An apparatus for measuring electromagnetic field distribution using a focused electron beam

Abstract: A disclosure is made on an apparatus for measuring an electromagnetic field distribution using a focused electron beam which can measure the electromagnetic field distribution in a specimen with a high resolution and a high reliability. The present apparatus comprises: a focused electron beam radiation system for irradiating a specimen with a focused electron beam; a specimen tilt mechanism for tilting a specimen by 180° with a tilt axis being perpendicular to the optical axis of the focused electron beam; an electron beam position detector for measuring the direction and the quantity of the deflection given to the focused electron beam when it is transmitted through the specimen; and a processing system for calculating the direction and the intensity of an electric field, and the direction and the intensity of a magnetic field separately at a point on the specimen through which the focused electron beam is transmitted from the data on the direction and the quantity of the deflection of the focused electron beam measured by the electron beam position detector before and after the turnover of the specimen by the specimen tilt mechanism. Owing to the present apparatus, an electric field and a magnetic field in a specimen can be separately observed independent of each other.

Description: The present invention relates to an apparatus for measuring an electromagnetic field distribution using an electron beam as a probe, in particular, it relates to an apparatus for measuring an electromagnetic field distribution in an extremely small area using a focused electron beam. A differential phase contrast method as a method for measuring an electromagnetic field distribution in an extremely small area inside a magnetic thin film is known to the public, in which a specimen is irradiated with a focused electron beam and the direction and the magnitude of a deflection (Lorentz deflection) of the beam caused by the magnetic field in the specimen when the beam is transmitted through the specimen is detected by an electron beam position detector. In this method, since an electron beam can be focused to a very fine beam (diameter: in the order of several nanometers), a magnetic field distribution can be measured with a very high resolution by scanning a specimen with the focused electron beam with a high precision. The differential phase contrast method is explained in, for example, "Materials Science and Engineering, B3 , 355-358 (1989)". In an apparatus for measuring a magnetic field distribution using the above-mentioned differential phase contrast method, a deflection phenomenon of an electron beam caused by a magnetic field is utilized, but the deflection of an electron beam is caused by not only a magnetic field but also by an electric field. Therefore, in a case where the influence of an electric field is large, or where a detailed magnetic field distribution is to be investigated without ambiguity, it is necessary to make measurement in separating the effect of an electric field and that of a magnetic field. In general, when a direction of incidence of an electron beam onto a specimen is reversed, in the Lorentz force given to an electron beam by an electromagnetic field, the direction of a force given by the electric field is not changed but the direction of a force given by the magnetic field is reversed. Therefore, it is made possible to find out an electric field distribution and a magnetic field distribution separately at a point at which the electron beam is transmitted by the combined analysis on the direction and the magnitude of a deflection of the electron beam caused by the Lorentz force when the electron beam is transmitted through a specimen in one direction, and on the direction and the magnitude of a deflection of the electron beam when it is transmitted through the specimen in the opposite direction to the above. It is described, for example, in the Physical Review, B34 , 3397-3402 (1986), that the separated measurement of an electric field and a magnetic field based on the principle as described in the above is actually possible. In the above case, after a specimen is observed in a direction in a holographic interference microscope which can detect the deflection of an electron beam caused by a Lorentz force, the specimen is once taken out of the microscope and turned over, and then it is put back to the microscope again for observation. The separation of the contribution of an electric field from that of a magnetic field is performed by comparing the observation results obtained before and after the turnover of the specimen. In the method as described in the above, however, a pretty long time is needed for the observation and besides there occurs a problem such as the shift of the observation area between the 2 cases, before and after the turnover of the specimen. For example, it is one of the methods for reversing the direction of incidence of an electron beam onto a specimen being different from the above-mentioned method to prepare two sets of measuring systems being composed of two electron sources and two electron beam position detectors, which are disposed to have opposite directions of incidence of electron beams onto a specimen and these systems will be used being properly selected. In the method, however, the apparatus is naturally made bulky