Abstract:
An inkjet nozzle assembly includes a nozzle chamber having a planar roof spaced apart from a floor. A heater element is suspended in the nozzle chamber and is configured as a planar beam extending longitudinally and parallel with a plane of the roof. A nozzle aperture defined in the roof has a centroid offset from a longitudinal centroid of the planar beam.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 11/764808 filed Aug. 12, 2007, now issued U.S. Pat. No. 7, 780,271, all of which is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of inkjet printers. In particular, the invention concerns printheads with heater elements that vaporize ink to eject an ink droplet from the nozzle. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     The following patents or patent applications filed by the applicant or assignee of the present invention are hereby incorporated by cross-reference. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
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                 7,661,803 
               
               
                 11/495,819 
                 7,771,029 
                 11/677,050 
                 7,658,482 
                 7,306,320 
               
               
                 7,111,935 
                 7,562,971 
                 7,735,982 
                 7,604,322 
                 7,261,482 
               
               
                 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 
               
               
                 6,920,704 
                 7,217,049 
                 7,607,756 
                 7,147,102 
                 7,287,828 
               
               
                 7,249,838 
                 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 
                 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 
                 7,726,785 
                 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 
               
               
                 7,695,204 
                 7,322,761 
                 11/223,021 
                 7,735,994 
                 7,079,292 
               
               
                   
               
             
          
         
       
     
