Abstract:
An inkjet printhead includes a wafer having a droplet ejection side and a liquid supply side opposite the droplet ejection side; a plurality of active ink ejection structures formed on the droplet ejection side of the wafer, each active ink ejection structure partially defining a nozzle chamber, and defining an ink ejection port; a plurality of individual fluid passages formed in the wafer, each fluid passage providing fluid to corresponding nozzle chambers; a plurality of static ink ejection structures formed on the droplet ejection side of the wafer and inside of corresponding active ink ejection structures, each static ink ejection structure defining a nozzle chamber with a corresponding active ink ejection structure; and droplet ejection actuators respectively corresponding to each active ink ejection structure. The droplet ejection actuators are formed on the droplet ejection side of the wafer and attached to respective active ink ejection structures. The droplet ejection actuators are operable to displace an active ink ejection structures towards a corresponding static ink ejection structure to effect ejection of fluid in the nozzle chamber through the ink ejection port.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. Ser. No. 10/728,784 filed Dec. 8, 2003, which is a Continuation In Part Application of U.S. Ser. No. 10/307,330 filed on Dec. 2, 2002, now issued U.S. Pat. No. 6,666,544, all of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the fabrication of fluid ejection chips. More particularly, this invention relates to fabrication techniques of fluid ejection chips that minimize the spacing between adjacent nozzles. 
       REFERENCED PATENT APPLICATIONS 
       [0003]    The following applications are incorporated by reference: 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 6,362,868 
                 6,227,652 
                 6,213,588 
                 6,213,589 
                 6,231,163 
                 6,247,795 
               
               
                 6,394,581 
                 6,244,691 
                 6,257,704 
                 6,416,168 
                 6,220,694 
                 6,257,705 
               
               
                 6,247,794 
                 6,234,610 
                 6,247,793 
                 6,264,306 
                 6,241,342 
                 6,247,792 
               
               
                 6,264,307 
                 6,254,220 
                 6,234,611 
                 6,302,528 
                 6,283,582 
                 6,239,821 
               
               
                 6,338,547 
                 6,247,796 
                 6,557,977 
                 6,390,603 
                 6,362,843 
                 6,293,653 
               
               
                 6,312,107 
                 6,227,653 
                 6,234,609 
                 6,238,040 
                 6,188,415 
                 6,227,654 
               
               
                 6,209,989 
                 6,247,791 
                 6,336,710 
                 6,217,153 
                 6,416,167 
                 6,243,113 
               
               
                 6,283,581 
                 6,247,790 
                 6,260,953 
                 6,267,469 
                 6,273,544 
                 6,309,048 
               
               
                 6,420,196 
                 6,443,558 
                 6,439,689 
                 6,378,989 
                 6,848,181 
                 6,634,735 
               
               
                 6,623,101 
                 6,406,129 
                 6,505,916 
                 6,457,809 
                 6,550,895 
                 6,457,812 
               
               
                 6,428,133 
                 6,755,509 
               
               
                   
               
             
          
         
       
     
