GB2621322A

GB2621322A - Actuator unit

Info

Publication number: GB2621322A
Application number: GB2211328.6A
Authority: GB
Inventors: Mardilovich Peter; Gardner Catherine; Leon Vella Andrew
Original assignee: Xaar Technology Ltd
Current assignee: Xaar Technology Ltd
Priority date: 2022-08-03
Filing date: 2022-08-03
Publication date: 2024-02-14
Also published as: JP2024022559A; GB202211328D0

Abstract

An actuator unit 100 for a MEMS device, the actuator unit comprises a substrate 110 having a first surface 110A, a membrane 140 provided on the substrate; a fluidic path, comprising a fluidic chamber 195, formed in the substrate, wherein the membrane provides one of the fluidic chamber walls; an electrical element 120 provided on the membrane and, at least in part, over the fluidic chamber; and a peripheral structure (168, Fig.6) arranged on each opposite end of the electrical element. The electrical element comprises a first electrode 121; a ceramic member 123; and a second electrode 122, so that the electrical element deforms upon the application of a predetermined electrical field. The electrical element being elongated in a first direction 510 and having two opposite ends spaced apart in the first direction, each end arranged in part on the substrate and in part over the fluidic chamber. The peripheral structure is arranged on each opposite end of the electrical element, wherein both peripheral structures have substantially the same elastic properties and are arranged relative to the fluidic chamber so that both ends of the electrical element experience substantially the same stress upon application of the predetermined electric field.

Description

ACTUATOR UNIT

FIELD OF THE INVENTION

The present invention relates to the field of microelectromechanical systems (MENIS) and, in particular, to an actuator unit of an electrical component for a NIEMS device, such as, but not limited to, an electromechanical actuator. It may find particularly beneficial application as an actuator element for a droplet ejection head. The actuator unit may be especially beneficial to improve reliability through stress and heat management.

BACKGROUND

The present disclosure is concerned with an actuator unit comprising an electric element for an electrical component for a microelectromechanical systems (MEMS) device, in particular, but not limited to, an electromechanical actuator. It may find particularly beneficial application as an actuator element for a droplet ejection head.

Droplet ejection heads are now in widespread usage, whether in more traditional applications, such as inkjet printing, or in 3D printing, or other materials deposition or rapid prototyping techniques. Accordingly, fluids used in droplet ejection heads often require novel chemical properties to adhere to new substrates and increase the functionality of the deposited material.

A variety of fluids may be deposited by a droplet ejection head. For instance, a droplet ejection head may eject droplets of fluid that travel towards a receiving medium, such as paper or card, ceramic tiles or shaped articles (e.g. cans, bottles etc.) to form an image.

Droplets of fluid may be used to build structures, for example, electrically active fluids may be deposited onto receiving media, such as a circuit board, to enable prototyping of electrical devices, or to deposit droplets of solution containing biological or chemical material onto a receiving medium such as a microarray of assay tubes.

Droplet deposition heads may be used in applications without a receiving medium. For example, a fine vapour or mist may be generated by droplet ejection heads to control humidity in greenhouse misting systems.

So as to be suitable for new and/or increasingly challenging ejection and deposition applications, droplet ejection heads continue to evolve and specialise. However, while a great many developments have been made, there remains room for improvements in the field of droplet ejection heads. Specifically, increasingly challenging specifications of droplet ejection heads often require improvements in their structure and manufacturing techniques. Electrical elements for MENIS devices are commonly manufactured through the deposition of a series of layers arranged on a first substrate, for example, through one or more techniques known in the thin film technology field. One or more intermediate layers are usually formed between the substrate and the electrical element, in order to provide a flexible membrane.

A typical electrical element may have a configuration where a ceramic member is interposed between two electrically conductive layers that form a first and a second electrode. The ceramic member may be, for example, a thin film of a ceramic material, showing fen-oelectric behaviour, like a piezoelectric material or a relaxoeferroelectric crossover material. Commonly employed ceramic materials include lead based ceramics with perovskite structure, especially lead zirconate titanate (PZT), doped PZT and PZT based solid solutions. They may be deposited through a number of deposition techniques known in the art, for example through sputtering, chemical vapour deposition (CVD) or chemical solution deposition (CSD).

