NL2003839C2

NL2003839C2 - Thermogravimetric device with thermal actuator.

Info

Publication number: NL2003839C2
Application number: NL2003839A
Authority: NL
Inventors: Alexander Willem Herwaarden; Elina Iervolino
Original assignee: Xensor Integration B V
Priority date: 2009-11-20
Filing date: 2009-11-20
Publication date: 2011-05-23
Also published as: WO2011062489A1

Description

Thermogravimetric device with thermal actuator

TECHNICAL FIELD

The present invention relates to a thermogravimetric device as defined in the preamble of claim 1.

5 BACKGROUND

ThermoGravimetric Analysis (TGA) is a thermal analysis technique used to determine changes in weight of a sample-under-test in relation to changes in temperature. The operational range for commercial TGA instruments ( e.g. TGA 4000, PerkinElmer) is 1 ug up to 1500 mg for the mass. MEMS (Micro Electro Mechanical 10 System) devices give an opportunity to shift the operational range towards lower masses.

In order to measures lower masses, the thermogravimetric device are made smaller and smaller. Thermogravimetric devices are now formed as MEMS or NEMS. These thermogravimetric devices experience the problem that a resonance sensing 15 element, for instance a piezo-element provided on the resonator element, suffers from crosstalk induced by the piezo-electric actuators that are used to put the resonator element in resonance, as these actuators are necessarily driven at the resonance frequency [1],

To overcome this problem, thermal actuators have been used to bring the 20 resonating element into resonance. The time-dependent heating of a stack of layers of different thermal expansion coefficient leads to movement and actuation of the resonating element. The advantage of this method is that the thermal actuator is driven at half the resonance frequency, as the conversion from electrical bias signal to thermal heating signal is accompanied by a frequency doubling. This method has been shown in 25 non-TGA MEMS in which silicon beams are brought into resonance [2, 3].

SUMMARY

It is an object to provide a thermogravimetric device, that solves the at least one of the above identified problems.

30 Therefore, according to an aspect there is provided a thermogravimetric device, as claimed in claim 1.

Using a thermal actuator has the advantage that electrical crosstalk between the actuator and the resonance sensing element is minimized. This is because the thermal 2 actuator can be electrically driven at half of the resonance frequency. So, crosstalk that might occur between the thermal actuator and the resonance sensing element has a frequency that is half the frequency that the resonance sensing element actually has to measure. This greatly simplifies the electronic measurement set up, and allows for 5 more accurate measurements.

According to an embodiment the thermal actuator is an integrated element of the thermogravimetric device 1. The term integrated is used her to indicate that the thermal actuator is made in the same lithographic process in which the thermogravimetric device is made. Providing an integrated thermal actuator allows for 10 a compact design that is relatively easy to manufacture. Using a thermal actuator allows for an integrated design, without influencing the measurements too much due to crosstalk.

According to an embodiment the thermal actuator is mechanically linked to the resonator element. The thermal actuator may be designed to be a part of the resonator 15 and thus automatically be linked to it, or it may be designed as a separate element of the thermogravimetric device, and mechanically linked to the resonator by a connecting element (31).

According to an embodiment the resonance sensing element 40 is provided on the resonator element 10. The resonance sensing element may be provided on the 20 resonator element, preferably close to the frame, to measure the resonance frequency. The resonance sensing element may be provided on the base of the beam forming the resonator element. The resonance sensing element may comprise piezo-resistive or piezo-electric elements to measure deformation of the resonator element.

According to an embodiment the thermal actuator is thermally isolated from the 25 resonance sensing element 40. Thermal isolation between the thermal actuator and the resonance sensing element may be provided to minimize thermal interaction between the thermal actuator and the resonance sensing element. The thermal isolation may be provided to minimize a change of temperature of the resonance sensing element to minimize thermal crosstalk.

30 Furthermore, thermal isolation may be provided to reduce a change of temperature of the resonator element, especially those parts of the resonator element close to the frame, due to heat generated by the thermal actuator, as this may change 3 the resonance frequency of the resonator element, thereby disturbing the measurement results.

The thermal actuator produces thermal AC-waves. These thermal AC-waves cause the resonator element to heat up to an elevated temperature with respect to the 5 ambient temperature (DC-heating) and may also result in a fluctuation of the temperature of the resonator element (AC-heating).

The thermal isolation may be provided to reduce DC-heating and/or AC-heating of the resonator element. However, because DC damping goes basically linear with distance, and AC damping goes exponentially with distance, the AC damping is the 10 dominant effect. DC damping is of less importance, as will be explained below.DC damping is also of less importance, because it does not interfere with signals at the resonance frequency.

DC isolation is more of interest to reduce the power needed to drive the thermal actuator and also reduce possible self-heating effects due to the actuator power. This is 15 also a reason why the thermal actuator is not directly attached to the frame along its entire edge, but with thermally isolating links (see references 72 and 73 explained below).

According to an embodiment a conducting path between the resonance sensing element and thermal actuator is characterized by a thermal dampening factor TDF = 20 (27ifD21 a)0'5 wherein ƒ = frequency of the thermal actuator , D = distance between thermal actuator and resonance sensing element and a = effective thermal diffusivity of the conducting path.

The conducting path is the path connecting the resonance sensing element and the thermal actuator through which heat may be conducted. It may for instance be 25 formed by a connecting element 31 and the applicable, relevant parts of resonator element 10. The value of the thermal dampening factor TDF may be at least above 10 or preferably above 12 or even 15.

The term effective thermal diffusivity of the conducting path is introduced to denote a thermal diffusivity that represent a thermal diffusivity of the entire conducting 30 path. It may be a weighted average value which takes into account the various parts of the conducting path.

A high value of TDF ensures that the thermal AC-waves generated by the thermal actuator are substantially dampened before reaching the resonance sensing 4 element. This dampening is preferably sufficient so that the temperature induced changes in the resonance sensing element (e.g. changes of resistance of the piezo-resistive elements of the resonance sensing element), are well, for instance a factor 10, below the stress induced changes in the resonance sensing element due to the 5 resonating movements of the resonance element 10.

Typically encountered stress-induced changes in resistance induce a signal of the order of 10 pV on a Wheatstone bridge voltage (resonance sensing element 40) of typically 1 V, or 10 ppm at the resonance frequency, and 1 ppm at significantly different frequencies. The temperature coefficient of resistance (TCR) for the piezo-10 resistive elements is typically 1000 ppm/K. This indicates, that the AC-thermal waves may be filtered down to 1 mK in order to reach a base level of cross-talk of only 1 ppm. With a typical thermal actuator AC-temperature amplitude of 100 K, this means a filtering of 105 times of the AC-thermal waves.

