Patent Grant
6886921
The invention relates to ink ejectors for ink jet printers and specifically to improved thin film heater resistors for ink jet printers.
Conventional ink jet printers make use of square or rectangular shaped heater resistors. The primary advantage of square or rectangular shaped thin film resistors is their electrical simplicity. In a square or rectangular shaped resistor, the direct current (DC) resistance is directly proportional to the length/width ratio (L/W), often referred to as the number of squares. By knowing the sheet resistance of the thin film and the L/W ratio, the DC resistance value of the thin film resistor can be calculated.
Unlike in a typical electronic application where a thin film resistor is a passive element in the circuit, the thin film resistor used as an ink ejector in an ink jet printer is an active element. The thermodynamics and hydrodynamics of the ink in conjunction with the thin film resistor make design of these devices much more complicated than if the thin film resistor were a passive element in the circuit. Accordingly, use of square or rectangular shaped resistors, while simplifying the construction, do not lead to the most energy efficient heater resistors. Furthermore, many resistor shapes, including square or rectangular shapes can contribute to ejector misfires due to air build up in ink chambers adjacent the resistors.
There continues to be a need for more energy efficient ink ejectors so that a higher density of ink ejectors can be placed on a printhead chip without excessively heating the chip. There is also a need for heater resistor designs which reduce misfiring caused by air build up in the ink chambers adjacent the chips.
With regard to the foregoing and other objects and advantages, the invention provides an improved ink jet printer ejector including a substantially decahedral-donut shaped thin film resistor having a first end, a second end opposite the first end, a major axis having a first length, and a minor axis having a second length less than the first length. The major axis extends between the first end and the second end thereof. Electrical conductors are attached to the first end and to the second end of the resistor for activating the ink ejector on command from the ink jet printer.
In another embodiment, the invention provides an ink ejector for an ink jet printer having a substantially uniform surface temperature profile and a substantially non-uniform current density distribution. The ink ejector includes a thin film resistor having a first segment and a second segment attached at an angle on a first end thereof to a first end portion disposed between the first and second segments, a third segment and a fourth segment attached at an angle on a first end thereof to a second end portion disposed between the third and fourth segments and on a second end thereof to the first and second segments. The resistor has a major axis having a first length, and a minor axis having a second length less than the first length. The major axis extends between the first end portion and the second end portion thereof. Electrical conductors are attached to the first end portion and to the second end portion of the resistor for activating the ink ejector on command from the ink jet printer.
In yet another embodiment, the invention provides ink ejector for an ink jet printer including a thin film resistor having opposed edges attached to conductors, a center portion disposed between the opposed edges, and a shape that promotes a non-uniform current density distribution in the thin film resistor and a first temperature adjacent the opposed edges that is greater than a second temperature of the center portion of the resistor.
The invention provides a number of specific advantages over conventional ink ejectors. For example, any air bubbles trapped in corners of the ink chamber are more readily forced out with the ink upon activation of the ink ejector because, as explained in more detail below, nucleation does not begin in the center section of the ink chamber. Another advantage is that ink ejection can be achieved with lower energy and correspondingly lower surface temperature since there is a more uniform heating of the thin film resistor used as the ink ejector. A high impedance thin film resistor can be made from conventional resistor material for use as the ink ejector thereby providing the ability to increase the impedance of power transistors connected to the ink ejector.
The above and other aspects and advantages of the invention will become further apparent by reference to the following detailed description of preferred embodiments when considered in conjunction with the accompanying drawings in which:
Ink ejection devices for ink jet printers include thin film resistor devices and piezoelectric devices. Both ink ejection devices have been in common use for a number of years. With the advent of higher speed, higher quality ink jet printers, improvements are constantly being sought to reduce power consumption, increase reliability, and increase the ejection device density on a printhead substrate. The ink ejection devices of the invention enable significant improvements to be made to thermal ink jet printers as described in more detail below.
With reference to
Conventional thin film resistor ink ejection devices for use in ink jet printheads 16 are shown in
In
One disadvantage of each of the ink ejection devices 26, 36 and 38 described above is that the current density is uniform over the surface of the thin film resistors. However, uniform current density leads to non-uniform heating of the thin film materials. In the case of square or rectangular-shaped ink ejection devices such as device 26 a hot spot is typically formed toward the center area 50 of the ink ejection devices while portions 52 and 54 adjacent edges 32 and 34 are relatively cooler. A temperature profile from the center area 50 to one edge 32 is shown in FIG. 5. As seen in
The purpose of a thermal type ink ejection device is to generate a vapor bubble in an ink chamber for ejection of ink through the nozzle holes 20 (FIG. 1).
