JP6106547B2 - Transmission electron microscope - Google Patents

Description

  The present invention relates to a transmission electron microscope, and more particularly to a transmission scanning electron microscope that performs calibration of defocus of an objective lens of a transmission electron microscope with high accuracy.

  An electron microscope typified by a transmission electron microscope has an autofocus function. In autofocus, the excitation current of the objective lens is adjusted so that the electron beam is focused on the sample using parameters such as the amount of blur of the sample image.

  The defocus calibration parameter representing the ratio of the change in the excitation current amount of the objective lens to the change amount of the focal length is determined by the following calibration method. First, after aligning the sample surface with the normal focus, the sample position is shifted by a unit amount by moving the stage, and the object lens is used again to adjust to the normal focus to obtain the current change amount.

JP 2012-28324 A

  In the above defocus parameter determination method, the accuracy of the defocus calibration parameter may be reduced due to the positioning accuracy of the stage. The defocus parameter shift may cause an autofocus shift, which may deteriorate the measurement accuracy of the transmission electron microscope.

  In particular, when measuring the aberration of a transmission electron microscope, it is necessary to accurately perform defocusing on the order of several tens of nanometers, and the deviation of the defocus parameter may deteriorate the measurement accuracy of the transmission electron microscope. .

  An object of the present invention is to provide a scanning transmission electron microscope having a function of determining a defocus calibration parameter of an objective lens without depending on the positioning accuracy of the stage.

  A typical configuration of the present invention is as follows. An electron source that emits an electron beam, an objective lens that focuses the electron source on the sample, a detector that detects electrons transmitted through the sample, and a display device that displays a Ronchigram image using a signal from the detector A change in the Ronchigram image when the focal position with respect to the sample is changed by changing the excitation current value of the objective lens in a scanning transmission electron microscope comprising the aberration corrector for correcting the aberration of the electron beam A scanning transmission electron microscope characterized in that the excitation current of the objective lens is changed based on a focal position change amount with respect to an excitation current amount change obtained from the equation (1).

  Since the transmission electron microscope of the present invention does not require stage movement during defocus calibration, defocus calibration can be performed with high accuracy without depending on the positioning accuracy of the stage. Thereby, it is possible to provide a transmission electron microscope capable of measuring with higher accuracy.

Configuration diagram showing the main configuration of a scanning transmission electron microscope Flow chart showing defocus calibration procedure Screen diagram showing an example of the screen displayed on the display Flow chart showing the procedure for calculating the index value for boundary determination

  First, the principle of the present invention will be described. In the present invention, a Ronchigram hexagonal pattern is used in a scanning transmission electron microscope equipped with an aberration corrector. The Ronchigram is disclosed in, for example, Patent Document 1. Utilizing the fact that the stripe-like stretch appearing in the hexagonal pattern of the Ronchigram due to astigmatism appears to overlap images with different orientations by 90 ° when the focal point is aligned inside the thick sample. That is, when the crossing of the stripes can be confirmed in the Ronchigram image, it can be determined that the focal position is at any position between the upper boundary surface (front surface) and the lower boundary surface (back surface) of the sample. The change in the focus position with respect to the excitation current of the objective lens can be determined from the change in the Ronchigram image when the width of the known sample and the focus position are shifted, and the defocus parameter can be determined.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a diagram illustrating a configuration example of a scanning transmission electron microscope apparatus (STEM) including an aberration corrector according to an embodiment of the present invention. In FIG. 1, the electron beam applied to the sample on the sample stage 6 from the electron gun 1 passes through the sample and is photographed by an imaging device such as a camera 9. The captured image signal is sent to the image processing device 13, held in the information processing device 15, and displayed on the display 14.

  The transmission scanning electron microscope includes a focusing lens 2 that focuses an electron beam irradiated from an electron gun 1, an objective lens 5 that focuses the electron beam on a sample, an aberration corrector 3 that corrects aberrations of the objective lens 5, and an electron beam as a sample. A scan coil 4 that scans above, a detector 8 that detects an electron beam transmitted, scattered, and diffracted through the sample, an electron gun controller 10 that applies a high voltage to the electron gun 1, and an electron beam control that drives the lens and coil An apparatus 11, an aberration corrector control device 12 that drives the aberration corrector, and an information processing device 15 that executes a program, gives instructions to each control device, and the like.

  FIG. 2 is a flowchart showing the procedure of defocus calibration in the present invention. Each step will be described below.

  (Step 201) First, aberration correction is performed by controlling the aberration corrector 3 until a flattened region can be confirmed in a Ronchigram image captured using the camera 9. At this time, a sample having a known thickness is used.

  (Step 202) When Step 201 is completed, a two-fold astigmatism is intentionally introduced using the aberration corrector 3. As a result, it becomes possible to observe a linear fringe shape whose directions are different by 90 ° in under focus and over focus.

  (Step 203) Next, the objective lens 5 is controlled to take an image (through focus image) taken with the focal length shifted little by little, and is held in the information processing device 15. The held image is displayed on the display device 14. The user discriminates the range in which the intersection of the stripes can be confirmed in the Ronchigram image from the displayed image, and the information processing apparatus 15 receives the input of the discrimination result of the user. An example of the screen at this time is shown in FIG.

  (Step 204) When the crossing of the stripes can be confirmed in the Ronchigram image, it can be determined that the focal position is at any position between the upper boundary surface (front surface) and the lower boundary surface (back surface) of the sample. This is because when the sample has a focal point, an underfocus image is formed on the surface side of the sample and an overfocus image is formed on the back side of the sample, and these images appear to overlap. In order to obtain the crossing of the stripes of the Ronchigram image, it is desirable that the thickness of the sample is as thick as possible within the range where the Ronchigram image can be observed. When prompting the user to determine the intersection of the stripes of the Ronchigram image, the information processing device 15 calculates a value that serves as an index for boundary determination and displays it on the display 14 together with the image and the shooting conditions.

