Patent Publication Number: US-7901051-B2

Title: Inkjet nozzle assembly having thermoelastic actuator beam disposed on nozzle chamber roof

Description:
CROSS TO RELATED APPLICATION 
     This is a continuation of U.S. application Ser. No. 11/607,979 filed Dec. 4, 2006, now issued U.S. Pat. No. 7,611,225, all of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to thermal bend actuators. It has been developed primarily to provide improved inkjet nozzles which eject ink via thermal bend actuation. 
     CO-PENDING APPLICATIONS 
     The following applications have been filed by the Applicant simultaneously with application Ser. No. 11/607,979: 
     
       
         
           
               
               
               
               
               
             
               
                   
               
             
            
               
                 11/607,976 
                 7,618,124 
                 7,654,641 
                 7,794,056 
                 7,794,055 
               
               
                 11/563,684 
               
               
                   
               
            
           
         
       
     
     The disclosures of these co-pending applications are incorporated herein by reference. 
     CROSS REFERENCES 
     The following patents or patent applications filed by the applicant or assignee of the present invention are hereby incorporated by cross-reference. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
             
            
               
                 6,988,841 
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                 6,464,325 
                 6,435,664 
                 6,412,914 
               
               
                 6,550,896 
                 6,439,695 
                 6,447,100 
                 7,381,340 
                 6,488,359 
               
               
                 6,623,108 
                 6,698,867 
                 6,488,362 
                 6,425,651 
                 6,435,667 
               
               
                 6,527,374 
                 6,582,059 
                 6,513,908 
                 6,540,332 
                 6,679,584 
               
               
                 6,857,724 
                 6,652,052 
                 6,672,706 
                 7,077,508 
                 7,207,654 
               
               
                 6,935,724 
                 6,927,786 
                 6,988,787 
                 6,899,415 
                 6,672,708 
               
               
                 6,644,767 
                 6,874,866 
                 6,830,316 
                 6,994,420 
                 7,086,720 
               
               
                 7,240,992 
                 7,267,424 
                 7,066,578 
                 7,101,023 
                 7,399,063 
               
               
                 7,159,965 
                 7,255,424 
                 7,137,686 
                 7,216,957 
                 7,461,923 
               
               
                 6,916,082 
                 6,786,570 
                 7,407,261 
                 6,848,780 
                 6,966,633 
               
               
                 7,179,395 
                 6,969,153 
                 6,979,075 
                 7,132,056 
                 6,832,828 
               
               
                 6,860,590 
                 6,905,620 
                 6,786,574 
                 6,824,252 
                 6,890,059 
               
               
                 7,246,881 
                 7,125,102 
                 7,028,474 
                 7,066,575 
                 6,986,202 
               
               
                 7,044,584 
                 7,032,992 
                 7,140,720 
                 7,207,656 
                 7,416,275 
               
               
                 7,008,041 
                 7,011,390 
                 7,048,868 
                 7,014,785 
                 7,131,717 
               
               
                 7,331,101 
                 7,182,436 
                 7,104,631 
                 7,556,358 
                 7,172,265 
               
               
                 7,284,837 
                 7,364,270 
                 7,152,949 
                 7,334,877 
                 7,326,357 
               
               
                 7,566,110 
                 7,637,594 
                 7,413,671 
                 7,571,983 
                 7,284,326 
               
               
                 7,284,834 
                 6,932,459 
                 7,032,997 
                 6,998,278 
                 7,004,563 
               
               
                 6,938,994 
                 7,188,935 
                 7,380,339 
                 7,134,740 
                 7,077,588 
               
               
                 6,918,707 
                 6,923,583 
                 6,953,295 
                 6,921,221 
                 7,168,167 
               
               
                 7,337,532 
                 7,322,680 
                 7,192,120 
                 7,168,789 
                 7,207,657 
               
               
                 7,152,944 
                 7,147,303 
                 7,101,020 
                 7,182,431 
                 7,252,367 
               
               
                 7,374,695 
                 6,945,630 
                 6,830,395 
                 6,641,255 
                 7,284,833 
               
               
                 6,666,543 
                 6,669,332 
                 6,663,225 
                 7,073,881 
                 7,155,823 
               
               
                 7,219,427 
                 7,347,952 
                 6,808,253 
                 6,827,428 
                 6,959,982 
               
               
                 6,959,981 
                 6,886,917 
                 6,863,378 
                 7,052,114 
                 7,001,007 
               
               
                 7,008,046 
                 6,880,918 
                 7,066,574 
                 7,156,495 
                 6,976,751 
               
               
                 7,175,775 
                 7,080,893 
                 7,270,492 
                 7,055,934 
                 7,367,729 
               
               
                 7,419,250 
                 7,083,263 
                 7,226,147 
                 7,195,339 
                 7,524,032 
               
               
                 7,350,901 
                 7,067,067 
                 6,776,476 
                 6,880,914 
                 7,086,709 
               
               
                 6,783,217 
                 7,147,791 
                 6,929,352 
                 6,824,251 
                 6,834,939 
               
               
                 6,840,600 
                 6,786,573 
                 7,144,519 
                 6,799,835 
                 6,938,991 
               
               
                 7,226,145 
                 7,140,719 
                 6,988,788 
                 7,022,250 
                 6,929,350 
               
               
                 7,004,566 
                 7,055,933 
                 7,144,098 
                 7,189,334 
                 7,431,429 
               
               
                 7,147,305 
                 7,325,904 
                 7,152,960 
                 7,441,867 
                 7,470,003 
               
               
                 7,401,895 
                 7,270,399 
                 6,866,369 
                 6,886,918 
                 7,204,582 
               
               
                 6,921,150 
                 6,913,347 
                 7,284,836 
                 7,093,928 
                 7,290,856 
               
               
                 7,086,721 
                 7,159,968 
                 7,147,307 
                 7,111,925 
                 7,229,154 
               
               
                 7,341,672 
                 7,278,711 
               
               
                   
               
            
           
         
       
     
