Abstract:
A process for forming an orifice plate for a thermal inkjet printhead involves the use of a photoimageable polymer and photolithography for forming a plastic orifice plate having a defined pattern of orifices therein. A substrate is used to support a photoimageable polymer layer (which ultimately becomes the orifice plate) during the photolithographic steps, which preserves the structural integrity of the polymer layer. The process allows high accuracy in the dimensioning, spacing and shaping of the orifices. A thermal inkjet print head assembly is also disclosed which involves bonding the plastic orifice plate to a polymer barrier layer of a thin film resistor heater structure using heat and pressure.

Description:
TECHNICAL FIELD 
     The present invention relates to thermal ink jet printing and more particularly to the manufacture of a plastic orifice plate for an inkjet printhead assembly, manufacture of an inkjet printhead assembly, provision of a plastic orifice plate and provision of an inkjet printhead assembly. 
     BACKGROUND 
     In thermal inkjet printing, localised heat transfer to a defined volume of ink, which is located adjacent to an ink jet orifice, vaporises the ink and causes it to expand thereby ejecting the ink from the orifice during the printing of characters on a print medium. The defined volume of ink is usually provided in a “barrier layer” which provides a plurality of ink reservoirs. These reservoirs are located between a corresponding plurality of resistive heater elements, usually provided by a thin film structure, and a corresponding plurality of orifices (which are effectively nozzles), provided by an “orifice plate”. 
     Thus orifice plates with multiple orifices aligned with thin film resistors are used to control the trajectory, drop weight and drop velocity of ink drops. Typically, these orifice plates are manufactured by electroforming processes and the metal that is commonly used is Nickel. Details of such metallic orifice plates and the functioning and manufacture of thermal inkjet printheads with orifice plates are described in the Hewlett-Packard Journal, Vol. 36, No.5, May 1985 and in U.S. Pat. No. 4,694,308 issued to C. S. Chan et al. 
     Use of plastic materials to fabricate orifice plates has certain advantages over metallic orifice plates. Some of the advantages of these plastic orifice plates are described in U.S. Pat. No. 4,829,319 issued to C. S. Chan et al. These include low cost of the orifice plates, transparency of the orifice plate which helps in viewing the fluid dynamics in the print cartridges, corrosion resistance to ink chemicals and the possibility of forming integral barrier layers on the thin film resistors. 
     U.S. Pat. No. 4,829,319 to Chan et al (hereafter US &#39;319) discloses a plastic orifice plate for an inkjet printhead and manufacturing process therefor which includes electroforming a metal die having raised sections thereon of predetermined centre-to-centre spacings, and using the die to punch out openings in a plastic substrate of a chosen thickness to form a plurality of closely spaced orifice openings in the substrate. However the process of US &#39;319 has a number of problems associated with it. First, it is difficult to preserve the structural integrity of thin plastic sheets during the die stamping operation. The thin plastic sheets are difficult to handle and are susceptible to tearing. Second, for most inkjet printing applications, a dimensional accuracy within sub-micron range is needed for the orifices and the US &#39;319 process may not give this level of accuracy. Third, the shape of the orifices is important in controlling the directionality of ink droplets and it is difficult to achieve a perfect shape definition with the US &#39;319 die stamping process. Fourth, the latest printheads require a high density of orifices in an orifice plate. This requires spacing consecutive orifices a distance of less than 10 microns apart, which spacing cannot be easily achieved using the US &#39;319 process. Fifth, the US &#39;319 process is rather complex involving many process steps, which may result in low yields in the process. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a process for manufacturing plastic orifice plates which reduces at least some of the above problems. The invention includes providing a plastic orifice plate as such and also providing an inkjet printhead assembly which incorporates a plastic orifice plate. 
     The invention involves the use of a photoimageable polymer and photolithography for forming a plastic orifice plate having a defined pattern of orifices therein. 
     In another aspect, in forming an inkjet printhead assembly, a thin film resistor structure having a plastic barrier layer is provided and a formed plastic orifice plate is bonded thereto using heat and pressure. 
     Use of a photolithographic technique according to the invention allows use of a substrate to support a photoimageable polymer layer for the photolithographic steps, thereby avoiding the problem of damaging the plastic sheets as in US &#39;319. Photolithography also allows for greater accuracy in the final product, both dimensionally and in orifice shapes, than is achievable in the US &#39;319 process. The invention also involves less process steps compared to the US &#39;319 process and thus should result in higher process yields. 
     For a better understanding of the invention and to show how it may be performed, embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 H are schematic cross-sectional views of steps in a preferred process for forming a plastic orifice plate according to the invention. 
