Thermal bend actuator having improved lifetime

Abstract

A thermal bend actuator includes: a thermoelastic beam for connection to drive circuitry; and a passive beam mechanically cooperating with the thermoelastic beam, such that when a current is passed through the thermoelastic beam, the thermoelastic beam expands relative to the passive beam resulting in bending of the actuator. The thermoelastic beam wherein the thermoelastic beam is comprised of an aluminium alloy. The aluminium alloy comprises a first metal which is aluminium, a second metal, and at least 0.1 at. % of a third metal selected from the group consisting of: copper, scandium, tungsten, molybdenum, chromium, titanium, silicon and magnesium.

Claims

1. A thermal bend actuator comprising: a thermoelastic beam for connection to drive circuitry; and a passive beam mechanically cooperating with the thermoelastic beam, such that when a current is passed through the thermoelastic beam, the thermoelastic beam expands relative to the passive beam resulting in bending of the actuator, wherein the thermoelastic beam is comprised of an aluminium alloy, the aluminium alloy comprising a first metal which is aluminium, a second metal, and at least 0.1 at. % of a third metal selected from the group consisting of: copper, scandium, tungsten, molybdenum, chromium, titanium, magnesium and silicon.

2. The thermal bend actuator of claim 1, wherein the second metal is selected from the group consisting of: vanadium, titanium, chromium, manganese, cobalt, nickel and scandium.

3. The thermal bend actuator of claim 1, wherein the second metal is vanadium.

4. The thermal bend actuator of claim 1, wherein the third metal is copper.

5. The thermal bend actuator of claim 1, wherein: an amount of aluminium is in the range of 80 to 95% at. %; an amount of the second metal is in the range of 2 to 18 at. %; and an amount of the third metal is in the range of 0.1 to 5 at. %.

6. The thermal bend actuator of claim 1, wherein the passive beam is multilayered or monolayered.

7. The thermal bend actuator of claim 6, wherein the passive beam comprises at least one material selected from the group consisting of: silicon oxide and silicon nitride.

8. The thermal bend actuator of claim 1, wherein the thermoelastic beam is fused or bonded to the passive beam.

9. The thermal bend actuator of claim 1, wherein the passive beam is cantilevered.

10. The thermal bend actuator claim 9, wherein the thermoelastic beam is connected to a pair of electrical terminals positioned at one end of the passive beam.

11. The thermal bend actuator of claim 10, wherein the thermoelastic beam comprises a plurality of legs interconnected by one or more turns.

12. An inkjet nozzle device comprising: a nozzle chamber having a nozzle opening and an ink inlet; and a thermal bend actuator for ejecting ink through the nozzle opening, said actuator comprising: a thermoelastic beam connected to drive circuitry; and a passive beam mechanically cooperating with the thermoelastic beam, such that when a current is passed through the thermoelastic beam, the thermoelastic beam expands relative to the passive beam resulting in bending of the actuator, wherein the thermoelastic beam is comprised of an aluminium alloy having at least 0.1 at. % copper.

13. The inkjet nozzle device of claim 12, wherein the nozzle chamber comprises a floor and a roof having a moving portion, whereby actuation of said actuator moves said moving portion towards said floor.

14. The inkjet nozzle device of claim 13, wherein the moving portion comprises the actuator.

15. The inkjet nozzle device of claim 14, wherein the nozzle opening is defined in the moving portion, such that the nozzle opening is moveable relative to the floor.

16. The inkjet nozzle device of claim 12 comprising a plurality of thermal bend actuators for ejecting ink through the nozzle opening.

17. An inkjet printhead comprising a plurality of inkjet nozzle devices according to claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic plan view of an inkjet nozzle device comprising thermal bend actuators;

(3) FIG. 2 is a cross-section along line 2-2 of the inkjet nozzle device shown in FIG. 1; and

(4) FIG. 3 is a perspective of a portion of an inkjet printhead comprising a plurality of the inkjet nozzle devices shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

(5) Referring to FIGS. 1 and 2, there is shown an inkjet nozzle device 1 incorporating a pair of opposed thermal bend actuators 3 according to one embodiment of the present invention. Suitable MEMS processes for fabricating nozzle devices of the type shown in FIGS. 1 and 2 are described in the Applicant's US 2008/0309728 and US 2008/0225077, the contents of which are herein incorporated by reference.

(6) The inkjet nozzle device 1 is fabricated on a passivation layer 5 of a silicon substrate 7 having a drive circuitry layer 8 for delivering current pulses to the thermal bend actuators 3. The inkjet nozzle device 1 comprises a nozzle chamber 9 having a nozzle opening 10, a roof 11 and sidewalls 13 extending between the roof and the silicon substrate 7. A blanket silicon oxide layer 15 deposited on the passivation layer 5 defines the sidewalls 13 of the nozzle chamber. Electrical connector posts 17 (e.g. copper posts) formed via a damascene process, as described in U.S. Pat. No. 7,819,503 (the contents of which are incorporated herein by reference), extend through the silicon oxide layer 15 to form an electrical connection to the drive circuitry layer 8 of the silicon substrate 7. As best shown in FIG. 1, a pair of connector posts 17 (power and ground) are provided at an anchored end of each cantilevered thermal bend actuator 3

(7) Each of the thermal bend actuators 3 is comprised of a lower passive beam 20 and an upper thermoelastic (‘active’) beam 22. Each passive beam 20 is formed via deposition of a suitable passive material onto a sacrificial scaffold (not shown), such that the passive beam at least partially defines the roof 11 of the nozzle chamber 9. In the embodiment shown in FIG. 2, each passive beam 20 is simply a monolayer of silicon oxide, although it will of course be appreciated that multilayered passive beams, as described in U.S. Pat. No. 8,079,668, are within the ambit of the present invention.

