EP2490896B1

EP2490896B1 - Crack-resistant thermal bend actuator

Info

Publication number: EP2490896B1
Application number: EP09848585.7A
Authority: EP
Inventors: Gregory John Mcavoy; Vincent Patrick Lawlor; Ronan Padraig Sean O'reilly
Original assignee: Memjet Technology Ltd
Current assignee: Memjet Technology Ltd
Priority date: 2009-08-25
Filing date: 2009-08-25
Publication date: 2016-05-25
Anticipated expiration: 2029-08-25
Also published as: EP2490896A4; SG178479A1; AU2009351617A1; EP2490896A1; AU2009351617B2; KR20120057608A; WO2011022750A1; JP2012530624A; JP5561747B2

Description

Field of the Invention

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.

Background of the Invention

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 US Patent No. 6,416,167 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 US Patent No. 6,260,953 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 US Patent No. 6,623,101 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 describe 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.
US2005046672 relates to a thermal actuator for a micro-electromechanical device, especially a liquid drop emitter for ink jet printing. The thermal actuator comprises a base element and a movable element extending from the base element and residing at a first position. The movable element includes a barrier layer constructed of a barrier material having low thermal conductivity material, bonded between a first layer and a second layer; wherein the first layer is constructed of a first material having a high coefficient of thermal expansion and the second layer is constructed of a second material having a high thermal conductivity and a high Young's modulus.; An apparatus is provided adapted to apply a heat pulse directly to the first layer, causing a thermal expansion of the first layer relative to the second layer and deflection of the movable element to a second position, followed by relaxation of the movable element towards the first position as heat diffuses through the barrier layer to the second layer.

Summary of the Invention

In view of the above, the present teaching provides a crack-resistant thermal bend actuator in accordance with the claims which follows.
In a first aspect, there is provided a thermal bend actuator 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,

The thermal bend actuator 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:

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:
- 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,

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.
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, wherein the moving portion comprises the actuator, and 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:
- an active beam connected 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,

Brief Description of the Drawings

Optional embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a side-sectional view of a partially-fabricated alternative inkjet nozzle assembly after a first sequence of steps in which nozzle chamber sidewalls are formed;
Figure 2 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 1;
Figure 3 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a second sequence of steps in which the nozzle chamber is filled with polyimide;
Figure 4 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 3;
Figure 5 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a third sequence of steps in which connector posts are formed up to a chamber roof;
Figure 6 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 5;
Figure 7 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a fourth sequence of steps in which conductive metal plates are formed;
Figure 8 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 7;
Figure 9 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a fifth sequence of steps in which an active beam member of a thermal bend actuator is formed;
Figure 10 is a perspective view of the partially-fabricated inkjet nozzle assembly shown in Figure 9;
Figure 11 is a side-sectional view of a partially-fabricated inkjet nozzle assembly after a sixth sequence of steps after coating with a polymeric layer, protecting with a metal layer and etching a nozzle opening;
Figure 12 is a side-sectional view of completed inkjet nozzle assembly, after backside MEMS processing and removal of photoresist; and
Figure 13 is a cutaway perspective view of the inkjet nozzle assembly shown in Figure 12.