  Finally, the change in the focus position relative to the excitation current of the objective lens can be determined from the change in the Ronchigram image when the width of the known sample and the focus position are shifted, that is, the change as shown in FIG. Parameters can be determined. The excitation current of the objective lens is changed using the obtained defocus parameter, and the electron beam can be focused at a desired position.

  It can be achieved by programming the above steps and executing them in the information processing apparatus 15.

  The defocus parameter determination method in step 204 will be described in more detail with reference to FIG. If this method is used, the determination of a defocus parameter can be automated. FIG. 4 is a flowchart of an index value calculation process for boundary determination.

  (Step 401) Smoothing processing is performed on each Ronchigram image at each focus position in FIG.

  (Step 402) Further, binarization processing is performed on the smoothed image. By the processing in steps 401 and 402, unnecessary information included in the image is reduced, and feature amounts can be easily extracted.

  (Step 403) Next, all the images are rotated at the same angle so that the component along the stripe and the component orthogonal thereto are along the coordinate axis of the image. Since the directions of the stripes at the time of underfocus and at the time of overfocus are orthogonal to each other, both of them may be rotated along the coordinate axis.

  (Step 404) Next, a difference filter or the like is applied to each image in the vertical and horizontal directions to extract edges. By the processing so far, an image obtained by extracting the component in the direction of the stripe and the direction orthogonal thereto is obtained. In steps 401 to 404, the detection accuracy and the like vary depending on the order, but the order is not necessarily required.

  (Step 405) When step 404 is completed, the average luminance value of the image from which the edge is extracted in each of the vertical and horizontal directions is calculated.

  (Step 406) Finally, the ratio of the vertical and horizontal average luminance values is calculated for each image, and used as an index value for boundary determination. This index value becomes a value close to 1 when the stripes of the Ronchigram image intersect and are evenly included. The magnitude of this value, the increase, or the point at which the phenomenon starts helps the boundary determination. Further, a predetermined value is set for the ratio between the average luminance value in the vertical and horizontal directions, and it can be determined that the electron beam is focused on the front or back surface of the sample.

  The objective lens current when the focus is on the sample surface and the sample back surface is obtained from the discrimination result of the presence or absence of crossing of the image stripes input in step 204. When the difference between the current values of the two objective lenses thus obtained is ΔI (μA) and the thickness of the sample used is d (nm), the defocus calibration parameter Calib (μA / nm) is Calib = ΔI / d. expressed.

  By using the calculated defocus calibration parameter Calib, it is possible to perform accurate autofocus independent of stage movement, and to improve measurement accuracy.

  According to the embodiment of the present invention, since the reference surface for focusing at the time of defocus calibration is the front surface and the back surface of the sample, the distance between the reference surfaces can be obtained without depending on the positioning accuracy of the stage. By obtaining the distance between the reference planes and the change in the current amount of the objective lens necessary for moving the focal position, it is possible to calibrate the defocus with high accuracy.

DESCRIPTION OF SYMBOLS 1 Electron gun 2 Converging lens 3 Aberration corrector 4 Scan coil 5 Objective lens 6 Sample stand 7 Projection lens 8 Detector 9 Camera 10 Electron gun control device 11 Electron beam control device 12 Aberration corrector control device 13 Image processing device 14 Display 15 Information processing device

Claims (6)

An electron source that emits an electron beam;
An objective lens for focusing the electron source on the sample;
A detector for detecting electrons transmitted through the sample;
A display device for displaying a Ronchigram image using a signal from the detector;
An aberration corrector for correcting the aberration of the electron beam;
In a scanning transmission electron microscope with
Changing the excitation current of the objective lens based on the amount of change in the focal position with respect to the change in the amount of excitation current obtained from the change in the Ronchigram image when the focal position with respect to the sample is changed by changing the excitation current value of the objective lens. Scanning transmission electron microscope. The scanning transmission electron microscope of claim 1,
The focal position change amount with respect to the excitation current amount change is obtained from the fluctuation of the vertical and horizontal components of the stripes appearing on the Ronchigram image when the focal position with respect to the sample is changed by changing the excitation current value of the objective lens. Scanning transmission electron microscope. The scanning transmission electron microscope of claim 2,
A scanning transmission electron microscope characterized in that a focal position change amount with respect to a change in excitation current amount is obtained from a change in a ratio of vertical and horizontal average luminance values of the Ronchigram image . An electron source that emits an electron beam;
An objective lens for focusing the electron source on the sample;
A detector for detecting electrons transmitted through the sample;
A display device for displaying a Ronchigram image using a signal from the detector;
An aberration corrector for correcting the aberration of the electron beam;
In a method for adjusting the focal position using a scanning transmission electron microscope comprising:
Changing the excitation current of the objective lens based on the amount of change in the focal position with respect to the change in the amount of excitation current obtained from the change in the Ronchigram image when the focal position with respect to the sample is changed by changing the excitation current value of the objective lens. A focal position adjustment method characterized by the above. The focal position adjustment method according to claim 4, wherein
The focal position change amount with respect to the excitation current amount change is obtained from the fluctuation of the vertical and horizontal components of the stripes appearing on the Ronchigram image when the focal position with respect to the sample is changed by changing the excitation current value of the objective lens. Focus position adjustment method. The focus position adjusting method according to claim 5, wherein
A focal position adjustment method for obtaining a focal position change amount with respect to a change in excitation current amount from a change in a ratio between vertical and horizontal average luminance values of the Ronchigram image .