JP7012349B2 - A scanning microwave microscope and a method for measuring the electrical characteristics of the surface of the object to be measured using the microscope. - Google Patents

Description

The present invention relates to a scanning microwave microscope, and more specifically, to a scanning microscope including a phase-variable resonance circuit, and a method for measuring electrical characteristics of the surface of an object to be measured using the scanning microscope.

A scanning microwave microscope (SMM, hereinafter also simply referred to as SMM) is an electric property evaluation device for a material surface using an electromagnetic wave based on an atomic force microscope (AFM, hereinafter also simply referred to as AFM). In SMM, an electromagnetic wave signal is irradiated to a sample from the needle tip (needle) at the tip of the AFM probe, the reflected signal is detected, and the reflection characteristic accompanying the impedance of the sample surface is measured. Further, since it is possible to measure while scanning the surface as a function similar to that of AFM, it is possible to measure the distribution of electrical characteristics such as conductivity, permittivity, and magnetic permeability of the sample surface. This makes it possible to observe and analyze the distribution of materials and the distribution of carrier concentrations of semiconductors and the like in the composite material as a sample.

Therefore, there is a need for a method for measuring electromagnetic wave signals from the surface of materials with high accuracy and stability. However, since the SMM uses a vector network analyzer to measure the electromagnetic wave signal, it is not possible to obtain sufficient detection sensitivity for minute changes in the low impedance and high impedance regions simply by connecting them. Further, since the device itself needs to be mounted on the vibration isolation table and the space and weight are limited, a simple detection circuit with a small number of parts is required.

As a prior art, a plurality of detection circuits have been proposed so far. For example, as an SMM including a relatively simple circuit structure, Non-Patent Document 1 discloses a reflection type SMM including a resonance circuit with a 50Ω resistor connected in parallel with a half-wavelength resonator. Further, Non-Patent Document 2 discloses a reflection type SMM including a resonance circuit (interferometer) by an impedance adjuster and a phase adjuster. However, these conventional SMMs have not been able to obtain sufficient detection sensitivity.

In order to improve the sensitivity, as another conventional technique, a plurality of transmissive SMMs including various resonance circuits (interferometers) have been proposed. However, these conventional SMMs require an amplifier (active element) that requires a power supply for their circuit configuration, and also use many parts, resulting in a complicated circuit configuration and a large circuit size. .. In general, in high-frequency precision measurement, the use of active elements and the complexity of circuits are factors such as deterioration of stability due to changes in ambient temperature and vibration. As a result, a method that achieves both sensitivity and ease of realization of the measurement / detection circuit has not been established.

H. Tanbakuchi, et al., Semiconductor Material and Device TEXT via Scanning Microwave Microscopy, “IEEE Compound Semiconductor Integrated Circuit Symposium (CSICS), Pages: 1-5, 2013. A. Lewandowski, et al., “Wideband measurement of extreme impedances with a multistate reflectometer,” 72nd ARFTG Microwave Measurement Symposium, Pages: 45-49, 2008.

An object of the present invention is to improve the sensitivity of a scanning microwave microscope using a resonance circuit (interferometer) that does not use an active element and reduces the number of parts.

The scanning microwave microscope according to one aspect of the present invention has an AFM probe capable of scanning the surface of an object to be measured, a T-branch having a first end connected to the AFM probe, and a variable phase microscope connecting the second end of the T-branch. It comprises a short circuit and a vector network analyzer connected to the AFM probe and the phase variable short circuit via the third end of the T-branch. The AFM probe and the phase variable short circuit block form one resonance circuit, and the phase variable short circuit controller determines the phase of the resonance frequency of the reflected wave signal of the electromagnetic wave that the vector network analyzer irradiates the surface of the object to be measured via the AFM probe. adjust.

