GB2521990A

GB2521990A - A microelectromechanical switch and related fabrication method

Info

Publication number: GB2521990A
Application number: GB1305326.9A
Authority: GB
Inventors: Haydn Wadsworth; Samuel Strahan
Original assignee: Schrader Electronics Ltd
Current assignee: Schrader Electronics Ltd
Priority date: 2013-03-22
Filing date: 2013-03-22
Publication date: 2015-07-15
Also published as: GB201305326D0; WO2014146993A1

Abstract

A microelectromechanical (MEMs) device comprises a substrate structure on which there is formed a body 250and a movable component 246 that is movable with respect to the body into and out of a contact position where it engages with a contact surface 242B,244B that is perpendicular to a major plane of the substrate structure and carries a layer of electrically conductive material. The device may be used as a switch that is responsive to acceleration of a vehicle wheel, where movable component 246 is a proof mass that is suspended in a cavity 262 by resilient biasing springs 248. Such a MEMS device may be manufactured by forming a recessed pattern, preferably by deep reactive ion etching (DRIE) that defines a component and has a wall (fig 7, 254) that is perpendicular to the substrate major plane; depositing an electrically conductive layer (fig 8, 276) onto the recessed pattern and the wall; and removing the conductive layer from portions of the substrate that are parallel to its major plane.

Description

A Microelectromechanical Switch and Related Fabrication Method

Field of the Invention

The present invention relates to microelectromechanical (MEM5) devices and methods of manufacturing same. The invention relates particularly but not exclusively to a micromechanical motion detection switch suitable for use in a tyre monitoring device.

Background to the Invention

Microelectromechanical (MEM5) devices are well known and comprise micro-scale structures, typically formed from a semiconductor substrate, commonly silicon. Micro-scale fabrication techniques, sometimes referred to as micromachining, are normally required for their manufacture. An advantage of MEMs devices is that they are relatively small and so facilitate miniaturisation.

In the field of tyre monitoring, detecting vehicle motion is a key feature of tyre monitoring devices, such as tyre pressure monitoring (TPM) wheel-mounted transmitter units. Reliable detection of motion is critical for conserving the battery life of the transmitter unit since, when a vehicle is stationary, the transmitter unit can be configured to measure pressure and transmit less frequently.

Various solutions for detecting motion in a TPM unit have been devised, including using a roll switch, an accelerometer or shock sensors. Such solutions have commonly suffered from problems relating to size, reliability, cost and/or power consumption. In addition, accelerometer and shock sensor based solutions both require a relatively complex electronic interface to function correctly in a TPM unit.

It would be desirable to use the benefits of MEMs technology to mitigate the problems outlined above.

Summary of the Invention

A first aspect of the invention provides a microelectromechanical device comprising a substrate structure on which there is formed a body, at least one contact surface and a movable component, the movable component being movable with respect to the body into and out of a contact position in which it engages with said at least one contact surface, wherein said at least one contact surface is substantially perpendicularly disposed with respect to the major plane of said substrate structure, and carries at least one layer of electrically conductive material.

A second aspect of the invention provides a wheel monitoring unit comprising a microelectromechanical motion detecting switch, said switch comprising a substrate structure on which there is formed: a body; first and second electrical contacts comprising, respectively, first and second contact surfaces; and a movable component, the movable component being movable with respect to the body into and out of a contact position in which it engages with said contact surfaces, wherein said contact surfaces are substantially perpendicularly disposed with respect to the major plane of said substrate structure, and carry at least one layer of electrically conductive material, the switch being operable between switch states by movement of said movable component into and out of its contact position, and wherein said movable component is responsive to acceleration of said switch to move into and out of its contact position.

A third aspect of the invention provides a tyre pressure monitoring system comprising at least one wheel monitoring unit according to the second aspect.

A fourth aspect of the invention provides a method of manufacturing a microelectromechanical device from a substrate structure, said method comprising: forming in an active layer of said substrate structure a recessed pattern defining at least one component of said device, wherein said recessed pattern is defined by at least one wall that is substantially perpendicularly disposed with respect to the major plane of said substrate structure; depositing a layer of electrically conductive material onto said substrate structure covering said recessed pattern and said at least one wall; and removing said electrically conductive material trom portions of said substrate structure that are substantially parallely disposed with the major plane of said substrate structure.

Preferred features are recited in the dependent claims appended hereto.

One aspect of the invention may be embodied as a micro scale switch for detecting vehicle motion in a wheel mounted 1PM unit or other wheel mounted monitoring unit. The switch may be fabricated at least partly using standard MEMs processing technology.

Another aspect of the invention may be embodied as a method for forming switch contacts on a wall, in particular a sidewall, of a semiconductor substrate.

In some embodiments, a micro scale switch comprises a proof mass, which may be mechanically tree, formed on a substrate, typically a Silicon on Insulator (SOI) substrate. The proof mass may be mechanically free such that it is able to move with respect to the substrate, especially in response to movement of the substrate. The proof mass may be suspended with respect to the substrate by at least one integrally formed resilient biasing member. When the switch is subjected to acceleration the mass deflects (moves) against the resilient bias of the biasing member(s) and makes contact with electrical switch contacts. When contact is made an electrical conduction path is formed between the electrical contacts. In preferred embodiments, the electrical contacts are formed from a metallic or otherwise electrically conductive layer deposited on a side wall of the substrate. The conductive material results in reduced electrical resistance compared to bare silicon or heavily doped bare silicon. In embodiments where the switch is incorporated into a wheel mounted unit, centrifugal acceleration caused during wheel rotation triggers the switch.

