Patent Abstract:
A method of fabricating a suspended beam in a MEMS process, said method comprising the steps of:
       (a) etching a pit in a substrate, said pit having a base and sidewalls;   (b) depositing sacrificial material on a surface of said substrate so as to fill said pit;   (c) removing said sacrificial material from a perimeter region within said pit and from said substrate surface surrounding said pit;   (d) reflowing remaining sacrificial material within said pit such that said remaining sacrificial material contacts said sidewalls;   (e) depositing beam material on said substrate surface and on said reflowed sacrificial material; and   (f) removing said reflowed sacrificial material to form said suspended beam.

Full Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of inkjet printers and discloses an inkjet printing system using printheads manufactured with micro-electromechanical systems (MEMS) techniques. 
     CO-PENDING APPLICATIONS 
     The following applications have been filed by the Applicant simultaneously with the present application: 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
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     The disclosures of these co-pending applications are incorporated herein by reference. 
     CROSS REFERENCES TO RELATED APPLICATIONS 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following US Patents/Patent Applications filed by the applicant or assignee of the present invention: 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
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     The disclosures of these applications and patents are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention involves the ejection of ink drops by way of forming gas or vapor bubbles in a bubble forming liquid. This principle is generally described in U.S. Pat. No. 3,747,120 (Stemme). Each pixel in the printed image is derived ink drops ejected from one or more ink nozzles. In recent years, inkjet printing has become increasing popular primarily due to its inexpensive and versatile nature. Many different aspects and techniques for inkjet printing are described in detail in the above cross referenced documents. 
     Completely immersing the heater element in ink dramatically improves the printhead efficiency. Much less heat dissipates into the underlying wafer substrate so more of the input energy is used to generate the bubble that ejects the ink. 
     A convenient way of suspending the heater element is to deposit it on sacrificial photoresist that is subsequently removed by a release etch. The sacrificial material (SAC) is deposited into a pit or trench etched into the substrate adjacent the electrodes. However, it is difficult to precisely match the mask with the sides of the pit. Usually, when the masked photoresist is exposed, gaps form between the sides of the pit and the SAC. When the heater material layer is deposited, it fills these gaps to form ‘stringers’ (as they are known). The stringers remain in the pit after the metal etch (that shapes the heater element) and the release etch (to finally remove the SAC). The stringers can short circuit the heater so that it fails to generate a bubble. 
     By making the mask bigger than the trench, the SAC will be deposited over the side walls so that no gaps form. Unfortunately, this produces a raised lip around top of the trench. When the heater material layer is deposited, it is thinner on the vertical or inclined surfaces of the lip. After the metal etch and release etch, these thin lip formations remain and cause ‘hotspots’ because the localized thinning increases resistance. These hotspots affect the operation of the heater and typically reduce heater life. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a method of fabricating a suspended beam in a MEMS process, said method comprising the steps of:
         (a) etching a pit in a substrate, said pit having a base and sidewalls;   (b) depositing sacrificial material on a surface of said substrate so as to fill said pit;   (c) removing said sacrificial material from a perimeter region within said pit and from said substrate surface surrounding said pit;   (d) reflowing remaining sacrificial material within said pit such that said remaining sacrificial material contacts said sidewalls;   (e) depositing beam material on said substrate surface and on said reflowed sacrificial material; and   (f) removing said reflowed sacrificial material to form said suspended beam.       

     Preferably said suspended beam is substantially planar. In a further preferred form, all parts of said suspended beam have substantially the same thickness. 
     Optionally, said suspended beam is an actuator for an inkjet nozzle. 
     In a first aspect the present invention provides a method of fabricating a suspended beam in a MEMS process, said method comprising the steps of:
         (a) etching a pit in a substrate, said pit having a base and sidewalls;   (b) depositing sacrificial material on a surface of said substrate so as to fill said pit;   (c) removing said sacrificial material from a perimeter region within said pit and from said substrate surface surrounding said pit;   (d) reflowing remaining sacrificial material within said pit such that said remaining sacrificial material contacts said sidewalls;   (e) depositing beam material on said substrate surface and on said reflowed sacrificial material; and   (f) removing said reflowed sacrificial material to form said suspended beam.       

     Optionally, said suspended beam is substantially planar. 
     Optionally, all parts of said suspended beam have substantially the same thickness. 
     Optionally, said suspended beam is an actuator for an inkjet nozzle. 
     Optionally, said actuator is a heater element. 
     Optionally, said heater element is suspended between a pair of electrodes. 
     Optionally, said substrate is a silicon wafer. 
     Optionally, said silicon wafer comprises at least one surface oxide layer. 
     Optionally, said sacrificial material is photoresist. 
     Optionally, said photoresist is removed by exposure through a mask followed by development. 
     Optionally, said perimeter region comprises an area adjacent at least two of said sidewalls. 
     Optionally, said perimeter region comprises an area adjacent all of said sidewalls. 
     Optionally, removal of said sacrificial material from said perimeter region results in a space of less than 1 micron between said remaining sacrificial material and at least two of said sidewalls. 
     Optionally, removal of said sacrificial material from said perimeter region results in a space of less than 1 micron between said remaining sacrificial material and all of said sidewalls. 
     Optionally, said reflowing is performed by heating said sacrificial material. 
     Optionally, said sacrificial material is treated to prevent further reflow prior to deposition of beam material. 
     Optionally, said treatment comprises UV curing. 
     Optionally, said beam material is etched into a predetermined configuration after deposition. 
     Optionally, further MEMS process steps are performed after deposition of said beam material and prior to said removal of said reflowed sacrificial material. 
     Optionally, said further MEMS process steps comprise forming an inkjet nozzle containing said suspended beam. 
     In a second aspect the present invention provides a method of fabricating a plurality of inkjet nozzles on a substrate, each nozzle comprising a nozzle chamber having a roof spaced apart from said substrate and sidewalls extending from said roof to said substrate, one of said sidewalls having a chamber entrance for receiving ink from an ink conduit extending along a row of nozzles, said ink conduit receiving ink from a plurality of ink inlets defined in said substrate, said method comprising the steps of:
         (a) providing a substrate having a plurality of trenches corresponding to said ink inlets;   (b) depositing sacrificial material on said substrate so as fill said trenches and form a scaffold on said substrate;   (c) defining openings in said sacrificial material, said openings being positioned to form said chamber sidewalls and said ink conduit when filled with roof material;   (d) depositing roof material over said sacrificial material to form simultaneously said nozzle chambers and said ink conduit;   (e) etching nozzle apertures through said roof material, each nozzle chamber having at least one nozzle aperture; and   (f) removing said sacrificial material.       

     Optionally, each nozzle chamber contains an actuator for ejecting ink through said nozzle aperture. 
     Optionally, said actuator is formed prior to fabrication of said nozzle chamber. 