Description of Optional Embodiments

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 US Application No. 12/142,779 , 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.
Figures 1 to 13 show a sequence of MEMS fabrication steps for an inkjet nozzle assembly 100 described in the Applicant's earlier US Publication No. US 2008/0309728 , The completed inkjet nozzle assembly 100 shown in Figures 12 and 13 utilizes thermal bend actuation, whereby a moving portion of a roof bends towards a substrate resulting in ink ejection.
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 Figures 1 and 2, a substrate 101 has an electrode 102 formed in an upper portion thereof. The electrode 102 is one of a pair of adjacent electrodes (positive and earth) for supplying power to an actuator of the inkjet nozzle 100. The electrodes receive power from CMOS drive circuitry (not shown) in upper layers of the substrate 101.
The other electrode 103 shown in Figures 1 and 2 is for supplying power to an adjacent inkjet nozzle. In general, the drawings shows MEMS fabrication steps for a nozzle assembly, which is one of an array of nozzle assemblies. The following description focuses on fabrication steps for one of these nozzle assemblies. However, it will of course be appreciated that corresponding steps are being performed simultaneously for all nozzle assemblies that are being formed on the wafer. Where an adjacent nozzle assembly is partially shown in the drawings, this can be ignored for the present purposes. Accordingly, the electrode 103 and all features of the adjacent nozzle assembly will not be described in detail herein. Indeed, in the interests of clarity, some MEMS fabrication steps will not be shown on adjacent nozzle assemblies.
In the sequence of steps shown in Figures 1 and 2, an 8 micron layer of silicon dioxide is initially deposited onto the substrate 101. The depth of silicon dioxide defines the depth of a nozzle chamber 105 for the inkjet nozzle. After deposition of the SiO₂ layer, it is etched to define walls 104, which will become sidewalls of the nozzle chamber 105.
As shown in Figures 3 and 4, the nozzle chamber 105 is then filled with photoresist or polyimide 106, which acts as a sacrificial scaffold for subsequent deposition steps. The polyimide 106 is spun onto the wafer using standard techniques, UV cured and/or hardbaked, and then subjected to chemical mechanical planarization (CMP) stopping at the top surface of the SiO₂ wall 104.
In Figures 4 and 5, a roof member 107 of the nozzle chamber 105 is formed as well as highly conductive connector posts 108 extending down to the electrodes 102. Part of the roof member 107 will be used to define a passive beam 116 for the thermal bend actuator 115 in the completed inkjet nozzle assembly, as shown in Figures 12 and 13. In the Applicant's previous inkjet nozzle designs, the roof 107 (and thereby the passive beam of the thermal bend actuator) consists of silicon dioxide. Silicon dioxide has poor thermal conductivity, which minimizes the amount of heat conveyed away from the active beam of the thermal bend actuator during actuation. By using a passive beam having poor thermal conductivity, the overall efficiency of the device is improved. However, silicon dioxide is susceptible to cracking both during MEMS fabrication and during operation of the completed inkjet nozzle assembly. A further disadvantage of silicon dioxide is that it has a degree of permeability to aqueous ions (e.g. chloride ions), resulting in contamination of the active beam layer over time via leaching of aqueous ions from hot ink in the nozzle chamber. This mechanism of contamination can lead to failure of the active beam and the thermal bend actuator, which is highly undesirable.
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 Figure 12, the layer of silicon dioxide 130 is sandwiched between the active beam 110 and the layer of silicon nitride 131 in the completed actuator 115. This arrangement improves MEMS fabrication, because the roof member 107, particularly the part of the roof member 107 defining the passive beam of the thermal bend actuator, is less susceptible to cracking when the actuator is 'released' by removing the sacrificial polyimide or photoresist 106. The passive beam 116, as well as the nozzle plate of the printhead defined by contiguous roof members 107, also has improved mechanical robustness in the completed printhead without appreciably compromising thermal efficiency. Moreover, the roof member 107 does not allow any leaching of aqueous ions from hot ink towards the active beam of the thermal bend actuator. Therefore, it will be appreciated that the dual layer passive beam improves both operation of the actuator and fabrication of the actuator.
Returning now to Figures 5 and 6, after deposition of the bilayered roof member 107, a pair of vias are formed in the wall 104 down to the electrodes 102 using a standard anisotropic DRIE. This etch exposes the pair of electrodes 102 through respective vias. Next, the vias are filled with a highly conductive metal, such as copper, using electroless plating. The deposited copper posts 108 are subjected to CMP, stopping on the bilayered roof member 107 to provide a planar structure. It can be seen that the copper connector posts 108, formed during the electroless copper plating, meet with respective electrodes 102 to provide a linear conductive path up to the roof member 107.
In Figures 7 and 8, metal pads 109 are formed by initially depositing a 0.3 micron layer of aluminium onto the bilayered roof member 107 and connector posts 108. Any highly conductive metal (e.g. aluminium, titanium etc.) may be used and should be deposited with a thickness of about 0.5 microns or less so as not to impact too severely on the overall planarity of the nozzle assembly. The metal pads 109 are positioned over the connector posts 108 and on the roof member 107 in predetermined 'bend regions' of the thermoelastic active beam member.
In Figures 9 and 10, a thermoelastic active beam member 110 is formed over the bilayered roof 107. By virtue of being fused to the active beam member 110, part of the roof member 107 functions as a lower passive beam member 116 of a mechanical thermal bend actuator, which is defined by the active beam 110 and the passive beam 116. The thermoelastic active beam member 110 may be comprised of any suitable thermoelastic material, such as titanium nitride, titanium aluminium nitride and aluminium alloys. As explained in the Applicant's earlier US Publication No. 2008/0129793 , vanadium-aluminium alloys are a preferred material, because they combine the advantageous properties of high thermal expansion, low density and high Young's modulus.
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 Figures 9 and 10, the active beam member 110 comprises a partial nozzle opening 111 and a beam element 112, which is electrically connected at each end to positive and ground electrodes 102 via the connector posts 108. The planar beam element 112 extends from a top of a first (positive) connector post and bends around 180 degrees to return to a top of a second (ground) connector post.
Still referring to Figures 9 and 10, the metal pads 109 are positioned to facilitate current flow in regions of potentially higher resistance. One metal pad 109 is positioned at a bend region of the beam element 112, and is sandwiched between the active beam member 110 and the passive beam member 116. The other metal pads 109 are positioned between the top of the connector posts 108 and the ends of the beam element 112.
Referring to Figure 11, a hydrophobic polymer layer 80 is deposited onto the wafer and covered with a protective metal layer 90 (e.g. 100 nm aluminum). After suitable masking, the metal layer 90, the polymer layer 80 and the bilayered roof member 107 are then etched to define fully a nozzle opening 113 and a moving portion 114 of the roof.
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 Figures 12 and 13, an ink supply channel 120 is etched through to the nozzle chamber 105 from a backside of the substrate 101. Although the ink supply channel 120 is shown aligned with the nozzle opening 113 in Figures 12 and 13, it could, of course, be positioned offset from the nozzle opening.
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.