       BACKGROUND OF THE INVENTION 
       [0004]    As set out in the above referenced applications/patents, the Applicant has spent a substantial amount of time and effort in developing printheads that incorporate micro electro-mechanical system (MEMS)—based components to achieve the ejection of ink necessary for printing. 
         [0005]    As a result of the Applicant&#39;s research and development, the Applicant has been able to develop printheads having one or more printhead chips that together incorporate up to 84 000 nozzle arrangements. The Applicant has also developed suitable processor technology that is capable of controlling operation of such printheads. In particular, the processor technology and the printheads are capable of cooperating to generate resolutions of 1600 dpi and higher in some cases. Examples of suitable processor technology are provided in the above referenced patent applications/patents. 
         [0006]    The Applicant has overcome substantial difficulties in achieving the necessary ink flow and ink drop separation within the ink jet printheads. 
         [0007]    It is generally beneficial to increase the nozzle densities on a printhead to enhance the print resolution. MEMS fabrication of the nozzles on silicon wafer allows very high nozzle density. However, the wafer is typically about 200 microns thick with the nozzle guards, ink chambers, ejection actuators and so on occupying a layer about 20 microns thick on one side. Ink supply passages must be formed through the wafer to the nozzles. 
         [0008]    It is not practical to form the ink supply passages from the nozzle side of the wafer through to the supply side. The fabrication of other nozzle structures would require the entire supply passage to be filled with resist while the other structures were lithographically form on top. The resist subsequently needs to be stripped out of the passage. To strip a 200-micron deep passage of resist would be difficult and time consuming. 
         [0009]    Forming the ink supply passages from the supply side of the wafer through to the nozzle side presents its own difficulties. Firstly, the precise alignment of the masking on the supply side with the ink chambers of each nozzle on the other side is difficult. At present, the best equipment available for aligning the mask have ±2 microns accuracy. Secondly, a deep etch will often deviate from a straight path because the ions in the etchant are influenced by any charged particles in the wafer. Thirdly, the plasma etchant will often track sideways along an interface between silicon wafer and dielectric material. 
         [0010]    Misalignment of the supply passage can lead to the plasma etch contacting and damaging other components of the nozzle, for example, the drive circuitry for the ejection actuator. Furthermore, the above causes of misalignment can compound into large inaccuracies which imposes limits on the size of the nozzle structure and the spacing between nozzles. This, of course, reduces the density of nozzles and lowers the resolution. 
         [0011]    It is an object of the present invention to provide a useful alternative to known printheads and the techniques for fabricating them. In particular the invention aims to provide a method of making printhead chips that accommodate the standard manufacturing tolerances involved while minimizing the spacing between adjacent nozzles. 
       SUMMARY OF THE INVENTION 
       [0012]    According to one aspect of the present disclosure, an inkjet printhead includes a wafer having a droplet ejection side and a liquid supply side opposite the droplet ejection side; a plurality of active ink ejection structures formed on the droplet ejection side of the wafer, each active ink ejection structure partially defining a nozzle chamber, and defining an ink ejection port; a plurality of individual fluid passages formed in the wafer, each fluid passage providing fluid to corresponding nozzle chambers; a plurality of static ink ejection structures formed on the droplet ejection side of the wafer and inside of corresponding active ink ejection structures, each static ink ejection structure defining a nozzle chamber with a corresponding active ink ejection structure; and droplet ejection actuators respectively corresponding to each active ink ejection structure. The droplet ejection actuators are formed on the droplet ejection side of the wafer and attached to respective active ink ejection structures. The droplet ejection actuators are operable to displace an active ink ejection structures towards a corresponding static ink ejection structure to effect ejection of fluid in the nozzle chamber through the ink ejection port. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    In the drawings, 
           [0014]      FIG. 1  is a schematic perspective view, partially cut away, of a unit cell of a printhead according to the invention; 
