Patent Description:
The present invention concerns vibration insulation, damping or suppression of mechanical vibrations in MEMS devices.

In general, the present invention can be applied to MEMS devices, in particular MEMS devices consisting of a plurality of solid-state components formed in or on a common substrate.

With the advantages of small size, low-cost batch fabrication and low power consumption, Micro Electro-Mechanical Systems (MEMS) are attracting increasing interest from the scientific and industrial communities. It is then of fundamental importance to optimize their functioning and to improve their performances.

The working principle of many MEMS devices such as gyroscopes, accelerometers, micromirrors, micropumps and clocks, relies on resonating components that are driven at their own natural frequencies. Unwanted external vibrations at certain frequencies can produce spurious signals that can even compromise the right functioning of the MEMS resonant devices. For example, increased sensitivity and phase noise due to external vibration in resonant MEMS accelerometers and tuning fork gyroscopes are widely reported.

External vibration in resonant MEMS devices undermines both short-term (noise) and long-term (stability) performances. Therefore, reducing the impact of external vibrations on the MEMS resonators is critically important to improve their performance.

Several approaches have been reported to isolate MEMS resonant devices from external vibrations:
One approach for compensating the displacements induced by vibration is to introduce differential architectures in the MEMS, such as tuning fork gyroscopes (TFGs) that operate in anti-phase mode to cancel out the impact of external vibrations. Nevertheless, due to unavoidable electromechanical non-idealities, part of these signals can manifest as unwanted output changes.

Another approach is to increase the resonant frequency of the MEMS device so that it is much higher than the frequency spectrum of the environmental excitation. However, the stiffened MEMS structure requires ultra-narrow gaps to be actuated and may cause less sensitivity for the device.

Document <CIT>) relates to suppression of spurious modes of vibration for resonators and related apparatus and methods. A device includes a MEMS resonating structure, a substrate, and anchors between the MEMS resonating structure and the substrate. The anchors are configured to suppress the response of the at least one spurious mode of vibration.

Document <CIT>) relates to a micromechanical vibro-insulator which isolates vibration of a MEMS resonator and includes a micromechanical resonator interposed between phononic bandgap mirrors.

Document <CIT>) relates to an acoustic metamaterial which at least partially surrounds an active region of a resonator, the resonator including a piezoelectric material with first and second electrodes.

Document <CIT>) relates to acoustically decoupled microelectromechanical system devices anchored upon phononic crystals. A device comprises a resonator, a handle layer, and a pedestal disposed between the resonator and the handle layer, the pedestal connecting the resonator to the handle layer.

Document <CIT>) relates to structured metamaterials that can reflect, absorb, and focus the propagation of both scalar acoustic and vector elastic waves.

Document <CIT>) relates to a monolithic phononic crystal for vibration isolation comprising: a two-dimensional array of a plurality of resonant masses, said resonant masses being connected by bridges; wherein transition regions between bridges and resonant masses have a concave shape in the plane of the two-dimensional array, respectively; wherein the resonant masses each have convex edges in the plane of the two-dimensional array; wherein the bridges are recessed with respect to the thickness of the resonant masses.

The publication "<NPL> Acta Mechanica Sinica <NUM>(<NUM>) DOI:<NUM>/s10409-<NUM>-<NUM>-<NUM> relates to elastic metamaterials for subwavelength wave propagation control with shunted piezoelectric materials and electrorheological elastomers.

Known solutions have the drawback of limiting performances of MEMS resonators, due to residual exposure to external vibrations.

Moreover, known solutions have the drawback of being scarcely effective in suppressing unwanted vibrations in MEMS devices.

Again, known solutions for suppressing unwanted vibrations in MEMS devices have limited applications and require a tuned design for each resonator, losing robustness to process variations and environmental conditions.

It is an objective of the present invention to solve drawbacks of the prior art.

In particular, it is an object of the present invention to provide a MEMS device which is improved with respect to the prior art.

It is a further object of the present invention to insulate MEMS devices from unwanted vibrations.

It is a further object of the present invention to increase performances of MEMS resonators.

It is a further object of the present invention to provide a MEMS device which is suitable for a wide range of applications.

It is a further object of the present invention to provide a solution compatible with standard MEMS manufacturing technologies, especially within the silicon industry.

A MEMS device according to the present invention is defined in claim <NUM>.

An idea underlying the present invention is to interpose a vibration insulating plate between the resonating element and a substrate of the MEMS device. The vibration-insulating plate is based on a planar periodic structure providing a two-dimensional phononic crystal around and/or below the resonating element.

In that, the present invention envisages a purely mechanical solution for vibration isolation of a MEMS device, which can be fully integrated in the fabrication process through the combination of phononic crystals/metamaterials and MEMS.

