Improved thermal ink jet heater design

Abstract: A heater element for a printhead for an ink jet printing system has a pit layer (22) which protects the overglaze passivation layer (20), PSG step region (10), portions of the Ta layer (14) and dielectric isolation layer (12) and junctions or regions susceptible to the cavitational pressures. The inner walls (23) of the pit layer define the effective heater area and dopant lines (9) within the heater resistor (8) define the actual heater area. In an alternative arrangement, the dopant lines define both the actual and effective heater areas, and in yet another arrangement, one inner wall and one dopant line define the effective heater area. When a plurality of the heater elements are incorporated into a printhead having either full pit channel geometry and open pit channel geometry, the operating lifetime of the printhead is extended because the added protection of the pit layer prevents: 1) passivation damage and cavitational damages of the heater elements; and 2) degradation of heater robustness, hot spot formations and heater failures well into the 109 pulse range. The printhead can be incorporated into a drop-on-demand printing system of a carriage type or a full width type.

Description: The present invention is directed to ink jet printing systems, and in particular to drop-on-demand ink jet printing systems having printheads with heater elements. Ink jet printing systems can be divided into two types. The first type is a continuous stream ink jet printing system and the second type is a drop-on-demand printing system. In a continuous stream ink jet printing system, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed so that the stream breaks up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct the ink droplets to a gutter for recirculation or to a specific location on a recording medium. In a drop-on-demand ink jet printing system, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless the droplet is to be placed on the recording medium. Because the drop-on-demand ink jet printing system requires no ink recovery, charging or deflection, such a system is much simpler than the continuous stream ink jet printing system. Thus, ink jet printing systems are generally drop-on-demand ink jet printing systems. Further, there are two types of drop-on-demand ink jet printing systems. The first type uses a piezoelectric transducer to produce a pressure pulse that expels a droplet from a nozzle. The second type uses thermal energy to produce a vapor bubble in an ink-filled channel to expel an ink droplet. The first type of drop-on-demand ink jet printing system has a printhead with ink-filled channels, nozzles at ends of the channels and piezoelectric transducers near the other ends to produce pressure pulses. The relatively large size of the transducers prevents close spacing of the nozzles, and physical limitations of the transducers result in low ink drop velocity. Low ink drop velocity seriously diminishes the tolerances for drop velocity variation and directionality and impacts the system's ability to produce high quality copies. Further, the drop-on-demand printing system using piezoelectric transducers suffers from slow printing speeds. Due to the above disadvantages of printheads using piezoelectric transducers, drop-on-demand ink jet printing systems having printheads which use thermal energy to produce vapor bubbles in ink-filled channels to expel ink droplets are generally used. A thermal energy generator or heater element, usually a resistor, is located at a predetermined distance from a nozzle of each one of the channels. The resistors are individually addressed with an electrical pulse to generate heat which is transferred from the resistor to the ink. The transferred heat causes the ink to be super heated, i.e., far above the ink's normal boiling point. For example, a water based ink reaches a critical temperature of 280°C for bubble nucleation. The nucleated bubble or water vapor thermally isolates the ink from the heater element to prevent further transfer of heat from the resistor to the ink. Further, the nucleating bubble expands until all of the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor which, of course, removes heat due to heat of vaporization. During the expansion of the vapor bubble, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. When the excess heat is removed from the ink, the vapor bubble collapses on the resistor, because the heat generating current is no longer applied to the resistor. