Patent Publication Number: US-6209991-B1

Title: Transition metal carbide films for applications in ink jet printheads

Description:
This application relates to the subject matter disclosed in commonly assigned copending U.S. application Ser. No. 08/811,404, filed herewith on Mar. 04, 1997, entitled “STRUCTURE TO EFFECT ADHESION BETWEEN SUBSTRATE AND INK BARRIER IN AN INK JET PRINTHEAD”, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The subject invention generally relates to ink jet printing, and more particularly to thin film ink jet printheads for ink jet cartridges and methods for manufacturing such printheads. 
     The art of ink jet printing is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines have been implemented with ink jet technology for producing printed media. The contributions of Hewlett-Packard Company to ink jet technology are described, for example, in various articles in the  Hewlett - Packard Journal , Vol. 36, No. 5 (May 1985); 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); all incorporated herein by reference. 
     Generally, an ink jet image is formed pursuant to precise placement on a print medium of ink drops emitted by an ink drop generating device known as an ink jet printhead. Typically, an ink jet printhead is supported on a movable carriage that traverses over the surface of the print medium and is controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to a pattern of pixels of the image being printed. 
     A typical Hewlett-Packard ink jet printhead includes an array of precisely formed nozzles in an orifice plate that is attached to an ink barrier layer which in turn is attached to a thin film substructure that implements ink firing heater resistors and apparatus for enabling the resistors. The ink barrier layer defines ink channels including ink chambers disposed over associated ink firing resistors, and the nozzles in the orifice plate are aligned with associated ink chambers. Ink drop generator regions are formed by the ink chambers and portions of the thin film substructure and the orifice plate that are adjacent to the ink chambers. 
     The thin film substructure is typically comprised of a substrate such as silicon on which are formed various thin film layers that form thin film ink firing resistors, apparatus for enabling the resistors, and also interconnections to bonding pads that are provided for external electrical connections to the printhead. The thin film substructure more particularly includes a top thin film layer of tantalum disposed over the resistors as a thermomechanical passivation layer. 
     The ink barrier layer is typically a polymer material that is laminated as a dry film to the thin film substructure, and is designed to be photodefinable and both UV and thermally curable. layer forms an oxidation and wear resistance layer and/or a barrier adhesion layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages and features of the disclosed invention will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein: 
     FIG. 1 is a schematic, partially sectioned perspective view of an ink jet printhead in accordance with the invention. 
     FIG. 1A is a schematic, partially sectioned perspective view of a further ink jet printhead in accordance with the invention. 
     FIG. 2 is an unscaled schematic top plan illustration of the general layout of the thin film substructure of the ink jet printhead of FIG.  1 . 
     FIG. 3 is an unscaled schematic top plan view illustrating the configuration of a plurality of representative heater resistors, ink chambers and associated ink channels. 
     FIG. 4 is an unscaled schematic cross sectional view of the ink jet printhead of FIG. 1 taken laterally through a representative ink drop generator region and illustrating an embodiment of the printhead of FIG.  1 . 
     FIG. 5 sets forth an unscaled schematic cross sectional view of the ink jet printhead of FIG. 1 taken laterally through a representative ink drop generator region and illustrating another embodiment of the printhead of FIG.  1 . 
     FIG. 6 is an unscaled schematic cross sectional view of the ink jet printhead of FIG. 1 taken laterally through a representative ink drop generator region and illustrating a further embodiment of the printhead of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The problem with tantalum as a bonding surface is due to the fact that while the tantalum layer is pure tantalum when it is first formed in a sputtering apparatus, a tantalum oxide layer forms as soon as the tantalum layer is exposed to an oxygen containing atmosphere. The chemical bond between an oxide and a polymer film tends to be easily degraded by water, since the water forms a hydrogen bond with the oxide that competes with and replaces the original polymer to oxide bond, and thus ink formulations, particularly the more aggressive ones, debond an interface between a metal oxide and a polymer barrier. 
     SUMMARY OF THE INVENTION 
     It would therefore be an advantage to provide an ink jet printhead having a thermomechanical passivation layer with increased wear resistance. 
     It would therefore be an advantage to provide an improved ink jet printhead that reduces delamination of the interface between the thin film substructure and the ink barrier layer. 
     A further advantage would be to provide in a ink jet printhead a bonding surface that provides bonding sites to which a polymer barrier layer can form a stable chemical bond. 
