The present invention relates to an electron beam biprism device, an electron microscope in which the electron beam biprism device is used, and an electron beam device.
The electron beam has a large interaction with substances and is used various measurements as a probe, such as a structural analysis of a substance by means of electron diffracted image observation, electron micrograph observation, etc. and an element analysis of a substance by electron spectroscopy (energy analysis of an electron beam after transmission of a specimen). The electron beam devices that serve these purposes utilize a characteristic that the electron beam interacts with an electric field and a magnetic field, and use electron lenses for imaging, deflectors for controlling a propagation azimuth of the electron beam, the electron biprisms for splitting the electron beam and making them interfere with each other, etc. These deflectors and electron biprisms can be configured to be of either an electric field type or a magnetic field type. Although a deflection effect to the electron beam is different in each type in that the effect lies in a direction of the electric field in the electric field type whereas it lies in a vertical direction to the magnetic field, the fundamental effect is equivalent. Therefore, although the electron beam device of the electric field type will be explained in this application, it is not limited to the electric field type.
Although in the electric field, an electron trajectory 27 draws a parabola, it goes straight after being emitted from the electric field region. As shown in
The plane on which the deflection point places and that is perpendicular to the optical axis plays an important role in constructing an interference system. Hereafter, unless otherwise noted in this application, let it be assumed for simplicity that the electron trajectory in the deflector is drawn by a straight line and the electron beam is given a predetermined deflection either at the deflection point or in a plane that includes the deflection point and is perpendicular to the optical axis. It is known that this assumption holds without any problem within a range of paraxial approximation that deals with a trajectory of the electron beam in the vicinity of the optical axis.
An imaginary trajectory 29 that goes back on the trajectory of the electron beam emitted from the second deflector while keeping its straight line, as it is, intersects an imaginary trajectory 28 of the incident electron beam along the optical axis 2 in a region where no electric field exists between the first deflector and the second deflector. That is, in the two-stage deflectors, by controlling the deflection angles β1 and β2 of the upper and lower deflectors, it is possible to control a deflection point 86 synthesized regardless of existence or absence of the electric field and a position in a plane 865 that includes the deflection point 86 and is perpendicular to the optical axis.
The electron biprism is an electrooptical device indispensable to the interference system as a beam splitter in the electron beam. It has a characteristic of separating an incident electron beam into two electron beams (22, 24) and deflecting the two electron beams to directions in which they approach mutually to the optical axis or to directions in which they separate mutually from the optical axis by the same angle α regardless of a distance from the optical axis 2.
Generally, the electric field type electron biprism is configured to include a filament electrode 9 made of conductive filament and parallel plate grounded electrodes 99 held in a form that sandwiches the electrode.
Since a characteristic that the electron biprism defects the electron beams in directions in which they face each other symmetrically to the optical axis 2 regardless of the eccentric distance from the optical axis 2 or in directions in which they separate from each other by the same angle corresponds to an effect of a biprism that combines two prisms in the optics, it is called an electron biprism. If the electron beams have coherence, interference fringes 8 will be observed in a region where the separated two electron beams (22, 24) superimpose on the downstream side of the electron biprism. An image obtained by making the electron beam having information of an object in the one side of the separated two electron beams interfere with the electron beam in the other side as an electron beam (a reference wave 23) having, for example, an already known phase distribution such as a plane wave is an interferogram (an electron beam hologram) [Nonpatent Literature 2].
Like the electron trajectory 27 shown in the deflector of
In this application, when describing “the electron biprism”, it is a general term of the conventional electron biprism including a filament electrode, and the electron biprism having also the deflection function that is considered to be an object of this application is called the “electron beam biprism device” including its deflection mechanism. Moreover, when referring to a strict position in the electron optical system, it is described, for example, as a “position of the filament electrode of the electron biprism.”
