JP5564292B2 - Phase plate and phase contrast electron microscope using the same - Google Patents

Description

  The present invention relates to a phase plate used in an electron microscope, particularly a transmission electron microscope, and a phase contrast electron microscope including the phase plate.

  A transmission electron microscope (TEM) irradiates an observation target material with electrons accelerated at a high voltage of several tens of kV or more, transmits it, and magnifies and forms an image using an electromagnetic lens to form an observation image with a high magnification. It is a device to obtain. As a mechanism for generating an image contrast by the transmission electron microscope image, there is one called a phase contrast. This is generated by mutual interference between the wave of electrons transmitted through the observation sample and scattered, and the wave of electrons not scattered. Therefore, in order to increase the image contrast, it is necessary to appropriately control the phase difference of the electron wave. In a normal electron microscope, the phase difference is controlled by adjusting the aberration and defocus amount of the objective lens. However, in the case of an observation sample having a relatively large internal structure (about 10 nm or more) such as a biological sample, a necessary phase difference cannot be obtained only by adjusting the defocus amount of the objective lens, and observation is performed with sufficient image contrast. The image could not be obtained.

  In order to solve this problem, technical developments have been made to realize the phase plate technology used in the field of optical microscopes for electron microscopes. The phase plate for an electron microscope is an optical element that changes the phase of an electron wave independently of the objective lens, and is installed on the back focal plane of the objective lens or in the vicinity thereof. The electron wave that has not been scattered by the observation sample travels along the optical axis of the electron microscope and converges to a size of about 1 μm in the vicinity of the back focal plane. On the other hand, the scattered electron wave passes through a position away from the optical axis in the vicinity of the back focal plane. Therefore, if the phase of the scattered wave and the non-scattered wave can be changed by different amounts by the phase plate disposed in the vicinity of the back focal plane, a phase difference can be generated between the two electron waves. For example, the phase difference required to maximize positive image contrast is 180 degrees in phase angle. In general, when observing very thin, light-element objects that approximate a weak phase object, electron wave scattering adds about 90 degrees of phase shift and must be generated by a phase plate. The phase difference amount is about 90 degrees. Examples of the structure of the phase plate for generating the phase difference of about 90 degrees are disclosed in Non-Patent Document 1, Non-Patent Document 2, and the like. These utilize the fact that an electron wave traveling in an electrostatic potential causes a phase change.

  As a technique for realizing the basic structure of the phase plate and adding controllability and the like, in Patent Document 1, an electrode to which a potential is applied is sandwiched between conductors through an insulating film, and an electrode is provided only in the vicinity where a non-scattered wave passes. A technique is disclosed in which a minute ring-shaped electrostatic lens having an exposed structure or a fine wire functions as a phase plate. In the disclosed technique, an electrostatic potential formed in the vicinity of an exposed electrode changes the phase of an electron wave, and generates a phase difference between electron waves. Patent Document 2 discloses a technique for using an amorphous carbon thin film with fine holes as a phase plate. In this technique, the internal potential inside the amorphous thin film is used as the origin of the phase change of the electron wave. Furthermore, as a technique that does not use an electrostatic potential, Patent Document 3 and Patent Document 4 disclose a phase plate technique that uses a phase change originating from a vector potential of an electromagnetic field by using a magnetic material. .

JP-A-9-237603 JP 2001-273866 A JP-A-60-7048 JP 2008-91312 A JP 2009-506485 A JP 2008-198612 A JP 2003-217498 A JP 2007-250541 A US Patent Application No. 20080035854

Zeitshift fur Natureforschung 2a, 615-633 (1947). Philosophical Transactions of the Royal Society of London 261, 95-104 (1971).

  However, the above known technique has various problems.

  For example, in the case of a phase plate using a ring electrode as shown in FIG. 8A, a non-scattered electron wave passes through a hole having a diameter of 1 μm to several μm. Since the diameter ring electrode shields the electron wave, there is a problem that specific information is lost. In order to reduce this problem, efforts have been made to reduce the size of the ring electrode as much as possible. However, the ring electrode has a multilayer structure including at least five layers including an insulating film, and further a wiring structure for supplying a voltage. Therefore, the structure is complicated and miniaturization has not progressed so much. For example, as shown in Patent Document 5, the production of a fine ring is an extremely complicated process. For this reason, there has been proposed a technique in which optical elements such as the configuration of an electromagnetic lens are significantly changed and added so as not to use an electrode for blocking an electron wave or to make it effectively small. There are technical disclosures of Patent Document 7 and Patent Document 8.

