Ink jet pen with a heater element having a contoured surface

Abstract: A thermal ink jet printhead (10) with a body (12) having an ink firing chamber (22) and an orifice (26). An electrically activated heating element (16) is connected to the body in thermal communication with the firing chamber, and includes a contoured surface portion coextensive with at least a portion of the heating element. The contoured surface portion of the firing chamber has a plurality of recesses (42).

Description: Field of the Invention This invention relates to thermal ink jet printing, and more particularly to heater elements for thermal ink jet printheads. Background and Summary of the Invention Ink jet 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 ink - jet printer typically includes a printhead having a multitude of independently addressable firing units. Each firing unit includes an ink chamber connected to a common ink source, and an ink outlet nozzle or orifice. A transducer within the chamber provides the impetus for expelling ink droplets through the nozzles. In thermal ink jet pens, the transducer is a resistive heater element that provides sufficient heat to rapidly vaporize a small portion of ink within the chamber, forming a bubble. The bubble displaces a droplet of liquid ink from the nozzle. For uniform and precise printer output, it is desirable that the timing, magnitude, rate, shape, and position of the bubble formation be as uniform as possible. Uniformity is desired from firing unit to firing unit, and between sequential droplets originating from the same nozzle. A particular uniformity concern relates to the boiling properties of fluids. Heterogeneous nucleate boiling, or bubble nucleation, normally occurs at defect sites on the surface of a heating element, or other heated surface. These defects may be cracks, discontinuities, and edges and vertexes where surfaces meet at angle. Heterogeneous nucleate boiling occurs more readily than homogenous or film boiling, which occurs after additional heat energy is added when sufficiently sized nucleation sites are not present. Therefore, it is the heterogeneous nucleation that has the greatest effect during the rapid and uniformity-sensitive boiling process that occurs during thermal ink jet printing. Existing thermal inkjet printheads have at least partially controlled the heterogeneous nucleate boiling process by providing each firing chamber with a heating element shaped with a single small recessed basin having sharp edges that provide nucleation sites. The basin is smaller than the respective orifice, and registered therewith so that all potential nucleation sites are directly below an open portion of the orifice. This avoids the risk that some firing chambers may lack any nucleation sites and require a higher energy to achieve homogeneous nucleation. The deliberate positioning of the sites in registration with the orifices also reduces the chance that an unintended defect offset from the centerline will generate off axis droplet ejection. However, these improved systems have not achieved ideal uniformity of performance. EP 638 424A discloses a thermal ink jet printhead and method of manufacture in which an ink feed opening in a thin film substrate is axially aligned with a central opening in a heater resistor disposed on the substrate. The ink feed opening is also aligned with a firing chamber. The uniformity disadvantages of prior art systems are reduced or overcome by providing a thermal ink jet as claimed in the appendent claims. The ink jet includes a body having an ink firing chamber and an orifice. An electrically activated heating element is connected to the body in thermal communication with the firing chamber, and includes a contoured surface portion coextensive with at least a portion of the heating element. The contoured surface portion of the firing chamber has a plurality of recesses. Brief Description of the Drawings Figure 1 is a sectional isometric view of a thermal ink jet printhead according to a preferred embodiment of the invention. Figure 2 is a sectional side view of the embodiment of Fig. 1 taken along line 2-2. Figure 3 is an enlarged sectional side view of the embodiment of Fig. 1 taken along line 2-2. Figure 4 is a sectional view of a first alternative embodiment of the invention. Figure 5 is a sectional view of a second alternative embodiment of the invention. Figure 6 is a sectional view of a third alternative embodiment of the invention. Figure 7 is a sectional view of a fourth alternative embodiment of the invention. Figure 8 is a sectional view of a fifth alternative embodiment of the invention. Figures 9A-9D illustrate a sequence of operation of a prior art apparatus. Figures 10A-10D illustrate a sequence of operation of the embodiment of Fig. 1. Detailed Description of a Preferred Embodiment Figure 1 illustrates a thermal ink jet printhead 10 having a rigid planar substrate 12 with a flat upper surface 14. A planar heater element 16 is applied to the upper surface of the substrate. A barrier layer 20 is applied to the substrate surrounding the heater element 16 to define a firing chamber 22 centered above the heater element. An orifice plate 24 is connected to the upper surface of the barrier layer to enclose the firing chamber, and defines an orifice 26 centered above the heater element. The firing chamber has at least one lateral opening 30 on at least one side to provide an inlet for ink supplied by an ink supply plenum 32. The printhead includes an elongated array of adjacent firing units having the same arrangement of heater element, chamber, and orifice, and supplied by the common plenum. As shown in Fig. 2, the heater element 16 is connected between a pair of conductive aluminum leads 34 that overlay the upper surface 14 of the substrate 12, and which are connected to a powered controller (not shown) that applies a voltage across the heater element when a droplet of ink is required to be expelled from the firing unit. As shown in Fig. 3, the heater element 16 has a flat lower surface 36 and a parallel flat upper surface 14. The heater element is a stack of three layers. The lowest layer is a TaAl resistor film 37 resting