     Optionally, said substrate is a silicon wafer. 
     Optionally, said silicon wafer comprises at least one surface oxide layer. 
     Optionally, said sacrificial material is photoresist. 
     Optionally, said openings are defined by exposing said photoresist through a mask followed by development. 
     Optionally, said photoresist is UV cured prior to deposition of said roof material, thereby preventing reflow of said photoresist during deposition. 
     Optionally, said photoresist is removed by plasma ashing. 
     In a further aspect there is provided a method further comprising the step of etching ink supply channels from an opposite backside of said substrate, said ink supply channels being in fluid communication with said ink inlets. 
     Optionally, each ink inlet has at least one priming feature extending from a respective rim thereof, and said method further comprises defining at least one opening corresponding to said at least one priming feature in said photoresist. 
     Optionally, said at least one priming feature comprises a column of roof material extending from said rim. 
     Optionally, each ink inlet has a plurality of priming features positioned about a respective rim thereof. 
     Optionally, said plurality of priming features together form a columnar cage extending from said rim. 
     Optionally, said chamber entrance includes at least one filter structure, and said method further comprises defining at least one opening corresponding to said at least one priming feature in said photoresist. 
     Optionally, said at least one filter structure comprises a column of roof material extending from said substrate to said roof. 
     Optionally, each chamber entrance includes a plurality of filter structures arranged across said entrance. 
     Optionally, each chamber entrance includes a plurality of rows of filter structures arranged across said entrance. 
     Optionally, said rows of filter structures are staggered. 
     In a third aspect there is provided a method of fabricating a plurality of inkjet nozzles on a substrate, each nozzle comprising a nozzle chamber having a roof spaced apart from said substrate and sidewalls extending from said roof to said substrate, said chamber having an entrance for receiving ink from at least one ink inlet defined in said substrate, said at least one ink inlet having at least one priming feature extending from a respective rim thereof, said method comprising the steps of:
         (a) providing a substrate having a plurality of trenches corresponding to said ink inlets;   (b) depositing sacrificial material on said substrate so as fill said trenches and form a scaffold on said substrate;   (c) defining openings in said sacrificial material, said openings being positioned to form said chamber sidewalls and said at least one priming feature when filled with roof material;   (d) depositing roof material over said sacrificial material to form simultaneously said nozzle chambers and said at least one priming feature;   (e) etching nozzle apertures through said roof material, each nozzle chamber having at least one nozzle aperture; and   (f) removing said sacrificial material.       

     Optionally, said at least one priming feature comprises a column of roof material extending from said rim. 
     Optionally, each ink inlet has a plurality of priming features positioned about a respective rim thereof. 
     Optionally, said plurality of priming features together form a columnar cage extending from said rim. 
     Optionally, each nozzle chamber contains an actuator for ejecting ink through said nozzle aperture. 
     Optionally, said actuator is formed prior to fabrication of said nozzle chamber. 
     Optionally, said substrate is a silicon wafer. 
     Optionally, said silicon wafer comprises at least one surface oxide layer. 
     Optionally, said sacrificial material is photoresist. 
     Optionally, said openings are defined by exposing said photoresist through a mask followed by development. 
     Optionally, said photoresist is UV cured prior to deposition of said roof material, thereby preventing reflow of said photoresist during deposition. 
     Optionally, said photoresist is removed by plasma ashing. 
     In a further aspect there is provided a method further comprising the step of etching ink supply channels from an opposite backside of said substrate, said ink supply channels being in fluid communication with said ink inlets. 
     Optionally, said chamber entrance is defined in one of said sidewalls of said nozzle chamber. 
     Optionally, said chamber entrance receives ink from an ink conduit extending along a row of nozzles, whereby step (c) further comprises defining further openings in said sacrificial material, said further openings being positioned to form said ink conduit when filled with roof material. 
     Optionally, said ink conduit receives ink from said at least one ink inlet. 
     In a fourth aspect the present invention provides a method of fabricating a plurality of inkjet nozzles on a substrate, each nozzle comprising a nozzle chamber having a roof spaced apart from said substrate and sidewalls extending from said roof to said substrate, one of said sidewalls having a chamber entrance for receiving ink from at least one ink inlet defined in said substrate, said chamber entrance including at least one filter structure, said method comprising the steps of:
         (a) providing a substrate having a plurality of trenches corresponding to said ink inlets;   (b) depositing sacrificial material on said substrate so as fill said trenches and form a scaffold on said substrate;   (c) defining openings in said sacrificial material, said openings being positioned to form said chamber sidewalls and said at least one filter structure when filled with roof material;   (d) depositing roof material over said sacrificial material to form simultaneously said nozzle chambers and said at least one filter structure;   (e) etching nozzle apertures through said roof material, each nozzle chamber having at least one nozzle aperture; and   (f) removing said sacrificial material.       

     Optionally, said filter structure comprises a column of roof material extending from said substrate to said roof. 
     Optionally, each chamber entrance includes a plurality of filter structures arranged across said entrance. 
     Optionally, each chamber entrance includes a plurality of rows of filter structures arranged across said entrance. 
     Optionally, said rows of filter structures are staggered. 
     Optionally, each nozzle chamber contains an actuator for ejecting ink through said nozzle aperture. 
     Optionally, said actuator is formed prior to fabrication of said nozzle chamber. 
     Optionally, said substrate is a silicon wafer. 
     Optionally, said silicon wafer comprises at least one surface oxide layer. 
     Optionally, said sacrificial material is photoresist. 
     Optionally, said openings are defined by exposing said photoresist through a mask followed by development. 
     Optionally, said photoresist is UV cured prior to deposition of said roof material, thereby preventing reflow of said photoresist during deposition. 
     Optionally, said photoresist is removed by plasma ashing. 
     In a further aspect there is provided a method further comprising the step of etching ink supply channels from an opposite backside of said substrate, said ink supply channels being in fluid communication with said ink inlets. 
     Optionally, said chamber entrance receives ink from an ink conduit extending along a row of nozzles, whereby step (c) further comprises defining further openings in said sacrificial material, said further openings being positioned to form said ink conduit when filled with roof material. 
     Optionally, said ink conduit receives ink from said at least one ink inlet. 
     In a fifth aspect the present invention provides a method of forming a low-stiction nozzle plate for an inkjet printhead, said nozzle plate having a plurality of nozzle apertures defined therein, each nozzle aperture having a respective nozzle rim, said method comprising the steps of:
         (a) providing a partially-fabricated printhead comprising a plurality of inkjet nozzle assemblies sealed with roof material;   (b) etching partially into said roof material to define simultaneously said nozzle rims and a plurality of stiction-reducing formations; and   (c) etching through said roof material to define said nozzle apertures, thereby forming said nozzle plate.       

     Optionally, each nozzle rim comprises at least one projection around a perimeter of each nozzle aperture. 