Compared to a mechanical roll switch a MEMs switch has a much reduced size and additionally a reduced cost. The preferred MEMs switch has only two operating states -ON' and OFF' -and as such requires a much less complicated electronic interface than an accelerometer or shock sensor. The preferred MEMs switch consumes relatively little power in comparison to prior alt devices, in particular not drawing power in its OFF state.

Further advantageous aspects of the invention will become apparent to those ordinarily skilled in the art upon review of the following description of a specific embodiment and with reference to the accompanying drawings.

Brief Description of the Drawings

An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which: Figure 1 is a block diagram of an embodiment of a tyre monitoring system shown in conjunction with parts of a vehicle; Figure 2A is a schematic representation of a micro-scale switch device embodying one aspect of the invention, incorporated into a wheel mountable monitor embodying another aspect of the invention, the switch being shown in an open state; Figure 2B is a schematic representation of a micro-scale switch device embodying one aspect of the invention, incorporated into a wheel mountable monitor embodying another aspect of the invention, the switch being shown in a closed state; Figure 3 is a plan view of the micro-scale switch device embodying one aspect of the invention (shown without a protective cap); Figures 4 to 10 are respective side views of the switch device of Figure 3 shown in respective stages of fabrication; Figure 11 is a side view of the switch device of Figures 3 to 10, shown with a protective cap; and Figure 12 is a side view of the switch device of Figures 3 to 10, shown with the protective cap and a wire connector.

Detailed Description of the Drawings

Referring now to Figure 1 of the drawings, there is shown, generally indicated as 102, a tyre monitoring system shown in situ on a vehicle 100. For reasons of clarity, only those portions of the vehicle 100 and system 102 that are helpful in understanding some aspects of the present invention are shown.

The vehicle 100 includes wheels 104, 106, 108, 110, each wheel including a tyre mounted on a rim. The system 102 includes a control unit 112 (such as a vehicle engine control unit (ECU), or a Body Control Module (BCM)) and wheel mounted (in use) monitors 124, 126, 128, 130, which are commonly referred to as sensors, transmitters, wheel units, tyre monitors or the like. The monitors 124, 126, 128, measure tyre and/or other wheel characteristics and transmit corresponding data for reception and processing by the control unit 112. Typically, a respective monitor is associated with each wheel of the vehicle 100. The monitors 124, 126, 128, 130 may be mounted on any suitable part of the respective wheel, for example the rim, the tyre or the valve stem.

In typical embodiments, the monitors are capable of measuring tyre pressure and of transmitting data to the control unit 112, including but not limited to data representing the measured tyre pressure and usually also identification information uniquely identifying the respective monitor. Each of the monitors 124, 126, 128, 130 includes a suitably powered wireless transmitter, conveniently a radio frequency (RF) transmitter, and typically a pressure sensor for measuring the pressure of the gas (usually air) within the tyre. In such embodiments, the system 102 may be referred to as a tyre pressure monitoring system (TPMS). A battery or other convenient power source is also provided. Any suitable control unit may be used in the system 102. By way of example, in the illustrated embodiment, the control unit 112 includes a controller 132 (e.g. the vehicle ECU), a memory device 134 and a receiver 136 for receiving wireless transmissions from the monitors.

Referring now to Figures 2A and 2B there is shown a simplified schematic representation of an exemplary one of the wheel mountable monitors 124, 126, 128, 130, including a controller 120, which may take any convenient form, e.g. a suitably programmed microprocessor, microcontroller or other processor. The wheel monitor 124, 126, 128, 130 includes a switch device 140 embodying one aspect of the invention. The switch device 140 is electrically connected to the controller 120 such that the state of the switch 140 is detectable by the controller 120. The switch 140 includes first and second electrical contacts 142, 144 and a movable component 146 that this movable into and out of a contact position (Figure 2B) in which it is in contact with each of the contacts 142, 144 to form an electrical connection between them. When the movable component 146 is out of the contact position (Figure 2A) the contacts 142, 144 are electrically isolated from one another. Hence, the switch 140 is operable between first and second states, namely an ON (or closed) state in which the component 146 is in the contact position, and an OFF (or open) state in which the component 146 is out of the contact position. The controller 120 is electrically connected to the contacts 142, 144 and can detect which state the switch 140 is in by determining, in any convenient manner, whether or not the contacts 142, 144 are electrically connected.

The switch device 140 preferably includes biasing means 148 for causing the movable component 146 to adopt a rest position in the absence of any overriding forces, for example forces caused by motion of the switch 140. The biasing means preferably provides resilient bias and may comprise one or more springs or other resilient biasing members as is described in more detail hereinafter. In preferred embodiments, movable member 146 is out of the contact position when in the rest position.

