Abstract:
A print head for ink-jet printing including an orifice plate with an intermediate layer of reinforcement material bonded thereto, an ink barrier layer, and an adhesion promoter located between the intermediate layer and the barrier layer. The adhesion promoter bonds the intermediate layer to the barrier layer. Exemplary adhesion promoters include organosilane compositions, polyacrylic acid, polymethylacrylic acid, and mixtures thereof. Representative intermediate layers include oxidizable metals (e.g. chromium, tantalum, and mixtures thereof) and silicon-containing compositions (e.g. silicon dioxide, silicon nitride, silicon carbide, and mixtures thereof). In a process for making a print head, an adhesion promoter is applied to the orifice plate/intermediate layer combination which is thereafter bonded to the barrier layer using the adhesion promoter. All of these components are secured together through the application of pressure and heat.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of co-pending U.S. patent application Ser. Ser. No. 08/742,118 filed Oct. 31, 1996 which is incorporated herein by reference for all that it discloses. 
    
    
     FIELD OF INVENTION 
     The present invention generally relates to ink-jet printing and, more particularly, to print heads for ink-jet print cartridges and methods for manufacturing such print heads. 
     BACKGROUND OF THE INVENTION 
     The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing printed media. Hewlett-Packard&#39;s contributions to this technology are described, 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). 
     Generally, an ink-jet image is formed when a precise pattern of dots is ejected from drop generating device known as a “print head” onto a printing medium. The typical ink-jet print head has an array of precisely formed nozzles in an orifice plate that is attached to an ink barrier layer on a thermal ink-jet print head substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorant dissolved or dispersed in a solvent) from an ink reservoir. Each chamber has a thin-film resistor, known as a “firing resistor”, located opposite each nozzle so ink can collect between the firing resistor and the nozzle. When electric printing pulses heat the thermal ink-jet firing resistor, a small volume of ink adjacent the firing resistor is heated, vaporizing a bubble of ink, and thereby ejecting a drop of ink from the print head. The droplets strike the printing medium and then dry to form “dots” that, when viewed together, form the printed image. 
     The physical arrangement of orifice plate, ink barrier layer, print head substrate, and various intermediate layers on the substrate is further described and illustrated at page 44 of the  Hewlett-Packard Journal  of February 1994, cited above. 
     In ink-jet print head technology, the orifice plate is expected to be permanently attached to the ink barrier layer on the print head substrate. Delamination of the interface between the orifice plate and the barrier layer has always been a problem but recently the problem has increased in significance. 
     Delamination principally occurs from environmental moisture and the ink itself. Environmental moisture develops from storing the print cartridge in a capping station on the printer, in normal, open room storage, or in shipping packages. Environmental moisture has become an increasing problem because print cartridges are increasingly being subjected to longer and longer periods of storage. As for ink, it has become a problem because some inks wick much more into the interface between the orifice plate and the barrier. Such inks contain surfactants and solvents that increase the capillary effect at the orifice plate-ink barrier interface. 
     Delamination of the orifice plate is manifested in several ways. Full delamination occurs when the orifice plate falls off the print cartridge. The print cartridge deprimes, and the electrical leads within the printer can be shorted out. When partial delamination occurs, print cartridge performance and print quality can degrade markedly. Delamination changes the architecture of the ink conduits and firing chambers. Fluidic isolation of the firing chambers can be lost, cross-talk between the firing chambers and ink conduits can develop, and if there are inks of different colors in adjacent chambers, color mixing can occur. 
     U.S. Pat. No. 5,493,320 entitled “Ink-jet Printing Nozzle Array Bonded to a Polymer Ink Barrier Layer” by Sandbach et al. issued on Feb. 20, 1996 recognizes the problem of orifice plate-ink barrier layer delamination. This patent, however, does not go far enough and does not contemplate the measures needed to be taken against very aggressive inks and increased storage times in printers, open room environment, or shipping packages. 
     It will be apparent from the foregoing that although there are many varieties of print cartridges and processes for making them, there is still a need for an approach that avoids both full and partial delamination of the orifice plate-ink barrier layer interface. 
     SUMMARY OF THE INVENTION 
     Briefly and in general terms, a print head for ink-jet printing according to the present invention includes an orifice plate comprised of at least one metal with an intermediate layer of reinforcement material bonded or otherwise secured thereto, an ink barrier layer, and an adhesion promoter located between the intermediate layer and the barrier layer. The adhesion promoter bonds the intermediate layer to the barrier layer thereby securing the orifice plate to the barrier layer. The intermediate layer of reinforcement material preferably involves at least one silicon-containing composition. Representative compounds suitable for this purpose include but are not limited to silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), silicon carbide (SiC), and mixtures thereof. In an exemplary embodiment, the intermediate layer of reinforcement material will have a uniform thickness of about 200-10,000 angstroms. 