     Optionally, each nozzle rim comprises a plurality of coaxial projections around a perimeter of each nozzle aperture. 
     Optionally, said at least one rim projection projects at least 1 micron from said nozzle plate. 
     Optionally, each stiction-reducing formation comprises a columnar projection on said nozzle plate. 
     Optionally, each columnar projection projects at least 1 micron from said nozzle plate. 
     Optionally, each columnar projection is spaced apart from an adjacent columnar projection by less than 2 microns. 
     Optionally, each stiction-reducing formation comprises an elongate wall projection on said nozzle plate. 
     Optionally, each wall projection projects at least 1 micron from said nozzle plate. 
     Optionally, said wall projections are positioned for minimizing color-mixing of inks on said nozzle plate. 
     Optionally, said wall projections extend along said nozzle plate parallel with rows of nozzles, each nozzle in a row ejecting the same colored ink. 
     Optionally, the positions of said nozzle rims and said stiction-reducing formations are defined by photolithographic masking. 
     Optionally, at least half of the surface area of said nozzle plate is tiled with stiction-reducing formations. 
     Optionally, said inkjet nozzle assemblies are formed on a silicon substrate and said nozzle plate is spaced apart from said substrate. 
     Optionally, said nozzle plate is comprised of silicon nitride, silicon oxide, silicon oxynitride or aluminium nitride. 
     Optionally, said nozzle assemblies are sealed by CVD or PECVD deposition of said roof material. 
     Optionally, said roof material is deposited onto a sacrificial scaffold. 
     Optionally, each inkjet nozzle assembly has at least one nozzle aperture associated therewith for ejection of ink. 
     Optionally, said nozzle plate is subsequently treated with a hydrophobizing material. 
     The printhead according to the invention comprises a plurality of nozzles, as well as a chamber and one or more heater elements corresponding to each nozzle. The smallest repeating units of the printhead will have an ink supply inlet feeding ink to one or more chambers. The entire nozzle array is formed by repeating these individual units. Such an individual unit is referred to herein as a “unit cell”.
         Also, the term “ink” is used to signify any ejectable liquid, and is not limited to conventional inks containing colored dyes. Examples of non-colored inks include fixatives, infra-red absorber inks, functionalized chemicals, adhesives, biological fluids, medicaments, water and other solvents, and so on. The ink or ejectable liquid also need not necessarily be a strictly a liquid, and may contain a suspension of solid particles.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: 
         FIG. 1  shows a partially fabricated unit cell of the MEMS nozzle array on a printhead according to the present invention, the unit cell being section along A-A of  FIG. 3 ; 
         FIG. 2  shows a perspective of the partially fabricated unit cell of  FIG. 1 ; 
         FIG. 3  shows the mark associated with the etch of the heater element trench; 
         FIG. 4  is a sectioned view of the unit cell after the etch of the trench; 
         FIG. 5  is a perspective view of the unit cell shown in  FIG. 4 ; 
         FIG. 6  is the mask associated with the deposition of sacrificial photoresist shown in  FIG. 7 ; 
         FIG. 7  shows the unit cell after the deposition of sacrificial photoresist trench, with partial enlargements of the gaps between the edges of the sacrificial material and the side walls of the trench; 
         FIG. 8  is a perspective of the unit cell shown in  FIG. 7 ; 
         FIG. 9  shows the unit cell following the reflow of the sacrificial photoresist to close the gaps along the side walls of the trench; 
         FIG. 10  is a perspective of the unit cell shown in  FIG. 9 ; 
         FIG. 11  is a section view showing the deposition of the heater material layer; 
         FIG. 12  is a perspective of the unit cell shown in  FIG. 11 ; 
         FIG. 13  is the mask associated with the metal etch of the heater material shown in  FIG. 14 ; 
         FIG. 14  is a section view showing the metal etch to shape the heater actuators; 
         FIG. 15  is a perspective of the unit cell shown in  FIG. 14 ; 
         FIG. 16  is the mask associated with the etch shown in  FIG. 17 ; 
         FIG. 17  shows the deposition of the photoresist layer and subsequent etch of the ink inlet to the passivation layer on top of the CMOS drive layers; 
         FIG. 18  is a perspective of the unit cell shown in  FIG. 17 ; 
         FIG. 19  shows the oxide etch through the passivation and CMOS layers to the underlying silicon wafer; 
         FIG. 20  is a perspective of the unit cell shown in  FIG. 19 ; 
         FIG. 21  is the deep anisotropic etch of the ink inlet into the silicon wafer; 
         FIG. 22  is a perspective of the unit cell shown in  FIG. 21 ; 
         FIG. 23  is the mask associated with the photoresist etch shown in  FIG. 24 ; 
         FIG. 24  shows the photoresist etch to form openings for the chamber roof and side walls; 
         FIG. 25  is a perspective of the unit cell shown in  FIG. 24 ; 
         FIG. 26  shows the deposition of the side wall and risk material; 
         FIG. 27  is a perspective of the unit cell shown in  FIG. 26 ; 
         FIG. 28  is the mask associated with the nozzle rim etch shown in  FIG. 29 ; 
         FIG. 29  shows the etch of the roof layer to form the nozzle aperture rim; 
         FIG. 30  is a perspective of the unit cell shown in  FIG. 29 ; 
         FIG. 31  is the mask associated with the nozzle aperture etch shown in  FIG. 32 ; 
         FIG. 32  shows the etch of the roof material to form the elliptical nozzle apertures; 
         FIG. 33  is a perspective of the unit cell shown in  FIG. 32 ; 
         FIG. 34  shows the oxygen plasma release etch of the first and second sacrificial layers; 
         FIG. 35  is a perspective of the unit cell shown in  FIG. 34 ; 
         FIG. 36  shows the unit cell after the release etch, as well as the opposing side of the wafer; 
         FIG. 37  is a perspective of the unit cell shown in  FIG. 36 ; 
         FIG. 38  is the mask associated with the reverse etch shown in  FIG. 39 ; 
         FIG. 39  shows the reverse etch of the ink supply channel into the wafer; 
         FIG. 40  is a perspective of unit cell shown in  FIG. 39 ; 
         FIG. 41  shows the thinning of the wafer by backside etching; 
         FIG. 42  is a perspective of the unit cell shown in  FIG. 41 ; 
         FIG. 43  is a partial perspective of the array of nozzles on the printhead according to the present invention; 
         FIG. 44  shows the plan view of a unit cell; 
         FIG. 45  shows a perspective of the unit cell shown in  FIG. 44 ; 
         FIG. 46  is schematic plan view of two unit cells with the roof layer removed but certain roof layer features shown in outline only; 
         FIG. 47  is schematic plan view of two unit cells with the roof layer removed but the nozzle openings shown in outline only; 
         FIG. 48  is a partial schematic plan view of unit cells with ink inlet apertures in the sidewall of the chambers; 
         FIG. 49  is schematic plan view of a unit cells with the roof layer removed but the nozzle openings shown in outline only; 
         FIG. 50  is a partial plan view of the nozzle plate with stiction reducing formations and a particle of paper dust; 
         FIG. 51  is a partial plan view of the nozzle plate with residual ink gutters; 
         FIG. 52  is a partial section view showing the deposition of SAC 1 photoresist in accordance with prior art techniques used to avoid stringers; 
         FIG. 53  is a partial section view showing the depositon of a layer of heater material onto the SAC1 photoresist scaffold deposited in  FIG. 52 ; and, 
         FIG. 54  is a partial schematic plan view of a unit cell with multiple nozzles and actuators in each of the chambers. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the description than follows, corresponding reference numerals relate to corresponding parts. For convenience, the features indicated by each reference numeral are listed below. 
