JP3053485B2 - Scanning probe microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning probe microscope for obtaining microscopic surface information of a sample using a probe.

[0002]

2. Description of the Related Art As a scanning probe microscope, there is a scanning tunneling microscope (ST).
M) and Atomic Force Microscope (A)
FM) are known.

[0003] STM was founded in 1982 by Binni.
g) and US Patent No. 4,343,9 by Rohrer et al.
With the fine surface shape measurement device proposed in No. 93, the surface shape of a conductive sample can be measured with atomic-level resolution.

When a sharp and sharp conductive probe is brought close to the surface of a conductive sample to a distance of about several nm and a bias voltage is applied between the probe and the sample, a tunnel current flows between the two. This tunnel current _IT is represented by the following equation.

I _T = B (V _T ) exp (−Aφ ^1/2 S) (1) where V _T is a bias voltage, B (V _T ) is a coefficient depending on the bias voltage, and A is about 10.25 (nm) (eV) ^1/2 ) ^-1 number coefficient,
φ is the barrier height, and S is the distance between the probe samples. Since the barrier height φ of the clean metal surface is about 1 to 5 eV,
When the probe-sample distance is 0.1nm changes, the tunneling current I _T is seen from the may vary about one digit (1). S
The TM probe is scanned (eg, rastered) along the surface of the sample, that is, the xy plane, by a fine movement element such as a piezoelectric body. During scanning, the probe or the sample is relatively moved in the direction perpendicular to the sample surface (that is, the z direction) by using a fine moving element, and the distance S between the probe and the sample is set to 0.1 nm so as to keep the tunnel current constant. Control with the following accuracy. Therefore, the tip of the probe moves on a curved surface that reflects the surface shape of the sample at a certain distance from the sample surface. By obtaining and recording the position of the tip of the probe on the xy plane and the position in the z direction from the voltage applied to the fine movement element, S indicating minute irregularities on the sample surface is obtained.
A TM image is obtained.

As can be seen from the equation (1), the tunnel current detected by the STM depends not only on the distance S between the sample probes but also on the barrier height reflecting the local electron state of the sample. . The barrier height φ is a trapezoidal approximation of the electrical barrier between the sample and the probe by the following equation.

Φ = (φ _TIP + φ _S ) / 2 (2) Here, φ _TIP and φ _S are the local work functions of the tip of the probe and the surface of the sample, respectively, which are necessary for extracting one electron from the surface. Energy in terms of voltage. The local work function φ _S on the sample surface varies depending on the charge density and constituent atoms on the surface. Therefore, the barrier height will reflect the distribution of local work function phi _S of the sample surface by using the same probe.

[0008] Methods for determining the barrier height φ from the value of the tunneling current I _T, which is detected by the STM, for example, Physical Review Letters (Physical Review Letters), Vol. 60, No. 12, 1988, first from pages 1166
Some are described on page 169. in this way,
At the time of measuring the surface unevenness, a constant minute vibration in a direction perpendicular to the sample surface is applied to the probe to detect a vibration component of the tunnel current. In this way, the barrier height φ, which is the distance derivative of the tunnel current, is obtained at the same time as the unevenness information of the sample. Note that the tunnel current detected by this method is a signal having a vibration component. However, since the frequency of this minute vibration is set higher than the cutoff frequency of the feedback control system, the feedback control is performed in the same manner as in a normal STM. Asperity information of the sample can be obtained from the output of the system.

AFM is disclosed in JP-A-62-130302.
No. 6,009,036. When you move the probe close to the sample,
A small force (atomic force) acts between the tip of the probe and the atoms on the sample surface. The interatomic force includes a repulsive force based on the exclusion rule, a Van der Waals force, a covalent force, and the like, and is represented by a Lennard-Jones potential having an attractive region and a repulsive region as shown in FIG.
The AFM detects a signal corresponding to an atomic force or a derivative thereof based on a deflection amount or a vibration amplitude of a cantilever, and scans a probe while controlling a distance between a probe and a sample so as to keep this signal constant. , The obtained tip position information is S
By processing in the same manner as in TM, an image showing the surface shape of the sample at an atomic level resolution can be obtained. AFM is ST
It is possible to measure not only a conductive sample but also an insulating sample like the M apparatus. As described above, SFM (Scanning Force Microscop) is a general term for a scanning probe microscope that measures a sample by detecting the force acting between the sample and the probe using the cantilever.
Called e), AFM is a form of SFM.

