Patent Description:
Transducers for converting an input signal from a sensor into an electrical output signal for processing are known. When miniaturizing transducers it would be useful to ensure that the electrical output signal has a high quality factor (Q-factor). However, this is difficult.

<CIT> describes a sensing device, including a piezoelectric diving board or cantilever attached to a substrate by tethers. The piezoelectric diving board can be constructed including materials such as quartz. The sensing device can be any sensor such as a monolithic crystal filter (MCF), among other things.

<CIT> describes a device consisting of a thin freestanding single-crystal Y-cut quartz resonator connected to a silicon substrate via legs.

<CIT> describes a mechanical tuning fork resonator having a quartz crystal sandwiched in between electrodes. The resonator is for measuring various properties of fluids.

According to the invention there is provided an apparatus comprising,.

In some, but not necessarily all examples the oscillator is sensitive to mass so that a frequency of oscillation of the oscillator provides an indication of mass accumulated on at least one of: the oscillator; the first electrode; or the second electrode.

The apparatus is configured to enable an analyte to enter the gap.

The side surface is functionalized to enable the analyte to accumulate on the side surface of the oscillator.

In some, but not necessarily all examples the tethers are attached to nodal points of the oscillator.

In some, but not necessarily all examples a sum of widths of the tethers is no greater than approximately <NUM>% of a nominal diameter of the oscillator.

In some, but not necessarily all examples the monolithic crystal comprises quartz.

In some, but not necessarily all examples the oscillator has a thickness of at least <NUM>.

In some, but not necessarily all examples the first electrode is on a first surface of the oscillator, and wherein the second electrode is on a second surface of the oscillator.

According to various, but not necessarily all, embodiments there is provided a transducer array comprising the apparatus,.

In some, but not necessarily all examples the electrodes within the first array and the second array comprise elongate traces, wherein the elongate traces within the second array are configured to cross over the elongate traces within the first array, and wherein the elongate traces are electrically connected to one or more contact pads and the one or more contact pads are located at locations where the electrodes in the second array cross over the electrodes in the first array.

In some, but not necessarily all examples the transducer array is configured to enable the oscillators to be activated individually.

In some, but not necessarily all examples the transducer array is configured to enable the oscillators to be activated in a sequence so that oscillators within a given distance of each other are not activated simultaneously.

In some, but not necessarily all examples the transducer array is configured to enable the oscillators to be activated in a sequence so that a first subset of oscillators can be activated at the same time while a second subset of oscillators are not activated.

In some, but not necessarily all examples the transducer array is configured to provide an output to an artificial intelligence module.

According to the invention there is provided a method comprising:.

In some, but not necessarily all examples providing the gap comprises etching the gap.

Examples of the disclosure relate to a transducer apparatus <NUM> and systems comprising a transducer apparatus <NUM>. The transducer apparatus <NUM> can be configured to transduce inputs from one or more sensors into an identifiable electrical output signal. This can enable parameters such as chemical analytes to be sensed and identified.

<FIG> schematically illustrates a side view of a transducer apparatus <NUM> showing a single transducer. In some, but not necessarily all examples the transducer apparatus <NUM> is a transducer array comprising a plurality of transducers. The transducer apparatus <NUM> comprises a monolithic crystal <NUM>, a first electrode <NUM>, a second electrode <NUM> and an oscillator <NUM>.

In at least some examples the oscillator <NUM> is sensitive to mass so that the resonant frequency of the oscillator <NUM> will be changed by any mass that is absorbed by a functionalized part such as the electrodes <NUM>, <NUM> and/or a surface of the oscillator <NUM>. The resonant frequency of the oscillator <NUM> can then be measured to provide an indication of accumulated mass. The transducers therefore transduce a detected mass into an identifiable electrical output signal.

The oscillator <NUM> is a portion of the monolithic crystal <NUM>. The oscillator <NUM> is surrounded by a gap <NUM> that extends through the entire thickness (z-axis) of the monolithic crystal <NUM>, or at least through enough of the thickness of the monolithic crystal <NUM> that the oscillator <NUM> has to be supported by something else.

The gap <NUM> separates the oscillator <NUM> from the bulk of the monolithic crystal <NUM>, referred to herein as a substrate portion <NUM>. The gap <NUM> exposes a side surface <NUM> of the oscillator <NUM>. One or more tethers <NUM> extend across the gap <NUM> so that the oscillator <NUM> is supported by the substrate portion <NUM> of the monolithic crystal <NUM>. The tethers <NUM> are shown in dashed lines to indicate that they do not fully surround the oscillator <NUM>. The gap <NUM> could be unfilled, enabling analyte such as chemicals to enter the gap <NUM>.

