Acoustic ink printheads

Abstract: An acoustic ink printhead with an integrated liquid level control layer has a spacer layer (27) fixed to a substrate (20). Apertures (28A, 28B) are created in the spacer layer, which is then used as a mask, to define acoustic lenses (36A) and ink supply channels (36B) in the substrate. The apertures in the spacer layer define self-aligned acoustic lenses and form cavities to hold the ink reservoirs for each ejector. The thickness of the spacer layer is set so that acoustic waves from the acoustic lens below are focused at the free surface of the ink which maintains its level at the top of the spacer layer by capillary action.

Description: This invention relates to acoustic ink printing and, specifically, to an acoustic ink printhead with an integrated liquid level control layer and method of manufacture therefor. In acoustic ink printing, acoustic radiation by an ejector is used to eject individual droplets on demand from a free ink surface. Typically several ejectors are arranged in a linear or two-dimensional array in a printhead. The ejectors eject droplets at sufficient speed in a pattern for the ink droplets to be deposited on a nearby record medium in the shape of an image. A droplet ejector employing a concave acoustic focusing lenses is described in US-A-4,751,529. These acoustic ink ejectors are sensitive to variations of their free ink surface levels. The size and speed of the ink droplets which are ejected are difficult to control unless the free ink surfaces remain within the effective depth focus of their droplet ejectors. Thus the free ink surface level of such a printer should be closely controlled. To maintain the free ink surfaces at more or less constant levels, various approaches have been proposed. One approach is the use of a closed loop servo system for increasing and decreasing the level of the free-ink surface under the control of an error signal which is produced by comparing the output voltage levels from the upper and lower halves of a split photo-detector. The magnitude and sense of that error signal are correlated with the free ink surface level by the reflection of a laser beam off the free ink surface to symmetrically or asymmetrically illuminate the opposed halves of the photo- detector depending upon whether the free ink surface is at a pre-determined level or not. This approach is somewhat costly to implement and requires that provision be made for maintaining the laser and the split photo-detector in precise optical alignment. Moreover, it is not well-suited for use with larger ejector arrays because the surface tension of the ink tends to cause the level of the free surface to vary materially when the free surface spans a large area. Therefore alternative approaches for controlling the ink levels of the free surface for the ejectors are desirable. The present invention provides for such an alternative approach. The present invention provides for an integrated acoustic ink printhead with liquid level control. The acoustic printhead has a substrate with an array of ejectors. Each ejector has a substrate surface area capable of radiating a free surface of ink with focused acoustic radiation to eject individual droplets of ink on demand, and the acoustic focal length of each ejector is approximately equal to the acoustic focal lengths of other ejectors. A plurality of channels in the substrate communicate with the substrate surface areas of the ejectors to supply ink thereto. Fixed to the substrate is a spacer layer with a first surface in contact with the substrate and a second surface remote from the first surface. The spacer layer has a predetermined thickness approximately equal to the difference between the ejector acoustic focal length and the radius of the acoustic lens. The spacer layer also has a first set of apertures through the spacer layer, each first aperture being aligned with one of the ejector substrate surface areas, and a second set of apertures through the spacer layer, each second aperture being aligned with one of the substrate ink supply channels. Thus the first set of apertures in the spacer layer form a control for the level of the free ink surface above each ejector substrate surface. The method of fabricating the integrated acoustic printhead comprises placing the spacer layer in fixed contact with the substrate. First and second sets of apertures are formed through the spacer layer. The first set of apertures is placed in locations corresponding to the locations of the ejectors on the substrate surface. The location of the second set of apertures corresponds to the location of ink supply channels for the ejectors. The ejectors and the ink supply channels are etched in the substrate with the spacer layer and its apertures serving as a mask. Thus the apertures are self-aligned with the ejectors. The first set of apertures in the spacer layer form a control for the level of the ink above the ejector substrate surface. The present invention will now be described by way of example with reference to the following drawings in which: Fig. 1 is a cross-sectional view of a known acoustic ink ejector, and Figs. 2-8 show the steps in manufacturing an ejector according to the present invention. Fig. 1 shows a known ejector of a printhead for an acoustic ink printer. In all the drawings, including Fig. 1, only a single ejector is shown. Typically the ejector is part of a closely spaced array, either linear or two- dimensional, in a substrate. During the printing operation, a record medium, such as paper, is moved relative to and above the ejector array. The drawings are not necessarily drawn to scale but facilitate an understanding of the present invention. The ejector is formed by part of a substrate 10, a concave surface 14 on the top surface 11 of the substrate 10 and a piezoelectric transducer 13 attached to the back surface 12 of the substrate 10. The spherically concave surface 14 is the microlens described in US-A-4,751,529 mentioned above. The surface 14 has a radius of curvature R centered about a point lying in the plane of the top surface 11 of the substrate 10. The ejector is covered by a pool of liquid ink 15 with a free surface 