EP4078808A1

EP4078808A1 - Clock device

Info

Publication number: EP4078808A1
Application number: EP20821204.3A
Authority: EP
Inventors: Marc SANSA PERNA; Philippe Robert
Original assignee: Commissariat a lEnergie Atomique CEA; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2019-12-16
Filing date: 2020-12-10
Publication date: 2022-10-26
Also published as: US20220416150A1; FR3104861A1; WO2021122331A1

Abstract

The present invention relates to a clock signal generator device (902) comprising: a micro-electromechanical resonant element (504); and at least one nano-electromechanical transduction element (512).

Description

DESCRIPTION

Clock device

The present patent application claims the priority of the French patent application 19/14510 which will be considered as forming an integral part of the present description.

Technical area

The present description relates generally to electronic devices, and, more particularly, clock signal generator devices, or clock devices.

Prior art

A clock device is an electronic device generally making it possible to produce a periodic signal of constant frequency, called a clock signal. In some applications, this clock signal is used as a synchronization signal. In other applications, for example in the field of telecommunications, advantage is taken of this clock signal to select transmission channels at precise frequencies.

Current clock devices, in particular based on quartz, are generally well suited to the generation of clock signals with a frequency below one megahertz. On the other hand, these devices are inefficient in frequency ranges greater than one megahertz, for example of the order of around ten megahertz, or even around one hundred megahertz.

Summary of the invention

[0004] There is a need to improve current clock devices.

[0005] One embodiment overcomes all or part of the drawbacks of known clock devices. [0006] One embodiment provides for a clock signal generator device comprising: a micro-electromechanical resonant element; and at least one nano electromechanical transducer element.

[0007] According to one embodiment, the resonant element, of planar shape, is parallel to a surface of a substrate.

[0008] According to one embodiment, the resonant element has breathing vibration modes parallel to the surface of the substrate.

[0009] According to one embodiment, the resonant element is linked to the substrate by the transduction element.

[0010] According to one embodiment, the resonant element is linked to the substrate by at least one beam, the beam being held, at each end, by anchors located on the surface of the substrate.

[0011] According to one embodiment, the ends of the transduction element are respectively linked to the substrate and to the beam, the transduction element being perpendicular to the beam.

According to one embodiment, the resonant element is further linked to the substrate by a pillar located, preferably, directly above the center of gravity of the resonant element.

According to one embodiment, the resonant element has, seen from above, a polygonal shape, preferably a parallelogram shape, even more preferably a square shape.

According to one embodiment, the resonant element has a natural frequency of between 1 MHz and 100 MHz, preferably between 10 MHz and 100 MHz, more preferably equal to approximately 20 MHz. According to one embodiment, the resonant element is, seen from above, square in shape and has: a side between 2 μm and 1 mm, preferably equal to approximately 200 μm; and a thickness between 200 nm and 500 µm, preferably equal to about 10 µm.

According to one embodiment, each transduction element forms a rectangular parallelepiped having: a length between 500 nm and 100 μm, preferably equal to approximately 5 μm; and a width of between 50 nm and 50 µm, preferably equal to about 250 nm; and a height between 50 nm and 50 μm, preferably equal to approximately 250 nm.

[0017] According to one embodiment, each transduction element is a piezoresistive strain gauge.

[0018] One embodiment provides for an electronic circuit comprising at least one device as described.

One embodiment provides a method of manufacturing a device as described, comprising a step consisting in: producing the micro-electromechanical resonant element from a first layer; and making the nano-electromechanical transducer element from a second layer, the second layer having a thickness at least ten times less, preferably about forty times less, than the thickness of the first layer.

Brief description of the drawings

These characteristics and advantages, as well as others, will be explained in detail in the following description of modes. particular embodiments made without limitation in relation to the accompanying figures including:

[0021] FIG. 1 represents, schematically and in the form of blocks, an example of an electronic circuit of the type to which apply, by way of example, the embodiments described;

FIG. 2 is a top view, schematic and partial, of an example of a clock device;

Figure 3 is a top view, schematic and partial, illustrating an example of operation of the clock device of Figure 2;

FIG. 4 is a top view, schematic and partial, of another example of a clock device;

FIG. 5 is a top view, schematic and partial, of an embodiment of a clock device;

Figure 6 is a sectional view, schematic and partial, of the clock device of Figure 5;

FIG. 7 is a top view, schematic and partial, of another embodiment of a clock device;

Figure 8 is a top view, schematic and partial, of yet another embodiment of a clock device;

Figure 9 is a top view, schematic and partial, of yet another embodiment of a clock device;

FIG. 10 is a top view, schematic and partial, symbolizing a mode of vibration of a resonant element; Figure 11 is a top view, schematic and partial, symbolizing another mode of vibration of the resonant element of Figure 10;

FIG. 12 is a top view, schematic and partial, symbolizing a mode of vibration of another resonant element;

FIG. 13 is a top view, schematic and partial, symbolizing a mode of vibration of yet another resonant element; and

FIG. 14 is a top view, schematic and partial, symbolizing a mode of vibration of yet another resonant element.

