JPH07103709A - Scan type tunnelling microscope - Google Patents

Abstract

PURPOSE:To prevent damage to a probe and a sample while obtaining a highly reliable data by accomplishing a scanning of a tunnel voltage for obtaining a spectrum at a speed higher than the response speed of a control system and continuously. CONSTITUTION:A voltage V (t) is applied between a sample 1 and a probe 2 with a voltage generation circuit 3. The voltage V (t) varies cyclically and continuously between optional voltages V1 and V2 and the frequencies of the voltages are higher than a response frequency of a control system formed with a control section 4 and a scanner 7. Here, a tunnel current flowing between the sample 1 and the probe 2 is detected by a current detector 5 to be sent to a difference signal computing section 6. At the computing section 6, the voltage V (t) is monitored from the voltage generation circuit 3 to detect a current value I (V0 (t)) when a reference voltage value V0 for controlling the height of the probe 2 is given and a difference signal DELTAI(t) from a target current value I0 is sent to control section 4. At the control section 4. a control signal is computed on the basis of the difference signal DELTAI(t) to control the scanner 7.

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 which obtains surface information by scanning a probe on the surface of a sample. The present invention relates to a scanning tunneling microscope that analyzes the electronic state and composition of a sample surface.

[0002]

2. Description of the Related Art Conventionally, a method for obtaining a voltage-dependent spectrum image of a tunnel current has been disclosed in Review. of. Scientific. Instruments, Volume 58, 10
Issue (1987), pages 1806-1810 (Re
v. Sci. Instrum., 58 (10) (1987), pp.180
6-1810) was used.

The above conventional example will be described with reference to FIG. Figure 2
FIG. 2A is a diagram showing the scanning voltage, and FIG. 2B is a diagram showing the timing for controlling the probe. First, the position of the probe is controlled so that the tunnel current reaches the target current value I _{0 at} the reference voltage V ₀ . That is, as shown in FIG. 2 (b), the control of the probe position only time t = 0 to t ₁ the voltage is V _0. Then, at times t _{1 to} t ₂ , the probe control is stopped, and the voltage is changed from the voltage values V ₁ to V ₄ at which the spectrum is to be acquired, as shown in FIG.
As shown in (a), it is changed stepwise. The current measured at this time is shown in FIG. By obtaining the current values I _{1 to} I ₄ at the respective voltages V _{1 to} V ₄ , X on the sample surface is obtained.
Spectrum I in _{1 (V i, X 1)} (i = 1~4) is obtained. In order to move to the next measurement point X ₂ , the tunnel voltage is first returned to the reference voltage V ₀ (FIG. 2 (a), time t ₂ ). Since the current is disturbed at this time, the control is started after the current becomes stable (time t ₃ ).

Thereafter, by repeating the operation at the measuring point X _{2 at} the measuring point X ₁ , the spectrum I (V _i , X
₂ ) (i = 1-4) is obtained. A spectral image was obtained by repeating these at all measurement points. In the following description, this method will be called conventional method 1.

Another method is eye. Bee. M. journal. of. research. and. Development, Volume 30, Issue 4 (1986) pp. 411-4
16 pages (IBM J. Res. Develop., 30 (4) (1986) p.
pp. 411-416).

This method will be described in detail with reference to FIG.
FIG. 3A is a diagram showing the scanning voltage. Reference voltage V
₀ is modulated at a frequency higher than the response frequency of the control system. At this time, the tunnel current as shown in FIG. 3B is detected. The current is also modulated at the same cycle as the voltage modulation. This is because the modulation frequency is higher than the response frequency of the control system. The probe is controlled by the average value I of the current. The spectrum is usually obtained by extracting the current component synchronized with the voltage modulation with a lock-in amplifier. In the following description, this method will be called conventional method 2.

[0007]

According to the conventional method 1, since the voltage changes stepwise, spike noise is likely to occur during the change and the current is disturbed. Therefore, it was necessary to wait for the current to settle down before starting the acquisition of the current at each voltage or the control of the probe. Therefore, the time required for spectrum acquisition becomes long, and while controlling the probe at each measurement point and scanning the voltage,
There is a problem that the position of the probe changes due to temperature drift due to thermal expansion of the sample holder and the like. The current spectrum needs to be obtained by fixing the relative position of the probe to the sample surface, but the reliability of the obtained data was low for the reasons described above. Furthermore, it is difficult to perform stable measurement because the probe or the sample may be damaged by the noise when the voltage changes (for example, it is possible to peel off the atoms on the surface with a pulse voltage of about 5 V. , Applied Physics, Vol. 62, No. 2 (1993), pages 155 to 159).

