JP2005216612A - Accurate axis adjustment method of transmission electron microscope and its device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To adjust the axis of a transmission electron microscope by using a crystalline sample. <P>SOLUTION: The crystalline sample 4 with a minute region irradiated with incident electrons 1 is set by shifting it from a regular focus sample position 3, and a TEM image 7 is observed on a conjugate surface at the regular focus sample position 3. A Bragg diffraction image 8 of the crystalline sample is shifted from a Gaussian image due to aberration. This application comprises a technique and its device for adjusting the axis by estimating an aberration coefficient from the shift. Since the axis can be adjusted by the crystalline sample, the axis can be adjusted on the scene as compared with a conventional technique. As a result, this method has excellent instancy, and can adjust the axis in a condition for observing a high-resolution image. The method speeds up and facilitates adjustment of a multipolar spherical aberration corrector and is considered to be essential for dissemination of spherical aberration. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an axis adjusting method for a transmission electron microscope and an apparatus for performing the same.

  2. Description of the Related Art In recent years, a transmission electron microscope (hereinafter referred to as TEM) has been actively used as a material evaluation method for a minute region. In order to observe an image with high spatial resolution by TEM, it is essential to perform appropriate axis adjustment. The axis adjustment is to appropriately set the astigmatism, the focal point, the incident direction of incident electrons, and the like for the objective lens used for observing the sample. In particular, since the direction and astigmatism of the incident electrons greatly affect the spatial resolution of the obtained TEM image, it is desired to adjust this quickly and accurately.

  Furthermore, in recent years, spherical aberration correction of an objective lens using a spherical aberration corrector composed of multipole elements such as hexapoles and octupoles has been put into practical use. In order to perform aberration correction using this multipole element, it is necessary to quickly and accurately measure the remaining aberration and adjust the multipole corrector.

  Conventionally, a method called a voltage axis adjustment method has been used for axis adjustment of a TEM (for example, Patent Document 1). This is because the TEM acceleration voltage is slightly modulated (for example, about 1 kV with respect to an acceleration voltage of 200 kV), the change in the position of the TEM image due to the voltage modulation is observed, and the incident electrons are minimized so that the position variation of the TEM image is minimized The direction is adjusted.

  In addition, a current axis adjustment method is also used in which the current of the TEM objective lens is modulated to adjust the direction of incident electrons so that the positional fluctuation of the TEM image is minimized. The voltage axis adjustment method and the current axis adjustment method are currently widely used because they can be performed on a fluorescent screen without requiring a special device.

  On the other hand, the need for a new axis adjustment method in place of voltage axis adjustment and current axis adjustment has been pointed out with the recent increase in accuracy and resolution of TEM (Non-patent Document 1). This is to adjust the direction of incident electrons in a direction where there is no coma aberration, and is called a coma-free axis adjustment method. Several methods have been proposed for the frame-free axis adjustment method (for example, Non-Patent Document 2).

  All of the methods proposed so far observe multiple TEM images by changing the direction of the incident electrons, and change the focus, astigmatism and position of the TEM image as the direction of the incident electrons changes. By detecting it, an appropriate incident direction of the electron beam is obtained (for example, Patent Documents 2 and 3 as similar methods).

  Further, a method in which the coma-free axis adjustment method is extended is used to measure the remaining aberration for adjusting the multipole spherical aberration corrector.

  In order to perform these coma-free axis adjustment methods and multipole spherical aberration corrector adjustment methods, a special device that adjusts the direction of incident electrons with high accuracy, astigmatism and the TEM image by Fourier transform A device for calculating the focus is needed.