16. Under the influence of electric pulses the piezoelectric transducer 13 generates planar acoustic waves 18 which travel in the substrate 10 toward the top surface 11. The waves 18 have a much higher speed in the substrate 10 than in the ink 15. Typically, the ink 15 has an acoustic speed of about 1 to 2 kilometers per second, while the substrate 10 has a velocity of 2.5 to 4 times the speed of sound in the ink. When the waves 18 reach the substrate top surface 11, they are focused at or near the free ink surface 16 by the concave surface 14. The acoustic waves 18 are concentrated as they travel through the ink 15. If sufficiently intense, the focused acoustic energy can drive a droplet of ink 17 from the surface 16 to impact a record medium (not shown) to complete the printing process. As described above, it is important that the level of the free surface be maintained in proper position so that the acoustic waves are focused on the surface. Otherwise, the acoustic energy is not efficiently utilized, the uniformity and speed of the ejected droplets become varied and the print quality deteriorates. The present invention provides for an acoustic ink printhead in which the acoustic lens and liquid level control layer of each ejector are integrated and precisely positioned. Control of the free surface level is provided by a spacer layer which is fixed to the substrate according to the present invention. Aligned with the ejectors in the substrate, apertures in the spacer layer provide a space for a pool of ink for each ejector. Capillary action of the ink meniscus, the free surface, causes the free surface to maintain itself at the top surface of the spacer layer. While the apertures are small enough to maintain the level of the ink surface by capillary action, the apertures are large enough so that the focused waist diameters of the acoustic waves from the aligned ejectors below are substantially smaller than the diameters of the apertures. The apertures have no material effect upon the size or speed of the ejected droplets. Fig. 2-8 illustrates the steps of making such an integrated acoustic printhead. Fig. 2 shows a substrate 21 which may be made of silicon, alumina, sapphire, fused quartz and certain glasses. The upper surface 21 of the substrate 20 is covered by a spacer layer 27 of any suitable material, such as silicon, amorphous silicon or glass, but which is different from that of the substrate 20. The spacer layer 27 may be placed on the substrate surface 21 by any conventional technique, such as thin film deposition, epitaxial growth, plating or anodic bonding techniques. The spacer layer 27 has a thickness H given by H = R[1/(1-V ink /V subs )-1] where R, typically 150 µm, is the radius of the spherically concave lens, and V ink and V subs are the acoustic speeds in ink and substrate respectively. The thickness H, typically 35 µm, of the spacer layer 27 is such that the acoustic waves are focused at distance it from the top surface 21 of the substrate 20. Stated differently, the thickness of the spacer layer 27 is such the distance from acoustic lens to the top of the spacer layer is approximately equal to the acoustic focal length of the lens. During operation of the acoustic printhead, the free surface of the ink is maintained at the top of spacer layer 27. To define features in the spacer layer 27 and the underlying substrate 20, a photoresist layer 29 is deposited over the spacer layer 27. By standard photolithographic techniques, apertures are defined in the spacer layer 27 as illustrated in Fig. 3A. Initial aperture 28A, in the shape of a circle, is used for the etching of the acoustic lenses in the substrate 20. Because the acoustic lens of each substrate is ideally a spherically concave surface, the aperture 28A should be small, so as to appear as a point source for an isotropic etch through the aperture 28A into the substrate 20. However, the initial aperture 28A cannot be so small that the aperture interferes with the movement of etchant and etched material through the aperture 28A. Thus the initial diameter of the aperture 28A should be approximately 75 µm, about 25% of the final diameter of the aperture 38. Apertures 28B are the etching aperture masks for the ink supply channels in the substrate 20. Fig. 3B is a top view of this stage of the manufacture. As can be seen from the drawing, each circular aperture 28A is part of a linear array parallel with the longitudinal apertures 28B for the ink supply channels for the ejectors in the printhead. The apertures 28B for the ink supply channels are spaced 2L apart with the apertures 28A centered between. The parameter L, approximately 250 µm, is chosen such that upon the completion of the etching for the ink supply channels and acoustic lenses in the substrate 20, the ink supply channels and acoustic lenses are connected. The substrate 20 is isotropically etched, with the spacer layer 27 and photoresist layer 29 used as masks in the etching operation. Fig. 4 illustrates the beginnings of cavities 26A and 26B in the substrate 20. The cavity 26A is the start of the concave-surfaced microlens of the ejector. The cavities 26B form the beginnings of the cylindrically- shaped bottoms of the ink supply channels which interconnect the ejectors of the completed printhead. The result of the etching operation is shown in Fig. 5. The ink supply channels, the cavities 36B, are now in communication with the ink reservoir, the cavity 36A, above the spherically concave surface 39 (with radius of curvature R) of the ejector microlens (with acoustic focal length F). A second etching operation with a new photoresist layer 41, using an etchant which specifically removes the exposed spacer material and not the material of the substrate 20, is then performed. The operation opens the initial aperture 28A in the spacer layer 27 to the final aperture 38 and its full size of 0.1 mm in diameter. Such an etching operation again relies on the fact that the material of the substrate 20 is different from the material of the spacer layer 27 so that only the spacer layer 27 material is removed, as shown in Fig. 5. Thus the final aperture 38 in the spacer layer 27 is self-aligned with