As in general the resonance frequency f is determined by design considerations 15 other than the dampening factor TDF, important design considerations for enlarging TDF are found in increasing the distance D between thermal actuator and resonance sensing element and/or decreasing the thermal diffiisivity a of the path between thermal actuator and resonance sensing element.

The thermal diffiisivity of a layer is given by its thermal conductivity, divided 20 by its volumetric specific heat: a = K/(pCp) , expressed in m2/s. As a rule of thumb, the volumetric specific heat of most solids at room temperature are very much the same, of the order of 1-4 MJ/m3K. Thus, to minimize the thermal diffiisivity a, materials of low thermal conductivity may be chosen.

Therefore, in designs where the distance D between thermal actuator and 25 resonance sensing element is small, the thermal conducting path may be made of low-conductivity materials. This has the advantage that not only the AC-thermal waves are dampened by filtering, but also the average, DC thermal wave is reduced. As an example, the conducting link (31) between the resonator element (10) and a separate thermal actuator element (30) as shown and explained in more detail below with 30 reference to Fig. 1 may be made of the SiN layer material, since this material shows a low value of a of about 1.3 mm2/sec.

5

According to transmission-line theory [4], the dampening in a beam of uniform width will occur typically in an exponential decay, where the TDF factor, with a minus sign, is the exponent: T/Tactuator ^e“TDF (1) 5 In order to achieve the desired dampening of 105, it is required that the TDF be of the order of 132. Because the factor 27if7a is approximately 5X1010 for a thermal wave at 10 kHz and for a connecting link made of SiN, the lower limit of 132 is achieved for a conducting path length D of about 50 pm.

A connecting link (31) of pure SiN effectively dampens AC-thermal waves.

10 Moreover, in the situation of Fig. 1 explained in more detail below, another effects adds to the dampening. As a matter of design, between the connecting link (31) and the resonance sensing element (40), strips of metal or poly-silicon may be positioned running from the heater (50) to the frame (20), offering a path of high thermal conductance to the silicon frame (20) for the AC-thermal waves, away from the 15 resonance sensing element (40). This will add further to the dampening of the AC thermal waves at the location of the resonance sensing element (40).

The situation is somewhat different if, instead of using a connecting link made of SiN, a beam or membrane is provided, further comprising metal strips and polysilicon strips. The volumetric specific heat will not change much, but the average 20 thermal conductance of the beam will be 4-9 times as high as for a pure SiN beam.

Then, the thermal diffusivity will be higher by that factor, and the distance for effective dampening of the AC-thermal waves would be 2-3 times as high. This means that if the thermal actuator element (30) and the resonance sensing element (40) are placed on the same beam, the required separation D is now about 100-150 pm.

25 For resonator elements made of mono-crystalline silicon, the required distance D is even larger, as the thermal diffusivity of silicon is about 70 times higher than that of SiN, thus D may be about 8 times longer, 400 pm instead of 50 pm. This is in practice prohibitive for single-beam resonator structures. Thus the low thermal-conductivity nature of the structural materials in the fabrication process is advantageous 30 for this approach.

According to an embodiment the thermal actuator is suspended from the frame by means of at least one suspension element allowing at least an actuation part of the thermal actuator to move relatively to the frame. This movement may be transferred 6 towards the resonator element (e.g. being a beam or a membrane), thereby actuating the resonator element. The actuation part of the thermal actuator may be connected to the beam. The actuation part of the thermal actuator may perform a movement which is substantially perpendicular to a top surface of the frame. This vertical movement is thus 5 transferred to the resonator element.

According to an embodiment the actuation part is provided on a first edge of the thermal actuator and the thermal actuator further comprises a suspension element provided on a second edge of the thermal actuator, the second edge being opposite to the first edge.

10 The function of element 72 is to provide a mechanically rigid (but thermally isolated) connection of the thermal actuator to the frame, so that the bending of the thermal actuator will result in a time-varying vertical force onto the connecting link 31, and via that link, to the resonator element (10). This time-varying force is then used to bring the resonator in resonance.

15 According to an embodiment the thermal actuator comprises further suspension elements, provided on a third and fourth edge of the thermal actuator, the third and fourth edge connecting the first and second edge. The further suspension elements limit movement of the thermal actuator in a direction substantially perpendicular to the surface of the thermal actuator, such that the induced movement is substantially 20 transferred towards the actuation part (77). Elements (73) further enhance the transfer of the bending motion towards connecting link (31).

According to an embodiment the sample platform is suspended from the frame by a single beam.

According to an embodiment the thermal actuator is positioned near a base part 25 of the beam interacting with the base part of the beam. The base part of the beam is the part of the beam closest to the frame, i.e. opposite a free part of the beam comprising the sample platform.

According to an embodiment the sample platform is suspended from the frame by two or more beams.

30 According to an embodiment the sample platform is suspended from the frame by two or more beams connecting a first edge of the sample platform with the frame.

According to an embodiment the sample platform is suspended from the frame by a first beam and a second beam, the first beam connecting a first edge of the 7 platform with the frame and the second beam connecting a second edge of the platform with the frame.

According to an embodiment one or more thermal actuators interact with a first subset of beams and one or more resonance sensing elements interact with a second 5 subset of beams, the beams of the second subset not being part of the first subset. The resonance sensing elements may be provided with respect to different beams than the thermal actuators, thereby reducing crosstalk between the resonance sensing element and the thermal actuators. In other word, the thermal dampening factor TDF = (27ifD2/a)°5, is increased by providing a relatively large value for D, being the distance 10 between the thermal actuator 30 and the resonance sensing element 40.

The embodiments with two or more beams may also be provided with one or more thermal actuators that are positioned near a base part of the beam interacting with the base part of the beam. The thermal actuator may be suspended from the frame by means of at least one suspension element allowing at least an actuation part of the 15 thermal actuator to move relatively to the frame. This movement may be transferred towards the beam, thereby actuating the beam. The actuation part of the thermal actuator may be connected to the beam. The actuation part of the thermal actuator may perform a movement which is substantially perpendicular to a top surface of the frame. This vertical movement is thus transferred to the resonator element.