  Further, in the case of a phase plate having a configuration in which an electrode to which a potential is applied is covered with a conductor via an insulating film as shown in FIG. 8B, even if the shape is a thin line shape, it is a ring shape. However, since the insulating film is exposed and close to the very vicinity of the optical axis through which the non-scattered electron wave passes, there is a problem that the insulating film is charged and causes image drift. When the insulating film is in a position where it can be seen from the electron wave path, the influence of charging is significant. In order to avoid this problem, there is a technical disclosure of Patent Document 9 as a structure of a phase plate that does not use an insulating film. In this disclosed technique, a fine electrode structure in which a potential-applied electrode is covered with the same separated conductor without using an insulating film is realized by an advanced process technique using the MEMS technique.

  Further, in the case of a phase plate using a carbon thin film having a fine hole of about 1 μm as shown in FIG. 8C, since the scattered electron wave passes through the carbon thin film, the thin film absorbs the electron beam. There is a problem that the number of electrons contributing to image formation is reduced and the S / N of the image is lowered. By increasing the electron acceleration voltage to 300 kV or more and increasing the energy of the electrons, the amount of absorption can be reduced, but the acceleration voltage that can be used by the phase plate is narrowed.

  As described above, the phase plate used in the phase-contrast electron microscope needs a function of adding a phase difference of about 90 degrees between the electron wave scattered by the observation sample and the electron wave not scattered. For the structure of the phase plate that uses the electrostatic potential for the physical origin that creates this phase difference, minimize the electrode structure to minimize the shielding of the electron wave, and avoid the influence of the charging of the insulating film. It is necessary to avoid a decrease in image S / N due to a decrease in electron dose caused by passing through the thin film. In addition, as proposed to improve these problems, the optical elements such as the configuration of electromagnetic lenses are changed and added significantly, and the structure that does not require an insulating film by advanced manufacturing technology. This is not necessarily a desirable method in consideration of commercial use.

  The first object of the present invention is to make a phase structure used in a phase-contrast electron microscope that can be easily manufactured without using an insulating material and without using a thin film that causes a decrease in electron dose, because the electrode structure constituting the phase plate can be miniaturized. To provide a board.

  A second object of the present invention is to provide a phase-contrast electron microscope that includes the above-described phase plate and does not involve a significant change in electro-optical elements such as addition of an electromagnetic lens.

  In order to achieve the above object, the present invention provides a phase plate used in an electron microscope, which is composed of a plurality of stacked conductive films.

  In order to achieve the above object, in the present invention, at least two kinds of conductor films having different work functions are used as the plurality of conductor films, and the plurality of conductor films are laminated in contact with each other. A phase plate is provided.

  Furthermore, in order to achieve the above object, the present invention provides a phase plate using a metal or a semiconductor as a plurality of stacked conductor films.

  That is, in the phase plate of the present invention, a film in which at least two kinds of conductors are contacted and laminated is used, and an electrostatic potential formed by a contact potential difference generated at the laminated interface is used. Preferably, a desired phase plate is realized by processing the laminated film into a ring shape or a thin line shape. Moreover, as a suitable phase-contrast electron microscope, the structure which installs the above-mentioned phase plate with the position adjustment apparatus in the back focal plane of the objective lens of a transmission electron microscope or its vicinity is provided.

  ADVANTAGE OF THE INVENTION According to this invention, the phase plate used for the phase-contrast electron microscope which can be manufactured easily can be provided. In addition, it is possible to provide a phase-contrast electron microscope that does not involve a significant change in electro-optical elements such as addition of an electromagnetic lens.

It is a figure which shows the structure of the phase plate concerning a 1st Example. It is the figure which showed the outline of arrangement | positioning of the optical system of a phase-contrast electron microscope, an electron beam, and a phase plate based on a 1st Example. It is a figure explaining the holding structure of the phase plate based on a 1st Example. It is a figure which shows an example of the manufacturing process of the phase plate based on a 1st Example. It is a figure which shows the structure of the phase plate based on a 2nd Example. It is a figure which shows the line profile of the power spectrum of the image which observed the amorphous film. It is a figure explaining the contact potential difference of a metal-metal interface and a metal-semiconductor interface. It is a figure which shows the example of the phase plate by a prior art.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, the principle of the phase plate of the present invention will be described. Note that in this specification, a conductor includes a good conductor such as a metal and a semiconductor, and is defined to be different from an insulator. The boundary between the semiconductor and the insulator is about 10 ⁻⁶ S / m, and the boundary between the semiconductor and the good conductor is about 10 ⁶ S / m.

