Inkjet printers use a printhead that includes an array of orifices through which ink is ejected on to paper or other print media. Ink filled channels, supplied from a reservoir, feed ink to a firing chamber at each orifice. In a piezoelectric type inkjet printhead, the deformation of a piezoelectric element coupled to one wall of the firing chamber alternately contracts and expands the volume of the firing chamber. During contraction, pressure in the chamber increases and ink is expelled from the chamber through the orifice. During expansion, pressure in the chamber decreases and ink refills the chamber through the channels from the reservoir(s), allowing for repetition of the ink expulsion sequence. One challenge in designing printheads with more dense orifice arrays and correspondingly smaller firing chamber dimension(s) is generating sufficient pressure differentials within the chamber volume to sustain adequate ink expulsion and refill. Thus, it may be desirable in some printhead designs to maximize the volume change in the firing chamber achieved by each deformation of the piezoelectric elements.
Embodiments of the present disclosure were developed in an effort to maximize the volume change in a piezoelectric inkjet printhead firing chamber induced by the piezoelectric actuator, thus facilitating the design of printheads with more dense orifice arrays and correspondingly smaller firing chamber dimension(s) while still generating sufficient pressure differentials within the chamber volume to sustain adequate ink expulsion and refill. Embodiments of the disclosure, therefore, will be described with reference to a piezoelectric inkjet ejector structure. Embodiments, however, are not limited to inkjet ejector structures, but may be implemented in other piezoelectric fluid ejector structures. Hence, the following description should not be construed to limit the scope of the disclosure.
Referring to
Ejection orifices 18 are formed in the exposed face 30 of cap 26. Cap 26, which is commonly referred to as an “orifice plate” or a “nozzle plate,” is usually formed in a silicon or metal sheet, although other suitable materials or configurations may be used. Membrane 24 may be formed, for example, on the underlying structure as a comparatively thin oxide layer. As an alternative to the “face shooter” shown in the figures, in which ejection orifices 18 are formed in face 30 of orifice plate 26, a so-called “edge shooter” could be used in which ink ejection orifices 18 are formed in an exposed edge 32 of orifice plate 26. Also, although the elements of only a single ejector structure 14 are shown and described in detail, the components of many such ejector structures 14 are typically formed simultaneously on a single wafer or on continuous sheets of substrate materials, along with the associated drive and control circuitry, and individual printhead dies 10 (
With continued reference to
Piezoelectric plates 34 are coupled to chamber membrane 24 through a flexible backing 60, a rigid post 62, and a rigid pusher plate 64. (For clarity, only piezoelectric plates 34 and post 62 are shown in the plan view of
A single elongated post 62 interposed between backing 60 and pusher 64 extends laterally across chamber 16 at free ends 46 of cantilever piezoelectric plates 34 such that post 62 transmits the movement of plates 34 toward chamber 16 to pusher plate 64 along a line extending laterally across chamber 16. For the bending cantilever plates 34 shown in
Other configurations are possible. For example, a series of discrete transmission posts 62 extending laterally across chamber 16 at cantilever ends 46 may provide a suitable alternative to a single elongated post 62 for some applications. For another example, where a smaller displacement of membrane 24 (and a corresponding smaller volume change in firing chamber 16) is desired, a narrower transmission post 62 and/or a less expansive pusher plate 64 may be appropriate. If the expanse of pusher 64 is too great, extending too close to the perimeter of membrane 24, the strain at the perimeter of membrane 24 may be large enough to cause a material failure in membrane 24. On the other hand, shrinking the expanse of pusher 64 away from the perimeter of membrane 24 reduces the displacement of membrane 24 and the corresponding volume and pressure changes in chamber 16. Also, the relatively larger uncovered perimeter area of membrane 24 acts as a compliance to absorb the fluid displaced above pusher 64. For a thin film membrane 24 on the order of 1 μm thick, such as might be used in a piezoelectric ejector structure 14, the strain in membrane 24 should be kept below a few percent to prevent fatigue failure. Thus, the thickness and perimeter area of membrane 24 not covered by pusher 64 should be selected to keep the strain in membrane 24 below the fatigue threshold while ensuring the compliance is not large enough to diminish the pressure in chamber 16.
“Flexible” and “rigid” as used herein are relative terms whose characteristics are determined in the context of the scale of deformation and movement in the elements of actuator 28 and in membrane 24. Although the actual scale may vary depending on the particular fluid ejector application or environment, it is expected that for a typical inkjet printing application for a ejector structure 14, the movement of the free end 46 of plates 34 will be on the order of tenths of a micro meter, μm (10−7 m) and the displaced volume of firing chamber 14 on the order of pico liters, pl (10−12l). Thus, it is desirable that backing 60 and membrane 24 are sufficiently flexible for micro meter displacements to allow comparatively free movement of piezoelectric plates 34 without comprising structural integrity. Similarly, post 62 and pusher 64 are sufficiently rigid to transmit fully, or substantially fully, micro meter movement of piezoelectric plates 34. It is expected that piezoelectric plates 34 and backing 60 will usually be configured to have comparable flexibility/stiffness to help ensure sufficient bending in cantilevers 65 in response to deformation of plates 34. The desired degree of flexibility and rigidity may be achieved, for example, through the relative thicknesses of the elements and/or the characteristics of the material used to form those elements.
