Abstract:
The systems and methods of this invention allows for an electrical contact structure of the drop ejecting transducer in an inkjet printhead to be designed in such a way that the relatively thick electrical contact lines are not in the ink drop ejection path between the drop ejector transducer and the corresponding nozzle. Such a design thereby minimizes any visible defects due to misdirected satellite drops.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates generally to the mechanical and electrical structure of the thermal inkjet drop ejectors. 
     2. Description of Related Art 
     A conventional thermal inkjet transducer array is essentially a large bank of thin-film resistive heaters electrically connected in parallel. In particular, a thermal inkjet printer comprises an array of drop ejectors. Each drop ejector has an ink channel having an inlet end and a nozzle end and contains a resistive heater. The nozzle end of each resistive heater in the array of drop ejectors is connected to a common electrical bus, which in turn is connected to an electrical power supply providing a printer operating voltage. Each individual drop ejector is driven to eject a droplet of ink by grounding an inlet end of the resistive heater through an individually-addressable driver transistor. 
     The common electrical bus should be narrow, so that the length of the ink nozzle can be kept as short as possible. This tends to increase drop ejection energy efficiency. To reduce the electrical series resistance of the common bus, it is desirable to make the common bus relatively thick. Often, the common bus will have two or more layers of metal and/or polysilicon. 
     SUMMARY OF THE INVENTION 
     However, this thick bus structure presents a “bump”-shaped obstacle in the nozzle that misdirects the ejected main drop and/or associated satellite droplets that are ejected with the main drop. The misdirected satellite drops tend to limit the print quality achievable with drop ejectors having this bump-shaped obstacle. Unfortunately, no reasonable alternative to these drop ejectors was previously available. 
     This invention provides an electrical contact structure that connects the resistive heaters of the drop ejectors to the common bus that avoid the bump-shaped mechanical structure of the conventional electrical contact structure. 
     This invention separately provides a mechanical structure for the electrical contact structure between the common bus and the resistive heater that avoids placing relatively thick electrical contact layers in an ink drop ejection path between the resistive heater of the drop ejector and the nozzle of that drop ejector. 
     This invention separately provides a low-topography inkjet printhead drop ejector array that avoids a large common bus structure in the front of the drop ejectors. 
     This invention separately provides a low-topography inkjet printhead drop ejector array that locates individual electrical feed-through lines between each drop ejector in the array. 
     This invention separately provides an inkjet printhead drop ejector array that reduces visible defects due to misdirected satellite drops. 
     This invention separately provides inkjet printhead drop ejector arrays that relocates the thick electrical contact lines from the ink drop ejection path between the drop ejector resistive heater and the corresponding nozzle. 
     In various exemplary embodiments of a thermal inkjet printhead according to this invention, the high-current common bus does not extend in front of the row of resistive heaters in the array of drop ejectors. Instead, a flat layer of highly-doped polysilicon forms the common bus. This flat, highly-doped polysilicon layer runs between the resistive heaters and is routed to interconnection pads for each ink drop ejector without placing a bump in the path of an exiting ink droplet. 
     In various exemplary embodiments, a floor of the ink channel is left more or less flat at the level of the resistive heater. A layer of passivation material, such as, for example, silicon nitride, can be added to the nozzle region of the ink channel to reduce any residual topography. By making the floor of the ink channel more or less coplanar from an inlet end of the resistive heater through the nozzle and out to the front face of the printhead, the topographic features that contribute to misdirecting main drops and/or creating satellite drops are reduced. 