  First, the contact potential difference will be described. All metals (including alloys) that are conductors have dependency on crystal plane orientation, but have a work function φ unique to the material and are distributed in a range of about 2 eV to 6 eV. Therefore, as shown in FIG. 7A, when metal A and metal B (or semiconductor B) are brought into contact with each other, the work function difference φA−φB (where φA and φB are the work functions of metal A and metal B). It is known that a contact potential difference equal to) occurs. For example, Journal of Applied Physics Vol. 48, 4729 (1977), the work function of Au is 5.1 eV, and the work function of Cu is 4.65 eV. Therefore, a contact potential difference of 0.45 V can be obtained by bringing both layers into contact with each other. Here, when Au is grounded, a state in which Cu is 0.45 V lower than the ground potential is realized. Due to this potential, an electrostatic potential distribution is formed around the Au / Cu laminated film.

  Next, a phase plate having a ring structure will be described. First, the Au / Cu laminated film is formed into a ring structure as shown in FIG. 1 as a three-layer structure of Au / Cu / Au. (A), (b) of the figure has each shown the perspective view and sectional drawing of a ring-shaped structure. The Au / Cu / Au laminated structure is exposed at the hole 2 inside the ring, and the outer part of the ring-like structure is covered with Au and grounded. If the potential of the grounded Au is 0 V, the potential of the Cu portion becomes −0.45 V, and a localized electric field is formed in the hole 2 portion of the ring. If a ring-shaped laminated film is arranged so that this hole is parallel to the optical axis of the electron beam, an extremely weak electrostatic lens (Einzel lens or unipotential lens) is realized. .

  Consider the amount of phase change when an electron wave travels in the hole 2 portion and the peripheral portion of the electrostatic lens. It is known that when an electron wave moves in the region of potential V by a distance S, it undergoes a phase change proportional to V × S. For example, when the thickness of Cu is 0.38 μm and the thickness of Au is sufficiently thick, a 100 kV electron beam passing through a hole portion of a ring-shaped electrostatic lens having a round hole parallel to the optical axis passes therethrough. Regardless of the position in the hole, the phase change of 90 degrees is received. On the other hand, since the electron beam passing outside the ring-shaped electrostatic lens is hardly affected by the lens electric field, the amount of phase change is small enough to be ignored. Therefore, the phase difference between the electron waves passing through the inside and outside of the lens is 90 degrees.

  In order to set the phase difference to a value different from 90 degrees, the value of the contact potential difference may be changed by changing the Cu film thickness or changing the combination of the laminated metals. Even when electrons having different acceleration voltages are used, the phase difference can be set to 90 degrees by changing the thickness of the Cu layer. To determine the thickness b,

から求めれば良い。

Find it from

  If the phase difference produced by the designed and manufactured phase plate is slightly different from 90 degrees as a result of the evaluation, the thickness of Cu is increased in proportion to the decrease (or increase). Needless to say (or reduce).

  In the above example, the contact potential difference due to the combination of Au and Cu is used, but any combination of metals (including alloys) having different work functions can be used. For example, a combination of Au—Ti, Au—Cr, Au—Mo, Au—Ru, Au—Ta, Au—W, Pt—Au, Pt—Mo, or the like may be used. In the case where the ring-shaped phase plate is configured by the combination of metals exemplified here, the first metal (Au or Pt) of each combination covers Cu, Ti, Cr, Mo, Ru, Ta, and Au. It is desirable to use by laminating. This is because the first metal of each combination has a work function larger than that of the second metal, and a negative potential can be obtained inside the lens hole. In the case of a negative potential, the non-scattered electron wave undergoes a phase change of about −90 degrees, but considering that the scattered electron wave originally undergoes a phase change of 90 degrees, the difference is about 180 degrees. This is because a phase difference of 180 degrees can be obtained with a small amount of phase change. Further, Au and Pt have high resistance to surface oxidation and are less likely to be charged. Therefore, it is desirable to use Au and Pt as the metal that covers the lens. When using a ferromagnetic metal, it must be considered that the vector potential due to the magnetization distribution affects the phase difference.

  In the above description, a phase plate using a conductor has been described, but a semiconductor such as Si can also be used. For example, a combination of Au-Si. In this case, as shown in FIG. 7B, since a depletion layer is formed at the interface with the metal Au in the semiconductor Si, contact is required unless the thickness of the depletion layer is sufficiently narrower than the thickness of Si. A potential difference cannot be generated. A high carrier concentration is required to realize a narrow depletion layer. The relationship between the Si film thickness D, the carrier concentration N, and the contact potential difference V is

とあわすことができる。

Can be said.

Here, e is the charge of electrons, and ε is the dielectric constant of Si. For example, in the case of Si, when the film thickness is 300 nm, the carrier concentration needs to be 10 ¹⁸ cm ⁻³ or more.

