DE102017202455B4 - MEMS- or NEMS-based sensor and method for operating such - Google Patents

Abstract

MEMS- or NEMS-based sensor, which is designed as a cantilever, which is constructed in the form of a strip and has at least three sections (1, 2, 3), wherein • the first section (1) is attached on one side in a holder or wall and on opposite end or near at least one electrical circuit which has a piezoresistive element for heating all three sections (1, 2, 3) and for measurement, which is electrically coupled to a voltage supply and an evaluation unit, and • the second Section (2) directly adjoins the first section (1) and is suitable for transferring heat to the third section (3), and • the third section (3) is at least partially multi-material, at least bi-material that the top and bottom of the third section (3) have different thermal expansion coefficients, so that a heating of the third section (3) d eat curvature and causes a deflection of the cantilever, and • a data processing device controls the generation of DC and AC voltage to operate the first section (1) and the data processing device also has an evaluation unit for recording, processing and storing the values recorded by the sensor , both actuation and reading of the deflection of the cantilever with the piezoresistive element takes place.

Description

The present invention relates to a MEMS- or NEMS-based sensor and a method for operating such a sensor. It is used for atomic force microscopy (AFM), mass detection and all conceivable areas of application for MEMS or NEMS operating in dynamic mode.

Known MEMS devices are able to recognize (detect) contact forces acting on the sensor. The effective force can be determined by measuring the deflection (bending) either with an integrated piezo-resistive readout element, capacitive, piezoelectric, by means of an optical measurement of the beam bending or similar measurement methods.

The presented invention is suitable for scanning probes that operate with high resolution and at high speed. Furthermore, the presented invention relates to a freely oscillating sensor which has at least two materials with different thermal expansion coefficients. With the present invention, the problems arising when using an external actuator can be solved, with shorter response times, high-frequency operation and improved resolution can be achieved.

An atomic force microscope is used to image surface structures in the nm range. It comprises a flexible Si leaf spring (also referred to as a cantilever or cantilever), on the underside of which a measuring tip with a radius of approx. 10 nm is attached, and a detection system for detecting the cantilever deflection. The measuring tip at the free end of the cantilever moves over a surface to be measured, whereby the bending of the cantilever depends on the surface structure of the sample and the distance between the sample and the tip. This bending or deflection can be measured with capacitive, piezoresistive, piezoelectric or optical sensors and typically represented as the force acting between the tip and the surface forces. The surface structure can then be reconstructed from this force.

The atomic force microscope can operate in various imaging modes (contact mode, non-contact mode, intermittent mode, etc.). It is therefore well suited for a wide variety of measurement tasks in the nano range. The scanning speed is typically between 0.5 and 100 lines per second.

The non-contact mode belongs to the group of dynamic modes in which the cantilever is vibrated by an external periodic force, the vibration frequency being close to the resonance frequency of the cantilever. The amplitude of the oscillation is used as a control variable for the regulation, which allows the measuring tip to scan the surface. When the tip approaches the surface, the interaction between tip and surface changes both the vibration amplitude and the phase of the vibrating cantilever.

A large number of capacitive, electromagnetic, piezoelectric and thermal actuators for MEMS devices, each with application-specific advantages and disadvantages, are known from the prior art.

With capacitive actuators, for example, an electrostatic field is used between the sensor and a stationary electrode to deflect the cantilever in the direction of the electrode. This type of actuator can be used in particular for certain biological measurements.

In contrast, with an electromagnetic actuator, magnetic field forces are used to deflect the cantilever. This type of excitation is particularly suitable when high force and deflection, bidirectional actuation, non-contact (remote) actuation, etc. are required.

The well known piezoelectric effect can be used to create a voltage in the sensor by applying an electric field across a piezoelectric film. The advantages of piezoelectric excitation lie in the very low power consumption, the possibility of self-actuation with high frequency and a feedback for the degree of deflection.

The thermomechanical actuators consist of two or more layers of material, each of which has different coefficients of thermal expansion. The deflection of the cantilever is proportional to the applied thermal power.

