Abstract:
An inkjet printer printhead utilizes a substrate, an orifice layer, and a directionally biased electrostrictive polymer ink actuator disposed between the orifice layer and the substrate to eject ink from the printhead. The electrostrictive polymer ink actuator has a passivation layer disposed on the substrate, a first compliant electrode disposed at least on a first portion of the passivation layer, an electrostrictive polymer membrane disposed on a first area of the first compliant electrode, a passivation constraint disposed on a second portion of the passivation layer and a second area of the first compliant electrode effectively surrounding, in contact with, but not covering the electrostrictive polymer membrane in the first area of the first compliant electrode, and a second compliant electrode disposed on the passivation constraint which is disposed on the second portion of the passivation layer and the electrostrictive polymer membrane which is disposed on the first area of the first compliant electrode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a (X) continuation of application Ser. No. 09/070,826 now U.S. Pat. No. 6,126,273 filed on Apr. 30 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to print cartridges for inkjet printers and more specifically to the expulsion of ink from an inkjet printer printhead. 
     Inkjet printing mechanisms use pens that shoot droplets of colorant onto a printable surface to generate an image. Such mechanisms may be used in a wide variety of applications, including computer printers, plotters, copiers, and facsimile machines. For convenience, the concepts of the invention are discussed in the context of a printer. An inkjet printer typically includes a printhead having a plurality of independently addressable firing devices. Each firing device includes a firing chamber connected to a common ink source, an ink propulsion device, and an ink expulsion nozzle. The ink propulsion device within the firing chamber provides the impetus for expelling ink droplets through the nozzles. 
     In thermal inkjet pens, the ink propulsion device is a resistor that provides sufficient heat to rapidly vaporize a small portion of ink within the firing chamber. The bubble expansion provides for the displacement of a droplet of liquid ink from the nozzle. The heat to which the ink is exposed in a thermal ink jet pen prevents the use of thermally unstable ink formulations that might otherwise provide desirable performance and value. Therefore, the available ink options are reduced to those that are not adversely affected by varying temperatures. 
     Conventional piezoelectric inkjet pens avoid the disadvantages of thermally stressing the ink by using a piezoelectric transducer in each firing chamber. The firing chamber dimensionally contracts in response to the application of a voltage to provide the displacement to expel a droplet of ink having a volume limited to the volume change of the piezoelectric material. Because of the very low displacement or equivalent strains (&lt;1%) of piezoelectric material, conventional piezoelectric transducers have limited volume displacement capability requiring relatively large crystals thereby reducing packing density. Furthermore, piezoelectric transducers are susceptible to degradation by direct exposure to some inks that might otherwise be desirably employed, and have other disadvantages related to limited miniaturization, cost, and reliability. 
     With the invention as described hereinafter, an ink expulsion actuator is manufacturable that has increased ink flexibility; is a more predictable and repeatable actuator by the elimination of thermal cycling used in conventional inkjet propulsion systems which eliminates unpredictable ink nucleation variations; and, allows discrete control of ink drop size through the control of voltage due to the increased displacement or strain (up to 30%) of electrostrictive polymer actuators over piezoelectric devices. 
     SUMMARY OF THE INVENTION 
     An inkjet printer printhead utilizes a substrate, an orifice layer, and a directionally biased electrostrictive polymer ink actuator disposed between the orifice layer and the substrate. The electrostrictive polymer ink actuator has a passivation layer disposed on the substrate, a first compliant electrode disposed at least on a first portion of the passivation layer, an electrostrictive polymer membrane disposed on a first area of the first compliant electrode, a passivation constraint disposed on a second portion of the passivation layer and a second area of the first compliant electrode effectively surrounding, in contact with, but not covering the electrostrictive polymer membrane in the first area of the first compliant electrode, and a second compliant electrode disposed on the passivation constraint which is disposed on the second portion of the passivation layer and the electrostrictive polymer membrane which is disposed on the first area of the first compliant electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be further understood by reference to the following description and attached drawings, which illustrate the preferred embodiment. 
     FIG. 1 is a perspective view of an inkjet printer print cartridge according to one embodiment of the present invention. 
     FIG. 2 is a perspective view of the top surface of the Tape Automated Bonded (TAB) printhead assembly (hereinafter “TAB head assembly”)removed from the print cartridge of FIG.  1  and exposing the printhead. 
     FIG. 3 is a view A from FIG. 2, expanded for clarity and a better perspective of the points of cross sectioning for FIG. 6A,  6 B and  7 . 
     FIG. 4A and 4B are illustrations of the basic structure of an embodiment of the invention in an unactuated ( 4 A) and an actuated ( 4 B) state. 
     FIG. 5A and 5B are illustrations of the basic structure of the preferred embodiment of the invention in an unactuated ( 5 A) and an actuated ( 5 B) state. 
