Inertial sensors and methods of operation thereof

Abstract

An inertial sensor comprises, a fixed support structure 110, one or more test mass-sensing optical microresonators 120a, b and one or more datum sensing optical microresonators 130a, b supported on th

Description

[0001] The present disclosure relates to inertial sensors for sensing acceleration or rate of rotation, and methods of operation thereof. In particular, the inertial sensors disclosed herein include micro-electromechanical structures for allowing compensation for errors resulting from undesired relative structural movements. BACKGROUND

[0002] Micro-electromechanical inertial sensors, whether an accelerometer or angular rate sensor gyroscope, use a micromechanical structure having a proof mass that is suspended by flexible elements or flexures, such that the test mass is designed to move under the influence of inertial loads. The flexures allow the test mass to move in the intended sense direction in the case of an accelerometer, while in the case of an angular rate sensor or gyroscope the flexures may be designed to allow the test mass to move in a direction perpendicular to the sense direction, to generate desired velocities. To detect movement of the test mass there are several transduction strategies, the most common is to form a capacitor between an adjacent facing static support structure and the movable test mass, and to use the sensed capacitance to indicate deflection of the test mass, and hence to reveal the inertial force acting on the test mass. A common issue for such devices is that the materials, package, and assembly tend to generate deformations appearing as relative movements between the static support structures surrounding the proof mass, and these relative movements appear m detected signal as a sensed movement of the test mass. This generates an error in the detected inertial force, rotation rate or acceleration.

[0003] In commonly used capacitive sensing arrangements, the test mass is designed to have relatively large surfaces (e.g. having areas greater than 500 pm2) that face a conductive capacitor plate electrode mounted on the stationary support structure, connected to the capacitive detection circuitry. This is so that a sufficiently large and accurate sensing signal is generated. Thus, due to the size, in a capacitive sensing arrangement, the relative movements between the stationary capacitor plate electrode and the test mass due to unintended movement from structural deformations, are difficult to remove. Possible techniques to reduce this error signal in capacitor sensing arrangements include suspending the stationary capacitor plate electrode from a location in close proximity to the anchor locations of the suspended test mass. Another approach is to co-locate the capacitor plate electrodes with the anchors of the test mass to average the test mass movement with the stationary electrode movements to effectively cancel relative movements in the detected signal. These techniques have limitations in their ability to isolate and remove the error signal.

[0004] Other detector arrangements for sensing the deflection of the test mass include optical transduction schemes in which light propagating through a specific waveguide structure (such as a whispering mode gallery resonator facing the moving test mass) has its resonant frequency modulated by the variations in the proximity of the test mass. The modulation is detected using a photodetector and current amplifier to generate an electronic signal. This mechanical to optical transduction scheme is prone to the same issues as the deformations in the structure of the inertial sensor due to package and materials shifts over time and temperature lead to relative movements between the optical waveguide structure and the suspended test mass.

[0005] It is in this context the present disclosure has been devised. SUMMARY OF THE DISCLOSURE

[0006] One notable difference between optical and capacitive transduction schemes in MEMS inertial sensors that has been realised is that the size of the area that needs to be provided on the test mass to face the optical waveguide structure is small (approximately <100pm2). That is considerably smaller than the size of the area that needs to be provided on the test mass for sensing by a capacitor plate electrode.

[0007] It has been realised that, because the optical sensor is very area-efficient, the mechanical design of the micro-electromechanical structure of the test mass can incorporate additional structures usable to accurately detect the undesired relative movements due to deformations in the structure of the inertial sensor, and thus reduce the error in the detected signal which is not due to the deflection of the test mass.

[0008] Thus, viewed from one aspect, the present disclosure provides an inertial sensor including a fixed support structure. The inertial sensor also includes one or more test masssensing microresonators supported on the fixed support structure. Each test mass sensing microresonator supports a corresponding optical resonance. The inertial sensor also includes one or more datum sensing microresonators supported on the fixed support structure. Each datum-sensing microresonator supports a corresponding optical resonance. The inertial sensor also includes a micro-electromechanical structure including a suspension structure anchored to the fixed support structure at an anchor point. The micro-electromechanical structure also includes one or more flexures coupled to the suspension structure. The microelectromechanical structure also includes a test mass suspended from the suspension structure by the one or more flexures to be deflectable under the application of an inertial force on the micro-electromechanical structure. The test mass is suspended to have respective deflection sense portions each facing and non-contiguous with one of the one or more test mass-sensing microresonators. The suspension structure includes one or more rigid protrusions extending to locations proximate to the deflection sense portions of the test mass to provide datum sense portions each facing and non-contiguous with one of the one or more datum sensing microresonators. The datum sense portions are fixed relative to the anchor point and the test mass is deflectable relative to the datum sense portions. A change in a spacing between the deflection sense portions and the test mass-sensing microresonators due to an inertial force acting on the test mass causes a change in the optical resonance characteristics of the test masssensing microresonators, detectable to generate a sensing signal indicative of the inertial force on the test mass. A change in a spacing between the datum sense portions and the datum sensing microresonators due to undesired relative structural movements causes a change in the optical resonance characteristics of the datum sensing microresonators, detectable to generate an error signal usable to correct the measurement of the inertial force by the test mass-sensing microresonators.

[0009] In this way, a design of a micro electromechanical inertial sensor is provided that achieves accurate optical detection of motion due to undesired structural deformations to generate an error signal, which can then be used to correct the sensing signal resulting from the optical detection of the deflection of the test mass. This is in particular achieved by the suspension structure of the micro-electromechanical structure, which is anchored to the fixed support structure (and from which the test mass is flexibly suspended), having rigid protrusions extending to locations proximate to the deflection sense portions of the test mass to provide datum sense portions. The datum sense portions are fixed relative to the anchor point, and the test mass is deflectable relative to the datum sense portions. As a result, measurements of the variation in displacement between the datum sensing portions and the datum sensing microresonators, being supported on the fixed support structure, will reveal the error signal resulting from the undesired structural deformations between the test mass and microresonators. The structural deformations acting on the rigid protrusions will also act on the test mass at the deflection sense portions thereof. Thus the error signal produced by the datum sensing microresonators can be used to accurately correct the measurement of the inertial force by the test mass-sensing microresonators by cancelling out the effect on the sensing signal of the undesired relative structural movements.

[0010] The inertial sensor may also include a test mass-sensing subsystem. The test mass-sensing subsystem may comprise one or more optical couplers each for coupling light into and out of a test mass-sensing microresonator. The test mass-sensing subsystem may comprise one or more detectors each for detecting the light coupled out of a test mass-sensing microresonator by an optical coupler. The detected light may generate a sensing signal indicative of the inertial force acting on the test mass. Advantageously, the inertial sensor may therefore provide an optical sensing scheme for efficiently and accurately outputting measurements indicative of the corrected inertial force on the test mass (i.e. corrected for any errors arising from undesired relative structural movements), by virtue of the test mass-sensing microresonators providing a very area-efficient optical sensing scheme allowing for room for the error-detecting datum sensing microresonators.

