JP3639744B2 - Signal detection device using scanning probe with piezoresistive cantilever - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a signal detection apparatus using a scanning probe applied to an observation apparatus, a recording / reproducing apparatus, or the like that uses a physical phenomenon generated by bringing a probe and a medium close to each other, and particularly works between a probe and a sample. The present invention relates to a displacement detection apparatus applied to a scanning probe microscope that measures physical forces and measures fine surface characteristics of the surface of an observation object from changes in those force signals.
[0002]
[Prior art]
In recent years, a scanning probe microscope (hereinafter referred to as SPM) that can directly observe the electronic structure of a material surface and the vicinity of the surface by using a physical phenomenon (tunnel phenomenon, atomic force, etc.) generated by bringing the probe close to the sample. (Abbreviated) has been developed, and real space images of various physical quantities can be measured with high resolution regardless of single crystal or amorphous.
In the industrial field, paying attention to the principle of high resolution of the atomic or molecular size of SPM, as disclosed in Japanese Patent Laid-Open Nos. 63-161552 and 63-161553, a recording layer is formed on a medium. The application and practical application to information recording / reproducing apparatus by using the computer is energetically advanced.
[0003]
Further, in the application to the apparatus as described above, in order to improve the throughput, parallel processing (multiple) of a plurality of probes is necessary, and development in that direction is also progressing. With respect to the probe manufacturing technique, a plurality of levers using an SOI substrate or the like is manufactured based on a conventional semiconductor process.
As for detection technology, a method using an optical lever that measures the amount of deflection of the probe from the positional deviation of the return light by applying a laser beam to the tip of the probe has been common, but in the case of signal detection from a multi-probe. However, using an AFM detection system with an optical lever not only makes the structure of the optical system very complex, but also the size of the system itself is considerably large. Therefore, a piezoresistor is currently used. A method has been proposed in which distortion detection is provided in an AFM lever so that the amount of bending of the lever is read by the resistance change of the piezoresistor.
[0004]
[Problems to be solved by the invention]
Conventionally, an AFM using a piezoresistor (hereinafter referred to as a piezoresistor AFM) has been performed using a resistance bridge circuit or the like in order to measure the resistance value, but as a major problem at that time, it depends on the temperature of the piezoresistor. There was a drift.
As the name suggests, a piezoresistor is a resistor that generates heat when an electric current is passed through it. Therefore, the resistor causes a temperature change and changes its own resistance value.
If the space containing the probe is stable without air flow, and there is no object in the vicinity, it takes a thermal equilibrium state and settles to a certain resistance value, but such a situation is conceivable in actual AFM. Absent. That is, when considering AFM in the atmosphere, the temperature change of the probe is caused by the flow of air and the like.
In addition, the SPM generally brings the probe close to the observation sample to a very small distance. In this process, heat exchange occurs between the probe and the sample, and the temperature of the probe changes.
The temperature change due to these phenomena shows a slower change in time than the concavo-convex signal of the sample, but in order to greatly change the resistivity of the piezoresistor itself, it is as small as the resistance change due to the bending of the lever. When detecting such a signal, the sensitivity and S / N ratio decrease as large low-frequency noise, which is a problem.
[0005]
Therefore, the present invention solves the above-described problems in the conventional one, and efficiently removes the offset current caused by the initial resistance in the unbent state when performing a resistance change accompanying the displacement of the piezoresistive lever by current measurement, It is an object of the present invention to provide a signal detection device using a scanning probe provided with a piezoresistive cantilever capable of detecting a signal with high sensitivity by detecting a current change accompanying a resistance change due to bending.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is characterized in that a signal detection apparatus using a scanning probe provided with a piezoresistive cantilever is configured as follows.
That is, a probe formed of a cantilever mounted with a piezoresistor is provided, the probe is brought close to the medium by an approach control unit, is scanned relative to the medium, and a constant bias is applied to the piezoresistor by a bias applying unit. A signal detection device using a scanning probe that measures changes in piezoresistance by means of signal detection means and measures surface characteristics of the medium, etc.
The signal detecting means includes current measuring means for measuring a current flowing through the piezoresistor, and an offset current removing means for removing an offset current included in the current flowing through the piezoresistor. A voltage source that generates a constant voltage, a transistor and a transistor control unit,
The transistor control unit controls the current value of the transistor with reference to the output of the current measuring means,
The transistor constitutes a feedback circuit that subtracts an offset current from a current that is controlled by the transistor control unit and input to the current measuring means .
In the signal detection device of the present invention, the transistor control unit is configured to detect a low frequency component of the current measuring unit and control a current value of the transistor .
In the signal detection device of the present invention, the transistor control unit is an integrator.
