Vibratory angular rate sensor

Abstract

A micromechanical vibratory angular rate sensor comprising: a substrate; a micromechanical structure anchored to the substrate and comprising a pair of Coriolis masses and a linkage coupling the pair of Coriolis masses to allow the pair of Coriolis masses to oscillate towards and away from each other in antiphase along a drive axis; an actuator configured to, in use, drive oscillatory motion of the linked pair of Coriolis masses in the drive axis such that angular rotation of the pair of Coriolis masses around an axis orthogonal to the drive axis causes the pair of Coriolis masses to vibrate in a direction along a sense axis orthogonal to the drive axis and rotation axis; and an optical sensor arranged to sense the magnitude of the vibration of the pair of Coriolis masses in a direction along the sense axis, the sensed magnitude being indicative of the rate of rotation around the rotation axis. The pair of Coriolis masses comprises a first Coriolis mass and a second Coriolis mass shaped to have an extent in directions along the drive axis to overlap alongside each other in the drive axis.

Description

VIBRATORY ANGULAR RATE SENSOR

[0001] TECHNICAL FIELD

[0002] The present disclosure relates to a micro electromechanical vibratory angular rate sensor for sensing rate of rotation.BACKGROUND

[0003] Vibratory angular rate sensor gyroscopes are a type of inertial microelectromechanical sensor (MEMS) used to detect the rate of angular rotation with respect to one or more axes. For angular rate sensing in both the X and Z directions, there are generally two categories of mechanical design approaches employed in the field: a two- mass tuning fork structure and a quad-mass vibratory gyroscope.

[0004] FIG. 1 shows a schematic view of an example of a typical two-mass vibratory gyroscope 100 in the art, which has a tuning fork design including two oscillating Coriolis masses 102 (or proof masses) coupled with links 104 to reduce common mode motion and consume low area while still providing reasonable isolation to linear accelerations. The two masses 102 are designed to be driven in anti-phase to oscillate 106 towards and away from one another along a drive axis 106 to generate desired velocities, and are also designed to oscillate 108 about a pivot point 112 along a sense axis 108 perpendicular to the drive axis 106 under the influence of inertial loads. This tuning fork structure 100 is often coupled to a second tuning fork structure such that multiple sensing axes can be achieved with the same driven oscillation frequency.

[0005] FIG. 2 shows a schematic view of an example of a typical quad-mass vibratory gyroscope 200 in the art, which uses a micromechanical structure having four Coriolis masses 202, 204, 206, 208 coupled together via links. The top two masses 202, 206 and the bottom two masses 204, 208 are driven out of phase as a two-mass tuning fork structure to oscillate 210 toward and away from the other along the drive axis 210, whilst the left two masses 202, 204, and the right two masses 206, 208 oscillate 212 toward and away from one another along the sense axis 212.

[0006] Typically, these micromechanical based vibratory gyroscopes use capacitive sensing schemes to detect movement of the masses, the most common of which is to provide a capacitor between an adjacent facing static support structure and the movable mass, and to use the sensed capacitance to indicate deflection of the mass, and hence to reveal the inertial force acting on the test mass.

[0007] Other detector arrangements for sensing the deflection of the masses include optical transduction schemes in which light propagating through a specific waveguide structure (such as a whispering mode gallery resonator facing the moving masses) has its resonant frequency modulated by the variations in the proximity of the masses. The modulation is detected using a photodetector and current amplifier to generate an electronic signal.

[0008] It is in this context the present disclosure has been devised.BRIEF SUMMARY

[0009] Tuning fork structures, such as the two-mass vibratory gyroscope 100 shown in FIG. 1, are typically small, but are susceptible to mechanical motion on the tuning fork structure 100, which is not intended to be read and is intended to be distinct from the angular rotation output. In particular, angular acceleration 110 arising from mechanical motion of the gyroscope 100 causes the two masses 102 to oscillate 108 about a pivot point 112 which is aligned with the sense axis 108, such that the sensor output couples the angular acceleration 110 to the angular rotation. Since it is difficult to decouple the angular acceleration 110 from the angular rotation, this means the sensor output is compromised by the angular acceleration 110, thereby skewing the sensor output, such that it is difficult to determine the angular rotation accurately. For example, mechanical sense movements such as external shock impacts on the tuning fork 100 may lead to the tuning fork 100 outputting high frequency readings arising from the angular acceleration levels 110 contained within external shock impacts. The torque T arising from the angular acceleration 110 may be determined as a product of the masses m of both Coriolis masses 102, the rate of angular acceleration fl and the separation distance d between the centres of gravity 114 of the two Coriolis masses 102:T = m - fl ■ d2

[0010] This torque is responsible for forcing the two Coriolis masses 102 to move in the sense direction 108 in the presence of angular acceleration 110. As torque is a function of the separation distance d, a smaller separation distance d between the two Coriolis masses 102 would lead to a lower torque input, such that even if the tuning fork 100 is exposed to mechanical sense movements, any angular acceleration 110 would have less influence on the sensor output, as compared with larger separation distances d between the two Coriolis masses 102. For example, in FIG. 1 it can be seen the separation distance d between the centres of gravity 114 of the two masses 102 is relatively large, resulting in designs that have the centres of gravity 114 of each Coriolis mass 102 separated by distances of approximately 1000pm.

[0011] Four-mass MEMS devices, such as the quad-mass vibratory gyroscope 200 shown in FIG. 2, attempt to address some of the issues with the tuning fork structure 100 of FIG. 1. Given that there are four masses 202, 204, 206, 208, any torque arising from mechanical motions (such as from external shock impacts) is countered by having four masses 202, 204, 206, 208 in the system. For example, the left two masses 202, 204 would react much like the two-mass gyroscope 100 of Fig. 1, in that angular acceleration 214 would impose a torque on the left two masses 202, 204 to move in the sense direction 212. However, the right two masses 206, 208 would have a torque from angular acceleration 214 that would counter the sense oscillations 212. By providing this configuration, the quad-mass vibratory gyroscope 200 may reduce the angular acceleration input 214. This improves accuracy in the angular rotation output of the gyroscope 200. However, such quad-mass vibratory gyroscope 200 designs typically lead to larger configurations by virtue of using more masses, and are typically used in higher performance products.

[0012] As discussed above, such micromechanical based vibratory gyroscopes 100, 200 may use capacitive or optical sensing schemes to detect movement of the masses. However, capacitive sensing schemes require significant area (typically areas greater than 500pm2) to be placed adjacent to the masses, which force the micromechanical structure to have complementary areas that are separated spatially. In addition, the sense capacitors also need to be placed symmetrically on the proof masses to help reduce the effects of package induced errors, again contributing to the spatial separation between masses. Furthermore, mechanical to optical transduction schemes are often prone to the same issues as capacitive sensing schemes, since the deformations in the structure of the inertial sensor (due to package and materials shifts over time and high temperatures) lead to relative movements between the optical waveguide structure and the suspended masses.

