Abstract:
An inkjet printhead comprising: an array of ink chambers, each having a nozzle and an actuator for ejecting ink through the nozzle; wherein, the nozzle has a nozzle rim defining a nozzle aperture and a localized irregularity on the nozzle rim extending toward the center of the nozzle aperture.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of micro-electromechanical systems (MEMS) devices and discloses an inkjet printing system using MEMS techniques. 
     CO-PENDING APPLICATIONS 
     The following applications have been filed by the Applicant simultaneously with the present application: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
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     The disclosures of these co-pending applications are incorporated herein by reference. 
     CROSS REFERENCES TO RELATED APPLICATIONS 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following US Patents/ Patent Applications filed by the applicant or assignee of the present invention: 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
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     The disclosures of these applications and patents are incorporated herein by reference. 
     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 (Stemme). Each pixel in the printed image is derived ink drops ejected from one or more ink nozzles. In recent years, inkjet printing has become increasing popular primarily due to its inexpensive and versatile nature. Many different aspects and techniques for inkjet printing are described in detail in the above cross referenced documents. 
     One of the perennial problems with inkjet printing is the control of drop trajectory as it is ejected from the nozzle. With every nozzle, there is a degree of misdirection in the ejected drop. Depending on the degree of misdirection, this can be detrimental to print quality. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides an inkjet printhead comprising:
         an array of ink chambers, each having a nozzle and an actuator for ejecting ink through the nozzle; wherein,   the nozzle has a nozzle rim defining a nozzle aperture and a localized irregularity on the nozzle rim extending toward the centre of the nozzle aperture.       

     A localized irregularity on the nozzle rim can be used to predetermine the magnitude and direction of the ‘misdirection’ of the drop ejected. In this sense, ‘misdirection’ refers to intention deviation of the drop trajectory away from the central axis of the nozzle aperture. This is a different approach to minimising the drop misdirection as discussed above. By intentionally misdirecting the drops ejected by every nozzle in the array by a controlled amount, the printed image is equivalent to one from a minimised drop misdirection printhead (albeit slightly offset from the nozzle array). 
     Preferably, the localized irregularity is a droplet stem anchor positioned so that a droplet stem will attach to it in preference to any other point on the nozzle rim. In a further preferred form, the localized irregularity is a lateral spur extending into the nozzle aperture from the nozzle rim. In some embodiments, the actuator is a thermal actuator with a suspended beam heater element for immersion in the ink. Preferably, all the spurs in the array are parallel and have the same position relative to the heater element. 
     In a first aspect the present invention provides an inkjet printhead comprising:
         an array of ink chambers, each having a nozzle, an elongate actuator for ejecting ink through the nozzle; wherein,   the nozzle has an elongate shape with its long dimension aligned with that of the elongate actuator.       

     Optionally, the nozzle is elliptical. 
     Optionally, the actuator is a thermal actuator with an elongate heater element that generate a vapour bubble to eject in through the nozzle. 
     Optionally, each ink chamber in the array has a plurality of elongate nozzles aligned with the elongate actuator. 
     Optionally, each ink chamber in the array has a plurality of elongate nozzles corresponding to a plurality of elongate actuators respectively. 
     In a further aspect there is provided an inkjet printhead according further comprising drive circuitry for providing actuator drive signals via a pair of electrodes for each actuator respectively, wherein the actuators are thermal actuators, each having an elongate heater element extending between two contacts on the pair of electrodes wherein the thermal actuators are all unitary planar structures. 
     Optionally, a trench etched into the drive circuitry extends between the electrodes. 
     Optionally, each of the ink chambers have a plurality of nozzles; wherein during use, the actuator simultaneously ejects ink through all the nozzles of the chamber. 
     Optionally, each of the ink chambers have two nozzles. 
     Optionally, the nozzles in each chamber are arranged in a line parallel to the length of the heater element with the central axes of the nozzles are regularly spaced along the heater element. 
     Optionally, the nozzles are elliptical. 
     Optionally, the major axes of the elliptical nozzles are aligned. 
     Optionally, the drive circuitry has a drive field effect transistor (FET) for each of the thermal actuators, the drive voltage of the drive FET being less than 5 Volts. 
     Optionally, the drive voltage of the drive FET is 2.5 Volts. 
     In a further aspect there is provided an inkjet printhead further comprising an ink conduit between the nozzle plate and the underlying wafer, the ink conduit being in fluid communication with the openings of a plurality of the ink chambers. 
     In a further aspect there is provided an inkjet printhead further comprising a plurality of ink inlets defined in the wafer substrate; wherein,
         each of the ink conduits is in fluid communication with at least one of the ink inlets for receiving ink to supply to the ink chambers.       

     Optionally, each of the ink conduits is in fluid communication with two of the ink inlets. 
     Optionally, each of the ink inlets has an ink permeable trap and a vent sized so that the surface tension of an ink meniscus across the vent prevents ink leakage; wherein during use,
         the ink permeable trap directs gas bubbles to the vent where they vent to atmosphere.       

     Optionally, the ink chambers have an elongate shape such that two of the sidewalls are long relative to the others, and the opening for allowing ink to refill the chamber is in one of the long sidewalls. 
     Optionally, the nozzles are arranged in rows such that the nozzle centres are collinear and the nozzle pitch along each row is greater than 1000 nozzles per inch. 
     In a second aspect the present invention provides an inkjet printhead comprising:
         an array of nozzles;   a plurality of actuators for ejecting ink through the nozzles such that a bulb of ink attached to a droplet stem forms prior to drop separation when the stem breaks;   a plurality of droplet stem anchors positioned between adjacent actuators; wherein during use,   the adjacent actuators eject ink simultaneously and the droplet stem anchors combine the ink simultaneously ejected by the adjacent nozzles into a single drop.       

     Optionally, the adjacent actuators are two thermal actuators ejecting ink through a single oval shaped nozzle. 
     Optionally, the thermal actuators are both heater elements connected in series for simultaneous actuation and ejection. 
     Optionally, the two heater elements are part of a single beam of heater material suspended at its ends and at it mid point. 
     Optionally, the heater elements have a tapered section where electrical resistance is at a maximum such that vapour bubbles initiate at the maximum resistance sections. 
     Optionally, the heater elements are on opposite sides of the droplet stem anchor so that the trajectory of the ink ejected by one heater element intersects with the trajectory of ink ejected by the other heater element. 
     Optionally, the heater elements are in adjacent ink chambers with the droplet stem anchor at an adjoining boundary. 
     Optionally, the heater elements are in a single ink chamber. 
     Optionally, the ink ejected by the adjacent actuators is in fluid communication prior to actuation. 
     Optionally, the heater elements are formed from TiAlN. 
     Optionally, the nozzles are elliptical. 
     Optionally, the major axes of the elliptical nozzles are aligned. 
     Optionally, the drive circuitry has a drive field effect transistor (FET) for each of the thermal actuators, the drive voltage of the drive FET being less than 5 Volts. 
     Optionally, the drive voltage of the drive FET is 2.5 Volts. 
     In a further aspect there is provided an inkjet printhead further comprising an ink conduit between the nozzle plate and the underlying wafer, the ink conduit being in fluid communication with the openings of a plurality of the ink chambers. 
     In a further aspect there is provided an inkjet printhead further comprising a plurality of ink inlets defined in the wafer substrate; wherein,
         each of the ink conduits is in fluid communication with at least one of the ink inlets for receiving ink to supply to the ink chambers.       

     Optionally, each of the ink conduits is in fluid communication with two of the ink inlets. 
     Optionally, each of the ink inlets has an ink permeable trap and a vent sized so that the surface tension of an ink meniscus across the vent prevents ink leakage; wherein during use,
         the ink permeable trap directs gas bubbles to the vent where they vent to atmosphere.       

     Optionally, the ink chambers have an elongate shape such that two of the sidewalls are long relative to the others, and the opening for allowing ink to refill the chamber is in one of the long sidewalls. 
     Optionally, the nozzles are arranged in rows such that the nozzle centres are collinear and the nozzle pitch along each row is greater than 1000 nozzles per inch. 
     In a third aspect the present invention provides an inkjet printhead comprising:
         an array of ink chambers, each having an ink refill aperture, a nozzle and an actuator for ejecting ink through the nozzle; and,   a fluid flow rectifying valve at the ink refill aperture for providing less hydraulic resistance to ink flowing into the chamber than ink flowing out of the chamber.       

     Optionally, the recifying valve is a Tesla valve with a main conduit and a secondary conduit and at least one secondary conduit; wherein during use, ink flow out of the chamber is split into a main flow and a secondary flow such that when ink flows out of the chamber the secondary flow is combined with the main flow so as to constrict the main flow. 
     Optionally, the Tesla valve has two secondary conduits, on opposite sides of the main conduit. 
     Optionally, during use, when ink flows into the chamber, the upstream openings of the secondary conduits are in plane parallel to the flow direction and the downstream openings direct any secondary flow parallel and adjacent to flow from the main conduit downstream opening. 
     Optionally, the downstream openings of the secondary conduits during ink flow out of the chamber are on opposing sides of the main conduit face transversely to the flow direction through the main conduit. 
     In a further aspect there is provided an inkjet printhead further comprising drive circuitry for providing actuator drive signals via a pair of electrodes for each actuator respectively, wherein the actuators are thermal actuators, each having an elongate heater element extending between two contacts on the pair of electrodes wherein the thermal actuators are all unitary planar structures. 
     Optionally, a trench etched into the drive circuitry extends between the electrodes. 
     Optionally, each of the ink chambers have a plurality of nozzles; wherein during use,
         the actuator simultaneously ejects ink through all the nozzles of the chamber.       