     FIG. 2 is a plan view of an orifice plate formed using the steps of FIGS. 1A to H. 
     FIGS. 3A to  3 C schematically illustrate in cross section further process steps for forming an inkjet printhead assembly involving attaching the orifice plate of FIG. 2 onto a thin film resistor wafer. 
     FIGS. 4A to  4 C illustrate alternative process steps to those of FIGS. 3A to  3 C. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIGS. 1A to  1 H, a surface  12  of a standard six inch silicon wafer substrate  10  for supporting a photoimageable polymer for forming a plastic orifice plate is first coated with a layer  14  of metal, which may be gold, tantalum/gold, or chromium/stainless steel, to a thickness of about 2000 Angstrom by a vacuum deposition process (see FIG.  1 B). Layer  14  acts as a seed layer for the subsequent electro-deposition of a Nickel layer  16 . Nickel layer  16  is electro-deposited to a thickness of about 5 microns in a Watts&#39; bath containing Nickel Sulphate, Nickel Chloride and Boric Acid in an aqueous solution along with organic additives such as saccharin, Aromatic Sulphonic acids, Sulfonamides and Sulphonimides. The Nickel layer  16  provides the required surface energy for the adhesion of a plastic material (from which the orifice plate is to be formed) during a lamination process onto the substrate  10  and it facilitates the release of the subsequently formed plastic orifice plate. 
     The silicon wafer  10  of FIG. 1C is preferably treated with an aqueous solution containing 30% Nitric acid and 4% Hydrogen peroxide for 30 seconds to increase the surface roughness (see Ref.  18  in FIG. 1D) of the Nickel layer  16  depending on the exposure time. Typically for a 30 second exposure an increase in surface roughness of around 20% can be observed. For example, the measured values of surface roughness from a Digital Instrumental Atomic Force microscope on the Nickel layer  16  before and after the acid treatment are 11.22 nm and 14.15 nm respectively. Such surface treatment by acid is found to increase the adhesion of a polymer material to the Nickel layer  16 . Thus the substrate  10  is provided having a surface with predetermined characteristics. 
     With reference to FIG. 1E, a layer  20  of a photoimageable polymer material of about 25 microns thickness is then provided on the surface  18  of substrate  10 . Polymer  20  may be a solid film which is pasted onto the substrate  10  either manually or using a standard laminating machine. Alternatively the polymer may be supplied as a liquid and spun onto substrate  10  using a spin coating machine. A photoimageable polymer includes three major components: a photo active compound that undergoes cross-linking polymerization reaction on exposure to the suitable radiation, a photo packaging compound that initiates the radical polymerization and a solvent or a binder that carries both the photo active and photo packaging compounds either in a liquid or in a solid form. In the present invention the photoimageable polymers referred by their trade names IJ5000 series Barrier material and SU-8 photoresists have been used. These chemicals are supplied by DuPont and Microchem companies respectively. Photoimageable polymers with the composition given below are suitable for the fabrication of orifice plates. 
     Photo active compounds: Methacrylate esters, Urethane derivatives and Epoxy derivatives. 
     Photo packaging compounds: Aryl sulfonium salts 
     Solvents and Binders: Polymethyl metacrylate, γ-Butyrolactone 
     A mask  22  which defines a required pattem of orifices  24  for the orifice plate is then provided (see FIG.  1 F). The mask  22  and silicon substrate  10  of the figures encompasses a number of “dies”, that is, they provide for simultaneous fabrication of a number of orifice plates, thus the mask  22  also provides a required pattern of orifice plates. 
     Mask  22  is appropriately aligned relative to substrate  10  and the photoimageable polymer layer  20  is then exposed to ultra-violet (UV) radiation  26  through mask  22  (see FIG.  1 G). Under typical operating conditions, an expose energy of 45 mJoules/cm 2  may be used. The expose energy can be varied between 40 to 600 mJoules/cm 2  depending on the nature of the polymer film used in the fabrication process. Instead of a single polymer layer  20  a dual polymer film coating using two different types of polymers to increase the total polymer layer thickness to 60 microns may be used. The main reason for using a dual polymer film is to increase the thickness of the plastic orifice plate. The typical thickness range of the orifice plates is between 20 to 60 microns while most of the commercially available photoimageable polymers are about 25 microns thick. Hence for orifice plates requiring higher thickness, it is necessary to coat more than one layer to attain the required thickness. 
     After the expose step, the polymer layer  20  is then developed using a suitable solvent such as a solution of N-methyl pyrrolidone and Diethylene Glycol resulting in a pattern of orifice plates  28  on the substrate  10  (see FIG.  1 H). The developing solvent can be a solution with a concentration of N-methyl pyrrolidone in the range of 50% v/v to 75% v/v and with Diethylene Glycol up to a concentration of 26% v/v. The plastic orifice plates  28  on the silicon wafer substrate  10  are then cured with UV radiation to complete the fabrication process. 
