JP4758405B2 - Sensor element and physical sensor device - Google Patents

Description

  The present invention relates to a sensor element and a physical sensor device including the sensor element.

  Conventionally, as this type of device, an atomic force microscope device that detects an atomic force by vibrating a probe having a probe by a comb electrode actuator is known (for example, see Patent Document 1). In this microscope apparatus, the probe is driven in the z direction so that the vibration state of the comb electrode actuator is kept constant, and unevenness information on the sample surface is obtained from the drive amount data.

JP 2007-93231 A

  However, if the probe that vibrates with the above-described comb electrode actuator is used to detect a sample in the liquid, the vibration of the probe is limited by the resistance of the liquid, and the detection sensitivity when contacting the sample decreases. There was a drawback that it would.

The sensor element according to the first aspect of the present invention has first and second comb electrodes formed in opposite directions, and is a vibration portion that is elastically supported by the base portion and vibrates in the direction of the concave and convex portions of the comb teeth. And a third comb electrode that meshes with the first comb electrode, a fixed portion fixed to the base portion, and a fourth comb electrode that meshes with the second comb electrode, And a movable portion that is elastically supported by the base portion and is displaced in the concave-convex direction of the comb teeth by the action of an external force.
According to a second aspect of the present invention, in the sensor element according to the first aspect, the fixed portion, the vibrating portion, and the movable portion are integrally formed on the SOI wafer by a photolithography method.
According to a third aspect of the present invention, there is provided a physical sensor device comprising: the sensor element according to the first or second aspect; and a voltage for driving the vibration unit to vibrate between the third comb electrode and the fourth comb electrode. And a detection unit that detects the displacement of the movable unit based on a change in the vibration state of the vibration unit.
According to a fourth aspect of the present invention, in the physical sensor device according to the third aspect, the detection unit detects the displacement of the movable unit based on a change in immittance of the system including the sensor element and the drive unit due to a change in the vibration state. It is what I did.
According to a fifth aspect of the present invention, in the physical sensor device according to the third aspect, the drive unit causes self-excited oscillation by using an electrical equivalent circuit of the sensor element as a feedback circuit, and the vibration unit is caused by the self-excited oscillation. An amplifier to be oscillated is provided.
According to a sixth aspect of the present invention, in the physical sensor device according to the fifth aspect, the displacement of the movable portion is detected based on a change in vibration frequency as a change in vibration state.

  ADVANTAGE OF THE INVENTION According to this invention, the detection sensitivity at the time of detecting the displacement of a movable part can be improved more.

  Hereinafter, the best mode for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of an atomic force microscope apparatus (hereinafter referred to as an AFM apparatus) using a sensor element according to the present invention. The AFM apparatus 100 vibrates the sensor 1, the support 4 to which the sensor 1 is fixed, the piezo actuator 5 that displaces the support 4 in the z direction, the stage 6 that moves the sample S in the xy direction, and the sensor 1. A drive unit 7 for driving, a control device 8, a stage drive control unit 9, and a monitor 10 are provided. The drive unit 7 is provided with a DC power supply 71 and an AC power supply 72. The control device 8 is provided with an admittance detector 81 and a control calculation unit 82 that performs various calculations and controls the entire apparatus.

  FIG. 2 is a perspective view showing the configuration of the sensor 1. The sensor 1 includes a base 14, a fixed part 11 formed on the base 14, a first movable part 12, and a second movable part 13. The movable portion 12 is elastically supported by four beams 121 formed between the movable portion 12 and the base 14. Similarly, the movable part 13 is elastically supported by four beams 131 formed between the movable part 13 and the base 14. Therefore, the movable parts 12 and 13 have a structure that can be displaced in the z direction.

The sensor 1 shown in FIG. 2 is integrally manufactured by a semiconductor microfabrication technique using a substrate having a three-layer structure composed of two upper and lower silicon layers sandwiching an insulating layer, for example, an SOI (Silicon on Insulator) wafer. The SOI wafer is a substrate having a three-layer structure in which an insulating SiO ₂ layer is sandwiched between two Si layers. The base 14 of the sensor 1 is formed by a thick lower Si layer, and the fixed portion 11, the movable portions 12, 13, and the beams 121, 131 are formed by an upper Si layer that is thinner than the lower Si layer. The SiO ₂ layer remains so as to be interposed between the base 14 and the fixed portion 11 and between the connection portions 121a and 131a of the beams 121 and 131 and the base 14.

