Segmented resistor inkjet drop generator with current crowding reduction

In order to overcome inefficient power dissipation in parasitic resistances and to provide economies in the power supply, a higher resistance value heater resistor is employed in a thermal inkjet printhead. Higher current densities in a high resistance segmented heater resistor are reduced by employing a shorting bar divided by a current balancing resistor.

BACKGROUND OF THE INVENTION
 The present invention relates generally to inkjet printing devices, and
 more particularly to an inkjet printhead drop generator that utilizes a
 high resistance heater resistor structure with current crowding reduction.
 The art of inkjet printing technology is relatively well developed.
 Commercial products such as computer printers, graphics plotters, copiers,
 and facsimile machines successfully employ inkjet technology for producing
 hard copy printed output. The basics of the technology has been disclosed,
 for example, in various articles in the Hewlett-Packard Journal, Vol. 36,
 No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October
 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and
 Vol. 45, No. 1 (February 1994) editions. Inkjet devices have also been
 described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices (R. C.
 Durbeck and S. Sherr, ed., Academic Press, San Diego, 1988, chapter 13).
 A thermal inkjet printer for inkjet printing typically includes one or more
 translationally reciprocating print cartridges in which small drops of ink
 are formed and ejected by a drop generator towards a medium upon which it
 is desired to place alphanumeric characters, graphics, or images. Such
 cartridges typically include a printhead having an orifice member or plate
 that has a plurality of small nozzles through which the ink drops are
 ejected. Beneath the nozzles are ink firing chambers, enclosures in which
 ink resides prior to ejection by an ink ejector through a nozzle. Ink is
 supplied to the ink firing chambers through ink channels that are in fluid
 communication with an ink supply, which may be contained in a reservoir
 portion of the print cartridge or in a separate ink container spaced apart
 from the printhead.
 Ejection of an ink drop through a nozzle employed in a thermal inkjet
 printer is accomplished by quickly heating the volume of ink residing
 within the ink firing chamber with a selectively energizing electrical
 pulse to a heater resistor positioned in the ink firing chamber. At the
 commencement of the heat energy output from the heater resistor, an ink
 vapor bubble nucleates at sites on thesurface of the heater resistor or
 its protective layers.
 The rapid expansion of the ink vapor bubble forces the liquid ink through
 the nozzle. Once the electrical pulse ends and ink is ejected, the ink
 firing chamber, refills with ink from the ink channel and ink supply.
 The electrical energy required to eject an ink drop of a given volume is
 referred to as "turn-on energy". The turn-on energy is a sufficient amount
 of energy to overcome thermal and mechanical inefficiencies of the
 ejection process and to form a vapor bubble having sufficient size to
 eject a predetermined amount of ink through the printhead nozzle.
 Following removal of electrical power from the heater resistor, the vapor
 bubble collapses in the firing chamber in a small but violent way.
 Components within the printhead in the vicinity of the vapor bubble
 collapse are susceptible to fluid mechanical stresses (cavitation) as the
 vapor bubble collapses, allowing ink to crash into the ink firing chamber
 components. The heater resistor is particularly susceptible to damage from
 cavitation. A protective layer, comprised of one or more sublayers, is
 typically disposed over the resistor and adjacent structures to protect
 the resistor from cavitation and from chemical attack by the ink. The
 protective sublayer in contact with the ink is a thin hard cavitation
 layer that provides protection from the cavitation wear of the collapsing
 ink. Another sublayer, a passivation layer, is typically placed between
 the cavitation layer and the heater resistor and associated structures to
 provide protection from chemical attack. Thermal inkjet ink is chemically
 reactive, and prolonged exposure of the heater resistor and its electrical
 interconnections to the ink will result in a chemical attack upon the
 heater resistor and electrical conductors. The protection sublayers,
 however, tend to increase the turn-on energy required for ejecting drops
 of a given size. Additional efforts to protect the heater resistor from
 cavitation and attack include separating the heater resistor into several
 parts and leaving a center zone (upon which a majority of the cavitation
 energy concentrates in a top firing thermal inkjet firing chamber) free of
 resistive material.