           [0015]      FIG. 2  shows a schematic, sectioned perspective of a unit cell of the type shown in  FIG. 1 , at an intermediate stage of its fabrication; 
           [0016]      FIG. 3  shows a schematic, sectioned perspective of a unit cell of the type shown in  FIG. 1 , at an intermediate stage of its fabrication; 
           [0017]      FIG. 4  shows a schematic, sectioned perspective of a unit cell of the type shown in  FIG. 1 , at an intermediate stage of its fabrication; 
           [0018]      FIG. 5  shows a schematic, sectioned perspective of the unit cell shown in  FIG. 1 , at an intermediate stage of its fabrication in accordance with the present invention; 
           [0019]      FIG. 6  shows a schematic, sectioned perspective of the unit cell shown in  FIG. 1 , at an intermediate stage of its fabrication in accordance with the present invention; 
           [0020]      FIG. 7  shows a schematic, sectioned perspective of the unit cell shown in  FIG. 1 , at an intermediate stage of its fabrication in accordance with the present invention; 
           [0021]      FIG. 8  shows a three-dimensional view of a nozzle arrangement of a thermal bend actuator embodiment of a printhead chip in accordance with the invention, for an ink jet printhead; 
           [0022]      FIG. 9  shows a three-dimensional sectioned view of the nozzle arrangement of  FIG. 8 ; 
           [0023]      FIG. 10  shows a transverse cross sectional view of a thermal bend actuator of the nozzle arrangement of  FIG. 8 ; 
           [0024]      FIG. 11  shows a three-dimensional sectioned view of the nozzle arrangement of  FIG. 8 , in an initial stage of ink drop ejection; 
           [0025]      FIG. 12  shows a three-dimensional sectioned view of the nozzle arrangement of  FIG. 8 , in a terminal stage of ink drop ejection; 
           [0026]      FIG. 13  shows a schematic view of one coupling structure of the nozzle arrangement of  FIG. 8 ; 
           [0027]      FIG. 14  shows a schematic view of a part of the coupling structure attached to an active ink ejection structure of the nozzle arrangement, when the nozzle arrangement is in a quiescent condition; 
           [0028]      FIG. 15  shows the part of  FIG. 14  when the nozzle arrangement is in an operative condition; 
           [0029]      FIG. 16  shows an intermediate section of a connecting plate of the coupling structure, when the nozzle arrangement is in a quiescent condition; 
           [0030]      FIG. 17  shows the intermediate section of  FIG. 16 , when the nozzle arrangement is in an operative condition; 
           [0031]      FIG. 18  shows a schematic view of a part of the coupling structure attached to a connecting member of the nozzle arrangement when the nozzle arrangement is in a quiescent condition; 
           [0032]      FIG. 19  shows the part of  FIG. 18  when the nozzle arrangement is in an operative condition; and 
           [0033]      FIG. 20  shows a plan view of a nozzle arrangement of a second embodiment of a printhead chip, in accordance with the invention, for an ink jet printhead. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    The present invention is applicable to printheads formed on and through silicon wafers by lithographic etching and deposition techniques, regardless of whether bubble forming heater elements or thermal bend actuators are used. 
         [0035]    Bubble Forming Heater Element Actuated Printheads 
         [0036]      FIG. 1  shows a nozzle of this type. The nozzles, ejection actuators, associated drive circuitry and ink supply passages are formed on and through a wafer using lithographically masked etching techniques described in great detail in U.S. Ser. No. 10/302,274. In the interests of brevity, the disclosure of the &#39;274 application is incorporated herein in its entirety. For convenience, the reference numerals on  FIGS. 1 to 7  accord with the reference numbering used in &#39;274. Corresponding features of the embodiments shown in  FIGS. 8 to 20  do not necessarily use the same reference numerals. 
         [0037]    The unit cell  1  is shown with part of the walls  6  and nozzle plate  2  cut-away, which reveals the interior of the chamber  7 . The heater  14  is not shown cut away, so that both halves of the heater element  10  can be seen. 
         [0038]    In operation, ink  11  passes through the ink inlet passage  31  (see  FIGS. 2-7 ) to fill the chamber  7 . Then a voltage is applied across the electrodes  15  to establish a flow of electric current through the heater element  10 . This heats the element  10 , to form a vapor bubble in the ink within the chamber  7  to eject a drop of ink. 
         [0039]    It is generally beneficial to increase the nozzle densities on a printhead to enhance the print resolution. MEMS fabrication of the nozzles on silicon wafer allows very high nozzle density. However, the wafer is typically about 200 microns thick with the nozzle guards, ink chambers, ejection actuators and so on occupying a layer about 20 microns thick on one side. These dimensions are indicated generally by A and B on  FIG. 1 . 
