JPH05223520A - Barrier-height measuring apparatus - Google Patents

Abstract

PURPOSE:To obtain a barrier-height measuring apparatus wherein an uneven image on a specimen and a local work function can be measured simultaneously. CONSTITUTION:A piezoelectric actuator 16 which supports a cantilever 12 which is equipped with a probe 14 arranged near a specimen 26 at its free end is moved to a direction perpendicular to the surface of the specimen when an AC voltage is fed from an oscillator 18. A displacement detector 24 optically detects the displacement of the cantilever 12; its displacement signal is fed to a lock-in amplifier 20 and a servo circuit 36. On the basis of the displacement signal, the servo circuit 36 detects a position with reference to the surface of the specimen in the center of the vibration of the cantilever 12, and the distance between the probe and the specimen is controlled by using an x-y-z actuator 28 so as to keep it definite. A bias power supply 30 feeds a voltage to the specimen 26; a tunneling current which flows by it is detected by a tunneling-current detection circuit 34. An operation device 38 computes the square root of a barrier height phi and outputs it.

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 which obtains microscopic surface information of a sample by scanning a probe brought close to the sample.

[0002]

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

The STM was developed by Binni in 1982.
g) and Rohrer et al., US Pat. No. 4,343,9
The fine surface profiler proposed in No. 93 can measure the surface profile of a conductive sample with atomic level resolution.

When a sharp and sharp conductive probe is brought close to the surface of a conductive sample by 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 I _T is expressed by the following equation.

I _T = B (V _T ) exp (−Aφ ^1/2 S) (1) Here, 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 a clean metal surface is about 1 to 5 eV,
It can be seen from the formula (1) that the tunnel current I _T changes by about one digit when the distance between the probe samples changes by 0.1 nm. S
The TM probe is scanned by a fine movement element such as a piezoelectric body along the surface of the sample, that is, the xy plane (for example, raster). 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 the fine movement element, and the probe sample-to-sample distance S 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 away from the sample surface by a certain distance and moves on a curved surface that reflects the surface shape of the sample. By recording 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 the fine unevenness on the sample surface is recorded.
A TM image is obtained.

By the way, the tunnel current detected by the STM depends on the barrier height reflecting the local electron state of the sample in addition to the sample probe distance S, as can be seen from the equation (1). . The barrier height φ is a value obtained by trapezoidally approximating 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 local work functions of the tip of the probe and the surface of the sample, respectively, and are required to extract one electron from the surface. The energy is expressed by voltage. The local work function φ _S of the sample surface differs depending on the surface charge density and constituent atoms. Therefore, if the same probe is used, the barrier height will reflect the local distribution of the work function φ _{S on} the sample surface.

A method for obtaining the barrier height φ from the value of the tunnel current I _T detected by the STM is, for example, Physical Review Letters, Volume 60, No. 12, 1988, pp. 1166 to 1st.
Some are described on page 169. in this way,
At the time of measuring the surface unevenness, a constant small vibration in the direction perpendicular to the sample surface is applied to the probe to detect the 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. The tunnel current detected by this method becomes a signal having an oscillating component, but the frequency of this minute oscillation is set higher than the cutoff frequency of the feedback control system, so feedback control is performed in the same way as in normal STM. The irregularity information of the sample can be obtained from the output of the system.

Further, the AFM is disclosed in JP-A-62-130302.
It is disclosed in Japanese Patent Publication No. When you bring the probe closer to the sample,
A small force (atomic force) acts between the tip of the probe and the atoms on the sample surface. Atomic force includes repulsive force, van der Waals force, covalent bond force, etc. based on the rule of exclusion, and is represented by Lennard-Jones potential with attractive region and repulsive region as shown in FIG.
The AFM detects a signal corresponding to the atomic force or its derivative based on the deflection amount or vibration amplitude of the cantilever, and scans the probe while controlling the sample-to-sample distance so as to keep this signal constant. , S for the position information of the obtained probe
By processing in the same manner as TM, an image showing the surface shape of the sample with atomic resolution is obtained. AFM is ST
It is possible to measure not only a conductive sample like the M device but also an insulating sample. As described above, the scanning probe microscopes that measure the sample by detecting the force acting between the sample and the probe using the cantilever are collectively called SFM (Scanning Force Microscop).
Called e), the AFM is a form of SFM.

