Scanning probe microscope and measurement method using the same

A scanning probe microscope that includes a probe, a positioning unit configured to position a probe on a measurement sample, an excitation unit configured to excite the measurement sample at a predetermined frequency, a resonance unit configured to output a frequency modulation signal by converting a change of a capacitance of the measurement sample, a lock-in amplifier configured to output a differential capacitance signal obtained by extracting a predetermined frequency component and a harmonic component of the predetermined frequency of the demodulated signal, a conversion unit configured to output data indicative of a relationship between a voltage applied to the measurement sample and the capacitance, a detecting unit that detects a voltage value corresponding to a feature point of the relationship data, and a main measurement control unit that measures electrical characteristics of the measurement sample subjected to a DC bias voltage substantially equal to the feature point voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to Japanese Patent Application No. 2016-051367, filed Mar. 15, 2016; the entire contents of which are incorporated herein by reference.

FIELD

An embodiment described herein relates generally to a scanning probe microscope and a measurement method using the same.

BACKGROUND

A scanning nonlinear dielectric microscope (“SNDM”) can be used for observing charge distribution in a semiconductor region, polarization distribution of a dielectric, and the like at a nanoscale level. A measurement sample is two-dimensionally scanned with a probe of the SNDM. However, there can be stray capacitance and a floating inductor, which can lead to disturbance, between the probe and the measurement sample, and between a stage on which the measurement sample is placed and the measurement sample. Since the influence of the disturbance is superimposed on the differential capacitance detected by the probe, measurement sensitivity is lowered, or a measurement value becomes variable.

In addition, when a polishing process, an oxide film forming process, and the like are performed with respect to the measurement sample, there is a case where a surface of the measurement sample is charged. In this case, a charged state of the surface of the measurement sample is changed, and a measured C-V curve is shifted in a voltage direction. Accordingly, a differential capacitance signal detected in the SNDM is changed.

As described above, the SNDM has a feature which can detect a small capacitance change. Therefore, the SNDM is likely to be affected by the ambient environment. Measurement results obtained using the SNDM may vary from day to day even for the same measurement sample and may greatly depend on the ambient environment.

SUMMARY

According to some embodiments, there is provided a scanning probe microscope and a measurement method using the scanning probe microscope capable of accurately measuring electrical characteristics of a measurement sample with reduced impact by disturbance.

In some embodiments according to one aspect, a scanning probe microscope includes a probe, a positioning unit configured to position the probe on a measurement sample or a standard sample, an excitation unit configured to excite the measurement sample, the standard sample, or the probe at an excitation frequency, a resonance unit configured to output a frequency modulation signal obtained by converting a change of a capacitance of the measurement sample or the standard sample into a change of a resonant frequency, and a frequency demodulator configured to output a demodulated signal obtained by demodulating the frequency modulation signal. The scanning probe microscope further includes a lock-in amplifier configured to output a differential capacitance signal obtained by extracting a frequency component and a harmonic component of the excitation frequency included in the demodulated signal, a conversion unit configured to output relationship data indicative of a relationship between a voltage applied to the measurement sample or the standard sample and the capacitance, based on the differential capacitance signal, a feature point voltage detecting unit configured to detect a voltage value corresponding to a feature point of the relationship data, and a main measurement control unit configured to measure electrical characteristics of the measurement sample, by operating the excitation unit, the resonance unit, the frequency demodulator, and the lock-in amplifier, in a state where a DC bias voltage corresponding to the voltage value detected by the feature point voltage detecting unit is applied to the measurement sample.

