JPH0769140B2 - Scanning tunneling microscope - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a scanning tunnel microscope that displays a concavo-convex image of a sample surface on an image display device by detecting a tunnel current by bringing a probe close to the sample.

[Prior art] nm where the atom at the tip of the probe and the electron cloud of the atom of the sample overlap
When the tip of the probe and the tip of the sample are brought close to the sample surface to the order, and a constant bias voltage is applied between the probe and the sample in this state, a tunnel current flows. This tunnel current is particularly sensitive to the distance between the probe and the sample,
By measuring the magnitude of the tunnel current, the distance between the sample and the probe can be measured ultra-precision.

The scanning tunneling microscope (STM) controls the height of the probe so that the tunnel current is constant, and the unevenness of the sample surface is determined by the trajectory of the probe height when the probe is scanned horizontally. It is a microscope device for observing the shape and attracting attention in observing the surface atomic arrangement.

In addition, the scanning tunneling microscope can not only observe the surface atomic arrangement (irregularities on the sample surface), but also the PHYSICAL REVIEW
LETTERS VOLUME56, NUMBER6, PP.608−611,10FEBRUARY198
By the method described in 6, it is possible to know the characteristics of the sample and the electronic state of the sample. That is, the characteristic of the sample is that the bias voltage V _T is changed from a negative value to a positive value with the probe fixed at an arbitrary position (x ₀ , y ₀ , z ₀ ) on the sample, and the tunnel current at that time is changed. measure the current I _T, it can be known by obtaining the I-V curve. FIG. 3 (a) shows an IV curve when the sample is a metal, and FIG. 3 (b) shows that when the sample is a semiconductor. Similarly, the electronic state of the sample can be known by fixing the probe at an arbitrary position and obtaining the change dI _T / dV _T of the tunnel current when the bias voltage V _T is changed by a unit amount.

[Problems to be Solved by the Invention] However, when the IV curve is obtained by fixing the probe to any one point of the sample as in the conventional case, the reliability of the characteristic of the sample obtained from the IV curve is reliable. The property is low and the wrong characteristics may be required. This is because the tunnel current is very small, and if noise is mixed in the tunnel current, the IV curve is greatly influenced by the noise.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a scanning tunneling microscope capable of accurately obtaining the characteristics of a sample even if noise is mixed in the tunnel current.

[Means for Solving the Problems] The present invention relates to XY axis direction driving means for driving a probe in the XY direction along a sample surface, and Z axis for driving the probe in the Z direction for changing the height with respect to the sample surface. Direction drive means, voltage application means for applying a bias voltage between the probe and the sample, tunnel current detection means between the probe and the sample, and a drive signal for the probe to generate an uneven image of the sample surface. In a scanning tunneling microscope having a signal processing means for outputting, a probe is brought close to a sample, a tunnel current is detected, and an uneven image of the sample surface is displayed on an image display device. Means for designating a plurality of measurement points, means for sequentially moving the probe onto each of the designated measurement points to fix the probe, and means for sweeping the bias voltage at each of the designated measurement points , The signal of the tunnel current measured at each measurement point It is characterized in that a signal processing means for processing averages.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings. First
FIG. 1 is a diagram showing a configuration of a scanning tunneling microscope according to an embodiment of the present invention, and FIG. 2 is a diagram showing an image display state of a sample image. In FIG. 1, 1 is a sample, 2 is a probe, 3 is a bias power source, 4x, 4y and 4z are piezoelectric elements, 5 is a scan generator, 6x, 6y and 6z are piezoelectric element driving high voltage circuits, and 7 is I / V amplifier, 8 is a log amplifier, 9 is a comparator, 10 is an integrator, 11 is an image amplifier, and 12 is a CPU monitor.