and expensive, and further the operation is made complicated; the apparatus cannot be considered to be good for a practical use. An object of the present invention is to offer an apparatus for measuring an electromagnetic field distribution with a focused electron beam which is able to measure the electric field distribution and the magnetic field distribution of a specimen separately with a relatively simple method and to measure the magnetic field distribution of an extremely small area in a specimen with a relatively high precision and reliability. In the present invention, in order to achieve the above-mentioned object, a set of a measuring system composed of an electron source and an electron beam position detector is used; in the measuring system, a mechanism for tilting a specimen by 180° to overturn the specimen with a tilt axis (y axis) being perpendicular to the optical axis (z axis) of the electron beam emitted from the electron source and also being in-plane with the specimen is provided, and an arrangement is prepared to be able to measure a point in a specimen in 2 directions, from the surface and from the back, with an identical electron beam. The direction and the quantity of the deflection for the optical axis (z axis) of an electron beam which is transmitted through an identical point in the specimen before and after the overturn of the specimen are measured with an electron beam position'detector provided under the specimen; thereby, the intensity and the direction of an electric field, and those of a magnetic field at the measurement point can be separately measured respectively. The specimen is scanned with the above-mentioned focused electron beam and the intensity of the transmitted electron beam through the specimen is detected and a scanning transmission electron microscope image of the specimen is obtained and displayed; at the same time the quantity and the direction of the deflection of the electron beam which is transmitted through the respective points of the specimen are detected and the electromagnetic field distribution in the specimen is obtained and it is also displayed as a scanning image. Further, when a specimen is turned over, sometimes, it is difficult to judge whether the measured points are an identical point or not; therefore, to facilitate the judgment, more than three markers are preset being disposed inside the observation field of the above-mentioned scanning transmission microscope image, and capable of being clearly confirmed that these points before and after the turnover of the specimen are identical points. A data processor is provided which has the functions to identify the measured points before and after the turnover of a specimen and to operate the electromagnetic field distributions at respective measurement points and output the operation results. The operation of an apparatus having a constitution according to the present invention will be explained in the following. At first, the intensity of an electric field and that of a magnetic field at respective measurement points on the specimen are divided into parallel components (component in y axis direction) being parallel to the specimen tilt axis (y axis) and perpendicular components (component in x axis direction), and the influence of deflection forces (Lorentz force) of respective components on the focused electron beam will be considered. The directions of the Lorentz forces by the components (components in y axis direction) parallel to the specimen tilt axis (y axis) of an electric field and a magnetic field are explained as follows: the direction of the Lorentz force by an electric field is parallel (y axis direction) to the specimen tilt axis (y axis) and that by a magnetic field is perpendicular (x axis direction) to the y axis. The directions of the Lorentz forces by the components (x axis direction) perpendicular to the specimen tilt axis of an electric field and a magnetic field are explained as follows: the direction of the Lorentz force by an electric field is perpendicular (x axis direction) to the specimen tilt axis and that by a magnetic field is parallel (y axis direction) to the specimen tilt axis. Therefore, Lorentz force which is parallel (y axis direction) to the specimen tilt axis is the resultant force of a force caused by an electric field component which is parallel to the specimen tilt axis (electric field component in the y axis direction) and a force caused by a magnetic field component which is perpendicular to the specimen tilt axis (magnetic field component in x axis direction). Similarly, Lorentz force which is perpendicular to the specimen tilt axis (x axis direction) is the resultant force of a force caused by an electric field component perpendicular to the specimen tilt axis (electric field component in x axis direction) and a force caused by a magnetic field component parallel to the specimen tilt axis (magnetic field component in y axis direction). When the specimen is turned over, the direction of an electric field component parallel to the tilt axis of the specimen (electric field component in y axis direction) is kept unchanged before and after the turnover of the specimen, so that in a combined Lorentz force which is generated in the direction of the specimen tilt axis (y axis direction), the direction of a component force caused by an electric field is kept unchanged before and after the turnover of the specimen, but