Claims

An inkjet nozzle assembly (100) comprising:
a nozzle chamber (105) comprising a floor, a roof (107) and an ink inlet, the roof having a nozzle opening (113) and a moving portion (114) comprising a thermal bend actuator (115), for ejecting ink through the nozzle opening (113), wherein the thermal bend actuator (115) comprises:
an active beam (110) disposed on an upper surface of a passive beam (116) relative to the floor, the active beam (110) being connected to drive circuitry such that when a current is passed through the active beam (110), the active beam (110) expands relative to the passive beam (116), resulting in bending of the actuator (115) towards the floor,

characterised in that the passive beam (116) comprises a lower first layer comprised of silicon nitride and an upper second layer comprised of silicon dioxide, said upper second layer being sandwiched between the lower first layer and the active beam (110).
The inkjet nozzle assembly (100) of claim 1, wherein said first layer is thicker than said second layer, optionally wherein said first layer is at least four times thicker than the second layer.
The inkjet nozzle assembly (100) of claim 1, wherein the second layer has a thickness in the range of 0.05 and 0.2 microns, and/or wherein the first layer has a thickness in the range of 1.0 and 2.0 microns.
The inkjet nozzle assembly (100) of claim 1, wherein the active beam (110) has a thickness in the range of 1.5 and 2.0 microns.
The inkjet nozzle assembly (100) of claim 1, wherein said active beam (110) is connected to said drive circuitry via a pair of electrical contacts positioned at one end of said actuator.
The inkjet nozzle assembly (100) of claim 1, wherein the active beam (110) is fused to the passive beam (116) by a deposition process.
The inkjet nozzle assembly (100) of claim 1, wherein the active beam (110) is comprised of a material selected from the group consisting of: titanium nitride, titanium aluminium nitride and an aluminium alloy, or wherein the active beam (110) is comprised of a vanadium-aluminium alloy.
The inkjet nozzle assembly (100) of claim 1, wherein the nozzle opening (113) is defined in the moving portion (114), such that the nozzle opening (113) is moveable relative to the floor.
The inkjet nozzle assembly (100) of claim 1, wherein the actuator (115) is moveable relative to the nozzle opening (113).
The inkjet nozzle assembly (100) of claim 1, wherein said roof (107) is coated with a polymeric material.
An inkjet printhead comprising a plurality of nozzle assemblies as claimed in any one of the preceding claims.