(8) Each thermoelastic beam 22 is formed via deposition of a thermoelastic material onto both the passive beam 20 and exposed upper surfaces of the connector posts 17 to thereby form an electrical connection to the drive circuitry layer 8. Etching of the thermoelastic material defines the thermoelastic beams 22, which are each configured as a pair of parallel legs 24 extending from respective power and ground terminals 26 (defined by upper surfaces of the connectors posts 17) towards the nozzle opening 10 and interconnected at respective distal ends by a turn 28. The thermoelastic material is typically a vanadium-aluminum-copper alloy, as will be described in more detail below.

(9) From the foregoing, it will therefore be appreciated that each thermal bend actuator 3 takes the form of a cantilevered paddle, which forms a moving portion of the roof 11 of the nozzle chamber 9. During actuation, the thermoelastic beam 22 of each thermal bend actuator 3 receives an electrical signal from the drive circuitry 8, which cause the thermoelastic beam to expand relative to the passive beam 20, thereby causing each thermal bend actuator to bend downwards towards the silicon substrate 7 in the direction indicated by arrows A. This bending motion increases pressure inside the nozzle chamber 9, thereby causing ejection of an ink droplet through the nozzle opening 10. The circular nozzle opening 10 has a semicircular portion defined in each of thermal bend actuators 3, such that the nozzle moves during actuation. Following droplet ejection, ink is replenished in the nozzle chamber via a pair of ink inlets 32, which receive ink from ink supply channels (not shown) defined in the silicon substrate.

(10) As shown in FIG. 2, a polymer layer 30 (e.g. polyimide layer) is superposed on the entire structure, including exposed portions of the passive beam and the thermoelastic beam, so as to protect the thermal bend actuators 3 from ink and to provide thermal insulation. The polymer layer 30 may include a dewetting coating (e.g. hydrophobic and/or oleophobic coating) to assist in preventing flooding and encourage stable droplet ejection. For clarity, the polymer layer 30 is not shown in FIG. 1.

(11) FIG. 3 shows an example of a pagewide inkjet printhead 100 incorporating MEMS inkjet nozzle devices 1, as described above.

(12) Improved Thermoelastic Material

(13) As described in U.S. Pat. No. 7,984,973, aluminium alloys are excellent candidates for use as the thermoelastic beam in thermal bend actuators, combining the properties of relatively high thermal expansion and a relatively high modulus of elasticity compared to other known thermoelastic materials. For example, vanadium-aluminium and titanium-aluminium alloys have been used by the present Applicant in the development of inkjet nozzle devices employing thermal bend actuation technology.

(14) However, there remains a need to improve the longevity of thermal bend actuators, whilst maintaining the above-mentioned desirable properties of aluminium alloys. Following an extensive review of materials and device configurations, it has now been found that the addition of small amounts of copper (e.g. up to about 5 at. %) to aluminium alloys dramatically improves longevity without compromising performance

(15) Table 1 shows the performance of two aluminium alloys used as the thermoelastic material in otherwise identical inkjet nozzle devices 1 of the type described above in connection with FIGS. 1 and 2. One aluminium alloy (“VA1”) consists of 90 at. % Al and 10 at. % V; the other aluminium alloy (“VAlCu”) consists of 89.9 at. % Al, 9.8 at. % V and 0.3 at. % Cu.

(16) TABLE-US-00001 TABLE 1 Comparison of VAI and VAICu as thermoelastic materials Measurement VAI VAICu Energy input (nJ) 698   696   Current density (A/m.sup.2)  5.73  5.71 Nozzles alive after 6.2 billion 17% 93% actuations Thermal bend response- −2.33 −2.31 heating to 180° C. (MPa/° C.) Thermal bend response- −2.61 −2.64 cooling from 180° C. (MPa/° C.) Maximum velocity during free air ca. −2.5 ca. −2.5 oscillation (m/s)

(17) The results in Table 1 clearly demonstrate that the addition of copper to an aluminum alloy produces a surprising improvement in longevity. With a similar energy input and current density, a mere 17% of the devices having a VA1 thermoelastic beam were still alive and actuating after about 6 billion actuations, whereas 93% of the devices having a VAlCu thermoelastic beam were still alive after the same number of actuations. This represents a remarkable and surprising fivefold improvement in lifetime.

(18) Furthermore, the performances of both thermal bend actuators were very similar in terms of their thermal bend response and maximum velocity during free air oscillation. Therefore, the addition of copper, whilst dramatically improving longevity, made negligible difference in terms of device performance. It was therefore concluded that aluminum alloys containing small amounts of copper were optimal for overall device performance and longevity.

(19) It will, of course, be appreciated that the present invention has been described by way of example only and that modifications of detail may be made within the scope of the invention, which is defined in the accompanying claims.