Description of the embodiments

The same elements have been designated by the same references in the various figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

For the sake of clarity, only the steps and elements useful for understanding the embodiments described have been shown and are detailed. In particular, the use which is made of the signals generated by the clock devices described is not detailed.

Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two connected elements (in English "coupled") between them, this means that these two elements can be connected or be linked through one or more other elements. In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "rear", "top", "bottom", "left", "right", etc., or relative, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc. ., Reference is made, unless otherwise specified, to the orientation of the figures.

Unless otherwise specified, the expressions "approximately", "approximately", "substantially", and "of the order of" mean within 10%, preferably within 5%.

FIG. 1 schematically shows in the form of blocks an example of an electronic circuit 102 of the type to which the described embodiments apply, by way of example. The electronic circuit 102 illustrated in FIG. 1 comprises: a calculation entity 104 (UC), for example a state machine, a microprocessor, a programmable logic circuit, etc. ; a memory 106 (MEM), for example a memory made up of one or more volatile and / or non-volatile storage areas making it possible to store program code instructions, variables, constants, etc. ; a clock device 108 (CLK) linked or connected to the calculation entity 104 and to the memory 106; one or more bus 110 for data, addresses and / or commands between different elements internal to circuit 102; and an input-output interface 112 (I / O) for communication with the outside of the circuit 102.

As shown in Figure 1, the electronic circuit 102 can further include various other circuits depending on the application. These circuits are, in FIG. 1, symbolized by a single functional block 114 (FCT).

The clock device 108 of circuit 102 is typically used to produce a clock signal, or synchronization signal, for example a periodic signal of constant frequency. This clock signal, generated by the device 108, makes it possible for example to clock data exchanges between the calculation entity 104 and the memory 106. The signal transmitted by the clock device 108 can in particular be used by the. computation entity 104 in order to perform operations consisting in writing, reading or erasing data in the memory 106.

FIG. 2 is a top view, schematic and partial, of an example of a clock device 202.

In the example of Figure 2, the clock device 202 comprises a plate 204, or plate, of square shape. Each corner, in other words each angle, of the plate 204 ends with a beam 206 arranged perpendicular to the diagonal of the square passing through this corner. The ends of each beam 206 are respectively held integral with a substrate 208 by anchors 210. The plate 204 and the beams 206 are, for example, made from the same layer. In this case, the plate 204 and the beams 206 jointly form a single mechanical part, generally called “seismic mass”.

In Figure 2, a hatched area 208 symbolizes a surface portion of the substrate. Viewed from above in Figure 2, the plate 204 is located on the substrate 208, parallel to the surface of the substrate 208. The plate 204 is held, for example, a few micrometers or a few millimeters above the surface of the substrate 208. , so that the plate 204 overhangs the surface of the substrate 208 without touching it. In the example of the clock device 202, the plate 204 and the beams 206 each have a micrometric thickness, for example equal to approximately 50 μm. The clock device 202 is then referred to as a microelectromechanical system (MEMS). The plate 204 and the beams 206 form, due to their micrometric thicknesses, a flexible structure capable of deforming under the effect of external stresses.

Figure 3 is a top view, schematic and partial, illustrating an example of operation of the clock device 202 of Figure 2.

In this example of operation, two electrodes 302 are arranged on either side of the plate 204, parallel to two opposite sides of the plate 204. The electrodes 302 are subjected to an excitation signal, or signal d 'actuation, symbolized, in FIG. 3, by arrows 304. This excitation signal 304 is, for example, a sinusoidal alternating electric voltage. Each electrode 302 then produces a variable electrostatic field, causing the plate 204 to vibrate. The plate 204 is then said to constitute a resonant element, or resonator, of the clock device 202.

Under the effect of the variable electrostatic field created by the electrodes 302, areas of the plate 204 extend and retract in turn, periodically, for example mainly in a plane parallel to the surface of the substrate 208, while that the thickness of the plate 204 remains substantially constant. The plate 204 is then said to vibrate in “breathing mode”. The plate 204 vibrates for example more precisely in a volume breathing mode. In this case, the electrostatic field imposes for example on the plate 204 movements of constriction and expansion, for example oscillatory, in the three dimensions of space. These movements have, for example, a greater amplitude in the two dimensions parallel to the surface of the substrate 208 than in the dimension orthogonal to the surface of the substrate 208.

In the example of Figure 3, two other electrodes 306 are arranged on either side of the plate 204, parallel to the two other opposite sides of the plate 204. Each electrode 306 forms, with the plate 204, a capacitor, one armature of which is formed by the considered electrode 306, and the other armature of which is formed by the plate 204. The electrodes 306, assumed to be fixed with respect to the substrate 208, are then referred to as capacitive detection electrodes.

The distance separating the plate 204 from each electrode 306, in other words the distance separating the two plates of each capacitor jointly formed by an electrode 306 and by the plate 204, varies periodically, at the rate of the extension and retraction movements imposed on the plate 204 by the excitation signal 304 applied to the electrodes 302. This causes a periodic variation of the capacitance of the capacitors formed by the electrodes 306 and by the plate 204, thus allowing the device 202 to produce a clock signal . The detection of this periodic variation in capacitance is symbolized, in FIG. 3, by arrows 308.

