JPH07248334A - Interatomic force microscope - Google Patents

Abstract

PURPOSE:To prevent any damage to a cantilever from occurring by stopping any movement of a sample when the measured frictional force has exceeded the preset reference value, reducing an amount of flexure to control the cantilever so as to make the frictional force become smaller than another reference value, and starting from measurement in shifting the sample. CONSTITUTION:A frictional force signal control circuit 15 feeds a scanner control circuit 9 with a scan stopping signal at a time when a frictional force signal, being outputted by a frictional force signal conversion circuit 12, is larger than the preset first reference value. After scanning stoppage, the control circuit 15 imparts an interruption signal to a central processing unit 14 which feeds a signal to the scanner control circuit 9 to drive a three-dimensional actuator 8 so to reduce an amount of flexure in a cantilever till the frictional force becomes smaller than the preset second reference value. In succession, in a state that the cantilever 2 is shifted to the right direction as long as the specified distance, sample scanning is carried out again. With this constitution, any possible damage to the cantilever is preventable from occurring.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an atomic force microscope, and in particular, when measuring the shape of a sample having a wall surface such as a groove shape with a cantilever having a tip at the tip, the tip is damaged by twisting. It relates to a means to prevent.

[0002]

2. Description of the Related Art Atomic force microscopes are designed so that the acting atomic force between a cantilever having a probe having a very sharp tip and a sample surface is constant, that is, the bending of the cantilever is constant. The amount is controlled to be always constant, and the sample and the cantilever are moved relatively. Then, the control amount is measured at the same time as the sample is scanned by the probe, and the surface shape of the sample is measured based on the control amount with a resolution of the atomic size level and displayed on the display device. This makes it possible to observe the shape of the sample.

The relative movement between the sample and the cantilever is
It is performed by applying a predetermined voltage to electrodes provided on an actuator using a piezoelectric element.

For example, when observing the surface shape of a mounted sample, the amount of movement of the actuator in each axial direction is controlled so that the sample is placed against the probe in a predetermined region on the XY plane on the sample surface. Relative movement is performed and scanning for measuring the shape of the sample is performed. By such scanning, a so-called interatomic force acts between the probe and the sample, the cantilever bends due to the magnitude of this force, and the probe is displaced in the Z-axis direction, so that the amount of bending of the cantilever is constant. In accordance with the feedback control amount, by scanning the sample while performing feedback control so that
The surface shape of the sample will be obtained.

In order to make the amount of bending of the cantilever constant, the light is emitted from the light source and condensed by the lens,
The laser light radiated and reflected on the reflecting surface of the cantilever,
Light is received by a four-division photodetector composed of four light receiving parts,
It suffices to perform a predetermined calculation based on the output signal from each light receiving unit to obtain the movement amount of the actuator, and perform control to move the sample in the Z direction so as to correspond to the movement amount. Note that such an optical system is usually called an optical lever system.

[0006] In addition, 4 which is composed of the four light receiving portions
A method is also known in which the amount of torsion of the cantilever is detected at the same time by performing a predetermined calculation on the output from each light receiving part of the split photodetector, and the frictional force acting between the probe and the sample surface is also detected at the same time. It is possible to

[0007]

The sample to be measured by the atomic force microscope is not limited to a tangible object having an atomic level size and a very flat shape.

For example, the depth of the groove is about 0.1 (μm), the pit of the magneto-optical disk, and the groove width is 0.5.
(μm) and a depth of about 1.5 (μm), there are many measurement targets provided with a groove having a shape like a contact hole.

In order to measure the surface shape of such a sample, it becomes necessary to perform the measurement using a cantilever having an extremely sharp tip and an elongated probe.

However, when the sample has a groove having a wall surface close to vertical, such as a contact hole,
When measuring using a cantilever equipped with the above-mentioned probe, the probe receives a force from the wall surface, and if a force greater than the limit elastic force in the torsion direction of the cantilever acts, the cantilever will be damaged. There was a point.

[0011]

In order to solve the above problems, the following means are considered.

