CH702402A2 - Microelectromechanical system resonator for microelectromechanical system oscillator used as timebase, has microstructure for electromechanical coupling between vibrating structure and channel to modulate conductivity of conduction element - Google Patents

Abstract

The resonator has a fixed structure (320) carried by a substrate. A space or gap is provided between the fixed structure and a vibrating structure (310). A caisson is integrated in the vibrating structure or in the fixed structure, so that the channel formed by a semiconducting region is extended next to the space. The space includes a microstructure (328) in a piezoelectric material provided to ensure an electromechanical coupling between the vibrating structure and the channel to allow modulating the conductivity of a conduction element by the vibration frequency of the vibrating structure.

Description

Field of the invention

The present invention relates to transistor MEMS resonators and MEMS oscillators comprising such transistor MEMS resonators. The present invention relates more particularly to oscillators comprising a resonant gate transistor, and in particular such oscillators used as a time base.

State of the art

Oscillators that use the resonance of an electromechanical resonator to produce a signal of a determined frequency are most often formed of an integrated oscillating circuit, and a piezoelectric coupling resonator which is not itself even integrated with the circuit. It will be understood that this construction undesirably increases the size and cost of such devices. A number of solutions have therefore been proposed to provide resonator oscillators that are fully integrated.

[0003] US Pat. No. 3,413,573 in particular describes an oscillator comprising an electromechanical resonator and which can in principle be entirely produced using the usual techniques for manufacturing integrated circuits. The proposed oscillator is built around a transistor resonator, more specifically around a resonant gate transistor. A resonant gate transistor is constituted by the combination of a surface-potential-controlled transistor and a resonator arranged facing the transistor and designed to produce an oscillating electric field on the surface of the transistor. By way of example, the surface-potential-controlled transistor may be a field-effect transistor (FET) made in a semiconductor substrate. The resonator, meanwhile, is constituted by a flexible recessed beam, which is made of a conductive material. The flexible beam extends above the substrate in which the transistor is formed, and is provided to vibrate under the effect of electrostatic excitation.

The free end of the beam moves opposite the FET transistor so as to allow the vibrations of the beam to control the transistor. For this purpose, the beam is biased with a constant voltage relative to the substrate in which the transistor is formed. The bias voltage must be high enough that the electric field emanating from the end of the beam influences the conductivity of the field effect transistor. When this condition is fulfilled and the end of the beam oscillates, diverging and moving closer to the transistor, the fluctuating intensity of the electric field on the surface of the transistor has the effect of modulating the conductivity of the latter; hence the name of transistor "resonant gate".

The transistor resonator, whose operating principle has just been described, has certain disadvantages. In particular, only a very small fraction of the electric field around the flexible beam acts on the FET transistor. The bias voltage of the flexible beam must therefore be relatively high; see even higher than the maximum voltage supported by the usual integrated circuits. On the other hand, the electromechanical coupling is not optimal, the dynamic resistance (in English "motional resistance") of the resonator is increased.

[0006] WO 2007/135 064 entitled "Motion sensitive device having at least one transistor" describes several variants of transistor MEMS resonators using a plurality of transistors for improved electromechanical coupling. This document also teaches that instead of producing the surface-potential-controlled transistor in the substrate, and to make the vibrating structure play the role of a gate, the body of the transistor can be equivalently integrated with the vibrating structure. and provide a fixed gate, so that the body of the transistor is movable facing the gate.

An object of the present invention is therefore to provide a MEMS oscillator comprising a resonant gate transistor which has an improved electromechanical coupling regardless of the number of transistors and which also has a reduced power consumption.

FIG. US Patent 4,873,871 illustrates an accelerometer operating by means of a mechanical gate transistor (MFET). Fig. 1 appended reproduces FIG. 5 of this prior document. The MFET (referenced 10) comprises a substrate 12 and a gate 52. The substrate 12 comprises a drain 20, a source 22, an oxide layer 24 and a plurality of connectors 46. Finally, the substrate 12 further comprises a zone 44 situated between the source and the drain and in which a concentration or a depletion of charge carriers is likely to occur.