     Furthermore, the invention includes a process for making a print head for ink-jet printing comprising the steps of providing an ink-jet orifice plate comprised of at least one metal, providing an intermediate layer of reinforcement material (e.g. at least one silicon-containing composition as noted above) beneath the orifice plate, providing a layer of an adhesion promoter beneath the intermediate layer of reinforcement material, providing a barrier layer beneath the layer of adhesion promoter, and laminating the orifice plate to the barrier layer with the above-listed components therebetween by applying pressure and heat. 
     The problem of delamination caused by aggressive inks and environmental moisture is addressed by using the intermediate layer of reinforcement material (e.g. at least one silicon-containing composition) and the selected adhesion promoter in combination between the orifice plate and the barrier layer. These materials chemically interact in a novel and effective manner which substantially improves the overall strength, durability, and structural integrity of the print head. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawing and graphs, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is side elevational view, in cross section, of an adhesion promoter bonding an orifice plate to a barrier layer in an ink-jet print head, embodying the principles of the invention. 
     FIG. 2 is a plot of the work of adhesion between a barrier layer and an orifice plate having a tantalum (Ta) layer bonded thereto, using various organosilane adhesion promoters versus the number of days that a print head was soaked in ink at 60° C. depicting the improvement in the resistance to delamination as a result of organosilane adhesion promoters. 
     FIG. 3 is a plot of the work of adhesion between a barrier layer and an orifice plate having a chromium (Cr) layer bonded thereto, using a polyacrylic acid (PAA) adhesion promoter versus the number of days that a print head was soaked in ink at 60° C. depicting the improvement in the resistance to delamination as a result of a PAA adhesion promoter. 
     FIG. 4 is a plot of the push strength between a barrier layer and an orifice plate having a chromium (Cr) layer bonded thereto, using a polyacrylic acid (PAA) adhesion promoter, versus the number of days that a print head was soaked in ink at 60° C. depicting the improvement in the resistance to delamination as a result of the PAA adhesion promoter. 
     FIG. 5 is a plot of the push strength between a barrier layer and an orifice plate having a tantalum (Ta) layer bonded thereto, using a polyacrylic acid (PAA) adhesion promoter, versus the number of hours that a print head was soaked in ink at 60° C. depicting the improvement in the resistance to delamination as a result of the PAA adhesion promoter. 
     FIG. 6 is a plot of the push strength between a barrier layer and an orifice plate having a tantalum (Ta) layer bonded thereto, using a polymethylacrylic acid (PMAA) adhesion promoter, versus the number of hours that a print head was soaked in ink at 60° C. depicting the improvement in the resistance to delamination as a result of the PMAA adhesion promoter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the drawings, tables and graphs, the invention is embodied in an improved print head for ink-jet printing and in a process for making such print heads. 
     The problem of delamination caused by aggressive inks and environmental moisture is addressed by the use of an intermediate layer of reinforcement material (discussed below) and a layer of adhesion promoter between the orifice plate and the ink barrier layer. 
     Referring to FIG. 1, reference numeral  12  generally indicates an ink-jet print head for ink-jet printing. The print head  12  includes an orifice plate  14  that is preferably fabricated from nickel and electroformed on a mandrel. The orifice plate  14  is about 50 microns thick. In a preferred and non-limiting embodiment, the orifice plate has a coating  16  of gold that is about 1.5 to about 3 microns thick. Other coatings  16  can be used including nickel, chromium and palladium. The orifice plate also contains a nozzle generally indicated by reference numeral  17 . On the lower surface of the orifice plate  14  on the gold coating  16  is an intermediate layer  18  of reinforcement material. The intermediate layer  18  is designed to provide an enhanced degree of strength, durability, and delamination-resistance in the completed print head  12 , particularly when used in cooperation with the adhesion promoters discussed below. Preferred materials used to fabricate the intermediate layer  18  of reinforcement material can be divided into two groups, both of which are related by their ability to chemically bond in an effective manner to both the selected adhesion promoter (discussed below) and other adjacent material layers in the print head  12 . The first group involves at least one oxidizable metal. In the first group, chromium, tantalum, and mixtures thereof represent preferred oxidizable metals (with tantalum providing optimum results). If an oxidizable metal is employed in connection with the intermediate layer  18 , it is preferably sputtered onto the coating  16  at a representative, non-limiting thickness of about 200-1300 angstroms. 
