Abstract:
An inkjet printhead includes a plurality of ink chambers fed by a common ink inlet, the plurality of ink chambers sharing a common roof structure; a plurality of heater elements respectively allocated to each of ink chambers; a plurality of nozzle apertures defined through the common roof structure and respectively aligned with the heater element of each ink chamber; and a bubble vent defined in the common roof of the ink chambers and aligned with the common ink inlet, the bubble vent sized to allow ink surface tension to restrict an egress of ink, and permit egress of gas bubbles therefrom.

Description:
CONTINUATION INFORMATION 
       [0001]    The present application is a Continuation of U.S. application Ser. No. 13/018,360 filed Jan. 31, 2011, which is a Continuation of U.S. application Ser. No. 12/267,615 filed Nov. 10, 2008, now issued U.S. Pat. No. 7,901,056, which is a Continuation-In-Part of U.S. application Ser. No. 11/482,953, filed Jul. 10, 2006, now issued U.S. Pat. No. 7,654,645, the entire contents of which are now incorporated by reference, which is in turn, a Continuation-In-Part of U.S. application Ser. No. 11/097,308 (now Abandoned) filed on Apr. 4, 2005 all of which is herein incorporated by reference. 
     
    
     CROSS REFERENCES TO RELATED APPLICATIONS 
       [0002]    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, the contents of which are incorporated herein by reference: 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 6,750,901 
                 6,476,863 
                 6,788,336 
                 7,249,108 
                 6,566,858 
                 6,331,946 
               
               
                 6,246,970 
                 6,442,525 
                 7,346,586 
                 7,685,423 
                 6,374,354 
                 7,246,098 
               
               
                 6,816,968 
                 6,757,832 
                 6,334,190 
                 6,745,331 
                 7,249,109 
                 7,197,642 
               
               
                 7,093,139 
                 7,509,292 
                 7,685,424 
                 7,743,262 
                 7,210,038 
                 7,401,223 
               
               
                 7,702,926 
                 7,716,098 
                 7,364,256 
                 7,258,417 
                 7,293,853 
                 7,328,968 
               
               
                 7,270,395 
                 7,461,916 
                 7,510,264 
                 7,334,864 
                 7,255,419 
                 7,284,819 
               
               
                 7,229,148 
                 7,258,416 
                 7,273,263 
                 7,270,393 
                 6,984,017 
                 7,347,526 
               
               
                 7,357,477 
                 7,465,015 
                 7,364,255 
                 7,357,476 
                 7,758,148 
                 7,284,820 
               
               
                 7,341,328 
                 7,246,875 
                 7,322,669 
                 7,445,311 
                 7,452,052 
                 7,455,383 
               
               
                 7,448,724 
                 7,441,864 
                 7,506,958 
                 7,472,981 
                 7,448,722 
                 7,575,297 
               
               
                 7,438,381 
                 7,441,863 
                 7,438,382 
                 7,425,051 
                 7,399,057 
                 7,695,097 
               
               
                 7,686,419 
                 7,753,472 
                 7,448,720 
                 7,448,723 
                 7,445,310 
                 7,399,054 
               
               
                 7,425,049 
                 7,367,648 
                 7,370,936 
                 7,401,886 
                 7,506,952 
                 7,401,887 
               
               
                 7,384,119 
                 7,401,888 
                 7,387,358 
                 7,413,281 
                 6,623,101 
                 6,406,129 
               
               
                 6,505,916 
                 6,457,809 
                 6,550,895 
                 6,457,812 
                 7,152,962 
                 6,428,133 
               
               
                 7,204,941 
                 7,282,164 
                 7,465,342 
                 7,278,727 
                 7,417,141 
                 7,452,989 
               
               
                 7,367,665 
                 7,138,391 
                 7,153,956 
                 7,423,145 
                 7,456,277 
                 7,550,585 
               
               
                 7,122,076 
                 7,148,345 
                 7,470,315 
                 7,572,327 
                 7,416,280 
                 7,252,366 
               
               
                 7,488,051 
                 7,360,865 
                 7,438,371 
                 7,465,017 
                 7,441,862 
                 7,654,636 
               
               
                 7,458,659 
                 7,455,376 
                 6,746,105 
                 7,744,195 
                 7,645,026 
                 7,322,681 
               
               
                 7,708,387 
                 7,753,496 
                 7,712,884 
                 7,510,267 
                 7,465,041 
                 7,857,428 
               
               
                 7,465,032 
                 7,401,890 
                 7,401,910 
                 7,470,010 
                 7,735,971 
                 7,431,432 
               
               
                 7,465,037 
                 7,445,317 
                 7,549,735 
                 7,597,425 
                 7,661,800 
                 7,712,869 
               
               
                 7,156,508 
                 7,159,972 
                 7,083,271 
                 7,165,834 
                 7,080,894 
                 7,201,469 
               
               
                 7,090,336 
                 7,156,489 
                 7,413,283 
                 7,438,385 
                 7,083,257 
                 7,258,422 
               
               
                 7,255,423 
                 7,219,980 
                 7,591,533 
                 7,416,274 
                 7,367,649 
                 7,118,192 
               
               
                 7,618,121 
                 7,322,672 
                 7,077,505 
                 7,198,354 
                 7,077,504 
                 7,614,724 
               
               
                 7,198,355 
                 7,401,894 
                 7,322,676 
                 7,152,959 
                 7,213,906 
                 7,178,901 
               
               
                 7,222,938 
                 7,108,353 
                 7,104,629 
                 7,303,930 
                 7,401,405 
                 7,464,466 
               
               
                 7,464,465 
                 7,246,886 
                 7,128,400 
                 7,108,355 
                 6,991,322 
                 7,287,836 
               
               
                 7,118,197 
                 7,575,298 
                 7,364,269 
                 7,077,493 
                 6,962,402 
                 7,686,429 
               
               
                 7,147,308 
                 7,524,034 
                 7,118,198 
                 7,168,790 
                 7,172,270 
                 7,229,155 
               
               
                 6,830,318 
                 7,195,342 
                 7,175,261 
                 7,465,035 
                 7,108,356 
                 7,118,202 
               
               
                 7,510,269 
                 7,134,744 
                 7,510,270 
                 7,134,743 
                 7,182,439 
                 7,210,768 
               
               
                 7,465,036 
                 7,134,745 
                 7,156,484 
                 7,118,201 
                 7,111,926 
                 7,431,433 
               
               
                 7,018,021 
                 7,401,901 
                 7,468,139 
                 7,128,402 
                 7,387,369 
                 7,484,832 
               
               
                 7,448,729 
                 7,246,876 
                 7,431,431 
                 7,419,249 
                 7,377,623 
                 7,328,978 
               
               
                 7,334,876 
                 7,147,306 
                 7,721,948 
                 7,079,712 
                 6,825,945 
                 7,330,974 
               
               
                 6,813,039 
                 6,987,506 
                 7,038,797 
                 6,980,318 
                 6,816,274 
                 7,102,772 
               
               
                 7,350,236 
                 6,681,045 
                 6,728,000 
                 7,173,722 
                 7,088,459 
                 7,707,082 
               
               
                 7,068,382 
                 7,062,651 
                 6,789,194 
                 6,789,191 
                 6,644,642 
                 6,502,614 
               
               
                 6,622,999 
                 6,669,385 
                 6,549,935 
                 6,987,573 
                 6,727,996 
                 6,591,884 
               
               
                 6,439,706 
                 6,760,119 
                 7,295,332 
                 6,290,349 
                 6,428,155 
                 6,785,016 
               
               
                 6,870,966 
                 6,822,639 
                 6,737,591 
                 7,055,739 
                 7,233,320 
                 6,830,196 
               
               
                 6,832,717 
                 6,957,768 
                 7,456,820 
                 7,170,499 
                 7,106,888 
                 7,123,239 
               
               
                 7,377,608 
                 7,399,043 
                 7,121,639 
                 7,165,824 
                 7,152,942 
                 7,818,519 
               
               
                 7,181,572 
                 7,096,137 
                 7,302,592 
                 7,278,034 
                 7,188,282 
                 7,592,829 
               
               
                 7,770,008 
                 7,707,621 
                 7,523,111 
                 7,573,301 
                 7,660,998 
                 7,783,886 
               
               
                 7,831,827 
                 7,171,323 
                 7,278,697 
                 7,369,270 
                 6,795,215 
                 7,070,098 
               
               
                 7,154,638 
                 6,805,419 
                 6,859,289 
                 6,977,751 
                 6,398,332 
                 6,394,573 
               