 The heater resistor of a conventional inkjet printhead utilizes a thin film
 resistive material disposed on an oxide layer of a semiconductor
 substrate. Electrical conductors are patterned onto the oxide layer and
 provide an electrical path to and from each thin film heater resistor.
 Since the number of electrical conductors can become large when a large
 number of heater resistors are employed in a high density (high DPI--dots
 per inch) printhead, various multiplexing techniques have been introduced
 to reduce the number of conductors needed to connect the heater resistors
 to circuitry disposed in the printer. See, for example, U.S. Pat. No.
 5,541,629 "Printhead with Reduced Interconnections to a Printer" and U.S.
 Pat. No. 5,134,425, "Ohmic Heating Matrix". Each electrical conductor,
 despite its good conductivity, imparts an undesirable amount of resistance
 in the path of the heater resistor. This undesirable parasitic resistance
 dissipates a portion of the electrical power which otherwise would be
 available to the heater resistor. If the heater resistance is low, the
 magnitude of the current drawn to nucleate the ink vapor bubble will be
 relatively large and the amount of energy wasted in the parasitic
 resistance of the electrical conductors will be significant. That is, if
 the ratio of resistances between that of the heater resistor and the
 parasitic resistance of the electrical conductors (and other components)
 is too small, the efficiency of the printhead suffers with the wasted
 energy.
 The ability of a material to resist the flow of electricity is a property
 called resistively. Resistively is a function of the material used to make
 the resistor and does not depend upon the geometry of the resistor of the
 thickness of the resistive film used to form the resistor. Resistively is
 related to resistance by:
EQU R=.rho.L/A
 where R=resistance (Ohms); .SIGMA.=resistively (Ohm-cm); L=length of
 resistor; and A=cross sectional area of resistor. For thin film resistors
 typically used in thermal inkjet printing applications, a property
 commonly known as sheet resistance (R.sub.sheet) is commonly used in
 analysis and design of heater resistors. Sheet resistance is the
 resistively divided by the thickness of the film resistor, and resistance
 is related to sheet resistance by:
EQU R=R.sub.sheet (L/W)
 where L=length of the resistive material and W=width of the resistive
 material. Thus, resistance of a thin film resistor of a given material and
 of a fixed film thickness is a simple calculation of length and width for
 rectangular and square geometries.
 Most of the thermal inkjet printers available today use heater resistors
 that are roughly of a square shape and have a resistance of 35 to 40 Ohms.
 If it were possible to use resistors with higher values of resistance, the
 energy needed to nucleate an ink vapor bubble would be transmitted to the
 thin film heater resistor at a higher voltage and lower current. The
 energy wasted in the parasitic resistances would be reduced and the power
 supply that provides the power to the heater resistors could be made
 smaller and less expensive. Realization of the higher values of
 resistance, however, may increase the current density despite the overall
 current reduction. High current density can reduce the life of electronic
 circuits by creating localized elevated temperatures and by generating
 high electric field strengths that induce electromigration in materials.
 Moreover, in applications where the current is switched on and off, such
 as in thermal inkjet heater resistors, extreme thermal cycling produces
 expansion and contraction, which results in fatigue failures.
 SUMMARY OF THE INVENTION
 A segmented heater resistor for an inkjet printer includes a first heater
 resistor segment and a second heater resistor segment. A coupling device
 provides a serial coupling between the first and second resistor segments.
 A current control device provides reduced current crowding in the coupling
 device.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
 There are three main techniques for obtaining a higher resistance heater
 resistor for use in a thermal inkjet printer application. First, a thinner
 resistance layer can be deposited on the substrate oxide. The downside of
 this approach is that as the films become thinner, they become susceptible
 to surface defects and, the thinner the film, the more difficult it
 becomes to control the film thickness. Second, a different material having
 a higher innate resistively than the well understood tantalum-aluminum
 film could be used. The extreme environmental conditions experienced by
 the heater resistor as well as the need for an inexpensive, low defect,
 thin film process reduces the short term desirability of this approach.
 Third, new configurations of thin film resistor geometries can result in
 higher resistance heater resistors. It is from this third technique that
 the present invention derives.