In Japanese Patent Application No. 2-313389, the present inventors apply a bias voltage between a sample and the tip of a conductive cantilever, and when the cantilever is vibrated, the displacement of the tip of the cantilever and the average of the tunnel current flowing therethrough A device for detecting the barrier height between the sample and the tip of the probe by simultaneously measuring the value and the amount of fluctuation was proposed.

[0011] Contacting the different metals together work function, a potential difference occurs phi _A -.phi _B given by the difference between the respective work function phi _A and [phi] B, a current flows between them as would equipotential. So if you give the metals are in opposition to place the potential difference VI _o to compensate for the potential difference between each other in the capacitor C _AB made between these metal charge Q as: not Tamarira.

Q = C _AB (φ _A −φ _B −V _I0 ) = 0 That is, V _I0 = φ _A −φ _B and the potential difference V _I0 is 2
Work function difference between two metals. A method of detecting a charge accumulated in a metal-to-metal capacitance and calculating a difference in work function is called a Kelvin method.

[0013]

SUMMARY OF THE INVENTION STM or AFM
In the barrier height measurement using, the barrier work height obtained by trapezoidal approximation of the tunnel barrier between the probe and the sample is measured, so if the work function of the probe is not known, the local work function of the sample is did not understand. Further, there is a similar problem in the work function difference measurement using the Kelvin method, and further, the local work function distribution cannot be measured.

An object of the present invention is to provide a scanning probe microscope capable of simultaneously measuring a sample uneven image and a local work function.

[0015]

A scanning probe microscope according to the present invention has a conductive probe at a free end, and a cantilever which is displaced when a force acts between the probe and a sample;
Vibrating means for vibrating the probe in a direction perpendicular to the sample surface;
Displacement detection means for detecting the displacement of the cantilever, servo means for keeping the position of the center of vibration of the cantilever relative to the sample surface, voltage applying means for applying a bias voltage between the sample and the probe, the probe and the sample Current detecting means for detecting a tunnel current flowing between the two, and voltage recording means for recording a voltage at which the tunnel current stops flowing when the bias voltage is changed.

Another scanning probe microscope of the present invention has a conductive probe at a free end, a cantilever that is displaced when a force acts between the probe and the sample, and a probe perpendicular to the surface of the sample. Vibration means for vibrating the probe in various directions, displacement detection means for detecting the displacement of the cantilever, servo means for keeping the amplitude of the vibration of the free end of the cantilever constant, and applying a bias voltage between the sample and the probe A voltage application unit, a current detection unit that detects a tunnel current flowing between the probe and the sample, and a voltage recording unit that records a voltage at which the tunnel current stops flowing when the bias voltage is changed.
Furthermore, the scanning probe microscope of the present invention and another scanning probe microscope of the present invention may be configured to include a calculation unit that calculates a barrier height from the displacement of the cantilever and the tunnel current.

[0017]

The probe is scanned along the sample surface while being vibrated in the z-direction at a specific frequency by vibrating means. Therefore, the free end of the cantilever that supports it also vibrates, and its displacement is always detected by the displacement detecting means. At this time, the center and amplitude of vibration of the free end of the cantilever change according to the strength of the interatomic force between the tip of the probe and the constituent atoms on the sample surface. Further, a bias voltage is applied between the probe and the sample, and a tunnel current flowing between the two is detected by the current detecting means.