The gate 52 comprises a piezoelectric material 54 and a seismic mass 56. When a force of inertia perpendicular to the substrate acts on the seismic mass, the piezoelectric material 54 and is compressed or stretched, which has the effect to polarize the piezoelectric material 54. The electric field associated with this polarization varies the concentration of charge carriers in the zone 44. In particular, when the piezoelectric material 54 is compressed, electrons are attracted to the surface of the zone 44 giving rise to a channel through which an electric current can flow between the drain 20 and the source 22. Since the electric field generated by the compression of the piezoelectric material is sufficient to change the concentration of charge carriers in the area 44, the gate 52 need not be connected to an external voltage source. It will be understood that the transistor described in this prior document is designed to perform the function of electromechanical transducer and that it is absolutely not a resonant gate transistor.

Brief description of the invention

An object of the present invention is to overcome the aforementioned drawbacks of the prior art. The present invention achieves this goal by providing a transistor MEMS resonator according to claim 1.

According to a first embodiment of the invention, the box comprising the source region, the drain region and the channel connecting the source region and the drain region, is integrated in the fixed structure. According to a first variant of this embodiment, the microstructure of a piezoelectric material is integral with the vibrating structure and separated from the channel by a space. According to an alternative variant of this embodiment, the microstructure of a piezoelectric material is integral with both the vibrating structure and the fixed structure.

According to a second embodiment of the present invention, the box is integrated in the vibrating structure. According to a variant of this embodiment, the microstructure of a piezoelectric material is integral with both the vibrating structure and the fixed structure.

The present invention also relates to a MEMS oscillator according to claim 11.

It will be understood that the MEMS oscillator according to the present invention is made around a transistor resonator according to the present invention. An advantage associated with the use of the transistor resonator according to the present invention is that the low dynamic resistance of the resonator makes it possible to produce oscillators having a low power consumption.

Brief descriptions of the figures

Other features and advantages of the present invention will appear on reading the description which follows, given solely by way of non-limiting example, and with reference to the accompanying drawings in which: <tb> - fig. 1 <sep> is the diagram of an accelerometer of the prior art, comprising a mechanical gate transistor; <tb> - fig. 2 <sep> is a schematic plan view of a transistor resonator according to a first embodiment of the present invention; the figure also shows two piezoelectric films provided to excite the resonator; <tb> - figs. 3A and 3B <sep> are diagrammatic sectional views of the resonator of FIG. 2, respectively according to A-A and B-B; <tb> - fig. 4 <sep> is a block diagram showing the deformations according to a particular vibration mode of the resonator of the transistor of FIGS. 2 and 3; <tb> - fig. <Sep> is a schematic plan view of a transistor resonator according to a variant of the embodiment shown in FIG. 2; <tb> - figs. 6A to 6H <sep> illustrate different steps of a manufacturing method for producing the transistor resonator shown in FIGS. 2 and 3; <tb> - fig. 7A <sep> is a perspective view of a transistor resonator according to a second embodiment of the present invention; <tb> - fig. 7B <sep> is a perspective diagram showing the deformations according to a particular vibration mode of the resonator of FIG. 7A; <tb> fig. 7C <sep> is a schematic perspective view showing the microstructure of a piezoelectric material provided to provide electromechanical coupling between the micromechanical resonator and the transistor channel of FIG. 7A.

Figs. 2, 3A, 3B and 4 illustrate a MEMS oscillator which corresponds to a first embodiment of the present invention. Some components of the oscillator that do not directly relate to the present invention have been omitted so as not to overload the figures. The MEMS oscillator of the present example is based on a transistor resonator according to the invention. The resonator 100 is entirely produced by etching in a wafer of crystalline or amorphous material, such as silicon. As can be seen from fig. 3A and 3B, in particular, a feature of this first embodiment is that the resonator and its transistor are formed in the same plane parallel to the wafer.

In the present example, the wafer is of the Silicon On Insulator (SOI) type and comprises a thick silicon substrate 102, an insulating silicon oxide intermediate layer 104, and a silicon top layer 106 in which is etched the resonator. Referring more particularly to FIG. 2, it can be seen that the resonator of the present example is mainly formed of a resonant frame 110 supported at both ends by supports 112 and 114. As can be seen in FIG. 3A, the support 114 is formed of a silicon dioxide foot surmounted by a silicon arm which comes from material with the resonant frame 110. The support 112 also comprises a silicon dioxide foot. However, unlike the support 114, the support 112 is not in direct contact with the frame 110. Indeed, as shown in the figures, a microstructure 126 of a piezoelectric material and an insulating oxide layer 124 are interposed between the support 112 and frame 110 (in this example, microstructure 126 may simply consist of a piezoelectric material thickness).

