WO2007102324A1

WO2007102324A1 - Phase lock-in type high-frequency scanning tunnel microscope

Info

Publication number: WO2007102324A1
Application number: PCT/JP2007/053428
Authority: WO
Inventors: Katsuyoshi Endo; Kouji Inagaki; Kenta Arima
Original assignee: Osaka Industrial Promotion Organization; Osaka University
Priority date: 2006-03-07
Filing date: 2007-02-23
Publication date: 2007-09-13
Also published as: JPWO2007102324A1; JP4590574B2

Abstract

Provided is a phase lock-in type high-frequency scanning tunnel microscope, which can observe an atomic image of the surface of an insulator or a wide-band-gap semiconductor. The microscope comprises a bias power source (8) for applying a high-frequency bias voltage of a rectangular or sinusoidal wave between a probe (1) and a specimen (2), a current amplifier (5) for amplifying a detection current, in which a tunnel current to flow between the probe and the specimen and a current derived from a stray capacity are mixed, and signal processing means (a lock-in amplifier (6) or a DSP) having phase lock-in functions to input the current amplified by the current amplifier and to synchronize a sampling signal with the frequency of the high-frequency bias voltage and lock the same with the frequency of the high-frequency bias voltage so that the current derived from the stray capacity is eliminated to generate a feedback signal exclusively of the tunnel current.

Description

Specification

Phase lock-in type high-frequency scanning tunneling microscope

Technical field

The present invention relates to a phase lock-in type high-frequency scanning tunneling microscope, and more specifically, a phase lock-in capable of observing an atomic image of an insulator or a wide band gap semiconductor surface including a conductor or a semiconductor. This is related to a high-frequency scanning tunneling microscope.

Background art

[0002] Since the development of Scanning Tunneling Microscopy (STM), it has become possible to observe the atomic structure of solid surfaces in real space. However, the conventional STM using a DC bias voltage uses a tunnel current, so that the atomic structure of the conductor or semiconductor through which the current flows can be observed, but not with an insulator. However, a surface having a function that affects the atomic arrangement of the surface, for example, many optical elements such as optical lenses and mirrors are insulators, and many insulating materials are used for semiconductor devices. Observation of the atomic structure on the body surface is strongly desired.

[0003] Since the Si practical surface currently used in the semiconductor industry is usually covered with a natural oxide film, it has been powerful enough to perform STM measurement without solution treatment. Therefore, in order to observe the surface of an insulator such as SiO using STM, an AC voltage was applied between the probe and the sample.

2

Attempts have been made to apply a sufficient DC voltage to the probe or to a thin Si native oxide film.

[0004] Non-Patent Documents 1 to 3 show that tunnel electrons flow between a probe sample and a small amount of electrons between the probe sample and the sample during a short period of a high-frequency voltage in the order of GHz. We are trying to perform STM operation on samples that do not have electrical conductivity by using the generated harmonics as feedback signals. However, the insulator material has only achieved a spatial resolution of up to about lnm. One of the causes is the charge on the sample surface. It is also a force that prevents tunneling of electrons from the probe by charging the tunnel electrons directly under the probe. The second is AC tunneling with an applied frequency f. Since the flow component is much larger than that, it is buried in the current component due to the capacitance between the sample and others, so harmonic components (such as 3f) instead of f are used as feedback signals.

1 1

It is because it uses it.

[0005] There are several reports on the observation of the Si surface with an oxide film, which is one of the targets of the present invention, and applying a DC bias between the probe and the sample to operate as a normal STM. Among them, Non-Patent Document 4 reports STM observation performed by applying a voltage of about 5 V to tunnel electrons to the conduction band on a Si surface having an oxide film of several nm. . However, the resolution of the atomic image size has not been obtained, and the oxide film is limited to a few nm or less. This is because the natural oxide film on Si is amorphous, so there are various defect levels in the band gap, and some of the tunneled electrons fall to the defect level, and the sample This is thought to be due to charging and obstructing the tunnel during STM observation.

[0006] In this way, V, the resolution of observing an atomic image even in the case of a deviation is appropriate. Therefore

In Patent Document 1, the present inventors set the distance between the tip of the probe and the sample extremely close to each other, generate a tunnel current by a bias voltage applied between the probe and the sample, and maintain this tunnel current constant. A scanning tunneling microscope that applies a feedback signal to the drive system under certain conditions to relatively scan the probe and the sample and observe an atomic scale image of the sample surface, with a high-frequency rectangular short between the probe and the sample. A pulse AC bias voltage is applied, and the time constant determined by the tunnel resistance and stray capacitance between the probe and the sample is substantially matched with the time constant determined by the input impedance of the current amplifier, and only the tunnel current component is detected. We have proposed a high-frequency pulsed scanning tunneling microscope that rectifies the feedback signal.