 An exemplary inkjet printing apparatus, a printer 101, that may employ the
 present invention is shown in outline form in the isometric drawing of
 FIG. 1A. Printing devices such as graphics plotters, copiers, and
 facsimile machines may also profitably employ the present invention. A
 printer housing 103 contains a printing platen to which an input print
 medium 105, such as paper, is transported by mechanisms that are known in
 the art. A carriage within the printer 101 holds one or a set of
 individual print cartridges capable of ejecting ink drops of black or
 color ink. Alternative embodiments can include a semipermanent printhead
 mechanism that is sporadically replenished from one or more
 fluidically-coupled, off-axis, ink reservoirs, or a single print cartridge
 having two or more colors of ink available within the print cartridge and
 ink ejecting nozzles designated for each color, or a single color print
 cartridge or print mechanism; the present invention is applicable to a
 printhead employed by at least these alternatives. A carriage 109, which
 may be employed in the present invention and mounts two print cartridges
 110 and 111, is illustrated in FIG. 1B. The carriage 109 is typically
 mounted on a slide bar or similar mechanism within the printer and
 physically propelled along the slide bar to allow the carriage 109 to be
 translationally reciprocated or scanned back and forth across the print
 medium 105. The scan axis, X, is indicated by an arrow in FIG. 1A. As the
 carriage 109 scans, ink drops are selectively ejected from the printheads
 of the set of print cartridges 110 and 111 onto the medium 105 in
 predetermined print swath patterns, forming images or alphanumeric
 characters using dot matrix manipulation. Generally, the dot matrix
 manipulation is determined by a user's computer (not shown) and
 instructions are transmitted to a microprocessor-based, electronic
 controller (not shown) within the printer 101. Other techniques employ a
 rasterization of the data in a user's computer prior to the rasterized
 data being sent, along with printer control commands, to the printer. This
 operation is under control of printer driver software resident in the
 user's computer. The printer interprets the commands and rasterized data
 to determine which drop generators to fire. The ink drop trajectory axis,
 Z, is indicated by the arrow. When a swath of print has been completed,
 the medium 105 is moved an appropriate distance along the print media
 axis, Y, indicated by the arrow in preparation for the printing of the
 next swath. This invention is also applicable to inkjet printers employing
 alternative means of imparting relative motion between printhead and
 media, such as those that have fixed printheads (such as page wide arrays)
 and move the media in one or more directions, those that have fixed media
 and move the printhead in one or more directions (such as flatbed
 plotters). In addition, this invention is applicable to a variety of
 printing systems, including large format devices, copiers, fax machines,
 photo printers, and the like.
 The inkjet carriage 109 and print cartridges 110, 111 are shown from the -Z
 direction within the printer 101 in FIG. 1B. The printheads 113, 115 of
 each cartridge may be observed when the carriage and print cartridges are
 viewed from this direction. In a preferred embodiment, ink is stored in
 the body portion of each printhead 110,115 and routed through internal
 passageways to the respective printhead. In an embodiment of the present
 invention which is adapted for multi-color printing, three groupings of
 orifices, one for each color (cyan, magenta, and yellow), is arranged on
 the foraminous orifice plate surface of the printhead 115. Ink is
 selectively expelled for each color under control of commands from the
 printer that are communicated to the printhead 115 through electrical
 connections and associated conductive traces (not shown) on a flexible
 polymer tape 117. In the preferred embodiment, the tape 117 is typically
 bent around an edge of the print cartridge as shown and secured. In a
 similar manner, a single color ink, black, is stored in the ink-containing
 portion of cartridge 110 and routed to a single grouping of orifices in
 printhead 113. Control signals are coupled to the printhead from the
 printer on conductive traces disposed on a polymer tape 119.
