JP2571625B2 - Scanning tunnel microscope - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning tunneling microscope for detecting a tunnel current by bringing a probe close to a sample and displaying an uneven image of the sample surface on an image display device.

[Prior Art] The atom cloud at the tip of the probe and the electron cloud of the sample atoms overlap
The tip of the probe and the tip of the sample are brought close to the sample surface to the order of nm, and in this state, a tunnel current for applying a bias voltage flows between the probe and the sample. Since the tunnel current is particularly sensitive to the distance between the probe and the sample, the distance between the sample and the probe can be measured very precisely by measuring the magnitude of the tunnel current.

The scanning tunneling microscope (STM) controls the height of the probe so that the tunnel current is constant, and uses the locus of the height of the probe when the probe is scanned in the horizontal direction to measure the surface of each sample. This is a microscope apparatus for observing the uneven shape and attracting attention for observing the surface atomic arrangement.

[Problem to be Solved by the Invention] The scanning tunneling microscope having the above-described configuration is
It can be widely used in observation of surface images of various samples.

Incidentally, a piezoelectric element or the like is used to move the probe in a plane (z direction) perpendicular to the sample surface according to the unevenness of the sample. This piezoelectric element expands and contracts when a voltage is applied, but there is a certain limit to the range in which the piezoelectric element can normally expand and contract. It is, for example, ± 50
The range is ± 1ηm to ± 1.2ηm, corresponding to the applied voltage of 0V. Beyond this range, the piezoelectric element is destroyed, and even if the absolute value of the voltage applied to the piezoelectric element is increased, the desired response cannot be obtained because the amount of expansion and contraction is saturated. Therefore, in the related art, consideration is made so that the piezoelectric element is driven in a state close to its natural length, that is, in a state where the voltage applied to the piezoelectric element is 0 V. However, when the entire sample surface is inclined, for example, the expansion and contraction imposed on the piezoelectric element in order to maintain a constant tunnel current flowing between the sample and the sample includes the unevenness of the sample surface due to the unevenness of the sample surface. Since the inclination is added, the piezoelectric element may be driven beyond the normal expansion and contraction operation range. In such a case, even if the piezoelectric element is destroyed or the piezoelectric element is not destroyed, an image containing an error due to the failure of the piezoelectric element to respond normally is erroneously recognized as an original image.

The present invention solves such a conventional problem and can monitor whether the Z drive voltage element is in a normal expansion / contraction operation range, thereby preventing the Z drive piezoelectric element from being broken, It is an object of the present invention to provide a scanning tunnel microscope that can prevent a tunnel microscope image in which a piezoelectric element deviates from a normal expansion and contraction operation range and is degraded as an original image.

Means for Solving the Problems According to a first aspect of the present invention, a Z-axis direction driving unit that drives a probe in a Z direction that changes the height with respect to a sample surface, and moves the probe relatively to the sample surface. Means for two-dimensional scanning; control means for controlling the Z-axis direction driving means so as to maintain a constant tunnel current flowing between the probe and the sample; and a sample based on the control amount of the control means. Image display means for displaying a surface unevenness image, and a range of a drive signal supplied to the Z-axis direction drive means corresponding to a movable range of the Z-axis direction drive means, for displaying together with the unevenness image on the image display means. And scanning means for displaying a waveform of the driving signal supplied to the Z-axis direction driving means on the image display means together with the display of the uneven image and the range of the driving signal supplied to the Z-axis direction driving means. It is characterized by a shaped tunnel microscope.

Further, a second invention provides a Z-axis direction driving means for driving a probe in a Z direction which changes the height with respect to a sample surface, and a means for two-dimensionally scanning the probe relative to the sample surface. A control means for controlling the Z-axis direction driving means so as to keep a tunnel current flowing between the probe and the sample constant; and displaying an uneven image of the sample surface based on a control amount of the control means. A scanning tunnel microscope comprising image display means and means for displaying the tunnel current together with the concave-convex image on the image display means with the scanning of the probe over time.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 1 is a view showing a configuration of a scanning tunneling microscope according to an embodiment of the present invention, FIG. 2 is a view for explaining a display example of a line profile, and FIG. 3 is an image display state of a sample image and a line profile. FIG. In FIG.
1 is a sample, 2 is a probe, 3 is a bias power source, 4x, 4y, 4z are piezoelectric elements, 5 is a scan generator, 6x, 6y, 6z are piezoelectric element driving high voltage circuits, 7 is an I / V amplifier, and 8 is Log amp, 9
Is a comparator, 10 is an indexer, 11 is an image amplifier, 12 is a waveform monitor circuit, and 13 is an image display device.