     The foregoing and other advantages are provided by the invention in an ink jet printhead that includes a thin film substrate including a plurality of thin film layers, a plurality of ink firing heater resistors defined in the plurality of thin film layers, a patterned tantalum carbide layer disposed on the plurality of thin film layers, an ink barrier layer disposed over the tantalum carbide layer, and respective ink chambers formed in the ink barrier layer over respective thin film resistors, each chamber formed by a chamber opening in barrier layer. The tantalum carbide 
     An example of the physical arrangement of the orifice plate, ink barrier layer, and thin film substructure is illustrated at page 44 of the  Hewlett - Packard Journal  of February 1994, cited above. Further examples of ink jet printheads are set forth in commonly assigned U.S. Pat. No. 4,719,477 and U.S. Pat. No. 5,317,346, both of which are incorporated herein by reference. 
     A consideration with the foregoing ink jet printhead architecture includes reduced heater resistor life due to accelerated oxidation of localized regions of the tantalum passivation layer. 
     Another consideration with the foregoing ink jet printhead architecture include delamination of the ink barrier layer from the thin film substructure. Delamination principally occurs from environmental moisture and the ink itself which is in continual contact with the edges of the thin film substructure/barrier interface in the drop generator regions. 
     It has been determined that the tantalum thermomechanical passivation layer offers the additional functionality of improving adhesion to the ink barrier layer. However, while the barrier adhesion to tantalum has proven to be sufficient for printheads that are incorporated into disposable ink jet cartridges, barrier adhesion to tantalum is not sufficiently robust for semipermanent ink jet printheads which are not replaced as frequently. Moreover, new developments in ink chemistry have resulted in formulations that more aggressively debond the interface between the thin film substructure and the barrier layer, as well as the interface between the barrier layer and the orifice plate. 
     In particular, water from the ink enters the thin film substructure/barrier interface by penetration through the bulk of the barrier and penetration along the thin film substructure/barrier interface, causing debonding of the interfaces through a chemical mechanism such as hydrolysis. 
     In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals. 
     Referring now to FIG. 1, set forth therein is an unscaled schematic perspective view of an ink jet printhead in which the invention can be employed and which generally includes (a) a thin film substructure or die  11  comprising a substrate such as silicon and having various thin film layers formed thereon, (b) an ink barrier layer  12  disposed on the thin film substructure  11 , and (c) an orifice or nozzle plate  13  attached to the top of the ink barrier  12  with a silicon carbide adhesion layer  14 . 
     The thin film substructure  11  is formed pursuant to integrated circuit fabrication techniques, and includes thin film heater resistors  56  formed therein. By way of illustrative example, the thin film heater resistors  56  are located in rows along longitudinal edges of the thin film substructure. 
     The ink barrier layer  12  is formed of a dry film that is heat and pressure laminated to the thin film substructure  11  and photodefined to form therein ink chambers  19  and ink channels  29  which are disposed over resistor regions which are on either side of a generally centrally located gold layer  62  (FIG. 2) on the thin film substructure  11 . Gold bonding pads  71  engagable for external electrical connections are disposed at the ends of the thin film substructure  11  and are not covered by the ink barrier layer  12 . As discussed further herein with respect to FIG. 2, the thin film substructure  11  includes a patterned gold layer  62  generally disposed in the middle of the thin film substructure  11  between the rows of heater resistors  56 , and the ink barrier layer  12  covers most of such patterned gold layer  62 , as well as the areas between adjacent heater resistors  56 . By way of illustrative example, the barrier layer material comprises an acrylate based photopolymer dry film such as the Parad brand photopolymer dry film obtainable from E. I. duPont de Nemours and Company of Wilmington, Del. Similar dry films include other duPont products such as the Riston brand dry film and dry films made by other chemical providers. The orifice plate  13  comprises, for example, a planar substrate comprised of a polymer material and in which the orifices are formed by laser ablation, for example as disclosed in commonly assigned U.S. Pat. No. 5,469,199, incorporated herein by reference. The orifice plate can also comprise, by way of further example, a plated metal such as nickel. 
