Poly-p-xylylene films as an orifice plate coating
TECHNICAL FIELD 
     The present invention relates to thermal ink-jet printers, and, more particularly, to printheads incorporating an improved orifice coating for securing the orifice plate to the barrier material used to form the ink firing chamber within the printhead. 
     BACKGROUND ART 
     Thermal ink-jet printers offer a low cost, high quality, and comparatively noise free option to other types of printers commonly used with computers. Such printers employ a resistor element in a chamber provided with an egress for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements are arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium. The entire assembly of printhead and reservoir comprise an ink-jet pen. 
     In operation, each resistor element is connected via a conductive trace to a microprocessor, where current carrying signals cause one or more selected elements to heat up. The heating creates a bubble of ink in the chamber, which is expelled through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alphanumeric characters, performs area fill, and provides other print capabilities on the medium. 
     The present construction of printheads in thermal ink-jet print cartridges employs nickel as the orifice plate material. Since inks corrode nickel, a gold layer (electroplated gold) is formed on the surface of the nickel as a corrosion barrier. The orifice plate is bonded to the barrier layer by a thermal cure cycle. 
     Problems also exist with the use of adhesives for securing the orifice plate. Adhesives do not provide a uniform coating and often cover the holes in the orifice plate. 
     A need remains for a material useful in the construction of thermal ink-jet printheads as an orifice coating which improves the securing of the orifice plate to the barrier material, improves corrosion resistance, and eliminates costly gold plating. 
     DISCLOSURE OF INVENTION 
     In accordance with the invention, a poly-p-xylylene coating has been found to demonstrate superior properties as a thermal ink-jet orifice coating material to improve securing the orifice plate to the barrier material in combination with resistor assemblies employed in thermal inkjet printheads. This coating material evidences better adhesion for securing the orifice plate to the barrier material used to form the ink firing chamber within the printhead. 
     Printheads made with the foregoing orifice coating exhibit improved corrosion resistance to ink and no longer require costly gold plating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a resistor and ink feed channel assembly in a thermal ink-jet printhead which shows the orifice plate coated with a layer of poly-p-xylylene adhering to the barrier layer; and 
     FIGS. 2A-C provide the structures and names of poly-p-xylylenes preferably employed in the practice of the invention. 
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Referring now to FIG. 1, a portion of a printhead is shown comprising a plurality of firing resistors, of which one resistor assembly 10 is depicted. The resistor assembly 10 comprises a barrier inlet channel 12, with a resistor element 14 adjacent to one end 12a thereof. Ink (not shown) is introduced at the opposite end 12b of the channel, as indicated by arrow &#34;A&#34; from an ink refill slot, generally indicated at 16. A constriction 18 near the fluid introduction end 12b may be used for control of hydraulic damping. 
     Associated with each resistor element 14 is an orifice 20, located above the resistor element in an orifice plate, nozzle plate or top-plate, 22. The diameter of the nozzles typically is in the range of about 25 to 75 μm, depending on the particular printhead design, which is a function of the frequency at which the printhead is designed to operate, ink viscosity, drop volume, hydraulic damping, etc. The orifice plate 22 typically comprises gold plated, electroformed nickel, about 25 to 75 μm thick. 
     Droplets of ink are ejected through the nozzle as indicated by arrow &#34;B&#34; upon heating of a quantity of ink by the resistor element 14. The orifice plate 22 is supported on barrier material 24, which is patterned to define the ink feed channel and the chamber 26 in which the resistor element 14 is located. The barrier material 24 itself is formed on a major surface of a substrate 28, such as silicon. 
     FIG. 1 depicts one configuration of the resistor assembly, in which an orifice coating layer coats all sides 30a and 30b of the orifice plate and enhances adhesion of the orifice plate 22 to the barrier layer 24. 
     In accordance with the invention, the coating material 30 consists essentially of a poly-p-xylylene. The poly-p-xylylene is applied to a nickel electroformed orifice plate 22 as a coating ranging from 0.5 to 5 μm and preferably about 1 to 2 μm. A coating process similar to chemical vapor deposition is used. The poly-p-xylylene starts as a dimer, it is pyrolyzed, volatilized, and then polymerized onto the nickel surface. The poly-p-xylylene evenly coats the nickel orifice plate, maintaining the openings formed therethrough. The poly-p-xylylene coating is then ashed using argon plasma consisting of 130 watts of forward power, 30 watts of reflected power and 100 volts cathode for 8 minutes or oxygen plasma treatment using oxygen and a REI asher for 5 minutes at 200 watts of power to roughen the surface. The orifice plate is then bonded to the barrier layer 24 by a thermal cure cycle. 
     FIGS. 2A-C depict the structures and names of poly-p-xylylenes preferably employed in the practice of the invention. FIG. 2A shows poly-p-xylylene (I), which is available under the tradename Parylene N. FIG. 2B shows poly(monochloro-p-xylylene) (II), which is available under the tradename Parylene C. FIG. 2C shows poly(dichloro-p-xylylene) (III), which is available under the tradename of Parylene D. Of these, poly(monochloro-p-xylylene) (II) is most preferred. In the Tables below, each reference to poly-p-xylylene is actually to the (II) compound. 
     Table I sets forth a comparison of the orifice plate adhering process of the invention with both the prior art gold plating process and the improved poly-p-xylylene coating process. 
     
