Capping structures for acoustic printing

Abstract: A droplet ejector (10) for an acoustic printer has an acoustically thin capping structure (12) that permits accurate location of the free surface of a liquid ink (30) to enable acoustically induced ink droplet ejection, and that prevents the ink from spilling from its well. Acoustically thin implies that the capping structure thickness is a small fraction of the wavelength of the applied acoustic energy. One capping structure is a thin wafer of porous silicon placed over the aperture of an ink filled ink well (28). Acoustic radiation pressure pushes liquid ink from the well through the pores (36) so that a thin ink film forms over the capping structure. Another capping structure is a solid membrane placed over the ink well aperture. An ink deposition means (60,90) deposits a thin film of ink over the capping structure. With either structure, applied acoustic energy from a transducer (20) can pass through the capping structure to cause droplet ejection from the free surface of the ink film.

Description: The present invention relates to techniques for retaining a fluid within a cavity while permitting droplets to be acoustically ejected. Various ink jet printing technologies have been or are being developed. One such technology, referred to as acoustic ink printing (AIP), uses focused acoustic energy to eject droplets from the free surface of a marking fluid (for example liquid ink) onto a recording medium. For a more detailed description of acoustic ink printing, reference may be made to U.S. Patents 4,308,547, 4,697,195, and 5,028,937, and the citations therein. A basic concern in AIP printing is the spatial relationship between the acoustic energy's focal area and the free surface of the marking fluid. Current practice dictates that the acoustic focal area be located within about one wavelength (typically about 10 micrometers) of the free surface. While this is difficult to do reliably, various techniques have been developed for accomplishing this task. See, for example, U.S. Patent 5,028,937 which discusses the use of a perforated membrane to control the subject spatial relationship. However, these techniques may not be optimum with regards to manufacturability, cost, and performance. Compounding the difficulty of accurately positioning the free surface of the marking fluid is the necessity of simultaneously preventing the marking fluid from spilling from its well while still permitting droplet ejection. Thus, a technique that permits accurate control of the location of the free surface of a marking fluid, that prevents spilling, and that enables droplet ejection would be beneficial. The present invention provides a capping structure for an ink well having an opening, said capping structure comprised of an acoustically thin wafer dimensioned to overlie the opening. The thin wafer thereby retards ink from spilling from the ink well. The wafer, which may be silicon, may include a plurality of pores through the thickness of said wafer so as to enable ink to pass though said wafer. The pores may be formed by etching. By the term "acoustically thin" herein, it is implied that the capping structure thickness is a small fraction of the wavelength of the applied acoustic energy. The present invention further provides a droplet ejector comprising: a transducer for converting input electrical energy into acoustic energy; a well for marking fluid, the well having an opening; a capping structure according to any of claims 1 to 4, a pool of marking fluid being formed, in use, on said thin wafer; and means for focusing said acoustic energy through the wafer into a focal area in the said pool; wherein said droplet ejector is dimensioned so that the said focal area is near the free surface of the said pool. Preferably, the well comprises an ink well for holding ink so that said acoustic energy passes through said ink and out of said opening. The thin wafer may have a plurality of pores that enable ink to pass through said capping structure, said capping structure dimensioned and disposed to overlie said opening so that ink that passes through said pores forms an ink pool having a free surface over said opening. In one embodiment, the droplet ejector includes a body having front and back surfaces, the acoustic energy passing through said body from said back surface; wherein said focal area is at a predetermined position in front of said body; wherein the well is in the form of an ink container comprised of a front wall, a back wall, an interior wall that defines an aperture through said ink container, and an interior ink chamber for holding a liquid ink, said front wall containing a plurality of pores adjacent to said aperture, said pores for enabling ink in said ink chamber to pass through said front wall, said back wall of said ink container joined to said front surface of said body such that said acoustic energy focused by said focusing means passes through said aperture; wherein the capping structure is joined to said front wall of said ink container and the thin wafer has a plurality of pores adjacent said aperture for enabling ink to pass from said pores of said ink container through said capping structure to form a pool of ink having a free surface in front of said capping structure so that said free