20 According to an embodiment the thermal actuator is mechanically linked by means of a connecting element having a low thermal conductance, less than half the conductance of the part of the resonator element between frame and a connection between the resonator element and the connecting element. This connecting element may connect the actuation part of the thermal actuator to the resonator element. This 25 connecting element may be made of a material with a relatively low thermal conductance, such as SiN, with a low value of the thermal conductance k of about 3 W/Km and a low value of the thermal diffusivity a of about 1 mm2/sec. The connecting element may also be provided with a relatively small cross-sectional area to reduce the thermal flow. The connecting element may further be made relatively long, to further 30 decrease the thermal conductance.

According to an embodiment the sample platform is formed by a membrane that is suspended from the frame substantially along its entire perimeter.

8

This structure is more robust and easier to make compared to structures with beams and/or paddles. Such a membrane has the advantage that a relatively high thermal isolation is obtained as a result of the structure of such a device. Based on [5], using a closed membrane with similar layers as the other embodiments described, 5 results in a temperature drop of 95% within approximately 200jum from the heating element 50.

According to an embodiment actuator is provided on the membrane on a first location and the resonance sensing element is positioned on the membrane on a second location, wherein the first and the second location are remote from each other. By 10 choosing the first and second location remote from each other, unwanted interaction between the first resonance sensing element and the thermal actuators is reduced. The distance between the first and second location may be chosen to be at least lA of the outer dimensions of the membrane. So, in case the membrane has a rectangular shape, the distance between the first and second location may be at least XA of a length or 15 width of the membrane. In case the membrane is substantially round, the distance between the first and second location may be at least lA of a diameter of the membrane.

According to an embodiment the conduction path between the actuator and the resonance sensing element comprises a heat shield, formed by leads of high thermal conductivity (such as metals). The leads may for instance be formed by leads 20 connecting the heating element and/or the temperature element with power supplies or control units provided on the frame of the thermo gravimetric device or being provided separately. The leads may run through the conducting path between the actuator and the resonance sensing element and may create high conductance paths directly to the frame. The leads are substantially perpendicular to the direction of the conduction path. 25

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: 30 Figs, la - c schematically depict a thermogravimetric device according to an embodiment,

Figs. 2 - 5b schematically depict thermogravimetric devices according to different embodiments.

9

DETAILED DESCRIPTION

Different embodiments will now be provided with reference to the Figures.

The embodiments presented provide device for thermogravimetric analysis 5 (TGA), which may be a Micro-Electro-Mechanical System (MEMS) or Nano-Electro-Mechanical System (NEMS) or the like, suitable for Thermo Gravimetric Analysis (TGA) and optionally simultaneous Fast Scanning Calorimetry (FSC). The embodiments provided comprise integrated mechanical (including sensing elements) and thermal elements required for, while cross-talk between these elements is reduced. 10 The embodiments presented here provide a Micro-Electro-Mechanical System (MEMS) device for Thermo Gravimetric Analysis (TGA) and optionally simultaneous Fast Scanning Calorimetry (FSC), exhibiting a low temperature dependence of the mass measurement.

The embodiments described may be formed as MEMS or NEMS devices. The 15 term MEMS is used to refer to a Micro Electro Mechanical System or micromechanical device. This term will readily be understood by a skilled person and relates to the technology of small devices, i.e. devices with typical dimensions in the range of 1 - 1000 micrometer. It will be understood that smaller dimensions are also conceivable. Such smaller devices may also be referred to as NEMS (Nano Electro 20 Mechanical System), with typical dimensions in the range of 10 - 1000 nanometer.

The embodiments incorporate a mass-sensing resonator comprising a sample platform which may be thermally isolated from the rest of the device, in particular from the parts of the resonator element which determine the resonance frequency. These parts may be the parts of the resonator element close to the frame. It incorporates an 25 integrated heating element in the sample platform, making it possible to heat up the sample platform with respect to the ambient in an arbitrary temperature-time program. In this way a sample under test on the sample platform can be subjected to this arbitrary temperature-time program. The resonator element is also a mechanical structure capable of being brought into resonance at a resonance frequency which is dependent 30 upon mass loading by the sample on the sample platform.

The embodiments incorporate a frame (which may be a silicon support frame) in which the resonator element is mechanically suspended and that also acts as heat sink, 10 and that together with the housing in which the device may be mounted supplies a stable ambient (base) temperature.

The embodiments may incorporate a piezo-resistive element at the (cold) side of the resonator where the resonator is suspended from the silicon support frame, forming 5 a resonance sensing element, that detects the resonance (frequency) of the resonator, thus allowing mass measurement of the sample.

The embodiments may incorporate a thermal actuator mechanically connected to the resonator element. The thermal actuator drives the resonator element into resonance using electronic feedback (the driving electronics can be off-chip, or can be made on-10 chip, or on a second ASIC chip). The mechanical link between thermal actuator and resonator is designed such that there is a good thermal isolation between thermal actuator and the resonance sensing elements (e.g. piezo-resistive element) to eliminate thermal cross-talk. Also electrical cross-talk is absent, because of the nature of thermal actuation.

15 The temperature of the sample platform is measured by a temperature-dependent element, either the heating element itself, or a second resistive element, or separately by a thermopile measuring the temperature increase of the sample platform 11 with respect to the ambient.

By using two thermogravimetric devices, either on separate chips or both 20 integrated in a single chip, and by using one of the two thermogravimetric devices as a reference device without sample, two additional functionalities are created. First of all it is possible to compare the resonance frequency of the reference device with the sample device, and measure only the differential frequency. In this way, any dependence of the resonance frequency on the temperature of the sample platform, 25 ambient temperature or other causes can be compensated for. Secondly, it is possible to perform FSC by comparing either the temperature if both devices receive the same power to the heating element, or by comparing the compensation power when the power to the sample device is controlled to give the same temperature-time program as the reference device.

30 The embodiments described here provide a thermogravimetric device, i.e. a mass sensing resonator capable of simultaneously measuring the mass of a sample, and heating the sample according to a predefined temperature program in time, comprising a resonator element 10, comprising a sample platform 11 which can be heated (hot 11 spot). Examples thereof are shown in the Figures, which will be described in more detail below.

