JPH10104245A - Minute displacement measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a minute displacement measuring device which can automatically adjust laser alignment of a cantilever displacement detection means of optical lever type. SOLUTION: This device 1 (atomic force microscope) has a cantilever 7 having a probe 9 which probes a surface of a sample 11 and a displacement detection means of optical lever type which detects displacement of the cantilever 7. This optical lever type displacement detection means is provided with a laser beam scanning means (acoustic to optical modulator 24) which scans a laser beam 23. A position where incident laser beam 31 hits on the cantilever 7 is adjusted so that the intensity of reflected light detected by a photodetector 35 becomes the maximum intensity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a micro-displacement measuring apparatus used in a scanning probe microscope capable of measuring information on a sample surface with high resolution. In particular, the present invention relates to a micro-displacement measuring apparatus having an optical lever-type cantilever displacement detecting means and improved so that the alignment of incident laser light of the means can be automatically adjusted.

[0002]

2. Description of the Related Art One of the scanning probe microscopes is an atomic force microscope (AFM). This is by contacting (or approaching) the cantilever made by microfabrication with the sample, and detecting the deflection of the cantilever due to the atomic force acting between the probe attached to the tip of the cantilever and the sample,
It measures the properties of the sample surface. Means for measuring the deflection of the cantilever include an optical lever method, an optical interferometer,
Piezoresistance and the like can be mentioned, but the optical lever method, which is the simplest means, is most often used.

A typical configuration example of a conventional AFM device will be described. FIG. 5 is a schematic diagram showing the entire configuration of a conventional typical AFM. The AFM 201 shown in FIG.
Is mounted on the sample table 212. The sample stage 212 is provided with an XYZ scanner (sample scanner) 21 existing thereunder.
3 and is scanned in the X, Y, and Z directions. In this specification, the bending displacement direction of the cantilever 207 (usually the vertical direction of the earth) is called a Z direction, and a direction perpendicular to the Z direction is called an XY direction. The scanner 213 is
Usually, it is a piezo actuator type. Scanner 21
3 is fixed on a table 215.

A cantilever 20 is placed on the surface of a sample 211.
A probe 209 having seven tips is applied. Cantilever 2
Reference numeral 07 is held on the column 203 via a highly rigid arm 205. The column 203 and the base 215 are firmly connected. A probe 209 is formed on the lower surface of the distal end portion of the cantilever 207 so as to protrude downward.

[0005] On the upper surface of the cantilever 207, a sample table 21 is provided.
Laser light (incident light 231) is applied from a laser oscillator 221 installed above the light source 2. The incident light 231 is reflected on the upper surface of the cantilever 207 (reflected light 233), and impinges on a photodetector (PD) 235 installed diagonally above the sample table 212. This PD23
5 is a vertical (AB) two-segment type (the optical sensor is
Divided), the incident position of the incident light 233 can be detected. For example, when the incident light 233 hits above,
The light amount of the upper (A) sensor is larger than that of the lower (B) sensor. The opposite occurs when the incident light 233 strikes below.

[0006] The signal of the PD 235 is sent to the Z feedback circuit 251. The signal of the Z feedback circuit 251 is transmitted to the XYZ scanner driver 253 and the display unit 255.
Sent to The control unit 257 controls the XYZ scanner driver and the display unit 255.

The operation of the AFM shown in FIG. 5 will be described. First, the position of the laser incident light 231 is adjusted so that the sum signal (A + B) of the two-divided PD 235 is maximized, and then the position of the PD 235 is adjusted so that the difference signal (A−B) is minimized. After these adjustments are completed, the cantilever 207
Is brought into contact with the sample, and in the case of a sample scan, the sample 21
1 is scanned with respect to the cantilever 207. At this time 2
By monitoring the difference signal of the divided PD 235, the shape of the surface of the sample 211 can be imaged on the display unit 255. It is also possible to image the shape of the sample surface while applying feedback so that the difference signal of the PD 235 is constant, that is, the force acting between the cantilever 207 and the sample 211 is constant (constant force mode). It is.

[0008]

In the conventional optical lever method,
In order to achieve optical alignment, it is necessary to adjust the positions of the incident light and the detector, and the alignment position is often misaligned during scanning due to various drifts and the like. If the alignment position is displaced, there arise problems such as the cantilever moving away from the sample even during scanning, and distortion of the taken image.