[0011] The inertial sensor may also include a datum sensing subsystem. The datum sensing subsystem may include one or more optical couplers each for coupling light into and out of a datum sensing microresonator. The datum sensing subsystem may comprise one or more detectors each for detecting the light coupled out of a datum sensing microresonator by an optical coupler. The detected light may generate an error signal indicative of the undesired relative structural movements in the inertial sensor. Advantageously, the inertial sensor may therefore provide a mechanical to optical transduction scheme for efficiently and accurately outputting measurements indicative of the error signal for correcting the inertial induced motion of the test mass, whilst being very area-efficient.

[0012] The inertial sensor may also include inertial force measurement circuitry. The inertial force measurement circuitry may be configured to receive a sensing signal generated from at least one test mass-sensing microresonator. The inertial force measurement circuitry may be configured to receive an error signal generated from at least one datum sensing microresonator. The inertial force measurement circuitry may be configured to, based on the sensing signal and the error signal, generate a sensed inertial force signal indicative of the inertial force acting on the test mass, including a correction to compensate the sensing signal for errors due to undesired relative structural movements in the inertial sensor. In doing so, the error signal can be accurately detected and the sensing signal can be corrected to give a sensed inertial force signal indicative of the inertial force acting on the test mass.

[0013] The one or more of the datum sense portions of the suspension structure may each be within 500 um of one of the deflection sense portions of the test mass. In doing so, the datum sense portions are arranged substantially proximate to the deflection sense portions of the test mass, so as to be arranged near the edge of the suspension structure, whilst the deflection sense portions are arranged near the edge of the test mass. Advantageously, this facilitates in accurately detecting displacement of both the test mass and the suspension structure.

[0014] The suspension structure of the micro-electromechanical structure may be anchored to the fixed support structure at a single anchor point. Advantageously, by anchoring the suspension structure to the fixed support structure via a single anchor point, this helps to reduce the impact of undesired structural deformations upon the suspended test mass, whilst providing a micro-electromechanical structure configuration designed to allow relative movements between the test mass and the suspension structure to be detected. The suspension structure may be anchored to the fixed support structure at two or more anchor points.

[0015] The datum sense portions of the suspension structure may extend in substantially the same plane as the deflection sense portions of the test mass. Advantageously, by extending in the same plane, this facilitates in accurately generating the sensing signal.

[0016] The test mass may have an average thickness of at least tens of microns. The distance between the deflection sense portions of the test mass and the one or more test mass-sensing microresonators may be less than a micron. By virtue of being so close to one another, the microresonators may accurately detect any movements in the respective sense portions.

[0017] The inertial sensor may be for sensing acceleration or rate of rotation. The inertial sensor may be driven at low frequency to be driven as an accelerometer. The inertial sensor may be driven at an operating frequency as a gyroscope, whereby the output signals are demodulated to generate an angular rotation output signal that is proportional to the angular rotation input. Advantageously, the inertial sensor may therefore provide an accelerometer or an angular rate gyroscope with improved accuracy.

[0018] The one or more microresonators may be whispering gallery mode microresonators. Advantageously, the whispering gallery mode microresonators may provide high-quality factor and low loss whilst being area efficient.

[0019] The test mass may be substantially planar. The flexures may be arranged to allow the test mass to deflect in a direction substantially perpendicular to the plane of the test mass. The flexures may be arranged to deflect in a direction substantially parallel to the plane of the test mass. The “plane of the test mass” corresponds to the plane defined by the planar test mass. When the flexures allow the test mass to deflect in a direction substantially perpendicular to the plane of the test mass, this facilitates in vertical sensing using a torsional pivot via the anchor. When the flexures allow the test mass to deflect in a direction substantially parallel to the plane of the test mass, this facilitates in in-plane sensing. As such, the inertial sensor can be adapted to provide sensing in a desired sensing direction.

[0020] The inertial sensor may also include one or more electrodes coupled to a second support structure arranged proximate to the test mass. The test mass may further include one or more finger portions. Each finger portion may be configured to extend between a pair of the one or more electrodes. The electrodes may be usable to apply a deflection force to the finger portions. The electrodes may be supported by one or more stationary structures arranged to face the test mass. In doing so, the electrodes may be self-actuating to apply a deflection force to the finger portions of the test mass. By providing self-actuating electrodes, the electrodes may be used to verify that the system is operating correctly, thereby providing self-diagnostics. The inclusion of actuation electrodes also increases the surface area of the stationary structures supporting the electrodes. In predetermined atmospheric conditions, the stationary structures having increased surface area with small gaps, which are used to apply electrostatic force, also impose squeeze film damping and this may act to damp the resonant frequency oscillations of the test mass. Damping thus makes the inertial sensor more robust to external shock and vibrations, and helps to reduce the impact of undesired ringing of the test mass.

[0021] The error signal may be usable to determine one or more control inputs for controlling the inertial sensor. The error signal may be output to a control system for generating the one or more control inputs. A driving frequency for actuating the test mass may be determined based on the error signal. A temperature system for controlling the temperature of the inertial sensor may determine a temperature based on the error signal. In doing so, the inertial sensor may be controlled to compensate for the error signal.

[0022] According to another aspect of the disclosure, there is provided a method of operating the inertial sensor as described herein. The method includes detecting light coupled out of the one or more test mass-sensing microresonators. The method also includes generating, based on the detected light a sensing signal indicative of the inertial force acting on the test mass. The method also includes detecting light coupled out of the one or more datum sensing microresonators. The method also includes generating, based on the detected light, an error signal indicative of undesired relative structural movements in the inertial sensor. The method also includes generating, based on the sensing signal and the error signal, a sensed inertial force signal indicative of the inertial force acting on the test mass, including a correction to compensate the sensing signal for errors due to undesired relative structural movements in the inertial sensor. Advantageously, the error signal produced by the datum sensing microresonators can be used to accurately correct the measurement of the inertial force by the test mass-sensing microresonators by cancelling out the effect on the sensing signal due to undesired relative structural movements.

[0023] Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

[0025] FIG. 1 shows a schematic view of an inertial sensor in accordance with examples of the disclosure.

[0026] FIG. 2 shows a flowchart of a method of operating the inertial sensor of Fig. 1.

[0027] FIG. 3 shows a schematic view of another inertial sensor in accordance with examples of the disclosure.

[0028] FIG. 4 shows a top view of a micro-electromechanical structure having a Z sense configuration in accordance with examples of the disclosure.

[0029] FIG. 5 shows a top view of a micro-electromechanical structure having an X sense configuration in accordance with examples of the disclosure.

[0030] FIG. 6 shows a flow chart of a method of manufacturing an inertial sensor in accordance with examples of the disclosure. DETAILED DESCRIPTION

[0031] Hereinafter, examples of the disclosure are described with reference to the accompanying drawings. However, it should be appreciated that the disclosure is not limited to the described examples, and all changes and / or equivalents or replacements thereto also belong to the scope of the disclosure. The same or similar reference denotations may be used to refer to the same or similar elements throughout the specification and the drawings.

[0032] As used herein, the terms “have,” “may have,” “include,” or “may include” a feature (e.g., a number, function, operation, or a component such as a part) indicate the existence of the feature and do not exclude the existence of other features. Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0033] As used herein, the terms “A or B,” “at least one of A and / or B,” or “one or more of A and / or B” may include all possible combinations of A and B. For example, “A or B,” “at least one of A and B,” “at least one of A or B” may indicate all of (1) including at least one A, (2) including at least one B, or (3) including at least one A and at least one B.

