JPH03210406A - Scanning type tunnel microscope instrument - Google Patents

Abstract

PURPOSE:To safely and surely stop a prove with respect to a specimen by making the probe approach a specimen surface while detecting whether a feedback signal agrees with a set value and stopping a drive when the feedback signal agrees with the set value. CONSTITUTION:Piezoelectric elements 1x, 1y and 1z constitute a three- dimensional scanner for driving X-, Y- and Z-axes, respectively. The piezoelectric elements 1x and 1y are driven by piezoelectric element driving high voltage circuits 12x and 12y, respectively, according to scanning signals generated in a scan generator in a CPU 13, a probe 2 is driven along a specimen surface in X and Y directions, a tunnel current flowing between the probe 2 and a specimen is measured, the tunnel current is fed back to a Z-axis piezoelectric element driving high voltage circuit 11 via a logarithmic amplifier 8, a comparator circuit 9 and an integrating circuit 10 and the Z-axis scanner 1z is controlled so that the tunnel current may be constant. Before beginning a measurement, the piezoelectric elements 1x - 1z are lowered by a moving mechanism 5 and, after the descent of the piezoelectric elements 1x - 1z is stopped in a position wherein the tunnel current takes a prescribed value, the measurement is begun.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is a scanning type device that detects a tunnel current generated when a probe approaches a sample and displays an uneven image of the sample surface on an image display device. The present invention relates to a tunneling microscope (STM) device.

[Prior art] The tip of the probe is brought close to the sample surface to a distance on the order of several nanometers where the atoms at the tip of the probe and the electron cloud of the atoms of the sample overlap, and in this state, a constant distance is maintained between the sample and the probe. When a bias voltage of is applied, a tunnel current flows. This tunneling current changes logarithmically according to the distance between the sample and the probe, so by measuring the magnitude of this tunneling current, the distance between the sample and the probe can be measured with extreme precision. STM
The device is configured such that this tunneling current is constant, i.e.
While controlling the height of the probe so that the distance between the sample surface and the probe is constant, the height of the probe is controlled by the locus of the height of the probe when the piezoelectric element scans the probe in the XY force direction (horizontal direction). It is used to observe the uneven shape of the surface, and is attracting attention because it allows the observation of the atomic arrangement on the sample surface. Also, this 8
The 7M device not only observes the surface atomic arrangement, but also has an analysis function that analyzes information on the depth direction of the sample and the electronic state based on information on the tunnel current.

[Problem to be solved by the invention 1] In order to obtain the above information, the probe must be placed a few nanometers above the sample surface.
You need to get close to the order. Therefore, when the probe is brought close to the sample using a manual manipulator, for example, there are many problems in which the probe collides with the sample when brought into the tunnel current region. This collision not only damages the sample surface, but also causes part of the sample to adhere to the probe surface, making the sample surface image unstable.

The present invention has been made in view of the above, and its main purpose is to provide a means for safely and reliably bringing a probe close to a sample and obtaining a stable sample surface image with a simple configuration. This is the purpose.

[Means for Solving the Problems] In order to achieve the above object, the scanning tunneling microscope device of the present invention includes an XY-axis direction driving means for moving an XY force direction probe along the sample surface, and Z-axis driving means for moving the probe in the Z direction to change the height, voltage application means for applying a bias voltage between the probe and the sample, and detecting the tunnel current flowing between the probe and the sample. In the scanning tunneling microscope device, the scanning tunneling microscope device comprises: a feedback circuit that generates a feedback signal for feedback controlling the Z-axis drive means so that the tunneling current is constant; and a display means to which the feedback signal is supplied. Probe, XY axis driving means, and Z
a second Z-axis direction drive unit that integrally brings the axial drive unit toward the sample surface in a continuous or intermittently manner; and a coincidence detection unit that detects whether or not the feedback signal matches a preset value. The invention is characterized in that the second two-axis driving means is stopped based on the output signal of the coincidence detecting means.