               
                 6,622,923 
                 6,747,760 
                 6,921,144 
                 7,092,112 
                 7,192,106 
                 7,457,001 
               
               
                 7,173,739 
                 6,986,560 
                 7,008,033 
                 7,551,324 
                 7,222,780 
                 7,270,391 
               
               
                 7,195,328 
                 7,182,422 
                 7,374,266 
                 7,427,117 
                 7,448,707 
                 7,281,330 
               
               
                 7,328,956 
                 7,735,944 
                 7,188,928 
                 7,093,989 
                 7,377,609 
                 7,600,843 
               
               
                 10/854,498 
                 7,390,071 
                 7,549,715 
                 7,252,353 
                 7,607,757 
                 7,267,417 
               
               
                 7,517,036 
                 7,275,805 
                 7,314,261 
                 7,281,777 
                 7,290,852 
                 7,484,831 
               
               
                 7,758,143 
                 7,832,842 
                 7,549,718 
                 7,866,778 
                 7,631,190 
                 7,557,941 
               
               
                 7,757,086 
                 7,266,661 
                 7,243,193 
                 7,163,345 
                 7,465,033 
                 7,452,055 
               
               
                 7,470,002 
                 7,722,161 
                 7,475,963 
                 7,448,735 
                 7,465,042 
                 7,448,739 
               
               
                 7,438,399 
                 7,467,853 
                 7,461,922 
                 7,465,020 
                 7,722,185 
                 7,461,910 
               
               
                 7,270,494 
                 7,632,032 
                 7,475,961 
                 7,547,088 
                 7,611,239 
                 7,735,955 
               
               
                 7,758,038 
                 7,681,876 
                 7,780,161 
                 7,448,734 
                 7,425,050 
                 7,364,263 
               
               
                 7,201,468 
                 7,360,868 
                 7,234,802 
                 7,303,255 
                 7,287,846 
                 7,156,511 
               
               
                 7,258,432 
                 7,097,291 
                 7,645,025 
                 7,083,273 
                 7,367,647 
                 7,374,355 
               
               
                 7,441,880 
                 7,547,092 
                 7,513,598 
                 7,198,352 
                 7,364,264 
                 7,303,251 
               
               
                 7,201,470 
                 7,121,655 
                 7,293,861 
                 7,232,208 
                 7,328,985 
                 7,344,232 
               
               
                 7,083,272 
                 7,621,620 
                 7,669,961 
                 7,331,663 
                 7,360,861 
                 7,328,973 
               
               
                 7,427,121 
                 7,407,262 
                 7,303,252 
                 7,249,822 
                 7,537,309 
                 7,311,382 
               
               
                 7,360,860 
                 7,364,257 
                 7,390,075 
                 7,350,896 
                 7,429,096 
                 7,384,135 
               
               
                 7,331,660 
                 7,416,287 
                 7,488,052 
                 7,322,684 
                 7,322,685 
                 7,311,381 
               
               
                 7,270,405 
                 7,303,268 
                 7,470,007 
                 7,399,072 
                 7,393,076 
                 7,681,967 
               
               
                 7,588,301 
                 7,249,833 
                 7,524,016 
                 7,490,927 
                 7,331,661 
                 7,524,043 
               
               
                 7,300,140 
                 7,357,492 
                 7,357,493 
                 7,566,106 
                 7,380,902 
                 7,284,816 
               
               
                 7,284,845 
                 7,255,430 
                 7,390,080 
                 7,328,984 
                 7,350,913 
                 7,322,671 
               
               
                 7,380,910 
                 7,431,424 
                 7,470,006 
                 7,585,054 
                 7,347,534 
                 7,441,865 
               
               
                 7,469,989 
                 7,367,650 
                 7,469,990 
                 7,441,882 
                 7,556,364 
                 7,357,496 
               
               
                 7,467,863 
                 7,431,440 
                 7,431,443 
                 7,527,353 
                 7,524,023 
                 7,513,603 
               
               
                 7,467,852 
                 7,465,045 
               
               
                   
               
             
          
         
       
     
       CO-PENDING APPLICATIONS 
       [0003]    The following applications have been filed by the Applicant with the parent application: 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 7,637,588 
                 7,648,222 
                 7,669,958 
                 7,607,755 
                 7,699,433 
                 7,658,463 
               
               
                 7,530,663 
                 7,467,846 
                 7,669,957 
                 7,771,028 
                 7,758,174 
                 7,695,123 
               
               
                 7,798,600 
                 7,604,334 
                 7,857,435 
                 7,708,375 
                 7,695,093 
                 7,695,098 
               
               
                 7,722,156 
                 7,703,882 
                 7,510,261 
                 7,722,153 
                 7,658,792 
                 7,709,633 
               
               
                 7,733,535 
                 7,934,092 
                 11/482,966 
                 7,645,033 
                 7,681,000 
                 7,530,446 
               
               
                 7,784,915 
                 7,571,906 
                 7,703,903 
                 7,645,034 
                 7,637,602 
               
               
                   
               
             
          
         
       
     
