Patent Publication Number: US-6905196-B2

Title: Polysilicon feed-through fluid drop ejector

Description:
This nonprovisional application claims the benefit of U.S. Provisional Application No. 60/378,398, filed May 8, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates generally to the mechanical and electrical structure of fluid 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. 
     Often, fluid ejection systems, such as inkjet printers, include an array of thin-film drop ejectors electrically connected in parallel. A drop ejector includes a channel into which fluid flows, a resistive heater to vaporize a portion of the fluid to form a bubble in the chamber, and a nozzle through which fluid downstream of the vapor bubble ejects from the chamber to form a drop projected towards a receiving medium. Vaporizing the fluid creates pressure in the channel forcing the fluid collected downstream of the heater out of the nozzle. 
     Each drop ejector in the array connects to a common electrical bus communicating with an electrical power supply. Each ejector is controlled by grounding an electrical supply end through an individually-addressable driver transistor. Design optimization encourages a narrow electrical bus for minimal nozzle length, and thick cross-section structures to minimize electrical resistance. Such a thick bus, underneath the channel (˜6-7 μm high), can present an obstacle over which the fluid must flow. Such a flow restriction can induce lateral forces to the fluid being ejected, creating potential directional biases and/or variations in the drop and/or satellite trajectories, thereby degrading ejection quality. 
     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. 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. 
     A fluid ejector having a low topography formed by rerouting the electrical conductors from underneath to adjacent to the chamber is disclosed in U.S. Pat. No. 6,227,657 (the 657 patent), which is incorporated herein by reference in its entirety. This low topography fluid drop ejector provides for an electrical contact structure to the resistive heater that avoids placing relatively thick electrical contact layers in a fluid drop ejection path between the resistive heater and the ejector nozzle. 
     SUMMARY OF THE INVENTION 
     However, manufacturing and operational considerations for fluid ejector designs have revealed limitations to the approach described in the 657 patent. The low topography fluid ejector disclosed in the 657 patent incorporates electrically conductive metal layers separated by a concatenation of insulating layers along selected regions. Unavoidable production flaws along the interface edges of these layers can reduce the operational longevity of the circuit and/or adversely affect production yield. Over-etching on one or more of these layers can exacerbate the variation in the electrical resistance far beyond the allowable design value tolerances. 
     While the layered electrical and structural configuration disclosed in the 657 patent diminishes the flow obstructions in the fluid ejection channel, the added complexity in depositing patterned layers makes it difficult to control quality and/or obtain commercially useful yields. Further, the connections between the metal layer and the heavily doped polysilicon layer penetrate the protective layer. 
     Because of the cross-layer interface, the edge junctions between these layers facilitate leak paths through which the fluid and/or fluid vapors can percolate, thus degrading performance and reliability. Additionally, such interfaces at these layer edges require additional patterned insulating layers, thickening the overall structure. 
     This invention provides a fluid channel having a low-topography using a relatively simple internal structure. 
     This invention separately provides a low-topography fluid channel that reduces the number of leak paths into the fluid channel. 
     This invention separately provides a low-topography fluid channel that can be manufactured at higher yield rates. 
     This invention separately provides a low-topography fluid channel having increased reliability. 
     This invention separately provides a low-topography fluid channel having a fluid channel having a relatively simple internal structure. 
     This invention separately provides a method for forming a tantalum-silicide layer in a fluid ejector. 
     By eliminating interface connections between metal and semiconductor layers, the cross-sectional structure may be further flattened relative to the low-topography fluid ejector device disclosed in the 657 patent. This can further improve droplet trajectory control. Additionally, enhanced reliability can result by reducing potential failure modes associated with over-etching along these interfaces. 
     In various exemplary embodiments, a thermal fluid ejector structure according to this invention includes a fluid channel having a resistive heater that terminates in a nozzle, and a common bus formed transverse to the fluid channel and extending between the resistive heater and the nozzle. The fluid ejector further includes a connection line that extends longitudinally adjacent to the fluid channel, and a connection structure that electrically connects the common bus with the resistive heater and the connection line. The connection structure includes a first set of one or more layers that electrically connects the connection line to the resistive heater and a second set of one or more layers that covers the common bus and the connection line. 