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as an ink droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity to expel the ink droplet towards a recording medium, such as paper, in a substantially straight line direction. The entire bubble expansion and collapse cycle takes about 20 microseconds (µs). The channel can be refired after 100 to 500 µs minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to be somewhat dampened. Figure 1 is an enlarged, cross-sectional view of a conventional heater element design. The conventional heater element 2 comprises a substrate 4, an underglaze layer 6, a resistive layer 8, a phosphosilicate glass (PSG) step region 10, a dielectric isolation layer 12, a tantalum (Ta) layer 14, addressing and common return electrodes 16, 18, an overglaze passivation layer 20, and a pit layer 22. The actual heater area is determined by the length L R of the resistive material. However, the effective heater area is determined by the distance L E between the inner slanted walls of the overglaze passivation layer. In another conventional heater element design (not shown), the side walls of the overglaze passivation do not overlap the side walls of the PSG step region, and the effective heater area is determined by the distance between the inner side walls of the PSG step region. Because there is a relatively large difference L D between the actual heater area and effective heater area, the heat generated at the unused heater areas is lost. Further, the overglaze passivation layer 20 or PSG step region 10 alone prevents exposure of the ionic and corrosive ink to the addressing and common return electrodes and/or resistor ends. It is generally recognized in the ink jet technology that the operating lifetime of an ink jet printhead is directly related to the number of cycles of vapor bubble expansion and collapse that the heater elements can endure before failure. Further, after extended usage, the heater robustness, i.e., the printhead's ability to produce well defined ink droplets, is degraded. Heater failures and degradation of heater robustness are due to extended exposure of the heater elements to high temperatures, frequency related thermal stresses, large electrical fields and significant cavitational pressures during vapor bubble expansion and collapse. Under such environmental conditions of the heater elements, the average heater lifetime is in the high 10 7 pulse range, i.e., number of ink droplets produced, with the first heater failure occurring as low as 3×10 7 pulse range. Further, the bulk of all heater failures does not occur on the resistors 8 which vaporize the ink, but rather occurs near the junction between the resistor 8 and electrodes 16, 18. Specifically, during the collapse phase of the vapor bubble, large cavitational pressures of up to 1000 atm. impact the regions near the PSG step region 10 and overglaze passivation layer 20 of the heater. The large cavitational pressures result in attrition damage to the tantalum (Ta) layer 14 and dielectric isolation layer 12 and also attrition damage, i.e., notch damage, to the overglaze passivation layer 20 covering the PSG step region 10. Moreover, the overglaze passivation layer 20 alone protects the electrodes 16, 18 from the ionic ink, which is corrosive. Eventually, a hole in the Ta layer 14, dielectric isolation layer 12 and/or passivation layer 20 allows the ionic and corrosive ink to contact the heater at the electrodes 16, 18 to cause degradation of heater robustness and hot spot formation and eventually to heater failures. Moreover, the heater failures are exacerbated by the problem of obtaining good conformal coverage of the Ta layer 14 over the PSG step region 10. The problem of obtaining good conformal coverage has been corrected by using an extra processing step to taper which consequentially extends the heater lifetime into the low 10 8 pulse range. However, heater failures are still located at the PSG step region 10 and/or the overglaze passivation layer 20, and the cost of fabrication is increased by an extra processing step to obtain good conformal coverage. Various printhead design approaches and heater constructions are disclosed in the following patents to mitigate the vulnerability of the heaters to cavitational pressures, but none of the patents discloses a heater design which removes the failure prone overglaze passivation layer 20 and/or PSG step region 10 from the region of final bubble collapse so that the PSG step region 10 and overglaze passivation layer 20 are no longer subject to the cycles of vapor bubble expansion and collapse and to the ionic and corrosive ink. U.S. Patent No. 4,951,063 to Hawkins et al. discloses a thermal ink jet printhead improved by a specific heating element structure and method of manufacture. The heating elements each have a resistive layer, a high temperature deposited plasma or pyrolitic silicon nitride thereover of predetermined thickness to electrically isolate a subsequently formed cavitational stress protecting layer of tantalum thereon. Such a construction lowers the manufacturing cost and concurrently provides a more durable printhead. U.S. Patent No. 5,041,844 to Deshpande discloses a thermal ink jet printhead having an ink channel geometry that controls the location of the bubble collapse on the heating elements. The ink channels provide the flow path between the printhead ink reservoir and the printhead nozzles. In one embodiment, the heating elements are located in a pit a predetermined distance upstream from the nozzle. The channel portion upstream from the heating element has a length and a cross-sectional flow area that is adjusted relative to the channel portion downstream from