JPH07198732A - Near-field microscope - Google Patents

Abstract

PURPOSE:To provide a near-field microscope which can perform high-resolution and high-sensitivity measurement against a refractive index distribution near the surface of a sample to be measured by detecting evanescent light locally existing near the surface of the sample without bringing the microscope into contact with the sample. CONSTITUTION:A near-field microscope is provided with a cantilever 26 equipped with a probe 24, evanescent light generating means 30, 32, 34, 36, and 38 which generate evanescent light near a sample 28 to be measured, vibrating means 40 and 42 which vibrate the cantilever 26 at a prescribed oscillation frequency, first detecting means 44, 45, 46, and 48 which detect the variation of the oscillation frequency, and servo means 50 and 52 which maintain the distance between the vibrational center of the cantilever 26 and the sample 28 at a fixed value. The microscope is also provided with a scanning means 54 which makes the probe 24 to scan the sample 28 in a non-contacting state, second detecting means 56 which detects the intensity of scattered light generated when the probe 24 comes into contact with the evanescent light, and image analyzing means 56, 58, 60, and 62 which analyzes the image of the refractive index distribution on the surface of the sample 28.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a near-field microscope for measuring the refractive index distribution of a measurement sample by detecting the intensity of evanescent light generated near the surface of the measurement sample.

[0002]

2. Description of the Related Art The resolution of an optical microscope is determined by the diffraction characteristics of light, and an image of an object smaller than a wavelength cannot be observed due to its diffraction limit.

However, it is known that a resolution exceeding the diffraction limit can be obtained by using a different type of light localized in a minute space instead of the above light. Such light is called evanescent light.

Since the latter half of the 1980s, an optical microscope having a resolution exceeding the diffraction limit by using an evanescent wave has been proposed. This microscope is called a near field microscope or a Photon-scanning tunneling microscope (PSTM).

PSTM utilizes the characteristic of an evanescent wave that "localizes in a region smaller than the wavelength and does not propagate in free space".

In the actual PSTM measurement, first, evanescent light localized near the surface of a micro object (ie, measurement sample) is scattered by scanning an optical pickup (ie, probe) and propagated light (ie, , Scattered light). Then, by detecting the scattered light and creating a map of the scattered light intensity, the resolution of the minute object is formed.

FIG. 2 schematically shows the structure of a general PSTM device.

As shown in FIG. 2, a measurement sample 2 is placed on a clean prism 6 having a surface 6a which is placed upside down on a three-dimensional moving stage 4.

In such a state, the semiconductor laser 8
The laser light guided through the lens 10 and the reflection mirror 12 is incident on the surface 6a of the prism 6 at an angle satisfying the condition of total reflection.

As a result, the measurement sample 2 placed on the surface 6a of the prism 6 is illuminated so that the background (background light) of scattered light, which will be described later, is minimized.

As a result, evanescent light (E) having the relationship shown in Table 1 below is generated near the surface of the measurement sample 2.

[0012]

[Table 1]

At this time, when the optical fiber probe 14 having a sharpened tip is brought close to the surface 6a of the prism 6,
The optical fiber probe 14 converts the evanescent light (E) into scattered light.

The scattered light is guided to the photodetector 16 via the optical fiber probe 14, and the change in the scattered light intensity is detected.

The change in scattered light intensity detected by the photodetector 16 is converted into a corresponding scattered light intensity signal,
It is output to the Z position control mechanism 18.

The Z position control mechanism 18 controls the movement of the three-dimensional movement stage 4 in the Z direction, that is, the direction of the optical fiber probe 14 based on the scattered light intensity signal, and moves the measurement sample 2 on the prism 6 to the optical fiber probe 14. Fix it in the same position as the tip of.

In such a state, the microcomputer 20 controls the X / Y movement of the three-dimensional movement stage 4 via the X / Y scanning device 22. As a result, the optical fiber probe 14 moves relative to the measurement sample 2 in the X direction.
Y scanning is performed.