     The other preferred group of materials which may be employed in connection with the intermediate layer  18  of reinforcement material involves the use of at least one silicon-containing composition. Exemplary and preferred silicon-containing compositions which are suitable for this purpose include but are not limited to silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), silicon carbide (SiC), and mixtures thereof. Preferred application methods involving these materials include, for example, plasma enhanced chemical vapor deposition (also known as “PECVD”) wherein the selected silicon-containing composition is optimally applied to the lower surface of the orifice plate (whether or not a gold coating  16  is employed) at an exemplary, non-limiting uniform thickness of about 200-10,000 angstroms. However, this value may be varied as needed in accordance with preliminary pilot testing. It should also be noted that this group of materials (e.g. silicon-containing compositions) will work effectively with all of the various adhesion promoters discussed in detail below. Likewise, all of the assembly techniques and parameters outlined below in connection with the print head  12  are equally applicable to both groups of materials employed in connection with the intermediate layer  18  of reinforcement material. 
     Reference numeral  24 , FIG. 1 generally indicates an ink barrier layer. The ink barrier layer  24  is fabricated from polymethylmethacrylate (PMMA) which is obtainable from E. I. du Pont de Nemours and Company of Wilmington, Del. 
     Laminated between the intermediate layer  18  on the orifice plateand the ink barrier layer  24  is an adhesion promoter  20 . The adhesion promoter  20  is formed from either organosilane, polyacrylic acid herein referred to as “PAA”, or polymethylacrylic acid herein referred to as “PMAA”. The silanes are obtainable from the Dow Coming Corporation of Midland, Mich. and are identified in FIG.  2  and Table A by their product numbers. The PAA and the PMAA are obtainable from Polysciences, Inc. of Warrington, Pa. 
     Referring to FIG. 1, reference numeral  27  generally indicates a plurality of intermediate layers of various materials which are deposited on a print head substrate  29  fabricated from silicon dioxide. The barrier layer  24 , the intermediate layers  27  and the substrate  29  define the firing chamber  32 . 
     ORGANOSILANE ADHESION PROMOTERS 
     To apply the adhesion promoter  20  to the orifice plate, the orifice plate is dipped in an aqueous solution of organosilane having a concentration of between about .01% to about 1.0% in water. The preferred silane concentration is about 0.1%. After the dipping process, the orifice plate is rinsed and rotated at about 1500 rpm to remove any excess silane. The orifice plate and the layer of silane promoter are then heated for 5 minutes at 70° C. to 100° C. 
     A wafer, not shown, is covered with a plurality of individual print head substrates  29 . Each substrate has a layer  24  of ink barrier material already cured thereon. The individual orifice plates  14  are placed on the ink barrier layers so that the orifice plates, adhesion promoter layers, and substrates are in registration. Registration is necessary so that the architecture of the firing chambers  32  is precisely obtained. 
     The wafer with the orifice plates and adhesion layers in place is placed in a laminator and compressed at a pressure of about 150 psi at about 200° C. for about 10 minutes. Thereafter, the wafer is placed in an oven at 220° C. for 30 minutes. Next, each print head is sawed off the wafer and the application process is completed. 
     The completed print heads were tested by soaking the print heads in a solution of ink at a temperature of 60° C. for differing periods of time. Ink at an elevated temperature was used for testing in order to accelerate the delamination process. At selected times an individual print head was removed from the ink and rinsed in water. Thereafter, the print head was push tested. A force was applied perpendicularly between the orifice plate and the substrate by a mechanical tool, not shown. The force was increased until the orifice plate separated from the substrate. The amount of applied force and the movement of the tool were measured. The work of adhesion was obtained by integrating the area under the curve of applied force and the movement of the tool. The work of adhesion is measured in newton-millimeters. The push strength is the maximum force necessary to separate the orifice plate from the substrate and is measured in pounds. It is desired that the work of adhesion and the push strength be maximized. 
     Referring to Table A below and FIG. 2, the results of the testing are tabulated and illustrated. The following organosilanes were tested: 
     aminoethyl aminopropyl trimethoxysilane, Dow Coming Z-6020 
     3-chloropropyl trimethoxysilane, Dow Coming Z-6026 
     glycidoxypropyl trimethoxysilane, Dow Coming Z-6040 
     gamma-aminopropyl triethoxysilane, Dow Coming Z-6011 
     methacryloxypropyl trimethoxysilane, Dow Coming Z-6030. 
     The controls were orifice plates without adhesion promoters and, in particular, orifice plates with either an intermediate layer  18  of Palladium (Pd) or an intermediate layer  18  of Tantalum (Ta). 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE A 
               