     BACKGROUND OF THE INVENTION 
     The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 to Stemme. 
     There are various known types of thermal inkjet (Bubblejet™ is owned by Canon K.K.) printhead devices. Two typical devices of this type, one made by Hewlett Packard and the other by Canon, have ink ejection nozzles and chambers for storing ink adjacent the nozzles. Each chamber is covered by a so-called nozzle plate which is mechanically secured to the walls of the chamber. These devices also include heater elements in thermal contact with ink that is disposed adjacent the nozzles, for heating the ink thereby forming gas bubbles in the ink. The gas bubbles generate pressures in the ink causing ink drops to be ejected through the nozzles. 
     Thermal inkjet printheads are traditionally prone to overheating. The rapid successive vaporization of ink during printing can build up heat in the printhead. If too much builds up in the printhead, the ink will boil in an uncontrolled manner. This heat is removed from the printhead either by an active cooling system or with heats sinks and the use of small nozzle arrays. The overheating problem has limited the firing frequency of the nozzles and printhead size, both of which reduce the print speed. 
     The Applicant has developed a range of pagewidth printheads that overcome the problem of excess heat generation. The large pagewidth arrays and high nozzle firing frequencies provide print speeds in excess of 60 pages per minute at full color 1600 dpi resolution. These printheads avoid excess heat generation by reducing the energy used by the heaters to eject the drops of ink. The heat input to the printhead by the heaters is removed from the printhead by the ejected drops of ink. 
     One aspect of reducing the energy required to eject drops of ink is a reduction in the mass of the ejected drop, and hence the volume of the drop. The Applicant&#39;s ‘self cooling’ printheads eject drops of about 1 pl to 2 pl (pico-liters). Unfortunately drop volumes this small are susceptible to trajectory misdirection. The trajectory of the ejected drop is particularly sensitive to the nozzle geometry and the shape of the bubble generated by the heater element. It will be appreciated that any misdirection of the ejected ink drops is detrimental to print quality. 
     Fluidic symmetry around the heater is not possible unless the heater is suspended directly over the ink inlet. The Applicant has developed printheads with this arrangement (see U.S. Pat. No. 6,755,509 filed Nov. 23, 2002—), however there are production efficiencies and nozzle density gains available if multiple ink chambers are supplied from a single ink supply channel through the supporting wafer. This requires that the individual chambers are supplied with ink through lateral inlets—that is, inlets extending parallel to the planes of the heaters and the nozzles. As the heater is laterally bounded by the chamber walls except for the ink inlet, the bubble generated by the heater is distorted by this asymmetry. The inlet can be lengthened and or narrowed to increase its fluidic resistance to back flow caused by the bubble. This will reduce the fluidic asymmetry caused by the inlet but also increase the chamber refill times because of the higher flow resistance. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a printhead for an inkjet printer, the printhead comprising: 
     an array of nozzles each defining a planar ejection aperture; 
     a plurality heater elements corresponding to each of the nozzles respectively, each heater element formed as a planar structure, the heater element having opposing sides positioned parallel to the plane of the ejection aperture, the opposing sides defining a two dimensional shape with two orthogonal axes of symmetry and during use the heater element generates a vapor bubble that is asymmetrical about at least one of the axes of symmetry; wherein, 
     the ejection aperture has a centroid that is offset from the centroid of the two dimensional shape of the heater element in a direction parallel to the plane of the ejection aperture. 
     The invention is predicated on the realization that misdirected drop trajectories caused by asymmetries in the vapor bubble can be compensated for by offsetting the nozzle centroid from the heater centroid. The ordinary worker in this field will understand that the centroid is a point at the geometric centre of a two dimensional shape. 
     The vapor bubble generated by the heater can be asymmetrical because of the configuration of the heater relative to the nozzle and the ink inlet. As the bubble grows, it not only forces ink from the nozzle but also creates a small back flow of ink through the ink inlet. The back flow is usually negligible compared to the ink ejected because the fluidic drag resisting flow out of the inlet compared to flow out of the nozzle is very high. If the ink inlet is at the side of the chamber (that is, the inlet flow is parallel to the plane of the heater and the nozzle), the small back flow of ink allows the bubble to skew towards the ink inlet. The pressure pulse through the ink is likewise skewed and meets one side of the ejection aperture slightly before the other side. 
     The ink drop ejected through the nozzle will trail a thin stem of ink behind it immediately after ejection. Eventually the momentum of the drop overcomes the surface tension in the trailing stem of ink to break the stem so that the drop completely separates from the printhead. With a skewed pressure pulse ejecting the drop, the trailing stem of ink pins to one particular side or part of the ejection aperture. Before the thin stem of ink between the nozzle and the ejected drop breaks, the surface tension in the stem can drag the droplet away from a trajectory normal to the plane of the nozzles. This causes consistent droplet misdirection. However, the invention addresses this by offsetting the heater and nozzle from each other so that the pressure pulse is much less skewed when it is incident on the nozzle aperture. 
