Imaging dielectric spectroscopy method and apparatus thereof

Abstract

Provided is an imaging dielectric spectroscopic technology for obtaining mapping of a dielectric property in a specimen using electron beam holography. An interference image formed by an interference optical system is stroboscopically captured by, while periodically applying a stimulus with a repetition period T to a specimen (103) of a transmission electron microscope in which the interference optical system is installed, performing repeated exposures on a camera (105) for only a short time in synchronization with the period T, N interference images having different delays are obtained by changing the delay time of the exposure with respect to the stimulus in N ways, and frequency dependence of a complex dielectric constant is obtained by obtaining a reproduced image from each of the interference images by a computer (106).

Description

Technical Field

[0001] The present invention relates to a technique for analyzing substances using electron beam holography, and particularly to an imaging dielectric spectroscopy technique for obtaining mapping of dielectric properties in a sample.

Background Art

[0002] Electron beam holography has applications such as observing the potential created by ion distribution in a solid electrolyte sample, as described in Non-Patent Document 1 for example. However, this was observing the potential in the steady state reached after applying a voltage to the sample. Although technical studies on high-speed imaging using electron beam holography have also been progressing, the application of time-resolved electron beam holography to an actual substance sample has been delayed (see Non-Patent Document 2).

[0003] Analysis methods for examining the dynamic response of a sample to an external electric field are called dielectric spectroscopy or impedance spectroscopy, and in recent years, their applications have also spread to the evaluation of cells flowing through a microchannel between microelectrodes, etc. However, the dielectric spectroscopy method measures the response of the entire volume between electrodes and cannot image the internal structure of the sample (see Non-Patent Document 3).

[0004] A scanning probe microscope (SPM) can also be given the function of measuring electrical properties such as dielectric constant, not only the unevenness of the sample, so it is necessary to scan using a probe-like microelectrode as a probe, and a microscopic technique capable of mapping the dielectric constant has been developed (Patent Document 1).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Non-Patent Documents

[0006]

Non-Patent Document 1

[0007] The conventional methods described above have already spawned a market for analytical services, and a roadmap for applications in electrochemistry is also being envisioned, indicating a high social demand for analytical and evaluation techniques such as dielectric property mapping or imaging. However, with probe electrode methods, it takes time for the probe to sequentially measure each point of the sample and complete the scan to obtain data for one screen, making it an incomplete method for capturing instantaneous, high-speed changes.

[0008] The object of the present invention is to solve the above problems and provide an imaging dielectric spectroscopy method and apparatus that enables the observation of dynamic changes in the potential distribution in a sample and allows the mapping of dielectric properties in a sample from the potential response at each point to an external field that fluctuates over time. [Means for solving the problem]

[0009] To achieve the above objective, the present invention provides an imaging dielectric spectroscopy method using an electron microscope equipped with an interference electron optical system, wherein the electron microscope periodically applies pressure to a sample with a repetition period T. While applying stimulation, the imaging device is repeatedly exposed to the camera for a duration of 1 / M of the period T, synchronized with the period T. This invention provides an imaging dielectric spectroscopy method for obtaining the frequency dependence of the complex dielectric constant by stroboscopically capturing interference images formed by an interference optical system, varying the exposure delay time in response to a stimulus in N ways to obtain N interference images with different time delays, and obtaining a reconstructed image from each of these interference images.

[0010] Furthermore, in order to achieve the above objective, the present invention provides an imaging dielectric spectrometer using an electron microscope equipped with an interference electron optical system, wherein the sample of the electron microscope is periodically stimulated with a repeating period, and exposure is repeated for 1 / M time of the period, synchronized with the period, The present invention provides an imaging dielectric spectrometer comprising: an imaging device that stroboscopically captures interference images formed by an interference optical system and obtains N interference images with different time delays by varying the exposure delay time in response to a stimulus in N ways; and a signal processing device that obtains a reconstructed image from the interference images captured by the imaging device to obtain the frequency dependence of the complex dielectric constant. [Effects of the Invention]

[0011] According to the present invention, the dynamic response of the electromagnetic field of a sample can be observed as a series of still images, and the distribution of the electrical properties of the sample can be visualized. [Brief explanation of the drawing]

[0012] [Figure 1] A diagram showing an example configuration of an imaging dielectric spectrometer according to Example 1. [Figure 2] A figure showing a modified example of the imaging dielectric spectrometer according to Example 1. [Figure 3] A schematic diagram showing the deflection device of the imaging dielectric spectrometer according to Example 1. [Figure 4] A diagram showing a specific configuration of the deflection device according to Example 1. [Figure 5] Figure showing interference fringes photographed by the apparatus according to Example 1. [Figure 6] Figure showing the sharpness of interference fringes photographed by the apparatus according to Example 1. [Figure 7] Figure showing a circuit for connecting an impedance matcher to the deflection device according to Example 1. [Figure 8] Figure showing a configuration example for accelerating electrostatic deflection according to Example 1. [Figure 9] Figure showing another configuration example for accelerating electrostatic deflection according to Example 1. [Figure 10] Figure showing the principle of differential blanking electrostatic deflection according to Example 1. [Figure 11] Figure showing a configuration example of a differential blanking electrostatic deflector according to Example 1. [Figure 12] Schematic diagram for explaining a captured image of the dynamic response of an electromagnetic field according to Example 1. [Figure 13] Figure showing a configuration example for monitoring the current flowing through a sample according to Example 1 and an example of discrete modeling of the sample. [Figure 14] Figure showing three configuration examples for measuring the current flowing through a sample according to Example 1. [Figure 15] Schematic diagram of a two-stage electron beam deflector installed along the path of an electron beam according to Example 2. [Figure 16] Figure showing a reproduced image of a varying electric field photographed by stroboscopic exposure according to Example 2. [Figure 17] Figure showing a phase reproduced image from a still image hologram photographed by continuous exposure according to Example 2. [Figure 18] Schematic diagram of an apparatus in which a two-stage electron beam deflector according to Example 2 is mounted on a common support mechanism. [Figure 19] Schematic diagram of an apparatus in which a convergent electron aperture and an electron beam deflector according to Example 2 are mounted on a common support mechanism. [Figure 20] Figure showing a comparison between Example 3 and the conventional method. [Figure 21] Figure showing a configuration for applying a voltage to a sample via a switching element according to Example 3. [Figure 22] This figure shows the cross-sectional shape of the switching element fabricated on the sample support according to Example 3. [Figure 23] A diagram showing a planar configuration in which capacitances are arranged in parallel with the sample according to Example 3. [Figure 24] A diagram illustrating the operation using the equivalent circuit according to Example 3. [Modes for carrying out the invention]

[0013] Hereinafter, embodiments for carrying out the present invention will be described sequentially with reference to the drawings, but prior to that, a preferred procedure for the imaging dielectric spectroscopy method and apparatus of the present invention will be described.