In the dynamic working mode, the cantilever (bending beam) oscillates continuously with its own resonance frequency. The changes in the amplitude and the frequency shift can be used as a measure of the force interaction between the cantilever tip and the surface and also as a control signal during the imaging of the surface.

wobei f die Resonanzfrequenz des Sensors ist, k die Federkonstante und m_eff die effektive Masse.According to the state of the art, high-frequency cantilevers are used for highly sensitive, sensitive measurements. For this purpose, the cantilever is sensitized to different active ingredients by coating it with various chemical substances. A molecule absorbed by the sensor influences the resonance frequency of the cantilever, which is detected with the help of suitable measuring electronics. For such cantilevers, the frequency shift can be compared with a calibrated mass sensitivity factor in order to obtain an actual mass value as a result. This principle of comparing the frequency shift on the peak of the amplitude after a mass change is carried out successfully. For this purpose it is necessary to reduce the effective mass of the cantilever to a few picograms. Therefore, the size of the cantilever has to be reduced in the micrometer range, whereby the resonance frequency can reach up to 10 MHz. For a rectangular bar-shaped sensor, the relationship between the physical parameters is simply given by the following formula: $$f=\frac{1}{2\pi }\sqrt{\frac{k}{m_{eff}}}$$

where f is the resonance frequency of the sensor, k is the spring constant and m _{eff is} the effective mass.

The DE 101 06 854 A1 describes a microprobe that has a bending beam with at least two sections. The first section is clamped on one side and extends linearly from this fixed point. At the end opposite the clamped end, a second section is arranged, which extends from the first section in the longitudinal or transverse direction (viewed in the plane in which the first section designed as a flat strip lies). At least one piezo element is arranged on each lever section, which is suitable for heating the section in the area in which it rests on the material of the respective lever section and thus to achieve a deformation of the respective lever section in the area of the overlying piezo element due to the different expansion coefficients. The piezo element on the first section is used to set the desired position perpendicular to the surface of the material to be examined. The piezo element on the second section is used both to record the measurement data and optionally to adjust the curvature of the second section by introducing a current through which it is heated.

The construction described is extremely complex and requires the arrangement of at least two piezo elements with the associated power supply lines.

WO 2003/036 767 A2 discloses a probe arrangement whose individual probes can be moved independently of one another by means of the thermo-bimetallic effect or electrostatic effects. This document describes in particular an arrangement which is heated by means of a heating device and which is able to deposit material on the surface of a substrate.

The DE 10 2008 049 647 A1 describes a micromechanical element in which a membrane-like functional element is held by four retaining strips. Two of the holding strips have piezoelectric drive elements which are used to stimulate the functional element electromechanically, the use of the holding strips both for actuation and for measurement being mentioned.

A micro-probe head and a device for measuring a sample surface are in the DE 101 06 854 A1 described. A microprobe is described here which enables fine feed due to a piezoresistive element which is formed on a cantilevered arm. A variant is also described in which the piezoresistive element is used to detect the deflection of a lever element.

In the DE 10 2014 212 311 A1 describes a scanning probe microscope with a scanning device in which a measuring tip is used to scan over a surface. The measuring tip can be pivoted, whereby a more precise examination of the surface can be achieved.

It's in the US 2006/0032289 A1 a cantilever described which has two piezoresistive paths. Heating is generated by means of one and a change in resistance is determined by means of the other, which change is caused by the bending of the cantilever and a change in its temperature.

Furthermore, the US 6 667 467 B2 a measuring probe for examining small surfaces in the nanometer range and a device for scanning surfaces using such a measuring probe.

In the solutions known from the prior art, the thermomechanical actuation and readout is implemented by separate units on the cantilever (bimaterial strips). A bi-material strip (bi means two or often more) consists of two strips made of different materials. If the temperature changes, the length of the layers (layers on the cantilever) changes. Due to the different expansion of the different metals with the same temperature change, the cantilever bends. The cantilever bends in the direction of the material that has the smaller coefficient of linear expansion. In order to implement the thermomechanical actuation and readout, a complex layout is required to manufacture the individual parts required for this on the MEMS unit (cantilever).