     MNN MPN SERIES PARTS LIST 
     
         
           1 . Nozzle Unit Cell 
           2 . Silicon Wafer 
           3 . Topmost Aluminium Metal Layer in the CMOS metal layers 
           4 . Passivation Layer 
           5 . CVD Oxide Layer 
           6 . Ink Inlet Opening in Topmost Aluminium Metal Layer  3 . 
           7 . Pit Opening in Topmost Aluminium Metal Layer  3 . 
           8 . Pit 
           9 . Electrodes 
           10 . SAC1 Photoresist Layer 
           11 . Heater Material (TiAlN) 
           12 . Thermal Actuator 
           13 . Photoresist Layer 
           14 . Ink Inlet Opening Etched Through Photo Resist Layer 
           15 . Ink Inlet Passage 
           16 . SAC2 Photoresist Layer 
           17 . Chamber Side Wall Openings 
           18 . Front Channel Priming Feature 
           19 . Barrier Formation at Ink Inlet 
           20 . Chamber Roof Layer 
           21 . Roof 
           22 . Sidewalls 
           23 . Ink Conduit 
           24 . Nozzle Chambers 
           25 . Elliptical Nozzle Rim
         25 ( a ) Inner Lip     25 ( b ) Outer Lip     
           26 . Nozzle Aperture 
           27 . Ink Supply Channel 
           28 . Contacts 
           29 . Heater Element. 
           30 . Bubble cage 
           32 . bubble retention structure 
           34 . ink permeable structure 
           36 . bleed hole 
           38 . ink chamber 
           40 . dual row filter 
           42 . paper dust 
           44 . ink gutters 
           46 . gap between SAC1 and trench sidewall 
           48 . trench sidewall 
           50 . raised lip of SAC1 around edge of trench 
           52 . thinner inclined section of heater material 
           54 . cold spot between series connected heater elements 
           56 . nozzle plate 
           58 . columnar projections 
           60 . sidewall ink opening 
           62 . ink refill opening
 
MEMS Manufacturing Process
 
       
    
     The MEMS manufacturing process builds up nozzle structures on a silicon wafer after the completion of CMOS processing.  FIG. 2  is a cutaway perspective view of a nozzle unit cell  100  after the completion of CMOS processing and before MEMS processing. 
     During CMOS processing of the wafer, four metal layers are deposited onto a silicon wafer  2 , with the metal layers being interspersed between interlayer dielectric (ILD) layers. The four metal layers are referred to as M1, M2, M3 and M4 layers and are built up sequentially on the wafer during CMOS processing. These CMOS layers provide all the drive circuitry and logic for operating the printhead. 
     In the completed printhead, each heater element actuator is connected to the CMOS via a pair of electrodes defined in the outermost M4 layer. Hence, the M4 CMOS layer is the foundation for subsequent MEMS processing of the wafer. The M4 layer also defines bonding pads along a longitudinal edge of each printhead integrated circuit. These bonding pads (not shown) allow the CMOS to be connected to a microprocessor via wire bonds extending from the bonding pads. 
       FIGS. 1 and 2  show the aluminium M4 layer  3  having a passivation layer  4  deposited thereon. (Only MEMS features of the M4 layer are shown in these Figures; the main CMOS features of the M4 layer are positioned outside the nozzle unit cell). The M4 layer  3  has a thickness of 1 micron and is itself deposited on a 2 micron layer of CVD oxide  5 . As shown in  FIGS. 1 and 2 , the M4 layer  3  has an ink inlet opening  6  and pit openings  7 . These openings define the positions of the ink inlet and pits formed subsequently in the MEMS process. 
     Before MEMS processing of the unit cell  1  begins, bonding pads along a longitudinal edge of each printhead integrated circuit are defined by etching through the passivation layer  4 . This etch reveals the M4 layer  3  at the bonding pad positions. The nozzle unit cell  1  is completely masked with photoresist for this step and, hence, is unaffected by the etch. 
     Turning to  FIGS. 3 to 5 , the first stage of MEMS processing etches a pit  8  through the passivation layer.  4  and the CVD oxide layer  5 . This etch is defined using a layer of photoresist (not shown) exposed by the dark tone pit mask shown in  FIG. 3 . The pit  8  has a depth of 2 microns, as measured from the top of the M4 layer  3 . At the same time as etching the pit  8 , electrodes  9  are defined on either side of the pit by partially revealing the M4 layer  3  through the passivation layer  4 . In the completed nozzle, a heater element is suspended across the pit  8  between the electrodes  9 . 
     In the next step ( FIGS. 6 to 8 ), the pit  8  is filled with a first sacrificial layer (“SAC1”) of photoresist  10 . A 2 micron layer of high viscosity photoresist is first spun onto the wafer and then exposed using the dark tone mask shown in  FIG. 6 . The SAC1 photoresist  10  forms a scaffold for subsequent deposition of the heater material across the electrodes  9  on either side of the pit  8 . Consequently, it is important the SAC1 photoresist  10  has a planar upper surface that is flush with the upper surface of the electrodes  9 . At the same time, the SAC1 photoresist must completely fill the pit  8  to avoid ‘stringers’ of conductive heater material extending across the pit and shorting out the electrodes  9 . 
     Typically, when filling trenches with photoresist, it is necessary to expose the photoresist outside the perimeter of the trench in order to ensure that photoresist fills against the walls of the trench and, therefore, avoid ‘stringers’ in subsequent deposition steps. However, this technique results in a raised (or spiked) rim of photoresist around the perimeter of the trench. This is undesirable because in a subsequent deposition step, material is deposited unevenly onto the raised rim—vertical or angled surfaces on the rim will receive less deposited material than the horizontal planar surface of the photoresist filling the trench. The result is ‘resistance hotspots’ in regions where material is thinly deposited. 