In recent years, significant effort has been put into the development of lead-free alternative materials such as (K,Na)Nb03-based materials, (Ba,Ca)(Zr,Ti)03-based materials and (Bi,Na,K)TiO3-based materials with similar deposition techniques.

Electrical elements formed from such materials are deposited layer by layer on said first substrate, commonly a wafer, accommodating several arrays of electrical elements, where the first and second electrodes may be described to form a lower electrode supporting the thin film of piezoelectric or relaxonferroelectric crossover material, and a top electrode covering the ceramic member.

The first, or lower, electrode may be a common electrode or it may be patterned to form arrays of individual electrodes, each associated with an individual electrical element. In the same way, the second, or top, electrode may be a common electrode or it may be patterned to form arrays of individual electrodes, each associated with an individual electrical element. The thin film material, similarly, may or may not be patterned.

Individual electrical elements, therefore, might comprise a patterned ceramic material thin film or a region of an unpatterned ceramic material thin film which is common to more than one electrical element. Individually addressable regions of electrical elements may be defined by at least one of the electrodes, of each electrical element, being patterned.

The electrical connection of the electrical element to the drive circuitry may be ensured using electrical traces that are directly connected to the electrodes of the electrical element.

One or more insulating or passivating layers are also deposited at various stages of the formation of an electrical element in order to ensure electrical insulation of the electrodes and electrical traces. The one or more insulating or passivating layers also protect the various features of the electrical element from chemical attacks caused by the external environment or by chemicals used either in the production process or during the electrical element operation.

A second substrate, such as a protective layer, for example a capping layer, is usually bonded to the first substrate in order to further protect the electrical elements from the external environment, especially moisture and other chemical species. The second substrate may have a recess to accommodate one or more electrical elements.

Usually, a plurality of electrical elements are formed on a surface of a substrate at the same time. The plurality of electrical elements is usually arranged in an array.

After the electrical elements have been formed, cavities, for example fluidic chambers, may be provided in the substrate, underneath the electrical elements, for example by etching the substrate from a side opposite the side on which the electrical elements are formed. The flexible membrane will form one of the walls of each fluidic chamber.

In operation, the ceramic member of an electrical element deforms in response to the application of a predetermined electric field through the first and second electrodes. Since the electrical element is, generally, formed in part above the substrate and in part above the fluidic chamber, the deformation of the ceramic member will produce different levels of stress at different locations of the electrical element. In particular, the region of the electrical element above the fluidic chamber will experience a lower stress with respect to the regions located at the interface between the substrate and the fluidic chamber. This is because the substrate will not itself deform as an effect of the ceramic member deformation, differently from the membrane which will deform together with the ceramic member. This can lead to delamination of the various components of the electrical element from each other and from the substrate.

One challenge of providing a reliable electrical component is to ensure that the stress experienced by the electrical element is managed properly and that each electrical element is subject to substantially the same level of stress at locations which are symmetrically arranged with respect to each other, for example, but not limited to, regions located at opposite ends of the electrical element in a specific direction. This will reduce the presence of weak spots which may more easily undergo failure, thus compromising the performance of the electrical element and, in turn, affecting the overall performance of the electrical component.

In some configurations, an example of which is depicted in Figures 1 to 3, the actuator unit 200 comprises a substrate 110 and an electrical element 120 formed on the first surface 110A of the substrate 110 and on the membrane 140. The electrical element has a ceramic member 123 interposed between a first (or lower) electrode 121 and a second (or top) electrode 122, in a thickness direction 505. The second electrode 122 and the ceramic member 123 are patterned. The ceramic member 123 (and the electrical element 120) extend in a first direction 510 and the second electrode 122 is patterned substantially in the same shape as the surface of the ceramic member 123 in contact with the second electrode 122, as seen in Figure 1. The actuator unit 200 further comprises a fluidic chamber 195 formed in the substrate 110. In some instances, the ceramic member 123 may be chamfered, i.e. having a sloping edge, as shown in Figures 1 and 2.

When the electrical element 120 is actuated, and the ceramic member 123 is deformed towards the corresponding fluidic chamber 195, the portion of the electrical element which overlaps the interface between the fluidic chamber and the substrate 110 is subject to high stress since it is fixed to the substrate, while the region above the fluidic chamber 195 may deform. This may cause failure of the electrical element by delamination.