The object of the present invention is to overcome the disadvantages of the prior art and to provide a MEMS- or NEMS-based sensor with a simplified structure and a method for its operation, in which the number of internal electrical contacts and connections for the actuation and the readout is reduced without, however, affecting the performance of the sensor (resolution and sensitivity).

According to the invention, this object is achieved with the features of the first claim. Advantageous refinements of the solution according to the invention are specified in the dependent claims.

• - Ein erster Abschnitt ist einseitig fixiert und so in einer Halterung oder Wandung befestigt. Am anderen Ende bzw. in dessen Nähe trägt der erste Abschnitt mindestens einen geeigneten elektrischen Schaltkreis, vorzugsweise ein piezoelektrisches Element. Dieser elektrische Schaltkreis dient sowohl der Durchführung der Messung als auch der Erwärmung aller drei Abschnitte des Cantilevers, wobei der Hauptbeitrag der Krümmung des Cantilevers vom dritten Abschnitt geliefert wird,
• - Ein zweiter Abschnitt schließt sich unmittelbar an den ersten Abschnitt an und dient der Wärmeleitung an den dritten Abschnitt,
• - Der dritte Abschnitt ist wenigstens bereichsweise multi-materiell, zumindest bi-materiell (bspw. bi-metallisch), ausgeführt. Dies bedeutet insbesondere, dass die Ober- und die Unterseite des dritten Abschnittes unterschiedliche thermische Ausdehnungskoeffizienten aufweisen, so dass eine Erwärmung des dritten Abschnitts dessen Krümmung zur Folge hat. Ganz besonders bevorzugt, ist keines der Materialien, die im dritten Abschnitt eingesetzt werden, als Messaufnehmer zur Krümmungsbestimmung oder als Heizelement ausgestaltet. So werden vorteilhaft die Zuführung elektrischer Spannung und die Ausführung dazu notwendiger Leiterbahnen im dritten Abschnitt vermieden. Der dritte Abschnitt weist an seinem vom zweiten Abschnitt entfernten Teil oder in dessen Nähe optional eine Messspitze auf, die der zu untersuchenden Materialoberfläche zugewandt ist.

The object is achieved according to the invention with a cantilever which is constructed in the form of a strip and has at least three sections:

• - A first section is fixed on one side and thus fastened in a holder or wall. At the other end or in its vicinity, the first section carries at least one suitable electrical circuit, preferably a piezoelectric element. This electrical circuit is used to carry out the measurement as well as to heat all three sections of the cantilever, with the main contribution to the curvature of the cantilever being provided by the third section,
• - A second section immediately follows the first section and serves to conduct heat to the third section,
• The third section is at least partially multi-material, at least bi-material (e.g. bi-metallic). This means in particular that the top and bottom of the third section have different coefficients of thermal expansion, so that heating of the third section results in its curvature. Most preferably none of the materials that are used in the third section are designed as measuring sensors for determining curvature or as heating elements. In this way, the supply of electrical voltage and the implementation of the conductor tracks required for this in the third section are advantageously avoided. The third section optionally has, on its part remote from the second section or in its vicinity, a measuring tip which faces the material surface to be examined.

The cantilever is designed in the manner of the constructions known from the prior art. It thus has an essentially strip-shaped (leaf spring-like) shape. The dimensions of the three sections may differ from one another. The total length of the cantilever is preferably in the range between 1 μm and 10 mm. The first section is preferably between 1 nm and 4 mm long. The length of the third section is preferably between 1 nm and 6 mm.

The total material thickness is preferably between 50 nm and 20 μm. In a preferred embodiment, it is the same over the entire length.

The simplest embodiment consists of exactly the three sections mentioned and a constant material thickness. More developed embodiments have additional sections. These can be, for example, additional sections that have the functionalities of the second and third sections.

The first section preferably has a conductive or semiconducting layer (piezoresistive layer) as the electrical circuit. The piezo-resistive element formed in this way is preferably integrated in the cantilever. Since the cantilever is preferably made of silicon or other materials of semiconductor technology, the formation of the piezo-resistive element in the first section is preferably carried out using known methods of semiconductor lithography. The piezo-residual element therefore preferably consists of one or more layers of doped silicon. Furthermore, it is preferably coated with 0.5-1.5 µm thick silicon oxide and wired with the aid of an approximately 1.0 µm thick Al film.