At this time, the evanescent light (E) generated near the surface of the measurement sample 2 is scattered by the optical fiber probe 14 and converted into scattered light.

The scattered light is converted into an electric signal corresponding to the light intensity by the photodetector 16, and then image processing such as noise and background removal is performed. Then, the scattered light intensity distribution with respect to the XY scanning amount is displayed as a PSTM image.

The optical fiber probe 14 may be replaced with a dielectric or metal needle having a sharpened tip or a minute opening, but it has an optical waveguide and can collect scattered light with high efficiency and can be manufactured. It has the advantage of being easy. Further, by chemically etching a single-mode optical fiber using hydrofluoric acid, the radius of curvature of the tip is 10 nm or less and the sharpness of the tip is 20 °.

To obtain the PSTM image,
The scattered light generated by scanning the probe tip with respect to the evanescent light (E) must be distinguishable from the background light component due to the large irregularities on the surface of the measurement sample 2. For this purpose, it is necessary that the sharpness of the tip of the probe is small and the radius of curvature thereof is about the radius of the measurement sample 2.

When the optical fiber probe 14 satisfying the above conditions is scanned with respect to the measurement sample 2 while keeping a distance of about the above radius of curvature, the evanescent light component having a short attenuation distance caused by the fine structure of the measurement sample 2 is obtained. Are removed. As described above, the PSTM is an apparatus for selectively extracting a component having a fineness corresponding to the radius of curvature of the probe tip, such as a bandpass filter, from the spatial frequency of fine irregularities on the surface of the measurement sample 2. It can be said that the resolution is determined by the radius of curvature and the sharpness of the probe tip.

[0023]

As described above, in the conventional PSTM method, the optical fiber probe 14 is kept in contact with the optical fiber probe 14 and the measurement sample 2.
Since the position control is performed, it is difficult to detect the evanescent light (E) localized near the surface of the measurement sample 2 with high accuracy.

Since the optical fiber probe 14 is in contact with the measurement sample 2, the optical fiber probe 1
There is also a problem that the shape of the tip of No. 4 changes and it becomes difficult to keep the measurement resolution constant.

Further, when the change in the refractive index of the surface of the measurement sample 2 is small, the steady change in the evanescent light intensity is also small, so that it becomes difficult to keep the S / N ratio within a certain allowable range. The problem occurs.

As a result of the above-mentioned problems, it becomes difficult to measure the refractive index distribution on the surface of the measurement sample 2 with high accuracy.

The present invention has been made to solve such a problem, and an object thereof is to perform emissive light localized in the vicinity of the surface of a measurement sample by performing non-contact scanning on the measurement sample. It is an object of the present invention to provide a near-field microscope that can detect with high accuracy and can measure the refractive index distribution on the surface of a measurement sample with high resolution and high sensitivity.

[0028]

In order to achieve such an object, the near-field microscope of the present invention is capable of setting a cantilever having a probe on its free end surface and a predetermined measurement sample, and Evanescent light generating means for generating evanescent light in the vicinity of the surface of the measurement sample, vibrating means for vibrating the cantilever at a predetermined vibration frequency, and irradiating the free end back surface of the cantilever with laser light and from the back surface of the free end. First detection means for detecting a change in the vibration frequency caused by interaction between the probe and the measurement sample based on the optical characteristics of the reflected laser light, and detection by the first detection means Servo means for maintaining a constant distance between the vibration center of the cantilever and the measurement sample based on the generated first detection signal. A scanning unit for scanning the probe relative to the measurement sample in a non-contact manner based on a first detection signal detected by the first detection unit; and the scanning unit generated near the surface of the measurement sample. The surface of the measurement sample based on second detection means for detecting the intensity of scattered light generated when the probe comes into contact with the evanescent light and the second detection signal detected by the second detection means. Image analysis means for image-analyzing the refractive index distribution of the.

