JPH0315704A - Scanning tunnelling microscope - Google Patents

Abstract

PURPOSE:To improve the measurement precision by providing a subtractor which outputs the difference between first and second tunnelling currents flowing between first and second probes and a sample. CONSTITUTION:A prescribed voltage is applied to a Z-direction driving piezoelectric body of an actuator 7 from a scanning circuit 8 to approximate a probe 1 to a position about 1nm distant from the face to be examined of the sample 3. When a voltage is applied between the probe 1 and the sample 3 from a power source 5, a first tunnelling current it1 flows between the probe 1 and the sample 3. This current it1 is inputted to the positive input terminal of a subtractor 13 after being amplified by an amplifier 11. A prescribed voltage is applied to an actuator 9 from a (z) position control circuit 10 to support a probe 2 about 1nm apart from the sample 3. Then, a voltage is applied between the probe 2 and the sample 3 from a power source 6, and a second tunnel current it2 flows between them and is amplified by an amplifier 12 and is inputted to the negative input terminal of the subtractor 13. The subtractor 13 outputs the difference signal as the result of subtraction between currents it1 and it2.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a scanning tunneling microscope.

C. Prior Art] A scanning tunneling microscope (STM) is a device that can observe the surface of a sample in units of atoms. When a metal probe with a sharp tip is placed as close as 1 nm to the surface of a conductive sample and a predetermined voltage is applied between the probe and the sample, a tunnel current flows between the probe and the sample. This tunneling current sensitively responds to changes in the distance between the probe and the sample and changes significantly. Taking advantage of this property of tunneling current, the probe is attached to an actuator that can move in three dimensions, and the probe is scanned so that the tunneling current remains constant. Trace. The STM outputs the position of the probe at this time as a three-dimensional image, thereby displaying an image of changes in shape on the sample surface in units of atoms.

Normally, even in a laboratory where there is no vibration source nearby, there is floor vibration with an amplitude of about 1 μm, mainly consisting of frequency components below lo011z. In addition, there are always acoustic vibrations that travel through the air, such as machine vibrations, human voices, and footsteps.

The distance between the STM probe and the sample easily changes due to the influence of these floor vibrations and acoustic vibrations. In particular, since STM has high measurement accuracy and detects changes in shape on an atomic basis, changes in the distance between the tip and sample greatly affect the measurement results. Therefore, vibration isolation of the device is important in STM.

In STM, STM is attached to the normally used air spring type vibration isolation table.
Merely placing the unit is not enough. For this reason,
Usually, a smaller vibration isolating mechanism is provided between the STM unit and the floor-standing air spring type vibration isolating table. This vibration isolation mechanism includes a method that combines magnetic levitation using a superconductor and a permanent magnet with eddy current braking, and a method that uses eddy current damping.
There are a tiered spring suspension method and a stack method in which several metal plates are stacked with rubber sandwiched between them.

[Problem to be solved by the invention] However, the air spring type vibration isolation table and vibration isolation mechanism prevent STM
The vibration of the unit is not completely eliminated. Therefore, the Jl determination result includes an error due to the influence of vibration. Moreover, improving the vibration isolation performance has the disadvantage that the measuring device becomes bulky.

An object of the present invention is to provide an STM device that can eliminate the influence of vibration and provide stable measurement results.

[Means for Solving the Problems] The scanning tunneling microscope of the present invention supports first and second probes having sharp tips and the first probe close to the inspection surface of the sample. , a first actuator that moves in a three-dimensional direction; a second actuator that supports the second probe close to the inspection surface and moves it at least in a direction perpendicular to the inspection surface; a first power supply that biases the probe and the sample, a second power supply that biases the second probe and the sample, and a first power supply that flows between the first probe and the sample. A subtracter is provided that outputs a difference between a tunnel current and a second tunnel current flowing between the second probe and the sample.

[Function] The first probe is supported close to the inspection surface of the sample to a distance where tunnel current flows ('tunnel distance M), and a predetermined distance between the first probe and the sample is supplied from the first power source. A voltage is applied and a tunnel current is caused to flow between the two. The first probe is scanned while its position is controlled by the first actuator during measurement. On the other hand, the second probe is supported by a second actuator so as to be close to the inspection surface within a tunneling distance, and the second probe and the sample are biased to cause a tunneling current to flow. When the first probe is scanned, the second probe remains supported at a predetermined position above the inspection surface and is not moved. In the calculator, a second tunnel current flowing between the second probe and the sample is subtracted from a first tunnel current flowing between the first probe and the sample and output.

The first tunneling current includes the influence of sample vibration. The change in the second tunneling current reflects the change in the probe-sample distance due to sample vibration. Therefore, the signal output from the subtracter is a signal obtained by removing the influence of sample vibration from the first tunnel current. While this signal is kept constant by the first actuator,
A probe is scanned along the inspection surface. As a result, the curved surface displayed as an image based on the position of the first probe accurately represents the unevenness of the inspection surface, with fluctuations in the distance between the probe and the sample due to vibrations of the inspection surface being removed.