         [0004]    The disclosures of these co-pending applications are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0005]    The invention relates to MEMS devices and in particular MEMS devices that vaporize liquid to generate a vapor bubble during operation. 
       BACKGROUND OF THE INVENTION 
       [0006]    Some micro-mechanical systems (MEMS) devices process, or use liquids to operate. In one class of these liquid-containing devices, resistive heaters are used to heat the liquid to the liquid&#39;s superheat limit, resulting in the formation of a rapidly expanding vapor bubble. The impulse provided by the bubble expansion can be used as a mechanism for moving liquid through the device. This is the case in thermal inkjet printheads where each nozzle has a heater that generates a bubble to eject a drop of ink onto the print media. In light of the widespread use of inkjet printers, the present invention will be described with particular reference to its use in this application. However, it will be appreciated that the invention is not limited to inkjet printheads and is equally suited to other devices in which vapor bubbles formed by resistive heaters are used to move liquid through the device (e.g. some ‘Lab-on-a-chip’ devices). 
         [0007]    The resistive heaters in inkjet printheads operate in an extremely harsh environment. They must heat and cool in rapid succession to form bubbles in the ejectable liquid—usually a water soluble ink with a superheat limit of approximately 300° C. Under these conditions of cyclic stress, in the presence of hot ink, water vapor, dissolved oxygen and possibly other corrosive species, the heaters will increase in resistance and ultimately go open circuit via a combination of oxidation and fatigue, accelerated by mechanisms that corrode the heater or its protective oxide layers (chemical corrosion and cavitation corrosion). 
         [0008]    To protect against the effects of oxidation, corrosion and cavitation on the heater material, inkjet manufacturers use stacked protective layers, typically made from Si 3 N 4 , SiC and Ta. In certain prior art devices, the protective layers are relatively thick. U.S. Pat. No. 6,786,575 to Anderson et al (assigned to Lexmark) for example, has 0.7 μm of protective layers for a ˜0.1 μm thick heater. 
         [0009]    To form a vapor bubble in the bubble forming liquid, the surface of the protective layers in contact with the bubble forming liquid must be heated to the superheat limit of the liquid (˜300° C. for water). This requires that the entire thickness of the protective layers be heated to (or in some cases above) the liquid superheat limit. Heating this additional volume decreases the efficiency of the device and significantly increases the level of residual heat present after firing. If this additional heat cannot be removed between successive firings of the nozzle, the ink in the nozzles will boil continuously, causing the nozzles to cease ejecting droplets in the intended manner. 
         [0010]    The primary cooling mechanism of printheads on the market is currently thermal conduction, with existing printheads implementing a large heat sink to dissipate heat absorbed from the printhead chip. The ability of this heatsink to cool the liquid in the nozzles is limited by the thermal resistance between the nozzles and the heatsink and by the heat flux generated by the firing nozzles. As the extra energy required to heat the protective layers of a coated heater contributes to an increased heat flux, more severe constraints are imposed on the density of the nozzles on the printhead and the nozzle firing rate. This in turn has an impact on the print resolution, the printhead size, the print speed and the manufacturing costs. 
         [0011]    The Applicant has developed a range of printheads without protective coatings added to the heaters to lower the energy needed to form a vapor bubble. These heaters form a thin surface oxide layer that has an oxygen diffusivity low enough to slow the rate of further oxidation to a level that gives the printhead an acceptable lifetime. However, the oxide layer does grow with time and in particular, with the number of drive pulses or actuations sent to the heater. This changes the heater resistance over the operation lifetime of the heater and therefore the drop ejection characteristics. It will be appreciated that this can be detrimental to print quality. 
       SUMMARY OF THE INVENTION 
       [0012]    According to an aspect of the present disclosure, an inkjet printhead comprises a plurality of ink chambers fed by a common ink inlet, the plurality of ink chambers sharing a common roof structure; a plurality of heater elements respectively allocated to each of ink chambers; a plurality of nozzle apertures defined through the common roof structure and respectively aligned with the heater element of each ink chamber; and a bubble vent defined in the common roof of the ink chambers and aligned with the common ink inlet, the bubble vent sized to allow ink surface tension to restrict an egress of ink, and permit egress of gas bubbles therefrom. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Preferred embodiments of the present invention will now be described, by way of example only with reference to the accompanying drawings in which: 
           [0014]      FIG. 1  is a schematic cross-sectional view through an ink chamber of a unit cell of a printhead with a suspended heater element at a particular stage during its operative cycle. 
           [0015]      FIG. 2  is a schematic cross-sectional view through the ink chamber  FIG. 1 , at another stage of operation. 
           [0016]      FIG. 3  is a schematic cross-sectional view through the ink chamber  FIG. 1 , at yet another stage of operation. 
           [0017]      FIG. 4  is a schematic cross-sectional view through the ink chamber  FIG. 1 , at yet a further stage of operation. 
           [0018]      FIG. 5  is a diagrammatic cross-sectional view through a unit cell of a printhead in accordance with an embodiment of the invention showing the collapse of a vapor bubble. 
           [0019]      FIG. 6  is a schematic cross-sectional view through an ink chamber of a unit cell of a printhead with a floor bonded heater element, at a particular stage during its operative cycle. 
           [0020]      FIG. 7  is a schematic cross-sectional view through the ink chamber of  FIG. 6 , at another stage of operation. 
           [0021]      FIG. 8  is a schematic cross-sectional view through an ink chamber of a unit cell of a printhead with a roof bonded heater element, at a particular stage during its operative cycle. 
           [0022]      FIG. 9  is a schematic cross-sectional view through the ink chamber of  FIG. 8 , at another stage of operation; 
           [0023]      FIGS. 10 ,  12 ,  14 ,  15 ,  17 ,  18 ,  20 ,  22 ,  23 ,  25 ,  27 ,  28 ,  30 ,  32 ,  34  and  36  are schematic perspective views of a unit cell of a printhead in accordance with a suspended heater embodiment of the invention, at various successive stages in the production process of the printhead; 
           [0024]      FIGS. 11 ,  13 ,  16 ,  19 ,  21 ,  24 ,  26 ,  29 ,  31 ,  33  and  35  are each schematic plan views of a mask suitable for use in performing the production stage for the printhead, as represented in the respective immediately preceding figures; 
           [0025]      FIGS. 37 and 38  are a schematic section view and perspective view respectively of a partially complete second embodiment of the invention, wherein the passivation layer has been deposited on the CMOS; 
           [0026]      FIGS. 39 ,  40  and  41  are a perspective, mask and section view respectively showing the etch through the passivation layer to the top layer of the CMOS of the second embodiment; 
           [0027]      FIGS. 42 and 43  are a perspective and section views respectively showing the deposition of the heater material of the second embodiment; 
           [0028]      FIGS. 44 ,  45  and  46  are a perspective, mask and section view respectively showing the etch patterning the heater material of the second embodiment; 
           [0029]      FIGS. 47 ,  48  and  49  are a perspective, mask and section view respectively showing the deposition of a photoresist layer and subsequent etch for the dielectric etch of the front ink hole; 
           [0030]      FIGS. 50 and 51  are a perspective and section view respectively showing the dielectric etch into the wafer for the front ink hole; 
           [0031]      FIGS. 52 and 53  are a perspective and section view respectively showing the deposition of a new photoresist layer; 
           [0032]      FIGS. 54 ,  55  and  56  are a perspective, mask and section view respectively showing the patterning of the photoresist layer; 
           [0033]      FIGS. 57 and 58  are a perspective and section view respectively showing the deposition of the roof layer; 
           [0034]      FIGS. 59 ,  60  and  61  are a perspective, mask and section view respectively showing the etch of the nozzle rims into the roof layer; 
           [0035]      FIGS. 62 ,  63  and  64  are a perspective, mask and section view respectively showing the etch of the nozzle openings; 
           [0036]      FIGS. 65 and 66  are a perspective and section view respectively showing the deposition of the protective photoresist overcoat; 
           [0037]      FIGS. 67 and 68  are a perspective and section view respectively showing the back etch of the wafer; 
           [0038]      FIG. 69  is a section view showing the release etch removing the remaining photoresist; 
           [0039]      FIG. 70  is plan view of the completed unit cell of the second embodiment; and, 
           [0040]      FIG. 71  is a Weibull chart showing the reliability of a Inconel™ 718 heater element with a nanocrystalline microstructure compared to a TiAlN heater. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    In the description that follows, corresponding reference numerals, or corresponding prefixes of reference numerals (i.e. the parts of the reference numerals appearing before a point mark) which are used in different figures relate to corresponding parts. Where there are corresponding prefixes and differing suffixes to the reference numerals, these indicate different specific embodiments of corresponding parts. 
         [0042]    Overview of the Invention and General Discussion of Operation 
         [0043]    With reference to  FIGS. 1 to 4 , the unit cell  1  of a printhead according to an embodiment of the invention comprises a nozzle plate  2  with nozzles  3  therein, the nozzles having nozzle rims  4 , and apertures  5  extending through the nozzle plate. The nozzle plate  2  is plasma etched from a silicon nitride structure which is deposited, by way of chemical vapor deposition (CVD), over a sacrificial material which is subsequently etched. 
         [0044]    The printhead also includes, with respect to each nozzle  3 , side walls  6  on which the nozzle plate is supported, a chamber  7  defined by the walls and the nozzle plate  2 , a multi-layer substrate  8  and an inlet passage  9  extending through the multi-layer substrate to the far side (not shown) of the substrate. A looped, elongate heater element  10  is suspended within the chamber  7 , so that the element is in the form of a suspended beam. The printhead as shown is a microelectromechanical system (MEMS) structure, which is formed by a lithographic process which is described in more detail below. 
         [0045]    When the printhead is in use, ink  11  from a reservoir (not shown) enters the chamber  7  via the inlet passage  9 , so that the chamber fills to the level as shown in  FIG. 1 . Thereafter, the heater element  10  is heated for somewhat less than 1 microsecond (μs), so that the heating is in the form of a thermal pulse. It will be appreciated that the heater element  10  is in thermal contact with the ink  11  in the chamber  7  so that when the element is heated, this causes the generation of vapor bubbles  12  in the ink. Accordingly, the ink  11  constitutes a bubble forming liquid.  FIG. 1  shows the formation of a bubble  12  approximately 1 μs after generation of the thermal pulse, that is, when the bubble has just nucleated on the heater elements  10 . It will be appreciated that, as the heat is applied in the form of a pulse, all the energy necessary to generate the bubble  12  is to be supplied within that short time. 
         [0046]    Turning briefly to  FIG. 35 , there is shown a mask  13  for forming a heater  14  (as shown in  FIG. 34 ) of the printhead (which heater includes the element  10  referred to above), during a lithographic process, as described in more detail below. As the mask  13  is used to form the heater  14 , the shapes of several of its parts correspond to the shape of the element  10 . The mask  13  therefore provides a useful reference by which to identify various parts of the heater  14 . The heater  14  has electrodes  15  corresponding to the parts designated  15 . 34  of the mask  13  and a heater element  10  corresponding to the parts designated  10 . 34  of the mask. In operation, voltage is applied across the electrodes  15  to cause current to flow through the element  10 . The electrodes  15  are much thicker than the element  10  so that most of the electrical resistance is provided by the element. Thus, nearly all of the power consumed in operating the heater  14  is dissipated via the element  10 , in creating the thermal pulse referred to above. 
         [0047]    When the element  10  is heated as described above, the bubble  12  forms along the length of the element, this bubble appearing, in the cross-sectional view of  FIG. 1 , as four bubble portions, one for each of the element portions shown in cross section. 
         [0048]    The bubble  12 , once generated, causes an increase in pressure within the chamber  7 , which in turn causes the ejection of a drop  16  of the ink  11  through the nozzle  3 . The rim  4  assists in directing the drop  16  as it is ejected, so as to minimize the chance of drop misdirection. 
         [0049]    The reason that there is only one nozzle  3  and chamber  7  per inlet passage  9  is so that the pressure wave generated within the chamber, on heating of the element  10  and forming of a bubble  12 , does not affect adjacent chambers and their corresponding nozzles. However, it is possible to feed ink to several chambers via a single inlet passage as long as pressure pulse diffusing structures are positioned between chambers. The embodiment shown in  FIGS. 37 to 70  incorporates these structures for the purpose of reducing cross talk to an acceptable level. 
         [0050]    The advantages of the heater element  10  being suspended rather than embedded in any solid material, are discussed below. However, there are also advantages to bonding the heater element to the internal surfaces of the chamber. These are discussed below with reference to  FIGS. 6 to 9 . 