The second electrode 122, in the present case, is connected to the driving circuit (not shown) through an electrical trace 160 which connects to the second electrode 122 at an electrical connecting point 161, through a via 161_a formed over the second electrode 122 at one end of the element 120. This is shown in Figure 2, which depicts the portion of the actuator unit circled in Figure 1. Figure 2 further shows that the electrical element 120 may also be provided with one or more passivation layers 150 and one or more insulation layers to protect the various parts of the electrical element 120. Figure 3 offers a top view of the two regions at opposite ends of the actuator unit 200 in the first direction 510. The ceramic member 123 and the passivation layer or layers 150 are omitted for the sake of clarity.

In some known configurations (not shown), the second electrode is designed to extend away from the ceramic member and on top of the substrate where the electric connection is established.

The configuration of Figures 2 and 3, by having the electrical connecting point 161 established on top of the electrical element 120, rather than on top of the substrate 110, allows space to be saved on the substrate 110 and reduces the overall size of the electrical component. This means that it may allow tighter packing of the electrical elements 120, thus increasing the number of electrical elements 120 which may be formed on the substrate 110. In the case of a droplet ejection head, this would result in a higher resolution.

On the other hand, the presence of the electrical connecting point 161 on top of the electrical element 120 as shown in Figure 2 and 3, may create unevenness in the distribution of the stress throughout the electrical element 120, with the region having the electrical connecting point 161, being subject to higher stress as a result of the additional stiffness provided by the electrical connecting point 161 and having higher probability of failing with respect to the region opposite to it in the first direction 510.

In order to ensure good reliability of the electrical element 120, it is important that the stress experienced by the electrical element 120 is managed so that the risk of delamination is reduced and that the stress experienced by regions of the electrical element which are largely symmetric to each other is as similar as possible. In this way, the stress is more uniformly distributed and the occurrence of weak spots prone to failure is reduced. The presence of the electrical connection, as described above, can also have an effect on the distribution of heat. Hot spots can be formed corresponding to electrical connecting point 161 rendering the electrical element 120 prone to thermal failure. It is, therefore, also important that the heat distribution is as even as possible throughout the electrical element 120.

The present invention provides an improved design of an actuator unit with an electrical element, a design that allows the unevenness in the distribution of stress and heat in the electrical element to be reduced so that the occurrence of weak spots is minimised and the reliability of the electrical element increased.

SUMMARY OF THE INVENTION

Aspects of the invention are set out in the appended independent claims, while particular variants of the invention are set out in the appended dependent claims.

The present invention provides, in one aspect, an actuator unit for a MEMS device, the actuator unit comprising: a substrate having a first surface, a membrane provided, at least in part, on the first surface of the substrate; a fluidic path comprising a fluidic chamber formed in the substrate, wherein the membrane provides one of the walls of the fluidic chamber; an electrical element provided on the membrane and, at least in part, over the fluidic chamber, the electrical element comprising: a first electrode arranged on the membrane and for connection to a first electrical trace; a ceramic member arranged on the first electrode, and a second electrode arranged on the ceramic member and for connection to a second electrical trace so that the electrical element deforms upon the application of a predetermined electrical field; the electrical element being elongated in a first direction and having two opposite ends spaced apart in the first direction, each end arranged in part on the substrate and in part over the fluidic chamber; a peripheral structure arranged on each opposite end of the electrical element; wherein both peripheral structures have substantially the same elastic properties and are arranged relative to the fluidic chamber so that both ends of the electrical element experience substantially the same stress upon application of the predetermined electric field.

In a second aspect the present invention provides is a droplet ejection head comprising the actuator unit according to the first aspect.

Further provided is a droplet ejection apparatus comprising a droplet ejection head according to the second aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now directed to the drawings, in which.

Figure 1 is a schematic diagram of a cross-section of an actuator unit which is not part of the invention; Figure 2 is a schematic diagram of a cross-section of the portion of the actuator unit depicted in the circle in Figure 1, Figure 3 is a schematic diagram of a top view of the actuator unit of Figure 1; Figure 4 is a schematic diagram of a cross-section of an actuator unit according to an embodiment of the invention, Figure 5 is a schematic diagram of a cross-section of a portion of the actuator unit depicted in the circles in Figure 4; Figures 6A to C are schematic diagrams of a top view of actuator units according to different embodiments of the invention; Figure 7 is a schematic diagram of a cross-section of a droplet ejection head according to an embodiment of the invention.