Upon activation of the ink ejection device 58, ink in the ink chamber 82 adjacent the cavitation layer 74 begins to boil and forms a vapor bubble that acts like a positive displacement pump to force ink out of the ink chamber 82 through nozzle hole 60 and onto a print media adjacent the printhead 56. Ideally all of the electrical energy input into the thin film resistor layer material 66 by means of the anode 68 and cathode 70 is converted to heat energy for heating ink in the ink chamber 82. However, because of passivation layer 72 and cavitation layer 74, additional energy is required heat the ink to the desired nucleation temperature. Accordingly, the thickness of layers 72 and 74 is typically minimized to reduce the energy required to eject a droplet of ink through nozzle hole 60.
In order to create a vapor bubble in ink chamber 82, a current pulse is applied to ink ejection device 58 for a period of time long enough to generate a temperature high enough to boil the ink on a surface 86 of the cavitation layer 74. In order to provide predictable droplet ejection, the surface temperature of cavitation layer 74 must boil the ink at its superheat limit. Many of the ink compositions used in ink jet printers are water based ink formulations. For water-based ink formulations, the superheat limit is about 320° C. So the ink ejection device 58 must generate a surface temperature of the surface 86 of at least about 320° C. for each and every ink droplet ejected through nozzle 60.
As explained above, the edge portions 52 and 54 of a conventional ink ejection device 26 are substantially cooler than the center portion 50 of the device 26. Accordingly, in order to generate a vapor bubble wherein most of the surface 86 of the ink ejection device 26 participates in bubble nucleation, the center portion 50 of the device 26 must be driven to a temperature well in excess of 320° C. so that the edge portions 52 and 54 will approach the desired nucleation temperature. In this case, it has been observed that the center portion 50 of the ink ejection device 26 must approach about 500° C. in order for the edge portions 52 and 54 to approach 320° C. Hence, considerable excess energy must be input to ejection devices 26, 36 and 38 for those devices to reliably eject a droplet of ink each time they are activated by an electrical pulse from the ink jet printer.
One problem associated with thermal ink ejection devices is that air dissolved in the ink formulation is forced out of solution when the ink is heated. A water-based ink formulation typically contains about 14.5 ppm dissolved air. As the ink is heated, less air remains in solution. For example, for ejection of from about 2 to about 5 nanograms of ink, about 1.4×10−18 moles of air comes out of solution on each ejector activation cycle. As the ink temperature increases more air is devolved from the ink formulation. Air bubbles 88 formed from the air coming out of solution tend to accumulate in corners or dead flow zones of the ink chamber 82 particularly in roof areas 90 toward the edges of the ink ejection device. If there is insufficient ink flow in the dead flow zone areas, the bubbles will continue to grow and affect ink flow into and out of the ink chamber 82. Periodic removal of air bubbles from the ink chamber 82 is required otherwise a 10 micron air bubble can form after 15,600 ejection cycles.
With reference now to
Like the embodiment shown in
Ink ejection device 110 is similar to ink ejection device 108 in that the device 110 has a first length L1 much greater than a second length L2 to provide a thin film heater having an equivalent of about 4 squares or more when the thin film resistor material used to make the ink ejection device is TaAl. The tab length TL is preferably less than about 10 percent of the major axis length L1 of the ink ejection device 110. The tab width TW is preferably about 3 times the TL. With respect to the open area 122, the major axis length MA1 is preferably about 2 to about 4 times the minor axis length MA2. Without desiring to be limited thereto, the following tables provide typical dimensions of ink ejection devices according to
An important advantage of the ink ejection devices 94, 108, and 110 is that there is substantially more uniform heating of the ink contact surface of the ejection devices so that the highest temperature portions of the device are closer to the edges thereof, such as edge 112 (FIG. 9), than for conventional ink ejection devices. A typical temperature distribution on the surface of a cavitation layer overlying ink ejection devices according to the invention is shown in FIG. 11. Contrasting
By providing more uniform surface temperature distribution, it is not necessary to overheat the central areas of the ink ejection devices 94, 108, and 110 in order to use more of the surface area of the ink ejection device for bubble nucleation. Since, less energy input is required for nucleation of ink using ink ejection devices according to the invention, the ink ejection devices of the invention may be operated at less than 0.2 microjoule per nanogram ink ejected.
