JP3482588B2 - Electron beam interference measurement system using biprism shift - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam interferometry system for quantitatively measuring information such as a thickness distribution of a sample, an electric field and a magnetic field by obtaining an interference image using an electron biprism.

[0002]

2. Description of the Related Art It was demonstrated in 1967 that holography can be performed by using an electron beam. However, a measuring method using the electron beam holography is a practical method that exhibits the characteristics of the resolution expected by using the electron beam. In order to be successful, the development of technology to obtain an electron beam with good coherence and high brightness, and the interference distance of the electron beam, that is, the area where the interference fringes occur are very small, so a very thin and uniform filament is created. It was necessary to develop the used biprism. With the successful development of the field emission electron gun and the successful development of the electron beam biprism having the characteristics as described above, the usefulness of using the electron beam holography for measurement has been proved and made practical. Then, topographical (contour-line) measurement of fine particles, direct visual observation of domain structure of magnetic material, dynamic observation of magnetic flux quantum trapped in a superconductor, and experimental study of Ahalano-Fouboom (AB) effect. The application of electron holography to confirmation has popularized the use of electron holography.

Further, it is also important to be able to holographically observe a weak magnetic field generated from an observation sample having a weak phase such as a biological sample and fine particles in a single region. In addition, a method of narrowing the stripe pattern of the hologram has been proposed in order to increase the spatial resolution, but this method has a disadvantage that a fine particle photographic film or a high pixel TV (CCD) camera is required. A phase-shifted electron holography method has been proposed that can determine the wavefront of the observed object regardless of the spread of the striped pattern (Reference: "Ultramicroscopy" Vol. 55 (1994) 209-220).
Q.Ru, G.Lai, K.Aoyama, J.Endo, A.Tonomura). This method uses a precisely controlled tilted electron beam for the same observation sample to create a series of three or more holograms in which the incident angle of the electron beam is changed, and these series of holograms are simply numerically calculated. It can be analyzed to determine the wavefront of the observed object. In this method, the angle of the tilted electron beam used for observation has an initial phase of exactly 2π (or more)
The above-mentioned series of holograms are created by shifting the holograms until they are shifted, that is, the hologram obtained by using the electron beam whose initial phase is shifted by 2π moves exactly by the stripe width with respect to the initially obtained hologram. It will be what you did.

Expressing this mathematically, S = Wtan θ≈Wθ initial phase φ0 = (2π / λ) S = (2π / λ) Wθ (where S = path difference, W = interference region width, θ = incident) Horn,
λ = wavelength of electron beam) Therefore, the incident angle θ corresponding to the initial phase shift of 2π
₀ = λ / W. The obtained hologram intensity data I, the initial phase data φ _0, and the wavefront of the reproduced observation object are I (x, y, n) = a (x, y) + b (x, y) cos [2πx / Tx + 2πy. / Ty + φ (x, y) + φ ₀ (n)] (1). Where a (x, y) is the background contrast proportional to the microscope intensity of the observed sample, and b
(X, y) is the fringe contrast proportional to the amplitude distribution of the observed object wave, φ (x, y) is the phase images Tx and Ty, the fringe spread in each direction, and n = a series of holograms. Therefore, the wavefront of the observed object can be reproduced by the numerical processing of the above equation (1).

With the establishment of a method of reconstructing the wavefront of an observed object from a series of holograms, any defect or stain is needed because the object wavefront at any point can be reconstructed from the intensity data at those points. Since it does not spread over the area, noise in measurement can be reduced. However, the numerical processing according to the equation (1) is based on the premise that a series of holograms obtained by the tilted electron beam is created under a fixed condition that is not out of focus. When the focus is off, the tilted electron beam causes a change in the image. Therefore, when trying to measure with atomic level resolution, it is impossible to perfectly focus due to spherical aberration, and there is a problem that the above assumption is broken. In addition, because the tilted electron beam may cause black diffraction on the plane of the atoms to change the image of the atomic array in the sample (especially when the atoms are multi-layered), the resolution at the atomic level is There was a problem that it was difficult to obtain.