In general there is provided a thermogravimetric device 1, comprising a resonator element 10 suspended from a frame 20, at least one actuator 30 for bringing the 5 resonator element 10 in a resonating state and a resonance sensing element 40 for sensing a resonance frequency of the resonator element 10, wherein the resonator element 10 comprises a sample platform 11 arranged to receive a sample and the thermogravimetric device 1 further comprises a heating element 50 for heating the sample when positioned on the sample platform 11, wherein the thermogravimetric 10 device 1 comprises a temperature element 60 sensing the temperature of the sample platform 11.

The resonator element 10 may comprise a first zone which is near the frame 20 and a second zone, which is in between the first zone and the sample platform 11. The first zone is the part of the resonator element 10 in which most stress and strain is 15 generated during resonance. The first zone may also be referred to as the base 19 of the resonator element 10, i.e. the part of the resonator element 10 that is attached to the frame. The second zone may also be referred to as a thermal isolation zone, as it forms an isolation zone between the sample platform 11 (which is heated during use) and the first zone of base 19 of the resonator element 10. The first zone 19 may cover 20 approximately half of the length of the resonator element, for instance half of the length of the beam 12. The second zone 24 may cover approximately the part of the resonator element 10 in between the heating element 50 and the first zone 19.

The sample platform 11 may incorporate an integrated heating element 50 and integrated temperature-sensing element 60 (which may be the same as heating element 25 50, or separate), making it possible to heat up the sample with respect to the ambient in an arbitrary temperature-time program. The heating element 50 and/or the temperaturesensing element 60 may also be provided as separate from the resonator element 10.

The thermogravimetric device 1 comprises a resonance sensing element 40 arranged to sense the resonance frequency of the resonator element 10. The resonance 30 sensing element 40 may comprise a piezo-resistive element at the first zone (the (cold) side) of the resonator element 10, i.e. near the suspension between the resonator element 10 and the frame 20 or where the resonator element 10 is suspended from the 12 frame 20, area 19. Resonance sensing element 40 may be positioned in the area of largest strain, i.e. at least in the first zone.

The thermogravimetric device 1 may further comprise one or more compensating piezo-resistive elements 41 on the frame 20. These compensating piezo-resistive 5 elements may be designed with an identical lay-out and process as the piezo-resistive element 40 on the resonator element 10, but now located beyond the resonator element 10 on the frame 20, e.g. on the solid silicon support frame, so that they do not experience any strain, but will experience the same ambient temperature fluctuations as element 40 on the resonator and will compensate for that when resonance sensing 10 element 40 and compensating resonance sensing element 41 are used in a bridge configuration. They are also of approximately the same resistance value, so that readout of the resonance-sensing element 40 using a bridge of Wheatstone is facilitated.

The resonance sensing element 40 is arranged to detect the resonance (frequency) of the resonator element 10. From the determined resonance frequency, or at least the 15 resonance frequency as a function of the temperature of the sample, information can be derived about the mass of the sample provided on the sample platform 11. Because of the relatively low temperature scanning rates used even in micro-TGA (below 10 K/s), it can safely be assumed that the temperature of the sample platform and the sample are substantially the same. The thermal lag in such small samples at such low scanning 20 rates will be negligible, as known from the thermal lag experienced in Fast Scanning Calorimetry with chips, where scanning rates can be many orders of magnitude higher,.

The thermogravimetric device 1 may further comprise a temperature-sensing element 60, i.e. a temperature-dependent element, to provide an indication of the temperature of the sample. The temperature-sensing element may for instance be used 25 to measure the temperature of the heating element 50, the sample platform 11 or the sample. Such a temperature-sensing element may for instance be formed by the heating element 50, by a separate resistive element, or by a thermopile measuring the temperature increase of the sample platform 11 or sample with respect to the ambient. The temperature sensing element may comprise a temperature-dependent resistive 30 element on or near the sample platform 11, made of a metallic or semiconductor material, said element 60 (see Fig. 5b) being different from the heater 50, or being one and the same element.

13

The temperature-sensing element may comprise a thermocouple made of one strip of 0.3 pm thick low-stress LPCVD (low-pressure chemical vapour deposition) polysilicon, boron-doped to a sheet resistance of 75 Ohm/square, and one strip of the same polysilicon, phosphorous-doped to a sheet resistance of 50 Ohm/square, giving a 5 overall resistance of 8 kOhm.

A thermopile may be formed of one or more thermocouples, laid out between the sample platform 11 and the frame 20, made of semiconductive layers of p-type and/or n-type doped silicon, silicon-germanium, bismuth-antimony or other types or combinations of layers of typically 0.1-1 pm thickness to serve as resistive heaters or in 10 combinations as thermocouples, and/or conductive layers such as aluminium, gold, platinum, chromium-nickel to serve as heaters or thermocouples, also in combination with semiconductor layers. These layers may be deposited onto or in between the structural layers of the resonator element 10. The thermocouple or thermopile may have hot junctions on or near the sample platform 11, and cold junction on the frame 20, 15 made of semiconductor and/or metallic leads.

The materials of which thermocouples and connection leads to the heating element 50 and other possible resistive elements are made usually have a much higher thermal conductivity than that of the structural materials of the resonator element 10.

Fig.’s la - c schematically depicts a thermogravimetric device 1 according to an 20 embodiment, comprising a resonator element 10 suspended from a frame 20, for instance a silicon frame 20. The embodiment shown in Figs, la - c may be a thermogravimetric device being formed by a 284 pm long Silicon nitride (SiN) resonator element 10, formed as a cantilever beam, which is the sensing cantilever.

The resonator element 10 as shown in Figs, la - c comprises a beam 12, fixed at 25 one end, with total length of approximately 284 pm, a width of 60 pm and a thickness of about 1 pm, and widened at the free end into a paddle to form the sample platform 11, of width 300 pm over a length of 70 pm, consisting of low-stress LPCVD SiN.

The resonator element 10 may comprise a sample platform 11 arranged to receive a sample. The sample platform 11 may be provided on the resonator element, remote 30 from the suspension of the resonator element 10 from the frame.

In Figs, la - c, the first zone or base 19 of the resonator element 10 and the second zone 24, in between the first zone 19 and the sample platform 11 are indicated.

14

As stated above the first zone may be approximately cover the first half of the resonator element 10, in this case approximately the first half of the beam 12. This dimension may be deduced from an elastic energy distribution analysis. However, in case the first zone is defined based on the a stress analysis, the first zone 19 may be 5 approximately 3/4 of the beam 12.