     BACKGROUND OF THE INVENTION 
     The present Applicant has described previously a plethora of MEMS inkjet nozzles using thermal bend actuation. Thermal bend actuation generally means bend movement generated by thermal expansion of one material, having a current passing therethough, relative to another material. The resulting bend movement may be used to eject ink from a nozzle opening, optionally via movement of a paddle or vane, which creates a pressure wave in a nozzle chamber. 
     Some representative types of thermal bend inkjet nozzles are exemplified in the patents and patent applications listed in the cross reference section above, the contents of which are incorporated herein by reference. 
     The Applicant&#39;s U.S. Pat. No. 6,416,167 describes an inkjet nozzle having a paddle positioned in a nozzle chamber and a thermal bend actuator positioned externally of the nozzle chamber. The actuator takes the form of a lower active beam of conductive material (e.g. titanium nitride) fused to an upper passive beam of non-conductive material (e.g. silicon dioxide). The actuator is connected to the paddle via an arm received through a slot in the wall of the nozzle chamber. Upon passing a current through the lower active beam, the actuator bends upwards and, consequently, the paddle moves towards a nozzle opening defined in a roof of the nozzle chamber, thereby ejecting a droplet of ink. An advantage of this design is its simplicity of construction. A drawback of this design is that both faces of the paddle work against the relatively viscous ink inside the nozzle chamber. 
     The Applicant&#39;s U.S. Pat. No. 6,260,953 (assigned to the present Applicant) describes an inkjet nozzle in which the actuator forms a moving roof portion of the nozzle chamber. The actuator is takes the form of a serpentine core of conductive material encased by a polymeric material. Upon actuation, the actuator bends towards a floor of the nozzle chamber, increasing the pressure within the chamber and forcing a droplet of ink from a nozzle opening defined in the roof of the chamber. The nozzle opening is defined in a non-moving portion of the roof. An advantage of this design is that only one face of the moving roof portion has to work against the relatively viscous ink inside the nozzle chamber. A drawback of this design is that construction of the actuator from a serpentine conductive element encased by polymeric material is difficult to achieve in a MEMS process. 
     The Applicant&#39;s U.S. Pat. No. 6,623,101 describes an inkjet nozzle comprising a nozzle chamber with a moveable roof portion having a nozzle opening defined therein. The moveable roof portion is connected via an arm to a thermal bend actuator positioned externally of the nozzle chamber. The actuator takes the form of an upper active beam spaced apart from a lower passive beam. By spacing the active and passive beams apart, thermal bend efficiency is maximized since the passive beam cannot act as heat sink for the active beam. Upon passing a current through the active upper beam, the moveable roof portion, having the nozzle opening defined therein, is caused to rotate towards a floor of the nozzle chamber, thereby ejecting through the nozzle opening. Since the nozzle opening moves with the roof portion, drop flight direction may be controlled by suitable modification of the shape of the nozzle rim. An advantage of this design is that only one face of the moving roof portion has to work against the relatively viscous ink inside the nozzle chamber. A further advantage is the minimal thermal losses achieved by spacing apart the active and passive beam members. A drawback of this design is the loss of structural rigidity in spacing apart the active and passive beam members. 
     There is a need to improve upon the design of thermal bend inkjet nozzles, so as to achieve more efficient drop ejection and improved mechanical robustness. 
     SUMMARY OF THE INVENTION 
     In a first aspect the present invention provides an inkjet nozzle assembly comprising:
         a nozzle chamber comprising a floor and a roof, said roof having a nozzle opening defined therein, said roof having a moving portion moveable towards the floor; and   a thermal bend actuator, having a plurality of cantilever beams, for ejecting ink through the nozzle opening, said actuator comprising:   a first active beam for connection to drive circuitry; and   a second passive beam mechanically cooperating with the first beam, such that when a current is passed through the first beam, the first beam expands relative to the second beam, resulting in bending of the actuator,
 
wherein the first active beam defines at least part of an exterior surface of said roof.
       

     Optionally, said moving portion comprises the actuator. 
     Optionally, the first active beam defines at least 30% of a total area of the roof. 
     Optionally, the nozzle opening is defined in the moving portion, such that the nozzle opening is moveable relative to the floor. 
     Optionally, the actuator is moveable relative to the nozzle opening. 
     Optionally, the first beam is defined by a tortuous beam element, said tortuous beam element having a plurality of contiguous beam members. 
     Optionally, the tortuous beam element comprises a plurality of longer beam members and at least one shorter beam member, each longer beam member extending along a longitudinal axis of the first beam and being interconnected by a shorter beam member extending across a transverse axis of the first beam. 
     Optionally, one of said plurality of beams is comprised of a porous material. 
     Optionally, said porous material is porous silicon dioxide having a dielectric constant of 2 or less. 
     Optionally, the thermal bend actuator further comprises a third insulation beam sandwiched between the first beam and the second beam. 
     Optionally, the third insulation beam is comprised of a porous material. 
     Optionally, the first beam is fused or bonded to the second beam. 
     Optionally, the second beam is comprised of a porous material. 
     Optionally, at least part of the first beam is spaced apart from the second beam. 
     Optionally, the first beam is comprised of a material selected from the group comprising: titanium nitride, titanium aluminium nitride and an aluminium alloy. 
     Optionally, the first beam is comprised of an aluminium alloy. 
     Optionally, said aluminium alloy comprises aluminium and at least one other metal having a Young&#39;s modulus of more than 100 GPa. 
     Optionally, said at least one metal is selected from the group comprising: vanadium, manganese, chromium, cobalt and nickel. 
     Optionally, said alloy comprises aluminum and vanadium. 
     Optionally, said alloy comprises at least 80% aluminium. 
     In a second aspect the present invention provides a thermal bend actuator, having a plurality of elements, comprising:
         a first active element for connection to drive circuitry; and   a second passive element mechanically cooperating with the first element, such that when a current is passed through the first element, the first element expands relative to the second element, resulting in bending of the actuator,
 
wherein the first element is comprised of an aluminium alloy.
       