In the example of the device 202, the electrodes 302 are separated from the plate 204 by an average distance typically between 1 μm and 10 μm. This allows the excitation electrodes 302 to generate their electrostatic field as close as possible to the plate 204, without interfering with its vibratory movements. A maximum amplitude of vibration is thus obtained.

Still in the example of the device 202, the electrodes 306 are separated from the plate 204 by a distance average typically less than 1 µm. This allows the electrodes 306 to detect the greatest possible variation in capacitance. However, the production of the electrodes 306 close to the plate 204 requires, in practice, high resolution photolithography techniques, which are often complex and expensive to implement.

One of the parameters defining the performance of the clock device 202 is called "clock jitter". Assuming that the clock device 202 outputs a frequency clock signal, denoted f0, substantially constant, the clock jitter corresponds to the frequency noise, denoted <5f / f0>, for a given measurement period . The clock jitter therefore sets a minimum resolution in frequency, or in time, that the clock device 202 is able to achieve.

The clock jitter of the device 202 generally depends on two types of preponderant factors: so-called "deterministic" factors, for example frequency drifts caused by temperature variations; and so-called “non-deterministic” factors, for example due to ambient noise.

We therefore seek to reduce the influence of these two types of factors as much as possible, in order to obtain a clock device 202 that is as precise and as stable in frequency as possible.

The non-deterministic factors are typically measured in terms of phase noise, denoted L (f), where L represents a Laplace transformation and where f represents a frequency. A low phase noise L (f) makes it possible to lower the frequency noise <5f / f0>, therefore to obtain a clock device 202 of better quality. The phase noise L (f) is inversely proportional to the signal-to-noise ratio (SNR) of the clock signal produced by the device 202. This signal-to-noise ratio is difficult to determine. foresee, because it depends in particular on the mechanical properties of the resonator 204 and on the electrical performance of a servo loop (not shown in FIG. 3) of the clock device 202. However, in order to increase the signal to noise ratio, it is necessary to generally seeks to optimize the properties of the resonator 204 as a priority. This makes it possible to impose less constraints on the control loop of the clock device 202.

For a given excitation signal 304, the level of the output signal of the resonator 204 depends mainly on two parameters: the quality factor, denoted Q, of the resonator 204, defined by a width at half the height of the response in frequency of resonator 204, this width at half height being linked to energy dissipation; and the transduction efficiency of the clock device

202.

In order to obtain a high signal level at the output of the clock device 202, it is therefore particularly important to improve the quality factor Q of the resonator 204. For this, the structure jointly formed by the plate 204 and the beams 206 in the vicinity of the resonant frequency, or natural frequency, of this structure. Vibratory movements of maximum amplitude are thus obtained for a given excitation signal 304. In a MEMS structure such as that described in relation to FIGS. 2 and 3, the resonant frequency is typically located around ten megahertz. A drawback of the clock device 202 is that the structure jointly formed by the plate 204 and the beams 206 generally produces low amplitude vibrations, even when this structure is excited in the vicinity of its resonant frequency. This complicates the detection, by the electrodes 306, of the movements of the plate 204, thus reducing the level of the clock signal at the output of the device 202.

Another drawback of the clock device 202 is that the anchors 210 tend to stiffen the structure jointly formed by the plate 204 and the beams 206. This causes mechanical losses, thus negatively impacting the quality factor Q of the clock device 202.

FIG. 4 is a top view, schematic and partial, of another example of a clock device 402. The clock device 402 of FIG. 4 comprises elements in common with the clock device 202 of Figure 3. These common elements will not be detailed again below. For the sake of clarity, the substrate 208 has not been shown in Figure 4.

The clock device 402 of Figure 4 differs from the clock device 202 of Figure 3 mainly in that, in the case of the clock device 402, the detection of the vibrations of the plate 204 is performed not thanks to the electrodes 306, but by piezoresistive anchors 404 arranged at the four corners of the plate 204. In the clock device 402, the capacitive detection electrodes 306 are omitted, which makes it possible for example to place an electrode 302 d excitation parallel to each side of the square formed by the plate 204.

In Figure 4, there is shown different areas of vibration of the plate 204 when the clock device 202 is in operation. More particularly: a non-hatched zone 204L symbolizes a region of the plate 204 subjected to low amplitude vibrations; hatched zones 204H symbolize regions of the plate 204 subjected to vibrations of high amplitude, that is to say of greater amplitude than that of the zone 204L; and another hatched zone 204M symbolizes a region of the plate 204 subjected to vibrations of average amplitude, that is to say of an amplitude between those of the zones 204L and 204H.

The zones 204L, 204M and 204H illustrate a mode of vibration of the plate 204. In order to optimize the detection in this mode of vibration, the piezoresistive anchors 404 of the clock device 402 are linked to the hatched zones 204H, otherwise said where the vibrations of plate 204 are most intense.

