JPH11108976A - Permittivity measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To evaluate a local ferroelectric substance characteristic of a thin film. SOLUTION: In a state where a signal voltage is applied to a sample 1, a change in the film thickness of a sample 1 is detected as deflection of a cantilever 4 by a beam light source 5 and a light position detector 6. An AC component of the signal voltage is used as control signal to perform an identical detection of the deflection with a lock-in amplifier 10, while an electrostatic capacitance between a probe 4a and a sample base 2 is detected by an electrostatic capacitance sensor 8, and the AC component of the signal voltage is used as control signal to perform an identical detection of the electrostatic capacitance with the lock-in amplifier 10. Thus, a value pertaining to a permittivity is determined by an information processor 12, based on the change in the thickness obtained by the identical detection with a lock-in amplifier 9, and a change in the electrostatic capacitance obtained by the identical detection with the lock-in amplifier 10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a type of a measuring device in which an object to be measured is provided between electrodes, one of which is a probe, and a voltage is applied. The present invention relates to a dielectric constant measuring apparatus for measuring a physical quantity indicating a differential dielectric constant, and more particularly to a dielectric constant measuring apparatus suitable for measuring a physical quantity indicating a local incremental dielectric constant of a thin film.

[0002]

2. Description of the Related Art In recent years, ferroelectric thin films have attracted attention because of their application to semiconductor devices such as high-density random access memories and nonvolatile random access memories. Such applications require high dielectric constant, high remnant polarization, and low coercive field. Among many ferroelectrics, Pb (Zr _x Ti _1-x ) O ₃ (PZT) is an example of a ferroelectric that satisfies these requirements. However, it is known that these characteristics cannot be obtained when the thickness of the thin film is reduced. The cause is not yet clear.

In order to clarify this characteristic, it is necessary to locally evaluate a ferroelectric thin film. For this purpose,
Conventionally, observation of a ferroelectric thin film using a scanning probe technique has been attempted. The observation method can be classified into the following three types.

[0004] A method classified into the first type is a method in which an electrostatic force exerted on a probe of a cantilever of an atomic force microscope is expressed by:
This is a method of measuring a probe while scanning the sample surface. According to this method, the distribution of the polarization charge appearing on the surface of the ferroelectric thin film is obtained, and from this, it is possible to obtain knowledge on the domain structure and the poling state of the ferroelectric thin film.

A method classified into the second type is such that a voltage is applied to the probe and a ferroelectric thin film immediately below the probe in a state where the probe of the cantilever of the atomic force microscope is in contact with the sample surface. This is a method of locally measuring a change in film thickness. In actual measurement, an electrode is provided on a dielectric thin film, an alternating voltage is applied between the electrode and the probe, and the movement of the probe due to a change in film thickness is detected as a change in bending of the cantilever.

[0006] The method classified into the third type is based on the second method.
In the same way as the method classified into the type, the probe is brought into contact with the sample surface, and a voltage is applied between the probe and the electrode below the ferroelectric thin film. Is a method of locally measuring the capacitance during the period. By mapping the measured capacitance or the voltage dependence of the capacitance, a two-dimensional distribution of ferroelectric characteristics on the sample surface can be obtained.

[0007]

As described above, according to the conventional technique, it is possible to obtain information on the distribution of the polarization charge, and the change in the film thickness and the change in the capacitance with the change in the applied voltage. Each piece of obtained information is used to evaluate ferroelectric characteristics.

However, in order to evaluate the ferroelectric characteristics, it is preferable to obtain the above-mentioned information on the dielectric constant, remanent polarization, and coercive electric field. According to the study of the present inventors, in order to obtain such information indicating the ferroelectric characteristics, it is effective to measure a physical quantity indicating a differential dielectric constant indicating a ratio of a change in a dielectric constant to a change in a voltage. It is considered to be.

An object of the present invention is to provide a dielectric constant measuring apparatus capable of measuring a physical quantity indicating a local differential dielectric constant of a thin film.

[0010]

According to one aspect of the present invention, a dielectric constant for measuring a characteristic of a dielectric constant with respect to a change in voltage in a local small area of a sample to be measured is provided. In the measuring device, a sample stage having electrodes, a conductive probe is provided at one end, and a cantilever having the other end fixed thereto, and a sample and the probe mounted on the sample stage are relatively moved. A driving mechanism, a signal source for generating a voltage having an arbitrary waveform to be applied between the probe and the electrode, and a displacement detection unit for detecting the displacement of the cantilever,
A first detection unit that detects a capacitance between the probe and the electrode and outputs a ratio of a change in the capacitance to a change in a voltage output from the signal source; A second detection that detects a waveform component that changes in the same cycle as the waveform output from the signal source, and outputs a ratio of the displacement of the cantilever to a change in the voltage output from the signal source. And a calculation unit that outputs a change in dielectric constant with respect to the voltage of the sample from a signal detected by the first detection unit and a signal detected by the second detection unit. Is provided.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

Referring to FIG. 1, a dielectric constant measuring apparatus according to the present embodiment includes a sample stage 2, a scanner 3, and a probe 4a.
, A beam light source 5, an optical position detector 6, a signal source 7, a capacitance sensor 8, lock-in amplifiers 9 and 10, and a scanner control device 1.
1, an information processing device 12, and a display device 13.

The sample table 2 is for mounting the sample 1 thereon. The sample table 2 is formed of a conductive material, and is electrically connected to the sample 1 to be mounted.

The scanner 3 is for relatively moving the sample 1 placed on the sample table 2 and the probe 4a. This movement is performed in a direction in which the distance between the sample 1 and the probe 4a is changed and a direction in which the surface of the sample 1 is scanned. The movement for scanning the surface of the sample 1 is as follows.
For example, it can be performed in two directions orthogonal to each other in a plane along the surface of the sample 1.

In this embodiment, a case will be described in which the scanner 3 is configured to movably support the sample stage 2. However, the scanner 4 is configured to movably support the holder 4b and to move the probe 4a. It is good also as a structure to make it.

The scanner 3 can be constituted by using, for example, a tube-type scanner using a tube-type piezo element. This realizes a scanner having a simple configuration capable of moving in the z direction that changes the distance between the sample 1 and the probe 4a and the x direction and the y direction that are orthogonal to each other in a plane along the surface of the sample 1. can do.

The cantilever 4 has a probe 4a at one end and the other end supported by a holder (not shown) by a holder 4b. Cantilever 4
Is configured such that electrical conduction can be obtained from the tip of the probe 4a to the holder 4b. In order to obtain conduction, for example, a conductive material can be coated on the surface. As the conductive substance, for example, a metal, more specifically, nickel chromium (NiCr) or the like can be used.

The beam light source 5 and the light position detector 6
Is for detecting the bending of the cantilever 4. The light beam emitted from the beam light source 5 to the cantilever 4 is reflected on the back surface of the cantilever 4 (the surface opposite to the surface on which the probe 4a is provided), and its position is detected by the light position detector 6. . Thereby, the probe 4
In a, the deflection of the cantilever 4 due to the interaction with the sample 1 can be detected.

The beam light source 5 only needs to be capable of emitting a light beam in the form of a beam. For example, a laser light source or a light source having a collimating optical system can be used.

The light position detector 6 only needs to receive the reflected light from the cantilever 4 and detect a change in the light receiving position. For example, the light position detector 6 can be configured using a two-division photo sensor.

One of the signal sources 7 is connected to the sample table 2. The other is grounded. Thereby, a signal voltage can be applied between the probe 4 a and the sample table 2 via the cantilever 4. In other words, an electric field can be locally formed by applying a voltage to the sample 1 placed on the sample stage 2.

The signal source 7 outputs a signal voltage in which an AC component of a desired frequency and amplitude voltage is superimposed on a specified bias voltage, and can apply the signal voltage between the probe 4 a and the sample table 2. . The designation of the frequency, the amplitude voltage, and the bias voltage may be performed by setting the signal source 7 in advance, or the signal source is controlled by an external information processing device 12 such as a computer. It may be.

Next, one of the capacitance sensors 8 is connected to the signal source 7 and the other is connected to the holder 4b. The capacitance sensor 8 detects the capacitance between the probe 4a and the sample table 2 via the cantilever 4, and outputs a signal corresponding to the magnitude of the capacitance.

Next, the lock-in amplifier 9 outputs an output signal related to the capacitance output from the capacitance sensor 8,
An output voltage applied between the sample stage and the probe from the signal source 7 is input. Then, the lock-in amplifier 9 detects a signal component having the same cycle as the output signal from the signal source 7 from the output signal from the capacitance sensor 8, and detects a change in the output signal from the signal source 7 on the sample stage. The amount of change in capacitance between the probes is detected.

The lock-in amplifier 10 receives an output signal from the optical position detector 6 relating to the displacement of the cantilever 4 and an output voltage from the signal source 7. Then, the lock-in amplifier 10 detects a signal component having the same cycle as the output signal from the signal source 7 from the signal output from the optical position detector 6, and detects the signal component of the sample with respect to the amount of change in the output voltage from the signal source 7. Detect the displacement.

Next, the scanner control device 11 controls the distance between the probe 4a and the sample 1 using the output signal from the optical position detector 6. That is, a signal of a component having a frequency lower than the predetermined threshold frequency is detected from the output from the optical position detector 6. This is to detect the bending of the cantilever that has changed due to the shape on the sample surface during scanning. Then, the scanner 3 is driven and the distance between the probe 4a and the sample 1 is feedback-controlled so that the detected signal always becomes a constant signal. Since the signal from the light position detector 6 indicates the amount of bending of the cantilever 4, the scanner 3 can be controlled so that the bending of the cantilever 4 is constant.

In general, the change in the unevenness of the sample surface with respect to the scanning speed has a long cycle with respect to the frequency of the AC voltage applied between the probe 4a and the sample table 2, so that the threshold frequency is determined by the signal source. 7 is set to a frequency lower than the frequency of the AC component of the signal voltage generated.

For example, as described above, the optical position detector 6
If a two-segment photodetector is used,
Since a voltage proportional to the amount of deflection of the cantilever 4 is output, the distance between the probe 4a and the sample 1 is controlled by the scanner 3 so that the voltage becomes a predetermined constant value.

The information processing device 12 includes a lock-in amplifier 9
On the dielectric constant of the sample 1 based on the change in the capacitance with respect to the change in the applied voltage detected by the lock-in amplifier 10 and the change in the deflection of the cantilever 4 with respect to the change in the applied voltage detected by the lock-in amplifier 10. It is for seeking. The calculation process for this will be described later.

The information processing device 12 can further output the control amount given to the scanner 3 in the vertical direction of the sample surface from the scanner control device 11 as information representing the undulation of the surface of the sample 1.

The present apparatus detects the change in the capacitance with respect to the change in the applied voltage, the change in the bending of the cantilever 4 with the change in the applied voltage, and the change in the bending of the cantilever 4 during scanning. In doing so, they can be detected in synchronization with each other. For this purpose, for example, such information can be received as an analog signal, and data digitized by an analog-digital converter whose sampling timing is synchronized with each other can be received. Such an analog
The digital converter may be provided as an interface in the information processing device 12, or may be provided as an interface in the information processing device 12.
May be provided in the preceding stage of.

By synchronizing the measurement in this way,
Simultaneous acquisition of such information can be guaranteed. Therefore, when the measurement is performed while the probe 4a scans the surface of the sample 1, the locality of the measurement site can be guaranteed. That is, it is possible to localize an area including a position where each piece of information is obtained. Therefore, a physical quantity for evaluating the ferroelectric characteristics of the sample 1 can be obtained for each of the localized regions.

For this purpose, scanning of the surface of the sample 1 is performed a plurality of times, and the above-described measurement may be performed in each scanning under the condition that these scanning positions are reproduced. Good.

The display device 13 is for displaying the result processed by the information processing device 12. The display device 13 is, for example, a VDU (visual displ.
ay unit). Further, the display device 13 can also be used as a man-machine interface for receiving an operation on the information processing device 12.

Further, an output device for outputting the processed result to a recording medium such as paper or a magnetic disk may be provided.

Next, the operation of the dielectric constant measuring apparatus to which the present invention is applied will be described with reference to FIG.

FIG. 2 shows a probe 4 according to the first embodiment.
a (see FIG. 1) is shown in an enlarged manner. In FIG.
A ferroelectric thin film 1 having an electrode 1a formed on one surface is placed on a sample table 2 as a sample. The cantilever 4 is supported with the probe 4a in contact with the other surface of the ferroelectric thin film 1.

Assuming that the probe 4a is an electrode having an area S, the capacitance C between the probe 4a and the electrode 1a under the ferroelectric thin film 1 becomes

[0039]

(Equation 1)

## EQU4 ## Here, z is the distance between the probe 4a and the electrode 1a, and a is the dielectric constant of the ferroelectric thin film 1. When a signal voltage V is applied between the probe 4a and the electrode 1a, a and z change. Therefore, taking the total derivative of C,

[0041]

(Equation 2)

Is as follows. That is, it is shown that the change in the capacitance induced when the signal voltage V is changed is the sum of the change due to the change in the dielectric constant and the change due to the change in the film thickness. .

In practice, the equation (1) is simplified, but basically, the capacitance is a function of the dielectric constant and the distance between the probe and the electrode. For the shape of
An approximate function can be obtained by simulation or the like. In Expression (2), if (dC / dV) and (∂z / ∂V) can be measured, (∂a / ∂V) is obtained. Here, since a is a quantity related to the dielectric constant ε, (∂a / ∂V) is a physical quantity indicating a differential dielectric constant, and information about the dielectric constant, remanent polarization, and coercive electric field can be obtained. Therefore, for the same local site in the ferroelectric thin film, (dC / dV) and (∂z / ∂
By simultaneously measuring V), local ferroelectric characteristics can be evaluated.

In actual measurement, the variation of the signal voltage, that is, the amplitude of the AC component is made small enough to ignore the non-linearity of the hysteresis curve, and can be regarded as differentiation.

Here, since the distance z between the probe a and the electrode 1 a is known, the variation of the signal voltage can be replaced by the variation of the electric field formed on the ferroelectric thin film 1.
Accordingly, from the variation of the quantity a indicating the dielectric constant measured by giving a finite voltage variation, it is possible to obtain the quantity indicating the differential dielectric constant (∂ε / ∂E) without taking the film thickness change into account. .

Further, by measuring (∂a / ∂V) while changing the DC bias component of the signal voltage,
The history curve of the ferroelectric thin film 1 can be examined. Then, from this hysteresis curve, remanent polarization, coercive electric field and the like can be obtained.

In the above description, the case where the ferroelectric thin film is evaluated has been described. However, the same measurement is performed for the antiferroelectric thin film, and the local property of the antiferroelectric thin film is measured. It may be applied to evaluation.

(Embodiment) Next, an embodiment of the present invention will be described with reference to FIG.

Sample 1 was formed by sputtering a platinum film on a silicon wafer to a thickness of 200 nm to form an electrode 1a (see FIG. 2).
A bTiO ₃ film was formed to a thickness of 100 nm by a sol-gel method.

The sample 1 was mounted on a metal sample stand 2. Thus, the electrode 1a (see FIG. 2) and the sample stage 2 were electrically connected.

The sample table 2 is attached to a scanner 3 configured using a tube-type scanner, so that the sample 1 can be moved in three directions of x, y, and z.

The cantilever 4 used had a probe 4a formed at the tip, and the other end was held by a holder 4b. The cantilever 4 is coated with metallic NiCr,
Electrical conduction was obtained from the tip of the probe 4a to the holder 4b.

The deflection of the cantilever 4 due to the interaction between the probe 4a of the cantilever 4 and the sample 1 is as follows.
The reflected light of the beam light emitted from the beam light source 5 by the cantilever 4 was monitored by the optical position detector 6. A laser light source is used as a beam light source, and the optical position detector 6
It was configured using a two-part photodetector.

The sample 1 was passed through the sample stage 2 at 100 kHz.
z, an AC voltage having a voltage amplitude of 4 V was applied by the signal source 7. The probe 4a of the cantilever 4 and the electrode 1a of the sample 1
(Refer to FIG. 2).
, And an AC component of the signal voltage from the signal source 7 was used as a reference signal, and the lock-in amplifier 9 performed the same wave detection and measured. As a result, a change in capacitance with respect to the AC component applied between the sample and the probe, that is, (dC / dV) was obtained.

The amount of deflection of the cantilever 4 was measured by detecting the output from the optical position detector 6 with the lock-in amplifier 10 for the same wave. Also in this case, the reference signal was an AC component of the signal voltage from the signal source 7. As a result, a change in film thickness (Δz / ΔV) with respect to the AC component was measured.

The scanner 3 was controlled by the scanner controller 11 so that the DC component of the output from the optical position detector 6 was constant. Here, since a voltage proportional to the amount of bending of the cantilever 4 is output from the two-segment photodetector used as the optical position detector 6, the bending of the cantilever 4 could be controlled to be constant. .

The setting of the amount of deflection of the cantilever 4, the signal given to the scanner 3, and the signals from the lock-in amplifiers 9 and 10 were centrally managed by the information processing device 12. An arithmetic operation was performed on the obtained data to obtain (∂a / ∂V). The obtained (∂a / ∂V) was mapped from the movement amount that supported the scanning by the scanner 3, and information indicating a two-dimensional differential dielectric constant distribution in the ferroelectric thin film 1 could be obtained. This distribution was displayed by the display device 13 configured using a CRT.

[0058]

According to the present invention, a physical quantity indicating a local differential dielectric constant of a ferroelectric thin film can be measured.

Also, while changing the DC bias component of the signal voltage applied to the sample and measuring the quantity related to the differential dielectric constant corresponding to each DC bias, the hysteresis curve, the dielectric constant, the remanent polarization, and the coercive electric field of the sample are measured. Knowledge can be obtained.

The findings thus obtained are as follows:
It can be used to evaluate local ferroelectric properties of thin films.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a dielectric constant measuring apparatus to which the present invention is applied.

FIG. 2 is an enlarged cross-sectional view in the vicinity of a probe of a dielectric constant measuring device to which the present invention is applied.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... sample, 1a ... electrode, 2 ... sample stand, 3 ... scanner,
4 cantilever, 4a probe, 4b holder, 5
... Beam source, 6 Optical position detector, 7 Signal source, 8 Capacitance sensor, 9, 10 Lock-in amplifier, 11
Scanner control device, 12 information processing device, 13 display device.

Claims (5)

[Claims] 1. A dielectric constant measuring device for measuring a characteristic of a dielectric constant with respect to a voltage change in a local minute region of a sample to be measured, wherein a sample stage having electrodes and a conductive probe are provided at one end. A cantilever having the other end fixed thereto, a driving mechanism for relatively moving the sample and the probe placed on the sample stage, and a voltage having an arbitrary waveform applied between the probe and the electrode. A signal source to be generated, a displacement detection unit that detects a displacement of the cantilever, a capacitance between the probe and the electrode, and a change in capacitance with respect to a change in voltage output from the signal source. A first detection unit that outputs a ratio, and among the signals detected by the displacement detection unit, a waveform component that changes in the same cycle as the waveform output from the signal source is detected and output from the signal source. Voltage change A second detector that outputs a ratio of displacement of the cantilever; a dielectric constant with respect to a voltage of the sample, based on a signal detected by the first detector and a signal detected by the second detector. A dielectric constant measuring device, comprising: a calculating unit that outputs a change in the dielectric constant. 2. The dielectric constant measurement device according to claim 1, wherein a waveform having a period different from a period of a waveform of a voltage emitted from the signal source is detected from signals detected by the displacement detection unit. And a drive control unit that controls the moving mechanism so that the amplitude of the different waveform becomes a predetermined value; a change in a dielectric constant with respect to a voltage of the sample output from the arithmetic unit; And a signal processing unit that simultaneously outputs a control signal output to the moving mechanism. 3. The dielectric constant measuring apparatus according to claim 1, wherein the arithmetic unit further receives from the drive control unit a control amount for controlling the moving mechanism by the drive control unit, and based on the received control amount, A dielectric constant measuring apparatus for determining an undulating shape of a sample placed on a table. 4. The dielectric constant measurement device according to claim 3, wherein the signal processing unit changes a dielectric constant with respect to a voltage of the sample output from the arithmetic unit and outputs the change from the drive control unit to the moving mechanism. A control signal to be output and a control amount controlled by the moving mechanism at the same time. 5. The dielectric constant measuring apparatus according to claim 1, wherein the signal source further comprises a DC voltage power supply for generating a DC bias voltage applied between the probe and the electrode. Permittivity measuring device.