voltage of the power supply) and the drain connects to a ground plane (the metal layer at zero voltage or ground). Also connected to the ground plane and the power plane are the electrodes for each of the actuators. 
         [0011]    The power plane is typically the uppermost metal layer and the ground plane is the metal layer immediately beneath (separated by a dielectric layer). The actuators, ink chambers and nozzles are fabricated on top of the power plane metal layer. Holes are etched through this layer so that the negative electrode can connect to the ground plane and an ink passage can extend from the rear of the wafer substrate to the ink chambers. As the nozzle density increases, so to does the density of these holes, or punctuations through the power plane. With a greater density of punctuations through the power plane, the gap width between the punctuations is reduced. The thin bridge of metal layer between these gaps is a point of relatively high electrical resistance. As the power plane is connected to a supply along one side of the printhead IC, the current to actuators on the non-supply side of the printhead IC may have had to pass through a series of these resistive gaps. The increased parasitic resistance to the non-supply side actuators will affect their drive voltage and ultimately the drop ejection characteristics from those nozzles. 
         [0012]    In light of the above, there are ongoing efforts to improve print resolution by increasing the density of nozzles on the printhead while maintaining consistent drop ejection characteristics. 
       SUMMARY OF THE INVENTION 
       [0013]    According to an aspect of the present invention there is provided a printhead for an inkjet printer, the printhead comprising: 
         [0014]    an elongate wafer substrate; 
         [0015]    an array of nozzles fabricated on the elongate wafer substrate, the array of nozzles being arranged in a first row and a second row extending parallel to each other and the longitudinal extent of the wafer substrate, each nozzle having an ejection aperture and an actuator for ejecting printing fluid through the ejection aperture, each of the actuators having a pair of electrodes spaced apart from each other in a direction transverse to a long edge of the wafer substrate such that one of electrode of each of the electrode pairs is proximate the long edge of the wafer substrate and the other electrode of each of the electrode pairs is remote from the long edge; and, 
         [0016]    drive circuitry formed on one surface of the wafer substrate for supplying current to the electrodes of the actuators, the drive circuitry being supplied with power and data along the long edge of the wafer substrate, 
         [0017]    wherein at least one of the electrodes remote from the long edge of the first row share a current path which conducts current from the drive circuitry to the electrodes with at least one of the electrodes proximate the long edge of the second row, the first row being nearer to the long edge than the second row. 
         [0018]    Other aspects are also disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1A  is a schematic representation of the linking printhead IC construction; 
           [0021]      FIG. 1B  shows a partial plan view of the nozzle array on a printhead IC according to the present invention; 
           [0022]      FIG. 2  is a unit cell of the nozzle array; 
           [0023]      FIG. 3  shows the unit cell replication pattern that makes up the nozzle array; 
           [0024]      FIG. 4  is a schematic cross section through the CMOS layers and heater element of a nozzle; 
           [0025]      FIG. 5A  schematically shows an electrode arrangement with opposing electrode polarities in adjacent actuator rows; 
           [0026]      FIG. 5B  schematically shows an electrode arrangement with typical electrode polarities in adjacent actuator rows; 
           [0027]      FIG. 6  shows the electrode configuration of the printhead IC of  FIG. 1 ; 
           [0028]      FIG. 7  shows a section of the power plane of the CMOS layers; 
           [0029]      FIG. 8  shows the pattern etched into the sacrificial scaffold layer for the roof/side wall layer; 
           [0030]      FIG. 9  shows the exterior surface of the roof layer after the nozzle apertures have been etched; 
           [0031]      FIG. 10  shows the ink supply flow to the nozzles; 
           [0032]      FIG. 11  shows the different inlets to the chambers in different rows; 
           [0033]      FIG. 12  shows the nozzle spacing for one color channel; 
           [0034]      FIG. 13  shows an enlarged view of the nozzle array with matching elliptical chamber and ejection aperture; 
           [0035]      FIG. 14  is a sketch of a photolithographic stepper; and, 
           [0036]      FIGS. 15A to 15C  schematically illustrate the operation of a photolithographic stepper. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]    The printhead IC (integrated circuit) shown in the accompanying drawings is fabricated using the same lithographic etching and deposition steps described in the U.S. Ser. No. 11/246,687 (Our Docket MNN001US) filed 11 Oct. 2005, the contents of which are incorporated herein by reference. The ordinary worker will understand that the printhead IC shown in the accompanying drawings have a chamber, nozzle and heater electrode configuration that requires the use of exposure masks that differ from those shown in Ser. No. 11/246,687 Figures. However the process steps of forming the suspended beam heater elements, chambers and ejection apertures remains the same. Likewise, the CMOS layers are formed in the same manner as that discussed Ser. No. 11/246,687 with the exception of the drive FETs. The drive FETs need to be smaller because the higher density of the heater elements. 
