JP7031851B2 - Atomic force microscope - Google Patents

Description

The present invention relates to an atomic force microscope, and more particularly to a technique for automatically initializing various control parameters of a control circuit such as a feedback circuit for obtaining a clear image.

Atomic Force Microscope (AFM) measures the shape of a sample by measuring the atomic force acting on the sample and the probe provided at the tip of the cantilever. The cantilever has a cantilever structure, and the probe and the sample surface are brought into contact with each other with a slight force, and the distance between the probe and the sample (distance in the Z direction) is feedback-controlled so that the amount of deflection of the cantilever is constant (distance in the Z direction). By scanning on a horizontal plane (on an XY plane) while controlling PID as an example, the surface shape is imaged.

In such an atomic force microscope, it is necessary to initially set the scanning conditions (scanning range, speed, resolution, etc.) and the gain (control parameter) of PID control before starting the measurement of the sample. Further, in order to obtain a clear image, a prescan is executed and the operator visually observes the image to adjust the gain of the PID control so as to be an optimum value.

Such adjustment of control parameters can be performed in a short time of about several minutes by a skilled person, but it may take a long time for an unskilled operator such as a beginner. However, there is a problem that a suitable setting cannot be made.

Further, for example, Patent Document 1 describes an initial setting method at the time of replacement of a cantilever, but does not disclose automatic adjustment of control parameters such as gain of PID control. Therefore, there has been a growing demand for somehow automating the setting of control parameters.

Japanese Unexamined Patent Publication No. 11-316243

The present invention has been made to solve such a conventional problem, and an object thereof is to provide an atomic force microscope capable of automatically setting control parameters of a control circuit. To do.

In order to achieve the above object, the present invention detects the surface shape of the sample by bringing the cantilever into contact with the sample placed on the sample table and scanning the sample so that the amount of deflection of the cantilever is constant. In an atomic force microscope, a Z-direction scanner that displaces the sample table in the deflection direction of the cantilever, a control circuit that controls the displacement of the Z-direction scanner, and a parameter setting unit that sets control parameters of the control circuit. The parameter setting unit scans the sample to acquire unevenness data on the surface of the sample, adjusts the control parameters, and the inclination of the edge of the unevenness data is larger than a preset first threshold value. It is characterized in that the control parameter when it becomes large is set as a control parameter .

In the atomic force microscope according to the present invention, since the control parameters of the control circuit are automatically set, it is possible to set the initial settings of the control parameters in a short time and with high accuracy regardless of the skill level of the operator.

FIG. 1 is a block diagram showing a configuration of an atomic force microscope according to an embodiment of the present invention. FIG. 2 is an explanatory diagram showing two photodetectors provided on the photodiode and the laser beam to be irradiated. FIG. 3A is a flowchart showing a procedure of setting processing when measuring a sample with an atomic force microscope according to an embodiment of the present invention. FIG. 3B is a flowchart showing a procedure of setting processing when measuring a sample with an atomic force microscope according to an embodiment of the present invention. 4A and 4B are views showing an AFM image obtained by prescan. FIG. 4A is a case where an image cannot be obtained, FIG. 4B is a case where the image is unclear, and FIG. 4C is a case where the image is clear. show. FIG. 5 is a diagram showing an example of an AFM image obtained by prescan. 6A and 6B are profiles obtained when the AFM image shown in FIG. 5 is scanned in the X direction. FIG. 6A shows a case where there is a tilt, and FIG. 6B shows a case where there is no tilt. 7A and 7B are diagrams showing an AFM image obtained by prescan and a profile obtained when scanning in the X direction. FIG. 7A is a case where the AFM image is unclear, and FIG. 7B is a case where the AFM image is clear. , (C) show the profile when the AFM image is unclear and when it is clear. FIG. 8 is a diagram showing profile data of the outward route and the inbound route in the X direction when the prescan is performed. FIG. 8A shows a low degree of profile matching, and FIG. 8B shows a high degree of profile matching. Show the case.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Structure explanation of this embodiment]
FIG. 1 is a block diagram showing a configuration of an atomic force microscope according to an embodiment of the present invention. The atomic force microscope 100 according to the present embodiment includes a scanner 15, a cantilever 11, a laser diode 17, and a photodiode 18, and the scanner 15 is tilted in the cantilever direction so that the amount of deflection of the cantilever 11 is constant. It is displaced in the Z direction (this is the Z direction), and the shape of the sample is measured based on this amount of displacement.