The phononic crystals correspond to a planar periodic structure fully compatible with standard MEMS fabrication technologies, that shows a 3D bandgap in the frequency range of interest, i.e. natural frequencies of the resonant components of the MEMS device to isolate from external vibrations.

According to the present invention, the MEMS device as defined in claim <NUM> comprises at least one resonating element, and a vibration-insulating plate centrally housing the at least one resonating element. The vibration-insulating plate comprises a planar periodic structure surrounding the at least one resonating element. The planar periodic structure comprises a plurality of adjacent unit cells, each unit cell comprising a respective movable mass element suspended by spring elements. The spring elements comprise folded-beam springs.

Advantageously, the MEMS device of the present invention is capable of increasing performances of its MEMS resonators.

Advantageously, the solution of the present invention is more effective in insulating MEMS devices from unwanted external vibrations. Advantageously, the solution of the present invention is effective for a wide range of the MEMS devices and is robust to process variability and environmental conditions.

Advantageously, the solution of the present invention is fully compatible with standard MEMS manufacturing technologies, preferably providing a common co-manufactured silicon substrate.

Other features and advantages of the invention will be apparent from the following description of preferred embodiments, and from the claims.

The invention will be now described with reference to the annexed drawings, provided as non-limiting examples of preferred embodiments, wherein:.

In the drawings referred to in the description, the same reference numerals will designate the same or equivalent elements.

Where a plurality of alike elements exists in a same figure, only one of them may be indicated by a reference in the drawings, to improve legibility, while the other alike elements will be intended as comprised by analogy.

The present invention relates to Micro Electro-Mechanical Systems (MEMS), which are generally also related to microelectronics, micromechatronics, micromachines and microsystems.

MEMS devices are a technology of microscopic devices, particularly with moving parts. In general, individual MEMS devices are made up of components typically between <NUM> and <NUM> micrometers in size (i.e., <NUM> to <NUM>), and individual MEMS devices have an overall size typically ranging from <NUM> micrometers to a few millimeters (i.e., <NUM> to <NUM> or more).

In the context of the present invention, as it will be further described, each "unit cell" comprises components which have microscale features, while the overall dimension of each "unit cell" is of about one or more millimeters in a non-limitative example. As a result, the typical overall size of a MEMS device according to a non-limitative example of the present invention may be of a few millimeters, including a plurality of "unit cells".

MEMS devices may merge at the nanoscale into nanoelectromechanical systems (NEMS) and nanotechnology. It is thus understood that, in the present context, MEMS or NEMS are to be considered as technically equivalent, since the teaching of the present invention could be applied to a smaller nanoscale, if required or if the technology so provides. <FIG> shows a first embodiment of MEMS device <NUM> which comprises at least one resonating element <NUM> and a vibration-insulating plate <NUM> centrally housing the at least one resonating element <NUM>.

In preferred embodiments, the least one resonating element <NUM> includes one or more of: MEMS gyroscopes, MEMS accelerometers, MEMS micromirrors, MEMS micropumps, MEMS clocks, or other MEMS elements. In particular, a non-limitative example of the present invention is particularly effective for resonating elements such as accelerometers and gyroscopes with a resonant frequency e.g. around <NUM>. The MEMS device of the present invention can nonetheless be designed and tuned for various frequency ranges.

In particular, as it can be seen, the at least one resonating element <NUM> is housed in the vibration-insulating plate <NUM> in its central part. The vibration-insulating plate <NUM> comprises a planar periodic structure <NUM> surrounding the at least one resonating element <NUM>.

In the present disclosure, the term "surround" technically indicates that the resonating element <NUM> applied to the vibration insulating plate <NUM> has at least one contact point with the vibration insulating plate <NUM> which is "included inside" the planar periodic structure <NUM>, so that the latter is capable of insulating vibrations, as it will be further described.

In a preferred embodiment, the planar periodic structure <NUM> comprises a plurality of adjacent unit cells <NUM>, which can overall be defined as phononic crystals and which will be further described. In a preferred but non-limiting embodiment, the at least one resonating element <NUM> is sized as one the unit cells <NUM> constituting the functional part of the vibration-insulating plate.

In particular, the planar periodic structure <NUM> comprises at least one complete surrounding pattern of adjacent unit cells <NUM>, in the case of the MEMS device <NUM>, two complete concentric surrounding patterns of adjacent unit cells <NUM>. In a variant, even more concentric patterns could be envisaged in order to tune performances of the device.

As it will become apparent, the vibration-insulating plate <NUM> acts as a metamaterial or two-dimensional phononic crystal adapted to attenuate external vibration in a 3D bandgap, for the range of frequencies needed to isolate the MEMS resonator, for example <NUM> to <NUM> for MEMS gyroscopes.