On the other hand, in the conventional method 2, since the modulation voltage applied is smaller than the reference voltage, the spectrum over a wide voltage range cannot be acquired. In particular, if the voltage is modulated over both polarities, the average current value may become 0, so it was impossible to acquire a spectrum over both polarities. Further, since the shape of the spectrum changes depending on the location of the sample, there is a problem that the voltage corresponding to the average value I of the current used for controlling the probe differs. For example, in FIG. 3B, the average value I of the current is
There's the I ₀ is at time 0 to t ₁ but is when the voltage is V _0, at time t ₂ ~t _3, a V ₀ '. The spectrum must be acquired with the height of the probe fixed at each measurement point, so the height of the probe is controlled so that the tunnel current has a target current value at a reference voltage. But,
The conventional method 2 has a big problem that the reliability of the measurement is lowered because this is not satisfied.

As described above, according to the conventional method, the control cannot be performed continuously, so the probe position may move (conventional method 1). There is a problem that the current value becomes ambiguous (conventional method 2), and the reliability of measurement becomes low regardless of which method is used.

[0010]

SUMMARY OF THE INVENTION The above-mentioned problem is to make tunnel voltage scanning for obtaining a spectrum faster and continuous than the response speed of a control system, and to provide a current at a reference voltage value of the scanning voltage. This is solved by detecting the value and using it as a probe control signal. Further, more reliable data can be obtained by obtaining a spectrum by performing voltage scanning a plurality of times.

[0011]

[Function] Since the scanning of the tunnel voltage is made faster than the response frequency of the control system, it is the same as controlling the probe substantially continuously, and the probe position shifts due to drift etc. during spectrum acquisition. Does not occur. Further, since the current value at the reference voltage is detected and used as the control signal, the measurement reliability is high. Furthermore, even when acquiring a spectrum over both polarities, a voltage that does not make the current value of the control signal zero can be selected as the control voltage, so that the probe can be stably controlled.
Further, since the voltage is continuously scanned, spike noise does not enter and the probe and the sample are not damaged.

[0012]

EXAMPLE An example of the present invention will be described with reference to FIG. Between the sample 1 and the probe 2, a voltage V is generated by the voltage generating circuit 3.
(t) is applied. This voltage V (t) is an arbitrary voltage V
It is characterized in that it periodically and continuously changes between ₁ and V ₂ , and its frequency is higher than the response frequency of the control system formed by the controller 4 and the scanner 7. At this time, the tunnel current flowing between the sample 1 and the probe 2 is the current detector 5
And is sent to the difference signal calculator 6. The difference signal calculation unit 6 monitors the voltage V (t) from the voltage generating circuit 3 and controls the height of the probe 2 at the reference voltage value V ₀ at the current value I (V ₀ (t)). Is detected and a difference signal ΔI (t) from the target current value I ₀ is sent to the control unit 4. The control unit 4 calculates a control signal based on the difference signal ΔI and controls the scanner 7.

On the other hand, the data storage unit 9 has a current detector 5
Current I (t) and the voltage V (t) from the voltage generation circuit 3
Are sent and stored in the memory 10 as a spectral image together with the scanning signals (X (t), Y (t)) sent from the X, Y scanning signal generator 8. Further, the uneven image is also formed from the Z (t) signal and the (X (t), Y (t)) signal from the control unit 4 at the same time and stored in the memory 11.

A feature of the present invention is that the voltage generated by the voltage generation circuit 3 is continuous including the reference voltage V ₀ , the frequency thereof is higher than the response frequency of the control system, and the reference signal is used as the control signal of the probe. The point is that the current value at the time of voltage V ₀ is detected and used. The control unit 4 calculates a control signal from the difference signal ΔI and controls the scanner 7. Since the difference signal ΔI is sent at a frequency higher than the response frequency,
In effect, it means that the required voltage and current values are continuously controlled.

Details of the voltage scanning and the probe control will be described with reference to FIG. Here, it is assumed that the probe position is controlled so that the tunnel current at the reference voltage value V ₀ becomes I ₀ . For simplicity, the current is detected at four points of the voltage V ₁ ~V _4.

FIG. 4 (a) is a diagram showing the time change of the tunnel voltage applied between the probe and the sample. It continuously changes between the voltages V _{1 to} V ₄ . This frequency is characteristically higher than the response frequency of the control system. The state of the tunnel current flowing between the sample and the probe at this time is shown in FIG. As described in FIG. 1, the difference between the current value at the voltage V ₀ and the target current value I ₀ is calculated as the difference signal ΔI. This difference signal ΔI is newly calculated for each voltage scan as shown in FIG. The probe is controlled based on the difference signal ΔI. FIG. 4D is a diagram showing the movement of the probe when the response speed of the control system is infinite. Since the difference signal ΔI is discontinuous, it moves in a polygonal line. However, in reality, since the response speed of the control system is finite, the probe is smoothly controlled as shown in FIG. By making the voltage scanning faster than the response speed of the control system, the control becomes equivalent to continuous control.