As shown in FIG. 1, the atomic force microscope 100 according to the present embodiment supports the sample table 13 on which the sample 12 to be measured is placed and the sample table 13 provided below the sample table 13. It is equipped with a scanner 15. Further, below the scanner 15, a motor 16 for moving the scanner in the Z direction is provided.

The scanner 15 includes a piezo actuator (Z-direction scanner, hereinafter referred to as “Z piezo 15a”) that displaces the sample table 13 in the Z direction. Further, it is provided with a piezo actuator (hereinafter referred to as "XY piezo 15b") that displaces the sample table 13 in the XY plane direction, which is a plane orthogonal to the Z direction (a plane having the Z direction as a normal).

Further, it includes a Z piezo control circuit 22 that outputs a control voltage to the Z piezo 15a, an XY piezo control circuit 23 that outputs the control voltage to the XY piezo 15b, and a motor control circuit 24 that outputs the control voltage to the motor 16. ..

By applying a control voltage to the Z piezo 15a and the XY piezo 15b, the sample table 13 can be finely displaced in the Z direction and the XY plane in the order of nanometers (nm). The Z direction is, for example, a vertical direction, and the XY plane is, for example, a horizontal plane.

A cantilever 11 having a cantilever spring structure is provided near the upper part of the sample table 13. At the tip of the cantilever 11, a probe 11a that comes into contact with the sample placed on the sample table 13 with a fine force is provided. Further, the rear end of the cantilever 11 is supported by a support member (not shown).

A laser diode 17 and a photodiode 18 are provided above the cantilever 11. The laser diode 17 is, for example, a semiconductor laser, and irradiates the back surface of the probe 11a of the cantilever 11 with a laser beam. The photodiode 18 receives the light reflected by the cantilever 11. A mirror or a beam splitter that reflects laser light is provided between the laser diode 17 and the cantilever 11, and between the cantilever 11 and the photodiode 18, but the description is omitted in FIG. 1.

The photodiode 18 has photodetectors 18a and 18b divided into two blocks as shown in FIG. 2A. The photodetector is not limited to two divisions, and a four division photo detector can also be used. The photodiode 18 is connected to the differential amplifier 19, and the differential amplifier 19 is connected to the amplitude detector 20.
The differential amplifier 19 detects the difference signal of the signals detected by the photodetectors 18a and 18b, and outputs the difference signal to the amplitude detector 20.

The amplitude detector 20 converts the difference signal output from the differential amplifier 19 into an RMS signal (Root Mean Square; effective value), and outputs the converted RMS signal to the control circuit 21. For example, as shown in FIG. 2A, when the center of the laser beam indicated by the dotted line substantially coincides with the center N1 of the photodiode 18, the difference signal between the detection signal Sa of the photodetector 18a and the detection signal Sb of the photodetector 18b. Is almost zero. On the other hand, as shown in FIG. 2B, when the center of the laser beam is deviated from the center N1 of the photodiode 18, the difference signal between the detection signals Sa and Sb becomes large, and eventually the RMS signal becomes large. That is, the RMS signal is an index indicating the amount of deviation of the laser beam irradiation position with respect to the center N1 of the photodiode 18.

The control circuit 21 includes a calculation unit 21a that compares the RMS signal with the target voltage output from the target voltage output unit 31 described later, and executes PID control to bring the RMS signal closer to the target voltage (difference). Is set to zero), and the control voltage of the Z piezo 15a is calculated. The calculated control voltage is output to the Z piezo control circuit 22. The control circuit 21 is, for example, a feedback circuit that executes PID control.