 As can be appreciated from FIG. 2, a single medium sheet is advanced from
 an input tray into a printer print area beneath the printheads by a medium
 advancing mechanism including a roller 207, a platen motor 209, and
 traction devices (not shown). In a preferred embodiment, the inkjet print
 cartridges 110, 111 are incrementally drawn across the medium 105 on the
 platen by a carriage motor 211 in the .+-.X direction, perpendicular to
 the Y direction of entry of the medium. The platen motor 209 and the
 carriage motor 211 are typically under the control of a media and
 cartridge position controller 213. An example of such positioning and
 control apparatus may be found described in U.S. Pat. No. 5,070,410
 "Apparatus and Method Using a Combined Read/Write Head for Processing and
 Storing Read Signals and for Providing Firing Signals to Thermally
 Actuated Ink Ejection Elements". Thus, the medium 105 is positioned in a
 location so that the print cartridges 110 and 111 may eject drops of ink
 to place dots on the medium as required by the data that is input to a
 drop firing controller 215 and power supply 217 of the printer. These dots
 of ink are formed from the ink drops expelled from selected orifices in
 the printhead in a band parallel to the scan direction as the print
 cartridges 110 and 111 are translated across the medium by the carriage
 motor 211. When the print cartridges 110 and 111 reach the end of their
 travel at an end of a print swath on the medium 105, the medium is
 conventionally incrementally advanced by the position controller 213 and
 the platen motor 209. Once the print cartridges have reached the end of
 their traverse in the X direction on the slide bar, they are either
 returned back along the support mechanism while continuing to print or
 returned without printing. The medium may be advanced by an incremental
 amount equivalent to the width of the ink ejecting portion of the
 printhead or some fraction thereof related to the spacing between the
 nozzles. Control of the medium, positioning of the print cartridge, and
 selection of the correct ink ejectors for creation of an ink image or
 character is determined by the position controller 213. The controller may
 be implemented in a conventional electronic hardware configuration and
 provided operating instructions from conventional memory 216. Once
 printing of the medium is complete, the medium is ejected into an output
 tray of the printer for user removal.
 A single example of an ink drop generator found within a printhead is
 illustrated in the magnified isometric cross section of FIG. 3. As
 depicted, the drop generator comprises a nozzle, a firing chamber, and an
 ink ejector. Alternative embodiments of a drop generator employ more than
 one coordinated nozzle, firing chamber, and/or ink ejectors. The drop
 generator is fluidically coupled to a source of ink.
 In FIG. 3, the preferred embodiment of an ink firing chamber 301 is shown
 in correspondence with a nozzle 303 and a segmented heater resistor 309.
 Many independent nozzles are typically arranged in a predetermined pattern
 on the orifice plate so that the ink which is expelled from selected
 nozzles creates a defined character or image of print on the medium.
 Generally, the medium is maintained in a position which is parallel to the
 external surface of the orifice plate. The heater resistors are selected
 for activation by the microprocessor and associated circuitry in the
 printer in a pattern related to the data presented to the printer by the
 computer so that ink which is expelled from selected nozzles creates a
 defined character or image of print on the medium. Ink is supplied to the
 firing chamber 301 via opening 307 to replenish ink that has been expelled
 from orifice 303 when ink has been vaporized by heat energy released by
 the segmented heater resistor 309. The ink firing chamber is bounded by
 walls created by an orifice plate 305, a layered semiconductor substrate
 313, and firing chamber wall 315. In a preferred embodiment, fluid ink
 stored in a reservoir of the cartridge housing 212 flows by capillary
 force to fill the firing chamber 301.
 Once the ink is in the firing chamber 301 it remains there until it is
 rapidly vaporized by the heat energy created by the electrically energized
 segmented heater resistor 309 disposed on the oxidized surface of
 substrate 313. The substrate is typically a semiconductor such as silicon.
 The silicon is treated using either thermal oxidation or vapor deposition
 techniques to form a thin layer of silicon dioxide thereon. The segmented
 heater resistor 309 is then created by depositing a patterned film of
 resistive material on the silicon dioxide. Preferably, the film is
 tantalum aluminum, TaAl, which is a well known resistive heater material
 in the art of thermal inkjet printhead construction. Next, a thin layer of
 aluminum is deposited to provide the electrical conductors.