[0007] By the method described in Patent Document 1, the HOPG atomic image can be observed relatively clearly up to a frequency of several tens of kHz, but a clear atomic image can be observed at a frequency of 100 kHz. I couldn't do it. In the method described in Patent Document 1, a high-frequency rectangular short pulse is applied as a bias voltage between the probe 101 and the sample 102 by the pulse generator 103 as shown in FIG. Due to the capacitance of the pulse, overcurrent (spike current) occurs at the rise and fall of the pulse, and the tunnel current is buried in the overcurrent, so it cannot be used as a feedback signal. Therefore, the impedance matching circuit 104 that also has the capacitor C and the variable resistance R force is provided, and the current amplifier 1 The signal is converted into a DC signal proportional to the tunnel current via 05 and the rectifier circuit 106 and input to the control system 107 as a feedback signal for driving the probe 101. However, if the matching circuit 104 is used, the stray capacitance changes due to the problem of poor frequency characteristics and the displacement of the probe. Therefore, even if an impedance matching circuit is provided, only the tunnel current component cannot be detected. In particular, for a bias voltage with a high frequency, it was difficult to obtain a clear atomic image with large noise.

[0008] In addition, the conventional STM has a structure in which observation is performed by placing the probe and the sample in an ultra-high vacuum chamber with strict electromagnetic shielding in order to reduce noise and increase detection sensitivity. Therefore, it is inevitable that the apparatus is expensive and it takes a long time before the sample can be set and observed.

Non-patent literature l: G.P.Kochanski Physical Review Letters, Vol. 62 (1989) p2285 Non-patent literature 2: W. Seifert et al. Ultramicroscopy, Vol. 42-44 (1992) p379

Non-Patent Document 3: B. Micheal et al. Rev. Sci. Instrum., Vol. 63, No. 9 (1992) p4080

Non-Patent Document 4: Watanabe et al. Applied Physics Letters, Vol.72, No.l6 (1998) pl987 Patent Document 1: JP 2002-365194 A

Disclosure of the invention

Problems to be solved by the invention

[0009] Thus, the insulator surface using the current STM (for example, SiO

In the observation in 2), high spatial resolution has not been obtained. This is because tunnel electrons are charged on the surface of the sample directly under the probe, preventing tunneling of electrons from the probe. Here, the conditions for observing the insulator or semiconductor surface with STM can be summarized as follows.

(1) A high frequency bias voltage is applied between the probe and the sample to prevent the sample from being charged.

(2) Only the tunnel current component is detected and used as a feedback signal that keeps the distance between the probe and the sample surface constant.

(3) To prevent the sample directly under the STM probe from being charged, it is necessary to minimize the amount of charge sent from the probe side to the sample side. For this purpose, the frequency of the high-frequency bias voltage is made as high as possible.

(4) Tunnel electrons to the conduction band of the insulator material. [0010] However, when a high-frequency bias voltage is applied between the probe and the sample, the tunnel current component is much larger than that, and it is buried in the current component generated by the stray capacitance between the probe and other samples. It is difficult to detect the tunnel current due to the effect of electromagnetic noise. For example, the current value (i = 2 w fCV) due to stray capacitance in the high-frequency STM can be obtained by setting the bias voltage frequency f to 100 kHz and bias voltage V to 200 mV, even if the smaller value 0. IpF is selected as the stray capacitance C. Can be estimated at about 12nA. In contrast, since the tunnel current value in STM observation is generally 0.1 to LnA, the current value derived from the floating capacitance is one to two orders of magnitude larger than that. However, stray capacitance varies depending on the device configuration. Since the 0.IpF is estimated to be sufficiently small, the current derived from stray capacitance is actually much larger than the tunnel current.

[0011] A high-frequency STM, a new measurement method that solves these problems, was proposed and developed to enable observation of insulator or semiconductor surfaces with higher spatial resolution. Atomic Force Microscope It can be expected to obtain information about electronic states that could not be done with; AFM). Of course, it is also possible to obtain information on the electronic state by observing the atomic image of the conductor in the same way as a general STM.