     The ink chambers  19  in the ink barrier layer  12  are more particularly disposed over respective ink firing resistors  56 , and each ink chamber  19  is defined by the edge or wall of a chamber opening formed in the barrier layer  12 . The ink channels  29  are defined by further openings formed in the barrier layer  12 , and are integrally joined to respective ink firing chambers  19 . By way of illustrative example, FIG. 1 illustrates an outer edge fed configuration wherein the ink channels  29  open towards an adjacent outer longitudinal edge  11   a  of the outer perimeter of the thin film substructure  11  and ink is supplied to the ink channels  29  and the ink chambers  19  around the outer longitudinal edges of the thin film substructure, for example as more particularly disclosed in commonly assigned U.S. Pat. No. 5,278,584, incorporated herein by reference, whereby the outer longitudinal edges  11   a  comprise feed edges. The invention can also be employed in a center edge fed ink jet printhead such as that disclosed in previously identified U.S. Pat. No. 5,317,346, and as schematically illustrated in FIG. 1A wherein ink channels  129  open towards an edge  111   a  formed by a slot  116  in the middle of the thin film substructure  111  in which heater resistors  156  are formed, whereby such edge comprises a feed edge. Similarly to the printhead of FIG. 1, the printhead of FIG. 1A includes an ink barrier layer  112 , ink chambers  119 , and an orifice plate  113  attached to the top of the ink barrier  112  with a silicon carbide adhesion layer  114 . 
     The orifice plate  13  includes orifices  21  disposed over respective ink chambers  19 , such that an ink firing resistor  56 , an associated ink chamber  19 , and an associated orifice  21  are aligned. An ink drop generator region is formed by each ink chamber  19  and portions of the thin film substructure  11  and the orifice plate  13  that are adjacent the ink chamber  19 . 
     Referring now to FIG. 2, set forth therein is an unscaled schematic top plan illustration of the general layout of the thin film substructure  11 . The ink firing resistors  56  are formed in resistor regions that are adjacent the outer longitudinal edges  11   a  the thin film substructure  11 . A patterned gold layer  62  comprised of gold traces forms the top layer of the thin film structure in a gold layer region located generally in the middle of the thin film substructure  11  between the resistor regions and extending between the ends of the thin film substructure  11 . Bonding pads  71  for external connections are formed in the patterned gold layer  62 , for example adjacent the ends of the thin film substructure  11 . The ink barrier layer  12  is defined so as to cover all of the patterned gold layer  62  except for the bonding pads  71 , and also to cover the areas between the respective openings that form the ink chambers and associated ink channels. Depending upon implementation, one or more thin film layers can be disposed over the patterned gold layer  62 . 
     Referring now to FIG. 3, set forth therein is an unscaled schematic top plan view illustrating the configuration of a plurality of representative heater resistors  56 , ink chambers  19  and associated ink channels  29 . As shown in FIG. 4, the heater resistors  56  are polygon shaped (e.g., rectangular) and are enclosed on at least two sides thereof by the wall of an ink chamber  19  which for example can be multi-sided. The ink channels  29  extend away from associated ink chambers  19  and can become wider at some distance from the ink chambers  19 . Insofar as adjacent ink channels  29  generally extend in the same direction, the portions of the ink barrier layer  12  that form the openings that define ink chambers  19  and ink channels  29  thus form an array of barrier tips  12   a  that extend toward an adjacent feed edge of the thin film substructure  11  from a central portion of the barrier layer  12  that covers the patterned gold layer  62  and is on the side of the heater resistors  56  away from the adjacent feed edge. Stated another way, ink chambers  19  and associated ink channels  29  are formed by an array of side by side barrier tips  12   a  that extend from a central portion of the ink barrier  12  toward a feed edge of the thin film substructure  11 . 
     In accordance with the invention, the thin film substructure  11  includes a patterned tantalum carbide layer  63  (FIGS. 4,  5 ,  6 ) that functions as a wear resistant layer over the heater resistors and/or an adhesion layer for the ink barrier layer  12 . As described further herein, the tantalum carbide layer can comprise (a) a blanket film that covers most of the thin film substructure (illustrated in FIG.  4 ), (b) subareas that are located beneath respective ink chambers (illustrated in FIG.  5 ), or (c) a generally blanket film that includes openings over the heater resistors so as to be absent from the heater resistor areas. 