In addition, the laser beam, the cantilever, and the PD need to have a fixed positional relationship. Therefore, in order to scan the cantilever relative to the sample, it is necessary to move the entire optical system or move the sample. However, there is a disadvantage that the device configuration becomes large.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved minute displacement measuring device capable of automatically adjusting the laser alignment of an optical lever type cantilever displacement detecting means. Still another object of the present invention is to provide a small displacement measuring device suitable for high frequency scanning.

[0011]

In order to solve the above-mentioned problems, a minute displacement measuring apparatus according to the present invention comprises a cantilever having a probe for probing a sample surface, and an optical lever-type displacement detecting the displacement of the cantilever. A micro-displacement measuring device including: a detecting unit; a basic scanning unit that scans a sample and / or a cantilever; and a control unit of the displacement detecting unit and the scanning unit. The optical lever-type displacement detecting unit outputs a laser beam. A laser beam emitting unit that irradiates the cantilever surface; and a reflected light intensity detecting unit that receives a laser beam reflected from the cantilever surface and detects the intensity of the laser beam. Further, it is characterized by comprising a laser beam scanning means for scanning the laser beam. That is,
Because of the means for scanning the laser beam, the laser beam alignment can be adjusted during the measurement.
Further, since the laser beam can be applied while tracking the cantilever, there is no need to move the entire optical coupling, and a small displacement measuring device suitable for high-frequency scanning can be obtained.

[0012]

In the minute displacement measuring apparatus according to the present invention, the control section controls the laser beam scanning means so that the intensity of reflected light detected by the reflected light intensity detecting means becomes maximum. It is preferable to adjust the position where the incident laser beam hits the cantilever. As a result, problems such as image distortion due to laser misalignment and cantilever separation from the sample during scanning can be solved.

In the minute displacement measuring device according to the present invention, the basic scanning means scans the cantilever in XY directions and Z directions, and the laser beam scanning means scans a laser beam in XY directions. Is preferred. That is, a cantilever scan is performed, and the laser beam of the cantilever displacement detecting means is scanned in the X-Y directions so that the laser beam can be applied while tracking the cantilever. Accordingly, a small displacement measuring device suitable for high-frequency scanning can be obtained without moving the entire optical coupling.

In the minute displacement measuring device according to the present invention, the laser beam scanning means is an acousto-optic modulator (AOM).
It is preferable to provide AOM is also used in laser microscopes and the like, and is suitable for high-speed precision scanning of a laser beam in the XY directions.

In the minute displacement measuring device according to the present invention, it is preferable that the reflected light intensity detecting means is a split photodetector, and that the intensity of the reflected light is detected as a sum signal of the photodetector. It is preferable that this division be divided into four. This is because, in addition to the bending displacement of the cantilever, a torsional displacement (falling) can be detected.

[0016]

Embodiments of the present invention will be described below. FIG.
AF according to the first embodiment (sample scan type) of the present invention
It is a figure which shows the whole structure of M typically. AFM in this figure
1 has a sample stage 12 on which a sample 11 is placed. Sample table 1
Numeral 2 is supported by an XYZ scanner (basic scanning means) 13 located below, and is scanned in the X, Y, and Z directions. In this specification, the bending displacement direction of the cantilever 7 (usually the vertical direction of the earth) is referred to as a Z direction, and a direction perpendicular to the Z direction is referred to as an XY direction. The scanner 13 is
Usually, it is a piezo-actuator type, and can perform scanning over about 100 μm with fine positioning accuracy on the order of nm. The scanner 13 is fixed on a table 15.

A probe 9 at the tip of the cantilever 7 is applied to the surface of the sample 11. The cantilever 7 is held on the column 3 via a highly rigid arm 5. Post 3
And the base 15 are firmly connected. FIG.
It is a perspective view which expands and shows the cantilever of FM. A cutout hole 7b is formed at the base of the cantilever 7, which is a triangular thin plate (the connection with the arm 5). A probe 9 is formed on the lower surface of the tip of the cantilever 7 so as to protrude downward. Near the top of the upper surface of the cantilever 7, a coating 7a for increasing the light reflectance is formed. This coating is to distinguish light reflected from other parts of the cantilever. This cantilever is generally manufactured by a fine processing technique used for manufacturing a semiconductor element.