[0034] As used herein, the terms “first” and “second” may modify various components regardless of importance and do not limit the components. These terms are only used to distinguish one component from another. For example, reference to a first component and a second component may indicate different components from each other regardless of the order or importance of the components.

[0035] It will be understood that when an element (e.g., a first element) is referred to as being (physically, operatively or communicatively) “coupled with / to,” or “connected with / to” another element (e.g., a second element), it can be coupled or connected with / to the other element directly or via a third element. In contrast, it will be understood that when an element (e.g., a first element) is referred to as being “directly coupled with / to” or “directly connected with / to” another element (e.g., a second element), no other element (e.g., a third element) intervenes between the element and the other element.

[0036] The terms as used herein are provided merely to describe some embodiments thereof, but not to limit the scope of other embodiments of the disclosure. It is to be understood that the singular forms “a,” “'an,” and “the” include plural references unless the context clearly dictates otherwise. All terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments of the disclosure belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0037] An inertial sensor in accordance with examples of the disclosure is described herein with reference to the accompanying FIG. 1. The sensing system includes an inertial sensor including a fixed support structure 110. The inertial sensor also includes one or more test masssensing microresonators 120a, 120b supported on the fixed support structure 110. Each test mass-sensing microresonator 120a, 120b supports a corresponding optical resonance. The inertial sensor also includes one or more datum sensing microresonators 130a, 130b supported on the fixed support structure 110. Each datum sensing microresonator 130a, 130b supports a corresponding optical resonance. The inertial sensor also includes a micro-electromechanical structure 140 including a suspension structure 150 anchored to the fixed support structure 110 at an anchor point. The micro-electromechanical structure 140 also includes one or more flexures 151 coupled to the suspension structure 150. The micro-electromechanical structure 140 also includes a test mass 160 suspended from the suspension structure 150 by the one or more flexures 151 to be deflectable under the application of an inertial force on the microelectromechanical structure 140. The test mass 160 is suspended to have respective deflection sense portions 161a, 161b each facing and non-contiguous with one of the one or more test mass-sensing microresonators 120a, 120b. The suspension structure 150 includes one or more rigid protrusions 162a, 162b extending to locations L proximate to the deflection sense portions 161a, 161b of the test mass 160 to provide datum sense portions 163a, 163b each facing and non-contiguous with one of the one or more datum sensing microresonators 130a, 130b. The datum sense portions 163a, 163b are fixed relative to the anchor point and the test mass 160 is deflectable relative to the datum sense portions 163a, 163b. A change in a spacing between the deflection sense portions 161a, 161b and the test mass-sensing microresonators 120a, 120b due to an inertial force acting on the test mass 160 causes a change in the optical resonance characteristics of the test mass-sensing microresonators 120a, 120b, detectable to generate a sensing signal indicative of the inertial force on the test mass 160. A change in a spacing between the datum sense portions 163a, 163b and the datum sensing microresonators 130a, 130b due to undesired relative structural movements causes a change in the optical resonance characteristics of the datum sensing microresonators 130a, 130b, detectable to generate an error signal usable to correct the measurement of the inertial force by the test mass-sensing microresonators 120a, 120b.

[0038] Here, “non-contiguous” means not in contact or not touching one another. Furthermore, it will be understood that the deflection sense portions 161a, 161b correspond to regions of the test mass 160 used for detecting the deflection of the test mass 160, and the datum sense portions 163a, 163b correspond to regions of the suspension structure 150 used for detecting any movements of the suspension structure 150.

[0039] In this way, a design of a micro electromechanical inertial sensor is provided that achieves accurate optical detection of motion due to structural deformations to generate an error signal, which can then be used to correct the sensing signal resulting from the optical detection of the deflection of the test mass 160. This is in particular achieved by the suspension structure 150 of the micro-electromechanical structure 140, which is anchored to the fixed support structure 110 (and from which the test mass 160 is flexibly suspended), having rigid protrusions 162a, 162b extending to locations L proximate to the deflection sense portions 161a, 161b of the test mass 160 to provide datum sense portions 163a, 163b. In the present embodiment, the fixed support structure 110 comprises a substrate arranged above or below the plane of the test mass 160, such that the test mass-sensing microresonators 120a, 120b, and the datum sensing microresonators 130a, 130b are supported on the substrate forming part of the support structure 110. The datum sense portions 163a, 163b are fixed relative to the anchor point via the rigid protrusions 162a, 162b, and the test mass 160 is deflectable relative to the datum sense portions 163a, 163b. As a result, measurements of the variation in displacement between the datum sensing datum sense portions 163a, 163b and the datum sensing microresonators 130a, 130b, being supported on the fixed support structure 110, will reveal the error signal resulting from the undesired structural deformations of the inertial sensor. The structural deformations acting on the rigid protrusions 162a, 162b will also act on the test mass 160 at the deflection sense portions 161a, 161b thereof. Thus the error signal produced by the datum sensing microresonators 130a, 130b can be used to accurately correct the measurement of the inertial force by the test mass-sensing microresonators 120a, 120b by cancelling out the effect on the sensing signal of the undesired relative structural movements.

[0040] In the example of FIG. 1, the test mass 160 is formed to be substantially planar and resonant so as to be sensitive to inertial forces, and has an average thickness of approximately 50 um. The disclosure is not limited to this however, and in other examples of the disclosure, the average thickness may be in the order of at least 10s of pm. Any suitable resonant material may be used, such as silicon or mono- or polycrystalline silicon, to form the test mass 160. For example, a layer of resonant material may be cut to provide the test mass 160 using a suitable cutting technique such as photolithography mask patterning and etching according to predetermined dimensions.

[0041] In the example of FIG. 1, the distance between the deflection sense portions 161a, 161b of the test mass 160 and the respective test mass-sensing microresonators 120a, 120b is less than a micron. Similarly, the distance between the datum sense portions 163a, 163b and the respective datum sensing microresonators 130a, 130b is also less than a micron. As such, the sense portions 161a, 161b, 163a, 163b are each facing and non-contiguous with the respective microresonators 120a, 120b, 130a, 130b, but are very close to one another by only being separated by a distance of less than one micron. By virtue of being so close to one another, the microresonators 120a, 120b, 130a, 130b may accurately detect any movements in the respective sense portions 161a, 161b, 163a, 163b. The datum sense portion 163a of the suspension structure 150 is within 50 pm of the deflection sense portion 161a of the test mass 160, and the datum sense portion 163b of the suspension structure 150 is within 50 pm of the deflection sense portion 161b of the test mass 160. In some examples of the disclosure, the datum sense portions may be arranged even closer to the deflection sense portions, for example within 20 pm. The disclosure is not limited to this however, and in other examples of the disclosure the deflection sense portions may be arranged within 500 pm of the datum sense portions. Advantageously, the datum sense portions 163a, 163b are arranged substantially proximate and adjacent to the deflection sense portions 161a, 161b of the test mass, so as to be arranged near the edge of the suspension structure 150, whilst the deflection sense portions 161a, 161b are arranged near the edge of the test mass 160. Specific examples of arrangements of the datum sense portions and deflection sense portions are described in relation to Figs. 4 and 5. In doing so, this facilitates in accurately detecting displacement of both the test mass 160 and the suspension structure 150.