[0012] Therefore, in view of the above situation, the present invention intends to solve an insulator such as SiO.

2

Another object is to provide a phase lock-in type high-frequency scanning tunneling microscope capable of observing atomic images on the surface of a wide band gap semiconductor or optical component.

Means for solving the problem

[0013] In order to solve the above-mentioned problems, the present invention makes the distance between the tip of the probe and the sample very close, generates a tunnel current by a bias voltage applied between the probe and the sample, and makes this tunnel current constant. A scanning tunneling microscope that applies a feedback signal to the drive system under the conditions to maintain and relatively scans the probe along the sample surface and observes an atomic scale image of the sample surface. A bias power source that applies a rectangular or sine wave high-frequency bias voltage between them, a current amplifier that amplifies the detection current that is a mixture of the tunnel current flowing between the probe and the sample and the current derived from the stray capacitance, and the current amplifier The sampled current is input, the sampling signal is synchronized with the frequency of the high frequency bias voltage and locked to the phase of the high frequency bias voltage, the current from the stray capacitance is removed, and the tunnel is removed. Current only to A phase lock-in type high-frequency scanning tunneling microscope comprising a signal processing means having a phase lock-in function for generating a feedback signal.

Here, the high-frequency bias voltage is a rectangular wave, and impedance matching that substantially matches a time constant determined by a tunnel resistance and stray capacitance between the probe and the sample and a time constant determined by an input impedance of a current amplifier. It is preferable to provide a circuit on the input side of the current amplifier.

In addition, the high-frequency bias voltage is a sine wave, and the phase of the sampling signal of the signal processing means is locked to the phase of the current derived from the stray capacitance outside the tunnel region where the distance between the probe and the sample is separated. It is more preferable that the output of the current derived from the stray capacitance is set to 0 by shifting the phase of the sampling signal, and then the approach between the probe and the sample is narrowed and the approach is started into the tunnel region where the tunnel current flows.

[0016] In this case, the phase of the sampling signal of the signal processing means is locked to the phase of the current derived from the stray capacitance outside the tunnel region, and then the phase of the sampling signal is shifted by -90 ° to The output is set to 0.

[0017] The frequency of the bias voltage is in the range of 100 kHz to 10 GHz, and the tunnel current used as the feedback signal is in the range of ΙρΑ to LONA.

[0018] Further, the distance between the probe and the sample is kept constant, a high frequency bias voltage of a sine wave is applied between the probe and the sample from the bias power source, and the phase of the sampling signal of the signal processing means is tunneled. The I-V characteristic is obtained by measuring dlZdV of the sample by locking to the phase of the current, and the phase of the sampling signal of the signal processing means is locked to the phase of the current due to the capacitance. It is preferable to obtain the CV characteristics by measuring the dCZdV of the sample.

Here, it is preferable that the signal processing means is a lock-in amplifier, and the sampling signal is a reference signal of the lock-in amplifier. Further, the signal processing means is a lock-in amplifier, the sampling signal is a reference signal for the lock-in amplifier, and the phase of the reference signal for the lock-in amplifier is automatically set outside the tunnel region, so that the current derived from the stray capacitance is Lock to phase.

[0020] Alternatively, the signal processing means is preferably a digital signal processor (DSP). Yes.

The invention's effect

[0021] According to the phase lock-in type high-frequency scanning tunneling microscope of the present invention configured as described above, the conduction band is prevented while applying a high-frequency bias voltage of a sine wave between the probe and the sample. Electrons can be tunneled to the STM image. Here, if the bias voltage is a sine wave, is a high frequency, and if a square wave is used, the tunnel current component is in the same phase as the bias voltage, but the current from the stray capacitance is 90 degrees behind the phase of the bias voltage. Therefore, by using a signal processing means with a phase lock-in function to lock the phase of the sampling signal to the tunnel current component, it is possible to cancel the current derived from the floating capacitance whose phase is shifted by 90 degrees. As a result, in the case of a sine wave, the matching circuit is not required, the performance of the current amplifier can be better drawn, and the tracking frequency up to the front of the signal processing means can be improved. Here, when the signal processing means is a lock-in amplifier and the sampling signal is a reference signal of a lock-in amplifier, a phase lock-in type high-frequency scanning tunneling microscope can be easily configured. Further, if the signal processing means is a digital signal processor (DSP), it can be configured at low cost by limiting the functions to the minimum necessary.