         [0051]      FIGS. 2 and 3  show the unit cell  1  at two successive later stages of operation of the printhead. It can be seen that the bubble  12  generates further, and hence grows, with the resultant advancement of ink  11  through the nozzle  3 . The shape of the bubble  12  as it grows, as shown in  FIG. 3 , is determined by a combination of the inertial dynamics and the surface tension of the ink  11 . The surface tension tends to minimize the surface area of the bubble  12  so that, by the time a certain amount of liquid has evaporated, the bubble is essentially disk-shaped. 
         [0052]    The increase in pressure within the chamber  7  not only pushes ink  11  out through the nozzle  3 , but also pushes some ink back through the inlet passage  9 . However, the inlet passage  9  is approximately 200 to 300 microns in length, and is only about 16 microns in diameter. Hence there is a substantial inertia and viscous drag limiting back flow. As a result, the predominant effect of the pressure rise in the chamber  7  is to force ink out through the nozzle  3  as an ejected drop  16 , rather than back through the inlet passage  9 . 
         [0053]    Turning now to  FIG. 4 , the printhead is shown at a still further successive stage of operation, in which the ink drop  16  that is being ejected is shown during its “necking phase” before the drop breaks off. At this stage, the bubble  12  has already reached its maximum size and has then begun to collapse towards the point of collapse  17 , as reflected in more detail in  FIG. 5 . 
         [0054]    The collapsing of the bubble  12  towards the point of collapse  17  causes some ink  11  to be drawn from within the nozzle  3  (from the sides  18  of the drop), and some to be drawn from the inlet passage  9 , towards the point of collapse. Most of the ink  11  drawn in this manner is drawn from the nozzle  3 , forming an annular neck  19  at the base of the drop  16  prior to its breaking off. 
         [0055]    The drop  16  requires a certain amount of momentum to overcome surface tension forces, in order to break off. As ink  11  is drawn from the nozzle  3  by the collapse of the bubble  12 , the diameter of the neck  19  reduces thereby reducing the amount of total surface tension holding the drop, so that the momentum of the drop as it is ejected out of the nozzle is sufficient to allow the drop to break off. 
         [0056]    When the drop  16  breaks off, cavitation forces are caused as reflected by the arrows  20 , as the bubble  12  collapses to the point of collapse  17 . It will be noted that there are no solid surfaces in the vicinity of the point of collapse  17  on which the cavitation can have an effect. 
         [0057]    Manufacturing Process for Suspended Heater Element Embodiments. 
         [0058]    Relevant parts of the manufacturing process of a printhead according to embodiments of the invention are now described with reference to  FIGS. 10 to 33 . 
         [0059]    Referring to  FIG. 10 , there is shown a cross-section through a silicon substrate portion  21 , being a portion of a Memjet™ printhead, at an intermediate stage in the production process thereof. This figure relates to that portion of the printhead corresponding to a unit cell  1 . The description of the manufacturing process that follows will be in relation to a unit cell  1 , although it will be appreciated that the process will be applied to a multitude of adjacent unit cells of which the whole printhead is composed. 
         [0060]      FIG. 10  represents the next successive step, during the manufacturing process, after the completion of a standard CMOS fabrication process, including the fabrication of CMOS drive transistors (not shown) in the region  22  in the substrate portion  21 , and the completion of standard CMOS interconnect layers  23  and passivation layer  24 . Wiring indicated by the dashed lines  25  electrically interconnects the transistors and other drive circuitry (also not shown) and the heater element corresponding to the nozzle. 
         [0061]    Guard rings  26  are formed in the metallization of the interconnect layers  23  to prevent ink  11  from diffusing from the region, designated  27 , where the nozzle of the unit cell  1  will be formed, through the substrate portion  21  to the region containing the wiring  25 , and corroding the CMOS circuitry disposed in the region designated  22 . 
         [0062]    The first stage after the completion of the CMOS fabrication process consists of etching a portion of the passivation layer  24  to form the passivation recesses  29 . 
         [0063]      FIG. 12  shows the stage of production after the etching of the interconnect layers  23 , to form an opening  30 . The opening  30  is to constitute the ink inlet passage to the chamber that will be formed later in the process. 
         [0064]      FIG. 14  shows the stage of production after the etching of a hole  31  in the substrate portion  21  at a position where the nozzle  3  is to be formed. Later in the production process, a further hole (indicated by the dashed line  32 ) will be etched from the other side (not shown) of the substrate portion  21  to join up with the hole  31 , to complete the inlet passage to the chamber. Thus, the hole  32  will not have to be etched all the way from the other side of the substrate portion  21  to the level of the interconnect layers  23 . 
         [0065]    If, instead, the hole  32  were to be etched all the way to the interconnect layers  23 , then to avoid the hole  32  being etched so as to destroy the transistors in the region  22 , the hole  32  would have to be etched a greater distance away from that region so as to leave a suitable margin (indicated by the arrow  34 ) for etching inaccuracies. But the etching of the hole  31  from the top of the substrate portion  21 , and the resultant shortened depth of the hole  32 , means that a lesser margin  34  need be left, and that a substantially higher packing density of nozzles can thus be achieved. 
         [0066]      FIG. 15  shows the stage of production after a four micron thick layer  35  of a sacrificial resist has been deposited on the layer  24 . This layer  35  fills the hole  31  and now forms part of the structure of the printhead. The resist layer  35  is then exposed with certain patterns (as represented by the mask shown in  FIG. 16 ) to form recesses  36  and a slot  37 . This provides for the formation of contacts for the electrodes  15  of the heater element to be formed later in the production process. The slot  37  will provide, later in the process, for the formation of the nozzle walls  6  that will define part of the chamber  7 . 
         [0067]      FIG. 21  shows the stage of production after the deposition, on the layer  35 , of a 0.5 micron thick layer  38  of heater material, which, in the present embodiment, is of titanium aluminium nitride. 
         [0068]      FIG. 18  shows the stage of production after patterning and etching of the heater layer  38  to form the heater  14 , including the heater element  10  and electrodes  15 . 
         [0069]      FIG. 20  shows the stage of production after another sacrificial resist layer  39 , about 1 micron thick, has been added. 
         [0070]      FIG. 22  shows the stage of production after a second layer  40  of heater material has been deposited. In a preferred embodiment, this layer  40 , like the first heater layer  38 , is of 0.5 micron thick titanium aluminium nitride. 
         [0071]      FIG. 23  then shows this second layer  40  of heater material after it has been etched to form the pattern as shown, indicated by reference numeral  41 . In this illustration, this patterned layer does not include a heater layer element  10 , and in this sense has no heater functionality. However, this layer of heater material does assist in reducing the resistance of the electrodes  15  of the heater  14  so that, in operation, less energy is consumed by the electrodes which allows greater energy consumption by, and therefore greater effectiveness of, the heater elements  10 . In the dual heater embodiment illustrated in  FIG. 42 , the corresponding layer  40  does contain a heater  14 . 
         [0072]      FIG. 25  shows the stage of production after a third layer  42 , of sacrificial resist, has been deposited. The uppermost level of this layer will constitute the inner surface of the nozzle plate  2  to be formed later. This is also the inner extent of the ejection aperture  5  of the nozzle. The height of this layer  42  must be sufficient to allow for the formation of a bubble  12  in the region designated  43  during operation of the printhead. However, the height of layer  42  determines the mass of ink that the bubble must move in order to eject a droplet. In light of this, the printhead structure of the present invention is designed such that the heater element is much closer to the ejection aperture than in prior art printheads. The mass of ink moved by the bubble is reduced. The generation of a bubble sufficient for the ejection of the desired droplet will require less energy, thereby improving efficiency. 
         [0073]      FIG. 27  shows the stage of production after the roof layer  44  has been deposited, that is, the layer which will constitute the nozzle plate  2 . Instead of being formed from 100 micron thick polyimide film, the nozzle plate  2  is formed of silicon nitride, just 2 microns thick. 
         [0074]      FIG. 28  shows the stage of production after the chemical vapor deposition (CVD) of silicon nitride forming the layer  44 , has been partly etched at the position designated  45 , so as to form the outside part of the nozzle rim  4 , this outside part being designated  4 . 1   
         [0075]      FIG. 30  shows the stage of production after the CVD of silicon nitride has been etched all the way through at  46 , to complete the formation of the nozzle rim  4  and to form the ejection aperture  5 , and after the CVD silicon nitride has been removed at the position designated  47  where it is not required. 
         [0076]      FIG. 32  shows the stage of production after a protective layer  48  of resist has been applied. After this stage, the substrate portion  21  is then ground from its other side (not shown) to reduce the substrate portion from its nominal thickness of about 800 microns to about 200 microns, and then, as foreshadowed above, to etch the hole  32 . The hole  32  is etched to a depth such that it meets the hole  31 . 
         [0077]    Then, the sacrificial resist of each of the resist layers  35 ,  39 ,  42  and  48 , is removed using oxygen plasma, to form the structure shown in  FIG. 34 , with walls  6  and nozzle plate  2  which together define the chamber  7  (part of the walls and nozzle plate being shown cut-away). It will be noted that this also serves to remove the resist filling the hole  31  so that this hole, together with the hole  32  (not shown in  FIG. 34 ), define a passage extending from the lower side of the substrate portion  21  to the nozzle  3 , this passage serving as the ink inlet passage, generally designated  9 , to the chamber  7 . 
         [0078]      FIG. 36  shows the printhead with the nozzle guard and chamber walls removed to clearly illustrate the vertically stacked arrangement of the heater elements  10  and the electrodes  15 . 
         [0079]    Bonded Heater Element Embodiments 
         [0080]    In other embodiments, the heater elements are bonded to the internal walls of the chamber. Bonding the heater to solid surfaces within the chamber allows the etching and deposition fabrication process to be simplified. However, heat conduction to the silicon substrate can reduce the efficiency of the nozzle so that it is no longer ‘self cooling’. Therefore, in embodiments where the heater is bonded to solid surfaces within the chamber, it is necessary to take steps to thermally isolate the heater from the substrate. 
         [0081]    One way of improving the thermal isolation between the heater and the substrate is to find a material with better thermal barrier properties than silicon dioxide, which is the traditionally used thermal barrier layer, described in U.S. Pat. No. 4,513,298. The Applicant has shown that the relevant parameter to consider when selecting the barrier layer, is the thermal product; (ρCk) 1/2 . The energy lost into a solid underlayer in contact with the heater is proportional to the thermal product of the underlayer, a relationship which may be derived by considering the length scale for thermal diffusion and the thermal energy absorbed over that length scale. Given that proportionality, it can be seen that a thermal barrier layer with reduced density and thermal conductivity will absorb less energy from the heater. This aspect of the invention focuses on the use of materials with reduced density and thermal conductivity as thermal barrier layers inserted underneath the heater layer, replacing the traditional silicon dioxide layer. In particular, this aspect of the invention focuses on the use of low-k dielectrics as thermal barriers 
         [0082]    Low-k dielectrics have recently been used as the inter-metal dielectric of copper damascene integrated circuit technology. When used as an inter-metal dielectric, the reduced density and in some cases porosity of the low-k dielectrics help reduce the dielectric constant of the inter-metal dielectric, the capacitance between metal lines and the RC delay of the integrated circuit. In the copper damascene application, an undesirable consequence of the reduced dielectric density is poor thermal conductivity, which limits heat flow from the chip. In the thermal barrier application, low thermal conductivity is ideal, as it limits the energy absorbed from the heater. 
         [0083]    Two examples of low-k dielectrics suitable for application as thermal barriers are Applied Material&#39;s Black Diamond™ and Novellus&#39; Coral™, both of which are CVD deposited SiOCH films. These films have lower density than SiO 2  (˜1340 kgm −3  vs ˜2200 kgm −3 ) and lower thermal conductivity (˜0.4 Wm −1 K −1  vs ˜1.46 Wm −1 K −1 ). The thermal products for these materials are thus around 600 Jm −2 K −1 s −1/2 , compared to 1495 Jm −2 K −1 s −1/2  for SiO 2  i.e. a 60% reduction in thermal product. To calculate the benefit that may be derived by replacing SiO 2  underlayers with these materials, models using equation 3 in the Detailed Description can be used to show that ˜35% of the energy required to nucleate a bubble is lost by thermal diffusion into the underlayer when SiO 2  underlayers are used. The benefit of the replacement is therefore 60% of 35% i.e. a 21% reduction in nucleation energy. This benefit has been confirmed by the Applicant by comparing the energy required to nucleate a bubble on:
       1. heaters deposited directly onto SiO 2  and   2. heaters deposited directly onto Black Diamond™       
 