[Operation] While detecting whether the feedback signal matches a preset value, the probe is brought closer to the sample surface continuously or intermittently, and when the feedback signal matches the set value, the probe is driven. I tried to stop it.

[Example] Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a schematic diagram showing the configuration of an apparatus according to an embodiment of the present invention. In FIG. 1, piezoelectric element lx.

1y and 1z constitute a three-dimensional scanner that drives the X-axis, Y-axis, and Z-axis, respectively, 2 is a probe, 3 is a sample, and 4 is a power supply that supplies a bias voltage between the probe 2 and sample 4. , 5 is a moving mechanism for bringing the probe 2 close to the surface of the sample 3, 6 is a drive circuit for driving the moving mechanism 5, 7 is an I/V conversion amplifier circuit, and 8 is an output signal of the I/V amplifier circuit 7. A logarithmic amplifier circuit 9 performs signal conversion (linearization) to linearly correspond to the height of the probe 2, and 9 converts the output value of the logarithmic amplifier circuit 8 to a reference voltage corresponding to the set value of the tunnel current. 10 is an integration circuit for integrating the output of the comparison circuit 9; 11 is a high-voltage circuit for driving a Z-axis pressure type element; 12x and 12y are high-voltage circuits for driving the X-axis and Y-axis pressure elements, respectively; 13 is a CPU device 14 is an image amplification circuit. 15 is a device including a monitor, a scan generator, a frame memory, and an AD/DA converter. 15 is a device that determines whether the signal from the Z-axis pressure type element drive high voltage circuit 11 matches a preset value. This is a coincidence detection circuit that detects whether

The scan generator included in the CPU device 13 is
It generates scanning signals for the piezoelectric elements 1x, ly on the Y-axis and the Y-axis, and the image display device. Piezoelectric element drive high voltage circuit 1
2x and 12y are for driving the piezoelectric elements 1x and ly of the three-dimensional scanner to scan the probe 2 along the sample surface in the X and Y directions according to the scanning signal generated by the scan generator in the CPU device 13. It is something. The Z-axis pressure type element drive high voltage circuit 11 drives the piezoelectric element 1z of the three-dimensional scanner using the output of the integrating circuit 10, and is variable within a range of, for example, ±150V. The first stage I/V amplifier circuit 7 connected to the sample converts the Teunel current 11 obtained from an arbitrary bias voltage = '4 jJ: supplied between the probe 2 and the sample 3 from the power source 4 into a voltage, This will further amplify it. Then, the tunneling current I7 is fed back to the Z-axis piezoelectric element drive high voltage circuit 111\ through the logarithmic amplifier circuit 8, the comparison circuit 9, and the integration circuit 1o to control the Z-axis piezoelectric element 1z of the three-dimensional scanner.
Tunnel current 1. is kept constant. The moving mechanism 5 is driven by a drive circuit 6 that is controlled by a signal from a Z-axis pressure type element drive high voltage circuit 11 and a drive signal from a CPU device 13.

Further, the signal fed back to the Z-axis pressure type element drive high voltage circuit 11 is supplied to the image amplification circuit 14, where it is brightness-modulated and displayed as a two-dimensional sample surface image.

In such a configuration, when a positive voltage is applied, the piezoelectric element 1z expands more than when no voltage is applied (0 volts),
It is assumed that it contracts when a negative voltage is applied. Further, the reference value of the coincidence detection circuit 15 is set to 0 volts. Then, when the operator issues a start command, the drive circuit 6 receives a command from the CPU module 13 and receives the command as shown in FIG.
As shown in a), time t. A drive signal is sent to the moving mechanism 5 from there. Therefore, the probe 2 has a piezoelectric element 1 x *
1 y * 1z, the probe is moved continuously or intermittently from a distant position toward the sample surface, and the distance between the probe 2 and the sample surface gradually decreases as shown in Figure 2 (b). .