The method for measuring the electrical characteristics of the surface of the object to be measured using the scanning microwave microscope according to one aspect of the present invention is as follows: (a) The surface of the object to be measured is irradiated with an electromagnetic wave having a frequency within a predetermined range via an AFM probe. The phase of the resonance frequency of the reflected electromagnetic wave is adjusted by using the steps, (b) the step of detecting the reflected electromagnetic wave from the surface of the object to be measured by the vector network analyzer via the AFM probe, and (c) the phase variable short circuit. This includes (d) determining the electrical characteristics of the surface of the object to be measured from the amplitude change or phase change of the reflected electromagnetic wave at the selected resonance frequency after the phase adjustment by the vector network analyzer.

According to the present invention, it is possible to improve the sensitivity of a scanning microwave microscope using a resonance circuit (interferometer) that reduces the number of parts without using an active element. Further, by using a phase variable short-circuit element as a component of the resonance circuit (interferometer), the resonance (interference) frequency to be observed can be freely selected. This makes it possible to realize highly sensitive / highly accurate and stable measurement according to the surface condition of the object to be measured, the properties of the material, and the like.

It is a figure which shows the structure of the scanning microwave microscope of one Embodiment of this invention. It is a figure which shows the flow of the measuring method of one Embodiment of this invention. It is a figure explaining the state before the phase adjustment of the resonance frequency of one Embodiment of this invention. It is a figure explaining the state after the phase adjustment of the resonance frequency of one Embodiment of this invention. It is a figure which shows the measurement result (cross-sectional SMM image by the amplitude change) of one Example of this invention. It is a figure which shows the measurement result (cross-sectional SMM image by a phase change) of one Example of this invention. It is a figure which shows the measurement result (amplitude change) of one Example of this invention. It is a figure which shows the measurement result (phase change) of one Example of this invention.

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of a scanning microwave microscope (SMM) according to an embodiment of the present invention. The SMM 100 includes an AFM probe 1, a T-branch (circuit) 3, a phase variable short circuit element (Sliding short) 7, and a vector network analyzer (VNA) 9. Further, as a part of the SMM 100 or as an external device (mechanism), a stage 11 having a drive mechanism (not shown) for mounting the object 12 to be measured and moving in the XYZ direction is provided.

The AFM probe 1 is also generally called a cantilever, has a probe 2 at the tip, and is made of a semiconductor, a metal, or the like. By moving the AFM probe 1 up and down or by moving the stage 11 up and down, the probe 2 at the tip of the AFM probe is adjusted to be in contact with or in the vicinity of the surface 13 of the object 12 in a non-contact manner. .. The first end 4 of the T-branch 3 is connected to the AFM probe 1 via a coaxial cable, the second end 5 is connected to the phase variable short-circuit element 7 via a coaxial cable, and the third end 6 is connected via a coaxial cable. Connect to one input / output port (PORT1) of VNA9. The AFM probe 1 and the phase variable short-circuit element 7 are connected in parallel to the input / output port of the VNA 9 via the T branch 3, and both form one resonance (resonance, interference) circuit, in other words, an impedance adjustment circuit. ..

The VNA 9 can transmit an electromagnetic wave having a frequency within a predetermined range (for example, a range of 1 to 20 GHz) to the AFM probe 1 and the phase variable short-circuit element 7 via a coaxial cable connected to the input / output port (PORT1). .. The electromagnetic wave is irradiated from the probe 2 at the tip of the AFM probe to the surface 13 of the object to be measured 12, and the reflected electromagnetic wave from the surface 13 is received from the probe 2 of the AFM probe. The VNA 9 receives a signal of the reflected electromagnetic wave from the AFM probe 1 (hereinafter, also referred to as a reflected electromagnetic wave signal) via a coaxial cable, processes the reflected electromagnetic wave signal, and performs arithmetic processing on the reflected electromagnetic wave signal to perform various electrical characteristics of the surface 13 of the object 12 to be measured. Is calculated. The measurement result of the electrical characteristics is displayed on the display unit 10 built in or external to the VNA 9.

