Microelectronic wireless transmission device

A microelectronic wireless transmission device including:

TECHNICAL FIELD

The invention relates to a microelectronic wireless transmission device, operating autonomously. Such a device is advantageously fitted with a sensor in order to form an autonomous detection structure, able to transmit measurement results by radio waves. The invention also relates to a method for producing such a microelectronic wireless transmission device.

STATE OF THE PRIOR ART

It is known to produce microelectronic measuring devices of the wireless transmitter type. Such wireless microsensors, which are therefore autonomous, are produced in the form of small-size components, each of which includes a sensor, a microprocessor, an energy source and a data transmission system. Their function is to measure one or more physical parameters (temperature, pressure, vibration, CO2, etc.), to convert these measurements into quantifiable values, and to transmit them by radio waves for processing. One advantage of these microsensors is that it is possible to disperse a large number of them without having to provide maintenance for them.

A proportion of the microsensor's elements are produced, for example, on a ceramic of the HTTC or LTTC type, or on a printed circuit, and then encapsulated in a polymer. These technologies are, however, complex and expensive to implement. In addition, the miniaturisation and pattern resolution which may be obtained with such microsensors are limited. And the performance characteristics of these devices at the envisaged transmission frequencies, which are between several tens of MHz and several tens of GHz, are very sensitive to the geometric factors of the devices. In addition, the thermal expansion difference between the support (ceramic or printed circuit) and the silicon electronic components may lead to failures of such microsensors over time.

The document “High-efficiency 60 GHz antenna fabricated using low-cost silicon micromachining techniques” by N. Hoivik et al., Antennas and Propagation Society International Symposium, 2007 IEEE, Honolulu, Hi., 9-15 Jun. 2007, pages 5043-5046, describes a microelectronic wireless transmitter device in which an integrated circuit and an antenna are produced on a silicon support. These elements are packaged by a silicon cap transferred on to the silicon support, in which a reflective cavity is formed, intended to be positioned opposite the antenna, and the back wall of which is covered with several metal layers.

Compared to devices including a ceramic support or a printed circuit, use of a silicon support enables the achievable miniaturisation to be improved, and therefore the performance characteristics of the device. Conversely, the solution presented in this document requires that a metallic conformal deposit is produced throughout the cap, which is a delicate step to implement, and which implies risks of dropouts in the metallic layer deposited in this manner.

DESCRIPTION OF THE INVENTION

Thus there is a need to propose a new type of microelectronic wireless transmission device which does not have the disadvantages of the devices of the prior art, i.e. which does not have the disadvantages relating to the use of a ceramic support or a printed circuit, and which does not require the use of metallic conformal deposits.

To accomplish this, one embodiment proposes a microelectronic wireless transmission device including at least:a substrate able to be traversed by radio waves intended to be emitted by the microelectronic device,an antenna,an electrical power supply,an integrated circuit, electrically connected to the antenna and to the electrical power supply, and able to transmit to the antenna electrical signals intended to be emitted by the antenna in the form of the said radio waves,a cap rigidly connected to the substrate and forming, with the substrate, at least one cavity in which the antenna and the integrated circuit are positioned, where the cap comprises an electrically conductive material connected electrically to an electrical potential of the electrical power supply and/or of the integrated circuit, and able to form a reflector with regard to the radio waves intended to be emitted by the antenna.

Integration of an autonomous miniature system is thus proposed, where this system is able to communicate with the external environment by radiofrequencies and including an integrated circuit, for example an ASIC (Application-Specific Integrated Circuit) electrically connected to a power source and an antenna, and a cap the functions of which are to protect the elements, or components, positioned in the cavity, and to form a radio wave reflector. The cap comprises one or more electrically conductive materials and is connected to an electrical potential of the electrical power supply and/or of the integrated circuit, in order to perform the function of wave reflector.

The cap's electrically conductive material is advantageously highly doped silicon (for example of approximately 1 mOhm·cm to several tens of mOhm·cm, or between approximately 1 and 100 mOhm·cm).

The microelectronic device may be of millimetric dimensions, and the device's communication frequencies band may be around 60 GHz.

The integrated circuit and the antenna may be positioned in two separated cavities formed between the cap and the substrate. Such a configuration can be advantageous in preventing the integrated circuit from disturbing the antenna, or in preventing the integrated circuit from being disturbed by the antenna. In this case, electrical connections between the integrated circuit and the antenna may pass from one cavity to the other.