[0013] An aim of the present disclosure may therefore be considered as one of reducing the distance between the centres of gravity between the two Coriolis masses for reducing the effect of torque induced from angular accelerations on the device, whilst reducing the size of vibratory angular rate sensors.

[0014] According to a first aspect of the disclosure, there is provided a micromechanical vibratory angular rate sensor. The vibratory angular rate sensor comprises a substrate. The vibratory angular rate sensor comprises a micromechanical structure anchored to the substrate. The micromechanical structure comprises a pair of Coriolis masses and a linkage coupling the pair of Coriolis masses to allow the pair of Coriolis masses to oscillate towards and away from each other in antiphase along a drive axis. The vibratory angular rate sensor comprises an actuator configured to, in use, drive oscillatory motion of the linked pair ofCoriolis masses in the drive axis such that angular rotation of the pair of Coriolis masses around a rotation axis orthogonal to the drive axis causes the pair of Coriolis masses to vibrate in a direction along a sense axis orthogonal to the drive axis and the rotation axis. The vibratory angular rate sensor comprises an optical sensor arranged to sense the magnitude of the vibration of the pair of Coriolis masses in a direction along the sense axis, the sensed magnitude being indicative of the rate of rotation around the rotation axis. The pair of Coriolis masses comprises a first Coriolis mass and a second Coriolis mass shaped to have an extent in directions along the drive axis to overlap alongside each other in the drive axis.

[0015] Advantageously, a micro electromechanical inertial sensor is provided that improves accuracy and precision in sensor output, whilst also reducing the size of the vibratory angular rate sensor. In particular, the accuracy is improved by providing an overlapping configuration for the pair of Coriolis masses, which reduces the distance between the centres of gravity of each Coriolis mass, as compared with typical gyroscopes in the art (such as those shown in FIG. 1 and FIG. 2). Here, the term “centres of gravity of the masses” is used interchangeably with the term “centres of mass”. Because the coupling mechanism of the angular acceleration and the resulting torque arising from undesired mechanical motion (such as shock impacts) imposed on the sense mode of the structure is a function of the radial distance between the centres of mass of the pair of Coriolis masses, the compression of this distance reduces the susceptibility of the vibratory angular rate sensor to angular accelerations. The impact of errors arising (for example from mechanical sense movements from angular accelerations caused by shock impacts) may be reduced as the centres of mass are closer together and therefore improve accuracy in the sensor output of the rate of angular rotation. Furthermore, since the optical transduction scheme is smaller than capacitive sensors in the art, this facilitates in nesting the pair of Coriolis masses together with their centres of mass close to a centre line of the structure. The optical sensor may also be placed away from asymmetry in the masses, since the optical sensor does not occupy a significant area. By being arranged substantially symmetrically with respect to the pair of Coriolis masses along a line of symmetry that is parallel to the drive direction, this helps reduce the effects of package induced errors.

[0016] The first Coriolis mass and the second Coriolis mass may be shaped so as to be individually asymmetric along directions parallel to the sense axis so as to allow the pair of Coriolis masses to overlap. Advantageously, by providing the pair of Coriolis masses with an overlapping configuration, the centres of gravity of each of Coriolis mass may be closertogether and therefore reduce the impact of undesired mechanical motion upon the sensor output to improve accuracy thereof.

[0017] The first Coriolis mass and the second Coriolis mass may be shaped in the plane containing the drive axis and sense axis so as to be not congruent. Advantageously, the first Coriolis mass and the second Coriolis mass may not be congruent so as to facilitate being interdigitated to nest together and be driven toward and away from one another, whilst bringing their centres of mass close together, thereby reducing the impact of undesired mechanical motion upon the sensor output to improve accuracy thereof.

[0018] The first Coriolis mass and the second Coriolis mass may be individually symmetrical with respect to the drive axis. Advantageously, the pair of Coriolis masses may therefore be stable when driven in the drive direction, thereby improving the accuracy of the sensor output.

[0019] The first Coriolis mass and the second Coriolis mass may be shaped so that each of one or more portions of the first Coriolis mass protruding in the drive direction may be nested within one or more complementary recesses formed in the second Coriolis mass. Advantageously, this provides an interdigitated configuration that brings the centres of mass closer to one another, thereby reducing the impact of undesired mechanical motion upon the sensor output to improve accuracy thereof.

[0020] The first Coriolis mass and the second Coriolis mass may each be shaped to extend in the drive direction to have one or more protrusions and one or more recesses such that the opposing protrusions and recesses of the pair of Coriolis masses may be interdigitated. Advantageously, this configuration brings the centres of mass closer to one another, thereby reducing the impact of undesired mechanical motion upon the sensor output to improve accuracy thereof.

[0021] The first Coriolis mass and the second Coriolis mass may both be shaped to extend across a line orthogonal to the drive axis bisecting the centre of mass of the first Coriolis mass and the centre of mass of the second Coriolis mass. Advantageously, this configuration brings the centres of mass closer to one another, thereby reducing the impact of undesired mechanical motion upon the sensor output to improve accuracy thereof.

[0022] The first Coriolis mass and the second Coriolis mass may both be shaped and arranged so as to cause the centres of mass of the pair of Coriolis masses to be separated from each other in the drive axis by a distance less than if the pair of Coriolis masses were separated and non-overlapping in directions along the drive axis. Advantageously, thisconfiguration brings the centres of mass closer to one another, thereby reducing the impact of undesired mechanical motion upon the sensor output to improve accuracy thereof.

[0023] A distance separating the centre of mass of the first Coriolis mass and the centre of mass of the second Coriolis mass may be less than 250 pm. Advantageously, this improves resilience against undesired mechanical motion, thereby improving accuracy and precision in the output measurements.

[0024] The linkage coupling the pair of Coriolis masses may be configured to suppress in- phase motion of the pair of Coriolis masses in the direction along the sense axis. Advantageously, this means that linear acceleration shocks on the vibratory angular rate sensor will be less prone to couple into the sensor output, thereby improving the accuracy and precision in the output measurements during shock events.

[0025] The actuator may be configured to drive the pair of Coriolis masses to vibrate in a resonant mode in antiphase. Advantageously, since both the first Coriolis mass and the second Coriolis mass are driven in antiphase, this means the primary resonance mode may be driven more efficiently by the actuator, reducing higher frequency disruptions and nonlinear stimulus acting upon the Coriolis masses. In doing so, the sensor may be more sensitive to any angular rotations arising from induced Coriolis forces.

