Patent Application
20110050806
The present invention relates to the field of MEMS devices and particularly inkjet printheads. It has been developed primarily to improve the robustness of thermal bend actuators, both during MEMS fabrication and during operation.
The following patents or patent applications filed by the applicant or assignee of the present invention are hereby incorporated by cross-reference.
The disclosures of these co-pending applications are incorporated herein by reference.
The present Applicant has described previously a plethora of MEMS inkjet nozzles using thermal bend actuation. Thermal bend actuation generally means bend movement generated by thermal expansion of one material, having a current passing therethough, relative to another material. The resulting bend movement may be used to eject ink from a nozzle opening, optionally via movement of a paddle or vane, which creates a pressure wave in a nozzle chamber.
The Applicant's U.S. Pat. No. 6,416,167 (the contents of which are incorporated herein by reference) describes an inkjet nozzle having a paddle positioned in a nozzle chamber and a thermal bend actuator positioned externally of the nozzle chamber. The actuator takes the form of a lower active beam of conductive material (e.g. titanium nitride) fused to an upper passive beam of non-conductive material (e.g. silicon dioxide). The actuator is connected to the paddle via an arm received through a slot in the wall of the nozzle chamber. Upon passing a current through the lower active beam, the actuator bends upwards and, consequently, the paddle moves towards a nozzle opening defined in a roof of the nozzle chamber, thereby ejecting a droplet of ink. An advantage of this design is its simplicity of construction. A drawback of this design is that both faces of the paddle work against the relatively viscous ink inside the nozzle chamber.
The Applicant's U.S. Pat. No. 6,260,953 (the contents of which are incorporated herein by reference) describes an inkjet nozzle in which the actuator forms a moving roof portion of the nozzle chamber. The actuator is takes the form of a serpentine core of conductive material encased by a polymeric material. Upon actuation, the actuator bends towards a floor of the nozzle chamber, increasing the pressure within the chamber and forcing a droplet of ink from a nozzle opening defined in the roof of the chamber. The nozzle opening is defined in a non-moving portion of the roof. An advantage of this design is that only one face of the moving roof portion has to work against the relatively viscous ink inside the nozzle chamber. A drawback of this design is that construction of the actuator from a serpentine conductive element encased by polymeric material is difficult to achieve in a MEMS process.
The Applicant's U.S. Pat. No. 6,623,101 (the contents of which are incorporated herein by reference) describes an inkjet nozzle comprising a nozzle chamber with a moveable roof portion having a nozzle opening defined therein. The moveable roof portion is connected via an arm to a thermal bend actuator positioned externally of the nozzle chamber. The actuator takes the form of an upper active beam spaced apart from a lower passive beam. By spacing the active and passive beams apart, thermal bend efficiency is maximized since the passive beam cannot act as heat sink for the active beam. Upon passing a current through the active upper beam, the moveable roof portion, having the nozzle opening defined therein, is caused to rotate towards a floor of the nozzle chamber, thereby ejecting through the nozzle opening. Since the nozzle opening moves with the roof portion, drop flight direction may be controlled by suitable modification of the shape of the nozzle rim. An advantage of this design is that only one face of the moving roof portion has to work against the relatively viscous ink inside the nozzle chamber. A further advantage is the minimal thermal losses achieved by spacing apart the active and passive beam members. A drawback of this design is the loss of structural rigidity in spacing apart the active and passive beam members.
The Applicant's US Publication No. 2008/0129795 (the contents of which are incorporated herein by reference) describes an inkjet nozzle comprising a nozzle chamber with a moveable roof portion having a nozzle opening defined therein. The moveable roof portion comprises a thermal bend actuator for moving the moveable roof portion towards a floor of the chamber. Various means for improving the efficiency of the actuator are described, including the use of porous silicon dioxide for the passive layer of the actuator.
There is a need to improve upon the design of thermal bend inkjet nozzles, so as to achieve more efficient drop ejection and improved mechanical robustness. Mechanical robustness is an important factor in terms of both the operational characteristics of the inkjet nozzle and its fabrication. Fabrication requires a sequence of MEMS fabrication steps to provide a printhead integrated circuit in high overall yield.
In a first aspect, there is provided a thermal bend actuator comprising:
The thermal bend actuator according to the present invention is advantageously robust and resistant to cracking whilst maintaining excellent thermal efficiency. The first layer of silicon nitride provides the crack-resistance whilst the second layer of silicon dioxide provides thermal insulation, which maintains a high overall efficiency. Cracking may be problematic in thermal bend actuators due to inevitable stresses in the active and passive beams, but especially the passive beam which is usually formed from silicon dioxide having good thermally insulating properties. The present invention addresses the problem of cracking by using the bilayered passive beam described herein.