The servo means detects the position of the center of vibration of the free end of the cantilever with respect to the sample surface based on the displacement of the cantilever obtained by the displacement detecting means. The relative position between the probe and the sample is controlled so as to be constant. Alternatively, the amplitude of the vibration of the free end of the cantilever is detected, and the relative position between the probe and the sample is controlled so as to keep the amplitude of the vibration constant. In any case, the servo signal for controlling the relative position between the probe and the sample becomes information indicating the unevenness of the sample surface.

The voltage recording means changes the bias voltage applied between the probe and the sample by the voltage applying means, and records the voltage at which the tunnel current does not flow, thereby recording the work function φ _S of the sample and the probe. Difference from work function φ _TIP (φ _TIP
−φ _S ) is detected.

Further, the scanning probe microscope of the present invention and another scanning probe microscope of the present invention each include an arithmetic means, so that the tunnel current detected by the current detecting means and the cantilever detected by the displacement detecting means are provided. Is input to the calculating means, and the barrier height is calculated.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, the scanning probe microscope of the first embodiment has a thin-film cantilever 12 made of a conductive material having a conductive probe 14 at its free end. The other end, that is, the fixed end, of the cantilever 12 is fixed to the piezoelectric actuator 16, and is supported such that the probe 14 is located near the sample 26 mounted on the xyz actuator 28. The piezoelectric actuator 16, the AC voltage of the angular frequency omega _c from the oscillator 18 is supplied to vibrate the cantilever 12 i.e. the probe 14. The AC voltage from the oscillator 18 is also input to the two lock-in amplifiers 20 and 22. The displacement of the free end of the oscillating cantilever 12 is optically detected by a displacement detector 24, and the displacement signal is transmitted to the lock-in amplifier 2
0 is supplied to the servo circuit 36. The servo circuit 36
Based on the input displacement signal, the position of the center of vibration of the free end of the cantilever with respect to the sample surface is detected, and the distance between the probe 14 and the sample 26 is controlled using the xyz actuator 28 so as to keep this constant. A bias voltage is applied to the sample 26 by a bias power supply 30.
As a result, a tunnel current flowing between the probe 14 and the sample 26 is detected by the tunnel current detection circuit 34 and input to the lock-in amplifier 22 and the data sampling circuit 32. The data sampling circuit 32 receives a bias voltage from a bias power supply 30 in addition to the tunnel current. The outputs of the displacement detector 24, the two lock-in amplifiers 20 and 22, and the tunnel current detection circuit 34 are input to an arithmetic unit 38 that calculates the square root of the barrier height φ.

Next, the operation for obtaining information on the unevenness of the sample surface will be described. When an alternating voltage of the angular frequency omega _c is applied from the oscillator 18 to the piezoelectric actuator 16 supporting the cantilever 12, the free end of the cantilever 12 is vibrated at an amplitude of about 0.1 nm. The displacement Z _TIP of the free end of the cantilever 12 is detected by an optical displacement detector 24 provided above the cantilever 12 and input to the lock-in amplifier 20 and the servo circuit 36. Here, when the cantilever 12, that is, the probe 14 is moved closer to the sample 26 using a z-direction fine movement mechanism (not shown), an atomic force corresponding to the mutual distance as shown in FIG. Works. The center of vibration of the free end of the cantilever 12, that is, the average of the displacement Z _TIP changes according to the interatomic force.
The servo circuit 36 calculates the average of the displacement Z _TIP , and supplies a servo signal Z _FB to the xyz actuator 28 for moving the sample 26 in the z direction (vertical direction in the drawing) so as to keep the average constant. The servo control is performed with a time constant slower than the angular frequency omega _c of the cantilever. As a result, the center of vibration of the free end of the cantilever 12 is
6 moves on a curved surface reflecting the unevenness of the surface. Therefore,
By recording the servo signal _ZFB in a data recording device (not shown) in synchronization with the xy position information at the time of scanning, an uneven image of the sample surface can be obtained.

Next, the operation of measuring the barrier height will be described. (1) According to the equation, the square root of barrier height φ is the distance differential of the tunneling current I _T is given by the following equation.