This 'isolated oscillator' design with the gap <NUM> enables a Q-factor improvement of more than two orders of magnitude compared to an oscillator <NUM> fabricated on a planar surface (e.g. by trenches or cavities). This provides a significant Q-factor 'budget' that can be 'spent' on miniaturization and forming high density transducer arrays. In addition, the exposed side surface <NUM> of the oscillator <NUM> facing the gap <NUM> can be functionalized to increase the available surface area for sensing without increasing footprint area. Further still, arrays could be stacked in layers without substantially increasing footprint area, wherein analytes can pass through the gap <NUM> in a first layer to reach an underlying layer.

The monolithic crystal <NUM> comprises a single crystal. This distinguishes the crystal material from substantially amorphous or substantially polycrystalline materials. The monolithic crystal <NUM> can be configured with a first surface <NUM> and a second surface <NUM>. The first surface <NUM> and the second surface <NUM> can be flat or substantially flat surfaces. The first surface <NUM> can occupy an x-y plane, orthogonal to the z-axis. The first surface <NUM> and the second surface <NUM> can be opposing surfaces that are positioned on opposite sides of the monolithic crystal <NUM>, separated from each other by the thickness (z-axis) of the monolithic crystal <NUM>. The first surface <NUM> and the second surface <NUM> can be parallel or substantially parallel surfaces.

The oscillator <NUM> can be in-plane with the substrate portion <NUM>, therefore the first surface <NUM>' of the oscillator <NUM> was a portion of the first surface <NUM> of the monolithic crystal <NUM> prior to providing the gap <NUM>, and/or the second surface <NUM>' of the oscillator <NUM> was a portion of the second surface <NUM> of the monolithic crystal <NUM> prior to providing the gap <NUM>.

The tethers <NUM> can extend through part or all of the thickness (z-axis) of the monolithic crystal <NUM>. The tethers <NUM> can be a portion of the monolithic crystal <NUM>. For example, the cutting of an incomplete gap <NUM> around the oscillator <NUM> will leave behind a bridging portion of monolithic crystal referred to herein as a tether <NUM>. This minimises fabrication requirements. Alternatively, tethers <NUM> can be provided separately.

In addition to supporting the oscillator <NUM>, the tethers <NUM> provide a passage on which electrical connections can be deposited to form a circuit to the electrodes <NUM>, <NUM>. For details, refer to <FIG>.

The monolithic crystal <NUM> can be any suitable size. In some examples the monolithic crystal <NUM> can have x-axis and/or y-axis dimensions of the order of <NUM>^<NUM> to <NUM>^<NUM> for the widths of the surface of the monolithic crystal <NUM>. Other sizes of monolithic crystal <NUM> could be used in other examples of the disclosure.

The monolithic crystal <NUM> can comprise a piezoelectric material. For example, the monolithic crystal <NUM> may comprise quartz or any other suitable type of material. Other piezoelectric materials include but are not limited to Lithium Niobate and Lithium Tantalate. Of these materials, quartz is most resilient to heating during fabrication and is readily available. Later methods of the present disclosure provide a technique that enables miniaturization of quartz while retaining a high Q-factor. The quartz can be AT-cut quartz, which is usable for frequencies between <NUM> and <NUM>. Other cuts could be used depending on the use case.

The first electrode <NUM> is provided at a first location of the oscillator <NUM> and the second electrode <NUM> is provided at a second different location of the oscillator <NUM>. In <FIG>, but not necessarily all examples the first electrode <NUM> is on the first surface <NUM> of the monolithic crystal <NUM> and the second electrode <NUM> is on the second surface <NUM> of the monolithic crystal <NUM>. Both electrodes <NUM>, <NUM> could instead be provided at different locations on the oscillator <NUM>.

The electrodes <NUM>, <NUM> can comprise any suitable conductive material such as gold. The material used for the electrodes <NUM>, <NUM> can also be selected so that sensors can be coupled to electrodes <NUM> and/or <NUM> to functionalize the electrodes.

In some examples a sensor can be coupled to the electrodes <NUM>, <NUM> and/or to a surface <NUM>, <NUM> and/or <NUM> of the oscillator <NUM> to functionalize said surface. The sensor can be configured to sense a parameter such as a chemical analyte. The parameter sensed by the sensor will affect the resonant frequency of the oscillator <NUM>. This change in frequency can be detected by addressing the pairs of electrodes <NUM>, <NUM> that are coupled to the oscillator <NUM> to activate the oscillator <NUM>. The change in resonant frequency provides an indication of the parameters sensed by the sensors. This therefore enables the transducer apparatus <NUM> to transduce the sensed chemical into an identifiable electrical output signal and so enable the chemical to be detected.