         [0040]      FIGS. 2 to 7  show the unit cell with the ink chamber  7  and heater element  10  removed for clarity. Ink is supplied to the chambers by passages  32  extending to the opposite side of the wafer. It would be convenient to etch these passages  32  from the nozzle side of the wafer as this side will be subject to etching and deposition to form the nozzle structures. Unfortunately, it is not practical to form the ink supply passages from the nozzle side of the wafer. The entire supply passage  32  would have to be filled with resist while the nozzle structures were lithographically formed. Stripping the resist out of a 200-micron deep passage of resist would be prohibitively difficult and time consuming. 
         [0041]    Forming the ink supply passages from the supply side of the wafer through to the nozzle side presents its own difficulties. These problems are schematically illustrated in  FIGS. 2 ,  3  and  4 . 
         [0042]    Referring to  FIG. 2 , the ink supply passage is etched through the wafer  21  to the CMOS metallisation layers of the interconnect  23 . The inlet  31  in the interconnect  23  provides a fluid connection between the supply passage  32  and the nozzle chamber (not shown) to be formed on the passivation layer  24 . Guard rings  26  prevent ink from diffusing from within the inlet  31  to the wiring in the interconnect  23  and the CMOS drive circuitry  22  between the wafer substrate  21  and the interconnect  23 . Unfortunately, the precise alignment of the masking on the supply side of the wafer with the ink chambers of each nozzle on the nozzle side is difficult. At present, the best equipment available for aligning the mask has ±2 microns accuracy. If the drive circuitry  22  is too close to the inlet  31 , a portion C of the circuitry  22  risks damage by the etchant due to misalignment of the passage  32 . 
         [0043]    Another problem is schematically shown in  FIG. 3 . A deep etch will often deviate from a straight path. Ions in the etchant are influenced by any charged particles in the wafer  21 . While the mask may be perfectly aligned on the supply side of the wafer  21 , the deep etch is slightly angled and can result in a significant misalignment at the interface of the wafer  21  and the interconnect  23 . Again, if the drive circuitry  22  is too close, a portion C may be destroyed by the oxygen plasma etchant. 
         [0044]      FIG. 4  illustrates another potential problem. The plasma etchant will often track sideways along an interface between silicon wafer  21  and dielectric material of the interconnect  23 . Once again, this can lead to inadvertent etching of the drive circuitry  22 . 
         [0045]    The above causes of misalignment can compound into large inaccuracies that imposes limits on the size of the nozzle structure and the spacing between nozzles. This, of course, reduces the density of nozzles and lowers the resolution. 
         [0046]    Referring to  5 ,  6  and  7 , the present invention addresses this by etching the inlet  31  through the interconnect  23  and into the wafer  21  so that the ink supply passage  32  can stop short of the interface between the dielectric  23  and the wafer  21 . As best shown in  FIG. 5 , the plasma does not get the opportunity to track along the interface and damage the CMOS drive circuitry. As the inlet hole  31  is relatively shallow, the removal of the resist is not overly difficult. This permits a more compact overall design and higher nozzle packing density. Using this technique, the sizes of the ink conduits are also relative small. Typically, the width of the inlet hole  31  is between 8 microns and 24 microns, and the width of the supply passage  32  is between 10 microns and 28 microns. 
         [0047]    Thermal Bend Actuated Printheads 
         [0048]    In  FIGS. 8 to 12 , reference numeral  10  generally indicates a nozzle arrangement of a printhead chip, for an ink jet printhead in accordance with a related aspect of the invention. 
         [0049]    The nozzle arrangement  10  is one of a plurality of such nozzle arrangements formed on a silicon wafer substrate  12  to define the printhead chip of the invention. As set out in the background of this specification, a single printhead can contain up to 84 000 such nozzle arrangements. For the purposes of clarity and ease of description, only one nozzle arrangement is described. It is to be appreciated that a person of ordinary skill in the field can readily obtain the printhead chip by simply replicating the nozzle arrangement  10  on the wafer substrate  12 . 