[0042] In the example of FIG. 1, the suspension structure 150 of the micro-electromechanical structure 140 is anchored to the fixed support structure 110 at a single anchor point, which may be provided in any suitable configuration to provide a pivot point for the test mass 160 to deflect. Specific examples of the anchor and pivot will be described in relation to FIG. 4 and FIG. 5. Advantageously, by anchoring the suspension structure to the fixed support structure, this provides a micro-electromechanical structure configuration designed to allow relative movements between the test mass 160 and the suspension structure 150 to be detected and provides a pivot point about which the test mass 160 may deflect. The disclosure is not limited to this however, and in some examples of the disclosure the flexures 151 may allow for linear translation movement of the test mass 160 with respect to the suspension structure 150 in addition to the pivot movement about the anchor point. Furthermore, in the example of Fig. 1, the flexures 151 have a suitable elasticity to facilitate suspending the test mass 160 to oscillate. The flexures 151 may be provided as spring elements, or any suitable material.

[0043] In the example of FIG. 1, the test mass-sensing microresonators 120a, 120b, and the datum sensing microresonators 130a, 130b, which are supported on a substrate forming part of the support structure 110, are whispering-gallery mode resonators formed in or on the substrate. Any suitable substrate may be used. The disclosure however is not limited to this. For example, the microresonators may be provided on a separate support structure, and / or other microresonators, such as integrated ring resonators may be used. In the example of FIG. 1, the microresonators 120a, 120b, 130a, 130b of the whispering-gallery mode type include waveguides that are wrapped around in a circular fashion to form a ring, so as to support a corresponding optical resonance, such that travelling waves propagating therein resonate with high Q factor. Any suitable material that is low-loss and transparent may be used for the microresonators. Advantageously, the inertial sensor may therefore provide an optical sensing scheme for efficiently and accurately outputting measurements indicative of the corrected inertial force on the test mass 160 (i.e. corrected for any errors arising from undesired relative structural movements). This is because the test mass-sensing microresonators 120a 120b provide a very area-efficient optical sensing scheme that gives sufficient room to allow for the error-detecting datum sensing microresonators 130a, 130b to be included in the inertial sensor without needing to increase the size of the inertial sensor.

[0044] In the example of FIG. 1, the inertial sensor includes an inertial force measurement system 170, including a test mass-sensing subsystem 171 and a datum sensing subsystem 172. The test mass-sensing subsystem 171 includes one or more detectors D and one or more optical couplers for coupling light into and out of the test mass-sensing microresonators 120a, 120b. Similarly, the datum sensing subsystem 172 includes one or more detectors D and one or more optical couplers for coupling light into and out of the datum sensing microresonators 130a, 130b. One or more suitable light sources, such as lasers, are also provided to introduce light into the microresonators 120a, 120b, 130a, 130b via the optical couplers. As shown in FIG. 1, the test mass-sensing microresonators 120a, 120b are in communication with detectors D of the test mass-sensing subsystem 171 and the datum sensing microresonators 130a, 130b are in communication with detectors D of the datum sensing subsystem 172, since each optical coupler couples the corresponding microresonator 120a, 120b, 130a, 130b to one or more detectors D, such that the test mass-sensing subsystem 171 and datum sensing subsystem 172 are arranged to output signals using a suitable photodetector and current amplifier to generate electronic signals. In particular, the test mass-sensing subsystem 171 is arranged to generate a sensing signal indicative of the deflection of the test mass 160 at the deflection sense portions 161a, 161b (and therefore the inertial force acting upon the test mass 160), whilst the datum sensing subsystem 172 is arranged to generate an error signal indicative of the deflection of the suspension structure 150 at the datum sense portions 163a, 163b (and therefore the undesired structural relative movements of the inertial sensor). In doing so, the raw data corresponding to the detected deflection of the test mass 160 at the deflection sense portions 161a, 161b amounts to a sum of (i) the deflection of the test mass 160 due to inertial forces acting thereupon, and (ii) the deflection of the test mass 160 due to undesired structural movements. Given that the detected deflection of the suspension structure 150 at the datum sense portions 163a, 163b is indicative of the deflection of the inertial sensor due to undesired structural movements, this means the error signal generated by the datum sensing subsystem 172 may compensate the sensing signal generated by the test mass-sensing subsystem 171 for undesired structural movements of the inertial sensor.

[0045] In practice, light propagating through the microresonators 120a, 120b, 130a, 130b is driven to be excited to resonance. The resonance frequency of light propagating through the test mass-sensing microresonators 120a, 120b is, in use, modulated in response to a change in a spacing between the deflection sense portions 161a, 161b and the test mass-sensing microresonators 120a, 120b. This is due to inertial forces acting upon the test mass 160 causing the test mass 160 to deflect in a direction substantially parallel to the plane of the test mass 160, which in turn causes a change in the optical resonance characteristics of the test masssensing microresonators 120a, 120b. This is for example shown m FIG. 1, whereby the test mass 160 deflects horizontally in a direction parallel to the plane of the test mass. However, it will be appreciated that in other examples of the disclosure, the test mass may be arranged to deflect in a direction substantially perpendicular to the plane of the test mass.

[0046] Based on the change in the optical resonance characteristics, the test mass-sensing microresonators 120a, 120b may then output the sensing signal so as to be indicative of the inertial force acting upon the test mass 160. Similarly, the resonance frequency of light propagating through the datum sensing microresonators 130a, 130b is, in use, modulated in response to a change in a spacing between the datum sense portions 163a, 163b and the datum sensing microresonators 130a, 130b, whereby the detected movements (that may be in the form of vibrations and / or oscillations) may then be indicative of the undesired relative structural movements in the inertial sensor. As such, the datum sensing subsystem 172 can generate an error signal for correcting the sensing signal.

[0047] In the example of FIG. 1, the inertial force measurement system 170 includes processing circuitry for receiving the sensing signal from the test mass-sensing microresonators 120a, 120b and the error signal from the datum sensing microresonators 130a, 130b. The processing circuitry is arranged to generate, based on the sensing signal and the error signal, a sensed inertial force signal indicative of the inertial force acting on the test mass 160, including a correction to compensate the sensing signal for errors due to undesired relative structural movements in the inertial sensor. The processing circuitry may perform signal processing on the received sensing signal and error signal, for example to improve the signal to noise ratio, to generate the sensed inertial force signal by any suitable means, such as a rules-based algorithm, for example by subtracting the error signal from the sensing signal, or by machine learning (e.g. a trained model). In doing so, the inertial sensor may accurately output sensed inertial force signals indicative of inertial forces acting upon the test mass 160 such as the rate of rotation or acceleration, which are corrected for undesired relative structural movements.

[0048] However, the disclosure is not limited to this, and the sensed inertial force signal may be generated externally by a separate processor rather than by the inertial sensor. In some examples of the disclosure, the test mass-sensing subsystem 171 and datum sensing subsystem 172 may include communication circuitry arranged to be in communication with an external device. In such examples of the disclosure, the test mass-sensing subsystem 171 and datum sensing subsystem 172 may then output the sensing signal and the error signal, respectively to the external device, so that the sensing signal and the error signal are processed in the external device to generate the sensed inertial force signal.