In some embodiments according to another aspect, a measurement method using a scanning probe microscope includes exciting a measurement sample, a standard sample, or a probe, in a state where the probe is positioned on the measurement sample or the standard sample, generating a frequency modulation signal obtained by converting a change of a capacitance of the measurement sample or the standard sample into a change of a resonant frequency, generating a demodulated signal obtained by demodulating the frequency modulation signal, generating a differential capacitance signal obtained by extracting a frequency component and a harmonic component included in the demodulated signal, and generating relationship data indicative of a relationship between a voltage applied to the measurement sample or the standard sample and a capacitance, based on the differential capacitance signal. The measurement method using a scanning probe microscope further includes detecting a voltage value corresponding to a feature point of the relationship data, and measuring electrical characteristics of the measurement sample, based on the differential capacitance signal, by sequentially performing excitation of the measurement sample or the probe, generation of the frequency modulation signal, and generation of the differential capacitance signal, in a state where a DC bias voltage corresponding to the detected voltage value is applied to the measurement sample.

Other aspects and embodiments of the disclosure are also encompassed. The foregoing summary and the following detailed description are not meant to restrict the disclosure to any particular embodiment but are merely meant to describe some embodiments of the disclosure.

DETAILED DESCRIPTION

According to some embodiments, there is provided a scanning probe microscope and a measurement method using the scanning probe microscope capable of accurately measuring electrical characteristics of a measurement sample with reduced impact by disturbance.

Hereinafter, with reference to the drawings, an example embodiment will be described.FIG. 1is a block diagram illustrating a schematic configuration of a scanning probe microscope1according to the embodiment. The scanning probe microscope1inFIG. 1is a scanning nonlinear dielectric microscope (SNDM).FIG. 1illustrates a characteristic configuration portion of the SNDM1, and other configuration portions thereof are omitted.

The SNDM1inFIG. 1includes a stage2, a stage driving unit3, a probe4, a positioning unit5, an excitation unit6, a resonance unit7, a frequency demodulator8, a lock-in amplifier9, and a control unit10.

A measurement sample11can be placed on the stage2. The stage driving unit3can move the measurement sample11on the stage2in the vertical direction (Z direction), and can adjust a distance between the measurement sample11and the probe4. In addition, the stage driving unit3can move the measurement sample11on the stage2in the horizontal direction (XY direction). Instead of providing the stage driving unit3, a driving unit that moves the probe4in an XYZ direction may be provided, and a position of the measurement sample11can be fixed. The probe4can be positioned on the measurement sample11or on a standard sample, or on a portion of the measurement sample11or on a portion of the standard sample, such that the probe4can engage with the measurement sample11or the standard sample.

A light emitting unit12that can irradiate the probe4with a laser light and a light receiving unit13that can receive a reflected light from the probe4are connected to the positioning unit5. The positioning unit5can perform positioning of the probe4, based on the laser light received in the light receiving unit13.

The excitation unit6can excite the measurement sample11in the Z direction. More specifically, the excitation unit6can apply an alternating current (AC) voltage of a predetermined frequency to the measurement sample11. With this, a capacitance of the measurement sample11is changed by a small capacitance.

The resonance unit7includes an LC resonance circuit7a(including an inductor (L) and a capacitor (C)) and an amplifier7b, and can convert the small change of the capacitance of the measurement sample11due to the excitation of the measurement sample11into a change of the resonant frequency. With this, the resonance unit7can output a frequency modulation signal of which a frequency is changed according to the small change of the capacitance. Since the resonance unit7is provided, it is possible to detect the small change of the capacitance of the measurement sample11as a change of the frequency.

The frequency demodulator8can output a demodulated signal obtained by demodulating the frequency modulation signal. The demodulated signal can be a voltage signal. The lock-in amplifier9can output a differential capacitance signal obtained by extracting a predetermined frequency component included in the demodulated signal and the harmonic components of the frequency component. The differential capacitance signal can be represented by the following equation (1).

Since Equation (1) is a function of the frequency, it is possible to perform Fourier series expansion as represented in Equation (2).

As represented in Equation (2), the differential capacitance signal has a high-order differential capacitance component with respect to a change due to the voltage of the capacitance. Equation (2) represents the high order differential capacitance component up to a fourth order. However, the differential capacitance signal output from the lock-in amplifier9also includes a differential capacitance component higher than the fourth order.