                       TABLE I                                                     
______________________________________                                    
Improved Poly-p-xylylene Coating Pro-                                     
cess of the Invention Compared to Prior                                   
Art Gold Coating Process.                                                 
                 ORIFICE ADHESION TEST                                    
                 AFTER INK SOAK                                           
THERMAL CURE     (1 wk @ 70° C.)                                   
TEMPERATURE  TIME    INITIAL    INK1  INK2                                
______________________________________                                    
Poly-p-xylylene coating                                                   
180° C.                                                            
             60 min. 2.6        1.4   1.7                                 
200° C.                                                            
             30 min. 3.8        1.0   1.6                                 
220° C.                                                            
             30 min. 4.8        1.7   1.5                                 
Gold Plating                                                              
180° C.                                                            
             60 min.            0.7   0.6                                 
200° C.                                                            
             30 min.            0.9   0.7                                 
220° C.                                                            
             30 min.            1.0   0.6                                 
______________________________________                                    
 
    
     The foregoing Table I provides comparative data for reduced adhesion obtained from an ink soak test, 1 week in duration at 70° C., using two inks with the following compositions: 
     
         ______________________________________                                    
                  WT %                                                    
______________________________________                                    
INK1 (BLACK INK)                                                          
Black dyes          2.2                                                   
Solvent: 2-pyrrolidone                                                    
                    7.5                                                   
Biocide: Proxel GXL 0.3                                                   
Buffers             0.4                                                   
Minor components    0.07                                                  
Water               89.5                                                  
INK2 (CYAN INK)                                                           
Cyan dye: Acid Blue 9                                                     
                    1.9                                                   
(sodium salt)                                                             
Solvents: dipropylene glycol                                              
                    3.0                                                   
trimethylol propane 15.0                                                  
Surfactant          0.5                                                   
Biocide             0.3                                                   
Water               79.3                                                  
______________________________________                                    
 
    
     The higher the ink soak test number, the better. It is seen that poly-p-xylylene results in significantly greater adhesion for all temperature cure conditions (180°, 200°, and 220° C.). 
     Table II sets forth a comparison of the orifice plate adhering process of the invention with both the prior art gold plating process and the improved poly-p-xylylene coating process where the poly-p-xylylene coating has been ashed prior to bonding to the barrier layer. 
     