     These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the low-topography inkjet printhead drop ejectors according to this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of this invention will be described in detail, with reference to the following figures, wherein: 
     FIG. 1 illustrates the effect of ink channel topography on ink drop formation; 
     FIG. 2 shows one exemplary embodiment of a low-topography inkjet printhead structure according to this invention; 
     FIG. 3, as shown by cross-section line III—III in FIG. 2, is a cross-sectional view of one exemplary embodiment of a common bus connecting line portion of a low-topography inkjet printhead structure according to this invention; 
     FIG. 4, as shown by the cross-section line IV—IV in FIG. 2, is a cross-sectional view of one exemplary embodiment of a connection structure between the common bus connection line portion and the common bus of the low-topography inkjet printhead structure according to this invention; 
     FIG. 5, as shown by the cross-section line V—V in FIG. 2, a second cross-sectional view of the exemplary embodiments of the common bus connection line portion and the common bus shown FIGS. 3 and 4; and 
     FIG. 6, as shown by the cross-section line VI—VI in FIG. 2, is a cross-sectional view of one exemplary embodiment of a portion of an inkjet drop ejector ink channel of the low-topography inkjet printhead structure according to this invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1 illustrates the effect on ink drop formation caused by the nozzle topography of a conventional inkjet printhead drop ejector that has the conventional bump-shaped common bus connection structure discussed above. As shown in FIG. 1, the conventional inkjet printhead drop ejector  10  includes a channel plate  12  and a heater plate  14 . The channel and heater plates  12  and  14  combine with a polymer spacer layer (not shown) to form an ink channel  20  extending laterally between the channel plate  12  and the heater plate  14 . A polysilicon resistive heater  16  is formed on or over the heater plate  14 . The common bus connection structure  18  connects the polysilicon resistive heater  16  to a high-voltage power supply. In particular, in conventional thermal inkjet printers, the high-voltage power supply is usually in the range of approximately 40 volts. 
     When the circuit including the polysilicon resistive heater  16  and the connection structure  18  is closed, current flows through the connection structure  18  and the polysilicon resistive heater  16 , causing resistive heating. This resistive heating pumps thermal energy into the ink contained within the ink channel  20 . Eventually, a portion of the ink in the ink channel  20  vaporizes, forcing ink past the bump  18  and through a nozzle  22 . A top of the nozzle  22  is defined by the channel plate  12 , while a bottom of the nozzle  22  is defined by the heater plate  14 , and the sides of he nozzle  22  are defined by the polymer spacer layer. In particular, the nozzle  22  is on the other side of the connection structure  18  from the polysilicon resistive heater  16 . Thus, the bump-shaped connection structure  18  tends to act as a flow-restriction-like member in the ink channel  20 . 
     The bubble formed in the ink channel  20  causes a portion of the ink  24  to extend out of the nozzle  22 . In particular, the force applied by the bubble on the incompressible ink  24  causes a main drop  30  to be ejected from the nozzle  22 . However, due to the shape and position of the bump-shaped connection structure  18 , one or both of two disadvantageous effects can occur as the main drop  32  is ejected from the ink channel  20 . 
     First, the main drop  32  can be misdirected (as shown by drop  30 ) as it is ejected out of the inkjet nozzle  22 . That is, the main drop  32  ideally exits the ink channel  20  in a direction that is perpendicular to the surface of the recording medium  40  at which the ink drop  32  is ejected. However, due to the bump-shaped connection structure  18 , the main drop  32  exits the ink channel  20  at an angle (as shown by drop  30 ) to the desired direction, reducing the accuracy of ink spot placement on the recording medium  40  from the desired location by a distance “d”. 
     Secondly, the bump-shaped connection structure  18  can cause disturbances in the flow of the ink as it exits the nozzle  22 . When the main drop  30  is ejected from the nozzle  22 , one or more small satellite drops  30  are generated which also impact the recording medium  40 . This disturbance causes one or more satellite drops  30  to depart from the trajectory of the main drop  32  as the ink is ejected from the nozzle  22 . In particular, the satellite drops  30  will be ejected at an angle  0  divergent relative to the main drop  32 . 
     Thus, the topography of the ink channel  20  created by the bump-shaped connection structure  18  induces one or more print defects in the images formed by the inkjet printer. As described above, these print defects are related to departures from the ideal flight path of the main drop  32  and differences in the flight paths between the main drop  32  and any satellite drops  30  that may have been ejected with the main drop  32 . These defects cause the resulting printed images to be fuzzy, to have elongated spot aspect ratios, to have banding, and/or to have spot width variations. For example, if the inkjet printer forms images by printing swaths in both a forward and a return direction, the motion vector of the printhead will alternately additively or subtractively add to the flight path vectors of the satellite drops, causing the satellite drops to alternatively extend outside of, or fall within, the main drop as it lands on the recording medium  40 . Thus, depending on which way the printhead carriage moves relative to the recording medium  40 , the size of the spot formed by the combination of the main drop  30  and any satellite drops  32  will change. 
