JP2014059273A - Near-field scattered light measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a near-field scattered light measuring device that can stably acquire measurement data on a scattered type near-field microscope in a prolonged measurement, and can improve an S/N ratio.SOLUTION: The near-field scattered light measuring device detecting near-field scattered light by detection means for evaluating a specimen includes: an excitation mechanism that oscillates a probe by a fixed frequency; a detector that detects the near-field scattered light as a correction signal; drift correction signal detection means that, as a drift correction signal for correcting an irradiation position of light in the vicinity of a measurement part of the specimen and a relative position of the probe, detects the correction signal as an input signal and a frequency twice as high as a frequency in oscillation of the probe as a reference signal by a lock-in detection; and a drift correction control mechanism that corrects the irradiation position of light in the vicinity of the measuring part and the relative position of the probe so as to make a measurement signal detected by the drift correction signal detection means maximum.

Description

  The present invention relates to a near-field scattered light measuring apparatus capable of performing spectroscopic measurement of scattered light from a sample using a scattering near-field microscope.

Typical methods for evaluating molecules and crystals on the sample surface by molecular vibration include Raman spectroscopy and infrared spectroscopy.
Raman spectroscopy is characterized by the fact that it can obtain knowledge about the structure of organic molecules and the crystallinity of inorganic materials, and these measurements can be performed in a non-contact and non-destructive manner without any special pretreatment of the sample. Can do.
In conventional Raman spectroscopy measurements, the optical theoretical resolution is limited to about half the wavelength used due to diffraction limitations.
However, in recent years, a technology called nanotechnology has been actively developed, and the demand for evaluating the state of molecules and crystals on the sample surface with a spatial resolution of several tens to several hundreds of nanometers exceeding the diffraction limit is increasing. .
In order to realize such a demand, a Raman scattering spectroscopic apparatus combined with a near-field microscope having a spatial resolution exceeding the diffraction limit has been vigorously developed.
Conventional near-field microscopes are roughly classified into fiber type and scattering type, but a scattering type near-field microscope is often used for measurement where it is difficult to obtain a signal light quantity such as Raman spectroscopy.

Scattering near-field microscopes include a transmission type apparatus that performs measurement by irradiating light from the back surface of a sample, and a reflection type apparatus that performs measurement by irradiating light from the surface of the sample.
The transmission-type scattering near-field microscope can have a highly sensitive apparatus configuration, but can measure only a sample that transmits light, but the reflection-type apparatus has a feature that it can measure any sample.
FIG. 4 is a diagram showing a schematic configuration in the case of measuring a Raman scattering spectrum with a conventional scattering type near-field microscope apparatus.
In order to maintain a constant distance between the tip of the probe 40 and the surface of the sample 41, the distance is generally controlled using the principle of an atomic force microscope (hereinafter AFM).
In FIG. 4, the distance control is performed by moving the XYZ fine movement stage 43.
Here, as an AFM method, a method in which distance control is performed by pressing a probe 40 attached to an AFM cantilever 42 against a sample 41 and detecting force from the deflection of the beam is referred to as a contact mode.
On the other hand, the one that performs amplitude control by exciting the cantilever 42 and performing force detection from the amount of change in the amplitude and the frequency shift is called a non-contact mode.

Light irradiation to the tip of the probe 40 is performed by condensing light from a light source 46 such as laser light near the tip of the probe 40 via a beam splitter 47, a mirror 48, and an objective lens 44.
In order to collect light at the tip of the probe 40, it is necessary to adjust the relative position between the position of the tip of the probe 40 and the light collection position. Therefore, the objective lens is installed on the XYZ stage 45. Yes.
Light scattered in the vicinity of the tip of the probe 40 due to light irradiation is collected using an objective lens 44 that is common to the irradiation light, and the mirror 48, beam splitter 47, edge filter 49, and lens 53 are moved along the reverse optical path. Through the spectroscope 50.
The scattered light dispersed by the spectroscope 50 is detected by a CCD detector 51, and a Raman scattering spectrum is recorded using a PC 52 or the like.