         [0050]    The printhead chip is the product of an integrated circuit fabrication technique. In particular, each nozzle arrangement  10  is the product of a MEMS—based fabrication technique. As is known, such a fabrication technique involves the deposition of functional layers and sacrificial layers of integrated circuit materials. The functional layers are etched to define various moving components and the sacrificial layers are etched away to release the components. As is known, such fabrication techniques generally involve the replication of a large number of similar components on a single wafer that is subsequently diced to separate the various components from each other. This reinforces the submission that a person of ordinary skill in the field can readily obtain the printhead chip of this invention by replicating the nozzle arrangement  10 . 
         [0051]    An electrical drive circuitry layer  14  is positioned on the silicon wafer substrate  12 . The electrical drive circuitry layer  14  includes CMOS drive circuitry. The particular configuration of the CMOS drive circuitry is not important to this description and has therefore not been shown in any detail in the drawings. Suffice to say that it is connected to a suitable microprocessor and provides electrical current to the nozzle arrangement  10  upon receipt of an enabling signal from said suitable microprocessor. An example of a suitable microprocessor is described in the above referenced patents/patent applications. It follows that this level of detail will not be set out in this specification. 
         [0052]    An ink passivation layer  16  is positioned on the drive circuitry layer  14 . The ink passivation layer  16  can be of any suitable material, such as silicon nitride. 
         [0053]    The nozzle arrangement  10  includes an ink inlet channel  18  that is one of a plurality of such ink inlet channels defined in the substrate  12 . 
         [0054]    The nozzle arrangement  10  includes an active ink ejection structure  20 . The active ink ejection structure  20  has a roof  22  and sidewalls  24  that depend from the roof  22 . An ink ejection port  26  is defined in the roof  22 . 
         [0055]    The active ink ejection structure  20  is connected to, and between, a pair of thermal bend actuators  28  with coupling structures  30  that are described in further detail below. The roof  22  is generally rectangular in plan and, more particularly, can be square in plan. This is simply to facilitate connection of the actuators  28  to the roof  22  and is not critical. For example, in the event that three actuators are provided, the roof  22  could be generally triangular in plan. There may thus be other shapes that are suitable. 
         [0056]    The active ink ejection structure  20  is connected between the thermal bend actuators  28  so that a free edge  32  of the sidewalls  24  is spaced from the ink passivation layer  16 . It will be appreciated that the sidewalls  24  bound a region between the roof  22  and the substrate  12 . 
         [0057]    The roof  22  is generally planar, but defines a nozzle rim  76  that bounds the ink ejection port  26 . The roof  22  also defines a recess  78  positioned about the nozzle rim  76  which serves to inhibit ink spread in case of ink wetting beyond the nozzle rim  76 . 
         [0058]    The nozzle arrangement  10  includes a static ink ejection structure  34  that extends from the substrate  12  towards the roof  22  and into the region bounded by the sidewalls  24 . The static ink ejection structure  34  and the active ink ejection structure  20  together define a nozzle chamber  42  in fluid communication with an opening  38  of the ink inlet channel  18 . The static ink ejection structure  34  has a wall portion  36  that bounds an opening  38  of the ink inlet channel  18 . An ink displacement formation  40  is positioned on the wall portion  36  and defines an ink displacement area that is sufficiently large so as to facilitate ejection of ink from the ink ejection port  26  when the active ink displacement structure  20  is displaced towards the substrate  12 . The opening  38  is substantially aligned with the ink ejection port  26 . 
         [0059]    The thermal bend actuators  28  are substantially identical. It follows that, provided a similar driving signal is supplied to each thermal bend actuator  28 , the thermal bend actuators  28  each produce substantially the same force on the active ink ejection structure  20 . 
         [0060]    In  FIG. 3  there is shown the thermal bend actuator  28  in further detail. The thermal bend actuator  28  includes an arm  44  that has a unitary structure. The arm  44  is of an electrically conductive material that has a coefficient of thermal expansion which is such that a suitable component of such material is capable of performing work, on a MEMS scale, upon expansion and contraction of the component when heated and subsequently cooled. The material can be one of many. However, it is desirable that the material has a Young&#39;s Modulus that is such that, when the component bends through differential heating, energy stored in the component is released when the component cools to assist return of the component to a starting condition. The Applicant has found that a suitable material is Titanium Aluminum Nitride (TiAlN). However, other conductive materials may also be suitable, depending on their respective coefficients of thermal expansion and Young&#39;s Modulus. 
