JP5363013B2 - Mechanical element and method for controlling vibration thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mechanical element increased in Q-value by controlling the vibration thereof by utilizing the piezoelectric and optical properties of the material thereof after the mechanical element is manufactured, and also to provide a method of controlling the vibration of the mechanical element. <P>SOLUTION: This mechanical element has a means having a beam formed of at least two layers with different characteristics and controlling the vibration of the beam by irradiating the beam with a laser beam of such a wavelength that provides the coefficients of absorption different from each other between the layers with different characteristics of the beam. Hence, the Q-value of the mechanical element having the beam manufactured by fine processing is easily controlled by adjusting the wavelength and the intensity of the emitted laser beam after the element is manufactured. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a micromechanical element or nanomechanical element such as a detector for detecting force or mass using vibration of a fine beam or a filter for selecting a frequency, and a method for controlling the vibration.

  A mechanical element including a beam manufactured by microfabrication is used as a high sensitivity detection element. An example of this element is created as follows. Using a microfabrication technique represented by lithography, a solid thin film such as silicon is processed into a beam shape to form a mechanical force detector beam. According to this mechanical force detector, a minute force applied to the detector is detected by detecting an elastic displacement of the beam electrically or optically. Such a mechanical force detector is widely used as an element for consumer equipment such as a cantilever or an acceleration sensor of a scanning probe microscope.

The sensitivity of the mechanical element used as the high sensitivity detection element is affected by the Q value. The Q value is a dimensionless number representing the vibration state, and is given as the energy stored in the system during one vibration period divided by the energy dissipated from the system. As a more intuitive interpretation, the maximum amplitude of the forced vibration at the resonance frequency ω ₀ is Q times the displacement due to the static forcing force. As shown in FIG. 3, the actually observed Q value is calculated from the resonance peak shape of the vibration spectrum, and is given by Q = ω ₀ / Δω using the resonance frequency and the half-value width Δω. A mechanical vibration system having a low Q value is a system in which dispersion of vibration energy is large, and vibration is damped (damped) within a short time. On the other hand, a mechanical vibration system having a high Q value is a system in which dispersion of vibration energy is small, and once the vibration is started, the vibration continues for a long time.

In general, a high detection sensitivity of a mechanical element used as a high sensitivity detection element is given by a high Q value in a resonance state. For example, it is known that the minimum detection force of a mechanical force detector is proportional to (1 / Q) ^0.5 (Non-Patent Document 1). That is, the larger the Q value, the smaller the force can be detected. Therefore, the improvement of the Q value is very important in the application of mechanical elements.

Physical Review, B, 69, 2004, 045403 (Physical Review B, Volume 69, 2004, 045403)

In general, the Q value of a mechanical element including a beam manufactured by microfabrication largely depends on the beam processing technique, the quality and shape of the beam, and is almost determined in the process of manufacturing the element. For this reason, an advanced manufacturing technique is required to obtain a high Q value. On the other hand, as a problem that cannot be avoided in processing, there is a decrease in the Q value accompanying the characteristics of the material itself constituting the beam, such as a thermal expansion effect. For this reason, there is a limit to increasing the Q value obtained by improving the manufacturing technique. In general, the Q value of a semiconductor cantilever is in the range of 10 ¹ to 10 ^{3 in} a room temperature atmosphere.

  The present invention solves the above-mentioned problems and, after fabrication of a mechanical element, utilizes the piezoelectric and optical properties of the material constituting the element to control the vibration of the mechanical element and improve the Q value of the element I will provide a.

In order to solve the above problems, a mechanical element of the present invention includes a beam and a light source each composed of at least two layers made of different semiconductors, wherein the semiconductor has piezoelectricity, and the light source is the different semiconductor. interior and be selectively with light having an excitation wavelength of one irradiating the beam, and the selectively excited one carrier from the semiconductor by the irradiation is generated, the carrier is the beam of the And generating polarization by spatially separating them, applying stress to the beam by generating a piezoelectric effect in the semiconductor, and warping of the beam due to the stress, and distortion corresponding to the warp of the beam. deformation potential effect appears gradient, the band gap changes in response to strain, by the band gap change, joined by additional force in the forward direction with respect to the vibration direction of the beam, the additional Forces by acting later in time with respect to the vibration of the beam acts positive feedback to the vibration of the beam, and performing and controlling the vibration of the beam.

  The mechanical element of the present invention may change the resonance frequency or Q value of the beam by irradiation with the laser beam, or may cause the beam to self-oscillate by irradiation with the laser beam. .