  The portions of the fixed portion 11 and the movable portion 13 that face each other have a comb-like uneven shape. The concave and convex portions of the fixed portion 11 and the movable portion 13 are such that the convex portion 112 of the fixed portion 11 enters the concave portion 133 of the movable portion 13 and the convex portion 132 of the movable portion 13 enters the concave portion 113 of the fixed portion 11. And meshing with a gap. Similarly, the opposing part of the movable part 12 and the movable part 13 has a comb-like uneven shape, and the convex part 122 of the movable part 12 enters the concave part 135 of the movable part 13 and the convex part 134 of the movable part 13. Is engaged with a gap so that the material enters the recess 123 of the movable portion 12. A probe 124 protruding in the x direction is formed on the movable portion 12, and a probe 125 is formed at the tip of the probe 124. By observing the surface of the sample S with this probe by AFM, the surface shape of the sample S can be measured.

A DC power supply 71 and an AC power supply 72 of the drive unit 7 are connected in series, and a voltage obtained by superimposing the AC voltage e on the DC voltage E ₀ is applied between the fixed unit 11 and the movable unit 12. Therefore, an electric field is formed in the gap space of the comb-shaped portion between the fixed portion 11 and the movable portion 13 and in the gap space of the comb-shaped portion between the movable portion 12 and the movable portion 13, and the angular frequency ω of the AC voltage e is increased. Accordingly, the movable parts 12 and 13 vibrate. The sensor 1 constitutes an electrostatic actuator capable of driving the movable parts 12 and 13 by changing the applied voltage, and changes in the masses and spring constants of the movable parts 12 and 13, that is, changes in the mechanical system, It can be measured as a change in electrical admittance. In addition, in order to distinguish from the conventional comb-tooth actuator, the electrostatic actuator of this application will be called a coupled comb-tooth actuator.

  In the present embodiment, the spring constant of the movable part 12 is set to be smaller than the spring constant of the movable part 13, and the movable part 13 is excited at or near the resonance frequency of the movable part 13. The movable part 13 is in a floating state in terms of direct current. On the other hand, the movable portion 12 is not excited because the resonance frequency is away from the driving frequency, and is displaced statically or quasi-statically by an external force.

  The admittance detector 81 shown in FIG. 1 detects the admittance of the electrical / mechanical coupling system, and the detection data is input to the control calculation unit 82. When the sample surface is scanned with the probe 125, the probe 125 is attracted to the sample surface by the atomic force between the probe 125 and the sample surface atoms, and the movable portion 12 is displaced in the x direction. The displacement of the movable portion 12 is detected by an admittance detector 81 as a change in admittance.

  The control calculation unit 82 drives the piezo actuator 5 in the x direction so that the change in admittance is zero, that is, the displacement of the movable unit 12 is zero. Then, the sample S is moved in the xy direction by the stage 6 while performing such control. The control signal at this time represents the surface state (unevenness) of the sample S, and an observation image is displayed on the monitor 10 based on the control signal.

FIG. 3 is a diagram illustrating an analysis model of the sensor 1. As described above, the voltage is applied between the fixed portion 11 and the movable portion 12, and the movable portion 13 is in a DC floating state. In FIG. 3, m ₁ and m ₂ are the masses of the movable parts 12 and 13, respectively, k ₁ and k ₂ are the spring constants of the movable parts 12 and 13, and r ₁ and r ₂ are the masses of the movable parts 12 and 13, respectively. Mechanical resistance (including fluid resistance associated with vibration). Moreover, X ₁ is the initial amount of overlap comb between the fixed portion 11 and movable portion 13, X ₂ is the initial amount of overlap comb between the movable portion 12 and the movable portion 13. ω is the angular frequency of the AC voltage e.

The Lagrangian L and the dissipation function F of the electromechanical coupling system as shown in FIG. 3 are expressed as the following equations (1) and (2), respectively. In Expression (1), x ₁ and x ₂ represent displacements of the movable parts 12 and 13 in the x direction. C (x ₁ , x ₂ ) is a capacitance C ₁ (x ₁ , x ₂ ) between the fixed portion 11 and the movable portion 13 and a capacitance C ₂ (x ₁ ) between the movable portions 12 and 13. , x ₂ ) and the combined capacity, which is expressed by the equation (3). R in the formula (2) is an electric resistance. In equation (3), n is the total number of protrusions, b is the y-direction gap dimension between the comb teeth shown in FIG. 3, d is the z-direction thickness dimension of the comb teeth (see FIG. 2), and ε ₀ is a vacuum. Is the dielectric constant. The stray capacitance C _stray will be described assuming that C _stray = 0.