In preferred embodiments, the switch device 140 is used to detect motion, especially but not exclusively rolling motion of the wheel 104, 106, 108, 110 in which the monitor 124, 126, 128, 130 is mounted during use. Typically, the state of the switch 140 is dependent on the acceleration experienced by the switch 140. Preferably, the switch 140 is responsive to changes in acceleration, especially a change from a rest state to a moving state and/or a change from a moving state to a rest state, to move between its on and off states. Accordingly, the preferred switch 140 is configured such that the movable component 146 is responsive to changes in acceleration of the switch 140 (especially resulting from a change from a rest state to a moving state and/or from a moving state to a rest state, or other change in the acceleration of the switch) to move into, or out of, its contact position. This may be achieved by any suitable means, for example by suspending the movable component 146 with respect to the body 150 of the switch 140, the contacts 142, 144 being provided on or otherwise fixed with respect to the body 150. Any suitable suspension means may be provided for this purpose, for example one or more suspension members coupling the movable component 146 to the body 150. In preferred embodiments, one or more biasing members 148 conveniently also serve to suspend the movable component 146 with respect to the body 150.

In preferred embodiments, the movable component 146 adopts a non-contact position when the switch 140 experiences acceleration below a threshold value.

In particular, the movable component 146 adopts its rest position when the switch experiences zero acceleration. The movable component 146 adopts the contact position when the switch 140 experiences acceleration above at or above the threshold value.

When the switch 140 is mounted in a wheel, it experiences angular acceleration and radial acceleration when the wheel is rolling. In the preferred embodiment, the switch 140 is positioned with respect to the wheel in order that it is responsive to the radial acceleration caused by rolling movement of the wheel, although in alternative embodiments it may be arranged to be responsive to angular acceleration. The movable component 146 is biased to adopt its rest position when the wheel is at rest (i.e. zero radial acceleration in this example). In the illustrated embodiment, the movable component 146 is out of its contact position when in the rest state. When the wheel is rolling the movable component 146 experiences radial acceleration that causes it to move from the rest position and, when the acceleration meets or exceeds the threshold value, into the contact position. In the preferred embodiment, the threshold value is selected such that the switch 140 changes state when the wheel starts to roll after having been at rest, and such that the changed state is maintained while the wheel continues to roll (at least at or above a speed that maintains the radial acceleration at or above the threshold value). When the wheel stops rolling (or drops to a speed at which the radial acceleration drops below the threshold value), the movable member 146 moves out of the contact position (and may return to the rest position). Hence, by monitoring the state of the switch the controller 120 can determine if the vehicle 102 is moving or stationary. More generally, depending on the acceleration threshold value, the controller 120 can determine if the vehicle 102 is moving above or below a threshold speed by monitoring the state of the switch 140.

The acceleration threshold value may be determined (at least in part) by the bias exerted on the movable component 146 by the suspension and/or biasing means 148, since this bias must be overcome in order that the movable component 146 can adopt and maintain the contact position.

In the illustrated embodiment, the switch 140 adopts its ON state in response to the movement described above, and its OFF state otherwise. In alternative embodiments, the switch 140 may be configured to adopt its OFF state in response to the movement described above, and its ON state otherwise, e.g. when at rest. This may for example be achieved by appropriate configuration of the biasing means.

Referring now to Figures 3, 10, 11 and 12 of the drawings, there is shown a preferred embodiment of the switch device, generally indicated as 240. The switch device 240 and its operation are similar to the switch 140, like numerals being used to denote like parts and the same or similar description applying as would be apparent to a skilled person unless otherwise stated herein.

The switch 240 is a micro-scale switch, advantageously implemented as a Microelectromechanical (MEMs) device.

The switch 240, or other device, comprises micro-scale switch components (which may be referred to as micromechanical components and which in this example comprise first and second contacts 242, 244, movable component 246 and resilient biasing and suspending members 248) integrally formed in a substrate structure 260. The switch 240 may be said to comprise an integrated microelectromechanical device.

The substrate structure 260 is typically formed from semiconductor material(s), although other materials such as polymers, metals or ceramics may alternatively be used. Most conveniently, the substrate structure 260 is silicon based, comprising one or more layers of silicon, suitably doped. In the following description of a specific embodiment, it is assumed that the substrate structure 260 comprises a silicon semiconductor structure, in particular a Silicon-on-Insulator (501) substrate.

The switch components are formed in the substrate structure 260 using fabrication techniques that typically include deposition of material layers onto one or more surfaces of the substrate, creation of one or more patterns in or on the layers, and etching the substrate and/or layers. The specific fabrication technique may vary depending on the material from which the substrate structure 260 is formed.

The movable component 246 comprises a proof mass. The resilient biasing and suspending members 248 may be described as springs. The proof mass 246 is suspended in a cavity 262 formed in the substrate structure 260 by the springs 248. The springs 248 couple the proof mass 246 to a body portion 250 of the substrate structure 260 such that the proof mass 246 is movable with respect to the body portion 250 in the direction indicated by arrow A. The proof mass 246, the springs 248 and the body 250 are integrally formed in the substrate structure 260.

In the illustrated embodiment four springs 248 are shown although in alternative embodiments more or fewer springs may be provided. The illustrated proof mass 246 is substantially rectangular in plan view. As can be seen from Figures 10 and 11, the illustrated proof mass 246 is substantially rectangular in transverse cross section (when viewed from the left or right sides in the orientation shown in Figure 3). The other transverse cross section (i.e. when viewed from the top or bottom sides in the orientation shown in Figure 3). It may also be substantially rectangular. The proof mass 246 may therefore be substantially cuboid in shape.

In alterative embodiments, any one or more of the transverse or plan cross sectional shapes may be other than rectangular.