<FIG> provides a three-dimensional view of a transducer apparatus <NUM> incorporating the features of <FIG>. The first and second electrodes are not shown. The illustrated geometry and its advantages will be discussed with the understanding that other geometries can still function as sensors (the Sauerbrey equation considers the area of <NUM> or <NUM> when determining an active crystal area).

To summarise the illustration, the first surface <NUM>' of the oscillator <NUM> can be rounded, e.g. circular. The oscillator <NUM> can be a circular cylinder such that the second surface <NUM>' of the oscillator <NUM> is also circular. The side surface <NUM> therefore comprises a single face that extends around the whole circumference of the oscillator <NUM>, orthogonal to the first and second surfaces <NUM>', <NUM>'. The circular design avoids sharp corners/vertices which could potentially become cleavage sites. The gap <NUM> is annular.

In another example the surfaces <NUM>', <NUM>' of the oscillator <NUM> form a different shape such as a shape with a plurality of vertices. Optionally, the vertices could be rounded corners. In further examples a non-rounded shape could be used. The side surface <NUM> could then be described as comprising a plurality of faces.

Since the illustration uses a circular cylindrical design, the geometry of the first and second surfaces <NUM>', <NUM>' of the oscillator <NUM> will be expressed as a nominal radius R or nominal diameter 2R. If a different or irregular shape is used, the nominal radius R would be taken as the radius of an equivalent circle.

The z-axis thickness of the oscillator <NUM> is labelled as T, not counting the electrodes <NUM>, <NUM>. This thickness T also refers to the depth of the gap <NUM>, assuming the gap <NUM> extends all the way through the thickness of the monolithic crystal <NUM>.

The thickness T could be at least a value that makes the oscillator <NUM> significantly thicker than a 'thin film'. It is therefore not necessary to use complex, low-volume thin film fabrication techniques. A thin film refers to thicknesses from a few <NUM> to a few µm. In at least some examples the thickness T of the oscillator <NUM> is at least <NUM> (<NUM>). If quartz is used, a thickness of at least <NUM> would make the quartz wafer robust and easy to handle during fabrication. Intuitively, a lower thickness T would normally result in higher Q-factor. However, examples of the present disclosure enable a high Q-factor with a substantial thickness T (e.g. ><NUM>).

The width of the gap <NUM> in the x-y plane is labelled as G. The gap width G is optionally constant or substantially constant around the whole circumference of the oscillator <NUM>. Assuming an annular gap <NUM>, the width of the gap G can be expressed as the difference between the outer radius and the inner radius of the annulus. The gap width G can be no greater than 2R to reduce stress on the tethers <NUM>. The minimum gap width depends on the precision of the manufacturing technique and the mechanical stability of the monolithic crystal <NUM>.

The width of a tether <NUM> in the x-y plane is labelled as WT, orthogonal to the span length of the tether <NUM>. The span length of the tether <NUM> equals the gap width G if the tether <NUM> spans wholly in the radial direction as shown. The tether width WT is shown as constant but could vary.

The tether width WT is thin compared to the diameter 2R of the oscillator <NUM>, to minimise dampening of the oscillator <NUM> through acoustic energy leakage. In at least some examples the tether width WT is less than the radius R of the oscillator <NUM>. In further examples the sum of all tether widths WT of all the tethers <NUM> (two shown) of the oscillator <NUM> is no greater than approximately <NUM>% of the diameter 2R. A ratio greater than <NUM>% would result in little Q-factor improvement and therefore a need for additional signal processing (e.g. radio frequency circuitry). Ratios lower than <NUM>% see increasing Q-factor gains, and the best results were below approximately <NUM>% (results discussed later).

Regarding tether locations, a tether <NUM> can be attached to a nodal point of the oscillator <NUM>. A nodal point is a zero-displacement node of the oscillator <NUM>, for example in a thickness shear (TS) mode or a lateral extensional mode (LEM). By attaching a tether <NUM> to a nodal point, dampening of the oscillator <NUM> through acoustic energy leakage is minimized. The node locations can be identified using Multiphysics simulations of the oscillator <NUM>.

To demonstrate the value of placing tethers <NUM> at nodal points, the Q-factor for an AT-cut quartz crystal having the geometry of <FIG> was calculated while rotating the orientation vector through two opposing tethers <NUM> (T) around a surface normal z through the centre of the oscillator <NUM>, and the results are shown in <FIG>. The vertical axis is Q-factor and the horizontal axis is the angle of the tethers <NUM> compared to the crystal x-axis. Note that the crystal x-axis is a crystallographic property.

As shown in <FIG>, the highest Q-factor (Q=<NUM>) was obtained when the angle (θ) between T and the x-axis was approximately <NUM>°. It is important to note that this target angle θ is different depending on the dimensions of the oscillator <NUM>.