In order to solve the above problems, the method for controlling vibration of a mechanical element according to the present invention controls vibration of a mechanical element including a beam and a light source each composed of at least two layers made of different semiconductors. A method in which the semiconductor has piezoelectricity and the beam is irradiated with light having a wavelength that selectively excites one of the different semiconductors from the light source, and is selectively excited by the irradiation. A carrier is generated from the formed semiconductor, the carrier is spatially separated inside the beam to generate polarization, and the polarization generates a piezoelectric effect on the semiconductor to apply stress to the beam. phase and, the step of warping the beam is caused by the stress, deformation potential effect appears by the distortion gradient in accordance with the warp of the beam, the step of the band gap varies according to the strain, The serial band gap change, the step with respect to the vibration direction of the beam applied is additional force of the forward, and by the additional force acts temporally delayed with respect to the vibration of the beam, beams And a step of controlling the vibration of the beam including a step of applying positive feedback to the vibration.

  According to the present invention, the Q value of a mechanical element including a beam manufactured by microfabrication can be easily controlled by adjusting the wavelength and intensity of an irradiation laser after the element is manufactured. Further, according to the present invention, the mechanical element can be effectively cooled by laser irradiation even in a room temperature environment, and thermal noise can be easily reduced without using a cryogen such as liquid helium. Thereby, highly sensitive mechanical detection becomes possible. Furthermore, according to the present invention, it is possible to oscillate the mechanical element by laser irradiation without any other vibration drive mechanism, for example, an electric, magnetic, or dynamic drive mechanism. The mechanical element can be easily vibrated.

Next, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic diagram illustrating a method for controlling vibration of a mechanical element according to the first embodiment of the present invention.
In FIG. 1, a mechanical element 100 includes a cantilever 101, electrodes 102 and 103, a laser light source 105, and a piezoelectric vibration element 104. The cantilever 101 is lithographically processed based on a semiconductor multilayer structure formed by thin film crystal growth such as molecular beam epitaxy. As shown in FIG. 2, the semiconductor multilayer structure is composed of an Al _0.7 Ga _0.3 As layer 203, an undoped GaAs layer 202, and a Si-doped GaAs layer 201 provided on a semi-insulating GaAs substrate 204. The Al _0.7 Ga _0.3 As layer 203 is a sacrificial layer removed by selective etching, and the cantilever 101 composed of the n-type GaAs conductive layer 201 and the undoped GaAs layer 202 is formed on the sacrificial layer. . In the present embodiment, as shown in FIG. 1, the cantilever 101 is designed to face in the [110] direction within the (001) plane. The electrodes 102 and 103 are made of an alloy material, and are in ohmic contact with the n-type GaAs conductive layer 202 by annealing. The light source 105 is a titanium sapphire laser, and the wavelength can be adjusted in the range of 700 to 840 nanometers. The piezoelectric vibration element 104 is made of a piezoelectric ceramic material, and the piezoelectric vibration element 104 expands and contracts vertically by applying an AC voltage to the element.

Here, the principle of the present invention will be described.
If the beam is formed of a two-layer structure of an intrinsic semiconductor and an n-type semiconductor having a slightly smaller band gap energy than the intrinsic semiconductor, the energy band of the semiconductor is as shown in FIG. Electrons 403 below the conduction band 401 spill into the degenerate surface energy levels appearing on both surfaces of the beam, and negative charges are accumulated on the beam surface. On the other hand, positive charges due to a large number of impurities are accumulated below the conduction band 401 from which the electrons 403 have escaped, and the neutral state of the entire system is maintained in an equilibrium state.

When a beam having such a band structure is irradiated with laser light, if the excitation energy is greater than or equal to the band gap energy of the n-type semiconductor and less than the band gap energy of the intrinsic semiconductor, only the n-type semiconductor portion is selectively excited. Then, a carrier is generated (FIG. 4b). At this time, the holes 404 generated in the valence band 402 move toward both surfaces of the beam, and polarization in the thickness direction occurs due to spatial separation of the electrons 403 and the holes 304 (shielding polarization). Now, when the semiconductor constituting the beam has piezoelectricity, a piezoelectric effect is produced by this shielding polarization. That is, due to the polarization change caused by the carrier excitation, an effective electric field is applied in the thickness direction of the beam, and as a result, stress is applied to the beam. Due to this piezoelectric stress, a force F _SC for bending the beam acts (FIG. 4b). If no change occurs in the band structure with respect to the beam displacement, the same piezoelectric power F _SC acts on any bending state of the beam.