     Optionally, each of the ink chambers have two nozzles. 
     Optionally, the nozzles in each chamber are arranged in a line parallel to the length of the heater element with the central axes of the nozzles are regularly spaced along the heater element. 
     Optionally, the nozzles are elliptical. 
     Optionally, the major axes of the elliptical nozzles are aligned. 
     Optionally, the drive circuitry has a drive field effect transistor (FET) for each of the thermal actuators, the drive voltage of the drive FET being less than 5 Volts. 
     Optionally, the drive voltage of the drive FET is 2.5 Volts. 
     In a further aspect there is provided an inkjet printhead further comprising an ink conduit between the nozzle plate and the underlying wafer, the ink conduit being in fluid communication with the openings of a plurality of the ink chambers. 
     In a further aspect there is provided an inkjet printhead further comprising a plurality of ink inlets defined in the wafer substrate; wherein,
         each of the ink conduits is in fluid communication with at least one of the ink inlets for receiving ink to supply to the ink chambers.       

     Optionally, each of the ink conduits is in fluid communication with two of the ink inlets. 
     Optionally, each of the ink inlets has an ink permeable trap and a vent sized so that the surface tension of an ink meniscus across the vent prevents ink leakage; wherein during use,
         the ink permeable trap directs gas bubbles to the vent where they vent to atmosphere.       

     Optionally, the ink chambers have an elongate shape such that two of the sidewalls are long relative to the others, and the opening for allowing ink to refill the chamber is in one of the long sidewalls. 
     Optionally, the nozzles are arranged in rows such that the nozzle centres are collinear and the nozzle pitch along each row is greater than 1000 nozzles per inch. 
     In a fourth aspect the present invention provides an inkjet printhead comprising:
         an array of ink chambers, each having a nozzle, a droplet stem anchor and an actuator for ejecting ink through the nozzle; wherein during use,   the ink ejected from the nozzle is attached to the droplet stem anchor by an ink stem until the stem breaks so that the ejected ink forms a separate drop.       

     Optionally, the droplet stem anchor is a columnar feature with one proximate the nozzle. 
     Optionally, the axis of the droplet stem anchor and the central axis of the nozzle are collinear. 
     Optionally, each ink chamber has two actuators, each actuators having a heater element for generating a vapour bubble to eject ink through the nozzle, and the droplet stem anchor being positioned between the heater elements. 
     Optionally, the actuator has a plurality of heater elements connected in parallel with a cross bracing structure extending between the heater elements, the cross bracing structure also providing the droplet stem anchor. 
     Optionally, the actuator has two heater elements in parallel and the cross bracing structure is a single beam with a surface irregularity to locate the droplet stem actuator. 
     In a further aspect there is provided an inkjet printhead further comprising drive circuitry for providing actuator drive signals via a pair of electrodes for each actuator respectively, wherein the actuators are thermal actuators, each having an elongate heater element extending between two contacts on the pair of electrodes wherein the thermal actuators are all unitary planar structures. 
     Optionally, a trench etched into the drive circuitry extends between the electrodes. 
     Optionally, each of the ink chambers have a plurality of nozzles; wherein during use,
         the actuator simultaneously ejects ink through all the nozzles of the chamber.       

     Optionally, each of the ink chambers have two nozzles. 
     Optionally, the nozzles in each chamber are arranged in a line parallel to the length of the heater element with the central axes of the nozzles are regularly spaced along the heater element. 
     Optionally, the nozzles are elliptical. 
     Optionally, the major axes of the elliptical nozzles are aligned. 
     Optionally, the drive circuitry has a drive field effect transistor (FET) for each of the thermal actuators, the drive voltage of the drive FET being less than 5 Volts. 
     Optionally, the drive voltage of the drive FET is 2.5 Volts. 
     In a further aspect there is provided an inkjet printhead further comprising an ink conduit between a nozzle plate and an underlying wafer, the ink conduit being in fluid communication with the openings of a plurality of the ink chambers. 
     In a further aspect there is provided an inkjet printhead further comprising a plurality of ink inlets defined in the wafer substrate; wherein,
         each of the ink conduits is in fluid communication with at least one of the ink inlets for receiving ink to supply to the ink chambers.       

     Optionally, each of the ink conduits is in fluid communication with two of the ink inlets. 
     Optionally, each of the ink inlets has an ink permeable trap and a vent sized so that the surface tension of an ink meniscus across the vent prevents ink leakage; wherein during use,
         the ink permeable trap directs gas bubbles to the vent where they vent to atmosphere.       

     Optionally, the ink chambers have an elongate shape such that two of the sidewalls are long relative to the others, and the opening for allowing ink to refill the chamber is in one of the long sidewalls. 
     In a fifth aspect the present invention provides an inkjet printhead comprising:
     an array of ink chambers, each having a nozzle and an actuator for ejecting ink through the nozzle;   wherein during use,
       the actuator initiates a quadrupole pressure pulse that is symmetrical about two orthogonal axes parallel to the plane of the nozzle, the orthogonal axes intersecting a mutually orthogonal axis extending through the centre of the nozzle.   
       

     Optionally, the actuator is a thermal actuator with heater elements that generate vapour bubbles to eject the ink. 
     Optionally, the actuator has two parallel current paths with two heater elements connected in series along each current path for initiating the quadrupole pressure pulse. 
     Optionally, the heater elements include bubble nucleation sections that heat more rapidly than other sections of the current path. 
     Optionally, the bubble nucleation sections are between sections of the current path with greater thermal inertia. 
     Optionally, the bubble nucleation sections are tight radius curves in between larger radius curves such that current crowding around the tight radius curves generates more resistive heating than the larger radius curves. 
     Optionally, the heater elements are suspended within the chamber. 
     Optionally, the actuator has a cross bracing structure extending between intermediate points on the parallel current paths. 
     Optionally, the cross bracing structure provides increased thermal inertia where it connects to each current path. 
     Optionally, the cross bracing structure provides a droplet stem anchor. 
     In a further aspect there is provided an inkjet printhead further comprising drive circuitry for providing actuator drive signals via a pair of electrodes for each actuator respectively, wherein the actuators are thermal actuators, each having an elongate heater element extending between two contacts on the pair of electrodes wherein the thermal actuators are all unitary planar structures. 
     Optionally, a trench etched into the drive circuitry extends between the electrodes. 
     Optionally, each of the ink chambers have a plurality of nozzles; wherein during use,
         the actuator simultaneously ejects ink through all the nozzles of the chamber.       

     Optionally, each of the ink chambers have two nozzles. 
     Optionally, the nozzles in each chamber are arranged in a line parallel to the length of the heater element with the central axes of the nozzles are regularly spaced along the heater element. 
     Optionally, the nozzles are elliptical. 
     Optionally, the major axes of the elliptical nozzles are aligned. 
     Optionally, the drive circuitry has a drive field effect transistor (FET) for each of the thermal actuators, the drive voltage of the drive FET being less than 5 Volts. 
     Optionally, the drive voltage of the drive FET is 2.5 Volts. 
     Optionally, the nozzles are arranged in rows such that the nozzle centres are collinear and the nozzle pitch along each row is greater than 1000 nozzles per inch. 
     In a sixth aspect the present invention provides an inkjet printhead comprising:
         an array of ink chambers, each having a nozzle and a thermal actuator for generating vapour bubbles to eject ink through the nozzle; wherein,   the thermal actuator has a pair of contacts and at least two parallel current paths between the contacts, each of the current paths having a plurality of heater elements for nucleating a vapour bubble.       

     Optionally, the heater elements nucleate their respective bubbles simultaneously with every activation of the actuator. 
     Optionally, the actuator has two parallel current paths with two heater elements connected in series along each current path. 
     Optionally, the heater elements include bubble nucleation sections that heat more rapidly than other sections of the current path. 
     Optionally, the bubble nucleation sections are between sections of the current path with greater thermal inertia. 
     Optionally, the bubble nucleation sections are tight radius curves in between larger radius curves such that current crowding around the tight radius curves generates more resistive heating than the larger radius curves. 
     Optionally, the heater elements are suspended within the chamber. 
     Optionally, the thermal actuator has a cross bracing structure extending between intermediate points on the parallel current paths. 
     Optionally, the cross bracing structure provides increased thermal inertia where it connects to each current path. 
     Optionally, the cross bracing structure provides a droplet stem anchor. 
     Optionally, the actuator initiates a quadrupole pressure pulse that is symmetrical about two orthogonal axes parallel to the plane of the nozzle, the orthogonal axes intersecting a mutually orthogonal axis extending through the centre of the nozzle. 
     Optionally, the thermal actuator is formed from TiAlN. 
     In a further aspect there is provided an inkjet printhead further comprising drive circuitry for providing actuator drive signals via a pair of electrodes for each actuator respectively, wherein the actuators are thermal actuators, each having an elongate heater element extending between two contacts on the pair of electrodes wherein the thermal actuators are all unitary planar structures. 
     Optionally, a trench etched into the drive circuitry extends between the electrodes. 
     Optionally, each of the ink chambers have a plurality of nozzles; wherein during use,
         the actuator simultaneously ejects ink through all the nozzles of the chamber.       

     Optionally, each of the ink chambers have two nozzles. 
     Optionally, the nozzles in each chamber are arranged in a line parallel to the length of the heater element with the central axes of the nozzles are regularly spaced along the heater element. 
     Optionally, the nozzles are elliptical. 
     Optionally, the major axes of the elliptical nozzles are aligned. 
     Optionally, the drive circuitry has a drive field effect transistor (FET) for each of the thermal actuators, the drive voltage of the drive FET being less than 5 Volts. 
     In a seventh aspect the present invention provides an inkjet printhead comprising:
         an array of ink chambers, each having a nozzle and a plurality of heater elements for generating vapour bubbles to eject ink through the nozzle, the heater elements being suspended for immersion in the ink; and,   a cross bracing structure for maintaining the spacing between the heater elements.       