the heating element, so that the upstream and downstream portions of the channel have substantially equal ink flow impedances. This results in controlling the location of the bubble collapse on the heating element to a location substantially in the center of the heating elements. U.S. Patent No. 4,532,530 to Hawkins discloses a carriage type bubble ink jet printing system having improved bubble generating resistors that operate more efficiently and consume lower power without sacrificing operating lifetime. The resistor material is heavily doped polycrystalline silicon which can be formed on the same process lines with those for integrated circuits to reduce equipment costs and achieve higher yields. Glass mesas thermally isolate the active portion of the resistor from the silicon supporting substrate and from the electrode connecting points so that the electrode connection points are maintained relatively cool during operation. A thermally grown dielectric layer permits a thinner electrical isolation layer between the resistor and its protective ink interfacing tantalum layer and thus increases the thermal energy transfer to the ink. U.S. Patent No. 4,774,530 to Hawkins discloses an improved printhead which comprises an upper and lower substrate that are mated and bonded together with a thick insulative layer sandwiched therebetween. One surface of the upper substrate has etched therein one or more grooves and a recess, which when mated with the lower substrate, will serve as capillary filled ink channels and an ink supplying manifold, respectively. Recesses are patterned in the thick layer to expose the heating elements to the ink, thus placing them in a pit and to provide a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels, thereby eliminating the fabrication steps required to open the groove closed ends to the manifold recess so that the printhead fabrication process is simplified. U.S. Patent No. 4,835,553 to Torpey et al. discloses an ink jet printhead comprising upper and lower substrates that are mated and bonded together with a thick film insulative layer sandwiched therebetween. A recess patterned in the thick layer provides a flow path for the ink from the manifold to the channels by enabling the ink to flow around the closed ends of the channels and increase the flow area to the heating elements. Thus, the heating elements lie at the distal end of the recesses so that a vertical wall of elongated recess prevents air ingestion while it increases the ink channel flow area and decreases refill time, resulting in an increase in bubble generation rate. U.S. Patent No. 4,935,752 to Hawkins discloses an improved thermal ink jet printhead using heating element structures which space the portion of the heating element structures subjected to the cavitational forces produced by the generation and collapsing of the droplet expelling bubbles from the upstream interconnection to the heating element. In one embodiment, this is accomplished by narrowing the resistive area where the momentary vapor bubbles are to be produced so that a lower temperature section is located between the bubble generating region and the electrode connecting point. In another embodiment, the electrode is attached to the bubble generating resistive layer through a doped polysilicon descender. A third embodiment spaces the bubble generating portion of the heating element from the upstream electrode interface, which is most susceptible to cavitational damage, by using a resistive layer having two different resistivities. U.S. Patent No. 4,638,337 to Torpey et al. discloses an improved thermal ink jet printhead for ejecting and propelling ink droplets along a flight path toward a recording medium spaced therefrom in response to the receipt of the electrical input signals representing digitized data signals. The recess walls containing the heating elements prevent the lateral movement of the bubbles through the nozzle and therefore the sudden release of vaporized ink to the atmosphere, known as blow out which causes ingestion of air and interrupts the printhead operation. It is an object of the present invention to provide, for the printhead of an ink jet printing system, a heater element for improving the heater robustness, thermal efficiency and drop generation. The present invention provides a heater element for use in a printhead of a printing system to expel ink onto a recording medium by expansion and collapse of a vapor bubble, the heater element comprising: a substrate; a resistive layer formed on top of said substrate; contact means coupled to said resistive layer; an insulating means formed on top of said resistive layer to prevent contact between said resistive layer and the ink, the top surface of the insulating means transferring heat energy generated by said resistive layer to the ink to form said vapor bubble thereon; a passivating layer covering said substrate, contact means, and insulating means, the passivating layer being patterned to expose the top surface of a center portion of the insulating means but leaving the outer portions thereof covered by said passivating layer; and