As a variant, the integrated circuit and the antenna may alternatively be positioned in two portions of the same cavity which are separated from one another by an electromagnetic screen, where such a screen enables electromagnetic disturbances between the integrated circuit and the antenna to be prevented. In this case, electrical connections between the integrated circuit and the antenna may pass under the electromagnetic screen.

The substrate able to be traversed by radio waves may comprise one or more materials able to be traversed by the said radio waves, such as for example non-doped silicon, SiO2, or polymer. As a variant, it is possible for a portion only of the substrate to comprise one or more materials able to be traversed by the radio waves. This portion of the substrate forms a “window” through which the radio waves can be emitted.

The electrical power supply may include at least one microbattery and/or may be positioned in the cavity or one of the cavities.

The electrical connections between the integrated circuit and the antenna, between the integrated circuit and the electrical power supply, and between the cap and the electrical power supply, may include wires and/or electrically conductive tracks positioned on the substrate.

The electrically conductive tracks may comprise at least one electrically conductive material, advantageously a metallic material, similar to at least one electrically conductive material of the antenna. The electrically conductive tracks can thus be produced from one or more metal layers which are also used to produce the antenna.

The side walls of the cavity may be formed by the cap.

The cap may be rigidly connected to the substrate by at least one sealing bead comprising at least one electrically conductive material.

In this case the electrical power supply and/or the integrated circuit may be electrically connected to the sealing bead which is in contact with the cap. The cap may thus be electrically connected to an electrical potential of the electrical power supply and/or of the integrated circuit via the sealing bead.

The sealing bead may comprise at least one metal material similar to at least one metal material of the antenna. The sealing bead may thus be produced from one or more metal layers which are also used to produce the antenna.

The device may also include at least one sensor positioned in the cavity, or one of the cavities, and electrically connected to the integrated circuit, such that the sensor is able to transmit at least one measuring signal to the integrated circuit, where at least one portion of the electrical signals transmitted by the integrated circuit to the antenna may depend on the measuring signal transmitted to the integrated circuit by the sensor. In such a configuration, in which the sensor is encapsulated in the cavity, the target applications may be those in which physical measurements are made without direct contact with the exterior, for example measurement of a vibration or of radiation. Such a microelectronic device may meet needs in the fields of industry, transport, housing and security, for example to anticipate the failure of mechanical parts by monitoring the vibrations. Such a device may also be a medical device allowing cardiac activity to be monitored (for example, positioned at the end of a cardiac stimulation probe).

Such a device may also include at least one energy recovery device electrically coupled to the electrical power supply.

Another embodiment proposes a method for producing a microelectronic wireless transmission device, including at least the following steps:integration, on a substrate able to be traversed by radio waves intended to be emitted by the microelectronic device, of an antenna, an electrical power supply and an integrated circuit electrically connected to the antenna and to the electrical power supply, and able to transmit to the antenna electrical signals intended to be emitted by the antenna in the form of the said radio waves,rigid connection of a cap to the substrate, forming at least one cavity in which the antenna and the integrated circuit are positioned, where the cap comprises an electrically conductive material connected electrically to an electrical potential of the electrical power supply and/or of the integrated circuit, and able to form a reflector with regard to the radio waves intended to be emitted by the antenna.

The term “integration” in this case refers to the production by transfer and/or by formation of the said abovementioned elements on the substrate.

The method may include at least, before the electrical power supply and the integrated circuit are integrated, steps of deposition, photolithography and etching of at least one electrically conductive layer on the substrate, forming the antenna and electrically conductive tracks, where the integrated circuit and the electrical power supply may be connected to at least one portion of the electrically conductive tracks by microbeads (for example comprising fusible material), or wired connections, where the electrically conductive tracks form at least a proportion of the electrical connections between the integrated circuit and the antenna, between the integrated circuit and the electrical power supply, and between the cap and the electrical power supply and/or the integrated circuit.

The steps of deposition, photolithography and etching of the said at least one electrically conductive layer on the substrate may also form at least a portion of a sealing bead, where the cap is rigidly connected to the substrate by the sealing bead.