The resonator element 10, i.e. the sample platform 11, may further comprise a heating element 50 for heating the sample when positioned on the sample platform 11. The heating element 50 may comprise a 7 kOhm resistance , made of 0.3 pm thick low-stress LPCVD poly-silicon, boron-doped to a sheet resistance of 75 Ohm/square, 10 covering the free end of the cantilever beam over an area of 128x38 pm.

The thermogravimetric device 10 further comprises at least one actuator 30 for bringing the resonator element 10 in a resonating state. The actuator 30 may be a thermal actuator which is mechanically linked to the resonator element 10 by means of a connecting element (31). The connecting element 31 may have a low thermal 15 conductance, e.g. less than half the conductance of the part of the resonator element 10 between frame 20 and a connection between the resonator element 10 and the connecting element 31.

The actuator 30 may be formed as a thermal actuator, which generates a bending motion by amplification of differences in thermal expansion of two layers, in this 20 embodiment the SiN layer and a metal layer on top of it. In general two layers from the available processes will be chosen having a significant different coefficient of thermal expansion and significant thicknesses to achieve bending with enough amplitude and force to be able to actuate an object. This principle is used to actuate the resonator element 10. The increase in temperature to generate thermal expansion may be 25 achieved internally by using electrical resistive heating or by using a separate heat source.

The embodiment shown in Figs, la - c may comprise two actuators 30, which are thermal actuators, formed as two membranes arranged symmetrically on both sides of the resonator element 10. The two membranes forming the thermal actuators may be 30 linked to the resonator element 10 by SiN bridges.

The two thermal actuators 30 may consist of a SiN-Platinum (Pt) stack, a SiN-Aluminum (Al) stack, a SiN-Gold (Au) stack, or another stack.

15

According to the embodiment shown in Figs, la - c, the thermal actuator may be suspended from the frame 20 by means of at least one suspension element 72, 73 allowing at least an actuation part 77 of the thermal actuator 30 to move relatively to the frame 20. The thermal actuator 30 may be formed by a number of layers as 5 explained above, thereby having a plate-shape. The thermal actuator 30 may be suspended from the frame 20 by means of suspension elements provided along the perimeter of the plate-shape of the thermal actuator. The thermal actuator may be suspended by means of suspension elements 72, 73 along a number of edges 82, 83, 84 of the thermal actuator 30 to limit the freedom of movement of the thermal actuator 10 along these edges, thereby concentrating movement of the thermal actuator 30 on an actuation part 77 of the actuator 30, which is provided along a further edge 81 of the thermal actuator.

The thermogravimetric device 1 may further comprise a resonance sensing element 40 for sensing a resonance frequency of the resonator element 10. The 15 resonance element 40 may comprise two piezo resistors provided in a Wheatstone bridge configuration to detect the bending of the resonator element 10, by detecting the bending of the resonator element 10.

The resonance sensing element 40 may be integrated in the resonator element 10, close to the frame 20. The resonance sensing element 40 may comprise a piezo-20 resistive element being of 1.25 kOhm resistance, made of 0.3 pm thick low-stress LPCVD poly-silicon, boron-doped to a sheet resistance of 75 Ohm/square, having a length of about 96 pm and a width of 6 pm, with one 180 degrees angle in it, and located near the fixed end of the cantilever.

The sample platform 11 may be thermally isolated from a suspension between the 25 resonator element 10 and the frame 20. In other words, a temperature increase caused by the heating element 50 leads to a change in resonance frequency in the absence of a sample smaller than or comparable to the reproducibility in the frequency measurement. The design may be such that it results in a decrease of the temperature coefficient of resonance frequency (TCRF) from of the order of 200 ppm/K (ppm = 30 parts per million) for changes in ambient temperature to less than 20 ppm/K, or at least less than 10 or 5 ppm/K, or preferably less than 2 ppm/K for heating of the sample platform 11. So, in case the temperature coefficient of resonance frequency (TCRF) is less than 10 ppm/K, this means that when the sample platform 11 is heated, while the 16 resonance frequency of the resonator element 10 is about 10.000 Hz, the resonance frequency of the resonator element 10 changes with less than 0,1 Hz per Kelvin, in the absence of mass changes.

The thermogravimetric device 1 as shown in Figs.la - c has a sample platform 11 5 that is suspended from the frame 20 by a single beam 11. The resonator element 10 is thus formed by a single beam 11 having a sample platform at its free end. This configuration may also be referred to as a cantilever design.

The beam 11 has at its fixed end a piezo resistor of about 1 kQ used as strain sensor forming the resonance sensing element 40, and at its free end near the sample 10 platform 11a heater resistor of about 7 kQ forming the heater element 50. The polysilicon heater on the sample platform covers the sample area of 128x38 pm2.

A compensation resistor may be located outside the sample platform 11 and may be designed to be identical to the strain sensor. The temperature of the heater resistor and sample is measured by a p-type vs. n-type polysilicon thermocouple with an 15 estimated sensitivity at room temperature of about 0.4 mV/K [6],

In Table 1 the major characteristics of a possible thermogravimetric device 1 according to Figs, la - c are listed.

Table 1 Summary of important characteristics of the TGA device.

Parameter Value

Cantilever or beam size [pm3] 214x61x0.9

Sample platform size [pm3] 70x300x0.9

Resonance frequency [kHz] 10.390

Actuator resistance [Q] 300

Heater resistance [kQ] 7

Strain sensor and compensation resistor [kQ] 1 Time constant [ms] 0.9

Transfer [V/W] 19

Thermal resistance of heater to ambient [kK/W] 58 Thermal resistance of actuator to ambient 15 [kK/W]

Thermocouple sensitivity [mV/K] 0.4 17

Mass detection range 100ngto20pg

Temperature range [°C] Room temperature - 650 5 10

Manufacturing

Thermogravimetric device 1 as described above with reference to the Figures, may be made using a thin-film bulk-micro-machining process. The starting material may be a 300-pm thick silicon wafer of 100 mm diameter with crystal orientation 15 <100> , on which 600 nm low-pressure chemical vapor deposition (LPCVD) SiN and 300 nm LPCVD low-stress poly-silicon (poly) are deposited.