[0026] The pair of Coriolis masses may be balanced and of equal mass such that they vibrate either side of a sensing pivot point mid-way between their centres of mass, providing a tuning fork gyroscope, the sensed angular rotation being around the sensing pivot point. Advantageously, by having a two-mass tuning fork type configuration, the vibratory angular rate sensor is low area-consuming, whilst the pair of Coriolis masses are more resistant to noise and undesired mechanical stimulus, including linear accelerations.

[0027] The optical sensor may be arranged to sense the magnitude of vibration of one or more of the Coriolis masses in the sense direction at the edge of the Coriolis masses in a central location facing the Coriolis masses. Advantageously, the optical sensor is centrally and substantially symmetrically aligned with respect to the Coriolis masses, thereby helping to reduce relative movements between the optical sensor and the Coriolis masses. Such relative movements may arise from deformations in the structure of the optical sensor due to package and materials shifts over time and temperature.

[0028] The optical sensor may comprise one or more microresonators facing the edge of one or more of the Coriolis masses in the sense direction. Each mi crore senator may support a corresponding optical resonance. A change in a spacing between the one or more of the Coriolis masses and the microresonators due to induced Coriolis forces causing vibrations inthe sense direction may cause a change in the optical resonance characteristics of the microresonators detectable to generate a sensing signal indicative of the rate of rotation of the vibratory angular rate sensor around the rotation axis. The change in the spacing between the one or more of the Coriolis masses and the microresonators may therefore indicate an amplitude of the oscillation movement of the Coriolis masses in the sense direction over a particular duration. Advantageously, the optical sensor provides an efficient transduction scheme for sensing the rate of rotation.

[0029] The microresonators may be whispering gallery mode microresonators. Advantageously, the microresonators provided as whispering gallery mode microresonators provide high-quality factor and low loss whilst being area efficient.

[0030] The vibratory angular rate sensor may further comprise one or more optical couplers each for coupling light into and out of a microresonator. The vibratory angular rate sensor may further comprise one or more detectors each for detecting the light coupled out of a microresonator by an optical coupler, the detected light generating a sensing signal indicative of the rate of rotation of the vibratory angular rate sensor around the rotation axis. The sensing signal may be demodulated, conditioned and calibrated by processing circuitry. Advantageously, optical sensor may therefore provide a mechanical to optical transduction scheme for efficiently outputting measurements.

[0031] Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims.

[0032] It will be appreciated from the foregoing disclosure and the following detailed description of the examples that certain features and implementations described as being optional in relation to any given aspect of the disclosure set out above should be understood by the reader as being disclosed also in combination with the other aspects of the present disclosure, where applicable. Similarly, it will be appreciated that any attendant advantages described in relation to any given aspect of the disclosure set out above should be understood by the reader as being disclosed as advantages of the other aspects of the present disclosure, where applicable. That is, the description of optional features and advantages in relation to a specific aspect of the disclosure above is not limiting, and it should be understood that the disclosures of these optional features and advantages are intended to relate to all aspects of the disclosure in combination, where such combination is applicable.BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Certain examples of the present disclosure will now be described, with reference to the accompanying drawings, in which:

[0034] FIG. l is a schematic view of an example of a two-mass gyroscope in the art;

[0035] FIG. 2 is a schematic view of an example of a quad-mass gyroscope in the art;

[0036] FIG. 3 A is a schematic view of an X vibratory angular rate sensor in accordance with a first example of the disclosure;

[0037] FIG. 3B is another schematic view of the X vibratory angular rate sensor of Fig. 3A;

[0038] FIG. 3C is a schematic view of a linkage of the X vibratory angular rate sensor of FIG. 3A;

[0039] FIG. 4A is a schematic view of a Z vibratory angular rate sensor in accordance with a second example of the disclosure;

[0040] FIG. 4B is a schematic view of a linkage of the Z vibratory angular rate sensor of FIG. 4A; and

[0041] FIG. 5 is a flow chart of a method of manufacturing a vibratory angular rate sensor in accordance with an example of the disclosure;DETAILED DESCRIPTION

[0042] 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.

[0043] 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.

[0044] 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,” “atleast 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] A micromechanical vibratory angular rate sensor in accordance with a first example of the disclosure is described herein with reference to the accompanying FIGs. 3 A, 3B and 3C (together referred to hereinafter as Fig. 3). The vibratory angular rate sensor 300 comprises a substrate. The vibratory angular rate sensor 300 comprises a micromechanical structure anchored to the substrate. The micromechanical structure comprises a pair of Coriolis masses 302, 304 and a linkage 306 coupling the pair of Coriolis masses 302, 304 to allow the pair of Coriolis masses 302, 304 to oscillate towards and away from each other in antiphase along a drive axis 308. The vibratory angular rate sensor 300 comprises an actuator 310 configured to, in use, drive oscillatory motion of the linked pair of Coriolis masses 302, 304 in the drive axis 308 such that angular rotation 311 of the pair of Coriolis masses 302, 304 around a rotation axis 312 orthogonal to the drive axis 308 causes the pairof Coriolis masses 302, 304 to vibrate in a direction along a sense axis 324 orthogonal to the drive axis 308 and the rotation axis 312. The vibratory angular rate sensor 300 comprises an optical sensor arranged to sense the magnitude of the vibration of the pair of Coriolis masses 302, 304 in a direction along the sense axis 324, the sensed magnitude being indicative of the rate of rotation 311 around the rotation axis 312. The pair of Coriolis masses 302, 304 comprises a first Coriolis mass 302 and a second Coriolis mass 304 shaped to have an extent 318, 320 in directions along the drive axis to overlap alongside each other in the drive axis.

[0049] Advantageously, a micro electromechanical inertial sensor 300 is provided that improves accuracy and precision whilst also reducing the size of the vibratory angular rate sensor 300. In particular, by providing a two-mass vibratory gyroscope 300, the vibratory angular rate sensor 300 benefits from being a tuning fork gyroscope by being area-efficient, whilst providing reasonable isolation to linear accelerations. Furthermore, the accuracy is improved by providing an overlapping configuration for the pair of Coriolis masses 302, 304, such that the distance between the centres of gravity 314, 316 of each Coriolis mass 302, 304 (hereinafter centres of mass 314, 316) is only approximately 200 pm, which is significantly smaller as compared with the centre of mass separation distances of typical gyroscopes known in the art (such as those 100, 200 shown in FIG. 1 and FIG. 2). Because the coupling mechanism of the angular accelerations through the torque arising from undesired mechanical motion, which are typically high frequency (such as shock impacts), is a function of the radial distance between the centres of mass 314, 316 of the pair of Coriolis masses 302, 304, the compression of this distance reduces the susceptibility of the vibratory angular rate sensor 300 to angular accelerations. The impact of errors arising (for example from mechanical sense movements from angular accelerations caused by shock impacts) may be reduced as the centres of mass 314, 316 are closer together and therefore improve stability and resilience against undesired mechanical motion. Furthermore, since the optical transduction scheme is smaller than capacitive sensors in the art, this facilitates in nesting the pair of Coriolis masses together with their centres of mass close to a centre line of the structure. For example, the X vibratory angular rate sensor 300 provides the Coriolis masses 302, 304 with a centre of mass 314, 316 separation distance of 200 pm, such that each Coriolis mass 302, 304 has a small area of the order of 100 pm2each for the optical sensor to detect movement of each mass 302, 304. The optical sensor may also be placed away from asymmetry in the masses since the optical sensor does not occupy a significant area. By being arranged substantially symmetrically with respect to the pair ofCoriolis masses 302, 304, the optical sensor's arrangement helps reduce the effects of package induced errors.

