JPH0668835A - Tunnel spectroscopy - Google Patents

Abstract

(57) [Summary] [Purpose] To provide tunnel spectroscopy that enables easy and accurate confirmation of analysis points. [Composition] A scanning tunneling microscope in which the probe is moved in the XY directions along the sample surface while controlling the probe in the Z direction with the sample surface so that the bias current is fixed to a constant value and the tunnel current becomes constant. In tunnel spectroscopy, in which the probe is fixed and the bias voltage is swept, the tunnel current flowing between the probe and the sample surface is detected to analyze the local area of the sample, the probe is positioned at the specified analysis point. When moving, the piezo scanners 5 and 12 are moved to the analysis point without changing the normal scanning in the XY directions, and after the analysis, the normal scanning is restarted from that point. As a result, the topograph before and after the analysis can be viewed, the deviation due to the hysteresis and the thermal drift can be reduced, and the positional accuracy to the designated analysis point can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention controls the probe in the Z direction with respect to the sample surface so that the tunnel current becomes constant by fixing the bias voltage to a constant value, and the XY direction is applied along the sample surface. The present invention relates to tunnel spectroscopy in which a probe is fixed and a bias voltage is swept to detect a tunnel current flowing between the probe and a sample surface to analyze a local region of the sample in a scanning tunneling microscope that moves in a direction.

[0002]

2. Description of the Related Art Tunneling microscope (STM: Scanning T)
The unneling Microscope) detects a tunnel current flowing between the sample and the probe when a bias voltage is applied to the probe to bring it closer to the pole surface of the sample, and z of the probe is adjusted so that the tunnel current becomes constant. By scanning the sample surface in the xy directions while feedback control of the direction (distance between the probe and the sample), an uneven image (topograph) of the sample surface is observed.

On the other hand, tunneling spectroscopy (STS: Scanning Tunneling Spectroscopy) is
While scanning the probe in the XY directions, control the Z direction to observe the uneven surface image (topograph) of the sample surface, and temporarily stop and fix the probe at a specified position to sweep the bias voltage. The electron state in the local region of the sample is analyzed by measuring the tunnel current by using the tunnel current.

[0004]

FIG. 5 is a diagram for explaining a conventional example of a method of moving a probe to an analysis point. by the way,
When performing a local region analysis by tunnel spectroscopy, usually, first, the probe or sample is moved to capture the topograph, and then the analysis point is selected on the topograph, and as shown in FIG. A method is adopted in which the probe or the sample is moved from the initial point to the analysis point, analysis is performed at that position, and the method returns to the initial point. As another method, as shown by the solid line in FIG. 5B, the probe or the sample is moved to search for the analysis point while capturing the topograph, and when the analysis point is specified, the capturing of the topograph is temporarily interrupted. This is a method in which the probe or sample is moved to that position for analysis, and then the probe or sample is returned to the interrupted position to continue capturing the topograph as indicated by the dotted line.

However, the conventional method of moving to the analysis point as described above has a problem that a positional deviation occurs. In other words, because tunnel spectroscopy has hysteresis and thermal drift of the piezo scanner that scans the probe or sample, even if you specify the analysis point on the topograph that was once captured, the actual moved position is specified on the topograph. There is a problem that the position is different from the point.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a tunnel spectroscopy which can be moved to an analysis point with high position accuracy and can be easily confirmed. .

[0007]

To this end, according to the present invention, the bias voltage is fixed to a constant value and the probe is controlled in the Z direction between the probe surface and the sample surface so that the tunnel current becomes constant. The scanning tunneling microscope that moves in the XY directions along the sample, fixes the probe at the specified analysis point, sweeps the bias voltage, and detects the tunnel current flowing between the probe and the sample surface to detect the local area of the sample. When moving the probe to the specified analysis point in tunnel spectroscopy for area analysis, move the probe to the analysis point by normal XY scanning, and resume the normal scan from that analysis point after the analysis is completed. It is characterized by having been made to do.

[0008]

In the tunnel spectroscopy of the present invention, when the probe is moved to the designated analysis point, it is moved to the analysis point by performing a normal scanning in the XY directions, and after the analysis is completed, a normal scanning is performed from that point. Since the scanning was restarted, the topograph immediately before and after the analysis can be seen, so the deviation due to hysteresis and thermal drift can be reduced (because even if there is, the analysis point can be easily estimated), the analysis point The position accuracy of can be improved.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an embodiment of tunnel spectroscopy of the present invention, and FIG. 2 is a diagram showing an example of movement of a probe on a sample.