A read circuit 406 connected to one of the anchors 404 of the device 402 makes it possible to extract, from the vibrations of the plate 204, the clock signal. In the example illustrated in FIG. 4, the read circuit 406 comprises: a resistor 406R connected between the anchor 404 and a node of application of a potential denoted Vd; and a capacitor 406C, connected between the anchor 404 and another node, through which a current denoted Iout flows.

In this example, the anchor 404 diagonally opposite the anchor 404 to which the read circuit 406 is connected is brought to a reference potential, here ground.

The clock device 402 is generally more efficient at high frequency and easier to manufacture than the clock device 202. However, a drawback of the clock device 402 is that the anchors 404 provide both mechanical strength and transduction functions, which tends to hinder the vibratory movements of the plate 204. We then speak of a feedback phenomenon, because the piezoresistive anchors 404 disturb the vibrations of the plate 204 which they are. themselves supposed to detect.

FIG. 5 is a top view, schematic and partial, of an embodiment of a clock device 502.

The clock device 502 comprises a resonant element 504 planar and having, seen from above in Figure 5, a preferably square shape. Each corner, in other words each angle, of the resonant element 504 ends with a beam 506 disposed perpendicular to the diagonal of the square passing through this corner. The ends of each beam 506 are respectively linked to a substrate 508 by anchors 510 perpendicular to the axis of the beams 506. The resonant element 504 and the beams 506 are, for example, made from the same layer. In this case, the resonant element 504 and the beams 506 jointly form a single mechanical part.

In Figure 5, a hatched area 508 symbolizes a surface portion of the substrate 508. Seen from above in Figure 5, the resonant element 504 is located on the substrate 508, parallel to the surface of the substrate 508. The element resonant 504 is maintained, for example, a few micrometers or a few millimeters above the surface of the substrate 508, so that the resonant element 504 overhangs the surface of the substrate 508 without touching it.

In the clock device 502, the resonant element 504 and the beams 506 have a thickness between 200 nm and 500 μm, preferably equal to approximately 10 μm. The resonant element, square in shape, has a side between 2 µm and 1 mm, preferably equal to about 200 µm. The beams 506 have, viewed from above in FIG. 5, a width of between 1 μm and 5 μm, preferably equal to approximately

3 µm, and a length between 10 µm and 70 µm, preferably equal to about 40 µm. Due to their micrometric thickness, the resonant element 504 and the beams 506 of the clock device 502 jointly form a microelectromechanical system, or MEMS. In particular, the resonant element 504 is qualified as a micro-electromechanical resonant element.

According to one embodiment, the resonant element 504 and the beams 506 have the same thickness. Alternatively, the beams 506 have a thickness less than that of the resonant element 504.

According to one embodiment, a transduction element 512, preferably a piezoresistive transduction element, is linked to each beam 506. As illustrated in FIG. 5, the transduction elements 512 are arranged perpendicular to the beams 506, in the extension of the diagonals of the square formed by the resonant element 504. Each transduction element is, moreover, linked to the substrate 508 by an anchoring 514. In the remainder of the description, the piezoresistive transduction elements 512 are also called piezoresistive gauges or strain gauges.

According to a preferred embodiment, the transduction elements 512 each form a rectangular parallelepiped having a length between 500 nm and 100 μm, preferably equal to approximately 5 μm, a width between 50 nm and 50 μm, of preferably equal to approximately 250 nm, and a height, or thickness, between 50 nm and 50 μm, preferably equal to approximately 250 nm. By length of the transducing element 512 is meant the dimension of the element 512 between the anchor 514 and the beam 506 and by height or thickness of the transducing element 512 means the dimension of the element 512 perpendicular to the surface of the substrate 508.

The transduction elements 512 preferably have identical dimensions, except for manufacturing dispersions. Because of their nanometric widths and heights, the transduction elements 512 of the clock device 502 are referred to as nanoelectromechanical systems (NEMS), or nanoelectromechanical transduction elements.

In order to produce a clock signal, the resonant element 504 is made to vibrate in a plane parallel to the surface of the substrate 508, in other words in "breathing mode" as explained above. In general, the vibration of the resonant element 504 is obtained by virtue of an electrical excitation signal (in current or in voltage). This signal is modulated so as to excite the resonant element 504 at a frequency approximately equal to its resonant frequency. The resonant element 504 has for example a natural frequency of between 1 MHz and 100 MHz, preferably between 10 MHz and 100 MHz, more preferably equal to approximately 20 MHz.

The device 502 is more particularly a device for generating clock signals in volume breathing mode. The application of the excitation signal causes, for example, deformations of the resonant element 504, for example oscillatory constriction and expansion movements, in all directions of space.

In the example shown, the transduction elements 512 are placed so as not to interfere with the breathing mode of the device 502. In addition, the elements of transduction 512 have dimensions that do not affect the dynamics of the device 502.

By way of example, the transduction elements 512 of the device 502 are such that the resonant frequency of the device 502 is substantially identical to within 5%, preferably within 1%, to the resonant frequency that the device would exhibit. device 502 devoid of transducing elements 512. Stated otherwise, the presence of transducing elements 512 results in a modification of the resonant frequency of device 502 of less than 5%, preferably less than 1%, compared to a case where the elements transduction 512 would be omitted.