The Q-factor dropped by half when the angle θ deviated ±<NUM>° from <NUM>° but was still very high. It is important to note that there is no set lower limit for Q-factor. A device with very low Q-factor can work with enough signal processing. Therefore, the present disclosure is not limited to θ=<NUM>°. <FIG> demonstrates that Q-factor improvement is available for angles θ between approximately <NUM>° and approximately <NUM>°, with peak improvements for angles θ within ±<NUM>° of <NUM>°. For angles outside these ranges, compromises may be useful elsewhere such as less miniaturization or more signal processing time.

<FIG> identifies an implementation in which extremely high Q-factor can be obtained: a cylindrical quartz oscillator <NUM>; one tether <NUM> at approximately +<NUM>° from the x-axis; the other tether <NUM> at approximately -<NUM>° from the x-axis.

The oscillator <NUM> of <FIG> also benefitted from thin tethers <NUM> (R=<NUM> and WT=<NUM> such that WT/R=<NUM>%). Of course, this ratio could be different.

The oscillator <NUM> of <FIG> also had sufficient radius (R=<NUM>) that TS nodal points spanned all the way through the thickness T of the oscillator <NUM> (T=<NUM>). If the nodal point does not extend through the full thickness, the fabrication could be modified to limit the thickness and vertical position of the tether <NUM> based on the size and location of the nodal point. However, fabrication would be quicker if the tether <NUM> could be left at thickness T. Calculations revealed that a radius R of at least approximately <NUM> enables a TS nodal point to span through the entire thickness (in this case T=<NUM>). Therefore, when R is approximately equal to or greater than T, TS nodal points become readily available. For geometries not suited to TS nodal points, LEM nodal points could be used. LEM modes can work for oscillators at least where T<<NUM> and where R<T.

Although two tethers <NUM> are shown in <FIG> and <FIG>, it would be understood that any number of tethers <NUM> could be used. If attaching tethers <NUM> to nodal points the number of tethers <NUM> may be no more than the number of nodal points. It may be more efficient to limit to no more than two tethers <NUM> even if there are more than two available nodal points. If only one tether <NUM> is provided, it may need to be stronger (e.g. thicker) than if two tethers are provided.

Some more insights into Q-factor can be derived by considering the results of a set of designs that varied radius R, tether width WT, and mode (TS or LEM). Table <NUM> below is visually complemented by <FIG> which uses simple schematic representations of the oscillator <NUM> and tethers <NUM> at correct relative scales, to highlight the geometric differences between the designs.

For Table <NUM>, the Saurbrey frequency shift Δf was calculated based on a computational simulation with an additional layer (<NUM>/cm<NUM>) of a thin film (<NUM>) on the first electrode <NUM>. All other constants were as described in relation to <FIG>. In fact, <FIG> represents design D.

It should be noted that all the Q-factors above are very good for a miniaturized quartz device. Observations on individual results are made below:.

Looking at the results as a whole, Table <NUM> demonstrates that reducing tether width (more specifically ∑WT/2R) correlates with an improvement in Q-factor, at least for radii smaller than design F. Observe that the Q-factor of design D increased relative to design B with roughly the same R, by halving WT. The same can be seen for design E compared to design C.

In some, but not necessarily all examples a tether width of <NUM>-<NUM> is recommended, providing a reasonable trade-off between performance, ease of fabrication using techniques discussed herein, and mechanical robustness.

Looking at the performance of TS relative to LEM, it is believed that the small out of plane movement in TS mode reduces energy lost to surrounding air molecules thus improving the Q-factor and therefore signal to noise ratio SNR. Although other modes (e.g. LEM) may achieve improved mass sensitivity and Q-factor, extra fabrication steps required for creating an oscillator <NUM> with LEM node tethers <NUM> may be needed. Therefore, in some implementations the tethers <NUM> are attached to TS nodes for fabrication reasons.

The three-dimensional nature of the oscillator <NUM> provides the opportunity to functionalize the side surface <NUM> to increase the effective sensing area of the oscillator <NUM> relative to the two-dimensional footprint of the oscillator <NUM>. This grants enhanced sensitivity compared to thin (conventional) MEMS (microelectromechanical system) design.

The side surface <NUM> can be thick enough that the three-dimensional surface area of the oscillator <NUM> is more than <NUM>% greater than the two-dimensional nominal cross-sectional area of the oscillator <NUM>. However, in the above designs, the side surface <NUM> can be much thicker. The 3D surface area of the oscillator <NUM> can be at least <NUM> times greater than the 2D cross-sectional area. In some cases, the side surface <NUM> could comprise more sensors than the other sensor locations <NUM>/<NUM>/<NUM>'/<NUM>'.