an insulative film overlying said passivating layer and having an upper surface which interfaces with the ink; characterised in that: said insulative film extends beyond the passivating layer which covers the outer portions of the insulating means to provide at least one inner wall, said at least one inner wall extending from the top surface of the insulating means to insulative film upper surface to form a pit, the at least one inner wall providing added protection to prevent damage of junctions and regions susceptible to cavitational pressures produced by the expansion and collapse of said vapor bubble on the insulating means. The present invention also provides a printhead as set out in claim 9. By way of example only, embodiments of the invention will be described with reference to the following drawings in which like reference numerals refer to like elements, and wherein: Figure 1 (already described) is an enlarged, cross-sectional view of a conventional heater element design; Figure 2 is a schematic perspective of a carriage- type drop-on-demand ink jet printing system; Figure 3 is an enlarged schematic isometric view of the printhead of the system illustrated in Figure 2; Figures 4A and 4B illustrate the expansion and collapse, respectively, of a vapor bubble in a full pit channel geometry printhead with a heater element in accordance with the present invention, along a view line A-A of Figure 3; Figures 5A and 5B are enlarged, cross-sectional views of heater elements in accordance with the present invention for use in printheads with full pit channel geometry; Figures 6A and 6B illustrate the expansion and collapse, respectively, of a vapor bubble in an open pit channel geometry printhead incorporating a heater element in accordance with the present invention along a view line A-A of Figure 3; and Figures 7A and 7B are enlarged, cross-sectional views of heater elements in accordance with the present invention for use in printheads with open pit channel geometry. Fig. 2 is a schematic perspective of a carriage- type drop-on-demand ink jet printing system 30 having a printhead 32. A linear array of ink droplet producing channels is housed in a printhead 32 of a reciprocating carriage assembly. Ink droplets 34 are propelled a preselected distance to a recording medium 36 which is stepped by a stepper motor 38 in the direction of an arrow 40 each time the printhead 32 traverses in one direction across the recording medium 36 in the direction of the arrow 42. The recording medium 36, such as paper, is stored on a supply roll 44 and stepped onto a roll 46 by the stepper motor 38 by means well known in the art. Further, it can be appreciated that sheets of paper can be used by using feeding mechanisms that are known in the art. The printhead 32 is fixedly mounted on a support base 48 to comprise the carriage assembly 50. The carriage assembly 50 is movable back and forth across the recording medium 36 in a direction parallel thereto by sliding on two parallel guide rails 52 and perpendicular to the direction in which the recording medium 36 is stepped. The reciprocal movement of the printhead 32 is achieved by a cable 54 and a pair of rotatable pulleys 56, one of which is powered by a reversible motor 58. The conduits 60 from a controller 62 provide the current pulses to the individual resistors in each of the ink channels. The current pulses which produce the ink droplets are generated in response to digital data signals received by the controller 62 through an electrode 64. A hose 66 from an ink supply 68 supplies the channel with ink during the operation of the printing system 30. Figure 3 is an enlarged schematic isometric view of the printhead 32 illustrated in Figure 2 which shows the array of nozzles 70 in a front face 71 of a channel plate 72 of the printhead 32. Referring also to Figures 4 and 6, which are cross-sectional views along a view line A-A, a lower electrically insulating substrate 4 has heater elements and terminals 82 patterned on a surface thereof while a channel plate 72 has parallel grooves 74 which extend in one direction and penetrate through a front face 71 of the channel plate 72. The other ends of grooves 74 terminate at a slanted wall 76. The surface of the channel plate 72 and grooves 74 are aligned and bonded to the substrate 4 so that the plurality of heater elements 1 is positioned in each channel 75 formed by the grooves 74 and the substrate 4 The printhead 32 is mounted on a metal substrate 78 containing insulated electrodes 80 which are used to connect the heater elements to the controller 62. The metal substrate 78 serves as a heat sink to dissipate heat generated within the printhead 32. The electrodes 16, 18 on the substrate 4 terminate at the terminals 82. The channel plate 72 is smaller than the substrate 4 in order that the electrode terminals 82 are exposed and available for connection to the controller 62 via the electrodes 80 on the metal substrate 78. An internal recess serves as an ink supply manifold 84 for the ink channels. The ink supply manifold 84 has an open bottom for use as an ink fill hole 86, and ink