A cantilever equipped with a probe for measuring the shape of a given sample, a sample stage on which the sample is placed, a light source, a lens for collecting the light emitted from the light source, and a collected light A light receiver having four divided light receiving portions for receiving the reflected light from the cantilever, a moving means capable of relatively moving the sample stage and the probe in three axis directions by a given signal, and the four Using the output signal from the divided light-receiving unit, a predetermined first calculation is performed to obtain a control amount given to the moving means in order to control the deflection amount of the cantilever to a predetermined amount, and based on the control amount, A first process for measuring the shape of the sample, and a second process for measuring the frictional force generated by the torsion of the cantilever by a second predetermined calculation using the output signals from the four divided light receiving parts. Processing It comprises at least processing means, output means for outputting at least the measured shape, and control means for giving a predetermined signal to the moving means and at least processing for moving the probe and the sample stage relatively. .

Then, the control means stops the relative movement between the sample stage and the probe when the frictional force measured in the second processing exceeds a predetermined first reference value, and the frictional force is reduced. The moving means is controlled so as to reduce the bending amount of the cantilever so that the force becomes smaller than a second predetermined reference value, and further, the sample stage is moved by a predetermined distance with respect to the sample, It is an atomic force microscope for starting the treatment of.

[0014]

The atomic force microscope according to the present invention can detect the atomic force and frictional force acting between the cantilever and the sample. When the measured frictional force exceeds a predetermined reference value, stop the movement of the sample and adjust the control amount of the cantilever so that the deflection amount of the cantilever is reduced until the frictional force becomes less than the predetermined reference value. The surface shape is obtained by reducing and repeating the measurement.

That is, when the measured frictional force exceeds a predetermined first reference value, the movement of the sample is stopped so that the frictional force becomes smaller than the predetermined second reference value. The control amount of the cantilever is reduced so as to reduce the bending amount of the cantilever, the sample is moved by a predetermined distance, and the shape measuring process is started.

When the measured frictional force exceeds a predetermined first reference value, the driving of the moving means is stopped,
In order to perform the shape measurement process, in the direction opposite to the relative movement direction of the cantilever with respect to the sample, by performing a process of relatively moving the cantilever and the sample by a predetermined distance,
Furthermore, the cantilever can be prevented from being damaged.

As described above, in the present invention, the frictional force signal is
Even when measuring a sample that has a wall surface that is close to vertical such as a groove shape so that it does not become larger than a predetermined reference value,
It is possible to prevent the cantilever from being damaged. Other,
Even when measuring a sample having a tapered shape or a sample having a rough surface and a surface not rough, the cantilever can be prevented from being damaged.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a constitutional example of an atomic force microscope according to the present invention.

FIG. 1 shows an example of a scanning atomic force microscope using the optical lever method.

1 is a probe, 2 is a cantilever, 3 is a sample,
5 is a laser diode, 6 is a condenser lens, 7 is a four-division photoelectric detector, 8 is a three-dimensional actuator, 9 is a scanner control circuit, 10 is a height signal conversion circuit, 11 is a height signal control circuit, and 12 is friction. Force signal conversion circuit, 13 is a memory, 14
Is a CPU, 15 is a frictional force signal control circuit, 16 is an image processing device, and 17 is a display device.

In addition, the cantilever 2 which causes bending is
It is attached to the cantilever fixing plate.

The three-dimensional actuator 8 is formed by forming a piezoelectric element such as PZT into a cylindrical shape, a common electrode on the inside, and a sample 3 on the surface in the X, Y, and Z axis directions. An electrode for moving the table 4 is provided.

By applying voltages having the same magnitude but different signs to the electrodes "X +" and "X-", the sample 3
Is displaced in the X-axis direction, so that the probe 1 can scan the sample 3 in the X-axis direction. Similarly, the electrode "Y +"
By applying voltages having the same magnitude but different signs to “Y−” (not shown), the sample 3 is displaced in the Y-axis direction, so that the sample 1 is moved in the Y-axis direction by the probe 1. Scanning is possible. In addition, the common electrode and all electrodes "X
It is also possible to expand / contract the piezoelectric element in the Z direction by applying a voltage between “+”, “X−”, “Y +” (not shown), and “Y−”.

The voltage applied to the three-dimensional actuator 8 for such scanning is given from the scanner control circuit 9 to a predetermined electrode according to an instruction from the CPU 14. In this way, the sample 3 and the probe 1 are moved relatively to each other, so that the sample 1 can be scanned in the X-axis and Y-axis directions.

Simultaneously with the above-mentioned scanning, the height signal control circuit 11 drives the scanner control circuit 9 to apply a voltage to a predetermined electrode so that the amount of bending of the cantilever 2 becomes constant, that is, The control is performed so that the interatomic force is constant, and the control amount (voltage applied to the electrode Z) is
The data is sequentially stored in the memory 13.