Bringing the RMS signal closer to the target voltage means aligning the position of the laser beam received by the photodiode 18 with the desired position, for example, as shown in FIG. 2A, the center of the laser beam aligns with N1. This means that the amount of deflection (displacement in the Z direction) of the cantilever 11 is kept constant.

By outputting the control voltage calculated by the control circuit 21 to the Z piezo control circuit 22, the Z piezo control circuit 22 controls to keep the amount of deflection of the cantilever 11 constant. At this time, the control characteristics are improved by adjusting the integrated gain (control parameter of the control circuit) of the PID control by the method described later.

Further, as shown in FIG. 1, the atomic force microscope 100 of the present embodiment includes a controller 30. The controller 30 includes a target voltage output unit 31, a parameter setting unit 32, an image generation unit 33, a drive command unit 34, and an image analysis unit 35.

Note that the controller 30 shows only the components for setting the control parameters of the control circuit 21 before starting the measurement of the sample 12, and the description of other control elements is omitted. Further, the controller 30 can be configured by, for example, a general-purpose microcontroller including a CPU, a memory, and an input / output unit.

The target voltage output unit 31 sets the target voltage of the RMS signal detected by the amplitude detector 20 and outputs it to the calculation unit 21a.

When starting the measurement of the sample 12, the drive command unit 34 outputs a drive command signal to the Z piezo control circuit 22, the XY piezo control circuit 23, and the motor control circuit 24.

The image generation unit 33 acquires the control voltage output from the control circuit 21 to the Z piezo control circuit 22, and images the surface shape of the sample 12 based on this control voltage.
That is, as described above, the probe 11a of the cantilever 11 and the surface of the sample 12 are brought into contact with each other with a minute force, and a control voltage is applied to the Z piezo 15a so that the amount of deflection of the cantilever 11 becomes constant. The distance between the sample 12 and the sample 12 is controlled. Further, the XY piezo 15b is scanned in a plane on the XY plane. Specifically, the entire XY plane is scanned by reciprocating the scan in the X direction (first direction) in a zigzag manner. Then, the amount of movement of the scanner 15 in the Z direction indicates the magnitude of the sample 12 in the Z direction, that is, the unevenness of the sample 12, and the unevenness data of the sample 12 on the XY plane can be obtained. In other words, the control voltage output from the control circuit 21 to the Z piezo control circuit 22 indicates the unevenness of the sample 12. Based on this control voltage, the image generation unit 33 imparts contrast and images the surface shape of the sample 12.

The parameter setting unit 32 analyzes the image generated by the image generation unit 33 at the time of setting before starting the measurement of the sample 12, and sets the control parameter of the PID control based on the analysis result of the image. Specifically, the integrated gain of PID control is set to be the optimum value. In this embodiment, an example in which only the integrated gain is set is shown as an example, but the proportional gain and the differential gain may be controlled so as to have appropriate numerical values.

[Explanation of operation of this embodiment]
Next, the processing procedure at the time of setting by the atomic force microscope 100 according to the present embodiment will be described with reference to the flowcharts shown in FIGS. 3A and 3B. This process is performed, for example, as a setting when a new sample 12 to be measured is placed on the sample table 13 and the measurement of the sample 12 is started.

First, when the sample 12 is placed on the sample table 13, in step S11, the motor control circuit 24 moves the motor 16 by a preset reference value in the Z direction. The reference value is set to a distance such that the cantilever 11 comes into contact with the surface of the sample 12 with a slight force.

In step S12, the parameter setting unit 32 sets the integrated gain (control parameter) of the control circuit 21 to the initial value and performs the prescan. That is, the XY piezo 15b is driven to scan the sample table 13 in a plane, and the Z piezo 15a is further driven so that the laser beam received by the photodiode 18 is at a desired position. A surface image of the sample table 13 is generated based on the control voltage output from the control circuit 21 at this time. In step S13, an AFM (Atomic Force Microscope) image is acquired. The prescan is continuously performed until the process of setting the control parameters is completed.