  When a phase plate is irradiated with an electron beam, a phenomenon called contamination in which a carbon-based substance adheres may occur. It is known that heating the phase plate is effective in order to slow down the progress of this contamination. When heating is performed, if a low melting point metal or a combination of metals having a large diffusion coefficient is used as a material constituting the phase plate, a problem occurs in the durability of the phase plate. A metal having a melting point of 1000 ° C. or higher such as Au or Cu is desirable, and a low melting point metal such as Sn is not desirable. When the contact potential difference between adhering carbon-based contamination and the metal is a problem and an attempt is made to reduce this, an alloy having the same work function as that of the contamination may be used. For example, an Au-Ag alloy is a solid solution alloy, and its work function is controlled to a value between the work function of Au (5.1 eV) and the work function of Ag (3.75 eV) depending on the mixing ratio. Can be used. For this reason, it can choose so that the difference of the work function with contamination may be made small.

  The value of the work function of a crystalline substance varies slightly depending on the orientation of the crystal plane. Therefore, when a film composed of polycrystalline particles is used for the phase plate, the magnitude of the contact potential difference varies depending on the crystal orientation of the contacting particles. In order to reduce the influence, the size of the crystal grains should be sufficiently smaller than the size of the hole of the ring-shaped phase plate. The influence of the crystal orientation of the crystal grains on the potential distribution inside the hole is sufficient if the crystal grains in the hole portion of the hole are considered. From this point of view, when a computer simulation was performed on the potential distribution inside the hole, it was found that for a 1 μm hole, if the particle size was 100 nm or less, the influence on the phase difference was 2% or less. In other words, if the metal film or the semiconductor film is composed of crystal grains having a size of 1/10 or less of the size of the hole, the influence of the variation in crystal orientation can be substantially ignored.

Next, a method for manufacturing a phase plate will be described with reference to FIGS. The phase plate is placed on the back focal plane of the electron microscope objective lens or in the vicinity thereof. Usually, a movable aperture plate called an objective diaphragm of a phase contrast electron microscope, which will be described later, is arranged at that position. The phase plate is formed on the aperture plate 5. First, as shown in FIG. 4A, a metal film 3 (for example, an Au film) is pasted so as to cover the hole formed in the aperture plate 5, and then the metal film 4 (for example, a Cu film) is used by using a vacuum deposition apparatus. ) Is deposited to a desired thickness, and then the metal film 3 is deposited again. Next, as shown in FIG. 4B, which is a cross-section in the AA direction shown in FIG. 3A, the disk shape that becomes the base of the ring-shaped electrode and the holding structure 32 are processed. The processing is performed from the direction perpendicular to the film surface. For example, if a focused ion beam apparatus is used, processing of an arbitrary shape can be easily performed. Next, as shown in FIG. 4C, a metal film 3 is deposited on the front and back surfaces and side surfaces of the shaped film to cover the front surface. Finally, as shown in FIG. 4D, the phase plate is completed when the hole 2 is formed in the center of the disk shape. When drilling with an ion beam is performed, the hole diameter on the ion beam incident side is often larger than the hole diameter on the ion beam emission side surface. For this reason, when used as a phase plate, it is desirable that the electron beam be installed so as to be incident from the side having a small hole diameter. This is to prevent the electron beam from being directly irradiated to the inside of the hole, to suppress the adhesion of contamination, and to reduce the influence of the variation in the generated potential due to the variation in crystal orientation. Note that the vacuum deposition apparatus and the focused ion beam apparatus, which are manufacturing apparatuses used here, are not particularly limited as long as they are film forming apparatuses capable of forming metal films and apparatuses capable of processing metal films. As the vacuum deposition apparatus, a resistance heating apparatus, an electron beam heating apparatus, a sputtering apparatus, a CVD apparatus, or the like may be used. An ion milling device may be used instead of the focused ion beam device. However, when an ion milling apparatus is used, a plurality of processes such as resist coating and exposure are added to process a desired shape.
As described above, by utilizing the contact potential difference of conductors (metals and alloys, etc.), a ring-shaped electrostatic lens constructed without using any insulator can be easily obtained without using advanced process technology. It is formed. Moreover, the thin film which absorbs an electron beam is not used. Further, since the structure is simple and does not have a wiring structure such as voltage supply from the outside, miniaturization is easy. That is, a phase plate that achieves the first object of the present invention can be provided. When this phase plate is formed on the aperture plate and attached to an objective movable diaphragm having a position adjusting function, it functions in the vicinity of the back focal plane of the electron microscope objective lens, and a desired phase difference can be obtained. That is, since there is no significant change in the electro-optic element such as addition of an electromagnetic lens, a phase contrast electron microscope that achieves the second object of the present invention can be provided.

  A ring phase plate will be described as the phase plate of the first embodiment. The structure of the ring phase plate of the present embodiment is as shown in FIG. 1 and is a ring-shaped phase plate 1 having holes 2.