     As shown in  FIG. 7 , the present process deliberately exposes the SAC1 photoresist  10  inside the perimeter walls of the pit  8  (e.g. within 0.5 microns) using the mask shown in  FIG. 6 . This ensures a planar upper surface of the SAC1 photoresist  10  and avoids any spiked regions of photoresist around the perimeter rim of the pit  8 . 
     After exposure of the SAC1 photoresist  10 , the photoresist is reflowed by heating. Reflowing the photoresist allows it to flow to the walls of the pit  8 , filling it exactly.  FIGS. 9 and 10  show the SAC1 photoresist  10  after reflow. The photoresist has a planar upper surface and meets flush with the upper surface of the M4 layer  3 , which forms the electrodes  9 . Following reflow, the SAC1 photoresist  10  is U.V. cured and/or hardbaked to avoid any reflow during the subsequent deposition step of heater material. 
       FIGS. 11 and 12  show the unit cell after deposition of the 0.5 microns of heater material  11  onto the SAC1 photoresist  10 . Due to the reflow process described above, the heater material  11  is deposited evenly and in a planar layer over the electrodes  9  and the SAC1 photoresist  10 . The heater material may be comprised of any suitable conductive material, such as TiAl, TiN, TAlN, TiAlSiN etc. A typical heater material deposition process may involve sequential deposition of a 100 Å seed layer of TiAl, a 2500 Å layer of TiAlN, a further 100 Å seed layer of TiAl and finally a further 2500 Å layer of TiAlN. 
     Referring to  FIGS. 13 to 15 , in the next step, the layer of heater material  11  is etched to define the thermal actuator  12 . Each actuator  12  has contacts  28  that establish an electrical connection to respective electrodes  9  on either side of the SAC1 photoresist  10 . A heater element  29  spans between its corresponding contacts  28 . 
     This etch is defined by a layer of photoresist (not shown) exposed using the dark tone mask shown in  FIG. 13 . As shown in  FIG. 15 , the heater element  12  is a linear beam spanning between the pair of electrodes  9 . However, the heater element  12  may alternatively adopt other configurations, such as those described in Applicant&#39;s U.S. Pat. No. 6,755,509, the content of which is herein incorporated by reference. For example, heater element  29  configurations having a central void may be advantageous for minimizing the deleterious effects of cavitation forces on the heater material when a bubble collapses during ink ejection. Other forms of cavitation protection may be adopted such as ‘bubble venting’ and the use of self passivating materials. These cavitation management techniques are discussed in detail in U.S. patent application (our docket MTC001US). 
     In the next sequence of steps, an ink inlet for the nozzle is etched through the passivation layer  4 , the oxide layer  5  and the silicon wafer  2 . During CMOS processing, each of the metal layers had an ink inlet opening (see, for example, opening  6  in the M 4  layer  3  in  FIG. 1 ) etched therethrough in preparation for this ink inlet etch. These metal layers, together with the interspersed ILD layers, form a seal ring for the ink inlet, preventing ink from seeping into the CMOS layers. 
     Referring to  FIGS. 16 to 18 , a relatively thick layer of photoresist  13  is spun onto the wafer and exposed using the dark tone mask shown in  FIG. 16 . The thickness of photoresist  13  required will depend on the selectivity of the deep reactive ion etch (DRIE) used to etch the ink inlet. With an ink inlet opening  14  defined in the photoresist  13 , the wafer is ready for the subsequent etch steps. 
     In the first etch step ( FIGS. 19 and 20 ), the dielectric layers (passivation layer  4  and oxide layer  5 ) are etched through to the silicon wafer below. Any standard oxide etch (e.g. O 2 /C 4 F 8  plasma) may be used. 
     In the second etch step ( FIGS. 21 and 22 ), an ink inlet  15  is etched through the silicon wafer  2  to a depth of 25 microns, using the same photoresist mask  13 . Any standard anisotropic DRIE, such as the Bosch etch (see U.S. Pat. Nos. 6,501,893 and 6,284,148) may be used for this etch. Following etching of the ink inlet  15 , the photoresist layer  13  is removed by plasma ashing. 
     In the next step, the ink inlet  15  is plugged with photoresist and a second sacrificial layer (“SAC2”) of photoresist  16  is built up on top of the SAC1 photoresist  10  and passivation layer  4 . The SAC2 photoresist  16  will serve as a scaffold for subsequent deposition of roof material, which forms a roof and sidewalls for each nozzle chamber. Referring to  FIGS. 23 to 25 , a ˜6 micron layer of high viscosity photoresist is spun onto the wafer and exposed using the dark tone mask shown in  FIG. 23 . 
     As shown in  FIGS. 23 and 25 , the mask exposes sidewall openings  17  in the SAC2 photoresist  16  corresponding to the positions of chamber sidewalls and sidewalls for an ink conduit. In addition, openings  18  and  19  are exposed adjacent the plugged inlet  15  and nozzle chamber entrance respectively. These openings  18  and  19  will be filled with roof material in the subsequent roof deposition step and provide unique advantages. in the present nozzle design. Specifically, the openings  18  filled with roof material act as priming features, which assist in drawing ink from the inlet  15  into each nozzle chamber. This is described in greater detail below. The openings  19  filled with roof material act as filter structures and fluidic cross talk barriers. These help prevent air bubbles from entering the nozzle chambers and diffuses pressure pulses generated by the thermal actuator  12 . 
     Referring to  FIGS. 26 and 27 , the next stage deposits 3 microns of roof material  20  onto the SAC2 photoresist  16  by PECVD. The roof material  20  fills the openings  17 ,  18  and  19  in the SAC2 photoresist  16  to form nozzle chambers  24  having a roof  21  and sidewalls  22 . An ink conduit  23  for supplying ink into each nozzle chamber is also formed during deposition of the roof material  20 . In addition, any priming features and filter structures (not shown in  FIGS. 26 and 27 ) are formed at the same time. The roofs  21 , each corresponding to a respective nozzle chamber  24 , span across adjacent nozzle chambers in a row to form a continuous nozzle plate. The roof material  20  may be comprised of any suitable material, such as silicon nitride, silicon oxide, silicon oxynitride, aluminium nitride etc. 
     Referring to  FIGS. 28 to 30 , the next stage defines an elliptical nozzle rim  25  in the roof  21  by etching away 2 microns of roof material  20 . This etch is defined using a layer of photoresist (not shown) exposed by the dark tone rim mask shown in  FIG. 28 . The elliptical rim  25  comprises two coaxial rim lips  25   a  and  25   b , positioned over their respective thermal actuator  12 . 
     Referring to  FIGS. 31 to 33 , the next stage defines an elliptical nozzle aperture  26  in the roof  21  by etching all the way through the remaining roof material  20 , which is bounded by the rim  25 . This etch is defined using a layer of photoresist (not shown) exposed by the dark tone roof mask shown in  FIG. 31 . The elliptical nozzle aperture  26  is positioned over the thermal actuator  12 , as shown in  FIG. 33 . 
