JPH05312564A - Scanning type probe microscope - Google Patents

Abstract

PURPOSE:To obtain a scanning type probe microscope which executes easily measurement of high resolution and high precision, while maintaining the rigidity and compactness of an apparatus. CONSTITUTION:A cantilever 4 has a probe at the fore end and a sample 3 set on the upper end part of a tube scanner 2 is disposed below the probe. A mirror 18 is fixed on the lower side of the upper end part of the tube scanner 2. Below the mirror, an optical fiber 14 is disposed so that the light emission end is opposed to the mirror 18. The optical fiber 14 is connected to an optical interferometer 15 which measures the position in the Z direction of the mirror 18, i.e. the sample 3. The result of measurement by the optical interferometer 15 is inputted to a computer 10 and used for calibration of an image constructed therein.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a scanning probe microscope.

[0002]

2. Description of the Related Art An atomic force microscope (AFM) can observe the surface shape of a sample with atomic level resolution by utilizing atomic force. The interatomic force acts between atoms that are very close to each other, and their magnitude changes depending on the interatomic distance. When a sharp probe supported by a flexible cantilever is brought close to the sample surface, an atomic force is generated between atoms at the probe tip and the sample surface, and the cantilever is displaced. In the AFM, the sample surface is scanned using a probe supported at a distance where an atomic force is generated. During scanning, the displacement of the cantilever caused by the unevenness of the sample surface is constantly measured, and the atomic force acting on the tips is detected at that time to obtain the distance between the tips and the unevenness image of the sample. To get Alternatively, during scanning, the displacement of the cantilever is kept constant by performing feedback control or the like, and an uneven image of the sample surface is obtained from the trajectory of the probe. In any case, the unevenness of the surface is obtained from the displacement of the cantilever using the atomic force.

The atomic force F (that is, the restoring force of the cantilever) is detected as the displacement δ of the cantilever. When the cantilever is a rectangular plate, this displacement δ can be expressed by the following equation, where L is length, a is thickness, b is width, and E is elastic modulus. δ = 4L ³ F / a ³ bE

As an example of the method of detecting the displacement δ,
There is an optical lever method in which an optical position detector provided on the back surface of the tip of a cantilever irradiates a laser beam to detect a reflection angle that changes according to the displacement of the cantilever with an optical position detector.

An atomic force microscope using this optical lever system will be briefly described with reference to FIG. The cantilever 4 is brought close to the sample 3 by the approaching motor 1 to a distance where an atomic force acts between the probe portion at the tip and the sample surface. X generated by the X scan signal generator 11 and the Y scan signal generator 12 in accordance with a command from the computer 10.
The scanning signal and the y scanning signal are amplified by the high voltage amplifier 13 and then supplied to the tube scanner 2 to scan the sample 3 in the XY directions. The laser beam emitted from the laser diode 5 is reflected by the mirror 6 and then reflected by the cantilever 4.
Incident on. After that, the laser beam cantilever 4
Is reflected by and is incident on the photodiode 7. The photodiode 7 has two light receiving portions, and each light receiving portion outputs a signal according to the intensity of incident light. Output A of these two light receiving parts
And B are input to the differential amplifier 8 and (AB) is output. In the device of this example, open loop control and feedback control are switched by the switch SW. During open loop control, the output from the differential amplifier 8 (A-
B) is directly taken into the computer 10, and this output (AB) is converted into information in the height direction of the sample 3. On the other hand, during feedback control, the output (AB) from the differential amplifier 8 is input to the servo circuit 9, and the servo circuit 9 keeps the output (AB) of the differential amplifier 8 constant. In order to control the position in the Z direction, the z signal supplied to the tube scanner 2 is output via the high voltage amplifier 13.
This z signal is taken into the computer 10 as information in the height direction of the sample 3.

By the way, the output of the differential amplifier 8 (A-
B) or the z signal from the servo circuit 9 at the time of feedback control must be calibrated in order to convert it into an absolute value of displacement. The following method is available as a calibration method during open loop control.

(1) The tube scanner 2 is moved in the Z direction while the cantilever 4 is in contact with the sample 3, and the relationship between the output of the differential amplifier 8 and the applied voltage of the tube scanner 2 is obtained. The relationship between the voltage applied to the tube scanner 2 and the absolute amount of its displacement is obtained in advance by another sensor, and the output of the differential amplifier 8 is converted into the absolute amount of displacement.