             
             
               
                   
               
               
                 Work of Adhesion (newton-mm) of Tantalum with 
               
               
                 Organosilane Adhesion Promoters 
               
             
          
           
               
                   
                 Coating 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 on 
               
               
                   
                 Orifice 
                 Adhesion 
                 Day 
                 Day 
                 Day 
                 Day 
                 Day 
                 Day 
               
               
                 Number 
                 Plate 
                 Promoters 
                 0 
                 1 
                 3 
                 6 
                 14 
                 30 
               
               
                   
               
               
                 1 
                 Ta 
                 6020 
                 18.31 
                 8.70 
                 7.16 
                 6.85 
                 6.03 
                 5.25 
               
               
                 2 
                 Ta 
                 6030 
                 18.28 
                 8.65 
                 7.08 
                 5.18 
                 4.97 
                 2.97 
               
               
                 3 
                 Ta 
                 6011 
                 15.46 
                 8.58 
                 5.83 
                 6.01 
                 5.91 
                 4.76 
               
               
                 4 
                 Ta 
                 6040 
                 17.35 
                 8.43 
                 6.52 
                 5.60 
                 5.56 
                 3.92 
               
               
                 control 
                 Ta 
                 none 
                 17.53 
                 7.14 
                 1.17 
                 0.50 
                 0.41 
                 — 
               
               
                   
               
             
          
         
       
     
     It should be appreciated from FIG. 2 that after about three days of soaking in ink at 60° C., the orifice plates without an adhesion promoter had essentially fallen off of the substrate. These were the orifice plates with just layers of Palladium (pd) and Tantalum (Ta) only. 
     POLYACRYLIC ACID (PAA) AND POLYMETHYLACRYLIC ACID (PMAA) ADHESION PROMOTERS 
     The PAA is applied to the orifice plates by first dipping the orifice plates in a 1% solution of PAA for 3 minutes and then drying the orifice plates in an oven at 150° C. for 5 minutes. The orifice plates are thereafter washed in deionized water at 50° C. for 30 minutes. During the washing process the orifice plates are agitated. Next, the orifice plates are air dried and laminated to the print head substrate as described above. 
     For PAA a molecular weight of between 90,000 and 250,000 daltons is used and a molecular weight of about 100,000 to 200,000 daltons is preferred. 
     In addition, a thickness of less than 5 monolayers of PAA on the orifice plate is preferred. This thin layer is obtained by controlling the concentration of PAA solution and the water rinse time as described above. A concentration of PAA of between 0.05% and 10% is used and a concentration of 1.0% in water is preferred. 
     FIGS. 3 and 4 and Tables B and C below illustrate Cr with PAA and also the correspondence of the work of adhesion with push strength for the same materials over the same periods of time. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE B 
               