the microlens, the concave surface 39 in the substrate 20. The photoresist layer 29 is then removed and as illustrated in Fig. 6, a sealing layer 31 is deposited over the substrate 20 and spacer layer 27. With another masking and etching operation, all of the material of the layer 31 is removed except that covering the apertures 28B. Thus, the ink supply channels are sealed. Typically, this layer 31 is formed by bonding a thin plate to the spacer layer 27, then etching away the undesired portion. Alternatively, the thin plate may be etched first and then bonded to the spacer layer 27. This is possible since the alignment between the plate and the spacer layer 27 is not particularly critical. If desired, an optional layer 30 may then deposited over the substrate 20, the spacer layer 27 and the sealing layer 31. This material, which can be silicon nitride, silicon dioxide or other material, is deposited by conventional techniques, such as sputtering, evaporation and chemical vapor deposition. The material should be different from the material of the spacer layer 27. Ideally the optional layer 30 should be more hydrophobic than the spacer layer 27. Note the word "hydrophobic" is used here with the presumption that the ink is water-based. "Hydrophobic" also has the meaning of ink-repellant in the more general sense. The layer 30 keeps the ink surface at the top surface height of the spacer layer 27. The hydrophobic layer 30 helps keep the top of the layer 30 from becoming wet and thereby drawing the ink surface up to a new level and out of focus of the acoustic beam. To help maintain the ink surface at this level, the spacer layer 27 may be cut back as shown by the dotted lines 32 in Fig.7 by an etchant specific to the spacer layer material. The ejector is completed by attaching a piezoelectric transducer on the bottom surface of the substrate 20. Of course, the piezoelectric transducer is aligned with the ejector cavity 26A and aperture 28A. Fig. 8 is a side view of the completed ejector which is more true to scale. With appropriate changes, some of the fabrication steps may be reversed in order. Furthermore, while exemplary dimensions and parameters have been disclosed, other dimensions and parameters may be used for particular operational characteristics as desired.

Claims: A method of fabricating an integrated acoustic ink printhead with liquid level control, the printhead having an array of ink droplet ejectors (24) in a substrate (20), each ejector including a surface (39) of the substrate capable of radiating focused acoustic radiation from within a body of ink towards the free surface thereof to eject individual droplets of ink from it on demand, each ejector having an acoustic focal length approximately equal to the acoustic focal lengths of each other ejector, the method comprising the steps of: placing a layer (27) of spacer material in intimate contact with the substrate (20), the layer having a thickness such that its outer surface is spaced from the ejector substrate surface (39) by a distance approximately equal to the focal length of the ejector; forming first (28A) and second (28B) sets of apertures through the spacer layer, with the location of the first set of apertures corresponding to the location of the ejectors on tie substrate surface, and with the location of the second set of apertures corresponding to the location of ink supply channels for the ejectors, and defining the ejectors and the ink supply channels in the substrate with the spacer layer and its apertures as a mask, whereby the first set of apertures is aligned with the ejectors to form a control for the level of ink above each ejector substrate surface. A method as claimed in claim 1, including the step of etching the first set of apertures larger after the ejector-defining step. A method as claimed in claim 1 or 2, in which the ejector-defining step includes forming the substrate surface area for each ejector by etching a top surface of the substrate into a concave spherical surface (36A) having a radius of curvature (R). A method as claimed in any preceding claim, in which the spacer layer has a thickness given by the formula: H = R[1/(1-V ink /V subs ) -1], where H is the thickness, and V ink and V subs are the speeds of sound in the ink and the substrate respectively. A method as claimed in any preceding claim, including the further step of depositing hydrophobic material on the spacer layer so that it extends around the peripheries of the first set of apertures. A method as claimed in claim 5, in which the deposited material is more hydrophobic than the material of the spacer layer. A method as claimed in claims 2 and 5, and any claim dependent therefrom, in which the first set of apertures is etched larger after the hydrophobic material deposition step. An integrated acoustic ink printhead with liquid level control, comprising: a substrate (20) having in one surface an array of ink droplet ejectors (24), each ejector having a concave surface capable of directing focused acoustic radiation from within a body of ink towards the free surface thereof to eject individual droplets of ink on demand, with all the ejectors having approximately the same acoustic focal length, the substrate (20) also having within it channels (36B) for supplying ink to the ejectors, and a layer (27) of spacer material in intimate contact with the substrate and being of a thickness approximately equal to the difference between the ejector acoustic focal length and the radius of the acoustic lens, the space layer having through it a first set of apertures (28A), of which each is aligned with one of the ejectors, and a second set of apertures (28B) through it of which each is aligned with an ink supply channel, whereby the first set of apertures forms a control for the level of the free surface of the ink above each ejector substrate surface. A printhead as claimed in claim 8, in which the material of the spacer layer is different from the material of the substrate.

Inventors: HADIMIOGLU BABUR B, KHURI-YAKUB BUTRUS T, HADIMIOGLU, BABUR B., KHURI-YAKUB, BUTRUS T.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1992
Main classification: B41J2
IPC codes: B41J2