                       TABLE II                                                    
______________________________________                                    
Improved Ashed Poly-p-xylylene Coat-                                      
ing Process of the Invention Compared                                     
to Prior Art Gold Coating Process.                                        
               ORIFICE                                                    
               ADHESION TEST AFTER                                        
THERMAL CURE 220° C.                                               
               INK SOAK (1 wk @ 70° C.)                            
ORIFICE TYPE   INITIAL   INK1    INK3  INK4                               
______________________________________                                    
Normal Gold    6.1       0.3     0.4   0.6                                
Poly-p-xylxylene Coated                                                   
               4.5       2.4     2.5   0.8                                
Nickel (as received)                                                      
Poly-p-xylxylene Coated                                                   
               5.0       3.8     3.6   4.0                                
Nickel (ashed)                                                            
______________________________________                                    
 
    
     The foregoing Table II provides comparative data for reduced adhesion obtained from an ink soak test, 1 week in duration at 70° C., using three inks with the following compositions: 
     
         ______________________________________                                    
                      WT %                                                
______________________________________                                    
INK3 (YELLOW INK)                                                         
Yellow dye: Acid Yellow 23,                                               
                        2.6                                               
tetramethyl ammonium salt                                                 
Solvents: dipropylene glycol                                              
                        3.0                                               
trimethylol propane     15.0                                              
Surfactant              0.5                                               
Biocide                 0.3                                               
Water                   78.6                                              
INK4 (BLACK PIGMENT-BASE INK)                                             
Pigment                 3.6                                               
Dispersant              1.8                                               
Solvents: diethylene glycol                                               
                        5.7                                               
LEG-1                   5.7                                               
Biocide                 0.3                                               
Water                   82.9                                              
______________________________________                                    
 
    
     (see U.S. Pat. No. 5,085,698). 
     As above, the higher the ink soak test number, the better. The ashed poly-p-xylylene results in a significantly greater adhesion than either the gold coating or the poly-p-xylylene coating without ashing. 
     The significantly improved adhesion of the orifice plate to the barrier layer results from the use of an ashed poly-p-xylylene coating applied to the orifice plate. The poly-p-xylylene coating replaces the costly gold coating and improves the reliability of the thermal ink-jet printhead. The improved chemical resistance of the poly-p-xylylene coating allows the use of a large number of thermal ink-jet inks for letter quality black and color text and graphics printing. The poly-p-xylylene coating material is very durable and abrasion resistant. Table III provides supporting data from a Wiper Test. 
     Wipers are an integral part of an ink-jet printer. The wiper is used to remove clogs in the ink-jet cartridge by wiping across the surface. The surface coating of the printhead must be durable and wear resistant to withstand this wiping action. A typical printhead might receive 5,000 wipes in its lifetime. Additional wipes are provided to ensure product margin. 
     
                       TABLE III                                                   
______________________________________                                    
Wiper Test.                                                               
No. of              No. of pens                                           
Wipes  Pen Type     tested     Test % Failure                             
______________________________________                                    
10,000 Gold         4          1    0 (pass)                              
       control                                                            
10,000 Poly-p-      4          1    0 (pass)                              
       xylylene coated                                                    
12,000 gold control 5          2    0 (pass)                              
12,000 poly-p-      5          2    0 (pass)                              
       xylylene coated                                                    
______________________________________                                    
 
    
     INK1 is very corrosive to nickel. An accelerated storage test revealed that a 1 μm thickness of the poly-p-xylylene coating material is required for corrosion resistance. Table IV provides data from an Accelerated Storage Life Test for INK1. 
     
                       TABLE IV                                                    
______________________________________                                    
Accelerated Storage Life Testing.                                         
poly-p-xylylene                                                           
             INK1                                                         
Thickness (μm)                                                         
             60° C., 6 weeks                                       
                           70° C., 3 weeks                         
______________________________________                                    
0.5          85% failure   60% failure                                    
1.0          10% failure   10% failure                                    
3.0          0             0                                              
1 μm Thick                                                             
             0             0                                              
Gold Controls                                                             
______________________________________                                    
 
    
     Table IV shows that if the poly-p-xylylene is too thin, approximately 1.0 micrometer or less thick, then corrosion failures occur. 
     INDUSTRIAL APPLICABILITY 
     Adhesive material comprising the poly-p-xylylene layer set forth herein is expected to find use in the manufacture of thermal ink-jet printheads. 
     Thus, there has been disclosed a process of fabricating thermal ink-jet printheads for thermal ink-jet printers. Various changes and modifications of an obvious nature will be readily apparent to those skilled in this art, and all such changes and modifications are considered to fall within the scope of the invention as defined by the appended claims.