     FIG. 6A and 6B are side elevation views in a cross-section taken along line A—A in FIG. 3 illustrating the relationship of the electrostrictive polymer ink propulsion device with respect to the layered components on a substrate on a TAB head assembly. 
     FIG. 7 is a side elevation view in a cross-section taken along line B—B in FIG. 3 illustrating the relationship of the electrostrictive polymer ink propulsion device and the ink feed into the device with respect to the layered components on a substrate on a TAB head assembly. 
     FIG. 8 is an illustration of a process flow for building the electrostrictive polymer ink propulsion device of the preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, reference number  101  generally indicates an inkjet printer print cartridge incorporating a printhead according to one embodiment of the present invention. Inkjet printer print cartridge  101  includes ink reservoir  105 , which holds the ink prior to expulsion, and printhead assembly  103 , where printhead assembly  103  is formed using Tape Automated Bonding (TAB) techniques. One conventional technique is described in U.S. Pat. No. 4,917,286 (Pollacek). Printhead assembly  103  (hereinafter “TAB head assembly  103 ”)includes ink expulsion nozzles  107  formed on substrate  201 . An alternate embodiment of the invention (not shown) has the ink expulsion nozzles  107  formed in flexible circuit  111  by, for example, laser ablation. 
     A back surface of flexible circuit  111  includes conductive traces (not shown) formed thereon, for example, using a photolithographic etching and/or plating process. Printer contact pads  109 , designed to interconnect with a printer, terminate these conductive traces on one end. The opposite ends are terminated, via TAB bond beams  113 , on a substrate  201  containing ink expulsion devices (FIG.  2 ). Inkjet printer print cartridge  101  is designed to be installed in a printer so that contact pads  109 , on the front surface of flexible circuit  111 , contact printer electrodes providing externally generated energization signals to TAB head assembly  103  to command firing of the desired ink expulsion device. 
     FIG. 2 is a perspective view of the top surface of a TAB head assembly  103  removed from inkjet printer print cartridge  101  of FIG.  1  and straightened out. Affixed to TAB head assembly  103  via TAB bond beams  113  through a TAB bond window  203  opening through the flexible circuit  111  is a semiconductor substrate  201  containing a plurality of individually energizable ink propulsion devices. Each ink propulsion device is fluidically coupled to a single ink expulsion nozzle  107  and expels a droplet of ink when selectively energized by one or more pulses or instructions applied to one or more contact pads  109 . The ink is supplied from ink reservoir  105  (FIG.  1 ). An alternate embodiment is contemplated where the ink is supplied from a remote ink reservoir connected to ink jet printer print cartridge  101  by a tube. In the preferred embodiment, the individually energizable ink propulsion devices are electrostrictive polymer actuators that are contained on the silicon substrate  201 . 
     FIG. 3 is a detailed view A from FIG. 2, expanded for clarity and a better perspective of the points of cross sectioning A—A and B—B which are detailed in FIG. 6A,  6 B and  7 . FIG. 3 provides a detailed top plan view of substrate  201  and the first four firing chambers  301  corresponding to the first four ink expulsion nozzles  107 . Each firing chamber  301  contains an electrostrictive polymer ink propulsion device  309  and associated first compliant electrode  303  and second compliant electrode  305 . These two electrodes overlap to create the circular shaped electrostrictive polymer ink A propulsion device  309  as shown. Although this device is pictured in a circular shape, it has been contemplated to make the devices other shapes such as oval or rectangular, depending upon the properties of the materials used and the desired response of the ink. Interposed between first compliant electrode  303  and second compliant electrode  305  is an electrostrictive polymer membrane. 
     The top surface of FIG. 3 is orifice layer  320 . Orifice layer  320  has the ink expulsion nozzles  107  defined in it and is the top, or ceiling, of firing chamber  301 . Ink feed channels  307  extend through substrate  201 , but not through orifice layer  320 . Ink feed channel  307  works as an ink supply duct between ink reservoir  105  and firing chamber  301  in order to supply ink to electrostrictive polymer ink propulsion device  309 . With orifice layer  320  atop substrate  201 , each ink expulsion nozzle  107 , in the preferred embodiment, would have an ink chamber entrance  313  and an ink chamber exit  311  defined in orifice layer  320  that would be aligned in a manner similar to that shown in FIG.  3 . Other embodiments have been contemplated where electrostrictive polymer ink propulsion device  309  is not in direct alignment with ink expulsion nozzle  107 , yet fluidically coupled thereby expulsion of ink is a result of a sudden decrease in the volume of firing chamber  301 . 