In an example, the frequency shift with and without side surface functionalization was compared, by simulating placement of a <NUM> thick uniform layer (<NUM>/cm<NUM>) on surfaces of a quartz oscillator <NUM> of R=<NUM> and T=<NUM>. The results are shown in Table <NUM>:.

The results of Table <NUM> show two orders of magnitude of improvement in mass loading sensitivity per unit area when the side surface <NUM> is exposed and utilised for mass sensing. By contrast, a side surface <NUM> is practically non-existent in a MEMS oscillator, so MEMS design typically considers components to be two-dimensional.

The preceding description has focused on a single oscillator <NUM> with potential application for transducer arrays. The following description provides an example of a transducer array. <FIG> provide example electrode layouts for a transducer array such as a quartz crystal microbalance (QCM) array or other array.

<FIG> shows that the first electrode <NUM> could be a first electrode array. <FIG> shows that the second electrode <NUM> could be a second electrode array. The electrodes in the arrays <NUM>, <NUM> can have any suitable shape and configuration. In some examples the electrodes can comprise elongate traces <NUM> that extend across the respective surfaces <NUM>, <NUM> of the monolithic crystal <NUM>. The elongate traces <NUM> in the first array <NUM> can extend towards a first direction and the elongate traces <NUM> in the second array <NUM> can extend towards a second direction where the second direction is not parallel to the first direction. This ensures that the electrodes within the respective arrays <NUM>, <NUM> cross over each other at a plurality of intersecting points. The elongate traces <NUM> in the second array <NUM> could extend in a direction that is perpendicular to, or substantially perpendicular to, the elongate traces <NUM> in the first array <NUM>, making the electrodes X-electrodes and Y-electrodes.

The arrays of electrodes <NUM>, <NUM> are positioned on the monolithic crystal <NUM> so that the electrodes in the first array <NUM> are positioned, at least partially, overlaying the electrodes in the second array <NUM>. This enables electrical connections to a plurality of oscillators <NUM>.

In these examples the elongate traces <NUM> in each array <NUM>, <NUM> are substantially parallel to each other. As shown in <FIG>, the elongate traces <NUM> in the second array <NUM> extend in a direction that is perpendicular, or substantially perpendicular to the elongate traces in the first array <NUM>. This enables a matrix of cross over points to be provided when the two arrays <NUM>, <NUM> of electrodes are added to the monolithic crystal <NUM>. In the example shown in <FIG> the elongate traces <NUM> in the first array <NUM> extend in a vertical direction while the elongate traces <NUM> in the second array <NUM> extend in a horizontal direction. It is to be appreciated that other orientations and configurations for the electrodes <NUM> could be used in other examples of the disclosure.

In the example shown in <FIG> the elongate traces <NUM> are electrically connected to one or more contact pads <NUM>. In the example shown the contact pads <NUM> comprise circular portions. The contact pads <NUM> provide an increased surface area of the electrodes <NUM>, <NUM> within the region of the cross over points. It is to be appreciated that other shapes and configurations could be used for the contact pads <NUM> in other examples of the disclosure. The contact pads <NUM> are on the first and second surfaces <NUM>', <NUM>' of the oscillator <NUM>.

As shown in <FIG>, the elongate traces <NUM> of the first array <NUM> can extend parallel to the orientation of the tethers <NUM>, electrically interconnecting contact pads <NUM> in a column. Accordingly, the elongate traces <NUM> of the first array <NUM> can extend along the tethers <NUM>, on the top surfaces <NUM> of the tethers <NUM>.

The elongate traces <NUM> of the second array <NUM> can electrically connect to the contact pads <NUM> of the second array <NUM> via the bottom surfaces <NUM> of the tethers <NUM>. Since the elongate traces <NUM> of the second array <NUM> may be at a different orientation, e.g. perpendicular, relative to the tethers <NUM>, the arrangement of <FIG> can be provided. <FIG> shows that the contact pads <NUM> of the second array <NUM> are branched off from the elongate traces <NUM> of the second array <NUM>, with each tether <NUM> forming a branch. An elongate trace <NUM> of the second array <NUM> can electrically interconnect a row of contact pads <NUM>.

In the example shown in <FIG> the arrays of electrodes <NUM>, <NUM> each comprise four elongate traces <NUM> each electrically connected to four contact pads <NUM>. It is to be appreciated that other numbers of elongate traces <NUM> and contact pads <NUM> could be provided in other examples of the disclosure.