[0022] In the conventional feedback control system, the force that was several tens of kHz was the limit to obtain the stability of the entire feedback loop. By using a lock-in amplifier or DSP, the feedback signal is converted to DC by the mouth-in amplifier or DSP. Therefore, the stability of the feed knock loop is not related to the frequency of the bias voltage, and is stable at high frequencies. In other words, the stability of the feedback loop greatly affects the output time constant of the lock-in amplifier or DSP, and the phase of the output current of the current amplifier immediately before the lock-in amplifier or DSP is 180 degrees higher than the phase of the noise voltage frequency. Even if it is more than a delay, the feedback loop is stable because the output time constant of the lock-in amplifier or DSP is stable. Therefore, it is possible to perform pulse STM observation at a higher frequency than before.

[0023] Furthermore, since the lock-in amplifier detects only a signal component having a frequency component near the reference signal, the lock-in amplifier is extremely strong against frequency noise having a frequency apart from the reference signal force, and has a high frequency. Since the measurement is performed with the frequency locked to the wave, the circuit system is extremely strong against electromagnetic noise such as 60 Hz of the commercial frequency, thereby improving the SZN ratio and performing feedback control even with a small tunnel current. An image can be obtained. A similar effect occurs when using DSP.

[0024] Also, according to the present invention, the sensitivity of the conductor is equivalent to that of the conventional STM even under the condition that the probe and the sample are kept open to the atmosphere without using the electromagnetic shield and ultra-high vacuum required by the conventional STM. The atomic image can be observed with this, and the cost of the apparatus can be greatly reduced. Similarly, the present invention is capable of observing an atomic image on the surface of an insulator or semiconductor, although it is affected by adsorbed molecules even in the open atmosphere.

Brief Description of Drawings

FIG. 1 is a simplified explanatory diagram between a probe and a sample showing a measurement principle according to the present invention.

FIG. 2 is a simplified circuit diagram showing a circuit configuration of a high-frequency tunneling microscope using a square-wave noise voltage.

FIG. 3 is an oscillograph showing waveforms at various parts of the circuit of FIG. 2 when a rectangular wave bias voltage with a frequency of 4 kHz is applied.

FIG. 4 is an STM image of the HOPG surface when a rectangular wave bias voltage with a frequency of 4 kHz is applied using the high-frequency tunneling microscope with the circuit configuration shown in FIG.

FIG. 5 is an oscillograph showing waveforms at various parts of the circuit of FIG. 2 when a rectangular wave bias voltage with a frequency of 100 kHz is applied.

FIG. 6 is an STM image of the HOPG surface when a rectangular wave bias voltage with a frequency of 1 OOkHz is applied using the high-frequency tunneling microscope with the circuit configuration shown in FIG.

FIG. 7 is a simplified circuit diagram showing a circuit configuration of a high-frequency tunneling microscope using a sinusoidal noise voltage.

FIG. 8 is a waveform diagram showing a general operating principle of a lock-in amplifier.

FIG. 9 is a waveform diagram showing a measurement principle in which only a tunnel current component is extracted by a lock-in amplifier from an input current in which a tunnel current component (measurement signal) and a capacitance component are mixed.

[Fig. 10] Shows the adjustment procedure when actually observing a sample using the high-frequency tunneling microscope of Fig. 7. (a) shows the phase of the reference signal of the lock-in amplifier outside the tunnel region. (B) shows the waveform when the phase of the reference signal is shifted by 90 ° and the output of the capacitive component is 0, and (c) shows the waveform when only the tunnel current component is detected. Show.

FIG. 11 is an STM image of the HOPG surface when a sinusoidal bias voltage with a frequency of 100 kHz is applied using the high-frequency tunneling microscope with the circuit configuration shown in FIG.

FIG. 12 is an STM image of the HOPG surface when a sinusoidal bias voltage with a frequency of 200 kHz is applied using the high-frequency tunneling microscope with the circuit configuration shown in FIG.

FIG. 13 is an STM image obtained by observing the Si (111) surface in the direct current mode in the atmosphere with the circuit configuration shown in FIG.

[Fig.14] An STM image of the Si (111) surface observed in the atmosphere using a sine wave bias voltage with a frequency of 100 kHz.

FIG. 15 is a simplified circuit diagram showing a circuit configuration of a conventional high-frequency pulse scanning tunneling microscope.