[0050] In the first example of the disclosure shown in Fig. 3 A, the vibratory angular rate sensor 300 is an example of an X vibratory angular rate sensor. To illustrate the axes, an X- Y-Z coordinate system is shown for reference, whereby in the first example of Fig. 3 A, the rotation axis 312 coincides with the X axis, the drive axis 308 coincides with the Y axis and the sense axis 324 coincides with the Z axis (the Z axis going into and out of the plane). In practice, when the first and second Coriolis masses 302, 304 are under the influence of Coriolis forces and rotate 311 around the rotation axis 312, the first Coriolis mass 302 vibrates along the sense axis 324 in a first direction 326 and the second Coriolis mass 304 vibrates along the sense axis 324 in a second direction 328 that is opposite the first direction 326 of the first Coriolis mass 302. In doing so, the pair of Coriolis masses 302, 304 vibrate out of phase with one another so as to move in opposing directions 326, 328 towards and away from one another along the sense axis 324. In the first example of the disclosure, a sense mode 322 is defined along the sense axis, whereby the optical sensor is arranged to sense the magnitude of the vibration of the pair of Coriolis masses 302, 304 along the sense axis 324, such that the sensed magnitude of the vibration is indicative of the rate of angular rotation 311. In particular, the first Coriolis mass 302 and the second Coriolis mass 304 are balanced and of equal mass, such that they vibrate either side of a sense pivot point 322 mid-way between their centres of mass 314, 316. In doing so, the X vibratory angular rate sensor 300 is provided with a two-mass tuning fork configuration, with the sensed angular rotation 311 coinciding with the sense pivot point 322. The sense mode pivot point 322 is shown at a centre point of the X vibratory angular rate sensor 300. As can also be seen from FIG. 3 A, the rotation axis 312 is also aligned with a centre line of the two masses 302, 304 with respect to and intersecting the drive axis 308, with the sense axis 324 also coinciding with the centre point sense mode 322.

[0051] In the first example of FIG. 3 A, the optical sensor comprises microresonators 330 facing the edge of the Coriolis masses 302, 304 in the sense axis 324. To illustrate this, Fig. 3B is presented, which shows a schematic view of a portion of the X vibratory angular rate sensor of Fig. 3 A, showing the relationship between the optical sensor and the first Coriolis mass 302. As shown in Fig. 3B, the microresonators 330 face respective sensing portions 338 of the first Coriolis mass 302. It will be appreciated that the description of the microresonators 330 in relation to the first Coriolis mass 302 equally applies to the second Coriolis mass 304, and is not repeated for the sake of conciseness. In the first example of FIG. 3, the microresonators 330 are whispering-gallery mode resonators. As whispering-gallery mode resonators, the microresonators 330 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 330. The disclosure however is not limited to whispering-gallery mode resonators and other microresonators with suitable shapes and sizes may be used. In the first example of the X vibratory angular rate sensor 300 of FIG. 3, the microresonators 330 include a first microresonator arranged proximate the centre of mass 314 of the first Coriolis mass 302 along the Z axis, and a second microresonator arranged proximate the centre of mass 316 of the second Coriolis mass 304 along the Z axis, such that the microresonators 330 are proximate the edge of each Coriolis mass 302, 304. For example, the first and second microresonators 330 may be arranged adjacent to the respective centres of mass 314, 316. In some examples of the disclosure, at least a portion of the first and second microresonators 330 may overlap the respective centres of mass 314, 316. By arranging the first and second microresonators 330 proximate the respective centres of mass 314, 316, the optical sensor uses differential detection to sense the magnitude of vibration of the Coriolis masses 302, 304 in the sense direction 324 (aligned with the Z axis in the X vibratory angular rate sensor in the first example of Fig. 3A) proximate or at the edge of the Coriolis masses 302, 304 in a central location facing the Coriolis masses 302, 304. For example, in examples where the microresonators 330 are arranged proximate the edge of the Coriolis masses 302, 304, the microresonators 330 are arranged to entirely overlap the Coriolis masses 302, 304 in the sense axis 324 so as to sense the magnitude of oscillation of the sensing portion 338 of the Coriolis masses 302, 304 that the microresonators 330 face in the sense direction 324. In examples where the microresonators 330 are arranged at the edge of the Coriolis masses 302, 304, at least a portion of the microresonators 330 overlaps the edge of the Coriolis masses 302, 304 in the sense direction 324, such that the sensing portion 338 may have at least a portion that is not overlapped by the body of the respective Coriolis masses 302, 304. In some examples of the disclosure, the microresonators 330 may be proximate without overlapping the respective centres of mass 314, 316. In other examples of the disclosure, the microresonators 330 may be proximate the respective centres of mass 314, 316 with at least a portion facing the linkage 306 in the sense direction 324. For example, the microresonators 330 may face a portion of the linkage 306, or entirely face the linkage 306. This arrangement of the microresonators 330 being proximate the centres of mass 314, 316 whilst facing the linkage 306 would still give rise to the benefits of the present invention, as a portion of the linkage 306 vibrates with the Coriolis masses 302, 304 (as discussed inrelation to Fig. 3C below), such that the magnitude of vibration of the sensed portion of the linkage 306 may be indicative of the magnitude of vibration of the Coriolis masses 302, 304.