The second section is used to conduct heat from the electrical circuit to the third section. It thus spatially separates the electrical circuit from the third section. It preferably consists of the same material as the first section (base material) in the area in which no electrical circuit is formed (base material, for example, undoped or lightly doped silicon).

The second section is very short in preferred embodiments and thus realizes a distance between the first and third section, preferably in the range between 3 nm and 50 nm, particularly preferably between 5 nm and 25 nm. A positive effect of the second section is the equalization of the heat flow across the width of the cantilever. This reduces the risk of an uneven deflection, in particular tilting, of the third section. In this embodiment, the second section consists of the basic material of the cantilever.

In an alternative preferred embodiment, the second section is designed, at least in some areas, as an electrically insulating cover which partially or completely covers the first section.

The third section is designed to be multi-material, preferably bi-material, at least in some areas. The material of the third section is preferably identical to that of the base material of the first and second sections. The material layer with a coefficient of thermal expansion that differs from the base material consists, for example, of a metal, preferably aluminum, or of an oxide of the base material (for example SiO in a silicon cantilever). Table 1 shows an overview of materials (from the third row) which, in combination with silicon (second row), are suitable for use as a material with a different coefficient of thermal expansion.

In the alternative embodiment, in which the second section is designed as an insulating cover, the material layer with the coefficient of thermal expansion of the third section that differs from the base material extends over this cover, so that it contributes to the conduction of heat from the first section into the third section. Table 1: Coefficients of linear expansion material Linear expansion coefficient [a] (10 ^-6 m / (m K)) Silicon (base material) 3rd aluminum 22.2 gold 14.2 chrome 6.2 copper 16.6 Gold - platinum 15.2 Silicon nitride Si ₃ N ₄ 3.3 Silicon dioxide 0.56

Due to the different coefficients of expansion of the at least two materials, the cantilever bends when heat is influxed. This curvature leads to a movement that leads out of the plane in which the flat extension of the cantilever extends, so that the measuring tip can be positioned at a distance relative to the material surface or can be made to vibrate, essentially perpendicular to the material surface.

The construction according to the invention is controlled by a data processing device, which continues to record and preferably also process the data. In particular, the Data processing device a suitable control algorithm that provides for the use of the natural Joule heating of the cantilever, which is caused by the appropriate electrical circuit (piezoelectric element) due to the voltage applied during thermomechanical actuation. At the same time, a novel electrical circuit is preferably used to read out the mechanical deflection of the cantilever. The data processing device contains an evaluation unit for acquiring, processing and storing the values acquired by the sensor during the measuring mode and controls the generation of the direct and / or alternating voltage that is used to operate the first section.

According to the invention, the operation of the cantilever according to the invention comprises a sequential (successive) actuation and reading with the same piezo-resistive element (piezoresistor) which is integrated in the first section in the cantilever or is arranged on it. Surprisingly, it has been shown that, despite the extensive heat conduction processes that are necessary for the sensor to function (the third section works completely passively, without its own heat source or sink), the heat conduction processes run fast enough to set the cantiolever vibrating at a frequency, which allows measurements to be taken in both static and dynamic modes.

Since the piezo-resistive element is an electrically conductive medium, the generated thermal energy is concentrated in it due to its Joule heating. Due to the high speed of thermal energy generation and the low thermal mass, the entire volume of the piezoresistor is responsible for energy generation. The thermal energy is then spread through diffusion in the rest of the cantilever until a thermal equilibrium is reached.