According to one embodiment, the vibration is caused by the application of a variable electrostatic field radiated by one or more electrodes positioned near the resonant element 504. According to another embodiment, the vibration is produced. by a layer of piezoelectric material located under the resonant element 504. As a variant, the resonant element 504 is made of a piezoelectric material capable of expanding and retracting with variations in a control signal. According to yet another embodiment, the beams 506 are made of a piezoelectric material, so that a potential applied to the beams 506 produces a stress on the beams 506 and on the resonant element 504.

As a variant, the beams 506 are made of an electrically conductive material which expands when the temperature increases, the beams 506 being arranged so as to apply a stress on the resonant element 504. An electric current passing through the beams 506 will then have tendency to cause a rise in temperature, therefore an expansion, of the beams 506. On the contrary, an absence of electric current passing through the beams 506 will then have tendency to cause a drop in temperature, therefore compression, of the beams 506. By periodically alternating between phases where an electric current passes through the beams 506 and other phases where the beams 506 are traversed by a negligible or zero current, it is possible succeed in causing the resonant element 504 to vibrate in the vicinity of its resonant frequency.

An advantage of the clock device 502 lies in the fact that the piezoresistive gauges 512 make it possible to achieve high transduction performance, notably superior to the performance of the capacitive and piezoresistive transduction systems respectively presented in relation to FIGS. 3 and 4 The clock device 502 is furthermore easier to manufacture than the capacitive sensing devices which, like the device 202 of FIG. 3, have electrodes 306 very close to the plate 204.

Another advantage of the device 502 lies in the fact that the piezoresistive gauges NEMS 512 have dimensions significantly smaller than those of the MEMS structure jointly formed by the resonant element 504 and by the beams 506. This makes it possible in particular to make so that the NEMS piezoresistive gauges 512 have a negligible influence on the vibrations of the resonant element 504, that is to say that the gauges 512 do not induce a feedback phenomenon.

In the case of the clock device 502, the mechanical maintenance, or anchoring, and transduction functions are in fact decoupled, unlike, for example, in the case of the clock device 402 of FIG. 4. This makes it possible in particular to optimize the mechanical performance of the resonant element 504 by reducing the losses linked to the anchors 510, and therefore to improve the quality factor Q of the clock device 502. More generally, the association of a resonant element 504 of MEMS type and transduction elements 512 of NEMS type allows the clock device 502 to achieve better performance both in terms of transduction. and in terms of quality factor Q. Thus, thanks to the clock device 502, a better quality clock signal is obtained than the clock signal capable of being produced, for example, by the clock devices 202 (Figures 2 and 3) and 402 (Figure 4).

FIG. 5 shows a clock device 502 comprising a MEMS resonant element 504 of square shape, linked to the substrate 508 by four NEMS piezoresistive gauges 512. As a variant, the clock device 502 comprises a MEMS resonant element 504 of any shape, in particular a polygonal shape, preferably a parallelogram shape. Further, the clock device 502 may include any number of NEMS piezoresistive gauges 512, these gauges 512 preferably being positioned so as to detect a maximum vibration amplitude of the MEMS resonant element 504.

Figure 6 is a sectional view along the plane AA, schematic and partial, of the clock device 502 of Figure 5.

As illustrated in the sectional view of Figure 6, the piezoresistive gauge 512 (NEMS) is interposed between on the one hand a structure (MEMS) jointly formed by the resonant element 504 and the beam 506, and on the other hand the anchor 514. The piezoresistive gauge NEMS 512 has a thickness, denoted T_NEMS, at least ten times less, preferably about forty times less, than the thickness, denoted T_MEMS, of the MEMS structure and of the anchor 514 The NEMS 512 piezoresistive gauge thus detects with great sensitivity the vibratory movements of the MEMS structure and does not disturb, or disturb little, these vibratory movements.

FIG. 7 is a top view, schematic and partial, of another embodiment of a clock device 702.

The clock device 702 of FIG. 7 comprises elements common to the device 502 of FIG. 5. These common elements will not be detailed again below. Device 702 of Figure 7 differs from device 502 of Figure 5 mainly in that device 702 has, at each corner of resonant element 504, two piezoresistive gauges 512a and 512b.

In Figure 7, only one corner of the resonant element 504 of the clock device 702 has been shown, it being understood that the other corners of the resonant element 504 may be identical, except for manufacturing dispersions, at the corner shown in figure 7.

According to one embodiment, the piezoresistive gauges 512a and 512b are eccentric with respect to the beam 506. This makes it possible to further reduce the influence of the piezoresistive gauges 512a and 512b on the vibratory movements of the resonant element 504 and to avoid or limit the appearance of common modes. The piezoresistive gauges 512a and 512b are respectively located face to face, on either side of the beam 506. Each piezoresistive gauge 512a, 512b is arranged perpendicular to the longitudinal direction of the beam 506 and is linked to the substrate 508 (not shown in Figure 7) by an anchor 514a, 514b. The beam 506 is always assumed to be linked to the substrate 508, that is to say kept integral with the substrate 508, by the anchors 510 located at each of its ends. In the orientation of FIG. 7, when the resonant element 504 extends upwards (arrow 702T), this causes compression of the piezoresistive gauge 512a and an extension of the piezoresistive gauge 512b. Conversely, when the resonant element 504 retracts downward (arrow 702B), this causes an extension of the piezoresistive gauge 512a and a compression of the piezoresistive gauge 512b. In this configuration, called differential measurement, the piezoresistive gauges 512a and 512b are subjected to stresses of substantially equal amplitude and of opposite sign.