(2) A standard sample whose shape and dimension are known is actually subjected to open loop measurement, the relationship between the output of the differential amplifier 8 and the absolute amount of displacement is obtained, and the output of the differential amplifier 8 is calculated as the absolute displacement. Convert to quantity. Further, as a calibration method during feedback control, there is a method described below.

(3) The displacement of the tube scanner 2 is measured by another sensor, the relationship between the applied voltage to the tube scanner 2 and the absolute value of the displacement (piezoelectric constant) is calculated, and the applied voltage to the tube scanner 2 and the displacement are determined. Find the relationship between absolute quantities.

(4) A standard sample of known shape and dimension is actually subjected to feedback measurement to find the relationship between the applied voltage of the tube scanner 2 and the absolute amount of displacement, and the applied voltage of the tube scanner 2 is determined as absolute displacement. Convert to quantity.

[0011]

The above-mentioned method has the following problems. (1) It is necessary to install a displacement sensor near the tube scanner 2 or install a standard sample on the sample stand each time calibration is performed.

(2) Unless the calibration is done for a long time with care, the operator may not be aware even if the piezoelectric constant of the tube scanner 2 changes due to aging, so that the measurement result is inaccurate. May become.

(3) Since the displacement sensor or the standard sample itself does not have an absolute value that serves as a reference for the length, the piezoelectric constant will be inaccurate if it is not calibrated accurately, and it is converted by this. The measurement result will be inaccurate. An object of the present invention is to provide a scanning probe microscope capable of easily performing high resolution and high accuracy measurement while maintaining the rigidity and compactness of the device.

[0014]

The scanning probe microscope of the present invention comprises a displacement detecting means for detecting the displacement of the probe at the tip of the cantilever, a means for moving the sample stage in a direction perpendicular to the sample surface, and a sample. An optical interferometer for measuring the amount of movement of the table and a means for comparing and calculating the output of the optical interferometer and the output of the displacement detecting means are provided.

[0015]

In the present invention, the signal corresponding to the displacement of the probe at the tip of the cantilever is converted into the absolute amount of actual displacement with reference to the wavelength of light used in the optical interferometer. As a result, the unevenness of the sample can be obtained accurately based on the wavelength of light.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will now be described with reference to the drawings. FIG. 1 shows the configuration of the scanning probe microscope of this embodiment, and FIG. 2 shows a partial sectional view of the displacement measuring section.

As shown in FIG. 2, the cantilever 4 having a probe portion at its tip is fixed to the lower surface of the head 19 by a leaf spring 22. A sample table 21 is provided below the cantilever 4.
The sample 3 placed on is placed. The sample table 21 is fixed to the sample table receiver 20 fixed to the upper end of the tube scanner 2. The tube scanner 2 has three balls 25
Scanner stand 2 supported stably by (only one shown)
It is fixed to 3 by adhesion. The mirror 18 is fixed to the lower side of the sample holder 20. Below that, the optical fiber 14 is arranged so that the light emitting end faces the mirror 18. The optical fiber 14 is fixed via a fiber support 16 fixed to the scanner table 23 by a screw 17. The optical fiber 14 is guided to the outside through a hole provided in the base 26 and connected to the optical interferometer 15 as shown in FIG. The optical interferometer 15 measures the position of the mirror 18, that is, the position of the sample 3 in the Z direction, and outputs the result to the computer 10.

Next, a procedure of calibrating the piezoelectric constant of the tube scanner before the AFM measurement by the feedback control will be described. First, a z signal for moving the tube scanner 2 in the Z direction is output from the computer 10. This z signal is amplified by the high-voltage amplifier 13 and supplied to the tube scanner 2, and the sample 3 is gradually moved in the Z direction. Meanwhile, the optical interferometer 15 monitors the displacement of the mirror 18 and outputs its output v to the computer 10.
The relationship between z and v at this time is as shown in FIG. 3, a voltage Z at which the mirror 18 moves by a distance corresponding to ¼ of the wavelength λ of light.
Is applied to the tube scanner 2, the output v of the optical interferometer 15 is increased from the maximum value to the minimum value or from the minimum value to the maximum value by V due to the interference between the reflected light from the surface of the mirror 18 and the reflected light from the fiber end surface. Change. Then, the computer 10 stores the voltage Z applied to the tube scanner 2 when the output v of the optical interferometer 15 changes by V.
An accurate piezoelectric constant (amount of displacement per unit applied voltage) can be obtained from the value of Z and the wavelength of light introduced into the optical fiber 14. By performing this calibration each time before the AFM measurement, the reliability of the AFM measurement is improved.