surface is in front of said aperture. In another embodiment, the droplet ejector includes a body having a front and a back, the acoustic energy passing through said body from said back and out said front; wherein said focal area is at a predetermined position in front of said body; an upper substrate having an aperture, said upper substrate joined to said body such that said aperture forms a cavity and such that said acoustic energy focused by said focusing means passes through said aperture; a volume of material filling said cavity; wherein the thin wafer has a front surface and a back surface, said back surface joined to said upper substrate such that said capping structure overlies and seals said cavity; wherein said well is in the form of an ink container comprised of a front wall, a back wall, an interior wall that defines an opening, and an interior ink chamber for holding a liquid ink, said back wall joined to said front surface of said capping structure so the said opening axially aligns with said cavity, said interior wall containing a plurality of pores for enabling ink in said ink chamber to pass into said opening to form a pool of ink having a free surface on said front surface; and means for locating said free surface near said focal area. In each case, the capping structure may be silicon, and the pores may be formed by etching. By way of example only, embodiments of the invention will be described with reference to the drawings, in which: FIG. 1 provides a simplified illustration of an acoustic droplet ejector that incorporates a first capping structure; FIG. 2 provides a simplified illustration of an acoustic droplet ejector that incorporates a second capping structure; and FIG. 3 provides a simplified illustration of an alternative acoustic droplet ejector that incorporates a third capping structure. In the drawings, like references designate like elements. Refer now to FIG. 1 where an acoustic droplet ejector 10 which incorporates a capping structure 12 is shown. The droplet ejector 10 includes a base 14 comprised of a 4" by 4" plate of 30 mil thick 7740 glass (pyrex) polished on both sides. To the back side 16 of the base is connected the front electrode 18 of a ZnO transducer 20. To generate acoustic energy, RF energy is applied to the ZnO transducer via the front electrode 18 and a gold plated back electrode 22. To the front side 24 of the base 14 is bonded an upper substrate 26 comprised of a 300 micron thick, 3" wafer of 〈100〉 silicon polished on both sides and having an etched aperture 28 formed therein. The upper substrate and the base form an ink well for ink 30 that is pumped into the aperture 28 via inlet and outlet ports (not shown). On the front surface 24 of the base 14, within the aperture 28, and axially aligned with the ZnO transducer 20 is an acoustic lens 32. The acoustic lens focuses acoustic energy that passes through the base 14 into a focal area which, as subsequently described, is located near the free surface of an ink pool. While a spherical acoustic lens could be used to focus the acoustic energy, a Fresnel lens is used in the droplet ejector of FIG. 1. The capping structure 12 attaches to the front surface 34 of the upper substrate 26 and is placed and dimensioned to completely overlie the front opening of the aperture 28. A plurality of pores 36 are formed width-wise through the capping structure. Since the capping structure 12 is silicon, the pores are beneficially formed using anisotropic etching techniques well known to those that specialize in fabricating microstructures in silicon. When acoustic energy is generated by the transducer 20, the acoustic radiation pressure forces the ink through the pores 36 to form a thin pool of ink 30 over the capping structure. The droplet ejector is dimensioned such that the acoustic focal area is located at, or adjacent to, the free surface 38 of the ink pool. Since a thin membrane, one much thinner than an acoustic wavelength, will move in the acoustic field almost in unison with the particles in the liquid, the acoustic energy readily passes through the capping structure. In the droplet ejector 10, the ratio of the acoustic wavelength to the thickness of the membrane, the capping structure, is equal to about 100. Thus, droplets are readily ejected from the free surface 38. When acoustic radiation stops, the ink seeps back through the pores 36. An alternative droplet ejector 50 that uses a capping structure 52 is shown in FIG. 2. The droplet ejector 50 is similar to the droplet ejector 10 of FIG. 1, with the differences being that (1) the capping structure 52, which does not have pores, replaces the porous capping structure 12; (2) an ink well 54 is added in front of the capping structure 52; and (3) the former cavity formed by the capping structure 12, the base 14, and the aperture 28 is now a sealed chamber 56. The ink well 54 includes an opening 57 formed by a side wall 58 and an internal chamber 59 containing ink 30 under pressure. The ink well 54 is positioned so that the opening 57 is located over the sealed chamber 56. A plurality of pores 60, which provide paths for ink to flow into the opening 57, are formed through the side wall 58 . In operation, the pressure forces the ink 30 through the pores 60 so as to create a thin pool of ink 30 on the capping