Then, the image processing device 16 includes a memory 13
A process for displaying the surface shape of the sample is performed based on the control amount stored in the display device 1 to display the surface shape of the sample.
Display on 7. Thereby, the surface shape of the sample can be observed. The memory 13 has a RAM and a ROM. The RAM stores the control amount and the like, and R
A program or the like for the CPU to perform a predetermined operation is built in the OM in advance.

By the way, in order to make the amount of bending of the cantilever 2 constant, the following processing may be performed.

The laser light emitted from the laser diode 5, condensed by the condenser lens 6, applied to the reflecting surface of the cantilever 2 and reflected is received by the four-division photoelectric detector 7.

The output current of each light receiving portion (four light receiving portions a, b, c and d are arranged as shown in the figure) constituting the four-division photoelectric detector 7 is represented by I, which is not shown. /
A V converter (built in the height signal conversion circuit 10) converts the voltage into a voltage, and the height signal conversion circuit 10 calculates a value represented by the following equation, and controls so that this value becomes equal. To do so, the height signal control circuit 11 drives the scanner control circuit 9 to move the sample 3 in the Z-axis direction.

When the output voltages corresponding to the respective light receiving portions a, b, c, d are Va, Vb, Vc, Vd, the height signal conversion circuit 10 performs an operation of H = (Va + Vb)-(Vc + Vd). Then, as described above, the height signal control circuit 11 drives the scanner control circuit 9 to move the sample 3 in the Z-axis direction in order to perform control so that this value (H) becomes equal. The control amount at this time is the control amount stored in the memory 13 described above.

The laser diode 5 emits light,
The optical system that collects the laser beam that is condensed by the condensing lens 6 and that is reflected by the reflecting surface of the cantilever 2 is received by the four-division photoelectric detector 7 constitutes a so-called "optical lever". The configuration as shown in 1 is called an atomic force microscope using an optical lever. Although not shown, for example, these constituent elements can be arranged at predetermined positions in the housing.

Further, the frictional force signal conversion circuit 12 outputs the output voltages corresponding to the light receiving portions a, b, c and d as Va, Vb and V.
If c and Vd, then F = (Va + Vb) − (Vc + V
The calculation of d) is performed, and this becomes the frictional force. It is assumed that the I / V converters corresponding to the light receiving parts a, b, c, d are built in the frictional force signal conversion circuit 12.

The frictional force signal control circuit 15 has a structure capable of setting a plurality of types of reference values in advance, and determines the magnitude of the frictional force of the cantilever 2 given from the frictional force signal conversion circuit 12 and the predetermined value. It is a means for performing a predetermined process including a stop of scanning of the sample 3 by the cantilever 2 by comparing it with a reference value. This operation will be described later with reference to the drawings.

In the structure shown in FIG. 1, the sample 3 is configured to be movable in the three-axis directions by the three-dimensional actuator 8. However, the cantilever 2 having the probe 1 is moved in the three-axis directions by the three-dimensional actuator 8. It goes without saying that a movable structure may be used to scan the probe 1 and measure the shape of the sample 3.

Now, the main operation of the present invention will be described in detail below.

FIGS. 2, 3 and 4 show the state of twisting of the cantilever when the cantilever is scanned rightward in each figure with respect to a sample having deep unevenness such as a contact hole, and at that time. Height signal, which occurs in
It shows the frictional force signal.

2 (b), 3 (b) and 4
(B) shows how the height signal changes. The height signal is output from the height signal conversion circuit 10.

Further, FIG. 2 (c), FIG. 3 (c), and
FIG. 4C shows how the frictional force signal changes. The frictional force signal is output from the frictional force signal conversion circuit 12.

Now, FIG. 2 shows a state in which the probe of the cantilever collides with the wall surface and immediately before the twist occurs.

From this state, when the scanning in the right direction of the drawing is further continued, as shown in FIG. 3A, the cantilever is twisted and a large frictional force signal is detected.

Therefore, a certain reference value 1 is preset in the frictional force signal control circuit 15. For such a value, a dip switch circuit may be provided and an arbitrary reference value may be set by setting the switch.