Since the interferogram by the electron beam includes an image and the interference fringes, techniques of the fringe analysis are usable for its analysis, and phase information extraction methods different from the Fourier transform method in principle (fringe scanning methods (Patent Literature 1) (Nonpatent Literature 3), a Moiré method (Nonpatent Literature 4), etc.) can be used. Especially, the fringe scanning method using multiple images obtained by controlling the phase of the interference fringes utilizing a phase difference of an object wave and a reference wave is a method that can achieve high resolution in a respect that spatial resolution of a reconstruction image does not depend on an interference fringe spacing. Its principle is to record M sheets of interferograms while the phase difference of the object wave and the reference wave is shifted by (2π)/M and obtain a phase distribution φ(x, y) of the object wave based on Formula 3 shown in
In the case where the basic interference fringes exist in the image like an electron beam interferogram, Formula 3 is modified a little bit to become like Formula 4 shown in
Especially, in an electron optical system, the method of controlling the phase difference of the object wave and the reference wave with high precision is not put in practical use, and there is tried no other methods better than the following methods: a method whereby a position of the specimen is moved and the movement of the position is corrected by an image processing after the recording; a method of moving the electron biprism in a direction perpendicular to both the optical axis and the filament electrode (in
In the conventional electron beam interference method, the electron biprism was placed on the optical axis and in a plane perpendicular to the optical-axis. For example, in the electric field type, the electron beams passing by the both sides of the filament electrode were deflected symmetrically to the optical axis in directions in which they faced each other or in directions in which they separated from each other, the two electron beams were superimposed on the downstream side of the electron biprism, and the interferogram was measured. Although this method was a simple method, the resolution of the phase image reproduced from an interferogram was three times as large as the recorded interference fringe spacing, and there was a theoretic restriction that the resolution remained at low spatial resolution.
One of measurement methods that are free from this restriction is a fringe scanning method. This is an interference measurement method whereby the phase difference is given between the object wave and the reference wave, and a phase image is made to reflect a spatial resolution of a recording system, as it is, from plural sheets (at least three sheets) of the interferograms such that only interference fringes superimposed on the specimen image are modulated by an arithmetic processing. The fringe scanning methods having been tried up to now include, for example, (1) a method whereby the position of the specimen is moved and the movement of position is corrected by an image processing after the recording, (2) a method whereby the electron biprism is moved in a direction perpendicular to the both the optical axis and the filament electrode, (3) a method whereby an incident angle of the electron beam to the specimen is changed, etc. However, these techniques had problems that real-timeness was lacking, an analysis processing after image recording became complicated, or accuracy sufficient for modulation of the phase difference was not achieved. Furthermore, when the methods of the above-mentioned (1) to (3) are performed in the conventional electron beam interferometer, since the Fresnel fringes superimposed on the interferogram are also modulated at the same time and generate a new artifact, the fact is that an accuracy expected from a principle of the fringe scanning method has not been achieved.
The present invention is made to provide an electron biprism for realizing a suitable fringe scanning method in the electron biprism interferometer, and is one that makes it possible to deflect the electron beams passing by right- and left-hand sides of the filament electrode by mutually different angles by adding a function of deflecting them to one direction to functions of the electron biprism. Its concrete structure is the electron biprism to which two-stage deflectors on the optical axis are added, and is characterized in that the electron beams are controlled so that the deflection points by the deflectors may be positioned in a plane on which the filament electrode is placed regardless of spatial locations of the deflectors by controlling magnitudes and directions of the deflection angles of the two-stage deflectors.
When performing the fringe scanning method in a double-biprism electron interference system, it is considered that an optical system of imaging a specimen image at its filament electrode position using the electron beam biprism device according to this application as an upper-stage electron biprism is most suitable. Since both of the plane including the deflection point by the electron biprism and the plane including the deflection point by the deflectors are in agreement with the image plane position, even if the electron beam is deflected, a position of the specimen image does not move on an observation and recording plane, which enables the fringe scanning method to be performed effectively.