[0029]

By operating the servo means through the scanning means while vibrating the cantilever, the probe is relatively contactlessly scanned with respect to the measurement sample, and the refractive index distribution on the entire surface of the measurement sample is measured. To be done.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A near field microscope according to an embodiment of the present invention will be described below with reference to FIG.

As shown in FIG. 1, the near-field microscope of this embodiment is capable of setting a cantilever 26 having a probe 24 on its free end surface and a predetermined measurement sample 28, and the surface of the measurement sample 28. Evanescent light generating means (30, 32, 34, 36, 38) for generating evanescent light (not shown) in the vicinity and the cantilever 2
Vibrating means (40, 4) for vibrating 6 at a predetermined vibration frequency.
2) and the above-mentioned vibration caused by the interaction between the probe 24 and the measurement sample 28 on the basis of the optical characteristics of the laser light that is irradiated onto the back surface of the free end of the cantilever 26 and reflected from the back surface of the free end. First to detect changes in frequency
Of the detection means (44, 45, 46, 48) and the first detection signal detected by the first detection means, the distance between the vibration center of the cantilever 26 and the measurement sample 28 is made constant. Servo means to maintain (50, 52)
And a scanning means 54 for relatively non-contact scanning the probe 24 with respect to the measurement sample 28 based on the first detection signal detected by the first detection means, and a vicinity of the surface of the measurement sample 28. Based on the second detection means 56 for detecting the intensity of scattered light generated by the probe 24 coming into contact with the generated evanescent light, and the second detection signal detected by the second detection means 56, measurement is performed. Image analysis means (56, 5) for image-analyzing the refractive index distribution on the surface of the sample 28.
8, 60, 62).

The probe 24 and the cantilever 26 are
It is made of a member (for example, silicon nitride) that is optically transparent to the light in the observation area.

The evanescent light generating means is the measurement sample 2
A prism 30 (for example, a 90 ° prism) having a surface 30a on which 8 can be set, a laser light source 32 such as a gas laser or a semiconductor laser, and the laser light source 3
The optical fiber cable 34 for guiding the laser light output from the optical fiber 2 toward the prism 30 and the collimator lens 36 for condensing the laser light emitted from the emission end of the optical fiber cable 34 on the surface 30a of the prism 30.
And the laser beam is incident on the surface 30a of the prism 30 at an angle satisfying the condition of total reflection.
0 and an adjusting mechanism 38 for adjusting the relative position between the output end of the optical fiber cable 34 and the incident angle of the laser light.

The measurement sample 28 is set on the surface 30a of the prism 30 through a matching oil (not shown) for matching the refractive index with the prism 30.

The vibrating means includes a Z-axis modulation signal generator 40 capable of outputting a predetermined vibration frequency signal (that is, a Z-axis modulation signal), and the cantilever 26 in the Z direction, that is, the measurement sample 28 based on the Z-axis modulation signal. And a piezoelectric actuator 42 that vibrates in a direction perpendicular to the.

The first detecting means is, for example, a semiconductor laser 44 capable of emitting a predetermined laser beam toward the back surface of the free end of the cantilever 26, and the efficiency of reflection of the laser beam from the semiconductor laser 44 and the laser beam. Of the cantilever 26 to prevent it from mixing with the evanescent light
A semiconductor laser reflecting surface (metal coat) 45 provided at the tip of the laser, and a two-divided photodetector 46 capable of receiving the laser light reflected from the semiconductor laser reflecting surface 45 and outputting an electric signal based on the change in the light quantity. And a lock-in amplifier 48 that detects a change in the vibration frequency of the cantilever 26 caused by the interaction between the probe 24 and the measurement sample 28 based on the electric signal output from the two-divided photodetector 46. There is.

The two-divided photodetector 46 is constructed so that the spot position of the laser light when the cantilever 26 is not displaced is the reference position (that is, on the two-division line).