Another advantage of the ink ejection devices according to the invention is that the devices promote the start of nucleation near the electrical conductors, such as conductors 116 and 118, of the devices rather than in the central portions of the devices due to the lack of resistive material in the open area, such as open area 120 (
Because the nozzle holes of an ink jet printhead are generally circular, and the ink chambers are generally elongate to correspond with high impedance ink ejection devices, such as device 94, there remains a substantial dead zone (DZ) in a roof area, such as roof area 90 (FIG. 6), of an ink chamber 152. As shown in
With reference to photomicrographs of actual ink ejection devices, ink ejection devices 26, 36, and 38 as set forth in
In contrast, nucleation of ink is biased toward the edges of the ink ejection devices of
More pronounced biasing of vapor bubble nucleation is shown in
Since the nucleation vapor bubbles 164 and 168 of devices 108 and 110 tend to grow from the edges of the ink ejection devices toward the center or open areas 120 and 122, the nucleation vapor bubbles are closer to the location of trapped air bubbles in the dead zones (DZ) of the ink chambers. Since the onset of nucleation is a vapor explosion generating pressures on the order of about 100 atmospheres, these vapor explosions are believed to contribute toward removal of air bubbles from the dead zone locations in the ink chamber. In contrast, a more centrally located vapor explosion as provided by ink ejection devices 26, 36, and 38 tends to force air bubbles into the dead zone areas of the ink chamber where they stay and accumulate.
It is believed that the use of ink ejection devices having open areas between segments provides significantly improved energy utilization whereby the ink ejection devices can be operated without heating any of the surface area of the ejection device significantly above the temperature required for ink nucleation. Accordingly, smaller, higher impedance ink ejection devices may be used to achieve ink ejection as compared to conventional ink ejection devices. By selecting resistor materials having higher sheet resistance values than TaAl, higher impedance ink ejection devices according to the invention may be formed. Increasing the impedance of the ink ejection devices has the added benefit of enabling use of smaller, higher impedance power field effect transistors (FET's) to drive the ink ejection devices. Decreasing the size of a power FET directly increases its DC impedance. However, the parasitic power loss of such a circuit is preferably designed to be less than about 15%. The parasitic power loss is defined as the ratio of the impedance of the circuit other than the ink ejection device to the total impedance of the circuit including the ink ejection device. Since about one third of the surface of the silicon substrate 62 (
The foregoing description of certain exemplary embodiments of the present invention has been provided for purposes of illustration only, and it is understood that numerous modifications, alterations, substitutions, or changes may be made in and to the illustrated embodiments without departing from the spirit and scope of the invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4339762 | Shirato et al. | Jul 1982 | A |
| 4712930 | Maruno et al. | Dec 1987 | A |
| 4725859 | Shibata et al. | Feb 1988 | A |
| 4870433 | Campbell et al. | Sep 1989 | A |
| 4914562 | Abe et al. | Apr 1990 | A |
| 4947189 | Braun et al. | Aug 1990 | A |
| 4947193 | Deshpande | Aug 1990 | A |
| 5559543 | Komuro | Sep 1996 | A |
| 5639386 | Burke et al. | Jun 1997 | A |
| 5694191 | Strathman et al. | Dec 1997 | A |
| 5808640 | Bhaskar et al. | Sep 1998 | A |
| 6123419 | Cleland | Sep 2000 | A |
| 6139130 | Takahashi et al. | Oct 2000 | A |
| 6276775 | Schulte | Aug 2001 | B1 |
| 20020008734 | Lee et al. | Jan 2002 | A1 |
| Number | Date | Country |
|---|---|---|
| 0385757 | May 1990 | EP |
| 58136462 | Aug 1983 | JP |
| 58162356 | Sep 1983 | JP |
| 61167573 | Jul 1986 | JP |
| 1087264 | Mar 1989 | JP |
| 2039955 | Feb 1990 | JP |
| 2080261 | Mar 1990 | JP |
| 3045359 | Feb 1991 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20040196334 A1 | Oct 2004 | US |