According to this application, the electron beams passing by right- and left-hand both sides of the filament electrodes are given deflection in one direction in addition to a deflection symmetrical to the optical axis and, as a result, it becomes possible to give the right- and left-hand electron beams emitted from the electron beam biprism device mutually different deflection angles. Therefore, in the image plane of a specimen on the downstream side of the electron beam biprism device, a control of a phase difference of an object wave and a reference wave becomes possible. That is, a relative spatial relationship of the image of the specimen recorded as an interferogram and interference fringes superimposed on the image can be modulated with high precision without changing the image of the specimen and its position, so that the fringe scanning method becomes implementable.
a) is a diagram showing the first sheet of the interferogram,
c) is a diagram showing the third sheet of the interferogram whose phase of the interference fringe is shifted from (b) by 2π/3 (whose phase is shifted from (a) by 4π/3).
a) is a schematic diagram showing a relationship among a source, a light ray, and the interference fringes when the conventional electron biprism is replaced with an optical biprism.
b) is a schematic diagram showing a relationship among a source, a light ray, and the interference fringes when the electron biprism according to the first embodiment is replaced with a corresponding optical biprism.
a) is a diagram showing the electron beam biprism device comprised of the electron beam biprism device, the first deflector, and the second deflector in this order from the upstream side of the electron beam in the travelling direction,
Hereafter, one example of an electron beam biprism device in the present invention will be described in accordance with specific examples.
At this time, with a downward direction (a travelling direction of the electron beam) of the optical axis being taken as a z-axis, and a clockwise direction to the travelling direction being defined as a positive direction of angle, the electron beam 22 on the left-hand side on the sheet of
Consequently, an angle difference of the two electron beams (22, 24) is 2α, which does not change from that after passing through a filament electrode 9 of the electron biprism. However, the two electron beams (22, 24) after being emitted from the electron beam biprism device are inclined to one direction by an angle β, becoming asymmetrical to the optical axis 2.
Only when the plane 855 that includes the deflection point 85 of the upstream side electron biprism and is perpendicular to the optical axis and a plane 865 that includes the deflection point 86 of the two-stage deflectors on the downstream side and is perpendicular to the optical axis coincide with each other, it becomes possible to describe a relation of the two deflection angles α and β by a simple relation like this. That is, a control of the position in the plane 865 that includes the deflection point 86 by the two-stage deflectors on the downstream side and is perpendicular to the optical axis is important for a control of a phase difference of the two electron beams (22, 24).
The light rays (22, 24) emitted from a real image 11 of the source are deflected in directions in which they face the optical axis 2 mutually by the biprism 45 placed in the propagation path on the optical axis 2. Consequently, these are equivalent to two light rays (22, 24) emitted from two virtual sources 12 and are made to superimpose on the downstream side of the biprism 45, generating interference fringes 8.
b) shows a biprism 46 corresponding to the electron beam biprism device according to this application and a situation of deflection thereby. Because angles of side parts of the biprism 46 are different between the right and the left, the deflection angles of right- and left-hand light rays are different. Therefore, positions of the virtual sources 12 become asymmetrical to the optical axis 2 and, as a result, the phase difference of right- and left-hand light rays (22, 24) makes fringe positions of the interference fringes 8 vary.
Then, configurations of the electron biprism and the two-stage deflectors will be explained below.
The electron beam biprism device of this example has a triple configuration comprised of the electron biprism 91 and the deflectors (81, 82) made in two stages. Therefore, three ways of configurations shown in
Here, d1 is a Z-coordinate value of the deflection point by the first deflector 81 when being seen from the Z-coordinate origin, d2 is a Z-coordinate value of the deflection point by the second deflector 82 when being seen from the Z-coordinate origin, β1 is a deflection angle by the first deflector 81, and β2 is a deflection angle by the second deflector 82. According to the definition, the variables take positive or negative values, respectively.
When controlling respective deflection angles of the two-stage deflectors (81, 82) based on Formula 5, the plane 865 that includes the synthesized deflection point 86 by the two-stage deflectors (81, 82) and is perpendicular to the optical axis always coincides with a plane perpendicular to the Z-coordinate origin. A synthesized deflection angle S by the two-stage deflectors (81, 82) at this time is β1+β2 (=β).