[0049] In the example of Fig. 1 where the inertial force measurement system 170 generates the sensed inertial force signal, the inertial force measurement system 170 comprises communication circuitry, such as a wireless module (e.g. WiFi, WLAN, Bluetooth®, etc.) that is arranged to output the sensed inertial force signal to another external device (not shown). The external device may for example include a display for displaying information corresponding to the corrected inertial force acting upon the test mass 160, based on the sensed inertial force signal. In some examples of the disclosure, the external device includes storage circuitry arranged to store data corresponding to the sensed inertial force signal in a database, so as to provide historical data of previously acquired sensed inertial force signals. The data corresponding to the sensed inertial force signal may be accompanied by a timestamp and / or location data. The storage circuitry may alternatively be provided on an external server in communication with the inertial sensor and / or the external device.

[0050] Turning now to FIG. 2, this shows a flowchart of a method 200 of operating the inertial sensor of FIG. 1 according to the example of the disclosure. In step 202, the method 200 includes detecting light coupled out of the one or more test mass-sensing microresonators 120a, 120b and generating, based on the detected light a sensing signal indicative of the inertial force acting on the test mass 160. As described above, the sensing signal correlates to the deflection of the test mass 160 and is therefore directly correlated to inertial forces acting upon the test mass 160, but is also directly correlated to any undesired structural movements of the test mass 160, such that the sensing signal is indicative of both of these causes of deflection of the test mass 160. In step 204, the method 200 includes detecting light coupled out of the one or more datum sensing microresonators 130a, 130b and generating, based on the detected light, an error signal indicative of undesired relative structural movements in the inertial sensor. In step 206, the method 200 includes generating, based on the sensing signal and the error signal, a sensed inertial force signal indicative of the inertial force acting on the test mass 160, including a correction to compensate the sensing signal for errors due to undesired relative structural movements in the inertial sensor. As such, the sensed inertial force signal corresponds to the sensing signal as corrected by the error signal. Step 206 may further include generating the sensed inertial force signal by processing the sensing signal and the error signal using a rules-based algorithm, as described above in relation to FIG. 1.

[0051] In some examples of the disclosure, the method further includes, prior to step 202, coupling light from a suitable light source, such as lasers, via the optical couplers into the test mass-sensing microresonators 120a, 120b and the datum sensing microresonators 130a, 130b. As described in relation to FIG. 1 above, light in practice propagating through the microresonators 120a, 120b, 130a, 130b is excited to resonance and any changes in the spacings between the respective sense portions 161a, 161b, 163a, 163b and microresonators 120a, 120b, 130a, 130b cause changes in the optical resonance frequencies of the microresonators 120a, 120b, 130a, 130b.

[0052] Step 206 may further include performing signal processing on the received sensing signal and error signal using the processing circuitry, for example to improve the signal-to-noise ratio, and generating the sensed inertial force signal based on the processed sensing signal and processed error signal. In some examples of the disclosure, the method further includes, after step 206 outputting the sensed inertial force signal to an external device, and displaying information corresponding to the corrected inertial force acting upon the test mass 160, based on the sensed inertial force signal, and / or storing data corresponding to the sensed inertial force signal in a database, as described above.

[0053] In some examples of the disclosure, the error signal generated in step 204 may be additionally output to a control system for controlling the inertial sensor, so as to be used in a feedback loop. In such examples of the disclosure, the control system is configured to generate control inputs for controlling the inertial sensor, whereby the control inputs are generated based on the error signal generated in step 204. In doing so, the control inputs of the inertial sensor may be adjusted in real time based on the error signal. For example, the position of the inertial sensor may be controlled based on the error signal to adjust the position of the test mass 160 relative to the test-mass sensing resonators 120a, 120b. The position of the test mass 160 may be adjusted by actuating the test mass with a driving frequency arranged to compensate for the error signal. (Actuation of the test mass 160 using electrodes is further described below in relation to Fig. 3.) In doing so, this provides a closed loop accelerometer with improved accuracy, whereby the error signal generated by the datum sensing microresonators 130a, 130b is used to determine the control inputs for controlling the inertial sensor, such as the position of the inertial sensor, whereby the driving frequency actuating the test mass 160 is determined based on the error signal. In doing so, the movement of the test mass 160 becomes more reliably responsive to inertial forces, with undesired structural movements providing less influence on the test mass 160 as the test mass’ movements are controlled to compensate for the undesired structural movements as indicated by the error signal. The disclosure is not limited to using feedback for controlling the position of the inertial sensor however, as the error signal may be used to determine any control inputs for controlling the inertial sensor. For example, in addition to or alternatively to the position of the test mass 160, a temperature control system may also use the error signal as feedback, such that the temperature of the inertial sensor may be controlled based on the error signal.

[0054] In further examples of the disclosure, the error signal may provide further information indicative of a scale of the error signal and whether the error signal is changing over time. For example, where the error signal becomes larger over time, this may indicate a structural issue that requires attention.

[0055] FIG. 3 shows another inertial sensor according to the disclosure. The inertial sensor of FIG. 3 includes all the same components as the inertial sensor of FIG. 1 (and although not shown, the inertial force measurement system 170 may also be included in the inertial sensor of FIG. 3), with the description in relation to FIG. 1 equally applying to FIG. 3. The inertial sensor of FIG. 3 only differs in that the inertial sensor of FIG. 3 further includes finger portions 302, electrodes 304 and a second support structure 306, as follows:

[0056] In the example of FIG. 3, the test mass 160 includes a plurality of finger portions 302. A plurality of electrodes 304 is coupled to a second support structure 306, such that the electrodes 304 are arranged proximate to the test mass 160. The second support structure 306 may be stationary and arranged to face the test mass 160. Each finger portion 302 is arranged to extend between a pair of the electrodes 304, such that the electrodes 304 are, in use, arranged to apply a deflection force to the finger portions 302. In particular, a voltage may be applied to the electrodes 304 to cause a potential difference between the electrodes 304 and the finger portions 302, so as to generate an electrostatic deflection force causing the test mass 160 to deflect via the finger portions 302. The voltage may be predetermined (for example by a user of the inertial sensor), such that the deflection detected by the test-mass sensing microresonators 120a, 120b in response to the applied predetermined voltage may be compared with the deflection expected to arise as determined for example during calibration. In doing so, the electrodes 304 may be self-actuating to apply a deflection force to the finger portions 302 of the test mass 160. By providing self-actuating electrodes 304 and accompanying finger portions 302, the electrodes 304 may be used to verify that the inertial sensor is operating correctly, thereby providing self-diagnostics. Furthermore, the electrodes 304 also increase the surface area of the second support structure 306, which in predetermined atmospheric conditions, means that the stationary second support structure 306 having an increased surface area with small gaps for applying electrostatic force, also imposes squeeze film damping, so as to help damp the resonant frequency oscillations of the test mass 160. Damping thus makes the inertial sensor more robust to external shock and vibrations, and helps to reduce the impact of undesired ringing of the test mass 160. It will be appreciated that under normal operation, the electrodes 304 only impose sufficient force to actuate the test mass 160, such that any undesired movement of the second support structure 306 does not influence the magnitude of the deflection of the test mass 160 read by the test mass-sensing microresonators 120a, 120b and couple with the test-mass sensing subsystem 171 signal output. Whilst the above electrodes 304 and fingers 302 have been described in relation to the present embodiment, the disclosure is not limited to this however, and other electrode configurations may be implemented to actuate the test mass. It will also be appreciated that if the electrodes 304 are applying electrostatic forces upon the test mass 160 and the second support structure 306 becomes subject to undesired stress or deformation, this would then impose an error signal on the test mass 160 that may not be compensated by the datum sensing subsystem 172.