On the other hand, a method using electrons scattered at a high angle has been proposed as a simple and easy frame-free axis adjustment method that does not require such a special device (Patent Document 4), but its accuracy is limited.
Japanese Unexamined Patent Publication No. 60-167248 (Japanese Patent Publication No. 03-39378, Japanese Patent No. 1672379) JP 60-91540 A Japanese Patent Publication No. 03-78738 JP 2003-197142 A F. Zemlin, et al., Ultramicroscopy, vol. 3, (1978), p49 K. Ishizuka, Ultramicroscopy, vol. 55 (1994) p. 407

  In order to observe a sample with high spatial resolution by TEM, it is necessary to adjust the frame-free axis. However, the coma-free axis adjustment method proposed so far requires an amorphous sample and cannot be used for a crystalline sample. Moreover, although the method using electrons scattered at a high angle is simple, there is a problem in accuracy.

  Furthermore, in order to observe a sample with high spatial resolution by TEM, it is necessary to correct spherical aberration of the objective lens. However, the proposed multipole spherical aberration corrector adjustment method, like the coma-free axis adjustment method, requires an amorphous sample and cannot be used for a crystalline sample.

  The present invention provides a quick and accurate coma-free axis adjustment method and multipole spherical aberration corrector adjustment method that can be applied to a crystalline sample by a principle and means different from conventional ones.

As a method for solving the above problems, the present invention provides an axis adjustment method based on a new aberration measurement method.

  That is, the present invention relates to a frame-free axis adjustment method using position information of a diffraction image, and in a transmission electron microscope that focuses an incident electron on a minute region of a sample and observes an electron microscope image, the sample is a crystalline sample. The incident direction of the incident electrons is set as the crystal zone axis of the sample, and the coma-free axis of the objective lens is adjusted based on the position information of the Bragg diffraction image observed in the electron microscope image. This is a method for adjusting the axis of a microscope.

  The present invention also relates to an astigmatism correction method using position information of a diffraction image. In a transmission electron microscope that focuses an incident electron on a minute region of a sample and observes an electron microscope image, the sample is a crystalline sample. The astigmatism correction amount of the objective lens is adjusted from the positional information of the Bragg diffraction image observed in the electron microscope image, with the incident direction of the incident electrons being substantially the zone axis of the sample. This is an axis adjustment method for an electron microscope.

  Further, the present invention relates to an axis adjustment method using position information of a diffraction image, and in a transmission electron microscope that focuses an incident electron on a minute region of the sample and observes an electron microscope image, the sample is a crystalline sample, Using the incident direction of the incident electrons as the crystal zone axis of the sample, and adjusting the coma-free axis adjustment of the objective lens and the astigmatism correction amount from the positional information of the Bragg diffraction image observed in the electron microscope image. This is a method for adjusting the axis of a transmission electron microscope.

  The present invention also relates to a method for adjusting a spherical aberration corrector using positional information of a diffraction image, and relates to a transmission electron microscope having a spherical aberration corrector that focuses incident electrons on a minute region of a sample and observes an electron microscope image. The spherical aberration corrector is adjusted based on the position information of the Bragg diffraction image observed in the electron microscope image, with the sample being a crystalline sample and the incident direction of the incident electrons being substantially the crystal axis of the sample. This is a method for adjusting the axis of a transmission electron microscope.

The present invention also relates to a method for adjusting a three-fold astigmatism using position information of a diffraction image. A three-fold astigmatism corrector for converging incident electrons in a minute region of a sample and observing an electron microscope image is provided. In the transmission electron microscope, the sample is a crystalline sample, the incident direction of the incident electrons is a substantially crystal zone axis of the sample, and from the position information of the Bragg diffraction image observed in the electron microscope image, the three times A method for adjusting an axis of a transmission electron microscope, comprising adjusting a point corrector.

  The present invention also relates to a method of adjusting a multipole aberration corrector using position information of a diffraction image, and a transmission having a multipole aberration corrector that focuses incident electrons on a minute region of a sample and observes an electron microscope image. In the electron microscope, the sample is a crystalline sample, the incident direction of the incident electrons is a substantially crystal axis of the sample, and the position information of the Bragg diffraction image observed in the electron microscope image is used to calculate the multipole aberration corrector. This is a method for adjusting the axis of a transmission electron microscope.

  Further, the present invention relates to handling of positional information of diffraction images, and in each of the above methods, the axis adjustment of a transmission electron microscope is characterized by using the sum and difference of vectors from transmission electron spots to Bragg diffraction spots. Is the method.