       Linking Printhead Integrated Circuits 
       [0038]    The Applicant has developed a range of printhead devices that use a series of printhead integrated circuits (ICs) that link together to form a pagewidth printhead. In this way, the printhead IC&#39;s can be assembled into printheads used in applications ranging from wide format printing to cameras and cellphones with inbuilt printers. The printhead IC&#39;s are mounted end-to-end on a support member to form a pagewidth printhead. The support member mounts the printhead IC&#39;s in the printer and also distributes ink to the individual IC&#39;s. An example of this type of printhead is described in U.S. Ser. No. 11/293,820, the disclosure of which is incorporated herein by cross reference. 
         [0039]    It will be appreciated that any reference to the term ‘ink’ is to be interpreted as any printing fluid unless it is clear from the context that it is only a colorant for imaging print media. The printhead IC&#39;s can equally eject invisible inks, adhesives, medicaments or other functionalized fluids. 
         [0040]      FIG. 1A  shows a sketch of a pagewidth printhead  100  with the series of printhead ICs  92  mounted to a support member  94 . The angled sides  96  allow the nozzles from one of the IC&#39;s  92  overlap with those of an adjacent IC in the paper feed direction  8 . Overlapping the nozzles in each IC  92  provides continuous printing across the junction between two IC&#39;s. This avoids any ‘banding’ in the resulting print. Linking individual printhead IC&#39;s in this manner allows printheads of any desired length to be made by simply using different numbers of IC&#39;s. 
         [0041]    The end to end arrangement of the printhead ICs  92  requires the power and data to be supplied to bond pads  98  along the long sides of each printhead IC  92 . These connections, and the control of the linking ICs with a print engine controller (PEC), is described in detail in Ser. No. 11/544,764 (Docket No. PUA001US) filed 10 Oct. 2006. 
       3200 DPI Printhead Overview 
       [0042]      FIG. 1B  shows a section of the nozzle array on the Applicants recently developed 3200 dpi printhead. The printhead has ‘true’ 3200 dpi resolution in that the nozzle pitch is 3200 npi rather than a printer with 3200 dpi addressable locations and a nozzle pitch less than 3200 npi. The section shown in  FIG. 1B  shows eight unit cells of the nozzle array with the roof layer removed. For the purposes of illustration, the ejection apertures  2  have been shown in outline. The ‘unit cell’ is the smallest repeating unit of the nozzle array and has two complete droplet ejectors and four halves of the droplet ejectors on either side of the complete ejectors. A single unit cell is shown in  FIG. 2 . 
         [0043]    The nozzle rows extend transverse to the media feed direction  8 . The middle four rows of nozzles are one color channel  4 . Two rows extend either side of the ink supply channel  6 . Ink from the opposing side of the wafer flows to the supply channel  6  through the ink feed conduits  14 . The upper and lower ink supply channels  10  and  12  are separate color channels (although for greater color density they may print the same color ink—eg a CCMMY printhead). 
         [0044]    Rows  20  and  22  above the supply channel  6  are transversely offset with respect to the media feed direction  8 . Below the ink supply channel  6 , rows  24  and  26  are similarly offset along the width of the media. Furthermore, rows  20  and  22 , and rows  24  and  26  are mutually offset with respect to each other. Accordingly, the combined nozzle pitch of rows  20  to  26  transverse to the media feed direction  8  is one quarter the nozzle pitch of any of the individual rows. The nozzle pitch along each row is approximately 32 microns (nominally 31.75 microns) and therefore the combined nozzle pitch for all the rows in one color channel is approximately 8 microns (nominally 7.9375 microns). This equates to a nozzle pitch of 3200 npi and hence the printhead has ‘true’ 3200 dpi resolution. 