  A specific configuration example and manufacturing method of the ring phase plate according to the present embodiment will be described with reference to FIGS. FIG. 3A shows a plan view, and FIG. 3B shows a cross-sectional view in the AA direction. In FIG. 3, 5 is an aperture plate, 31 is a hole, and 32 is a support bar which is a holding structure.

  This phase plate is placed on the back focal plane of the electron microscope objective lens or in the vicinity thereof, and usually a movable aperture plate called an objective diaphragm of a phase contrast electron microscope, which will be described later, is arranged at that position. The phase plate of this embodiment is formed on the aperture plate 5.

  First, as shown in FIG. 4A, a metal film 3 (for example, an Au film) is pasted so as to cover the hole formed in the aperture plate 5, and then the metal film 4 (for example, a Cu film) is used by using a vacuum deposition apparatus. ) Is deposited to a desired thickness, and then the metal film 3 is deposited again. Next, as shown in FIG. 4B, which is a cross-section in the AA direction shown in the plan view, the disk shape serving as the base of the ring-shaped electrode and its holding structure 32 are processed. The processing is performed from the direction perpendicular to the film surface. For example, if a focused ion beam apparatus is used, processing of an arbitrary shape can be easily performed.

  Next, as shown in FIG. 4C, a metal film 3 is deposited on the front and back surfaces and side surfaces of the shaped film to cover the front surface. Finally, as shown in FIG. 4D, the phase plate is completed when the hole 2 is formed in the center of the disk shape.

  When drilling with an ion beam is performed, the hole diameter on the ion beam incident side is often larger than the hole diameter on the ion beam emission side surface. For this reason, when used as a phase plate, it is desirable that the electron beam be installed so as to be incident from the side having a small hole diameter. This is to prevent the electron beam from being directly irradiated into the hole 2 to suppress the adhesion of contamination, and to reduce the influence of the variation in the generated potential due to the variation in crystal orientation.

  Note that the vacuum deposition apparatus and the focused ion beam apparatus, which are manufacturing apparatuses used here, are not particularly limited as long as they are film forming apparatuses capable of forming metal films and apparatuses capable of processing metal films. As the vacuum deposition apparatus, a resistance heating apparatus, an electron beam heating apparatus, a sputtering apparatus, a CVD apparatus, or the like may be used. An ion milling device may be used instead of the focused ion beam device. However, when an ion milling apparatus is used, a plurality of processes such as resist coating and exposure are added to process a desired shape.

  In the ring-shaped phase plate of this example, in accordance with the above manufacturing process, first, four holes of about φ0.1 mm are formed in an aperture plate (5 mm × 31 mm) made of Mo having a thickness of 300 μm. An Au thin film (0.4 μm) is pasted using an adhesive. Thereafter, a Cu thin film was deposited on the surface side to which the Au film was attached using a resistance heating type vacuum vapor deposition apparatus, and then about 0.4 μm was deposited, and then an Au film was further deposited on both the front and back surfaces of the aperture plate by about 0.5 μm. A self-supporting multilayer thin film having an Au / Cu / Au configuration is formed in the hole portion of the aperture plate.

  Next, the multilayer thin film portion is processed into a phase plate shape using a focused ion beam apparatus. Using Ga + accelerated to 40 kV, as shown in FIG. 3A, a structure is formed in which a disk-like plate of about φ1.5 μm is held near the center of a hole of about 0.05 mm. (At this time, the disk-shaped plate has not been processed into a ring shape yet) The disk-shaped plate near the center is held using a support rod having a width of about 0.5 μm. In FIG. 3 (a), the disk is held by two support bars that are symmetric with respect to the center of the disk. However, as shown in FIG. 3 (b), the disk is supported by non-centrosymmetric support bars. More desirable.

  This is because the diffraction / scattered electron distribution on the back focal plane of the objective lens in an electron microscope is generally centrally symmetric, so that information on a specific scattering direction is completely blocked by a support bar having a centrally symmetric structure. This is to avoid this. In addition, in FIG. 3, it is supported by two support rods, but there is no problem with the strength of supporting the disk, such as one, three, etc. A support rod may be used. Further, the support bar 32 may not have a uniform width. For example, the portion connected to the ring portion may be thin, and the portion connected to the edge of the 0.05 mm hole may be formed thick. Thereby, the strength of the support structure can be increased.

  Next, an Au film (about 0.2 μm) is deposited on the side surface portions of the multilayer thin film processed by the focused ion beam and on the front and back surfaces of the aperture plate. At this time, the entire aperture plate is covered with the Au film, and there is no exposed portion of Cu. The diameter of the disk-shaped plate increases by the thickness of the covered Au, and becomes less than φ2 μm (radius is less than 1 μm). Finally, a ring phase plate is completed by making a hole of about 1 μm in the center of the held disc-shaped plate using a focused ion beam device.