[0025]

(Equation 1) Here, <I _T > is the average value of the tunnel current, ΔI _T is the amplitude of the tunnel current, and ΔZ _TIP is the amplitude of the vibration of the free end of the cantilever.

A bias voltage V _T is applied to the sample 26 from a bias power supply 30. Since the cantilever 12 is at the zero potential, the potential difference between the probe 14 and the sample 26 becomes V _T. Tunneling current I _T represented by in formula (1) according to the potential difference V _T to flow between the probe 14 and the sample 26. Since the free end of the cantilever 12 as described above is vibrating at an amplitude of about 0.1nm by the angular frequency omega _c, tunneling current I _T to be detected angular oscillation about its mean value <I _T> becomes a signal that oscillates at a number ω _c. This tunnel current _IT is input to the lock-in amplifier 22. Lock-in amplifier 2
2, the output signal of the oscillator 18 as a reference signal, and outputs to detect the amplitude [Delta] I _T of the tunneling current. The displacement Z _TIP of the cantilever 12 detected by the displacement detector 24 is input to the lock-in amplifier 20. Lock-in amplifier 2
0 detects and outputs the amplitude ΔZ _TIP of the free end of the cantilever 12 using the output signal of the oscillator 18 as a reference signal.
The tunnel current I _T , its amplitude ΔI _T, and the cantilever amplitude ΔZ _TIP are input to the arithmetic unit 38, and the arithmetic unit 38 performs the operation of the equation (3), and outputs a signal φ ^1/2 corresponding to the square root of the barrier height. I do. The via height φ is obtained by squaring this signal. φ ^1/2 or φ
Is recorded in synchronization with the xy scanning signal, a barrier height image of the sample surface can be obtained.

Finally, the measurement of the contact potential difference between the sample surface and the tip of the probe will be described. While the distance S between the sample probes is kept constant by the servo circuit 36, the sample 26
When alters as shown in FIG. 2 (a) a bias voltage V _T which is applied to the tunneling current I _T to be detected FIG
The result is as shown in FIG. Here, the bias voltage at which the tunnel current becomes zero exactly, equal to the contact potential difference V _I0 = φ _{TI P} -φ _S of sample and the probe tip. The data sampling circuit 32 samples the bias voltage at the timing when the tunnel current signal _IT becomes 0, and
A signal shown in (c), that is, a signal corresponding to the contact potential difference at each point is output. By recording this signal in synchronization with the XY scanning signal, the distribution of the contact potential difference on the sample surface can be obtained.

Next, a scanning probe microscope according to a second embodiment of the present invention will be described with reference to FIG. In the drawings, the same members as those in the above-described embodiment are denoted by the same reference numerals, and description thereof will be omitted. This embodiment is different from the first embodiment in that the output signal of the lock-in amplifier 20 is used for the input signal to the servo circuit 36 instead of the output signal of the displacement detector 24. Therefore, the operation of measuring the barrier height on the sample surface and the operation of measuring the contact potential difference between the sample surface and the tip of the probe are exactly the same as those in the first embodiment described above, except for the operation of obtaining the uneven image of the sample surface. It is. For this reason, the description of the operation of measuring the barrier height and the operation of measuring the contact potential difference is omitted, and only the operation of obtaining an uneven image of the sample surface will be described below.

The free end of the cantilever 12 is vibrated at an amplitude of about 0.1nm at a constant angular frequency omega _c by the piezoelectric actuator 16 as in the first embodiment. The displacement detector 24 detects and outputs a displacement Z _TIP of the free end of the cantilever 12. This displacement Z _TIP is input to the lock-in amplifier 20, which detects and outputs the amplitude ΔZ _TIP of the free end of the cantilever 12. Cantilever 1
The amplitude ΔZ _TIP of 2 changes according to the strength of the interatomic force that changes depending on the probe sample distance S. Servo circuit 36
Controls the probe-to-sample distance using the xyz actuator 28 so as to keep the amplitude ΔZ _TIP constant.
As a result, the probe (accurately, the center of its vibration) moves on a curved surface reflecting the unevenness of the sample surface. Therefore, using a data recording device (not shown), the servo signal Z _FB (ie, the position information of the probe in the z direction) from the servo circuit 36 is recorded in synchronization with the xy scanning signal (ie, the position information of the probe in the xy direction). By doing so, an uneven image of the sample surface is obtained.