  In addition, the present invention relates to an apparatus incorporating an axis adjustment method, and performs each of the axis adjustment methods described above in a transmission electron microscope including an image acquisition device for detecting an electron microscope image and an analysis device for the electron microscope image. Therefore, the transmission electron microscope has a function of detecting the position of the Bragg diffraction image.

  In the axis adjustment method provided by the present invention, the amount of adjustment necessary for axis adjustment is automatically calculated with high accuracy by an electronic computer based on the positional information of the Bragg diffraction image. If the transmission electron microscope can be externally controlled by an electronic computer, the adjustment amount necessary for the axis adjustment can be automatically set in the transmission electron microscope.

 Conventionally, for axis adjustment of an electron microscope, an electron beam is tilted and a plurality of images are observed and compared. In the present invention, information necessary for axis adjustment can be obtained from one image. In addition, the present invention is a method that has not been performed at all in terms of using a crystalline sample generally used for electron microscope observation and further converging an electron beam on the sample surface. From the analysis of the position of the Bragg diffraction image observed under this condition, there has been no report example that the axis of the objective lens can be adjusted, and there has been no report example examined.

  Since the axis can be adjusted with a crystalline sample, the axis can be adjusted on the spot as compared with the conventional method. As a result, it is excellent in immediacy, and the axis can be adjusted under conditions for observing a high resolution image. In addition, this method is considered to be essential for the widespread use of spherical aberration by making the adjustment of the multipole spherical aberration corrector quick and easy.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings, taking frame free axis adjustment as an example. FIG. 1 is an example of an embodiment of axis adjustment of a transmission electron microscope (TEM) according to the present invention. In the present invention, a minute region in the vicinity of the optical axis 2 of the sample 4 is irradiated with the incident electrons 1, and a TEM image 7 that is largely out of focus than that normally used in TEM is observed. In order to observe a greatly defocused TEM image, in this embodiment, the crystalline sample 4 is brought closer to the objective lens 6 side than the positive focus sample position 3 (position where the focus is achieved).

  For example, in the case of the method and apparatus of the present invention, the size of the minute region to be irradiated is several nm. Further, the defocus amount is about several μm. This value is generally larger than the defocus amount used for high-resolution observation with TEM. By moving the sample 4 to the objective lens 6 side, the observed TEM image 7 becomes an underfocus image.

The electrons 5a transmitted through the sample are observed as transmitted electron spots 8a in the TEM image 7. The scattered electrons 5b, 5c, 5d, and 5e scattered by the crystalline sample are observed as Bragg diffraction images 8b, 8c, 8d, and 8e in the TEM image 7. At this time, the sample is made to have a substantially zone axis with respect to the incident electron beam so that the Bragg diffraction spots appear strongly.

  The positions of these Bragg diffraction images appear out of the ideal position due to the incident direction of the incident electrons and the aberration of the objective lens. In the case of an axially symmetric objective lens, if the incident direction of the electron beam to the objective lens is adjusted so that the Bragg diffraction image appears symmetrically with respect to the transmission electron spot, the frame-free axis adjustment is performed.

  In the case of an objective lens having coma aberration, using a complex expression of lens aberration (K. Ishizuka, Ultramicroscopy, vol. 55 (1994) p. 407), the direction of w = αexp (iφ) from the optical axis 2 The position d of the Bragg diffraction image scattered by is shown as follows in consideration of the third-order aberration.

Here, Cs is a spherical aberration, z is a defocus amount, a ₂ and a ₃ are astigmatisms 2 and 3, b is a coma aberration of the lens itself, and d is a constant shift. The bar above the symbol indicates the complex conjugate of that number.

Next, the expression based on the transmission electron spot is replaced. If the transmission electron spot is at w _m from the optical axis, and the direction of the Bragg diffraction image measured from the transmission electron spot is w _t , then w = w _m + w _t . Further, when the distance obtained by relatively measuring the position of the Bragg diffraction image from the position of the transmission electron spot is d, the following is obtained.