Therefore, in this state, the current values output from both light receiving surfaces (not shown) are equal to each other. When the cantilever 26 is displaced and the spot position is shifted, the current values output from both light receiving surfaces are relatively changed.

The lock-in amplifier 48 has a function of detecting the displacement of the cantilever 26 based on the changed current value and outputting the detection data to the servo means (50, 52) described later.

The servo means is a scanner 50 that holds the prism 30 upside down so that it can move three-dimensionally, and the distance between the vibration center of the cantilever 26 and the measurement sample 28 based on the detection data output from the lock-in amplifier 48. And a Z-servo circuit 52 for driving and controlling the scanner 50 so as to maintain the constant value.

The scanning means is provided with an XY scanning circuit 54 for driving and controlling the scanner 50 to relatively non-contact scan the probe 24 with respect to the measurement sample 28.

The second detecting means receives the scattered light generated by the contact of the probe 24 with the evanescent light generated near the surface of the measurement sample 28, and outputs an electric signal corresponding to the amount of light. A conversion element 56 is provided.

Further, in order to improve the light collection efficiency of the photoelectric conversion element 56, the photoelectric conversion element 56 is arranged substantially coaxially with the optical axis of the objective lens 64, and further, the scattered light passing through this objective lens 64 is The distance between the photoelectric conversion element 56 and the objective lens 64 is adjusted so that the light receiving surface of the photoelectric conversion element 56 is focused. The distance between the photoelectric conversion element 56 and the objective lens 64 can be adjusted manually, or
This is done by providing an adjusting mechanism (not shown) for adjusting this.

The image analysis means detects an AC component (AC component) synchronized with the vibration frequency of the cantilever 26 so as to detect the relative value of the intensity of the evanescent light based on the electric signal output from the photoelectric conversion element 56. Based on the lock-in amplifier 58 for detection and the electric signal output from the photoelectric conversion element 56, a frequency component (that is, a DC component) sufficiently lower than the vibration frequency of the cantilever 26 is detected so as to detect the intensity of the entire evanescent light. (DC component)) and an image that images the refractive index distribution on the surface of the measurement sample 28 by performing image processing on the detection signal output from the lock-in amplifier 58 or the low-pass filter 60 that selectively detects only the (DC component)). And an analysis device 62.

The operation of this embodiment will be described below.

First, it is assumed that the laser light incident on the surface 30a of the prism 30 is constantly adjusted by the adjusting mechanism 38 so as to be incident on the surface 30a of the prism 30 at an angle satisfying the condition of total reflection. .

In such a state, the laser light source 3
The laser light emitted from 2 is the optical fiber cable 3
The light is guided to the collimator lens 36 via 4.

The collimator lens 36 converts the laser light into parallel light having a predetermined diameter, and then the surface 30 of the prism 30.
It is incident on a.

The evanescent light (not shown) is generated near the surface of the measurement sample 28 by the laser light incident on the surface 30a of the prism 30.

At this time, based on the vibration frequency signal output from the Z-axis modulation signal generator 40, the piezoelectric actuator 42 causes the cantilever 26 to vibrate in the Z direction (that is, substantially perpendicular to the measurement sample 28) by a predetermined vibration. Vibrate at a frequency.

At this time, the laser light emitted from the semiconductor laser 44 toward the semiconductor laser reflecting surface 45 of the cantilever 26 is reflected by the semiconductor laser reflecting surface 45, and then is irradiated on the two-divided photodetector 46.
It is converted into an electric signal corresponding to the change in light quantity.

The lock-in amplifier 48, based on the vibration frequency signal output from the Z-axis modulation signal generator 40,
A change in vibration frequency of the cantilever 26 is detected and the detection data is output to the Z servo circuit 52.

The Z servo circuit 52 drives and controls the scanner 50 based on the detection data so as to keep the distance between the vibration center of the cantilever 26 and the measurement sample 28 constant.

As a result, the probe 24 is vibrated in a non-contact state with the measurement sample 28.