In the signal detection device of the present invention, the transistor control unit is constituted by an integrator, an adder, and an amplifier.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
In the SPM probe having a piezoresistor as a strain detector, the present invention has an offset caused by an initial resistance in a non-deflected state when a resistance change accompanying a displacement of the piezoresistive lever is performed by current measurement. A signal can be detected with high sensitivity by efficiently removing the current by feedback and detecting a current change accompanying a resistance change due to bending.
[0008]
【Example】
Examples of the present invention will be described below.
[Example 1]
FIG. 2 is a block diagram showing an outline of the piezoresistive AFM apparatus according to the first embodiment of the present invention.
In FIG. 2, reference numeral 201 denotes a cantilever type displacement detection probe in which a piezoresistor is mounted as a deflection detection element.
A specific configuration of this lever-type probe is shown in FIG. In FIG. 3, reference numeral 301 denotes a conical probe disposed on the lever tip, which is disposed in electrical contact with the lever tip piezoresistor 304. 303 is a silicon substrate, 302 is a wiring pattern, and 305 is a silicon oxide film for electrically insulating the substrate and the wiring. The size of the probe was 20 μm for W, 100 μm for L, and 2 μm for H for the cantilever fabricated this time. The resonance frequency of this lever was approximately 15 kHz.
[0009]
Next, the operation of the piezoresistive AFM device will be described with reference to FIG.
First, the lever-type probe 201 and the observation sample 202 are brought close to each other by the approach control unit 209 so as to exert a force on each other by an interatomic force or the like. After the probe and the sample 202 are brought close to a predetermined force, a scanning signal is generated by the scanning signal generation unit 207, and when the signal is applied to the XY driving mechanism of the stage 203 through the amplifier 208, the stage is moved to the sample. Driven in the in-plane direction. As a result, the probe 201 relatively scans within the sample surface, receives force change information, that is, unevenness information on the sample surface as deflection of the lever, and outputs it in the form of a change in piezoresistance.
That is, the change of the piezoresistance is measured by the signal measuring unit 204 by applying a constant bias to the piezoresistor by the bias applying means, and the information is transmitted to the control unit 210.
The control unit 210 mainly controls the approach control unit 209 and the scanning signal generation unit 207 based on the setting value input by the operator or the like through the setting unit 206. In addition, an image signal for visualizing the signal from the signal measuring unit 204 is generated and sent to the monitor 205.
[0010]
Next, details of the signal measuring unit 204 that is characteristic of the present invention will be described.
The signal measuring unit 204 is a part that detects the amount of bending of the piezoresistive lever from a change in piezoresistive value.
Conventionally, a resistance bridge method as shown in FIG. 7 is generally used to measure the amount of change in resistance. There, the resistor R is inserted in series with the piezoresistor, a reference bias for the bias applied to the piezoresistor is set by newly provided R and _RREF , and the difference is amplified by an amplifier.
However, according to this method, it is necessary to set R _REF in accordance with the piezoresistance value of the lever, and further adjustment is required when the piezoresistance changes due to a temperature change or the like and the balance is lost. In addition, there is a problem in that it is necessary to set the gain of the differential amplifier in the subsequent stage in view of those changes, and thus the gain cannot be increased.
[0011]
Therefore, in the circuit according to the present invention, the deflection signal is detected by measuring only the component due to the lever deflection of the current value output by applying a constant voltage to the piezoresistor. As shown in FIG. 1, the lever portion piezoresistor is composed of a piezoresistor Rs when there is no deflection and a resistance change ΔRs when it is bent. Therefore, the current flowing therethrough can also be divided into a current i due to Rs and a current Δi due to ΔRs. The sum of both currents is input to the current-voltage conversion unit, from which only the i component is subtracted by the current set by the power source -Vs and the MOS transistor (P channel). At this time, the gate voltage Vc of the MOS is determined by the current control circuit 101 with reference to the output Vo of the current-voltage converter. That is, a feedback circuit is configured. Here, since the current control circuit 101 is a circuit that detects a low frequency component of Vo, various devices such as various low-pass filters and amplifiers can be considered. In this embodiment, an integrator as shown in FIG. 4 is used. . Here, since the gate voltage / drain current characteristics of the P-channel MOS transistor have a form as shown in FIG. 5A, a non-inverting integrator is prepared, and a variable resistor for adjusting the loop gain. Rc is provided. Further, the resistance Rf of the current-voltage converter in FIG. 1 is 10 MΩ, and the conversion gain is 10 ⁶ .