After implantation of the poly to make low-resistive n-type and p-type regions (50Q/sq and 75 fl/sq, respectively) the poly is patterned and covered by another 300 nm SiN. The contact openings to the poly are made and an anti-diffusion barrier (40 nm 20 Titanium (Ti) and 80 nm Titanium Nitride (TiN)) is deposited. The TiTiN layer is then patterned and RTA is performed to get Titanium Silicide (TiSi2). After the silicidation step 10 nm Tantalum (Ta) and 0.2 pm Pt are deposited and patterned. Then SiN membranes are etched by anisotropic KOH-etching from the back side. Plasma etching from the front side is used to release the final structures. After sawing the chips are 25 ready for assembly.

Alternatively, below the 600 nm SiN layer, first a sacrificial layer of 200 nm SiN and 100 nm SiCL can be deposited. Then, before freeing membranes by KOH-etching, the trenches in the membranes are etched which delineate the cantilever beams, sample platform 11 (e.g. including paddles) and thermal actuators 30, by plasma etching the 30 stack of layers, but stopping on the 100 nm Si02 sacrificial layer. After this, KOH

etching is performed, where the front side of the wafer is still separated from the KOH-etch solution by the sacrificial layers of SiN and Si02. After this, the sacrificial layers of 200 nm SiN and 100 nmSi02 are removed at the backside of the beam 12 and 18 actuators 30 by plasma etching and/or wet etching, and the wafer is ready for sawing. This removes the need to process the wafer when fragile membranes are present, and when metals such as gold or platinum are present that may not be desired in a process line.

5 Alternatively, the wet etching may also be performed from the front side using TMAOH etching solution that etches similarly to KOH etching solutions but is less aggressive towards oxides and metals. Then, the trenches in the SiN layers to delineate the beams and other structures in the membrane are used as openings for the TMAOH to access the silicon to be etched. The membranes are then formed by removing the 10 silicon from underneath the beams and other structures by underetching.

Fig. 2 schematically depicts a thermogravimetric device 1 according to an alternative embodiment. According to this embodiment, the sample platform 11 is suspended from the frame 20 by two or more beams 12, 13, wherein the two or more 15 beams 12, 13 providing a connection between a first edge 15 of the sample platform 11. Of course, more than two beams may be provided for connecting the first edge 15 of the sample platform 11 to the frame 20.

By providing the beams 12, 13 all on the same edge 15 of the sample platform 11, a resonator element 10 is formed that can resonate in a direction substantially 20 perpendicular with respect to a plane defined by the longitudinal body axes of the beams 12, 13, without inducing tensional stress in the beams 12, 13.

According to this embodiment, the at least one actuator 30 may be provided to the first beam 12, i.e. being mechanically coupled to the first 12 to actuate the first beam 12, while the resonance sensing element 40 may be provided to sense a resonance 25 frequency of the second beam 14. This provides more accurate measurements, as electrical crosstalk and thermal interaction between the actuator 30 and the resonance sensing element 40 is minimized.

Unwanted thermal interaction between the actuator 30 and the resonance sensing element 40 may be specifically relevant in case the actuator is a thermal actuator 30 as 30 described above. By ensuring that enough distance exists between thermal actuator 30 and resonance sensing element 40, this thermal interaction is avoided. For the technology used, enough distance means e.g. at least 100-150 pm.

19

Figures 3 and 4 schematically depict a thermogravimetric device 1, wherein the sample platform 11 is suspended from the frame 20 by a first beam 12 and a second beam 14, the first beam 12 connecting a first edge of the platform 1 with the frame 20 and the second beam 14 connecting a second edge 16 of the sample platform 11 with 5 the frame 20. Such a design may also be referred to as a floating membrane device or a bridge design.

Fig. 3 shows an embodiment wherein the first and second edge 15, 16 are parallel, forming opposite edges of the sample platform 11. Fig. 4 shows an embodiment wherein the first and second edge 15, 16 are adjacent edges, being at an 10 angle with respect to each other. As can be seen in Fig. 4, the thermogravimetric device 1 may comprise further beams 12’, 14’ connecting further edges of the sample platform 11, i.e. edges 17, 18 with the frame 20.

The embodiments described with reference to Fig.’s 3 and 4 allow for an even further separation between the actuators 30 and the resonance sensing element(s) 40, 15 allowing an even more accurate determination of the mass of the sample by further reducing crosstalk .

According to a further embodiment, a closed membrane device is provided, i.e. a thermogravimetric device, wherein the sample platform is formed by a membrane 22 20 that is suspended from the frame 20 substantially along its entire perimeter.

Fig. 5a schematically depicts a perspective exploded view of a thermogravimetric device according to this embodiment. Instead of providing one or more beams to suspend the sample platform 11 from the frame 20, a membrane 22 is provided that is suspended along its perimeter like a drumhead or a drum skin. Such a membrane 22 25 can function as a resonator element 10. In the centre of the membrane 22 a sample platform 11 may be provided. Fig. 5b shows a top view of this embodiment.

The actuator 30 may be a thermal actuator, which is positioned on the membrane 22 near the frame 20. The thermal actuator 30 may comprise three main layers.

A first layer is formed by the structural layer of the membrane 22 (e.g. SiN-30 layer).

A second layer, which is a heating layer, may be provided directly on the first layer which can be heated, for instance by AC-Joule heating.

20 A third layer may be provided on top of the second layer, may be a metal layer with a distinct different coefficient of thermal expansion compared to the first layer.

Also, a temperature-sensing element 60 may be provided as a separate element, as is only shown in Fig. 5b by way of example.

5

Theory

To perform TGA, a resonator element 10 may be provided with a mass-dependent resonance frequency, elements to subject the sample under test to a time-programmed temperature profile, in general a heating element 50 and a temperature-10 sensing element 60 for the thermal analysis, and also elements to bring the resonator element into resonance and to read out the resonance frequency, in general an actuator (30) and a resonance sensing element (40).

The actuator for resonant elements may be a piezo-electric actuator. This generally needs to be driven with high ac-voltages (tens of Volts) at the resonance 15 frequency. Because this gives a significant electrical cross-talk, optical read-out is often used instead of piezo-resistive elements in a bridge of Wheatstone configuration. The latter, however, is very easy to implement in the embodiment provided. Moreover, just as external optical elements, piezo-electric actuators are mostly external components as well, although the technology exists to make integrated piezo-electric actuators, for 20 instance by adding layers of ZnO. This requires exotic processing, and still leaves the problem of electrical cross-talk.