In the Figures, like elements are indicated by like reference numerals throughout. It should be noted that the drawings are not to scale and that certain features may be shown with exaggerated sizes so that these are more clearly visible.

DETAILED DESCRIPTION

Exemplary embodiments of the invention and their variants will now be described with reference to Figures 4, 5, 6A, 6B and 6C.

Figure 4 shows a cross-section of an actuator unit 100 according to the first aspect of the invention. Figure 5 show a cross-section of the portion depicted in one of the circles of Figure 4 and Figures 6A, B and C represent top views of the portions located at opposite ends of the actuator unit of Figure 4, according to different embodiments of the invention.

The actuator unit 100 for a IVIEMS device, comprises: a substrate 110 having a first surface 110A; a membrane 140 provided, at least in part, on the first surface 110A of the substrate 110; a fluidic path comprising a fluidic chamber 195, formed in the substrate 110, wherein the membrane 140 provides one of the walls of the fluidic chamber 195; an electrical element 120, provided on the membrane 140 and, at least in part, over the fluidic chamber 195. The electrical element 120 comprises: a first electrode 121 arranged on the membrane 140 and for connection to a first electrical trace (not shown); a ceramic member 123 arranged on the first electrode 121 and a second electrode 122 arranged on the ceramic member 123 and, for connection, to a second electrical trace 160, so that the electrical element 120 deforms upon the application of a predetermined electrical field, The electrical element 120 is elongated in a first direction 510 and has two opposite ends (circled in Figure 4 and shown in more detail in Figure 5 and in top view in Figure 6A to C) spaced apart in the first direction 510. Each end is arranged in part on the substrate 110 and in part over the fluidic chamber 195. A peripheral structure 168 is arranged on each opposite end in the first direction 510 of the electrical element 120, wherein both peripheral structures 168 have substantially the same elastic properties and are arranged, relative to the fluidic chamber 195, so that both ends of the electrical element 120 experience substantially the same stress upon application of the predetermined electric field.

Attention is drawn to Figure 5 where the end portion of the actuator unit 100 of the embodiment of Figure 4 is shown in greater detail. Figure 5 depicts the substrate 110, with the first surface 110A, and the fluidic chamber 195 having a membrane 140 as one of its walls. The material of the substrate is not particularly limited, for example, the substrate 110 may be a silicon wafer. The membrane 140 may be formed of sub-layers 141 and 142 deposited on top of each other in the thickness direction 505. The sub-layer 141 may, for example, comprise silica (Si02) formed by thermal oxidation of the silicon wafer. The sub-layer 142 may, for example, comprise alumina (A1203) deposited by atomic layer deposition (ALD). Any other materials and/or deposition method known in the art may be used to form the membrane 140.

The electrical element 120 is formed on the membrane 140 and partially superposed to the fluidic chamber 195 and partially to the substrate 110. The first electrode 121 may be formed, for example, of platinum (Pt). The ceramic member 123 may, for example, comprise Nb doped PZT (PNZT), deposited by sol-gel deposition. The second electrode 122 may be another Pt electrode. Alternatively, the second electrode 122 may be compositionally different to the first electrode 121, for instance, a combination of iridium (Tr) and iridium oxide (Ir02) layers. One or more passivation layers 150 are provided on the electrical element 120. The passivation layer 152 may, for example, be an alumina layer deposited by ALD The passivation layer 153 may, for example, be a silica layer deposited by plasma enhanced chemical vapour deposition (PE-CVD).

The electrical element 120 comprises two peripheral structures 168 located at opposite ends of the electrical element 120 in the first direction 510. Both peripheral structures 168 have essentially the same elastic properties so that both ends of the electrical element 120 experience the same stress upon application of the predetermined electric field.

The peripheral structures 168 are symmetrically arranged over the electrical element.

They extend in part above the fluidic chamber 195 and in part above the substrate 110.