[0012]
In this example, when AFM measurement was performed with the AFM measuring device configured as described above, high sensitivity not found in the conventional piezoresistor AFM, that is, 0.1 nm as a resolution in the direction perpendicular to the observation surface was obtained. . The piezoresistance value of the lever produced at that time was 10 kΩ, and the ratio of the resistance change to the measured deflection was ΔRs / Rs = 10 ⁻⁶ (per displacement of 0.1 nm). For this reason, the value of i in FIG. 1 is about 0.1 mA and the value of Δi is 0.1 nA.
In this embodiment, even when the bipolar transistor shown in FIG. 5B is used in the portion using the MOS transistor in this circuit configuration, the same operation can be performed and the same resolution can be obtained. .
In this embodiment, an example of an atomic force was performed as an interaction caused by the approach between the probe and the sample. However, in the present invention, the probe is bent by a physical force acting between the probe and the sample. It does not limit the force and interatomic force. For example, force interaction due to electrostatic force or magnetic force can be considered.
[0013]
[Example 2]
An embodiment in which the MOS transistor is a junction field effect transistor (JFET) shown in FIG. 6B in the circuit shown in the embodiment 1 will be described.
In this embodiment, a P-channel JFET is used. In this case, since the characteristics of the gate voltage and drain current of the transistor are as shown in FIG. 6A, the current control circuit 101 shown in FIG. 1 is replaced with that shown in FIG. .
This is a circuit in which a constant voltage is added to the integrator, and the offset voltage V _off shown in FIG. _6A can be canceled by adjusting V _D and R _{1 to} R ₄ in the figure. .
When AFM measurement was performed using the same piezoresistive cantilever as in Example 1 as a probe, a resolution of 0.1 nm in the direction perpendicular to the observation surface was obtained. Other constants such as resistance values in FIG. 1 were set in exactly the same manner.
[0014]
【The invention's effect】
As described above, according to the present invention, when the resistance change accompanying the displacement of the piezoresistive lever is performed by current measurement, the offset current caused by the initial resistance in the unbent state is efficiently removed by feedback, and the bending is performed. By detecting the current change accompanying the resistance change due to, a signal can be detected with high sensitivity.
Further, according to the present invention, the piezo resistance value of the lever varies among individual probes due to manufacturing process errors, etc., but even in that case, the offset current can be appropriately removed, so that the piezo resistance value is adjusted to the specific resistance value of the probe. Complexity such as setting of element values of the circuit can be eliminated.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing the configuration of a piezoresistance measurement circuit according to Embodiment 1 of the present invention.
FIG. 2 is a block diagram showing an outline of a piezoresistive AFM apparatus according to Embodiment 1 of the present invention.
FIG. 3 is a configuration diagram of a piezoresistive lever according to Embodiment 1 of the present invention.
FIG. 4 is a diagram illustrating a current control circuit according to the first embodiment of the invention.
FIG. 5 is a characteristic diagram of the P-channel MOS transistor according to the first embodiment of the invention.
FIG. 6 is a characteristic diagram of a JFET according to Example 2 of the present invention.
FIG. 7 is a diagram for explaining resistance change measurement by a conventional bridge;
FIG. 8 is a diagram for explaining a current control circuit according to a second embodiment of the invention.
[Explanation of symbols]
101: current control circuit 201: lever type probe 202: sample 203: stage 204: signal measuring unit 205: monitor 206: setting unit 207: scanning signal generating unit 208: amplifier 209: approach control unit 210: control unit 301: probe 302: Wiring pattern 303: Silicon substrate 304: Silicon oxide film

Claims (4)

A probe formed by a cantilever mounted with a piezoresistor is provided, the probe is brought close to the medium by an approach control unit, is relatively scanned with respect to the medium, and a constant bias is applied to the piezoresistor by a bias applying means. A signal detection device using a scanning probe that measures changes in the signal by means of signal detection and measures the surface characteristics of the medium, etc.
The signal detecting means includes current measuring means for measuring a current flowing through the piezoresistor, and an offset current removing means for removing an offset current included in the current flowing through the piezoresistor. A voltage source that generates a constant voltage, a transistor and a transistor control unit,
The transistor control unit controls the current value of the transistor with reference to the output of the current measuring means,
A signal detection apparatus using a scanning probe , wherein the transistor constitutes a feedback circuit that subtracts an offset current from a current that is controlled by the transistor control unit and input to the current measuring means . 2. The signal detection apparatus using a scanning probe according to claim 1, wherein the transistor control unit is configured to detect a low frequency component of the current measuring unit to control a current value of the transistor . 3.   3. The signal detection apparatus using a scanning probe according to claim 1, wherein the transistor control unit is an integrator.   The signal detection apparatus using a scanning probe according to claim 1 or 2, wherein the transistor control unit includes an integrator, an adder, and an amplifier.

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