In a scattering near-field microscope, in order to obtain a strong near-field scattering signal, a position where the near-field scattering between the probe tip and the sample surface efficiently occurs, and a position where the electric field strength is strong in the focused spot of the irradiated light. It is necessary to repeat.
In a scattering near-field microscope using visible light, it is required to measure in a state where the light condensing position and the probe tip position are overlapped with an accuracy of about 0.1 μm.
However, the phenomenon of drift, in which the relative position between the position of the probe tip and the region where the electric field strength is strong changes over time due to the rigidity of the parts fixing the mirror and AFM and fluctuations due to temperature, etc. Occur.
Due to this drift, there is a problem that the signal intensity fluctuates with time, and in many cases the signal decreases with time.
This problem becomes a big problem because it is necessary to take a long measurement time especially when measuring weak light such as Raman scattering, which causes the signal intensity to attenuate.
In order to deal with these problems, in Patent Document 1, after performing spectroscopic measurement, the scattered light intensity is measured periodically, and the relative position between the probe tip and the light condensing position is set so that the intensity becomes maximum. There has been proposed a near-field scattered light measurement apparatus that performs drift correction by adjusting the distance.

JP 2005-147745 A

In a scattering near-field microscope, when light is applied to a probe that is close to the sample, not only the near-field scattered light generated between the probe and the sample, but also the light scattered by the probe and the sample is detected as scattered light. The
In particular, in the reflection-type scattering near-field microscope, the intensity of the near-field scattered light is weaker than the intensity of the scattered light from the probe or sample, so the intensity of the scattered light and the intensity of the near-field scattered light are not a simple proportional relationship. .
Therefore, when the relative position between the light irradiation position and the probe position is corrected using the scattered light intensity disclosed in Patent Document 1, the following problem occurs.
That is, the relative position is corrected not to the position where the near-field scattered light is strongest but to the position where the scattered light intensity from the probe or the sample is strong, and accuracy when performing drift correction is insufficient.
Furthermore, in the conventional technology, it is necessary to perform the measurement by appropriately distributing the scattered light to be measured and the scattered light to be used for drift correction with a mirror. It is necessary to acquire the signal.
Therefore, it is not always satisfactory to improve the S / N ratio of the signal as compared with the signal integration time. For this reason, a measurement such as Raman scattering requires a drift correction mechanism with improved positional accuracy while maintaining the S / N ratio in order to perform measurement efficiently.

  In view of the above problems, the present invention can stably acquire measurement data of a scattering near-field microscope and can improve the S / N ratio in long-time measurement. The purpose is to provide a measuring device.

The near-field scattered light measurement apparatus of the present invention includes a probe, causes the probe to be close to the measurement site on the surface of the sample, and generates near-field scattered light by irradiating light in the vicinity of the measurement site. A near-field scattered light measuring device for detecting field scattered light by a detection means for evaluating the sample,
An excitation mechanism for vibrating the probe at a constant frequency, a detector for detecting the near-field scattered light as a correction signal,
As a drift correction signal for correcting the light irradiation position in the vicinity of the measurement site and the relative position of the probe, the correction signal is used as an input signal, and a frequency twice the frequency of the probe vibration is used as a reference signal for lock-in. Drift correction signal detection means for detecting by detection;
A drift correction control mechanism that corrects the light irradiation position and the relative position of the probe in the vicinity of the measurement site so that the drift correction signal is maximized.

  According to the present invention, there is provided a near-field scattered light measurement apparatus that can stably acquire measurement data of a scattering near-field microscope and can improve the S / N ratio in a long-time measurement. Can be realized.

The figure explaining the structural example of the reflection-type scattering near-field microscope provided with the drift correction mechanism in embodiment and Example 1 of this invention. The figure explaining the relationship between the signal intensity | strength when detecting the component modulated by frequency 2omega among scattered light intensity | strength with the lock-in amplifier in embodiment of this invention. (A) is a figure explaining the case where a drift correction mechanism is not used, (b) demonstrates the case where a drift correction mechanism is used. FIG. 5 is a diagram illustrating a configuration example of a transmission type scattering near-field microscope including a drift correction mechanism according to the first embodiment of the present invention. The figure explaining the structure of the scattering type near field microscope in a prior art example.