The phase-variable short-circuit element 7 includes, for example, a cylindrical or rectangular parallelepiped cavity resonator 7. The variable short-circuit plate 8 in the cavity resonator 7 moves in the vertical or horizontal direction (vertical direction in FIG. 1) to change the cavity distance D, so that the variable short-circuit plate 8 is input from the VNA 9 into the cavity resonator 7 via the T-branch 3. The phase of the resonance frequency of the electromagnetic wave to be generated can be adjusted. The movement of the variable short circuit plate 8 in the cavity resonator 7 can be performed manually or by an accompanying drive mechanism. By adjusting the phase of the resonance frequency in the phase-variable short-circuit element 7, the phase of the resonance frequency of the reflected electromagnetic wave signal received from the AFM probes 1 connected in parallel forming one resonator together is adjusted as described in detail later. Can be adjusted.

FIG. 2 is a diagram showing a flow of a method for measuring electrical characteristics of the surface of an object to be measured using the scanning microwave microscope according to the embodiment of the present invention. The following description of the measurement flow of FIG. 2 shows an example when the SMM 100 of the embodiment of FIG. 1 is used, but this measurement flow is similarly executed when the SMM of another embodiment of the present invention is used. It is possible.

In step S1 of FIG. 2, the object to be measured 12 is set (placed) on the stage 11. The object 12 to be measured can basically include a substrate made of an arbitrary material (semiconductor, metal, magnetic material, dielectric, etc.) whose surface electrical characteristics can be measured by the AFM probe 1. In step S2, by moving the stage 11 and the AFM probe 1, the probe 2 of the AFM probe 1 is set at the measurement start point (start position of the measurement region) on the surface 13 of the object to be measured 12. The probe 2 of the AFM probe 1 is set in contact with or non-contact with the surface 13 of the object to be measured 12 depending on the object to be measured, measurement conditions, and the like.

In step S3, VNA 9 irradiates an electromagnetic wave from the probe 2 of the AFM probe 1 to the surface 13 of the object to be measured 12 while sweeping the frequency within a predetermined range (for example, in the range of 1-20 GHz). At the same time, an electromagnetic wave having the same frequency range is incident on the phase variable short-circuit element 7 from the VNA 9 via the second end 5 of the T branch 3. In step S4, the VNA 9 receives the reflected electromagnetic wave signal from the surface 13 of the object to be measured 12 via the probe 2 of the AFM probe 1.

In step S5, the phase of the resonance frequency of the reflected electromagnetic wave signal measured by the VNA 9 is adjusted by the phase variable short-circuit element 7, and the frequency of the reflected electromagnetic wave signal for which the VNA 9 measures (calculates) the electrical characteristics is set. The phase (frequency) adjustment will be described with reference to FIGS. 3 and 4. 3 (a) and 4 (a) are both equivalent circuits of the AFM probe 1, the T-branch 3, and the phase variable short-circuit element 7 on the object 12 to be measured. The object to be measured 12 has an impedance (resistance R1 + capacitance C2) that changes according to its surface state. The VNA 9 measures (changes in) its impedance as a reflected electromagnetic wave signal. The AFM probe 1 has a probe capacity C1 that is determined according to the material and the like. The phase variable short-circuit element 7 adjusts the phase of the resonance frequency in the cavity resonator 7 by changing the equivalent inductance L1 by changing the position (distance D) of the variable short-circuit plate 8 in the cavity resonator 7. do.

FIG. 3B is a diagram illustrating a state before phase adjustment of the resonance frequency. FIG. 4B is a diagram illustrating a state after phase adjustment of the resonance frequency. In the relationship diagram between the frequency and the amplitude or the reflection coefficient S11 parameter of the reflected electromagnetic wave signal measured by VNA9 before the phase adjustment in FIG. 3B, the resonance frequency is set to _f0 . Since the resonance frequency to be measured in order to obtain the electrical characteristics is fs, it is necessary to shift (delay) the phase and adjust _f0 to fs. Therefore, as described above, by changing the equivalent inductance L1 of the phase variable short-circuit element 7 of FIG. 4A to adjust the phase of the resonance frequency in the cavity resonator 7, FIG. 4B is shown. As shown, the resonance frequency measured by VNA9 can be set to fs.