However, in practice, a deformation potential effect appears due to a strain gradient corresponding to the warp of the beam, and the band gap changes according to the strain. When the band gap on the n-type semiconductor side is narrowed by this deformation potential effect and the band gap on the intrinsic semiconductor side is widened, the light absorption in the n-type semiconductor region increases, and as a result, the shielding polarization also increases (FIG. 4c). . As a result, the force to bend the beam is also strengthened to F _SC + ΔF _SC (FIG. 4c). On the other hand, when considering the case where the beam is bent in the opposite direction, the deformation potential effect is the opposite effect, and the band gap on the n-type semiconductor side widens (FIG. 4d). For this reason, the light absorption of the n-type semiconductor region is reduced, and the shielding polarization is also reduced. Therefore, the force to bend the beam is also weakened to F _SC -ΔF _SC (FIG. 4d). That is, a force ΔF _{SC in the} reverse direction effectively acts on F _SC .

Considering the above-described effects with respect to the vibration state of the beam, an additional force ΔF _{SC in the} forward direction with respect to the vibration direction of the beam is applied by carrier excitation. When this force ΔF _SC acts with a time delay with respect to the change in the position of the beam, an effect of suppressing vibration damping of the beam is produced. That is, positive feedback acts on the vibration of the beam and the Q value is improved. On the other hand, if the beam has the property that the piezoelectric force acts in the opposite direction (−ΔF _SC ) due to the same shielding polarization, a force of −F _SC −ΔF _SC acts in the bent state of FIG. Further, in the state of FIG. 3d, a force of -F _SC + ΔF _SC acts. That is, in this case, an additional force ΔF _{SC in the} direction opposite to the vibration direction of the beam is applied by carrier excitation. Accordingly, when this force ΔF _SC acts on the beam position change with a time delay, an effect of promoting vibration damping of the beam is produced. That is, negative feedback acts on the vibration of the beam, and the Q value decreases.

  Whether positive or negative feedback acts on the vibration of the beam depends on the crystal orientation of the semiconductor constituting the beam. For example, in a zinc-blende structure (such as GaAs) having piezoelectricity, when the beam structure is formed in the (001) plane and the longitudinal direction of the beam is the [110] direction, positive feedback works. As shown in FIG. 5A, the Q value is improved. On the other hand, in the case of the [−110] direction in which the longitudinal direction of the beam is different by 90 degrees, negative feedback works and the Q value decreases as shown in FIG. On the other hand, when the longitudinal direction of the beam is the [100] direction which is 45 degrees different, the piezoelectric effect is canceled due to the symmetry of the crystal. Therefore, in this case, feedback does not work, and the Q value does not change as shown in FIG.

  Thus, the control of the Q value of the mechanical element in the present invention is based on appropriate selection of the longitudinal direction of the beam and the irradiation laser wavelength. In other words, the essence of the present invention is that by utilizing the difference in the light absorption coefficient of the constituent materials and selectively exciting only a part of the materials, the electric polarization state of the beam is changed, and the piezoelectric stress due to this is changed. By using it, the vibration of the mechanical element and the Q value can be controlled.

  Furthermore, the reason why a highly sensitive mechanical detector can be provided by the above-described method is as follows. Now, if the intensity of the irradiation laser is increased with respect to a mechanical element in which the vibration damping of the beam is inhibited by light irradiation by adjusting the wavelength of the irradiation laser, the Q value of the element increases. When the intensity of the irradiation light is sufficiently large, the vibration damping is almost extinguished and the dispersion of the vibration energy becomes a very small value. As a result, an extremely large Q value can be obtained. By adjusting the mechanical vibration system in such a state, acceleration, magnetism, charge, displacement, mass, molecule, or atom can be detected with high sensitivity. For example, the molecular weight attached to the beam can be detected by detecting the change in the resonance frequency based on the mass change of the entire beam due to the molecule attached to the beam.

  Further, when the above method is used, the mechanical element can be effectively cooled. The reason is as follows. When negative feedback is exerted on the beam motion by laser irradiation, that is, when vibration damping increases, the dispersion of vibration energy increases. That is, vibration energy is dissipated outside. With this energy dissipation, the effective temperature of the beam itself decreases. That is, the beam can be cooled by laser irradiation.

  On the other hand, when the above method is used, the mechanical element can be self-oscillated. The reason is as follows. When the excitation intensity is set to a certain threshold value or higher for a mechanical element in which vibration damping of the beam is reduced by laser irradiation, the dispersion of vibration energy disappears and a sharp spectrum appears. In this state, the vibration damping is completely lost, and the beam vibrates without being attenuated. That is, the mechanical element self-oscillates by laser irradiation. Thus, self-excited oscillation can be continued by continuing to irradiate light to the solid material constituting the beam.