[Example] FIG. 1 is a block diagram showing the configuration of a scanning tunneling microscope according to an example of the present invention. The first probe 1 and the second probe 2 are metal probes of the same size and have tips with a radius of curvature of about 0.1μ, and are formed by processing tungsten by electrolytic polishing, for example. Ru. The first probe 1 is fixed to a first actuator 7 that is movable in three axes (x, y, z axes), and is placed a predetermined distance MM above the sample 3 placed a on the sample stage 4. It is supported by The first actuator 7 has, for example, three prismatic piezoelectric bodies each several square square and length +1, which are orthogonal to each other, and two parallel prismatic piezoelectric bodies
It consists of a so-called tri-bod with electrodes facing each other on the sides. The first actuator 7 includes a scanning circuit 8 that independently applies a predetermined voltage to each opposing electrode of the prismatic piezoelectric body.
connected to. When a voltage is applied to the opposing electrodes of the prismatic piezoelectric bodies, each prismatic piezoelectric body moves in its longitudinal direction (X+Y+
z direction). As a result, the first probe 1 fixed to the first actuator 7 is scanned in three-dimensional directions.

On the other hand, the second probe 2 is fixed to a second actuator 9 that can move at least in the Z-axis direction perpendicular to the inspection surface, and, like the first probe, is supported above the sample 3 at a predetermined interval. be done. The second actuator 9 is preferably constructed of a tri-bod of the same size as the first actuator.

The second actuator 9 is connected to a two-position control circuit 10, and a predetermined voltage is applied to the piezoelectric body for bidirectional driving to control the position of the second probe 2 in two directions.

By applying a predetermined voltage from the scanning circuit 8 to the bidirectional driving piezoelectric body of the first actuator 7, the first probe 1 is brought close to a distance of about ins from the inspection surface of the sample 3. When a voltage is applied between the first probe 1 and the sample 3 by the first electric current [5], a first tunnel current flows between the first probe 1 and the sample 3. The first tunnel current is amplified in the amplifier 11 and then input to the positive input terminal of the subtracter 13 .

The second probe 2 is also supported at a distance of about lnm from the sample 3 by applying a predetermined voltage from the two-position control circuit 10 to the second actuator 9. When a voltage is applied between the second probe 2 and the sample 3 by the second power supply 6, a second tunnel current flows between them. Second
After the tunnel current is amplified by the amplifier 12, the subtracter 1
Human power is applied to the negative human power terminal of 3.

The first tunnel current and the second tunnel current manually input to the subtracter 13
The tunnel current is subjected to subtraction processing in the subtracter 13,
A signal representing the difference between the first tunnel current and the second tunnel current is output from the subtracter 13.

The first tunnel current detected by the first probe 1 is
This includes the influence of variations in the probe-sample distance due to vibrations on the inspection surface of sample 3. Further, the change in the second tunneling current detected by the second probe 2 reflects the change in the distance between the probe and the sample due to the vibration of the sample surface of the sample 3. The first probe 1 and the first actuator 7 are connected to the second probe 1 and the first actuator 7, respectively.
Since the size of the probe 2 and the second actuator 9 are the same, the variation in the distance between the first probe 1 and the sample 3 due to vibration is due to the change in the distance between the second probe 2 and the sample 3. Equals variation. Therefore, the signal output from the subtracter 13, which is obtained by subtracting the second tunnel current from the first tunnel current, is:
This is a signal that accurately reflects the distance between the first probe 1 and the inspection surface of the sample 3, with the vibration of the sample 3 removed from the first tunneling current.

During measurement, so that this signal remains constant,
A predetermined voltage is applied to the piezoelectric body of the first actuator 7 for driving in two directions by the scanning circuit 8, and the position of the first probe 1 in two directions is controlled, as well as for driving in the x and y directions. A predetermined voltage is applied to each piezoelectric body, and the first probe 1 is scanned along the inspection surface of the sample 3. Therefore, X+
The curved surface obtained as coordinate values of the voltages applied to each piezoelectric body for driving in the Y and Z directions accurately depicts the unevenness of the inspection surface, eliminating fluctuations in the distance between the probe and the sample due to vibration of the sample 3. It is an image to express.

According to the scanning tunneling microscope of the present invention, the subtracter 13 outputs a signal from which fluctuations in the distance between the probe and the sample due to vibration have been removed from the tunnel current flowing between the first probe 1 and the sample 3. Since it is output, stable measurements against vibrations can be performed, which is effective in improving measurement accuracy.

[Effects of the Invention] According to the scanning tunneling microscope of the present invention, the vibration of the inspection surface of the sample is detected by the second probe as a second tunneling current, and the vibration of the inspection surface detected by the first probe is detected by the second probe as a second tunneling current. The second tunnel current is subtracted from the first tunnel current reflecting the surface state and output from the subtracter. Therefore, a tunnel current from which the influence of vibration has been removed is output. As a result, stable measurement against vibrations can be performed, which is effective in improving measurement accuracy. Alternatively, measurement results can be obtained with considerable accuracy without using a large-scale vibration isolator.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a scanning tunneling microscope according to an embodiment of the invention. DESCRIPTION OF SYMBOLS 1... First probe, 2... Second probe, 3... Sample, 5... First power source, 6... Second power source, 7...
- First actuator 9... Second actuator 13... Castle calculator.

Claims (1)

[Claims] First and second probes having sharp tips; a first actuator that supports the first probe close to the inspection surface of the sample and moves it in three dimensions; , a second actuator that supports the second probe close to the inspection surface and moves it at least in a direction perpendicular to the inspection surface; and a first actuator that biases the first probe and the sample. a power source; a second power source that biases the second probe and the sample; a first tunnel current flowing between the first probe and the sample; A scanning tunneling microscope characterized by comprising: a subtracter that outputs a difference between a sample and a second tunneling current flowing between the sample and the second tunneling current.