[0014] (1) Interferential electrons in a transmission electron microscope so that holographic observation (photography and exposure) can be performed. Install an optical system.

[0015] (2) A transmission electron microscope can periodically stimulate a sample with a repetition period T, and repeatedly accumulate exposures on the camera for a short time of 1 / M of the period T, synchronized with the period of that stimulation. It features a high-speed shutter mechanism. However, the number of M values ​​is approximately M > 50. It is desirable.

[0016] (3) The electron beam hologram (interference image) formed by the interference optical system is used by the shutter mechanism By using this method to take stroboscopic images, a hologram of a sample responding to a stimulus at a specific moment (a very short period, less than a fraction of the period T) is obtained.

[0017] (4) The time delay at the moment of exposure in response to stimulation of the sample is changed in N ways to obtain N different time delays. A hologram is obtained. A reconstructed image can be obtained from each hologram using a standard procedure, and the electromagnetic field can be determined. In this way, the dynamic response of the electromagnetic field of the sample over a period T can be observed as a series of N frame images.

[0018] (5) For a sinusoidal external stimulus with period T, the magnitude of the potential change and the phase lag at each point of the sample can be determined based on the observations described above. (Current density) = (Potential gradient) / (Complex resistivity) If the relationship is considered to hold, the complex resistivity at this frequency f (f = 1 / T) at each point can be determined from the boundary conditions for the potential and the total current flowing through the sample monitored by appropriate means.

[0019] (6) By performing measurements with varying frequency f, i.e., period T, and the above analysis, The frequency dependence of the complex resistivity at each point in the sample, or the complex permittivity through conversion, can be determined. Since the real and imaginary parts of the permittivity are given for each point, the spatial distribution of the sample's electrical properties can be visualized by mapping it to coordinates and by comparisons using frequency as an argument.

[0020] Therefore, (a) The dynamic response of the electromagnetic field of the sample can be observed as a series of still images. (b) For example, imaging the spatial distribution of electrical properties such as dielectric constant of a sample at a certain frequency. It is possible. [Examples]

[0021] Example 1 is an example of an imaging dielectric spectrometer and a method used to obtain a stroboscopic hologram for a short time.

[0022] Figure 1 shows a specific configuration example of the imaging dielectric spectrometer of this embodiment, and Figure 2 shows another configuration example. In the configuration of Figure 2, a pulsed electron source 107 is used, so the electron source itself has the function of short exposure, but the coherence of the electron beam is still under improvement. In the configuration of Figure 1, the coherence of the electron beam has been demonstrated in still image acquisition by combining a cold cathode field emission electron gun with an electrostatic deflector, so the following explanation will use the configuration of Figure 1.

[0023] In the configuration shown in the figure, a deflection device 102, which uses electrostatic force to deflect the electron beam from the electron source 101 so that it does not reach the camera 105, is placed upstream of the sample 103 in the sample chamber to act as a shutter for short exposure times. It can also be placed downstream of the sample 103. The electron beam that has passed through the sample passes through the interference optical system inside the microscope body and is input to the camera 105, which is the imaging device. In the figure, elements that are normally present in a transmission electron microscope, such as the illumination optical system, imaging / magnification optical system, and lenses that constitute the projection optical system, are not shown.

[0024] The arrows in the diagram indicate the flow of signals. For example, signal generator 1 sends a sinusoidal signal of frequency f to stimulate the sample 103 in the electron microscope, and signal generator 2 sends a polarized signal. A pulse is sent to the target device 102 to initiate exposure. The dashed lines indicate information sharing between devices, such as the transfer of a hologram image from the camera 105 (an imaging device) to the computer 106 (a signal processing device), and the recognition of the frequency and phase delay of the modulation signal used during each hologram capture by the computer 106.

[0025] The deflection device 102, which switches the electron beam ON / OFF, can be materialized by two parallel plate electrodes that sandwich the electron beam path from both sides, as will be described below. A current measuring device 104 is provided to measure the current flowing through the sample, in order to enable observation of the dielectric properties of the sample. Alternatively, as shown in the figure, it may be provided in the middle of the wire that provides external stimulation to the sample.

[0026] Figure 3 shows a schematic diagram of the deflection device 102 of the imaging dielectric spectrometer shown in Figure 1. Deflection electrodes 1 and 2, consisting of electrode plates parallel to the xz-plane, are placed on either side of an electron beam incident in the z-direction along the optical axis (z-axis), and an electric field in the y-direction is generated by applying an external voltage between the electrodes. Therefore, the electrons are deflected in the y-direction or -y-direction. If the deflection is large enough that the electron beam does not reach the camera 105 located downstream on the optical axis, this deflector acts as a shutter for the camera.

[0027] Pulsed voltage signals are supplied to the deflection electrodes 1 and 2 from the signal generator 2 via conductors 1 and 2. Since the magnitude of the deflection depends on the voltage, images obtained with the deflected electron beam are also accumulated on the camera while the amount of deflection is small during the rising edge of the pulse. If the number of electrons accumulated on the camera 105 in this beam deflection transition state can be ignored compared to the number of electrons accumulated on the camera in the undefended state, then a sharp ON / OFF switch can be said to be achieved. If the contribution is not negligible, the image will become blurred.