     Optionally, the heater elements nucleate their respective bubbles simultaneously with every activation of the actuator. 
     Optionally, the ink chamber has a pair of contacts with two parallel current paths extending between the contacts, each current path having two of the heater elements connected in series. 
     Optionally, the heater elements include bubble nucleation sections that heat more rapidly than other sections of the current path. 
     Optionally, the bubble nucleation sections are between sections of the current path with greater thermal inertia. 
     Optionally, the cross bracing structure is integrally formed with the hater elements and extends between intermediate points on the parallel current paths. 
     Optionally, the cross bracing structure provides sections of greater thermal inertia in the current paths. 
     Optionally, the heater elements initiate a quadrupole pressure pulse that is symmetrical about two orthogonal axes parallel to the plane of the nozzle, the orthogonal axes intersecting a mutually orthogonal axis extending through the centre of the nozzle. 
     Optionally, the thermal elements and the contacts are formed from TiAlN. 
     Optionally, the cross bracing structure provides a droplet stem anchor. 
     Optionally, the actuator initiates a quadrupole pressure pulse that is symmetrical about two orthogonal axes parallel to the plane of the nozzle, the orthogonal axes intersecting a mutually orthogonal axis extending through the centre of the nozzle. 
     In a further aspect there is provided an inkjet printhead further comprising drive circuitry for providing actuator drive signals via a pair of electrodes for each actuator respectively, wherein the actuators are thermal actuators, each having an elongate heater element extending between two contacts on the pair of electrodes wherein the thermal actuators are all unitary planar structures. 
     Optionally, a trench etched into the drive circuitry extends between the electrodes. 
     Optionally, each of the ink chambers have a plurality of nozzles; wherein during use,
         the actuator simultaneously ejects ink through all the nozzles of the chamber.       

     Optionally, each of the ink chambers have two nozzles. 
     Optionally, the nozzles in each chamber are arranged in a line parallel to the length of the heater element with the central axes of the nozzles are regularly spaced along the heater element. 
     Optionally, the nozzles are elliptical. 
     Optionally, the major axes of the elliptical nozzles are aligned. 
     Optionally, the drive circuitry has a drive field effect transistor (FET) for each of the thermal actuators, the drive voltage of the drive FET being less than 5 Volts. 
     Optionally, the drive voltage of the drive FET is 2.5 Volts. 
     In an eighth aspect the present invention provides an inkjet printhead comprising:
         an array of ink chambers, each having a nozzle and an actuator for ejecting ink through the nozzle; wherein,   the nozzle has a nozzle rim defining a nozzle aperture and a localized irregularity on the nozzle rim extending toward the centre of the nozzle aperture.       

     Optionally, the localized irregularity is a droplet stem anchor positioned so that a droplet stem will attach to it in preference to any other point on the nozzle rim. 
     Optionally, the localized irregularity is a lateral spur extending into the nozzle aperture from the nozzle rim. 
     Optionally, the actuator is a thermal actuator with a suspended beam heater element for immersion in the ink. 
     Optionally, all the spurs in the array are parallel and have the same position relative to the heater element. 
     In a further aspect there is provided an inkjet printhead further comprising drive circuitry for providing actuator drive signals via a pair of electrodes for each actuator respectively, wherein the actuators are thermal actuators, each having an elongate heater element extending between two contacts on the pair of electrodes wherein the thermal actuators are all unitary planar structures. 
     Optionally, a trench etched into the drive circuitry extends between the electrodes. 
     Optionally, each of the ink chambers have a plurality of nozzles; wherein during use,
         the actuator simultaneously ejects ink through all the nozzles of the chamber.       

     Optionally, each of the ink chambers have two nozzles. 
     Optionally, the nozzles in each chamber are arranged in a line parallel to the length of the heater element with the central axes of the nozzles are regularly spaced along the heater element. 
     Optionally, the nozzles are elliptical. 
     Optionally, the major axes of the elliptical nozzles are aligned. 
     Optionally, the drive circuitry has a drive field effect transistor (FET) for each of the thermal actuators, the drive voltage of the drive FET being less than 5 Volts. 
     Optionally, the drive voltage of the drive FET is 2.5 Volts. 
     In a further aspect there is provided an inkjet printhead further comprising an ink conduit between the nozzle plate and the underlying wafer, the ink conduit being in fluid communication with the openings of a plurality of the ink chambers. 
     In a further aspect there is provided an inkjet printhead further comprising a plurality of ink inlets defined in the wafer substrate; wherein,
         each of the ink conduits is in fluid communication with at least one of the ink inlets for receiving ink to supply to the ink chambers.       

     Optionally, each of the ink conduits is in fluid communication with two of the ink inlets. 
     Optionally, each of the ink inlets has an ink permeable trap and a vent sized so that the surface tension of an ink meniscus across the vent prevents ink leakage; wherein during use,
         the ink permeable trap directs gas bubbles to the vent where they vent to atmosphere.       

     Optionally, the ink chambers have an elongate shape such that two of the sidewalls are long relative to the others, and the opening for allowing ink to refill the chamber is in one of the long sidewalls. 
     Optionally, the nozzles are arranged in rows such that the nozzle centres are collinear and the nozzle pitch along each row is greater than 1000 nozzles per inch. 
     The printhead according to the invention comprises a plurality of nozzles, as well as a chamber and one or more heater elements corresponding to each nozzle. The smallest repeating units of the printhead will have an ink supply inlet feeding ink to one or more chambers. The entire nozzle array is formed by repeating these individual units. Such an individual unit is referred to herein as a “unit cell”. 
     Also, the term “ink” is used to signify any ejectable liquid, and is not limited to conventional inks containing colored dyes. Examples of non-colored inks include fixatives, infra-red absorber inks, functionalized chemicals, adhesives, biological fluids, medicaments, water and other solvents, and so on. The ink or ejectable liquid also need not necessarily be a strictly a liquid, and may contain a suspension of solid particles. 
    
    
     