The method may also include integration (i.e. production by transfer and/or by formation on the substrate) of a sensor, such that it is positioned in the cavity and electrically connected to the integrated circuit, where the sensor is able to transmit at least one measuring signal to the integrated circuit, where at least a proportion of the electrical signals transmitted by the integrated circuit to the antenna depends on the measuring signal transmitted to the integrated circuit by the sensor.

The method may also include at least integration (i.e. production by transfer and/or by formation on the substrate) of an energy recovery device electrically coupled to the power supply.

Identical, similar or equivalent parts of the various figures described below have the same numerical references, to make it easier to move from one figure to another.

The various parts represented in the figures are not necessarily represented at a uniform scale, in order to make the figures more readable.

The various possibilities (variants and embodiments) must be understood as not being mutually exclusive, and being able to be combined with one another.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

Reference is firstly made toFIG. 1, which represents a profile section view of a microelectronic wireless transmission device100, according to a first embodiment.

Device100includes a substrate102on which various components, or elements, of device100are produced, or integrated. The material or materials of substrate102are in this case chosen such that the radio waves (represented symbolically inFIG. 1and bearing reference103) intended to be emitted by device100are able to traverse, or cross, substrate102. In this first embodiment, substrate102comprises silicon and/or glass and/or polymer. The thickness (dimension in the Z axis) of substrate102is, for example, between approximately 50 μm and 500 μm. The length (dimension in the X axis) of substrate102, which may be equal to the total length of device100, and/or the width (dimension in the Y axis) of substrate102, which may be equal to the total width of device100, are for example between approximately 0.5 mm and 20 mm.

Several electrically conductive tracks104, comprising for example one or more metals, are produced on the front face of substrate102, on which are positioned the various components of device100. These electrically conductive tracks104in particular enable the components of device100to be electrically connected to one another. Electrically conductive tracks104are produced from one or more metal layers deposited on substrate102, for example by electrolytic means, by plasma deposition (for example of the PECVD type, i.e. a Plasma Enhanced Chemical Vapour Deposition), or by evaporation. This or these metal layers are then subject to one or more steps of photolithography and of wet and/or dry etching, depending on the desired pattern. These metal layers comprise, for example, titanium, and/or tungsten nitride and/or nickel and/or aluminium and/or copper and/or gold. Electrically conductive tracks104are for example between approximately 100 nm and 5 μm thick. Finally, tracks104are separated, in order to be electrically insulated from one another. Tracks104are also advantageously deposited on a layer of dielectric material, for example an oxide such as SiO2and/or a polymer, positioned on the front face of substrate102when the latter is electrically conductive. In addition, tracks104may be covered by an electrically insulating layer, for example comprising a silicon oxide or a polymer.

The pattern of the metal layer or layers deposited on substrate102and etched includes advantageously, in addition to electrically conductive tracks104, a portion able to form an antenna106of device100able to emit and possibly receive radio waves. The area occupied by antenna106on substrate102depends in particular on the frequency or frequencies of the radio waves intended to be emitted and possibly received by device100. For example, to accomplish wave emission at approximately 60 GHz, the area of antenna106may be between approximately 0.01 mm2and a few mm2, i.e. less than 10 mm2. In addition, for such an emission frequency, antenna106may be approximately 2.5 mm in length.

Components of device100are transferred by the flip-chip technique on the side of the front face of substrate102(where the active faces of these components which include electrical contacts are located on the side of the front face of substrate102), in particular on electrically conductive tracks104and/or on metal docking terminals, where at least a proportion of these docking terminals are electrically connected to electrically conductive tracks104. This transfer, which is accomplished by means of microbeads of fusible material, comprising for example SnAgCu and/or SnAu and/or indium, or by means of micro-inserts, for example comprising nickel, enable the components of device100to be rigidly connected to substrate102, and also enable them to be electrically connected to one another, and/or to other elements of device100.

These components of device100are an integrated circuit108, for example of the ASIC type, a sensor110, for example of the MEMS and/or NEMS type, and an electrical power supply112, produced in this case in the form of a microbattery. Sensor110is intended to measure one or more physical parameters (for example a vibration and/or radiation external to device100). In order to be able to transmit signals by radio waves dependent on the measurements made by sensor110, sensor110is electrically connected to integrated circuit108via one or more of electrically conductive tracks104. The measurements made by sensor110are thus transmitted to integrated circuit108via these tracks104. Integrated circuit108, which is for example a microprocessor of the ASIC type, may, for example, transform these measurements into quantifiable values and deliver at output electrical signals corresponding to these measurements. Integrated circuit108is electrically connected to antenna106by one or more electrically conductive tracks104, where the output signals of integrated circuit108are transmitted to antenna106via this or these tracks104. The output signals of integrated circuit108may be the measurements made by sensor110, or more generally may depend on the measurement values made (device100may emit by radio waves data which is not directly equal to the measurements made by sensor110but, for example, data the transmission of which is conditional on a certain measurement value). Antenna106then emits radio waves103which match the signals sent by integrated circuit108.