A solution to this problem is provided by using thermal actuators. Thermal actuators are easily implemented in the TGA-MEMS process, as they require no extra processing steps, and they circumvent the problem of electrical cross-talk, because they 25 are driven at half the resonance frequency. The conversion of driving voltage to thermal power signal involves a frequency doubling, and it is easy to filter out the halffrequency electrical signal. This approach has been shown before [1,2], but not for TGA-MEMS.

To further increase the performance of the embodiments presented, thermal 30 cross-talk may be reduced . If the AC thermal signals of the thermal actuator 30 reach the resonance sensing element 40, they may cause cross-talk through the thermal domain, and still deteriorate the measurements. This may happen if the product of the AC thermal signals and the temperature coefficient of resistance of the piezo-resistive 21 elements used as resonance sensing element is larger than the stress-induce signals due to the resonating movements.

For instance, in the embodiment according to Figs, la - c , typically encountered stress-induced changes in resistance induce a signal of the order of 10 pV on a 5 Wheatstone bridge voltage of typically 1 V, or 10 ppm at the resonance frequency, and 1 ppm at significantly different frequencies. The temperature coefficient of resistance (TCR) for the poly-silicon piezo-resistive elements is typically 1000 ppm/K [4], This indicates, that the AC-thermal waves need to be filtered down to 1 mK in order to reach a base level of cross-talk of only 1 ppm. With a typical thermal actuator AC-10 temperature amplitude of 100 K, this means a filtering of 105 times of the AC-thermal waves.

To analyze the filtering that is achieved for these ac thermal signals, the thermal conducting path between the thermal actuator (30) and the resonance sensing element (40) is considered as a linear transmission line. In the frequency domain, the 15 propagation of the ac thermal signals is characterized by a propagation constant y, where y2 = /?s (Gp + jcoC), where Rs ,GP and C are the transmission line series resistance, shunt conductance and capacitance per unit length. The shunt conductance here is ignored, as the dc behaviour is not of interest, and since Gp is much smaller than co C at the frequencies under consideration. Then y2 = i?scoC. In terms of the TGA-20 devices, this may be rewritten as y2 = 2nf! a, wherein ƒ = frequency of the thermal actuator and a = effective thermal diffusivity of the conducting path, and D = length of the conducting path between thermal actuator 30 and resonance sensing element 40. If further the Thermal Dampening Factor TDF = yD, then, according to transmission-line theory [5], the dampening in a beam of uniform width will occur typically in an 25 exponential decay, where the TDF factor, with a minus sign, is the exponent: T/Tactuator ~e'TDF (1)

Of course, for conducting paths of various sections with different values of the thermal diffusivity or even geometry, a more detailed analysis is required. However, as this is an exponentially decaying phenomenon, a first, simple analysis will yield 30 essential insight.

As generally the resonance frequency ƒ is determined by design considerations other than the dampening factor TDF, important design considerations for enlarging TDF are found in increasing the distance D between thermal actuator 30 and resonance 22 sensing element 40 and/or decreasing the thermal diffusivity a of the path between thermal actuator 30 and resonance sensing element 40.

The thermal diffusivity of a layer is given by its thermal conductivity, divided by its volumetric specific heat: a = K/(pCp) , expressed in m2/s. As a rule of thumb, the 5 volumetric specific heat of most solids at room temperature are very much the same, of the order of 1-4 MJ/m3K. Thus, to minimize the thermal diffusivity a, one needs to choose materials of low thermal conductivity.

Therefore, in designs where the distance D between thermal actuator 30 and resonance sensing element 40 is small, the thermal conducting path may be made of 10 low-conductivity materials. This has the advantage that not only the AC-thermal waves are dampened by filtering, but also the average, DC thermal wave is reduced. As an example, the conducting link 31 between the resonator element 10 and a separate thermal actuator element 30 as shown in Fig.1 will be made of the SiN layer material, since this material shows a low value of a of about 1.3 mm2/sec.

15 In order to achieve the desired dampening of 105, Eq.(l) indicates that it is required that the TDF be of the order of 12. Because the factor Inf/o. is approximately 5xl010 for a thermal wave at 10 kHz and for a connecting link made of SiN, the lower limit of 12 is achieved for a conducting path length D of about 50 pm.

A connecting link 31 of pure SiN effectively dampens AC-thermal waves.

20 Moreover, in the situation of Fig. 1, another effects adds to the dampening. As a matter of design, between the connecting link 31 and the resonance sensing element 40, strips of metal or poly-silicon may be positioned running from the heater 50 to the frame 20, offering a path of high thermal conductance to the silicon frame 20 for the AC-thermal waves, away from the resonance sensing element 40. This will add further to the 25 dampening of the AC thermal waves at the location of the resonance sensing element 40.

The situation is somewhat different if, instead of using a connecting link made of SiN, a beam or membrane containing also metal strips and poly-silicon strips is provided. The volumetric specific heat will not change much, but the average thermal 30 conductance of the beam will be 4-9 times as high as for a pure SiN beam. Then, the thermal diffusivity will be higher by that factor, and the distance for effective dampening of the AC-thermal waves would be 2-3 times as high. This means that if the 23 thermal actuator element 30 and the resonance sensing element 40 are positioned on the same beam, the required separation D is now about 100-150 pm.

For resonator elements made of mono-crystalline silicon, the required distance D is even larger, as the thermal diffusivity of silicon is about 70 times higher than that 5 of SiN, thus D should be about 8 times longer, 400 pm instead of 50 pm. This is in practice difficult to achieve for single-beam resonator structures. Thus the low thermal-conductivity nature of the structural materials in the fabrication process is essential for this approach to be feasible.

The thermal diffusivity of SiCk is actually 25% lower than that of SiN, but the material 10 is not so stiff, cancelling out this advantage.

Another point of interest is the efficiency of the thermal actuator 30. The thermal actuator 30 may be designed as large as possible to obtain maximum forces for bending the resonator element 10. However, there are limits to the size, connected to the 15 resonance frequency to be actuated. For the time-varying bending, it is needed that the thermal actuator 30 is heated and cooled periodically with the resonance frequency. For this to occur quickly enough, the thermal time constant of the actuator should not be too large. In particular, for the embodiment provided with respect to Figs, la - c, a resonance frequency of 10 kHz is found, and then a time constant of about 16 ps is 20 desired. In this case, a thermal resistance to the ambient is about 15 kK/W (see Table 1), and a volume is about 70><60x 1 pm3, and assuming a volumetric specific heat of 2 MJ/m3K, this yields a heat capacitance of about 8 nJ/K. The RC product is then about 120 ps, which is a bit high compared to the preferred value. So, in order to optimize the design of the thermal actuator from a thermal point of view, either the size or geometry, 25 or preferably, the thermal resistance to the ambient, may be decreased. In general, the RC-time constant of the thermal actuator may be designed to be of the order of Mlnf, as this will ensure optimum performance.