     FIG. 2 is a top plane view of one exemplary embodiment of a low-topography inkjet printhead structure  100  according to this invention. In particular, FIG. 2 shows a top plane view of the heater plate  102  of the low-topography inkjet printhead structure  100  according to this invention. As shown in FIG. 2, a plurality of ejector structures  110  is interleaved with a plurality of common bus connection portions  120 . Each of the ejector structures  110  includes an address line connection portion  112  that connects that ejector structure  110  to a high-voltage driver transistor that selectively connects and disconnects the ejector structure  110  to ground. The address line connection portion  112  is located at an inlet end of a resistive heater  114 . A polymer nozzle structure  116  is formed over the resistive heater  114  and ends in a nozzle  118 . 
     It should be appreciated that, as outlined below, the resistive heater  114  can be formed by a layer of doped polysilicon. However, it should also be appreciated that the resistive heater  114  can also be formed using a thin-film resistor in place of the doped polysilicon layer within the ink channel  20 . It should further be appreciated that the thin-film resistor can be formed using any appropriate process, such as, for example, sputtering. 
     Each of the common bus connection structures  120  forms a connection structure  124  that connects a common bus portion  130  to a drive voltage bus that is held at the drive voltage. In general, for most common thermal inkjet printers, the drive voltage is 40 volts. The common bus portion  130  extends across a front portion of the heater plate  102  and connects to each of the resistive heaters  114 . In the exemplary embodiment shown in FIG. 2, the common bus connection portion  120  connects to the drive voltage bus at a location behind the ejector structures  1  relative to the nozzles  118 . In particular, the common bus connection portion  120  includes a linear connection portion  122  connected to the common bus  130  through the connection structure  124 . 
     FIG. 3, as shown by cross-section line III—III in FIG. 2, is a cross-sectional view of the linear connection portion  122  taken across the long axis of the linear connection portion  122 . As shown in FIG. 3, a field oxide layer  200  forms at least a portion of the heater plate  102 . A relatively lightly-doped (N + ) layer  210  is formed on or over the field oxide layer  200 . In the region of the linear portion  122  of the connection portion  120 , the relatively lightly-doped polysilicon layer  210  is patterned to form a plurality of the resistive heaters. A first insulative layer  220  is formed and patterned to act as an insulative layer between adjacent resistive heater portions of the patterned polysilicon layer  210  and a protective layer  230  formed on or over the insulative layer  220  and the relatively lightly-doped polysilicon layer  210 . 
     As shown in FIG. 3, in various exemplary embodiments, the protective layer  230  is a multi-layer protective layer  230 . In various exemplary embodiments, the multi-layer protective layer  230  comprises a pair of layers. In particular, the multilayer protective layer  230  comprises a lower silicon nitride  232  layer formed using a chemical vapor deposition process and an upper beta-phase tantalum layer  235 . 
     In various exemplary embodiments, the multi-layer protective layer  230  should overlap the first insulative layer  220  by approximately 2 μm to reduce the likelihood that, outside of the ink channel, the beta phase tantalum layer  235  does not terminate on the polysilicon layers  220 , described above, and  270 , described below. Otherwise, if the tantalum layer  235  terminates in electrical contact with one of the polysilicon layers  220  or  270 , the polysilicon becomes damaged near the edge of the tantalum layer  235  and unacceptably low polysilicon-tantalum breakdown voltages occur. 
     The protective layer  230  is used both to protect against the cavitation forces generated within the ink channel  20  as vapor bubbles of the ink form and collapse within the ink channel  20  to eject ink drops from the ejector structures  110 , and to provide electrical isolation between the polysilicon heater structure  210 , which is held at the drive voltage, and the ink  24  in contact with the tantalum layer  235 . 
     It should be appreciated that, in other various exemplary embodiments, any other known or later developed protection layer, whether a single layer structure or a multi-layer structure, can be used in place of the multi-layer protective layer  230  described above, so long as that protective layer is able to adequately protect the resistive heater  114  against chemical attack by the ink or by the cavitation forces and/or thermal forces generated by the ink bubbles as they form and collapse within the ink channels  20 . It should also be appreciated that the protection structure outside of the ink channels  20 , whether the protective layer is the multi-layer protective layer  230  described above or any other known or later developed protective layer, can be patterned away from any regions outside of the ink channels  20 . In this case, a separate planarizing layer can be put down in place of the protective layer in order to reduce the topography of the low-topography inkjet printhead structure  100  according to this invention. 