In FIG. 1, a clock generation circuit 1 is for generating a clock having a frequency set by a control circuit (computer) 8. The counter 2 counts the clock and outputs the count value as a scanning signal in the X direction, and outputs an overflow signal for generating a scanning signal in the Y direction. The piezo scanner 5 scans the probe in the X direction, and as a signal therefor, an analog signal obtained by converting the count value of the counter 2 by the DA converter 3 is supplied through the drive amplifier 4. The counter 9 counts the overflow signal of the counter 2 and outputs the count value as a scanning signal in the Y direction. The piezo scanner 12 scans the probe in the Y direction, and as a signal therefor, an analog signal obtained by converting the count value of the counter 9 by the DA converter 10 is supplied through the drive amplifier 11.

The analysis point address buffer 6 stores the address of a specified analysis point, and the comparator 7 is
The address stored in the analysis point address buffer 6 is compared with the values of the counters 2 and 9, and the count of the counters 2 and 9 is stopped by the coincidence signal. Control circuit 8
Is for controlling the clock generation circuit 1, the counters 2 and 9, the analysis point address buffer 6, and the comparator 7 according to the uneven image observation mode and the local region analysis mode.

Next, the operation will be described. First, the case of the uneven image observation mode will be described. In this case, the control circuit 8 limits the operations of the analysis point address buffer 6 and the comparator 7 so that the counters 2 and 9 generate the scanning signals of the entire region. Then, the operations of the clock generation circuit 1 and the counters 2 and 9 are started. As a result, the counters 2 and 9 count the clocks in the normal operation, and therefore the probe is moved to the X-axis in the piezo scanners 5 and 12 which operate by the output thereof.
Scanning is performed in the Y direction, and the topograph is captured.

On the other hand, in the local area analysis mode, the control circuit 8 sets the address of the specified analysis point in the analysis point address buffer 6 and sets the clock generation circuit 1, the counters 2 and 9, and the comparison. Operate the vessel 7. Then, the counters 2 and 9 continue the normal operation until the analysis point, but the operation is stopped by the output of the comparator 7 at the analysis point. The control circuit 8 detects this operation stop, executes the analysis process, and when the analysis process ends, resets the comparator 7 to release the stopped state of the counters 2 and 9,
Restart counting. That is, according to the present invention, as shown in FIG. 2, the operation of designating an analysis point, performing normal scanning up to the analysis point, temporarily stopping at the analysis point, and performing the analysis is repeated. .

FIG. 3 is a diagram showing an embodiment of tunnel spectroscopy including a tunnel current detection system, and FIG. 4 is a waveform diagram for explaining the operation of tunnel spectroscopy.

In FIG. 3, the scanning circuit 21 is a circuit including the clock generation circuit 1, the counters 2 and 9, the DA converters 3 and 11, the analysis point address buffer 6 and the comparator 7 shown in FIG. 41, Y drive amplifier 4
2, X piezo scanner 51, Y piezo scanner 52
Correspond to the drive amplifiers 4 and 11 and the piezo scanners 5 and 12 shown in FIG. 1, respectively. The bias control circuit 26 applies a bias voltage to the sample 25, which is fixed to a constant bias voltage in the unevenness image observation mode, and is swept in the local region analysis mode.
Due to this bias, when the distance between the probe 24 and the sample 25 approaches to about 1 nm, a tunnel current flows. The preamplifier 28 detects this tunnel current, and the feedback amplifier 29 returns the tunnel current to the Z piezo scanner 53 through the Z drive amplifier 43 so that the tunnel current detected by the preamplifier 28 becomes constant. . By the feedback control to the Z piezo scanner 53, the Z piezo scanner 53 expands and contracts in the direction of the sample 25, and the distance between the probe 24 and the sample 25 is controlled to be constant. Further, the feedback amplifier 29 has a switch 91, and keeps the switch 91 closed in the concavo-convex image observation mode and opens the switch 91 in the local area analysis mode. The former is the sample state and the latter is the hold state.