[0052] Each microresonator 330 includes one or more optical couplers for coupling light into and out of the microresonator 330, whereby each optical coupler couples the corresponding microresonator 330 to one or more detectors 332 arranged to output a sensing signal using a suitable photodetector and current amplifier to generate an electronic sensing signal. This is illustrated in Fig. 3B, which shows a signal system 340 including the detectors 332 coupled to the microresonators 330, whereby the signal system 340 further includes the above-mentioned suitable photodetector and current amplifier. In the first example of the disclosure, the signal system 340 is in communication with processing circuitry, for example in an external device, for processing the sensing signal by any suitable means. The sensing signal may be processed by any suitable means, including but not limited to demodulating the sensing signal based on the drive frequency, and conditioning and / or calibrating the sensing signal. In the first example of the disclosure, the signal system 340 further comprises communication circuitry to communicate with processing circuitry in the external device. However, it will be appreciated that the disclosure is not limited to the sensing signal being processed by an external device, and in some examples of the disclosure, the signal system 340 may further comprise processing circuitry, such that the X vibratory angular rate sensor 300 may be arranged to perform processing upon the sensing signal via its signal system 340. One or more suitable light sources are also provided to introduce light into the microresonators 330 via the optical couplers.

[0053] In practice, light propagating through the microresonators 330 is excited to resonance, whereby its resonant frequency is modulated in response to a change in a spacing between the Coriolis masses 302, 304 and the microresonators 330 due to induced Coriolis forces cause vibrations 326, 328 in the sense direction 324, which in turn causes a change in the optical resonance characteristics of the microresonators 330. The change in spacing is indicated by the arrow between the microresonator 330 and sensing portion 338 in Fig. 3B. The detected vibrations 326, 328 may then be indicative of the rate of rotation 311 of the X vibratory angular rate sensor 300 around the rotation axis 312, since the rate of rotation 311 is a function of the amplitude of the vibration 326 of the Coriolis masses in the sense direction 324 over a particular duration. As such, the microresonators 330 are arranged to detect any vibrations 326, 328 in the sense axis 324 to generate and output via the detector332 a sensing signal indicative of the rate of rotation of the X vibratory angular rate sensor 300 around the rotation axis 312.

[0054] In the first example of the disclosure as shown in Fig. 3A, the first Coriolis mass 302 and the second Coriolis mass 304 each have profiled surfaces facing one another in the drive direction 308, whereby the profiled surfaces include extents 318, 320 and recesses for providing an interdigitated configuration, where the two masses 302, 304 are nested together. The first Coriolis mass 302 has two extents 318 that protrude in the drive direction 308 toward and into two complimentary recesses formed in the second Coriolis mass 304. The second Coriolis mass 304 also has two extents 320 that protrude in the drive direction 308 toward and into two recesses formed in the first Coriolis mass 302. In particular, the extents 318, 320 each extend across a centre line that is orthogonal to the drive axis 308 and bisects the centres of mass 314, 316 of the Coriolis masses 302, 304, whereby in the case of FIG. 3 A, the centre line is aligned with the rotation axis 312. The recesses are adjacent the respective extents 318, 320. The first Coriolis mass 302 and the second Coriolis mass 304 are individually symmetrical with respect to the drive axis 308, and also individually asymmetric along directions parallel to the rotation axis 312 so as to allow the pair of Coriolis masses 302, 304 to overlap. In the X-Y plane, which in the case of FIG. 3A coincides with the plane containing the drive axis 308 and the rotation axis 312, the first Coriolis mass 302 and the second Coriolis mass 304 are not congruently shaped. In the first example of the disclosure, the width of the extents 318 of the first Coriolis mass 302 are greater than the width of the extents 320 of the second Coriolis mass 304. The “widths” here refer to the dimension aligned with the X axis. It will however be appreciated that in other examples of the disclosure, the surface profiles of the two masses 302, 304 are not limited to the specific configuration in FIG. 3A, and two or more extents 318, 320 and complimentary depressions may be present in the surface profile of each mass 302, 304. For example, in some examples of the disclosure, the extents 318 of the first Coriolis mass 302 may have a width equal to the extents 320 of the second Coriolis mass 320.

[0055] In the first example of the disclosure, the extents 318 of the first Coriolis mass 302 are separated by a central recess so as to be arranged towards the sides of the first Coriolis mass 302. Similarly, the second Coriolis mass 304 includes a central recess between the two recesses complimenting the two extents 318 of the first Coriolis mass 302, whereby the central recess of the second Coriolis mass 304 is differentiated by being shallower than its neighbouring two recesses in the drive axis 308, such that the central recess of the second Coriolis mass 304 extends closer toward the centre line (coinciding with the rotation axis 312) than its neighbouring two recesses. In doing so, the extents 318 of the first Coriolismass 302 may nest into the recesses of the second Coriolis mass 304. In doing so, the central recesses provided in the surface profiles of both masses 302, 304 define a central gap where the surface profiles of the two masses 302, 304 do not meet in the drive direction 308. The linkage 306 is arranged within this central gap and substantially centrally in the X- Y plane (which in the first example of FIG. 3 A corresponds to the plane containing the rotation axis 312 and the drive axis 308), such that the centre line aligned with the X axis bisects the linkage 306. Each end of the linkage 306 is connected to a central portion of the central recess of the surface profile of each respective mass 302, 304, such that the linkage 306 couples the two masses 302, 304 along a line of symmetry bisecting the drive axis 308 in the X-Y plane. The linkage 306 thus provides a pivot about which the Coriolis masses rotate 311. The linkage 306 is configured to suppress in-phase motion (also referred to as “common mode motion”) of the pair of Coriolis masses in the direction along the sense axis 324. By suppressing in-phase motion of the masses 302, 304 via the linkage 306 coupling, this means that linear acceleration shocks on the X vibratory angular rate sensor 300 will be less prone to couple into the sensor output, thereby improving the accuracy and precision in the output measurements during shock events.

[0056] Fig. 3C shows the linkage 306 according to the first example of the disclosure, in the context of the Coriolis masses 302, 304. The linkage 306 includes an anchor 342, a body 344 extending from the anchor 342, body beams 346 connecting the anchor 342 to the body 344, and mass beams 348 for connecting the body 344 to the Coriolis masses 302, 304. In the first example of the disclosure, the anchor 342 is arranged substantially centrally within the linkage 306, with the body beams 346 extending along the rotation axis 312 from the anchor 342 into the linkage body 344, whereby at least the ends of the body beams 346 are formed integrally with the anchor 342 and body 344 to form a continuous surface therebetween. In the first example of FIG. 3 A, the linkage body 344 is substantially cruciform in the X-Y plane, whereby the linkage body 344 extends along the X axis and has arms extending into the Coriolis masses 302, 304 along the drive axis 308. The mass beams 348 extend in a direction aligned with the X axis to connect the linkage 306 with the Coriolis masses 302, 304, whereby at least the ends of the mass beams 348 are integrally coupled with the Coriolis masses 302, 304 so as to form a continuous surface therebetween, such that the linkage 306 couples the Coriolis masses 302, 304 together. By providing the linkage 306 with this design, the linkage 306 provides a pivot point with a high stiffness in the sense direction 324, with its body beams 346 and mass beams 348 allowing the Coriolis masses 302, 304 to oscillate in the sense direction 324 out of phase, whilst suppressing in- phase movement. To facilitate this, the linkage body 344 is formed of a stiff material andmay be cut from the same material as the Coriolis masses 302, 304, with springs arranged between the substrate and the linkage body 344 so as to allow the drive oscillations 308. However, the disclosure is not limited to the configuration shown in Fig. 3C, and in other examples of the disclosure, the linkage 306 may take on any suitable shape that facilitates a pivot point around the rotation axis 312 and anti-phase movement of the Coriolis masses 302, 304, whilst suppressing in-phase movement in the sense direction 324. Although anchored, the linkage body 344 still vibrates with the Coriolis masses 302, 304 in response to Coriolis forces, such that as described above, the microresonators 330 may face a portion of the linkage 306 whilst being proximate the centres of mass 314, 316 to still benefit from the present invention.