Optionally, the first layer is thicker than the second layer. The first layer of silicon nitride may be between 2 and 20 times thicker than the second layer of silicon dioxide, optionally between 8 and 20 times thicker.
Optionally, the first layer is at least two times thicker than the second layer, optionally at least four time thicker or optionally at least eight times thicker.
Optionally, the second layer has a thickness in the range of 0.01 and 0.5 microns, optionally in the range of 0.02 and 0.3 microns, optionally in the range of 0.05 and 0.2 microns, or optionally about 0. 1 microns.
Optionally, the first layer has a thickness in the range of 0.05 and 5.0 microns, optionally in the range of 1.0 and 2.0 microns, or optionally about 1.4 microns.
Optionally, the active beam has a thickness in the range of 0.05 and 5.0 microns, optionally in the range of 1.0 and 3.0 microns, optionally in the range of 1.5 and 2.0 microns, or optionally about 1.7 microns.
Optionally, the active beam is connected to the drive circuitry via a pair of electrical contacts positioned at one end of the actuator.
Optionally, the active beam is fused to the passive beam by a deposition process.
Optionally, the active beam is comprised of a conductive thermoelastic material, which is optionally selected from the group consisting of: titanium nitride, titanium aluminium nitride and an aluminium alloy.
Optionally, the active beam is comprised of a vanadium-aluminium alloy.
In a second aspect, there is provided an inkjet nozzle assembly comprising:
an active beam for connection to drive circuitry; and
a passive beam mechanically cooperating with the active beam, such that when a current is passed through the active beam, the active beam expands relative to the passive beam, resulting in bending of the actuator,
wherein the passive beam comprises a first layer comprised of silicon nitride and a second layer comprised of silicon dioxide, the second layer being sandwiched between the first layer and the active beam.
In addition to the advantages discussed above in respect of the first aspect, a further advantage of inkjet nozzle assemblies according to the second aspect is that the second layer of silicon nitride is an impermeable barrier to the fluid contained in the nozzle chamber. Accordingly, aqueous ions are unable to leach through the passive beam and contaminate the active beam, which may result in nozzle failure. Leaching of aqueous ions from hot ink has been identified by the present Applicants as a failure mechanism for thermal bend actuators having a passive beam comprised of silicon dioxide only.
Optionally, the nozzle chamber comprises a floor and a roof having a moving portion, whereby actuation of the actuator moves the moving portion towards the floor.
Optionally, wherein the moving portion comprises the actuator.
Optionally, the active beam is disposed on an upper surface of the passive beam relative to the floor of the nozzle chamber.
Optionally, the nozzle opening is defined in the moving portion, such that the nozzle opening is moveable relative to the floor.
Optionally, the actuator is moveable relative to the nozzle opening.
Optionally, the roof is coated with a polymeric material, such as a polymerized siloxane described in further detail herein.
In a third aspect, there is provided an inkjet printhead comprising a plurality of nozzle assemblies, each nozzle assembly comprising:
a nozzle chamber having a nozzle opening and an ink inlet; and
a thermal bend actuator for ejecting ink through the nozzle opening, the actuator comprising:
In a fourth aspect, there is provided a MEMS device comprising one or more thermal bend actuators, each thermal bend actuator comprising:
Examples of such MEMS devices include LOC valves and LOC pumps (as described in the Applicant's U.S. application Ser. No. 12/142,779), sensors, switches etc. The skilled person would be well aware of the plethora of applications for MEMS devices comprising thermal bend actuators.
In a fifth aspect, there is provided a method of fabricating a thermal bend actuator comprising the steps of:
Optionally, the sacrificial scaffold is comprised of photoresist or polyimide.
Optionally, the sacrificial scaffold is removed by an oxidative plasma, known in the art as ‘ashing’. Ashing may be achieved using an O2 plasma, an O2/N2 plasma or any other suitable oxidizing plasma.
Optionally, residual stresses in the passive beam after release of the thermal bend actuator reside predominantly in the first layer.
Optionally, the method forms at least part of a MEMS fabrication process for an inkjet nozzle assembly.
Optionally, the first and second layers define a roof of a nozzle chamber.
Optionally, the roof comprises a moving portion, the moving portion including the thermal bend actuator.
Optionally, a nozzle opening is defined in the roof prior to release of the thermal bend actuator.
Optionally, the nozzle opening is defined in the moving portion of the roof.