         [0061]    The arm  44  has a pair of outer passive portions  46  and a pair of inner active portions  48 . The outer passive portions  46  have passive anchors  50  that are each made fast with the ink passivation layer  16  by a retaining structure  52  of successive layers of titanium and silicon dioxide or equivalent material. 
         [0062]    The inner active portions  48  have active anchors  54  that are each made fast with the drive circuitry layer  14  and are electrically connected to the drive circuitry layer  14 . This is also achieved with a retaining structure  56  of successive layers of titanium and silicon dioxide or equivalent material. 
         [0063]    The arm  44  has a working end that is defined by a bridge portion  58  that interconnects the portions  46 ,  48 . It follows that, with the active anchors  54  connected to suitable electrical contacts in the drive circuitry layer  14 , the inner active portions  48  define an electrical circuit. Further, the portions  46 ,  48  have a suitable electrical resistance so that the inner active portions  48  are heated when a current from the CMOS drive circuitry passes through the inner active portions  48 . It will be appreciated that substantially no current will pass through the outer passive portions  46  resulting in the passive portions heating to a significantly lesser extent than the inner active portions  48 . Thus, the inner active portions  48  expand to a greater extent than the outer passive portions  46 . 
         [0064]    As can be seen in  FIG. 3 , each outer passive portion  46  has a pair of outer horizontally extending sections  60  and a central horizontally extending section  62 . The central section  62  is connected to the outer sections  60  with a pair of vertically extending sections  64  so that the central section  62  is positioned intermediate the substrate  12  and the outer sections  60 . 
         [0065]    Each inner active portion  48  has a transverse profile that is effectively an inverse of the outer passive portions  46 . Thus, outer sections  66  of the inner active portions  48  are generally coplanar with the outer sections  60  of the passive portions  46  and are positioned intermediate central sections  68  of the inner active portions  48  and the substrate  12 . It follows that the inner active portions  48  define a volume that is positioned further from the substrate  12  than the outer passive portions  46 . It will therefore be appreciated that the greater expansion of the inner active portions  48  results in the arm  44  bending towards the substrate  12 . This movement of the arms  44  is transferred to the active ink ejection structure  20  to displace the active ink ejection structure  20  towards the substrate  12 . 
         [0066]    This bending of the arms  44  and subsequent displacement of the active ink ejection structure  20  towards the substrate  12  is indicated in  FIG. 4 . The current supplied by the CMOS drive circuitry is such that an extent and speed of movement of the active ink displacement structure  20  causes the formation of an ink drop  70  outside of the ink ejection port  26 . When the current in the inner active portions  48  is discontinued, the inner active portions  48  cool, causing the arm  44  to return to a position shown in  FIG. 1 . As discussed above, the material of the arm  44  is such that a release of energy built up in the passive portions  46  assists the return of the arm  44  to its starting condition. In particular, the arm  44  is configured so that the arm  44  returns to its starting position with sufficient speed to cause separation of the ink drop  70  from ink  72  within the nozzle chamber  42 . 
         [0067]    On the macroscopic scale, it would be counter-intuitive to use heat expansion and contraction of material to achieve movement of a functional component. However, the Applicant has found that, on a microscopic scale, the movement resulting from heat expansion is fast enough to permit a functional component to perform work. This is particularly so when suitable materials, such as TiAlN are selected for the functional component. 
         [0068]    One coupling structure  30  is mounted on each bridge portion  58 . As set out above, the coupling structures  30  are positioned between respective thermal actuators  28  and the roof  22 . It will be appreciated that the bridge portion  58  of each thermal actuator  28  traces an arcuate path when the arm  44  is bent and straightened in the manner described above. Thus, the bridge portions  58  of the oppositely oriented actuators  28  tend to move away from each other when actuated, while the active ink ejection structure  20  maintains a rectilinear path. It follows that the coupling structures  30  should accommodate movement in two axes, in order to function effectively. 
