JPH0712824A - Scanning tunneling microscope with potential distribution measurement function - Google Patents

Abstract

(57) [Abstract] [Purpose] Using a scanning tunneling microscope, the shape and potential distribution in a microscopic region of the sample surface can be measured simultaneously with high precision and S / N ratio. A probe, a moving means for moving the probe along the sample surface, and a first AC voltage applied between the probe and the sample to flow between the probe and the sample surface. A scanning tunneling microscope having a control means for detecting a tunnel current and feedback-controlling the distance between the probe and the sample surface so as to keep the current constant, and a frequency different from the first AC voltage. Voltage applying means for applying the second AC voltage between both electrodes of the sample, detecting means for obtaining the amplitude of the component of the tunnel current synchronized with the second AC voltage, and means for recording and displaying the magnitude of the amplitude. Have.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning tunneling microscope (STM) for simultaneously measuring unevenness and potential distribution in a region of a sample surface of several .mu.m or less.

[0002]

2. Description of the Related Art A scanning tunneling microscope (STM) is a microscope capable of measuring the shape of the surface of a fine structure element with an angstrom order resolution by utilizing the tunneling phenomenon of electrons. STM applies a bias voltage between the probe with a sharp tip and the sample, scans the probe while keeping the tunnel current flowing between the probe and the sample constant, and reflects the unevenness of the sample from the position of the probe. STM image can be obtained.

However, since the tunnel current depends on the distance between the sample and the probe and the potential difference, a normal STM image contains not only the shape of the sample surface but also potential distribution information. Therefore, the method of separating and displaying the potential information on the sample surface from the tunnel current is a method of scanning tunneling potential measurement (Scanning Tunneling).
Potentiometry: STP).

The STP for applying the STM to measure the potential distribution inside the element has been proposed by P. Murart of IBM and the like. (IBM Journal of Reseach and Developement, pp
443-450, vol.30, No.5, 1986) In this method, an AC voltage is applied between the sample and the probe, and a potential gradient due to the DC potential is applied to the surface of the sample to be measured. Using the AC component of the tunnel current, keep the distance between the probe and the sample constant, apply a potential measurement signal to the electrode of the sample so that the DC component of the tunnel current becomes 0, and measure this signal voltage. By doing so, the potential inside the element is determined.

A method has also been proposed in which the potential is measured in two states, that is, a state in which a current is not applied to the sample and a state in which a current is applied, and the potential distribution is determined from the difference between the two.

[0006]

STP can measure the potential distribution on the sample surface and calculate the local potential gradient on the sample surface from the measured potential distribution. This potential gradient reflects the local charge mobility of the sample.

However, since the potential change in a minute region (μm or less) is very small, the S / N is poor when the potential distribution is measured by a DC voltage. Furthermore, the method of P. Murart et al. Requires a complicated feedback circuit.

Further, in the case of the method of determining the potential distribution from the difference between the state in which the current is not applied to the sample and the state in which the current is applied, in the measurement in the atmosphere, the measurement position changes due to thermal drift or the like. It was not possible to eliminate the resulting measurement error.

Therefore, the present invention solves the conventional problems and provides a scanning tunneling microscope capable of measuring unevenness and potential distribution in a region of a sample surface of several μm or less with a simple circuit with high accuracy and S / N ratio. With the goal.

[0010]

A scanning tunneling microscope of the present invention is a scanning tunneling microscope that detects a tunnel current between a sample and a probe, and applies an alternating voltage 1 having a predetermined waveform that changes at a predetermined cycle. A voltage applying unit that applies between the sample and the probe, a detecting unit that obtains the amplitude of a component of the tunnel current that is synchronized with the AC voltage 1, and the sample and the probe according to the amplitude. With means for controlling the distance between. At the same time, a voltage applying means for applying an alternating voltage 2 having a predetermined waveform having a frequency different from the alternating voltage 1 between both electrodes of the sample, and a detecting means for obtaining the amplitude of a component of the tunnel current synchronized with the alternating voltage 2. And means for measuring and recording the magnitude of the amplitude.