     A second insulative layer  240  is formed on or over the protective layer  230  and positioned generally vertically over the space formed between the relatively lightly-doped polysilicon layers  210 . A conductive metal layer  250  is then formed on or over the second insulative layer  240 . An insulative passivation layer  260  is formed on or over the conductive metal layer  250 , the second insulative layer  240  and partially over the protective layer  230  to completely encapsulate the second insulation layer  240  and the conductive metal layer  250 . 
     As indicated above, the protective layer  230  is thus only absolutely necessary within the ink channels  20 . However, the protective layer  230  is also used outside of the ink channels  20  as an electrical isolation and surface passivation layer in the nozzle  118 . That is, in the regions outside the ink channels  20 , the protective layer  230  can be utilized to provide electrical, mechanical, and chemical protection to underlying circuit elements of the heater wafer  102  without adding additional topographical structures above the top surface plane of the resistive heater  114  formed by the top surface of the protective layer  230 . In contrast, using the second insulating layer  240  or the passivation layer  260  for these purposes, as is generally done in prior art devices, would generate undesirable additional topographical structures. 
     In various exemplary embodiments, the field oxide layer  200  acts as an electrical and thermal insulation layer and is approximately 1.5 μm thick. In various exemplary embodiments, the field oxide layer  200  is formed using a thermal steam oxide process. In various exemplary embodiments, the relatively lightly-doped polysilicon layer  210  is approximately 4500 Å thick and is formed using any appropriate chemical vapor deposition or physical vapor deposition process. The first insulative layer  220 , in various exemplary embodiments, includes a silicon oxide layer approximately 1,000 Å thick and a 7,000 Å thick doped glass layer. In various exemplary embodiments, the silicon oxide layer is formed using a thermal dry-oxygen process, while the doped glass layer is formed using a low-pressure chemical vapor deposition process with a subsequent oxygen high-temperature reflow process. In particular, in various exemplary embodiments, this doped glass layer has a phosphorous (P) content of approximately 7.2 percent by weight. 
     The multi-layer protective layer  230 , in various exemplary embodiments, has a lower silicon nitride layer  232  and an upper tantalum layer  235 . The silicon nitride layer  232  is formed using a pyrolytic low-pressure chemical vapor deposition process and is approximately 1500 Å thick. The tantalum layer  235  is approximately 2500 Å thick. The tantalum layer  235  is deposited as beta-phase tantalum and is formed by sputtering. The second insulative layer  240 , in various exemplary embodiments, includes a silicon oxide layer that is approximately 1.0 μm thick and formed using a plasma-enhanced chemical vapor deposition process and a TEOS (tetra-ethyl-ortho-silicate) precursor. 
     The conductive metal layer is approximately 1.25 μm thick. In various exemplary embodiments, the conductive metal layer  250  is an aluminum-silicon alloy having 1 percent by weight silicon and is formed by sputtering. Prior to depositing the conductive metal layer  250 , the exposed surfaces of the various layers are etched using an radio frequency sputter etch process to clean the exposed silicon surfaces to improve the contact resistance of the conductive metal layer  250 . The passivation layer  260  is, in various exemplary embodiments, approximately 1500 Å thick and is formed using plasma enhanced chemical vapor deposition using a TEOS (tetra-ethylortho-silicate) precursor. The passivation layer  260  also includes a 1.0 μm silicon nitride layer formed by plasma-enhanced chemical vapor deposition. 
     As mentioned above, the protective layer  230  acts as a heater protection layer providing both chemical and mechanical protection to the resistive heater  114  in the ejector structure  110 . The passivation layer  260  also acts as a mechanical and chemical protection layer. Because the passivation layer  260  encapsulates the conductive metal layer  250 , the passivation layer  260  also provides electrical protection. 
     It should be appreciated that, while the various layers described above are formed, in various exemplary embodiments, using the various thicknesses and processes described above, that any known or later developed process for forming one of the above-outlined layers can be used so long as it results in a layer having the proper mechanical and/or electrically properties as discussed herein. Thus, while various materials, thicknesses and/or deposition processes have been discussed above with respect to layers  200 - 260 , it should be appreciated that this discussion is illustrative only, and not intended to be limiting of the scope of this invention. 
