JP4874863B2 - TEM defocus amount measurement method - Google Patents

Description

  The present invention relates to a TEM defocus amount measurement method, and more particularly, to a defocus amount measurement method capable of stably measuring a defocus amount.

  In image observation with a TEM (transmission electron microscope), it is important to determine the defocus amount. Here, the defocus amount refers to a deviation in excitation of the objective lens. Since the frequency component of the information included in the image differs depending on the defocus amount, it is necessary to realize a strictly determined defocus amount in order to realize the target analysis. This “realizing a strictly determined defocus amount” is hereinafter referred to as “focus adjustment”. Even if the lens state of the TEM is constant, when the position of the sample is moved, the defocus amount changes due to a slight tilt or unevenness of the sample. For this reason, the focusing is usually not performed once by observation of one sample, but is usually performed immediately before taking an image by moving the visual field.

  There are various methods for focusing, but the most widely used method is called a beam tilt method or an image wobble method. This method uses the phenomenon that when an electron beam is tilted and the sample is exposed, the image does not move when the focus is normal, but the image moves when compared with the case where the sample is not tilted when the focus is not correct. .

  FIG. 5 is a conceptual diagram showing how the image moves due to the inclination of the electron beam. In the figure, 1 is a sample and 2 is an objective lens. Point A indicates the state of just focus. B indicates an underfocus state. C indicates an overfocus state. It can be seen that the position of the focused image is shifted between under focus and over focus. At this time, the upper arrow represents that the electron beam is inclined. When observing at the normal focal point, the image does not move even if the electron beam is tilted. At this time, if the tilt angle is constant, the defocus amount and the image movement amount are proportional to each other (see FIG. 4, which will be described in detail later).

  When the beam tilt method is used while observing an image with a fluorescent screen normally mounted on a TEM, the electron beam is not tilted, and it is tilted by a certain amount (or opposite to a certain amount in a certain direction). Sweep the direction in the direction). Then, when the focal point is not the normal focus, the image observed on the fluorescent screen appears to tremble, and the magnitude of the tremor is proportional to the defocus amount. Focus adjustment can be performed by adjusting the defocus amount (adjusting the excitation current value of the objective lens) while observing this shaking.

  When a computer is connected to the TEM for automatic focusing, a digital camera is attached to the TEM so that an image can be captured in the computer, and the electron beam is not tilted and the electron beam is tilted by a certain amount (or a certain amount) Each image is captured one by one in a state tilted in a certain direction and tilted in the opposite direction), and the positional deviation between the two images is measured. When measuring the amount of deviation between two images, a method such as normalized correlation is often used.

  FIG. 6 is a conceptual diagram for detecting a positional deviation between two images by normalized correlation. (A) is a reference image, (b) is an input image, and (c) is a normalized correlation image thereof. The peak of the normalized correlation image (the part indicated by the arrow) corresponds to the positional deviation of the input image with respect to the reference image. The figure shows an example of a main image in a display screen displayed on the display in the embodiment of the present invention as a photograph of a halftone image.

  The normalized correlation equation is expressed by the following equation.

Here, g (X, Y) is a normalized correlation value at the coordinate value (X, Y), h (x, y) is a luminance value at the coordinate value (x, y) of the reference image, and f (x, y). ) Is a luminance value at the coordinate value (x, y) of the input image, and Σ indicates that a sum is taken for the entire image. When the input image f is an image shifted by a in the x direction and b in the y direction with respect to the reference image h, the value of g is the maximum at (a, b). That is, by obtaining (X, Y) that maximizes the value of g, it is possible to know how much the input image f is deviated from the reference image h.

As a conventional method of this type, a cross-correlation between the image A before tilting the electron beam and the image B after tilting is taken, and the movement vector appearing in the cross-correlation represents the shift amount δ between the two images and the direction of the shift. A technique is known in which focusing is performed by changing the excitation of the objective lens based on the obtained shift amount δ (see, for example, Patent Document 1).
JP 2004-55143 A (paragraphs 0019 to 0035, FIG. 1)

  As described above, the method for automatically obtaining the misregistration requires two images, which causes various problems. In order to minimize image noise, a type of camera called a slow scan camera is often used, but when such a camera is used, it takes several seconds to several tens of seconds to capture one image. For this reason, there is a time between capturing two images, and when the sample is drifting, image movement that does not depend on the electron beam tilt also occurs.