the direction of a component force caused by a magnetic field which is perpendicular to the specimen tilt axis (magnetic field component in x axis direction) is reversed before and after the turnover of the specimen, so that in a combined Lorentz force which is generated in the direction of the specimen tilt axis (y axis direction), the direction of a component force caused by the magnetic field is reversed. In contrast to this, the direction of an electric field component which is perpendicular to the specimen tilt axis (electric field component in x axis direction) is reversed before and after the turnover of the specimen, so that in a combined Lorentz force which is perpendicular to the specimen tilt axis (x axis direction), the direction of a component force caused by an electric field is reversed before and after the turnover of the specimen, but the direction of a component force caused by a magnetic field component which is parallel to the specimen tilt axis (magnetic field component in y axis direction) is kept unchanged before and after the turnover of the specimen, so that in a combined Lorentz force which is generated in the perpendicular direction to the specimen tilt axis (x axis direction), the direction of a component force caused by the magnetic field is kept unchanged before and after the turnover of the specimen. Therefore, when the Lorentz force which is in the direction parallel to the specimen tilt axis (y axis direction) is measured before and after the turnover of the specimen, the Lorentz force component caused by the magnetic field can be separately obtained by finding the difference between the Lorentz force data obtained before and after the turnover of the specimen, and the Lorentz force component caused by the electric field can be separately obtained by finding the summation of the Lorentz force data obtained before and after the turnover of the specimen. Similarly, when the combined Lorentz force which is perpendicular to the specimen tilt axis (x axis direction) is measured before and after the turnover of the specimen, and the Lorentz force component caused by an electric field can be separately obtained by finding the difference between the data of the combined Lorentz forces obtained before and after the turnover of the specimen, and the Lorentz force component caused by a magnetic field can be separately obtained by finding the summation of the data of the combined Lorentz forces obtained before and after the turnover of the specimen. The combined Lorentz force caused by an electromagnetic force can be obtained from the direction and the quantity of a deflection of an electron beam which is transmitted through a specimen. The direction and the quantity of a deflection of an electron beam can be measured quantitatively by an electron beam position detector which is provided under the specimen, so that according to the above-mentioned principle an electric field component and a magnetic field component in respective directions can be separately obtained by measuring the direction and the quantity of a deflection of an electron beam before and after the turnover of a specimen. The distribution of an electromagnetic field can be displayed as a scanning image together with the scanning transmission electron microscope image of a specimen by scanning the specimen with a focused electron beam. Owing to this, the shape of a specimen and a state of distribution of an electromagnetic field of a specimen can be observed in comparing them. When markers are provided at more than three positions on a specimen, identification of the positions of measuring points when a specimen is turned over can be made certain by utilizing these markers as reference points. Further, the shape of a specimen and the distribution of an electromagnetic field in the specimen can be automatically observed simultaneously by providing a data processor which has functions to identify the measurement points and to operate the electromagnetic field distribution at the measurement point and output the operation results. In the accompanying drawings: Fig. 1 is a schematic structural view showing the outline of the constitution of an apparatus for measuring an electromagnetic field distribution according to the present invention. Fig. 2 is an illustrative view for explaining the principle of the measurement of an electromagnetic field distribution according to the present invention. Example embodiments of the invention will be explained in detail referring to the drawings in the following. Fig. 1 shows the outline of the constitution of an embodiment of an apparatus for measuring of an electromagnetic field distribution using a focused electron beam according to the present invention. The constitution and the operation of the apparatus according to the present invention will be explained using Fig. 1 in the following. In Fig. 1, an electron beam 2 which is output from an electron source 1 composed of an electron gun and an electron accelerating system is focused on a specimen 4 by an illumination electron optics 3 and used for scanning a specimen. The electron beam 2 is transmitted through the specimen 4 composed of a thin film having a thickness through which the electron beam 2 can be transmitted, and reaches a deflection detector 5 through a projection electron optics. Therefore, an apparatus in the present embodiment is substantially a scanning transmission