     FIG. 2 shows a plan view of an orifice plate  28  with orifices  24 . 
     The adhesion of the plastic orifice plates  28  thus fabricated to the Nickel layer  16 - 18  on the silicon wafer substrate  10  is very strong at this stage. In order to release the orifice plates  28  from the substrate  10  for subsequent processing, the Nickel layer  16 - 18  is oxidised by a “dip” step. In this step, the substrate  10  with plastic orifice plates  28  is dipped in a solution of pH 4 and at a temperature of 55° C. for 15 minutes. Operating conditions for the “dip” process for the pH can vary between 2 to 5 and for the solution temperature between 50° C. to 70° C. The Watts&#39; bath solution described hereinbefore may be used for this “dip” step, which is for oxidizing the surface  18  of Nickel layer  16  for weakening the Nickel 16-barrier material  22  adhesion. The plastic orifice plates  28  after this dip step can be released from the silicon wafer substrate using a blue sticky tape. 
     Subsequent processing steps to form an inkjet printhead assembly involve attaching an orifice plate  28  to a thin film structure, which structure provides a plurality of resistive heater elements. Such a thin film structure will have a plastic barrier layer thereon which defines ink reservoirs aligned over the resistive heater elements. Provision of such a thin film structure having a plastic barrier layer is known. Two methods for attaching an orifice plate  28  to such a thin film structure are shown in FIGS. 3A to  3 C and FIGS. 4A to  4 C respectively. 
     With reference to FIGS. 3A to  3 C, orifice plates  28  are singly attached to a thin film resistor structure  30  which is a wafer. Each orifice plate  28  is attached onto a barrier layer  32  of each die pattern  34  of thin film wafer  30 . This is done by placing thin film wafer  30  on a heater chuck  36  for heating the barrier layers  32  to a temperature above the glass transition temperature Tg of the barrier layer  32  which is about 90° C. The barrier layer  32  material comprises two main components, a thermoplastic component and a thermoset component. Above the temperature Tg, the thermoplastic component starts to soften and causes the barrier layer  32  to get sticky. A plastic orifice plate  28  is brought above a die  34  of thin film wafer  30  and is aligned with the die pattern on the thin film wafer (see FIG.  3 A). Once aligned the orifice plate  28  is pressed onto the die  34  and barrier layer  32  using a place chuck  38  (see FIG.  3 B). As the barrier layer  32  is above its Tg temperature, the plastic orifice plate  28  will bond to the barrier layer  32  due to the pressure applied by place chuck  38 . The place chuck  38  is then retracted (see FIG. 3C) to proceed to the next plastic orifice plate  28  and die  34 . 
     With reference to FIGS. 4A to  4 D, a wafer to wafer attachment method involves (as in FIG. 3) placing the thin film wafer  30  having barrier layers  32  on a heater block  36  and heating to above the glass transition temperature Tg of the barrier layer  32  material. However, in this method the silicon substrate  10  and attached plastic orifice plates  28  of FIG. 1H (after the oxidation step) is positioned above the thin film wafer  30  for alignment. The alignment can be done by using a pair of matching patterns on the thin film wafer  30  and the silicon wafer  10 , with that on the silicon wafer  10  being associated with an etched “see through” hole—as indicated at  40  and  42 . Once aligned, the silicon wafer  10  with plastic orifice plates  28  is pressed via place chuck  44  onto the barrier layers  32  of thin film wafer  30 . Upon withdrawal of place chuck  44  and because the adhesion between the Nickel layer  16  and the plastic orifice plates  28  is weaker than that between the barrier layer  32  and the plastic orifice plates  28 , the silicon wafer  10  gets separated from the plastic orifice plates  28  leaving them attached to barrier layer  32  (see FIG.  4 D). Inkjet printhead assemblies are then provided by removing the thin film wafer  30  from heater chuck  36  and individualizing the thin film dies. 
     Using the above described process steps, plastic orifice plates having diameters less than 25 microns with size distributions within one micron, and having a pitch between orifices of less than 10 microns, can be provided. Important features of the orifices, such as their shapes, can be controlled to sub-micron accuracy. The invention includes providing orifice plates having different orifice shapes, both circular and non-circular. 
     By choosing the same material for the plastic orifice plates  28  and for the barrier layers  32  of the thin film resistor structure  30 , the adhesion and corrosion resistance properties of the thin film dies  34  can be improved.