The silicon top portion of the support 114 constitutes a p-type semiconductor box for a transistor. A conduction element of the transistor consisting of a source region 108, a drain region 109, and an n-doped region (unreferenced) forming a channel for connecting the source region and the drain region is formed in the well. The insulating oxide layer 124, already mentioned, covers the channel region of the conduction element, and the thickness of piezoelectric material 126 is interposed between the oxide layer 124 and the frame 110. The horizontal stack formed by the oxide layer 124 and the piezoelectric material thickness 126 constitutes what is called a "stack stack" (or "stack stack" in English). According to variants of the invention, the structure of the gate stack may be more complex than that just described. Firstly, it may be advantageous to form a layer of silicon dioxide on either side of the piezoelectric material (This variant corresponds to the structure illustrated in the series of FIGS. 6A to 6H). On the other hand, the thickness of piezoelectric material may for example be sandwiched between two layers of a high dielectric constant material, so as to improve the electromechanical coupling. Finally, it can be provided, as is also the case with the embodiment illustrated in FIG. 7, that the gate stack comprises a gap, that is to say an empty space separating two successive layers of the stack. It should further be pointed out that, as has already been said, in the present example, the silicon forming the support 112 has received a p-type doping, and the source 108 and the drain 109 are regions which have subsequently been n-doped. However, the invention is obviously not limited to this variant.

As further shown in FIG. 2, the resonator substantially in the form of a rectangular frame 110 held in place by its two small sides which are connected to the supports 112 and 114. The long sides of the frame are free to deform. Comparing FIG. 2 with fig. 4 which represents the same resonator, it can be seen that the frame 110 has a very elongated shape, and that it has been amputated from its central portion in FIGS. 2and 3A. From a functional point of view, the resonator of FIG. 2 can be likened to a tuning fork resonator which would be closed at both ends or, better still, a pair of tuning forks placed end to end and whose arms would join. Fig. 4illustrates the deformations undergone by the frame 110 in a particular vibration mode. It will be understood that in the vibration mode shown, the two arms of the resonator are always symmetrical, which has the advantage of avoiding any displacement of the center of gravity of the structure transversely to its longitudinal axis. It will be understood, however, that the present invention is not limited to a particular vibration mode. In addition, the present invention is not limited to the particular type of resonator described in this example.

Referring now to FIG. 3B, it can be seen that in addition to the two supports 112, 114, the resonant frame 110 is also supported on two structures 116, 118. Like the supports 112, 114, the two structures 116, 118 are each formed of a foot silicon dioxide and a silicon arm. As shown in fig. 2, the arms of the supports 116, 118 are each connected to one of the long sides of the frame 110. It is further seen that a thickness of piezoelectric material 122 is interposed between each arm and the corresponding side of the frame 110. The structures 116 , 118 are provided to vibrate the two long sides of the frame 110 through the piezoelectric effect. For this purpose, the two structures 116, 118 receive an excitation signal whose function is to alternately expand and contract the material thicknesses 122 by inverse piezoelectric effect. Since the amplitude of the thickness variation of the piezoelectric material is very limited, the structures 116 and 118 are preferably placed at one end of the long sides of the frame 110, close to the support 114. It will be understood that this arrangement makes it possible to profit lever effect.

When the frame 110 vibrates under the effect of the excitation signal, the degree of curvature of the long sides varies symmetrically. It will be understood that the resultant of this symmetrical variation is to alternately lengthen and shorten the frame 110. As shown in FIG. 2, one end of the frame 110 is held fixed by the combined action of the support 114 and the two structures 116, 118. Under these conditions, any change in the length of the frame 110 will be reflected in the support 112. C ' is the thickness of piezoelectric material 126 which will absorb the length variations of the frame 110 by deforming. The piezoelectric material 126 will be alternately stretched and compressed, which has the effect of making it cyclically polarize. Finally, in a manner known per se, the conductivity of the channel connecting the source region 108 to the drain region 109 will be modulated according to the degree of polarization of the piezoelectric material 126.