Explanation of symbols

1 tip

2 Sample

3 Pulse generator (bias power supply)

4 Impedance matching circuit

5 Current amplifier

6 Lock-in amplifier (Signal processing means)

7 Control system

8 AC power supply (bias power supply)

101 probe

102 samples

103 Pulse generator (bias power supply)

104 Impedance matching circuit

104 matching circuit

105 current amplifier 106 Rectifier circuit

107 Control system

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention will be described in more detail based on the embodiments shown in the accompanying drawings. First, the measurement principle of the high-frequency scanning tunneling microscope, which is the premise of the present invention, will be described with reference to FIG. The object of the present invention is to obtain an atomic image of the surface of an insulator or semiconductor. Fig. 1 schematically shows the exchange of electrons between the probe 1 and the sample 2. A high frequency bias voltage whose polarity changes between positive and negative of a sine wave or a rectangular wave is applied between the probe 1 and the sample 2. In the present invention, it is particularly preferable to apply a sinusoidal high-frequency bias voltage between the probe 1 and the sample 2. Here, the sample 2 is assumed to be Si (ll l) having an oxide film (SiO 2) formed on the surface. The frequency of the bias voltage is 1

2

The range is 00 kHz to 10 GHz, and the tunnel current used as a feedback signal is in the range of 1 pA to LONA.

FIG. 1 (a) shows a state where electrons are tunneled and flow into the conduction band of sample 2 when a negative bias voltage is applied to probe 1. Figure 1 (b) shows that some of the tunneled electrons remain on the sample surface just below the probe even when the bias voltage becomes 0, and the sample surface is charged. FIG. 1 (c) shows how the charged electrons move to the probe 1 when the positive bias voltage is applied to the probe 1 and the charge on the surface of the sample 2 is relaxed. Fig. 1 (d) shows that the charged electrons have almost disappeared and the bias voltage has returned to 0. Next, when a negative bias voltage is applied to probe 1, the electron tunnel is not obstructed. Shows how to recover and show.

However, when a high frequency bias voltage is applied to the probe as shown in FIG. 1, the electrons are ideally tunneled and current is detected, but when the sample is an insulator or semiconductor, the tunnel is Since the current is very small, it is hidden by the current component and noise caused by stray capacitance, so a feedback signal cannot be obtained simply. The phenomenon that a large current flows between the probe and the sample due to this stray capacitance has become a problem that does not become a problem in the conventional DC bias type STM.

[0030] The high-frequency scanning tunneling microscope with the rectangular wave bias voltage shown in Fig. 2 applies a rectangular wave bias voltage between the probe 1 and the sample 2 with the pulse generator 3 (bias power supply) force. It is a method to add. The current flowing between the probe 1 and the sample 2 is detected by the current amplifier 5 through the impedance matching circuit 4 including the capacitor C and the variable resistance R force. Here, C is set to 100pF and R is set to 0 to 2Ω, so the time constant (CR) can be adjusted in the range of 0 to 200 μs. The output of the current amplifier 5 is input to the control system 7 that performs feedback control of the probe 1 through the lock-in amplifier 6. In the present invention, since the conventional STM is a DC bias voltage system, the use of the lock-in amplifier 6 that has never been used eliminates noise due to stray capacitance, etc., and greatly improves SZN. It was planned. In the present embodiment, the same applies to the case where a force digital signal processor (DSP), which explains the case where the input amplifier 6 is used, is used as the signal processing means having the phase lock-in function in the present invention. In other words, the lock-in amplifier is generally configured using a DSP. However, in the present invention, of the functions of a general lock-in amplifier, the current amplified by the current amplifier 5 is input, and the reference signal is a high-frequency bias voltage. Equivalent function because only the phase lock-in function is used, which synchronizes with the frequency and locks to the phase of the high-frequency bias voltage to remove the stray capacitance-derived current and generate a feedback signal based only on the tunnel current. This is because DSP can be realized.

FIG. 3 shows signal waveforms in the apparatus having the circuit configuration shown in FIG. The bias voltage of the square wave is 4kHz, ± 200mV, and the waveform is shown in the upper part of Fig.3. Since 4 kHz is 250 s in terms of period, the matching circuit 4 is adjusted so that only components that change faster than this can be removed. The output waveform when the gain of current amplifier 5 is 10 ⁷ is shown in the middle part of Fig. 3, and the output waveform of lock-in amplifier 6 where the gain is 10 and the output time constant is 100 s is shown in the lower part of Fig. 3. Yes. Here, since the lock-in amplifier 6 functions as a band-pass filter, it is sampled in synchronism with the frequency of the bias voltage, excluding the influence of stray capacitance that appears late at the rising and falling parts of the bias voltage. And output. Fig. 4 is an ATM image of the surface of HOPG as a sample. Fig. 4 (a) is an observation region of lnm x lnm, Fig. 4 (b) is an observation region of 2nm x 2nm, and Fig. 4 (c) is 5 This is an observation region of nm X 5 nm.