  Here, by obtaining Equation (2) for a plurality of Bragg diffraction images and solving the simultaneous equations, each aberration coefficient can be determined.

Further, the sum and difference of vectors from the transmission electron spot to the Bragg diffraction spot facing each other are _expressed as Δδ _t = (δ (w _t ) + δ (−w _t )) / 2 and <δ _t > = (δ (w _t ), respectively. When defined as −δ (−w _t )) / 2, the vector sum Δδ _t of the Bragg diffraction image is as follows.

Here, B = b + 2Csw _m corresponds to the apparent coma aberration. Expression (3) is obtained for a plurality of Bragg diffraction images, and apparent coma aberration B and three astigmatisms a ₃ can be obtained. Next, attention is paid to the vector difference <δ _t > of the Bragg diffraction image as follows.

Expression (4) is obtained for a plurality of Bragg diffraction images, and spherical aberration Cs, defocus amount z, and astigmatism a ₂ can be obtained. In the equations (3) and (4), it is approximated that the misalignment w _{m in} the incident direction of the incident electrons is sufficiently small with respect to w _t .

  FIG. 2 shows an embodiment according to the present invention. In the state where the coma remains, the Bragg diffraction image does not appear symmetrically with respect to the transmission electron spot as shown in (a). This deviation is measured, and the apparent coma is calculated by equation (3). By tilting the incident electrons from the current incident direction by B / 2Cs, the coma is canceled and symmetrical as shown in (b). Distribution.

  In the above embodiment, the sample position is changed in order to remove the focus of the TEM image. However, the present invention is not limited to this, but the focus of the intermediate lens is changed, the acceleration voltage is changed, and the objective lens is changed. A method such as changing the focal length of the lens may be used.

  In the above-described embodiment (FIG. 2), the TEM image is observed with a hyperfocal point, but the same effect can be obtained by analyzing the position of the Bragg diffraction image of the scattered electron observed with the underfocal point.

  FIG. 3 shows another embodiment. In addition to the TEM 19, an image acquisition device 32 and an image analysis device 33 are provided. The image acquisition device 32 acquires the TEM image 29 and transfers the TEM image 29 to the image analysis device 33. The image analyzer 33 detects the position of the Bragg diffraction image in the TEM image 29. Further, the image analysis device 33 calculates a spherical aberration coefficient, an out-of-focus amount, a coma aberration, two times and three times astigmatism from the deviation amount based on the equation (2).

  Based on the calculation result, the image analysis device 33 instructs the deflection means control device 22 about the necessary deflection amount, whereby the deflection means 21 is driven, the direction of the incident electrons 23 by the electron gun 20 is set, and the frame-free axis adjustment is performed. Is done. Further, the image analysis device 33 instructs the astigmatism correction means control device 31 to change the necessary amount, whereby the astigmatism correction means 30 is driven to perform astigmatism correction. By using the image acquisition device 32, the frame-free axis adjustment and astigmatism correction can be performed with higher accuracy than the operator can visually observe.

  Next, a method for measuring an aberration coefficient necessary for adjusting the multipole spherical aberration corrector will be described. Since the multipole is non-rotationally symmetric, non-rotationally symmetric aberration is generated by using the multipole. Therefore, in the multipole spherical aberration corrector, it is necessary to correct the spherical aberration and adjust the plurality of multipoles so as to minimize the non-rotationally symmetric aberration. The wavefront aberration including this higher-order non-rotationally symmetric aberration is expressed as follows.

Here, c _nm is an aberration coefficient, α is a scattering angle, and φ is a rotation angle. Further, n represents the order of aberration (order), and m represents the symmetric shaft rotation number (fold). Where complex number representation

Is used, the wavefront aberration is expressed as follows.

  From this, the position d of the Bragg diffraction image easily scattered in the direction of w from the optical axis is shown as follows.