Figure 6A offers a top view of the fluidic path of the embodiment of Fig. 4, including the fluidic chamber 195 and, optionally, a restrictor 194, the lower electrode 121, the top electrode 122 and the peripheral structures 168. The ceramic member 123 and the passivation 150 are omitted for the sake of clarity. As depicted in figure 6A, the peripheral structures 168 have a tri-lobed shape but this shape is not limiting and other shapes may be chosen. The shape of each peripheral structure 168 is symmetrical with respect to the symmetry line a of the electrical element 120 along the first direction 510. The peripheral structures 168 are also arranged symmetrically with respect to a symmetry line b in the same plane rotated 90 degrees (i.e. perpendicular) relative to the symmetry line a.

The shape of the peripheral structure 168 provides sufficient and evenly distributed rigidity to the respective end of the electrical element 120 so that the occurrence of delamination upon deformation of the electrical element 120 is reduced by strengthening the regions of the electrical element that are subject to higher stress, i.e. the regions at the interface between the fluidic chamber 195 and the substrate 110. This is achieved by ensuring that the peripheral structures 168 extend symmetrically above such interface regions. It is preferred that each peripheral structure 168 extends above the substrate 110 to an extent that ensures enough strengthening of the end portion of the electrical element 120 by anchoring the end portion to the substrate 110 effectively. The person skilled in the art will understand that the extension of the peripheral structure 168 over the substrate 110 will influence the shape of the peripheral structure 168 as a function of the overall geometry of the electrical element 120 and of the degree of deformation of the ceramic member 123 upon application of the predefined electric field.

Both peripheral structures 168 of Figure 6A are preferably formed of the same material or materials. Each peripheral structures168 may be formed of a plurality of layers preferably of different materials in order to fine tune the elastic properties. They may comprise at least a layer made of an electrically conductive material. An electrically conductive material may provide electrical connection between the electrical trace 160 and the second electrode 122. A material which is also thermally conductive, would, additionally, help with the heat management by providing a route for the dissipation of the heat produced by the electrical element 120 upon actuation. This could help by reducing the occurrence of hot spots at the ends of the electrical element 120 where the stress is the highest. The peripheral structure 168 may also comprise a layer of insulating material which may assist with the fine tuning of the elastic and/or thermal properties.

In some embodiments, at least one of the peripheral structures 168 comprises an electrically conductive material forming an electrical connection at the connecting point 161 between the second electrode 122 and the second electrical trace160. Preferably, the second electrical trace 160 connects the second electrode 122 to a driving circuit (not shown) through a via 161_a formed, for example by etching, in the passivation layer, or layers, 150, as shown in Figure 5 and 6A A passivation layer 150 is interposed between the electrical element 120 and the peripheral structure 168 at least around the via 161 a. In the example depicted the passivation layer 150 is a stack of a plurality of layers 151-152, which protect the electrical element 120 and the electrical trace 160 and insulate the second electrical trace 160 from the lower electrode 121.

It will be understood that where the peripheral structure 168 and the second electrical trace 160 are directly connected (formed of the same material) on only one end of the electrical element (as for example shown in Figure 6A and 6C), the peripheral structure 168 itself will still have the same shape as the peripheral structure 168 at the opposite end of the actuator 120 in the vicinity of the electrical element. The presence of the connected electrical trace 160 will have no significant effect on the stress experienced at the specific end of the electrical element 120, since that electrical trace material is located away from the regions at the interface between the fluidic chamber 195 and the substrate 110. In preferred embodiments, the peripheral structures 168 are made of the same material as the electrical trace 160, so that they can be formed in the same process steps and no further process steps need to be added to the manufacturing process. Suitable materials are aluminium (Al), gold (Au), copper (Cu), platinum (Pt), nickel (Ni) and the like or combinations thereof Thin adhesion layers may also be deposited prior to and/or after the formation of the electrical trace. The second electrical trace 160 and the peripheral structures 168 may be formed by sputtering followed by patterning the metallic layer to the desired shape using standard photolithography and etching processes.

A further passivation layer 151 may be deposited over the entire electrical element 120. The passivation layer 151 may, for example, be made of silica deposited by PE-CVD.

The passivation layer 151 is also deposited on the second electrical trace 160.

