Thermal ink jet print head assembly.

0001This invention relates generally to thermal ink jet printing and more particularly to a new and improved thermal ink jet printhead assembly.

0002Thermal ink jet printing has been described in many technical publications, and one such publication relevant to this invention is the <u style="single">Hewlett Packard Journal</u>, Volume 36, Number 5, May 1985, incorporated herein by reference.

0003In the art of thermal ink jet printing, it is known to provide a plurality of electrically resistive ele­ments on a common thin film substrate for the purpose of heating a corresponding plurality of adjacent ink reservoirs during the ink ejection and printing process. Using such an arrangement, the adjacent ink reservoirs are typically pro­vided as cavities in a barrier layer above the substrate for properly concentrating thermal energy emanating from the resistive elements to predefined volumes of ink. Also, a plurality of ink ejection orifices are provided above these cavities and provide exit paths for ink during the printing process.

0004In constructing the above type of printhead assembly, one practice has been to drill vertical holes in a common substrate in order to provide ink flow paths from a common ink reservoir to the individual reservoir cavities within the barrier layer. However, the use of multiple holes (vertical cylindrical channels) in a single substrate possesses several disadvantages. One of these disadvantages is that the boring bit used for drilling holes in the sub­strate places a substantial pressure on the substrate material and thus can cause fracturing of this material. On the other hand, if laser drilling is utilized, the laser beam will leave channels with fratured side walls as a result of heating, and thus produce a weakened substrate structure.

0005The per se creation of multiple vertical channels in the silicon substrate weakens the printhead structure, and with some types of prior art printhead structures, these channels are used to provide ink flow to a plurality of resistive heater elements located at different distances from the channels. In such a structure, these varying ink-­flow distances produce corresponding different pressure drops in the ink flow paths. That is, the pressure drop along a liquid ink flow path is proportional to the cube of the distance of the path. This fact has sometimes resulted in pressure drops over large ink flow distances sufficiently great as to prevent adequate vaporization during ink jet propulsion from the ink jet orifice.

0006Another disadvantage of using small diameter ver­tical channels to supply ink to the ink reservoirs is that these channels simply do not have the capacity to adequately respond to certain ink volume demands at the required increasingly higher frequencies of operation.

0007A further disadvantage of using a plurality of ink flow channels in a common substrate is that they normally require a special routing of conductive leads on the sub­strate surface. In addition to the added costs associated with this special routing, this requirement also greatly reduces the achievable packing density because of the sur­face area required to accomodate such special routing schemes.

Disclosure of Invention

0008The general purpose of this invention is to pro­vide a new and improved ink jet printhead assembly which eliminates the above problems associated with the use of drilled holes through a common printhead substrate member. In this new assembly, a single elongated slot is cut in the substrate and provides ink flow to a plurality of ink reser­voirs associated with resistive heater elements formed above the top surface of the substrate. These heater elements are spaced around the periphery of the slot at predetermined distances therefrom. Conductive leads are provided on the substrate between each resistive heater element and external electrical connections, and a barrier layer and orifice plate member covers all of the resistive heater elements and defines a plurality of individual ink reservoirs repectively above each of the resistive heater elements.

0009The above described slotted geometry structure greatly increases the packing density of heater resistors on the common printhead substrate. This increase in packing density is partially a result of the fact that, in the prior art multiple hole printhead structures, the conductive traces to the individual resistor elements had to be routed around the holes, thus increasing the required substrate area. Thus, by using the elongated slot arrangement of this invention instead of vertical holes in the prior art struc­tures, a packing density increase of 8:1 to 10:1 may be achieved.

0010After the orifice plate and associated barrier layer member are secured to the thin film substrate, the substrate is die bonded to a header manifold member. This manifold member has an elongated slot therein for passing ink from a well section of the header manifold and through the substrate slot to the individual reservoirs of the barrier layer and orifice plate member.

0011Accordingly, it is an object of the present inven­tion to provide a new and improved thermal ink jet printhead assembly having an improved packing density for the heater resistors and their associated ink jet orifices and reservoirs.

0012Another object is to provide a new and improved manufacturing process for realizing this assembly using latest state-of-the-art semiconductor processing techniques.

0013A novel feature of this invention is the provision of improved control of ink flow pressures from a common ink supply source and through a single slot in a thin film resistor structure and then through a common ink flow path simultaneously to a plurality of ink reservoirs in the printhead assembly.

0014These and other objects and features of this invention will become more readily apparent from the following description of the accompanying drawing.