wobei T_base die Temperatur an der Cantilever-Basis ist, I die Länge des Piezowiderstands ist und $$C_1=\frac{\delta V^2{A}_{prz}}{{\rho }_{eT0}{k}_{eff}{L}_{prz}{V}_{prz}}$$

und $$C_2=\frac{\left(1-\delta T_0\right)V^2{A}_{prz}}{{\rho }_{eT0}{k}_{eff}{L}_{prz}{V}_{prz}}$$

ist, wobei 6 der Widerstandstemperaturkoeffizient ist, V die angelegte Spannung, ρ_eT0 der elektrische Widerstand bei einer definierten Temperatur T0, k_eff die effektive Wärmeleitfähigkeit und L_prz der Gesamtstrompfad ist und A_prz die Querschnittsfläche und V_prz das gesamte Cantilever-Volumen (einschließlich der Piezowiderstände) ist.When the piezo-resistive element and the rest of the cantilever are in thermal equilibrium, the maximum temperature of the cantilever corresponds to that generated in the piezoresistor: $$T_{\text{Max}}=\frac{T_{base}}{2}\left[e^{\sqrt{\mathrm{C.}1l}}+e^{-\sqrt{\mathrm{C.}1l}}\right]+\frac{{\mathrm{C.}}_2{\mathrm{<figure-callout\; id="2"\; label="l"\; filenames="DE102017202455B4\_0009.png,DE102017202455B4\_0010.png"\; state="\{\{state\}\}">l</figure-callout>}}^2}{2}$$

where T _{base is} the temperature at the cantilever base, I is the length of the piezoresistor, and $${\mathrm{C.}}_1=\frac{\delta {\mathrm{V.}}^2{\mathrm{A.}}_{prz}}{{\rho }_{eT0}{k}_{eff}{\mathrm{L.}}_{prz}{\mathrm{V.}}_{prz}}$$

and $${\mathrm{C.}}_2=\frac{\left(1-\delta T_0\right){\mathrm{V.}}^2{\mathrm{A.}}_{prz}}{{\rho }_{eT0}{k}_{eff}{\mathrm{L.}}_{prz}{\mathrm{V.}}_{prz}}$$

where 6 is the temperature coefficient of resistance, V is the applied voltage, ρ _{eT0 is} the electrical resistance at a defined temperature T0, k _{eff is} the effective thermal conductivity and L _{prz is} the total current _{path and A prz is} the cross-sectional area and V _{prz is} the total cantilever volume (including the piezoresistors).

und $$K_z=\frac{\left({\alpha }_1-{\alpha }_2\right)\left(t_1+t_2\right)\frac{1}{t_2^2}}{4+6\left(\frac{t_1}{t_2}\right)+4{\left(\frac{t_1}{t_2}\right)}^2+{\left(\frac{t_1}{t_2}\right)}^3\left(\frac{E_1}{E_2}\right)+\left(\frac{t_2{E}_2}{t_1{E}_1}\right)}$$

wobei t die Schichtdicke, α der thermische Ausdehnungskoeffizient und E das Elastizitätsmodul ist.On the other hand, the maximum displacement z _T at the free end of the cantilever with a certain length I at a constant ambient temperature T is: $$z_T=\mathrm{3rd}{K}_{\mathrm{<figure-callout\; id="2"\; label="z\; T\; l"\; filenames="DE102017202455B4\_0009.png,DE102017202455B4\_0010.png"\; state="\{\{state\}\}">z</figure-callout>}}T{l}^{}{}^2$$

and $$K_z=\frac{\left({\alpha }_1-{\alpha }_2\right)\left({\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="DE102017202455B4\_0010.png"\; state="\{\{state\}\}">t</figure-callout>}}_1+t_2\right)\frac{1}{{\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="DE102017202455B4\_0009.png,DE102017202455B4\_0010.png"\; state="\{\{state\}\}">t</figure-callout>}}_2^2}}{\mathrm{4th}+\mathrm{6th}\left(\frac{t_1}{t_2}\right)+\mathrm{4th}{\left(\frac{t_1}{t_2}\right)}^2+{\left(\frac{t_1}{t_2}\right)}^{\mathrm{3rd}}\left(\frac{{\mathrm{E.}}_1}{{\mathrm{E.}}_2}\right)+\left(\frac{{\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="DE102017202455B4\_0009.png,DE102017202455B4\_0010.png"\; state="\{\{state\}\}">t</figure-callout>}}_2{\mathrm{E.}}_2}{{\mathrm{<figure-callout\; id="1"\; label="t"\; filenames="DE102017202455B4\_0010.png"\; state="\{\{state\}\}">t</figure-callout>}}_1{\mathrm{E.}}_1}\right)}$$

where t is the layer thickness, α is the coefficient of thermal expansion and E is the modulus of elasticity.