The electrode arrays <NUM>, <NUM> and components <NUM>, <NUM> within the arrays <NUM>, <NUM> can have any suitable dimensions. For example, the contact pads <NUM> could have a diameter between <NUM> to <NUM> and could be spaced by between <NUM> to <NUM>. Different ranges could be used in other examples of the disclosure. For instance, in some examples the arrays of electrodes <NUM>, <NUM> could be designed on a nanometre scale, if smaller oscillators are used. This could enable a large number of electrodes to be provided within each of the arrays of electrodes <NUM>, <NUM> and so could provide a large number of transducers within the transducer apparatus <NUM>.

In some examples, the transducer apparatus <NUM> can be mounted to a circuit board which drives the oscillation via soldering, gold bumps, gold wire bonding, anisotropic conductive film (ACF) or anisotropic conductive elastomer.

<FIG> illustrate the electrode arrays <NUM>, <NUM> coupled to the monolithic crystal <NUM> and configured to create a transducer array <NUM>. In this example the first array of electrodes <NUM> is provided on the top surface <NUM> of the monolithic crystal <NUM> and the second array of electrodes <NUM> is provided on the bottom surface <NUM> of the monolithic crystal <NUM>. In this illustration the first array of electrodes <NUM> extend in a horizontal direction and the second array of electrodes <NUM> extend in a vertical direction.

In the example of <FIG> each of the arrays of electrodes <NUM>, <NUM> comprises four electrodes <NUM>, for illustrative purposes only. This creates sixteen cross over points arranged in a 4X4 matrix. The cross over points of the electrodes <NUM> enable oscillators <NUM> to be created within the monolithic crystal <NUM> as described above and so provide a transducer array <NUM>.

In the example shown in <FIG> a first multiplexer <NUM> is coupled to the electrodes in the first array <NUM> and a second multiplexer <NUM> is coupled to the electrodes in the second array <NUM>. The multiplexers <NUM>, <NUM> can be configured to enable the electrodes within the arrays <NUM>, <NUM> to be addressed individually so that a signal can be provided to a first electrode within an array <NUM>, <NUM> without providing a signal to other electrodes within the same array <NUM>, <NUM>. A passive matrix addressing scheme allows ease of fabrication and minimizes front-end circuitry. According to a passive matrix addressing scheme, log<NUM>(mn) control signals can address an m x n matrix.

The oscillator design described herein works at low fundamental frequencies (e.g. <<NUM>) so radio frequency circuitry may not be needed. This enables the use of field programmable gate arrays (FPGA) or similar which operate at lower frequencies.

In some examples the transducer array <NUM> can be configured to enable the oscillators <NUM> to be activated in a sequence so that oscillators <NUM> within a given distance of each other are not activated simultaneously. The given distance can ensure that there is no cross talk between the oscillators <NUM> that affects the measurements of the resonant frequencies. The size of the distance that is necessary to avoid the cross talk will depend on the size of the contact pads <NUM> within the electrode arrays <NUM>, <NUM> and the spacing between the electrodes.

In some examples the transducer array <NUM> can be configured to enable the oscillators <NUM> to be activated in a sequence so that a first subset of oscillators <NUM> can be activated at the same time while a second subset of oscillators <NUM> are not activated. For example, two or more oscillators <NUM> that are separated by more than a minimum distance can be activated simultaneously while any oscillators <NUM> within the minimum distance could remain de-activated so as to avoid cross talk.

In the example shown in <FIG> the transducer array <NUM> is configured to enable the oscillators <NUM> to be activated individually by addressing the electrodes within the arrays <NUM>, <NUM> individually. This reduces cross talk between oscillators <NUM> as only one oscillator <NUM> is activated at a given time.

<FIG> schematically illustrates a system <NUM> comprising a transducer apparatus <NUM> such as a transducer array, a plurality of sensors <NUM> and an artificial intelligence module <NUM>. The system <NUM> shown in <FIG> can be used to sense parameters such as chemicals. This could be used as an artificial nose or other similar application. It is to be appreciated that the transducer array <NUM> could also be used in other types of systems. An artificial nose benefits from a miniaturized transducer array because of the potentially large number of sensors and higher required sensitivity, so the ability to miniaturize quartz while retaining a high Q-factor is beneficial.

The transducer array <NUM> can be as described above. Corresponding reference numerals are used for corresponding features. In the example shown the transducer array <NUM> comprises sixteen pixels where each pixel comprises an oscillator <NUM> formed from a portion of monolithic crystal <NUM> between the cross over points of two contact pads <NUM> from the electrode arrays <NUM>, <NUM>.

A plurality of sensors <NUM> are coupled to the transducer array <NUM> so that the frequency of oscillation of the oscillators <NUM> is dependent upon one or more chemicals sensed by the plurality of sensors <NUM>. Each oscillator <NUM> may comprise a different sensor <NUM>, that is, a sensor configured to sense a different analyte.