structure over the sealed chamber 56. The droplet ejector 50 is dimensioned such that the free surface 38 of the ink pool is at or is near the acoustic focal area. Since the capping structure 52 is acoustically thin, acoustic energy can readily eject droplets from the ink pool. Another droplet ejector 80, which uses a capping structure 82, is illustrated in FIG. 3. The base 14, the transducer 20 (and its electrodes 18 and 22), and the acoustic lens 32 are the same as those illustrated in the previous embodiments. However, the solid upper substrate 26 in those embodiments is replaced by an ink container 84 filled with ink 30. The container 84 also includes an aperture 86. The aperture 86, whose location and function is analogous to the aperture 28 of FIG. 1, is formed by container walls 88. The capping structure 82 is placed above the container 84 such that it seals the aperture 86. Adjacent to the aperture 86 are a plurality of pores 90 that enable ink 30 to pass from the container 84 and through the capping structure 82 so to form a pool of ink 30 over the aperture 86. The droplet ejector 80 is dimensioned such that the free surface 38 of the pool of ink is at or is near the acoustic focal area. Since the capping structure 82 is acoustically thin, the acoustic energy from the transducer 20 can readily pass through the capping structure to eject droplets from the ink film. Although the above description refers to the use of a liquid ink as a marking fluid, it will be appreciated that other suitable marking fluids could be used. In Figs. 2 and 3 the sealed chamber 56,86 could contain a material other than the marking fluid and, in each of those droplet ejectors, the pores 60,90 in the marking fluid container 59,84 could be formed by etching. The droplet ejectors shown in the drawings (beneficially employed within a print head) have acoustically thin capping structures that permit accurate location of the free surface of a liquid ink, that permit acoustically induced ink droplet ejection, and that prevent the ink from spilling from its ink well. A first capping structure is an acoustically thin slab of porous silicon placed over the aperture of an ink filled ink well. In operation, acoustic radiation pressure pushes liquid ink through the pores so that a thin ink film forms over the capping structure. A second capping structure is a solid membrane placed over the aperture of an ink well which is in close proximity to an ink deposition means that deposits a thin film of ink over the capping structure. In either case, acoustic energy readily passes through the capping structure to eject a droplet from the free surface of the ink film.

Claims: A capping structure (12) for a well of marking fluid, the well having an opening, said capping structure comprising an acoustically thin wafer dimensioned to overlie the opening. A capping structure according to claim 1, wherein said wafer is silicon. A capping structure according to claim 1 or claim 2, wherein said wafer includes a plurality of pores (36) through the thickness of said wafer so as to enable marking fluid to pass through said wafer. A capping structure according to claim 3, wherein said pores are formed by etching. A droplet ejector (10) comprising: a transducer (20) for converting input electrical energy into acoustic energy; a well (28) for marking fluid, the well having an opening; a capping structure (12) according to any of claims 1 to 4, a pool of marking fluid being formed, in use, on said thin wafer; and means (32) for focusing said acoustic energy through the wafer into a focal area in the said pool; wherein said droplet ejector is dimensioned so that the said focal area is near the free surface of the said pool. A droplet ejector according to claim 5, wherein the focusing means is located in the well in axial alignment with the transducer. A droplet ejector according to claim 5 or claim 6, wherein the wafer is silicon. A droplet ejector according to any one of claims 5 to 7, wherein the wafer has a plurality of pores (36) through which marking fluid from the well can pass to form the said pool. A droplet ejector according to any one of claims 5 to 7, wherein the wafer seals the well and wherein a container (59, 84) for marking fluid is located adjacent the wafer, the container having a plurality of pores (60, 90) through which marking fluid from the container can pass to form the said pool. A droplet ejector according to claim 9, wherein the container defines an opening (57) aligned with the well (54), into which opening (57) the marking fluid from the container passes to form the said pool. A droplet ejector according to claim 9, wherein the well (86) is defined within the container and wherein the wafer also has a plurality of pores (90) through which marking fluid from the container can pass to form the said pool. A droplet ejector according to any one of claims 9 to 11, wherein the well contains a material other than marking fluid. A droplet ejector according to any one of claims 8 to 12, wherein the pores (36, 60, 90) are formed by etching.

Inventors: AKAMINE SHINYA, HADIMIOGLU BABUR B, KHURI-YAKUB BUTRUS T, QUATE CALVIN F, AKAMINE, SHINYA, HADIMIOGLU, BABUR B., KHURI-YAKUB, BUTRUS T., QUATE, CALVIN F.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: B41J2
IPC codes: B41J2