         [0086]    The latter required 20% less energy for the onset of bubble nucleation, as determined by viewing the bubble formation stroboscopically in an open pool boiling configuration, using water as a test fluid. The open pool boiling was run for over 1 billion actuations, without any shift in nucleation energy or degradation of the bubble, indicating the underlayer is thermally stable up to the superheat limit of the water i.e. ˜300° C. Indeed, such layers can be thermally stable up to 550° C., as described in work related to the use of these films as Cu diffusion barriers (see “Physical and Barrier Properties of Amorphous Silicon-Oxycarbide Deposited by PECVD from Octamethylcycltetrasiloxane”, Journal of The Electrochemical Society, 151 (2004) by Chiu-Chih Chiang et. al.). 
         [0087]    Further reduction in thermal conductivity, thermal product and the energy required to nucleate a bubble may be provided by introducing porosity into the dielectric, as has been done by Trikon Technologies, Inc. with their ORION™ 2.2 porous SiOCH film, which has a density of ˜1040 kgm −3  and thermal conductivity of ˜0.16 Wm −1 K −1  (see IST 2000 30043, “Final report on thermal modeling”, from the IST project “Ultra Low K Dielectrics For Damascene Copper Interconnect Schemes”). With a thermal product of ˜334 Jm −2 K −1 s −1/2 , this material would absorb 78% less energy than a SiO 2  underlayer, resulting in a 78*35%=27% reduction in the energy required to nucleate a bubble. It is possible however that the introduction of porosity may compromise the moisture resistance of the material, which would compromise the thermal properties, since water has a thermal product of 1579 Jm −2 K −1 s −1/2 , close to that of SiO 2 . A moisture barrier could be introduced between the heater and the thermal barrier, but the heat absorption in this layer would likely degrade overall efficiency: in the preferred embodiment the thermal barrier is directly in contact with the underside of the heater. If it is not in direct contact, the thermal barrier layer is preferably no more than 1 μm away from the heater layer, as it will have little effect otherwise (the length scale for heat diffusion in the ˜1 μs time scale of the heating pulse in e.g. SiO 2  is 
         [0088]    An alternative for further lowering thermal conductivity without using porosity is to use the spin-on dielectrics, such as Dow Corning&#39;s SiLK™, which has a thermal conductivity of 0.18 Wm −1 K −1 . The spin-on films can also be made porous, but as with the CVD films, that may compromise moisture resistance. SiLK has thermal stability up to 450° C. One point of concern regarding the spin-on dielectrics is that they generally have large coefficients of thermal expansion (CTEs). Indeed, it seems that reducing k generally increases the CTE. This is implied in “A Study of Current Multilevel Interconnect Technologies for 90 nm Nodes and Beyond”, by Takayuki Ohba, Fujitsu magazine, Volume 38-1, paper 3. SiLK, for example, has a CTE of ˜70 ppm·K −1 . This is likely to be much larger than the CTE of the overlying heater material, so large stresses and delamination are likely to result from heating to the ˜300° C. superheat limit of water based ink. SiOCH films, on the other hand, have a reasonably low CTE of ˜10 ppm·K −1 , which in the Applicant&#39;s devices, matches the CTE of the TiAlN heater material: no delamination of the heater was observed in the Applicant&#39;s open pool testing after 1 billion bubble nucleations. 
         [0089]    Since the heater materials used in the inkjet application are likely to have CTEs around ˜10 ppm·K −1 , the CVD deposited films are preferred over the spin-on films. 
         [0090]    One final point of interest relating to this application relates to the lateral definition of the thermal barrier. In U.S. Pat. No. 5,861,902 the thermal barrier layer is modified after deposition so that a region of low thermal diffusivity exists immediately underneath the heater, while further out a region of high thermal diffusivity exists. The arrangement is designed to resolve two conflicting requirements:
       1. that the heater be thermally isolated from the substrate to reduce the energy of ejection and   2. that the printhead chip be cooled by thermal conduction out the rear face of the chip.       
 