The control unit10can include a capacitance-voltage (CV) curve generating unit14, a feature point voltage detecting unit15, and a main measurement control unit16. The CV curve generating unit (conversion unit)14can generate data, such as a CV curve, representing a relationship between an applied voltage to the measurement sample11and a capacitance, based on the differential capacitance signal. Generally speaking, the conversion unit14can output data representing a relationship between the applied voltage to the measurement sample11and the capacitance, based on the differential capacitance signal. The feature point voltage detecting unit15can detect a voltage value corresponding to a feature point on the CV curve. The main measurement control unit16can operate the excitation unit6, the resonance unit7, the frequency demodulator8, and the lock-in amplifier9, and can measure electrical characteristics of the measurement sample11, in a state where a voltage substantially equal to the voltage detected in the feature point voltage detecting unit15is applied to the measurement sample11as a direct current (DC) bias voltage. In the description of some embodiments, when referring to two values or characteristics as being substantially the same or equal, the terms can refer to a first value or characteristic being precisely the same or equal to a second value or characteristic, as well as cases where the first value or characteristic is within a range of variation of less than or equal to ±5% of the second value or characteristic, such as less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, or less than or equal to ±1%. The measured electrical characteristics include, for example, the charge distribution, the carrier concentration distribution, and the like within the measurement sample11. The control unit10can be implemented in hardware using associated circuits, such as a CV curve generating circuit, a feature point voltage detecting circuit, and a main measurement control circuit. Functionality of the control unit10also can be implemented using a processor and a memory (or other non-transitory computer-readable storage medium) storing instructions executable by the processor.

Here, an amplitude of the AC voltage applied to the measurement sample11by the excitation unit6should be large enough to allow for obtaining a significant portion of the CV curve, as explained below.FIG. 2Ais a waveform diagram of the differential capacitance ΔC when the amplitude of the AC voltage is small, andFIG. 2Bis a waveform diagram of the differential capacitance ΔC when the amplitude of the AC voltage is greater than that inFIG. 2A.

When the amplitude of the AC voltage is small, as illustrated inFIG. 2A, although the waveform distortion of the differential capacitance ΔC is small, a differential capacitance ΔC corresponding to an approximately linear region in the center portion of the CV curve is obtained. Meanwhile, when the amplitude of the AC voltage is great, as illustrated inFIG. 2B, although the waveform of the differential capacitance ΔC in the peak value is distorted, the differential capacitance ΔC corresponding to the entirety of regions including both ends20of the CV curve is obtained.

The CV curve generating unit14can obtain the differential capacitance component up to a high order differential capacitance component higher than that of the differential capacitance signal represented in the above-described Equation (2), and can generate the CV curve. With this, the characteristics of both end regions in the CV curve inFIG. 2Bcan be accurately reproduced. However, since the value of terms in Equation (2) becomes small as the order of the differential capacitance component is high, the CV curve can be generated by obtaining up to approximately the sixth order differential capacitance component for practical purposes. However, generating a CV curve by obtaining up to first, second, third, fourth, fifth, seventh, eighth, ninth, tenth, or other order terms may be appropriate in some circumstances.

When a surface of the measurement sample11is charged due to the influence of the disturbance in the periphery of the measurement sample11, as described above, there is a possibility that the CV curve is shifted in the voltage direction.FIG. 3illustrates an example in which the CV curve of the measurement sample11is shifted in the voltage direction due to the charge of the surface. In this case, a slope SL1of the CV curve is changed into a slope SL2at a voltage zero. That is, this means that the differential capacitance signal output from the lock-in amplifier9is changed such that the electrical characteristics of the measurement sample11may not be precisely measured. Accordingly, in the embodiment, when the measurement of the electrical characteristics of the measurement sample11is performed in the SNDM1, a voltage shift amount of the CV curve can be detected, and, in a state where a DC bias voltage set based on the detected voltage shift amount is applied to the measurement sample11, then the measurement of the SNDM1is performed.