Although the voltage is scanned in a triangular wave in FIG. 4, a sine wave may be used as long as it is continuous. Further, the difference signal is calculated for each scan, but it may be calculated at the time of reciprocating the voltage scan depending on how to take the reference voltage.

According to the present invention, it is possible to obtain a spectrum while constantly controlling the tunnel current to a target current at a reference voltage.

Next, the method of spectrum acquisition in the present invention will be described. FIG. 5 shows an example thereof. Figure 5 (a)
Shows the scanning voltage, and FIG. 5B shows the tunnel current measured at that time. FIGS. 5 (c) and 5 (d) are diagrams showing data acquisition timings, and FIGS. 5 (e) and 5 (f) are diagrams showing scanning signals in the in-plane direction of the probe.

First, an example in which the data acquisition system is faster than the control system will be described. The timing of data acquisition is all possible timings indicated by circles in FIG. 5 (c). When the data acquisition system is high-speed, necessary data can be acquired only at the first voltage scan at each measurement point on the sample surface. That is, the data at X ₁ on the sample surface is X
All can be acquired at the first voltage scan in ₁ , that is, at the time indicated by the black circle in FIG. On the other hand, if the data acquisition system is not very fast, all the data cannot be acquired in one voltage scan, so data will be collected for one measurement point on the sample surface by multiple voltage scans. .

For example, as shown in FIG. 5D, the current value at the voltage V ₁ is acquired by the first voltage scan, and the current value at the voltage V ₂ is acquired by the second voltage scan. Is. At this time, it is not necessary to scan in the in-plane direction while acquiring the data of one measurement point (see FIG. 5).
(e)), or (FIG. 5 (f)).

In the case where data continuously changes between pixels as in the atomic level measurement, the position of the probe can be easily operated. Therefore, it is better to scan the probe in the in-plane direction of the sample. good. This is because the obtained data originally has a resolution of only one pixel, so that the X resolution in FIG.
_{This is because 11} and X ₁₂ can be regarded as the same point. However, in the case of micron-order measurement, measurement points X ₁₁ and X ₁₂
Then, since the physical properties of the sample may be greatly different, it is desirable to stop the scanning.

It is also possible to obtain a spectrum by averaging the current values of a plurality of voltage scans. That is, the current value at the voltage V ₄ can be obtained as the average value of the current values I (V ₄ , t ₁₁ ) to I (V ₄ , t ₁₃ ) obtained by the three voltage scans. Further, since the voltage scanning is fast, the problem can be solved by averaging the reciprocating current when the history of the current spectrum appears in the reciprocating scanning.

[0024]

According to the present invention, since the spectrum can be acquired while continuously controlling the current at the reference voltage to the target current value, highly reliable data can be acquired.
Moreover, since spike noise does not enter, data can be collected without damaging the probe and sample.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram of conventional method 1.

FIG. 3 is an explanatory diagram of conventional method 2.

FIG. 4 is a detailed explanatory diagram of the present invention.

FIG. 5 is an explanatory diagram of spectrum acquisition according to the embodiment of the present invention.

[Explanation of symbols]

1 ... Sample, 2 ... Tip, 3 ... Voltage generation circuit, 4 ... Control unit,
5 ... Current detector, 6 ... Difference signal calculation unit, 7 ... Scanner, 8
... X, Y scanning signal generating section, 9 ... Data storage section, 10 ... Spectral image storage section, 11 ... Concavo-convex image storage section.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takashi Furukawa 2520 Akanuma, Hatoyama-cho, Hiki-gun, Saitama Stock company Hitachi Research Laboratory

Claims (6)

[Claims] 1. A scanning tunneling microscope that obtains surface information by scanning a probe on the surface of a sample, wherein the voltage is higher than the response frequency of a control system when a voltage-dependent spectrum of the tunnel current is acquired. A scanning tunneling microscope characterized by performing continuous scanning in a fast cycle, and at the same time detecting a current at a reference voltage of the scan voltage in each cycle and using it as a control signal for a probe. 2. The scanning tunneling microscope according to claim 1, wherein the control signal of the probe has a time coefficient that changes with each cycle of the voltage scanning. 3. A scanning tunneling microscope according to claim 1, wherein a spectrum for one point on the sample surface is obtained by performing voltage scanning a plurality of times. 4. The current at the voltage value according to claim 3, wherein the voltage scanning is performed a plurality of cycles to average a plurality of current values obtained for each cycle for one voltage value. Scanning tunneling microscope with value. 5. The method according to claim 3, wherein the current value for at least one voltage value is acquired in one voltage scan, and the current value for all voltage values is obtained by performing the voltage scan a plurality of cycles. A scanning tunneling microscope that obtains a spectrum for one point on the sample surface. 6. A scanning tunneling microscope according to claim 1, 2, 3, 4, or 5, wherein a current value at the time of one round trip of voltage scanning is averaged to obtain a spectrum.