     Optionally, said aluminium alloy comprises aluminium and at least one other metal having a Young&#39;s modulus of more than 100 GPa. 
     Optionally, said at least one metal is selected from the group comprising: vanadium, manganese, chromium, cobalt and nickel. 
     Optionally, said alloy comprises aluminum and vanadium. 
     Optionally, said alloy comprises at least 80% aluminium. 
     Optionally, said first and second elements are cantilever beams. 
     Optionally, the first beam is fused or bonded to the second beam along a longitudinal axis thereof. 
     Optionally, at least part of the second beam is spaced apart from the first beam, thereby insulating the first beam from at least part of the second beam. 
     Optionally, one of said plurality of elements is comprised of a porous material. 
     Optionally, said porous material has a dielectric constant of about 2 or less. 
     Optionally, said porous material is porous silicon dioxide. 
     Optionally, a third insulation beam is sandwiched between the first beam and the second beam. 
     Optionally, the third insulation beam is comprised of a porous material. 
     Optionally, the second beam is comprised of a porous material. 
     In a further aspect the present invention provides an inkjet nozzle assembly comprising:
         a nozzle chamber having a nozzle opening and an ink inlet; and   a thermal bend actuator, having a plurality of cantilever beams, for ejecting ink through the nozzle opening, said actuator comprising:   a first active beam for connection to drive circuitry; and   a second passive beam mechanically cooperating with the first beam, such that when a current is passed through the first beam, the first beam expands relative to the second beam, resulting in bending of the actuator,
 
wherein the first beam is comprised of an aluminium alloy.
       

     Optionally, the nozzle chamber comprises a floor and a roof having a moving portion, whereby actuation of said actuator moves said moving portion towards said floor. 
     Optionally, the moving portion comprises the actuator. 
     Optionally, the first active beam defines at least 30% of a total area of the roof. 
     Optionally, the first active beam defines at least part of an exterior surface of said nozzle chamber. 
     Optionally, the nozzle opening is defined in the moving portion, such that the nozzle opening is moveable relative to the floor. 
     In a third aspect the present invention provides a thermal bend actuator, having a plurality of elements, comprising:
         a first active element for connection to drive circuitry; and   a second passive element mechanically cooperating with the first element, such that when a current is passed through the first element, the first element expands relative to the second element, resulting in bending of the actuator,
 
wherein one of said plurality of elements is comprised of a porous material.
       

     Optionally, said porous material has a dielectric constant of about 2 or less. 
     Optionally, said porous material is porous silicon dioxide. 
     Optionally, said first and second elements are cantilever beams. 
     In a further aspect there is provides a thermal bend actuator further comprising a third insulation beam sandwiched between the first beam and the second beam. 
     Optionally, the third insulation beam is comprised of a porous material. 
     Optionally, the first beam is fused or bonded to the second beam along a longitudinal axis thereof. 
     Optionally, the second beam is comprised of a porous material. 
     Optionally, the first element is comprised of a material selected from the group comprising: titanium nitride, titanium aluminium nitride and an aluminium alloy. 
     Optionally, the first element is comprised of an aluminium alloy. 
     Optionally, said aluminium alloy comprises aluminium and at least one other metal having a Young&#39;s modulus of more than 100 GPa. 
     Optionally, said at least one metal is selected from the group comprising: vanadium, manganese, chromium, cobalt and nickel. 
     Optionally, said alloy comprises aluminum and vanadium. 
     Optionally, said alloy comprises at least 80% aluminium. 
     In another aspect the present invention provides an inkjet nozzle assembly comprising:
         a nozzle chamber having a nozzle opening and an ink inlet; and   a thermal bend actuator, having a plurality of cantilever beams, for ejecting ink through the nozzle opening, said actuator comprising:   a first active beam for connection to drive circuitry; and   a second passive beam mechanically cooperating with the first beam, such that when a current is passed through the first beam, the first beam expands relative to the second beam, resulting in bending of the actuator,
 
wherein one of said plurality of beams is comprised of a porous material.
       

     Optionally, the nozzle chamber comprises a floor and a roof having a moving portion, whereby actuation of said actuator moves said moving portion towards said floor. 
     Optionally, the moving portion comprises the actuator. 
     Optionally, the first active beam defines at least 30% of a total area of the roof. 
     Optionally, the first active beam defines at least part of an exterior surface of said nozzle chamber. 
     Optionally, the nozzle opening is defined in the moving portion, such that the nozzle opening is moveable relative to the floor. 
     In a fourth aspect the present invention provides an inkjet nozzle assembly comprising:
         a nozzle chamber comprising a floor and a roof, said roof having a nozzle opening defined therein, said roof having a moving portion moveable towards the floor; and   a thermal bend actuator, having a plurality of cantilever beams, for ejecting ink through the nozzle opening, said actuator comprising:   a first active beam for connection to drive circuitry; and   a second passive beam mechanically cooperating with the first beam, such that when a current is passed through the first beam, the first beam expands relative to the second beam, resulting in bending of the actuator,
 
wherein said moving portion comprises the actuator.
       

     Optionally, the first active beam defines at least 30% of a total area of the roof. 
     Optionally, the first active beam defines at least part of an exterior surface of said roof. 
     Optionally, the nozzle opening is defined in the moving portion, such that the nozzle opening is moveable relative to the floor portion. 
     Optionally, the actuator is moveable relative to the nozzle opening. 
     Optionally, the first beam is defined by a tortuous beam element, said tortuous beam element having a plurality of contiguous beam members. 
     Optionally, the plurality of contiguous beam members comprises a plurality of longer beam members extending along a longitudinal axis of the first beam, and at least one shorter beam member extending across a transverse axis of the first beam and interconnecting longer beam members. 
     Optionally, one of said plurality of beams is comprised of a porous material. 
     Optionally, said porous material is porous silicon dioxide having a dielectric constant of 2 or less. 
     Optionally, the thermal bend actuator further comprises a third insulation beam sandwiched between the first beam and the second beam. 
     Optionally, the third insulation beam is comprised of a porous material. 
     Optionally, the first beam is fused or bonded to the second beam. 
     Optionally, the second beam is comprised of a porous material. 
     Optionally, at least part of the first beam is spaced apart from the second beam. 
     Optionally, the first beam is comprised of a material selected from the group comprising: titanium nitride, titanium aluminium nitride and an aluminium alloy. 
     Optionally, the first beam is comprised of an aluminium alloy. 
     Optionally, said aluminium alloy comprises aluminium and at least one other metal having a Young&#39;s modulus of more than 100 GPa. 
     Optionally, said at least one metal is selected from the group comprising: vanadium, manganese, chromium, cobalt and nickel. 
     Optionally, said alloy comprises aluminum and vanadium. 
     Optionally, said alloy comprises at least 80% aluminium. 
     In a fifth aspect the present invention provides an inkjet nozzle assembly comprising:
         a nozzle chamber comprising a floor and a roof, said roof having a nozzle opening defined therein, said roof having a moving portion moveable towards the floor; and   a thermal bend actuator, having a plurality of cantilever beams, for ejecting ink through the nozzle opening, said actuator comprising:   a first active beam for connection to drive circuitry; and   a second passive beam mechanically cooperating with the first beam, such that when a current is passed through the first beam, the first beam expands relative to the second beam, resulting in bending of the actuator,       

