1. Field of the Invention
The present invention relates to an ink-jet printhead. More particularly, the present invention relates to a thermally driven monolithic ink-jet printhead in which a metal nozzle plate is formed integrally with a substrate and a manufacturing method thereof.
2. Description of the Related Art
Ink-jet printheads are devices for printing a predetermined color image by ejecting a small droplet of a printing ink at a desired position on a recording sheet. Ink-jet printheads are largely categorized into two types depending on the ink droplet ejection mechanisms: a thermally driven ink-jet printhead, in which a heat source is employed to form and expand bubbles in ink causing an ink droplet to be ejected, and a piezoelectrically driven ink-jet printhead, in which a piezoelectric crystal bends to exert pressure on ink causing an ink droplet to be expelled.
An ink ejection mechanism of the thermally driven ink-jet printhead will now be described in detail. When a current pulse flows through a heater consisting of a resistive heating material, heat is generated by the heater to rapidly heat ink near the heater to approximately 300° C. thereby causing the ink to boil and form bubbles. The formed bubbles expand to exert pressure on ink contained within an ink chamber. This pressure causes a droplet of ink to be ejected through a nozzle from the ink chamber.
A thermally driven ink-jet printhead can be further subdivided into top-shooting, side-shooting, and back-shooting types depending on the direction in which the ink droplet is ejected and the direction in which a bubble expands. While the top-shooting type refers to a mechanism in which an ink droplet is ejected in a direction the same as a direction in which a bubble expands, the back-shooting type is a mechanism in which an ink droplet is ejected in a direction opposite to a direction in which a bubble expands. In the side-shooting type, the direction of ink droplet ejection is perpendicular to the direction of bubble expansion.
Thermally driven ink-jet printheads need to meet the following conditions. First, a simple manufacturing process, low manufacturing cost, and mass production must be provided. Second, to produce high quality color images, the distance between adjacent nozzles must be as small as possible while still preventing cross-talk between the adjacent nozzles. More specifically, to increase the number of dots per inch (DPI), many nozzles must be arranged within a small area. Third, for high speed printing, a cycle beginning with ink ejection and ending with ink refill must be as short as possible. That is, the heated ink and heater should cool down quickly to increase an operating frequency.
FIG. 1A illustrates a partial cross-sectional perspective view showing a structure of a conventional thermally driven printhead. FIG. 1B illustrates a cross-sectional view of the printhead of FIG. 1A for explaining a process of ejecting an ink droplet.
Referring to FIGS. 1A and 1B, a conventional thermally driven ink-jet printhead includes a substrate 10, a barrier wall 14 disposed on the substrate 10 for defining an ink chamber 26 filled with ink 29, a heater 12 disposed in the ink chamber 26, and a nozzle plate 18, having a nozzle 16 for ejecting an ink droplet 29′. If a current pulse is supplied to the heater 12, the heater 12 generates heat to form a bubble 28 in the ink 29 within the ink chamber 26. The bubble 28 expands to exert pressure on the ink 29 present in the ink chamber 26, which causes an ink droplet 29′ to be expelled through the nozzle 16. Then, the ink 29 is introduced from a manifold 22 through an ink feed channel 24 to refill the ink chamber 26.
The process of manufacturing a conventional top-shooting type ink-jet printhead configured as above involves separately manufacturing the nozzle plate 18 equipped with the nozzle 16 and the substrate 10 having the ink chamber 26 and ink feed channel 24 formed thereon and bonding them to each other. These required steps complicate the manufacturing process and may cause a misalignment during the bonding of the nozzle plate 18 with the substrate 10. Furthermore, since the ink chamber 26, the ink channel 24, and the manifold 22 are arranged on the same plane, there is a restriction on increasing the number of nozzles 16 per unit area, i.e., the density of nozzles 16. This restriction makes it difficult to implement a high printing speed, high resolution ink-jet printhead.
Recently, in an effort to overcome the above problems of conventional ink-jet printheads, ink-jet printheads having a variety of structures have been proposed. FIGS. 2A and 2B illustrate a conventional monolithic ink-jet printhead. FIGS. 2A and 2B illustrate a plan view showing an example of a conventional monolithic ink-jet printhead and a vertical cross-sectional view taken along line A–A′ of FIG. 2A, respectively.