Material of the resonator element/beam

The resonator element 10, especially the beams and the membrane may be made 30 of dielectric layers of low thermal conductivity such as SiN, SiC and Si02 and combinations thereof, deposited by LPCVD and/or PECVD (Plasma-enhanced chemical vapor deposition) fabrication methods onto a silicon wafer substrate, with total layers thicknesses of typically 0.5-2 pm. LPCVD is preferred for its better high- 24 temperature resistance and mechanical properties. Materials of low conductivity may be used to thermally isolate the thermal actuator (30) from the resonance sensing element 40.

The descriptions above are intended to be illustrative, not limiting. It will be 5 apparent to the person skilled in the art that alternative and equivalent embodiments of the invention can be conceived and reduced to practice, without departing from the scope of the claims set out below.

List of references 10 [1] J. Lee and W.P. King, Rev. Sci. Instr., 79 (2008), pp. 1-6 [2] C. Hagleitner et al., IEEE J. of Solid-state circuits, vol. 37, no. 12 (2002), pp. 1867-1878 [3] D. Lange, A. Koll, O. Brand, and H. Baltes, Proc. SPIE , vol. 3328, (1998),pp. 233-243 [4] G.C.M. Meijer and A.W. van Herwaarden, editors., Thermal Sensors, AdamHilger, 15 Bristol (1994) 304 pgs, ISBN 0-7503-0220-8, pp. 54-58 [5] A. Minakov, J. Morikawa, T. Hashimoto, H. Huth and C. Schick, “Temperature distribution in a thin-film chip utilized for advanced nanocalorimetry”,Meas. Sci.

Techn. 17 (2006) pp. 199-207 [6] E. Iervolino, A.W. van Herwaarden, P.M. Sarro, Proc. EUROSENSORS 2008, pp. 773- 20 776

Claims

A thermogravimetric device (1) comprising a resonator element (10) suspended from a frame (20), at least one actuator (30) for bringing the resonator element (10) into a resonating state and a resonance sensing element (40) for observing a resonance sequence of the resonator element (10), wherein the resonator element (10) comprises a sample platform (11) adapted to receive a sample and wherein the thermogravimetric device (1) further comprises a heating element (50) for heating of the sample when placed on the sample platform (11), wherein the thermogravimetric device (1) comprises a temperature element (60) which detects the temperature of the sample platform (11), characterized in that the actuator (30) is a thermal actuator (30).

2. Thermogravimetric device (1) according to claim 1, wherein the thermal actuator is an integrated part of the thermogravimetric device (1).

The thermogravimetric device (1) according to any of the preceding claims, wherein the thermal actuator is mechanically connected to the resonator element (10).

A thermogravimetric device (1) according to any one of the preceding claims, wherein the resonance sensing element (40) is provided on the resonator element (10).

5. Thermogravimetric device (1) according to one of the preceding claims, wherein the thermal actuator is thermally insulated relative to the resonance sensing element (40).

6. Thermogravimetric device (1) according to one of the preceding claims, wherein a conductive path between the resonance sensing element (40) and the thermal actuator (30) is characterized by a thermal damping factor TDF = (2n / D2 / a) 0'5 where ƒ = frequency of the thermal actuator (30), D = distance between thermal actuator (30) and resonance sensing element (40) and a = effective thermal diffusivity of the conductive path.

7. Thermogravimetric device according to one of the preceding claims, wherein the thermal actuator is suspended from the frame (20) by means of at least one suspension element (72, 73) that allows at least one drive part (77) of the thermal actuator (30) can move relative to the frame (20).

8. Thermogravimetric device according to claim 7, wherein the drive part (77) is provided on a first edge (81) of the thermal actuator (30) and the thermal actuator further comprises a suspension element (72) provided on a second edge (82) of the thermal actuator (30), the second edge (82) being opposite the first edge (81).

9. Thermogravimetric device according to any of claims 7-8, wherein the thermal actuator comprises further suspension elements (73) provided on a third and fourth edge (83, 84) of the thermal actuator (30), the third and fourth edge (83, 84) connect the first and second edges (81, 82).

10. Thermogravimetric device according to one of the preceding claims, wherein the sample platform (11) is suspended from the frame (20) by a single beam (12).

The thermogravimetric device according to claim 10, wherein the thermal actuator (30) is located near a base portion (19) of the beam (12) interacting with the base portion (19) of the beam (12). 25

A thermogravimetric device according to any of claims 1-9, wherein the sample platform (11) is suspended from the frame (20) by two or more beams (12, 13, 14).

The thermogravimetric device of claim 12, wherein the sample platform (11) is suspended from the frame by two or more beams (12, 13) connecting a first edge of the sample platform (11) to the frame (20). Tl

A thermogravimetric device according to claim 12, wherein the sample platform (11) is suspended from the frame by a first beam (12) and a second beam (14), the first beam (12) having a first edge of the platform (11) connects to the frame (20) and the second beam (14) connects a second edge of the platform 5 to the frame.

15. Thermogravimetric device according to any of claims 12-14, wherein one or more thermal actuators (30) interact with a first subset of beams (12, 12 ') and one or more resonance sensing elements (40) interact with a second subset of beams (14, 14 '), wherein the beams from the second subset are not part of the first subset.

16. Thermogravimetric device (1) as claimed in any of the foregoing claims, wherein the thermal actuator (30) is mechanically connected by means of a connecting element (31) with a low thermal conductivity, less than half the conductivity of the part of the resonator element (10) between frame (20) and a connection between the resonator element (10) and the connection element (31).

17. Thermogravimetric device according to any of claims 1-6, wherein the sample platform (11) is formed by a membrane suspended from the frame substantially along its entire circumference.

18. Thermogravimetric device according to claim 16, wherein the actuator (30) is provided on the membrane at a first location / position and the resonance sensing element (40) is placed on the membrane at a second location / position, the first and second location / position far apart.

A thermogravimetric device (1) according to any one of the preceding claims, wherein the conduction path between the actuator (30) and the resonance sensing element (40) comprises a heat shield formed by pipes with a high thermal conductivity (such as metals).