The multiplexer 30 is an AD converter 3
The signal input to 1 is fed back to the feedback amplifier 29 in the sample state.
Select the signal to be returned to the Z piezo scanner 53 from
In the hold state, the tunnel current detected by the preamplifier 28 is selected, and the AD converter 31 converts the selected signal into a digital signal and outputs it to the computer 32. Computer 32 is AD
The digital signal input from the converter 31 is subjected to necessary processing, and then the uneven image of the sample 5 and local area analysis information are displayed on the display device 33. The clock control circuit 27 clock-controls the operation timing of the scanning circuit 21, the bias control circuit 26, the feedback amplifier 29, the multiplexer 30, etc., and switches between the concavo-convex image observation mode and the local region analysis mode by this control.

Next, description will be given of each of the operation in the concavo-convex image observation mode as the tunnel microscope and the operation in the local area analysis mode as the tunnel spectroscopy.

In the operation in the concavo-convex image observation mode, the scanning circuit 21 causes the X drive amplifier 41 and the Y drive amplifier 42 to pass the X piezo scanner 51 and the Y piezo scanner 52.
Is controlled to scan the probe 24 in the XY directions, a constant bias voltage is applied to the sample 25 from the bias control circuit 26, and the switch 91 of the feedback amplifier 29 is closed (sampled) to make the tunnel current constant. As described above, the Z piezo scanner 53 is expanded and contracted through the Z drive amplifier 43 so that the distance between the probe 24 and the sample 25 is controlled to be constant. Then, the multiplexer 30 selects the feedback amplifier 29 side and outputs the feedback signal to the AD converter 3
1 is converted and sent to the computer 32, so that the computer 32 displays the surface of the sample 25 on the display device 33.
An uneven image obtained by scanning in the Y direction is displayed.

When shifting from the uneven image observation mode to the local area analysis mode of the designated analysis point, first, as shown in FIG. 4, the probe 24 reaches a point on the sample 25 to be analyzed at t1. The clock control circuit 27 detects this and temporarily opens (holds) the switch 91 in the feedback amplifier 29. By this hold, the feedback signal sent to the Z piezo scanner 53 through the Z drive amplifier 43 is held, and the distance between the probe 24 and the sample 25 is fixed.
At the same time, the multiplexer 30 also switches the input from the feedback amplifier 29 side to the preamplifier 28 side. Subsequently, the bias voltage applied to the sample 25 from the bias control circuit 26 is swept according to the timing created by the clock control circuit 27. Thereby, the change of the tunnel current is measured by the computer 32 from the multiplexer 30 through the AD converter 31, and the electron state at the analysis point is analyzed.

The present invention is not limited to the above embodiment, but various modifications can be made. For example, in the above embodiment, one analysis point has been described, but a plurality of addresses may be set in the analysis point address buffer, or a new address may be rewritten each time the analysis is completed. The probes may be sequentially stopped at the analysis points. Further, although the description has been given of the case where the probe is moved, it goes without saying that the sample may be moved. Further, it is possible to improve the measurement accuracy by repeating bias voltage scanning and tunnel current detection at the same analysis point a required number of times and averaging them.

[0021]

As is apparent from the above description, according to the present invention, when the probe is moved to the designated analysis point,
Since the normal scanning in the XY directions is moved to the analysis point, the topograph before and after that can be observed, and the displacement of the analysis point due to hysteresis and thermal drift is small, and the position can be easily confirmed. .

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of tunnel spectroscopy of the present invention.

FIG. 2 is a diagram showing an example of movement of a probe on a sample.

FIG. 3 is a diagram showing an example of tunnel spectroscopy including a tunnel current detection system.

FIG. 4 is a waveform diagram for explaining the operation of tunnel spectroscopy.

FIG. 5 is a diagram for explaining a conventional example of a method of moving a probe to an analysis point.

[Explanation of symbols]

1 ... Clock generation circuit, 2, 9 ... Counter, 3, 10 ...
DA converter, 4, 11 ... Drive amplifier, 5, 12 ... Piezo scanner, 6 ... Analysis point address buffer, 7 ... Comparator, 8 ... Control circuit

Claims (1)

[Claims] 1. A probe is fixed to a sample surface so that the bias voltage is fixed to a constant value so that the tunnel current becomes constant.
With a scanning tunneling microscope that moves in the XY directions along the sample surface while controlling the direction, the probe is fixed at a specified analysis point to sweep the bias voltage, and the tunnel current that flows between the probe and the sample surface. In the case of moving the probe to the specified analysis point in tunnel spectroscopy, which detects the sample and analyzes the local area of the sample, the probe is moved to the analysis point by normal XY scanning, and after the analysis, the analysis is performed. Tunnel spectroscopy, characterized by restarting normal scanning from a point.