[0057] In the example of FIG. 3, the test mass 160 includes a surface profile having a plurality of recesses, whereby each pair of neighbouring recesses defines a respective finger portion 302. As such, each electrode 304 protrudes into a respective recess between a pair of neighbouring finger portions 302. For illustration purposes, FIG. 3 shows the test mass 160 as including two finger portions 302, and the second support structure 306 as including three electrodes 304 extending into respective recesses between the finger portion 302, but it will be appreciated that n number of finger portions 302 and n+1 number of electrodes 304 may be provided, where n is an integer equal to or greater than 1. In the example of FIG. 3, the finger portions 302 are arranged on one side of the test mass 160. Whilst FIG. 3 shows the finger portions 302 as being defined by the recesses in the surface profile of the test mass 160, the disclosure is not limited to this and in other examples of the disclosure, the finger portions may for example be protrusions extending from a side of the test mass. Furthermore, whilst FIG. 3 shows the plurality of electrodes 304 and finger portions 302 being arranged on one side of the test mass 160, the disclosure is not limited to this, and in some examples of the disclosure, a plurality of arrays of electrodes 304 and finger portions 302 may be provided, with one or more arrays arranged on one or more sides of the test mass 160. In further examples of the disclosure, finger portions may be enclosed in the test mass 160. In such examples, a plurality of electrodes may be arranged in a substrate of the fixed support structure 110, such that the finger portions extend between the corresponding pairs of electrodes so as to provide an electrostatic drive comb configuration. In doing so, the electrodes pull the test mass 160 to actuate in the X or Y direction, requiring zero input voltage and facilitating self-actuation. In yet further examples of the disclosure, a plurality of electrodes may be arranged to face the plane of the test mass 160 so as to pull the test mass 160 to actuate in the Z direction. In such cases, a voltage bias may be applied.

[0058] FIG. 4 shows a top view of a micro-electromechanical structure having a Z sense configuration according to the disclosure, which may provide the micro-mechanical structure of the inertial sensor described in the examples above, including those described in FIGS. 1, 2 and 3. The micro-electromechanical structure of FIG. 4 includes a suspension structure 150, flexure 151, test mass 160, deflection sense portion 161a, deflection sense portion 161b, rigid protrusion 162a, rigid protrusion 162b, datum sense portion 163a, and datum sense portion 163b, which may each be as substantially described above in relation to FIGS. 1 to 3. The micro-electromechanical structure of FIG. 4 is provided with a vertical Z sense configuration, whereby taking an X-Y-Z coordinate system, the test mass 160 is arranged in the X-Y plane and is allowed to move in the Z direction perpendicular to the plane of the test mass 160 (as indicated by the arrow of motion aligned with the Z axis), by virtue of the flexure 151 of the suspension structure 150. The micro-electromechanical structure of Fig. 4 is anchored to the support structure 110, which is arranged below the micro-mechanical structure in the Z direction.

[0059] In the example of FIG. 4, the suspension structure 150 is arranged within the perimeter of the test mass 160, such that the test mass 160 encompasses and surrounds the suspension structure 150 withm the X-Y plane. In particular, the test mass 160 includes an opening dimensioned to be slightly larger than the suspension structure 150, such that the suspension structure 150 is arranged within the opening, with the perimeter of the suspension structure 150 being proximate the edge defined by the opening of the test mass 160. It will be appreciated that whilst Fig. 4 shows the opening to be substantially cruciform-shaped, the disclosure is not limited to this and may take any suitable shape that allows the test mass 160 to surround the perimeter of the suspension structure 150 in the X-Y plane. The perimeter of the test mass 160 is substantially planar and paddle-shaped, having a quadrilateral-shaped portion 402 and a protruding portion 404. The protruding portion 404 protrudes from a central portion from one side of the test mass 160. The suspension structure 150 is shaped such that the rigid protrusions 162a, 162b extend in opposing directions from a centre point of the suspension structure 150, with one rigid protrusion 162b extending into the quadrilateral-shaped portion 402 of the test mass 160, and the other rigid protrusion 162a extending into the protruding portion 404 of the test mass 160. The flexure 151 is arranged to couple the test mass 160 to the suspension structure 150. In the example of FIG. 4, the flexure 151 is a spring element that is relatively elongated in the Y direction. The length (extending in the Y direction) of the flexure 151 determines the stiffness of the test mass 160, as the longer the length of the flexure 151, the more the test mass 160 is allowed to deflect relative to the anchor point 406. Although Fig. 4 shows that the length (in the Y direction) of the flexure 151 corresponds to almost the width (in the Y direction) of the test mass 160, this is merely an example and it will be appreciated that the length of the flexure 151 is set according to a predetermined design based on the desired inertial force to be measured and the deflection properties of the configuration. It will however be appreciated that the disclosure is not limited to this and any suitable flexure 151 may be provided that is arranged to suspend the test mass 160 and couple to the suspension structure 150.

[0060] The deflection sense portion 161a is arranged substantially proximate to the datum sense portion 163a, and the deflection sense portion 161b is arranged substantially proximate to the datum sense portion 163b. In particular, the deflection sense portion 161a is arranged proximate and adjacent an end of the rigid protrusion 162a so as to be proximate the datum sense portion 163a. Similarly, the deflection sense portion 161b is arranged in a region of the test mass 160 that is proximate and adjacent an end of the rigid protrusion 162b so as to be proximate the datum sense portion 163b. Although not shown, test-mass sensing microresonators 120a, 120b are arranged under the test mass 160 in the Z direction so as to face the deflection sensing portions 161a, 161b to detect deflection of the test mass 160, and datum sensing microresonators 130a, 130b are arranged similarly in the Z direction to face the datum sensing portions 163a, 163 b, so as to detect any movement of the suspension structure 150, in the same manner described above in relation to Figs. 1 to 3. Furthermore, although not shown, electrodes 304 are arranged under the quadrilateral-shaped portion 402 of the test mass 160 so as to provide a deflection force to pull the test mass 160 downward.

[0061] By providing the configuration shown in Fig. 4, the micro-electromechanical structure of FIG. 4 is adapted for vertical sensing in the Z direction by using a torsional pivot with an asymmetrically mounted test mass 160. In the example of FIG. 4, the suspension structure 150 is anchored to the fixed support structure via a single anchor point 406 provided at a substantially central location of the suspension structure 150 between the rigid protrusions 162a, 162b. It will be appreciated that the lengths of the rigid protrusions 162a, 162b and the anchoring of the suspension structure 150 are predetermined to reduce any differential structural deformations where the suspension structure 150 deforms differently to the test mass 160 in the presence of undesired structural movements, which risk introducing additional errors into the inertial sensor output. For example, by using the single anchor point 406, this helps to reduce the above described differential structural deformations between the suspension structure 150 and the test mass 160 as the deformation of the suspension structure 150 may deform in substantially the same manner as the test mass 160 in the presence of undesired structural movements. This means that the error signal generated by the datum sensing subsystem 171 may accurately correct the sensing signal generated by the test mass-sensing subsystem 172, for generating the resulting inertial force measurement signal. This helps to provide stability, although it will be appreciated the disclosure is not limited to this, for example the entire length of the suspension structure from the rigid protrusion 162a to the rigid protrusion 162b may be coupled to the fixed support structure 110 to anchor thereto in other examples of the disclosure.