As illustrated in FIG. 7, in operation, a voltage source 704a (V) applies, on the anchor 514a, a direct electric potential, of positive value denoted + Vb. Similarly, a voltage source 704b (V) applies, to the anchor 514b, a direct electric potential, of negative value denoted -Vb. A resistor 706 (RL) is connected between one of the anchors 510 of the beam 506 (the anchor 510 closest to the piezoresistive gauges 512a and 512b, in FIG. 7) and the ground. The value of the resistance RL is assumed to be much greater, for example a thousand times greater, than the resistance of the piezoresistive gauges 512a and 512b, denoted RG.

In this configuration, a voltage of equal amplitude Vb, but of opposite sign, is applied to each piezoresistive gauge 512a, 512b. One thus obtains, on the anchor 510 connected to the resistor 706, a variable potential denoted Vout, of approximately zero average value, for example between -0.1 V and +0.1 V, preferably zero, and of which l The amplitude is proportional to the amplitude of vibration of the resonant element 504.

As a variant, the voltage Vb is an alternating electric voltage of frequency denoted fb, the frequency fb being close to the vibration frequency fO of the resonant element 504. In this variant, the output voltage Vout has a frequency component equal to fO - fb. This advantageously makes it possible to use read circuits operating at low frequency.

According to one embodiment, the MEMS resonant element 504 consists of silicon. The NEMS piezoresistive gauges 512a and 512b of the clock device 702 are preferably made of the same material as the MEMS resonant element 504. Alternatively, the NEMS piezoresistive gauges 512a and 512b are made of a piezoresistive material other than that. of which the MEMS 504 resonant element is made.

[0100] FIG. 8 is a top view, schematic and partial, of yet another embodiment of a clock device 802.

The clock device 802 of FIG. 8 comprises elements common to the device 502 of FIG. 5. These common elements will not be detailed again below. Device 802 of Figure 8 differs from device 502 of Figure 5 primarily in that, in device 802, beams 506 are omitted. Each corner of the resonant element 504 of the clock device 802 is thus linked to the substrate 508 (not shown in FIG. 8) by a piezoresistive gauge 512 of resistance RG. As in the device 502 of FIG. 5, each piezoresistive gauge 512 of the device 802 is kept integral with the substrate 508 by an anchor 514.

[0102] According to one embodiment, the resonant element 504 is linked to the substrate 508 by a central pillar (not shown in FIG. 8). This advantageously makes it possible to decouple the mechanical anchoring function, performed by the central pillar, and the transduction function, provided by the gauges. piezoresistive 512. In other words, in this case, the gauges 512 do not participate, or participate little, in the mechanical anchoring function.

In the clock device 802, a voltage source 804 (V) imposes a positive voltage on one of the anchors 514, denoted + Vb. Another anchor 514 of device 802 is connected to ground via a resistor 806 of value RL. The value RL of resistor 806 is chosen so as to be equal to approximately twice the resistance RG of a piezoresistive gauge 512 (RL = 2.RG). An output voltage Vout is measured across resistor 806.

The two gauges 512 respectively connected to the voltage source 804 and to the resistor 806 are then said to be connected in the "half Wheatstone bridge" configuration, the resistor 806 here acting as the second resistor of the bridge. When the resonant element 504 vibrates, the output voltage Vout of the Wheatstone half-bridge will have an oscillating component of amplitude proportional to the mechanical vibration of the resonant element 504.

As explained in relation to FIG. 7, the voltage Vb can be a direct voltage or an alternating voltage.

[0106] The transduction of the vibratory movement of the resonant element 504 of the clock device 802 is effected by virtue of the piezoresistive gauges 512. Under the action of the vibrations of the resonant element 504, all the piezoresistive gauges 512 alternately undergo substantially identical tensile and compressive stresses. These constraints cause periodic variations in resistance RG of the piezoresistive gauges 512, which make it possible to produce a clock signal.

[0107] The piezoresistive gauges 512 of the clock device 802 fulfill both a transduction function and a mechanical holding function. However, unlike the clock device 402 of FIG. 4, here it is ensured that all of the piezoresistive gauges 512 have negligible compressive stiffness, that is to say at least ten times lower than the stiffness of the piezoresistive gauge. mode of vibration of the resonant element 504. This eliminates the feedback problems previously described in relation to FIG. 4.