<FIG> illustrates an exemplary section of a MEMS device <NUM>. The vibration-insulating plate <NUM> is interposed between the at least one resonating element <NUM> and a further silicon substrate <NUM> of the MEMS device <NUM>. In addition, a package <NUM> protecting the MEMS device <NUM> is preferably provided.

The vibration-insulating plate <NUM> or metaplate <NUM> is inserted between the main silicon layer where the MEMS resonating element <NUM> is located and the silicon substrate <NUM>. The metaplate <NUM> is then directly connected to the overlying MEMS resonator <NUM> in the center, and to the underlying substrate <NUM>, preferably at its four corners, preferably by co-fabrication. The full MEMS device <NUM> made by the resonating element <NUM>, the metaplate <NUM> and the substrate <NUM> is then packaged in a ceramic package <NUM> as usually done in MEMS devices.

When an unwanted external vibration acts on the MEMS package <NUM>, it directly propagates to the metaplate <NUM> through the anchors, placed as an example at the four corners, but it does not reach the MEMS resonating element <NUM> thanks to the vibration isolation properties of the metaplate <NUM>.

In that, the present invention as defined in claim <NUM> provides for periodic structures or phononic crystals which attenuate unwanted frequency ranges acting on the MEMS device <NUM>.

<FIG> highlights a specific unit cell <NUM> in the first embodiment of MEMS device <NUM>, within the plurality of unit cells making up the vibration-insulating plate <NUM>. All the unit cells are preferably all alike in the vibration-insulating plate <NUM>, but they could be of different types within a same vibration-insulating plate <NUM>.

<FIG> shows a first embodiment 300a of unit cell <NUM>. The unit cell <NUM> comprises a respective mass element <NUM> suspended by spring elements <NUM>. The spring elements <NUM> are elastic elements which tend to return to their original configuration after (dynamic) deformation.

The spring elements <NUM> are configured for mechanically connecting the respective mass element <NUM> to an outer frame <NUM> of the unit cell <NUM>. In particular, the outer frame <NUM> is integral with the vibration-insulating plate <NUM>.

In that, the unit cell <NUM> can be defined as unit cell of a phononic crystal. Phononic crystals are devices adapted to control the propagation of elastic/acoustic waves; they are artificial periodic structures that consist of periodically arranged scattering centers embedded in a homogeneous background matrix. One of the most important characteristics of phononic crystals is the existence of phononic bandgaps, frequency ranges where the propagation of elastic/acoustic waves is prohibited.

The metaplate <NUM>, according to its application in MEMS resonant devices, is constituted by periodically arranged unit-cells <NUM>. In an example, the metaplate <NUM> has overall dimensions of <NUM> × <NUM> × <NUM> and is fabricated in single-crystal silicon through a MEMS fabrication process.

The unit cell <NUM> can be schematized as a 'frame-spring-mass' system, where the block <NUM> represents the mass and the folded beams <NUM> represent the springs. The springs <NUM> are designed to allow the movement of the mass <NUM> along the three orthogonal directions, thus guaranteeing a full 3D bandgap of the metaplate <NUM>.

<FIG> respectively show a top view and a perspective view of a second embodiment 300b of unit cell <NUM>.

As mentioned, the spring elements <NUM> are configured for mechanically connecting their respective mass element <NUM> to the outer frame <NUM> of the unit cell <NUM> which is integral with the vibration-insulating plate <NUM>.

The folded-beam springs <NUM> comprise respective planar spring lines, each including a plurality of angled segments. In particular, the angled segments provide complete <NUM>° turns in the folded-beam springs <NUM>.

Each unit cell <NUM> is configured for movement of its respective mass element <NUM> along all spatial directions. The mass element <NUM> is thus a movable mass element.

In particular, the mass element <NUM> is flat, preferably of polygonal shape, and is surrounded by a plurality of radially arranged spring elements <NUM>, preferably on every side of the polygon.

<FIG> respectively show a top view and a perspective view of a third embodiment 300c of unit cell <NUM>.

The spring elements <NUM> comprise folded-beam springs with respective planar spring lines, each including a plurality of angled segments. The mass element <NUM> is circular, but it could be polygonal for example octagonal as there are eight spring elements <NUM>.

<FIG> respectively show a top view and a perspective view of a fourth embodiment 300d of unit cell <NUM>.

The spring elements <NUM> comprise folded-beam springs with respective planar spring lines, each including a plurality of angled segments. In this non-limiting example, the mass element <NUM> is square and there are four spring elements <NUM>. In other variants (not shown) the mass elements could have yet other shapes.

<FIG> respectively show a top view and a perspective view of a fifth embodiment 300e of unit cell <NUM>.

The spring elements <NUM> comprise two slightly different designs of folded-beam springs 302a and 302b, with <NUM>° degrees turns of different lengths. In this non-limiting example, the mass element <NUM> is rectangular and there is a first pair of spring elements 302a and a second pair of different spring elements 302b. In other variants (not shown) the mass elements could have yet other shapes.