     Optionally, the first active beam defines at least 30% of a total area of the roof. 
     Optionally, said moving portion comprises the actuator. 
     Optionally, the first active beam defines at least part of an exterior surface of said roof. 
     Optionally, the nozzle opening is defined in the moving portion, such that the nozzle opening is moveable relative to the floor. 
     Optionally, the actuator is moveable relative to the nozzle opening. 
     Optionally, the first beam is defined by a tortuous beam element, said tortuous beam element having a plurality of contiguous beam members. 
     Optionally, the plurality of contiguous beam members comprises a plurality of longer beam members extending along a longitudinal axis of the first beam, and at least one shorter beam member extending across a transverse axis of the first beam and interconnecting longer beam members. 
     Optionally, one of said plurality of beams is comprised of a porous material. 
     Optionally, said porous material is porous silicon dioxide having a dielectric constant of 2 or less. 
     Optionally, the thermal bend actuator further comprises a third insulation beam sandwiched between the first beam and the second beam. 
     Optionally, the third insulation beam is comprised of a porous material. 
     Optionally, the first beam is fused or bonded to the second beam. 
     Optionally, the second beam is comprised of a porous material. 
     Optionally, at least part of the first beam is spaced apart from the second beam. 
     Optionally, the first beam is comprised of a material selected from the group comprising: titanium nitride, titanium aluminium nitride and an aluminium alloy. 
     Optionally, the first beam is comprised of an aluminium alloy. 
     Optionally, said aluminium alloy comprises aluminium and at least one other metal having a Young&#39;s modulus of more than 100 GPa. 
     Optionally, said at least one metal is selected from the group comprising: vanadium, manganese, chromium, cobalt and nickel. 
     Optionally, said alloy comprises aluminum and vanadium. 
     Optionally, said alloy comprises at least 80% aluminium. 
     In a sixth aspect the present invention provides a thermal bend actuator, having a plurality of elongate cantilever beams, comprising:
         a first active beam for connection to drive circuitry, said first beam being defined by a tortuous beam element, said tortuous beam element having a plurality of contiguous beam members; and   a second passive beam mechanically cooperating with the first beam, such that when a current is passed through the first beam, the first beam expands relative to the second beam, resulting in bending of the actuator,
 
wherein the plurality of contiguous beam members comprises a plurality of longer beam members extending along a longitudinal axis of the first beam, and at least one shorter beam member extending across a transverse axis of the first beam and interconnecting longer beam members.
       

     Optionally, said first beam is connected to said drive circuitry via a pair of electrical contacts positioned at one end of said actuator. 
     Optionally, a first electrical contact is connected to a first end of said tortuous beam element and a second electrical contact is connected to a second end of said tortuous beam element. 
     Optionally, one of said plurality of beams is comprised of a porous material. 
     Optionally, said porous material is porous silicon dioxide having a dielectric constant of 2 or less. 
     In a further aspect there is provided a thermal bend actuator further comprising a third insulation beam sandwiched between the first beam and the second beam. 
     Optionally, the third insulation beam is comprised of a porous material. 
     Optionally, the first beam is fused or bonded to the second beam. 
     Optionally, the second beam is comprised of a porous material. 
     Optionally, at least part of the first beam is spaced apart from the second beam. 
     Optionally, the first beam is comprised of a material selected from the group comprising: titanium nitride, titanium aluminium nitride and an aluminium alloy. 
     In a further aspect the present invention provides an inkjet nozzle assembly comprising:
         a nozzle chamber having a nozzle opening and an ink inlet; and   a thermal bend actuator, having a plurality of cantilever beams, for ejecting ink through the nozzle opening, said actuator comprising:   a first active beam for connection to drive circuitry, said first beam being defined by a tortuous beam element, said tortuous beam element comprising a plurality of contiguous beam members; and   a second passive beam mechanically cooperating with the first beam, such that when a current is passed through the first beam, the first element expands relative to the second beam, resulting in bending of the actuator,
 
wherein the plurality of contiguous beam members comprises a plurality of longer beam members extending along a longitudinal axis of the first beam, and at least one shorter beam member extending across a transverse axis of the first beam and interconnecting longer beam members.
       