       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: 
         FIG. 1  shows a partially fabricated unit cell of the MEMS nozzle array on a printhead according to the present invention, the unit cell being section along A-A of  FIG. 3 ; 
         FIG. 2  shows a perspective of the partially fabricated unit cell of  FIG. 1 ; 
         FIG. 3  shows the mark associated with the etch of the heater element trench; 
         FIG. 4  is a sectioned view of the unit cell after the etch of the trench; 
         FIG. 5  is a perspective view of the unit cell shown in  FIG. 4 ; 
         FIG. 6  is the mask associated with the deposition of sacrificial photoresist shown in  FIG. 7 ; 
         FIG. 7  shows the unit cell after the deposition of sacrificial photoresist trench, with partial enlargements of the gaps between the edges of the sacrificial material and the side walls of the trench; 
         FIG. 8  is a perspective of the unit cell shown in  FIG. 7 ; 
         FIG. 9  shows the unit cell following the reflow of the sacrificial photoresist to close the gaps along the side walls of the trench; 
         FIG. 10  is a perspective of the unit cell shown in  FIG. 9 ; 
         FIG. 11  is a section view showing the deposition of the heater material layer; 
         FIG. 12  is a perspective of the unit cell shown in  FIG. 11 ; 
         FIG. 13  is the mask associated with the metal etch of the heater material shown in  FIG. 14 ; 
         FIG. 14  is a section view showing the metal etch to shape the heater actuators; 
         FIG. 15  is a perspective of the unit cell shown in  FIG. 14 ; 
         FIG. 16  is the mask associated with the etch shown in  FIG. 17 ; 
         FIG. 17  shows the deposition of the photoresist layer and subsequent etch of the ink inlet to the passivation layer on top of the CMOS drive layers; 
         FIG. 18  is a perspective of the unit cell shown in  FIG. 17 ; 
         FIG. 19  shows the oxide etch through the passivation and CMOS layers to the underlying silicon wafer; 
         FIG. 20  is a perspective of the unit cell shown in  FIG. 19 ; 
         FIG. 21  is the deep anisotropic etch of the ink inlet into the silicon wafer; 
         FIG. 22  is a perspective of the unit cell shown in  FIG. 21 ; 
         FIG. 23  is the mask associated with the photoresist etch shown in  FIG. 24 ; 
         FIG. 24  shows the photoresist etch to form openings for the chamber roof and side walls; 
         FIG. 25  is a perspective of the unit cell shown in  FIG. 24 ; 
         FIG. 26  shows the deposition of the side wall and risk material; 
         FIG. 27  is a perspective of the unit cell shown in  FIG. 26 ; 
         FIG. 28  is the mask associated with the nozzle rim etch shown in  FIG. 29 ; 
         FIG. 29  shows the etch of the roof layer to form the nozzle aperture rim; 
         FIG. 30  is a perspective of the unit cell shown in  FIG. 29 ; 
         FIG. 31  is the mask associated with the nozzle aperture etch shown in  FIG. 32 ; 
         FIG. 32  shows the etch of the roof material to form the elliptical nozzle apertures; 
         FIG. 33  is a perspective of the unit cell shown in  FIG. 32 ; 
         FIG. 34  shows the oxygen plasma release etch of the first and second sacrificial layers; 
         FIG. 35  is a perspective of the unit cell shown in  FIG. 34 ; 
         FIG. 36  shows the unit cell after the release etch, as well as the opposing side of the wafer; 
         FIG. 37  is a perspective of the unit cell shown in  FIG. 36 ; 
         FIG. 38  is the mask associated with the reverse etch shown in  FIG. 39 ; 
         FIG. 39  shows the reverse etch of the ink supply channel into the wafer; 
         FIG. 40  is a perspective of unit cell shown in  FIG. 39 ; 
         FIG. 41  shows the thinning of the wafer by backside etching; 
         FIG. 42  is a perspective of the unit cell shown in  FIG. 41 ; 
         FIG. 43  is a partial perspective of the array of nozzles on the printhead according to the present invention; 
         FIG. 44  shows the plan view of a unit cell; 
         FIG. 45  shows a perspective of the unit cell shown in  FIG. 44 ; 
         FIG. 46  is schematic plan view of two unit cells with the roof layer removed but certain roof layer features shown in outline only; 
         FIG. 47  is schematic plan view of two unit cells with the roof layer removed but the nozzle openings shown in outline only; 
         FIG. 48  is a partial schematic plan view of unit cells with ink inlet apertures in the sidewall of the chambers; 
         FIG. 49  is schematic plan view of a unit cells with the roof layer removed but the nozzle openings shown in outline only; 
         FIG. 50  is a partial plan view of the nozzle plate with stiction reducing formations and a particle of paper dust; 
         FIG. 51  is a partial plan view of the nozzle plate with residual ink gutters; 
         FIG. 52  is a partial section view showing the deposition of SAC1 photoresist in accordance with prior art techniques used to avoid stringers; 
         FIG. 53  is a partial section view showing the deposition of a layer of heater material onto the SAC1 photoresist scaffold deposited in  FIG. 52 ; 
         FIG. 54  is a partial schematic plan view of a unit cell with multiple nozzles and actuators in each of the chambers; 
         FIGS. 55 to 59  are schematic cross sections of the ink chamber shown in  FIG. 44  at sequential stages of drop ejection; 
         FIG. 60  is a schematic perspective of a nozzle with droplet stem anchor as shown in  FIG. 61 ; 
         FIG. 61  is a plan view of nozzle apertures with centrally disposed droplet stem anchors; 
         FIG. 62  is schematic plan view of a unit cell with the roof layer removed showing a simple ‘theta’ heater element; 
         FIG. 63  shows a theta heater element with a sudden reduction in cross section on the cross bar to locate the droplet stem; 
         FIG. 64  shows a theta heater element with a formation in cross section on the cross bar to locate the droplet stem; 
         FIG. 65  shows a dual bar, four kink heater element; 
         FIG. 66  is schematic plan view of a unit cell with a Tesla valve to rectify the ink flow through the chamber inlets; and, 
         FIG. 67  is a schematic perspective of a nozzle with a spur extending into the nozzle aperture for controlled drop misdirection. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the description than follows, corresponding reference numerals relate to corresponding parts. For convenience, the features indicated by each reference numeral are listed below.
       1 . Nozzle Unit Cell     2 . Silicon Wafer     3 . Topmost Aluminium Metal Layer in the CMOS metal layers     4 . Passivation Layer     5 . CVD Oxide Layer     6 . Ink Inlet Opening in Topmost Aluminium Metal Layer  3 .     7 . Pit Opening in Topmost Aluminium Metal Layer  3 .     8 . Pit     9 . Electrodes     10 . SAC1 Photoresist Layer     11 . Heater Material (TIALN)     12 . Thermal Actuator     13 . Photoresist Layer     14 . Ink Inlet Opening Etched Through Photo Resist Layer     15 . Ink Inlet Passage     16 . SAC2 Photoresist Layer     17 . Chamber Side Wall Openings     18 . Front Channel Priming Feature     19 . Barrier Formation at Ink Inlet     20 . Chamber Roof Layer     21 . Roof     22 . Sidewalls     23 . Ink Conduit     24 . Nozzle Chambers     25 . Elliptical Nozzle Rim
         25 ( a ) Inner Lip     25 ( b ) Outer Lip         26 . Nozzle Aperture     27 . Ink Supply Channel     28 . Contacts     29 . Heater Element.     30 . Bubble cage     32 . bubble retention structure     34 . ink permeable structure     36 . bleed hole     38 . ink chamber     40 . dual row filter     42 . paper dust     44 . ink gutters     46 . gap between SAC1 and trench sidewall     48 . trench sidewall     50 . raised lip of SAC1 around edge of trench     52 . thinner inclined section of heater material     54 . cold spot between series connected heater elements     56 . nozzle plate     58 . columnar projections     60 . sidewall ink opening     62 . ink refill opening     64 . ink     66 . bubble     68 . bulging ink meniscus     70 . ink bulb     72 . droplet stem     74 . droplet stem attachment point     76 . nozzle centre-line     78 . drop misdirection     80 . drop     82 . satellite drop     84 . droplet stem anchor     86 . maximum resistance section or ‘hotspot’     88 . shots either side of droplet stem anchor     90 . semi-circular current path     92 . ‘cold spot’     94 . central bar     96 . larger radius curve     98 . tight radius curve     100 . outside edge of tight radius curve     102 . inside edge of tight radius curve     104 . ink refill aperture     106 . rectifying valve (Tesla valve)     108 . main conduit     110 . secondary conduit     112 . lateral spur from nozzle rim
 