     In various exemplary embodiments, the first set of one or more layers can be formed on or over a field oxide layer, and can further include a heavily doped polysilicon layer formed on or over the field oxide layer and a tantalum-silicide layer formed on or over the heavily doped polysilicon layer. In various exemplary embodiments, a second set of one or more layers can be formed on or over the first set of one or more layers and can further include a nitride layer formed on or over the first set of one or more layers, and a tantalum layer formed on or over the nitride layer. 
     In various exemplary embodiments, a fluid channel can be formed on or over the field oxide layer and can further include a heavily doped polysilicon layer formed on or over the field oxide layer upstream of the nozzle, a tantalum-silicide layer formed on or over the heavily doped polysilicon layer, a lightly doped polysilicon layer formed on or over the field oxide layer adjacently upstream of and electrically connected with the heavily doped polysilicon layer, and a set of protective layers formed on or over the tantalum-silicide layer and the lightly doped polysilicon layer. The set of protective layers can further include a nitride layer formed on or over the tantalum-silicide layer and the lightly doped polysilicon layer, and a tantalum layer formed on or over the nitride layer. 
     In various exemplary embodiments, a common bus can be formed on or over the field oxide layer and can further include a first set of one or more layers formed on or over the field oxide layer, a lightly doped polysilicon layer formed on or over the field oxide layer adjacent to and electrically separated from the first set of one or more layers, an insulating layer formed on or over the first set of one or more layers and a first portion of the lightly doped polysilicon layer, and a second set of one or more layers formed on or over the insulating layer and a second portion of the lightly doped polysilicon layer. The first set of one or more layers can further include a heavily doped polysilicon layer formed on or over the field oxide layer and a tantalum-silicide layer formed on or over the first doped polysilicon layer. The second set of one or more layers can further include a nitride layer formed on or over the tantalum-silicide layer and the second doped polysilicon layer, and a tantalum layer formed on or over the nitride layer. 
     Various exemplary embodiments of a method to produce a low topography fluid ejector according to this invention include forming a fluid channel having a resistive heater and terminating in a nozzle, forming a common bus transverse to the fluid channel and between the resistive heater and the nozzle, forming a connection line longitudinally adjacent to the fluid channel, forming a first set of one or more layers that electrically connects the common bus with the resistive heater and the connection line, and forming a second set of one or more layers that covers the common bus and the connection line. Forming the first set of one or more layers over the field oxide layer further includes forming a first doped polysilicon layer on or over the field oxide layer, and optionally forming a tantalum-silicide layer on or over the first doped polysilicon layer. Forming the second set of one or more layers over the first set of the one or more layers further includes forming a nitride layer on or over the first set of the one or more layers, and forming a tantalum layer on or over the nitride layer. 