[0028] Figure 4 shows a specific configuration example of a deflection device 102, consisting of parallel plate electrodes. Figures (a) and (b) show a plan view and a side view, respectively. The deflection electrodes 1 and 2 are connected to mutually insulated support rings 1 and 2 on a base placed in the vacuum of the electron microscope. The electrode support rings 1 and 2 and the insulating spacers can rotate together around the z-axis on the base. The microscope has a mechanism (not shown in the illustration) that allows the direction of application of the electric field to the electron beam to be changed within the xy-plane. To allow voltage to be applied to the electrode 1 support ring from outside the microscope, a slider, for example, pressed by a spring, or a flexible wire that allows the ring to rotate is in contact with it. If the potential of electrode 2 is also to be controlled independently of the base, then similar insulation from the base and a power supply line should be provided for it as well.

[0029] Having electrodes 1 and 2 with length in the z-axis direction along the electron beam allows for greater deflection at lower applied voltages. In the experiment, the electrodes had a z-length of 4 mm and a spacing of 1.2 mm. For a 300 kV electron beam, deflection was obtained with a voltage of 6 V when the electrodes were placed on the optical axis near the condenser aperture.

[0030] Figure 5 shows electron beam holograms (interference fringes) 501, obtained by deflecting the beam using a signal generator with a frequency limit of 15 MHz and changing the exposure window width. (a) in the figure shows interference The images show the fringes captured with continuous exposures of 4 seconds each. (b) was captured by repeating exposures of 5 μs 800,000 times for a net exposure of 4 seconds, (c) by repeating exposures of 1 μs 4 million times, and (d) by repeating exposures of 0.5 μs 8 million times. As the exposure time interval per exposure shortens, the influence of the ON / OFF transition state, which is the window boundary, increases (with increasing exposure count) and affects the interference fringes. You can see that the clarity deteriorates.

[0031] The clarity of interference fringes 501 obtained from the image shown in Figure 5 is measured per pass. When plotted against exposure time (exposure window width), the change in interference fringe clarity with respect to exposure window width was as shown in Graph 601 of Figure 6. In this experimental setup, it was found that the clarity of the interference fringe was comparable to that of continuous exposure when the exposure window width was 5 μs or more.

[0032] To achieve high temporal resolution imaging with a smaller exposure window width, it is desirable to use a signal generator capable of higher frequencies and to configure the circuit 701 as shown in Figure 7. In circuit 701, in addition to conductors 1 and 2 that supply pulses from the signal generator 2, which is a pulse generator, to the parallel plate electrodes placed in the vacuum of the electron microscope, two more conductors 3 and 4 are provided facing outwards from the vacuum. By connecting an impedance matching circuit, which is an appropriate termination matching device, to the ends of conductors 3 and 4, it becomes possible to suppress pulse reflection and waveform distortion, and to shorten the pulse rise time that impairs image clarity.

[0033] For faster deflection, it is desirable to use a pulse generator faster than the signal generator mentioned above, and to provide two systems of four wires connecting the inside and outside of the microscope to prevent pulse reflection. From the standpoint of eliminating unnecessary stray impedance in the transmission path, a termination matching device is also desirable. It is advantageous to place the deflection electrode in a vacuum close to it. On the other hand, to proceed with matching adjustments while confirming the effect, it is advantageous to place it in a location that is easy to operate outside of a vacuum.

[0034] Next, a method for increasing the speed of electrostatic deflection will be explained. Figure 8 shows one example configuration. As shown in the figure, a pulsed laser from a pulsed laser light source 802 is input to a photoconductive switch 801 installed on conductor 1 of the circuit 701 in Figure 7. As the photoconductive switch, a semiconductor is used that causes a rapid increase in conductivity due to carriers excited by the pulsed laser. In this configuration, a voltage is applied to the parallel plate electrodes when the photoconductive switch 801 is turned on. In order to prevent degradation of the pulse waveform in the transmission line from the signal generator placed outside the microscope to the deflection electrode, the photoconductive switch 801 is connected to the deflection electrode. It can also be placed in the immediate vicinity and operated.

[0035] Figure 9 illustrates another method for increasing the speed of electrostatic deflection. In this figure, a photoconductive switch 901 is installed between conductors 1 and 2 of circuit 701. It is also possible to use the photoconductive switch 901 to short-circuit the bias voltage applied to the deflection electrode. In this configuration, a bias voltage is applied to the deflection electrode when the switch is off, and the switch The deflection electrodes are short-circuited only when the switch is turned on. Whether the electron beam reaches the camera when a voltage is applied to the deflection electrodes, or is deflected away from the camera when a voltage is applied, can be arbitrarily selected using the deflector included in the standard optical system of the electron microscope.

[0036] Next, the principle of differential blanking electrostatic deflection and an example configuration of a differential blanking electrostatic deflector will be explained using Figures 10 and 11. In order to perform sharp, short-time exposures with electrostatic blanking, a fast pulse rise is required so that the electron beam is instantly repelled and leaves no trace on the camera during the deflection process. At the same time, when the beam is not being repelled (during exposure), stability of the voltage applied to the deflection electrodes is required to prevent beam blurring. However, short-pulse generators are not good at creating rectangular pulses with flat peaks and troughs. Therefore, short-pulse "exposure" is not as easy as "blanking" with short pulses.

[0037] To avoid this inconvenience, an exposure sequence combining two types of pulse waveforms, one long and one short, was devised. This is called the "differential blanking method." Figure 10 shows the short pulse width τ1. By adding both a pulse of length τ2 and taking the difference between the image captured with waveform C in Figure 10 and the image captured with only the pulse of length τ2 in waveform B, the total amount of τ1 pulses on each pixel can be calculated. The aim is to obtain a large number of images. Since the peak of the exposure pulse is flat, the image is stable. Since the count during τ2 is subtracted, the S / N ratio does indeed deteriorate, but compared to the case of ideally rectangular pulse exposure of τ1, (S / N ratio in differential blanking) / (S / N ratio in ideal pulse exposure with τ1) = [τ1 / (2τ2)] 1 / 2 This is estimated to be the extent of the issue. In this way, the problem of image distortion due to the peaks of short pulses not being flat is avoided.