In this state, the scattered light generated by the probe 24 coming into contact with the evanescent light is condensed on the photoelectric conversion element 56 by the objective lens 64 and converted into an electric signal corresponding to the amount of light.

The electric signal output from the photoelectric conversion element 56 is input to the lock-in amplifier 58 or the low-pass filter 60.

The lock-in amplifier 58 detects an AC component (AC component) synchronized with the vibration frequency of the cantilever 26 so as to detect the relative value of the intensity of the evanescent light based on the input electric signal.

The detection signal output from the lock-in amplifier 58 is input to the image analysis device 62.

The image analysis device 62 subjects the input detection signal to image processing to image the refractive index distribution on the surface of the measurement sample 28.

On the other hand, the low-pass filter 60 detects a total intensity of the evanescent light on the basis of the input electric signal so that the frequency component (that is, the DC component (DC component) sufficiently lower than the vibration frequency of the cantilever 26 is detected. ))
Only select and detect.

The detection signal output from the low-pass filter 60 is input to the image analysis device 62 and similarly subjected to image processing.

Such processing is performed over the entire scanning area. That is, while vibrating the cantilever 26,
By driving and controlling the scanner 50 through the XY scanning circuit 54, the probe 24 is relatively contactlessly scanned with respect to the measurement sample 28, and the refractive index distribution on the entire surface of the measurement sample 28 is measured.

As described above, according to the near-field microscope of the present embodiment, the surface of the measurement sample 28 is maintained while the distance between the vibration center of the cantilever 26 and the measurement sample 28 is kept constant by the servo means (50, 52). The evanescent light localized in the can be detected with high accuracy.
As a result, the refractive index distribution on the surface of the measurement sample 28 can be measured with high resolution and high sensitivity.

It is needless to say that the present invention is not limited to the configuration of the above-mentioned embodiment and can be variously modified within the scope obvious to those skilled in the art.

[0065]

According to the near-field microscope of the present invention, the evanescent light localized on the surface of the measurement sample is enhanced while the distance between the vibration center of the cantilever and the measurement sample is kept constant by the servo means. It can be detected accurately. As a result, it becomes possible to measure the refractive index distribution on the surface of the measurement sample with high resolution and high sensitivity.

[Brief description of drawings]

FIG. 1 is a block diagram schematically showing the overall configuration of a near-field microscope according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing a configuration of a conventional near-field microscope.

[Explanation of symbols]

24 ... Probe, 26 ... Cantilever, 28 ... Measurement sample, 3
0, 32, 34, 36, 38 ... Evanescent light generating means, 40, 42 ... Vibrating means, 44, 45, 46, 48 ...
First detecting means, 50, 52 ... Servo means, 54 ... Scanning means, 56 ... Second detecting means, 56, 58, 60, 62
… Image analysis means.

Claims (1)

[Claims] 1. A cantilever having a probe on the surface of its free end, an evanescent light generating unit capable of setting a predetermined measurement sample and generating evanescent light in the vicinity of the surface of the measurement sample, and the predetermined cantilever. Vibrating means for vibrating at a vibration frequency of, and based on the optical characteristics of the laser light that irradiates the free end back surface of the cantilever with laser light and is reflected from the free end back surface, between the probe and the measurement sample. A first detection unit that detects a change in the vibration frequency caused by an interaction between the first and second detection units, and a vibration center of the cantilever and the measurement sample based on a first detection signal detected by the first detection unit. The probe is moved forward based on the servo means for keeping the distance between them constant and the first detection signal detected by the first detection means. Scanning means for relatively non-contact scanning with respect to the measurement sample, and second detection means for detecting the intensity of scattered light generated by the probe coming into contact with the evanescent light generated near the surface of the measurement sample. A near-field microscope, comprising: and an image analysis unit for image-analyzing the refractive index distribution of the surface of the measurement sample based on the second detection signal detected by the second detection unit.