A laser beam 23 (incident light 31) from a laser oscillator (laser beam emitting means) 21 provided above the sample table 12 hits the upper surface (reflection coating portion) of the cantilever 7. However, unlike the conventional AFM shown in FIG. 5, between the laser oscillator 21 and the cantilever 7, an X-direction AOM 25 and a Y-direction AOM 27, which are laser beam scanning means 24, are provided. The laser beam 23 is deflected in the X and Y directions by the two AOMs 25 and 27, and is scanned toward an arbitrary position.

The incident light 31 hitting the reflective coating on the upper surface of the cantilever 7 is reflected on the upper surface of the cantilever 7 (reflected light 33) and is incident on a photodetector (PD) 35 installed diagonally above the sample stage 12. .

FIG. 4 is a diagram showing a divided structure of the PD of the AFM in FIG. As shown in this figure, the PD 35 is divided into four parts, up, down, left, and right. That is, it is divided into four blocks: an upper left A block, an upper right B block, a lower left C block, and a lower right D block. The signal of each block is
_A , I _B , I _C , and I _D, and an arithmetic circuit 37 described later
Etc., the following signal processing is performed. That is, I <sub>A
+ I B + I C + I</sub> D is the sum signal. The difference signal 1 is I _A
+ I _B - a (I _C + I _D), indicating the reflected light 33 from the cantilever 7 is displaced above or below. The difference signal 2, I _A + I _C - a (I _B + I _D), indicating the reflected light 33 from the cantilever 7 are shifted in any of the right and left. After the difference signal 1 is processed, the image is displayed as a surface shape (topography) on a display unit described later.

Returning to FIG. 1, the description will be continued. The signal of the PD 35 is sent to an arithmetic circuit 37, which calculates the above-described sum signal and difference signal. The sum signal is sent to the peak detection circuit 39 and sent to the AOM driver 43 through the laser feedback circuit 41. The difference signal is Z
The signal is converted into a signal corresponding to the raising and lowering of the Z-axis of the scanner in the feedback circuit 51 and sent to the XYZ scanner driver 53 and the display unit 55. The XYZ scanner driver 57 and the display unit 55 are controlled by the control unit 57.

The operation of the AFM according to the embodiment shown in FIG. 1 will be described.
First, P is adjusted so that the sum signal (A + B + C + D) of the PD is maximized and the difference signal ((A + B)-(C + D)) is minimized.
Adjust the position of D35. When this adjustment is completed, the cantilever 7 is brought into contact with the sample surface, and the scanner 13 is driven to start measurement. At the same time, the laser beam is scanned in the X and Y directions using the AOM driver 43. The scanning speed of the laser beam must be faster than the scanning speed of the measurement, and the sum signal (A +
Feedback is applied to the X and Y AOM drivers 43 so that (B + C + D) is maximized. Further, by passing the difference signal ((A + B)-(C + D)) through a Z feedback circuit, the shape of the sample surface can be measured (constant force mode). When the incident laser light is scanned, a peak appears in the sum signal of the four-divided PD at the same cycle as the cycle of the laser light. Only the difference signal at the time of this peak is used for feedback and image creation. At this time, synchronization detection may be performed using a lock-in amplifier or the like. Further, the difference signal ((A +
By detecting and imaging (C)-(B + D)), an LFM (Lateral Force Microscope) image of the sample can be measured.

FIG. 2 is a diagram schematically showing the overall configuration of an AFM according to a second embodiment (cantilever scan type) of the present invention. In FIGS. 2 and 1, reference numerals having the same last two digits (for example, 121 and 21) have the same names except for those specially described below.

The AFM in FIG. 2 differs from the AFM in FIG. 1 in that the cantilever 107 is scanned in the X, Y, and Z directions (lever scan). That is, the projecting portion 10 projecting in the direction of
3a is provided, and the scanner 10
4 is hanging. A cantilever 107 is connected to a lower end of the scanner 104 via an arm 105. The laser beam 131 for the cantilever displacement detecting means also scans in the X-Y direction in the same manner as in the embodiment of FIG. At this time, the control signal from the control unit 157 is AO
It is sent to the M driver 143.