[0038] Figure 11 shows an example configuration of a differential blanking electrostatic deflector that utilizes this principle. While the τ2 pulse in the figure, i.e., waveform A in Figure 10, is applied, the deflection electrodes are short-circuited by a photoconductive switch for a time τ1, thereby applying waveform C in Figure 10 to the deflection electrodes.

[0039] Figure 12 schematically shows an example of a "frame image of the dynamic response of the electromagnetic field" obtained as described above, as an example of time-resolved observation. Figure 12(a) schematically shows the current response 1201 of the ion conductor after voltage application, and Figure 12(b) schematically shows the equipotential surfaces that converge over time.

[0040] As shown in the figure, when a voltage that changes in a step-like manner at a certain time is applied to a sample consisting of an ion conductor and a metal electrode, ions are first attracted according to their polarity and begin to move toward the electrode. The moved ions accumulate at the electrode interface, and when an electric double layer is finally formed, the electric field within the bulk of the ion conductor is shielded by this double layer.

[0041] If we observe this process using a potential distribution, a uniform potential gradient will be generated in the sample immediately after voltage application. However, as ions move, the equipotential surfaces will converge at the electrode interface, as shown in figure (b), and this change should be observed in the "dynamic response images of the electromagnetic field" obtained with this configuration.

[0042] Incidentally, Figure 12(a) shows the current response when a rectangular wave voltage is applied to an ionic liquid sample. The current peak immediately after the voltage rise indicates the contribution of ion movement toward bilayer formation, and it was found that the time until bilayer formation is completed is relatively slow, at several tens of microseconds. Therefore, it is expected that if this ionic liquid is used as an ionic conductor, the convergence process of equipotential surfaces can be observed with short exposure times of several microseconds.

[0043] While changes in potential distribution in response to a step-like external electric field, as in the example above, provide insights into electrical properties such as ion mobility, the inventors propose a combination of time-resolved electron holography and dielectric spectroscopy as a method for systematically observing and analyzing samples whose behavior is unknown beforehand.

[0044] Dielectric spectroscopy, which simply measures the frequency dependence of dielectric constant, has been used to analyze battery materials. This is a powerful technique that can provide various insights, such as into the degradation of materials and the types of living cells. Normally, dielectric constant measurement detects the response of the entire volume between a pair of electrodes, so in order to map it in space, it is necessary to scan the sample with miniaturized electrodes, as is done with scanning probe microscopy. On the other hand, with time-resolved electron holography, the external field response of equipotential surfaces can be measured simultaneously and in parallel at each point in the sample, making mapping possible without scanning such probes. For example, in a sample consisting of electrodes and an ion conductor, any abnormalities in the electrode reaction can be identified without being overlooked in the image.

[0045] Since the sample used in a transmission electron microscope is a thin, sheet-like section with a thickness t through which electrons can pass, Consider the distribution of resistivity ρ, ρ(x, y), within the xy plane. Figure 13 shows the signal generator 1. A sinusoidal signal of frequency f is applied to the sample inside the electron microscope, and a current flows through the circuit passing through the sample. The electron microscope is equipped with a means 1301 for monitoring the current. This current monitoring means 1301 may be a measuring instrument placed outside the electron microscope, or it may be a resistor with a known resistance value R placed adjacent to the sample inside the electron microscope and photographed in the field of view of the hologram at the same time as the sample. In this case, the current is obtained by dividing the potential difference across the resistive element, measured by time-resolved holography, by R. At each point on the sample, a proportional relationship (Ohm's law) is assumed between the current density i and the potential gradient. To determine;

[0046]

number

[0047]

number

[0048] To describe a time-varying electromagnetic field, equations (1) and (2) are treated separately for common frequency components. Assuming that both the current density and potential have a time dependence of exp(2πj ft) (where j is the imaginary unit), then ρ at this frequency is a single complex number at each point. ρ(x, y) can be numerically determined from the boundary conditions of the sample in the xy-plane and the total current flowing through the observation area measured using an appropriate monitoring method. In other words, the complex resistivity at this frequency is determined at each point based on time-resolved observations of the potential surface. This can also be converted to complex permittivity and complex conductivity, and a mapping image can be created on the sample surface. By performing measurements at different frequencies, the frequency-dependent spectrum of the permittivity can be derived at each point, i.e., dielectric spectroscopy can also be performed.

[0049] The plate-shaped sample 1302 containing electrodes is modeled and treated as a discrete network 1303 with m rows × n columns, as shown in the lower part of Figure 13. The potential at the nodes of the network is given by measurement using electron holography. The values ​​of the resistances (more precisely, complex impedances) represented by rectangles and the currents flowing through each resistor are unknown. For large m and n, the number of resistors N is approximately N ≈ 2m × n Therefore, the total number of unknowns is approximately 4mn.

[0050] If the sample contains precipitates or other substances identified as having a known resistivity, it is not necessary to solve the network by treating their resistivity as an unknown. The current flowing through them can be directly determined from the potential difference across the ends measured by holography.

[0051] Even when it is difficult to numerically solve for network parameters, elements that act as current sensors or current monitors can be artificially created as follows. For example, by using a focused ion beam apparatus and replacing a portion of the sample with tungsten or carbon with known resistivity, a current monitor can be placed where needed.

[0052] The constraint equation for the parameter is, (Resistance) × (Current) = (Potential difference across the resistor) There are as many equations of this type as there are resistors. That is, approximately 2mn equations. This is not enough to cover the number of unknowns, but considering the continuity of the sample, for each resistance, (Resistance value) ≈ (Average value of nearby resistances) This constraint can be imposed. Since this constraint also applies to the total number of resistors, ~2mn, if we combine both... With approximately 4mn of these variables, the number of unknowns is comparable to the number of unknowns, and it is expected that the resistance value and current distribution can be determined. .