[0011]

[Function] In order to obtain an accurate uneven surface image of the sample surface, only the change in tunnel current due to the unevenness is detected without being affected by the potential distribution in the sample surface, and the tip of the probe does not collide with the sample surface. So that the probe has to be scanned. In the scanning tunneling microscope of the present invention, the AC voltage 1 that changes in a predetermined cycle is used as the bias voltage, and the amplitude of the component of the tunnel current that is synchronized with the AC voltage 1 is obtained. The component of the tunnel current that synchronizes with the AC voltage 1 is
Since it strongly depends on the distance between the probe and the sample surface and does not include other potentials and noises, the scanning tunneling microscope of the present invention can accurately obtain an uneven image of the sample surface without the probe colliding with the sample. You can

Further, in the scanning tunneling microscope of the present invention, at the same time as the above operation, an alternating voltage 2 having a predetermined waveform having a frequency different from the alternating voltage 1 is applied between the electrodes of the sample, and the tunnel current is synchronized with this alternating voltage 2. The amplitude of the component is calculated. Since the component of the tunnel current that is synchronized with the AC voltage 2 depends on the potential of the sample surface and does not include other electrical signals or noise, the scanning tunneling microscope of the present invention accurately determines the potential distribution on the sample surface. , Can be obtained at the same time as the uneven surface image of the sample surface.

Any means can be used as the detecting means as long as it can extract a component synchronized with the AC voltage. For example, the detected tunnel current value is converted into a voltage signal, and this voltage signal and AC A product that is obtained by multiplying the product signal with the voltage signal by a multiplier and outputs a component having a frequency equal to or lower than a certain frequency can be favorably used.

[0014]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of a scanning tunneling microscope according to an embodiment of the present invention. Sample 1 is a carbon resistor in which conductive carbon and an insulating binder are mixed. Further, a Pt-Ir electric field polishing needle is used for the probe 7. A probe 7 having a sharp tip is arranged to face the sample 1 for measurement. The surface of the sample 1 and the tip of the probe 7 are close to each other to the extent that a tunnel current flows.

The probe 7 is held by the probe driving piezoelectric element 15. The piezoelectric element 15 is, for example, a cylindrical piezoelectric actuator, and can move the probe 7 arbitrarily in three directions of x, y, and z. Here, the x and y directions are the in-plane directions of the surface of the sample 1, and the z direction is the direction perpendicular to the surface of the sample 1. The piezoelectric element 15 is driven by the output of the feedback circuit 13 described later to move the probe 7 to x, y, z.
It is designed to be displaced in each direction.

The sample 1 is provided with a first electrode 2 and a second electrode 3. Also, the sine wave V ₁ (t) = V _T sin ω ₁ t
There is provided an oscillator 5 for generating. One end of this oscillator 5 is connected to the first electrode 2 of the sample 1, and the other is grounded. Further, a rectangular wave having the same period as the sine wave applied to the sample 1 is output from the oscillator 5 to the multiplier 10 as the reference signal 1.

Meanwhile oscillator 6 for generating a sine wave _{V 2 (t) = V P} sinω 2 t of frequency different from the frequency oscillator 5
Is provided. One end of the oscillator 6 is connected to the adder circuit 4, and the other is grounded. In the adder circuit 4, the sine wave from the oscillator 5 and the sine wave from the oscillator 6 are added and output to the second electrode 3 of the sample 1. Further, a rectangular wave having the same period as the sine wave applied to the adder 4 is output from the oscillator 6 to the multiplier 17 as the reference signal 2.

A current-voltage conversion circuit 8 for detecting a tunnel current flowing from the probe 7 to the sample 1 and converting it into a voltage signal is connected to the probe 7, and the output of the current-voltage conversion circuit 8 is passed through an amplifier circuit 9. It is connected to one input terminal of the multiplier 10. The reference signal 1 from the oscillator 5 is supplied to the other input terminal of the multiplier 10.