In Japanese Patent Application No. 2-313389, the present inventors applied a bias voltage between the sample and the tip of the conductive cantilever to oscillate the cantilever, and the average displacement of the tip of the cantilever and the flowing tunnel current. We proposed a device to detect the barrier height between the sample and the tip of the probe by measuring both the value and the variation at the same time.

Further, when metals having different work functions are brought into contact with each other, a potential difference φ _A -φ _B given by the difference between the work functions φ _A and φ _B is generated, and an electric current flows between them so as to attain 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 this potential difference V _I0 is 2
The work function difference between the two metals. The method of detecting the charge accumulated in the capacitance formed between the metals and obtaining the difference in work functions is called the Kelvin method.

[0013]

[Problems to be Solved by the Invention] STM or AFM
In the barrier height measurement using, the barrier 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 unknown, the local work function of the sample is did not understand. In addition, there is a similar problem in the work function difference measurement using the Kelvin method, and it is impossible to measure the local work function distribution.

An object of the present invention is to provide a barrier height measuring apparatus capable of simultaneously measuring the uneven image of a sample and the local work function.

[0015]

The barrier height measuring apparatus of the present invention has a conductive probe disposed near the surface of a sample and a probe at its free end, and a force is applied between the probe and the sample. When the action of the cantilever is applied, the vibration means that vibrates the probe in the direction perpendicular to the sample surface, the displacement detection means that detects the displacement of the cantilever, and the position of the center of vibration of the cantilever with respect to the sample surface is kept constant. Servo means for maintaining, voltage applying means for applying a bias voltage between the sample and the probe,
Current detection means to detect the tunnel current flowing between the probe and the sample, operation means to calculate the barrier height from the displacement of the cantilever and the tunnel current, and the voltage at which the tunnel current stops flowing when the bias voltage is changed. And a voltage recording means for operating.

Another via-height measuring device of the present invention has a conductive probe disposed near the surface of the sample and a probe at the free end, and when a force is applied between the probe and the sample. A cantilever that displaces, a vibrating means that vibrates the probe in a direction perpendicular to the sample surface, a displacement detecting means that detects the displacement of the cantilever, a servo means that keeps the vibration amplitude of the free end of the cantilever constant, and a sample. A voltage applying means for applying a bias voltage between the probe and the probe, a current detecting means for detecting a tunnel current flowing between the probe and the sample, a computing means for computing the barrier height from the displacement of the cantilever and the tunnel current, Voltage recording means for recording the voltage at which the tunnel current stops flowing when the bias voltage is changed.

[0017]

The probe is scanned along the surface of the sample while being vibrated in the z direction at a specific frequency by the vibrating means. Therefore, the free end of the cantilever supporting it also vibrates, and its displacement is always detected by the displacement detecting means. At this time, the center and amplitude of vibration at the free end of the cantilever change depending on the strength of the interatomic force between the tip of the probe and the constituent atoms on the sample surface. A bias voltage is applied between the probe and the sample, and the 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 detection means, and the position of this vibration center is always relative to the sample surface. The relative position between the probe and the sample is controlled so as to be constant. Alternatively, the amplitude of the vibration at 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 this vibration constant. In any case, the servo signal for controlling the relative position between the probe and the sample becomes the information indicating the unevenness of the sample surface.

The tunnel current detected by the current detecting means and the displacement of the cantilever detected by the displacement detecting means are input to the calculating means, and the barrier height is calculated.