In step S14, the image analysis unit 35 determines whether or not the acquired AFM image is a clear image. Whether or not the image is clear can be determined by using a well-known image processing technique based on factors such as image contrast, noise, and brightness change at the edge portion.

When a clear image is obtained (YES in step S14), the position of the sample table 13 in the Z direction and the integrated gain (control parameter) set in the control circuit 21 are appropriately set. Then, in step S34, the controller 30 shifts to the execution of the main scan for actually measuring the sample 12. That is, the setting of the control parameter is completed.

On the other hand, when a clear image cannot be obtained (NO in step S14), in step S15, the image analysis unit 35 determines whether or not an image is obtained. For example, when it is clear that the image generated by the image generation unit 33 has no edges and no unevenness is detected, that is, as shown in FIG. 4A, the surface of the sample 12 cannot be confirmed. If it is an image, it is determined that there is no image. If it is determined that there is no image, the process proceeds to step S16. If it is determined that there is an image, the process proceeds to step S20 in FIG. 3B.

In step S16, the drive command unit 34 outputs a drive command to the motor control circuit 24. The motor control circuit 24 drives the motor 16 to bring the sample table 13 closer to the cantilever 11. Further, in step S17, the Z piezo control circuit 22 sets the Z piezo 15a to the initial position.

In step S18, an AFM image is generated based on the data acquired by the prescan. In step S19, it is determined whether or not the image exists again, and if there is no image, the process returns to step S15. When the sample table 13 approaches the cantilever 11 by driving the motor 16, for example, the unevenness of the surface of the sample 12 is imaged as shown in FIG. 4 (b). Further, when the sample table 13 reaches the optimum position, the unevenness of the surface of the sample 12 is clearly imaged as shown in FIG. 4 (c).

Then, when the surface image becomes clear and it is determined in step S19 that there is an image, the process proceeds to step S20. In step S20, the image generation unit 33 reads the profile when scanning along one line in the X direction. FIG. 5 is an image showing an example of the surface of the sample 12, and the black circular portion indicates the recess. For example, the profile when scanning along the straight line M1 is read.

In step S21, it is determined whether or not the inclination (X tilt) of the X line of the profile is zero. For example, as shown in FIG. 6A, when the profile shown by the reference numeral k1 is obtained, the center position of each uneven portion is inclined, and the line m1 connecting the center positions is in the horizontal axis direction of the image. On the other hand, it is inclined. In such a case, it is determined that X tilt = 0. In step S22, the Z piezo control circuit 22 drives the Z piezo 15a so that X tilt = 0. As shown in FIG. 6B, when the profile shown by the reference numeral k2 is obtained and the line m2 is substantially parallel to the horizontal axis direction of the image, it is determined that X tilt = 0.

When X tilt = 0, in step S23, it is determined whether or not the inclination (Y tilt) of the Y line of the profile is zero. If Y tilt = 0, in step S25, the Z piezo control circuit 22 drives the Z piezo 15a so that Y tilt = 0.

When Y tilt = 0, in step S26, it is determined again whether or not X tilt and Y tilt are zero based on the profile obtained by prescan, and when it is zero, step S28 is determined. Proceed to the process. By the processing of steps S20 to S27, the inclination of the image can be corrected.

In step S28, the image generation unit 33 reads the X-line profile of the acquired image.
In step S29, the parameter setting unit 32 adjusts the integrated gain of the control circuit 21.

In step S30, the image analysis unit 35 is set to the control circuit 21 based on at least one factor of the inclination of the edge of the uneven portion, the contrast, and the comparison of the reciprocating profile based on the image obtained by the prescan. It is determined whether or not the integrated gain is an appropriate value, and if it is not appropriate, the integrated gain is adjusted so that it is an appropriate value. Hereinafter, it will be described in detail.