[0006]

Therefore, according to the present invention, a series of initial phase-shifted holograms that can be processed by the numerical analysis described above can be produced without changing the image of the atomic sequence, and the hologram of atomic level can be obtained. The aim is to establish electron beam interferometry with improved resolution and improved sensitivity.

[0007]

SUMMARY OF THE INVENTION The gist of the invention of claim 1 of the present application is that a voltage applied to a piezo element is set by placing an electron beam biprism on a tube piezo element across an opening of the tube piezo element through an insulator. Is sequentially changed by a control parameter to move the biprism in a direction perpendicular to the wire axis and the optical axis of the incident electron beam, thereby sequentially changing the initial phase in the interference pattern formed by the lower lens, This is an electron beam interferometry system by biprism shift that captures sequentially changed holograms as frame images and mathematically analyzes the captured series of holograms to obtain the phase distribution.Control of voltage applied to the piezo element Based on the average fringe width obtained from the hologram obtained in advance, the parameter is set to 2 as the initial phase shift. Determine the maximum amount of voltage becomes higher,
It is an electron beam interferometry system by biprism shift, which is characterized by setting the increment of the voltage amount applied to the piezo sequentially and the length of time that the piezo element is at each voltage by inputting it to a computer. The gist of the invention of claim 2 is to execute the shift of the initial phase by the control parameter by setting a set value of the length of time that the piezo element is at each voltage and a scan signal of a synchronization signal of the television camera. An electron beam by a biprism shift, wherein the scan signal of the synchronizing signal of the television camera is prioritized, and the execution / stop of the shift is automatically corrected by replacing it with an approximate value of the control parameter. It is an interferometric measurement system.

According to the present invention, the electron beam biprism is moved in the direction perpendicular to the wire axis of the biprism and the optical axis of the incident electron beam until the total shift of the initial phase becomes 2π or more. The holograms are sequentially shifted to create three or more holograms having different initial phases. With this method of changing the initial phase, the angle of the electron beam with respect to the observation sample does not change even if the initial phase is changed. The above problems caused by changing the incident angle of the line and changing the initial phase can be theoretically eliminated. By the way, in order to realize the method, in the direction perpendicular to the wire axis of the electron beam biprism and the optical axis of the incident electron beam,
Precise means for sequentially moving the initial phase until the total shift becomes 2π or more is required.
It is necessary to automate the creation of a series of holograms, aligned with the means for capturing the holograms at each initial phase. Therefore, the present inventors have placed the electron beam biprism on the tube piezo element via an insulator so as to cross the opening, that is, so that the wire of the biprism is in the diameter direction of the opening, and the piezo element A predetermined voltage is applied to the piezoelectric element to generate distortion in the piezoelectric element to realize precise movement of the electron beam biprism.

The hologram intensity data obtained by moving the electron beam biprism and shifting the initial phase shifts x in the equation (1) relating to the shift of the initial phase to x-nΔx. Therefore, if this is substituted into the equation (1), I (x, y, n) = a (x, y) + b (x, y) cos [2π (x−nΔx) / Tx + 2πy / Ty + φ (X, y)] (2), and the equation (2) is as follows: I (x, y, n) = a (x, y) + b (x, y) cos [2πx / Tx + 2πy / Ty + φ (x, y) ) -NΔx / Tx] (3) can be transformed to -nΔx / Tx in the expression (1) by φ ₀ (n)
Is treated numerically as a term corresponding to, and the wavefront of the reconstructed observation object can be obtained by the numerical processing described in the above document.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail. Electron beam biprism B. P is a tube piezo element P.P. Z
(Manufactured by JEOL Ltd.) on top of the insulating layer.
The tube piezo element has an electrode P.P. for applying a predetermined voltage, for example, −130 v to 130 v to the element. Z. E and the earth electrode Et inside the tube piezo element are attached. When a voltage is applied to the piezo element, the element causes distortion in response to the voltage, which causes the biprism to be in atomic order (for example, in 0.1 nm increments or continuously) in a direction perpendicular to the axis (x direction in the figure). Shift to. The maximum voltage applied to the piezo element is the voltage at which the interference fringe shifts the biprism by that width (by 2π in the unit of initial phase), and the voltage is calculated by calculating the average fringe width from the hologram created in advance. Change in the amount of voltage applied to the piezo in sequence because it is necessary to take three or more holograms that are determined based on the width and sequentially shifted (or continuously shifted) until the maximum charge is reached for numerical analysis. (Corresponding to ΔV in VE in the figure) and the length of time that the piezo element is at each voltage (Δt in VE in the figure)
Is input to the computer as a control parameter for controlling the piezo element by the computer, and the computer gives priority to the scanning time required to capture an image of one frame by the video camera (CCD camera) and controls the piezo element. Provided is an electron beam interferometry system that corrects parameters and controls a piezo element.