In the clock device 802, the resonant element 504 has for example a mode of vibration at a frequency fO equal to approximately 20 MHz, with a stiffness equal to approximately 3.6.10 ⁷ N.nr ¹ . The four piezoresistive gauges 512 together have a compression stiffness, for example equal to approximately 5.10 ³ N.nr ¹ , in other words a rigidity several thousand times lower than that of the mode of vibration of the resonant element 504. More generally, the rigidity of the resonant element 504 is at least a thousand times greater than that of a piezoresistive gauge 512.

FIG. 9 is a top view, schematic and partial, of yet another embodiment of a clock device 902.

[0110] The clock device 902 of FIG. 9 comprises elements common to the device 802 of FIG. 8. These common elements will not be detailed again below. Device 902 of Figure 9 differs from device 802 of Figure 8 primarily in that device 902 is bonded to substrate 508 by only two diagonally opposed piezoresistive gauges 512.

[0111] According to one embodiment, the resonant element 504 is further linked to the substrate 508 by a pillar 904 shown, in FIG. 9, by a dotted circle. The pillar 904 is preferably a cylindrical central pillar, located directly above the center of gravity of the resonant element 504. [0112] The pillar 904 is advantageously linked to a zone where the resonant element 504 undergoes minimal deformation in the mode of vibration considered. This makes it possible in particular to ensure that the pillar 904 has a negligible impact on the mode of vibration of the resonant element 504, in other words that the pillar 904 practically does not hamper the vibratory movements of the resonant element 504. This is limited. thus the losses due to the connection of the resonant element 504 to the substrate 508, while improving the mechanical strength of the clock device 902.

FIG. 9 shows a clock device 902 comprising a MEMS resonant element 504 of square shape, linked to the substrate 508 by two NEMS piezoresistive gauges 512. As a variant, the clock device 902 comprises a MEMS resonant element. 504 of any shape, in particular a polygonal shape, preferably a parallelogram shape. Further, the clock device 902 may include any number of NEMS piezoresistive gauges 512, these gauges 512 preferably being positioned so as to detect a maximum vibration amplitude of the MEMS resonant element 504.

The clock devices 502, 702, 802 and 902 respectively described in relation to FIGS. 5, 7, 8 and 9 are preferably produced by implementing a method such as that described in the patent application No. W02011048132, which is incorporated by reference to the extent permitted by law. An embodiment of this method comprises in particular a step where the resonant element 504 and the beams 506 are preferably obtained from a first layer of micrometric thickness while the piezoresistive gauges 512 are obtained from '' a second layer of nanometric thickness, that is to say at least ten times thick lower, preferably about forty times lower, than that of the first layer.

The use, in the circuit 102 of FIG. 1, of a clock device 108 similar to the clock devices 502, 702, 802 and 902 advantageously makes it possible to produce a high quality clock signal at a frequency for example equal to approximately 20 MHz.

[0116] FIGS. 10 to 14 illustrate, in top view, embodiments of resonant elements 504 of various shapes and symbolize associated modes of vibration. Figures 10 to 14 include an orthonormal coordinate system (O, c, g, z) whose z axis is perpendicular to the surface of the substrate 508 (not shown), Figures 10 to 14 all showing views located in a plane ( 0, x, y) perpendicular to the z axis.

In Figures 10 to 14, the resonant element extends and retracts in the plane (0, x, y) parallel to the substrate 508 (not shown). The resonant element 504, on the other hand, does not undergo any transformation along the z axis, or a transformation along the z axis of negligible amplitude with respect to the transformations undergone along the x and y axes.

FIG. 10 is a top view, schematic and partial, symbolizing a mode of vibration 504_M1 of a resonant element 504 of square shape, for example the resonant element 504 of the clock device 902 of FIG. 9. The resonant element 504 is assumed to be linked to the substrate 508 (not shown) by the pillar 904.

During each period of the mode of vibration 504_M1 symbolized in FIG. 10, the resonant element 504 extends along the x and y axes, then retracts along the x and y axes.

[0120] FIG. 11 is a top view, schematic and partial, symbolizing another mode of vibration 504_M2 of the resonant element 504 of FIG. 10. [0121] During each period of the vibration mode 504_M2, the resonant element 504 extends along the x axis while it retracts along the y axis, then retracts along the x axis while ' it extends along the y axis.

In general, when it is desired to take advantage of the resonant element 504 to produce a clock signal, the nano-electromechanical transduction element (s) 512 is advantageously placed where the amplitude of the vibration is maximum. for a given vibration mode. To make the best use of the mode of vibration 504_M1 symbolized in FIG. 10, it is for example advantageous to place a transduction element 512 at each corner of the resonant element 504, in the extension of the diagonals of the square formed by the element 504. On the other hand, to take advantage of the mode of vibration 504_M2 symbolized in FIG. 11, it will for example be better to provide transduction elements 512 perpendicular to the sides parallel to the y axis of the square formed by the element 504.

[0123] FIG. 12 is a top view, schematic and partial, symbolizing the mode of vibration 504_M1 in an embodiment where the resonant element 504 is of circular shape.

During each period of the mode of vibration 504_M1 symbolized in FIG. 12, the diameter of the disc formed by the upper surface of the resonant element 504 increases and then decreases.