Piezoelectric plates 34 may be formed, for example, from a high density type 5A or 5H piezoceramic material commercially available from a variety of sources. Backing 60 may be formed, for example, as a layer of silicon oxynitride or another dielectric material with suitable material properties that can be deposited uniformly at low temperature. To help match material stress characteristics and reduce interface constraints, it may be desirable to form post 62 and pusher 64 from the same material, polysilicon for example, or another suitably rigid material. Where the same materials are used, the thickness of each layer may be adjusted to develop the desired performance characteristics for the part. In any event, since the bending stiffness (rigidity) of post 62 and pusher 64 is a cubic function of thickness, thickness has a comparatively greater influence on the bending stiffness of each part. Backing 60, post 62 and pusher 64 may be prefabricated as a thin film stack that is glued to plates 34, for example, or backing, post and pusher layers may be deposited over piezoelectric plates 34 and selectively removed (patterned and etched for example) to form the desired backing 60, post 62 and pusher 64 structures. Also, although post 62 and pusher 64 are depicted as rectilinear structures, other shapes may be possible.
In one example configuration, a rectangular firing chamber 16 approximately 1 mm (1,000 λm) long and 70 μm wide enables an array density of about 300 orifices per inch. For a chamber depth of 30 μm, a volume change in firing chamber 16 on the order of 5-10 pl expels an ink drop through orifice 18. It is expected that the desired volume change in chamber 16 may achieved, for example, with 10 volts applied to piezoelectric plates 34 using a polysilicon post 62 about 0.5 μm thick and a polysilicon plate 64 about 3.0 μm thick where plate 64 covers approximately 80% of the area of membrane 24 within chamber 16. Thus, in the above noted chamber configuration, a 56 μm×984 μm rectangular plate 64 covers 79% of the 70 μm×1,000 μm rectangular membrane 24 (leaving an 8 μm perimeter of membrane 24 surrounding plate 64). Further, in this example, a 3.0 μm silicon oxynitride backing 60 covers 10 μm thick piezoelectric ceramic plates 34. Metal electrodes 52 and 54 typically will be 0.1 μm thick. Gap 51 should be deep enough to minimize or eliminate “squeeze film” damping by the air in gap 51. Gap 51 should also be large enough to dilute water vapor out gassed from chamber 16, keeping the vapor pressure low in gap 51, to help prevent water vapor permeating piezoelectric plates 34. Thus, for a typical configuration for ejector structure 14 such as that described above, gap 51 should be at least 10 μm deep and, if possible, more than 100 μm deep.
The use of multiple piezoelectric elements means that shorter piezoelectric elements running at higher vibration frequencies, in the range of 1 MHz for example, may be used without regard to the length of the firing chamber since more (or fewer) elements may be incorporated into the piezoelectric actuator for each chamber to achieve both the required volume change and the desired operating frequency. Also, each piezoelectric element is operatively coupled to the chamber membrane by a rigid transmission structure. Thus, the displacement of the piezoelectric element (due to bending or other modes) is transmitted to the chamber membrane in a rigid, or substantially rigid, piston-like manner that helps maximize displacement of the membrane and the corresponding volume change in the firing chamber. This combination of features facilitates the design of piezoelectric printheads with more dense orifice arrays and correspondingly smaller firing chamber dimension(s) while still generating sufficient pressure differentials within the chamber volume to sustain adequate ink expulsion and refill.
As used in this document, no limitation on aspect ratio is intended for a “plate.” A “plate” may range from being long and narrow (an aspect ratio much greater or much smaller than 1) to short and wide (an aspect ratio about 1). Also, a “plate” as used herein may be rectilinear (e.g., a rectangle) or curvilinear (e.g., a circle).
No directional limitation is intended from the use of “up” and “down” and other terms indicating directional orientation. Such terms are used herein for convenience only based on the orientation depicted in the figures. The actual orientation may be different from that depicted in the figures. Also, as used in this document, forming one part “over” or “overlaying” or “covering” another part does not necessarily mean forming one part above the other part. A first part formed over, overlaying or covering a second part will mean the first part formed above, below and/or to the side of the second part depending on the orientation of the parts. Also, “over” or “overlaying” or “covering” includes forming a first part on a second part or forming the first part above, below or to the side of the second part with one or more other parts in between the first part and the second part.
As noted at the beginning of this Description, the example embodiments shown in the figures and described above illustrate but do not limit the disclosure. Other forms, details, and embodiments may be made and implemented. Therefore, the foregoing description should not be construed to limit the scope of the disclosure, which is defined in the following claims.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US09/31440 | 1/20/2009 | WO | 00 | 6/9/2011 |