The piezoelectric elements 4x, 4y, and 4z constitute a three-dimensional scanner that drives the X axis, the Y axis, and the Z axis, respectively. The scan generator 5 generates scanning signals for the probe 2 and the image display device 12. is there. The piezoelectric element driving high voltage circuits 6x and 6y drive the piezoelectric elements 4x and 4y of the three-dimensional scanner according to the scanning signal generated by the scan generator 5, and the piezoelectric element driving high voltage circuit 6z It drives the piezoelectric element 4z of the dimensional scanner. The first stage I / V amplifier 7 connected to the sample converts the tunnel current It into a voltage and further amplifies it. The log amplifier 8 connected to the next stage outputs the output signal of the I / V amplifier 7. Signal conversion (linear ratio) is performed so as to correspond linearly to the height of the probe. Comparator 9
The output value of the log amplifier 8 is compared with a reference value (DC Source) corresponding to the set value of the tunnel current. The integrator 10 integrates the output of the comparator 9 and uses this output as the Z-axis piezoelectric force of the three-dimensional scanner. This is a control value for the element 4z. In this way, the tunnel current It is fed back to the Z-axis piezoelectric element driving high voltage generating circuit 6z through the I / V amplifier 7, the log amplifier 8, the comparator 9, and the integrator 10 to control the Z-axis piezoelectric element 4z of the three-dimensional scanner. The height of the probe 2 to control the tunnel current
It is constant.

In the configuration described above, the present invention provides a waveform monitor circuit 12, supplies a signal input to the waveform monitor circuit 12 to an image display device 13, and inputs an input signal together with an uneven image (scanning tunnel microscope) of the sample surface. Is displayed in the line profile. The signals input to the waveform monitor circuit 12 include:
There are an output signal of the comparator 9, an output signal of the integrator 10, an output signal of the image amplifier 11, and the like. FIG. 2 (a) shows the image amplifier supplied to the waveform monitor circuit 12.
This is a waveform obtained based on the output signal of No. 11, and from this waveform, information on the sample unevenness in the height direction can be obtained. Here, the output signal of the image amplifier 11 is also supplied to the image display device 13, and the signal supplied to the image display device is luminance-modulated, and the image display device 13 is scanned by the scan signal from the scan generator 5. Is displayed as a two-dimensional sample surface image.

FIG. 2B shows a display example of another waveform displayed on the display device 13 based on the output signal of the integrator 10 supplied to the waveform monitor circuit 12. FIG. 2 (b)
In the display example shown in FIG. 7, broken lines displayed above and below the center line indicate the upper and lower limits of the normal expansion and contraction operation range of the piezoelectric element 4z of the three-dimensional scanner. In this embodiment, these limits are from ± 1 μm to ± 1.2 corresponding to an applied voltage of ± 500V.
It corresponds to the expansion and contraction amount of about μm. The waveform of the voltage applied to the piezoelectric element 4z obtained based on the output signal of the integrator 10 is superimposed on the display of the normal telescopic operation range. This waveform corresponds to the driving state (expanded state) of the piezoelectric element 4z of the three-dimensional scanner that changes with time according to the scanning by the probe 2.

When the operator instructs the probe 2 to start scanning the sample surface, the display device 13 displays a tunnel scan image of the sample. On the other hand, the lower part of the display screen of the display device 13 along with the scanning by the probe 2 is enlarged as shown in FIG.
Since a voltage waveform applied to the piezoelectric element 4z indicating the driving state (expanded / retracted state) of the piezoelectric element 4z during scanning is displayed, by monitoring this display, it is determined whether the piezoelectric element 4z deviates from the normal expansion / contraction operation range. The observation of the scanning tunneling microscope image can be performed while monitoring whether or not the image is observed. If the piezoelectric element 4z deviates from a favorable range of expansion and contraction due to a large inclination of the entire surface of the sample, the scanning by the probe 2 is stopped, the sample is set again, and the observation is performed again. To start. In this way, it is possible to prevent the piezoelectric element 4z from being destroyed, and there is also a situation where an image containing an error obtained when the piezoelectric element 4z deviates from a favorable expansion / contraction operation state is mistaken for an original image. Can be prevented.

