The project leading to this application has received funding from the European Union's Horizon 2020 Research and Innovation programme under grant agreement No 636820.

BACKGROUND TO THE INVENTION

Field of the Invention

The present invention relates to a method of operation of a resonator. The invention has particular, but not exclusive, applicability to a bulk acoustic wave resonator (BAWR), such as a thin film bulk acoustic wave resonator (FBAR) or a solidly mounted resonator (SMR). Such resonators are of use in sensor applications, such as in gravimetric-based sensing.

Related Art

Acoustic resonators can be used for sensing of various chemical and biological agents in air. Such resonators are typically small and highly sensitive to mass changes, and have a relatively low manufacturing cost. For a typical acoustic mass sensor, the resonant frequency of the acoustic-wave resonator changes in response to the mass change on the resonator surface. Typically, such sensors are not suitable for sensing applications in liquid, because the thickness longitudinal mode (TLM) experiences a substantial reduction in Q factor. This reduction is typically large, usually more than 90%. The reduction of the Q factor is considered to be due to damping caused by loss of acoustic energy into the liquid.

Zhang and Kim (2005) disclose using an FBAR sensor in longitudinal mode. The sensor has a Q factor of 200-300 at about 1 GHz in air, allowing a vapour mass change of 10−9g/cm2at its surface. In air, the sensor is operated in fundamental resonance mode. However, when the sensor is operated in liquid, the Q factor is reduced by several orders of magnitude, due to damping. Zhang and Kim (2005) propose instead to operate the sensor in liquid at its second harmonic resonance, at about 2 GHz, and report a Q factor of about 40. Therefore operating at the second harmonic resonance reduces the effects of damping by the liquid, but there is still more than 88% reduction in Q factor. Pottigari and Kwon (2009) form an FBAR device using a piezoelectric ZnO film sandwiched between Al electrodes as a transducer. A layer of parylene is supported over the uppermost electrode on supporting posts, with a vacuum gap between the parylene layer and the uppermost electrode. This approach was used in order to investigate the sensitivity of the FBAR device to sensing in liquid environments, with the parylene layer constituting a passive sensing diaphragm. In effect, the supporting posts guide the acoustic waves from the transducer into the passive sensing diaphragm. An effect is shown which is that the Q factor of the driving resonator is reduced by only 9% when sensing in water compared with air. The posts are essential to support the parylene layer and to withstand the pressure differential between the vacuum and the external atmosphere and they are the only way to transmit acoustic waves from the resonator to the sensible area. Additionally, the device structure proposed by Pottigari and Kwon (2009) is fragile and not robust enough for practical applications.

Xu et al (2011) discloses an FBAR sensor used in conjunction with microfluidic confinement in order to limit the available damping provided by the liquid. The thickness of the microfluidic channel is critical to this reduced damping and substantially limits the utility of the sensor to sample a suitable volume of liquid.

SUMMARY OF THE INVENTION

The present inventors have realised that it would be advantageous to provide a resonator device that is operable in liquid and which provides useful effects compared with prior art resonator devices, and propose a method of sensing a target analyte in a liquid sample using the device.

Accordingly, in a first preferred aspect, the present invention provides a method of sensing a target analyte in a liquid sample including the steps: (i) providing a bulk acoustic wave resonator device, (ii) placing a liquid sample on the bulk acoustic wave resonator device, (iii) operating the bulk acoustic wave resonator device to generate bulk acoustic waves, (iv) measuring a shift in a resonant frequency of the bulk acoustic wave resonator device.

The bulk acoustic wave resonator device comprises a resonator structure and an acoustic decoupling layer, the resonator structure comprising a piezoelectric material layer, electrodes arranged to apply a driving signal to the piezoelectric material layer to generate bulk acoustic waves, and a resonator structure surface. The resonator structure has a resonator structure acoustic impedance and the liquid sample has a liquid sample acoustic impedance. The acoustic decoupling layer is formed over the resonator structure surface. The acoustic decoupling layer has an acoustic decoupling layer acoustic impedance, the acoustic decoupling layer acoustic impedance being up to ⅕ times or not less than 5 times the resonator structure acoustic impedance, and up to ⅕ times or not less than 5 times the liquid sample acoustic impedance. In other words, the acoustic impedance of this layer is, by a factor of at least 5, smaller or larger than the respective acoustic impedances of the resonator structure and of the liquid sample.