Conveniently, a respective spring 248 is provided at least one, or a respective, corner of the proof mass 246, especially with respect to a plan view of the proof mass 246. The springs 248 may altematively be located elsewhere, for example between the respective corners of one or more sides of the proof mass 246 (with respect to a plan view of the proof mass), and/or above and/or below the proof mass (with respect to the side view of Figures 10 and 11). In any case one or more springs (or other suspending devices) are provided to suspend the proof mass 246 in the cavity 262 from the body 250 (or otherwise to couple the proof mass to the body), and to allow movement of the proof mass 246 with respect to the body 250 into and out of its contact position.

The illustrated spring 248 each comprises a length of substrate material extending between the body 250 and the proof mass 246. Each spring 248 may be limited, typically suspended, in a respective cavity 252 formed in the substrate structure 260. Each spring 248 is shaped and dimensioned to provide a resilient bias, for example shaped to define at least one turn. The shape and dimensions of the length of substrate material, together with the inherent resilience of the substrate material, allows the length of the material to act as a spring, as well as suspending the proof mass from the body 250. In alternative embodiments, the means for suspending the proof mass 246 may be provided separately from the means for biasing the proof mass into its rest position. For example, the length of substrate material connecting the proof mass to the body may be shaped and dimensioned to allow movement of the proof mass into and out of the contact position, but without providing sufficient resilient bias to return the proof mass to a rest position. Separately, one or more springs, or other resilient biasing devices (not shown) may be coupled between the proof mass and the body.

The proof mass 246 is coupled to the body 250 such that it is capable of movement with respect to the body 250 into and out of the contact position in the direction indicated by arrow A in Figures 3 and 10. Advantageously, the direction of movement A is substantially parallel with the major plane of the substrate structure 260, i.e. substantially parallel with the respective plane of the, or each layer of the substrate structure. To facilitate this movement, it is preferred that the springs 248, or other suspension devices, are also disposed substantially parallel with the major plane of the substrate structure 260.

In the illustrated embodiment, the proof mass 246 is biased to adopt a rest position in which it is out of its contact position, as shown in Figure 3. This may be achieved by any suitable arrangement of the springs 248 (e.g. by any suitable configuration of the shape, position and/or orientation of the springs). When the switch 240 is subjected to acceleration, the proof mass 246 tends to move towards the contact position, i.e. towards the contacts 242, 244. When the acceleration meets or exceeds the threshold level, the proof mass 246 overcomes the bias exerted by the springs 248 and contacts the contacts 242, 244 and makes electrical contact between the contacts 242, 244. When the acceleration drops below the threshold level, the proof mass returns to the rest position under the bias of the springs 248 (in the absence of any other overriding forces).

In an alternative embodiment (not illustrated), the proof mass 246 is biased to adopt a rest position in which it is in its contact position. This may be achieved by suitable arrangement of the springs 248 (e.g. by any suitable configuration of the shape, position and/or orientation of the springs). When the switch 240 is subjected to acceleration, the proof mass 246 tends to move away from the contact position, i.e. away from the contacts 242, 244. When the acceleration meets or exceeds the threshold level, the proof mass 246 overcomes the bias exerted by the springs 248, and breaks contact with the contacts 242, 244 to break electrical contact between the contacts 242, 244. When the acceleration drops below the threshold level, the proof mass returns to the rest position under the bias of the springs 248 (in the absence of any other overriding forces).

The contacts 242, 244 are preferably formed from electrically conductive material, typically metal. Each contact 242, 244 includes a respective first portion 242A, 244A disposed in a plane that is substantially parallel with the major plane of the substrate structure 260 (substantially horizontal as viewed in Figure 10).

The first portion 242A, 244A is amenable to being formed by conventional fabrication techniques for depositing layers of (e.g. metallic) material on a substrate (substantially parallel with the major plane of the substrate). The contacts 242, 244 include a respective second portion 242B, 244B disposed in a plane that is substantially perpendicular with the major plane of the substrate structure 260 (substantially vertical as viewed in Figure 10). The second portion 242B, 244B may comprise one or more deposited layers of (e.g. metallic) material. In the preferred embodiment, each contact 242, 244 is formed on a respective separate block surrounded by a cavity 253 formed in the body 250.

The respective first and second portions 242A, 242B and 244A, 244B of each contact may be joined or spaced apart. In either case, an electrical path is provided between the respective first and second portions 242A, 242B and 244A, 244. Where the portions are joined, the electrical path may be along the respective portions (i.e. along the surface of the substrate). Where the portions are spaced apart, the electrical path may be through the active layer of the substrate 260.

Advantageously, the proof mass 246 makes contact with the respective second portions 242B, 244B when in its contact position. Conveniently a side face 254 or one or more contacting portions of the side face 254, of the proof mass 246 makes contact with the second portions 242B, 244B. The side face 254 (or relevant contacting portions thereof) is advantageously disposed in a plane that is substantially perpendicular with the major plane of the substrate structure 260.

Depending on the electrical conductivity of the portion of the substrate structure that forms the proof mass, an electrical connection may be made between the contacts 242, 244 by the bare side face portions 254. Preferably however an electrical contact 255 is provided on the face 254, for example comprising one or more layers 256 of electrically conductive material (typically metal) deposited on the side face 254 shaped and dimensioned to extend between the contact portions 242B, 244B when the proof mass 246 is in its contact position, thereby creating good electrical contact between the contacts 242, 244.

The proof mass 246 may include one or more through-holes 258 for facilitating the preferred fabrication process, as described in more detail hereinafter.