     Optionally, the nozzle chamber comprises a floor and a roof having a moving portion, whereby actuation of said actuator moves said moving portion towards said floor. 
     Optionally, the moving portion comprises the actuator. 
     Optionally, the first active beam defines at least 30% of a total area of the roof. 
     Optionally, the first active beam defines at least part of an exterior surface of said nozzle chamber. 
     Optionally, the nozzle opening is defined in the moving portion, such that the nozzle opening is moveable relative to the floor. 
     Optionally, the actuator is moveable relative to the nozzle opening. 
     In a further aspect there is provided an inkjet nozzle assembly further comprising a pair of electrical contacts positioned at one end of said actuator, said electrical contacts providing electrical connection between said tortuous beam element and said drive circuitry. 
     Optionally, a first electrical contact is connected to a first end of said tortuous beam element and a second electrical contact is connected to a second end of said tortuous beam element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of a bi-layered thermal bend actuator comprising an active beam formed from aluminium-vanadium alloy; 
       FIGS.  2 (A)-(C) are schematic side sectional views of an inkjet nozzle assembly comprising a fused thermal bend actuator at various stages of operation; 
         FIG. 3  is a perspective view of the nozzle assembly shown in  FIG. 2(A) ; 
         FIG. 4  is a perspective view of part of a printhead integrated circuit comprising an array of nozzle assemblies, as shown in  FIGS. 2(A) and 3 ; 
         FIG. 5  is a cutaway perspective view of an inkjet nozzle assembly comprising a spaced apart thermal bend actuator and moving roof structure; 
         FIG. 6  is a cutaway perspective view of the inkjet nozzle assembly shown in  FIG. 5  in an actuated configuration; 
         FIG. 7  is a cutaway perspective view of the inkjet nozzle assembly shown in  FIG. 5  immediately after de-actuation; 
         FIG. 8  is a side sectional view of the nozzle assembly shown in  FIG. 6 ; 
         FIG. 9  is a side sectional view of an inkjet nozzle assembly comprising a roof having a moving portion defined by a thermal bend actuator; 
         FIG. 10  is a cutaway perspective view of the nozzle assembly shown in  FIG. 9 ; 
         FIG. 11  is a perspective view of the nozzle assembly shown in  FIG. 10 ; 
         FIG. 12  is a cutaway perspective view of an array of the nozzle assemblies shown in  FIG. 10 ; 
         FIG. 13  is a side sectional view of an alternative inkjet nozzle assembly comprising a roof having a moving portion defined by a thermal bend actuator; 
         FIG. 14  is a cutaway perspective view of the nozzle assembly shown in  FIG. 13 ; 
         FIG. 15  is a perspective view of the nozzle assembly shown in  FIG. 13 ; 
         FIG. 16  is a schematic side view of a tri-layered thermal bend actuator comprising a sandwiched insulating beam formed of porous material; and 
         FIG. 17  is a schematic side view of a bi-layered thermal bend actuator comprising a passive beam formed of porous material. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Thermoelastic Active Element Comprised of Aluminium Alloy 
     Typically, a MEMS thermal bend actuator (or thermoelastic actuator) comprises a pair of elements in the form of an active element and a passive element, which constrains linear expansion of the active element. The active element is required to undergo greater thermoelastic expansion relative to the passive element, thereby providing a bending motion. The elements may be fused or bonded together for maximum structural integrity or spaced apart for minimizing thermal losses to the passive element. 
     Hitherto, we described titanium nitride as being a suitable candidate for an active thermoelastic element in a thermal bend actuator (see, for example, U.S. Pat. No. 6,416,167). Other suitable materials described in, for example, Applicant&#39;s U.S. Pat. No. 6,428,133 are TiB 2 , MoSi 2  and TiAlN. 
     In terms of its high thermal expansion and low density, aluminium is strong candidate for use as an active thermoelastic element. However, aluminum suffers from a relatively low Young&#39;s modulus, which detracts from its overall thermoelastic efficiency. Accordingly, aluminium had previously been disregarded as a suitable material for use an active thermoelastic element. 
     However, it has now been found that aluminium alloys are excellent materials for use as thermoelastic active elements, since they combine the advantageous properties of high thermal expansion, low density and high Young&#39;s modulus. 
     Typically, aluminium is alloyed with at least one metal having a Young&#39;s modulus of &gt;100 GPa. Typically, aluminium is alloyed with at least one metal selected from the group comprising: vanadium, manganese, chromium, cobalt and nickel. Surprisingly, it has been found that the excellent thermal expansion properties of aluminium are not compromised when alloyed with such metals. 
     Optionally, the alloy comprises at least 60%, optionally at least 70%, optionally at least 80% or optionally at least 90% aluminium. 
       FIG. 1  shows a bimorph thermal bend actuator  200  in the form of a cantilever beam  201  fixed to a post  202 . The cantilever beam  201  comprises a lower active beam  210  bonded to an upper passive beam  220  of silicon dioxide. The thermoelastic efficiencies of the actuator  200  were compared for active beams comprised of: (i) 100% Al; (ii) 95% Al/5% V; and (iii) 90% Al/10% V. 
     Thermoelastic efficiencies were compared by stimulating the active beam  210  with a short electrical pulse and measuring the energy required to establish a peak oscillatory velocity of 3 m/s, as determined by a laser interferometer. The results are shown in the Table below: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                   
                 Energy 
               
               
                   
                   
                 Required 
               
               
                   
                   
                 to Reach Peak 
               
               
                   
                 Active Beam 
                 Oscillatory 
               
               
                   
                 Material 
                 Velocity 
               
               
                   
                   
               
             
            
               
                   
                 100% Al 
                 466 nJ 
               
               
                   
                 95% Al/5% V 
                 224 nJ 
               
               
                   
                 90% Al/10% V 
                 219 nJ 
               
               
                   
                   
               
            
           
         
       