       Unit Cell 
       [0045]      FIG. 2  is a single unit cell of the nozzle array. Each unit cell has the equivalent of four droplet ejectors (two complete droplet ejectors and four halves of the droplet ejectors on both sides of the complete ejectors). The droplet ejectors are the nozzle, the chamber, drive FET and drive circuitry for a single MEMS fluid ejection device. The ordinary worker will appreciate that the droplet ejectors are often simply referred to as nozzles for convenience but it is understood from the context of use whether this term is a reference to just the ejection aperture or the entire MEMS device. 
         [0046]    The top two nozzle rows  18  are fed from the ink feed conduits  14  via the top ink supply channel  10 . The bottom nozzle rows  16  are a different color channel fed from the supply channel  6 . Each nozzle has an associated chamber  28  and heater element  30  extending between electrodes  34  and  36 . The chambers  28  are elliptical and offset from each other so that their minor axes overlap transverse to the media feed direction. This configuration allows the chamber volume, nozzle area and heater size to be substantially the same as the 1600 dpi printheads shown in the above referenced U.S. Ser. No. 11/246,687 (Our Docket MNN001US) filed 11 Oct. 2005. Likewise the chamber walls  32  remain 4 microns thick and the area of the contacts  34  and  36  are still 10 microns by 10 microns. 
         [0047]      FIG. 3  shows the unit cell replication pattern that makes up the nozzle array. Each unit cell  38  is translated by its width x across the wafer. The adjacent rows are flipped to a mirror image and translated by half the width: 0.5x=y. As discussed above, this provides a combined nozzle pitch for the rows of one color channel ( 20 ,  22 ,  24  and  26 ) of 0.25x. In the embodiment shown, x=31.75 and y=7.9375. This provides a 3200 dpi resolution without reducing the size of the heaters, chambers or nozzles. Accordingly, when operating at 3200 dpi, the print density is higher than the 1600 dpi printhead of U.S. Ser. No. 11/246,687 (Our Docket MNN001US) filed 11 Oct. 2005, or the printer can operate at 1600 dpi to extend the life of the nozzles with a good print density. This feature of the printhead is discussed further below. 
       Heater Contact Arrangement 
       [0048]    The heater elements  30  and respective contacts  34  and  36  are the same dimensions as the 1600 dpi printhead IC of U.S. Ser. No. 11/246,687 (Our Docket MNN001US) filed 11 Oct. 2005. However, as there is twice the number of contacts, there is twice the number of FET contacts (negative contacts) that punctuate the ‘power plane’ (the CMOS metal layer carrying the positive voltage). A high density of holes in the power plane creates high resistance through the thin pieces of metal between the holes. This resistance is detrimental to overall printhead efficiency and can reduce the drive pulse to some heaters relative to others. 
         [0049]      FIG. 4  is a schematic section view of the wafer, CMOS drive circuitry  56  and the heater. The drive circuitry  56  for each printhead IC is fabricated on the wafer substrate  48  in the form of several metal layers  40 ,  42 ,  44  and  45  separated by dielectric material  41 ,  43  and  47  through which vias  46  establish the required inter layer connections. The drive circuitry  56  has a drive FET (field effect transistor)  58  for each actuator  30 . The source  54  of the FET  58  is connected to a power plane  40  (a metal layer connected to the position voltage of the power supply) and the drain  52  connects to a ground plane  42  (the metal layer at zero voltage or ground). Also connected to the ground plane  42  and the power plane  40  are the electrodes  34  and  36  or each of the actuators  30 . 