FIG. 5 shows a waveform when the bias voltage of the rectangular wave is 100 kHz and p−p 200 mV in the apparatus having the circuit configuration shown in FIG. In this case as well, the upper part of FIG. The first voltage waveform, the middle row shows the output waveform of the current amplifier 5, and the lower row shows the output waveform of the lock-in amplifier 6. Fig. 6 (&) shows an STM image when the square wave bias voltage is 5 (¾1¾, p-p200mV), and Fig. 6 (b) shows an STM image when the square wave bias voltage is 100kHz and p-p200mV. Both of these are observation areas of 5 nm × 5 nm, which shows that the present invention has made it possible to observe a much clearer atomic image compared to FIG. The effect of using an amplifier became obvious.

[0033] Here, when looking at Fig. 5, the output waveform of the current amplifier becomes a waveform close to a sine wave (sine wave) when the frequency increases to 1 OOkHz even though a rectangular wave bias voltage is applied. It is assumed that the atomic image can be observed even when the bias voltage is a sine wave. On the other hand, in order to prevent charging of the sample surface, it is desirable to apply a bias voltage having a frequency as high as possible. In addition, since the problem of spike current is eliminated by using a sine wave as the bias voltage, the impedance matching circuit 4 in FIG. 2 is not necessary. As a result, a high-frequency scanning tunneling microscope with a sine wave bias voltage with a simple circuit configuration as shown in Fig. 7 was reached.

The phase lock-in type high-frequency scanning tunneling microscope shown in FIG. 7 applies a sinusoidal bias voltage from the AC power source 8 (bias power source) between the probe 1 and the sample 2, The current flowing between the sample 2 and the current amplifier 5 is amplified by the current amplifier 5, and the output is input to the lock-in amplifier 6. The lock-in amplifier 6 generates a feedback signal based only on the tunnel current and inputs it to the control system 7. Then, the probe 1 is feedback-controlled. In this case, since the impedance matching circuit 4 is not provided, it is possible to prevent the frequency characteristics from being deteriorated due to the matching circuit.

Next, based on FIGS. 8 and 9, description will be given of a measurement principle that uses the lock-in amplifier 6 to remove a current component due to stray capacitance and extract only a weak tunnel current. First, a general operation principle of the lock-in amplifier will be described with reference to FIG. As shown in Fig. 8 (a), when a measurement signal with sinusoidal force is input to the lock-in amplifier and the phase difference between the measurement signal and the lock-in amplifier reference signal is 0 °, the reference signal is added to the measurement signal. The multiplied PSD output is a waveform in which the negative half-wave of the measurement signal is positively folded (positive full-wave rectification). The smoothed LPF output is positive DC. On the other hand, as shown in Figure 8 (b) If the phase difference between the measurement signal and the reference signal of the lock-in amplifier is 90 °, the PSD output will be a symmetrical waveform, and the LPF output will be zero. Also, as shown in Fig. 8 (c), if the phase difference between the measurement signal and the reference signal of the lock-in amplifier is 180 °, the PSD output becomes an inverted waveform when the phase difference is 0 °, and the LPF output Becomes negative direct current. The lock-in amplifier can detect the measurement signal by locking the phase of the reference signal to the phase of the measurement signal. In addition, in measurement signal detection, components with a phase difference of 90 ° from the measurement signal are not output.

Next, the probe 1 and the sample 2 are replaced with a circuit in which a resistance for flowing a tunnel current and a floating capacity are connected in parallel in an equivalent circuit. As shown in Fig. 9, the current passed between the probe and the sample is the force that flows through the lock amplifier while mixing the tunnel current component (measurement signal) and the capacitive component. The capacitive component is 90 ° out of phase with the tunnel current component. It's off. Therefore, when the phase of the reference signal is locked to the phase of the tunnel current component by the lock-in amplifier, the capacitive component shifted by 90 ° is not output, and only the tunnel current component can be detected as shown in FIG. By using this as a feedback signal, STM observation can be performed.