enters the manifold 84 through the fill hole 86 and fills each channel 75 by capillary action. The ink at each nozzle 70 forms a meniscus at a slight negative pressure which prevents the ink from weeping therefrom. Figures 4A and 6A illustrate the growth of ink droplet ejecting vapor bubbles of ink jet printhead with a full pit channel geometry and open pit channel geometry, respectively, incorporating a heater element in accordance with the present invention. Further, Figures 4B and 6B illustrate the cavitational pressure producing collapse in a printer having full pit channel geometry and open pit channel geometry, respectively, incorporating a heater element in accordance with the present invention. In a full pit channel geometry as shown in Figure 4A and 4B, which incorporates the heater element of Figure 5A, the thick film insulative layer 22, i.e., pit layer, is patterned to form a common recess 88 and a pit 24 (Fig. 5A) that exposes the heater element 1 to the ink. The channel 75 comprises a front channel length (L f ) downstream of the heating element, a rear channel length (L r ) upstream of the heating elements, and a pit length (L p ) covering the portion of the channel 75 containing the heater element 1. During the expansion of a vapor bubble 90, the ink is pushed away from the pit so that the ink flows out through the front channel portion and also flows towards the reservoir at the end of the rear channel portion as indicated by the arrows 92. The ink flow to the front channel portion causes the ink to bulge from the nozzle as a protrusion 34A. As the vapor bubble 90 collapses, an ink droplet 34 is ejected as shown in Figure 4B. Further, the ink moves into the pit 24 from both the front and rear channel portions as shown by arrows 94, and from the manifold 84 as shown by an arrow 96. Because L r is larger than L f and they both have the same flow area, the ink flowing from the rear channel portion has higher flow resistance than ink flowing from the front channel portion. As a result, more ink moves into the pit 24 from the front channel portion and such ink flow pushes the collapsing vapor bubble 90 to the junction between the resistor 8 and addressing electrode 16 and the region near the PSG step region 10 (Figs. 5A and 5B). Thus, the overglaze passivation layer 20, PSG step region 10 and portions of Ta and dielectric isolation layers 12, 14 near the PSG step region 10 of the addressing electrode 16 are subjected to large cavitational pressures. Figures 5A and 5B are enlarged, cross-sectional views of heater elements in accordance with the present invention. The heater element is formed on an underglaze layer 6 of a substrate 4, in the following manner Polysilicon is deposited on top of the underglaze layer and etched to form a resistor 8. The resistor has a lightly doped n-type region 8A with two heavily doped n-type regions 88 formed at ends of the lightly doped n-type region 8A. The interfaces between the heavily doped and lightly doped regions define dopant lines 9. The dopant lines 9 define the actual heater region L R of the heater element. A reflow phosphosilicate glass (PSG) is formed on top of the resistor 8 and etched to form the PSG step regions 10 which expose a top surface of the resistor 8 and electrode vias 17, 19 for the addressing and common return electrodes 16, 18. A dielectric isolation layer 12 is formed on top of the resistor 8 to electrically isolate the resistor 8 from the ink. A tantalum (Ta) layer 14 is sputter deposited on the dielectric isolation layer 12 to protect the dielectric isolation layer 12 from the heat and cavitational pressures. The dielectric isolation and Ta layers 12, 14 are etched and aluminum (Al) is metallized and etched to form the addressing electrode 16 and common return electrode 18. For an overglaze passivation layer 20, a thick composite layer of phosphorus doped CVD silicon dioxide and Si 3 N 4 is deposited over the entire substrate and etched to expose the Ta layer 14. Finally, a thick insulative layer is deposited over the entire substrate and etched to form the pit layer 22 and define the pit 24 and pit length L p . In both of the heater elements illustrated at 5A and 5B, the pit length L p is defined by the inner walls 23 of the pit layer 22. Further, the pit layer 22 has an inner wall height H p which is higher than the inner wall height of conventional heater element designs. In the preferred embodiment, the inner wall height is about 35 µm. Further, the inner walls 23 of the pit layer 22 extend beyond the inner ends of the overglaze passivation layer 20, Ta layer 14, dielectric isolation layer 12 and PSG step region 10 to provide an added protection to prevent damage of junctions and regions susceptible to the cavitational pressures. Further, PSG step region 10 and the overglaze passivation 20 no longer define the effective heater area. In the preferred embodiment, the inner walls 23 of the pit layer 22 define the effective heater region L E and the dopant lines 9 define the actual heater region L R . In Figure 5A, the difference L D between the actual heater region and effective heater