FIG. 4is a flowchart illustrating an example measurement procedure by the SNDM1inFIG. 1. First, the measurement sample11is placed on the stage2, and by using the positioning unit5, the probe4is positioned on the measurement sample11(step S1). Next, the AC voltage is applied to the measurement sample11by the excitation unit6(step S2). Next, a small change of the capacitance of the measurement sample11due to the application of the AC voltage to the measurement sample11is converted into a change of the resonant frequency in the resonance unit7(step S3). The resonance unit7outputs the frequency modulation signal that is converted from the resonant frequency according to the small change of the capacitance of the measurement sample11. The frequency demodulator8outputs the demodulated signal obtained by demodulating the frequency modulation signal output from the resonance unit7(step S4). Next, the lock-in amplifier9extracts the differential capacitance signal from the demodulated signal (step S5).

Next, the CV curve generating unit14within the control unit10generates the CV curve, by performing the inverse Fourier conversion of the differential capacitance signal (step S6). Here, “the generation of the CV curve” can include that the CV curve itself is drawn or generated, and also a function and a matrix corresponding to the CV curve are generated. Data underlying the generated CV curve, the function, and the matrix can each be considered to be relationship data indicative of a relationship between a voltage applied to the measurement sample or the standard sample and a capacitance. Next, the feature point voltage detecting unit15detects the DC bias voltage corresponding to a feature point on the CV curve (step S7). As described below, the feature point is a point suitable for offsetting the change of the CV curve due to the disturbance of the measurement sample11.

Next, the main measurement control unit16controls the excitation unit6, the resonance unit7, the frequency demodulator8, and the lock-in amplifier9, and performs the measurement for measuring the electrical characteristics of the measurement sample11, in a state where the DC bias voltage detected in step S7is applied to the measurement sample11(step S8).

In above-described steps S1to S7, an example in which the DC bias voltage is detected by using the measurement sample11is described. However, the DC bias voltage may be detected by using a standard or a reference sample having the same material and carrier concentration as the measurement sample11, and a process of step S8may be performed by replacing the standard sample on the stage2to the measurement sample11. When the standard sample is used, the standard sample can be one in which a flat band voltage VFB between the standard sample and the probe is already obtained.

A curve shape of the CV curve is different when the measurement sample11is a sample of an n-type semiconductor region, a sample of a p-type semiconductor region, or a sample having a depletion layer, respectively.FIG. 5A,FIG. 5B, andFIG. 5Cillustrate the CV curves when the measurement sample11is the sample of the n-type semiconductor region, the sample of the p-type semiconductor region, and the sample having the depletion layer. The CV curve of the n-type semiconductor region becomes a curve shape in which the capacitance monotonically decreases according to the increment of voltage as illustrated inFIG. 5A. The CV curve of the p-type semiconductor region becomes a curve shape in which the capacitance monotonically increases according to the increment of voltage as illustrated inFIG. 5B. The CV curve of a case having the depletion layer becomes a curve shape having a minimum point in which the capacitance is minimized at a voltage illustrated inFIG. 5C.

A process of the feature point voltage detecting unit15is different according to the curve shape of the CV curve generated in the CV curve generating unit14.FIG. 6is a diagram illustrating a processing procedure of the feature point voltage detecting unit15in a case where the measurement sample11is the sample of the n-type semiconductor region.FIG. 6illustrates a first order differential curve, a second order differential curve, and a third order differential curve in a voltage of the CV curve. The feature point voltage detecting unit15detects a point, as the feature point, in which the first order differential curve becomes a minimum value and the second order differential curve becomes zero, and detects a corresponding voltage value as the DC voltage. The voltage value is equal to a point in which an absolute value of the slope of the CV curve is maximized, and is equal to an inflection point of the CV curve in another perspective.

The difference between the voltage value corresponding to the feature point and the zero voltage becomes a voltage to be applied as the DC bias voltage.

In practice, a point slightly shifted from the above-described points may be used as the feature point. That is, when the measurement sample11is the sample of the n-type semiconductor region, as illustrated in the curve inFIG. 6, the feature point can be set at a point between a maximum point and a minimum point of the second order differential curve of the CV curve.