In step S6 of FIG. 2, the amplitude and phase of the reflected electromagnetic wave signal at the resonance frequency phase-adjusted in step S5 are measured. Specifically, the VNA 9 measures (detects) the amplitude and phase of the reflected electromagnetic wave signal at the resonance frequency fs after the phase adjustment. The amplitude and phase reflect the reflection characteristic, that is, the impedance of the surface 13 of the object to be measured 12. In step S7, the electrical characteristics (capacitance, conductivity, dielectric constant, magnetic permeability, impurity level, etc.) of the surface 13 of the object to be measured 12 are selectively selected from the amplitude and phase obtained in step S6 as necessary. calculate.

In the next step S8, steps S3 to S7 are executed while scanning (moving) the AFM probe 1 or the stage 11, and the amplitude of the reflected electromagnetic wave signal in the measurement region (part or all) of the surface of the object to be measured 12. And the phase are measured, and various required electrical characteristics are calculated. As an alternative flow, step S8 is inserted between steps S6 and S7, and steps S3 to S6 are executed while scanning the AFM probe 1 or stage 11, and the measurement region on the surface of the object to be measured 12 (1) The amplitude and phase of the reflected electromagnetic wave signal in part or all) may be measured.

In step S9, the distribution of the electrical characteristics in the measurement region of the object to be measured 12 obtained in step S8 is displayed on the display unit 10 as an image. This image distribution corresponds to the distribution of impedance changes (differences) that reflect the surface state of the measurement region of the object to be measured 12.

The surface of the sample of the object to be measured was actually measured by using the SMM exemplified in FIG. 1 and the measurement method flow exemplified in FIG. 2 described above. In the measurements below, a Si substrate containing a Si layer doped in a region where an N-type dopant was selected as an impurity was used as the sample.

5 and 6 are SMM images of cross sections of Si layer structures having different doping concentrations. The horizontal axis of each image is the distance (μm) in the XY direction, and the vertical axis is the distance (μm) in the Z (depth) direction. FIG. 5 is a measurement image corresponding to the amplitude change, and FIG. 6 is a measurement image corresponding to the phase change. In both figures, (a) is a measurement result using a reflection type SMM including a half-waver resonator exemplified in the conventional non-patent document 1 and a resonance circuit with a 50Ω resistor, and (b) is a conventional non-patented measurement result. It is the measurement result using the reflection type SMM including the resonance circuit (interferometer) by the impedance regulator and the phase adjuster exemplified in Document 2, and (c) is the measurement result by the SMM of this invention.

In each image, the white part indicates the region where the dopant concentration is high. In the image of FIG. 6B, black and white are inverted due to image processing, and the black portion indicates a region having a high dopant concentration. In both figures, by comparing the conventional images (a) and (b) with the image of the present invention (c), the image (c) of the present invention is more than the conventional images (a) and (b). The region (white region) where the image blur (noise) is less and the dopant concentration is higher is shown, indicating that the detection sensitivity (S / N ratio is also high) is high.

7 and 8 are graphs showing the measurement results of the amplitude and phase of one line of Si layer structures having different doping concentrations. The horizontal axis of each graph in FIG. 7 is the distance (μm) in the XY direction, and the vertical axis is the amplitude (dB). The horizontal axis of each graph in FIG. 8 is the distance (μm) in the XY direction, and the vertical axis is the phase (deg). The range of the vertical axis of the graphs (a) and (b) in FIG. 7 is −0.004 to 0.02 dB, and the range of the vertical axis of the graph of FIG. 7 (c) is −0.4 to 2.0 dB. It is 100 times larger than the graphs of (a) and (b). Similarly, the range of the vertical axis of the graphs (a) and (b) of FIG. 8 is −0.01 to 0.03 deg and −0.5 to 0.3, respectively, and the vertical axis of the graph of (c) is vertical. The range of the axis is -1 to 4 deg, which is 100 times or 10 times larger than the graphs of (a) and (b). In both figures, (a) is a measurement result using a reflection type SMM including a half-waver resonator exemplified in the conventional non-patent document 1 and a resonance circuit with a 50Ω resistor, and (b) is a conventional non-patented measurement result. It is the measurement result using the reflection type SMM including the resonance circuit (interferometer) by the impedance regulator and the phase adjuster exemplified in Document 2, and (c) is the measurement result by the SMM of this invention.