     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 systems and methods according to this invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments of the systems and methods 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  is a plan view of a conventional low-topography fluid ejector; 
         FIG. 3  is a transverse cross-sectional view of a conventional linear connection taken along a plane III—III of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view through a conventional connection structure taken along a plane IV—IV of  FIG. 2 ; 
         FIG. 5  is a longitudinal cross-sectional view of a conventional bus connection structure taken along a plane V—V of  FIG. 2 ; 
         FIG. 6  is a longitudinal cross-sectional view of a conventional resistive heater taken along a plane VI—VI of  FIG. 2 ; 
         FIG. 7  is a plan view of one exemplary embodiment of a low-topography fluid ejector, according to the invention; 
         FIG. 8  is a cross-sectional view showing in greater detail a first exemplary embodiment of a connection structure taken along a plane VIII—VIII of  FIG. 7 , according to the invention; 
         FIG. 9  is a cross-sectional view showing in greater detail a first exemplary embodiment of a linear connection structure taken along a plane IX—IX of  FIG. 7 , according to the invention; 
         FIG. 10  is a longitudinal cross-sectional view showing in greater detail a first exemplary embodiment along a resistive heater taken along a plane X—X of  FIG. 7 , according to the invention; 
         FIG. 11  is a cross-sectional view showing in greater detail a second exemplary embodiment of a connection structure taken along the plane VIII—VIII of  FIG. 7 , according to the invention; 
         FIG. 12  is a cross-sectional view showing in greater detail a second exemplary embodiment of the linear connection structure taken along the plane IX—IX of  FIG. 7 , according to the invention; 
         FIG. 13  is a longitudinal cross-sectional view showing in greater detail a second exemplary embodiment along the resistive heater taken along the plane X—X of  FIG. 7 , according to the invention; 
         FIG. 14  is a cross-sectional view showing in greater detail a third exemplary embodiment of a connection structure taken along the plane VIII—VIII of  FIG. 7 , according to the invention; 
         FIG. 15  is a cross-sectional view showing in greater detail a third exemplary embodiment of the linear connection structure taken along the plane IX—IX of  FIG. 7 , according to the invention; and 
         FIG. 16  is a longitudinal cross-sectional view showing in greater detail a third exemplary embodiment along the resistive heater taken along the plane X—X of  FIG. 7 , according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following detailed description of various exemplary embodiments of the fluid ejection systems according to this invention are directed to one specific type of fluid ejection system, an inkjet printer, for sake of clarity and familiarity. However, it should be appreciated that the principles of this invention, as outlined and/or discussed below, can be equally applied to any known or later developed fluid ejection systems, beyond the inkjet printer specifically discussed herein. 
       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 between approximately 12 and 50 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 the 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  30  is ejected from the ink channel  20 . 
     First, the main drop  32  can be misdirected as it is ejected out of the inkjet nozzle  22 . That is, the main drop  30  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  30  is ejected. However, due to the bump-shaped connection structure  18 , the main drop  30  exits the ink channel  20  at an angle to the desired direction, reducing the accuracy of ink spot placement on the recording medium  40  from the desired location. 
     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  32  are generated which also impact the recording medium  40 . This disturbance causes one or more satellite drops  32  to depart from the trajectory of the main drop  30  as the ink is ejected from the nozzle  22 . In particular, the satellite drops  32  will be ejected at an angle θ divergent relative to the main drop  30 . 
     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  30  and differences in the flight paths between the main drop  30  and any satellite drops  32  that may have been ejected with the main drop  30 . 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 embodiment of a low-topography inkjet printhead structure  100  ejector, as disclosed in the 657 patent. In particular,  FIG. 2  shows a top plane view of the heater plate  102  of the low-topography inkjet printhead structure  100 , as disclosed in the 657 patent. 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 on or over the resistive heater  114  and ends in a nozzle  118 . 
     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 typically between 12 and 50 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 . As shown in  FIG. 2 , the common bus connection portion  120  connects to the drive voltage bus at a location behind the ejector structures  110  relative to the nozzles  118 . In particular, the common bus connection portion  120  includes a linear connection portion  122  connected to the common bus portion  130  through the connection structure  124 . 
       FIG. 3  is a cross-sectional view of the linear connection portion  122  taken across the long axis, i.e., a plane III—III of  FIG. 2 , of the linear connection portion  122 , as disclosed in the 657 patent. 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 + ) polysilicon 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 and relatively lightly-doped 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 , and as disclosed in the 657 patent, the protective layer  230  is a multi-layer protective layer  230 . The multi-layer protective layer  230  comprises a pair of layers. In particular, the multi-layer protective layer  230  comprises a lower silicon-nitride  232  layer formed using a chemical vapor deposition process and an upper β-phase tantalum layer  235 . 
     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 β-phase tantalum layer  235  does not terminate on the polysilicon layers  210 , described above, and  270 , described below. Otherwise, if the tantalum layer  235  terminates in electrical contact with one of the polysilicon layers  210  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 . 
     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 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. 