As is clear on comparing
Although the deflection angle of each deflector can be altered based on Formula 5 at the time of an experiment, on the other hand, distances among the deflectors and the biprism, electrode sizes of the deflectors, etc. are constants decided at the time of design of the mechanism. That is, as is clear from Formula 5, since the deflection angle S and an applied voltage VBD to the deflector are in a proportional relationship, what is necessary is just to control the applied voltages so that a ratio of the applied voltage to the first deflector 81 and the applied voltage to the second deflector 82 may become a predetermined constant value. Incidentally, the deflection angle S by these two-stage deflectors (81, 82) and the deflection angle α by the electron biprism 91 are independent.
Then, one example of a configuration of the fringe scanning method in the two-stage electron biprism interferometer will be described below.
In the two-stage electron biprism interferometer, the electron beam biprism device 93 is used as an upper-stage electron biprism, and an image plane 71 of a specimen is configured to coincide with the plane 855 perpendicular to the optical axis including the position of the filament electrode of the electron biprism 91, namely the deflection point 85 by the electron biprism 91. As a result, the specimen image plane 71, the plane 855 that includes the deflection point 85 by the electron biprism 91 and is perpendicular to the optical axis, and the plane 865 that includes the deflection point 86 by the two-stage deflectors and is perpendicular to the optical axis are configured to be in the same plane where all the planes electrooptically coincide with one another. This means in a design that the image plane 71 can be fitted into the plane 855 that is defined mechanically, includes the deflection point 85 by the electron biprism, and is perpendicular to the optical axis using an objective lens 5, and the plane 865 that includes the deflection point 86 by the two-stage deflectors (81, 82) and is perpendicular to the optical axis can be fitted into the fitted plane through adjustment of the deflection angles by the first deflector 81 and the second deflector 82 in an independent manner, respectively.
In performing the fringe scanning method, the following procedure will be taken: (1) An interference fringe spacing and an interference width of the interferogram (8 and 32) are decided by the upper-stage electron biprism (the electron biprism 91 inside the electron beam biprism device 93 of this application) and a lower-stage electron biprism 95; and subsequently, (2) the fringe positions of the interference fringes 8 are modulated by controlling the phase difference of two electron waves (21, 23) with the two-stage deflectors (81, 82) by this application. That is, since the imaging of a specimen 3 by the objective lens 5 and a modulation operation of the deflection angle by the two-stage deflectors (81, 82) after the deflection for interference by the upper-stage electron biprism 91 was done are performed in this order, the configuration with an order of the electron biprism 91, the first deflector 81, and the second deflector 82 is the most suitable configuration for the fringe scanning method.
Regarding the interferogram 88 whose interference fringe spacing and interference width have been decided by the first and second electron biprisms (91, 95), the phase difference of the two electron waves is controlled by a deflection action of the two-stage deflectors (81, 82) and the fringe positions of the interference fringes 8 are modulated. The interferogram 88 of the specimen decided to be under predetermined interference conditions is controlled to be in a predetermined magnification through first, second, third, and fourth imaging lenses (61, 62, 63, and 64), and is recorded in an image observation and recording medium 79 (for example, a TV camera and a CCD camera) on an observation recording plane 89.
Then, it is reproduced as an amplitude image, a phase image, etc. by an arithmetic processing unit 77 and is displayed, for example, on a monitor 76 etc.
Although
Next, another example of the configuration of the fringe scanning method in the two-stage electron biprism interferometer will be described below.
The electron beam biprism device 93 uses a mechanism of a triple configuration comprised of the first deflector 81, the electron biprism 91, and the second deflector 82 from the upstream side shown in
A respect that the specimen image plane 71, the plane 855 that includes the deflection point 85 by the electron biprism and is perpendicular to the optical axis, and the plane 865 that includes the deflection point 86 by the two-stage deflectors and is perpendicular to the optical axis are configured to be in an electrooptically coinciding plane is the same as that of the configuration example of the second embodiment. Therefore, the image plane 71 is made to fit to the plane 855 that includes the deflection point 85 by the electron biprism defined mechanically and is perpendicular to the optical axis using the objective lens 71, and regarding the plane 865 that includes the deflection point 86 by the two-stage deflectors (81, 82) and is perpendicular to the optical axis, the fitting is achieved each independently by adjustment of the deflection angles by the first deflector 81 and the second deflector 82, which are made to be the same as the above.