[0062] FIG. 5 shows a top view of another micro-electromechanical structure having an X sense configuration according to the disclosure, which may provide the micro-mechanical structure of the inertial sensor described in the examples above, including those described in FIGS. 1, 2 and 3. The micro-electromechanical structure of FIG. 5 includes test-mass sensing microresonators 120a, 120b, datum sensing microresonators 130a, 130b, suspension structure 150, flexure 151, test mass 160, deflection sense portion 161a, deflection sense portion 161b, rigid protrusion 162a, rigid protrusion 162b, datum sense portion 163a, and datum sense portion 163b, which may each be as substantially described above in relation to FIGs. 1 to 3. The micro-electromechanical structure of FIG. 5 is provided with an in-plane sense configuration, whereby taking an X-Y-Z coordinate system, the test mass 160 is arranged in the x-y plane and is allowed to move in the X direction parallel to the plane of the test mass 160, by virtue of being suspended by the flexure 151 of the suspension structure 150. The micro electromechanical structure of Fig. 5 is anchored to the support structure 110, which is arranged below the micro-mechanical structure in the Z direction.

[0063] As shown in FIG. 5, the test mass 160 is substantially planar and quadrilateral-shaped and by contrast to FIG. 4, does not contain the suspension structure 150 within its perimeter, but rather the suspension structure 150 in FIG. 5 is arranged to extend from one side of the test mass 160. In particular, the suspension structure 150 is substantially T-shaped, whereby a base 502 of the suspension structure 150 extends from the one side of the test mass 160, the rigid protrusions 162a, 162b extend alongside the edge of the one side of the test mass 160 and the flexure 151 extends from the base 502 to the test mass 160. In the example of FIG. 5, the anchor point 504 is provided in the base 502 of the suspension structure 150 so as to be arranged outside of the test mass 160. The flexure 151 in the example of FIG. 5 is provided as an elongated spring element. The rigid protrusions 162a, 162b are such that they each extend in opposing directions from a line bisecting the base 502 of the suspension structure 150. However, the disclosure is not limited to this, for example the suspension structure 150 may be anchored to the fixed support structure 110 via coupling the base 502 and rigid protrusions 162a, 162b of the suspension structure 150 to the fixed support structure 110 via a plurality of anchor points in other examples of the disclosure, rather than only the single anchor point 504.

[0064] The test mass 160 of FIG. 5 includes an elongated recess running along a portion of the edge of the one side of the test mass 160, such that the rigid protrusions 162a, 162b may extend alongside the edge of the one side of the test mass 160 within the recess, so as to be in parallel with the edge of the test mass 160. As such, the deflection sense portion 161a is arranged substantially proximate to the datum sense portion 163a, and the deflection sense portion 161b is arranged substantially proximate to the datum sense portion 163b. In particular, the deflection sense portion 161a is arranged proximate and adjacent an end of the rigid protrusion 162a so as to be proximate the datum sense portion 163a. Similarly, the deflection sense portion 161b is arranged in a region of the test mass 160 that is proximate and adjacent an end of the rigid protrusion 162b so as to be proximate the datum sense portion 163b. The test-mass sensing microresonators 120a, 120b are arranged adjacent the deflection sense portions 161a, 161b in the Y direction and the datum sensing microresonators 130a, 130b are arranged adjacent the datum sensing portions 163a, 163b in the Y direction. By providing this configuration, the test mass 160 may oscillate using a pivot and a single fixed anchor position 504 while allowing for the proximate datum sensing.

[0065] In the example of FIG. 5, the micro-electromechanical structure includes finger portions 302 arranged to extend between a pair of electrodes 304 (shown in Fig. 3 but not in Fig. 5), such that the electrodes 304 are, in use, arranged to apply a deflection force to the finger portions 302, as described in relation to FIG. 3. In the particular example of FIG. 5, the micro-electromechanical structure includes a plurality of arrays of finger portions 302, with an array arranged on each of the other sides of the test mass 160 from the one side anchored to the substrate via the suspension structure 150. As described in relation to FIG. 3 however, the disclosure is not limited to this and in other examples of the disclosure, one or more finger portions may be provided to extend between corresponding pairs of electrodes.

[0066] Whilst Figs. 4 and 5 show particular arrangements of the deflection sense portions 161a, 161b and datum sense portions 163a, 163b, it will be understood that the disclosure is not limited to these configurations. For example, the deflection sense portion may be provided as an edge but not an outer edge of the proof mass. In some examples of the disclosure, the deflection sense portion is for example a hole cut in the test mass that has an edge but is arranged substantially centrally with respect to the test mass, similarly to the deflection sense portion 161b shown in Fig. 4. In some examples of the disclosure, the suspension structure also resides within the perimeter of the test mass, similarly to the suspension structure 150 shown in Fig. 4. In further examples of the disclosure, the test mass may include an extension upon which the deflection sense portion is located, similarly to the protrusion 404 shown in Fig. 4.

[0067] FIG. 6 shows a flow chart of a method 600 of manufacturing an inertial sensor, such as those described in relation to the examples in Figs. 1 and 3 above, and those having the micro-el ectromechanical structures described in relation to the examples in Figs. 4 and 5 above.

[0068] The method 600 comprises step 602 of forming a layered structure comprising a plurality of layers. The plurality of layers includes a substrate layer, a sacrificial layer and a device layer, whereby the substrate layer forms the base layer of the layered structure and the sacrificial layer is arranged between the substrate layer and the device layer so as to be layered in the X-Y plane.

[0069] In examples of the method, the substrate layer is provided as a wafer comprising a suitable semiconducting compound, such as silicon. The sacrificial layer comprises a suitable sacrificial material (e.g. silicon dioxide) for etching away from the substrate and device layers. The device layer includes a suitable resonant material, such as silicon (including crystalline silicon e.g. single crystal silicon or deposited poly crystalline silicon), electroplated metal, a dielectric like silicon nitride, or aluminum nitride.

[0070] In the method 600, step 602 includes forming the layered structure by forming the sacrificial layer on the substrate layer and forming the device layer on the sacrificial layer. It will be appreciated however that the disclosure is not limited to this and the layered structure may be formed in any suitable manner, such as by providing the individual layers and bonding them together, or depositing films corresponding to respective layers.

[0071] The method 600 comprises step 604, which includes cutting the layered structure to provide the X-Y profile of an inertial sensor, which may be substantially as described in relation to Figs. 4 and 5 above, such that the X-Y profile of a test mass and suspension structure may be defined. In particular, the layered structure may be cut using a suitable cutting device according to pre-determined dimensions, for example using a mask and photolithography step followed by a Deep Reactive Ion Etch to cut the structure layer.

[0072] The method 600 comprises step 606, which includes arranging one or more test masssensing microresonators and one or more datum-sensing microresonators either below or above the device layer, substantially as described in relation to Figs. 4 and 5. In the method 600, step 606 comprises arranging the microresonators on the cut layered structure so as to be arranged on top of the device layer, and further comprises forming a photonics layer on the cut layered structure. The photonics layer may include any suitable conductive material such that in use the optical sensor may be coupled to an electrical input, for example to allow optical couplers to introduce light into the microresonators. However, the disclosure is not limited to this. For example, the photonics layer may be formed and the microresonators may be arranged as part of step 602 when forming the layered structure. In other examples of the method, the microresonators may be arranged below the device layer, so as to be formed in or on the substrate. In such examples, the microresonators may be formed in or on the substrate layer before the layered structure is formed, and the substrate layer may incorporate photonics to allow optical couplers to introduce light into the microresonators. In further examples, a segment of the photonics layer may be attached to the device layer so as to provide lateral coupling with the microresonators.