The passivation layers 151-153 may be etched, for example by lithography, over the electrical element 120 This is especially advantageous for reducing any inhibitive effect of the passivation layer 150 on the displacement of the electrical element 120 during operation.

A continuous insulating layer 170 may also be deposited over the electrical element 120. The insulating layer 170 may be, for example, a stack of silica and tantala layers deposited on top of each other in the thickness direction 505 by atomic layer deposition (ALD) The insulating layer 170 may further protect the electrical element 120 from the external environment and may improve the robustness of the passivation layer 150 by plugging pinholes and other defects.

In some embodiments of the invention, an example of which is depicted in Figure 6A, the second electrode 122 may be formed of a material with high electrical conductivity, such as, but not limited to platinum (Pt) and/or gold (Au).

In other embodiments, examples of which are depicted in Figures 6B and 6C, the second electrode 122 may be formed of materials of comparatively lower conductivity, such as a bi-layer of iridium (Ir) and iridium oxide (Ir02) The choice of the material of which the electrodes are formed may be driven by the capability of a material to achieve specific requirements, for example, good adhesion to the ceramic member or the like, at the partial expense of the conductivity. In such cases, a way of reducing the contribution of the second electrode to the resistance of the electrical connection is to provide more than one electrical connecting points 161, between each second electrode 122 and the corresponding second electrical trace 160. In those embodiments, both peripheral structures 168 may comprise conductive material forming an electrical connection at the electrical connecting point 161 between the second electrode 122 and the second electrical trace 160. In preferred embodiments, both peripheral structures 168 are made of the same material as the second electrical trace 160.

If two electrical connecting points 161 are provided at opposite ends of the second electrode 122, the second electrical trace 160 may be provided with a connecting portion 162 between the two electrical connecting points 161. In the embodiment of Figure 6B the connecting portion 162 is provided on the substrate 110, along the electrical element 120 in the first direction 510. In other embodiments, as shown in Figure 6C, the second electrical trace 160 comprises a connecting portion 162 overlying the electrical element 120 and extending between said electrical connecting points 161.

It will be understood that in the example depicted in Figure 6B, as described above, the presence of the electrical trace material connecting the peripheral structures 168 to the second electrical trace 160 has no effect on the stress experienced by the ends of the electrical element 120, since that material is located away from regions at the interface between the fluidic chamber 195 and the substrate 110, which regions experience high stress when the electrical element 120 deforms.

As already discussed, the peripheral structures 168 may also be useful to assist with the management of heat produced by the electrical element upon application of the predetermined electric field. In some embodiments, therefore, said peripheral structures 168 comprise a material with a thermal conductivity higher than the thermal conductivity of the ceramic member 123, for example, but not limited to a metal, a combination of metals, an alloy, a polymer, a resin or the like.

In order to further strengthen the end portions of the electrical element 120, an additional layer (not shown) may be provided on the peripheral structures 168 on a side of the peripheral structures 168 opposite to the side of the peripheral structures 168 facing the electrical element 120, so as to encapsulate the end portions of the electrical element 120 In preferred embodiments, the additional layer comprises a material which affords sufficient rigidity as to minimize the stress experienced by the regions at the interface between the fluidic chamber 195 and the substrate 110. The additional layer may comprise, for example, a curable polymeric material. In one implementation, the additional layer comprises a polymerisable alkene exhibiting some ring strain, such as a cyclobutene. The cyclic alkene may, in particular, comprise a benzo-cyclobutene or bisbenzocyclobutene such as those which are available under the trade mark Cyclotene (BCB) In another implementation, the additional layer comprises a polymerisable epoxide which is partially curable. Suitable partially curable epoxides include Novolac epoxides such as those known as SU8 negative photoresists. In still another implementation, the additional layer comprises a partially cured polyimide, for example, a partially cured aliphatic or aromatic polyimide. Suitable polyimides include those which are available under the trade mark RD Microsystems (for example, P1-5878G). In preferred embodiments the additional layer comprises a pattemable polymeric material such as, but not limited to, BCB The material of the additional layer may be locally provided on the end regions of the electrical element 120 and by any method known in the art, for example printing, lithography and stamping, followed by curing.