         [0093]    Such an arrangement is unnecessary in the Applicant&#39;s nozzles, which are designed to be self cooling, in the sense that the only heat removal required by the chip is the heat removed by ejected droplets. Formally, ‘self cooled’ or ‘self cooling’ nozzles can be defined to be nozzles in which the energy required to eject a drop of the ejectable liquid is less than the maximum amount of thermal energy that can be removed by the drop, being the energy required to heat a volume of the ejectable fluid equivalent to the drop volume from the temperature at which the fluid enters the printhead to the heterogeneous boiling point of the ejectable fluid. In this case, the steady state temperature of the printhead chip will be less than the heterogenous boiling point of the ejectable fluid, regardless of nozzle density, firing rates or the presence or otherwise of a conductive heatsink. If a nozzle is self cooling, the heat is removed from the front face of the printhead via the ejected droplets, and does not need to be transported to the rear face of the chip. Thus the thermal barrier layer does not need to be patterned to confine it to the region underneath the heaters. This simplifies the processing of the device. In fact, a CVD SiOCH may simply be inserted between the CMOS top layer passivation and the heater layer. This is now discussed below with reference to  FIGS. 6 to 9 . 
         [0094]    Roof Bonded and Floor Bonded Heater Elements 
         [0095]      FIGS. 6 to 9  schematically show two bonded heater embodiments; in  FIGS. 6 and 7  the heater  10  is bonded to the floor of the chamber  7 , and  FIGS. 8 and 9  bonded the heater to the roof of the chamber. These figures generally correspond with  FIGS. 1 and 2  in that they show bubble  12  nucleation and the early stages of growth. In the interests of brevity, figures corresponding to  FIGS. 3 to 5  showing continued growth and drop ejection have been omitted. 
         [0096]    Referring firstly to  FIGS. 6 and 7 , the heater element  10  is bonded to the floor of the ink chamber  7 . In this case the heater layer  38  is deposited on the passivation layer  24  after the etching the passivation recesses  29  (best shown in  FIG. 10 ), before etching of the ink inlet holes  30  and  31  and deposition of the sacrificial layer  35  (shown in  FIGS. 14 and 15 ). This re-arrangement of the manufacturing sequence prevents the heater material  38  from being deposited in the holes  30  and  31 . In this case the heater layer  38  lies underneath the sacrificial layer  35 . This allows the roof layer  50  to be deposited on the sacrificial layer  35 , instead of the heater layer  38  as is the case in the suspended heater embodiments. No other sacrificial layers are required if the heater element  10  is bonded to the chamber floor, whereas suspended heater embodiments need the deposition and subsequent etching of the second sacrificial layer  42  above described with reference to  FIGS. 25 to 35 . To maintain the efficiency of the printhead, a low thermal product layer  25  can be deposited on the passivation layer  24  so that it lies between the heater element  10  and the rest of the substrate  8 . The thermal product of a material and its ability to thermally isolate the heater element  10  is discussed above and in greater detail below with reference to equation 3. However, in essence it reduces thermal loss into the passivation layer  24  during the heating pulse. 
         [0097]      FIGS. 8 and 9  show the heater element  10  is bonded to the roof of the ink chamber  7 . In terms of the suspended heater fabrication process described with reference to  FIGS. 10 to 36 , the heater layer  38  is deposited on top of the sacrificial layer  35 , so the manufacturing sequence is unchanged until after the heater layer  38  is patterned and etched. At that point the roof layer  44  is then deposited on top of the etched heater layer  38 , without an intervening sacrificial layer. A low thermal product layer  25  can be included in the roof layer  44  so that the heater layer  38  is in contact with the low thermal product layer, thereby reducing thermal loss into the roof  50  during the heating pulse. 
         [0098]    Bonded Heater Element Manufacturing Process 
         [0099]    The unit cells shown in  FIGS. 6 to 9  are largely schematic and purposely correspond to the unit cells shown in  FIGS. 1 to 4  where possible so as to highlight the differences between bonded and suspended heater elements.  FIGS. 37 to 70  show the fabrication steps of a more detailed and complex bonded heater embodiment. In this embodiment, the unit cell  21  has four nozzles, four heater elements and one ink inlet. This design increases the nozzle packing density by supplying a plurality of nozzle chambers from a single ink inlet, using elliptical nozzle openings, thinner heater elements and staggering the rows of nozzles. The greater nozzle density affords greater print resolution. 
         [0100]      FIGS. 37 and 38  show the partially complete unit cell  1 . In the interests of brevity, this description begins at the completion of the standard CMOS fabrication on the wafer  8 . The CMOS interconnect layers  23  are four metal layers with interlayer dielectric in between. The topmost metal layer, M4 layer  50  (shown in dotted line) has been patterned to form heater electrode contacts covered by the passivation layer  24 . M4 layer is in fact made up of three layers; a layer if TiN, a layer of Al/Cu (&gt;98% Al) and another layer of TiN which acts as an anti-reflective coating (ARC). The ARC stops light from scattering during subsequent exposure steps. A TiN ARC has a resistivity suitable for the heater materials (discussed below). 
         [0101]    The passivation layer may be a single silicon dioxide layer is deposited over the interconnect layers  23 . Optionally, the passivation layer  24  can be a silicon nitride layer between two silicon dioxide layers (referred to as an “ONO” stack). The passivation layer  24  is planarised such that its thickness on the M4 layers  50  is preferably 0.5 microns. The passivation layer separates the CMOS layers from the MEMS structures and is also used as a hard mask for the ink inlet etch described below. 
         [0102]      FIGS. 39 and 41  show the windows  54  etched into the passivation layer  24  using the mask  52  shown in  FIG. 40 . As usual, a photoresist layer (not shown) is spun onto passivation layer  24 . The clear tone mask  52 —the dark areas indicate where UV light passes through the mask—is exposed and the resist developed in a positive developing solution to remove the exposed photoresist. The passivation layer  24  is then etched through using an oxide etcher (for example, a Centura DPS (Decoupled Plasma Source) Etcher by Applied Materials). The etch needs to stop on the top, or partly into the TiN ARC layer but not the underlying Al/Cu layer. Then the photoresist layer (not shown) is stripped with O 2  plasma in a standard CMOS asher. 
         [0103]      FIGS. 42 and 43  show the deposition of a 0.2 micron layer of heater material  56 . Suitable heater materials, such as TiAl, TiAlN and Inconel™ 718, are discussed elsewhere in the specification. As shown in  FIGS. 44 and 46 , the heater material layer  56  is patterned using the mask  58  shown in  FIG. 45 . As with the previous step, a photoresist layer (not shown) is exposed through the mask  58  and developed. It will be appreciated that mask  58  is a clear tone mask, in that the clear areas indicate where the underlying material is exposed to UV light and removed with developing solution. Then the unnecessary heater material layer  56  is etched away leaving only the heaters. Again, the remaining photoresist is ashed with O 2  plasma. 
         [0104]    After this, a layer photoresist  42  is again spun onto the wafer  8  as shown in  FIG. 47 . The dark tone mask  60  (dark areas block the UV light) shown in  FIG. 48 , exposes the resist which is then developed and removed to define the position of the ink inlet  31  on the passivation layer  24 . As shown in  FIG. 49 , the removal of the resist  42  at the site of the ink inlet  31  exposes the passivation layer  24  in preparation for the dielectric etch. 
         [0105]      FIGS. 50 and 51  shows the dielectric etch through the passivation layer  24 , the CMOS interconnect layers  23  and into the underlying wafer  8 . This is a deep reactive ion etch (DRIE) using any standard CMOS etcher (e.g. Applied Materials Centura DPS (Decoupled Plasma Source) Etcher), and extends about 20 microns to 30 microns into the wafer  8 . In the embodiment shown, the front side ink inlet etch is about 25 microns deep. The accuracy of the front side etch is important as the backside etch (described below) must be deep enough to reach it in order to establish an ink flow path to the nozzle chamber. After the front side etch of the ink inlet  31 , the photoresist  42  is ashed away with O 2  plasma (not shown). 
         [0106]    Once the photoresist layer  42  is removed, another layer of photoresist  35  is spun onto the wafer as shown in  FIGS. 52 and 53 . The thickness of this layer is carefully controlled as it forms a scaffold for the subsequent deposition of the chamber roof material (described below). In the present embodiment, the photoresist layer  35  is 8 microns thick (except where it plugs the ink inlet  31  as best shown in  FIG. 53 ). Next the photoresist layer  35  is patterned according to the mask  62  shown in  FIG. 55 . The mask is a clear tone mask in that the dark areas indicate the areas of exposure to UV light. The exposed photoresist is developed and removed so that the layer  35  is patterned in accordance with  FIG. 54 .  FIG. 56  is a section view of the patterned photoresist layer  35 . 
         [0107]    With the photoresist  35  defining the chamber roof and support walls, a layer of roof material, such as silicon nitride, is deposited onto the sacrificial scaffolding. In the embodiment shown in  FIGS. 57 and 58 , the layer of roof material  44  is 3 microns thick (except at the walls or column features). 
         [0108]      FIGS. 59 ,  60  and  61  show the etching of the nozzle rims  4 . A layer of photoresist (not shown) spun onto the roof layer  44  and expose under the clear tone mask  64  (the dark areas are exposed to UV). The roof layer  44  is then etched to a depth of 2 microns leaving the raised nozzle rims  4  and the bubble vent feature  66 . The remaining photoresist is then ashed away. 
         [0109]      FIGS. 62 ,  63  and  64  show the nozzle aperture etch through the roof layer  44 . Again, a layer of photoresist (not shown) is spun onto the roof layer  44 . It the then patterned with the dark tone mask  68  (clear areas exposed) and then developed to remove the exposed resist. The underlying SiN layer is then etched with a standard CMOS etcher down to the underlying layer of photoresist  35 . This forms the nozzle apertures  3 . The bubble vent hole  66  is also etched during this step. Again the remaining photoresist is removed with O 2  plasma. 
         [0110]      FIGS. 65 and 66  show the application of a protective photoresist overcoat  74 . This prevents the delicate MEMS structures from being damaged during further handling. Likewise, the scaffold photoresist  35  is still in place to provide the roof layer  44  with support. 
         [0111]    The wafer  8  is then turned over so that the ‘backside’  70  (see  FIG. 67 ) can be etched. Then the front side of the wafer  8  (or more specifically, the photoresist overcoat  74 ) is stuck to a glass handle wafer with thermal tape or similar. It will be appreciated that wafers are initially about 750 microns thick. To reduce the thickness, and therefore the depth of etch needed to establish fluid communication between the front and the back of the wafer, the reverse side  70  of the wafer is ground down until the wafer is about 160 microns thick and then DRIE etched to remove any pitting in the ground surface. The backside is then coated with a photoresist layer (not shown) in preparation for the channel  32  etching. The clear tone mask  72  (shown in  FIG. 68 ) is positioned on the back side  70  for exposure and development. The resist then defines the width of the channel  32  (about 80 microns in the embodiment shown). The channels  32  are then etched with a DRIE (Deep Reactive Ion Etch) down to and marginally beyond the plugged front side ink inlets  31 . The photoresist on the backside  72  is then ashed away with O 2  plasma, and the wafer  8  is again turned over for the front side ashing of the protective overcoat  74  and the scaffold photoresist  35 .  FIGS. 69 and 70  show the completed unit cell  1 . While  FIG. 70  is a plan view, the features obscured by the roof have been shown in full line for the purposes of illustration. 
         [0112]    In use, ink is fed from the backside  70  into the channel  32  and into the front side inlet  31 . Gas bubbles are prone to form in the ink supply lines to the printhead. This is due to outgassing where dissolved gasses come out of solution and collect as bubbles. If the bubbles are fed into the chambers  7  with the ink, they can prevent ink ejection from the nozzles. The compressible bubbles absorb the pressure generated by the nucleating bubbles on the heater elements  10  and so the pressure pulse is insufficient to eject ink from the aperture  3 . As the ink primes the chambers  7 , any entrained bubbles will tend to follow the columnar features on either side of the ink inlet  31  and be pushed toward the bubble vent  66 . Bubble vent  66  is sized such that the surface tension of the ink will prevent ink leakage, but trapped gas bubbles can vent. Each heater element  10  is enclosed on three sides by chamber walls and by additional columnar features on the fourth side. These columnar features diffuse the radiating pressure pulse to lower cross-talk between chambers  7 . 
         [0113]    Superalloy Heaters 
         [0114]    Superalloys are a class of materials developed for use at elevated temperatures. They are usually based on elements from Group VIIA of the Periodic Table and predominantly used in applications requiring high temperature material stability such as jet engines, power station turbines and the like. Their suitability in the thermal inkjet realm has until now gone unrecognized. Superalloys can offer high temperature strength, corrosion and oxidation resistance far exceeding that of conventional thin film heaters (such as tantalum aluminium, tantalum nitride or hafnium diboride) used in known thermal inkjet printheads. The primary advantage of superalloys is that they can have sufficient strength, oxidation and corrosion resistance to allow heater operation without protective coatings, so that the energy wasted in heating the coatings is removed from the design—as discussed in the parent specification U.S. Ser. No. 11/097,308. 
         [0115]    Testing has indicated that superalloys can in some cases have far superior lifetimes compared to conventional thin film materials when tested without protective layers.  FIG. 71  is a Weibull Plot of heater reliability for two different heater materials tested in open pool boiling (the heaters are simply actuated in an open pool of water i.e. not within a nozzle). Skilled artisans will appreciate that Weibull charts are a well recognized measure of heater reliability. The chart plots the probability of failure, or unreliability, against a log scale of the number of actuations. It should be noted that the Key shown in  FIG. 71  also indicates the number of failed and suspended data points for each alloy. For example, F=8 below Inconel 718 in the key indicates that eight of the heaters used in the test were tested to the point of open circuit failure, while S=1 indicates that one of the test heaters was suspended or in other words, still operating when the test was suspended. The known heater material, TiAlN is compared with the superalloy Inconel 718. The registered trademark Inconel is owned by Huntington Alloys Canada Ltd 2060 Flavelle Boulevard, Mississauga, Ontario L5K 1Z9 Canada. 
         [0116]    The applicant&#39;s prior work indicates that oxidation resistance is strongly correlated with heater lifetime. Adding Al to TiN to produce TiAlN greatly increased the heater&#39;s oxidation resistance (measured by Auger depth profiling of oxygen content after furnace treatment) and also greatly increased heater lifetime. The Al diffused to the surface of the heater and formed a thin oxide scale with a very low diffusivity for further penetration of oxygen. It is this oxide scale which passivates the heater, protecting it from further attack by an oxidative or corrosive environment, permitting operation without protective layers. Sputtered Inconel 718 also exhibits this form of protection and also contains Al, but has two other advantageous properties that further enhance oxidation resistance; the presence of Cr, and a nanocrystalline structure. 
         [0117]    Chromium behaves in a similar fashion to aluminium as an additive, in that it provides self passivating properties by forming a protective scale of chromium oxide. The combination of Cr and Al in a material is thought to be better than either in isolation because the alumina scale grows more slowly than the chromia scale, but ultimately provides better protection The Cr addition is beneficial because the chromia scale provides short term protection while the alumina scale is growing, allowing the concentration of Al in the material required for short term protection to be reduced. Reducing the Al concentration is beneficial because high Al concentrations intended for enhanced oxidation protection can jeopardize the phase stability of the material. 
         [0118]    X-ray diffraction and electron microscope studies of the sputtered Inconel 718 showed a crystalline microstructure, with a grain size less than 100 nm (a “nanocrystalline” microstructure). The nanocrystalline microstructure of Inconel 718 is beneficial in that it provides good material strength yet has a high density of grain boundaries. Compared to a material with much larger crystals and a lower density of grain boundaries, the nanocrystalline structure provides higher diffusivity for the protective scale forming elements Cr and Al (more rapid formation of the scale) and a more even growth of the scale over the heater surface, so the protection is provided more rapidly and more effectively. The protective scales adhere better to the nanocrystalline structure, which results in reduced spalling. Further improvement in the mechanical stability and adherence of the scale is possible using additives of reactive metal from the group consisting of yttrium, lanthanum and other rare earth elements. 
         [0119]    It should be noted that superalloys are typically cast or wrought and this does not yield a nanocrystalline microstructure: the benefits provided by the nanocrystalline structure are specific to the sputtering technique used in the MEMS heater fabrication of this application. It should also be noted that the benefits of superalloys as heater materials are not solely related to oxidation resistance: their microstructure is carefully engineered with additives to encourage the formation of phases that impart high temperature strength and fatigue resistance. Potential additions comprise the addition of aluminium, titanium, niobium, tantalum, hafnium or vandium to form the gamma prime phase of Ni based superalloys; the addition of iron, cobalt, chrome, tungsten, molybdenum, rhenium or ruthenium to form the gamma phase or the addition of C, Cr, Mo, W, Nb, Ta, Ti to form carbides at the grain boundaries. Zr and B may also be added to strengthen grain boundaries. Controlling these additives, and the material fabrication process, can also act to suppress undesirable age-induced Topologically Close Packed (TCP) phases, such as sigma, eta, mu phases which can cause embrittlement, reducing the mechanical stability and ductility of the material. Such phases are avoided as they may also act to consume elements that would otherwise be available for the favoured gamma and gamma prime phase formation. Thus, while the presence of Cr and Al to provide oxidation protection is preferred for the heater materials, superalloys in general can be considered a superior class of materials from which selection of heater material candidates may be made, since considerably more effort has been put into designing them for high temperature strength, oxidation and corrosion resistance than has been put into improving the conventional thin film heater materials used in MEMS. 
         [0120]    The Applicant&#39;s results indicate that superalloys:
       a Cr content between 2% by weight and 35% by weight;   a Al content of between 0.1% by weight and 8% by weight;   a Mo content of between 1% by weight and 17% by weight;   a Nb+Ta content of between 0.25% by weight and 8.0% by weight;   a Ti content of between 0.1% by weight and 5.0% by weight;   a Fe content of up to 60% by weight;   a Ni content of between 26% by weight and 70% by weight; and or,   a Co content of between 35% by weight and 65% by weight;   are likely to be suitable for use as a thin film heater element within a MEMS bubble generator and warrant further testing for efficacy within the specific device design (e.g. suspended heater element, bonded heater element and so on).       
 