• • Modus 1: Anregung: während dieses Betriebsmodus wird die Versorgungsdiagonale der Wheatstone-Brücke sowohl mit Gleichspannung als auch mit Wechselspannung versorgt. Anstelle der Wechselspannung kann alternativ auch eine gepulste Gleichspannung eingesetzt werden. Die Ausgangsspannung der Brücke wird nicht gemessen.
• • Modus 2: Messung: während dieses Betriebsmodus wird die Brücke nur mit Gleichspannung versorgt. Die Ausgangsspannung der Brücke wird gemessen und für die Ermittlung der Wechselwirkung zwischen der Spitze des Cantilevers und der Oberfläche der Probe verwendet.

In a particularly preferred embodiment, a bimetallic resonance beam as a cantilever with or without an integrated piezo-resistive Wheatstone bridge is excited in the first section with a special algorithm in which two operating modes can be distinguished:

• • Mode 1: Excitation: during this operating mode, the supply diagonal of the Wheatstone bridge is supplied with both direct voltage and alternating voltage. Instead of the alternating voltage, a pulsed direct voltage can alternatively be used. The output voltage of the bridge is not measured.
• • Mode 2: Measurement: during this operating mode, the bridge is only supplied with direct voltage. The output voltage of the bridge is measured and used to determine the interaction between the tip of the cantilever and the surface of the sample.

wobei R_H der Widerstand des integrierten Piezo-Resistors ist.During mode 1, a current flows that consists of direct and alternating current components (AC and DC). The Wheatstone bridge causes alternating Joule heating or power loss of the piezoresistors integrated in the first section of the cantilever. The induced mechanical stress in the multilayer material of the first and third sections causes the movement of the cantilever. The power loss is calculated using the following formula: $$\begin{array}{l}{P}_{prz}=\frac{1}{{\mathrm{R.}}_{prz}}{\left[U_{dc}+U_{ac}\text{sin}{\omega }_{prz}t\right]}^2=\frac{1}{{\mathrm{R.}}_{prz}}\left[\left(U_{dc}^2+\frac{1}{2}{U}_{ac}^2\right)+2U_{ac}{U}_{dc}\text{sin}{\omega }_{prz}t-\left(\frac{1}{2}{U}_{a\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="DE102017202455B4\_0009.png,DE102017202455B4\_0010.png"\; state="\{\{state\}\}">c</figure-callout>}}^2\text{cos}\left(2{\omega }_{prz}t\right)\right)\right]=\\ =\left[P_{dc}+P\left({\omega }_H\right)+P\left(2{\omega }_{prz}\right)\right]\end{array}$$

where R _{H is} the resistance of the integrated piezo resistor.

The maximum oscillation amplitude is reached when the mechanical system is in resonance, i.e. the excitation generator is adapted to the resonance frequency of the cantilever. The oscillation amplitude depends to a large extent on the interaction between the tip of the cantilever and the effect on the tip of the external forces (on the scanned material surface or on the deposited molecules - in connection with the application). The forces cause a change in some parameters of the cantilever. For example, the closer the tip is to the scanned surface, the stronger the damping of the oscillation, which leads to a drastic decay of the oscillation amplitude. In a further exemplary application, the change in the sensor mass (especially at the end of the third section) as a result of deposited or removed material leads to a change in the resonance frequency of the sensor.