Referring now to Figures 4 to 12 of the drawings, there is described an embodiment of a method of manufacturing a microelectromechanical device embodying a further aspect of the invention. The embodiment is described in the context of manufacturing the switch device 240, but the invention is not limited to such.

Referring initially to Figure 4, the substrate structure 260 conveniently comprises a Silicon on Insulator (501) substrate, although other substrates could alternatively be used. The preferred substrate structure 260 comprises a top layer 264 which may be referred to the active' layer (also known as the "device layer"). The active layer 264 is used to form the active moving, or other mechanical, components of the switch device 240 (or other MEMs device). Any electrical connections and/or circuitry may also be formed on the top layer 264.

In this example the top layer 264 is formed from semiconductor material, conveniently suitably doped silicon, although other materials may be used.

Typically the top layer 264 is approximately 10 to 50 microns in thickness.

A second layer 266 is provided beneath the top layer 264 and serves as an etch stop layer. The second layer 266 is formed from an electrically insulating material, for example silicon dioxide, and may be referred to as a buried oxide or BOX' layer. The box layer 266 has a thickness typically from a few hundred nanometres up to a few microns.

A third layer 268, which may be referred to as the "mechanical layer", provides mechanical support to the structure 260. The third layer 288 is conveniently formed from silicon and may be, for example, approximately 200 to 500 microns in thickness.

In Figures 4 to 12, the substrate structure 260 is shown in side view (from the right as viewed in Figure 3). The major plane of the structure 260 is therefore horizontally disposed as viewed in Figures 4 to 12. The layers 264, 266, 268 are substantially parallel with one another and lie substantially in or parallel with the major plane of the structure 260.

With reference to Figure 5, the preferred fabrication process is started by depositing an electrically conductive, typically metallic, layer on a top surface 270 of the top layer 264. The purpose of this deposition is to form an electrical contact to the top surface 270, which in this example is used to form the first portion 242A, 244A of contacts 242, 244. The conductive layer is deposited on the top surface 270, using any suitable conventional deposition technique. A mask (not shown) is provided over the layer and the layer is then etched, typically by any suitable conventional chemical etching process, to leave a conductive pattern on the top surface 270 which is determined by the shape of the mask. In this example, the mask is configured such that the pattern defines electrical pads 272 (only one visible) for use as the contact portions 242A, 244A.

Referring now to Figure 6, the next step of the preferred process is to deposit a sacrificial layer 274 onto the top surface 272. The sacrificial layer 274 acts as an etch mask to the active silicon layer 264, which is to be etched in subsequent steps. The sacrificial layer 274 may be blanket deposited on the top surface 270 by any suitable conventional technique, for example Chemical Vapour Deposition (CVD), covering the exposed top surface 270 and any layers deposited thereon, in this case the pads 272. The sacrificial layer is pattern etched such that it is shaped to match the desired shape of the structures of the device being fabricated, which in this example comprise the proof mass and springs, to allow those structures to be defined by the subsequent fabrication processes. The sacrificial layer 274 also protects the metal contact pads 272 from any unwanted etching. The sacrificial layer 274 may be formed from any suitable material, for example silicon dioxide, silicon nitride or suitable types of photo-resist. The masking and etching of the sacrificial layer to create the desired pattern may be performed by any suitable conventional fabrication techniques.

Once the sacrificial layer 274 has been patterned, it defines the required shapes and dimensions of the micromechanical structures of the MEMs device being created (at least in plan as in Figure 3). With reference to Figure 7, the same pattern is then etched in the active layer 264. Advantageously an anisotropic etching process is used, preferably involving plasma (or ionic) etching. In the preferred embodiment, an etching process known as Deep Reactive Ion Etching (DRIE) is employed for this purpose. DRIE involves successive etching and deposition steps in Sulphur Hexaflouride (SF6) and Octafluoro-cyclobutane (04F5) and, advantageously, facilitates the formation of relatively tall vertical etch profiles, which may for example possess an aspect ratio of up to 50:1. A suitable embodiment of the DRIE process is commonly known as the Bosch process, a description of which is available in United States Patent 5,501,893. The buried oxide layer 266 acts as a highly effective etch stop against the DRIE process.

Alternatively, any other suitable conventional pattern forming fabrication technique may be used.

With the desired micromechanical structures (in this case the proof mass 246 and springs 248) defined in plan, a further blanket deposition is performed to create an electrically conductive, typically metallic, layer 276 over the sacrificial layer 274 and any exposed portions of the active layer 264 and etch stop layer 266 as applicable. As can be seen from Figure 8, the conductive layer 276 covers not only surfaces that are parallel with the major plane of the structure 260, but also surfaces that are perpendicular to it, including in this example the side walls 278 of the etched sacrificial and active layers 274, 264.

It is advantageous to the operation of the switch device 240 that the metal layer 276 is deposited such that the side walls 278 are covered with a substantially uniform thickness. It is preferred, therefore, to use a sputtering or CVD metal deposition process to deposit the layer 276. Other metal deposition techniques such as evaporation, which is a common process in the silicon industry, may be used, but may be less suitable for the present application since the sidewall metal coverage may not be adequate.

The conductive material used to form the layer 276 is selected to suit the etching procedures to which it is exposed during the fabrication process. For example, as is described in more detail below with reference to Figure 10, the preferred fabrication process includes a mechanical release step in which hydrofluoric (HF) acid is used. Some metals are not sufficiently resistant to HF acid to withstand this process. For example, aluminium does not stand up to a prolonged HF etching procedure. It is preferred therefore to use a metal such as tungsten to form the layer 276 since tungsten's etch rate in HF is close to zero. For similar reasons, tungsten may be used to form the contact pads 272.