       FIG. 4  is a cross-sectional view along a plane IV—IV of FIG.  2  and illustrates how the conductive metal layer  250  is electrically connected to a relatively highly-doped (N ++ ) polysilicon layer  270  forming the common bus portion 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  is a cross-sectional view from the 657 patent of the common bus connection portion  120 , i.e., a plane V—V of  FIG. 2 , along the long dimension of the common bus connection portion  120 , showing both the structure of the linear connection portion  122  and the contact portion  124 . 
       FIG. 6  is a cross-sectional view along the long axis, i.e., the line VI—VI of  FIG. 2 , 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 linear connection portion  122 , the connection structure  124 , the common bus portion  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  210 . 
     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  210 . This tends to cause most of the resistive heating to occur in the relatively lightly-doped polysilicon layer  210 , and relatively little of the resistive heating to occur in the relatively heavily-doped polysilicon layer  270 . 
     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. 
       FIG. 7  is a schematic plan view of an exemplary embodiment of the polysilicon feed-through heater, according to this invention. As shown in  FIG. 7 , a printhead structure  300  includes a heater plate  302  onto which fluid ejector structures  310  and bus connections  320  are formed laterally adjacently to each other. Each ejector structure  310  includes an address line connection  312  communicating with a contact  313  of a resistive heater  314 . A polymer nozzle structure  316  ending in a nozzle  318  is formed above and forward of the resistive heater  314 . The fluid is ejected from the nozzle  318  in the direction indicated by the upward-pointing arrow. 
     As shown in  FIG. 7 , each bus connection  320  includes a connection line  322  communicating over a common bus  330  to a drive voltage source. The common bus  330  extends along and adjacent to the forward edge of the heater plate  302 , transversely to the ejector structures  310  and the bus connections  320 , and connects to each resistive heater  314 . The contact  313  completes the circuit by connecting the resistive heater  314  to the address line  312  in communication with the drive voltage source. These structures may be fabricated using conventional techniques known in the semiconductor and microelectronics industries. 
       FIGS. 8-10  provide cross-section elevation views through selected structures for a first exemplary embodiment of the polysilicon feed-through heater.  FIG. 8  is a cross-section through the first exemplary embodiment of the common bus  330  along a plane VIII—VIII shown in FIG.  7 .  FIG. 9  is a cross-section across the first exemplary embodiment of the connection line  322  along a plane IX—IX shown in FIG.  7 .  FIG. 10  is a cross-section through the first exemplary embodiment of the resistive heater  314  along a plane X—X shown in FIG.  7 . 
       FIG. 8  is a cross-sectional view showing in greater detail the first exemplary embodiment of the common bus  330 . As shown in  FIG. 8 , a polysilicon layer is deposited on or over one surface of the field oxide layer  400  for the heater plate  302 . A relatively heavily-doped polysilicon layer  410  is produced from the polysilicon layer. A tantalum-silicide layer  420  is formed on or over the relatively heavily-doped polysilicon layer  410  to produce a set of electrically conductive layers  450 . 
     As shown in  FIG. 8 , an electrically insulating layer  440  is deposited on or over the tantalum-silicide layer  420 . A set of protective layers  430  is formed on or over the insulating layer  440 . The set of protective layers  430  include a lower silicon-nitride insulating layer  432  and an upper β-phase tantalum coating layer  435 . 
       FIG. 9  is a transverse cross-sectional view showing in greater detail the first exemplary embodiment of the bus connection  320 . As shown in  FIG. 9 , a polysilicon layer is deposited along a pattern over one surface of the field oxide layer  400  that forms the substrate for the heater plate  302 . This polysilicon layer is patterned to form the bus connection  320  and the resistive heaters  314 . 
     As shown in  FIG. 9 , the polysilicon layer  410  is formed by relatively heavily-doping (N ++ ) this polysilicon layer to increase its electrical conductivity to as large a value as possible. A polysilicon layer  460  is formed by relatively lightly-doping (N + ) this polysilicon layer to increase its electrical conductivity to a value appropriate to act as a resistive heating element for ejecting fluid droplets. In various exemplary embodiments, the thicknesses of the relatively heavily-doped and relatively lightly-doped polysilicon layers  410  and  460  are typically between 500 Å and 6000 Å. The relatively heavily-doping of the patterned polysilicon layer is relative to the relatively lightly-doped polysilicon layer  460 . The relatively lightly-doping of the patterned polysilicon layer is relative to the relatively highly-doped polysilicon layer  410 . 