     Preferably, the printhead further comprising a plurality of chambers in fluid communication with each of the nozzles respectively, each of the chambers adapted to hold printing fluid in contact with each of the heater elements respectively, wherein the chamber has a printing fluid inlet that defines a fluid path that extends parallel to the plane of the heater element. In a further preferred form, the chambers defines walls extending generally transverse to the plane of the heater element, the walls surrounding the heater element except for an opening defining one end of the printing fluid inlet. In a particularly preferred form, the ejection aperture centroid is offset from the centroid of the two dimensional shape of the heater element in a direction away from the printing fluid inlet. 
     Optionally, the ejection aperture is elliptical. In another option, the heater element is a rectangular beam. In some embodiments, the major axis of the elliptical ejection aperture is parallel to the longitudinal extent of the rectangular beam heater element. 
     Preferably, the heater element is a rectangular beam suspended in the chamber. In a further preferred form, the vapor bubble vents to atmosphere through the ejection aperture. 
     Preferably, the ejection aperture centroid is offset from the centroid of the two dimensional shape of the heater element in a direction parallel to the major axis of the ejection aperture. 
     Preferably, the nozzle is formed in a roof layer that partially defines the chamber, and the roof layer and the walls of the chamber are integrally formed. 
     In some embodiments, the heater element is a rectangular beam and the chamber is less than 40 microns wide in a direction transverse to the rectangular beam, and less than 80 microns long in the elongate direction of the rectangular beam. In these embodiments, it is preferable when the vapor bubble ejects a drop of printing fluid through the ejection aperture, the drop having a volume between 1 pl and 2 pl. 
     Preferably the offset is less than 20 microns. In a further preferred form, the offset is less than 5 microns. In a particularly preferred form, the offset is between 1 micron and 3 microns. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 
         FIGS. 1 to 5  schematically shows the ejection of a drop of ink from a prior art printhead without any offset between the nozzle and the heater; 
         FIG. 6  is a partial plan view of a printhead with offset heater and nozzle; 
         FIG. 7  is a partial section view taken along line  7 - 7  of  FIG. 6 ; and, 
         FIGS. 8 to 13  schematically shows the ejection of a drop of ink from a printhead with the nozzle and the heater offset from each other. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 to 5  sketch the ejection stages of a misdirected drop of ink from a prior art printhead. The printhead structure is a simplified representation of the printheads described in detail in U.S. Ser. No. 11/246,687 filed Oct. 11, 2005, the contents of which are incorporated herein by reference. While the invention is described here with reference to this particular printhead design, it will be appreciated that this is purely illustrative and in no way restrictive on the printheads to which the invention can be applied. 
     Referring to  FIG. 1 , a unit cell of an inkjet printhead  2  is shown. The unit cell is the smallest repeatable unit making up the printhead—in this case the ink supply channel  4  extending from the supply side  6  of the wafer substrate  10 , to the ejection side  8  of the wafer substrate, the nozzle  14 , the chamber  16 , the suspended beam heater  18  with its contacts  20  and associated CMOS drive circuitry  12 . 
     The heater  18  is a thin rectangular strip suspended as a beam over a trench  24  in the floor of the chamber  16 . The centroid of the top surface rectangle shape of the heater  18  is simply the intersection of the rectangle&#39;s diagonals. The nozzle  14  is an ellipse so the centroid is simply the intersection of the major and minor axes. As described in the above referenced U.S. Ser. No. 11/246,687 filed Oct. 11, 2005 the roof layer  22  is formed by CVD of silicon nitride and the nozzles  14  subsequently etched. Hence the centroids of the nozzle and the heater are closely aligned. 
       FIG. 1  shows the nucleation of the vapor bubble  26  around the heater  18 . It begins with film boiling of the ink directly in contact with the heater surface. In  FIG. 2 , the vapor bubble  26  has grown and has forced a bulb of ink  28  through the nozzle  14 . A stem  30  of ink trails behind the bulb  28  and pins to the edges of the nozzle  14 . The pressure pulse in the chamber  16  also causes a small backflow  34  of ink through the chamber inlet  32 . 
       FIG. 3  shows the bubble  26  immediately before it vents to atmosphere through the nozzle  14 . The ejected drop  28  is still connected to the ink in the chamber by the thin stem of ink  30 . The backflow  34  of ink through the chamber inlet  32  has allowed the bubble  26  to widen and flatten on the inlet side  40 , while the side  42  constrained by the chamber walls  44  has grown to the roof layer  22  and one side  38  of the nozzle  14 . The bubble surface  40  is still spaced from the opposing side  38  of the nozzle  14 . 