[0073] In examples of the method, step 604 of cutting the layered structure also includes defining holes through which the sacrificial layer may be etched away.

[0074] The method 600 further comprises step 608, which includes cutting the sacrificial layer to form the inertial sensor. In examples of the method, the cutting of the sacrificial layer includes etching a portion of the sacrificial layer (for example using an etching technique using hydrofluoric acid) away from the device layer and the substrate layer so as to define an anchor that anchors and couples the inertial sensor to the substrate, substantially as described in relation to Figs. 4 and 5. As such, the anchor at least partially comprises the sacrificial material of the sacrificial layer (e.g. silicon dioxide). By cutting away the sacrificial layer, this allows the anchor to be defined and release the movable elements of the inertial sensor from the substrate.

[0075] Where functional units have been described as circuitry, the circuitry may be general purpose processor circuitry configured by program code to perform specified processing functions. The circuitry may also be configured by modification to the processing hardware. Configuration of the circuitry to perform a specified function may be entirely in hardware, entirely in software or using a combination of hardware modification and software execution. Program instructions may be used to configure logic gates of general purpose or special-purpose processor circuitry to perform a processing function.

[0076] Circuitry may be implemented, for example, as a hardware circuit comprising custom Very Large Scale Integrated, VLSI, circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. Circuitry may also be implemented in programmable hardware devices such as field programmable gate arrays, FPGA, programmable array logic, programmable logic devices, A System on Chip, SoC, graphics processing units, GPU, or the like.

[0077] Machine readable program instructions may be provided on a transitory medium such as a transmission medium or on a non-transitory medium such as a storage medium. Such machine readable instructions (computer program code) may be implemented in a high level procedural or object oriented programming language. However, the program(s) may be implemented in assembly or machine language, if desired. In any case, the language may be a compiled or interpreted language, and combined with hardware implementations. Program instructions may be executed on a single processor or on two or more processors in a distributed manner.

[0078] Features, integers, characteristics or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. In particular, any dependent claims may be combined with any of the independent claims and any of the other dependent claims.

[0079] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

1. An inertial sensor comprising:a fixed support structure;one or more test mass-sensing microresonators supported on the fixed support structure, each test mass sensing microresonator supporting a corresponding optical resonance;one or more datum sensing microresonators supported on the fixed support structure, each datum-sensing microresonator supporting a corresponding optical resonance;a micro-electromechanical structure including:a suspension structure anchored to the fixed support structure at an anchor point; one or more flexures coupled to the suspension structure;a test mass suspended from the suspension structure by the one or more flexures to be deflectable under the application of an inertial force on the microelectromechanical structure, the test mass suspended to have respective deflection sense portions each facing and non-contiguous with one of the one or more test mass-sensing microresonators;the suspension structure comprising one or more rigid protrusions extending to locations proximate to the deflection sense portions of the test mass to provide datum sense portions each facing and non-contiguous with one of the one or more datum sensing microresonators, the datum sense portions being fixed relative to the anchor point and the test mass being deflectable relative to the datum sense portions;wherein a change in a spacing between the deflection sense portions and the test mass-sensing microresonators due to an inertial force acting on the test mass causes a change in the optical resonance characteristics of the test mass-sensing microresonators, detectable to generate a sensing signal indicative of the inertial force on the test mass; andwherein a change in a spacing between the datum sense portions and the datum sensing microresonators due to undesired relative structural movements causes a change in the optical resonance characteristics of the datum sensing microresonators, detectable to generate an error signal usable to correct the measurement of the inertial force by the test mass-sensing microresonators.

2. The inertial sensor of claim 1, further comprising:a test mass-sensing subsystem comprising:one or more optical couplers each for coupling light into and out of a test masssensing microresonator; andone or more detectors each for detecting the light coupled out of a test masssensing microresonator by an optical coupler, the detected light generating a sensing signal indicative of the inertial force acting on the test mass.

3. The inertial sensor of claim 1 or 2, further comprising:a datum sensing subsystem comprising:one or more optical couplers each for coupling light into and out of a datum sensing microresonator; andone or more detectors each for detecting the light coupled out of a datum sensing microresonator by an optical coupler, the detected light generating an error signal indicative of the undesired relative structural movements in the inertial sensor.

4. The inertial sensor of any one of claims 1 to 3, further comprising inertial force measurement system configured to:receive a sensing signal generated from at least one test mass-sensing microresonator;receive an error signal generated from at least one datum sensing microresonator;based on the sensing signal and the error signal, generate a sensed inertial force signal indicative of the inertial force acting on the test mass, including a correction to compensate the sensing signal for errors due to undesired relative structural movements in the inertial sensor.

5. The inertial sensor of any one of claims 1 to 4, wherein the one or more of the datum sense portions of the suspension structure are each within 500 pm of one of the deflection sense portions of the test mass.

6. The inertial sensor of any one of claims 1 to 5, wherein the suspension structure of the micro-electromechanical structure is anchored to the fixed support structure at a single anchor point.

7. The inertial sensor of any one of claims 1 to 6, wherein the datum sense portions of the suspension structure extend in substantially the same plane as the deflection sense portions of the test mass.

8. The inertial sensor of any one of claims 1 to 7, wherein the test mass has an average thickness of at least tens of microns, and wherein the distance between the deflection sense portions of the test mass and the one or more test mass-sensing microresonators is less than a micron.

9. The inertial sensor of any one of claims 1 to 8, wherein the inertial sensor is for sensing acceleration or rate of rotation.

10. The inertial sensor of any one of claims 1 to 9, wherein the one or more microresonators are whispering gallery mode microresonators.

11. The inertial sensor of any one of claims 1 to 10, wherein the test mass is substantially planar and the flexures are arranged to allow the test mass to deflect in a direction substantially perpendicular to the plane of the test mass, or to deflect in a direction substantially parallel to the plane of the test mass.

12. The inertial sensor of any one of claims 1 to 11, further comprising one or more electrodes coupled to a second support structure arranged proximate to the test mass, the test mass further comprising one or more finger portions, each finger portion configured to extend between a pair of the one or more electrodes, the electrodes being usable to apply a deflection force to the finger portions.

13. The inertial sensor of any one of claims 1 to 12, wherein the error signal is usable to determine one or more control inputs for controlling the inertial sensor.

14. A method of operating the inertial sensor of any one of claims 1 to 13, comprising: detecting light coupled out of the one or more test mass-sensing microresonators and generating, based on the detected light a sensing signal indicative of the inertial force acting on the test mass;detecting light coupled out of the one or more datum sensing microresonators and generating, based on the detected light, an error signal indicative of undesired relative structural movements in the inertial sensor; andgenerating, based on the sensing signal and the error signal, a sensed inertial force signal indicative of the inertial force acting on the test mass, including a correction tocompensate the sensing signal for errors due to undesired relative structural movements in the inertial sensor.

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