A low-pass filter 11 is provided on the output side of the multiplier 10. The cutoff frequency of the low-pass filter 11 is set to a value sufficiently smaller than the frequency of the sine wave generated by the oscillator 5. Low pass filter 11
The output of is input to the feedback circuit 13 via the amplifier circuit 12. The feedback circuit 13 outputs drive signals for the x, y, and z axes in order to drive the probe 7 in the directions for the x, y, and z axes.
In this case, the feedback circuit 13 outputs the z-axis drive signal according to the output of the amplifier circuit 12. Each driving signal is input to the piezoelectric element 15 and is also supplied to the recording / display device 14, and the recording / display device 14 can three-dimensionally display and observe the unevenness of the sample surface.

Further, the output of the current-voltage conversion circuit 8 is also connected to one input terminal of the multiplier 17 via the amplifier circuit 16. The reference signal 2 is supplied from the oscillator 6 to the other input terminal of the multiplier 17.

A low-pass filter 18 is provided on the output side of the multiplier 17. The cutoff frequency of the low-pass filter 18 is set to a value sufficiently smaller than the frequency of the sine wave generated by the oscillator 6. Low pass filter 1
The output of 8 is input to the voltage measuring circuit 20 via the amplifier circuit 19. The voltage measured by the voltage measuring circuit 20 and the x and y axis drive signals output from the feedback circuit 13 are supplied to the recording / display device 21, and the recording / display device 21 three-dimensionally determines the potential distribution on the sample surface. Can be displayed and observed.

Next, the operating principle of this scanning tunneling microscope will be described with reference to FIG. The oscillator 5 is shown in FIG.
As shown in (a), the sine wave AC voltage V ₁ (t) is output, and the oscillator 6 outputs the sine wave AC voltage V ₂ (t) shown in FIG. 2D. The AC voltage V ₁ (t) is applied as a bias voltage between the sample 1 and the probe 7 via the first electrode 2. AC voltage V ₁ (t) + V ₂ is applied to the second electrode 3.
(T) is applied. By applying such an AC voltage, a tunnel current I _t (t) as shown in FIG. 2B flows between the probe 7 and the sample 1. The tunnel current I _t (t) is the tunnel current I ₁ due to the voltage V ₁ (t).
(T) component and V ₂ (t) by a tunneling current I ₂ (t)
It consists of components [I _t (t) = I ₁ (t) + I ₂
(T)].

By the way, the tunnel current is actually detected.
In this case, since the tunnel current itself is very small, superimposing noise
I can't avoid it. Here, as shown in FIG.
Noise component I_n (T) is the tunnel current I_t Superimposed on (t)
It shall be. As a result, current-voltage conversion circuit 8 and amplification
The sum of the tunnel current and noise I through the circuits 9 and 16 _t 
(T) + I_{n (t)}Is proportional to the voltage of the multipliers 10 and 17
It will be input to the other terminal.

When k is a proportional constant, the multiplier 10 sums V ₁ (t) of the reference signal 1, tunnel current and noise k.
The product of (I _t (t) + I _n (t)) is output. Since the reference signal 1 is a rectangular wave signal having a single frequency, the output of the multiplier 10 has a frequency component corresponding to the sum and difference between each component frequency of the sum signal of the tunnel current and noise and the frequency of the reference signal. Will be a signal having.

Therefore, the component of the sum signal which is synchronized with the reference signal 1 is converted into DC, and the other frequency components are converted into AC. Since the low-pass filter 11 having a sufficiently low cutoff frequency is provided on the output side of the multiplier 10, only the DC component of the output of the multiplier 10 is extracted. This DC component is a component of the sum signal that is synchronized with the reference signal 1 of the oscillator 5, and this synchronized component is the tunnel current I _t that flows through the probe 7 and the sample 1.
Since there is nothing but the component corresponding to I ₁ (t) in (t), the output of the low-pass filter 11 represents only the amplitude of the tunnel current I ₁ (t) as shown in FIG. 2 (e). Will be.