The electrical power of integrated circuit108is provided by microbattery112, these two elements being electrically connected to one another by one or more electrically conductive tracks104. Such a microbattery112forms a compact electrical power supply system, capable of delivering sufficient peak power levels, in particular when information is transmitted by antenna106, and of providing great autonomy (over several years) for device100. Such a microbattery includes an all-solid-state architecture having the following properties: a long lifetime, a high number of charge/discharge cycles, little self-discharge, high energy efficiency, and low volume. Due to its all-solid-state architecture, and the materials used (for example comprising an inorganic solid electrolyte), use of such a microbattery112in device100, which is intended to operate autonomously for a long operational period, is particularly advantageous. Such a microbattery is described, for example, in the document “Microbatteries: microsources d'energie en couches minces” [Micro-batteries: microsources of energy in thin layers], by Levasseur A., Pecquenard B., Vinatier P., Salot R., Le Cras F. and Martin M.,Techniques de I'Ingénieur. Energie 2009, n° D3342.

Although use of microbattery112in device100is particularly advantageous, the function of supplying the electrical power of the components of device100may be performed by any electrical power source of the accumulator type, providing an autonomous electrical power supply of the elements of device100.

In device100described in connection withFIG. 1, all the components of device100are encapsulated in a cavity114formed between substrate102and a cap116. Cap116comprises in this case silicon which is electrically conductive due to a high doping, and in which cavity114has previously been etched. Cap116may comprise any electrically conductive material, for example a metal or a metal alloy. However, for reasons of thermal expansion, cap116and substrate102are advantageously produced from materials having close or similar thermal expansion coefficients, for example silicon. Cap116is rigidly connected to substrate102via a sealing bead118, of which at least one portion is, for example, made from the metal layers used to produce electrically conductive tracks104, and also antenna106, produced around elements106,108,110and112of device100. The pattern of the metal layer or layers deposited on substrate102and etched therefore includes, in addition to conductive tracks104and antenna106, sealing bead118. The thickness (dimension in the Z axis) of cap116is, for example, between approximately 50 μm and 500 μm. The length (dimension in the X axis) of cap116, which may be equal to the total length of device100, and/or the width (dimension in the Y axis) of cap116, which may also be equal to the total width of device100, are for example between approximately 0.5 mm and 20 mm. In the example ofFIG. 1, the length and the width of cap116are roughly similar to those of substrate102, and are equal to the total length and width of device100.

The material or materials of sealing bead118are chosen such that they enable a mechanical connection to be provided between substrate102and cap116, and an electrical connection with cap116. Sealing bead118comprises, for example, titanium and/or chromium, and/or tungsten nitride and/or nickel and/or gold, and/or an alloy of gold and tin and/or an alloy of gold and silicon. Sealing bead118is, for example, between approximately 100 nm and 5 μm thick, and between approximately 10 μm and 1 mm in width.

In addition to the function of mechanical protection of the components performed by cap116, the fact that cap116comprises an electrically conductive material also enables it to perform the function of reflector with regard to the radio waves emitted by antenna106. The radio waves emitted from antenna106in cavity114are thus reflected by cap116, and these reflected waves are emitted outside, then traversing substrate102. In order for cap116to be able to perform this reflector function, it is electrically connected to an electrical potential of reference. In device100this reference potential is one of the electrical potentials of microbattery112. To produce this connection, microbattery112is electrically connected to one of conductive tracks104, which is itself electrically connected to sealing bead118. Due to the fact that sealing bead118and cap116are both electrically conductive and in contact with one another, cap116is therefore electrically connected to one of the electrical potentials of microbattery112. As a variant, the electrical potential of reference to which cap116is electrically connected may be an electrical potential of integrated circuit108. In this case, integrated circuit108is electrically connected to one of conductive tracks104, which is itself electrically connected to sealing bead118.