  The vibration control of the mechanical element according to the first embodiment of the present invention is performed as follows. First, with a constant current flowing between the electrode 102 and the electrode 103, the frequency of the AC voltage applied to the piezoelectric vibration element 104 is swept, and the potential difference between the electrode 102 and the electrode 103 is monitored as a function of frequency. To do. When strain is applied to the semiconductor, the electrical resistance changes due to the piezoresistance effect that is a characteristic of the semiconductor. Accordingly, a large potential difference is generated between the electrode 102 and the electrode 103. That is, in this embodiment, the resonance state of the beam is detected through a change in the electrical resistance of the semiconductor due to strain, that is, a piezoresistance.

  Here, when the semiconductor is irradiated with a laser having a wavelength (about 830 to 840 nm) corresponding to energy slightly smaller than the band gap energy of GaAs in a state where the beam is resonated, as described above, positive feedback with respect to vibration is given. Works and vibration damping is suppressed. The Q value of the mechanical element can be improved. This phenomenon is shown in the black circle plot of FIG.

Next, a second embodiment of the present invention will be described.
FIG. 7 is a schematic diagram for explaining a method of cooling a mechanical element according to the second embodiment of the present invention.
In FIG. 7, the mechanical element 700 includes a cantilever 701 having a two-layer structure of an n-type GaAs layer and an undoped GaAs layer, electrodes 702 and 703 made of an alloy material, and titanium sapphire, as in the first embodiment. A laser light source 705 and a piezoelectric ceramic vibration element 704 are provided. However, the present embodiment is different from the first embodiment in that the cantilever beam 701 has a configuration in which the longitudinal direction of the cantilever 701 faces the [−110] direction in the (001) plane.

  Here, when the semiconductor is irradiated with a laser having a wavelength (approximately 830 to 840 nm) corresponding to energy slightly smaller than the band gap energy of GaAs, negative feedback with respect to vibration works, and the cantilever beam 701 increases as the intensity of irradiation light increases. Vibration damping increases. As a result, the Q value decreases. This phenomenon is shown in the black square plot of FIG. That is, the dispersion of vibration energy increases and the dissipation of energy increases. Along with this, the effective temperature of the cantilever 701 decreases. That is, the cantilever 701 can be cooled by laser irradiation. When the temperature of the cantilever 701 changes, the thermal vibration amplitude of the cantilever 701 due to Brownian motion also changes. Therefore, the effective temperature of the cantilever beam 701 can be known by thermal noise spectrum measurement.

Next, a third embodiment of the present invention will be described.
FIG. 8 is a schematic diagram for explaining a method of self-exciting a mechanical element according to the third embodiment of the present invention.
In FIG. 8, the mechanical element 800 includes a cantilever beam 801 having a two-layer structure of an n-type GaAs layer and an undoped GaAs layer, and a titanium sapphire laser light source 802, as in the first embodiment. The longitudinal direction of the cantilever 801 is configured to face the [110] direction in the (001) plane, as in the first embodiment. However, this embodiment is different from the first embodiment in that a piezoelectric vibration element is not particularly required and an electrode material is not particularly required.

  Here, when the semiconductor is irradiated with a laser having a wavelength (about 830 to 840 nm) corresponding to an energy slightly smaller than the band gap energy of GaAs in a state where the beam is stationary, the intensity of the irradiation light becomes a threshold (about 5 to 10 μW). When exceeding, the beam self-oscillates. That is, the mechanical element can be easily vibrated by light irradiation without requiring another electric, magnetic, or dynamic drive mechanism. In order to stop the vibration, the light source may be turned off or the intensity may be lowered below the threshold value. Alternatively, the driving and stopping of vibration can be controlled by changing the wavelength of the irradiation laser 802.

  As described above, the embodiments of the present invention have been described in detail. However, the specific configuration is not limited to these embodiments, and even if there is a design change or the like without departing from the gist of the present invention, It is included in the present invention. For example, in these embodiments, GaAs is used as the semiconductor material constituting the beam, but it goes without saying that any material having piezoelectricity can be used.

  In these embodiments, a beam composed of a two-layer structure of an n-type semiconductor and an intrinsic semiconductor is used. However, as long as the electric dipole in the thickness direction of the beam can be changed according to the deflection of the beam. Any kind of piezoelectric material can be used. It is also possible to use a multilayer structure of two or more layers. Even if a p-type semiconductor is used instead of an n-type semiconductor, the essence is not lost. In this case, since majority carriers become holes, the piezoelectric stress that contributes to vibration damping acts in contrast to the case where an n-type semiconductor is used.