electron microscope (STEM), and a scanning transmission electron microscope image (STEM image) of the specimen 4 can be obtained. The specimen 4 is held with a specimen holder 8 which is provided with a bi-axial image shift mechanism 6 for selecting an observation area and a tilt mechanism 7 which is able to tilt the specimen 4 by 180°. The tilt mechanism 7 is so adjusted that the tilt axis is always positioned in the center of the scanning area on the specimen not to let a scanning image being observed go outside the observation area when the specimen 4 is turned over. The deflection detector 5 has a function to detect the direction and the magnitude of a deflection of the electron beam 2 which is transmitted through the specimen 4. In order to realize the function, the present embodiment comprises a constitution in which, as shown in Fig. 1, 4 independent detectors are disposed to have respective boundaries to make right angles with each other. When the electron beam 2 is not deflected at all by the specimen 4, the projection electron optics 9 is so adjusted that even if the specimen 4 is scanned with the electron beam 2, the electron beam 2 is always incident onto the center of the deflection detector 5 (the point of intersection of boundaries of the 4 independent detectors making right angles with each other). Since the electron beam 2 which is incident onto the deflection detector 5 has a circular profile, the direction and the magnitude of the deflection of the electron beam 2 can be obtained by processing the signal intensity of the electron beam 2 distributed to the above-mentioned 4 independent detectors. Hereinafter the center of the circular profile of the electron beam 2 will be referred to as a deflection point of the electron beam 2. In this place, the deflection detector 5 having four detector portions is used, but it is also possible to use deflection detectors based on the other principles such as a semiconductor position detector or a micro channel plate. Next, the principle for measuring the intensity of an electric field and the intensity of a magnetic field separately according to the present invention will be explained using Fig. 2. It is assumed tentatively that an electromagnetic field vector 10 which exists at an observation point on the specimen 4 is perpendicular to the specimen tilt axis 11 as shown in Fig. 2(a). Here in this place, the direction of the electromagnetic field vector 10, the direction of the specimen tilt axis 11 and the direction of a current of the electron beam 2 incident onto the specimen 4 are respectively designated to be x axis, y axis and z axis. Generally, when the projection electron optics 9 is constituted with a magnetic lens, the electron beam 2 is rotated centering the optical axis, but the quantity of rotation can be completely corrected, so that coordinate axes x, y and z can be set on the deflection detector 5 having the same directions as those on the specimen plane. In this case, the direction of the deflection of the electron beam 2 is, as shown in Fig. 2(a), in the negative direction of x axis when the electromagnetic field vector is an electric field vector, and it is in the positive direction of y axis when the electromagnetic field vector is a magnetic field vector. Next, when the specimen 4 is turned over centering the specimen tilt axis 11, as shown in Fig. 2(b), the direction of the deflection of the electron beam 2 is in the positive direction of x axis when the electromagnetic field vector 10 is an electric field vector, and when it is a magnetic field vector the direction of the deflection is in the negative direction of y axis. When the electromagnetic field vector 10 is parallel to the specimen tilt axis 11, the direction of the deflection of the electron beam 2 is not changed by the turnover of the specimen 4 independent of the kind of the electromagnetic field vector, whether it is an electric field vector or a magnetic field vector. Therefore, when it is assumed that the deflection points of the electron beam 2 on the detection plane of the deflection detector 5 before and after the turnover of the specimen are respectively (X1, Y1) and (X2, Y2), the components of the electric field and a magnetic field, Ex, Ey and Bx, By, being in-plane with the specimen are as shown below: Ex = e(-X1 + X2), Ey = e(-Y1 - Y2), Bx = b(+Y1 - Y2), By = b(-X1 - X2), where e and b are positive integers, and they can be found by the measurement of a standard specimen of which the magnitude of the electromagnetic field is known. When the electromagnetic field vector 10 is 0, the deflection point of the electron beam 2 is positioned at the origin (X = 0, Y = 0). As described in the above, in the present invention, the deflection points of the electron beam 2 which is transmitted through the same observation point on the specimen from the directions opposite to each other are compared by comparing the scanning images of the specimen before and after it is turned over. Therefore, the identification of the observation points in the scanning image before and after the turnover of the specimen has to be performed with certainty. Actually however, various kinds of position corrections are needed because of the occurrences of facts such as no constitutional distinct feature can be found in the objective observation area, tilting precision of the specimen 4 is not enough, or there is distortion in a scanning image. In