Now, when the frictional force signal output from the frictional force signal conversion circuit 12 is larger than the reference value 1, the frictional force signal control circuit 15 instructs the scanner control circuit 9 to stop the scanning ( Send a STOP signal). In addition, standard value 1
Can be determined in advance by experiments or the like to find the magnitude of the frictional force when the cantilever breaks, and determine based on this.

Next, at the point where the scanning is stopped, the frictional force signal control circuit 15 causes the CPU 14 to interrupt the CPU 14 so as to perform an interrupt process.
An interrupt signal is given to the PU 14. As shown in FIG. 4, the CPU 14 drives the three-dimensional actuator so that the bending amount of the cantilever is reduced until the frictional force signal becomes smaller than a predetermined reference value 2 similar to the reference value 1. Therefore, a signal is sent to the scanner control circuit 9. CPU
According to the instruction of 14, for example, to move the sample 3 relatively so that the cantilever is pulled up in the figure,
An applied voltage may be applied from the scanner control circuit 9 to the three-dimensional actuator 8.

Then, the sample is scanned again from the state where the cantilever is moved rightward by a predetermined distance. In addition, in order to move the cantilever rightward by a predetermined distance, the sample 3 may be moved leftward. The movement of such a predetermined distance is performed when the scanning is started from a point where the cantilever is relatively pulled up in the figure, and when the cantilever is pulled down relatively, the friction force is the reference value. This is to avoid rescanning the points exceeding 1.

Thus, when the frictional force applied to the cantilever exceeds the predetermined first reference value, the movement of the sample is stopped and the frictional force becomes smaller than the predetermined second reference value. After reducing the amount of bending of the cantilever,
By moving the sample for a predetermined distance and starting scanning,
It is possible to prevent damage to the cantilever.

Further, as a reliable method, after a large frictional force is applied to the cantilever, the frictional force signal is
The scanning is returned until it becomes 0 or smaller than the reference value 2 (the cantilever is moved in the opposite direction relative to the sample). Then, the cantilever can be prevented from being broken by reducing the amount of bending of the cantilever.

In other words, when the frictional force exceeds the predetermined first reference value, the drive of the three-dimensional actuator is stopped and the relative movement direction of the cantilever with respect to the sample for performing the surface shape measurement process. By moving the cantilever and the sample in the opposite direction relative to each other by a predetermined distance, it is possible to reliably prevent the cantilever from being broken.

By imaging only the height signal in a series of operations, the shape of the sample surface having deep irregularities can be measured. If a continuous height signal cannot be obtained due to the separation process between the cantilever and the sample, for example, the shape at the point where a large frictional force of 1 or more is applied to the cantilever has a depth of 1.5. (μ
It is conceivable that the image processing device 16 performs image processing, and that a section where height data is interrupted is interpolated by a polynomial.

FIG. 5 is a flow chart showing the processing until the cantilever is separated from the sample when the cantilever collides with the wall surface of deep unevenness and twists.

In step 100, it is assumed that the frictional force measured by the frictional force signal conversion circuit 12 exceeds the reference value 1 set in the frictional force signal control circuit 15.

At this time, in step 200, the frictional force signal control circuit 15 sends a signal for stopping the activation to the scanner control circuit 9, and the scanning for shape measurement is stopped.

Next, in step 300, the three-dimensional actuator 8 is driven until the frictional force becomes equal to or less than a predetermined value.

Specifically, by the interrupt processing of the CPU 14, after the three-dimensional actuator 8 is driven so that the frictional force becomes smaller than the predetermined second reference value and the amount of bending of the cantilever is reduced, Further, the sample is moved by a predetermined distance. At this time, it is also possible to completely separate the cantilever and the sample.

Further, as a reliable method, after a large frictional force is applied to the cantilever, the frictional force signal is
The scan is returned to "0" (that is, the sample and the cantilever are separated) or becomes smaller than the reference value 2 (the cantilever is moved in the opposite direction relative to the sample). Then, it is possible to separate the cantilever and the sample.

Then, in step 400, scanning for measuring the shape of the sample is started.

When a sample having deep irregularities such as a contact hole is measured by the above series of processing,
It is possible to prevent the probe from breaking.

In the present invention, FIG. 6 shows scanning of a cantilever with respect to a sample having a tapered shape. That is, if the frictional force exceeds the predetermined reference value 1 during the scanning of the cantilever, the series of processes described with reference to FIG. 5 may be performed. This makes it possible to prevent the probe from breaking even with respect to the sample as shown in FIG. 6 as in the case of measuring a sample having deep unevenness.