The output of the low-pass filter 11 is the amplifier circuit 1.
It is input to the feedback circuit 13 via 2 and is used to keep the distance between the probe 7 and the surface of the sample 1 constant (that is, the amplitude of the tunnel current constant). As a result, the feedback circuit 13
The value of the z-axis driving signal output by the above accurately reflects the unevenness of the surface of the sample 1. The z-axis drive signal is recorded in the recording / display circuit 14.

On the other hand, in the multiplier 17, _assuming that k is a proportional constant, the product of V ₂ (t) of the reference signal 2 and the sum k (I _t (t) + I _n (t)) of the tunnel current and noise is obtained. Output.

Therefore, as in the case of the multiplier 10, the component of the sum signal that is synchronized with the reference signal 2 of the oscillator 6 is converted into DC, and the other frequency components are converted into AC. Since the output side of the multiplier 17 is provided with a low-pass filter 18 having a sufficiently low cutoff frequency, the multiplier 1
Of the 7 outputs, only the DC component will be extracted. This DC component is a component of the sum signal that is synchronized with the reference signal 2 of the oscillator 6, and this synchronized component is I of the tunnel current I _t (t) flowing through the probe 7 and the sample 1.
_Since it is nothing but the component corresponding to ₂ (t), the output of the low-pass filter 18 represents only the amplitude of the tunnel current I ₂ (t) as shown in FIG. 2 (f).

The output of the low pass filter 18 is the amplifier circuit 1.
It is input to the voltage measuring circuit 20 via 9 and the measured voltage value is recorded in the recording / display circuit 21 together with the x and y axis drive signals output from the feedback circuit 13. Therefore, by recording the amplitude (proportional to the potential) of the tunnel current I ₂ (t) together with the position where the probe is present, it is possible to obtain a potential distribution map on the sample surface.

By this method, it is possible to simultaneously measure the unevenness of the surface of the carbon resistor and the potential distribution in the atmosphere, and as a result, there are two parts, that is, the part containing only carbon and the part containing the binder and carbon. all right.

Since alternating current is used to measure the potential distribution in this method, it is possible to measure a minute potential distribution and a minute change in the potential as compared with the case of using direct current, and a more accurate potential distribution map can be obtained. Obtainable. Further, even if the measurement is performed in the atmosphere, since the shape and the potential distribution are simultaneously measured, an error does not occur at the measurement position due to thermal drift or the like.

Next, as an application example of the present invention, a measurement result of a gold vapor deposition thin film will be described.

The morphology of the sample used in this application example is shown in FIG. 22 and 23 are gold electrodes having a thickness of 1500 angstroms formed by vapor deposition on a quartz glass substrate 24, and the distance between both electrodes is 40 μm. Further, a gold thin film 25 having a thickness of 100 angstrom is vapor-deposited between the electrodes. When a current is applied to this sample through the gold electrodes 22 and 23, the gold thin film 25 is locally destroyed along the electrodes over a width of about 1 μm, and a discontinuous gold thin film 26 is formed. It is known that the sample produced by such a method exhibits non-ohmic electrical characteristics as well as electron emission and light emission phenomena (for example, Ohmsha's "Thin Film Handbook", pp. 463).

The quartz glass substrate 24 is partially exposed in the discontinuous thin film 26 formed by this local destruction, and the electric conductivity is extremely poorer than other portions, so that the tunnel current flowing through the probe is extremely large. Due to its small size, it is very difficult to measure the surface shape of this portion by ordinary scanning tunneling microscopy. Similarly, it has been difficult to measure the potential distribution of this portion with a good S / N ratio by the potential distribution measuring methods used so far.