Where the resonator structure includes a series of layers, the relevant resonator structure acoustic impedance may be taken as the acoustic impedance of the uppermost layer of the resonator structure.

By providing an acoustic decoupling layer with such an acoustic mismatch with respect to the acoustic impedance of the liquid sample, and of the resonator, the layer can act as an acoustic isolation layer. The acoustic impedance of the acoustic decoupling layer may be a factor of more than 10 times smaller or larger than the resonator structure acoustic impedance, and of the liquid sample acoustic impedance. However, the acoustic mismatch is preferably not so high as to prevent propagation of the wave through the acoustic decoupling layer for sensing. This is confirmed by the expression R=(Z/Z0−1)/(Z/Z0+1), where R is the reflection coefficient. If Z/Z0is too large, the intensity of the wave passing through the interface is too low and can be vanished in the layer due to acoustic losses. A mismatch of 10 (i.e. 10 times larger or smaller) gives a transmission of 19% of the intensity, which is considered to be sufficient transmission to allow for useful sensing.

The acoustic impedance of the acoustic decoupling layer, resonator, and liquid sample may be measured in a manner well-known to the skilled person. Generally, to measure acoustic impedance, the sound velocity of the selected mode in the material is determined, and multiplied by mass density of the material. As the skilled person will be aware, the acoustic impedance of a material strongly depends on the propagation mode. In the present invention, therefore the acoustic impedance of the different elements is assessed with respect to the propagation mode used for the bulk acoustic wave resonator device. The acoustic impedance of a material typically does not vary with frequency in the range of operation of devices discussed herein. The bulk acoustic wave resonator device proposed herein may be an SMR type device. Alternatively, the device may be an FBAR type device. The fundamental resonant frequency of the bulk acoustic wave resonator device may be in the range 1-5 GHz.

The following references set out suitable methods for determining the acoustic impedance as used in this disclosure: DeMiguel-Ramos et al (2015); Iborra et al (2013).

Further optional features of the invention are set out below. These may be applied singly or in any suitable combination.

Preferably, the thickness of the acoustic decoupling layer is between ⅛λ and 50λ. λ is the wavelength in the decoupling layer at a fundamental resonant frequency of the resonator. More preferably, the upper limit of the thickness range of the acoustic decoupling layer may be selected to be e.g. 20λ, 10λ, or 2λ. The lower limit of the thickness range may be e.g. ⅛λ, ¼λ or ½λ. Preferably, the thickness of the acoustic decoupling layer is a multiple of ½λ.

The acoustic decoupling layer may be formed from a number of materials, given that they fulfil the acoustic impedance requirements discussed above. Preferably, the acoustic decoupling layer comprises carbon nanotubes. Other examples of possible materials suitable for use as an acoustic decoupling layer may include, but are not limited to, polydimethylsiloxane (PDMS), parylene, and SU-8. Typical acoustic impedance values for different materials are set out further below.

As the skilled person will readily understand, the density of the acoustic impedance layer will have an effect on the sound velocity of the selected mode in the layer. With carbon nanotubes, the density of the layer will be affected by the number density of carbon nanotubes. Accordingly, where the acoustic impedance layer comprises carbon nanotubes, the density of carbon nanotubes in the layer may be at least 0.3×1010cm−2, alternatively may be at least 0.5×1010cm−2, at least 1×1010cm−2, or at least 2×1010cm−2.

The acoustic decoupling layer may include binding sites specific for the analyte in the liquid sample. For example, the acoustic decoupling layer may be functionalised to promote attachment of specific analytes.

The liquid sample may be an aqueous liquid, e.g. deionised (DI) water or a biological buffer. The mass density of typical biological buffers varies in the range 1 to 1.3 times of that of DI-water. Sound velocity between such liquids typically varies less than 3%. Therefore the acoustic impedance of a typical liquid sample is in the range of 1.48 MRayl (corresponding to pure water) to 2 MRayl for a highly concentrated buffer. These values are for typical buffers used in biological detection. Blood, for instance, has an acoustic impedance of 1.66 MRayl. The liquid sample may partially penetrate the acoustic decoupling layer, although this is not necessary.