     With all the MEMS nozzle features now fully formed, the next stage removes the SAC1 and SAC2 photoresist layers  10  and  16  by O 2  plasma ashing ( FIGS. 34 to 35 ). After ashing, the thermal actuator  12  is suspended in a single plane over the pit  8 . The coplanar deposition of the contacts  28  and the heater element  29  provides an efficient electrical connection with the electrodes  9 . 
       FIGS. 36 and 37  show the entire thickness (150 microns) of the silicon wafer  2  after ashing the SAC1 and SAC2 photoresist layers  10  and  16 . 
     Referring to  FIGS. 38 to 40 , once frontside MEMS processing of the wafer is completed, ink supply channels  27  are etched from the backside of the wafer to meet with the ink inlets  15  using a standard anisotropic DRIE. This backside etch is defined using a layer of photoresist (not shown) exposed by the dark tone mask shown in  FIG. 38 . The ink supply channel  27  makes a fluidic connection between the backside of the wafer and the ink inlets  15 . 
     Finally, and referring to  FIGS. 41 and 42 , the wafer is thinned 135 microns by backside etching.  FIG. 43  shows three adjacent rows of nozzles in a cutaway perspective view of a completed printhead integrated circuit. Each row of nozzles has a respective ink supply channel  27  extending along its length and supplying ink to a plurality of ink inlets  15  in each row. The ink inlets, in turn, supply ink to the ink conduit  23  for each row, with each nozzle chamber receiving ink from a common ink conduit for that row. 
     Features and Advantages of Particular Embodiments 
     Discussed below, under appropriate sub-headings, are certain specific features of embodiments of the invention, and the advantages of these features. The features are to be considered in relation to all of the drawings pertaining to the present invention unless the context specifically excludes certain drawings, and relates to those drawings specifically referred to. 
     Low Loss Electrodes 
     As shown in  FIGS. 41 and 42 , the heater element  29  is suspended within the chamber. This ensures that the heater element is immersed in ink when the chamber is primed. Completely immersing the heater element in ink dramatically improves the printhead efficiency. Much less heat dissipates into the underlying wafer substrate so more of the input energy is used to generate the bubble that ejects the ink. 
     To suspend the heater element, the contacts may be used to support the element at its raised position. Essentially, the contacts at either end of the heater element can have vertical or inclined sections to connect the respective electrodes on the CMOS drive to the element at an elevated position. However, heater material deposited on vertical or inclined surfaces is thinner than on horizontal surfaces. To avoid undesirable resistive losses from the thinner sections, the contact portion of the thermal actuator needs to be relatively large. Larger contacts occupy a significant area of the wafer surface and limit the nozzle packing density. 
     To immerse the heater, the present invention etches a pit or trench  8  between the electrodes  9  to drop the level of the chamber floor. As discussed above, a layer of sacrificial photoresist (SAC)  10  (see  FIG. 9 ) is deposited in the trench to provide a scaffold for the heater element. However, depositing SAC  10  in the trench  8  and simply covering it with a layer of heater material, can lead to stringers forming in the gaps  46  between the SAC  10  and the sidewalls  48  of the trench  8  (as previously described in relation to  FIG. 7 ). The gaps form because it is difficult to precisely match the mask with the sides of the trench  8 . Usually, when the masked photoresist is exposed, the gaps  46  form between the sides of the pit and the SAC. When the heater material layer is deposited, it fills these gaps to form ‘stringers’ (as they are known). The stringers remain in the trench  8  after the metal etch (that shapes the heater element) and the release etch (to finally remove the SAC). The stringers can short circuit the heater so that it fails to generate a bubble. 
     Turning now to  FIGS. 52 and 53 , the ‘traditional’ technique for avoiding stringers is illustrated. By making the UV mask that exposes the SAC slightly bigger than the trench  8 , the SAC  10  will be deposited over the side walls  48  so that no gaps form. Unfortunately, this produces a raised lip  50  around top of the trench. When the heater material layer  11  is deposited (see  FIG. 53 ), it is thinner on the vertical or inclined surfaces  52  of the lip  50 . After the metal etch and release etch, these thin lip formations  52  remain and cause ‘hotspots’ because the localized thinning increases resistance. These hotspots affect the operation of the heater and typically reduce heater life. 
     As discussed above, the Applicant has found that reflowing the SAC  10  closes the gaps  46  so that the scaffold between the electrodes  9  is completely flat. This allows the entire thermal actuator  12  to be planar. The planar structure of the thermal actuator, with contacts directly deposited onto the CMOS electrodes  9  and suspended heater element  29 , avoids hotspots caused by vertical or inclined surfaces so that the contacts can be much smaller structures without acceptable increases in resistive losses. Low resistive losses preserves the efficient operation of a suspended heater element and the small contact size is convenient for close nozzle packing on the printhead. 
     Multiple Nozzles for Each Chamber 
     Referring to  FIG. 49 , the unit cell shown has two separate ink chambers  38 , each chamber having heater element  29  extending between respective pairs of contacts  28 . Ink permeable structures  34  are positioned in the ink refill openings so that ink can enter the chambers, but upon actuation, the structures  34  provide enough hydraulic resistance to reduce any reverse flow or fluidic cross talk to an acceptable level. 
     Ink is fed from the reverse side of the wafer through the ink inlet  15 . Priming features  18  extend into the inlet opening so that an ink meniscus does not pin itself to the peripheral edge of the opening and stop the ink flow. Ink from the inlet  15  fills the lateral ink conduit  23  which supplies both chambers  38  of the unit cell. 
     Instead of a single nozzle per chamber, each chamber  38  has two nozzles  25 . When the heater element  29  actuates (forms a bubble), two drops of ink are ejected; one from each nozzle  25 . Each individual drop of ink has less volume than the single drop ejected if the chamber had only one nozzle. By ejecting multiple drops from a single chamber simultaneously improves the print quality. 
     With every nozzle, there is a degree of misdirection in the ejected drop. Depending on the degree of misdirection, this can be detrimental to print quality. By giving the chamber multiple nozzles, each nozzle ejects drops of smaller volume, and having different misdirections. Several small drops misdirected in different directions are less detrimental to print quality than a single relatively large misdirected drop. The Applicant has found that the eye averages the misdirections of each small drop and effectively ‘sees’ a dot from a single drop with a significantly less overall misdirection. 
     A multi nozzle chamber can also eject drops more efficiently than a single nozzle chamber. The heater element  29  is an elongate suspended beam of TiAlN and the bubble it forms is likewise elongated. The pressure pulse created by an elongate bubble will cause ink to eject through a centrally disposed nozzle. However, some of the energy from the pressure pulse is dissipated in hydraulic losses associated with the mismatch between the geometry of the bubble and that of the nozzle. 