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 to record the work function φ _S of the sample and that of the probe. Difference from work function φ _TIP (φ _TIP
−φ _S ) is detected.

[0021]

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

As shown in FIG. 1, the via height measuring apparatus of the first embodiment comprises 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 so that the probe 14 is located near the sample 26 placed on the xyz actuator 28. The piezoelectric actuator 16 is supplied with an AC voltage having an angular frequency ω _c from an oscillator 18, and vibrates the cantilever 12 or 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 vibrating cantilever 12 is optically detected by the displacement detector 24, and the displacement signal is the lock-in amplifier 2
0 and supplied to the servo circuit 36. The servo circuit 36 is
The position of the center of vibration of the free end of the cantilever with respect to the sample surface is detected based on the input displacement signal, 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 the bias power supply 30.
As a result, the 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. In addition to the tunnel current, the bias voltage from the bias power supply 30 is input to the data sampling circuit 32. 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 the arithmetic unit 38 that calculates the square root of the barrier height φ.

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

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

[0025]

[Equation 1] Here, <I _T> is the average value of the tunnel current, the [Delta] I _T amplitude of the tunnel current, [Delta] Z _TIP is the amplitude of oscillation 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 0 potential, the potential difference between the probe 14 and the sample 26 is V _T. A tunnel current I _T represented by the equation (1) flows between the probe 14 and the sample 26 according to the potential difference V _T. As described above, since the free end of the cantilever 12 vibrates at an angular frequency ω _c with an amplitude of about 0.1 nm, the detected tunneling current I _T also has an angular vibration around its average value < _IT >. The signal oscillates at a number ω _c . This tunnel current I _T is input to the lock-in amplifier 22. Lock-in amplifier 2
2 detects and outputs the amplitude ΔI _{T of the} tunnel current by using the output signal of the oscillator 18 as a reference signal. Further, 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 arithmetic operation of the equation (3) and outputs a signal φ ^1/2 corresponding to the square root of the barrier height. To do. The via height φ is obtained by squaring this signal. φ ^1/2 or φ
By recording the signal of 1 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 tips is kept constant by the servo circuit 36, the sample 26
When the bias voltage V _T applied to the gate is changed as shown in FIG. 2A, the detected tunnel current I _T is as shown in FIG.
As shown in (b). 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 the data sampling circuit 32 shown in FIG.
The signal shown in (c), that is, the 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 barrier height measuring apparatus according to the second embodiment of the present invention will be described with reference to FIG. In the figure, the same members as those in the above-described embodiment are designated by the same reference numerals, and the 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 as the input signal to the servo circuit 36 instead of the output signal of the displacement detector 24. Therefore, only the operation of obtaining the uneven image of the sample surface is different, and the operation of measuring the barrier height of 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. Is. Therefore, the description of the barrier height measurement operation and the contact potential difference measurement operation 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 oscillates at a constant angular frequency ω _c with an amplitude of about 0.1 nm by the piezoelectric actuator 16 as in the first embodiment. The displacement detector 24 detects and outputs the displacement Z _TIP of the free end of the cantilever 12. This displacement Z _TIP is input to the lock-in amplifier 20, and the lock-in amplifier 20 detects and outputs the amplitude ΔZ _TIP of the free end of the cantilever 12. Cantilever 1
The amplitude ΔZ _TIP of 2 changes depending on the strength of the interatomic force that changes depending on the probe sample distance S. Servo circuit 36
Controls the inter-probe sample distance using the xyz actuator 28 so that this amplitude ΔZ _TIP is kept constant.
As a result, the probe (to be exact, the center of its vibration) moves on a curved surface that reflects the unevenness of the sample surface. Therefore, using a data recording device (not shown), the servo signal Z _FB from the servo circuit 36 (that is, position information of the probe in the z direction) is recorded in synchronization with the xy scanning signal (that is, position information of the probe in the xy direction). By doing so, an uneven image of the sample surface can be obtained.