         [0050]    The power plane  40  is typically the uppermost metal layer and the ground plane  42  is the metal layer immediately beneath (separated by a dielectric layer  41 ). The actuators  30 , ink chambers  28  and nozzles  2  are fabricated on top of the power plane metal layer  40 . Holes  46  are etched through this layer so that the negative electrode  34  can connect to the ground plane and an ink passage  14  can extend from the rear of the wafer substrate  48  to the ink chambers  28 . As the nozzle density increases, so to does the density of these holes, or punctuations through the power plane. With a greater density of punctuations through the power plane, the gaps between the punctuations are reduced. The thin bridge of metal layer though these gaps is a point of relatively high electrical resistance. As the power plane is connected to a supply along one side of the printhead IC, the current to actuators on the non-supply side of the printhead IC may have had to pass through a series of these resistive gaps. The increased parasitic resistance to the non-supply side actuators will affect their drive current and ultimately the drop ejection characteristics from those nozzles. 
         [0051]    The printhead uses several measures to address this. Firstly, adjacent rows of actuators have opposite current flow directions. That is, the electrode polarity in one rows is switched in the adjacent row. For the purposes of this aspect of the printhead, two rows of nozzles adjacent the supply channel  6  should be considered as a single row as shown in  FIG. 5A  instead of staggered as shown in the previous figures. The two rows A and B extend longitudinally along the length of the printhead IC. All the negative electrodes  34  are along the outer edges of the two adjacent rows A and B. The power is supplied from one side, say edge  62 , and so the current only passes through one line of thin, resistive metal sections  64  before it flows through the heater elements  30  in both rows. Accordingly, the current flow direction in row A is opposite to the current flow direction in row B. 
         [0052]    The corresponding circuit diagram illustrates the benefit of this configuration. The power supply V+ drops because of the resistance R A  of the thin sections between the negative electrodes  34  of row A. However, the positive electrodes  36  for all the heaters  30  are at the same voltage relative to ground (V A =V B ). The voltage drop across all heaters  30  (resistances R HA  and R HB  respectively) in both rows A and B is uniform. The resistance R B  from the thin bridges  66  between the negative electrodes  34  of row B is eliminated from the circuit for rows A and B. 
         [0053]      FIG. 5B  shows the situation if the polarities of the electrodes in adjacent rows are not opposing. In this case, the line of resistive sections  66  in row B are in the circuit. The supply voltage V+ drops through the resistance R A  to V A —the voltage of the positive electrodes  36  of row A. From there the voltage drops to ground through the resistance R HA  of the row A heaters  30 . However, the voltage V B  at the row B positive electrodes  36  drops from V A  though R B  from the thin section  66  between the row B negative electrodes  34 . Hence the voltage drop though the row B heaters  30  is less than that of row A. This in turn changes the drive pulse and therefore the drop ejection characteristics. 
         [0054]    The second measure used to maintain the integrity of the power plane is staggering adjacent electrodes pairs in each row. Referring to  FIG. 6 , the negative electrodes  34  are now staggered such that every second electrode is displaced transversely to the row. The adjacent row of heater contacts  34  and  36  are likewise staggered. This serves to further widen the gaps  64  and  66  between the holes through the power plane  40 . The wider gaps have less electrical resistance and the voltage drop to the heaters remote from the power supply side of the printhead IC is reduced.  FIG. 7  shows a larger section of the power plane  40 . The electrodes  34  in staggered rows  41  and  44  correspond to the color channel feed by supply channel  6 . The staggered rows  42  and  43  relate to one half the nozzles for the color channels on either side—the color fed by supply channel  10  and the color channel fed by supply channel  12 . It will be appreciated that a five color channel printhead IC has nine rows of negative electrodes that can induce resistance for the heaters in the nozzles furthest from the power supply side. Widening the gaps between the negative electrodes greatly reduces the resistance they generate. This promotes more uniform drop ejection characteristics from the entire nozzle array. 
       Efficient Fabrication 
       [0055]    The features described above increase the density of nozzles on the wafer. Each individual integrated circuit is about 22 mm long, less than 3 mm wide and can support more than 10000 nozzles. This represents a significant increase on the nozzle numbers (70,400 nozzles per IC) in the Applicants 1600 dpi printhead ICs (see for example MNN001US). In fact, a true 3200 dpi printhead nozzle array fabricated to the dimensions shown in  FIG. 12 , has 12,800 nozzles. 