on the substrate's upper surface 40. An electrically insulative passivation layer 38 overlays the resistor, and a mechanically protective tantalum cavitation barrier 39 overlays the passivation layer. The cavitation barrier 39 protects the resistor from the stresses of bubble formation and collapse during printing. A plurality of separate square recesses 42 are defined in an array that extends across most of the surface of the heater element 16. The recesses have flat bottoms or floors 44 parallel to the upper and lower surfaces of the heater element, and extend to a limited depth so that the passivation layer 38 is not exposed within the recesses; in alternative embodiments, the passivation layer may be exposed. The periphery of each recess is defined by a vertical side wall 46 that provides a step between the level of the recess floors 44 and the upper surface 40 of the heater element. The side wall 46 meets the recess floor 44 at a sharp corner or interior edge 50, and meets the upper surface 40 at a rim edge 52. Both edges are sharp right angles, although variations will be discussed below. In practice, the sharp edges are slightly radiused due to inherent limitations of the etching process. These sharp edges provide nucleation sites 53 where boiling will tend to occur most rapidly, and at the lowest energy. In addition, the reduced thickness at the floor of a recess may provide a higher heat due to its proximity to the resistor and the reduced thermal gradient across the cavitation barrier 39 to further expedite boiling at the lower nucleation sites 50. In the preferred embodiment the recesses 42 are arranged on a grid, with individual recesses positioned at alternate locations in the manner of a checkerboard. The recesses each have a width and length less than or equal to the pitch of the grid on which they are arranged, such that they are spaced apart at their comers to avoid intersecting with the comers of adjacent recesses. The etching process used to form the recesses after application of the heater element to the substrate yields recesses with rounded corners as viewed from above, providing separation even between recesses arranged on a grid having the same pitch as the width of the recesses. In the preferred embodiment, the recesses have a width of between 5 and 10 µm, although the advantages of the invention will be realized as further miniaturization becomes practical. In the illustrated embodiment, 13 recesses are provided, although more or fewer may be provided in other arrangements Figures 4-8 illustrate alternative heater element surface contour patterns. Figure 4 shows an alternative printhead with a heater element having a pattern of concentric ring-shaped recesses 56, each having a cross section similar to the recesses 42 of the preferred embodiment. The outer rings are large, and each ring functions as many recesses to provide many nucleation sites by having a substantial length of edges 50, 52 for a given major area of the heater element. It is also possible modify the illustrated pattern to form a spiral shape having similar recess and land widths over the same area. Although not literally a plurality of recesses, such a recess is considered a "plurality" for the purposes of this application because of the high ratio of edges 50, 52 to the overall area and linear dimensions of the heater element and of the recessed area. For the purpose of this application, a single "recess" or recess segment shall be defined as a basin, pocket, channel, groove, or segment thereof having edges surrounding more then half its periphery. Thus, an elongated channel may be considered as segmented into multiple individual recesses, each having a length only slightly longer than its width. As shown in Fig. 5, an alternative embodiment heater element has a rectangular array of separate square plateaus 60 defined by a grid of parallel channels 62. As noted above with respect to Fig. 4, the grid of channels is considered to be a plurality of recesses, because the edges of the plateaus have a cumulative length much greater than the cumulative length of the edges of a single regular recess such as a square, circle, oblong, or similar simple shape of the same area. While the preferred embodiment is discussed in terms of recesses having perpendicular side walls and a rectangular profile, Figs. 6, 7, and 8 illustrate alternative channel profiles. In Fig. 6, the side walls 64 are undercut, such that they form an acute angle with respect to the flat floor 44. An acute lower nucleation site 66 is positioned at least partly below the side wall for enhanced nucleation. Side walls may be effectively offset from the perpendicular by any amount, including obtuse angles. Figure 7 shows a variation on the undercut of Fig. 6 in which a conventional etching process yields a channel 70 with a curved or elliptical profile, at least at the edges. The nucleation sites 72 are located similarly to those in Fig 6, although the illustrated etching technique may provide more acute upper edges 52 to further favor nucleation. Fig. 8 illustrates a further alternative embodiment in which the heater element surface 40 is primarily a flat plane, with an array of protruding ridges 74 spaced apart across the surface. The ridges of the Fig. 8 embodiment and the recesses of the Fig. 5 and 6 embodiments may all be formed in any of the patterns discussed above and illustrated in Figs. 1-5. Operation of Prior Art Figs. 9A-9D show the limitations of a prior art thermal ink jet printhead 110. It is constructed essentially the same as the preferred embodiment of the invention, except that it has a flat heater element surface instead of a contoured surface. The prior art printhead 110 has a pair of firing chambers 112, 114. In chamber 114, the heater element has an unintended defect crack 116 offset randomly from a central axis 120 passing through the orifice 122. The heater element in the right firing chamber 114 is free of defects. Consequently, as shown in Fig. 9B, simultaneous application of energy to both heater elements results initially in the nucleation of a bubble 124 at the defect 116 in the left chamber 112. Because higher energy is required without