[0053] As a means of measuring the total current flowing through a sample, for example, a high-speed ammeter 1401, as shown in Figure 14, can be placed in the middle of a wire that electrically stimulates the sample from outside the microscope. This method has the advantage of not requiring space for a highly sensitive circuit device.

[0054] Furthermore, an element with a known resistance, such as a thin film of metal, can be placed in contact with the sample and used as a current monitor. For example, in Figure 14, the electrode thin film 1402 formed of a known metal material plays this role. That is, the potential difference across the ends of this thin film can be determined from electron holography, and the current value can be determined by dividing it by the resistance value. This is advantageous when phase shift in the transmission path from an ammeter placed outside the microscope to the sample is a concern, as it allows monitoring in close proximity to the sample.

[0055] If the sample contains precipitates or other substances identified as having a known resistivity, it is not necessary to solve the network by treating their resistivity as an unknown. The current flowing through them can be directly determined from the potential difference across the ends measured by holography.

[0056] Even when it is difficult to numerically solve for network parameters, elements that act as local current sensors or current monitors can be artificially created, such as the current sensor element 1403 shown in the sample in Figure 14. For example, by using a focused ion beam apparatus to replace a part of the sample with tungsten or carbon with known resistivity, current monitors can be placed where needed. [Examples]

[0057] In Example 1, a differential blanking method was described in which two types of pulses, long and short, are superimposed and supplied to a deflector used to deflect the electron beam and limit the exposure time. In this example, in order to bring the advantages of the differential blanking method closer to ideal, instead of superimposing both pulses on a single set of deflection electrodes, two sets of deflection electrodes are provided, and the long pulse and short pulse are supplied to two separate deflectors, respectively.

[0058] Figure 15 shows a schematic diagram of the first and second stage deflectors installed along the electron beam path. Since the two pulse systems for differential blanking are supplied to the deflection electrodes of separate stages, the pulse source outputs of both systems are interconnected, preventing damage to the output circuit of the other. Furthermore, impedance matching between the pulse generator and the deflector can be optimized separately for each of the two pulse systems, more completely suppressing voltage waveform distortion due to reflected waves. This allows for faster transitions between shutter opening and closing (electron beam ON / OFF). This enables improved exposure time resolution.

[0059] In Figure 15, both the first stage deflection electrodes 11 and 12 and the second stage deflection electrodes 21 and 22 are A configuration is depicted that is parallel to the xz plane and generates an electric field in the y direction, but the first stage and the second stage The directions of the two deflection electric fields do not necessarily have to be parallel. If a configuration is used in which the deflection electrodes are mounted on a rotatable support ring as shown in Figure 4 of Example 1, the direction of the deflection electric field is the z-axis ( It can be freely selected around the incident beam direction.

[0060] If the directions of the first and second stage deflection electric fields are chosen to be perpendicular, the electron beam can be deflected like a cathode ray tube oscilloscope, so that while one deflection electrode sweeps the electron beam's focus spot position in the x-direction on the camera, the other... By applying y-direction deflection with a deflection electrode, the speed of y-direction deflection can be determined based on the trajectory of the spot on the camera and the speed of the x-direction sweep. This is useful because it allows us to know the speed of deflection when the deflection electrode and cable are actually mounted on the microscope.

[0061] The electron microscope equipped with a two-stage electron beam deflector in this embodiment houses the deflector shown in Example 1. In this case, the two stages of the deflector are arranged adjacent to each other along the flow of the electron beam, but optical elements of the electron microscope, such as electron lenses and diaphragms, may be sandwiched between the first and second stages of the deflector. Alternatively, one of the two stages of the deflector may be a type of deflector that deflects the electron beam using a magnetic field. Electrostatic deflection is superior in terms of performance for deflection in a short time, but magnetic field deflection has the advantage of easily obtaining a large deflection angle. In particular, in the case of an electron microscope designed without anticipating the introduction of multiple electrostatic deflectors, the magnetic field deflector normally provided for optical axis adjustment can be used as one of the deflectors for differential blanking by superimposing a signal current for blanking on it.

[0062] Figure 16 shows a reconstructed image of the fluctuating electric field captured by stroboscopic exposure, which switches the electron beam ON / OFF using electrostatic deflection. As shown in the hologram on the left of the figure, two opposing tungsten probes were used as the observation target, and the potential of one probe (bottom of the figure) was modulated with a sawtooth waveform (±2 V) with a period of 100 μs, as shown in the inset graph. The hologram of the modulated sample region was captured using a stroboscopic exposure with a time window of 5 μs synchronized with the modulation period of the sample. The images were captured using robotic exposure. The exposure window has the timing relationship with respect to the modulated waveform as shown in circles 1 through 5 of the inset graph.

[0063] The phase images reconstructed from holograms captured at different timings are shown in Figure 16, labeled with circles 1 through 5. For comparison, Figure 17 shows the phase reconstruction image from a still image hologram captured by normal continuous exposure while the probe potential was kept constant without modulation.

[0064] As the probe's potential approaches 0 V from -2 V, the phase gradient (the "black" in the grayscale image) changes. The steepness of the change from "white" decreases, but as it subsequently increases towards +2 V, the phase gradient reverses, and the absolute value of the gradient becomes larger. This change reflects the potential difference between the probes, and this trend obtained from still images is also commonly observed in Figure 16, which shows a stroboscopic image of an electric field modulated at a frequency of 10 kHz, demonstrating that time-resolved observation by holography has been achieved.

[0065] By mounting two-stage deflectors on a common support mechanism, it becomes possible to introduce both deflectors into the electron microscope using only one opening, without requiring separate, dedicated openings for each deflector in the electron microscope housing. Furthermore, by mounting the electron beam deflector together with the electron diaphragm and its support mechanism, it becomes possible to introduce and install both the diaphragm and the deflector into the electron microscope through a single opening.