(Inclination of the edge of the uneven part)
For example, as shown in FIG. 7 (a), when the image obtained by prescan is slightly unclear, the profile of the X line (horizontal line of the image) is shown by reference numeral p2 in FIG. 7 (c). It becomes a curve that becomes smooth at the edge part like this. As shown by reference numeral R1, the slope of the rising edge or the falling edge of the curve p2 at the edge of the uneven portion is gentle. The image analysis unit 35 calculates the inclination d1 of the edge, and determines whether or not the inclination d1 is larger than the preset threshold value dth (first threshold value). When d1 <dth, it is determined that the integrated gain is not an appropriate numerical value (NG).

Then, the integrated gain is changed in the direction in which the gradient d1 increases, and when d1 ≧ dth, it is determined that the integrated gain has become an appropriate numerical value. For example, when the slope d1 becomes large and the curve shown by the reference numeral p1 in FIG. 7C is obtained, it is determined that the integrated gain is appropriate. In this case, the image obtained by the prescan is a clear image as shown in FIG. 7B.

(contrast)
When the image is slightly unclear as in FIG. 7 (a) described above, the X-line profile has a small difference Q1 in amplitude between the concave portion and the convex portion (see FIG. 7 (c)). Therefore, the contrast when imaged is low. The image analysis unit 35 calculates the difference Q1 of the amplitude and determines whether or not the difference Q1 is larger than the preset threshold value Qth (second threshold value). When Q1 <Qth, it is determined that the integrated gain is not an appropriate value (NG).

Then, the integrated gain is changed in the direction in which the difference Q1 increases, and when Q1 ≧ Qth, it is determined that the integrated gain has become an appropriate numerical value. For example, when the difference Q1 becomes large and the amplitude of the curve shown by the reference numeral p1 in FIG. 7C is obtained, it is determined that the integrated gain is appropriate.

(Comparison of round-trip profiles)
As described above, when scanning the sample 12 with the cantilever 11, the sample table 13 is scanned in a zigzag manner in the X direction on the XY plane. In this case, although the outbound and inbound routes in the X-axis direction are slightly displaced in the Y direction, almost the same X-line is scanned, so there should be no significant difference in profile between the outbound and inbound routes. Therefore, by acquiring the outbound and inbound profiles and comparing them, it is determined whether or not the integrated gain is appropriate.

In FIG. 8A, reference numeral q1 shows an example of an outward profile, and reference numeral Q2 shows an example of a return profile. As is clear from FIG. 8A, there is a large deviation between the curves q1 and q2. In such a case, the integrated gain is adjusted so that the curves q1 and q2 match or almost match. Then, as shown in FIG. 8B, when the difference between the curves q1 and q2 becomes almost zero, it is determined that the integrated gain is appropriate. As an example, the determination method is to calculate the degree of matching between the outbound profile and the inbound profile by calculating a correlation coefficient or the like, and when the degree of matching is larger than a preset third threshold value, the outbound profile and the inbound profile are determined. Judge that the profiles of are matched.
As described above, it is not necessary to compare the inclination of the edge of the uneven portion, the contrast, and the reciprocating profile, and the integrated gain can be set by performing at least one of these.

Then, in step S31 of FIG. 3B, when it is determined that the integrated gain is appropriate by the above-mentioned method, it is determined that it is “OK”, and the process proceeds to step S32.
In step S32, the image generation unit 33 generates an AFM image from the data obtained by the prescan. In step S33, it is determined whether or not the generated AFM image is a clear image, and if a clear image is obtained, the setting of the integrated gain is terminated and the operation proceeds to the main scan (step S34). ).

In this way, the integral gain (control parameter) set by the control circuit 21 before starting the measurement of the sample 12 can be automatically set.

In this way, the atomic force microscope 100 according to the present embodiment can achieve the following effects.
(1)
A prescan is executed at the time of measuring the sample, and the control parameter of the control circuit 21 (for example, the integrated gain of PID control) is automatically set based on the unevenness data of the sample surface. Therefore, since the control parameters can be set regardless of the skill level of the operator, the control parameters can be optimally set in a short time.