In order for cap116to be able to perform its function as a reflector, cavity114in which antenna106is positioned is dimensioned such that a space (in which the waves can be propagated) is present between antenna106and the wall of cavity114opposite antenna106(where this wall is formed by cap116). The height of this space between cap116and antenna106depends in particular on the permittivity of the element, or material, between antenna106and cap116. When air separates antenna106from cap116, as is the case in the example ofFIG. 1, this height is equal to a multiple of λ, and advantageously λ/4, where λ is equal to the wavelength of the waves intended to be emitted. To reduce this distance, a dielectric having a permittivity higher than that of air may be deposited on antenna106or against cap116, inside cavity114. In the case of a device100able to emit waves of frequency equal to approximately 60 GHz, this distance between antenna106and the wall of cavity114formed by cap116opposite antenna106is, for example, between approximately 200 μm and 1500 μm.

Cap116is made, for example, from a second substrate, comprising for example a semiconductor such as highly doped silicon. Deposition (for example by electrolytic means, by plasma deposition or by evaporation), and structuring of one or more metal layers intended to form at least one portion of sealing bead118, are firstly accomplished on a face of the second substrate intended to be on the side of substrate102. A thermal treatment may then be applied in order to diffuse metal species (derived from the deposited metal layers) in the material of the second substrate, and by this means to improve the electrical contact between the deposited metal material or materials and the second substrate. Cavity114is then formed in the second substrate, for example by chemical etching from a solution of KOH and/or of TMAH, or by plasma etching of the DRIE type (Deep Reactive Ion Etching), thus completing the production of cap116. Etching of the DRIE type has the advantage in particular that it forms cavity114such that it has very straight side walls. Depending on the etching depth required, i.e. the height of cavity114, the second substrate may be etched through a photosensitive resin mask and/or an oxide mask.

Cap116is then rigidly connected to substrate102, preferably at the wafer scale, in order to encapsulate components of several devices similar to device100simultaneously. This rigid connection may, however, be accomplished at the scale of the chip (corresponding to device100on its own). The rigid connection is accomplished, for example, by melting and/or thermocompression, the parameters of which (temperature, pressure, etc.) are dependent in particular on the nature of the materials forming sealing bead118. In the example described above, thermocompression is therefore applied between the metal materials present on cap116and the metal materials present on substrate102, where these materials have been etched with the desired pattern of sealing bead118.

When device100is produced it may in certain cases be advantageous to separate the manufacture of electrically conductive tracks104from the manufacture of antenna106and/or from that of sealing bead118, in order to use different materials and/or different material thicknesses for these different elements. Similarly, metal docking terminals for the microbeads of fusible material used to rigidly connect microbattery112, integrated circuit108and sensor110may be produced independently of these elements. It is also possible to apply a thinning and a polishing of the rear face of substrate102if it is desired to reduce the initial thickness of substrate102.

As a variant, it is possible for a portion only of substrate102opposite antenna106to comprise one or more materials able to be traversed by the radio waves. In this case, this portion of substrate102forms a window through which the radio waves can be emitted.

Previously described device100is a microelectronic wireless emitter device able to measure a parameter via sensor110, and to transmit this measurement by radio waves. As a variant, such a microelectronic wireless emitter device may not include this measurement function, and may be used only to transmit (emission and possibly reception) data by radio waves. A second embodiment of a microelectronic wireless transmitter device200is represented inFIGS. 2A and 2B, which are respectively profile and top section views of device200.

Unlike device100, device200has no elements or components enabling the device to make a measurement. Device200thus includes substrate102, on which are positioned antenna106, integrated circuit108and the electrical power supply formed by microbattery112. These elements are encapsulated in cavity114formed between electrically conductive cap116and substrate102, which are rigidly connected to one another by sealing bead118. As in device100, microbattery112is electrically connected to integrated circuit108, in order to power electrically integrated circuit108. Integrated circuit108is also connected electrically to antenna106in order to transmit the data to be emitted by radio waves.