Even when the measurement sample11is the sample of the p-type semiconductor region, the DC voltage is detected by a similar procedure. When the measurement sample11is the sample of the p-type semiconductor region, since the CV curve monotonically increases as illustrated inFIG. 5B, each of waveforms of the first order differential curve, the second order differential curve, and the third order differential curve is opposite to that inFIG. 6. However, the DC voltage can be detected by a procedure similar to that described above in reference toFIG. 6.

Summarizing the result ofFIG. 6, the feature point is a point between the maximum point and the minimum point of values obtained by second order differentiating with respect to the voltage of the CV curve, when the CV curve monotonically increases or decreases. More specifically, the feature point is the maximum point or the minimum point of a value obtained by first order differentiating with respect to the voltage of the CV curve, and a point obtained by the second order differentiating with respect to the voltage of the CV curve becomes zero, when the CV curve monotonically increases or decreases. Further specifically, the feature point becomes a point in which the absolute value of the slope of the CV curve is maximized or the inflection point of the CV curve, when the probe4is placed on the p-type semiconductor region or n-type semiconductor region within the measurement sample11.

FIG. 7is a diagram illustrating a process procedure of the feature point voltage detecting unit15when the measurement sample11has the depletion layer.FIG. 7illustrates the first order differential curve and the second order differential curve in the voltage of the CV curve. The feature point voltage detecting unit15detects a point, as the feature point, in which the first order differential curve becomes zero and the second order differential curve becomes a maximum value, and a corresponding voltage value as the DC voltage. The voltage value is equal to the minimum value of the CV curve.

Also, in a case ofFIG. 7, in practice, a point slightly shifted from the above-described point may be used as the feature point. That is, the feature point is set at the point between the maximum point and the minimum point of the first order differential curve of the CV curve, when the measurement sample11has the depletion layer, as illustrated in the curve of the drawing.

Summarizing the result ofFIG. 7, the feature point is a point between the maximum point and the minimum point of values obtained by first order differentiating the CV curve with respect to the voltage, when the CV curve has a peak. More specifically, the feature point is a point obtained by the first order differentiating with respect to the voltage of the CV curve becomes zero and values obtained by the second order differentiating with respect to the voltage of the CV curve becomes the maximum value or the minimum value, when the CV curve has a peak. Further specifically, the feature point is a point in which the slope of the CV curve is minimized, or the minimum point of the CV curve, when the probe4is placed on the depletion layer of the measurement sample11.

Referring to the CV curves depicted inFIG. 6andFIG. 7, the DC bias voltage can be set to be substantially equal to a difference in voltage between a voltage corresponding to the feature point on the CV curve and the zero voltage. However, this is because it is assumed that the flat band voltage VFB=0 V. When the flat band voltage VFB is not 0 V, the DC bias voltage can be set to be substantially equal to the difference in voltage between the voltage corresponding to the feature point on the CV curve and the flat band voltage VFB.

As described above, in the embodiment, the CV curve is generated by measuring the differential capacitance by using the measurement sample11, and the voltage value corresponding to the feature point on the CV curve is set as the DC bias voltage. Accordingly, in a state where the DC bias voltage is applied to the measurement sample11, the electrical characteristics of the measurement sample11are measured. With this, even if the surface of the measurement sample11is charged due to the effect of the disturbance, it is possible to accurately measure the electrical characteristics of the measurement sample11. And thus, according to the embodiment, it is unlikely that measurement results vary day by day even in the same measurement sample11.

In addition, in the embodiment, since a method for setting the DC bias voltage is changed depending on the measurement sample11being the sample of the p-type semiconductor region, the sample of the n-type semiconductor region, or the sample having the depletion layer, an optimal DC bias voltage based on a type of the measurement sample11can be set, and accuracy improvement of the electrical characteristics of the measurement sample11is attained.

In the above description, an example for detecting the DC bias voltage of a case where the differential capacitance is measured in the SNDM1is described. However, the embodiment can be applied also when the differential capacitance is measured by using a scanning capacitance microscope (SCM). In addition, the embodiment can be applied also when the differential capacitance is measured by using a scanning probe microscope of other various types.