  The material, shape, and manufacturing method used for the ring phase plate described here can be substituted as long as they meet the purpose of the present invention. For example, the material of the aperture plate may be Ta or the like, and the thickness may be any thickness that does not easily deform and does not easily transmit the electron beam. The number and size of the holes to be opened in advance is not limited to four and φ0.1 mm, and may be, for example, one square hole of 2.5 mm × 10.5 mm. In this case, there is an advantage that the position where the phase plate is formed can be arbitrarily set. In order to maintain the strength of the Au film to be attached, a thick film of about 2.5 μm is preferably used. As described in the means for solving the problem, a combination other than Au / Cu / Au can be used for the metal multilayer film.

  As shown in FIG. 2, the aperture plate having the processed phase plate is mounted on a movable objective aperture mechanism of a commercially available transmission electron microscope (acceleration voltage 100 kV, equipped with LaB6 electron gun) to constitute a phase difference electron microscope. Is done. The objective aperture mechanism can adjust the position and retract in the vicinity of the rear focal plane 6 of the objective aperture. In the electron microscope, an electron beam 19 taken out and accelerated from the electron source 11 irradiates the sample placed on the sample surface 15 by the irradiation system lens 12. The electron wave transmitted through the sample forms an enlarged image on the image plane 18 by the magnifying lens 14 while forming the rear focal plane 6 and the intermediate image plane 17 by the objective lens 13. In the phase contrast electron microscope, the phase plate 1 is mounted on the back focal plane 6 of the objective lens. The hole 2 of the phase plate 1 is adjusted by an electromagnetic alignment mechanism such as a position adjusting function of a movable diaphragm and a deflector so as to be on the optical axis 20 of the electron microscope. Since the hole diameter of the ring phase plate is about 1 μm, it is desirable that the position adjustment function has an accuracy of 1 μm or less.

  Electrons irradiated on the sample are divided into scattered waves (or diffracted waves) and non-scattered waves. The non-scattered wave travels along the optical axis 20 and passes through the hole 2 of the phase plate 1. On the other hand, the scattered wave passes outside the ring phase plate. Since the potential distribution is different between inside and outside of the hole 2 of the ring phase plate 1, the amount of phase change received by each differs, and a phase difference occurs. The phase difference caused by the phase plate is equal to the phase change amount of the electron wave passing through the hole 2 when the phase change amount of the scattered wave is negligible.

となる。

It becomes.

  Where ΔΦ is the phase difference (unit radians), λ is the wavelength of the electron wave, U is the acceleration voltage, e is the elementary charge, m is the mass of the electron, c is the speed of light, z is the coordinate along the optical axis, V ( z) is the value of the potential at the coordinate z. Since the energy of electrons used in the transmission electron microscope is generally large, it can be approximated that the electrons travel in parallel along the optical axis, and the integration range is the traveling range of the electrons along the z axis. Although the value of V (z) differs depending on the position through which the hole 2 passes, it is known that the integrated value does not depend on the position. Therefore, the phase change amount is proportional to the potential distribution of the ridges of the holes 2, that is, the product of the contact potential difference and the sandwiched metal film (Cu) thickness (corresponding to the integral part of Equation 3).

  Now, the ring portion of the phase plate 1 blocks the electron wave. As a result, a part of the information regarding the sample structure cannot be transmitted to the image plane 18. When the size of the structure in the sample is d, the focal length of the objective lens 13 is f, and the distance from the optical axis 20 on the back focal plane 16 of the objective lens is r,

の関係があることが知られている。

It is known that there is a relationship.

  That is, it can be seen that the structural information of 10 nm in the sample is located at a distance of 1 μm from the optical axis 20 on the back focal plane 16 when the acceleration voltage is 100 kV (wavelength 0.0037 nm) and the focal length is 2.7 mm. . Further, structural information larger than 10 nm in the sample is located closer to the optical axis 20 than 1 μm. Therefore, in order to transmit even larger structural information of the sample, it is necessary to make the radius of the ring phase plate as small as possible. The radius of the ring phase plate described above is a little less than 1 μm, and the structural information of the sample can be transmitted to a little over 10 nm.

  The size of the hole 2 of the ring phase plate 1 is set to about φ1 μm by processing of the focused ion beam. However, if the outer diameter of the ring is not increased, it is made larger, for example, φ1.5 μm. It is desirable. The non-diffracted electron wave converged by the objective lens passes through the hole 2, but its size is about 1 μm, so as to avoid the concern of partial blocking of the non-diffracted electron wave.