Hereinafter, a near-field scattered light measurement apparatus according to an embodiment of the present invention will be described.
The near-field scattered light measurement apparatus of this embodiment generates a near-field scattered light by bringing a probe close to the measurement site on the surface of the sample and irradiating light in the vicinity of the measurement site. Detection is performed by detection means for evaluating the sample.
A configuration example of a reflection-type scattering near-field microscope provided with a drift correction mechanism in the present embodiment will be specifically described with reference to FIG.
In the following description, the vertical direction of the paper surface is the Z axis, the horizontal direction is the X axis, and the direction perpendicular to the paper surface is the Y axis.
In the description relating to the drawings, elements used with the same reference numerals represent elements having the same function.
In FIG. 1, a force sensor with a sharpened metal attached to a tuning fork 12 is used as a force sensor for AFM distance control.
As an alternative force sensor, a cantilever for AFM coated with metal may be used.

The distance control is performed by detecting the force according to the principle of AFM and moving the XYZ fine movement stage 14.
The AFM method uses a non-contact mode in which the distance between the tip of the probe 10 and the surface of the sample 11 is periodically modulated.
More specifically, the tuning fork 12 to which the probe 10 is attached is excited by an actuator (excitation mechanism) 13 so that the probe 10 is oscillated at a constant frequency.
The actuator 13 is driven by a function generator 26. A signal corresponding to the amplitude output from the tuning fork 12 is amplified by the preamplifier 29 and input to the reference signal of the lock-in amplifier 27.

The light is condensed on the tip of the probe 10 as follows.
The probe is brought close to the measurement site on the surface of the sample, and the light from the light source 17 such as a laser beam is condensed near the measurement site by the objective lens 15 via the beam splitter 18, the beam splitter 19, and the mirror 20. To generate near-field scattered light.
In order to overlap the light collecting position of the probe 10 with the light collecting position, the objective lens 15 is installed on the XYZ stage 16, and the XYZ stage 16 is moved in the in-plane direction so that the light collecting position and the tip of the probe 10 are aligned. Adjust so that the positions overlap.
The relative position between the probe tip and the light condensing position may be adjusted by optical scanning using a galvano mirror or a mechanism for moving the entire AFM instead of the XYZ stage 16.
Scattered light generated by irradiating the sample 11 and the tip of the probe 10 with light is condensed again by the objective lens 15 and guided to the beam splitter 19 along the reverse optical path.
The scattered light is divided into two optical paths by the beam splitter 19.
The beam splitter 19 may be a dichroic beam splitter whose reflectance is increased by a specific wavelength.

One of the divided scattered lights enters the spectroscope 22 through the edge filter 21 and the lens 30. The split light is detected by the CCD detector 23, and the spectrum is recorded using the spectrum recording PC 24 or the like.
Instead of the CCD detector 23, an avalanche photodiode or a photomultiplier tube may be used.
On the other hand, the other of the divided scattered light is detected by the photomultiplier tube 25 via the beam splitter 18 and the lens 31 that are installed next. Instead of the photomultiplier tube 25, a photodiode or the like may be used.
As a drift correction signal that corrects the irradiation position of light in the vicinity of the measurement site and the relative position of the probe, as will be described in detail below, a frequency twice the frequency of probe vibration is used as a reference signal, and photoelectron multiplication is performed. The detection signal obtained by the tube 25 is input to the lock-in amplifier 27 as an input signal for detection.

In the scattering near-field microscope, the intensity of the near-field scattered light attenuates exponentially with respect to the distance between the probe and the sample.
In such a system, when there is a sinusoidal modulation of the frequency ω between the distance between the probe and the sample and the time, the near-field scattered light intensity is an integer multiple (2ω, 3ω. There is a correlation with the one modulated by.
On the other hand, the intensity of scattered light scattered by the probe or the sample is modulated with the same period (ω) as the modulation period of the distance between the probe and the sample. Therefore, if a signal modulated at 2ω or more of the scattered light is detected by the lock-in amplifier, the position where the intensity of the detected signal is strong matches the position where the near-field scattered light is generated.
That is, as a drift correction signal, a component modulated at 2ω or more of the scattered light is detected by lock-in detection, and the irradiation position and the probe tip position are overlapped so that the intensity of the detected signal is maximized. Adjust. Thereby, the position accuracy of drift correction can be improved.
Therefore, lock-in detection is performed using a reference signal that is at least twice the frequency at which the probe 10 is vibrating, and the detected signal is input to a drift control PC (drift correction control mechanism) 28.