From the Lagrangian L and the dissipation function F described above, Lagrangian equations relating to the external forces f ₁ and f ₂ of the mechanical system and the AC voltage e of the electrical system as shown in (4), (5) and (6) are derived. Expressions (4) and (5) are expressions relating to the movable part 12 and the movable part 13, respectively, and Expression (6) is an expression relating to the electric circuit. F ₁ and F ₂ indicate forces due to the DC bias voltage E ₀ .

Here, assuming that the magnitude of the AC voltage e at the angular frequency ω is sufficiently small with respect to the bias voltage E ₀ and R≈0, Expressions (4) to (6) are expressed by Expression (7). It is expressed by the linear approximation basic equation. In Equation (7), three equations are expressed using a matrix. Further, C ₀ , C ₁ , and C ₂ are capacitances when the shift amounts x ₁ and x ₂ are zero, and are expressed by the following equations. Both the second term of the external force f ₁ expression and the first term of the external force f ₂ expression enclosed by the broken line include a factor of (1 / C ₁ −1 / C ₂ ). It can be seen that the movable part 12 and the movable part 13 are coupled to each other via an electric field.

Regarding the expression (7), when the external forces f ₁ and f ₂ are set to zero and the voltage e is obtained using the Cramer formula, the following expression (8) is obtained. Note that when obtaining the equation (8), the calculation is performed by regarding the mechanical resistances r ₁ and r ₂ as zero. By using the equation of voltage e, impedance Z is expressed as equation (9). Note that | H | appearing in the denominator of Expression (8) is a determinant relating to the matrix on the right side of Expression (7).

Further, the admittance Y is expressed as in Expression (10). In the equations (8) to (10), the constants A, B, D, and G are as follows, and ω ₁ ² = k ₁ / m ₁ and ω ₂ ² = k ₂ / m ₂ .

  As described above, a change in the vibration state of the sensor 1, that is, a change in the vibration system can be detected by measuring a change in the impedance Z or admittance Y of the electric system (more generally, a change in immittance). The absolute value of admittance Y can be directly measured by an impedance analyzer. Note that the admittance detection method is described in detail in Japanese Patent Application Laid-Open No. 2007-93231, and the description thereof is omitted here.

  As described above, in this embodiment, the spring constant of the movable part 12 is set smaller than the spring constant of the movable part 13. For this reason, even when the movable portion 13 is excited at or near the resonance frequency, the movable portion 12 does not follow the vibration of the movable portion 13 and remains almost stationary unless an external force is applied. When the movable part 12 is displaced by an external force, the strength of the coupling between the movable part 12 and the movable part 13 changes, and the change appears as a change in the electrical system. By detecting this electrical fluctuation as an admittance change by an electronic circuit, for example, the displacement of the movable portion 12 can be detected.

When the spring constant of the movable part 12 is small and the movable part 12 does not vibrate, the Lagrangian L and the dissipation function F are established with v _x = 0. Equations (11) and (12) are obtained, respectively. Then, when the Lagrangian L and the dissipation function F are solved for x1, v2, and e, the movable part 12 (f ₁ ), the movable part 13 (f ₂ ), and the electric circuit system (i) as given by the equation (13) A matrix corresponding to each system is obtained.

In this case as well, as in the case described above, regarding the equation (13), when the external forces f ₁ and f ₂ are set to zero and the voltage e is obtained using the Cramer formula, the following equation (14) is obtained. By using the equation of voltage e, impedance Z is expressed as equation (15).

  FIG. 4 shows experimental data, (a) shows data when the portion of the coupled comb actuator of FIG. 2 is replaced with a conventional comb actuator, and (b) shows the coupled comb actuator. The case of the sensor 1 using this is shown. This experimental data shows that the probe 125 is not brought into contact with the Si substrate while the comb actuator is excited by the drive unit 7 and the movable unit 12 is displaced by pushing the probe 125 toward the substrate. Show. 4A shows the relationship between the drive frequency and the admittance value | Y | measured by the impedance analyzer. In the case of FIG. 4B, the drive frequency And the current value measured by the lock-in amplifier.