Referring to FIGS. 2A and 2B, a hemispherical ink chamber 32 and a manifold 36 are formed on a front surface, i.e., an upper surface, and a rear surface, i.e., a lower surface, of a silicon substrate 30, respectively, and an ink channel 34 connects the ink chamber 32 with the manifold 36 at a bottom of the ink chamber 32. A nozzle plate 40 comprised of a plurality of stacked material layers 41, 42, and 43 is formed integrally with the substrate 30. The nozzle plate 40 has a nozzle 47 at a location corresponding to a central portion of the ink chamber 32. A heater 45 connected to a conductor 46 is disposed around the nozzle 47. A nozzle guide 44 extends along an edge of the nozzle 47 toward the ink chamber 32. Heat generated by the heater 45 is transferred through an insulating layer 41 to ink 48 within the ink chamber 32. The ink 48 then boils to form bubbles 49. The created bubbles 49 expand to exert pressure on the ink 48 contained within the ink chamber 32, which causes an ink droplet 48′ to be expelled through the nozzle 47. Then, the ink 48 flows through the ink channel 34 from the manifold 36 due to surface tension of the ink 48 contacting the air to refill the ink chamber 32.
A conventional monolithic ink-jet printhead configured as above has an advantage in that the silicon substrate 30 is formed integrally with the nozzle plate 40 thereby simplifying the manufacturing process and eliminating the chance of misalignment. Another advantage is that the nozzle 47, the ink chamber 32, the ink channel 34, and the manifold 36 are arranged vertically to increase the density of nozzles 47 as compared with the ink-jet printhead of FIG. 1A.
In the conventional monolithic ink-jet printhead shown in FIGS. 2A and 2B, the material layers 41, 42, and 43 disposed around the heater 45 are made from low heat conductive insulating materials, such as an oxide or a nitride, to provide electrical insulation. Thus, a significant time must elapse for the heater 45, the ink 48 in the ink chamber 32, and the nozzle guide 44, all of which are heated for ejection of the ink 48, to sufficiently cool down and return to an initial state, thereby making it difficult to increase the operating frequency of the printhead to a sufficient level.
Another drawback of the conventional ink-jet printhead is that there is a restriction on the thickness of the material layers 41, 42, and 43 of the nozzle plate 40 since they are formed by a chemical vapor deposition (CVD) process. That is, since the nozzle plate 40 has a thickness as small as about 5 μm, it is difficult to secure a sufficient length of the nozzle 47. A small length of the nozzle 47 not only decreases the directionality of the ink droplet 48′ ejected but also prohibits stable high speed printing since a meniscus in the surface of the ink 48, which cannot be formed in the nozzle 47 after ejection of the ink droplet 48′, moves into the ink chamber 32. In an effort to solve these problems, the conventional ink-jet printhead has the nozzle guide 44 formed along the edge of the nozzle 47. However, if the nozzle guide 44 is too long, this not only makes it difficult to form the ink chamber 32 by etching the substrate 30 but also restricts expansion of the bubbles 49. Thus, use of the nozzle guide 44 causes a restriction on sufficiently providing the length of the nozzle 47.
Furthermore, in the conventional ink-jet printhead, an outlet of the nozzle 47 has a curved edge instead of a sharp edge. This shape decreases the ejection performance of the ink droplet 48′ and makes the outer surface of the nozzle plate 40 vulnerable to becoming wet with the ink 48.
FIGS. 3 and 4 illustrate alternate examples of conventional thermally driven ink-jet printheads. Referring to FIG. 3, heater elements 51 are located on a substrate 50, and a passivation layer 52 is formed over the heater elements 51. An ink chamber 53 defined by a barrier wall 54 is constructed on the substrate 50, on top of which is an orifice plate 56 having a plurality of orifices 57. An ink feed hole 55 for supplying ink to the ink chamber 53 is formed by penetrating the substrate 50. The ink-jet printhead configured above has an advantage in that it has an integrated overall structure by forming the barrier wall 54 and the orifice plate 56 by metallic plating. However, since the ink-jet printhead has the ink chamber 53 constructed atop the substrate 50 and defined by the barrier wall 54 and uses a top-shooting ejection mechanism by locating the heater elements 51 under the ink chamber 53, it is different from an ink-jet printhead according to the present invention, which will be described later, in terms of structure, ink ejection mechanism, and manufacturing method.
FIG. 4 illustrates a conventional orifice plate of an ink-jet printhead. Referring to FIG. 4, an orifice plate 60 has a composite structure comprised of two metal layers 61 and 62 and is bonded to a substrate having heater elements located thereon after separate manufacturing. Thus, it differs from a monolithic ink-jet printhead according to the present invention.