In the case of this lever scan, the weight of the structure to be scanned is greatly reduced as compared with the sample scan of FIG. Therefore, the resonance frequency of the scan is also several kHz.
Thus, the scanning frequency (speed) can be increased to increase the observation area per unit time.

To perform the lever scan, the AOM combines a low-speed scanner synchronized with the movement of the cantilever and a high-speed scanner that scans only the vicinity thereof. Alternatively, a low-speed and high-speed scanner can be constituted by a single AOM by adding a signal obtained by adding a low-speed scan signal and a high-speed scan signal to the AOM driver. At this time, since the positional relationship between the laser, the cantilever, and the PD changes, the signal from the PD has an offset proportional to the position of the cantilever on the sample surface. The signal may be processed by an arithmetic circuit at a later stage that can be determined and then passed through a Z feedback circuit.
In this case, in order to prevent light from deviating from the light receiving section of the PD, (distance between cantilever and PD) / (AOM ~
Cantilever distance) x (cantilever scan range)
It is sufficient to use a PD having a light receiving area that is sufficiently larger than that.

[0027]

As is clear from the above description, according to the present invention, it is possible to measure a minute displacement without worrying about the alignment of laser light. Further, when laser beam scanning is performed by lever scanning or AOM, high-speed scanning is possible because it is possible to perform scanning at a high speed of about several tens of MHz.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating an overall configuration of an AFM according to a first embodiment (sample scan type) of the present invention.

FIG. 2 is a diagram schematically illustrating an overall configuration of an AFM according to a second embodiment (cantilever scan type) of the present invention.

FIG. 3 is an enlarged perspective view showing a cantilever of the AFM of FIG. 1;

FIG. 4 is a diagram showing a divided configuration of a PD of the AFM of FIG. 1;

FIG. 5 is a schematic diagram showing the entire configuration of a conventional typical AFM.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Atomic force microscope (AFM, micro displacement measuring device) 3 Column 5 Arm 7 Cantilever 7a Reflective coating part 7b Drilled hole 9 Probe 11 Sample 12 Sample table 13 XYZ scanner (basic scanning means) 15 Base 21 Laser oscillator (laser beam) Emission means) 23 Laser beam 24 Acousto-optic modulator period (AOM, laser beam scanning means) 25 X direction AOM 27 Y direction AOM 31 Incident light 33 Reflected light 35 Quadrant photodetector (Reflected light intensity detecting means) 37 Arithmetic circuit 39 Peak Detection circuit 41 Laser feedback circuit 43 AOM driver 51 Z feedback circuit 53 XYZ scanner driver 55 Display unit 57 Control unit 103 a Projection unit 104 XYZ scanner

Claims (7)

[Claims] 1. A cantilever having a probe for probing a sample surface, an optical lever-type displacement detecting means for detecting a displacement of the cantilever, a basic scanning means for scanning a sample and / or a cantilever, and a displacement detecting means And a control unit of a scanning unit, wherein the optical lever type displacement detecting unit applies a laser beam to the surface of the cantilever, and a laser beam reflected from the surface of the cantilever. And reflected light intensity detecting means for detecting the intensity of the laser beam by receiving the laser beam. The optical lever type displacement detecting means further comprises laser beam scanning means for scanning the laser beam. Micro displacement measuring device. 2. The control section controls the laser beam scanning means, and adjusts the position at which the incident laser beam hits the cantilever so that the intensity of the reflected light detected by the reflected light intensity detection means is maximized. The minute displacement measuring device according to claim 1, wherein: 3. The laser beam scanning means according to claim 1, wherein said laser beam scanning means is arranged in a plane (X-
The minute displacement measuring device according to claim 1, wherein a laser beam is scanned in the Y direction). 4. The basic scanning means according to claim 1, wherein
-While scanning in the Y direction and the Z direction, the laser beam scanning means scans the laser beam X-
3. The small displacement measuring device according to claim 1, wherein scanning is performed in the Y direction. 5. The laser beam scanning means according to claim 1, further comprising an acousto-optic modulator (AOM).
4. The minute displacement measuring device according to claim 1. 6. The reflected light intensity detecting means is a split photodetector, and detects the intensity of the reflected light as a sum signal of the photodetectors.
The minute displacement measuring device according to claim 1. 7. An atomic force microscope comprising the minute displacement measuring device according to any one of claims 1 to 6.