         [0069]    Details of one of the coupling structures  30  are shown in  FIG. 13 . It will be appreciated that the other coupling structure  30  is simply an inverse of that shown in  FIG. 13 . It follows that it is convenient to describe just one of the coupling structures  30 . 
         [0070]    The coupling structure  30  includes a connecting member  74  that is positioned on the bridge portion  58  of the thermal actuator  28 . The connecting member  74  has a generally planar surface  80  that is substantially coplanar with the roof  22  when the nozzle arrangement  10  is in a quiescent condition. 
         [0071]    A pair of spaced proximal tongues  82  is positioned on the connecting member  74  to extend towards the roof  22 . Likewise, a pair of spaced distal tongues  84  is positioned on the roof  22  to extend towards the connecting member  74  so that the tongues  82 ,  84  overlap in a common plane parallel to the substrate  12 . The tongues  82  are interposed between the tongues  84 . 
         [0072]    A rod  86  extends from each of the tongues  82  towards the substrate  12 . Likewise, a rod  88  extends from each of the tongues  84  towards the substrate  12 . The rods  86 ,  88  are substantially identical. The connecting structure  30  includes a connecting plate  90 . The plate  90  is interposed between the tongues  82 ,  84  and the substrate  12 . The plate  90  interconnects ends  92  of the rods  86 ,  88 . Thus, the tongues  82 ,  84  are connected to each other with the rods  86 ,  88  and the connecting plate  90 . 
         [0073]    During fabrication of the nozzle arrangement  10 , layers of material that are deposited and subsequently etched include layers of TiAlN, titanium and silicon dioxide. Thus, the thermal actuators  28 , the connecting plates  90  and the static ink ejection structure  34  are of TiAlN. Further, both the retaining structures  52 ,  56 , and the connecting members  74  are composite, having a layer  94  of titanium and a layer  96  of silicon dioxide positioned on the layer  74 . The layer  74  is shaped to nest with the bridge portion  58  of the thermal actuator  28 . The rods  86 ,  88  and the sidewalls  24  are of titanium. The tongues  82 ,  84  and the roof  22  are of silicon dioxide. 
         [0074]    When the CMOS drive circuitry sets up a suitable current in the thermal bend actuator  28 , the connecting member  74  is driven in an arcuate path as indicated with an arrow  98  in  FIG. 13 . This results in a thrust being exerted on the connecting plate  90  by the rods  86 . One actuator  28  is positioned on each of a pair of opposed sides  100  of the roof  22  as described above. It follows that the downward thrust is transmitted to the roof  22  such that the roof  22  and the distal tongues  84  move on a rectilinear path towards the substrate  12 . The thrust is transmitted to the roof  22  with the rods  88  and the tongues  84 . 
         [0075]    The rods  86 ,  88  and the connecting plate  90  are dimensioned so that the rods  86 ,  88  and the connecting plate  90  can distort to accommodate relative displacement of the roof  22  and the connecting member  74  when the roof  22  is displaced towards the substrate  12  during the ejection of ink from the ink ejection port  26 . The titanium of the rods  86 ,  88  has a Young&#39;s Modulus that is sufficient to allow the rods  86 ,  88  to return to a straightened condition when the roof  22  is displaced away from the ink ejection port  26 . The TiAlN of the connecting plate  90  also has a Young&#39;s Modulus that is sufficient to allow the connecting plate  90  to return to a starting condition when the roof  22  is displaced away from the ink ejection port  26 . The manner in which the rods  86 ,  88  and the connecting plate  90  are distorted is indicated in  FIGS. 14 to 19 . 
         [0076]    For the sake of convenience, the substrate  19  is assumed to be horizontal so that ink drop ejection is in a vertical direction. 
         [0077]    As can be seen in  FIGS. 18 and 19 , when the thermal bend actuator  28  receives a current from the CMOS drive circuitry, the connecting member  74  is driven towards the substrate  12  as set out above. This serves to displace the connecting plate  90  towards the substrate  12 . In turn, the connecting plate  90  draws the roof  22  towards the substrate  12  with the rods  88 . As described above, the displacement of the roof  22  is rectilinear and therefore vertical. It follows that displacement of the distal tongues  84  is constrained on a vertical path. However, displacement of the proximal tongues  82  is arcuate and has both vertical and horizontal components, the horizontal components being generally away from the roof  22 . The distortion of the rods  86 ,  88  and the connecting plate  90  therefore accommodates the horizontal component of movement of the proximal tongues  82 . 