     FIG. 4A and 4B are illustrations of the basic structure of an embodiment of the invention in a power off (FIG. 4A) and a power on (FIG. 4B) state. The first compliant electrode  303  and the second compliant electrode  305  together act as a parallel plate capacitor in the area where they overlap. In the overlapped area there is interposed an electrostrictive polymer membrane  405 . This overlapped area forms an electrostrictive polymer ink propulsion device  309 . When a voltage difference is applied between first compliant electrode  303  and second compliant electrode  305 , electrostrictive polymer membrane  405  is squeezed in thickness and stretched in length and width. Due to the otherwise incompressible nature of electrostrictive polymer materials, electrostrictive polymer membrane  405  will expand in an unconstrained way in an effort to conserve total volume. This is illustrated in FIG. 4B by polymer membrane bulges  407 . 
     In FIG. 5A and 5B, passivation constraint  503  is added to constrain electrostrictive polymer membrane  405  from expanding in a horizontal direction upon actuation. FIG. 5B illustrates the squeezing and stretching of electrostrictive polymer membrane  405  when a voltage difference is applied between first compliant electrode  303  and second compliant electrode  305 . Instead of expanding horizontally as shown in FIG. 4B, the flexible properties of first compliant electrode  303  and second compliant electrode  305 , coupled with horizontal constraint provided by passivation constraint  503 , the layers are forced to buckle into a domed shape as depicted in FIG.  5 B. The action created by alternating between the powered off state in FIG.  5 A and the powered on state of FIG. 5B creates the actuating movement of electrostrictive polymer ink propulsion device  309  of FIG.  3 . 
     The cross-sectional view of a firing chamber  301  at line A—A of FIG. 3 is shown in FIG.  6 A. This view shows the relative positions of substrate  201 , passivation layer  501  and passivation constraint  503 , first compliant electrode  303 , electrostrictive polymer membrane  405 , second compliant electrode  305  and orifice layer  320 . The layering area common to first compliant electrode  303 , electrostrictive polymer membrane  405 , and second compliant electrode  305  defines electrostrictive polymer ink propulsion device  309 . FIG. 6A is an illustration of electrostrictive polymer ink propulsion device  309  in an unactuated state with firing chamber  301  filled with ink at rest within ink expulsion nozzle  107 . In the preferred embodiment of the invention, electrostrictive polymer ink propulsion device  309  is slightly curved in order to precamber or bias electrostrictive polymer ink propulsion device  309  to assure expulsion of the ink droplet in the direction of ink expulsion nozzle  107 . The ink stays within firing chamber  301  when unactuated due to surface tension at ink expulsion nozzle  107  and backpressure in the ink delivery system of ink reservoir  105 . FIG. 6B depicts electrostrictive polymer ink propulsion device  309  in an actuated state with the ink held within firing chamber  301  being forced out of ink expulsion nozzle  107  by the volume displacement in firing chamber  301 . This displacement is created by the actuating movement of the electrostrictive polymer ink propulsion device  309  buckling toward the ink expulsion nozzle  107  thereby creating and shooting ink droplet  617  onto the media beyond. 
     The cross-sectional view of firing chamber  301  at line B—B of FIG. 3 is shown in FIG.  7 . Ink channels  307  are excavated through substrate  201  on both sides of electrostrictive polymer ink propulsion device  309 . The ink chamber entrance  313  is of a size large enough to encompass both ink channels  307  and electrostrictive polymer ink propulsion device  309 . Ink is supplied to electrostrictive polymer ink propulsion device  309  from ink reservoir  105 . The ink flows through ink feed channels  307 , into ink firing chamber  301  and ultimately into ink expulsion nozzle  107  to await expulsion by electrostrictive polymer ink propulsion device  309 . Other embodiments of this system have been contemplated where orifice hole  107  and its associated ink nozzle  607  are located on a side wall of firing chamber  301  rather than the top wall, or ceiling, of firing chamber  301 . 
     FIG. 8A through 8H illustrate the steps to construct an electrostrictive polymer ink propulsion device  309  in the preferred embodiment of the invention. The fabrication of an electrostrictive polymer ink actuator for an inkjet printer pen may be performed on a scale small enough to create small pitch nozzle arrays using current photolithography patterning techniques. Another embodiment of the present invention fabricates an electrostrictive polymer ink actuator using thin film deposition and patterning techniques such as suggested in HP Journal, May 1985, pg. 27 or pg. 35; HP Journal, August 1988, pg. 28; and HP Journal, February 1994, page 41. FIG. 8A shows the initial step of spin coating a first layer of passivation constructing passivation layer  501  to a substrate  201 . The passivation layer is then patterned by application of a photo-chemically reactive resist, masking the desired shape, electromagnetic radiation exposure, and finally etching in the shape of the perimeter of electrostrictive polymer ink propulsion device  309  as depicted by FIG.  8 B. 