[0125] FIG. 13 is a top view, schematic and partial, symbolizing the mode of vibration 504_M1 in an embodiment where the resonant element 504 is in the form of a crown. According to this embodiment, the pillar 904 connecting the resonant element 504 to the substrate 508 (not shown) is in the form of a hollow cylinder. During each period of the mode of vibration 504_M1 symbolized in FIG. 13, the outer diameter of the ring formed by the upper surface of the resonant element 504 increases and then decreases.

[0127] FIG. 14 is a top view, schematic and partial, symbolizing the mode of vibration 504_M1 in an embodiment where the resonant element 504 is rectangular in shape. According to this embodiment, the pillar 904 connecting the resonant element 504 to the substrate 508 (not shown) is omitted. In this case, the resonant element 504 is linked to the substrate 508 by an anchor 514 located at one of the ends, or one of the short sides, of the element 504.

During each period of the mode of vibration 504_M1 symbolized in FIG. 14, the resonant element 504 extends and then retracts mainly in its longitudinal direction y ·

To make the best use of the mode of vibration 504_M1 symbolized in FIG. 14, it is for example advantageous to place a transduction element perpendicular to the end of the resonant element 504 which is not linked to the substrate 508 by the anchor 514 (the upper end of resonant element 504, in the orientation of Figure 14).

[0130] The embodiments described above in relation to FIGS. 10 to 14 are not limiting. In particular, the person skilled in the art is capable of transposing these embodiments to configurations in which the resonant elements 504 are of any shape and / or in which the vibration modes may be different from those described above.

Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these various embodiments and variants could be combined, and other variants will be apparent to those skilled in the art. In particular, the person skilled in the art is able to adjust the geometry of the micro-electromechanical resonant element 504 as a function of the frequency f0 to be reached. The person skilled in the art is furthermore capable of positioning the nano-electromechanical transduction element (s) 512 in order to optimize the quality of the clock signal produced.

Finally, the practical implementation of the embodiments and variants described is within the abilities of those skilled in the art based on the functional indications given above.

Claims

A device (502; 702; 802; 902) for generating breath mode clock signals comprising: a micro-electromechanical resonant element

(504); and at least one nano electromechanical transducer element (512; 512a, 512b).

2. Device according to claim 1, wherein the breathing mode is a volume breathing mode.

3. Device according to claim 1 or 2, wherein the resonant element (504), of planar shape, is parallel to a surface of a substrate (508).

The device of claim 3, wherein the resonant element (504) has breathing vibration modes parallel to the surface of the substrate (508).

5. Device according to claim 3 or 4, wherein the resonant element (504) is linked to the substrate (508) by the transducing element (512; 512a, 512b).

6. Device according to claim 3 or 4, wherein the resonant element (504) is linked to the substrate (508) by at least one beam (506), the beam being held at each end by anchors (510). located on the surface of the substrate (508).

7. Device according to claim 6, wherein the ends of the transducing element (512; 512a, 512b) are respectively linked to the substrate (508) and to the beam (506), the transducing element being perpendicular to the beam.

8. Device according to any one of claims 3 to 7, wherein the resonant element (504) is further linked to the substrate (508) by a pillar (904) located, preferably, directly above the center of gravity of the resonant element.

9. A device according to any one of claims 1 to 8, wherein the resonant element (504) has, seen from above, a polygonal shape, preferably a parallelogram shape, even more preferably a square shape.

10. Device according to claims 6 and 9, comprising several beams (506) each located in a corner of the resonant element (504).

11. Device according to any one of claims

1 to 10, in which the resonant element (504) has a natural frequency of between 1 MHz and 100 MHz, preferably between 10 MHz and 100 MHz, more preferably equal to approximately 20 MHz.

12. Device according to any one of claims 1 to 11, wherein the resonant element (504) is, seen from above, square in shape and has: a side between 2 μm and 1 mm, preferably equal to approximately 200 µm; and a thickness (T_MEMS) of between 200 nm and 500 µm, preferably equal to approximately 10 µm.

13. Device according to any one of claims 1 to 12, wherein each transduction element (512; 512a, 512b) forms a rectangular parallelepiped having: a length between 500 nm and 100 pm, preferably equal to approximately 5 pm ; and a width of between 50 nm and 50 µm, preferably equal to about 250 nm; and a height (T_NEMS) comprised between 50 nm and 50 μm, preferably equal to approximately 250 nm.

14. Device according to any one of claims 1 to 13, wherein each transducing element (512; 512a, 512b) is a piezoresistive strain gauge.

15. Device according to any one of claims

1 to 14, in which the presence of the transducing element (s) (512; 512a, 512b) results in a change in the resonant frequency of the device (502; 702; 802; 902) of less than 5%, preferably less than 1 %.

16. Electronic circuit (102) comprising at least one device (502; 702; 802; 902) according to any one of claims 1 to 15.

17. A method of manufacturing a device (502; 702;

802; 902) according to any one of claims 1 to

15, comprising a step of: forming the micro electromechanical resonant element (504) from a first layer; and making the nano electromechanical transducer element (512; 512a, 512b) from a second layer, the second layer having a thickness at least ten times less, preferably about forty times less, than the thickness of the first layer.