  As described above, the function forms related to w of the contribution to the position d of the Bragg diffraction image by each aberration are all different from each other. For this reason, as in the above-described frame-free axis adjustment, each aberration coefficient can be determined by obtaining Equation (8) for a plurality of Bragg diffraction images and solving the simultaneous equations.

FIG. 1 is a conceptual diagram showing an embodiment of an axis adjustment method according to the present invention. FIG. 2 is a drawing-substituting photograph showing a TEM image when (a) is before the axis adjustment and (b) is completed in the embodiment of the axis adjustment method according to the present invention. FIG. 3 is a conceptual diagram showing another embodiment according to the present invention.

Explanation of symbols

1 ... Incident electrons
2 ... Optical axis
3 ... Focus sample position
4 ... Crystalline sample
5a ... Transmission electron
5b-5e ... Scattered electrons (Bragg diffraction wave)
6 ... Objective lens
7 ... TEM image
8a ... Transmission electron spots
8b-8e ... Bragg diffraction image
19 ... Transmission electron microscope
20 ... electron gun
21: Deflection means
22 ... Deflection means control device
23 ... Incident electrons
24 ... Transmission electron
25 ... scattered electrons
26 ... Sample
27… Objective lens
28 ... imaging lens system
29 ... TEM image
30 ... Astigmatism correction means
31 ... Astigmatism correction means control device
32 ... Image acquisition device
33 ... Image analysis device

Claims (8)

In a transmission electron microscope, which converges incident electrons on a very small region of the sample and observes an electron microscope image, the sample is a crystalline sample, the incident direction of the incident electrons is substantially the zone axis of the sample, and the electron microscope A method for adjusting an axis of a transmission electron microscope, comprising: adjusting a coma-free axis of an objective lens based on position information of a Bragg diffraction image observed in an image. In a transmission electron microscope that focuses an incident electron on a minute region of the sample and observes an electron microscope image, the sample is a crystalline sample, the incident direction of the incident electron is a substantially crystal zone axis of the sample, and the electron microscope A method for adjusting an axis of a transmission electron microscope, comprising adjusting an astigmatism correction amount of an objective lens based on position information of a Bragg diffraction image observed in an image. In a transmission electron microscope that focuses an incident electron on a minute region of the sample and observes an electron microscope image, the sample is a crystalline sample, the incident direction of the incident electron is a substantially crystal zone axis of the sample, and the electron microscope A method for adjusting an axis of a transmission electron microscope, comprising adjusting a coma-free axis of an objective lens and an astigmatism correction amount based on position information of a Bragg diffraction image observed in the image. In a transmission electron microscope having a spherical aberration corrector that converges incident electrons on a minute region of the sample and observes an electron microscope image, the sample is a crystalline sample, and the incident direction of the incident electron is substantially the crystal zone of the sample. A method for adjusting an axis of a transmission electron microscope, characterized in that the spherical aberration corrector is adjusted based on positional information of a Bragg diffraction image observed in the electron microscope image. In a transmission electron microscope having a three-fold astigmatism to focus incident electrons on a very small region of the sample and observe an electron microscope image, the sample is a crystalline sample, and the incident direction of the incident electron is substantially the same as that of the sample. A method for adjusting the axis of a transmission electron microscope, comprising adjusting the three-fold astigmatism as a zone axis and adjusting positional information of a Bragg diffraction image observed in the electron microscope image. In a transmission electron microscope having a multipole aberration corrector for converging incident electrons in a minute region of the sample and observing an electron microscope image, the sample is a crystalline sample, and the incident direction of the incident electron is substantially a crystal of the sample. An axis adjustment method for a transmission electron microscope, characterized in that the multipole aberration corrector is adjusted based on position information of a Bragg diffraction image observed in the electron microscope image as a band axis. 7. The method according to claim 1, wherein a vector sum and a difference from a transmission electron spot to a Bragg diffraction spot are used. In a transmission electron microscope provided with an image acquisition device for detecting an electron microscope image and an analysis device for the electron microscope image, in order to perform the axis adjustment method according to any one of claims 1 to 7, A transmission electron microscope having a function of detecting a position.

Images