JPH1130623A - Scanning capacitance type microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To optimize measuring sensitivity by reading the characteristic change of a resonance circuit, adjusting the oscillation frequency of an oscillating circuit or the resonance frequency of the resonance circuit, and modulating the oscillation frequency. SOLUTION: In the state of a probe of a cantilever 1 and a sample 5 being in proximity to each other, alternating voltage of about 100 kHz, 100 mVp-p, for instance, is applied to a variable capacity diode in an oscillating circuit. DC voltage applied to the variable capacity diode in the oscillating circuit is made to sweep, and components of about 100 kHz of signals of a capacitance sensor 4 are monitored in a first channel of a lock-in amplifier 11. Oscillation frequency of a detection signal with large value of the lock-in amplifier 11 is selected, alternating voltage is applied to the sample 5, and the probe and the sample 5 are brought into contact to measure the sample 5. It can also be so constituted that DC voltage applied to the variable capacity diode in the oscillating circuit is made constant, while DC voltage applied to a variable capacity diode in a resonance circuit is changed to obtain an optimum measuring condition.

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 capable of detecting capacitance.

[0002]

2. Description of the Related Art Various types of scanning probe microscopes have been proposed as a technique for examining information on a sample surface at a high atomic-level spatial resolution by scanning the sample surface with a sharp tip. Typical examples of the scanning probe microscope include a scanning tunneling microscope (STM) using a tunnel current between a probe and a sample and an atomic force microscope (STM) using a small force acting between the probe and the sample. Hereinafter, “AF
M "), and a scanning capacitance microscope for measuring the capacitance between the probe and the sample. Scanning capacitance microscopes are mainly based on the AFM, and simultaneously acquire capacitance information while scanning the probe surface with the AFM using the AFM.
Probe with capacitance sensor consisting of detection circuit-
A change in capacitance between samples is detected as a change in resonance frequency of the LC resonance circuit.

The oscillation frequency of a capacitance sensor is approximately 930 to 1050 by turning a trimmer of a variable capacitor in an oscillation circuit using a driver or the like.
It changes in the range of MHz. Further, a varicap diode is provided in the LC resonance circuit of the capacitance sensor, and has a mechanism for adjusting the resonance frequency by feeding back a voltage from the detection circuit.

[0004]

The scanning capacitance microscope has the highest sensitivity when measured at a steep slope of the LC resonance circuit. However, the resonance frequency of the LC resonance circuit depends on the cantilever used, the sample to be measured and The oscillation frequency needs to be set each time because the frequency changes sequentially due to the positional relationship. For that purpose, it is necessary to monitor some signal to know the resonance circuit characteristics. Conventionally, a DC voltage peak-detected by a detection circuit is monitored, but there is a problem that S / N is poor due to DC.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and provides a scanning capacitance microscope having a capacitance sensor including an oscillation circuit, a resonance circuit, and a detection circuit. A scanning capacitance microscope comprising: means for reading; means for adjusting the oscillation frequency of the oscillation circuit or the resonance frequency of the resonance circuit so as to optimize the measurement sensitivity; and means for modulating the oscillation frequency. I will provide a.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a resonance characteristic measuring mechanism will be described with reference to FIG. FIG. 2A shows an oscillation circuit,
FIG. 3 is a schematic diagram of a capacitance measuring mechanism including a resonance circuit and a detection circuit. The high-frequency signal generated by the oscillation circuit is transmitted to the detection circuit via the resonance circuit, and the high-frequency amplitude is peak-detected. FIG. 2B shows the resonance characteristics of the resonance circuit.
The horizontal axis represents the frequency of the high-frequency signal, and the vertical axis represents the amplitude of the high-frequency signal subjected to peak detection. The amplitude of the high-frequency signal transmitted to the detection circuit is represented by the frequency of the high-frequency signal equal to the resonance frequency (ω) of the resonance circuit.
r), and becomes the maximum when the frequency of the high frequency signal deviates from the resonance frequency. For example,
Assuming that the oscillation frequency of the high-frequency signal is ωo, the curve of the resonance characteristic has a downward-sloping slope at ωo shown in FIG. 2B, and the resonance frequency and the resonance characteristic are increased by the increase of the capacitance in the resonance circuit. Curve left (low frequency)
And the detected amplitude becomes smaller. On the other hand, when the oscillation frequency (ωo) is on the left side of the resonance frequency (ωr), the detected amplitude increases conversely due to an increase in capacitance. Further, the sensitivity of detecting a change in capacitance increases as the slope of the resonance characteristic curve becomes steeper. FIG. 2C shows a state of the capacitance change at the frequency of the AC voltage detected by the rocks-in amplifier when the AC voltage is applied to the variable capacitance diode in the resonance circuit having the resonance characteristic of FIG. 2B ( dC / dV), which is the differential value of FIG. 2B. Figure 2 shows the change in capacitance detected by the lock-in amplifier.
In (b), the greater the slope of the resonance characteristic curve, the greater the absolute value. At this point, the sensitivity as a scanning capacitance microscope is maximized.