The sensors <NUM> can be coupled to the contact pads, or any other suitable portion, of one or both of the electrode arrays <NUM>, <NUM>. The sensors <NUM> could be coupled to the side surfaces <NUM> of the oscillators <NUM> as mentioned. The sensors <NUM> can be coupled to contact pads so that the output signal provided by the oscillator <NUM> is dependent upon whether or not the sensors <NUM> have sensed a chemical.

In some examples plurality of sensors <NUM> could comprise genetically modified sensors. The genetically modified sensors <NUM> could comprise at least one of viral particles, desiccation tolerant cells, synthetic peptides, randomized DNA, proteins and receptors or any other suitable biological material. The genetically modified sensors <NUM> can be modified to sense chemicals in a gas phase and/or a liquid phase.

The genetically modified sensors <NUM> may be adsorbed to the contact pads <NUM> of the electrodes <NUM>. The adsorption could be chemical adsorption or physical adsorption. The chemical bond formed in a chemical adsorption process could be a covalent bond, a partially covalent bond or any other suitable type of bond. The type of coupling that is used to couple sensors <NUM> to the contact pads <NUM> would be dependent upon the types of sensors <NUM> that are used and the materials used for the electrodes <NUM> and contact pads <NUM>.

In other examples the plurality of sensors <NUM> can comprise chemical modification of the surface of the contact pads <NUM> and/or electrodes <NUM>. The chemical modification could comprise a chemical coating such as a polymer provided on the surface of the contact pad <NUM> and/or electrode <NUM>. The chemical coating could be configured to enable chemicals to be accumulated on the surface of the contact pad <NUM>.

The artificial intelligence module <NUM> is coupled to the transducer array <NUM> and configured to receive an electrical output signal from the transducer array <NUM>.

The output provided by each of the pixels within the transducer array <NUM> is dependent upon the chemicals that are sensed by the sensors <NUM> coupled to the pixels. The transducer array <NUM> therefore provides an electrical output signal that is dependent upon one or more chemicals being sensed by the plurality of sensors <NUM>. Different pixels can have different types of sensors <NUM> coupled to them so that different pixels provide different electrical output signals. The different electrical output signals from the pixels within the transducer array <NUM> can be combined so as to provide an identifiable electrical output signal <NUM>.

The identifiable electrical output signal <NUM> comprises information that enable one or more chemicals or types of chemicals to be identified. The identifiable electrical output signal <NUM> can comprise information that enables a plurality of different chemicals to be identified. In some examples the identifiable electrical output signal <NUM> can comprise information that enables a concentration of the chemicals or types of chemicals to be determined. The chemicals can comprise any suitable types of chemicals including bio-chemicals.

The identifiable electrical output signal <NUM> from the transducer array <NUM> is provided to the artificial intelligence module <NUM> so as to enable the identifiable electrical output signal <NUM> to be classified. The artificial intelligence module <NUM> can be configured to use a pattern recognition algorithm, or any other suitable type of algorithm, to classify the identifiable electrical output signal <NUM>. Classifying the electrical output signal <NUM> comprises one or more of: determining an identity of a chemical, determining a class of chemicals, determining concentration of a chemical. The artificial intelligence module may comprise instructions implemented by an electronic processor.

The artificial intelligence module may use machine learning which can include statistical learning. Machine learning is a field of computer science that gives computers the ability to learn without being explicitly programmed. The computer learns from experience E with respect to some class of tasks T and performance measure P if its performance at tasks in T, as measured by P, improves with experience E. The computer can often learn from prior training data to make predictions on future data. Machine learning includes wholly or partially supervised learning and wholly or partially unsupervised learning. It may enable discrete outputs (for example classification, clustering) and continuous outputs (for example regression). Machine learning may for example be implemented using different approaches such as cost function minimization, artificial neural networks, support vector machines and Bayesian networks for example. Cost function minimization may, for example, be used in linear and polynomial regression and K-means clustering. Artificial neural networks, for example with one or more hidden layers, model complex relationship between input vectors and output vectors. Support vector machines may be used for supervised learning. A Bayesian network is a directed acyclic graph that represents the conditional independence of a number of random variables.

<FIG> is a flowchart for a method of fabricating a transducer apparatus <NUM>. The method comprises:.

In some, but not necessarily all examples, providing the gap <NUM> may comprise etching the gap <NUM>. This is a more efficient and scalable technique than deposition/growth-based techniques. However, some applications could instead call for another technique such as deposition.

Etching the gap <NUM> may comprise providing a mask such as a dry film photoresist, and etching around the mask.

The same etching step could both create the gap <NUM> and leave behind the tethers <NUM>, performing both blocks <NUM> and <NUM>.

<FIG> illustrates a transducer apparatus <NUM> in different states of fabrication, according to an example fabrication process.