During mode 2, the output from the Wheatstone bridge is used to determine the actual amplitude and / or oscillation frequency. The bridge is only supplied with direct voltage and the alternating voltage is switched off. The output voltage of the measuring diagonal of the Wheatstone bridge is then proportional to the oscillation amplitude of the cantilever.

wobei fsw die Schaltfrequenz, f_AC die Frequenz der Wechselstromkomponente, n_act die Periodenanzahl der Wechselstromkomponente in dem Betriebsmodus der Anregung und n_meas Periodenanzahl der Wechselstromkomponente in dem Betriebsmodus Detektion ist.The switching frequency between the two operating modes can vary in a relatively wide range, preferably in the range from 1/2 to 50 periods of the alternating current component that supplies the bridge. A further reduction in the switching frequency between the two modes is possible up to the specific limit at which the damping of the oscillation amplitude causes an increase in the measurement errors. $$f_{\mathrm{S.}\mathrm{W.}}=\frac{f_{\mathrm{A.}\mathrm{C.}}}{\left(n_{act}+n_{meas}\right)}$$

where fsw is the switching frequency, f _{AC is} the frequency of the alternating current component, n _{act is} the number of periods of the alternating current _{component in the excitation operating mode and n meas is the number of} periods of the alternating current component in the detection operating mode.

• 1a und 1b: schematische Darstellung der erfindungsgemäßen Lösung in beiden Betriebsmodi mit einem Cantilever, bei dem der zweite Abschnitt zwischen erstem Abschnitt und dritten Abschnitt entlang der Längserstreckung des Cantilevers angeordnet ist.
• 1c und 1d: schematische Darstellung der erfindungsgemäßen Lösung in den beiden Betriebsmodi mit einem Cantilever, bei dem der zweite Abschnitt (Abschnitt 2) sich teilweise oberhalb des ersten Abschnitts erstreckt
• 2 : Zeitdiagramm der anliegenden Steuerspannung
• 3: Blockdiagramm eines Anwendungsbeispiels der Erfindung für ein AFM
• 4: beispielhaftes elektrisches Schaltbild für thermisches Aktuieren und Auslesen der Verbiegung des Cantilevers

The invention is explained in more detail below with reference to drawings. It shows:

• 1a and 1b : Schematic representation of the solution according to the invention in both operating modes with a cantilever, in which the second section is arranged between the first section and the third section along the longitudinal extension of the cantilever.
• 1c and 1d : Schematic representation of the solution according to the invention in the two operating modes with a cantilever, in which the second section (section 2 ) extends partially above the first section
• 2 : Time diagram of the applied control voltage
• 3rd : Block diagram of an application example of the invention for an AFM
• 4th : Exemplary electrical circuit diagram for thermal actuation and readout of the bending of the cantilever

In 1a and 1b the solution according to the invention is shown schematically, wherein in 1a the operating mode of the excitation is shown and in 1b the mode of operation of the measurement. In 1a are also the first section 1 , the second section 2 and the third section 3rd , the cantilever 4th marked. A MEMS with a bi-material structure made of materials with different thermal expansion coefficients α ₁ and α _{2 is shown} in the third section 3rd . The current flows through the piezoresistive layer, whereby Joule heat is dissipated. This heat is passed through the second section 2 headed into the third 3. Here a mechanical tension is induced, which causes a deflection of the cantilever 4th causes.

In the "Measurement" operating mode of the same bimaterial structure ( 1b) the applied current is used to reduce the deflection of the cantilever caused during the excitation time interval 4th to eat.

• • Im Betriebsmodus „Anregung“ ist die angelegte Spannung eine Überlagerung von Gleich- und Wechselstrom-Komponenten (AC und DC);
• • Im Betriebsmodus „Messung“ hat die angelegte Spannung nur eine GleichstromKomponente.

2 shows the graphical representation of a timing used in the present invention:

• • In the "Excitation" operating mode, the applied voltage is a superposition of direct and alternating current components (AC and DC);
• • In the "Measurement" operating mode, the applied voltage has only one direct current component.

The switching frequency between the first and the second operating mode is preferably in the range between less than one and several periods of the alternating current component. The switching frequency (that is, the frequency with which the two operating modes are switched over) is particularly preferably between 0.5 periods and 100 periods, very particularly preferably between one period and 10 periods. A switching frequency between one period and five periods has proven to be extremely favorable. The periods of time that the electrical circuit remains in each of the two operating modes are preferably identical. In a further preferred embodiment, the electrical circuit can remain in one of the two operating modes for longer than in the other. In particular, it is preferred to make the periods in which the electrical circuit is in an operating mode dependent on the desired curvature of the cantilever or the achievement of a stable resonance frequency or other conditions adapted to the measurement task.