With reference to Figure 9, once the conductive layer 276 is deposited, a further etching procedure is performed. Advantageously an anisotropic etching process is used, preferably involving plasma (or ionic) etching. In the preferred embodiment, a DRIE procedure is employed. The anisotropic nature of the etching process causes etching to be performed faster in the vertical direction than in the horizontal direction. Therefore, with the structure 260 disposed substantially horizontally (as viewed in Figure 9), the further DRIE procedure is performed such that only the portions 280 (Figure 8) of the conductive layer 276 on substantially horizontally disposed surfaces are removed (completely), and that the portions 282 of the conductive layer 276 on the side walls 278 remain.

This may be achieved by controlling the timing of the, or each, step of the DRIE process.

During a typical fabrication process, a suitable thickness for layer 276 is chosen such that good adhesion is achieved to the underlying substrate. The thickness may depend on the equipment being used to deposit the conductive material, and/or on processing parameters such as temperature and chamber pressure, and/or on any substrate treatment prior to deposition. An approximate thickness of 200 nanometres is typical.

The timing of the etch performed as part of the second DRIE process may be determined by the etch rate of the material used to form the conductive layer 276 and/or on etching conditions such as process pressure and temperature. A suitable etch rate can readily be determined experimentally. With the etch rate determined, a suitable etch time can be determined by calculating how long it takes to etch the relevant thickness of layer 276 (e.g. 200 nanometres). A small amount of over etching may be caused to ensure complete removal of metal from the horizontal surfaces.

The formation of the conductive portions 282 on the side walls 278 facilitates the operation of the switch device 240. In particular, respective conductive portions 282' (only one visible) on the side wall 278' adjacent the contact pads 272 serve as the contact portions 242B, 244B. It is noted that the respective portion 282' for each contact 242B, 244B are provided separately on the wall 278' as a result of the earlier pattern steps whereby the contacts 242, 244 are defined on separate blocks in cavity 253. The conductive portion 282" on the side wall 278" opposite the side wall 278' serves as the electrical contact 255 of the proof mass 246.

In Figure 3, contact portions 242B and 244B are shown comprising a plurality of projections or bumps. This is intended to illustrate that the contact surfaces of portions 242B and 244B (and/or of contact 255) are preferred not to be flat, since two flat surfaces, once engaged, may become stuck together (known as "stiction" in MEMs fabrication and operation). Stiction can be reduced by the provision of projections which reduce the contact surface area. Reducing the surface area of the contact region also causes the contacting parts to come into contact with each other with a greater pressure, ensuring a more reliable ohmic contact. The number of bumps does not have to be 3 (as illustrated) -it could be any number from 1 upwards.

Referring now to Figure 10, the next step of the preferred process is the mechanical release of the micromechanical structures, in this case the proof mass 246 and springs 248. This is achieved by removing the etch stop layer 266 from underneath the structures 246, 248. Removal of the etch stop layer 266 from underneath these structures allows the respective portions of the active layer 264 to be mechanically free with respect to the substrate structure 260, more particularly the body 250. As such, the proof mass 246 is able to move with respect to the body 250 in the direction of arrow A. The etch stop layer 266 layer may be removed using any suitable conventional etching process, for example using either liquid or vapour hydrofluoric acid (HF).

Through-holes 258 facilitate this process. Conveniently, any remaining portions of the sacrificial layer 274 can be removed simultaneously if possible. For example, HF acid removes the sacrificial layer 274 if it is made from a suitable material, e.g. silicon dioxide. Alternatively the sacrificial layer 274 may be removed with an additional step, preferably prior to the removal of the etch stop layer 266 (but after the second DRIE procedure).

It is noted that in the illustrated example, the box layer 266 is not removed from underneath the contact pad 272 due to the absence of any through-holes in the active layer 264 in the region of the pad 272 (and similarly for the opposite (left as viewed) end of the structure 260). The box layer 266 beneath the springs 248 is removed since the springs are formed by relatively thin sections of substrate material and so do not require through-holes.

Referring now in particular to Figures 11 and 12, to protect the switch device 240 (or other MEMs device) from its operating environment it is preferred to cover the MEMs structure, conveniently by provision of a cap 290, typically a cap wafer 290. The cap wafer 290 may have one or more cavities 292 etched in its underside and being shaped and dimensioned to ensure that the cap 290 does not interfere with movement of the movable MEMs components, in this case the proof mass 246 and springs 248. The cap wafer 290 is typically bonded, for example using a glass frit material (or any other suitable bonding material), to the substrate structure 260, as indicated by bonding material 294 in Figures 11 and 12.

A contact window 296 is provided (e.g. pre-formed, or formed (typically etched) after the cap 290 has been bonded to the substrate 260) to expose the contact pads 272. A respective wire 298 (only one visible) is connected to the contact pads 272 to enable electrical connection of the respective contacts 242, 244 to external circuitry, for example the controller 120.