In addition, unlike device100, in which the components (electrical power supply112, sensor110and integrated circuit108) are connected mechanically to substrate102and electrically to conductive tracks104by metal microbeads (transfer by flip-chip), the components of device200are rigidly connected, for example by bonding, directly to substrate102, and not on metal portions present on the substrate via microbeads of fusible material. In device200it is the rear faces of integrated circuit108and of electrical power supply112, i.e. those which do not have the electrical means of access to these components, which are rigidly connected to substrate102. At least a proportion of the electrical connections between power supply112and integrated circuit108, between integrated circuit108and antenna106, and between one of the electrical potentials of power supply112and sealing bead118are made not by electrically conductive tracks, but by electric wires202wired between these elements.

FIG. 3represents a microelectronic wireless emitter device300according to a third embodiment.

Device300includes all the elements of previously described device100, i.e. substrate102, conductive tracks104, antenna106, integrated circuit108, sensor110, electrical power supply112, cavity114, cap116and sealing bead118. However, unlike device100, electrical power supply112, which is for example a microbattery, is positioned on substrate102outside cavity114. Only sensor110, antenna106and integrated circuit108are protected by conductive cap116and positioned in cavity114.

As with device100, cap116is electrically connected to one of the electrical potentials of power supply112, via sealing bead118. Thus, in the example ofFIG. 3, an electrically conductive track302electrically connects one of the electrical potentials of power supply112to sealing bead118, where this conductive track302is positioned outside cavity114. This electrically conductive track is made, for example, of the same metal layer or layers used to produce the other conductive tracks114and/or antenna106and/or sealing bead118. At least one of conductive tracks104also connects electrical power supply112to integrated circuit108, in order to power electrically integrated circuit108. This or these electrical tracks extend from outside cavity114as far as the interior of cavity114, and are electrically insulated from sealing bead118, for example by inserting an electrically insulating material between these tracks104and sealing bead118, as will be described below in connection withFIGS. 4A and 4B.

As a variant, device300may not include sensor110, as in device200. In this case, only antenna106and integrated circuit108are protected by conductive cap116, and positioned in cavity114. In addition, at least a proportion of the different electrical connections between the components of device300may be made by electrical wires202, as in device200.

FIGS. 4A and 4Brepresent a device400according to a fourth embodiment. As represented in these figures, the electrical connection between electrical power supply112and integrated circuit108is made by two electric wires402, connected to two electrically conductive tracks404, extending on substrate102between the inside and outside of cavity114, and which are electrically insulated from sealing bead118, due to a layer of dielectric material407completely covering tracks404and positioned between tracks404and sealing bead118. This layer of dielectric material407is advantageously planarised such that sealing bead118is not discontinuous. An electric wire402also connects one of the electrical potentials of power supply112to sealing bead118. Electric wires406also make the electrical connections between antenna106and integrated circuit108, between integrated circuit108and conductive tracks404, and between integrated circuit108and sensor110.

In this fourth embodiment, electrical power supply112is coupled to an energy recovery device408, based for example on an energy conversion technique which may be photovoltaic and/or mechanical (for example by the vibrations to which the device is subject) and/or thermal (for example, by a temperature gradient on the device using the Seebeck effect).

FIGS. 5A and 5Brepresent a device500according to a fifth embodiment. Device500includes a first cavity114, formed between substrate102and cap116, in which are encapsulated electrical power supply112and integrated circuit108(which, in this embodiment, also performs the role of sensor110), and a second cavity514, in which antenna106is encapsulated. A portion of cap116forms a separation502between the two cavities114,514, and enables any electromagnetic disturbance of components112,108towards antenna106to be prevented. This separation502is advantageously produced by a portion of cap116, since the latter is electrically conductive. Separation502between the two cavities114,514is not necessarily in contact with substrate102. In this case, it is not necessary to protect, and in particular to insulate electrically, conductive tracks104connecting integrated circuit108and antenna106, since separation502does not come into contact with these tracks104. As a variant, separation502could be produced from a portion of material not belonging to cap116, i.e. which is transferred on to cap116to form an electromagnetic screen between the two cavities114,514. Finally, in device500, cap116is electrically connected, via sealing bead118, to an electrical potential of integrated circuit108.

In all the previously described embodiments and variants, it is possible for cavity114not to be formed in cap116, but in substrate102(where the cap is in this case an electrically conductive flat substrate), or partly in substrate102and partly in cap116, or again it is possible that at least a proportion of the side walls of cavity114are formed by portions of electrically conductive material independent of substrate102and of cap116, and electrically connected, like cap116, to an electrical potential of power supply112.