     Next, in FIG. 8C illustrates the preferred embodiment of the invention where a sacrificial photoresist bump  803  is formed in the area of the removed passivation shown in FIG.  8 B. Photoresist bump  803  is constructed by spinning on the photoresist material, patterning the material in the desired shape, then heating the photoresist material so that it reflows in a slightly “domed” shape. This shape is the foundation shape of the electrostrictive polymer ink propulsion device  309 . By forming photoresist bump  803  in a dome, when electrostrictive polymer ink propulsion device  309  is actuated, the domed shape will act as a bias, or precamber, that will promote the buckling and displacement (see FIG. 6A and 6B) to occur in the direction of ink expulsion nozzle  107 , in order to expel ink droplet  617  onto the media beyond. Other methods of biasing have been contemplated such as pre-stressing the layers of the electrostrictive polymer ink propulsion device  309 , inducing differing fluidic pressures on either side of the device, inducing differing horizontal compressive forces in each compliant electrode or patterning the surface of the substrate prior to the first layer. Each of these alternatives would encourage the electrostrictive polymer ink propulsion device  309  to buckle in the direction of least resistance, as opposed to an arbitrary direction. 
     In FIG. 8D, an electrically conductive first compliant electrode  303  is spun on atop and conforming to photoresist bump  803 . As illustrated in FIG. 3, first compliant electrode  303  is patterned in a strip that terminates in the shape of one half the exterior shape defined by electrostrictive polymer ink propulsion device  309 . In the preferred embodiment of the invention, this shape is a semicircle. The shaped end of first compliant electrode  303  is adjacent to passivation layer. FIG. 8E shows electrostrictive polymer membrane  405  constructed directly above photoresist bump  803  while first compliant electrode  303  is between electrostrictive polymer membrane  405  and photoresist bump  803 . Electrostrictive polymer membrane  405  is of approximately the same shape and size as photoresist bump  803 . 
     In FIG. 8F, passivation constraint  503  layer is deposited in a fashion similar to that used for passivation layer  501  and patterned to act as a mechanical constraint for electrostrictive polymer membrane  405  forcing it to buckle, rather than horizontally bulge, when deformed. In FIG. 8G, second compliant electrode  305  is layered atop electrostrictive polymer membrane  405  and terminated in the same shape as first compliant electrode  303 , covering electrostrictive polymer membrane  405 , but extending outward a direction opposite that of first compliant electrode  303  as illustrated in FIG.  3 . The overlapped layers of first compliant electrode  303 , and second compliant electrode  305  with electrostrictive polymer membrane  405  interposed between the two compliant electrodes, forms electrostrictive polymer ink propulsion device  309 . 
     In FIG. 8H, photoresist bump  803  is removed by excavating, for example by laser ablation, through substrate  201  and photoresist bump  803 , leaving the layers of first compliant electrode  303 , electrostrictive polymer membrane  405 , and second compliant electrode  305  free to move upon actuation. 
     In the preferred embodiment of the invention, electrostrictive polymer membrane  405 , first compliant electrode  303 , and second compliant electrode  305  are spin coated on silicon substrate  201  and patterned using conventional masking and etching technology. These electrodes are approximately 0.25 microns thick and approximately 40 microns in width. Passivation layer  501  and passivation constraint  503  are silicon nitride in the preferred embodiment and are approximately 0.5 microns thick. First compliant electrode  303  and second compliant electrode  305  are constructed from ultra-thin gold (100-200 Å) in the preferred embodiment; however, other materials such as carbon fibers and conductive rubber have been contemplated. The ideal electrode would be perfectly compliant and patternable, and could be made thin relative to the electrostrictive polymer membrane  405  thickness. 
     In the preferred embodiment, electrostrictive polymer membrane  405  is made from a silicone rubber approximately one micron thick and 40 microns in diameter with a Young&#39;s modulus of 0.7 Mpa and a dielectric constant of  10 . Acceptable variations of silicone rubber for electrostrictive polymer membrane  405  have a thickness of 0.25-2.1 microns, a diameter of 10-70 microns, a Young&#39;s modulus of 0.2-2.0 Mpa, and a dielectric constant of 1-14. 
     The technology of the present invention is comparable to piezoelectric transducers for use in ink drop propulsion. A voltage potential is applied to the actuator resulting in mechanical deformation. In principle it provides similar advantages as piezoelectric over thermal inkjet, such as no thermal cycling, control over drop size (more voltage=more deflection), higher ink independence and more repeatable performance. However, the disclosed invention provides an advantage over piezoelectric transducer in that these electrostrictive polymer materials can supply 30% strains as opposed to the piezoelectric strains of &lt;1%. 
     In the previously described drawings, a new method and apparatus for ink drop propulsion has been presented that has advantages over current thermal and piezoelectric technology. This invention eliminates thermal cycling used in current thermal inkjet propulsion systems, thereby eliminating unpredictable nucleation variations in the ink. Without concern for the unpredictable ink nucleation due to thermal cycling, flexibility in useable inks and repeatability of drop firing are increased, and the problem of thermal fatigue on thin films is no longer an issue.