[0066] Figure 18 shows a schematic diagram of a device in which two electron beam deflectors are mounted on a common support mechanism. Normally, two openings are required, one above and one below, corresponding to the number of deflector stages, to introduce and install the deflector from the air into the vacuum inside the electron microscope. However, as shown in this figure, if two deflectors are mounted on a single support mechanism, only one opening is needed to introduce the support mechanism from the air into the vacuum. In Figure 18, the deflection electric fields of both the first and second stages are set to be in the y-direction. The deflection electrodes 11, 12, 21, and 22 are arranged as shown, but as already mentioned, the electrode plates may be arranged so that the deflection electric fields of the two stages are perpendicular to each other, or they may be held on a rotating support ring.

[0067] Figure 19 shows a schematic diagram of a device in which a focusing electron diaphragm and an electron beam deflector are mounted on a common support mechanism. Multiple electron diaphragms are used in an electron microscope, and the electron microscope housing has openings for the introduction of each. If an electron beam deflector consisting of deflection electrodes 11 and 12 is also mounted on the support mechanism of one of these diaphragms, it becomes unnecessary to provide a dedicated opening for that deflector. In other words, by introducing this support mechanism into a conventional or existing electron microscope, time-resolved exposure functionality can be added without major modifications to the housing.

[0068] Furthermore, for electron microscopes that already have a single-stage electron beam deflector, combining this deflector with an aperture allows for ideal differential blanking using a two-stage deflector. Electron microscopes have several apertures, including focusing apertures, objective apertures, and field-limited apertures. However, combining a deflector with a focusing aperture upstream of the sample has the advantage of making it easier to achieve blanking even with a lower voltage applied to the deflection electrode compared to other apertures. This is because a small beam displacement upstream is amplified by the imaging and magnification optics downstream of the sample, resulting in a large displacement on the camera plane.

[0069] While it is arbitrary whether the aperture plate (or aperture hole) or the deflection electrodes are placed upstream of the electron beam, placing the aperture plate upstream as shown in Figure 19 offers the following advantages: Since the narrow beam passing through the aperture hole is incident on the central part between the deflection electrodes, the beam is not scattered by the non-uniform electric field near the edge of the electrode plate and receives uniform deflection. In addition, the electrode support bases 15 and 16, made of insulating material, are covered by the conductive aperture plate, aperture guide, and support frame, so that they are not exposed to the electron beam, thus avoiding charging and the resulting image distortion.

[0070] According to this embodiment, it is no longer necessary to supply the output of a signal generator that generates two types of voltage pulses, long and short, to the same deflection electrode. This prevents unintended fluctuations in electrode potential due to interference between the two outputs and damage to the output circuits of both devices, and also makes it possible to optimize impedance matching separately for each pulse sequence. Furthermore, it is not necessary to create multiple dedicated apertures to install the deflector in the electron microscope, and in some cases, time-resolved observation can be achieved using only the number of apertures originally provided in the electron microscope. [Examples]

[0071] When performing electromagnetic field observations using electron beam interferometry while applying an electrical signal to a sample, the external circuits and wires supplying the signal to the minute sample of the electron microscope are large, and the field due to their potential extends to the space around the sample with a magnitude that cannot be ignored. Furthermore, the phase of the electron beam, which is the basis for electromagnetic field measurement, is cumulatively affected not only by the thin-piece sample but also by the entire field along the path the electron beam passes through, so the influence of the field formed by the external circuits in the wide space around the sample is serious.

[0072] To avoid such effects, conventional methods have involved electrostatically shielding areas around the sample that differ from the ground potential. Figure 20 schematically shows a shielding plate 2002, etc., in the center, which protects against an external field extending from electrode 1, which has a non-zero potential, to the electron path. The shielding plate 2002 needs to be insulated from electrode 1, and the grounding wire from the grounding electrode 2 to the shielding plate 2002 must be provided in a way that does not interfere with the electron beam along the way, requiring a difficult task.

[0073] This embodiment attempts to observe the sample under voltage application while suppressing disturbances to the electron beam by returning the signal from the external circuit to zero potential, by placing a switching element 2003, such as a diode with switching properties, near the sample 2001, as shown on the right side of Figure 20. In Figure 20, 2104 indicates a conductive pattern.

[0074] In a specific embodiment, the method involves observing the response of a sample 2001 to an applied voltage using an electron microscope image including an electron beam interference image, wherein the voltage is applied to the sample from an external circuit via a switching element 2003.

[0075] Furthermore, the switching element 2003 is formed on the sample support by microfabrication using a focused ion beam or the like. In addition, the sample support for observing electron microscope images including electron beam interference images has an electrical capacitance (capacitor) connected in parallel with the sample, and a voltage from an external circuit is applied to the sample 2001 and the electrical capacitance via the switching element 2003.

[0076] As a result, after applying a voltage to the sample from an external circuit, if the switching element 2003 is opened and the external voltage is returned to zero, the charge stored in the sample's own capacitance remains. When the field during this time period is observed using time-resolved electron holography, the potential field created by the external circuit is absent, and it becomes possible to observe only the field created by the charge distribution in the region from the switching element 2003 to the sample.

[0077] By forming the switching element 2003 near the sample on the support using microfabrication, the region of the charge distribution can be narrowed and localized, and the reference wave used for electron beam holography will not be disturbed and will be closer to a plane wave, thus enabling more quantitative analysis. Furthermore, as shown in the center and right of Figure 24, even if the sample is connected in parallel with a capacitance C1 that is larger than the capacitance C0 of the sample itself, and the external voltage is reduced to zero after charging via the switching element 2003, the sample will still have a charge distribution. The voltage charged to C1 continues to be applied. In other words, C1 is used as a minute voltage source near the sample, allowing the sample's response to voltage to be observed without being affected by the field created by the external circuit.

[0078] According to this embodiment, a means for observing the response of a sample to an applied voltage using electron beam interference imaging is provided, and the voltage can be applied to the sample from an external circuit via a switching element. Instead of the diode mentioned above, semiconductor elements such as transistors or MEMS (Micro Electro-Mechanical System) can be used as the switching element.