MEMS Manufacturing Process
   

     The MEMS manufacturing process builds up nozzle structures on a silicon wafer after the completion of CMOS processing.  FIG. 2  is a cutaway perspective view of a nozzle unit cell  1  after the completion of CMOS processing and before MEMS processing. 
     During CMOS processing of the wafer, four metal layers are deposited onto a silicon wafer  2 , with the metal layers being interspersed between interlayer dielectric (ILD) layers. The four metal layers are referred to as M 1 , M 2 , M 3  and M 4  layers and are built up sequentially on the wafer during CMOS processing. These CMOS layers provide all the drive circuitry and logic for operating the printhead. 
     In the completed printhead, each heater element actuator is connected to the CMOS via a pair of electrodes defined in the outermost M 4  layer. Hence, the M 4  CMOS layer is the foundation for subsequent MEMS processing of the wafer. The M 4  layer also defines bonding pads along a longitudinal edge of each printhead integrated circuit. These bonding pads (not shown) allow the CMOS to be connected to a microprocessor via wire bonds extending from the bonding pads. 
       FIGS. 1 and 2  show the aluminium M 4  layer  3  having a passivation layer  4  deposited thereon. (Only MEMS features of the M 4  layer are shown in these Figures; the main CMOS features of the M 4  layer are positioned outside the nozzle unit cell). The M 4  layer  3  has a thickness of 1 micron and is itself deposited on a 2 micron layer of CVD oxide  5 . As shown in  FIGS. 1 and 2 , the M 4  layer  3  has an ink inlet opening  6  and pit openings  7 . These openings define the positions of the ink inlet and pits formed subsequently in the MEMS process. 
     Before MEMS processing of the unit cell  1  begins, bonding pads along a longitudinal edge of each printhead integrated circuit are defined by etching through the passivation layer  4 . This etch reveals the M 4  layer  3  at the bonding pad positions. The nozzle unit cell  1  is completely masked with photoresist for this step and, hence, is unaffected by the etch. 
     Turning to  FIGS. 3 to 5 , the first stage of MEMS processing etches a pit  8  through the passivation layer  4  and the CVD oxide layer  5 . This etch is defined using a layer of photoresist (not shown) exposed by the dark tone pit mask shown in  FIG. 3 . The pit  8  has a depth of 2 microns, as measured from the top of the M 4  layer  3 . At the same time as etching the pit  8 , electrodes  9  are defined on either side of the pit by partially revealing the M 4  layer  3  through the passivation layer  4 . In the completed nozzle, a heater element is suspended across the pit  8  between the electrodes  9 . 
     In the next step ( FIGS. 6 to 8 ), the pit  8  is filled with a first sacrificial layer (“SAC1”) of photoresist  10 . A 2 micron layer of high viscosity photoresist is first spun onto the wafer and then exposed using the dark tone mask shown in  FIG. 6 . The SAC1 photoresist  10  forms a scaffold for subsequent deposition of the heater material across the electrodes  9  on either side of the pit  8 . Consequently, it is important the SAC1 photoresist  10  has a planar upper surface that is flush with the upper surface of the electrodes  9 . At the same time, the SAC1 photoresist must completely fill the pit  8  to avoid ‘stringers’ of conductive heater material extending across the pit and shorting out the electrodes  9 . 
     Typically, when filling trenches with photoresist, it is necessary to expose the photoresist outside the perimeter of the trench in order to ensure that photoresist fills against the walls of the trench and, therefore, avoid ‘stringers’ in subsequent deposition steps. However, this technique results in a raised (or spiked) rim of photoresist around the perimeter of the trench. This is undesirable because in a subsequent deposition step, material is deposited unevenly onto the raised rim—vertical or angled surfaces on the rim will receive less deposited material than the horizontal planar surface of the photoresist filling the trench. The result is ‘resistance hotspots’ in regions where material is thinly deposited. 
     As shown in  FIG. 7 , the present process deliberately exposes the SAC1 photoresist  10  inside the perimeter walls of the pit  8  (e.g. within 0.5 microns) using the mask shown in  FIG. 6 . This ensures a planar upper surface of the SAC1 photoresist  10  and avoids any spiked regions of photoresist around the perimeter rim of the pit  8 . 
     After exposure of the SAC1 photoresist  10 , the photoresist is reflowed by heating. Reflowing the photoresist allows it to flow to the walls of the pit  8 , filling it exactly.  FIGS. 9 and 10  show the SAC1 photoresist  10  after reflow. The photoresist has a planar upper surface and meets flush with the upper surface of the M 4  layer  3 , which forms the electrodes  9 . Following reflow, the SAC1 photoresist  10  is U.V. cured and/or hardbaked to avoid any reflow during the subsequent deposition step of heater material. 
       FIGS. 11 and 12  show the unit cell after deposition of the 0.5 microns of heater material  11  onto the SAC1 photoresist  10 . Due to the reflow process described above, the heater material  11  is deposited evenly and in a planar layer over the electrodes  9  and the SAC1 photoresist  10 . The heater material may be comprised of any suitable conductive material, such as TiAl, TiN, TiAlN, TiAlSiN etc. A typical heater material deposition process may involve sequential deposition of a 100 Å seed layer of TiAl, a 2500 Å layer of TiAlN, a further 100 Å seed layer of TiAl and finally a further 2500 Å layer of TiAlN. 
     Referring to  FIGS. 13 to 15 , in the next step, the layer of heater material  11  is etched to define the thermal actuator  12 . Each actuator  12  has contacts  28  that establish an electrical connection to respective electrodes  9  on either side of the SAC1 photoresist  10 . A heater element  29  spans between its corresponding contacts  28 . 
     This etch is defined by a layer of photoresist (not shown) exposed using the dark tone mask shown in  FIG. 13 . As shown in  FIG. 15 , the heater element  12  is a linear beam spanning between the pair of electrodes  9 . However, the heater element  12  may alternatively adopt other configurations, such as those described in Applicant&#39;s U.S. Pat. No. 6,755,509, the content of which is herein incorporated by reference. For example, heater element  29  configurations having a central void may be advantageous for minimizing the deleterious effects of cavitation forces on the heater material when a bubble collapses during ink ejection. Other forms of cavitation protection may be adopted such as ‘bubble venting’ and the use of self passivating materials. These cavitation management techniques are discussed in detail in U.S. patent application Ser. No. 11/097,308. 
     In the next sequence of steps, an ink inlet for the nozzle is etched through the passivation layer  4 , the oxide layer  5  and the silicon wafer  2 . During CMOS processing, each of the metal layers had an ink inlet opening (see, for example, opening  6  in the M 4  layer  3  in  FIG. 1 ) etched therethrough in preparation for this ink inlet etch. These metal layers, together with the interspersed ILD layers, form a seal ring for the ink inlet, preventing ink from seeping into the CMOS layers. 
     Referring to  FIGS. 16 to 18 , a relatively thick layer of photoresist  13  is spun onto the wafer and exposed using the dark tone mask shown in  FIG. 16 . The thickness of photoresist  13  required will depend on the selectivity of the deep reactive ion etch (DRIE) used to etch the ink inlet. With an ink inlet opening  14  defined in the photoresist  13 , the wafer is ready for the subsequent etch steps. 
     In the first etch step ( FIGS. 19 and 20 ), the dielectric layers (passivation layer  4  and oxide layer  5 ) are etched through to the silicon wafer below. Any standard oxide etch (e.g. O 2 /C 4 F 8  plasma) may be used. 
     In the second etch step ( FIGS. 21 and 22 ), an ink inlet  15  is etched through the silicon wafer  2  to a depth of 25 microns, using the same photoresist mask  13 . Any standard anisotropic DRIE, such as the Bosch etch (see U.S. Pat. Nos. 6,501,893 and 6,284,148) may be used for this etch. Following etching of the ink inlet  15 , the photoresist layer  13  is removed by plasma ashing. 
     In the next step, the ink inlet  15  is plugged with photoresist and a second sacrificial layer (“SAC2”) of photoresist  16  is built up on top of the SAC1 photoresist  10  and passivation layer  4 . The SAC2 photoresist  16  will serve as a scaffold for subsequent deposition of roof material, which forms a roof and sidewalls for each nozzle chamber. Referring to  FIGS. 23 to 25 , a ˜6 micron layer of high viscosity photoresist is spun onto the wafer and exposed using the dark tone mask shown in  FIG. 23 . 
     As shown in  FIGS. 23 and 25 , the mask exposes sidewall openings  17  in the SAC2 photoresist  16  corresponding to the positions of chamber sidewalls and sidewalls for an ink conduit. In addition, openings  18  and  19  are exposed adjacent the plugged inlet  15  and nozzle chamber entrance respectively. These openings  18  and  19  will be filled with roof material in the subsequent roof deposition step and provide unique advantages in the present nozzle design. Specifically, the openings  18  filled with roof material act as priming features, which assist in drawing ink from the inlet  15  into each nozzle chamber. This is described in greater detail below. The openings  19  filled with roof material act as filter structures and fluidic cross talk barriers. These help prevent air bubbles from entering the nozzle chambers and diffuses pressure pulses generated by the thermal actuator  12 . 
     Referring to  FIGS. 26 and 27 , the next stage deposits 3 microns of roof material  20  onto the SAC2 photoresist  16  by PECVD. The roof material  20  fills the openings  17 ,  18  and  19  in the SAC2 photoresist  16  to form nozzle chambers  24  having a roof  21  and sidewalls  22 . An ink conduit  23  for supplying ink into each nozzle chamber is also formed during deposition of the roof material  20 . In addition, any priming features and filter structures (not shown in  FIGS. 26 and 27 ) are formed at the same time. The roofs  21 , each corresponding to a respective nozzle chamber  24 , span across adjacent nozzle chambers in a row to form a continuous nozzle plate. The roof material  20  may be comprised of any suitable material, such as silicon nitride, silicon oxide, silicon oxynitride, aluminium nitride etc. 
     Referring to  FIGS. 28 to 30 , the next stage defines an elliptical nozzle rim  25  in the roof  21  by etching away 2 microns of roof material  20 . This etch is defined using a layer of photoresist (not shown) exposed by the dark tone rim mask shown in  FIG. 28 . The elliptical rim  25  comprises two coaxial rim lips  25   a  and  25   b , positioned over their respective thermal actuator  12 . 
     Referring to  FIGS. 31 to 33 , the next stage defines an elliptical nozzle aperture  26  in the roof  21  by etching all the way through the remaining roof material  20 , which is bounded by the rim  25 . This etch is defined using a layer of photoresist (not shown) exposed by the dark tone roof mask shown in  FIG. 31 . The elliptical nozzle aperture  26  is positioned over the thermal actuator  12 , as shown in  FIG. 33 . 
     With all the MEMS nozzle features now fully formed, the next stage removes the SAC1 and SAC2 photoresist layers  10  and  16  by O 2  plasma ashing ( FIGS. 34 to 35 ). After ashing, the thermal actuator  12  is suspended in a single plane over the pit  8 . The coplanar deposition of the contacts  28  and the heater element  29  provides an efficient electrical connection with the electrodes  9 . 
       FIGS. 36 and 37  show the entire thickness (150 microns) of the silicon wafer  2  after ashing the SAC1 and SAC2 photoresist layers  10  and  16 . 
     Referring to  FIGS. 38 to 40 , once frontside MEMS processing of the wafer is completed, ink supply channels  27  are etched from the backside of the wafer to meet with the ink inlets  15  using a standard anisotropic DRIE. This backside etch is defined using a layer of photoresist (not shown) exposed by the dark tone mask shown in  FIG. 38 . The ink supply channel  27  makes a fluidic connection between the backside of the wafer and the ink inlets  15 . 