         [0078]    In particular, the rods  86  bend and the connecting plate  90  rotates partially as shown in  FIG. 19 . In this operative condition, the proximal tongues  82  are angled with respect to the substrate. This serves to accommodate the position of the proximal tongues  82 . As set out above, the distal tongues  84  remain in a rectilinear path as indicated by an arrow  102  in  FIG. 15 . Thus, the rods  88  that bend as shown in  FIG. 15  as a result of a torque transmitted by the plate  90  resist the partial rotation of the connecting plate  90 . It will be appreciated that an intermediate part  104  between each rod  86  and its adjacent rod  88  is also subjected to a partial rotation, although not to the same extent as the part shown in  FIG. 19 . The part shown in  FIG. 15  is subjected to the least amount of rotation due to the fact that resistance to such rotation is greatest at the rods  88 . It follows that the connecting plate  90  is partially twisted along its length to accommodate the different extents of rotation. This partial twisting allows the plate  90  to act as a torsional spring thereby facilitating separation of the ink drop  70  when the roof  22  is displaced away from the substrate  19 . 
         [0079]    At this point, it is to be understood that the tongues  82 ,  84 , the rods  86 ,  88  and the connecting plate  90  are all fast with each other so that relative movement of these components is not achieved by any relative sliding movement between these components. 
         [0080]    It follows that bending of the rods  86 ,  88  sets up three bend nodes in each of the rods  86 ,  88 , since pivotal movement of the rods  86 ,  88  relative to the tongues  82 ,  84  is inhibited. This enhances an operative resilience of the rods  86 ,  88  and therefore also facilitates separation of the ink drop  70  when the roof  22  is displaced away from the substrate  12 . 
         [0081]    In  FIG. 20 , reference numeral  110  generally indicates a nozzle arrangement of a second embodiment of a printhead chip, in accordance with the invention, for an ink jet printhead. With reference to  FIGS. 8 to 19 , like reference numerals refer to like parts, unless otherwise specified. 
         [0082]    The nozzle arrangement  110  includes four symmetrically arranged thermal bend actuators  28 . Each thermal bend actuator  28  is connected to a respective side  112  of the roof  22 . The thermal bend actuators  28  are substantially identical to ensure that the roof  22  is displaced in a rectilinear manner. 
         [0083]    The static ink ejection structure  34  has an inner wall  116  and an outer wall  118  that together define the wall portion  36 . An inwardly directed ledge  114  is positioned on the inner wall  116  and extends into the nozzle chamber  42 . 
         [0084]    A sealing formation  120  is positioned on the outer wall  118  to extend outwardly from the wall portion  38 . It follows that the sealing formation  120  and the ledge  114  define the ink displacement formation  40 . 
         [0085]    The sealing formation  120  includes a re-entrant portion  122  that opens towards the substrate  12 . A lip  124  is positioned on the re-entrant portion  122  to extend horizontally from the re-entrant portion  122 . The sealing formation  120  and the sidewalls  24  are configured so that, when the nozzle arrangement  10  is in a quiescent condition, the lip  124  and a free edge  126  of the sidewalls  24  are in horizontal alignment with each other. A distance between the lip  124  and the free edge  126  is such that a meniscus is defined between the sealing formation  120  and the free edge  126  when the nozzle chamber  42  is filled with the ink  72 . When the nozzle arrangement  10  is in an operative condition, the free edge  126  is interposed between the lip  124  and the substrate  12  and the meniscus stretches to accommodate this movement. It follows that when the chamber  42  is filled with the ink  72 , a fluidic seal is defined between the sealing formation  120  and the free edge  126  of the sidewalls  24 . 
         [0086]    The Applicant believes that this related aspect of the invention provides a means whereby substantially rectilinear movement of an ink-ejecting component can be achieved. The Applicant has found that this form of movement enhances efficiency of operation of the nozzle arrangement  10 . Further, the rectilinear movement of the active ink ejection structure  20  results in clean drop formation and separation, a characteristic that is the primary goal of ink jet printhead manufacturers.