In each graph, the peak waveform portion where the amplitude of the waveform is large indicates the region where the dopant concentration is high. In the graph of FIG. 8B, the peak waveform is inverted due to image processing, and the waveform portion of the lower peak of the waveform shows a region where the dopant concentration is high. In both figures, by comparing the conventional graphs (a) and (b) with the graph of the present invention (c), the graph of the present invention (c) is more than the conventional graphs (a) and (b). The amplitude of the waveform is about 10 to 100 times larger, the noise on the waveform is reduced, and the detection sensitivity (S / N ratio) in the region where the dopant concentration is high is high.

An embodiment of the present invention has been described with reference to the drawings. However, the present invention is not limited to these embodiments. Further, the present invention can be carried out in a mode in which various improvements, modifications and modifications are added based on the knowledge of those skilled in the art without departing from the spirit of the present invention.

The scanning microwave microscope of the present invention uses a resonance circuit (interferometer) in which the number of parts is reduced without using an active element, and has high sensitivity / accuracy according to the surface state of the object to be measured, the properties of the material, and the like. It can be used as a scanning microwave microscope that can realize stable measurement.

1: AFM probe 2: AFM probe probe (tip)
3: T branch (circuit)
4: 1st end of T-branch (circuit) 5: 2nd end of T-branch (circuit) 6: 3rd end of T-branch (circuit) 7: Phase variable short circuit (cavity resonator)
8: Movable short circuit plate 9: Vector network analyzer (VNA)
10: Display 11: Stage 12: Object to be measured 100: Scanning microwave microscope (SMM)

Claims (5)

With an AFM probe capable of scanning the surface of the object to be measured,
A T-branch whose first end connects to the AFM probe,
A phase variable short circuit connected to the second end of the T-branch,
It comprises an AFM probe and a vector network analyzer connected to the phase variable short circuiter via the third end of the T-branch.
The AFM probe and the phase variable short circuit block form one resonance circuit, and the phase variable short circuit controller determines the phase of the resonance frequency of the reflected wave signal of the electromagnetic wave that the vector network analyzer irradiates the surface of the object to be measured via the AFM probe. A scanning microwave microscope to adjust. The vector network analyzer irradiates the surface of the object to be measured with an electromagnetic wave via the AFM probe, detects the reflected wave signal of the electromagnetic wave, and selects the resonance frequency after phase adjustment by the phase variable short circuit. The scanning microwave microscope according to claim 1 , wherein the electrical characteristics of the surface of the object to be measured are obtained from the amplitude change or the phase change in the above. It is a method of measuring the electrical characteristics of the surface of the object to be measured using a scanning microwave microscope.
(A) A step of irradiating the surface of the object to be measured with an electromagnetic wave having a frequency within a predetermined range via an AFM probe.
(B) A step of detecting the reflected electromagnetic wave from the surface of the object to be measured by a vector network analyzer via an AFM probe, and
(C) A step of adjusting the phase of the resonance frequency of the reflected electromagnetic wave using a variable phase short circuit, and
(D) A measurement method including a step of obtaining the electrical characteristics of the surface of an object to be measured from the amplitude change or phase change of the reflected electromagnetic wave at a selected resonance frequency after phase adjustment by a vector network analyzer. The measuring method according to claim 3 , wherein the AFM probe and the phase variable short circuit device are connected in parallel to the vector network analyzer. 3. The third aspect of the present invention further comprises a step of performing the steps (a) to (d) while scanning the surface of the object to be measured with the AFM probe to obtain a distribution of electrical characteristics on the surface of the object to be measured. Or the measuring method according to 4 .

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