The lock-in amplifier used in the above embodiment was used to measure the amplitude of a signal having a specific angular frequency contained in the signal. Therefore, another device that performs the same function may be used. For example, a bandpass filter and a detector may be used.

Next, a third embodiment of the present invention will be described with reference to FIGS. In the drawing, the same members as those in the first embodiment are denoted by the same reference numerals, and the description thereof will be omitted.
The basic configuration of the present embodiment is the same as that of the first embodiment, and is different in that the analog arithmetic unit 40 is used for measuring the barrier height without using the lock-in amplifier. Therefore, the operation of measuring the unevenness of the sample surface and the operation of measuring the contact potential difference are exactly the same as those in the first embodiment, and a description thereof will be omitted.

As shown in FIG. 5, the analog operation unit 40 has a differential operation unit 42 to which the displacement Z _{TIP of the} cantilever 12 detected by the displacement detector 24 is input. The analog arithmetic unit 40 has a logarithmic operation unit 44 and the differential calculator 46, the tunneling current I _T detected by the tunnel current detection circuit 34 is input serially to these computing units 44 and 46. Further, the analog arithmetic unit 40 has a divider 48 for dividing the output X of the differential arithmetic unit 42 and the output Y of the differential arithmetic unit 46, that is, calculating and outputting Y / X. Accordingly, the analog arithmetic unit 40 performs the operation shown in the following equation as a whole, and as a result, outputs a signal proportional to the square root of the barrier height φ.

[0033]

(Equation 2) As can be seen from equation (4), even when an analog arithmetic unit is used without using a lock-in amplifier, the unevenness of the sample surface, the barrier height, and the contact potential difference can be measured simultaneously. This embodiment has an advantage that the device configuration is simpler than the above-described embodiment.

[0034]

According to the present invention, it is possible to provide a scanning probe microscope capable of simultaneously measuring the surface unevenness and the local work function of a sample.

[Brief description of the drawings]

FIG. 1 shows the configuration of a first embodiment of the scanning probe microscope of the present invention.

FIG. 2 is a time chart for explaining measurement of a contact potential difference.

FIG. 3 shows a configuration of a second embodiment of the scanning probe microscope of the present invention.

FIG. 4 shows the configuration of a third embodiment of the scanning probe microscope of the present invention.

FIG. 5 shows a configuration of the analog operation circuit 40 shown in FIG.

FIG. 6 is a graph showing characteristics of an atomic force with respect to a probe sample distance S.

Claims (3)

(57) [Claims] 1. A cantilever which has a conductive probe at a free end and is displaced when a force acts between the probe and a sample, and means for vibrating the probe in a direction perpendicular to the surface of the sample. Displacement detection means for detecting displacement of the cantilever; servo means for keeping the center of vibration of the cantilever relative to the sample surface; voltage applying means for applying a bias voltage between the sample and the probe; A scanning probe microscope comprising: current detecting means for detecting a tunnel current flowing between a sample and a sample; and voltage recording means for recording a voltage at which the tunnel current stops flowing when a bias voltage is changed. 2. A cantilever having a conductive probe at a free end portion thereof when a force acts between the probe and a sample, and means for vibrating the probe in a direction perpendicular to the surface of the sample. Displacement detection means for detecting the displacement of the cantilever; servo means for keeping the amplitude of vibration of the free end of the cantilever constant; voltage application means for applying a bias voltage between the sample and the probe; A scanning probe microscope comprising: current detection means for detecting a tunnel current flowing between the two; and voltage recording means for recording a voltage at which the tunnel current stops flowing when a bias voltage is changed. 3. The scanning probe microscope according to claim 1, further comprising a calculating means for calculating a barrier height from a displacement of the cantilever and a tunnel current.