Alternatively, the material of the additional layer may be provided as a layer on the whole first surface 110A, and in the electrical element or elements 120, partially or fully cured and patterned in order to be removed from the regions where it is not required, for example from the region above the second electrode 122 In some embodiments, the additional layer is also the bonding layer used to bond the first substrate to the capping layer. In these embodiments it is important that the thickness of the additional layer is controlled so that upon bonding, when the additional or bonding layer deforms upon the application of the bonding force, the material of the additional layer covers the peripheral structures 168 but not the region of the top electrode 122 from where the passivation 150 has been removed.

In a second aspect of the invention, there is provided a droplet ejection head comprising an actuator unit as described above.

Figure 7 depicts a droplet ejection head 400, according to an embodiment of the invention, in which a nozzle plate 196 is provided at the side of the fluidic chamber 195 opposite to the side on which the electrical element 120 is formed, in the thickness direction 505. A nozzle 197 is formed in the nozzle plate 196 to allow ejection of fluid droplets from the fluidic chamber 195. A second substrate 103 defines a recess 106 for the electrical element 120. Such recess 106 may be sealed in a fluid-tight manner so as to prevent fluid within the fluidic chamber 195 and inlet passageways 131 and outlet passageways 132, at either end of the fluidic chamber 195, in the first direction 510, from entering the recess 106.

Further provided is a droplet ejection apparatus comprising a droplet ejection head according to the second aspect of the invention.

The embodiments and their variants described above may be used alone or in combination, as dictated by the specific application requirements, to achieve an improved electrical component according to the invention.

Claims

Claims 1. An actuator unit for a NIEMS device, the actuator unit comprising: - a substrate having a first surface, - a membrane provided, at least in part, on the first surface of the substrate; - a fluidic path comprising a fluidic chamber formed in the substrate, wherein the membrane provides one of the walls of the fluidic chamber; - an electrical element provided on the membrane and, at least in part, over the fluidic chamber, the electrical element comprising: i) a first electrode arranged on the membrane and for connection to a first electrical trace; ii) a ceramic member arranged on the first electrode, and iii) a second electrode arranged on the ceramic member and for connection to a second electrical trace, so that the electrical element deforms upon the application of a predetermined electrical field; the electrical element being elongated in a first direction and having two opposite ends spaced apart in the first direction, each end arranged in part on the substrate and in part over the fluidic chamber, - a peripheral structure arranged on each opposite end of the electrical element, wherein both peripheral structures have substantially the same elastic properties and are arranged relative to the fluidic chamber so that both ends of the electrical element experience substantially the same stress upon application of the predetermined electric field.
2. The actuator unit according to Claim I, wherein the peripheral structures are arranged in a symmetrically overlying relationship to the electrical element.
3. The actuator unit according to Claim 1 or Claim 2, wherein each peripheral structure is symmetrical with respect to a centre line of the electrical element extending in the first direction
4. The actuator unit according to any preceding claim, wherein the peripheral structures are symmetrical with respect to a centre line of the electrical element extending across the electrical element perpendicular to the first direction.
5 The actuator unit according to any preceding claim, wherein at least one of the peripheral structures comprises a conductive material forming an electrical connection between the second electrode and the second electrical trace.
6. The actuator unit according to any preceding claim, wherein the actuator unit further comprises a passivation layer interposed between the electrical element and the peripheral structures.
7 The actuator unit according to any preceding claim, wherein both peripheral structures comprise conductive material forming an electrical connection between the second electrode and the second electrical trace.
8. The actuator unit according to Claim 7, wherein said trace comprises a connecting portion overlying the electrical element and extending between said electrical connections.
9. The actuator unit according to any preceding claim, wherein said peripheral structures comprise a material with a thermal conductivity higher than the thermal conductivity of the ceramic member.
10. The actuator unit according to any preceding claim wherein the peripheral structures are formed of a plurality of layers.
11. The actuator unit according to any preceding claim, wherein an additional layer is provided on the peripheral structures on a side of the peripheral structures opposite to the side of the peripheral structures facing the electrical element.
12. The actuator unit according to Claim 11, wherein the additional layer comprises a patternable polymeric material.
13. A droplet ejection head comprising an actuator unit according to any preceding claim.
14. A droplet ejection apparatus comprising a droplet ejection head according to Claim 13.