<FIG> respectively show a top view and a perspective view of a sixth example 300f of unit cell <NUM>.

The example 300f comprises springs <NUM> which are non-folded, i.e. with respective planar spring lines, each including a single curved or straight segment. The spring elements <NUM> are configured for mechanically connecting their respective mass element <NUM> to the outer frame <NUM>.

<FIG> shows a second embodiment of MEMS device <NUM>.

The MEMS device <NUM> comprises at least one resonating element <NUM> and a vibration-insulating plate <NUM> centrally housing the at least one resonating element <NUM> in a central part.

The vibration-insulating plate <NUM> comprises a planar periodic structure <NUM> surrounding the at least one resonating element <NUM>. The planar periodic structure <NUM> comprises a plurality of adjacent unit cells <NUM>, each unit cell <NUM> comprising a respective mass element <NUM> suspended by spring elements <NUM>.

In particular, the planar periodic structure <NUM> comprises at least one complete surrounding pattern of adjacent unit cells <NUM>, preferably two or more complete concentric surrounding patterns of adjacent unit cells <NUM>.

In the example of MEMS device <NUM>, the unit cells <NUM> are rectangular with an overall orthogonal development of the vibration-insulating plate <NUM>.

<FIG> shows a third embodiment of MEMS device <NUM>.

In the example of MEMS device <NUM>, the unit cells <NUM> are hexagonal with an overall hexagonal development of the vibration-insulating plate <NUM>.

<FIG> shows a fourth embodiment of MEMS device <NUM>.

In particular, the planar periodic structure <NUM> comprises only one complete surrounding pattern of adjacent unit cells <NUM>, while other outer adjacent unit cells <NUM> do not define a closed pattern but are merely juxtaposed on the sides of the vibration-insulating plate <NUM>. In particular, the at least one resonating element <NUM> in the central part is sized as a block-like plurality (specifically, four-by-four) of the unit cells <NUM> constituting the functional part of the vibration-insulating plate <NUM>.

In an example, the metaplate <NUM> is made by a periodic repetition of a unit cell <NUM> that consists of a square block <NUM> connected to the external frame <NUM> through folded beams <NUM>, properly designed to achieve a full 3D bandgap at low frequency. Four anchor points <NUM> and a central area are also added to the metaplate <NUM>.

<FIG> is a diagram illustrating the vibration-insulating performances of a MEMS device.

The figure provides a comparison of the transmission diagrams measured in experimental vibrational tests performed "with metaplate" (dashed line <NUM>) and "without metaplate" (solid line <NUM>).

It can be seen that the signal <NUM> measured in absence of the MEMS metaplate <NUM> is significantly higher than the signal <NUM> measured in the presence of the metaplate <NUM>, thus proving the vibration isolation property of the present invention.

The MEMS device of the present invention is fully compatible with standard MEMS fabrication processes. Advantageously, the mechanical vibration isolator or metaplate can be co-fabricated with the MEMS device.

The present invention as defined in claim <NUM> represents a mechanical solution for vibration isolation of MEMS resonant devices, including a vibration-insulating plate or metaplate relying on a periodic 'frame-springs-mass' system in a unit cell, which shows a complete 3D bandgap in the range of frequency of interest (i.e. resonant frequencies of the resonant devices to be isolated from external vibrations).

The metaplate shows remarkable vibration isolation properties in the frequency range <NUM> - <NUM>, but it is not limited thereto. An attenuation of around -<NUM> dB of external vibration in a wide range of frequency is then possible through the solution of the present invention. The metaplate of the MEMS device can be designed with ad-hoc bandgap properties for the resonant device one wants to isolate and be fabricated together with the device inside the package through standard MEMS fabrication process.

Claim 1:
MEMS device (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprising:
- at least one resonating element (<NUM>, <NUM>, <NUM>, <NUM>), and
- a vibration-insulating plate (<NUM>, <NUM>, <NUM>, <NUM>) centrally housing said at least one resonating element (<NUM>, <NUM>, <NUM>, <NUM>);
wherein said vibration-insulating plate (<NUM>, <NUM>, <NUM>, <NUM>) comprises a planar periodic structure (<NUM>, <NUM>, <NUM>, <NUM>) surrounding said at least one resonating element (<NUM>, <NUM>, <NUM>, <NUM>),
wherein said planar periodic structure (<NUM>, <NUM>, <NUM>, <NUM>) comprises a plurality of adjacent unit cells (<NUM>, <NUM>, <NUM>, <NUM>; <NUM>), each unit cell (<NUM>, <NUM>, <NUM>, <NUM>; <NUM>) comprising a respective movable mass element (<NUM>) suspended by spring elements (<NUM>),
characterised in that
the said spring elements (<NUM>) comprise folded-beam springs.