At this time, the feedback loop is operating, but the distance is large and no tunnel current flows! .

-〇, so the feedback signal is maximum. Therefore, Z
The voltage Vz supplied from the axial pressure type element drive high voltage circuit 11 to the piezoelectric element 1z is +150 volts as shown in FIG. 2(c), and the piezoelectric element 1z is in an elongated state.

FIG. 2(d) shows a voltage signal (I↑) corresponding to the output of the logarithmic amplifier circuit, that is, the tunnel current IT. Time t
1 and the probe 2 approaches the sample surface sufficiently, a tunnel current begins to flow, and as the probe 2 approaches the sample surface, the tunnel current gradually increases. Then, at time t2, V
When (I 2(C). At this stage, the probe 2 and the piezoelectric elements lx, ly, lz are still being sent toward the sample surface by the moving mechanism 5. (2) Since the piezoelectric element 1z gradually contracts from the fully extended state due to the decrease in 2, the distance between the tip of the probe 2 and the sample surface is maintained at the predetermined distance described above.Then, at time t, the voltage Vzh< When the voltage reaches 0 volts, the coincidence detector 15 generates a coincidence pulse as shown in FIG. is completed.

At this point, the tunneling current is obtained in an optimal state 1, and the voltage Vz supplied to the piezoelectric element is 0 volts. Therefore, after that, piezoelectric elements 1x, ly are used for imaging.
When the probe starts two-dimensional scanning along the sample surface by supplying a scanning signal to the sample surface, the probe can correspondingly move along the unevenness of the sample surface with an equal dynamic range both upward and downward.

In addition, in the above embodiment, the supply voltage to the piezoelectric element 1z is +
Although it is set to 150 volts, the reference voltage may not necessarily be 0 volts as long as it is near the center of the dynamic range of the voltage supplied to the piezoelectric element 1z. Further, in the above embodiment, the piezoelectric element 1z is used in both the elongated direction and the contracted direction, but depending on the case, only one of the piezoelectric elements 1z may be used.

In that case as well, the reference point of the coincidence detection circuit may be set to a value near the center of the dynamic range of the voltage Vz supplied to the piezoelectric element.

[Effect] As detailed above, according to the present invention, the probe is brought closer to the sample surface while detecting whether or not the feedback signal matches a preset value, so that the feedback signal matches the preset value. Since the driving is stopped when the value coincides with , the probe can be brought close to the sample safely and reliably, and a stable image of the sample surface can be obtained.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the configuration of an apparatus according to an embodiment of the present invention;
FIG. 2 is a diagram for explaining the operation of an apparatus according to an embodiment of the present invention. lx, ly, lzl: Piezoelectric element 2: Probe 3: Sample 4ria source 5: Moving mechanism 6: Drive circuit 7: I/V conversion amplifier circuit 8: Logarithmic amplifier circuit 9: Comparison circuit 10: A minute path 11: Z-axis pressure type element drive high voltage circuit 12x, 12y: Piezoelectric element drive high voltage circuit 13: CP
U device 14: Image amplification circuit 15 - coincidence detection circuit

Claims (1)

[Claims] An XY-axis drive means that moves the probe in the XY directions along the sample surface, a Z-axis drive means that moves the probe in the Z direction to change the height with respect to the sample surface, and a bias between the probe and the sample. Voltage application means for applying a voltage, means for detecting the tunnel current flowing between the probe and the sample, and feedback for generating a feedback signal for feedback controlling the Z-axis driving means so that the tunnel current is constant. In a scanning tunneling microscope device comprising a circuit and a display means to which a feedback signal is supplied, the probe, the XY-axis direction drive means, and the Z-axis direction drive means are integrally directed toward the sample surface continuously or intermittently. a second Z-axis direction driving means for moving the feedback signal closer to the target; and a coincidence detection means for detecting whether or not the feedback signal matches a preset value. Z
A scanning tunneling microscope device characterized in that an axial drive means is stopped.