FIG. 2 (c) shows a display example of another waveform obtained based on the output signal of the comparator 9 supplied to the waveform monitor circuit 12. In the display shown in FIG. 2C, a change in the tunnel current accompanying the scanning by the probe 2 is displayed with time. Since the Z-axis piezoelectric element driving high voltage circuit 6z is controlled so that the tunnel electrode is kept constant, the tunnel current is kept constant with almost no temporal change in a normal operation. However,
If the piezoelectric element 4z deviates from the normal telescopic operation range,
Since the expansion and contraction of the piezoelectric element 4z for maintaining the tunnel current constant becomes impossible, the tunnel current increases or decreases from a constant value up to that point, causing a noticeable shift. Thus, the operator can also know whether or not the piezoelectric element 4z is in the normal expansion / contraction operation range by monitoring the display in FIG. 2 (c), and as a result, as shown in FIG. 2 (b). The same effects as in the illustrated embodiment can be achieved.

FIG. 3 shows the waveforms described with reference to FIGS. 2 (a), 2 (b) and 2 (c) below the display screen of the display device 13 displaying the scanning tunneling microscope. Are displayed side by side. According to this display example,
There is an advantage that correspondence with a tunnel microscope image can be easily made.

In addition, if each output signal of the waveform monitor circuit 12 in the above-described embodiment is used, it is also possible to automatically operate and control the scanning tunnel microscope itself such as a coarse movement mechanism.

According to the first aspect of the present invention, the range of the drive signal supplied to the Z-axis direction drive unit corresponding to the movable range of the Z-axis direction drive unit is displayed on the image display unit together with the uneven image. And a means for displaying a waveform of the drive signal supplied to the Z-axis direction drive means on the image display means together with the display of the uneven image and the range of the drive signal supplied to the Z-axis direction drive means. With this arrangement, it is possible to monitor whether or not the Z-axis direction driving means deviates from a normal expansion / contraction operation range as the sample is scanned by the probe, thereby preventing the Z-direction driving means from being destroyed. At the same time, it is possible to prevent a tunnel microscope in which the Z-drive piezoelectric element has deviated from the normal expansion / contraction operation range and has been degraded as an original image.

Further, according to the second aspect of the present invention, since there is provided a means for displaying the tunnel current together with the uneven image on the image display means, the driving in the Z-axis direction is performed so that the tunnel current is kept constant. If the tunnel current deviates from a certain value under the control of the means by the control means, it is possible to know that the Z-drive piezoelectric element has deviated from the normal expansion / contraction operation range. A similar effect can be achieved.

[Brief description of the drawings]

FIG. 1 is a view showing a configuration of a scanning tunneling microscope according to an embodiment of the present invention, FIG. 2 is a view for explaining a display example of a line profile, and FIG. 3 is an image display state of a sample image and a line profile. FIG. 1: sample, 2: probe 3: bias power supply 4x, 4y, 4z: piezoelectric element 5: scan generator 6x, 6y, 6z: piezoelectric element driving high voltage circuit 7: I / V amplifier, 8: log amplifier 9: comparator, 10: Integrator 11: Image amplifier, 12: Waveform monitor circuit 13: Image display device

Claims (2)

(57) [Claims] 1. Z-axis direction driving means for driving a probe in a Z direction which changes the height from a sample surface, means for two-dimensionally scanning the probe relative to the sample surface, and a probe A control unit for controlling the Z-axis direction driving unit so as to maintain a constant tunnel current flowing between the sample and the sample unit; and an image display unit for displaying an uneven image of the sample surface based on a control amount of the control unit. A means for displaying a range of a drive signal supplied to the Z-axis direction driving means corresponding to a movable range of the Z-axis direction driving means together with the uneven image on the image display means; A scanning tunnel microscope comprising means for displaying a waveform of the supplied drive signal on the image display means together with the display of the range of the supplied drive signal to the uneven image and the Z-axis direction drive means. 2. A Z-axis direction driving means for driving a probe in a Z direction changing a height with respect to a sample surface, a means for two-dimensionally scanning the probe relative to the sample surface, and a probe. A control unit for controlling the Z-axis direction driving unit so as to maintain a constant tunnel current flowing between the sample and the sample unit; and an image display unit for displaying an uneven image of the sample surface based on a control amount of the control unit. A scanning tunnel microscope comprising means for displaying the tunnel current together with the concave and convex image on the image display means over time as the probe scans.