         [0130]    Superalloy&#39;s having the generic formula MCrAlX where:
       M is one or more of Ni, Co, Fe with M contributing at least 50% by weight;   Cr contributing between 8% and 35% by weight;   Al contributing more than zero but less than 8% by weight; and,   X contributing less than 25% by weight, with X consisting of zero or more of Mo, Re, Ru, Ti, Ta, V, W, Nb, Zr, B, C, Si, Y, Hf;   provide good results in open pool testing (described above).       
 
         [0136]    In particular, superalloys with Ni, Fe, Cr and Al together with additives comprising zero or more of Mo, Re, Ru, Ti, Ta, V, W, Nb, Zr, B, C, Si, Y, or Hf, show superior results. 
         [0137]    Using these criteria, suitable superalloy material for thermal inkjet printhead heaters may be selected from:
       INCONEL™ Alloy 600, Alloy 601, Alloy 617, Alloy 625, Alloy 625LCF, Alloy 690, Alloy 693, Alloy 718, Alloy X-750, Alloy 725, Alloy 751, Alloy MA754, Alloy MA758, Alloy 783, Alloy 925, or Alloy HX;   INCOLOY™ Alloy 330, Alloy 800, Alloy 800H, Alloy 800HT, Alloy MA956, Alloy A-286, or Alloy DS;   NIMONIC™ Alloy 75, Alloy 80A, or Alloy 90;   BRIGHTRAY® Alloy B, Alloy C, Alloy F, Alloy S, or Alloy 35; or,   FERRY® Alloy or Thermo-Span® Alloy       
 