     Spacing several nozzles  25  along the length of the heater element  29  reduces the geometric discrepancy between the bubble shape and the nozzle configuration through which the ink ejects. This in turn reduces hydraulic resistance to ink ejection and thereby improves printhead efficiency. 
     Ink Chamber Re-Filled Via Adjacent Ink Chamber 
     Referring to  FIG. 46 , two opposing unit cells are shown. In this embodiment, unit cell has four ink chambers  38 . The chambers are defined by the sidewalls  22  and the ink permeable structures  34 . Each chamber has its own heater element  29 . The heater elements  29  are arranged in pairs that are connected in series. Between each pair is ‘cold spot’  54  with lower resistance and or greater heat sinking. This ensures that bubbles do not nucleate at the cold spots  54  and thus the cold spots become the common contact between the outer contacts  28  for each heater element pair. 
     The ink permeable structures  34  allow ink to refill the chambers  38  after drop ejection but baffle the pressure pulse from each heater element  29  to reduce the fluidic cross talk between adjacent chambers. It will be appreciated that this embodiment has many parallels with that shown in  FIG. 49  discussed above. However, the present embodiment effectively divides the relatively long chambers of  FIG. 49  into two separate chambers. This further aligns the geometry of the bubble formed by the heater element  29  with the shape of the nozzle  25  to reduce hydraulic losses during drop ejection. This is achieved without reducing the nozzle density but it does add some complexity to the fabrication process. 
     The conduits (ink inlets  15  and supply conduits  23 ) for distributing ink to every ink chamber in the array can occupy a significant proportion of the wafer area. This can be a limiting factor for nozzle density on the printhead. By making some ink chambers part of the ink flow path to other ink chambers, while keeping each chamber sufficiently free of fluidic cross talk, reduces the amount of wafer area lost to ink supply conduits. 
     Ink Chamber with Multiple Actuators and Respective Nozzles 
     Referring to  FIG. 54 , the unit cell shown has two chambers  38 ; each chamber has two heater elements  29  and two nozzles  25 . The effective reduction in drop misdirection by using multiple nozzles per chamber is discussed above in relation to the embodiment shown in  FIG. 49 . The additional benefits of dividing a single elongate chamber into separate chambers, each with their own actuators, is described above with reference to the embodiment shown in  FIG. 46 . The present embodiment uses multiple nozzles and multiple actuators in each chamber to achieve much of the advantages of the  FIG. 46  embodiment with a markedly less complicated design. With a simplified design, the overall dimensions of the unit cell are reduced thereby permitting greater nozzle densities. In the embodiment shown, the footprint of the unit cell is 64 μm long by 16 μm wide. 
     The ink permeable structure  34  is a single column at the ink refill opening to each chamber  38  instead of three spaced columns as with the  FIG. 46  embodiment. The single column has a cross section profiled to be less resistive to refill flow, but more resistive to sudden back flow from the actuation pressure pulse. Both heater elements in each chamber can be deposited simultaneously, together with the contacts  28  and the cold spot feature  54 . Both chambers  38  are supplied with ink from a common ink inlet  15  and supply conduit  23 . These features also allow the footprint to be reduced and they are discussed in more detail below. The priming features  18  have been made integral with one of the chamber sidewalls  22  and a wall ink conduit  23 . The dual purpose nature of these features simplifies the fabrication and helps to keep the design compact. 
     Multiple Chambers and Multiple Nozzles for Each Drive Circuit 
     In  FIG. 54 , the actuators are connected in series and therefore fire in unison from the same drive signal to simplify the CMOS drive circuitry. In the  FIG. 46  unit cell, actuators in adjacent nozzles are connected in series within the same drive circuit. Of course, the actuators in adjacent chambers could also be connected in parallel. In contrast, were the actuators in each chamber to be in separate circuits, the CMOS drive circuitry would be more complex and the dimensions of the unit cell footprint would increase. In printhead designs where the drop misdirection is addressed by substituting multiple smaller drops, combining several actuators and their respective nozzles into a common drive circuit is an efficient implementation both in terms of printhead IC fabrication and nozzles density. 
     High Density Thermal Inkjet Printhead 
     Reduction in the unit cell width enables the printhead to have nozzles patterns that previously would have required the nozzle density to be reduced. Of course, a lower nozzle density has a corresponding influence on printhead size and/or print quality. 
     Traditionally, the nozzle rows are arranged in pairs with the actuators for each row extending in opposite directions. The rows are staggered with respect to each other so that the printing resolution (dots per inch) is twice the nozzle pitch (nozzles per inch) along each row. By configuring the components of the unit cell such that the overall width of the unit is reduced, the same number of nozzles can be arranged into a single row instead of two staggered and opposing rows without sacrificing any print resolution (d.p.i.). The embodiments shown in the accompanying figures achieve a nozzle pitch of more than 1000 nozzles per inch in each linear row. At this nozzle pitch, the print resolution of the printhead is better than photographic (1600 dpi) when two opposing staggered rows are considered, and there is sufficient capacity for nozzle redundancy, dead nozzle compensation and so on which ensures the operation life of the printhead remains satisfactory. As discussed above, the embodiment shown in  FIG. 54  has a footprint that is 16 μm wide and therefore the nozzle pitch along one row is about 1600 nozzles per inch. 
     Accordingly, two offset staggered rows yield a resolution of about 3200 d.p.i. 
     With the realisation of the particular benefits associated with a narrower unit cell, the Applicant has focused on identifying and combining a number of features to reduce the relevant dimensions of structures in the printhead. For example, elliptical nozzles, shifting the ink inlet from the chamber, finer geometry logic and shorter drive FETs (field effect transistors) are features developed by the Applicant to derive some of the embodiments shown. Each contributing feature necessitated a departure from conventional wisdom in the field, such as reducing the FET drive voltage from the widely used traditional 5V to 2.5V in order to decrease transistor length. 
     Reduced Stiction Printhead Surface 
     Static friction, or “stiction” as it has become known, allows dust particles to “stick” to nozzle plates and thereby clog nozzles.  FIG. 50  shows a portion of the nozzle plate  56 . For clarity, the nozzle apertures  26  and the nozzle rims  25  are also shown. The exterior surface of the nozzle plate is patterned with columnar projections  58  extending a short distance from the plate surface. The nozzle plate could also be patterned with other surface formations such as closely spaced ridges, corrugations or bumps. However, it is easy to create a suitable UV mask for the pattern columnar projections shown, and it is a simple matter to etch the columns into the exterior surface. 
     By reducing the co-efficient of static friction, there is less likelihood that paper dust or other contaminants will clog the nozzles in the nozzle plate. Patterning the exterior of the nozzle plate with raised formations limits the surface area that dust particles contact. If the particles can only contact the outer extremities of each formation, the friction between the particles and the nozzle plate is minimal so attachment is much less likely. If the particles do attach, they are more likely to be removed by printhead maintenance cycles. 