 In FIG. 4, a cross section of the firing chamber 301 and the associated
 structures are shown. The substrate 313 comprises, in the preferred
 embodiment, a silicon base 401, treated using either thermal oxidation or
 vapor deposition techniques to form a thin layer 403 of silicon dioxide
 and a thin layer 405 of phospho-silicate glass (PSG) thereon. The silicon
 dioxide and PSG forms an electrically insulating layer approximately 17000
 Angstroms thick upon which a subsequent discontinuous layer 407 of
 tantalum-aluminum (TaAl) of resistive material is deposited. The tantalum
 aluminum layer is deposited to a thickness of approximately 900 Angstroms
 to yield a resistively of approximately 30 Ohms per square. In a preferred
 embodiment, the resistive layer is conventionally deposited using a
 magnetron sputtering technique and then masked and etched to create
 discontinuous and electrically independent areas of resistive material
 such as areas 409 and 411. Next, a layer of aluminum-silicon-copper
 (AlSiCu) alloy conductor is conventionally magnetron sputter deposited to
 a thickness of approximately 5000 Angstroms atop the tantalum aluminum
 layer areas 409, 411 and etched to provide discontinuous and independent
 electrical conductors (such as conductors 415 and 417) and interconnect
 areas. To provide protection for the heater resistors, a composite layer
 of material is deposited over the upper surface of the conductor layer and
 resistor layer. A dual layer of passivating materials includes a first
 layer 419 of silicon nitride approximately 2500 Angstroms thick which is
 covered by a second layer 421 of inert silicon carbide approximately 1250
 Angstroms thick. This passivation layer (419, 421) provides both good
 adherence to the underlying materials and good protection against ink
 corrosion. It also provides electrical insulation. An area over the heater
 resistor 309 and its associated electrical connection to electrical
 conductors is subsequently masked and a cavitation layer 423 of tantalum
 3000 Angstroms thick is conventionally sputter deposited. A gold layer 425
 may be selectively added to the cavitation layer in areas where electrical
 interconnection to an interconnection material is desired. An example of
 semiconductor processing for thermal inkjet applications may be found in
 U.S. Pat. No. 4,862,197, "Process for Manufacturing Thermal Inkjet
 Printhead and Integrated Circuit (IC) Structures Produced Thereby." An
 alternative thermal inkjet semiconductor process may be found in U.S. Pat.
 No. 5,883,650, Thin-Film Printhead Device for an InkJet Printer."
 In a preferred embodiment, the sides of the firing chamber 301 and the ink
 feed channel are defined by a polymer barrier layer 315. This barrier
 layer is preferably made of an organic polymer plastic that is
 substantially inert to the corrosive action of ink and is conventionally
 deposited upon substrate 313 and its various protective layers. To realize
 the desired structure, the barrier layer is subsequently
 photolithographically defined into desired shapes and then etched.
 Typically the barrier layer 315 has a thickness of about 15 micrometers
 after the printhead is assembled with the orifice plate 305.
 The orifice plate 305 is secured to the substrate 313 by the barrier layer
 315. In some print cartridges the orifice plate 305 is constructed of
 nickel with plating of gold to resist the corrosive effects of the ink. In
 other print cartridges, the orifice plate is formed of a polyamide
 material that can be made into a common electrical interconnect structure.
 In an alternative embodiment, the orifice plate and barrier layer is
 integrally formed on the substrate.
 In a preferred embodiment of the present invention, a heater resistor
 having a higher value of resistance is employed to overcome the problems
 stated above, in particular the problems of undesired energy dissipation
 in the parasitic resistance and of the necessity of having a high current
 capacity in the power supply. Here, the implementation of a higher value
 resistance resistor is that of revising the geometry of the heater
 resistor, specifically that of providing two segments having a greater
 length than width. Since it is preferred to have the heater resistor
 located in one compact spot for optimum vapor bubble nucleation in a
 top-shooting (ink drop ejection perpendicular to the plane of the heater
 resistor) printhead, the resistor segments are disposed long side to long
 side as shown in FIG. 5. As shown, heater resistor segment 501 is disposed
 with one of its long sides essentially parallel to the long side of heater
 resistor segment 503. Electrical current I.sub.in is input via conductor
 505 to an input port 507 of the resistor segment 501 disposed at one of
 the short sides (width) edges of resistor segment 501. The electrical
 current, in the preferred embodiment, is coupled to the input port 509 of
 the resistor segment 503 disposed at one of the short side (width) edges
 of resistor segment 503 by coupling device that has been termed a
 "shorting bar" 511. The shorting bar is a portion of conductor film
 disposed between the output port 513 of heater resistor segment 501 and
 the input port 509 of heater resistor segment 503. The electrical current
 I.sub.out is returned to the power supply via conductor 515 connected to
 the output port 517 of heater resistor segment 503. As shown, with no
 additional electrical current sources or sinks, I.sub.in =I.sub.out. The
 output ports 513 and 517 of heater resistor segments 501 and 503,
 respectively, are disposed at the opposite short side (width) edges of the
 heater resistor segments from the input ports.