     FIG. 4, as shown by the cross-section line IV—IV in FIG. 2, is a cross-sectional view illustrating how the conductive metal layer  250  is electrically connected to a relatively highly-doped (N ++ ) polysilicon layer  270  forming the common bus structure  130  for the ejector structures  110 . As shown in FIG. 4, the relatively highly-doped polysilicon layer  270  is formed on or over the field oxide layer  200  and under the first and second insulation layers  220  and  240  and the protective layer  230 . In particular, the conductive metal layer  250  contacts the relatively heavily-doped polysilicon layer  270  either directly or through one or more conductive barrier structures. 
     FIG. 5, as shown by the cross-section line V—V in FIG. 2, is a cross-sectional view of the common bus connection portion  120  along the long dimension of the common bus connection portion  220 , showing both the structure of the common bus connection portion  122  and the contact portion  124 . 
     FIG. 6, as shown by the cross-section line VI—VI in FIG. 2, is a cross-sectional view along the long axis of the resistive heater  114  and extending through the nozzle  118 . As shown in FIG. 6, the common bus portion  130 , formed by the relatively heavily-doped polysilicon layer  270 , and the resistive heater portion  114 , formed by the relatively lightly-doped polysilicon layer  210 , are positioned laterally adjacent to each other to form a conductive path from the drive voltage bus to ground through the common bus connection portion  122 , the connection structure  124 , the common bus  130 , the resistive heater  114  and the address line connection portion  112  to ground. Thus, current flows through the relatively heavily-doped polysilicon layer  270  and into the relatively lightly-doped polysilicon layer  220 . 
     This current flow through the relatively lightly-doped polysilicon layer  210  causes resistive heating in the relatively lightly-doped polysilicon layer  210 . In particular, the relatively heavily-doped polysilicon layer  270  has a resistivity that is less than the resistivity of the relatively lightly-doped polysilicon layer  220 . In various exemplary embodiments, the resistivity of the relatively heavily-doped polysilicon portion layer is on the order of 20 Ω/□. In contrast, relatively lightly-doped polysilicon layer  210  has a resistivity on the order of 200-3000 Ω/□. In general, the relatively lightly-doped polysilicon layer  210  should have a resistivity that is 1 to 2 orders of magnitude greater that the resistivity of the relatively heavily-doped polysilicon layer  270 . This tends to cause most of the resistive heating to occur in the relatively lightly-doped polysilicon layer  220 , and relatively little of the resistive heating to occur in the relatively heavily-doped polysilicon layer  270 . 
     In various exemplary embodiments, the polysilicon layers  210  and  270  are doped using phosphorus. Phosphorus is particularly useful because phosphorus reduces roughening of the surface of the polysilicon layers  210  and  270 . However, it should be appreciated that any other known or later developed dopant can be used, including arsenic, and even p-type dopants, such as boron. 
     The heat created by the resistive heating in the relatively lightly-doped polysilicon layer  210  flows through the thermally conductive protective layer  230  and heats the ink in the ink channel  20  sufficiently to cause the ink to vaporize and eject a drop through the nozzle  118 . 
     As shown in FIG. 6, the passivation layer  260  and the protective layer  230  form a generally flat topography. In particular, the connection structure  118  shown in FIG. 1 is moved out of the ejector structure  110  to a portion of the heater plate  102  that is laterally adjacent to the ejector structure  110 , as shown in FIG.  2 . Thus, while the complex, multi-layer contact structure  124  that is located at the front of the heater plate  102  is required for each ejector structure  110 , this complex, multi-layer contact structure  124  avoids introducing any additional topography into the ejector structure  110  and especially avoids ejecting any additional topography into the ejector structure  110  at locations close to the nozzle  118 . 
     In particular, as shown in FIG. 6, the surface of the resistive heater  114  is essentially or substantially flat. It should be appreciated that, to the extent the resistive heater shown in FIG. 6 is not completely flat, a portion of the passivation layer  260  can be added to the ejector structure  110  at a region near the nozzle  118  to remove any residual topography that may be created by the field oxide  200  and the polysilicon layers  210  and  270 . It should be appreciated that, in various exemplary embodiments of the ink jet printhead according to this invention, for a given ink formulation, drop size, and/or drop ejection frequency, the most defect-free image formed on the recording medium  40  is obtained when the portion of the passivation layer  260  is provided in the ink channel  20  between the protective layer  230  and the nozzles  22 . In contrast, in various other exemplary embodiments of the ink jet printhead according to this invention, for other ink formulations, drop sizes, and/or drop ejection frequencies, the most defect-free image formed on the recording medium  40  is obtained when this portion of the passivation layer  260  is omitted from the ink channel  20 . 