In preferred embodiments, the fabrication is performed such that, when the cap 290 is in place, the proof mass 244 is enclosed in a partial vaccum. The fabrication process can be controlled in order to set the pressure of the partial vaccum and so to determine a level of mechanical damping for the movement of the proof mass 244, which in turn may effect the sensitivity of the device 240. A relatively high pressure within the cavity 262 creates a relatively high level of mechanical damping. Relatively high damping makes the operation of the device 240 more resistant to vibration and large shock loads. A relatively low pressure in the cavity 262 causes a lower level of damping. Lower damping may cause the device 240 to be more susceptible to damage from shock loads, but may also cause it to respond more quickly to applied acceleration. It is noted that the number and/or arrangement of through-holes 258 in the proof mass 244 may effect damping in the z direction (perpendicular to motion). More through-holes in the proof mass causes reduced damping and vice versa.

In the illustrated embodiment, each contact 242, 244 comprises a vertical (as viewed) conductive layer or pad 282', electrically isolated from one another, as the respective second portion 242B, 244B. The contacts 242, 244 are provided on respective blocks from in the substrate 264 but each being separate from the main body 250. In an alternative embodiment (not illustrated) one or other or both of the contacts 242, 244 may be provided on the main body 250 rather than on a separate block formed from the main body. At least one and optionally both of the contacts 242, 244 may electrically isolated from the main body 250. This may be achieved by providing the, or each, contact on blocks that are physically separated from the body 250 by, for example, the aforementioned cavity. In further alternative embodiments (not illustrated), either one or other of the contacts 242, 244 may be provided by other electrical contact means (instead of a vertical conductive layer), e.g. a side wall of the substrate 264 itself, or by the pad 272 (assuming that it is substantially flush with the sidewall 278') in cases where the side wall 278" carries the conductive contact 282/255. Accordingly, the device 240 may have a single vertical (as viewed) contact pad, the other contact being made to the substrate itself or some other electrical contact means.

It will be apparent that aspects of the present invention may be used to manufacture MEMs devices other than switches, especially where at least one substantially vertically orientated side wall carries one or more electrically conductive pads or other contacts.

In alternative embodiments, the switch device 240 or other MEMs device, may be made by other fabrication processes than described herein. For example, the MEMs structure may be formed by electroplating a substrate, or other deposition based process, in which case the structure may be built up in layers rather than etched from the substrate.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims

CLAIMS1. A microelectromechanical device comprising a substrate structure on which there is formed a body, at least one contact surface and a movable component, the movable component being movable with respect to the body into and out of a contact position in which it engages with said at least one contact surface, wherein said at least one contact surface is substantially perpendicularly disposed with respect to the major plane of said substrate structure, and carries at least one layer of electrically conductive material.
2. A microelectromechanical device as claimed in claim 1, wherein said at least one contact surface is provided on or otherwise fixed with respect to said body.
3. A microelectromechanical device as claimed in claim 1 or 2, wherein said at least one contact surface provides at least one of first and second electrical contacts.
4. A microelectromechanical device as claimed in claim 3, wherein said at least one contact surface comprises first and second spaced apart contact surfaces each being substantially perpendicularly disposed with respect to the major plane of said substrate structure, and carrying at least one layer of electrically conductive material, and wherein each of said first and second contact surfaces provides a respective one of said first and second electrical contacts.
5. A microelectromechanical device as claimed in claim 4, wherein said first and second electrical contacts each includes a respective top contact surface being substantially parallely disposed with respect to the major plane of said substrate structure, carrying at least one layer of electrically conductive material and being electrically connected to the first and second contact surfaces respectively.
6. A microelectromechanical device as claimed in claim 4 or 5, wherein at least one of, and preferably both of, said first and second electrical contacts is provided on a respective separate block formed on said substrate structure.
7. A microelectromechanical device as claimed in any preceding claim, wherein said movable component comprises a third contact surface for engaging with said at least one contact surface, said third contact surface being electrically conductive.
8. A microelectromechanical device as claimed in claim 8, wherein said third contact surface is substantially perpendicularly disposed with respect to the major plane of said substrate structure, and carries at least one layer of electrically conductive material.
9. A microelectromechanical device as claimed in claim 7 or 8 when dependent on any one of claims 4 to 6, wherein said third contact surface makes electrical contact between said electrical contacts when said movable component is in said contact position.
10. A microelectromechanical device as claimed in any preceding claim, wherein said movable component is movable into and out of said contact position in a direction that is substantially parallel with said major plane.
11. A microelectromechanical device as claimed in any preceding claim, further including suspension means for suspending said movable component with respect to said body.
12. A microelectromechanical device as claimed in claim 11, wherein said movable component is suspended by said suspension means in a cavity formed in said substrate structure.
13. A microelectromechanical device as claimed in any preceding claim, wherein further including resilient biasing means for resiliently biasing said movable component to a rest position.
14. A microelectromechanical device as claimed in claim 13, wherein said movable component is out of said contact position when in said rest position.
15. A microelectromechanical device as claimed in claim 1301 14 when dependent on claim 11 or 12, wherein said suspension means is configured to provide said resilient biasing means.
16. A microelectromechanical device as claimed in claim 15, wherein said suspension means comprises at least one spring structure coupled between said body and said movable component.
17. A microelectromechanical device as claimed in any one of claims 11 to 16, wherein said suspension means comprises at least one length of material formed from said substrate structure.
18. A microelectromechanical device as claimed in any one of claims 11 to 17, wherein said movable component, said suspension means and said body are integrally formed in said substrate structure.
19. A microelectromechanical device as claimed in any preceding claim, wherein said movable component comprises a proof mass.
20. A microelectromechanical device as claimed in any preceding claim, wherein said movable component is substantially cuboid in shape.
21. A microelectromechanical device as claimed in any one of claims 11 to 20, wherein said suspension means are disposed substantially in a plane that is parallel with said major plane.
22. A microelectromechanical device as claimed in any preceding claim, wherein said substrate structure comprises an active layer supported by a support layer, preferably with an etch stop layer located between said active layer and said support layer, and wherein said body, said movable component and said at least one contact surface are formed in said active layer.
23. A microelectromechanical device as claimed in claim 22, wherein said active layer is formed from semiconductor material, for example semiconducting silicon.
24. A microelectromechanical device as claimed in any one of claims 4 to 23, wherein said device is operable as a switch whereby said switch is operable between switch states by movement of said movable component into and out of its contact position.
25. A microelectromechanical device as claimed in claim 24, wherein said device is operable as a motion detecting switch whereby said movable component is responsive to acceleration of said device to move into and out of its contact position.
26. A microelectromechanical device as claimed in any preceding claim, further including a cover enclosing at least said movable component and said at least one contact surface, and preferably also said body.
27. A microelectromechanical device as claimed in claim 26, wherein said cover comprises a cap fitted to said substrate structure.
28. A microelectromechanical device as claimed in claim 26 or 27 when dependent on any one of claims 3 to 25, wherein said cover includes a window positioned to expose said electrical contacts.
29. A microelectromechanical device as claimed in any one of claims 26 to 28, wherein said movable component is located in a cavity defined by said cover and said substrate structure, in which cavity is provided a partial vacuum.
30. A wheel monitoring unit comprising a microelectromechanical motion detecting switch, said switch comprising a substrate structure on which there is formed: a body; first and second electrical contacts comprising, respectively, first and second contact surfaces; and a movable component, the movable component being movable with respect to the body into and out of a contact position in which it engages with said contact surfaces, wherein said contact surfaces are substantially perpendicularly disposed with respect to the major plane of said substrate structure, and carry at least one layer of electrically conductive material, the switch being operable between switch states by movement of said movable component into and out of its contact position, and wherein said movable component is responsive to acceleration of said switch to move into and out of its contact position.
31. A wheel monitoring unit as claimed in claim 30, wherein said movable component is responsive to acceleration of said switch caused by rolling movement of a wheel on which said unit is mounted in use to move into and out of its contact position.
32. A wheel monitoring unit as claimed in claim 30 or 31, wherein said movable component is responsive to radial and/or angular acceleration of said switch to move into and out of its contact position.
33. A wheel monitoring unit as claimed in any one of claims 30 to 32, wherein said movable component is responsive to acceleration corresponding to transitions between non-rolling and rolling states of a wheel on which said unit is mounted in use to move into and out of its contact position.
34. A tyre pressure monitoring system comprising at least one wheel monitoring unit as claimed in any one of claims 30 to 33.
35. A method of manufacturing a microelectromechanical device from a substrate structure, said method comprising: forming in an active layer of said substrate structure a recessed pattern defining at least one component of said device, wherein said recessed pattern is defined by at least one wall that is substantially perpendicularly disposed with respect to the major plane of said substrate structure; depositing a layer of electrically conductive material onto said substrate structure covering said recessed pattern and said at least one wall; and removing said electrically conductive material from portions of said substrate structure that are substantially parallely disposed with the major plane of said substrate structure.
36. A method as claimed in claim 35, wherein said removing of electrically conductive material involves using an anisotropic etching process directed to etch in a direction substantially perpendicular to said major plane.
37. A method as claimed in claim 35 or 36, wherein said removing of electrically conductive material involves using a plasma etching process.
38. A method as claimed in any one of claims 35 to 37, wherein said removing of electrically conductive material involves using Deep Reactive Ion Etching (DRIE).
39. A method as claimed in any one of claims 35 to 38, performed on a substrate structure in which said active layer is formed from semiconductor material, for example semiconducting silicon, or metal, or a polymer.
40. A method as claimed in any one of claims 35 to 39, wherein said electrically conductive material comprises metal.
41. A method as claimed in any one of claims 35 to 40, wherein said forming of said recessed pattern involves using an anisotropic etching process directed to etch in a direction substantially perpendicular to said major plane.
42. A method as claimed in any one of claims 35 to 41, wherein said forming of said recessed pattern involves using a plasma etching process.
43. A method as claimed in any one of claims 35 to 42, wherein said forming of said recessed pattern involves using Deep Reactive Ion Etching (DRIE).
44. A method as claimed in any one of claims 35 to 43, wherein said forming of said recessed pattern involves depositing a sacrificial layer on said active layer, forming a pattern in said sacrificial layer, said pattern defining said at least one component of said device and exposing portions of said active layer corresponding to said pattern, and etching said exposed portions of said active layer.
45. A method as claimed in claim 44, wherein said depositing of electrically conductive material involves depositing said layer of electrically conductive material over the patterned sacrificial layer and etched portions of said active layer.
46. A method as claimed in any one of claims 35 to 45, wherein said substrate structure includes an etch stop layer beneath said active layer, and wherein said forming of said recessed pattern involves etching said active layer to expose portions of said etch stop layer corresponding to said recessed pattern.
47. A method as claimed in claim 46, further including removing the exposed portions of said etch stop layer and portions of said etch stop layer located beneath at least one portion of said active layer corresponding to at least one of said at least one components.