[0079] Next, Figures 21 to 23 will be used to describe specific configurations suitable for applying voltage to a sample via a switching element. Figure 21 shows a configuration for applying voltage to a sample via a switching element according to this embodiment 3, Figure 22 shows a cross-sectional view in which the switching element according to this embodiment is fabricated in a sample support, Figure 23 shows a planar view in which a capacitor is arranged in parallel with the sample according to this embodiment, and Figure 24 is an explanatory diagram of the operation using the equivalent circuit according to this embodiment.

[0080] The upper part of Figure 21 is a plan view, and the lower part is a cross-sectional view, showing the conductive material on the Si-N insulating film which is the supporting substrate. The linear pattern 2104 is powered via a ROHM diode element, RB751V40TE17, which acts as a switching element.

[0081] After applying a voltage to sample 2102 from an external circuit, even when the external voltage is returned to zero, the reverse current is blocked by the switching element 2103, such as a diode, so the charge stored in the sample's own capacitance remains. When the field during this time period is observed using time-resolved electron holography, it becomes possible to observe only the field created by the charge distribution in the region from the diode to the sample, without the potential field created by the external circuit. Note that the effect of leaving charge in sample 2102 by opening the switching element 2103 can also be obtained by placing the switching element on the ground side of the sample.

[0082] In particular, as shown in Figure 22, if the switching element is formed by microfabrication near the sample on the support, the charge distribution region can be narrowed and localized, and the reference wave used in electron holography will not be disturbed and will be closer to a plane wave, thus enabling more quantitative analysis.

[0083] While the range of microfabrication techniques is broad, including photolithography and electron beam lithography, using a focused ion beam (FIB) system makes it possible to implant ions into fine details. It is suitable for forming small switching elements.

[0084] An example of the procedure for forming the switching element 2103 in the sample support is explained using Figure 22. (1) Ion implantation is performed on the insulating support substrate so as to cover the disconnected area of ​​the metal wiring from the external circuit to the sample, thereby imparting conductivity to the support substrate. (2) A first metal deposition is carried out so as to cover one of the wirings and half of the ion implantation area. (3) Heat treatment is used to form an ohmic contact between the first metal deposit and the ion implantation region of the supporting substrate. (4) A second metal deposit is made on the ion implantation area so as to cover the wiring opposite the first metal deposit and the ion implantation area. A Schottky barrier is formed between this ion implantation area and the second metal deposit, which provides the diode effect.

[0085] As shown in Figures 23 and 24, in a sample support for electron microscope image observation including electron beam interference images, which has a capacitance (capacitor) connected in parallel with the sample, and in a configuration in which a voltage from an external circuit is applied to the sample and the capacitance via a switching element, a capacitance C1 that is larger than the capacitance C0 of the sample itself is connected in parallel with the sample, and a diode is used as the switching element. Even if the external voltage is reduced to zero after charging via the diode, the voltage charged to C1 continues to be applied to the sample. In other words, C1 is used as a minute voltage application source near the sample, and the sample's response to voltage can be observed without being affected by the field created by an external circuit that is larger than the sample.

[0086] In this embodiment, a capacitance C1, which is larger than the capacitance C0 of the sample itself, is connected in parallel with the sample. Even when the external voltage is reduced to zero after charging via a switching element, the voltage charged to C1 continues to be applied to the sample. In other words, C1 is used as a minute voltage source near the sample, and the sample's response to voltage can be observed without being affected by the field created by an external circuit that is larger than the sample.

[0087] The procedure for using the configuration of this embodiment, as well as its effects, will be explained using the circuit model shown in Figure 24. First, as shown on the left side of Figure 24, the area enclosed by the dashed line represents the equivalent circuit of a sample, such as an ion conductor, using parallel capacitance C0 and electrical resistance R0. First, the signal source A voltage V0 is applied through a closed switch, then the switch is opened, and the electromagnetic field of the sample is observed with the signal source output at zero potential. At this time, since the signal source output is zero, the field created by the external circuit and conductors around the sample is small and does not interfere with the observation. On the other hand, because the switch is open, the instantaneous outflow of charge accumulated in capacitance C0 to the zero-potential signal source side is suppressed, and the voltage across the ends of the sample is maintained for a certain period of time and then decays with a time constant τ = R0C0. The present invention's "method for capturing still images" allows for time-resolved observation of the electromagnetic field of a sample during the process. Thus, by opening the switch and then returning the signal source voltage to zero, observation becomes possible while the sample voltage is maintained to a certain extent, which is an advantage of the present invention.

[0088] In this embodiment, a capacitance C1 larger than the sample is connected as shown in the center of Figure 24, or as shown on the right side of the same figure. Alternatively, an electrical resistor R1 may be connected. Close the switch and apply a voltage V0 from the signal source. After opening the switch, return the output of the signal source to zero and observe the electromagnetic field of the sample. The charge charged to C0 and C1 from the signal source is discharged through the parallel resistors R0 and R1, so the time constant is given by the product of the parallel resistance and the parallel capacitance, as shown in Equation 3.

[0089]

number

[0090] The capacitance C0 of a sample can vary over a wide range depending on the material, shape, and size of the sample. For example, if a sample has C0 = 6 pF, a dielectric material with C1 of several tens of pF is held on the support. If R1 is not specifically defined (R1→∞), the time constant given by equation A is for the sample alone. This becomes ten times greater than in the previous case, making it possible to observe the response while slowly changing the voltage across the sample.

[0091] Using a focused ion beam apparatus, it is possible to create electrical resistances ranging from several orders of magnitude or more using fine metal deposits. This is convenient because a larger capacitor (C1) can be placed on the support, and the time constant of the voltage change can be adjusted by adjusting the value of R1 while checking the observation results.

[0092] The present invention is not limited to the various embodiments described above, but further includes various modifications. For example, the embodiments and modifications described above are described in detail for the purpose of clearly illustrating the present invention, and are not necessarily limited to those having all the configurations described. Furthermore, it is possible to replace a part of the configuration of one embodiment or modification with the configuration of another embodiment or modification, and it is also possible to add the configuration of another embodiment or modification to the configuration of one embodiment or modification.

