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The present invention relates to a printhead for a thermal ink-jet printer, and more particularly to an ejector in which droplets of distinctly selectable sizes may be ejected.
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In ink-jet printing, it has been difficult to create an apparatus in which the size of a droplet ejected by a particular ejector can be selected. Generally, ejectors in thermal ink-jet printheads are capable of ejecting a droplet of generally one size only. However, there exist any number of printing situations where it would be desirable to be able to have a single ejector capable of selectably emitting a droplet of one of a plurality of selectable droplet sizes. Such situations in which a selectable droplet size would be highly useful include creation of half-tone images such as derived from photographs, and the creation of offset-quality alphanumeric characters.
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In the prior art, US-A-4,251,824 discloses a thermal ink-jet printhead wherein each ejector includes a plurality of independently-controlled heating elements. By selecting a particular combination of elements, one can select,the size of the ejected droplet. US-A-4,740,796 discloses basic principles of bubble nucleation in ink-jet printing. It is also generally known that one technique for manipulating the size of ink droplets is to control the temperature of the liquid ink just before nucleation, such as by preheating the liquid ink to a predetermined temperature which will yield a known droplet size.
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According to the present invention, there is provided an ejector for an ink-jet printing apparatus. A structure defines a channel that retains a quantity of liquid ink therein. An opening is associated with the channel, through which a quantity of liquid ink may be ejected. A heating element defines the heating surface within the channel, the heating element dissipating heat into the channel when voltage is applied thereto, thereby nucleating a bubble in liquid ink in the channel. The heating surface defines a first portion adapted to dissipate heat at a first power density and a second portion adapted to dissipate heat at a second power density.
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The present invention will be described further, by way of examples, with reference to the accompanying drawings, in which:-
- Figure 1 is a simplified perspective view showing the essential portions of a thermal ink-jet printhead incorporating the present invention;
- Figure 2 is a plan view of a portion of a heater chip incorporating the present invention;
- Figures 3 and 4 are plan views of a heating element according to one embodiment of the present invention, showing resulting ink nucleation under different sets of conditions;
- Figure 5 is an example graph showing the resulting volume of nucleation of liquid ink as a function of voltage applied to a heating element according to the present invention;
- Figure 6 is a plan view of a heating surface of a heating element according to an alternate embodiment of the present invention; and
- Figure 7 is a plan view of a heating surface of a heating element according to an alternate embodiment of the present invention.
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Figure 1 is a highly simplified perspective view showing the portions of an ejector for a thermal ink-jet printhead incorporating the present invention. Although only one ejector is shown, it will be understood that a practical thermal ink-jet printhead will include 100 or more such ejectors, typically spaced at 118 to 336 ejectors per cm (300 to 600 ejectors per inch). Illustrated in Figure 1 is the general configuration of what is known as a "side-shooter" printhead wherein the channels forming the ejectors are created between two chips which are bound together. The printhead shown in Figure 1 comprises a heater chip 10, which is bound on a main surface thereof to a "channel plate" indicated in phantom as 12. The heater chip 10 is generally a semiconductor chip design as known in the art, and defines therein any number of heating elements, such as indicated as 14, on a main surface thereof. There will typically be provided one heating element 14 for every ejector in the printhead. Adjacent each ejector 14 on the main surface of heater chip 10 is a channel 16 which is formed by a groove in channel plate 12. Channel plate 12 can be made of any number of ceramic, plastic, or metal materials known in the art. When the chip 10 is abutted against the channel plate 12, each channel 16 forms a complete channel with the adjacent surface of the heater chip 10, and one heating element 14 disposes a heating surface on the inside of the channel so formed, as shown in Figure 1.
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Figure 1 shows a highly simplified version of a practical thermal ink-jet printhead, and that any number of ink supply manifolds, intermediate layers, pit layers, etc., would be provided in a practical printhead. However, what is illustrated in Figure 1 are the essential elements necessary to practice the present invention, and the addition of further elements to make a fully practical printhead will not detract from the claimed invention as described in detail below.
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In operation, an ink supply manifold (not shown) provides liquid ink which fills the capillary channel 16 until it is time to eject ink from the channel 16 onto a print sheet. In order to eject a droplet of ink from channel 16, a small voltage is applied to heating element 14 in heater chip 10. As is familiar in the art of ink-jet printheads, heating element 14 is typically a portion of a semiconductor chip which is doped to a predetermined resistivity. Because heating element 14 is essentially a resistor, heating element 14 dissipates power in the form of heat through its heating surface (the heating surface being defined as the surface of heating element 14 disposed within channel 16), thereby vaporizing liquid ink immediately adjacent the heating surface. This vaporization creates a bubble of ink vapor within the channel, and the expansion of this bubble in turn causes liquid ink to be expelled out of the channel 16 and onto a print sheet to form a spot in a desired image being printed. As shown in the view of Figure 1, it is intended that the ink supply manifold be disposed behind the printhead, so that the ejected ink droplet will be ejected out of the page according to the perspective of Figure 1.