     
     Thus, the 95% Al/5% V alloy required 2.08 times less energy than the comparable 100% Al device. Further, the 90% Al/10% V alloy required 2.12 times less energy than the comparable 100% Al device. It was therefore concluded that aluminium alloys are excellent candidates for use as active thermoelastic elements in a range of MEMS applications, including thermal bend actuators for inkjet nozzles. 
     Inkjet Nozzles Comprising a Thermal Bend Actuator 
     There now follows a description of typical inkjet nozzles, which may incorporate a thermal bend actuator having an active element comprised of aluminium alloy. 
     Nozzle Assembly Comprising Fused Thermal Bend Actuator 
     Turning initially to  FIGS. 2(A) and 3 , there are shown schematic illustrations of a nozzle assembly  100  according to a first embodiment. The nozzle assembly  100  is formed by MEMS processes on a passivation layer  2  of a silicon substrate  3 , as described in U.S. Pat. No. 6,416,167. The nozzle assembly  100  comprises a nozzle chamber  1  having a roof  4  and sidewall  5 . The nozzle chamber  1  is filled with ink  6  by means of an ink inlet channel  7  etched through the substrate  3 . The nozzle chamber  1  further includes a nozzle opening  8  for ejection of ink from the nozzle chamber. An ink meniscus  20  is pinned across a rim  21  of the nozzle opening  8 , as shown in  FIG. 2(A) . 
     The nozzle assembly  100  further comprises a paddle  9 , positioned inside the nozzle chamber  1 , which is interconnected via an arm  11  to an actuator  10  positioned externally of the nozzle chamber. As shown more clearly in  FIG. 2 , the arm extends through a slot  12  in nozzle chamber  1 . Surface tension of ink within the slot  12  is sufficient to provide a fluidic seal for ink contained in the nozzle chamber  1 . 
     The actuator  10  comprises a plurality of elongate actuator units  13 , which are spaced apart transversely. Each actuator unit extends between a fixed post  14 , which is mounted on the passivation layer  2 , and the arm  11 . Hence, the post  14  provides a pivot for the bending motion of the actuator  10 . 
     Each actuator unit  13  comprises a first active beam  15  and a second passive beam  16  fused to an upper face of the active beam. The active beam  15  is conductive and connected to drive circuitry in a CMOS layer of the substrate  3 . The passive beam  16  is typically non-conductive. 
     Referring now to  FIG. 2(B) , when current flows through the active beam  15 , it is heated and undergoes thermal expansion relative to the passive beam  16 . This causes upward bending movement of the actuator  10 , which is magnified into a rotational movement of the paddle  9 . 
     This consequential paddle movement causes a general increase in pressure around the ink meniscus  20  which expands, as illustrated in  FIG. 1(B) , in a rapid manner. Subsequently the actuator is deactivated, which causes the paddle  9  to return to its quiescent position ( FIG. 2(C) ). 
     During this pulsing cycle, a droplet of ink  17  is ejected from the nozzle opening  8  and at the same time ink  6  reflows into the nozzle chamber  1  via the ink inlet  7 . The forward momentum of the ink outside the nozzle rim  21  and the corresponding backflow results in a general necking and breaking off of the droplet  17  which proceeds towards a print medium, as shown in  FIG. 2(C) . The collapsed meniscus  20  causes ink  6  to be sucked into the nozzle chamber  1  via the ink inlet  7 . The nozzle chamber  1  is refilled such that the position in  FIG. 2(A)  is again reached and the nozzle assembly  100  is ready for the ejection of another droplet of ink. 
     Turning to  FIG. 3 , it will be seen that the actuator units  13  are tapered with respect to their transverse axes, having a narrower end connected to the post  14  and a wider end connected to the arm  11 . This tapering ensures that maximum resistive heating takes place near the post  14 , thereby maximizing the thermoelastic bending motion. 
     Typically, the passive beam  16  is comprised of silicon dioxide or TEOS deposited by CVD. As shown in the  FIGS. 2 to 4 , the arm  11  is formed from the same material. 
     In the present invention, the active beam  15  is comprised of an aluminum alloy, preferably an aluminum-vanadium alloy as described above. 
     Nozzle Assembly Comprising Spaced Apart Thermal Bend Actuator 
     Turning now to  FIGS. 5 to 8 , there is shown a nozzle assembly  300 , in accordance with a second embodiment. Referring to  FIGS. 5 to 7  of the accompanying drawings, the nozzle assembly  300  is constructed (by way of MEMS technology) on a substrate  301  defining an ink supply aperture  302  opening through a hexagonal inlet  303  (which could be of any other suitable configuration) into a chamber  304 . The chamber is defined by a floor portion  305 , roof portion  306  and peripheral sidewalls  307  and  308  which overlap in a telescopic manner. The sidewalls  307 , depending downwardly from roof portion  306 , are sized to be able to move upwardly and downwardly within sidewalls  308  which depend upwardly from floor portion  305 . 
     The ejection nozzle is formed by rim  309  located in the roof portion  306  so as to define an opening for the ejection of ink from the nozzle chamber as will be described further below. 
     The roof portion  306  and downwardly depending sidewalls  307  are supported by a bend actuator  310  typically made up of layers forming a Joule heated cantilever which is constrained by a non-heated cantilever, so that heating of the Joule heated cantilever causes a differential expansion between the Joule heated cantilever and the non-heated cantilever causing the bend actuator  310  to bend. 
     The proximal end  311  of the bend actuator is fastened to the substrate  301 , and prevented from moving backwards by an anchor member  312  which will be described further below, and the distal end  313  is secured to, and supports, the roof portion  306  and sidewalls  307  of the ink jet nozzle. 
     In use, ink is supplied into the nozzle chamber through passage  302  and opening  303  in any suitable manner, but typically as described in our previously referenced co-pending patent applications. When it is desired to eject a drop of ink from the nozzle chamber, an electric current is supplied to the bend actuator  310  causing the actuator to bend to the position shown in  FIG. 6  and move the roof portion  306  downwardly toward the floor portion  305 . This relative movement decreases the volume of the nozzle chamber, causing ink to bulge upwardly through the nozzle rim  309  as shown at  314  ( FIG. 6 ) where it is formed to a droplet by the surface tension in the ink. 
     As the electric current is withdrawn from the bend actuator  310 , the actuator reverts to the straight configuration as shown in  FIG. 7  moving the roof portion  306  of the nozzle chamber upwardly to the original location. The momentum of the partially formed ink droplet  314  causes the droplet to continue to move upwardly forming an ink drop  315  as shown in  FIG. 7  which is projected on to the adjacent paper surface or other article to be printed. 
     In one form of the invention, the opening  303  in floor portion  305  is relatively large compared with the cross-section of the nozzle chamber and the ink droplet is caused to be ejected through the nozzle rim  309  upon downward movement of the roof portion  306  by viscous drag in the sidewalls of the aperture  302 , and in the supply conduits leading from the ink reservoir (not shown) to the opening  302 . 
     In order to prevent ink leaking from the nozzle chamber during actuation ie. during bending of the bend actuator  310 , a fluidic seal is formed between sidewalls  307  and  308  as will now be further described with specific reference to  FIGS. 7 and 8 . 