  In particular, in the case of a sample that is easily damaged by electron beam irradiation, it is necessary to irradiate the electron beam only at the time of photographing, and not to irradiate it while transferring the photographed image to the computer over several seconds to several tens of seconds. Has been done. However, it is known that an image moves due to charge accumulation in the sample immediately after the start of electron beam irradiation, particularly in a state of low electrical conductivity such as a frozen sample. Shooting is started after this movement is settled, but once the irradiation is stopped and then irradiation is started for the next shooting, it moves again for that purpose. That is, even in this case, there is a problem that image movement is not caused by electron beam tilt.

  The present invention has been made in view of such a problem, and provides a defocus amount measurement method capable of stably measuring a defocus amount by eliminating image movement not caused by electron beam inclination. The purpose is that.

(1) According to the first aspect of the present invention, an electron beam at a certain angle is irradiated onto a sample in a TEM to stabilize an image at that time, and then the same amount of electrons in the direction opposite to the electron beam inclination is used. When the sample is irradiated with a tilted line and the image is stabilized, the electron beam irradiation image is read by the image pickup means, and the amount of deviation of the image is obtained by taking autocorrelation with the electron beam irradiation image, A defocus amount is obtained from the obtained image shift amount.
(2) The invention described in claim 2 is characterized in that the image to be read is stabilized by allowing the image pickup means to wait until an image with sufficient contrast is formed in an inclined state of the electron beam. The TEM defocus amount measuring method according to claim 1.
(3) The invention described in claim 3 is characterized in that autocorrelation is performed on the electron beam irradiation image, a peak other than the central peak is searched for, and the defocus amount is estimated from the position. .

(1) According to the first aspect of the present invention, it is possible to stably measure the defocus amount without the image movement not depending on the electron beam inclination.
(2) According to the second aspect of the present invention, since the image is picked up in a state where sufficient contrast is obtained, the defocus amount can be accurately measured.
(3) According to the invention described in claim 3, since the autocorrelation is taken with respect to the electron beam irradiation image, it is possible to eliminate the drift of the sample and stably measure the defocus amount.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In FIG. 1, 1 is a TEM. Reference numeral 4 denotes a digital camera for capturing a sample transmission image provided in the TEM 1, and 3 denotes a computer. 2 is an image capturing cable for connecting the digital camera 4 and the computer 3, and 5 is a control cable for connecting the TEM 1 and the computer 3. Reference numeral 10 denotes software for executing the present invention provided in the computer 3. In the computer 3, the software 10 according to the present invention is operated to acquire an image from the digital camera 4 through the image capturing cable 2, control the TEM 1 through the control cable 5, and use the captured image as the data. A calculation for estimating the focus amount is performed. The operation of the apparatus configured as described above will be described as follows.

FIG. 2 is an operation flow of the present invention. Hereinafter, this operation flow will be used to describe the present invention in detail. As a precondition of the present invention, it is assumed that the relationship between the defocus amount and the image shift amount when the electron beam is tilted by a certain amount in the lens state (magnification, etc.) to be used is known.
S1. Activating the Software of the Present Invention The operator activates the software 10 of the present invention on the computer 3.
S2. Tilt the electron beam to one side The software 10 of the present invention operates a TEM deflector (not shown) via the TEM control cable 5 to direct the electron beam to a sample in a certain direction and a certain direction. Start irradiating.
S3. The exposure of the digital camera is started The software 10 of the present invention starts exposure using the digital camera 4 via the image capturing cable 2 after a sufficient time has passed for the sample fluctuation immediately after the start of irradiation to settle. .
S4. Waiting for a certain period of time The software 10 of the present invention waits for a certain period of time so that an image with sufficient contrast is formed on the image element of the digital camera 4 with the electron beam tilt set in step S2. This waiting time can be incorporated in the software 10 by measuring the time until a sufficient contrast appears in the image in the previous stage. As the amount of this waiting time, for example, about 0.1 second is used.
S5. Tilt the electron beam to the opposite side The software 10 of the present invention tilts the electron beam by the same amount to the side opposite to the electron beam tilt set in step S2 without stopping the exposure of the digital camera 4, and continues the exposure.
S6. Waiting for a certain period of time The software 10 of the present invention waits for a certain period of time so that an image with sufficient contrast is formed on the image element of the digital camera 4 with the electron beam tilt set in step S5. This waiting time may be incorporated into the software 10 in the manner set in step S4. Thereby, the image set in step S2 and the image set in step S5 are overlapped and exposed.
S7. Completing the exposure of the digital camera and capturing the image The software 10 of the present invention terminates the exposure by the digital camera 4 and captures the double-exposed image from the digital camera 4 via the image capturing cable 2.
S8. Obtaining the peak position of the autocorrelation image and estimating the defocus amount The software 10 of the present invention calculates the autocorrelation of the image captured in step S7, searches for a peak other than the central peak, and determines the defocus amount from that position. Is fed back to the objective lens current value so as to obtain an arbitrary defocus amount if necessary.