A scanning capacitance microscope according to an embodiment of the present invention will be described below with reference to FIG. 1, but the present invention is not limited to this embodiment. The scanning capacitance microscope according to the embodiment of the present invention includes a cantilever 1, a cantilever holder 2, a conducting wire 3, a capacitance sensor 4, a sample 5, a sample holder 6, a tube-type piezo element 7, and semiconductor lasers 8 and 2. It comprises a split photodetector 9, an interface box 10, a lock-in amplifier 11, a piezo drive power supply 12, a computer 13 and a CRT 14.

The cantilever 1 is formed by coating a silicon cantilever having a probe near the tip with cobalt and chromium, and is electrically connected to a capacitance sensor 4 by a conducting wire 3. The cantilever holder 2 is made of polycarbonate which is an insulator for reducing the stray capacitance, but may be made of another insulator such as ceramic. The sample 5 is a silicon substrate, the front surface of which is covered with an oxide film, and the back surface of which is formed with an aluminum electrode. The sample 5 is attached to a sample holder 6 made of aluminum by a conductive paste, and the sample holder 6 is attached to a tube type piezoelectric element 7 for scanning the sample 5 with respect to the cantilever 1. The bending of the cantilever 1 is detected by an optical lever method using a semiconductor laser 8 and a two-part photodetector 9. Sample 5 is
The cantilever 1 is scanned by applying a voltage to the tube-type piezo element 7 by the piezo drive power supply 12.
A feedback voltage is applied to the tube-type piezo element 7 so that the bending of the piezo element becomes constant. Sample 5 obtained at this time
The information on the surface shape is taken into the computer 13 together with the information on the capacitance.

To measure the capacitance, a DC bias and a modulation voltage of 100 kHz are applied to the sample 5 from the lock-in amplifier 11 having a built-in oscillator through the interface box 10 and the sample holder 6.
The sample 5 is applied by the voltage between the cantilever 1 and the ground potential.
A capacitance change at a modulation frequency between the probe and the sample due to a change in the thickness of a depletion layer formed therein is detected by the capacitance sensor 4 and the lock-in amplifier 11. The lock-in amplifier 11 has an AC voltage output of 100 kHz and four -10.5 V to +10.5
Those having a DC output of V were used. Further, since the lock-in amplifier 11 is a two-phase lock-in amplifier, it has two signal outputs, and
X) or amplitude (R) is assigned to channel 2 (CH)
In 2), Y or phase can be selected. Note that the present inventors have made the lock-in amplifier 11
Model SR830 DSP Lock-in am from ford Reseach Systems
plifier is used.