Next, a procedure for actually performing STM observation using the apparatus having the circuit configuration shown in FIG. 7 will be briefly described. When AC is used for the bias voltage, if the probe is in the tunnel region, it can be considered that the resistance and capacitance are in parallel between the probe and the sample, but the probe is not in the tunnel region. In this case (see Fig. 10 (a)), the circuit between the probe and the sample is only a capacitance. That is, when approaching, the force that only the capacitive component flows between the probe and the sample at the beginning, the mixed current of the capacitive component and the tunnel current component flows when the probe enters the tunnel region. First, as shown in Fig. 10 (a), the phase of the reference signal can be locked to the phase of the capacitive component by automatically setting the phase of the reference signal of the lock-in amplifier outside the tunnel region. Here, the reference signal and the current of the capacitive component have the same phase. From there, the approach is started by shifting the phase of the reference signal by 90 ° and setting the output of the capacitive component to 0 (see Fig. 10 (b)). In this way, as shown in Fig. 10 (c), the output appears only when the tunnel current component flows, and only the tunnel current can be detected. In Fig. 10 (c), the current flowing through the tunnel gap is the sum of the tunnel current component and the capacitance component. As the force frequency increases, the capacitance component becomes much larger than the signal level of the tunnel current component, so the waveform of the capacitance component becomes dominant. In this way, only the tunnel current component can be detected by adjusting the output of the capacitive component flowing before the broaching to 0 using the lock-in amplifier.

11 and 12 show STM images obtained by actually observing the surface of the HOPG with the phase lock-in type high-frequency scanning tunneling microscope having the circuit configuration shown in FIG. Figure 11 shows the result of HOPG surface observation using a sine wave bias voltage with a frequency of 100 kHz and p-p200 mV, with a lock-in amplifier time constant of 100 μs, and (a) shows the observation area of 2 nm X 2 nm. , (B), the observation region is 5 nm X 5 nm, and an atomic image is obtained. Figure 12 shows the result of HOPG surface observation using a sine wave bias voltage of 200 kHz and p-p200 mV with a lock-in amplifier time constant of 100 s. (A) shows the observation area of 2.5 nm. X 2.5 nm, (b) has an observation area of 5 nm X 5 nm, and although it is unclear, an atomic image is obtained.

[0039] In observation using a sinusoidal bias voltage with a frequency of 200 kHz, the maximum operating frequency of the lock-in amplifier used was 100 kHz, so measurement was performed with a frequency extender inserted before the lock-in amplifier. It was. It can be easily predicted that a clearer atomic image can be obtained by using a wide-band current amplifier and lock-in amplifier.

Finally, FIG. 13 and FIG. 14 show the results of observation of the step terrace structure on the Si (l l l) surface. First, Fig. 13 is an STM image of the Si (111) surface observed in the DC mode in the atmosphere with the circuit configuration shown in Fig. 7. (a) is an observation region of 150 nm X 150 nm, and (b) is 500 nm X It is an observation region of 500 nm. An STM image of the step terrace structure on the Si (l l l) surface was obtained in the DC mode even in the atmosphere.

[0041] FIG. 14 is an STM image obtained by observing the Si (ll 1) surface in the atmosphere using a sinusoidal bias voltage with a frequency of 100 kHz, (a) is an observation region of 200 nm × 200 nm, and (b) is This is an observation region of 500 nm X 500 nm. In this way, changes in the Si (lll) surface were also observed in high-frequency STM observation. This change appears to be smaller than in the positive case where the bias voltage in DC mode is greater than in the negative case. This is because the surface changes due to the influence of alternating positive and negative because the polarity of the bias voltage changes while alternating between positive and negative. it is conceivable that. However, it can be seen that a step-terrace structure is observed in the high-frequency STM image. As a result, it was confirmed that high-frequency STM observation was possible even for samples with a band gap.

[0042] As described above, by selecting the component having the same phase as the high-frequency noise voltage by the lock-in amplifier, the influence of the stray capacitance is minimized, and the distance between the probe and the sample is controlled only by the tunnel current component. It is possible to observe an insulator or a semiconductor sample by applying a sinusoidal bias voltage having a higher frequency.