The drift control PC 28 is configured so that the XYZ stage 16 can be operated. The XY axis of the XYZ stage 16 is moved so that the lock-in detected signal is maximized, and the light condensing position and the probe 10 are adjusted. The drift is corrected so that the relative position of the tip is optimized.
Here, a CCD detector 23 attached to the spectroscope 22 detects one of the lights divided by the beam splitter 19 provided to branch the optical path of the light irradiating the sample and the scattered light. The other of the divided lights is detected by the photomultiplier tube 25 via the beam splitter 18 and used as a signal for drift correction. Therefore, a signal for drift correction can be acquired at the same time without attenuating the amount of signal for actual measurement.
Therefore, a strong signal state can be maintained for a long time, and the S / N ratio of measurement data can be improved.

Examples of the present invention will be described below.
[Example 1]
As Example 1, a configuration example using a reflective scattering near-field microscope will be described with reference to FIG.
In this embodiment, a share force type apparatus using a tuning fork is used as the AFM.
A gold wire having a diameter of 0.1 μm was sharpened by electropolishing and cut into a length of about 0.5 μm was used as the probe 10.
The probe 10 was attached to a tuning fork 12 and attached to a folder with an actuator 13. The resonance frequency of the tuning fork 12 to which the probe 10 was attached was measured and found to be 23 kHz.
The AFM measurement was performed in a non-contact mode in which the probe was oscillated at the resonance frequency and the distance was controlled so that the attenuation of the amplitude width was constant.

Condensing light on the tip of the probe 10 was performed as follows.
An argon laser was used as the light source 17 and a wavelength of 514 nm was used.
The output laser beam was guided to the objective lens 15 by the beam splitter 18, the dichroic beam splitter 19, and the mirror 20, and condensed at the tip of the probe 10 by the objective lens 15.
Scattered light obtained as a result of focusing on the tip of the probe 10 was collected using the same objective lens 15 used for light irradiation.
The condensed light was guided to the dichroic beam splitter 19 via the mirror 20.
The dichroic beam splitter 19 reflects light having the same wavelength as that of the light source 17 but transmits light having a longer wavelength.
The light scattered by the sample 11 and the probe 10 includes Rayleigh scattered light having the same wavelength as the light source 17 and Raman scattered light having a longer wavelength than the laser light. That is, most of the Raman scattered light among the scattered light passes through the dichroic beam splitter 19 and the Rayleigh light is reflected.

Of the scattered light, the light transmitted through the dichroic beam splitter 19 passes through the edge filter 21 in order to attenuate the partially transmitted Rayleigh light, and is collected by the lens 30 and enters the spectroscope 22.
The dispersed light was detected by the CCD detector 23, and the detected signal was processed by the spectrum recording PC 24 to obtain a Raman scattering spectrum.
On the other hand, the light reflected from the dichroic beam splitter 19 out of the scattered light follows an optical path opposite to the light from the light source 17 and is divided into transmitted light and reflected light by the beam splitter 18.
Since the transmitted light side is separated from the optical path from the light source 17, the divided scattered light is detected by the photomultiplier tube 25.

The signal detected by the photomultiplier tube 25 is input to the input signal of the lock-in amplifier 27,
The signal corresponding to the amplitude of the probe 10 output from the tuning fork 12 is input to the reference signal of the lock-in amplifier 27 via the preamplifier 29 to perform lock-in detection.
At the time of lock-in detection, in order to remove signals other than near-field scattered light from the scattered light, the frequency of the reference signal was doubled (46 kHz) input to the reference signal. The detected signal intensity was monitored by a PC 28 for drift control.
A program for scanning the XY axes of the XYZ stage 16 on which the objective lens 15 is installed is incorporated in the drift control PC 28 so as to maximize the signal detected by lock-in.