         [0056]    The lithographic fabrication of this many nozzles (more than 10,000) is efficient because the entire nozzle array fits within the exposure area of the lithographic stepper or scanner used to expose the reticles (photomasks). A photolithographic stepper is sketched in  FIG. 14 . A light source  102  emits parallel rays of a particular wavelength  104  through the reticle  106  that carries the pattern to be transferred to the integrated circuit  92 . The pattern is focused through a lens  108  to reduce the size of the features and projected onto a wafer stage  110  the carries the integrated circuits  92  (or ‘dies’ as they are also known). The area of the wafer stage  110  illuminated by the light  104  is called the exposure area  112 . Unfortunately, the exposure area  112  is limited in size to maintain the accuracy of the projected pattern—whole wafer discs can not be exposed simultaneously. The vast majority of photolithographic steppers have an exposure area  112  less than 30 mm by 30 mm. One major manufacturer, ASML of the Netherlands, makes steppers with an exposure area of 22 mm by 22 mm which is typical of the industry. 
         [0057]    The stepper exposes one die, or a part of a die, and then steps to another, or another part of the same die. Having as many nozzles as possible on a single monolithic substrate is advantageous for compact printhead design and minimizing the assembly of the ICs on a support in precise relation to one another. The invention configures the nozzle array so that more than 10,000 nozzles fit into the exposure area. In fact the entire integrated circuit can fit into the exposure area so that more than 14,000 nozzles are fabricated on a single monolithic substrate without having to step and realign for each pattern. 
         [0058]    The ordinary worker will appreciate that the same applies to fabrication of the nozzle array using a photolithographic scanner. The operation of a scanner is sketched in  FIG. 15A to 15C . In a scanner, the light source  102  emits a narrower beam of light  104  that is still wide enough to illuminate the entire width of the reticle  106 . The narrow beam  104  is focused through a smaller lens  108  and projected onto part of the integrated circuit  92  mounted in the exposure area  112 . The reticle  106  and the wafer stage  110  are moved in opposing directions relative to each other so that the reticle&#39;s pattern is scanned across the entire exposure area  112 . 
         [0059]    Clearly, this type of photo-imaging device is also suited to efficient fabrication of printhead ICs with large numbers of nozzles. 
       Flat Exterior Nozzle Surface 
       [0060]    As discussed above, the printhead IC is fabricated in accordance with the steps listed in cross referenced U.S. Ser. No. 11/246,687 (Our Docket MNN001US) filed 11 Oct. 2005. Only the exposure mask patterns have been changed to provide the different chamber and heater configurations. As described in U.S. Ser. No. 11/246,687 (Our Docket MNN001US) filed 11 Oct. 2005, the roof layer and the chamber walls are an integral structure—a single Plasma Enhanced Chemical Vapor Deposition (PECVD) of suitable roof and wall material. Suitable roof materials may be silicon nitride, silicon oxide, silicon oxynitride, aluminium nitride etc. The roof and walls are deposited over a scaffold layer of sacrificial photoresist to form an integral structure on the passivation layer of the CMOS. 
         [0061]      FIG. 8  shows the pattern etched into the sacrificial layer  72 . The pattern consists of the chamber walls  32  and columnar features  68  (discussed below) which are all of uniform thickness. In the embodiment shown, the thickness of the walls and columns is 4 microns. These structures are relatively thin so when the deposited roof and wall material cools there is little if any depression in the exterior surface of the roof layer  70  (see  FIG. 9 ). Thick features in the etch pattern will hold a relatively large volume of the roof/wall material. When the material cools and contracts, the exterior surface draws inwards to create a depression. 
         [0062]    These depressions leave the exterior surface uneven which can be detrimental to the printhead maintenance. If the printhead is wiped or blotted, paper dust and other contaminants can lodge in the depressions. As shown in  FIG. 9 , the exterior surface of the roof layer  72  is flat and featureless except for the nozzles  2 . Dust and dried ink is more easily removed by wiping or blotting. 
       Refill Ink Flow 
       [0063]    Referring to  FIG. 10 , each ink inlet supplies four nozzles except at the longitudinal ends of the array where the inlets supply fewer nozzles. Redundant nozzle inlets  14  are an advantage during initial priming and in the event of air bubble obstruction. 