     Finally, and referring to  FIGS. 41 and 42 , the wafer is thinned 135 microns by backside etching.  FIG. 43  shows three adjacent rows of nozzles in a cutaway perspective view of a completed printhead integrated circuit. Each row of nozzles has a respective ink supply channel  27  extending along its length and supplying ink to a plurality of ink inlets  15  in each row. The ink inlets, in turn, supply ink to the ink conduit  23  for each row, with each nozzle chamber receiving ink from a common ink conduit for that row. 
     Features and Advantages of Particular Embodiments 
     Discussed below, under appropriate sub-headings, are certain specific features of embodiments of the invention, and the advantages of these features. The features are to be considered in relation to all of the drawings pertaining to the present invention unless the context specifically excludes certain drawings, and relates to those drawings specifically referred to. 
     Low Loss Electrodes 
     As shown in  FIGS. 41 and 42 , the heater element  29  is suspended within the chamber. This ensures that the heater element is immersed in ink when the chamber is primed. Completely immersing the heater element in ink dramatically improves the printhead efficiency. Much less heat dissipates into the underlying wafer substrate so more of the input energy is used to generate the bubble that ejects the ink. 
     To suspend the heater element, the contacts may be used to support the element at its raised position. Essentially, the contacts at either end of the heater element can have vertical or inclined sections to connect the respective electrodes on the CMOS drive to the element at an elevated position. However, heater material deposited on vertical or inclined surfaces is thinner than on horizontal surfaces. To avoid undesirable resistive losses from the thinner sections, the contact portion of the thermal actuator needs to be relatively large. Larger contacts occupy a significant area of the wafer surface and limit the nozzle packing density. 
     To immerse the heater, the present invention etches a pit or trench 8 between the electrodes  9  to drop the level of the chamber floor. As discussed above, a layer of sacrificial photoresist (SAC)  10  (see  FIG. 9 ) is deposited in the trench to provide a scaffold for the heater element. However, depositing SAC  10  in the trench  8  and simply covering it with a layer of heater material, can lead to stringers forming in the gaps  46  between the SAC  10  and the sidewalls  48  of the trench  8  (as previously described in relation to  FIG. 7 ). The gaps form because it is difficult to precisely match the mask with the sides of the trench  8 . Usually, when the masked photoresist is exposed, the gaps  46  form between the sides of the pit and the SAC. When the heater material layer is deposited, it fills these gaps to form ‘stringers’ (as they are known). The stringers remain in the trench  8  after the metal etch (that shapes the heater element) and the release etch (to finally remove the SAC). The stringers can short circuit the heater so that it fails to generate a bubble. 
     Turning now to  FIGS. 52 and 53 , the ‘traditional’ technique for avoiding stringers is illustrated. By making the UV mask that exposes the SAC slightly bigger than the trench  8 , the SAC  10  will be deposited over the side walls  48  so that no gaps form. Unfortunately, this produces a raised lip  50  around top of the trench. When the heater material layer  11  is deposited (see  FIG. 53 ), it is thinner on the vertical or inclined surfaces  52  of the lip  50 . After the metal etch and release etch, these thin lip formations  52  remain and cause ‘hotspots’ because the localized thinning increases resistance. These hotspots affect the operation of the heater and typically reduce heater life. 
     As discussed above, the Applicant has found that reflowing the SAC  10  closes the gaps  46  so that the scaffold between the electrodes  9  is completely flat. This allows the entire thermal actuator  12  to be planar. The planar structure of the thermal actuator, with contacts directly deposited onto the CMOS electrodes  9  and suspended heater element  29 , avoids hotspots caused by vertical or inclined surfaces so that the contacts can be much smaller structures without acceptable increases in resistive losses. Low resistive losses preserves the efficient operation of a suspended heater element and the small contact size is convenient for close nozzle packing on the printhead. 
     Multiple Nozzles for each Chamber 
     Referring to  FIG. 49 , the unit cell shown has two separate ink chambers  38 , each chamber having heater element  29  extending between respective pairs of contacts  28 . Ink permeable structures  34  are positioned in the ink refill openings so that ink can enter the chambers, but upon actuation, the structures  34  provide enough hydraulic resistance to reduce any reverse flow or fluidic cross talk to an acceptable level. 
     Ink is fed from the reverse side of the wafer through the ink inlet  15 . Priming features  18  extend into the inlet opening so that an ink meniscus does not pin itself to the peripheral edge of the opening and stop the ink flow. Ink from the inlet  15  fills the lateral ink conduit  23  which supplies both chambers  38  of the unit cell. 
     Instead of a single nozzle per chamber, each chamber  38  has two nozzles  25 . When the heater element  29  actuates (forms a bubble), two drops of ink are ejected; one from each nozzle  25 . Each individual drop of ink has less volume than the single drop ejected if the chamber had only one nozzle. By ejecting multiple drops from a single chamber simultaneously improves the print quality. 
     With every nozzle, there is a degree of misdirection in the ejected drop. Depending on the degree of misdirection, this can be detrimental to print quality. By giving the chamber multiple nozzles, each nozzle ejects drops of smaller volume, and having different misdirections. Several small drops misdirected in different directions are less detrimental to print quality than a single relatively large misdirected drop. The Applicant has found that the eye averages the misdirections of each small drop and effectively ‘sees’ a dot from a single drop with a significantly less overall misdirection. 
     A multi nozzle chamber can also eject drops more efficiently than a single nozzle chamber. The heater element  29  is an elongate suspended beam of TiAlN and the bubble it forms is likewise elongated. The pressure pulse created by an elongate bubble will cause ink to eject through a centrally disposed nozzle. However, some of the energy from the pressure pulse is dissipated in hydraulic losses associated with the mismatch between the geometry of the bubble and that of the nozzle. 
     Spacing several nozzles  25  along the length of the heater element  29  reduces the geometric discrepancy between the bubble shape and the nozzle configuration through which the ink ejects. This in turn reduces hydraulic resistance to ink ejection and thereby improves printhead efficiency. 
     Elliptical Nozzle 
     Similarly, the hydraulic resistance to droplet ejection can be reduced by using an elliptical nozzle. As shown in  FIG. 44 , the vapour bubbles generated by the heater elements  29  are elongated. The heater elements are designed to heat uniformly along most of their length so bubble nucleation and growth is likewise substantially uniform along the length. With an elliptical nozzle  25  centred over the heater element  29  such that its major axis is parallel with the centre-line of the element, the geometry of the bubble roughly corresponds to that of the nozzle. Hence the ink pushed along by the pressure pulse is not changing direction sharply and generating high fluidic drag before ejecting through the nozzle. With less power required for droplet ejection, the printhead is more efficient. 
     The elliptical nozzle is also thinner than a circular nozzle of equivalent aperture area. Hence the spacing between adjacent nozzles is reduced. This helps to increase nozzle pitch and therefore improve print resolution. 
     Ink Chamber Re-Filled Via Adjacent Ink Chamber 
     Referring to  FIG. 46 , two opposing unit cells are shown. In this embodiment, unit cell has four ink chambers  38 . The chambers are defined by the sidewalls  22  and the ink permeable structures  34 . Each chamber has its own heater element  29 . The heater elements  29  are arranged in pairs that are connected in series. Between each pair is ‘cold spot’  54  with lower resistance and or greater heat sinking. This ensures that bubbles do not nucleate at the cold spots  54  and thus the cold spots become the common contact between the outer contacts  28  for each heater element pair. 
     The ink permeable structures  34  allow ink to refill the chambers  38  after drop ejection but baffle the pressure pulse from each heater element  29  to reduce the fluidic cross talk between adjacent chambers. It will be appreciated that this embodiment has many parallels with that shown in  FIG. 49  discussed above. However, the present embodiment effectively divides the relatively long chambers of  FIG. 49  into two separate chambers. This further aligns the geometry of the bubble formed by the heater element  29  with the shape of the nozzle  25  to reduce hydraulic losses during drop ejection. This is achieved without reducing the nozzle density but it does add some complexity to the fabrication process. 
     The conduits (ink inlets  15  and supply conduits  23 ) for distributing ink to every ink chamber in the array can occupy a significant proportion of the wafer area. This can be a limiting factor for nozzle density on the printhead. By making some ink chambers part of the ink flow path to other ink chambers, while keeping each chamber sufficiently free of fluidic cross talk, reduces the amount of wafer area lost to ink supply conduits. 
     Ink Chamber with Multiple Actuators and Respective Nozzles 
     Referring to  FIG. 54 , the unit cell shown has two chambers  38 ; each chamber has two heater elements  29  and two nozzles  25 . The effective reduction in drop misdirection by using multiple nozzles per chamber is discussed above in relation to the embodiment shown in  FIG. 49 . The additional benefits of dividing a single elongate chamber into separate chambers, each with their own actuators, is described above with reference to the embodiment shown in  FIG. 46 . The present embodiment uses multiple nozzles and multiple actuators in each chamber to achieve much of the advantages of the  FIG. 46  embodiment with a markedly less complicated design. With a simplified design, the overall dimensions of the unit cell are reduced thereby permitting greater nozzle densities. In the embodiment shown, the footprint of the unit cell is 64 μm long by 16 μm wide. 
     The ink permeable structure  34  is a single column at the ink refill opening to each chamber  38  instead of three spaced columns as with the  FIG. 46  embodiment. The single column has a cross section profiled to be less resistive to refill flow, but more resistive to sudden back flow from the actuation pressure pulse. Both heater elements in each chamber can be deposited simultaneously, together with the contacts  28  and the cold spot feature  54 . Both chambers  38  are supplied with ink from a common ink inlet  15  and supply conduit  23 . These features also allow the footprint to be reduced and they are discussed in more detail below. The priming features  18  have been made integral with one of the chamber sidewalls  22  and a wall ink conduit  23 . The dual purpose nature of these features simplifies the fabrication and helps to keep the design compact. 
     Multiple Chambers and Multiple Nozzles for each Drive Circuit 
     In  FIG. 54 , the actuators are connected in series and therefore fire in unison from the same drive signal to simplify the CMOS drive circuitry. In the  FIG. 46  unit cell, actuators in adjacent nozzles are connected in series within the same drive circuit. Of course, the actuators in adjacent chambers could also be connected in parallel. In contrast, were the actuators in each chamber to be in separate circuits, the CMOS drive circuitry would be more complex and the dimensions of the unit cell footprint would increase. In printhead designs where the drop misdirection is addressed by substituting multiple smaller drops, combining several actuators and their respective nozzles into a common drive circuit is an efficient implementation both in terms of printhead IC fabrication and nozzles density. 