  The autocorrelation is calculated assuming that the reference image and the input image are the same in the normalized correlation described above. That is, when both the reference image and the input image in equation (1) are calculated as f (x, y), the value k (X, Y) of the normalization function at this time is expressed by the following equation.

The autocorrelation value k is maximum when (X, Y) = (0, 0), but a certain image and an image obtained by shifting the image by a in the x direction and b in the y direction are doubled. In the image exposed to k, k becomes a maximum also in (a, b) and (-a, -b). In other words, if there is a coordinate where k is a maximum other than (0, 0), the same shape exists in the image, and the amount of deviation is obtained from the maximum position.

  FIG. 3 is a conceptual diagram for detecting a positional shift in one image by autocorrelation. This figure is a diagram showing an example of a main image in a display screen displayed on a display according to an embodiment of the present invention as a photograph of a halftone image. (A) is an input image, (b) is the autocorrelation image. The autocorrelation image has a maximum peak (small arrow) at the center, but the same contrast exists in the input image with a shift, so there is another maximum at the position corresponding to the shift (large arrow).

  The software 10 estimates the defocus value from this large arrow displacement. FIG. 4 is an explanatory diagram for obtaining the defocus amount from the positional shift amount. In the figure, the vertical axis represents the image shift amount, and the horizontal axis represents the defocus amount. It is assumed that the software 10 incorporates the characteristic J in advance. In a region where the defocus amount is relatively small, this characteristic can be regarded as a linear expression. Therefore, when the peak value P1 is obtained, this P1 is substituted into the characteristic J, and the defocus amount Q1 at that time is obtained. In this way, the defocus amount corresponding to the positional deviation amount can be obtained.

  As described above in detail, according to the present invention, it is possible to provide a defocus amount measuring method capable of stably measuring a defocus amount without the image movement not depending on the electron beam tilt. Can be practically effective.

It is a figure which shows the system configuration example to which this invention is applied. It is a figure which shows the operation | movement flow of this invention. It is a conceptual diagram which detects the position shift in one image by autocorrelation. It is explanatory drawing at the time of calculating | requiring a defocus amount from a positional offset amount. It is a conceptual diagram of a mode that an image moves with the inclination of an electron beam. It is a conceptual diagram which detects the position shift of two images by normalization correlation.

Explanation of symbols

1 TEM
2 Image capture cable 3 Computer 4 Digital camera 5 Control cable 10 Software

Claims (3)

Irradiate an electron beam at a certain angle to the sample in TEM to stabilize the image at that time,
Next, tilt the electron beam by the same amount in the direction opposite to the electron beam tilt and irradiate the sample,
When the image is stabilized, the electron beam irradiation image is read by the image pickup means,
Taking the autocorrelation with the electron beam irradiation image to determine the amount of image shift,
Find the defocus amount from the obtained image shift amount,
TEM defocus amount measuring method characterized by the above.   2. The defocus amount measurement of a TEM according to claim 1, wherein the image to be read is stabilized by waiting until an image with sufficient contrast is formed on the image pickup means in an electron beam tilt state. Method.   2. The TEM defocus amount measurement according to claim 1, wherein autocorrelation is performed on the electron beam irradiation image, a peak other than a central peak is searched for, and a defocus amount is estimated from the position. Method.