 By placing the two resistor segments in a compact area, it is necessary for
 the electric current to change direction by way of the coupling device or
 shorting bar portion 511. Because the path of the electrons comprising the
 electric current is shorter between the two proximate comers of the heater
 resistor segments (causing the parasitic resistance of the shorter path to
 be less than the longer path), more of the electric current flows in this
 shorter path, illustrated by arrow 521 in FIG. 5, than any other path,
 illustrated by arrow 523. This concentration of current has been termed
 "current crowding". High current density produced by such current crowding
 will reduce the life of electronic circuits because it creates locally
 elevated temperatures and creates high electric field strengths that
 induce electromigration. In applications where the electric current is
 cycled on and off, such as in a thermal inkjet printhead, the rapid
 thermal variation causes expansion and contraction of the printhead
 substrate and the thin film layers disposed thereon. In areas having
 differential thermal expansion and contraction amounts because of the
 differences in thermal expansion rates of different materials, such as at
 the junction of a heater resistor segment and the conductor shorting bar,
 material fatigue stresses will cause an early failure.
 To address the current crowding problem, a feature of the present invention
 causes the current flow to spread more uniformly through the shorting bar.
 This is accomplished by enhancing the shorting bar with a current control
 device 600. This current control device comprises a modified and/or
 missing portion of the conductive film that serially connects resistor
 segments 501 and 503. Preferably, the control device 600 is a portion of
 coupling device 511 having varying degrees of sheet resistance to reduce
 problems with current concentrations or current crowding in coupling
 device 511. Preferably, the current control device 600 includes a higher
 sheet resistance region of coupling device 511 positioned in the shorter
 current path 521 region of coupling device 511. In a theoretical limit,
 removing a portion of the conductive sheet in the shorter current path 521
 region is equivalent to an infinite sheet resistance in that region. In a
 preferred embodiment, the current control device 600 is realized as a
 current balancing element created in association with the shorting bar. As
 shown in FIG. 6B, a balancing resistor 601 separates the shorting bar
 portion into two shorting bar segments, segment 511a and segment 511b. In
 a preferred embodiment where the resistive material is deposited first on
 the oxide layer of the semiconductor substrate then overlain with an
 electrical conductor film, balancing resistor 601 is preferably created by
 etching shorting bar portion conductive film in the balancing resistor 601
 area, thereby exposing the resistive material layer and creating a
 resistor (unshorted by the conductive layer disposed atop the resistive
 material layer). Alternatively the conductive film may be selectively
 deposited in masking and deposition steps. Although the balancing element
 is preferably a resistor, other elements, such as a parallel arrangement
 of diodes, or similar current restrictive devices may be employed in the
 present invention.
 Balancing resistor 601, in the preferred embodiment, is created with a
 trapezoidal or triangular-shaped tapered geometry in which the widest
 (base) end is positioned in the area of the shorting bar which previously
 experienced current crowding. The balancing resistor is further created
 with its narrowest (apex) end furthest from the area furthest from the
 area of current crowding. This tapered geometry, arranged as shown in FIG.
 6B, produces a resistor that has its highest incremental resistance at its
 base and its lowest incremental resistance at its apex. Incremental
 resistance, as used herein, is a magnitude of resistance which would be
 measured on an essentially linear path from a point on the edge of an
 input port 603 of balancing resistor 601 to a point on the edge of an
 output port 605 of balancing resistor 601 without any parallel resistance
 effects from any other path across balancing resistor 601. When the path
 lengths for current flowing through the shorting bar segment 511a, the
 balancing resistor 601, and the shorting bar segment 511b are taken into
 consideration, the resistance encountered by an electric current flowing
 from the output port 513 of heater resistor segment 501 to the input port
 509 of heater resistor segment 503 is essentially the same.