             
             
               
                   
               
               
                 Work of Adhesion (newton-millimeters) of Chromium 
               
               
                 with and without Polyacrylic Acid (PAA) Adhesion Promoters 
               
             
          
           
               
                   
                 Coating on 
                   
                   
                   
                   
                   
               
               
                   
                 Orifice 
                 Day 
                 Day 
                 Day 
                 Day 
                 Day 
               
               
                 Number 
                 Plate 
                 0 
                 1 
                 3 
                 6 
                 16 
               
               
                   
               
             
          
           
               
                 1 
                 Cr 
                 11.91 
                 6.34 
                 3.7 
                 1.55 
                 1.39 
               
               
                 2 
                 Cr + PAA 
                 16.9 
                 9.25 
                 8.91 
                 3.34 
                 4.7 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE C 
               
             
             
               
                   
               
               
                 Push Strength (lbs.) Of Chromium with and without Polyacrylic Acid 
               
               
                 (PAA) Adhesion Promoters 
               
             
          
           
               
                   
                 Coating on 
                   
                   
                   
                   
                   
               
               
                   
                 Orifice 
                 Day 
                 Day 
                 Day 
                 Day 
                 Day 
               
               
                 Numbers 
                 Plate 
                 0 
                 1 
                 3 
                 6 
                 16 
               
               
                   
               
             
          
           
               
                 1 
                 Cr 
                 9.52 
                 4.82 
                 2.13 
                 1.06 
                 0.76 
               
               
                 2 
                 Cr + PAA 
                 10.3 
                 6.6 
                 4.72 
                 2.43 
                 2.2 
               
               
                   
                   
                 2 
               
               
                   
               
             
          
         
       
     
     The ink used in Table E and FIG. 6 was different from the ink used in the other tables and figures. This other ink was used in all tests except Table E and FIG. 6 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE D 
               
             
             
               
                   
               
               
                 Push Strength (lbs.) Of Tantalum with and without 
               
               
                 Polyacrylic Acid (PAA) Adhesion Promoters 
               
             
          
           
               
                   
                 Coating on 
                 Hours 
                 Hours 
                 Hours 
                 Hours 
               
               
                 Number 
                 Orifice Plate 
                 71 
                 23 
                 172 
                 336 
               
               
                   
               
               
                 1 
                 Ta 
                 2.1 
                 1.8 
                 1.6 
                 0.6 
               
               
                 2 
                 Ta + PAA 
                 8.5 
                 6.4 
                 5.2 
                 2.0 
               
               
                   
               
             
          
         
       
     
     For the PMAA adhesion promoter, Table E, the orifice plates are prepared and the PMAA is applied in the same manner as described above. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE E 
               
             
             
               
                   
               
               
                 Push Strength (lbs.) Of Palladium and Polymethylacrylic 
               
               
                 Acid (PMAA) Adhesion Promotor on Tantalum 
               
             
          
           
               
                   
                 Coating 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 of 
               
               
                   
                 Orifice 
                 Hrs. 
                 Hrs. 
                 Hrs. 
                 Hrs. 
                 Hrs. 
                 Hrs. 
                 Hrs. 
                 Hrs. 
               
               
                 No. 
                 Plate 
                 52 
                 169 
                 336 
                 405 
                 504 
                 692 
                 836 
                 1005 
               
               
                   
               
             
          
           
               
                 1 
                 Pd 
                 7.8 
                 6.1 
                 2.5 
                 1.4 
                 0.4 
                 0.22 
                 0.21 
                 0.21 
               
               
                 2 
                 PMAA 
                   
                 9.3 
                 7.7 
                   
                 7.7 
                 7.5 
                 6.5 
                 7.0 
               
               
                   
                 + Ta 
               
               
                   
               
             
          
         
       
     
     Table E compares ink soak testing of Palladium only orifice plates to PMAA on tantalum sputtered on Palladium coated orifice plates. 
     Although specific embodiments and processes of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangement of parts so described and illustrated. The invention is limited only by the claims.