region is reduced relative to the conventional heater element design. In Figure 5B and Figure 7B, both the effective and actual heater regions L E ,L R are defined by the dopant lines 9 and thus, the unused heater area is eliminated. Such efficient use of the heater increases the efficiency of the heater elements because less of the heat generated by the heater is lost and the heat generating pulse currents are efficiently used. In the open pit channel geometry as shown in Figures 6A and 6B, which incorporate the heater element of Figure 7A, the rear channel portion has a larger cross-sectional flow area than the front channel portion because the thick insulative layer 22 is removed from the rear channel portion. The ink is pushed away through both front and rear channel portions as in the full pit geometry of Figure 5A and shown by arrows 92. However, the ink flow is different during the bubble collapse. In the open pit channel geometry, the ink in the rear channel portion has a lower fluid flow resistance than the ink in the front channel portion. As a result, more ink moves into the pit from the rear portion and such ink flow pushes the collapsing vapor bubble to the junction between the resistor 8 and the common return electrode 18 and regions near the PSG step region 10. Thus, the overglaze passivation layer 20 and PSG step region 10 and portions of Ta and dielectric isolation layers 12, 14 near the PSG step region 10 of the common return electrode 18 are subjected to large cavitational pressures. Figures 7A and 7B are enlarged, cross-sectional views of heater elements in acordance with the present invention for use in an open pit channel geometry. As shown, the designs are nearly identical to Figures 5A and 5B except that the pit layer 22 over the addressing electrode 16 has been removed. As discussed, the remaining inner wall 23 of the pit layer provides added protection to prevent damages to junctions and regions susceptible to the cavitational forces. Further, in Figure 7A, the effective heater region L E is defined by the inner wall 23 of the pit layer and the dopant line 9 of the addressing electrode 16 and thus, the unused heater region L D is relatively small. In Figure 7B, the effective and actual heater regions L E ,L R are defined by the dopant lines 9 as in Figure 6B. In the heater elements of Figures 5A, 5B, 7A and 7B, the use of the dopant lines 9 and inner wall(s) 23 of the pit layer 22 adds additional flexibility to the design of the heater elements 1. For example, the dopant lines 9 are laterally movable dependent upon the size of the mask to form the heavily doped n-type region. Further, the or each inner wall 23 of the pit layer 22 is laterally movable. By laterally moving the dopant lines 9 and inner wall(s) 23, various heater elements requiring different heater area can be quickly and easily designed for different printheads. The following describes the various methods and materials used to form the heater elements of designs illustrated in Figures 5A, 5B, 7A and 7B. The heater element design of Figures 5A and 5B and Figures 7A and 7B are substantially similar except for the pit layer. In the heater element designs, the substrate 4 is silicon. Silicon is preferably used because it is electrically insulative and has good thermal conductivity for the removal of heat generated by the heater elements. The substrate is a (100) double side polished P-type silicon and has a thickness of 525 micrometers (µm). Further, the substrate 4 can be: lightly doped, for example, to a resistivity of 5 ohm-cm; degenerately doped to a resistivity between 0.01 to 0.001 ohm-cm to allow for a current return path; or degenerately doped with an epitaxial, lightly doped surface layer of 2 to 25 µm to allow fabrication of active field effect or bipolar transistors. The underglaze layer 6 is preferably made of silicon oxide (SiO 2 ) which is grown by thermal oxidation of the silicon substrate. However, it can be appreciated that other suitable thermal oxide layers can be used for the underglaze layer 6. The underglaze layer 6 has a thickness between 1 to 2 µm and in the preferred embodiment has a thickness of 1.5 µm. A resistive material is deposited on top of the underglaze by a chemical vapor deposition (CVD) of polysilicon up to a thickness between 1,000 to 6,000 angstroms (Å) to form the resistor 8. In the preferred embodiment, the resistor 8 has a thickness between 4,000Å to 5,000Å and preferably has a thickness of 4,500Å . Polysilicon is initially lightly doped using either ion implantation or diffusion. Then, a mask is used to further heavily dope the ends of the resistor 8 by ion implantation or diffusion. Either wet or dry etching is used to remove excess polysilicon to achieve the proper length of the resistor 8. Further, the polysilicon can be simultaneously used to form elements of associated active circuitry, such as, gates for field effect transistors and other first layer metallization. The PSG step region 10 is formed of 7.5 wt.% PSG. To form the PSG, SiO 2 is deposited by CVD or is grown by thermal oxidation and the SiO 2 is doped with 7.5 wt.