 Stated another way and with reference to FIG. 7, a resistor model can be
 configured to help explain the operation of this facet of the present
 invention. Current flows into heater resistor segment 501' (having a
 resistance value of R.sub.H) via conductor 505'. At the output of heater
 resistor segment 501', the current divides into a multiplicity of
 paths--two of which are deemed to be path 701 and path 703. In path 701, a
 component of the current flows 30 through a physically short path 705
 (having a parasitic resistance value of r.sub.1) of shorting bar segment
 511a, through a physically long path 707 (having a resistance value of
 R.sub.A) of balancing resistor 601, and through another physically short
 path 709 (having a parasitic resistance value of r.sub.1) of shorting bar
 segment 511b. In path 711, another component of the current flows through
 a physically long path (having a parasitic resistance value of r.sub.2) of
 shorting bar segment 511a, through a physically short path 713 (having a
 resistance value of R.sub.B) of balancing resistor 601, and through
 another physically long path (having a parasitic resistance value of
 r.sub.1) of shorting bar segment 511b. The current recombines at the input
 to heater resistor segment 503' (having a resistance value of R.sub.H) and
 is returned via conductor 515'. In order that the current be balanced and
 current crowding be avoided, the balancing resistor 601 and the shorting
 bar segments 511a, and 511b are designed so that:
EQU r.sub.1 &lt;r.sub.2,
EQU R.sub.H &gt;R.sub.A &gt;R.sub.B, and
EQU R.sub.A +2r.sub.1 =R.sub.B +2r.sub.2.
 The component of the current flowing through path 701 is therefore made
 essentially equal to the component of current flowing through path 703 and
 current crowding is avoided.
 The physical implementation of a preferred embodiment of the present
 invention uses a heater resistor having a total (R.sub.H +R.sub.H)
 resistance value of approximately 140 ohms. As diagrammed in a preferred
 embodiment illustrated in FIG. 6B, the balancing resistor has a total
 measurable resistance value of 4 ohms with physical dimensions of
 b.congruent.2.3 .mu.m at the base, a.congruent.1.8 .mu.m at the truncated
 apex, and a truncated triangle height of h.congruent.25 .mu.m, which is
 related to the lengths of the triangle sides. The heater resistor segments
 501 and 503 each have a width of w.congruent.9 .mu.m and a length
 l.congruent.20 .mu.m. The tantalum-aluminum thin film of the heater
 resistor segments and the balancing resistor has a thickness of
 approximately 900 Angstroms. It should be noted that as the height, h,
 becomes larger (that is, as the shorting bar becomes wider) the current
 distribution becomes greater (more individual electron paths are
 available) and the total measurable resistance value increases.
 In an alternative embodiment where the heater resistor need not be
 concentrated in a confined area (such as in a distributed or multiple
 coordinated nozzle configuration) but in which a turn or corner is
 necessary in the shorting bar portion, an application of the present
 invention may be employed to minimize the effects of current crowding in
 the shorting bar. A ninety degree turn is necessary in the shorting bar
 for the heater resistor configuration of FIG. 8. The heater resistor
 consists of two resistor segments 801, 803 joined by a shorting bar
 conductor separated into two portions 805a and 805b by balancing resistor
 807.
 Other ways of balancing the current in a coupling device using a current
 control device can be considered, as illustrated in FIG. 9. For example,
 the current control device 600 can be a missing or higher resistance
 portion 901 of coupling device 511 that is positioned in the region of
 current crowding. Portion 901 is depicted to be of any or geometry that
 reduces current crowding in coupling device 511 to an acceptable level.
 Alternatively, coupling device 511 may have a graded or varying resistance
 level that increases with distance from resistor segments 501 and 503 to
 minimize the maximum current density in coupling device 511. Stated
 another way, coupling device 511 can comprise a sheet 511 of varying sheet
 resistance wherein the sheet resistance has a higher value where coupling
 device contacts resistor segments 501 and 501. In that event, this
 variation of sheet resistance can be referred to as a current control
 device aspect of coupling device 511.
 Thus, a thermal ink drop generator has been described which enables a
 higher value of resistance to be realized by improving the heater resistor
 geometry of segmented resistors. Current crowding is reduced by employing
 a balancing resistor as part of the shorting bar conductor.