As mentioned above, some components that are usually part of an oscillator are not shown in the drawings and have not been described. Among these components, there may be mentioned in particular a supply circuit for establishing a polarization between the source region 108 and the drain region 109, and a feedback circuit for providing an excitation signal to the structure 116 and 118 in response to an electric current between the source region 108 and the drain region 109. The circuits that have just been mentioned have not been described in detail because their realization can be quite conventional, and does not require explanations. for the skilled person. Furthermore, the resonator according to the invention obviously includes pads or connection pads (not shown) for connecting the oscillator with the outside, and tracks for electrically connecting the various components of the oscillator between them. The tracks (not shown) may for example be formed close to the surface of the upper silicon layer 106 by doping the silicon. Alternatively, the tracks may be formed by metallizations deposited on the silicon.

On the other hand, according to an advantageous variant of the present example, the MEMS oscillator may further comprise means for imposing an offset voltage for centering, on the point of operation of the channel, the interval inside. which varies the bias voltage of the piezoelectric material. For this purpose, the frame 110 and the support 114 can be made conductive by appropriate doping. Thus, by connecting the support 114 to a voltage source, it is possible to adjust the bias voltage of the frame 110. The thickness of the piezoelectric material being integral with the frame, the bias voltage of the frame constitutes a reference potential around which oscillates the polarization of the piezoelectric material.

FIG. 5 represents a MEMS oscillator 200 according to the invention. This oscillator corresponds to a variant of the embodiment described with reference to FIGS. 2 to 4. As in the previous example, the resonator 200 is entirely made by etching in a silicon wafer, and the resonator and transistor are formed in the same plane parallel to the wafer. The resonator comprises a resonant frame 210 supported at both ends by supports 212 and 214. These supports each consist of a silicon dioxide foot surmounted by a silicon arm. The supports 212, 214 are not in direct contact with the frame 210. Indeed one can see on the one hand, in FIG. 5, an insulating oxide layer 224 is interposed between the frame 210 and the support 214, and on the other hand, a piezoelectric material thickness 226 and an insulating oxide layer 224 are interposed between the frame and the support 212.

The two short sides of the frame 210 each constitute a p-type semiconductor box for a field effect transistor. These transistors each comprise a conduction element consisting of a source region (referenced respectively 208, 208), a drain region (referenced respectively 209, 209) and an n doped region (not referenced) forming a channel for connecting the source region and the drain region. The insulating oxide layers (referenced 224, 224 respectively) overlap the channel region of each transistor. As was the case in the previous example, the silicon forming the short sides of the frame 210 may for example have received a p-type doping, and the source regions 208, 208 and drain 209 , 209 have been doped n. The support 214 serves as a gate for one of the transistors, while the thickness of piezoelectric material 226 interposed between the frame 210 and the support 212 day the role of gate for the other transistor. The field effect transistor associated with the support 214 may be inactive, simply playing the role of variable resistance controlled according to the polarization of the support 214.

In a manner similar to that which was the case for the oscillator of the first example, two structures 216, 218 are provided to vibrate the two long sides of the frame 210. For this purpose, the two structures 216, 218 receive a excitation signal whose function is to alternately expand and contract the thicknesses of material 222 by inverse piezoelectric effect. According to what has already been explained in the context of the first example, the vibration of the frame 210 is accompanied by a cyclic variation in its length, the thickness of the dielectric material 226 is therefore alternately stretched and compressed, causing a cyclic polarization of the transistor gate.

Figs. 6A to 6H are sectional views which illustrate, by way of example only, the main steps of a manufacturing process for producing the resonator 100 shown in FIGS. 2, 3 and 4, and making it possible to produce, in particular, the thickness of piezoelectric material 126 (FIG 2).

As already mentioned, the resonator 100 can be entirely made by etching in a wafer of the Silicon On Insulator (SOI) type. The wafer comprises a thick silicon substrate 102, an insulating silicon oxide intermediate layer 104, and a silicon top layer 106 in which the components of the MEMS oscillator according to the invention are formed, and in particular the resonator 100. The structure of the resonator may first be etched in the silicon top layer 106 to arrive at the result illustrated in FIG. 6A. The skilled person can achieve the engraving by any means that it seems appropriate. On the other hand, although fig. 6A does not show them, it is preferably during this first step that the pads or connection pads and the conductive tracks used to connect the components of the oscillator are made between them. As already mentioned, the tracks may for example be formed by surface doping silicon. Alternatively, the tracks may for example be formed by metallizations deposited on the silicon.