         [0143]    Brightray, Ferry and Nimonic are the registered trademarks of Special Metals Wiggin Ltd Holmer Road HEREFORD HR49FL UNITED KINGDOM. 
         [0144]    Thermo-Span is a registered trademark of CRS holdings Inc., a subsidiary of Carpenter Technology Corporation. 
         [0145]    Titanium Aluminium Alloy Heaters 
         [0146]    Titanium aluminium (TiAl) alloys exhibit superior strength, low creep and light weight; properties which have seen these alloys widely used in the aeronautical and automotive industries. It&#39;s resistance to oxidation at extremely high temperatures makes it a suitable refractory coating for furnaces, kilns and the like (see “Oxidation Resistance of Refractory γ-TiAlW Coatings”, L. Kaczmarck et al. Surface &amp; Coatings Technology 201 (2007) 6167-6170). 
         [0147]    The Applicant&#39;s work has revealed that TiAl is also well suited to use as heater material in inkjet printheads. The alloy can provide surface oxide that is a uniform, thin and dense coating of predominantly Al 2 O 3  and very little TiO 2 . Al 2 O 3  has low oxygen diffusivity while TiO 2  has a much higher diffusivity. Accordingly, the native (i.e. naturally forming) oxide layer passivates the heater to guard against oxidative failure while remaining thin enough not thermally insulate the heater from the ink. This preserves the low energy ejection of drops that is necessary for large (pagewidth), high density nozzle arrays, without compromising the operational life of the heater. Tests using a 0.2 micron thick TiAl heater have achieved 180 millions ejections with good print quality. 
         [0148]    Other elements can be added to the alloy to further suppress the formation of TiO 2  and/or increase Al diffusivity to the heater surface (and therefore preferential formation of Al 2 O 3 ). Ag, Cr, Mo, Nb, Si, Ta and W, individually or in combination, enhance Al 2 O 3  and suppress the less protective TiO 2 . The additives should not exceed 5% by weight of the TiAl alloy in total. Of these, W provides the alloy with an oxide scale having the best oxidative resistance. Adding W in the range 1.7 wt. % to 4.5 wt. % provides superior results. 
         [0149]    Another advantage of adding W is that it is already used during integrated circuit fabrication. The vias through the inter-layer dielectric material (between the metal layers) of the CMOS are typically W. Using W in the ejection heaters is less likely to cause detrimental contamination of other components in the integrated circuit or MEMS. 
         [0150]    The microstructure of the TiAl is another important aspect. Gamma phase TiAl provides a lattice substrate that is complementary to alpha phase Al 2 O 3  (known as corundum). Hence the adherence of the oxide layer to the underlying metal is strong. Also the grain size of the microstructure should be nanocrystalline range. The nanocrystalline structure gives a high density of grain boundaries which promotes diffusivity of Al to the surface. This further promotes a dense and mechanically stable oxide scale. It will be appreciated that a nanocrystalline structure is easily achieved by magnetron sputtering the heater material such that grain sizes are below 100 nano-meters. 
         [0151]    The thin, dense Al 2 O 3  layer gives the heater an operational life comparable to that of existing inkjet printheads. While the oxygen diffusivity through the oxide is low, some oxygen will continue to reach. However, the operational life can be extended, albeit with some compromise on ejection efficiency, by adding a thin protective coating on the TiAl heater. A very thin protective coating (less than 0.5 microns thick) in conjunction with the protection of the native oxide scale will greatly enhance the operation life without substantially reducing the energy efficiency of droplet ejection. The protective coating can be a single layer or a laminate of different materials. Silicon oxides, silicon nitrides and silicon carbides form suitable protective coatings for inkjet heater elements. 
         [0152]    Increasing Drive Pulse to Counter Heater Oxide Growth 
         [0153]    If no protective coatings are used, and the heater relies solely on a dense surface oxide layer, then the drop ejection characteristics can change over the operational life of the printhead. The Applicant&#39;s work has found the resistivity of uncoated heaters changes over time. The surface oxide may have a low oxygen diffusivity, but any oxygen diffusivity will allow the heater material to continuously oxidize for the duration of its operational life. As the oxide layer grows, so does the heater resistance. With increasing resistance, the amount of energy that the heater transfers into the chamber is reduced (because the energy provided by the heater is the drive pulse voltage squared, divided by the heater resistance, multiplied by the pulse duration). Lower energy into the ink within the chamber produces a smaller vapour bubble in the ink. The smaller bubble affects droplet size and velocity. 
         [0154]    To counter the effects of oxide growth on the heaters, the print engine controller increases the energy of the drives pulses over the duration of a heaters operational life. Increasing pulse energy is most easily achieved by increasing the pulse duration. The number of pulses sent to a heater is a good gauge of the oxide growth. The environment created in a chamber when vaporising ink to eject droplets is extremely oxidative and so far outweighs the oxidation that occurs when a heater is inactive. Accordingly, the pulse duration for each heater can be incrementally increased after a set number of heater actuations. Alternatively, the controller can monitor the resistance of the heater (by incorporating it into a Wheatstone Bridge circuit within the CMOS) and extend the pulse duration when the measured resistance exceeds particular thresholds. 
         [0155]    The peak temperature of the heater decreases as the energy transferred to the ink during actuation decreases. Using the printhead temperature sensors, the operational temperature can be used as a trigger for increasing the duration of drive pulses to individual heaters. 
         [0156]    Compensating for oxide growth on the heaters reduces the variation in drop ejection characteristics from each nozzle over the operational life of the printhead. With more uniform drop ejection characteristics, there is less deterioration in the print quality during the life of each printhead. 
         [0157]    The present invention has been described herein by way of example only. Ordinary workers in this field will readily recognize many variations and modifications which do not depart from the spirit and scope of the broad inventive concept.