  In the case of the conventional comb-shaped actuator shown in FIG. 4A, a typical admittance curve is obtained in a free state (curve a) in which the probe 125 is not brought into contact. When displacement occurs in 12, the shape of the waveform of the peak portion is crushed as shown by curves b to d. This occurs because the vibration of the movable portion of the comb-shaped actuator is restricted when the probe 125 comes into contact with the substrate. The larger the amount of pressing, the more the waveform collapses. Therefore, there has been a drawback that the sensitivity of displacement measurement is reduced.

  On the other hand, in the measurement result shown in FIG. 4B, even when the probe 125 is in contact with the substrate and the movable portion 12 is displaced (b to d), the waveform collapse is not measured, and the contact amount (= displacement amount). The change in capacitance when changing is clearly shown in the waveform. Therefore, the displacement of the movable part 13 can be measured with high sensitivity. The contact amount is the same as the push amount. In the data shown in FIG. 4B, the current value greatly changes with respect to the change in the contact amount in the frequency range of f = 10.8 kHz to 11 kHz. Therefore, the movable part 13 is excited in this frequency range (for example, 10.9 kHz), and the displacement amount of the movable part 12 can be estimated with high accuracy from the amount of change in the current value. Of course, the correspondence relationship between the displacement amount and the external force applied to the movable portion 12 may be prepared in advance as a table, and the magnitude of the external force may be obtained from the displacement amount and the table.

  FIG. 5 is a diagram showing a case where the sensor 1 is applied to a touch sensor. FIG. 5A is a diagram showing a case where a conventional comb actuator is used as a touch sensor, and FIG. 5B shows a case where the sensor 1 is applied as a contact detection sensor. In the case shown in FIG. 5A, an arm 138 for detecting contact is formed on the movable portion 13 that is vibrating. When the arm 138 comes into contact with the sample 50, the vibration state of the movable portion 13 changes. By measuring the change in the vibration state as an admittance change, the contact with the sample 50 can be detected.

  On the other hand, when the coupled comb shown in FIG. 5B uses an actuator, a contact arm 128 is formed on the movable portion 12 in a stationary state. When the arm 128 comes into contact with the sample 50 and the movable part 12 is displaced in the direction of the movable part 13, the coupling strength of the electrode part changes. As a result, it is possible to detect that the vibration state of the movable portion 13 has changed and the arm 128 has contacted the sample 50.

  However, when measuring the sample 50 in water, in the example shown in FIG. 5A, the vibration state of the movable portion 13 changes due to the viscous resistance of water when a part of the arm 138 enters the water. For this reason, it is difficult to determine whether the change in admittance is caused by static displacement of the vibrating movable portion 13 or by viscous resistance. Furthermore, since the vibration is limited by the viscous resistance, the detection sensitivity is also lowered.

  On the other hand, in the example shown in FIG. 5B, since the movable portion 12 is not vibrating, the vibration state of the movable portion 13 does not change even when a part of the arm 128 enters the water. When the arm 128 comes into contact with the sample 50 and the movable part 12 is displaced to the left in the figure, the coupling strength of the electrode part changes and is measured as an admittance change. Therefore, even when the arm 128 is in water or in the atmosphere, contact can be detected with the same detection accuracy.

  Another example of using the sensor 1 as a physical sensor is a membrane type pressure sensor. In this case, the movable part 12 is provided so as to be in close contact with the membrane, and the deformation of the membrane due to pressure is detected as the displacement of the movable part 12.

[Modification]
FIG. 6 is a diagram showing a modification of the present embodiment, and is a block diagram showing the sensor 1 and the drive unit 7. In the above-described embodiment, the drive unit 7 is provided with the AC power source 72 to excite the movable unit 13. However, in the modification, the coupled comb actuator (electrostatic actuator) of the sensor 1 is a resonance circuit, and its output Is fed back via an amplifier 23 to cause self-oscillation. And the displacement of the movable part 12 is detectable by detecting the change of the vibration state of the sensor 1 which carries out self-oscillation as a frequency change, for example.