     Referring now to FIG. 4, set forth therein is an unscaled schematic cross sectional view of the ink jet printhead of FIG. 1 taken through a representative ink drop generator region and a portion of the centrally located gold layer region, and illustrating a specific embodiment of the thin film substructure  11 . The thin film substructure  11  of the ink jet printhead of FIG. 4 more particularly includes a silicon substrate  51 , a field oxide layer  53  disposed over the silicon substrate  51 , and a patterned phosphorous doped oxide layer  54  disposed over the field oxide layer  53 . A resistive layer  55  comprising tantalum aluminum is formed on the phosphorous oxide layer  54 , and extends over areas where thin film resistors, including ink firing resistors  56 , are to be formed beneath ink chambers  19 . A patterned metallization layer  57  comprising aluminum doped with a small percentage of copper and/or silicon, for example, is disposed over the resistor layer  55 . 
     The metallization layer  57  comprises metallization traces defined by appropriate masking and etching. The masking and etch of the metallization layer  57  also defines the resistor areas. In particular, the resistive layer  55  and the metallization layer  57  are generally in registration with each other, except that portions of traces of the metallization layer  57  are removed in those areas where resistors are formed. In this manner, the conductive path at an opening in a trace in the metallization layer includes a portion of the resistive layer  55  located at the opening or gap in the conductive trace. Stated another way, a resistor area is defined by providing first and second metallic traces that terminate at different locations on the perimeter of the resistor area. The first and second traces comprise the terminal or leads of the resistor which effectively include a portion of the resistive layer that is between the terminations of the first and second traces. Pursuant to this technique of forming resistors, the resistive layer  55  and the metallization layer can be simultaneously etched to form patterned layers in registration with each other. Then, openings are etched in the metallization layer  57  to define resistors. The ink firing resistors  56  are thus particularly formed in the resistive layer  55  pursuant to gaps in traces in the metallization layer  57 . 
     A composite passivation layer comprising a layer  59  of silicon nitride (Si 3 N 4 ) and a layer  60  of silicon carbide (SiC) is disposed over the metallization layer  57 , the exposed portions of the resistive layer  55 , and exposed portions of the oxide layer  53 . A tantalum passivation layer  61  is disposed on the composite passivation layer  59 ,  60  over most of the thin film substructure  11  so as to be disposed over the heater resistors  56  and extending beyond the ink chambers  19 . The tantalum passivation layer  61  can also extend to areas over which the patterned gold layer  62  is formed for external electrical connections to the metallization layer  57  by conductive vias  58  formed in the composite passivation layer  59 ,  60 . A tantalum carbide layer  63  is disposed on the tantalum layer  61  and functions as wear layer in the ink chambers  19  and as an adhesion layer in areas where it is in contact with the barrier layer  12 . Thus, to the extent that tantalum carbide to barrier adhesion in desired in the vicinity of the ink chambers and ink channels, the interface between the tantalum carbide layer  63  and the barrier  12  can extend for example from at least the region between the resistors  56  and the patterned gold layer  62  to the ends of the barrier tips  12   a . To the extent that the increased resistivity of tantalum carbide in the vias is not suitable, the tantalum carbide can be etched from the vias. 
     Referring now to FIG. 5, set forth therein is an unscaled schematic cross sectional view of the ink jet printhead of FIG. 1 taken laterally through a representative ink drop generator region and a portion of the patterned gold layer  62 , and illustrating another specific embodiment of the an ink jet printhead in accordance with the invention. The ink jet printhead of FIG. 5 is similar to the ink jet printhead of FIG. 4, except that a tantalum carbide layer  163  is limited to tantalum subareas  163   a  that are beneath ink chambers  19  and portions of associated ink channels  29  adjacent the ink chambers  19 . As shown in plan view in FIG. 3, the subareas  163   a  extend beyond the ink chamber  19  and the ink channels  29 , and in this manner, the tantalum carbide subareas  163   a  function as an oxidation and wear resistance layer in the ink chambers  19 , and as a barrier adhesion layer in the vicinity of the ink chambers  19  and the ink channels  29 . As a minimum, the tantalum carbide subareas  63   a  extend into areas that are subject to bubble collapse to provide mechanical passivation for the ink firing resistors by absorbing the cavitation pressure of the collapsing drive bubble. 