     However, it should be appreciated that, if it is provided in the ink channel  20 , this portion of the passivation layer  260  is nonetheless spaced from the protective layer  230  and the relatively heavily-doped polysilicon portion  270 , thus forming a valley or divot  280  in the topography of the resistive heater  114 . It should be appreciated that this valley or divot  280 , in various exemplary embodiments, is approximately 1 μm wide along the resistive heater  114 , and, in the various exemplary embodiments, it is approximately 0.5 μm deep. While this valley or divot  280  may generate de-minimis disturbances in the flow of ink through the nozzles  118 , removing this valley or divot  280  by attempting to butt the passivation layer  260  directly up against the protective layer  230  and the relatively heavily-doped polysilicon portion  270  tends to create a ridge that extends into the ink channel  20  and thus creates exactly the type of bump-shaped obstacle in the ink channel  10  that this invention was directed to reduce. 
     In particular, referring back to FIG. 1, the ink channel  20  has a height h 1  that is, in various exemplary embodiments, on the order of 20 μm. In contrast, the bump-shaped connection structure  18  has a height h 2  that is approximately 7 μm. Thus, the bump-shaped connection structure  18  has a height that is one-third or more of the height of the ink channel  20  itself In contrast, the valley or divot  280 , which has a depth of approximately 0.5 μm, is only 2.5% of the height h 1  of the ink channel  10 . Even if the portion of the passivation layer  260  is emitted at the front of the ink channel  20  near the nozzle  22 , the topography encountered by ink flowing from the protection layer  230  through the nozzle  22  is only a small 0.5 μm drop on to a smooth nozzle floor, as opposed to the large 7 μm constriction in a 20 μm nozzle present in the conventional ink channel. 
     Thus, by making the portion of the ejector structure  110  formed on the heater plate  102  more or less coplanar from the portion of the resistive heater  114  adjacent the address line connection portion  112  through to the nozzle  118 , the topographical features in the conventional thermal inkjet printhead that contribute to main and satellite drop misdirection are reduced, if not minimized or even fully eliminated. 
     It should also be appreciated that the relatively complex multi-layer contact structure  124  shown in FIGS. 4 and 5 provides a good, low-resistance electrical connection between the drive voltage bus and the common bus  130 . 
     It should be appreciated that additional complexity in the multi-layer contact structure shown in FIGS. 4 and 5 arises because the tantalum layer  235  should not be allowed to make electrical contact with either the high voltage drive power supply or to ground. That is, the protective layer  230  should be electrically floating. In particular, if the tantalum layer  235  is inadvertently connected to the 40V drive voltage, the ink can electrolyze and known print defects associated with electrolyzed ink will occur. In contrast, if the tantalum layer  235  is inadvertently connected to ground, high electric fields will be induced that will eventually result in failure of the resistive heaters in the regions of these high electric fields. 
     It should be appreciated that, in the various exemplary embodiments outlined above, the various exemplary materials and thicknesses of the various exemplary layers have been particularly selected to improve the chemical resistance against the ink and to improve the ability of the various electrical connection structures to operate at voltage levels up to 50 volts. 
     It should be appreciated that the thermal inkjet printer having the ejection structure  110  and connection structure  124  described above can be used in any known or later developed image forming device, such as a copier, a printer, a facsimile machine, or the like. It should be appreciated that the low-topography thermal inkjet printhead drop ejector structure  100  according to this invention allows the ejector structures to be packed at a high density without introducing topographical features that are detrimental to print quality. At the same time, the low-topography thermal inkjet printhead drop ejector structures  100  according to this invention does not compromise the resistance in the drop ejectors to the corrosive operating environment of a thermal inkjet printer. Finally, it should be appreciated that the low-topography thermal inkjet printhead drop ejectors structure  100  according to this invention reduce voltage variations that can occur from one end of the ejector array to the other end of the ejector array, which tend to introduce variations in ink drop size from one end of the array to the other. 
     While this invention has been described in conjunction with the exemplary embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.