[0093] The shutter mechanism described in the examples can be used in general for methods and apparatus for obtaining time-resolved frame images using an electron microscope. For example, it can be used for time-resolved electron holography observation of the magnetization process of magnetic materials, not for dielectric spectroscopy purposes. [Explanation of symbols]

[0094] 11,12,21,22 Deflection electrode 13,14,23,24 Conductor 15,16,25,26 Electrode support stand 30 support slots 40 Ground Bulkhead 50 Ground cover 60 aperture plate 61 Aperture Guide 101 Electron source 102 Deflection device 103 Samples 104 Current measuring device 105 Camera 106 Computers 107 Pulsed electron source 501 Interference fringes 601 Graph 701 Circuit 801, 901, 1101 Photoconductive Switches 802, 902, 1102 pulsed laser light source 1201 Current Response 1301 Current monitoring means 1302 Plate-shaped sample 1303 Discretization model of the sample 1401 High speed ammeter 1402 Metal electrodes with known resistivity and dimensions 1403 Current sensor element located inside the sample 2001 Sample 2002 Shielding plate 2003 Switching element

Claims

1. An imaging dielectric spectroscopy method using an electron microscope equipped with an interferometric electron optical system, By periodically stimulating the sample in the electron microscope with a repetition period T, and repeatedly exposing it on the imaging device for 1 / M time of the period T, synchronized with the period T, the interference image formed by the interference electron optical system is captured stroboscopically, interference images with different exposure delay times relative to the stimulation are obtained, and a reconstructed image is obtained from each of the interference images, thereby obtaining the frequency dependence of the complex dielectric constant. An imaging dielectric spectroscopy method characterized by the following:

2. The imaging dielectric spectroscopy method according to claim 1, The aforementioned delay time is varied in N ways to obtain N interference images with different time delays, a reconstructed image of each interference image is obtained, the electromagnetic field is determined, and the dynamic response of the electromagnetic field of the sample during the period T is observed as a series of N frame images. An imaging dielectric spectroscopy method characterized by the following:

3. The imaging dielectric spectroscopy method according to claim 1, The electron microscope has a deflection device for deflecting the electron beam from the electron source, A pulse is applied to the deflection device to turn on the irradiation of the electron beam. An imaging dielectric spectroscopy method characterized by the following:

4. The imaging dielectric spectroscopy method according to claim 1, A pulsed electron source is used as the electron source for the aforementioned electron microscope. An imaging dielectric spectroscopy method characterized by the following:

5. The imaging dielectric spectroscopy method according to claim 1, The aforementioned M is M > 50. An imaging dielectric spectroscopy method characterized by the following:

6. An imaging dielectric spectrometer using an electron microscope equipped with an interferometric electron optical system, An imaging device that periodically applies stimulation to a sample in the electron microscope at a repeating period, and repeatedly exposes it for 1 / M time of the period, synchronized with the period, thereby stroboscopically capturing the interference image formed by the interference electron optical system, and obtaining interference images with different exposure time delays in response to the stimulation, The system includes a signal processing device that obtains a reconstructed image from the interference image captured by the aforementioned imaging device, thereby obtaining the frequency dependence of the complex dielectric constant. An imaging dielectric spectrometer characterized by the following features.

7. The imaging dielectric spectrometer according to claim 6, The electron microscope has a shutter mechanism that is synchronized with the period and accumulates repeated exposures on the camera for a time of 1 / M of the period. An imaging dielectric spectrometer characterized by the following features.

8. An imaging dielectric spectrometer according to claim 7, The shutter mechanism has parallel plate electrodes, An imaging dielectric spectrometer characterized by the following features.

9. An imaging dielectric spectrometer according to claim 7, The system includes a signal generator that provides the aforementioned stimulus, and applies the generated signal to the sample via a current measuring device. An imaging dielectric spectrometer characterized by the following features.

10. The imaging dielectric spectrometer according to claim 6, The electron microscope is equipped with a pulsed electron source. An imaging dielectric spectrometer characterized by the following features.

11. The imaging dielectric spectrometer according to claim 6, The aforementioned M is M > 50. An imaging dielectric spectrometer characterized by the following features.

12. The imaging dielectric spectrometer according to claim 8, The pulse generator outputs to the parallel plate electrodes for a duration τ ２ In addition to supplying the exposure pulse, this τ ２ During the pulse duration of τ ２ Shorter τ １ A deflection potential can be applied to interrupt the exposure for a certain amount of time. An imaging dielectric spectrometer characterized by the following features.

13. The imaging dielectric spectrometer according to claim 8, The shutter mechanism is configured using two stages of electrostatic deflectors, each consisting of parallel plate electrodes. An imaging dielectric spectrometer characterized by the following features.

14. An imaging dielectric spectrometer according to claim 13, Long pulses and short pulses are supplied to separate stages of the two-stage electrostatic deflector, which consists of parallel plate electrodes. An imaging dielectric spectrometer characterized by the following features.

15. The imaging dielectric spectrometer according to claim 8, The shutter mechanism is equipped with two stages of deflectors using parallel plate electrodes, Since the two stages of the deflector are held on a common support base, the two stages of the deflector can be introduced through a single opening provided in the vacuum chamber of the electron microscope. An imaging dielectric spectrometer characterized by the following features.

16. The imaging dielectric spectrometer according to claim 8, The deflector, consisting of the parallel plate electrodes, is held on a common support base with the electron beam aperture. An imaging dielectric spectrometer characterized by the following features.

17. The imaging dielectric spectrometer according to claim 6, An imaging dielectric spectrometer comprising a switching element, wherein a voltage is applied to the sample from an external circuit via the switching element, and the response of the sample to the applied voltage is observed by an electron microscope image including the interference image.

18. An imaging dielectric spectrometer according to claim 17, An imaging dielectric spectrometer characterized in that the switching element is formed on a sample support by microfabrication using a focused ion beam or the like.

19. An imaging dielectric spectrometer according to claim 17, The sample support that supports the sample has a capacitance connected in parallel with the sample, and a voltage from the external circuit is applied to the sample and the capacitance via the switching element. An imaging dielectric spectrometer characterized by the following features.

Images