% phosphorus. The PSG is heated to reflow the PSG and create a planar surface to provide a smooth surface for aluminum metallization for the address and common return electrodes 16, 18. The PSG layer is etched to provide the vias 17, 19 for the addressing and common return electrodes 16, 18 and to provide the surface for the dielectric isolation and Ta layers 12, 14. The dielectric isolation layer 12 is formed by pyrolytic chemical vapor deposition of silicon nitride (Si 3 N 4 ) and etching of the Si 3 N 4 . The Si 3 N 4 layer, which has been directly deposited on the exposed polysilicon resistor, has a thickness of 500 to 2,500 Å and preferably about 1,500 Å. The pyrolytic silicon nitride has a very good thermal conductivity for efficient transfer of heat between the resistor and the ink when directly deposited in contact with the resistor. Alternatively, the dielectric isolation layer 12 can be formed by thermal oxidation of the polysilicon resistors to form SiO 2 . The SiO 2 dielectric layer can be grown to a thickness of 500 Å to 1 µm and in the preferred embodiment has a thickness from 1,000 to 2,000 Å. The Ta layer 14 is sputter deposited on top of the dielectric isolation layer 12 by chemical vapor deposition and has a thickness between 0.1 to 1.0 µm. The Ta layer 14 is masked and etched to remove the excess tantalum and then the dielectric isolation layer 12 is also etched prior to metallization of the addressing and common return electrodes 16, 18. The addressing and common return electrodes 16, 18 are formed by chemical vapor deposition of aluminum into the vias 17, 19 and etching the excess aluminum. The addressing and common return electrode terminals 82 are positioned at predetermined locations to allow clearance for electrical connection to the control circuitry after the channel plate 72 is attached to the substrate 4. The addressing and common return electrodes 16, 18 are deposited to a thickness of 0.5 to 3 µm, with a preferred thickness being 1.5 µm. The overglaze passivation layer 20 is formed of a composite layer of PSG and Si X N Y . The cumulative thickness of the overglaze passivation layer can range from 0.1 to 10 µm, the preferred thickness being 1.5 µm. A PSG having preferably with 4 wt.% phosphorus is deposited by low temperature chemical vapor deposition (LOTOX) to a thickness of 5,000Å. Next, silicon nitride is deposited by plasma assisted chemical vapor deposition to a thickness of 1.0 µm. Using a passivation mask, the silicon nitride is plasma etched and the PSG is wet etched off the heater element to expose the Ta layer 14 and terminals 82 of the addressing and common return electrodes 16, 18 for electrical connection to the controller 62. In an alternative embodiment, the overglaze passivation layer 20 can be formed entirely of PSG. Further, the overglaze passivation layer 20 can be formed of either of the above arrangements with an additional composite layer of polyimide with 1 to 10 µm thickness deposited over the PSG or silicon nitride layer(s). Next, a thick film insulative layer such as, for example, RISTON®, VACREL®, PROBIMER 52®, or polyimide is formed on the entire surface of the substrate. The thick insulative layer 22 is photolithographically processed to enable the etching and removal of those portions of the thick insulative layer over each heater element 1 and comprises a pit layer 22 for each heater element 1. In the heater element designs of Figures 5A and 5B, the thick film insulative layer 22 is removed to form the pit 24 and the common recess 88. In the heater designs of Figure 7A and 7B, the thick film insulative layer 22 is removed to form part of the pit 24 and the channels 75. Further, the inner walls 23 of the pit layer 22 inhibit lateral movement of each vapor bubble 90 generated by the heater and thus prevents the phenomenon of blow-out. As discussed above, the inner walls 23 of the pit layer 22 extend beyond the side walls of the PSG step region 10 and the overglaze passivation layer 20 to provide added protection against cavitational pressures. With the heater elements of Figures 5A, 5B, 7A and 7B, the ink droplet characteristics and stability at 10 9 pulse range remained essentially unchanged from the initial ink droplet characteristics and stability. For a particular geometry tested, which is shown in Fig. 5A, after 1.6 × 10 9 pulse, the droplet characteristics were: 1) velocity of 10 m/s; 2) drop volume of 130 picoliters; 3) velocity jitter of less than 4%; 4) transit time variability across the printhead of less than 5%; and 5) crisp threshold response with a slight increase of threshold value of about 9%. Further, the heater elements showed no signs of heater failures caused by cavitational pressure well into the 10 9 pulse range. Moreover, the heater elements are more efficient because they produce larger ink droplets 10-15% faster, when the same amount of heat generating pulse currents is applied, than conventional heater elements. It will be appreciated that heater elements in accordance with the present invention, as described above, are also applicable to printing systems which use a full-width printhead.