However, by using the scanning tunneling microscope of the present invention, the potential distribution could be accurately measured. From the measurement cross section, the surface potential is 2
It can be seen that 5 is sharply changed in the portion of the discontinuous thin film 26 formed by local destruction.

Further, an example in which the scanning tunneling microscopy of the present invention is applied to a semiconductor sample will be described.

The sample used in this application example is a metal-semiconductor interface made by vapor deposition of aluminum on arsenic-doped n-type silicon. It is generally known that at the interface between the metal and the n-type semiconductor, the movement of charged particles occurs, an energy barrier called a Schottky barrier occurs, and a rectifying effect is observed. When a voltage is applied to this interface in the forward direction, the potential distribution near the interface is not uniform due to the effect of this energy barrier.

Therefore, when the surface shape and the potential distribution of the cross section of the sample are simultaneously measured by the scanning tunneling microscope of the present invention, it is observed that the voltage drop occurs gently on the semiconductor side from the metal-semiconductor interface. I was able to.

In the case of such a semiconductor sample, although caution is required for quantitative evaluation in the voltage region in which the tunnel current between the probe and the sample shows non-ohmic characteristics, the scanning tunneling microscope of the present invention can be used for the metal sample. It can be seen that it can be applied not only to semiconductor samples.

[0040]

The scanning tunneling microscope having the potential distribution measuring function of the present invention can accurately measure a minute potential change in a minute region. Further, according to this method, the shape and the potential distribution are simultaneously measured. Therefore, there is an effect that the shape and the potential distribution at the same position on the sample surface can be simultaneously measured.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a scanning tunneling microscope according to an embodiment.

FIG. 2 is a diagram for explaining the operating principle of the scanning tunneling microscope of the embodiment, (a) a waveform diagram showing the output of the first oscillator,
(B) Waveform diagram showing tunnel current, (c) Waveform diagram showing noise component, (d) Waveform diagram showing output of second oscillator,
(E) A waveform diagram showing the amplitude of the tunnel current 1 obtained as the output of the multiplier, (f) A waveform diagram showing the amplitude of the tunnel current 2 obtained as the output of the multiplier.

FIG. 3 is a diagram showing a form of a sample used in an application example.

[Explanation of symbols]

 1 Measurement sample 2 1st electrode 3 2nd electrode 4 Addition circuit 5 1st oscillator 6 2nd oscillator 7 Probe 8 Current-voltage conversion circuit 9,16 Amplification circuit 10,17 Multiplier 11,18 Low-pass filter 12,19 Amplifier circuit 13 Feedback circuit 14,21 Recording / display circuit 15 Probe driving piezoelectric element 20 Voltage measurement circuit 22,23 Gold electrode 24 Quartz glass substrate 25 Gold thin film 26 Discontinuous thin film

Claims (2)

[Claims] 1. A probe, scanning control means for scanning the probe along a sample surface, first voltage applying means for applying a first AC voltage between the probe and the sample, and the probe. First detecting means for obtaining the amplitude of a component of the tunnel current flowing between the needle and the sample that is synchronized with the first AC voltage, the position of the probe along the sample surface, and the output of the first detecting means. Image forming means for forming a surface image of the sample according to the above, a control means for controlling the distance between the probe and the sample according to the amplitude, and a second AC voltage having a frequency different from the first AC voltage. Voltage applying means for applying to the two electrodes provided on the surface of the sample, second detecting means for obtaining the amplitude of the component of the tunnel current synchronized with the second AC voltage, and the sample surface of the probe. Of the surface of the sample according to the position along the line and the output of the second detecting means. A scanning tunneling microscope having a potential distribution measuring function comprising an image forming means for forming a potential distribution map. 2. A signal conversion circuit for converting the tunnel current into a voltage signal by the detection means, a multiplier to which the voltage signal and the AC voltage are input, and a low frequency band provided on the output side of the multiplier. A scanning tunnel microscope having a potential distribution measuring function according to claim 1, further comprising a pass filter.