     As shown in  FIG. 9 , the tantalum-silicide (TaSi 2 ) layer  420  is deposited on or over the relatively heavily-doped polysilicon layer  410 . In various exemplary embodiments, the layer  420  is formed, for example, by sputtering, followed by high-temperature sintering. In various exemplary embodiments, the thickness of the tantalum-silicide layer  420  is 2000 Å. The tantalum-silicide layer  420  can also be produced by depositing an elemental tantalum (Ta) layer on or over the relatively heavily-doped polysilicon layer  410  and reacting and annealing the structure in a high-temperature, inert environment so that the two layers alloy to form an intermetallic tantalum-silicide layer on or over the relatively heavily-doped polysilicon layer  410 . 
     As shown in  FIG. 9 , the electrically insulating oxide layer  440  is deposited on or over the tantalum-silicide layer  420 . A set of protective layers  430  is deposited on or over the insulating layer  440 . The set of protective layers  430  includes the silicon-nitride layer  432 , which electrically insulates the layers beneath from any layer above. The tantalum layer  435  is provided on or over the silicon-nitride layer  432  to mechanically and/or chemically isolate the heater plate layers from fluid vapors generated when the fluid to be ejected is vaporized. In various exemplary embodiments, the thickness of the electrically insulating layer  440  is 1000 Å of dry oxide plus 7000 Å of doped glass. 
     As indicated above, the set of electrically conductive layers  450  includes the relatively heavily-doped polysilicon layer  410  and the tantalum-silicide layer  420 . The set of electrically conductive layers  450  can be used, according to this invention, in place of a separately-routed layer of one or more high-conductivity materials, such as, for example, copper (Cu), and/or aluminum (Al). 
       FIG. 10  is a cross-sectional view showing in greater detail the first exemplary embodiment of the resistive heater  314 , with fluid flowing along a direction shown by the left-facing arrow through the nozzle  318 . A polysilicon layer is deposited on or over one surface of the field oxide layer  400  for the heater plate  302 . The relatively lightly-doped polysilicon layer  460  and the relatively heavily-doped polysilicon layer  410  can be formed adjacently from the deposited polysilicon layer by selective doping. The relatively lightly-doped polysilicon layer  460  and the set of electrically conductive layers  450  are overlaid by the set of protective layers  430 , including the silicon-nitride insulating layer  432  and the tantalum coating layer  435 . 
     It should be appreciated that, as outlined below, the resistive heater  314  can be formed by the relatively lightly-doped layer of doped polysilicon  460 . However, it should also be appreciated that the resistive heater  314  can also be formed using a thin-film resistor in place of the relatively lightly-doped polysilicon layer  460  within the ink channel. It should further be appreciated that the thin-film resistor can be formed using any appropriate process, such as, for example, sputtering. 
     In various exemplary embodiments, by forming the common bus and interconnection structures with tantalum-silicide, rather than by simply using relatively highly-doped polysilicon, the interconnect line resistance can be reduced from 2 mΩ-cm to approximately 50 μΩ-cm. With the known highly-doped polysilicon interconnection structures, parasitic resistances prevent efficient drop ejector operation for heater resistivities less than approximately 3000 Ω/□ (ohms/square). In contrast, in various exemplary embodiments according to this invention, the silicide interconnection structures enable efficient operation with heater resistances of 300 Ω/□ or less. Additionally, in various exemplary embodiments of this invention, by replacing the known superstructure of the metal layer  250  and the accompanying insulating layers  220 ,  232  and  240  and the passivation layer  260 , with the set of electrically conductive layers  450  and the set of protective layers  430 , the cross-sectional profile through the drop ejector nozzle region may be flattened. This tends to further improve the directional flow consistency of ejected fluid droplets. Finally, replacing the complex cross-sectional structures represented in  FIGS. 3-6  eliminates a number of material interfaces and electrical connections which penetrate the protective tantalum layer, resulting in a structure that is much more resistant to attack by corrosive fluids and vapors, and that is far more tolerant to manufacturing variations. 