The Raman scattering spectrum was obtained by the following procedure.
After the AFM measurement of the sample was started, the light from the light source 17 was condensed and irradiated on the tip of the probe 10.
At the time of irradiation, in order to adjust the relative position between the position of the tip of the probe 10 and the condensing position, first, the XYZ stage 16 was positioned near the tip while observing light scattering by the probe 10.
Then, the XYZ stage 16 was finely adjusted to a position where the signal intensity was maximized while monitoring the signal that was finally detected lock-in.
FIG. 2A shows the result of measuring the change over time of the signal obtained by lock-in detection after fixing the position of the XYZ stage 16 after the adjustment. As is clear from FIG. 2A, the signal intensity attenuated with time.
Next, FIG. 2B shows the result of measuring the change over time of the signal obtained by the lock-in detection by operating the mechanism that scans the XYZ stage so as to maximize the signal obtained by the lock-in detection. Show. Compared with before the scanning mechanism was operated, the attenuation of the signal detected by lock-in could be suppressed.
Further, the S / N ratio of the Raman scattering spectrum was improved when the scanning mechanism was operated than when the scanning mechanism was not operated.

[Example 2]
As Example 2, a configuration example using a transmission type scattering near-field microscope will be described with reference to FIG.
Note that the configuration of the apparatus will be described only for parts different from the first embodiment.
As the sample 11, a glass substrate that transmits visible light was used.
Condensing light to the tip of the probe 10 is performed as follows. The light from the light source 17 is reflected by the dichroic beam splitter, collected by the objective lens 15, transmitted through the back surface of the sample 11, and collected at the tip of the probe 10.
The light scattered at the tip of the probe 10 passed through the sample 11, collected by the objective lens 15, and then guided to the dichroic beam splitter 19.

The light transmitted through the dichroic beam splitter was detected by the CCD detector 23 via the spectroscope 22 to obtain a Raman scattering spectrum.
The light reflected from the dichroic beam splitter was transmitted through the next beam splitter 18 and detected by the photomultiplier tube 25.
Of the signals detected by the photomultiplier tube 25, a component for modulation with a period twice as long as the vibration period of the probe is detected by the lock-in amplifier 27, thereby obtaining a drift correction signal.
The drift correction was performed by moving the XYZ stage 16 with the drift control PC 28 so that the acquired signal was maximized.
As a result, the S / N ratio of the obtainable Raman scattering spectrum was improved.

10: Probe 11: Sample 12: Tuning fork 13: Actuator 14: XYZ fine movement stage 15: Objective lens 16: XYZ stage 17: Light source 18: Beam splitter 19: Beam splitter 20: Mirror 21: Edge filter 22: Spectrometer 23: CCD detector 24: spectrum recording PC
25: Photomultiplier tube 26: Function generator 27: Lock-in amplifier 28: PC for drift control
29: Preamplifier 30: Lens 31: Lens

Claims (4)

Detection that includes a probe, causes the probe to be close to the measurement site on the surface of the sample, generates near-field scattered light by irradiating the vicinity of the measurement site, and evaluates the near-field scattered light. A near-field scattered light measuring device for detecting by means,
An excitation mechanism for vibrating the probe at a constant frequency;
A detector for detecting the near-field scattered light as a correction signal;
As a drift correction signal for correcting the light irradiation position in the vicinity of the measurement site and the relative position of the probe, the correction signal is used as an input signal, and a frequency twice the frequency of the probe vibration is used as a reference signal for lock-in. Drift correction signal detection means for detecting by detection;
A drift correction control mechanism for correcting the irradiation position of light in the vicinity of the measurement site and the relative position of the probe so that the drift correction signal is maximized;
A near-field scattered light measuring apparatus comprising:   The near-field scattered light measurement apparatus according to claim 1, wherein the reference signal is an integer multiple of twice or more a frequency in vibration of the probe. Near-field scattered light generated by irradiating light in the vicinity of the measurement site is divided by a beam splitter,
The one of the divided near-field scattered lights is guided to a detection means for evaluating the sample through a spectroscope,
3. The near-field scattered light measuring apparatus according to claim 1, wherein the other divided near-field scattered light is guided to the correction signal detecting means.   The drift correction control mechanism is configured to adjust the irradiation position of light in the vicinity of the measurement site and the relative position of the probe by moving the sample in an in-plane direction. The near-field scattered light measuring apparatus according to any one of claims 1 to 3.

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