     The ink is retained in the nozzle chamber during relative movement of the roof portion  306  and floor portion  305  by the geometric features of the sidewalls  307  and  308  which ensure that ink is retained within the nozzle chamber by surface tension. To this end, there is provided a very fine gap between downwardly depending sidewall  307  and the mutually facing surface  316  of the upwardly depending sidewall  308 . As can be clearly seen in  FIG. 8  the ink (shown as a dark shaded area) is restrained within the small aperture between the downwardly depending sidewall  307  and inward faces  316  of the upwardly extending sidewall by the proximity of the two sidewalls which ensures that the ink “self seals” across free opening  317  by surface tension, due to the close proximity of the sidewalls. 
     In order to make provision for any ink which may escape the surface tension restraint due to impurities or other factors which may break the surface tension, the upwardly depending sidewall  308  is provided in the form of an upwardly facing channel having not only the inner surface  316  but a spaced apart parallel outer surface  18  forming a U-shaped channel  319  between the two surfaces. Any ink drops escaping from the surface tension between the surfaces  307  and  316 , overflows into the U-shaped channel where it is retained rather than “wicking” across the surface of the nozzle strata. In this manner, a dual wall fluidic seal is formed which is effective in retaining the ink within the moving nozzle mechanism. 
     Referring to  FIG. 8 , it will been seen that the actuator  310  is comprised of a first, active beam  358  arranged above and spaced apart from a second, passive beam  360 . By spacing apart the two beams, thermal transfer from the active beam  358  to the passive beam  360  is minimized. Accordingly, this spaced apart arrangement has the advantage of maximizing thermoelastic efficiency. In the present invention, the active beam  358  may be comprised of an aluminium alloy, as described above, such as aluminium-vanadium alloy. 
     Thermal Bend Actuator Defining Moving Nozzle Roof 
     The embodiments exemplified by  FIGS. 5 to 8  showed a nozzle assembly  300  comprising a nozzle chamber  304  having a roof portion  306  which moves relative to a floor portion  305  of the chamber. The moveable roof portion  306  is actuated to move towards the floor portion  305  by means of a bi-layered thermal bend actuator  310  positioned externally of the nozzle chamber  305 . 
     A moving roof lowers the drop ejection energy, since only one face of the moving structure has to do work against the viscous ink. However, there is still a need to increase the amount of power available for drop ejection. By increasing the amount of power, a shorter pulse width can be used to provide the same amount of energy. With shorter pulse widths, improved drop ejection characteristics can be achieved. 
     One means for increasing actuator power is to increase the size of the actuator. However, in the nozzle design shown in  FIGS. 5 to 8 , it is apparent that an increase in actuator size would adversely affect nozzle spacing, which is undesirable in the manufacture of high-resolution pagewidth printheads. 
     A solution to this problem is provided by the nozzle assembly  400  shown in  FIGS. 9 to 12 . The nozzle assembly  400  comprises a nozzle chamber  401  formed on a passivated CMOS layer  402  of a silicon substrate  403 . The nozzle chamber is defined by a roof  404  and sidewalls  405  extending from the roof to the passivated CMOS layer  402 . Ink is supplied to the nozzle chamber  401  by means of an ink inlet  406  in fluid communication with an ink supply channel  407  receiving ink from backside of the silicon substrate. Ink is ejected from the nozzle chamber  401  by means of a nozzle opening  408  defined in the roof  404 . The nozzle opening  408  is offset from the ink inlet  406 . 
     As shown more clearly in  FIG. 10 , the roof  404  has a moving portion  409 , which defines a substantial part of the total area of the roof. Typically, the moving portion  409  defines at least 20%, at least 30%, at least 40% or at least 50% of the total area of the roof  404 . In the embodiment shown in  FIGS. 9 to 12 , the nozzle opening  408  and nozzle rim  415  are defined in the moving portion  409 , such that the nozzle opening and nozzle rim move with the moving portion. 
     The nozzle assembly  400  is characterized in that the moving portion  409  is defined by a thermal bend actuator  410  having a planar upper active beam  411  and a planar lower passive beam  412 . Hence, the actuator  410  typically defines at least 20%, at least 30%, at least 40% or at least 50% of the total area of the roof  404 . Correspondingly, the upper active beam  411  typically defines at least 20%, at least 30%, at least 40% or at least 50% of the total area of the roof  404 . 
     As shown in  FIGS. 9 and 10 , at least part of the upper active beam  411  is spaced apart from the lower passive beam  412  for maximizing thermal insulation of the two beams. More specifically, a layer of Ti is used as a bridging layer  413  between the upper active beam  411  comprised of TiN and the lower passive beam  412  comprised of SiO 2 . The bridging layer  413  allows a gap  414  to be defined in the actuator  410  between the active and passive beams. This gap  414  improves the overall efficiency of the actuator  410  by minimizing thermal transfer from the active beam  411  to the passive beam  412 . 
     However, it will of course be appreciated that the active beam  411  may, alternatively, be fused or bonded directly to the passive beam  412  for improved structural rigidity. Such design modifications would be well within the ambit of the skilled person and are encompassed within the scope of the present invention. 
     The active beam  411  is connected to a pair of contacts  416  (positive and ground) via the Ti bridging layer. The contacts  416  connect with drive circuitry in the CMOS layers. 
     When it is required to eject a droplet of ink from the nozzle chamber  401 , a current flows through the active beam  411  between the two contacts  416 . The active beam  411  is rapidly heated by the current and expands relative to the passive beam  412 , thereby causing the actuator  410  (which defines the moving portion  409  of the roof  404 ) to bend downwards towards the substrate  403 . This movement of the actuator  410  causes ejection of ink from the nozzle opening  408  by a rapid increase of pressure inside the nozzle chamber  401 . When current stops flowing, the moving portion  409  of the roof  404  is allowed to return to its quiescent position, which sucks ink from the inlet  406  into the nozzle chamber  401 , in readiness for the next ejection. 
     Accordingly, the principle of ink droplet ejection is analogous to that described above in connection with nozzle assembly  300 . However, with the thermal bend actuator  410  defining the moving portion  409  of the roof  404 , a much greater amount of power is made available for droplet ejection, because the active beam  411  has a large area compared with the overall size of the nozzle assembly  400 . 
     Turning to  FIG. 12 , it will be readily appreciated that the nozzle assembly  400  (as well as all other nozzle assemblies described herein) may be replicated into an array of nozzle assemblies to define a printhead or printhead integrated circuit. A printhead integrated circuit comprises a silicon substrate, an array of nozzle assemblies (typically arranged in rows) formed on the substrate, and drive circuitry for the nozzle assemblies. A plurality of printhead integrated circuits may be abutted or linked to form a pagewidth inkjet printhead, as described in, for example, Applicant&#39;s earlier U.S. application Ser. Nos. 10/854,491 filed on May 27, 2004 and 11/014,732 filed on Dec. 20, 2004, the contents of which are herein incorporated by reference. 