  The verification that the phase difference is generated by the phase plate of this embodiment can be performed by observing an amorphous substance such as a carbon thin film as a sample and analyzing the power spectrum of the obtained image. The power spectrum of an image can be easily obtained by commercially available image processing software, and can be displayed substantially in real time in a transmission electron microscope equipped with an imaging device such as a CCD camera system. This power spectrum is represented by the product of a component reflecting the sample structure and a component obtained by squaring the phase contrast transfer function. The phase contrast transfer function expresses the intensity of interference between a scattered electron wave and a non-scattered electron wave including a sign, and changes greatly (oscillates) depending on the reciprocal (spatial frequency) of the size of the structure in the sample. The state of this vibration changes sensitively depending on both the phase difference caused by the aberration and defocus amount of the objective lens and the phase difference caused by the phase plate. On the other hand, the component reflecting the sample structure constituting the power spectrum changes only monotonously when an amorphous sample is observed. Therefore, the vibration component of the power spectrum reflects the vibration component of the phase contrast transfer function. Comparing images acquired under different phase difference conditions, if the aberration and defocus amount of the objective lens have not changed, the change in vibration of the phase contrast transfer function (peak position deviation) is considered to be due to the phase plate. Therefore, it is possible to verify whether a phase change has occurred.

  FIG. 6 shows two phase plates formed on the same aperture plate: (a) Au / Cu (0.4 μm) / Au, (b) Au / Cu (0.8 μm) / Au. A line profile diagram in which only a vibration component is extracted from a comparison with a theoretical curve calculated based on an out-of-focus amount, which is substantially equal to a certain aberration coefficient, for the line profile of the power spectrum of an amorphous carbon thin film image. Although both FIG. 6A and FIG. 6B vibrate greatly, it can be seen that the peak positions of the vibrations are different. Since there is almost no difference in the component of the phase contrast transfer function that depends on the aberration and the amount of defocus, the difference between (a) and (b) can be estimated mainly due to the phase plate, and the phase difference is caused by the phase plate. It can be confirmed that it has occurred.

  The configuration of the thin wire phase plate according to the second embodiment is shown in FIG. The phase plate according to the prior art of FIG. 8B is constituted by a metal multilayer thin film without using any insulating film.

  The thin wire phase plate was prepared as follows. First, a 2.5 mm × 10.5 mm square hole is made in a 300 μm thick Mo plate (5 mm × 31 mm), and a Pt film (thickness: 2.5 μm) is attached thereto with an epoxy adhesive. Thereafter, an Au film (0.1 μm) is deposited on both the front and back surfaces by a resistance heating vacuum deposition apparatus. Next, a shape in which a thin wire having a width of about 1 μm is formed in a round hole of φ50 μm is processed by a focused ion beam apparatus. That is, a semicircular hole is processed into a shape in which the side portion is abutted. Next, an Au film is vapor-deposited to a thickness of about 0.1 μm on the back and front and side surfaces of the aperture plate so as to wrap the processed ridges. This can be easily realized by placing an aperture plate on a sample stage where the sample can be tilted and rotated at the same time, and vapor deposition while tilting and rotating. Finally, side surface processing for exposing Pt as shown in FIG. 5A is performed near the center of the fine wire (near the center of the φ50 μm hole). The length of the exposed portion along the fine line is about 1 μm. Since Pt has a positive contact potential difference with respect to Au, a positive potential distribution centering on the exposed portion is formed in a substantially concentric spherical shape except in the vicinity of the exposed portion.

  When the phase plate formed on the aperture plate is mounted on the objective movable diaphragm of the electron microscope shown in FIG. 2 and installed near the rear focal plane 16 of the objective lens, a phase contrast electron microscope is configured. More specifically, the position of the objective movable diaphragm is adjusted so that the center of the electric potential spreading in a substantially concentric sphere on the thin wire phase plate is slightly away from the optical axis 20 (about 0.5 μm to 1 μm). This is for avoiding that the center of the potential distribution is within the fine line and the non-scattered electron beam traveling on the optical axis 20 is shielded by the fine line.

  The mechanism by which the phase difference is caused by the thin wire phase plate is slightly different from that of the ring phase plate, which will be described. Since the size of the potential generation portion of the fine wire phase plate is smaller than the spread of the potential spreading from there, the potential distribution created by the fine wire phase plate is created by a fine sphere (radius R) given the potential V. Assume that the distribution is approximately equal. Unlike the case of the ring phase plate, the electric potential spreads widely, and both the scattered wave and the non-scattered wave undergo a phase change. In this case, when the non-scattered electron wave travels a distance r from the center of the microsphere on the back focal plane, the phase difference ΔΦ (unit radians) from the scattered electron wave is