As was explained in
Next, another example of a configuration of the fringe scanning method in the two-stage electron biprism interferometer will be described below.
In the two-stage electron biprism interferometer, the electron beam biprism device 93 is used as the upper-stage electron biprism, and the image plane 71 of the specimen is constructed so as to coincide with the plane 855 that includes the filament electrode position of the electron biprism 91, i.e., the deflection point 85 by the electron biprism 91 and is perpendicular to the optical axis. A respect that the specimen image plane 71, the plane 855 that includes the deflection point 85 by the electron biprism 91 and is perpendicular to the optical axis, and the plane 865 that includes the deflection point 86 by the two-stage deflectors and is perpendicular to the optical axis are configured to be in an electrooptically coinciding plane is the same as those of the second and third embodiments.
In addition, a respect that the image plane 71 can be made to fit to the plane 855 that includes the deflection point 85 by the electron biprism defined mechanically and is perpendicular to the optical axis using the objective lens 5, a respect that fitting can be performed to the plane 865 that includes the deflection point 86 by the two-stage deflectors and is perpendicular to the optical axis by adjustment of the deflection angles by the first and second deflectors (81, 82) each independently, and other respects are the same as those of the configuration examples in the second and third embodiments.
For example, in the case of the device having the one-body mechanism explained in
Since the image plane 71 of the specimen is located on the most downstream side as compared with the second embodiment and the third embodiment, it is possible to make a magnification ratio of the specimen image 31 by the objective lens 5 larger than that of the first configuration example in the third embodiment and that of the second configuration example in the fourth embodiment. Moreover, it is possible for this configuration to obtain the interferograms (8 and 32) having the narrowest fringe spacing among the above-mentioned three configuration examples. A situation of how it is mounted on the electron microscope is the same as
Incidentally, although it was decided that the second deflector on the downstream side of the two-stage deflectors was used in this configuration example, even with a plane 835 perpendicular to the optical axis including a deflection point 83 by the first deflector 81 or with the plane 865 that includes the deflection point 86 and is perpendicular to the optical axis synthesized by the first and second deflectors, if it is made to coincide with the image plane 71 of the specimen, the same effect will be obtained.
A respect that only the electron beam biprism device 93 in this application is used as the electron biprism and a respect that only one-stage of the deflector in this application is also used are the same as those of the first configuration example in the fifth embodiment.
Here, since the position of the electron biprism 91 is between the specimen image 31 and the imaging lens 61, the voltage applied to the filament electrode 9 in order to produce interference is a negative voltage, and the polarity of the applied voltage is different from that of the optical system in the sixth embodiment. However, this is not an essential difference. The electron biprism has a configuration where the propagation directions of the object wave 21 and the reference wave 23 that have not yet generated interference are deflected by the second deflector 82 located on the image plane of the specimen 3. Since this deflection is deflection that is given at the image plane position of the specimen, the positions of the specimen images 31 and 32 do not change fundamentally, and only the phase difference of the two electron beams of the object wave 21 and the reference wave 23 is altered after passing through the imaging lens 61.
That is, the fringe scanning method is possible. However, since this optical system is a conventional interference system, a control of the interference fringe spacing and the interference width that is an advantage of the two-stage electron biprism interferometer and elimination of superimposition of the Fresnel fringes on the interferogram, etc. are unrealizable. Incidentally, although it was decided that the second deflector 82 on the downstream side of the two-stage deflectors was used in this configuration example, even with the plane 835 that includes the deflection point 83 by the first deflector and is perpendicular to the optical axis or the plane 865 that includes the synthesized deflection point 86 by the first and second deflectors and is perpendicular to the optical axis, if it is made to coincide with the image plane 71, the same effect will be obtained. These respects are the same as those of the fifth embodiment.
Number | Date | Country | Kind |
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2009 281123 | Dec 2009 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2010/071826 | 12/6/2010 | WO | 00 | 6/8/2012 |