     In  FIG. 4 , the thin stem of ink  30  is shown immediately before the momentum of the ejected drop  28  overcomes the surface tension of the ink and breaks the connection to the side  32  of the nozzle  14 . The bubble  26  has vented to atmosphere through the nozzle  14 . However, as the bubble is always first incident on the nozzle aperture at the side  38 , the stem  30  invariably pins to the side  32 . The side  32  is spaced from the centre line  50  of the nozzle  14 . The surface tension acting on the stem has a component acting normal to the centre line  50 . As a result, the centre of mass  46  of the drop  28  is pulled away from the centre line  50  until the stem  30  breaks. The drop trajectory  48  now deviates from the centre line  50  by the angle A. 
       FIG. 5  shows the now separated drop  28  continuing along it&#39;s deviated trajectory  48 . The bubble has become an ink meniscus  52  in the chamber  16  rapidly shrinking toward the nozzle  14  under the action of surface tension. This draws a refill flow  54  of ink through the inlet  32  and the process repeats when the heater  18  is next actuated. 
     The invention takes the asymmetry of the bubble into account and offsets the heater and nozzle accordingly.  FIGS. 6 and 7  show this arrangement. The plan view shown in  FIG. 6 , the nozzle aperture centroid  56  is slightly offset from the heater centroid  58  by a distance D. The offset D of the nozzle  14  is away from the chamber inlet  32  to counter the bubble asymmetry caused by ink back flow. 
     As seen in  FIG. 7 , the spacing between the plane of the heater and the plane of the nozzle is not the relevant offset—only the displacement of the heater centroid  58  relative to the nozzle centroid  56  in the plane of the nozzle aperture  14 . It will also be appreciated that centroid of the heater is a reference to the entire heater element structure. It may be the case that the heater has several parallel beams extending between the electrodes  20 . The bubbles generated by each individual beam will coalesce into a single bubble that ejects the ink from the nozzle. Accordingly, the nozzle centroid  56  is to be offset from a centroid of the overall two dimensional shape of the heater element(s) that generate the coalesced bubble. 
       FIGS. 8 to 13  schematically illustrates the drop ejection process using a printhead according to the present invention.  FIG. 8  shows the unit cell  2  in the quiescent state. The chamber  16  is primed with ink which completely immerses the heater  18 . The heater  18  is powered by contacts  20  in the CMOS drive circuitry  12 . The CMOS  12  is supported on the underlying silicon wafer  10 . The ink supply channel  4  fluidically connects the supply side  6  and the ejection side  8  of the printhead IC. Ink flows to the individual chamber  16  via the inlets  32 . The nozzles  14  are etched into the roof layer  22  such that the heater centroid  58  is offset from the nozzle centroid  56  by a distance D in the plane of the nozzle aperture. 
     In  FIG. 9 , the heater  18  has received a drive pulse and film boiling at the heater surface nucleates the bubble  26 . The increased pressure in the chamber forces the ink meniscus at the nozzle  14  to bulge outwardly and begin forming the drop  28 . In  FIG. 10 , the bubble  26  grows and forces more ink from the chamber  16  out of the nozzle  16 . It also starts a small back flow  34  in the inlet  32 . As the bubble  26  expands further (see  FIG. 11 ) the side  40  facing the inlet  32  is unconstrained and has a flatter, broader profile. In contrast, the side  44  facing the away from the inlet  32  is constrained so the bubble has a taller profile on this side. However, as the nozzle  14  is offset away from the inlet  32  by the distance D, the bubble  26  is approximately the same distance from the nozzle edge  36  as it is from the nozzle edge  38 . 
     If the printhead is of the type that vents the bubble  26  through the nozzle to avoid the cavitation corrosion of a bubble collapse point, the bubble will ideally contact all points on the nozzle&#39;s periphery simultaneously. This is shown in  FIG. 12 . As the bubble  26  touches the edge  36  and the edge  38  at the same time so the stem  30  trailing the drop  28  is not induced to pin itself at one specific location on the nozzle periphery. Consequently, as shown in  FIG. 13 , when the stem  3  breaks and the drop  28  separates, it has not been dragged away from the centroidal axis  50  of the nozzle by surface tension in the ink. The ejection trajectory stays on the centroidal axis of the nozzle  14 . 
     Also shown in  FIG. 13 , the vented bubble becomes an ink meniscus  52  within the chamber  16 . Surface tension drives the meniscus to the smallest surface area possible so it rapidly contracts to span the nozzle aperture  14 . This draws the refill flow  54  of ink through the inlet  32 . 
     The magnitude of nozzle offset will depend on a large number of variables such as chamber configuration, the dimensions of the heater, nozzle, and roof layer height and the nozzle shape. However, in most cases the offset need only be relatively small. For example, the unit cell of the printhead described in the above referenced U.S. Ser. No. 11/246,687 filed Oct. 11, 2005, has chambers of 32 microns wide and less than 80 microns from the ink supply channel to outside of the chamber end wall (opposite the inlet). In these printheads, offsetting the nozzle centroid from the heater centroid by less than 5 microns was sufficient to address instances of drop misdirection. As these printhead unit cells are particularly small relative to other prior art printhead unit cells, the maximum offset necessary for the vast majority of so called ‘roof-shooter’ printheads would be 20 microns. In the Applicant&#39;s range of printheads, most offsets would be between 1 and 3 microns. 
     The present invention has been defined herein by way of example only. The skilled addressee would readily recognize many variations and modifications which do not depart from the spirit ad scope of the broad invention concept.