Next, AFM measurement by feedback control will be described with reference to FIG. First, the approaching motor 1 brings the cantilever 4 close to the sample 3 to a distance where an atomic force acts between the probe portion at the tip of the cantilever 4 and the sample surface. While maintaining this state, the computer 10 causes the X scan signal generator 11 and the Y scan signal generator 12 to generate x scan signals and y scan signals, which are amplified by the high voltage amplifier 13 and supplied to the tube scanner 2. Scanning in the XY directions. The displacement of the cantilever 4 is kept constant by feedback control during scanning. That is, the servo circuit 9 outputs the z signal for controlling the position of the sample 3 in the Z direction so that the output (AB) from the differential amplifier 8 that is input is kept constant. The z signal is amplified by the high voltage amplifier 13 and supplied to the tube scanner 2.
The computer 10 takes in this z signal as height information of the sample 3 and processes it in synchronization with the x scanning signal and the y scanning signal to construct an uneven image of the sample 3. This z
When converting the signal to the absolute value of the displacement length in the Z direction,
As described above, the piezoelectric constant of the tube scanner 2 calculated before the measurement is used. An image obtained by using this piezoelectric constant has high reliability.

Next, AFM by open loop control
The calibration of the output of the differential amplifier before the measurement will be described. The tube scanner 2 is displaced in the Z direction while the cantilever 4 is in contact with the sample 3. At that time, the output (AB) from the differential amplifier 8 corresponding to the displacement of the cantilever 4 is taken into the computer 10. At the same time, the output v of the optical interferometer 15 corresponding to the displacement of the mirror 18 provided on the lower surface of the tube scanner 2 in the Z direction is taken into the computer 10. The relationship between the output (AB) of the differential amplifier 8 and the output v of the optical interferometer is as shown in FIG. 4, when the cantilever 4 is displaced corresponding to ¼ of the wavelength λ of light. The output of 14 changes by V, and the output of the differential amplifier 8 changes by P. From the relationship between V and P, the output of the differential amplifier when the position of the cantilever 4 changes by the unit length can be calculated.

In AFM measurement by open loop control, the computer 10 is operated while the sample 3 is scanned in the XY directions.
Takes the output (A-B) from the differential amplifier 8 as height information and converts the value into the absolute value of the length by the calibration value obtained before the measurement, so that an accurate uneven image of the sample can be obtained. can get.

[0022]

According to the present invention, there is provided a scanning probe microscope capable of obtaining a highly accurate concave-convex image of a sample based on the wavelength of light.

[Brief description of drawings]

FIG. 1 shows a configuration of a scanning probe microscope according to an embodiment of the present invention.

2 is a partial cross-sectional view of a displacement measuring unit of the scanning probe microscope of FIG.

FIG. 3 shows the relationship between the z signal supplied to the tube scanner and the output v of the optical interferometer.

FIG. 4 shows the relationship between the output (AB) of the differential amplifier and the output v of the optical interferometer.

FIG. 5 shows a configuration of a conventional scanning probe microscope.

[Explanation of symbols]

2 ... Tube scanner, 4 ... Cantilever, 5 ... Laser diode, 7 ... Photodiode, 8 ... Differential amplifier, 10 ... Computer, 15 ... Optical interferometer.

Claims (1)

[Claims] 1. A displacement detecting means for detecting a displacement of a probe at the tip of a cantilever, a means for moving a sample stage in a direction perpendicular to a sample surface, an optical interferometer for measuring a moving amount of the sample stage, and an optical element. A scanning probe microscope comprising a means for comparing and calculating the output of an interferometer and the output of a displacement detecting means.