       FIGS. 11-13  provide cross-section elevation views through selected structures for a second exemplary embodiment of the polysilicon feed-through heater shown in FIG.  7 .  FIG. 11  is a cross-section through the second exemplary embodiment of the common bus  330  along the plane VIII—VIII shown in FIG.  7 .  FIG. 12  is a cross-section across the second exemplary embodiment of the connection line  322  along the plane IX—IX shown in FIG.  7 .  FIG. 13  is a cross-section through the second exemplary embodiment of the resistive heater  314  along the plane X—X shown in FIG.  7 . 
     As shown in  FIG. 11 , a polysilicon layer is deposited on or over one surface of the field oxide layer  400  for the heater plate  302 . A relatively heavily-doped polysilicon layer  410  is produced from the polysilicon layer to produce an electrically conductive layer. As shown in  FIG. 11 , an electrically insulating layer  440  is deposited on or over the relatively heavily-doped polysilicon layer  410 . A set of protective layers  430  is formed on or over the insulating layer  440 . The set of protective layers  430  include a lower silicon-nitride insulating layer  432  and an upper β-phase tantalum coating layer  435 . 
     As shown in  FIG. 12 , the polysilicon layer  410  is formed by relatively heavily-doping (N ++ ) this polysilicon layer to increase its electrical conductivity to as large a value as possible. A polysilicon layer  460  is formed by relatively lightly-doping (N + ) this polysilicon layer to increase its electrical conductivity to a value appropriate to act as a resistive heating element for ejecting fluid droplets. As indicated above, the relatively heavily-doped polysilicon layer  410  can be used, according to this invention, in place of a separately-routed layer of one or more high-conductivity materials, such as, for example, copper (Cu), and/or aluminum (Al). 
       FIG. 13  is a cross-sectional view showing in greater detail the second exemplary embodiment of the resistive heater  314 , with fluid flowing along a direction shown by the left-facing arrow through the nozzle  318 . A polysilicon layer is deposited on or over one surface of the field oxide layer  400  for the heater plate  302 . The relatively lightly-doped polysilicon layer  460  and the relatively heavily-doped polysilicon layer  410  can be formed adjacently from the deposited polysilicon layer by selective doping. The relatively lightly-doped polysilicon layer  460  and the relatively heavily-doped polysilicon layer  410  are overlaid by the set of protective layers  430 , including the silicon-nitride insulating layer  432  and the tantalum coating layer  435 . 
       FIGS. 14-16  provide cross-section elevation views through selected structures for a third exemplary embodiment of the polysilicon feed-through heater shown in FIG.  7 .  FIG. 14  is a cross-section through the third exemplary embodiment of the common bus  330  along the plane VIII—VIII shown in FIG.  7 .  FIG. 15  is a cross-section across the third exemplary embodiment of the connection line  322  along the plane IX—IX shown in FIG.  7 .  FIG. 16  is a cross-section through the third exemplary embodiment of the resistive heater  314  along the plane X—X shown in FIG.  7 . 
     As shown in  FIG. 14 , a polysilicon layer is deposited on or over one surface of the field oxide layer  400  for the heater plate  302 . A relatively thick polysilicon layer  470  is produced from the polysilicon layer to produce an electrically conductive layer. The relatively thick polysilicon layer  470  in  FIG. 14  is thicker than the relatively heavily-doped polysilicon layer  410  shown in  FIGS. 8 and 11  so that the relatively thick polysilicon layer  470  can conduct current with less electrical resistance than otherwise. An electrically insulating layer  440  is deposited on or over the relatively thick polysilicon layer  470 . A set of protective layers  430  is formed on or over the insulating layer  440 . The set of protective layers  430  include a lower silicon-nitride insulating layer  432  and an upper β-phase tantalum coating layer  435 . 