As shown in FIG. 6B, a dielectric layer 132 is then formed on the silicon, including on the vertical walls. The dielectric 132 is provided in particular to constitute the gate dielectric 124 (FIG 2). The dielectric layer 132 may be, for example, silicon dioxide formed by thermal oxidation of silicon. This oxide can then be removed locally at locations where it is not useful. Silicon dioxide is only one possibility among others for producing the dielectric layer 132. The SiO 2 can be replaced or combined with other dielectric material to, on the one hand, improve the dielectric constant, and on the other hand, to improve selectivity during etching.

As shown in FIG. 6C, a dielectric material 134 is then grown in the openings previously etched in the upper layer of silicon 106. The wafer surface is then configured with a photoresist 136 as illustrated in FIG. 6D. The photosensitive resin layer is provided to protect the piezoelectric material 134 at locations where it is useful. The remainder of the piezoelectric material 134 is removed by etching to arrive at the result illustrated in FIG. 6E.

A new etching mask 138 is then formed to protect the dielectric layer 132 which covers the vertical walls (Fig. 6F). The etched SOI wafer is then immersed in a bath to dissolve the exposed silicon dioxide 104, so as to partially release the silicon and the piezoelectric material 134. An ideal exposure time having been previously determined by means of a few tests, the result of the step of dissolving the silicon dioxide gives the result illustrated in FIG. 6G. Finally, the resin 136, 138 is eliminated to arrive at the result illustrated in FIG. 6H.

Figs. 7A, 7B and 7C are perspective views of a second embodiment of the transistor resonator 300 according to the invention. Unlike the resonators 100 and 200 of the previous examples, the resonator 300 is designed to extend preferably in a vertical plane (perpendicular to a substrate).

[0033] Referring more particularly to FIG. 7A, we can see that the resonator 300 is mainly formed of a vibrating beam 310 supported at both ends by supports 312 and 314. The supports 312, 314 each comprise a foot 312a, 314a doped silicon surmounted by a superstructure 312b, 314b. Each of the superstructures 312b, 314b comprises a piezoelectric material thickness 326. The two parts 326 of piezoelectric material are provided to vibrate the beam 310 in a particular vibration mode. For this purpose, the two parts 326 receive an excitation signal whose function is to alternately expand and contract each of the two thicknesses of piezoelectric material. As the amplitude of the variation in thickness of the piezoelectric material is very limited, the two parts 326 are preferably placed substantially at the ends of the beam 310.

As shown in FIGS. 7Bet 7C, the transistor resonator 300 further comprises a piezoelectric sensing element 328 which is arranged in the center of the beam 310. The piezoelectric sensing element 328 has a substantially planar shape and is integral with the lower surface of the beam 310 The vibrating beam is designed to be excited at the first harmonic of its fundamental frequency. Under these conditions, in accordance with what is shown in FIG. 7B, the standing wave produced by the vibrations of the beam has two bellows separated by a node in the center of the beam. Thus, when the beam vibrates, its central part curves substantially, but, in principle, does not move. Thus, the piezoelectric element 328 integral with the center of the beam is subjected to considerable stretching constraints, while remaining virtually immobile.

We still see in FIG. 7A semiconductor casing 330 is arranged under the beam 310, facing the piezoelectric sensing element 328. A source region 308 and a drain region 309, as well as a channel structure (not referenced) located between the source region and the drain region, are formed in the box 330. It is still seen in FIG. 7A an insulating silicon oxide layer 324 overlies the channel structure that connects the source region 308 to the drain region 309. The oxide layer 324 faces the piezoelectric sensing element 328, both of which elements not in direct contact, but separated by a small space (or "gap" in English).

FIG. 7C schematically illustrates the polarization of the piezoelectric sensing element 328 when it is deformed by the vibration of the beam 310. When the middle of the beam passes through successive phases of bending and straightening, the piezoelectric element 328 polarizes and depolarizes alternately. Since the space between the piezoelectric element and the well 330 is very narrow, the conductivity of the channel connecting the source region 308 to the drain region 309 is modulated by the cyclic electric field associated with the polarization of the piezoelectric element 328.