     High Density Thermal Inkjet Printhead 
     Reduction in the unit cell width enables the printhead to have nozzles patterns that previously would have required the nozzle density to be reduced. Of course, a lower nozzle density has a corresponding influence on printhead size and/or print quality. 
     Traditionally, the nozzle rows are arranged in pairs with the actuators for each row extending in opposite directions. The rows are staggered with respect to each other so that the printing resolution (dots per inch) is twice the nozzle pitch (nozzles per inch) along each row. By configuring the components of the unit cell such that the overall width of the unit is reduced, the same number of nozzles can be arranged into a single row instead of two staggered and opposing rows without sacrificing any print resolution (d.p.i.). The embodiments shown in the accompanying figures achieve a nozzle pitch of more than 1000 nozzles per inch in each linear row. At this nozzle pitch, the print resolution of the printhead is better than photographic (1600 dpi) when two opposing staggered rows are considered, and there is sufficient capacity for nozzle redundancy, dead nozzle compensation and so on which ensures the operation life of the printhead remains satisfactory. As discussed above, the embodiment shown in  FIG. 54  has a footprint that is 16 μm wide and therefore the nozzle pitch along one row is about 1600 nozzles per inch. Accordingly, two offset staggered rows yield a resolution of about 3200 d.p.i. 
     With the realisation of the particular benefits associated with a narrower unit cell, the Applicant has focussed on identifying and combining a number of features to reduce the relevant dimensions of structures in the printhead. For example, elliptical nozzles, shifting the ink inlet from the chamber, finer geometry logic and shorter drive FETs (field effect transistors) are features developed by the Applicant to derive some of the embodiments shown. Each contributing feature necessitated a departure from conventional wisdom in the field, such as reducing the FET drive voltage from the widely used traditional 5V to 2.5V in order to decrease transistor length. 
     Reduced Stiction Printhead Surface 
     Static friction, or “stiction” as it has become known, allows dust particles to “stick” to nozzle plates and thereby clog nozzles.  FIG. 50  shows a portion of the nozzle plate  56 . For clarity, the nozzle apertures  26  and the nozzle rims  25  are also shown. The exterior surface of the nozzle plate is patterned with columnar projections  58  extending a short distance from the plate surface. The nozzle plate could also be patterned with other surface formations such as closely spaced ridges, corrugations or bumps. However, it is easy to create a suitable UV mask for the pattern columnar projections shown, and it is a simple matter to etch the columns into the exterior surface. 
     By reducing the co-efficient of static friction, there is less likelihood that paper dust or other contaminants will clog the nozzles in the nozzle plate. Patterning the exterior of the nozzle plate with raised formations limits the surface area that dust particles contact. If the particles can only contact the outer extremities of each formation, the friction between the particles and the nozzle plate is minimal so attachment is much less likely. If the particles do attach, they are more likely to be removed by printhead maintenance cycles. 
     Inlet Priming Feature 
     Referring to  FIG. 47 , two unit cells are shown extending in opposite directions to each other. The ink inlet passage  15  supplies ink to the four chambers  38  via the lateral ink conduit  23 . Distributing ink through micron-scale conduits, such as the ink inlet  15 , to individual MEMS nozzles in an inkjet printhead is complicated by factors that do not arise in macro-scale flow. A meniscus can form and, depending on the geometry of the aperture, it can ‘pin’ itself to the lip of the aperture quite strongly. This can be useful in printheads, such as bleed holes that vent trapped air bubbles but retain the ink, but it can also be problematic if stops ink flow to some chambers. This will most likely occur when initially priming the printhead with ink. If the ink meniscus pins at the ink inlet opening, the chambers supplied by that inlet will stay unprimed. 
     To guard against this, two priming features  18  are formed so that they extend through the plane of the inlet aperture  15 . The priming features  18  are columns extending from the interior of the nozzle plate (not shown) to the periphery of the inlet  15 . A part of each column  18  is within the periphery so that the surface tension of an ink meniscus at the ink inlet will form at the priming features  18  so as to draw the ink out of the inlet. This ‘unpins’ the meniscus from that section of the periphery and the flow toward the ink chambers. 
     The priming features  18  can take many forms, as long as they present a surface that extends transverse to the plane of the aperture. Furthermore, the priming feature can be an integral part of other nozzles features as shown in  FIG. 54 . 
     Side Entry Ink Chamber 
     Referring to  FIG. 48 , several adjacent unit cells are shown. In this embodiment, the elongate heater elements  29  extend parallel to the ink distribution conduit  23 . Accordingly, the elongate ink chambers  38  are likewise aligned with the ink conduit  23 . Sidewall openings  60  connect the chambers  38  to the ink conduit  23 . Configuring the ink chambers so that they have side inlets reduces the ink refill times. The inlets are wider and therefore refill flow rates are higher. The sidewall openings  60  have ink permeable structures  34  to keep fluidic cross talk to an acceptable level. 
     Inlet Filter for Ink Chamber 
     Referring again to  FIG. 47 , the ink refill opening to each chamber  38  has a filter structure  40  to trap air bubbles or other contaminants. Air bubbles and solid contaminants in ink are detrimental to the MEMS nozzle structures. The solid contaminants can obvious clog the nozzle openings, while air bubbles, being highly compressible, can absorb the pressure pulse from the actuator if they get trapped in the ink chamber. This effectively disables the ejection of ink from the affected nozzle. By providing a filter structure  40  in the form of rows of obstructions extending transverse to the flow direction through the opening, each row being spaced such that they are out of registration with the obstructions in an adjacent row with respect to the flow direction, the contaminants are not likely to enter the chamber  38  while the ink refill flow rate is not overly retarded. The rows are offset with respect to each other and the induced turbulence has minimal effect on the nozzle refill rate but the air bubbles or other contaminants follow a relatively tortuous flow path which increases the chance of them being retained by the obstructions  40 . 
     The embodiment shown uses two rows of obstructions  40  in the form of columns extending between the wafer substrate and the nozzle plate. 
     Intercolour Surface Barriers in Multi Colour Inkjet Printhead 
     Turning now to  FIG. 51 , the exterior surface of the nozzle  56  is shown for a unit cell such as that shown in  FIG. 46  described above. The nozzle apertures  26  are positioned directly above the heater elements (not shown) and a series of square-edged ink gutters  44  are formed in the nozzle plate  56  above the ink conduit  23  (see  FIG. 46 ). 
     Inkjet printers often have maintenance stations that cap the printhead when it&#39;s not in use. To remove excess ink from the nozzle plate, the capper can be disengaged so that it peels off the exterior surface of the nozzle plate. This promotes the formation of a meniscus between the capper surface and the exterior of the nozzle plate. Using contact angle hysteresis, which relates to the angle that the surface tension in the meniscus contacts the surface (for more detail, see the Applicant&#39;s co-pending USSN (our docket FND007US) incorporated herein by reference), the majority of ink wetting the exterior of the nozzle plate can be collected and drawn along by the meniscus between the capper and nozzle plate. The ink is conveniently deposited as a large bead at the point where the capper fully disengages from the nozzle plate. Unfortunately, some ink remains on the nozzle plate. If the printhead is a multi-colour printhead, the residual ink left in or around a given nozzle aperture, may be a different colour than that ejected by the nozzle because the meniscus draws ink over the whole surface of the nozzle plate. The contamination of ink in one nozzle by ink from another nozzle can create visible artefacts in the print. Gutter formations  44  running transverse to the direction that the capper is peeled away from the nozzle plate will remove and retain some of the ink in the meniscus. While the gutters do not collect all the ink in the meniscus, they do significantly reduce the level of nozzle contamination of with different coloured ink. 
     Bubble Trap 
     Air bubbles entrained in the ink are very bad for printhead operation. Air, or rather gas in general, is highly compressible and can absorb the pressure pulse from the actuator. If a trapped bubble simply compresses in response to the actuator, ink will not eject from the nozzle. Trapped bubbles can be purged from the printhead with a forced flow of ink, but the purged ink needs blotting and the forced flow could well introduce fresh bubbles. 
     The embodiment shown in  FIG. 46  has a bubble trap at the ink inlet  15 . The trap is formed by a bubble retention structure  32  and a vent  36  formed in the roof layer. The bubble retention structure is a series of columns  32  spaced around the periphery of the inlet  15 . As discussed above, the ink priming features  18  have a dual purpose and conveniently form part of the bubble retaining structure. In use, the ink permeable trap directs gas bubbles to the vent where they vent to atmosphere. By trapping the bubbles at the ink inlets and directing them to a small vent, they are effectively removed from the ink flow without any ink leakage. 
     Multiple Ink Inlet Flow Paths 
     Supplying ink to the nozzles via conduits extending from one side of the wafer to the other allows more of the wafer area (on the ink ejection side) to have nozzles instead of complex ink distribution systems. However, deep etched, micron-scale holes through a wafer are prone to clogging from contaminants or air bubbles. This starves the nozzle(s) supplied by the affected inlet. 
     As best shown in  FIG. 48 , printheads according to the present invention have at least two ink inlets  15  supplying each chamber  38  via an ink conduit  23  between the nozzle plate and underlying wafer. 
     Introducing an ink conduit  23  that supplies several of the chambers  38 , and is in itself supplied by several ink inlets  15 , reduces the chance that nozzles will be starved of ink by inlet clogging. If one inlet  15  is clogged, the ink conduit will draw more ink from the other inlets in the wafer. 
     Droplet Stem Anchors 
     The droplet stem that attaches the ejected ink to the ink in the chamber immediately prior to drop separation, can be a cause of drop misdirection.  FIGS. 55 to 59  show sequential stages of the drop ejection process from a nozzle. In  FIG. 55 , the heater element  29  is rapidly heated and vaporises the ink  64  in immediate contact with its surface to nucleate a bubble  66 . This causes the ink meniscus  68  across the nozzle aperture  26  to start bulging outwardly. 
     In  FIG. 56 , the bubble  66  continues to grow as the heater element  29  vaporises more of the ink  64  in the chamber  38 . This pressure pulse from the growing bubble pushes the ink meniscus further out of the nozzle aperture  26 . In  FIG. 57 , the bubble  66  continues to grow and the ejected ink starts to become a bulb  70  connected to the ink  64  in the chamber  38  by a relatively thick droplet stem  72 . 
     In  FIG. 58 , the bubble has grown to the point where it vents to atmosphere through the nozzle aperture  26 . This is an important mechanism for avoiding cavitation corrosion of the heater element  29 . 
     Cavitation corrosion occurs when a bubble collapses back to a single point on the heater element surface. As the bubble reaches the singularity of a collapse point, the surface tension creates severe hydraulic forces that can abrade the heater material. By venting the bubble, there is no collapse point on the heater element. 