Optionally, the roof is coated with a polymeric material prior to releasing the thermal bend actuator.
Optionally, the polymeric material is protected with a metal layer prior to releasing the thermal bend actuator.
Optionally, the polymeric material is coated on the roof by a spin-on process.
Optionally, the polymeric material is a polymerized siloxane, such as polydimethylsiloxane, polymethylsilsesquioxane or polyphenylsilsesquioxane.
Of course, it will be appreciated that optional aspects described in connection with the thermal bend actuator according to the first aspect are equally applicable to the second, third, fourth and fifth aspects.
Optional embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:
It will be appreciated that the present invention may be used in connection with any thermal bend actuator having an active beam fused to a passive beam. Such thermal bend actuators find uses in many MEMS devices, including inkjet nozzles, switches, sensors, pumps, valves etc. For example, the Applicant has demonstrated the use of thermal bend actuators in lab-on-a-chip devices as described in U.S. application Ser. No. 12/142,779, the contents of which are herein incorporated by reference, and a plethora of inkjet nozzles described in the cross-referenced patents and patent applications identified herein. Although MEMS thermal bend actuators find many different uses, the present invention will be described herein with reference to one of the Applicant's inkjet nozzle assemblies. However, it will, of course, be appreciated that the present invention is not limited to this particular device.
The starting point for MEMS fabrication is a standard CMOS wafer having CMOS drive circuitry formed in an upper portion of a silicon wafer. At the end of the MEMS fabrication process, this wafer is diced into individual printhead integrated circuits (ICs), with each IC comprising drive circuitry and plurality of nozzle assemblies.
As shown in
The other electrode 103 shown in
In the sequence of steps shown in
As shown in
In
Silicon nitride is less susceptible to cracking and allows a greater range of residual stresses compared to silicon dioxide—both compressive and tensile stresses. Silicon nitride is also completely impermeable, which minimizes nozzle failure via leaching of ions from ink in the nozzle chamber. However, silicon nitride has a much higher thermal conductivity than silicon dioxide, resulting in poorer efficiency of the bend actuator. Hence, silicon nitride is usually not used as the passive beam, despite having better mechanical properties than silicon dioxide.
In the present invention, the roof member 107, which defines the passive beam for the completed actuator, comprises a relatively thick layer (about 1.4 microns) of silicon nitride 131 and a relatively thin layer (about 0.1 microns) of silicon dioxide 130. Referring briefly to
Returning now to
In
In
To form the active beam member 110, a 1.5 micron layer of a conductive thermoelastic active beam material is initially deposited by standard PECVD. The beam material is then etched using a standard metal etch to define the active beam member 110. After completion of the metal etch and as shown in
Still referring to
Referring to
The moving portion 114 comprises a thermal bend actuator 115, which is itself comprised of the active beam member 110 and the underlying passive beam member 116. The nozzle opening 113 is defined in the moving portion 114 of the roof so that the nozzle opening moves with the actuator during actuation. Configurations whereby the nozzle opening 113 is stationary with respect to the moving portion 114, as described in US Publication No. 2008/0129793, are also possible and within the ambit of the present invention.
A perimeter region 117 around the moving portion 114 of the roof separates the moving portion from a stationary portion 118 of the roof. This perimeter region 117 allows the moving portion 114 to bend into the nozzle chamber 105 and towards the substrate 101 upon actuation of the actuator 115. The hydrophobic polymer layer 80 fills the perimeter region 117 to provide a mechanical seal between the moving portion 114 and stationary portion 118 of the roof 107. The polymer has a sufficiently low Young's modulus to allow the actuator to bend towards the substrate 101, whilst preventing ink from escaping through the gap 117 during actuation.
The polymer layer 80 is typically comprised of a polymerized siloxane, which may be deposited in a thin layer (e.g. 0.5 to 2.0 microns) using a spin-on process and hardbaked. Examples of suitable polymeric materials are poly(alkylsilsesquioxanes), such as poly(methylsilsesquioxane); poly(arylsilsesquioxanes), such as poly(phenylsilsesquioxane); and poly(dialkylsiloxanes), such as a polydimethylsiloxane. The polymeric material may incorporate nanoparticles to improve its durability, wear-resistance, fatigue-resistance etc.
In the final MEMS processing steps, and as shown in
Following the ink supply channel etch, the polyimide 106, which filled the nozzle chamber 105, is removed by ashing in an oxidizing plasma and the metal film 90 is removed by an HF or H2O2 rinse to provide the nozzle assembly 100.
It will be appreciated by ordinary workers in this field that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.