In 3rd Figure 3 shows a block diagram of an example of an application of the invention to an AFM. It is shown that an additional electrical circuit is not required for the excitation (just as no additional lines are required for a built-in actuator or piezo actuator). Common electrical lines are used for actuating and reading out the cantilever. The cantilever is made from the silicon cantilever chip 5 held.

The main advantage of the present invention is therefore also the significant reduction in the number of internal connections when used in multi-sensor arrangements (multi-sensor arrays).

The 4th shows an example of a possible basic circuit diagram for realizing the arrangement according to the invention. The measurement setup is shown schematically. The actuator and the measured value recording circuit (Wheatstone bridge) are implemented in the case shown by a common sensor circuit.

The arrangement according to the invention allows the number of connections required to be reduced to two. The same circuit, thanks to the mode switch used, enables the use of cantilevers with separate actuation and read-out.

List of reference symbols

1

first section of the cantilever

2

second section of the cantilever

3

third section of the cantilever

4th

Cantilever

5

Cantilever chip

Claims (11)

MEMS- or NEMS-based sensor, which is designed as a cantilever, this being constructed in the form of a strip and having at least three sections (1, 2, 3), wherein • the first section (1) is fastened on one side in a holder or wall and at the opposite end or in its vicinity at least one electrical circuit which has a piezoresistive element for heating all three sections (1, 2, 3) and for measurement carries, which is electrically coupled to a voltage supply and an evaluation unit, and • the second section (2) directly adjoins the first section (1) and is suitable for transferring heat to the third section (3), and • the third section (3) is at least partially multi-material, at least bi-material, designed so that the top and bottom of the third section (3) have different thermal expansion coefficients, so that a heating of the third section (3) whose curvature results and causes a deflection of the cantilever, and • a data processing device controls the generation of DC and AC voltage to operate the first section (1) and the data processing device also has an evaluation unit for capturing, processing and storing the values captured by the sensor, with both actuation and reading of the deflection of the cantilever takes place with the piezoresistive element. MEMS- or NEMS-based sensor according to Claim 1 , characterized in that the electrical circuit has a piezo-resistive Wheatstone bridge. MEMS- or NEMS-based sensor according to Claim 1 , characterized in that the first, the second and the third section (1, 2, 3) are arranged one behind the other along the leaf-spring-like structure of the cantilever (4). MEMS- or NEMS-based sensor according to Claim 1 , characterized in that the second section (2) is designed as an electrically insulating cover which at least partially covers the first section (1) and a material layer of the third section (3) extends over this cover. MEMS- or NEMS-based sensor according to Claim 1 characterized in that it is designed as an AFM micro-leaf spring and on the end of the third section (3) facing away from the second section (2) has a measuring tip which faces the surface of a material to be examined. MEMS- or NEMS-based sensor according to Claim 1 characterized in that it is designed as a mass-sensitive sensor or as an acceleration sensor. Method for operating a MEMS- or NEMS-based sensor according to one of the Claims 1 to 4th characterized in the dynamic mode is that a superimposed with a DC biased AC voltage is in a first mode of operation applied to the electrical circuit, the piezoresistive element is heated by the flow of current, and the third region of the sensor is heated due to heat conduction and curves due to the heating and The cantilever is deflected and, in a second operating mode, a direct voltage is applied to the electrical circuit and the output signal of the sensor is detected as a change in resistance of the electrical circuit. Procedure according to Claim 7 characterized in that the alternating voltage is implemented as a pulsed direct voltage. Procedure according to Claim 7 characterized in that the vibration of the free end of the third section (3) is influenced by external forces. Procedure according to Claim 7 characterized in that the free end of the third section (3) is changed in its resonance frequency by a change in mass due to accumulation or erosion. Method according to one of the Claims 7 to 10 , characterized in that the switching frequency between the first and the second operating mode is in the range between less than one and several periods of the alternating current component.

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