     As shown in  FIG. 58 , when the bubble vents, the droplet stem  72  can attach itself to a point  74  on the nozzle rim. As the attachment point  74  is not on the centre-line  76  of the nozzle aperture  26 , the ink bulb  70  is deflected  78  away from the centre-line because of the surface tension&#39;s tendency to reduce surface area. 
     Referring to  FIG. 59 , the stem  52  eventually breaks and the ink drop  80  forms and continues on its trajectory to the print media. However, the misdirection  78  remains for the ink drop  80  as well as any satellite drops  82 . The vented bubble has become an extended ink meniscus that helps to draw ink back into the chamber as it contracts to the nozzle aperture  26 . 
       FIGS. 60-67  show nozzle designs with droplet stem anchors that positively locate where the droplet stem attaches. Knowing where the stem will attach reduces the misdirection, or in some cases, controls the misdirection so that all nozzles are misdirected in the same direction by roughly the same amount. 
     However, the droplet stem anchors can also perform secondary functions and these will now be discussed below. 
     Combining Ink Ejected from Adjacent Actuators 
     Referring to  FIGS. 60 and 61 , the nozzle design shown has two actuators  29  ejecting ink through a single oval shaped nozzle  25 . The actuators are both heater elements connected in series for simultaneous actuation and ejection. Both actuators  29  are part of a single beam of heater material such as TiAlN which is suspended at its ends and at it mid point. Both heater elements  29  have a tapered section  86  where electrical resistance is at a maximum. During actuation, the vapour bubbles initiate at these maximum resistance sections or ‘hotspots’  86 . 
     The ink covering both heater elements  29  is connected by the slots  88 . The slots can be dimensioned so that they damp fluidic cross talk to the extent that the heater elements are in two separate ink chambers, or they can be large enough to that both elements  29  are considered to be in the same chamber  38 . 
     The heater elements  29  are positioned relative to the droplet stem anchor  84  so that as the ink ejected by each actuator forms a bulb attached by a stem, the ink surface tension, seeking to occupy the least surface area, will attach the stem to the anchor in preference to any other point on the nozzle rim  25 . As the hotspots  86  are on diametrically opposed sides of the anchor  84 , the bulbs of ink attached to respective droplet stems will be misdirected toward each other. Eventually they meet directly above the anchor and the opposing misdirections cancel each other out, or at least, the resultant misdirection is very small. 
     Quadrupolar Actuation 
       FIGS. 62-65  show several embodiments of nozzles with quadrupolar actuation. Quadrupolar actuation initiates the pressure pulse at positions in the ink chamber that are symmetrical about two orthogonal axes. As the pulses converge within the chamber, the symmetry about two axes pushes the ink in a direction that is normal to both axes, at least in the ideal case. In reality, slight asymmetries mean the drop direction may be not be exactly normal, but it will typically be much closer than if the pressure pulse initiated from a single point in the chamber. 
     Referring to  FIG. 62 , the unit cell shows two nozzles  25  in respective chambers  38 , each having a quadrupole thermal actuator  12 . The heater element portion  29  of each actuator  12  is shaped similar to the Greek letter ‘theta’. Each actuator has two semi-circular current paths  90  between the contacts  28 . A central bar  94  extends between the mid points of each current path. The entire theta-shaped structure is suspended in the chamber  38  to minimise heat dissipation into the wafer substrate and maximise heater transfer to the ink. 
     The central bar  94  serves multiple purposes. Firstly, it provides the heater element with structural rigidity and bracing. Without it, the cyclical heating and cooling of the semi-circular current paths would cause some buckling into or out of the page of  FIG. 62 . This could be addressed by supporting the semi-circles on the chamber floor, or even by a single support at each mid-point. However, this increases contact with the underlying wafer substrate and therefore increases heat dissipation. The central bar  94  provides resistance to buckling while keeping the heater element suspended within the chamber. 
     The central bar  94  also provides a ‘cold spot’  92  at the mid-point of each semi-circle. The thermal mass of the bar provides a small heat sink so the junction between the bar and the semi-circular current path heats to bubble nucleation temperature more slowly than the sections either side of the junction. 
     Likewise, the contacts  28  act as heat sinks so bubble nucleation is directed to the middle of the arc between the contact and the junction with the central car  94 . This ensures that the vapour bubbles nucleate at four positions on the theta shape and that these positions have quadrupole symmetry about two orthogonal axes. 
     Finally, the central bar also provides a droplet stem anchor for additional control of misdirection. If the position of the central bar  94  below the nozzle  25  is such that the area of the surface tension is minimised if the droplet stem attaches to the bar instead of a point on the nozzle  25 , then the drop trajectory will be more closely aligned with the central axis extending normal to the nozzle aperture  26 . 
     In  FIG. 63 and 64 , the central bar  94  has a latch point  96  for locating the base of the droplet stem. The latch point is simply a surface irregularity that the surface tension of the ink can ‘pin’ itself to. If the central bar  94  is not parallel to the plane of the nozzle aperture  26 , or there is some asymmetry in the position of the bubble nucleation sites, the droplet stem may latch to an off centre part of the centre bar  94 . A surface irregularity  96  on the central bar  94  tends to snag on the surface tension of the droplet stem and anchor it to the middle of the bar. The surface irregularity  96  can be a sudden reduction in cross section as shown in  FIG. 63 , or a boss such as that shown in  FIG. 64 . In either case, the droplet stem originates from the middle of the central bar  94  and so any misdirection in the drop trajectory is minimised. 
     Dual Bar, Four Kink, Heater Element 
       FIG. 65  shows another quadrupole thermal actuator  12 . Again it has two current paths  90  provided by separate beams extending between the contacts  28 . For clarity, the other features of the unit cell have been omitted. 
     The beams  90  are suspended in the chamber  38  to minimise heat dissipation into the wafer substrate and each beam has two tight radius curves or kinks  98 , between curves of larger radius  96 . In this embodiment, the tight radius kinks  98  act as hotspots where the vapour bubbles nucleate. This is because the current flow around the kinks  98  will concentrate towards the radially inner side of the element  102  and away from the outside radius  100 . This acts like a localised reduction in cross section which increases the resistance at these points. In the large radius curves  96 , the difference in current density between the inside edge and the outside edge is much less so the increase in resistance is small compared to that in the tight kinks  98 . 
     The tight kinks  98  have a relatively low bending resistance so the longitudinal expansion of the beam  90  during actuation is accommodated without buckling inot or out of the plane of the page. This makes the position of the hotspots in the chamber  38  relatively stable thereby maintaining the quadrupole symmetry and minimising drop misdirection. 
     Rectifying Valve at Ink Chamber Inlet 
     The unit cell shown in  FIG. 66  has a rectifying valve  106  at the ink refill aperture  104  to each chamber  38 . The particular rectifying valve shown is known as a Tesla valve. A rectifying valve provides less hydraulic resistance to ink flowing into the chamber  38  than ink flowing out of the chamber. This can be used to reduce fluidic cross talk between chambers  38 , while not retarding ink refill times (and therefore print speeds). 
     For the purposes of this example, the heater element  29  is a simple beam suspended in the chamber  38  between the contacts  28 . Also for clarity, the nozzle rim has been omitted, however the skilled worker will appreciate that it is centrally disposed over the heater element  29 . Alternatively, the chambers  38  could have several nozzles each, as discussed above. 
     The chambers  38  are supplied with ink from the ink inlet  15  via the lateral ink conduit  23 . The Tesla valve  106  at each refill aperture  104  has a main conduit  108  between a pair of smaller secondary conduits  110 . As ink flows into the chamber  38 , there is little resistance to the flow through the main conduit  108  other than fluidic drag against the walls of the conduit itself. The upstream openings of the secondary conduits  110  do not face into the flow so little of the main flow is diverted into them. The downstream openings direct any flow parallel and adjacent to the flow from the main conduit  108  downstream opening. Therefore, the secondary conduits  110  have negligible impact on ink flow into the chamber  38 . 
     Upon actuation, the pressure pulse can create a back flow of ink out of the chamber  38  and back into the lateral ink conduit  23 . Back flow is detrimental to drop ejection as it uses some of the energy from the pressure pulse. The back flow can also create fluidic cross talk that affects the ejection characteristics of adjacent chambers. 
     The Tesla valve  106  resists any back flow by using flow from the secondary conduits  110  to constrict flow through the main conduit  108 . During back flow, the upstream openings of the secondary conduits  110  are facing the flow direction. So to is the upstream opening to the main conduit  108 . The pressure pulse forces ink along the main and secondary conduits however, the downstream openings of the secondary conduits  110  direct their ink flow across and counter to the main flow direction. These conflicting flows create turbulence and a hydraulic constriction in the main conduit  108 . Hence back flow through the main conduit  108  and the secondary conduits  110  is stifled. With a high resistance to back flow, a greater portion of the pressure pulse is used to eject the ink drop through the nozzle and fluidic cross talk is reduced. 
     Controlled Drop Misdirection 
       FIG. 67  is a schematic perspective of a nozzle with controlled drop misdirection. This is a different approach to minimising the drop misdirection as discussed above. By intentionally misdirecting the drops ejected by every nozzle in the array by a controlled amount, the printed image is equivalent to one from a minimised drop misdirection printhead (albeit slightly offset from the nozzle array). 
     As with minimising drop misdirection, this approach uses a droplet stem anchor  74  is positioned so that the droplet stem will attach to it in preference to any other point on the nozzle rim  25  or heater element  29 . However, in nozzle designs that do not allow the drop to form symmetrically around the droplet stem anchor, so the drop trajectory is not normal to the plane of the nozzle aperture, the anchor can be positioned at a point that will cause a known misdirection that is the same magnitude and direction as every other nozzle in the array. 
     The embodiment shown in  FIG. 67  provides a droplet stem anchor at the end of a lateral spur  112  extending into the nozzle aperture  26  from the side of the nozzle rim  25 . This nozzles uses a simple suspended beam heater element  29  which is easier to deposit and etch than a theta heater (described above), but still controls drop misdirection with a droplet stem anchor. It will be appreciated that the spur  112  is an obstruction that deflects the drop from the normal trajectory. However, if all the spurs in the nozzle array are parallel and have the same position relative to the heater element, the misdirection across the whole array will be uniform. 
     Although the invention is described above with reference to specific embodiments, it will be understood by those skilled in the art that the invention may be embodied in many other forms.