Next, referring to FIG. 3, a description will be given of a signal flow between the capacitance sensor 4, the sample 5, the interface box 10, and the lock-in amplifier 11. The capacitance sensor 4 has a variable capacitance diode in the oscillation circuit, and has a function of changing the oscillation frequency by an external voltage. This technique is described in detail in Japanese Patent Application No. 9-101665, which is an undisclosed technique filed by the present applicant. The variable capacitance diode in the resonance circuit is mounted such that a depletion layer is generated when a positive voltage is applied from an external circuit. Therefore, a voltage obtained by adding +10 V to the voltage from the DC output 1 of the lock-in amplifier 11 in the interface box is applied to the variable capacitance diode. Since the variable capacitance diode in the oscillation circuit is mounted so that a depletion layer is generated when a positive voltage is applied from an external circuit, +10 V is applied to the voltage from the DC output 2 of the lock-in amplifier 11 in the interface box. The applied voltage is applied to the variable capacitance diode. Further, by switching a switch in the interface box 11, it is possible to apply an AC voltage together with a DC voltage to the variable capacitance diode in the oscillation circuit. Here, the switch has a function of selecting and applying the AC voltage output from the lock-in amplifier 11 to either the resonance circuit or the sample. Note that the scanning capacitance microscope according to the embodiment of the present invention has a DC voltage output of 3
And an AC voltage output can be applied. A signal output of the capacitance sensor 4 is connected to a signal input terminal of the lock-in amplifier 11 via the interface box 10.

By the way, the capacitance sensor 4 is
It has the circuit configuration described with reference to FIG. The resonance circuit in the capacitance sensor 4 is connected to the conducting wire 3 and the cantilever 1, and the resonance frequency of the resonance circuit changes due to the stray capacitance between the cantilever 1 and the surroundings.
Therefore, it is necessary to measure the resonance frequency and the resonance characteristics and determine the oscillation frequency in accordance with the resonance characteristics.
Hereinafter, the tuning process of the oscillation frequency and the resonance circuit characteristics for obtaining the optimum measurement conditions will be described.

First, in a state where the probe of the cantilever 1 and the sample 5 are brought close to each other, the switch shown in FIG.
Hz, an AC voltage of 100 mVp-p is applied. In this state, the DC voltage applied to the variable capacitance diode in the oscillation circuit is swept,
At 1, the signal of the capacitance sensor 4
Monitor the kHz component. A large oscillation frequency of a signal detected by the lock-in amplifier 11 is selected, an AC voltage is applied to the sample by a switch shown in FIG. 3, and the sample 5 is measured by bringing the probe into contact with the sample 5. .

[0013] Further, by keeping the DC voltage applied to the variable capacitance diode in the oscillation circuit constant and changing the DC voltage applied to the variable capacitance diode in the resonance circuit, an optimum measurement condition is obtained. It is also possible.

[0014]

According to the present invention, the function of efficiently measuring the resonance characteristics of the resonance circuit of the capacitance sensor and setting the oscillation frequency or the resonance frequency of the resonance circuit in accordance with the resonance characteristics is provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a scanning capacitance microscope of the present invention.

FIG. 2 is a schematic diagram for explaining the principle of a capacitance sensor and a resonance characteristic measuring function.

FIG. 3 is a schematic diagram for explaining a circuit configuration of the present embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Cantilever 2 ... Cantilever holder 3 ... Lead wire 4 ... Capacitance sensor 5 ... Sample 6 ... Sample holder 7 ... Tube type piezo element 8 ... Semiconductor laser 9 ... -2 split photodetector 10-Interface box 11-Lock-in amplifier 12-Piezo drive power supply 13-Computer 14-CRT (monitor)

Claims (5)

[Claims] In a scanning capacitance microscope having a capacitance sensor including an oscillation circuit, a resonance circuit, and a detection circuit, a means for reading a change in the resonance circuit characteristic and a circuit for controlling the oscillation circuit so that measurement sensitivity is optimized. A scanning capacitance microscope comprising: means for adjusting an oscillation frequency or a resonance frequency of a resonance circuit; and means for modulating the oscillation frequency. 2. The method according to claim 1, wherein the oscillation frequency is modulated.
A scanning capacitance microscope, wherein the scanning is performed by applying a direct current and an alternating voltage to a variable capacitance diode in an oscillation circuit. 3. The scanning capacitance microscope according to claim 1, further comprising a function of detecting a change in an output signal of the capacitance sensor at a modulation frequency by a lock-in amplifier. 4. The method according to claim 3, wherein the output of the lock-in amplifier is fetched into a storage device while sweeping the oscillation frequency, and a signal change peak detected by a detection circuit with respect to the oscillation frequency is displayed on a display device. A scanning capacitance microscope having a function of selecting a suitable oscillation frequency. 5. The scanning capacitance microscope according to claim 3, further comprising switching means for applying an AC voltage to the oscillation circuit or the sample.