The lock-in amplifier used in the above-mentioned embodiment was used to measure the amplitude of a signal having a specific angular frequency contained in the signal. Therefore, other materials may be used as long as they have the same function, 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 figure, the same members as those in the first embodiment are designated by the same reference numerals, and the description thereof is omitted.
The basic configuration of this embodiment is the same as that of the first embodiment, except that the analog calculator 40 is used for measuring the barrier height without using the lock-in amplifier. Therefore, the measurement operation of the unevenness of the sample surface and the measurement operation of the contact potential difference are exactly the same as those in the first embodiment, and the description thereof will be omitted.

The analog calculator 40 has a differential calculator 42 as shown in FIG. 5, and the displacement Z _{TIP of the} cantilever 12 detected by the displacement detector 24 is input to the differential calculator 42. The analog computing unit 40 has a logarithmic computing unit 44 and a differential computing unit 46, and the tunnel current I _T detected by the tunnel current detecting circuit 34 is input to these computing units 44 and 46 in series. Further, the analog computing unit 40 has a divider 48 that divides the output X of the differential computing unit 42 and the output Y of the differential computing unit 46, that is, calculates and outputs Y / X. Therefore, the analog calculator 40 performs the calculation shown in the following formula 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 the equation (4), even when the analog calculator is used without using the lock-in amplifier, the unevenness of the sample surface, the barrier height, and the contact potential difference can be measured at the same time. The present embodiment has an advantage that the device configuration is simpler than that of the above-described embodiments.

[0034]

According to the present invention, there is provided a barrier height measuring apparatus capable of simultaneously measuring the unevenness of the sample surface, the barrier height and the contact potential difference.

[Brief description of drawings]

FIG. 1 shows the configuration of a first embodiment of a barrier height measuring device of the present invention.

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

FIG. 3 shows the configuration of a second embodiment of the barrier height measuring device of the present invention.

FIG. 4 shows the configuration of a third embodiment of the barrier height measuring apparatus of the present invention.

5 shows a configuration of an analog arithmetic circuit 40 shown in FIG.

FIG. 6 is a graph showing the characteristics of the atomic force with respect to the distance S between the probe samples.

[Explanation of symbols]

12 ... Cantilever, 14 ... Probe, 16 ... Piezoelectric actuator, 18 ... Oscillator, 24 ... Displacement detector, 32 ... Data sampling circuit, 34 ... Tunnel current detection circuit,
36 ... Servo circuit, 38 ... Arithmetic device.

Claims (2)

[Claims] 1. A conductive probe disposed near the surface of the sample, a cantilever having the probe at its free end and displaced when a force acts between the probe and the sample, and a sample surface Means for vibrating the probe in the direction perpendicular to the direction, displacement detection means for detecting the displacement of the cantilever, servo means for keeping the center position of the vibration of the cantilever with respect to the sample surface constant, and bias between the sample and the probe. A voltage applying means for applying a voltage, a current detecting means for detecting a tunnel current flowing between the probe and the sample, a calculating means for calculating a barrier height from the displacement of the cantilever and a tunnel current, and a bias voltage when changing the bias voltage. A barrier height measuring device comprising: a voltage recording means for recording a voltage at which a tunnel current stops flowing. 2. A conductive probe disposed near the surface of the sample, a cantilever having the probe at its free end and displaced when a force acts between the probe and the sample, and a sample surface. Means for oscillating the probe in a direction perpendicular to the direction, displacement detection means for detecting the displacement of the cantilever, servo means for keeping the amplitude of vibration at the free end of the cantilever constant, and bias voltage between the sample and the probe. The voltage applying means, the current detecting means for detecting the tunnel current flowing between the probe and the sample, the calculating means for calculating the barrier height from the displacement of the cantilever and the tunnel current, and the tunnel when the bias voltage is changed. A barrier height measuring device, comprising: a voltage recording means for recording a voltage at which current stops flowing.