(2)
A profile in the X direction was generated from the data obtained by prescanning the sample surface, and further, the control parameters of the control circuit 21 were adjusted, and the slope of the edge obtained from the profile exceeded the threshold value dth (first threshold value). Obtain the control parameter of the time and set this as the control parameter. The large inclination of the edge of the uneven portion means that the image is clearly displayed, and it is possible to set the control parameter to an appropriate numerical value. Therefore, it is possible to appropriately set the control parameters in a short time regardless of the skill level of the operator.

(3)
A profile in the X direction is generated from the data obtained by prescanning the sample surface, the control parameters of the control circuit 21 are adjusted, and the difference Q1 in the amplitude of the uneven portion obtained from the profile is the threshold value Qth (second threshold value). ) Is obtained, and this is set as the control parameter. The fact that the difference in the amplitude of the uneven portion is large means that the contrast of the image is large and the image is clearly displayed, and it is possible to set the control parameter to an appropriate numerical value. Therefore, it is possible to appropriately set the control parameters in a short time regardless of the skill level of the operator.

(4)
From the data obtained by prescanning the sample surface, a one-way profile in the X direction is generated, and the control parameters of the control circuit 21 are adjusted, so that the outbound profile and the inbound profile have a high degree of agreement (third). If it is larger than the threshold value of), the control parameter at this time is obtained and set as the control parameter. The fact that the profiles match on the outward trip and the return trip means that the scan has been performed accurately and the profile has been acquired, and it is possible to set the control parameters to appropriate numerical values. Therefore, it is possible to appropriately set the control parameters in a short time regardless of the skill level of the operator.

Although embodiments of the present invention have been described above, the statements and drawings that form part of this disclosure should not be understood to limit the invention. This disclosure will reveal to those skilled in the art various alternative embodiments, examples and operational techniques.

11 Cantilever 11a Probe 12 Sample 13 Sample stand 15 Scanner 15a Z Piezo (Z direction scanner)
15b XY piezo 16 motor 17 laser diode 18 photodiode 18a, 18b photodetector 19 differential amplifier 20 amplitude detector 21 control circuit 21a arithmetic unit 22 Z piezo control circuit 23 XY piezo control circuit 24 motor control circuit 30 controller 31 target voltage output unit 32 Parameter setting unit 33 Image generation unit 34 Drive command unit 35 Image analysis unit 100 Atomic force microscope

Claims (4)

In an atomic force microscope in which a cantilever is brought into contact with a sample placed on a sample table and the sample is scanned so that the amount of deflection of the cantilever is constant to detect the surface shape of the sample.
A Z-direction scanner that displaces the sample table in the deflection direction of the cantilever, and
A control circuit that controls the displacement of the Z-direction scanner,
A parameter setting unit that sets the control parameters of the control circuit,
Have,
When the parameter setting unit scans the sample to acquire unevenness data on the surface of the sample and adjusts the control parameters so that the inclination of the edge of the unevenness data becomes larger than a preset first threshold value. To set the control parameter of
Atomic force microscope featuring. In an atomic force microscope in which a cantilever is brought into contact with a sample placed on a sample table and the sample is scanned so that the amount of deflection of the cantilever is constant to detect the surface shape of the sample.
A Z-direction scanner that displaces the sample table in the deflection direction of the cantilever, and
A control circuit that controls the displacement of the Z-direction scanner,
A parameter setting unit that sets the control parameters of the control circuit,
Have,
When the parameter setting unit acquires unevenness data on the surface of the sample obtained by scanning the sample and adjusts the control parameters so that the amplitude of the unevenness data becomes larger than a preset second threshold value. To set the control parameter of
Atomic force microscope featuring . The parameter setting unit adjusts the control parameters , measures the degree of agreement between the outbound and inbound profiles when the sample is scanned in the first direction on a plane orthogonal to the deflection direction of the cantilever, and the degree of agreement. To set the control parameter when becomes larger than the preset third threshold value as the control parameter.
The atomic force microscope according to claim 1 or 2. The atomic force microscope according to any one of claims 1 to 3 , wherein the control circuit is a PID circuit, and the control parameter is an integral gain .

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