  In the above-described embodiment, the biped cantilever structure is used. However, a simple cantilever beam, a double-end cantilever structure, or a beam structure having any other shape may be used. In the above embodiment, a titanium sapphire laser is used as the light source. However, any light source having a wavelength corresponding to the band gap energy of the material constituting the beam may be used.

  In the first and second embodiments, the alloy material is used as the ohmic electrode. Needless to say, any other kind of metal that enables ohmic contact with the semiconductor can be used. In the first and second embodiments, the vibration motion of the beam is detected through a change in the electrical resistance of the semiconductor. However, when an optical detection method such as an optical lever method is used, a highly sensitive detection element is similarly used. Can be manufactured.

It is a schematic diagram explaining the method of controlling the vibration of the mechanical element which is the 1st Embodiment of this invention. It is a cross-sectional schematic diagram of the element explaining the semiconductor multilayer structure which comprises the mechanical element in the 1st Embodiment of this invention. It is a schematic diagram which shows the mode of dispersion | distribution of the vibration energy which appears in a resonance spectrum. FIGS. 4A and 4B are schematic diagrams for explaining a semiconductor band structure constituting a beam in the first embodiment of the present invention. FIG. 4A shows a case where there is no laser light irradiation, and FIG. FIG. 4 (c) shows a case where the beam is warped in the state of laser light irradiation, and FIG. 4 (d) shows a case where the beam is warped in the opposite direction to FIG. 4 (c). Indicates the case. It is a schematic diagram which shows the relationship between the intensity | strength of the light irradiated to the beam in the 1st Embodiment of this invention, and Q value of an element, FIG.5 (a) shows the case where the longitudinal direction of a beam is [110], FIG. 5B shows the case where the longitudinal direction of the beam is [−110], and FIG. 5C shows the case where the longitudinal direction of the beam is [100]. It is a figure which shows the relationship between the Q value of a GaAs cantilever and a laser intensity | strength, a black (circle) plot shows the case of the cantilever which faced the [110] direction, and the black square plot faced the [-110] direction The case of a cantilever is shown. It is a schematic diagram explaining the method of cooling the mechanical element by the 2nd Embodiment of this invention. It is a schematic diagram explaining the method of carrying out self-oscillation of the mechanical element by the 3rd Embodiment of this invention.

Explanation of symbols

101, 701, 801 Cantilever 102, 103, 702, 703 Ohmic electrode 104, 704 Piezoelectric excitation element 105, 705, 802 Laser light source 201 Si-doped GaAs layer 202 Undoped GaAs layer 203 Al _0.7 Ga _0.3 As layer 204 Semi-insulating GaAs substrate

Claims (5)

Comprising a beam and a light source each consisting of at least two layers of different semiconductors,
The semiconductor has piezoelectricity,
The light source irradiates the beam with light of a wavelength that selectively excites one of the different semiconductors ;
Generation of carriers from one semiconductor selectively excited by the irradiation ;
The carriers are spatially separated within the beam to cause polarization ;
Stress is applied to the beam by the polarization creating a piezoelectric effect in the semiconductor ;
The warp occurs in the beam due to the stress, the deformation potential effect appears due to the strain gradient according to the warp of the beam, the band gap changes according to the strain, and the forward direction with respect to the vibration direction of the beam due to the band gap change joined by additional force of, by said additional force is applied later in time with respect to the vibration of the beam acts positive feedback to the vibration of the beam, and controlling the oscillation of the beam
The mechanical element characterized by performing .   The mechanical element according to claim 1, wherein at least one of the layers constituting the beam is a compound semiconductor.   The mechanical element according to claim 1, wherein the resonance frequency or Q value of the beam is changed by the light irradiation.   The mechanical element according to claim 1, wherein the beam is self-oscillated by the light irradiation. A method of controlling the vibration of a mechanical element comprising a beam and a light source each composed of at least two layers made of different semiconductors,
The semiconductor has piezoelectricity,
Irradiating the beam with light of a wavelength that selectively excites one of the different semiconductors from the light source;
Generating carriers from one semiconductor selectively excited by the irradiation;
The carrier is spatially separated within the beam to cause polarization;
Applying stress to the beam by the polarization creating a piezoelectric effect in the semiconductor;
A stage in which the beam is warped by the stress, a deformation potential effect appears by a strain gradient according to the warp of the beam, a stage in which a band gap changes in accordance with the strain, and a change in the band gap causes a vibration direction of the beam. A step of applying an additional force in the forward direction, and a step of applying a positive feedback to the vibration of the beam by the additional force acting with a time delay on the vibration of the beam. A method of controlling vibration of a mechanical element comprising the step of controlling vibration of a beam.

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