a nucleation site, the right chamber has not yet commenced bubble formation. As shown in Fig 9C, the left bubble 124 has grown sufficiently to begin displacing a droplet 126 from the left orifice 122. Meanwhile, a bubble 130 has formed in the right chamber, but is smaller than the left bubble 120 because of its delayed formation due to the higher energy to begin bubble formation without the benefit of a nucleation site. In Fig. 9D, the left droplet has been ejected on a path that deviates from the axis because of the off-center location of the bubble. The left droplet 126 is spatially deviated from the location where it is intended to impact the print medium (not shown). A right droplet 132 happens to be ejected on axis, but is temporally deviated from its position. As the printhead rapidly traverses over the print medium, delayed droplets will be deposited farther down range along the printhead path than if they were timely expelled. Operation of Preferred Embodiment As shown in Figures 10A-10D, the preferred embodiment suffers from neither temporal nor spatial droplet deviation. In Fig. 10B, nucleation occurs at many or most of the recesses 42. Even if nucleation does not occur at every site, or in every recess, there are sufficient recesses and sites so that at least some nucleation sites will promptly begin bubble formation, avoiding temporal deviation. The number of sites also ensures that the resulting bubbles are well distributed, even if all sites are not effective for nucleation, avoiding spatial deviation. The quantity and wide distribution of recesses provides an accelerated transition from nucleate to film boiling, further improving uniformity. Fig. 10C shows small bubbles 82 forming in most or all recesses 42. In Fig. 10D the bubbles 82 have coalesced in each firing chamber. The resulting bubbles 84 have a flatter surface or "wave front" to eject droplets 86 reliably on axis. By providing a flat wave front, the sensitivity of orifice position is reduced. This is an improvement over prior art systems in which positioning the orifice slightly off axis relative to a spherically expanding bubble can generate turbulence and lateral flow in the ink near the orifice. In the preferred embodiment, the heater element 16 has an overall thickness of about 8-10µm. Typically, the resistor 37 is 0.10 µm thick, the passivation 38 is 0.75µm thick, and the cavitation barrier 39 is 0.6µm thick. The aluminum leads 43 are about 0.7mm thick, and are positioned between the resistor layer and the passivation layer. In a typical application, adjacent firing units are spaced apart 40-80µm on center, each orifice having a diameter of 10-50µm, and spaced above the heater element surface by 14-25µm. The heater element is a square about 20-60µm on a side, and the firing chamber has a width or diameter of about 16µm greater than the resistor. Although the recess are formed by etching after the sequential photoimaging of the resistor and other layers, and before the addition of the barrier and orifice plate, the cavitation barrier may be imaged in two steps: first, imaging a continuous flat layer, and second, imaging a perforated layer to define the recesses.

Claims: A method of manufacturing a thermal ink jet printhead (10) comprising the steps: providing a substrate (12); applying a heating element (16) to the substrate; forming a contoured surface (40,42) of the heating element (16), the contoured surface having a plurality of first surface portions (40) at a first level, and a plurality of second surface portions (42) at a different second level, wherein the step of forming the contoured surface (40,42) comprises defining a plurality of edges (50, 52), each defining a boundary between a first surface portion and a second surface portion. A method of manufacturing a thermal inkjet printhead (10) according to claim 1 wherein the step of forming the contoured surface comprises removing material from the heating element (16). A method of manufacturing a thermal ink jet printhead (10) according to claim 1 or claim 2 wherein the step of forming the contoured surface comprises etching the heating element (16). A method of manufacturing a thermal ink jet printhead (10) according to any preceding claim wherein the step of defining a plurality of edges (50,52) includes undercutting the first surface portions such that grooves (66) are defined below the edges (50,52). A thermal ink jet printhead (10) comprising: a body defining a plurality of ink firing chambers (22); for each firing chamber (22), the body defining an orifice (26) providing fluid communication from the firing chamber (22) to a location outside of the printhead (10); a plurality of electrically activated heating elements (16), each connected to the body in thermal communication with each firing chamber (22), and each heating element (16) having a contoured surface (40, 42) defining a contoured surface portion of the firing chamber (22); each contoured surface portion of the firing chamber (22) defining a pattern of first surface portions (40) at a first level, and second surface portions (42) at a different second level. A thermal ink jet printhead according to claim 5 wherein each second surface portion (42) abuts a first surface portion (40) at an edge wall (46) having at least an edge wall portion offset at an angle from the adjacent surface portions. A thermal ink jet printhead according to claim 5 or claim 6 wherein the wall portion (46) is offset from at least one of the adjacent surface portions (40, 42) by at least 90 degrees, such that the edge wall portion is perpendicular to or acute to the at least one of the adjacent surface portions. A thermal ink jet printhead according to any one of claim 5 to 7 wherein each of the heating elements (16) has substantially the same contoured surface pattern such that uniform heater performance is provided. A thermal ink jet printhead according to any of claims 5 to 8 wherein the second surface portions (42) are evenly spaced apart to form a regular array.

Inventors: ELSHAIK NAGWA M, HESS ULRICH, ELSHAIK, NAGWA M., HESS, ULRICH
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1996
Main classification: B41J2
IPC codes: B41J2