In <FIG> the electrodes <NUM>, <NUM> are illustrated as being provided prior to etching the gap <NUM>. However, in an example alternative implementation the electrodes <NUM>, <NUM> are provided after etching the gap <NUM>.

The first electrode array <NUM> can be created on the monolithic crystal <NUM> using a lift-off process. The second electrode array <NUM> can be created using a lift-off process on the rear side of the monolithic crystal <NUM>. This results in the illustrated State (A).

A first mask <NUM> is provided on the second surface <NUM>, optionally when the second electrode array <NUM> is being provided. The first mask <NUM> could be a layer of photoresist. The photoresist can be dry film photoresist. The photoresist layer can be thick, such as thicker than <NUM> (e.g. approximately <NUM>). This results in the illustrated State (B).

A second mask <NUM> can be provided on the first surface <NUM> of the monolithic crystal <NUM>. The same mask type can be used. Gaps <NUM> in the second mask <NUM> can be formed by deposition and patterning. This results in the illustrated State (C). The illustrated gaps <NUM> in the second mask <NUM> form a pattern defining the locations for the gaps <NUM> in the monolithic crystal <NUM>.

Then, the pattern in the second mask <NUM> is etched through to create the gaps <NUM> in the monolithic crystal <NUM>. The etching technique may comprise anisotropic etching. In a first example, plasma etching is a suitable technique to produce small vertical features in quartz. An example technique is reactive ion etching, optionally using a SF<NUM>-based plasma. This results in the illustrated state (D).

The reactive ion etching step can be optionally performed in two steps, where each surface <NUM>, <NUM> of the monolithic crystal <NUM> (front & back) is etched halfway to ensure complete etching of through-hole structures (gaps <NUM>).

The low selectivity of regular photoresist masks (<NUM> to <NUM>) or the added complexity of metal masks with better selectivity (><NUM>) can make it difficult to etch features through the whole thickness of quartz wafers (<NUM>-<NUM>). However, by using thick dry photoresist film masks <NUM>, <NUM> the difficulty can be obviated. Conservatively, dry film photoresists will produce a resolution similar to their thickness of approximately <NUM> to approximately <NUM>, which makes it compatible with the example gap sizes G and tether widths WT described above. Therefore, in an example the photoresist film thickness is no more than approximately <NUM>% of the smallest required resolution (G or WT).

Stripping the masks <NUM>, <NUM> finishes the transducer apparatus <NUM>, as shown in State (E).

The transducer apparatus <NUM> can then be functionalized. An example process uses silane-based chemistry (e.g. <NUM>-aminopropyltriethoxysilane). If biomolecule biosensors are to be provided, the amine group can be used to couple biomolecules.

The above method uses reactive ion etching. However, other methods are available including:.

The blocks illustrated in the <FIG> may represent steps in a method. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some blocks to be omitted.

In some but not necessarily all examples, the transducer apparatus <NUM> may be part of the Internet of Things forming part of a larger, distributed network. In some examples the transducer apparatus <NUM> is a wearable device or a module thereof. As used here 'module' refers to a unit or apparatus that excludes certain parts/components that would be added by an end manufacturer or a user.

The sensing and processing of the data, whether local or remote, may be for the purpose of health monitoring, data aggregation, patient monitoring, vital signs monitoring or other purposes.

The above described examples find application as enabling components of:
automotive systems; telecommunication systems; electronic systems including consumer electronic products; distributed computing systems; media systems for generating or rendering media content including audio, visual and audio visual content and mixed, mediated, virtual and/or augmented reality; personal systems including personal health systems or personal fitness systems; navigation systems; user interfaces also known as human machine interfaces; networks including cellular, non-cellular, and optical networks; ad-hoc networks; the internet; the internet of things; virtualized networks; and related software and services.

The term 'a' or 'the' is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a/the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use 'a' or 'the' with an exclusive meaning then it will be made clear in the context. In some circumstances the use of 'at least one' or `one or more' may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning.

The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and also to features that achieve substantially the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result.

Claim 1:
An apparatus for detecting an analyte (<NUM>) comprising,
a monolithic crystal (<NUM>) comprising a substrate portion (<NUM>) and at least one oscillator (<NUM>);
a first electrode (<NUM>) provided at a first location of the oscillator;
a second electrode (<NUM>) provided at a second location of the oscillator;
a gap (<NUM>) separating the oscillator from the substrate portion, exposing a side surface (<NUM>) of the oscillator; and
one or more tethers (<NUM>) that extend across the gap so that the oscillator is supported by the substrate portion,
characterized in that the apparatus is configured to enable the analyte to enter the gap, and wherein the side surface is functionalized to enable the analyte entering the gap to accumulate on the side surface of the oscillator.