In FIG. 6, the sensor 1 is drawn based on the L, C, R resonance circuit that it inherently has. That is, the sensor 1 is shown as a passive two-terminal element. The drive unit 7 includes an amplifier 73 having the sensor 1 as a feedback circuit and a DC power source 71 that applies a DC bias voltage E ₀ to the sensor 1.

The internal resistance of the DC power source 71 for applying a DC bias voltage E ₀ is so small, it is necessary to prevent the feedback signal does not pass through the DC power supply 71 side. Therefore, in the present embodiment, the high resistance R _high is inserted in series with the DC power supply 71. This prevents the DC power supply 71 from affecting the feedback path. Further, at the same time that the DC power supply 71 is brought into a floating state as a DC circuit from the circuit system including the amplifier 73, the blocking capacitor C _B is prevented so that a DC voltage is not directly applied to the terminal (output terminal & input terminal) of the amplifier 73. Is inserted.

  By configuring an oscillation circuit with the amplifier 73 and the sensor (electrostatic actuator) 1, when a voltage is applied to the sensor 1, the movable portion 13 is self-excited at a resonance frequency. When a displacement occurs in the movable part 12 due to an external force, R, C, and L of the equivalent circuit change, and the amplitude and frequency of vibration change. An output signal from the amplifier 73 is input to a detection unit 60 that detects a change in vibration state as a frequency change. For example, the detection unit 60 is provided with a frequency comparator, and the frequency of the signal output from the amplifier 23 is monitored by the frequency comparator.

  In the embodiment described above, the change in the vibration state is detected as a change in admittance, but it may be detected as a change in current value as shown in FIG. In the correspondence between the embodiment described above and the elements of the claims, the movable portion 12 constitutes a movable portion, the movable portion 13 constitutes a vibrating portion, and the base 14 constitutes a base portion. The above description is merely an example, and the present invention is not limited to the above embodiment as long as the characteristics of the present invention are not impaired.

It is a figure which shows schematic structure of the atomic force microscope apparatus using the sensor element which concerns on this invention. 1 is a perspective view showing a configuration of a sensor 1. FIG. It is a figure which shows the analysis model of the sensor 1 (coupled comb-tooth actuator). It is a figure which shows experimental data, (a) shows the case of the sensor using the conventional comb-tooth actuator, (b) shows the case of the sensor 1 using a compound comb-tooth actuator. It is a figure which shows the case where the sensor 1 is applied to a touch sensor, (a) shows the case where the conventional comb-tooth actuator is used, (b) shows the case where the sensor 1 is applied to a touch sensor. It is a block diagram which shows a modification.

Explanation of symbols

  1: sensor, 5: piezo actuator, 6: stage, 7: drive unit, 8: control unit, 9: stage drive control unit, 11: fixed unit, 12, 13: movable unit, 14: base, 50: sample , 60: detection unit, 71: DC power supply, 72: AC power supply, 73 amplifier, 81: admittance detector, 82: control calculation unit, 121, 131: beam, 128, 138: arm

Claims (6)

A vibrating portion having first and second comb electrodes formed in opposite directions, elastically supported by the base portion and vibrating in the uneven direction of the comb teeth;
A third comb electrode that meshes with the first comb electrode, and a fixed portion fixed to the base portion;
A fourth comb electrode that meshes with the second comb electrode, and a movable portion that is elastically supported by the base portion and is displaced in the concave and convex direction of the comb tooth by the action of an external force. A characteristic sensor element. The sensor element according to claim 1, wherein
A sensor element, wherein the fixed portion, the vibrating portion, and the movable portion are integrally formed on an SOI wafer by a photolithography method. The sensor element according to claim 1 or 2,
A drive unit that applies a voltage for driving the vibration unit to vibrate between the third comb electrode and the fourth comb electrode;
A physical sensor device comprising: a detection unit that detects a displacement of the movable unit based on a change in a vibration state of the vibration unit. The physical sensor device according to claim 3,
The detection unit detects a displacement of the movable unit based on a change in immittance of a system including the sensor element and the drive unit due to a change in the vibration state. The physical sensor device according to claim 3,
The physical sensor device, wherein the driving unit includes an amplifier that causes self-excited oscillation by using an electrical equivalent circuit of the sensor element as a feedback circuit and vibrates the vibrating unit by the self-excited oscillation. The physical sensor device according to claim 5,
The detection unit detects a displacement of the movable unit based on a change in vibration frequency as a change in the vibration state.

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