Further, the same effect can be obtained for a sample having a rough surface and a non-rough surface even if the surface is flat.

Since it is possible to prevent a large twist of the cantilever, the accuracy of measuring the surface shape is also improved.

[0060]

As described above, according to the present invention, it is possible to prevent the probe from breaking when measuring a sample having deep irregularities such as a contact hole. Further, it becomes possible to measure the surface shape with high accuracy.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an atomic force microscope according to the present invention.

FIG. 2 is an explanatory diagram of a state immediately before a probe included in a cantilever hits a wall surface of a sample and an output signal at that time.

FIG. 3 is an explanatory diagram of a state in which a probe provided in a cantilever hits a wall surface of a sample and is twisted, and an output signal at that time.

FIG. 4 shows a state after reducing the amount of bending of the cantilever,
It is explanatory drawing of the output signal at that time.

FIG. 5 is a flowchart showing a process according to the present invention.

FIG. 6 is an explanatory diagram of the effect of the present invention on a sample having a tapered shape.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Probe, 2 ... Cantilever, 3 ... Sample, 5 ... Laser diode, 6 ... Condensing lens, 7 ... Quadrant photoelectric detector, 8
... three-dimensional actuator, 9 ... scanner control circuit, 10
... Height signal conversion circuit, 11 ... Height signal control circuit, 12 ...
Frictional force signal conversion circuit, 13 ... Memory, 14 ... CPU, 1
5 ... Friction force signal control circuit, 16 ... Imaging processing device, 17
... Display device

Claims (3)

[Claims] 1. In an atomic force microscope, a cantilever having a probe for measuring the shape of a given sample, a sample stage for mounting the sample, a light source, and a lens for condensing light emitted from the light source. And a light receiver having four divided light-receiving units for receiving the reflected light of the condensed light by the cantilever, and the sample stage and the probe can be relatively moved in three-axis directions by a given signal. The moving amount and the output signals from the four divided light receiving units are used to calculate the control amount to be given to the moving unit in order to control the bending amount of the cantilever to a predetermined amount by a first predetermined calculation. Generated when the cantilever is twisted by the first process of measuring the shape of the sample based on the control amount and the output signal from the four divided light-receiving units, and the predetermined second calculation. Suma A processing unit that performs at least a second process for measuring the rubbing force, an output unit that outputs at least the measured shape, and a predetermined signal to the moving unit to relatively move the probe and the sample stage. And a control means for performing at least the processing, wherein the control means, when the frictional force measured in the second processing exceeds a predetermined first reference value, the sample stage and the probe. Atomic force microscope characterized by stopping the relative movement of the cantilever and separating the cantilever from the sample. 2. In an atomic force microscope, a cantilever provided with a probe for measuring the shape of a given sample, a sample stage on which the sample is placed, a light source, and a lens for condensing light emitted from the light source. And a light receiver having four divided light-receiving units for receiving the reflected light of the condensed light by the cantilever, and the sample stage and the probe can be relatively moved in three-axis directions by a given signal. The moving amount and the output signals from the four divided light receiving units are used to calculate the control amount to be given to the moving unit in order to control the bending amount of the cantilever to a predetermined amount by a first predetermined calculation. Generated when the cantilever is twisted by the first process of measuring the shape of the sample based on the control amount and the output signal from the four divided light-receiving units, and the predetermined second calculation. Suma A processing unit that performs at least a second process for measuring the rubbing force, an output unit that outputs at least the measured shape, and a predetermined signal to the moving unit to relatively move the probe and the sample stage. And a control means for performing at least the processing, wherein the control means, when the frictional force measured in the second processing exceeds a predetermined first reference value, the sample stage and the probe. Of the cantilever is controlled so that the frictional force becomes smaller than the second reference value set in advance, and the moving means is controlled so that the frictional force becomes smaller than the predetermined second reference value. An atomic force microscope characterized by moving the distance and starting the first process. 3. The control device according to claim 1, wherein the control means sets the frictional force measured in the second process when the frictional force exceeds a predetermined first reference value. 1
The atomic force microscope characterized in that the cantilever and the sample stage are moved relative to each other by a predetermined distance in a direction opposite to the relative movement direction of the cantilever with respect to the sample stage.