the present embodiment, the procedure of an observation point correction as shown below is performed by an image processor 12. At least 3 markers which can be confirmed to be identical points respectively before and after the turnover of the specimen will be provided on the specimen 4 inside the observation area of the scanning image. A micro grid for holding a thin film specimen, a part having a distinct shape in the thin film specimen itself, etc. can be used as markers. There is another way: before the specimen is turned over three points which do not interfere the observation will be selected in the desired observation area, and these points will be irradiated for a proper duration of time with the electron beam 2 to form deposit spots at the selected points; these spots can be utilized as markers. A method for identifying the observation points in a scanning image before and after the turnover of a specimen using these markers will be explained. Let the coordinates of these markers before the turnover of the specimen be (x1, y1), (x2, y2) and (x3, y3), and let those after the turnover of the specimen be (xa, ya), (xb, yb) and (xc, yc) in the same order. Let the coordinates of an arbitrary point on the scanning image before the turnover of the specimen be, (x, y), and let those of the point after the turnover of the specimen be, (xi, yi). Then (xi, yi) can be expressed using the coordinates of the markers as shown below: xi = xc + A(x - x3) + B(y - y3), yi = yc + C(x - x3) + D(y - y3), then, xα = x1 - x3, yα = y1 - y3, xβ = x2 - x3, yβ = y2 - y3, xγ = xa - xc, yγ = ya - yc, xδ = xb - xc, yδ = yb - yc, E = (xα·yβ - yα·xβ)(yα·xβ - xα·yβ), thus, A = {xγ(yα·xβ·yβ - xα·yβ 2 ) + xδ(xα·yα·yβ - yα 2 ·xβ)}/E, B = {xγ(xα·xβ·yβ - yα·xβ 2 ) + xδ(xα·yα·xβ - xα 2 ·yβ)}/E, C = {yγ(-yα·xβ·yβ - xα·yβ 2 ) + yδ(xα·yα·yβ + yα 2 ·xβ)}/E, D = {yγ(xα·xβ·yβ + yα·xβ 2 ) + yδ(-xα·yα·xβ - xα 2 ·yβ)}/E. As described in the above, the identification of the measurement points before and after the turnover of the specimen 4 can be automatically performed. The intensity of an electric field and the intensity of a magnetic field at a measuring point can be separately obtained; in the present embodiment, all kinds of operation are performed by the image processor 12, and the results, together with the scanning transmission electron microscope image of the specimen, are displayed being visualized by image display methods such as the vectorial plots of the electric field and the magnetic field, the contour plots of intensity, and the gray scale presentation of each component. As explained in the above, in an apparatus for measuring an electromagnetic field distribution using a focused electron beam according to the present invention, it is possible to measure the distribution of an electric field and the distribution of a magnetic field separately and to decide the distribution of an electric field and that of a magnetic field in an extremely small area without ambiguity, and the states of respective distributions, together with a scanning transmission electron microscope image having a high resolution, are visualized and displayed.

Claims: An apparatus for measuring an electromagnetic field distribution using a focused electron beam comprising: a focused electron beam radiation means (1) for irradiating a specimen (4) with an electron beam (2); a specimen tilt mechanism (7) for turning-over said specimen by 180° in respect to a tilt axis being perpendicular to the optical axis of said focused electron beam (2); an electron beam position detector (5) for measuring the direction and magnitude of the deflection given to said focused electron beam (2) when it is transmitted through said specimen (4); and an operation means (12) for calculating the direction and intensity of an electric field and the direction and intensity of a magnetic field separately at a point on said specimen (4) through which said focused electron beam (2) is transmitted from the measurement data of the direction and magnitude of the deflection of said focused electron beam (2) measured by said electron beam position detector (5) before and after the turnover of said specimen (4) performed by said specimen tilt mechanism (7). An apparatus for measuring an electromagnetic field distribution using a focused electron beam according to claim 1, further comprising: a means (3) for scanning said specimen with said focused electron beam (2); and a means (12) for displaying a scanning transmission electron microscope image of said specimen (4) by measuring the intensity of said focused electron beam (2) transmitted through respective scanning points on said specimen (4). An apparatus for measuring an electromagnetic field distribution using a focused electron beam according to claim 1, further comprising: a means (3) for scanning said specimen with said focused electron beam (2); and a means (12) for displaying an electromagnetic field distribution in said specimen (4) as a scanning image by measuring the direction and magnitude of the deflection for said optical axis given to said focused electron beam (2) when it is transmitted through respective scanning points on said specimen (4).

Inventors: HOSOKI SHIGEYUKI, ICHIKAWA MASAKAZU, TAKAHASHI YOSHIO, YAJIMA YUSUKE, HOSOKI, SHIGEYUKI, ICHIKAWA, MASAKAZU, TAKAHASHI, YOSHIO, YAJIMA, YUSUKE
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: G01R31
IPC codes: G01R31, G01R33, G01N23