のようにあらわされる。

It appears like

  Here, U is the acceleration voltage, λ is the wavelength of the electron wave, e is the elementary charge, m is the mass of the electron, c is the speed of light, f is the focal length of the objective lens, and d is the size of the structure in the sample. If the phase difference ΔΦ according to Equation 5 is 90 degrees, it functions as a phase plate, but the adjustable parameters are increased by one to V, R, and r. V and R are the contact potential difference of Au—Pt and the thickness of the Pt film, respectively, and r is the distance between the thin line and the optical axis. The thickness of the Pt film shown in the above preparation procedure was 2.5 μm. At this time, the phase difference estimated by Equation 5 is five times larger than 90 degrees. However, the actual potential distribution is smaller than the case where microspheres are assumed due to the influence of the Au film remaining above and below the Pt film and the influence of the fine wire serving as the support rod. Therefore, in order to adjust the phase difference to 90 degrees, the remaining parameter r is used to finely adjust the position of the phase plate. For the position adjustment, a fine movement mechanism of a movable objective aperture may be used, or an electromagnetic alignment mechanism such as a deflector may be used. The accuracy of the position adjustment is desirably 1 μm or less because the size of the portion where the potential is generated is about 1 μm. Adjustment of the phase difference to 90 degrees can be obtained from power spectrum analysis of an amorphous image. However, it is not always necessary to aim to adjust the phase difference to 90 degrees, and the purpose of the phase contrast electron microscope is to improve the phase contrast. Therefore, the position of the thin wire phase plate should be adjusted so that the contrast of the observation image is maximized. Just do it.

  The embodiment of the thin wire phase plate has been described above, but the form is not limited to the form shown in FIG. For example, as shown in FIG. 5 (b), the metal 6 having a different work function may be partially placed near the center of the thin metal 7 wire. By laminating Pt on a thin wire (about 0.5 μm wide) made of the same material as the aperture plate (Mo), a substantially spherically symmetrical potential distribution is formed, and the principle explained based on FIG. The phase difference can be generated by the same mechanism. The form of the constructed phase plate is very simple.

  The present invention is useful for an electron microscope, particularly a phase plate used in a transmission electron microscope, and further a phase contrast electron microscope equipped with the phase plate.

DESCRIPTION OF SYMBOLS 1 ... Phase plate 2 ... Hole 3 ... Conductor film (metal film)
4 ... Conductor film (metal film)
5 ... Aperture plate 6 ... Conductor (metal)
7 ... Conductor (metal)
DESCRIPTION OF SYMBOLS 11 ... Electron source 12 ... Irradiation system lens 13 ... Objective lens 14 ... Magnifying lens 15 ... Sample surface 16 ... Rear focal plane 17 of objective lens ... Intermediate image surface 18 ... Image surface 19 ... Electron beam 20 ... Optical axis 31 ... Hole 32 ... support bar 51 ... ring phase plate 52 ... fine wire phase plate 53 ... insulating film 54 ... electrode 55 ... thin film phase plate.

Claims (7)

A phase plate used in an electron microscope,
Consists of a plurality of laminated conductor films,
The plurality of conductor films are at least two kinds of conductor films having different work functions, and the plurality of conductor films are laminated in contact with each other,
The phase plate has a shape with an opening or a fine line shape,
The plurality of stacked conductor films are configured such that at least one kind of conductor film is wrapped with a conductor film different from the conductor film,
A phase plate, wherein a plurality of the conductor films are exposed on a part of an inner surface of the shape having the opening or a part of the thin line shape. A phase plate used in an electron microscope,
Consists of a plurality of laminated conductor films,
The plurality of conductor films are at least two kinds of conductor films having different work functions, and the plurality of conductor films are laminated in contact with each other,
The plurality of stacked conductor films are stacked at a substantially center of a thin line composed of at least one kind of conductor film, and a conductor film different from the conductor film is stacked,
The phase plate, wherein the laminated portion is exposed. The phase plate according to claim 1 or 2,
The phase plate , wherein the plurality of stacked conductor films are metal or semiconductor . A phase plate according to claim 3 ,
The plurality of stacked conductor films are made of any metal of Au, Pt, Cu, Ti, Cr, Mo, Ru, Ta, and W, or an alloy containing the metal as a main component. Feature phase plate. The phase plate according to claim 1 ,
The conductor film is a polycrystalline grain film made of crystal grains, and in the case of the shape having the opening, the average crystal grain size of the crystal grain is the inner diameter of the opening having the shape having the opening. A phase plate characterized by being 1/10 or less . A phase contrast electron microscope comprising a phase plate,
The phase plate according to any one of claims 1 to 5, wherein the phase plate is provided in the vicinity of a point where an objective lens rear focal plane of the phase-contrast electron microscope and an optical axis intersect.
A phase-contrast electron microscope . The phase-contrast electron microscope according to claim 6 ,
A phase-contrast electron microscope comprising a mechanism capable of finely moving the position of the phase plate .

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