As was already the case with the first embodiment, some elements that are usually part of an oscillator are not shown in the drawings and have not been described. Among these components, there may be mentioned in particular a supply circuit for establishing a bias between the source region 308 and the drain region 309, and a feedback circuit for providing an excitation signal to the portions 326 in response to a request. electrical current through the channel structure connecting the source region 308 and the drain region 309, As in the previous example, the realization of the components not described can be quite conventional, and does not require explanations for the skilled person.

It will further be understood that various modifications and / or improvements obvious to a person skilled in the art can be made to the embodiments which are the subject of the present description without departing from the scope of the present invention defined by the appended claims. In particular, it is not necessary to use piezoelectric means to actuate the vibrating structure of the resonator according to the invention. The piezoelectric means described above, 122 (FIG 2, 3A and 3B), 222 (FIG 5) and 326 (FIG 7A and 7B), could for example be replaced in a manner known per se by capacitive means.

Claims (11)

A transistor MEMS resonator comprising: a well in which a conduction element consisting of a source region (108; 208; 308), a drain region (109; 209; 309), and a semiconductor region forming a channel connecting the source and drain region; a vibrating structure (110; 210; 310) integral with a substrate while being free to vibrate; - a fixed structure (112; 212; 330) carried by the substrate, a gap (or gap) being arranged between the fixed structure and the vibrating structure; the box being integrated either in the vibrating structure (210) or in the fixed structure (112; 330), so that the channel extends opposite said space; characterized in that said space includes a microstructure (126; 226; 328) of a piezoelectric material provided for electromechanical coupling between the vibrating structure (110; 210; 310) and the channel so as to allow the conductivity of the channel to be modulated. conduction element at the vibration frequency of the vibrating structure. 2. transistor MEMS resonator according to claim 1, characterized in that the box is integrated in the fixed structure. (112; 330) 3. transistor MEMS resonator according to claim 2, characterized in that the microstructure (328) of a piezoelectric material is integral with the vibrating structure (310) and is separated from said channel by a clear space. 4. transistor MEMS resonator according to claim 2, characterized in that the microstructure (126) of a piezoelectric material is integral with both the vibrating structure (110) and the box (112). 5. transistor MEMS resonator according to claim 1, characterized in that the box is integrated in the vibrating structure (210). 6. MEMS resonator transistor according to claim 5, characterized in that the microstructure (226) of a piezoelectric material is secured to both the fixed structure (212) and the box. 7. MEMS resonator transistor according to one of the preceding claims, characterized in that the microstructure (126; 226; 328) of a piezoelectric material is integral with a conductive portion (110; 212; 310) of the resonator, said portion conductor being connected to a voltage source so as to adjust a reference potential for the structure of a piezoelectric material. Transistor MEMS resonator according to one of the preceding claims, characterized in that a main plane of the resonator, which passes through the source region (108; 208), the drain region (109; 209), the channel the microstructure of a piezoelectric material (126; 226) and the vibrating structure (110; 210) is parallel to the substrate (102). Transistor MEMS resonator according to one of claims 1 to 7, characterized in that a main plane of the resonator, which passes through the source region (308), the drain region (309), the channel, the microstructure of a piezoelectric material (328) and the vibrating structure (310) is perpendicular to the substrate. 10. Transistor MEMS resonator according to one of the preceding claims, characterized in that at least one piezoelectric material part (122; 222; 326) is fixed to the vibrating structure (110; 210; 310) so as to allow an excitation signal to vibrate the vibrating structure. 11. MEMS oscillator comprising: - a box (112; 230) in which is formed a conduction element consisting of a source region (108; 208; 308), a drain region (109; 209; 309), and a semiconductor region forming a channel connecting the source region and the drain region; a power supply arranged to establish a bias between the source region and the drain region; a vibrating structure (110; 210; 310) integral with a substrate while being free to vibrate; a feedback circuit provided to provide an excitation signal to the vibrating structure in response to an electric current through the conduction element; - a fixed structure (112; 212; 330) carried by the substrate, a gap (or gap) being arranged between the fixed structure and the vibrating structure; the box being integrated either in the vibrating structure (210) or in the fixed structure (112; 330), so that the channel extends opposite said space; characterized in that said space includes a microstructure (126; 226; 328) of a piezoelectric material provided for electromechanical coupling between the vibrating structure (110; 210; 310) and the channel so as to allow the conductivity of the channel to be modulated. conduction element at the vibration frequency of the vibrating structure.