     Referring now to FIG. 6, set forth therein is an unscaled schematic cross sectional view of the ink jet printhead of FIG. 1 taken laterally through a representative ink drop generator region and a portion of the patterned gold layer  62 , and illustrating another specific embodiment of the an ink jet printhead in accordance with the invention. The ink jet printhead of FIG. 6 is similar to the ink jet printhead of FIG. 4, with the modification that a tantalum carbide layer  263  comprises a blanket barrier adhesion layer that covers most of the thin film substructure except areas over the heater resistors  56 . In other words, the tantalum carbide layer  263  includes openings over the heater resistors  56 . 
     The foregoing printhead is readily produced pursuant to standard thin film integrated circuit processing including chemical vapor deposition, photoresist deposition, masking, developing, and etching, for example as disclosed in commonly assigned U.S. Pat. No. 4,719,477 and U.S. Pat. No. 5,317,346, both previously incorporated herein by reference. 
     By way of illustrative example, the foregoing structures can be made as follows. Starting with the silicon substrate  51 , any active regions where transistors are to be formed are protected by patterned oxide and nitride layers. Field oxide  53  is grown in the unprotected areas, and the oxide and nitride layers are removed. Next, gate oxide is grown in the active regions, and a polysilicon layer is deposited over the entire substrate. The gate oxide and the polysilicon are etched to form polysilicon gates over the active areas. The resulting thin film structure is subjected to phosphorous predeposition by which phosphorous is introduced into the unprotected areas of the silicon substrate. A layer of phosphorous doped oxide  54  is then deposited over the entire in-process thin film structure, and the phosphorous doped oxide coated structure is subjected to a diffusion drive-in step to achieve the desired depth of diffusion in the active areas. The phosphorous doped oxide layer is then masked and etched to open contacts to the active devices. 
     The tantalum aluminum resistive layer  55  is then deposited, and the aluminum metallization layer  57  is subsequently deposited on the tantalum aluminum layer  55 . The aluminum layer  57  and the tantalum aluminum layer  55  are etched together to form the desired conductive pattern. The resulting patterned aluminum layer is then etched to open the resistor areas. 
     The silicon nitride passivation layer  59  and the SiC passivation layer  60  are respectively deposited. A photoresist pattern which defines vias to be formed in the silicon nitride and silicon carbide layers  59 ,  60  is disposed on the silicon carbide layer  60 , and the thin film structure is subjected to overetching, which opens vias through the composite passivation layer comprised of silicon nitride and silicon carbide to the aluminum metallization layer. 
     As to the implementation of FIG. 4 wherein the tantalum layer  61  and the tantalum carbide layer  63  are similarly patterned, such layers are formed for example by sputtering. Tantalum targets are sputtered in an inert gas such as argon or krypton to form the tantalum layer. After the desired tantalum thickness is obtained, a hydrocarbon containing gas such as acetylene or methane is mixed with the inert gas which allows the formation of the tantalum carbide layer. By way of illustrative example, the tantalum layer has a thickness of approximately 5000 Angstroms, and the tantalum carbide layer has a thickness of about 1000 Angstroms. The tantalum and tantalum carbide layers are then etched in the same pattern, and the gold layer  62  for external connections is deposited and etched. 
     As to the implementation of FIG. 5, the tantalum layer  61  and the tantalum carbide layer  63  are formed for example by sputtering as described above. The tantalum carbide layer is then etched to define the tantalum carbide layers, and the exposed tantalum layer is etched to define the tantalum areas. 
     As to the implementation of FIG. 6, the tantalum layer  61  is formed and etched to define the tantalum areas. The gold layer  62  is then deposited and etched, and the tantalum carbide layer is formed, for example by sputtering, and then etched. 
     After the thin film substructure  11  is formed, the ink barrier layer  12  is heat and pressure laminated onto the thin film substructure. The silicon carbide layer  14  is formed on the orifice plate  13 , and the orifice plate  13  with the silicon carbide layer  14  is laminated onto the laminar structure comprised of the silicon carbide layer  14 , the ink barrier layer  12 , and the thin film substructure  11 . 
     While the foregoing embodiments include a tantalum passivation layer over the heater resistors, it should be appreciated that a single tantalum carbide layer can replace the tantalum and tantalum carbide layers. The invention further contemplates other transition metal carbide films such as tungsten carbide and titanium carbide. 
     The foregoing has thus been a disclosure of an ink jet printhead having a transition metal carbide layer as a wear resistance layer and/or a barrier adhesion layer, and which provides a further advantage of improved print quality by functioning as a kogation limiter in the ink chambers. 
     Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims.