JP2006275875A - Scanning probe microscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem wherein a risk of displaying an inaccurate contact potential image exists when a voltage dependency curve is abnormal, and a problem wherein a dedicated constitutive component is required to increase a cost, when using an SKPM. <P>SOLUTION: The contact potential is obtained based on a signal as to static electricity provided by sweeping a bias voltage, in this scanning probe microscope of the present invention provided with a conductive probe opposed to a sample, an excitation means for exciting the probe, a scanner for scanning two-dimensionally the sample relatively to the probe, and for changing a distance between the sample and the probe, a bias voltage impressing means for impressing the bias voltage between the sample and the probe, a bias voltage sweeping means for sweeping the bias voltage, and an electrostatic signal detecting means for detecting the signal as to the static electricity from the probe. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a scanning probe microscope that observes a sample surface by detecting a physical quantity acting between a probe and a sample.

  In recent years, the atomic force acting between the probe and the sample can be achieved by scanning the surface of the sample with the probe, with the cantilever with the probe and the sample facing each other, the distance between the probe and the sample being several nanometers or less. 2. Description of the Related Art Scanning probe microscopes that measure physical quantities such as magnetic force or electrostatic force and obtain a concavo-convex image, magnetic image, spectroscopic image, etc. of a sample surface based on the measurement have attracted attention. Further, in order to image a contact potential difference (hereinafter referred to as CPD) between different kinds of substances in a local region, a scanning Kelvin probe force microscope (hereinafter referred to as SKPFM) was developed by applying an atomic force microscope. It is said).

FIG. 1 shows a conventional technique for SKPM using the FM detection method. The back surface of the cantilever 1 is irradiated with laser light from the laser light source 2, and the reflected light is detected by the photodetector 3. By this optical lever system, the vibration displacement of the cantilever 1 is detected and is electrically amplified by the preamplifier 4 having a built-in bandpass filter. A loop is formed in which the displacement signal of the cantilever 1 converted into an electric signal is input to the piezo element 6 for vibration via the attenuator 5. Although not shown, a phase shifter is also incorporated in this loop, and is set to positively oscillate at the natural frequency of the cantilever 1. The attenuator 5 is controlled by AGC or the like so that the vibration amplitude of the cantilever or the voltage amplitude input to the excitation piezo element 6 is constant.

  The frequency shift that is held constant is set by the reference voltage 12. The signal (the output of the filter 9) that controls the Z movement at this time corresponds to the surface unevenness image signal 18 (Topography Signal), and the scanner 11 is scanned two-dimensionally by the scan signal (X, Y Scan). An uneven image can be obtained by using the Z motion at that time as a luminance signal.

The natural frequency f ₀ of the cantilever 1 is also shifted by the electrostatic force between the probe and the sample. When a voltage [V _DC + V _AC sin (ωt)] obtained by adding the voltages from the oscillator 13 and the error amplifier 14 is applied to the conductive cantilever 1, there is a potential difference between the grounded sample 15 and the tip of the cantilever 1 tip. The electrostatic force is generated. If the frequency of the AC component of the applied voltage is set to a value that is smaller than the operating band of the FM demodulator 7 and exceeds the band of the feedback 1 (a value that cannot be followed), the output (V _f0 ) of the FM demodulator 7 Modulation due to AC component appears. When this signal is detected by the lock-in amplifier 16 with ω as a reference signal, an output corresponding to the amplitude of the ω component is obtained. When this output is zero, the electrostatic force between the tip of the cantilever 1 and the sample is minimized, and a DC voltage (V _DC ) that maintains this state is supplied from the error amplifier 14 via the adder 17 to the cantilever 1 (probe). ) As a potential (feedback 2). This _VDC is the CPD of the sample surface with respect to the probe, and the concavo-convex image signal 18 and the contact potential image signal 19 are simultaneously observed by displaying simultaneously with the concavo-convex signal on the sample surface.

However, when a bias voltage is applied between the sample and the probe, in principle, the voltage dependence curve of the electrostatic force draws a parabola, but in reality the voltage at the minimum point corresponding to CPD is an unclear voltage. Dependency curves are obtained and may be abnormal. In SKPM in the prior art, a CPD image can be obtained by electric circuit feedback regardless of the curve shape. Therefore, when the voltage dependency curve is abnormal, there is a possibility that an inaccurate potential image is represented.
Further, in order to perform the SKPM measurement, components such as the oscillator 13, the error amplifier 14, and the lock-in amplifier 16 in FIG. 1 are required in addition to the configuration of the normal non-contact atomic force microscope, which leads to an increase in cost.

  As a conventional technique, there is a scanning probe microscope that applies a signal between a sample and a probe by a DC bias circuit and detects a contact potential difference corresponding to the signal (for example, Patent Document 1).

JP 2004-294218 A

  The problem to be solved by the present invention is that there is a possibility that an inaccurate contact potential image was displayed when the voltage dependency curve is abnormal. In addition, when SKPM is used, dedicated components are required, leading to an increase in cost.

  According to the first aspect of the present invention, the conductive probe facing the sample, the excitation means for exciting the probe, the sample and the probe are relatively two-dimensionally scanned, and the sample and the probe are scanned. A scanner that changes a distance between the needle, a bias voltage applying unit that applies a bias voltage between the sample and the probe, a bias voltage sweeping unit that sweeps the bias voltage, and static electricity from the probe A scanning probe microscope comprising: an electrostatic signal detection means for detecting a signal, wherein a contact potential difference is obtained based on a signal related to static electricity obtained by sweeping the bias voltage. is there.

  According to a second aspect of the present invention, there is provided a contact potential difference based on a change in brightness according to the contact potential difference obtained by fixing a distance between the sample and the probe for each measurement point of the sample and sweeping the bias voltage. The scanning probe microscope according to claim 1, wherein an image is obtained.

  The invention according to claim 3 is the scanning probe microscope according to claim 1 or 2, wherein the signal related to static electricity is a frequency shift signal, and the potential difference is obtained by approximating the frequency shift signal by a quadratic expression. The scanning probe microscope according to claim 1 or 2, wherein

  According to the present invention, since the contact potential is obtained based on the voltage dependency curve, the abnormality of the voltage dependency curve can be detected in advance, and in this case, the contact potential is not represented. Further, the contact potential can be obtained without using dedicated components.

  The configuration of the present invention will be described with reference to FIG. A sample 15 is placed on the upper surface of a scanner 11 which is composed of a cylindrical piezo element and can be displaced in the XYZ directions. A cantilever 1 having a probe 24 at the tip is mounted facing the sample 15. The cantilever 1 and the probe 24 are conductive, and the surface is made of an elastic body such as silicon coated with a conductive material such as gold or platinum, or silicon doped with phosphorus, boron or the like. A vibrating piezo element 6 is installed at the fixed end of the cantilever 1. The laser beam emitted from the laser light source 2 irradiates the back surface of the cantilever 1, and the reflected spot of the laser beam reflected from the tip of the cantilever 1 is detected by the photodetector 3.

The photodetector 3 is electrically connected to a preamplifier 4 having a built-in band-pass filter. The preamplifier 4 is connected to a vibrating piezo element 6 installed in the cantilever 1 via an attenuator 5.
Further, the preamplifier 4 branches in the middle and is connected to an FM demodulator 7 constituted by a PLL, and the FM demodulator 7 is connected to an error amplifier 8. The FM demodulator 7 branches in the middle and is connected to the CPD image calculation device 20.

  The error amplifier 8 is connected to the Z scanner driver 10 through the filter 9 and the feedback hold device 21. The Z scanner driver 10 is a driver that controls the displacement of the scanner. Further, the filter 9 branches in the middle and is connected to the concavo-convex image calculation device 22.

The configuration of each unit in FIG. 2 has been described above. Next, the operation will be described.
In the non-contact mode of the atomic force microscope, observation and measurement are performed using the atomic attractive force between the sample surface and the probe. Since the interatomic attractive force is detected and observed from the non-contact distance, the damage to the sample 15 is small, and the outermost surface of the sample 15 can be accurately observed.

A cantilever 1 having a probe 24 at the tip is installed facing the sample 15. The cantilever 1 has a natural frequency of several tens of kHz to several hundreds of kHz depending on its length and thickness. When this natural vibration frequency f ₀ is applied to the piezo element 6 for vibration, a probe 24 is formed. The free end moves up and down about several nanometers at the natural vibration frequency.

When the state at this time is set to a steady state and the probe 24 is brought close to the sample 15, an interatomic attractive force acts between the probe and the sample at the lowest point. When the probe 24 receives an interatomic attractive force, the spring constant of the cantilever 1 is apparently changed, and becomes a frequency f ₀ ′ lower than the vibration frequency f ₀ in the steady state (the vibration period becomes longer). The frequency difference at this time is the frequency shift Δf.

  The tip of the cantilever 1 is irradiated with laser light from the laser light source 2, and the laser light reflected from the cantilever 1 is detected by the photodetector 3. When the cantilever 1 is displaced, the reflected spot of the laser light incident on the photodetector 3 is shifted. In accordance with this, the output of the photodetector 3 changes, and the displacement of the cantilever 1 can be detected.

  The detected signal is processed by the circuit as follows. That is, the vibration displacement of the cantilever 1 is detected by this optical lever system and is electrically amplified by the preamplifier 4. This signal is transmitted to the piezo element 6 for vibration through the attenuator 5, and the vibration at the natural frequency of the cantilever 1 is maintained by the positive feedback source loop reaching the piezo element 6 for cantilever 1 vibration. . The vibration amplitude of the cantilever 1 is set by weakening a voltage having a constant amplitude from a waveform converter (not shown) to an appropriate magnitude by the attenuator 5.

The signal from the preamplifier 4 is branched halfway and input to the FM demodulator 7, and a voltage corresponding to the signal is output (F / V conversion).

That is, among the frequency components included in the laser light reflected from the cantilever 1, using the frequency shift Δf as a feedback signal, the scanner 11 is displaced in the Z direction so that Δf is kept constant, and the probe-sample interval Is controlling the distance.
In the detection of the feedback signal, the slope detection that indirectly detects the frequency shift is generally used in the atmosphere, and the FM detection method that directly detects the frequency shift is used in the vacuum.

  When obtaining a concavo-convex image, the piezo scanner 11 scans two-dimensionally with a preset scan signal, and the concavo-convex image arithmetic unit 22 outputs a signal (output of the filter 9) that controls the Z motion at this time as luminance. An uneven image is obtained by processing as a signal.

  Next, a case where a contact potential difference image is obtained will be described. When the contact potential difference mode is designated by the operator, the central controller (not shown) displays the above uneven image for each measurement point (P (1, 1)... P (n, n)) as shown in FIG. First, control is performed to collect the concavo-convex image signal under the collecting condition, and a signal for operating the feedback hold device 21 is output from this state. That is, the scanner driving voltage at the time of collecting the concavo-convex image is held. Immediately thereafter, the central control device (not shown) sends a signal to the sample bias voltage sweeper 23 to gradually increase the sample bias voltage in 128 steps.

The signal V _f0 from the FM demodulator 7 is used for measurement of the frequency shift (Δf) at that time. As shown in FIG. 5, the Δf−V curve measured at P (1, 1) is approximated by the quadratic expression Δf = av ² + bv + c by the CPD image calculation device 20. Next, the voltage V _CPD which is a displacement of the minimum value corresponding to the contact potential difference (CPD) is calculated by the CPD image calculation device 20 from the quadratic equation approximated as shown in FIG. In the case of a sample bias, V _CPD = b / (2a). At this time, if the sample bias voltage shows an abnormal value, the central processing unit (not shown) notifies the operator of the abnormality and interrupts the operation. The V _CPD is converted into a luminance signal by the CPD image calculation device 20. By scanning the sample 15 with the scanner 11 in accordance with an instruction from a central processing unit (not shown), this luminance signal is sequentially obtained at other measurement points, thereby obtaining a contact potential difference image.

  FIG. 7 is a contact potential difference image obtained by the present invention. It can be seen that an image equivalent to the CPD image obtained by the conventional SKPM was obtained. FIG. 6 shows an uneven image (upper left) and a Δf image at each bias voltage swept in 128 steps. The number of Δf images is determined by the number of sample bias voltage steps to be swept.

  Of course, the central processing unit (not shown), the CPD image calculation device 20, and the concavo-convex image calculation device 22 may be integrated by using a computer. Alternatively, data at each measurement point may be collected first, and the data may be collectively processed to obtain a contact potential difference image.

  Although the operation has been described above, according to the present invention, since the contact potential is obtained based on the voltage dependency curve, an effect that the abnormality of the voltage dependency curve can be detected in advance can be obtained. Further, the contact potential can be obtained without using dedicated components.

In addition, this invention is not limited to the said embodiment, A various deformation | transformation is possible. For example, a contact potential difference image may be obtained by calculating a value of the contact potential difference by a statistical method such as an approximate expression, maximum, minimum, or limit according to a change in frequency shift to be measured.
Furthermore, in order to increase the measurement accuracy, a contact potential difference image may be obtained by performing a smoothing process by performing statistical processing using data of not only one measurement point but also a plurality of measurement points.

It is the schematic of the scanning Kelvin probe force microscopic copy in a prior art. 1 is a schematic view of an apparatus according to the present invention. It is a schematic diagram of the sample measurement point by this invention. It is a figure which shows the voltage corresponded to the contact potential difference by this invention. It is a figure which approximates the change of the frequency shift by this invention by a quadratic equation. It is (DELTA) f image in each bias voltage by this invention. 2 is a contact potential difference image according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cantilever 2 Laser light source 3 Photo detector 4 Preamplifier 5 Attenuator 6 Piezo element for vibration 7 FM demodulator 8 Error amplifier 9 Filter 10 Z scanner driver 11 Scanner 12 Reference voltage 13 Oscillator 14 Error amplifier 15 Sample 16 Lock-in amplifier 17 Addition Instrument 18 Concavity and convexity image signal 19 Contact potential image signal 20 CPD image computing device 21 Feedback hold device 22 Concavity and convexity image computation device 23 Bias voltage sweeper 24 Probe

Claims (3)

A conductive probe facing the sample;
Vibration means for exciting the probe;
A scanner that relatively two-dimensionally scans the sample and the probe, and changes a distance between the sample and the probe;
Bias voltage applying means for applying a bias voltage between the sample and the probe;
Bias voltage sweeping means for sweeping the bias voltage;
Electrostatic signal detection means for detecting a signal relating to static electricity from the probe;
A scanning probe microscope comprising:
A scanning probe microscope characterized by obtaining a contact potential difference based on a signal relating to static electricity obtained by sweeping the bias voltage.   A distance between the sample and the probe is fixed for each measurement point of the sample, and a contact potential difference image based on a luminance change according to the contact potential difference obtained by sweeping the bias voltage is obtained. The scanning probe microscope according to claim 1. The scanning probe microscope according to claim 1 or 2, wherein the signal related to static electricity is a frequency shift signal,
The scanning probe microscope according to claim 1, wherein the potential difference is obtained by approximating the frequency shift signal by a quadratic expression.

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