     As shown in  FIG. 15 , the relatively thick polysilicon layer  470  is formed by either preferably relatively heavily-doping (N ++ ) or else alternately relatively lightly-doping (N + ) this polysilicon layer to increase its electrical conductivity. A polysilicon layer  460  is formed by relatively lightly-doping (N + ) this polysilicon layer to increase its electrical conductivity to a value appropriate to act as a resistive heating element for ejecting fluid droplets. The greater cross-sectional area of the relatively thick polysilicon layer  470  will produce a lower current density and thus greater conductivity than the relatively lightly-doped polysilicon layer  460 . As indicated above, the relatively thick polysilicon layer  470  can be used, according to this invention, in place of a separately-routed layer of one or more high-conductivity materials, such as, for example, copper (Cu), and/or aluminum (Al). 
       FIG. 16  is a cross-sectional view showing in greater detail the third exemplary embodiment of the resistive heater  314 , with fluid flowing along a direction shown by the left-facing arrow through the nozzle  318 . A polysilicon layer is deposited on or over one surface of the field oxide layer  400  for the heater plate  302 . The relatively lightly-doped polysilicon layer  460  at the thinner portion and the relatively thick polysilicon layer  470  at the thicker portion can be formed adjacently from the deposited polysilicon layer by selective doping. The relatively lightly-doped polysilicon layer  460  and the relatively thick polysilicon layer  470  are overlaid by the set of protective layers  430 , including the silicon-nitride insulating layer  432  and the tantalum coating layer  435 . 
     One exemplary embodiment of a method for forming the polysilicon feed-through heater includes forming the fluid channel  316  with a resistive heater  314 , forming the common bus  330  transverse to the fluid channel  316 , forming the connection line  322 , forming the first set of the one or more layers  450  that electrically connects the common bus  330  with the resistive heater  314  and the connection line  322 , and forming the second set of the one or more layers  440  and  430  that covers the common bus  330  and the connection line  322 . In various exemplary embodiments, forming the electrically conductive first set of the one or more layers  450  includes forming the heavily-doped polysilicon layer  410  or the relatively thick polysilicon layer  470  on or over the field oxide layer  400 , and optionally forming the tantalum-silicide layer  420  on or over the relatively heavily-doped polysilicon layer  410 . In various exemplary embodiments, forming the protective second set of the one or more layers  430  includes forming the nitride layer  432  on or over the electrically conductive first set of the one or more layers  450 , and forming the tantalum layer  435  on or over the nitride layer  432 . 
     Another exemplary embodiment of a method for forming the fluid channel  316  includes forming the electrically-conductive first set of the one or more layers  450  having the relatively heavily-doped polysilicon layer  410  or  470  on or over the field oxide layer  400 , forming the tantalum-silicide layer  420  on or over the heavily doped polysilicon layer  410 , forming the relatively lightly-doped polysilicon layer  460  on or over the field oxide layer  400  so that the relatively lightly-doped polysilicon layer  460  is electrically connected to the relatively heavily-doped polysilicon layer  410  or the relatively thick polysilicon layer  470 , and forming the protective second set of the one or more layers  430  on or over the tantalum-silicide layer  420  and the relatively lightly-doped polysilicon layer  460 . 
     Yet another exemplary embodiment of a method for forming the common bus includes forming an electrically conductive first set of the one or more layers  450  on or over a first portion of the field oxide layer  400 , forming the relatively lightly-doped polysilicon layer  460  on or over a second portion of the field oxide layer  400 , forming the insulating layer  440  on or over the electrically conductive first set of the one or more layers  450  and on or over the relatively lightly-doped polysilicon layer  460 , and forming a protective second set of the one or more layers  430  on or over the tantalum-silicide layer  420  and the relatively lightly-doped polysilicon layer  460 . 
     While this invention has been described in conjunction with the exemplary embodiments outlined above, 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 can be made without departing from the spirit and scope of the invention.