[0057] As described above, the Coriolis masses 302, 304 have surface profiles with extents 318, 320 that overlap each other in the drive axis 308 and nest together with complimentary opposing recesses when the two Coriolis masses 302, 304 oscillate toward one another. By providing the first Coriolis mass 302 and second Coriolis mass 304 with these surface profiles, this provides an efficient shape for nesting the surface profiles together with an overlapping configuration when driven toward and away from one another. In doing so, this brings the centres of gravity 314, 316 very close together, as compared with typical gyroscopes (such as those of FIG. 1 and FIG. 2). As can be seen in FIG. 3 A, the centre of gravity 314 of the first Coriolis mass 302 is proximate the end of the linkage 306, with the centre of gravity 316 of second Coriolis mass 304 being proximate the other end of the linkage 306. By bringing the centres of mass so close together, this provides a two-mass tuning fork that reduces size, whilst improving stability and resilience against undesired mechanical motion.

[0058] In the first example of the disclosure of the X vibratory angular rate sensor 300 in Fig. 3, the actuator 310 includes a body and an array of electrostatic drive combs coupled to the actuator body. The electrostatic drive comb array includes an array of finger-like electrodes extending in the drive direction from a base extending in the X direction and is arranged to drive the actuator according to an input frequency, and may be coupled to the actuator body via any suitable means. For example, the electrostatic drive comb array may be formed integrally with the X vibratory angular rate sensor 300 for example by forming a layered structure that is etched into to define the different components including the electrostatic drive comb array using e.g. a mask and photolithography step followed by a Deep Reactive Ion Etch to cut the layered structure. Such a method is described in more detail in relation to Fig. 5 below. Other suitable means for coupling the electrostatic drive comb array to the actuator body (where for example the electrostatic drive comb array is notformed integrally with the actuator body) may include attachment or joining means, such as adhesion, welding, etc. The electrostatic drive comb array may be aligned with the X axis. The actuator body includes a plurality of drive springs arranged to allow the actuator 310 to move in the drive direction 308, whilst suppressing movement in the rotation axis 312 and sense axis 324. The drive springs may have a suitable elasticity and resilience to facilitate this, and may be arranged as elongated spring arrays aligned with the X axis. The actuator 310 is coupled to surfaces of each mass 302, 304 opposite the respective profiled surfaces. In the first example of the disclosure, the actuator 310 is coupled to each mass 302, 304 via coupling structures, which include a suitable arrangement of rotation torsion beams and coupling springs. The rotation torsion beams are aligned with the rotation axis 312, so as to provide a pivot point for the masses 302, 304 to rotate about the rotation axis 312. The coupling springs of the actuator coupling structure are arranged to allow the Coriolis masses to oscillate in the sense direction 324, whilst maintaining relatively high stiffness in the drive direction 308 such that common mode movement of the Coriolis masses 302, 304 is suppressed in the sense axis 324. The actuator 310 is therefore configured to drive the pair of Coriolis masses 302, 304 to vibrate in a resonant mode in antiphase. It will be appreciated however that the disclosure is not limited to the above actuator configuration and any suitable actuator 310 may be used. By driving the first Coriolis mass 302 and second Coriolis mass 304 in antiphase, this means the primary resonance mode may be driven more efficiently by the actuator 310, reducing higher frequency disruptions and nonlinear stimulus acting upon the Coriolis masses 302, 304, thereby providing more accurate sensor readings of the rate of angular rotations arising from induced Coriolis forces.

[0059] FIGs. 4A and 4B (together referred to hereinafter as Fig. 4) show a schematic view of a second example of a vibratory angular rate sensor 400, and more particularly a Z vibratory angular rate sensor 400. The Z vibratory angular rate sensor 400 includes a pair of Coriolis masses 302, 304, an actuator 310 and an optical sensor including microresonators 330, which may each be as described in relation to the first example of the X vibratory angular rate sensor 300. In particular, the optical sensor comprises microresonators 330 arranged proximate the respective centres of gravity of Coriolis masses 302, 304, with the description in relation to the optical sensor in the X vibratory angular rate sensor 300 of Fig. 3 equally applying to the optical sensor in the Z vibratory angular rate sensor 400 in Fig. 4. The axes in the second example of FIG. 4A have the same orientation as the first example of FIG. 3 A, whereby the X axis is horizontal, the Y axis is vertical and the Z axis goes into and out of the plane of the page as perpendicular to both the X and Y axes.

[0060] However, the Z vibratory angular rate sensor 400 in the second example of the disclosure shown in FIG. 4 differs from the X vibratory angular rate sensor 300 of FIG. 3 in that the Z vibratory angular rate sensor 400 is designed to sense the rate of angular rotation 412 around the Z axis (going into and out of the X-Y plane). In order to achieve this, the Z vibratory angular rate sensor 400 differs in its rotation axis 414, sense axis 402 and linkage401 coupling the Coriolis masses 302, 304 as compared to the X vibratory angular rate sensor 300 of Fig. 3, as follows:.

[0061] Whilst the drive direction 308 is aligned with the Y axis as the first Coriolis mass 302 and the second Coriolis mass 304 are both driven to oscillate along the drive axis 308, the Z vibratory angular rate sensor 400 has its rotation axis 414 aligned with the Z axis and the sense axis 402 aligned with the X axis. More particularly, the actuator 310 is arranged to drive oscillatory motion of the pair of Coriolis masses 302, 304 in the drive axis 308 such that angular rotation 412 of the pair of Coriolis masses 302, 304 around the rotation axis 414 (which is orthogonal to the drive axis 308) causes the pair of Coriolis masses 302, 304 to vibrate 404, 406 along the sense axis 402 in antiphase. In practice, when the first and second Coriolis masses 302, 304 are under the influence of Coriolis forces and rotate 412 around the rotation axis 414, the first Coriolis mass 302 vibrates along the sense axis 402 in a first sense direction 404 and the second Coriolis mass 304 vibrates along the sense axis402 in a second sense direction 406 that is opposite the first sense direction 404 of the first Coriolis mass 302. In doing so, first and second Coriolis masses 302 and 304 vibrate in opposing directions 404, 406 towards and away from one another along the sense axis 402.