         [0064]    As shown by the flow lines  74 , the refill flow to the chambers  28  remote from the inlets  14  is longer than the refill flow to the chambers  28  immediately proximate the supply channel  6 . For uniform drop ejection characteristics, it is desirable to have the same ink refill time for each nozzle in the array. 
         [0065]    As shown in  FIG. 11 , the inlets  76  to the proximate chambers are dimensioned differently to the inlets  78  to the remote chambers. Likewise the column features  68  are positioned to provide different levels of flow constriction for the proximate nozzle inlets  76  and the remote nozzle inlets  78 . The dimensions of the inlets and the position of the column can tune the fluidic drag such that the refill times of all the nozzles in the array are uniform. The columns can also be positioned to damp the pressure pulses generated by the vapor bubble in the chamber  28 . Damping pulses moving though the inlet prevents fluidic cross talk between nozzles. Furthermore, the gaps  80  and  82  between the columns  68  and the sides of the inlets  76  and  78  can be effective bubble traps for larger outgassing bubbles entrained in the ink refill flow. 
       Extended Nozzle Life 
       [0066]      FIG. 12  shows a section of one color channel in the nozzle array with the dimensions necessary for 3200 dpi resolution. It will be appreciated that ‘true’ 3200 dpi is very high resolution—greater than photographic quality. This resolution is excessive for many print jobs. A resolution of 1600 dpi is usually more than adequate. In view of this, the printhead IC sacrifice resolution by sharing the print data between two adjacent nozzles. In this way the print data that would normally be sent to one nozzle in a 1600 dpi printhead is sent alternately to adjacent nozzles in a 3200 dpi printhead. This mode of operation more than doubles the life of the nozzles and it allows the printer to operate at much higher print speeds. In 3200 dpi mode, the printer prints at 60 ppm (full color A4) and in 1600 dpi mode, the speed approaches 120 ppm. 
         [0067]    An additional benefit of the 1600 dpi mode is the ability to use this printhead IC with print engine controllers (PEC) and flexible printed circuit boards (flex PCBs) that are configured for 1600 dpi resolution only. This makes the printhead IC retro-compatible with the Applicant&#39;s earlier PECs and PCBs. 
         [0068]    As shown in  FIG. 12 , the nozzle  83  is transversely offset from the nozzle  84  by only 7.9375 microns. They are spaced further apart in absolute terms but displacement in the paper feed direction can be accounted for with the timing of nozzle firing sequence. As the 8 microns transverse shift between adjacent nozzles is small, it can be ignored for rendering purposes. However, the shift can be addressed by optimizing the dither matrix if desired. 
       Bubble, Chamber and Nozzle Matching 
       [0069]      FIG. 13  is an enlarged view of the nozzle array. The ejection aperture  2  and the chamber walls  32  are both elliptical. Arranging the major axes parallel to the media feed direction allows the high nozzle pitch in the direction transverse to the feed direction while maintaining the necessary chamber volume. Furthermore, arranging the minor axes of the chambers so that they overlap in the transverse direction also improves the nozzle packing density. 
         [0070]    The heaters  30  are a suspended beam extending between their respective electrodes  34  and  36 . The elongate beam heater elements generate a bubble that is substantially elliptical (in a section parallel to the plane of the wafer). Matching the bubble  90 , chamber  28  and the ejection aperture  2  promotes energy efficient drop ejection. Low energy drop ejection is crucial for a ‘self cooling’ printhead. 
       Conclusion 
       [0071]    The printhead IC shown in the drawings provides ‘true’ 3200 dpi resolution and the option of significantly higher print speeds at 1600 dpi. The print data sharing at lower resolutions prolongs nozzle life and offers compatibility with existing 1600 dpi print engine controllers and flex PCBs. The uniform thickness chamber wall pattern gives a flat exterior nozzle surface that is less prone to clogging. Also the actuator contact configuration and elongate nozzle structures provide a high nozzle pitch transverse to the media feed direction while keeping the nozzle array thin parallel to the media feed direction. 
         [0072]    The specific embodiments described are in all respects merely illustrative and in no way restrictive on the spirit and scope of the broad inventive concept.