Brief Description of Drawings

0015<ul id="ul0001" list-style="none"><li>Figure 1A is an isometric view of the slotted thin film resistor die (substrate) used in a preferred embodiment of the invention.</li><li>Figure 2 is an exploded view showing the die placement, the external lead attachment, and the orifice plate attachment steps used in fabricating the complete thermal ink jet printhead assembly in a preferred embodiment of the invention.</li><li>Figures 3A and 3B are fragmented and greatly enlarged plan and cross section views respectively, of the novel slot and lateral ink feed sections of the above printhead structure.</li></ul>

Best Mode for Carrying Out the Invention

0016Referring now to Figure 1, there is shown a thin film resistor substrate 10 for a thermal ink jet printer and including a metal orifice plate 12 thereon. The orifice plate 12 is typically constructed of nickel and includes a plurality of ink ejection openings or nozzles 14 spaced uniformly around the edges of an ink feed slot 16 indicated by the dotted lines in Figure 1.

0017Referring now Figure 2, the thin film resistor substrate 10 will be mounted on the top, I-beam shaped surface 18 of a header manifold 20. The header manifold 20 will include an ink reservoir (not shown) within the con­fines thereof which communicates with an ink feed slot 22. The slot 22 is aligned with the ink feed slot 16 in the thin film resistor substrate 10. The header manifold 20 further includes contoured walls 24 which have been shaped to match corresponding contoured walls of an ink jet printer carriage assembly (not shown) for receiving the printhead structure of Figure 2 when completely assembled.

0018When this printhead structure is completed and all the piece parts shown in Figure 2 brought together, the thin film resistor substrate 10 is positioned directly on the upper surface 18 of the header 20, and a flexible, tape automated bond (TAB) circuit 26 is brought into electrical contact with conductive traces on the top surface of the thin film resistor substrate 10. A plurality of thin conductive leads 28 overlie the contoured side walls 24 of the header 20, and the interior leads 30 of the tab bond flex circuit 26 are thermocompression bonded to conductive traces on the thin film resistor substrate 10.

0019In addition, the orifice plate 12 will be brought into alignment with the thin film resistor substrate 10 by means of an orifice plate and barrier layer manufacturing process.

0020Referring now to Figures 3A and 3B, the thin film resistor substrate 10 will typically include a silicon sub­strate 32 upon which is deposited a thin layer 34 of silicon dioxide for passivating and insulating the surface of the silicon substrate 32. A plurality of heater resistors 36 and 38 are formed on the upper surface of the silicon dioxide layer 34 and will typically be either tantalum aluminum or tantalum pentoxide and fabricated using known photolithographic masking and etching techniques. Aluminum trace conductors 40 make electrical contact to the heater resistors 36 and 38 for providing electrical pulses thereto during an ink jet printing operation, and these conductors are formed from a layer of aluminum previously evaporated on the upper surface of the silicon layer 34 using conventional metal evaporation processes.

0021After the formation of the aluminum conductors 40 is completed, a surface barrier layer 42, typically of silicon carbide or silicon nitride, is deposited over the upper surfaces of the conductors 40 and the heater resistors 36 and 38 to protect these members from cavitation wear and the ink corrosion which would otherwise be caused by the highly corrosive ink located in the reservoirs directly above these heater resistors. The silicon carbide layer 42, as well as the previously identified S_iO₂ surface layer 34, resistors 36 and 38 and aluminum conductors 40 are all formed using semiconductor processes well known to those skilled in thermal ink jet and semiconductor processing arts and for that reason are not described in detail herein. However, for a further detailed discussion of such processes, reference may be made to the above <u style="single">Hewlett</u><u style="single">Packard Journal</u>, Volume 36, Number 5, May 1985.

0022A nickel orifice plate 44 is positioned as shown on top of the silicon carbide layer 42 and includes ink reservoir areas 46 and 48 located directly above the heater resistors 36 and 38 for receiving ink therein by way of the horizontal slot 16. These ink reservoirs 46 and 48 extend vertically upward of the substrate 10 as shown and merge into the output ink ejection orifices defined by the conver­gent contoured walls 50 and 52. These contoured walls 50 and 52 have been designed to reduce cavitational wear and prevent "gulping" during an ink jet printing operation as described in more detail in the above identified copending Chan et al. application.

0023During an ink jet printing operation, ink will flow along the path indicated by the arrow 54 and laterally along the path 56 and into the ink flow ports 58, 60, 62, 64, 66 and 68 as identified on the left-hand portion of the structure of Figures 3A and 3B. Likewise, ink will enter the ink flow ports 70 , 72, 74, 76, 78 and 80 on the right-­hand portion of the structure of Figure 3B. By flowing ink form a common ink reservoir into the plurality of flow ports identified above, the pressure drops in the ink from the ink feed slot 16 to the individual heater resistors, such as 36 and 38, will be equal and thus insure proper ink bubble evaporation and firing during an ink jet printing operation. The advantages of this feature of the invention in contrast to the prior art have been previously discussed above.