Piezoelectric elements 4x, 4y, and 4z drive the X-axis, Y-axis, and Z-axis, respectively 3
The scan generator 5 constitutes a two-dimensional scanner, and the scan generator 5 generates scanning signals for the probe 2 and the image display device 12. Piezoelectric element drive high voltage generation circuit 6x, 6y,
The piezoelectric elements 4x and 4y of the three-dimensional scanner are driven according to the scanning signal generated by the scan generator 5, and the piezoelectric element driving high voltage generating circuit 6z is an integrator.
The output of 10 drives the piezoelectric element 4z of the three-dimensional scanner. First stage I / V amplifier 7 connected to the sample
Is for converting the tunnel current I _T into a voltage and further amplifying it. The log amp 8 connected to the next stage outputs the output signal of the I / V extension width device 7 to the height of the probe. Signal conversion (linearization) is performed so as to correspond to linearity. The comparator 9 compares the output value of the log amp 8 with a reference value (DC Source) corresponding to the set value of the tunnel current, and the integrator 10 integrates the output of the comparator 7 and outputs this output to the three-dimensional scanner. Is a control value for the Z-axis voltage element 4z. In this way, I / V amplifier 7, log amplifier 8, comparator 9, integrator 10
The tunnel current I _T is fed back through the Z-axis piezoelectric element driving high-voltage generating circuit 6z to control the Z-axis piezoelectric element 4z of the three-dimensional scanner to control the height of the probe 2 and keep the tunnel current It constant. .

Further, the signal fed back to the Z-axis piezoelectric element driving high voltage generation circuit 6z is supplied to the image amplifier 11, and the output signal from the image amplifier 11 is supplied to the CPU monitor 12 to be brightness-modulated and the two-dimensional sample. The image is displayed as a surface image.

In the configuration as described above, the present invention is provided with the position designating circuit 13 for designating a plurality of measurement points on the sample surface corresponding to the sample image two-dimensionally displayed on the CPU monitor 12. A pointing device 14 such as a mouse or a light pen for designating an arbitrary point on the screen is connected to the position designating circuit 13. When the operator designates a plurality of measurement points on the sample surface with the pointing device, a signal indicating the position is held in the position designation circuit 13, and the designated points are set as shown in FIG. It is displayed on the sample image.

In FIG. 2, a tunnel microscope image of the 7 × 7 structure of the silicon (111) surface is displayed, and dangling points a _{1 to} a _n in the sample image are measured as measurement points.

Then, the signal indicating the measurement point held in the position specifying circuit 13 is supplied to the piezoelectric element driving high voltage generating circuits 6x and 6y. The piezoelectric element driving high voltage generating circuits 6x and 6y drive the piezoelectric elements 4x and 4y of the three-dimensional scanner based on the signals, and sequentially move the probe 2 to the measurement point on the sample surface. Then, at each measurement point, the bias power supply 3 is controlled so that the probe Fermi level is scanned from sufficiently below to above the Fermi level of the sample,
The bias voltage V _T is changed positively and negatively. Tunnel current due to change in bias voltage V _T at this time
I _T is detected, and the detected tunnel current I _T is supplied to the CPU monitor 12. The tunnel current I _T accompanying the change in the bias voltage V _T at the measurement points a _{1 to} a _n supplied to the CPU 12 is
The calculation is performed in 2 to obtain the average of the tunnel currents. Then, the electronic state of the sample can be obtained by obtaining the change in the tunnel current with respect to the change in the unit amount of the bias voltage.

[Effect] According to the present invention, the probe is fixed on a plurality of measurement points of the sample, the bias voltage is swept at each measurement point to measure the tunnel current, and the average of the tunnel current measured at each measurement point is measured. Therefore, the characteristics of the sample can be accurately obtained even if noise is mixed in the tunnel current.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a scanning tunneling microscope according to an embodiment of the present invention, FIG. 2 is a diagram showing an image display state of a sample image, and FIG. 3 is an IV curve showing characteristics of the sample. It is a figure. 1: Sample, 2: Probe 3: Bias power supply 4x, 4y, 4z: Piezoelectric element 5: Scan generator 6x, 6y, 6z: Piezoelectric element drive high voltage circuit 7: I / V amplifier, 8: Log amplifier 9: Comparator , 10: Integrator 11: Image amplifier, 12: CPU monitor 13: Positioning circuit 14: Pointing device

Claims (1)

[Claims] 1. An XY for driving a probe in the XY directions along a sample surface.
Axial direction drive means, Z axis direction drive means for driving the probe in the Z direction that changes the height with respect to the sample surface, voltage application means for applying a bias voltage between the probe and the sample, probe and sample And a signal processing means for generating a probe driving means to output an uneven image of the sample surface, and bring the probe close to the sample to detect the tunnel current to detect an uneven image of the sample surface. In a scanning tunnel microscope displayed on an image display device, means for designating a plurality of measurement points on the sample surface corresponding to the sample image, and a probe by sequentially moving the probe to each of the designated measurement points. Means for fixing the needle, means for sweeping the bias voltage at each of the designated measurement points, and signal processing means for averaging the tunnel current signals measured at each of the measurement points are provided. Scanning tunneling microscope.