[0011]

【Example】

Example 1 Using a 200 kv electron beam (λ = 0.0025 nm),
The electron beam biprism is set to a voltage-shift (200
The electron beam interference device was assembled by mounting it on a tube piezo element equipped with an electrode connected to a voltage variable power supply controlled by a control parameter input to a computer having a (nm) characteristic. From the hologram obtained in advance, the average fringe width is 0.2 nm in terms of the sample surface, so the maximum voltage is set to 10 V and the voltage amount for taking 100 frame images on the electrodes attached to the tube piezo element. Input the change step ΔV = 0.1v, and take the scanning speed of the camera into consideration, the length of time Δt = 0.1 at each voltage.
The seconds were entered to set the control parameters. The device was started, the electron beam interference image was converted from the fluorescent plate, and this was taken by a TV camera with a scanning speed of 1/30 seconds and taken into a frame memory to obtain 100 holograms. When a series of initial phase holograms are lined up with their stripe directions aligned,
As shown in FIG. 2, a hologram with a striped pattern shifted was obtained. A reproduced image was prepared by a known method and numerically analyzed, and the wavefront of the sample was reproduced. For the sensitivity, for example, a phase difference of about 2π / 100 can be measured.

[0012]

INDUSTRIAL APPLICABILITY Since the initial phase can be shifted without changing the image of the atomic array in the sample due to black diffraction on the atomic plane, the resolution is improved to the atomic level and the sensitivity is improved. Was also able to be greatly improved. Therefore, it can be expected that the field in which the electron beam interferometry can be applied is greatly expanded.

[Brief description of drawings]

FIG. 1 shows a system for automatically creating a series of holograms with an initial phase shift by placing a biprism on a tube piezo element.

FIG. 2 shows a series of holograms with an initial phase aligned with the directions of stripe patterns.

[Explanation of symbols]

E. B electron beam S. P sample 0. L Objective lens B. P biprism P. Z tube piezo element P.P. Z. E Electrode for applying biprism shift voltage to the tube piezo element Et Opposed earth electrode V. of electrode for applying biprism shift voltage to the tube piezo element E Voltage switching power supply S. S sync signal I.S. I interference image V. D video camera (CCD camera) F.D. M frame memory C. P Computer K. B keyboard

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-2-298983 (JP, A) JP-A-3-163400 (JP, A) JP-A-10-208684 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G03H 5/00 H01J 37/26

Claims (2)

(57) [Claims] 1. An electron beam biprism is mounted on a tube piezo element via an insulator so as to traverse an opening of the tube piezo element, and a voltage applied to the piezo element is sequentially changed by a control parameter so that the biprism is connected to a wire axis and By moving in a direction perpendicular to the optical axis of the incident electron beam, the initial phase in the interference pattern formed by the lower lens is sequentially changed, and the hologram in which the initial phase is sequentially changed is captured as a frame image Is a biprism shift electron beam interferometry system that numerically analyzes the hologram to obtain the phase distribution, wherein the control parameter of the voltage applied to the piezo element is the average fringe width from the hologram obtained in advance. The maximum voltage amount at which the initial phase shift is 2π or more is determined based on the An electron beam interferometry system using a biprism shift, characterized in that a step of a change in voltage applied to a child and a length of time each piezoelectric element has at each voltage are input to a computer and set. 2. The execution of the shift of the initial phase according to the control parameter, the set value of the length of time that the piezo element is at each voltage and the scan signal of the synchronizing signal of the television camera, 2. The biprism shift according to claim 1, wherein the scan signal of the synchronization signal of 1 is given priority, and the execution / stop of the shift is automatically corrected by replacing it with the approximate value of the control parameter. Electron beam interferometry system.