     The nozzle assembly  500  shown in  FIGS. 13 to 15  is similar to the nozzle assembly  400  insofar as a thermal bend actuator  510 , having an upper active beam  511  and a lower passive beam  512 , defines a moving portion of a roof  504  of the nozzle chamber  501 . Hence, the nozzle assembly  500  achieves the same advantages, in terms of increased power, as the nozzle assembly  400 . 
     However, in contrast with the nozzle assembly  400 , the nozzle opening  508  and rim  515  are not defined by the moving portion of the roof  504 . Rather, the nozzle opening  508  and rim  515  are defined in a fixed portion of the roof  504  such that the actuator  510  moves independently of the nozzle opening and rim during droplet ejection. An advantage of this arrangement is that it provides more facile control of drop flight direction. 
     It will of course be appreciated that the aluminium alloys, with their inherent advantage of improved thermal bend efficiency, may be used as the active beam in either of the thermal bend actuators  410  and  510  described above in connection with the embodiments shown in  FIGS. 9 to 15 . 
     The nozzle assemblies  400  and  500  may be constructed using suitable MEMS technologies in an analogous manner to inkjet nozzle manufacturing processes exemplified in the Applicant&#39;s earlier U.S. Pat. Nos. 6,416,167 and 6,755,509, the contents of which are herein incorporated by reference. 
     Active Beam Having Optimal Stiffness in a Bend Direction 
     Referring now to  FIGS. 11 and 15 , it will be seen that the upper active beams  411  and  511  of the actuators  410  and  510  are each comprised of a tortuous beam element having either a bent (in the case of beam  411 ) or serpentine (in the case of beam  511 ) configuration. The tortuous beam element is elongate and has a relatively small cross-sectional area suitable for resistive heating. In addition, the tortuous configuration enables respective ends of the beam element to be connected to respective contacts positioned at one end of the actuator, simplifying the overall design and construction of the nozzle assembly. 
     Referring specifically to  FIGS. 14 and 15 , an elongate beam element  520  has a serpentine configuration defining the elongate active cantilever beam  511  of the actuator  510 . The serpentine beam element  520  has a planar, tortuous path connecting a first electrical contact  516  with a second electrical contact  516 . The electrical contacts  516  (positive and ground) are positioned at one end of the actuator  510  and provide electrical connection between drive circuitry in the CMOS layers  502  and the active beam  511 . 
     The serpentine beam element  520  is fabricated by standard lithographic etching techniques and defined by a plurality of contiguous beam members. In general, beam members may be defined as solid portions of beam material, which extend substantially linearly in, for example, a longitudinal or transverse direction. The beam members of beam element  520  are comprised of longer beam members  521 , which extend along a longitudinal axis of the elongate cantilever beam  511 , and shorter beam members  522 , which extend across a transverse axis of the elongate cantilever beam  511 . An advantage of this configuration for the serpentine beam element  520  is that it provides maximum stiffness in a bend direction of the cantilever beam  511 . Stiffness in the bend direction is advantageous because it facilitates bending of the actuator  510  back to its quiescent position after each actuation. 
     It will be appreciated that the bent active beam configuration for the nozzle assembly  400  shown in  FIG. 11  achieves the same or similar advantages to those described above in connection with nozzle assembly  500 . In  FIG. 11 , the longer beam members, extending longitudinally, are indicated as  421 , whilst the interconnecting shorter beam member, extending transversely, is indicated as  422 . 
     Use of Porous Material For Improving Thermal Efficiency 
     In all the embodiments described above, as well as all other embodiments of thermal bend actuators described by the present Applicant, the active beam is either bonded to the passive beam for structural robustness (see  FIGS. 1 and 2 ), or the active beam is spaced apart from the passive beam for maximum thermal efficiency (see  FIG. 8 ). The thermal efficiency provided by an air gap between the beams is, of course, desirable. However, this improvement in thermal efficiency is usually at the expense of structural robustness and a propensity for buckling of the thermal bend actuator. 
     U.S. Pat. No. 6,163,066, the contents of which is incorporated herein by reference, describes a porous silicon dioxide insulator, having a dielectric constant of about 2.0 or less. The material is formed by deposition of silicon carbide and oxidation of the carbon component to form porous silicon dioxide. By increasing the ratio of carbon to silicon, the porosity of the resultant porous silicon dioxide can be increased. Porous silicon dioxide are known to be useful as a passivation layer in integrated circuits for reducing parasitic resistance. 
     However, the present Applicant has found that porous materials of this type are useful for improving the efficiency of thermal bend actuators. A porous material may be used either as an insulating layer between an active beam and a passive beam, or it may be used as the passive beam itself. 
       FIG. 16  shows a thermal bend actuator  600  comprising an upper active beam  601 , a lower passive beam  602  and an insulating layer  603  sandwiched between the upper and lower beams. The insulating beam is comprised of porous silicon dioxide, while the active and passive beams  601  and  602  may be comprised of any suitable materials, such as TiN and SiO 2 , respectively. 
     The porosity of the insulating layer  603  provides excellent thermal insulation between the active and passive beams  601  and  602 . The insulating layer  603  also provides the actuator  600  with structural robustness. Hence, the actuator  600  combines the advantages of both types of thermal bend actuator described above in connection with  FIGS. 1 ,  2  and  8 . 
     Alternatively, and as shown in  FIG. 17 , the porous material may simply form the passive layer of a bi-layered thermal bend actuator. Accordingly, the thermal bend actuator  650  comprises an upper active beam  651  comprised of TiN, and a lower passive beam  652  comprised of porous silicon dioxide. 
     It will, of course, be appreciated that thermal bend actuators of the types shown in  FIGS. 16 and 17  may be incorporated into any suitable inkjet nozzle or other MEMS device. The improvements in thermal efficiency and structural rigidity make such actuators attractive in any MEMS application requiring a mechanical actuator or transducer. 
     The thermal bend actuators of the types shown in  FIGS. 16 and 17  are particularly suitable for use in the inkjet nozzle assemblies  400  and  500  described above. The skilled person would readily appreciate that appropriate modifications of the thermal bend actuators  410  and  510  would realize the above-mentioned improvements in thermal efficiency and structural robustness. 
     It will be further appreciated that the active beam members  601  and  651  in the thermal bend actuators  600  and  650  described above may be comprised of an aluminum alloy, as described herein, for further improvements in thermal bend efficiency. 
     It will, of course, be appreciated that the present invention has been described by way of example only and that modifications of detail may be made within the scope of the invention, which is defined in the accompanying claims.