[0043] Further, the distance between the probe and the sample is kept constant, a high frequency bias voltage of a sine wave is applied between the probe and the sample from the bias power source, and the phase of the reference signal of the lock-in amplifier is changed to the tunnel current. The dlZdV of the sample is measured by locking to the phase to obtain the I-V characteristics, or the dCZdV of the sample is measured by locking the phase of the reference signal of the lock-in amplifier to the phase of the current derived from the capacitance. By acquiring the C-V characteristics, the electrical characteristics in a minute region of the nm order on the semiconductor surface can be investigated. Here, the distance between the probe and the sample is kept constant, a sine wave high frequency bias voltage is applied between the probe and the sample from the noise power source, and the phase of the reference signal of the lock-in amplifier is changed to the phase of the current derived from the capacitance. By locking to, current fluctuations due to stray capacitance are eliminated, and therefore only changes in current due to capacitive components based on the physical properties of the sample can be detected. The frequency at which the dlZdV or dCZdV of the sample is measured may be different from the frequency used to control the probe-sample distance.

Claims

The scope of the claims

[1] The distance between the tip of the probe and the sample is very close, and a tunnel current is generated by the bias voltage applied between the probe and the sample. The feedback signal is sent to the drive system under the condition that this tunnel current is maintained constant. A scanning tunneling microscope for observing an atomic-scale image of the sample surface by relatively scanning the probe along the sample surface, and a high-frequency bias voltage of a square wave or a sine wave between the probe and the sample A bias power supply for applying a current, a current amplifier for amplifying a detection current obtained by mixing a tunnel current flowing between the probe and the sample and a current derived from a stray capacitance, and a current amplified by the current amplifier are input, and a sampling signal is input to a high-frequency bias. This is a feedback signal based only on the tunnel current by synchronizing with the frequency of the source voltage and locking to the phase of the high frequency bias voltage to remove the current from the floating capacitance. Generating, phase-lock-type high frequency scanning tunneling microscope, characterized in that a signal processing unit having a phase lock-in feature.

[2] The high-frequency bias voltage is a rectangular wave, and an impedance matching circuit that substantially matches the time constant determined by the tunnel resistance and stray capacitance between the probe and the sample and the time constant determined by the input impedance of the current amplifier is a current amplifier. The phase lock-in type high-frequency scanning tunneling microscope according to claim 1, wherein the phase-lock-in type high-frequency scanning tunneling microscope is provided on the input side.

[3] After the high-frequency noise voltage is a sine wave and the phase of the sampling signal of the signal processing means is locked to the phase of the current derived from the stray capacitance outside the tunnel region where the distance between the probe and the sample is separated, The phase lock according to claim 1, wherein the phase lock of the sampling capacitance is set to 0 by shifting the phase of the sampling signal, and then the approach is started into the tunnel region where the tunnel current flows by narrowing the distance between the probe and the sample. In-type high-frequency traveling tunnel tunnel microscope.

[4] Outside the tunnel region, the phase of the sampling signal of the signal processing means is locked to the phase of the current derived from the stray capacitance, and then the current of the stray capacitance is output by shifting the phase of the sampling signal by -90 ° The phase lock-in type high-frequency scanning tunneling microscope according to claim 3, wherein

[5] The frequency of the bias voltage is in a range of 100 kHz to 10 GHz, and a tunnel current used as a feedback signal is in a range of ΙρΑ to LONA. Phase lock-in type high-frequency scanning tunneling microscope.

[6] The distance between the probe and the sample is kept constant, a high frequency bias voltage of sine wave is applied between the probe and the sample from the bias power source, and the phase of the sampling signal of the signal processing means is changed to the phase of the tunnel current. The dlZdV of the sample is measured by locking the signal to obtain the IV characteristic, or the dCZdV of the sample is obtained by locking the phase of the sampling signal of the signal processing means to the phase of the current derived from the capacitance. 2. The phase lock-in type high-frequency scanning tunneling microscope according to claim 1, wherein CV characteristics are obtained by measurement.

7. The phase lock-in type high-frequency scanning tunneling microscope according to claim 1, wherein the signal processing means is a lock-in amplifier, and the sampling signal is a lock-in amplifier reference signal.

[8] The signal processing means is a lock-in amplifier, the sampling signal is a lock-in amplifier reference signal, and the stray capacitance is obtained by automatically setting the phase of the reference signal of the lock-in amplifier outside the tunnel region 5. The phase lock-in type high-frequency scanning tunneling microscope according to claim 3, wherein the phase lock-in type high-frequency scanning tunneling microscope is locked to a phase of a current derived therefrom.

9. The phase lock-in type high-frequency scanning tunneling microscope according to claim 1, wherein the signal processing means is a digital signal processor (DSP).