Claims: A heater element (1) for use in a printhead (32)of a printing system to expel ink onto a recording medium (36)by expansion and collapse of a vapor bubble (90), the heater element comprising: a substrate (4,6); a resistive layer (8) formed on top of said substrate; contact means (16,18) coupled to said resistive layer; an insulating means (12,14) formed on top of said resistive layer to prevent contact between said resistive layer and the ink, the top surface of the insulating means transferring heat energy generated by said resistive layer to the ink to form said vapor bubble thereon; a passivating layer (20) covering said substrate, contact means, and insulating means, the passivating layer being patterned to expose the top surface of a center portion of the insulating means but leaving the outer portions thereof covered by said passivating layer; and an insulative film (22) overlying said passivating layer and having an upper surface which interfaces with the ink; characterised in that: said insulative film extends beyond the passivating layer which covers the outer portions of the insulating means to provide at least one inner wall (23), said at least one inner wall extending from the top surface of the insulating means to insulative film upper surface to form a pit (24), the at least one inner wall providing added protection to prevent damage of junctions and regions susceptible to cavitational pressures produced by the expansion and collapse of said vapor bubble on the insulating means. A heater element as claimed in claim 1, wherein said resistive layer comprises a polysilicon layer having a lightly doped region (8A) and a heavily doped region (8B) at each end of said lightly doped region, said heavily doped regions being coupled to said contact means and interfaces between said lightly doped region and said heavily doped regions defining first and second dopant lines (9), the lightly doped region between the dopant lines defining a region of heat energy generation in the resistive layer. A heater element as claimed in claim 2, wherein said at least one inner wall of said insulative film extends beyond said first dopant line into the lightly doped region of the resistive layer, so that the at least one inner wall defines a region of energy transfer between said resistive layer and the ink. The heater element as claimed in claim 3, wherein the heater element is located in a channel (75) comprising a front channel portion (L f ) and a rear channel portion (L r ), the front channel portion having a nozzle (70) at an end opposite to an end adjacent the heater element, and the rear channel portion being in fluid communication with an ink supply manifold (84) at an end opposite to an end adjacent the heater element; and wherein the rear channel portion has a lower flow resistance than the ink in the front channel portion, so that the vapor bubble collapses on the insulating means adjacent said at least one inner wall. A heater element as claimed in claim 3, wherein said insulative film has a second inner wall which extends beyond said second dopant line into the lightly doped region of the resistive layer, so that damage of junctions and regions susceptible to cavitational pressures produced by the expansion and collapse of the vapor bubble is prevented without regard to ink flow resistance in portions (L f ,L r ) of a channel (75) in which the heater element resides. A heater element as claimed in claim 2, wherein said at least one inner wall or both said one inner and second inner wall, of the insulative film is or are aligned with said first or first and second dopant lines, so that the effective and actual heat transfer regions are equal as defined by the dopant lines. A heater element as claimed in any one of claims 1 to 6, wherein said insulative film prevents passivation and cavitational damages of said heater element well into the 10 9 pulse range. A heater element as claimed in any one of claims 1 to 6, wherein said insulative film prevents degradation of heater robustness, hot spot formations and heater failures well into the 10 9 pulse range. A printhead for use in a printing system to expel ink droplets onto a recording medium by expansion and collapse of vapor bubbles comprising: a plurality of heater elements, each as claimed in any one of the preceding claims, on a common substrate; a channel plate (72) having a plurality of channels (75) and having a manifold (84) for supplying ink to said channels, first ends of said plurality of channels forming nozzles (70) for expelling the ink droplets and second ends of said plurality of channels being in communication with said ink manifold to supply ink to said plurality of channels, the channel plate being coupled to the common substrate with each heater element being located in a respective channel at a predetermined distance from the nozzle; and a second substrate (78) coupled to said common substrate and opposite of said channel plate, said second substrate having a plurality of terminals (80) coupled to the contact means of the heater elements and to a controller (62) for sending electrical pulses to selected resistive layers of said plurality of heater elements, said resistive layers generating heat in response to the electrical pulses and causing the expansion and growth of vapor bubbles for ejection of the ink droplets at said nozzles of said printhead. A printing system for recording onto a surface of a medium comprising: a printhead having a plurality of nozzles and having a plurality of heater elements, each as claimed in any one of claims 1 to 8, for causing expansion and collapse of vapor bubbles to expel the ink from said nozzles onto the medium; means for supplying ink to said printhead; and means for controlling the ejection of ink coupled to said printhead, said controlling means applying electrical pulses to said contact means of said heater elements selected in accordance with signals received by said controlling means, said electrical pulses causing said resistive layers of selected heater elements to generate energy for transfer to the ink and the energy causing expansion and collapse of vapor bubbles to expel ink at said nozzles of said printhead to the surface of the medium.

Inventors: BURKE CATHIE J, DESHPANDE NARAYAN V, HAWKINS WILLIAM G, IMS DALE R, KNEEZEL GARY A, O'HORO MICHAEL P, REZANKA IVAN, TELLIER THOMAS A, BURKE, CATHIE J., DESHPANDE, NARAYAN V., HAWKINS, WILLIAM G., IMS, DALE R., KNEEZEL, GARY A., O'HORO, MICHAEL P., REZANKA, IVAN, TELLIER, THOMAS A.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: B41J2
IPC codes: B41J2