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Figure 2 is a plan view of a portion of the main surface of heater chip 10, showing in detail the heating surface provided by one heating element 14, as would be found in a single ejector according to the present invention. Also shown in Figure 2, in phantom lines, are the borders of the channel formed by channel 16 in channel plate 12 when it is bound to the main surface of heater chip 10.
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It will be seen that there are two conductive lines associated with each single heating element 14. An input line is shown as 20, and creates a first terminal where input line 20 attaches to heating element 14. Also attached to each heating element 14 is a ground line 22, forming a second terminal where it meets heating element 14, which connects heating element 14 to a common ground 24. In a typical design of a thermal ink-jet printhead, the ground lines 22 associated with all of the heating elements 14 on a particular chip share common ground line 24. Therefore, any control over the timing and manner of applying a voltage to any heating element 14 must be via input line 20, which is why an ejector of the present design can be considered a "single-terminal" ejector. As is known in the art, an input line such as 20 is ultimately controlled by electrical signals relating to digital data representative of an image desired to be printed.
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According to the illustrated embodiment of the present invention, the heating element 14 defines a heating surface of a general "bottle" shape, here defining two distinct, and generally rectangular, portions. A first portion of the heating surface is generally indicated as 30, while a second portion of the heating surface is generally indicated as 32. The two portions 30, 32 are generally aligned along an axis formed by input line 20 and ground line 22, an axis which also is parallel to the general direction of the channel formed by channel 16, and thereby aligned with the opening formed by channel 16.
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According to the present invention, the heating element 14 provides a heating surface within channel 16 of such properties that each portion dissipates energy, in the form of heat, at a particular power density which is different from the power density of another portion. "Power density" can be defined as the amount of energy dissipated from the heating surface per unit area.
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In thermal ink-jet printing, nucleation of liquid ink in a channel is brought about essentially as follows. A voltage must be applied to the heating element sufficient to drive the liquid ink adjacent the heating surface to vaporization (the "burn voltage"), and this burn voltage must be applied for a suitable duration of time ("pulse width"). Within some constraints, a relatively low burn voltage can be compensated for by extending the pulse width and vice-versa. Significantly, nucleation will occur only around that portion of the heating surface having the necessary power density by which a combination of burn voltage and pulse width will be sufficient to nucleate ink immediately adjacent the heating surface. Power density multiplied by time equals energy density, and nucleation occurs only over those surfaces having this sufficent energy density.
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The layout of the heating surface of heating element 14, which is an area of a particular shape doped to a uniform resistivity, provides two distinct portions 30 and 32 which will satisfy the requirements of burn voltage and/or pulse width separately. Turning first to Figure 3, if a first predetermined voltage V1 is applied to heating element 14, for a predetermined pulse width, the layout of the heating surface will be such that only the area over the portion 30 of the heating surface will be able to meet the requirements of pulse width and burn voltage; the second portion 32, being of larger area, will have no particular area therein which meets the requirements of power dissipation for nucleation. In other words, first portion 30, dissipating the energy of V1 over a smaller area, will provide a higher power density than the larger second portion 32 being subjected to the same voltage V1. Therefore, at a certain voltage V1, only the liquid ink immediately adjacent portion 30 of the heating surface will nucleate, resulting in a relatively small ink vapor bubble, indicated as 40.
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In contrast, looking at Figure 4, if a substantially higher voltage V2 is applied to heating element 14, the relationship of applied voltage to power dissipation will be such that essentially the entire heating surface, meaning both portions 30 and 32, of the heating element 14 will meet the conditions of power dissapation for nucleation. Thus, there will be formed a liquid ink vapor bubble which substantially corresponds to the entire combined surface area of the heating surface, as shown by bubble 42.
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As a general rule, the size of the nucleated liquid ink bubble in a channel is roughly proportional to the amount of liquid ink that is expelled by creation of the bubble. Therefore, the larger the nucleated liquid ink bubble, the larger the droplet of ink that will be ejected, and the larger the resulting spot size on the print sheet. By selecting the size of the nucleation bubble, the size of the resulting droplet ejected through the opening of channel 16 can be selected, in this case between a small droplet (nucleation bubble 40 in Figure 3) or a large droplet (nucleation bubble 42 in Figure 4).
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It will be noted that the two portions 30, 32 in the heating surface are demarcated by a discrete step represented by the "shoulders" of the general "bottle" shape of the heating surface. According to one embodiment of the present invention, this discrete demarcation between the two portions, as opposed to providing a heating surface with for example a uniform taper or other monotonic shape, facilitates the selectable spot size which is desirable with the present invention. Figure 5 is a simplified graph showing the relationship of the voltage applied to the heating element 14, on the x axis thereof, to the resulting nucleation bubble volume on the y axis. The numbers 40, 42 on the y axis represent the respective volumes of the nucleation bubbles 40, 42 shown in Figures 3 and 4. The basic line of the graph is a "stairstep" function which flattens in two distinct areas, which correspond on the x axis to acceptable ranges for V1 and V2 respectively.