[0062] In order to facilitate this, the linkage 401 coupling the Z vibratory angular rate sensor 400 is arranged to provide a pivot that allows movement in the rotation axis 414 and sense axis 402, whilst suppressing movement in the drive axis 308. Similarly to the linkage 306 in Fig. 3, the linkage 401 in Fig. 4 is arranged within the central gap defined between the surface profiles of the pair of Coriolis masses 302, 304 in the X-Y plane and substantially centrally in the X-Y plane (which in the Z vibratory angular rate sensor 400 of the second example shown in FIG. 4A corresponds to the plane containing the sense axis 402 and the drive axis 308), such that the centre line (coinciding with the sense axis 402 in Fig. 4A and aligned with the X axis) bisects the linkage 401. Each end of the linkage 401 is connected to a central portion of the central recess of the surface profile of each respective mass 302, 304, such that the linkage 401 couples the two masses 302, 304 along a line of symmetry bisecting the drive axis 308 (and coinciding with the sense axis 402 in Fig. 4A). The linkage 401 thus provides a pivot about which the Coriolis masses rotate 412. The linkage 401 is configured to suppress in-phase motion of the pair of Coriolis masses in thedirection along the sense axis 402. By suppressing in-phase motion of the masses 302, 304 via the linkage 401 coupling, this means that linear acceleration shocks on the Z vibratory angular rate sensor 400 will be less prone to couple into the sensor output, thereby improving the accuracy and precision in the output measurements during shock events.

[0063] Fig. 4B shows the linkage 401 according to the second example of the disclosure, in the context of the Coriolis masses 302, 304. The linkage 401 includes an anchor 342, a body 344 extending from the anchor 342, body beams 416 connecting the anchor 342 to the body 344, and mass beams 348 for connecting the body 344 to the Coriolis masses 302, 304. The anchor 342, body 344 and mass beams 348 may be as described in relation to the first example of the disclosure in Fig. 3. However, in the second example of the disclosure, the body beams 416 extend along the sides of the anchor 342 so as to be aligned with the drive axis 308 to couple the anchor 342 to the linkage body 344. With this design, the linkage 401 provides a pivot point with a high stiffness in the sense direction 402, with its body beams 416 and mass beams 348 allowing the Coriolis masses 302, 304 to oscillate in the sense direction 402 out of phase, whilst suppressing in-phase movement in the sense direction 402. However, it will be appreciated that the disclosure is not limited to this configuration shown in Fig. 3C, and in other examples of the disclosure, the linkage 401 may take on any suitable material and shape that facilitates a pivot point around the rotation axis 414 and anti-phase movement of the Coriolis masses 302, 304, whilst suppressing in-phase movement in the sense direction 402.

[0064] Furthermore, in addition to the linkage 401, more than one anchored linkage may be provided along the sense axis 402 to help suppress the common mode movement of the Coriolis masses 302, 304 in the drive axis 308 and the sense axis 402. For example, the Z vibratory angular rate sensor may include a second anchored linkage arranged at one side of the Z vibratory angular rate sensor and a third anchored linkage arranged at the other side of the Z vibratory angular rate sensor. In such examples of the disclosure, each of the second and third anchored linkages include springs elastically resilient in the sense direction 402 so as to allow the Coriolis masses 302, 304 to vibrate in the sense direction 402.

[0065] The actuator 310 in the Z vibratory angular rate sensor 400 in Fig. 4 includes electrostatic drive combs and a body including torsion beams and drive springs which have the same configuration as the actuator 310 of the X vibratory angular rate sensor 300 in Fig. 3. The actuator 310 in the Z vibratory angular rate sensor 400 in Fig. 4 includes coupling structures for coupling the actuator 310 to the Coriolis masses 302, 304 to allow the Coriolis masses 302, 304 to move in the rotation axis 414 and sense axis 402 via a suitablearrangement of rotation torsion beams aligned with the rotation axis 414 and coupling springs arranged to be elastically resilient in the sense direction 402.

[0066] In the second example of the disclosure, the microresonators 330 of the Z vibratory angular rate sensor 400 are arranged proximate the centres of mass 314, 316 and facing at least a portion of the Coriolis masses 302, 304 and / or the linkage 306. The microresonators 330 may be arranged in any suitable manner to detect movement in the sense direction 402 aligned with the X axis. Given that the sense direction 402 is aligned with the X axis in the second example of the disclosure, this may be achieved by the microresonators 330 being supported by the substrate for example via a substrate extension to suspend the microresonators 330 so as to be arranged laterally along an edge of each respective Coriolis mass 302, 304. By coupling the microresonators 330 with the substrate to suspend the microresonators 330 along the edges of the Coriolis masses 302, 304 in this way, the magnitude of vibration of the Coriolis masses 302, 304 may be detected.

[0067] In some examples of the disclosure, a multi-axis vibratory angular rate sensor is provided, which includes both the X vibratory angular rate sensor 300 and the Z vibratory angular rate sensor 400 coupled together via linkages and driven with the same frequency. In doing so, the multi-axis vibratory angular rate sensor may advantageously be driven with the same frequency to detect Coriolis angular rotations around both the X and Z axes.

[0068] FIG. 5 shows a flow chart of a method 500 of manufacturing a vibratory angular rate sensor, such as the X vibratory angular rate sensor 300 and Z vibratory angular rate sensor 400 in the first and second examples of the disclosure described above.

[0069] The method 500 comprises step 502 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.

[0070] In a first example of the method, the substrate layer is provided as a wafer comprising a suitable semiconducting compound (e.g. 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. In the first example of the method 500, step 502 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 500 comprises step 504, which includes cutting the layered structure to provide the X-Y profile of a vibratory angular rate sensor, which may be substantially as described in relation to Figs. 3 and 4 above, such that the X-Y profiles of a pair of Coriolis masses coupled together by a linkage and coupled to an actuator may be defined. In particular, the layered structure is cut to define a first Coriolis mass and a second Coriolis mass, each having complimenting extents and recesses so as to overlap alongside each other in the drive axis, in use. In particular, the layer may be cut using a suitable cutting device according to pre-determined dimensions, in order to provide extents and recesses substantially as described above in relation to the extents 318, extents 320 and recesses, respectively, in the first and second examples of the disclosure. Any suitable cutting technique may be used to cut the layered structure, for example, this may include using a mask and photolithography step followed by a Deep Reactive Ion Etch to cut the layered structure.