     Inlet Priming Feature 
     Referring to  FIG. 47 , two unit cells are shown extending in opposite directions to each other. The ink inlet passage  15  supplies ink to the four chambers  38  via the lateral ink conduit  23 . Distributing ink through micron-scale conduits, such as the ink inlet  15 , to individual MEMS nozzles in an inkjet printhead is complicated by factors that do not arise in macro-scale flow. A meniscus can form and, depending on the geometry of the aperture, it can ‘pin’ itself to the lip of the aperture quite strongly. This can be useful in printheads, such as bleed holes that vent trapped air bubbles but retain the ink, but it can also be problematic if stops ink flow to some chambers. This will most likely occur when initially priming the printhead with ink. If the ink meniscus pins at the ink inlet opening, the chambers supplied by that inlet will stay unprimed. 
     To guard against this, two priming features  18  are formed so that they extend through the plane of the inlet aperture  15 . The priming features  18  are columns extending from the interior of the nozzle plate (not shown) to the periphery of the inlet  15 . A part of each column  18  is within the periphery so that the surface tension of an ink meniscus at the ink inlet will form at the priming features  18  so as to draw the ink out of the inlet. This ‘unpins’ the meniscus from that section of the periphery and the flow toward the ink chambers. 
     The priming features  18  can take many forms, as long as they present a surface that extends transverse to the plane of the aperture. Furthermore, the priming feature can be an integral part of other nozzles features as shown in  FIG. 54 . 
     Side Entry Ink Chamber 
     Referring to  FIG. 48 , several adjacent unit cells are shown. In this embodiment, the elongate heater elements  29  extend parallel to the ink distribution conduit  23 . Accordingly, the elongate ink chambers  38  are likewise aligned with the ink conduit  23 . Sidewall openings  60  connect the chambers  38  to the ink conduit  23 . Configuring the ink chambers so that they have side inlets reduces the ink refill times. The inlets are wider and therefore refill flow rates are higher. The sidewall openings  60  have ink permeable structures  34  to keep fluidic cross talk to an acceptable level. 
     Inlet Filter for Ink Chamber 
     Referring again to  FIG. 47 , the ink refill opening to each chamber  38  has a filter structure  40  to trap air bubbles or other contaminants. Air bubbles and solid contaminants in ink are detrimental to the MEMS nozzle structures. The solid contaminants can obvious clog the nozzle openings, while air bubbles, being highly compressible, can absorb the pressure pulse from the actuator if they get trapped in the ink chamber. This effectively disables the ejection of ink from the affected nozzle. By providing a filter structure  40  in the form of rows of obstructions extending transverse to the flow direction through the opening, each row being spaced such that they are out of registration with the obstructions in an adjacent row with respect to the flow direction, the contaminants are not likely to enter the chamber  38  while the ink refill flow rate is not overly retarded. The rows are offset with respect to each other and the induced turbulence has minimal effect on the nozzle refill rate but the air bubbles or other contaminants follow a relatively tortuous flow path which increases the chance of them being retained by the obstructions  40 . The embodiment shown uses two rows of obstructions  40  in the form of columns extending between the wafer substrate and the nozzle plate. 
     Intercolour Surface Barriers in Multi Colour Inkjet Printhead 
     Turning now to  FIG. 51 , the exterior surface of the nozzle  56  is shown for a unit cell such as that shown in  FIG. 46  described above. The nozzle apertures  26  are positioned directly above the heater elements (not shown) and a series of square-edged ink gutters  44  are formed in the nozzle plate  56  above the ink conduit  23  (see  FIG. 46 ). 
     Inkjet printers often have maintenance stations that cap the printhead when it&#39;s not in use. To remove excess ink from the nozzle plate, the capper can be disengaged so that it peels off the exterior surface of the nozzle plate. This promotes the formation of a meniscus between the capper surface and the exterior of the nozzle plate. Using contact angle hysteresis, which relates to the angle that the surface tension in the meniscus contacts the surface (for more detail, see the Applicant&#39;s co-pending USSN (our docket FND007US) incorporated herein by reference), the majority of ink wetting the exterior of the nozzle plate can be collected and drawn along by the meniscus between the capper and nozzle plate. The ink is conveniently deposited as a large bead at the point where the capper fully disengages from the nozzle plate. Unfortunately, some ink remains on the nozzle plate. If the printhead is a multi-colour printhead, the residual ink left in or around a given nozzle aperture, may be a different colour than that ejected by the nozzle because the meniscus draws ink over the whole surface of the nozzle plate. The contamination of ink in one nozzle by ink from another nozzle can create visible artefacts in the print. Gutter formations  44  running transverse to the direction that the capper is peeled away from the nozzle plate will remove and retain some of the ink in the meniscus. While the gutters do not collect all the ink in the meniscus, they do significantly reduce the level of nozzle contamination of with different coloured ink. 
     Bubble Trap 
     Air bubbles entrained in the ink are very bad for printhead operation. Air, or rather gas in general, is highly compressible and can absorb the pressure pulse from the actuator. If a trapped bubble simply compresses in response to the actuator, ink will not eject from the nozzle. Trapped bubbles can be purged from the printhead with a forced flow of ink, but the purged ink needs blotting and the forced flow could well introduce fresh bubbles. 
     The embodiment shown in  FIG. 46  has a bubble trap at the ink inlet  15 . The trap is formed by a bubble retention structure  32  and a vent  36  formed in the roof layer. The bubble retention structure is a series of columns  32  spaced around the periphery of the inlet  15 . As discussed above, the ink priming features  18  have a dual purpose and conveniently form part of the bubble retaining structure. In use, the ink permeable trap directs gas bubbles to the vent where they vent to atmosphere. By trapping the bubbles at the ink inlets and directing them to a small vent, they are effectively removed from the ink flow without any ink leakage. 
     Multiple Ink Inlet Flow Paths 
     Supplying ink to the nozzles via conduits extending from one side of the wafer to the other allows more of the wafer area (on the ink ejection side) to have nozzles instead of complex ink distribution systems. However, deep etched, micron-scale holes through a wafer are prone to clogging from contaminants or air bubbles. This starves the nozzle(s) supplied by the affected inlet. 
     As best shown in  FIG. 48 , printheads according to the present invention have at least two ink inlets  15  supplying each chamber  38  via an ink conduit  23  between the nozzle plate and underlying wafer. 
     Introducing an ink conduit  23  that supplies several of the chambers  38 , and is in itself supplied by several ink inlets  15 , reduces the chance that nozzles will be starved of ink by inlet clogging. If one inlet  15  is clogged, the ink conduit will draw more ink from the other inlets in the wafer. 
     Although the invention is described above with reference to specific embodiments, it will be understood by those skilled in the art that the invention may be embodied in many other forms.

Technology Classification (CPC): 1