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An advantage of the stairstep function, which is provided by the discrete demarcation between portions 30, 32 of the heating surface, is that the necessary burn voltages V1 or V2 can be provided within reasonably broad ranges as shown on the x axis of Figure 5. Whether, for example, the actual value of applied V1 is closer to the left or right of the range in the graph of Figure 5, the size of the nucleation bubble which results will not significantly vary. This tolerance for a wide range of actual applied voltages is an important attribute in semiconductor-based heater chips 10, with the actual on-chip voltage (or the power density in response to a given voltage) can vary significantly from chip to chip and on a single chip over time. If the overall heating surface of heating element 14 were a monotonic shape, such as a single triangle or an ovoid, it is likely the function of voltage to volume would not be the stairstep shown in Figure 5, but rather a substantially linear function. While a substantially linear function would have theoretical advantages of providing a continuously-variable spot size as a function of voltage, the ejector-to-ejector and chip-to-chip variances in performance would make such a continuously-variable system impractical, particularly in a printer having a number of printhead chips.
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Significantly, it should also be pointed out that although Figure 5 shows a change in bubble volume as a function of applied voltage and presuming a constant pulse width, a generally similar function to Figure 5 will result if the applied voltage is held constant and the pulse width, or duration, of the voltage is varied. In other words, the same principle shown in Figures 3 and 4 with different voltages will also be generally apparent if the same voltage is applied to the heating element 14 at two different pulse widths. Therefore, systems could be provided which are able to choose the bubble size, and therefore droplet size, by selecting either the burn voltage or the pulse width, or both.
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Figure 6 is a simplified plan view of another possible embodiment of the present invention, wherein a different principle is used to,obtain the desired result of different power densities in different portions of the heating element. Here a heating element indicated as 14', which functions in the same way between control line 20 and ground line 22 as in the above embodiment, comprises different distinct resistive sections 50, 52, 54, 56, and 58, each of which is made of doped polysilicon, but wherein each individual section 50-58 is doped to a different resistivity. Generally speaking, in a preferred embodiment, the sections such as 50 closer to the ground line 22 and opening in the channel are doped to a higher resistivity, with the resistivities becoming progressively lower toward input line 20. Sections doped to a higher resistivity will tend to nucleate the adjacent liquid ink at lower voltages, or at shorter pulse widths. The relative sizes of the differently-doped sections 50-58 can be manipulated to obtain different selectable bubble sizes, which result in different resulting spot sizes. It will be noted that the present example in Figure 6 shows five distinct different-doped sections, which correspond to a single heating element 14' which is responsive to five different applied voltages or pulse widths to cause the output of five distinctly different droplet sizes. In one typical possible embodiment of a heating element 14' as shown in Figure 6, the overall dimension of the heating surface, including all of the portions thereof, is approximately 200 micrometers by 20 micrometers.
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When using a heating element 14', such as in Figure 6, with differently-doped resistive portions, one possible practical problem relates to the fact that a preferred protective covering within the channel 16 for a heating element such as 14 is tantalum. Tantalum is a good thermal conductor, which would act against the desired effect of restricting nucleation to specific portions of the heating surface. Possible solutions to this problem include using a protective material other than tantalum, thinning the tantalum layer, or even providing "shims" (not shown) within the protective layer over the borders between adjacent portions, these shims acting as thermal insulators. Possible materials for such shims include phosphosilicate glass, or polyimide.
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Figure 7 shows a heating element 14 which is demarcated into four distinct portions, adding portions 34 and 36 to the portions 30 and 32 described in detail above. Once again, such a heating element 14 would be responsive to four burn voltages or pulse widths and therefore be able to nucleate bubbles of four distinct sizes. It will be noted that the two general principles of defining separate portions of a heating surface, such as demarcating a uniformly-doped heating surface with "shoulders" or doping different portions of the heating surface for different resistivities, could be combined in a heating element which incorporates both techniques.
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Although the preferred embodiment of a heating surface as shown and described comprises substantially rectangular portions, it will be apparent that other general shapes for individual portions, such as round or triangular, could be provided. These alternate shapes are functional equivalents to the preferred rectangular shapes. For practical purposes of the invention, the important principle is that the overall shape of the heating surface, or the profile of relative resistivities, have portions of the heating surface organized in discrete sections, as opposed to a gradual or monotonic change. Although the illustrated embodiment shown depicts different portions such as 30, 32 or 50, 52 directly bordering each other in a single-piece heating element, it is not a necessity that the different portions directly abut each other. The different portions could be spaced apart within channel 16 and only be connected through a connector within the heater chip 10. Similarly, it is conceivable that different portions of what is electrically a single heating element 14 could be provided on different surfaces (such as provided by two separate chips) forming some of the walls of the channel.