[0072] The method 500 comprises step 506, which includes arranging an optical sensor either below or above the device layer. The optical sensor may be substantially as described in relation to Figs. 3 and 4, such that the method includes arranging microresonators proximate or at the centres of gravity of the Coriolis masses. In the first example of the method 500, step 506 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 optical sensor may be arranged as part of step 502 when forming the layered structure. In other examples of the method, the optical sensor may be arranged below the device layer, so as to be formed in or on the substrate. In such examples, the optical sensor 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. This applies particularly to examples of the disclosure where the method is for forming a Z vibratory angular rate sensor (such as the Z vibratory angular rate sensor 400 described above) where the microresonators are arranged laterally along an edge ofeach Coriolis mass, as the structured layer incorporates the photonics so as to couple light into the microresonators.

[0073] The method 500 further comprises step 508, which includes cutting the sacrificial layer to form the vibratory angular rate sensor. In the first example 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 vibratory angular rate sensor to the substrate. The anchor is arranged at substantially centrally with respect to the linkage as described in relation to Figs. 3 and 4, and at least partially comprises the sacrificial material of the sacrificial layer (e.g. silicon dioxide). By cutting away the sacrificial layer, the Y-Z profile of the vibratory angular rate sensor is formed, so as to define the anchor and release the movable elements of the vibratory angular rate sensor from the substrate. More particularly, the actuator, in use, is arranged to move in the drive direction, with suppressed movement in the rotation and sense axes, whilst the pair of Coriolis masses are arranged to move in the drive, rotation and sense axes, with the linkage suppressing common mode motion in the drive and sense axes. As such, the method 500 may be used to form the X and Z vibratory angular rate sensors according to their defined rotation and sense axes.

[0074] In the first example of the method, step 504 of cutting the layered structure also includes defining holes through which the sacrificial layer may be etched away.

[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. 1

[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

CLAIMS1. A micromechanical vibratory angular rate sensor comprising: a substrate; a micromechanical structure anchored to the substrate and comprising a pair of Coriolis masses and a linkage coupling the pair of Coriolis masses to allow the pair of Coriolis masses to oscillate towards and away from each other in antiphase along a drive axis; an actuator configured to, in use, drive oscillatory motion of the linked pair of Coriolis masses in the drive axis such that angular rotation of the pair of Coriolis masses around a rotation axis orthogonal to the drive axis causes the pair of Coriolis masses to vibrate in a direction along a sense axis orthogonal to the drive axis and the rotation axis; and an optical sensor arranged to sense the magnitude of the vibration of the pair of Coriolis masses in a direction along the sense axis, the sensed magnitude being indicative of the rate of rotation around the rotation axis; wherein the pair of Coriolis masses comprises a first Coriolis mass and a second Coriolis mass shaped to have an extent in directions along the drive axis to overlap alongside each other in the drive axis.

2. The vibratory angular rate sensor of claim 1, wherein the first Coriolis mass and the second Coriolis mass are shaped so as to be individually asymmetric along directions parallel to the sense axis so as to allow the pair of Coriolis masses to overlap.

3. The vibratory angular rate sensor of claim 1 or 2, wherein the first Coriolis mass and the second Coriolis mass are shaped in the plane containing the drive axis and sense axis so as to be not congruent.

4. The vibratory angular rate sensor of any one of claims 1 to 3, wherein the first Coriolis mass and the second Coriolis mass are individually symmetrical with respect to the drive axis.

5. The vibratory angular rate sensor of any one of claims 1 to 4, wherein the first Coriolis mass and the second Coriolis mass are shaped so that each of one or more portions of the first Coriolis mass protruding in the drive direction are nested within one or more complementary recesses formed in the second Coriolis mass.

6. The vibratory angular rate sensor of any one of claims 1 to 5, wherein the first Coriolis mass and the second Coriolis mass are each shaped to extend in the drive direction to have one or more protrusions and one or more recesses such that the opposing protrusions and recesses of the pair of Coriolis masses are interdigitated.

7. The vibratory angular rate sensor of any one of claims 1 to 6, wherein the first Coriolis mass and the second Coriolis mass are both shaped to extend across a line orthogonal to the drive axis bisecting the centre of mass of the first Coriolis mass and the centre of mass of the second Coriolis mass.

8. The vibratory angular rate sensor of any one of claims 1 to 7, wherein the first Coriolis mass and the second Coriolis mass are both shaped and arranged so as to cause the centres of mass of the pair of Coriolis masses to be separated from each other in the drive axis by a distance less than if the pair of Coriolis masses were separated and non-overlapping in directions along the drive axis.

9. The vibratory angular rate sensor of any one of claims 1 to 8, wherein a distance separating the centre of mass of the first Coriolis mass and the centre of mass of the second Coriolis mass is less than 250 pm.

10. The vibratory angular rate sensor of any one of claims 1 to 9, wherein the linkage coupling the pair of Coriolis masses is configured to suppress in-phase motion of the pair of Coriolis masses in the direction along the sense axis.

11. The vibratory angular rate sensor of any one of claims 1 to 10, wherein the actuator is configured to drive the pair of Coriolis masses to vibrate in a resonant mode in antiphase.

12. The vibratory angular rate sensor of any one of claims 1 to 11, wherein the pair of Coriolis masses are balanced and of equal mass such that they vibrate either side of a sensing pivot point mid-way between their centres of mass, providing a tuning fork gyroscope, the sensed angular rotation being around the sensing pivot point.

13. The vibratory angular rate sensor of any one of claims 1 to 12, wherein the optical sensor is arranged to sense the magnitude of vibration of one or more of the Coriolis masses in the sense direction at the edge of the Coriolis masses in a central location facing the Coriolis masses.

14. The vibratory angular rate sensor of any one of claims 1 to 13, wherein the optical sensor comprises one or more microresonators facing the edge of one or more of the Coriolis masses in the sense direction, each microresonator supporting a corresponding optical resonance, wherein a change in a spacing between the one or more of the Coriolis masses and the microresonators due to induced Coriolis forces causing vibrations in the sense direction causes a change in the optical resonance characteristics of the microresonators detectable to generate a sensing signal indicative of the rate of rotation of the vibratory angular rate sensor around the rotation axis.

15. The vibratory angular rate sensor of claim 14, wherein the microresonators are whispering gallery mode microresonators.

16. The vibratory angular rate sensor of claim 14 or 15, further comprising one or more optical couplers each for coupling light into and out of a microresonator; and one or more detectors each for detecting the light coupled out of a microresonator by an optical coupler, the detected light generating a sensing signal indicative of the rate of rotation of the vibratory angular rate sensor around the rotation axis.

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