JP4634324B2 - Transmission electron microscope - Google Patents

Description

  The present invention relates to a transmission electron microscope, and more particularly to a transmission electron microscope having an autofocus function.

  In a transmission electron microscope, a search in a wide range causes a shift in the Z direction, and the brightness of an image changes when an autofocus is operated. For example, when searching for microparticles with a transmission electron microscope, the search efficiency decreases due to a change in the brightness of the image. As a result, the reliability of the observation data and analysis data decreases.

  An object of the present invention is to provide a transmission electron microscope in which the brightness of an image does not change even when an autofocus is operated.

  According to the transmission electron microscope of the present invention, the brightness of the image is adjusted so that the brightness of the image does not change even when the current value of the objective lens changes due to autofocus.

  According to the transmission electron microscope of the present invention, the brightness of the image does not change even when the autofocus is operated.

  An example of a transmission electron microscope according to the present invention will be described with reference to FIG. The transmission electron microscope of this example includes an electron gun 1, first and second converging lens coils 2 and 3, first and second deflection coils 4 and 5, objective lens coil 6, first and second electromagnetic types. Sample image moving coils 7 and 8, first and second intermediate lens coils 9 and 10, first and second projection lens coils 11 and 12, excitation power supplies 13 to 23, digital analog converter (DAC) 24 to 34, a microprocessor 35, a storage device 36, a computing device 37, a CRT controller 38, a CRT (monitor) 39, an interface (I / F) 40 to 41, a magnification switching rotary encoder 42, an input rotary encoder 43, and a keyboard 44. Mouse 45, RAM 46, ROM 47, image capture interface 48, TV camera control unit 49, and TV camera 50. The TV camera 50 includes an optical lens 51 and a CCD 52. A sample stage 53 and a scintillator 54 are provided on the optical axis.

  When searching for particles, the observer uses the keyboard 44 and mouse 45 to move the field of view and search for particles to be searched. The transmission electron image lens data stored in the ROM 47 is read out and supplied to the digital-analog converters (DACs) 24 to 34. Digital-analog converters (DACs) 24 to 34 convert lens system data into analog signals and supply them to excitation power supplies 13 to 23. The excitation power supplies 13 to 23 output current to the lens coils 2, 3, 6, 9 to 12 of each lens system.

  The electron beam generated by the electron gun 1 is converged by the first and second converging lens coils 2 and 3, deflected by the first and second deflecting coils 4 and 5, and imaged by the objective lens coil 6. The sample on the sample stage 53 is irradiated.

  The electron beam that has passed through the sample is converted into first and second electromagnetic sample image moving coils 7 and 8, first and second intermediate lens coils 9 and 10, and first and second projection lens coils 11. , 12 and projected onto the scintillator 54. The scintillator 54 has a phosphor coated on transparent glass. Therefore, the phosphor emits a transmission image of the sample. The transmitted transmission image is transmitted through the transparent glass and is received by the CCD 52 through the optical lens 51. The CCD 52 converts the received transmission image into an electric signal and transmits it to the TV camera control unit 49. The TV camera control unit 49 transmits the image data to the microprocessor 35, the RAM 46, the storage device 36, and the arithmetic device 37 via the image capture interface 48. The transmission image is stored in the RAM 46 and the storage device 36 and displayed by a CRT (monitor) 39.

  The transmission electron microscope of this example has an autofocus function and a brightness adjustment function for correcting the brightness of an image. The autofocus mechanism of a transmission electron microscope is well known and will not be described in detail here. The transmission electron microscope of this example further has a pattern matching function, which is also a well-known technique. The brightness adjustment function performs control so that the brightness of the image does not change when the current value of the objective lens changes due to autofocus. That is, control is performed so that the brightness of the image does not change before and after autofocus.

  The autofocus function, the pattern matching function, and the brightness adjustment function may be configured by a computer-readable program, and are realized by the microprocessor 35, the RAM 46, the storage device 36, and the arithmetic device 37.

  When the current value of the objective lens changes due to autofocus, first, the first brightness adjustment is performed. The brightness of the image after the first brightness adjustment is compared with the brightness of the image before autofocus. If the degree of coincidence between the two is low, the initial brightness adjustment is insufficient, and the brightness adjustment is performed again. In this way, the brightness adjustment is performed until the brightness of the image subjected to the brightness adjustment matches the brightness of the image before the autofocus.

  In the first brightness adjustment, the correction amount is determined based on a comparison between the brightness of the image before autofocus and the brightness of the image after autofocus. In the second and subsequent brightness adjustments, the correction amount is determined based on a comparison between the brightness of the image before autofocus and the brightness of the image after brightness adjustment.

  When comparing the brightness of images, the luminance distribution of the images may be compared, the peak positions of the luminance distribution of the images may be compared, or the contrast of the images may be compared. Further, the current values of the CCD cameras may be compared. When comparing the luminance distribution of images, the mobility or coincidence of images using image correlation may be calculated.

  The brightness of the image is basically determined by the electron beam density on the sample. Therefore, in order to correct the brightness of the image, the convergent lens current may be changed. You may adjust the electron beam density on a sample using a diaphragm. Instead of changing the convergent lens current, the electron gun emission current, the CCD camera gain, the exposure time, and the like may be adjusted.

  The relationship between the objective lens current and the convergent lens current whose image brightness does not change may be obtained in advance. When the objective lens current value is changed by autofocus, the brightness can be adjusted by obtaining the convergent lens current from such a relationship. Similarly, the relationship between the objective lens current that does not change the brightness of the image and the electron gun emission current may be obtained in advance. When the objective lens current value is changed by autofocus, the brightness can be adjusted by obtaining the electron gun emission current from such a relationship.

  When searching for fine particles by a transmission electron microscope according to the present invention, the following effects are obtained. (1) The image searched for particles and the reliability can be displayed. (2) Particle search efficiency can be improved. (3) Brightness that varies due to autofocus can be corrected. (4) Correct the fluctuating brightness to improve the accuracy of autofocus. Therefore, the reliability of the observation data can be proved when searching for fine particles by the transmission electron microscope according to the present invention.

  A first example of a method for searching for microparticles using a transmission electron microscope according to the present invention will be described with reference to FIG. In step S101, search accuracy and a pattern of search target particles are set. For example, the search accuracy, the diameter of the search target particle, and the ellipticity of the search target particle are input on the input screen shown in FIG. As the search accuracy, a preset value may be used. In step S102, the field of view to be searched is set. As shown on the left side of the input screen shown in FIG. 5, one of the sample meshes is designated. In step S103, the visual field is moved. As the method of moving the visual field, the sample stage 53 may be used as shown in FIG. 23, but the visual field may be moved electromagnetically using the deflection coils 4 and 5 as shown in FIG. In step S104, the movement of the visual field is stopped.

  In step S 105, the image I 1 is captured and stored in the RAM 46. The image I1 is an image before autofocus. A luminance distribution LUT1 of the image I1 is obtained and stored in the RAM 46. As shown in FIG. 13, the luminance distribution LUT1 is a luminance distribution curve (histogram) when gradation is taken on the horizontal axis and the number of pixels is taken on the vertical axis.

  Auto focus is performed in steps S106 to S112. First, in step S106, the electron beam is tilted by the tilt angle α, the image I1α is captured, and stored in the RAM 46. For example, the tilt angle α is input on the input screen shown in FIG.

  When the normal focus is on the sample, the image does not move even if the electron beam is tilted. However, when the focus is shifted, the image moves by tilting the electron beam. Use this phenomenon.

  In step S107, the movement amount and coincidence between the two images I1 and I1α are calculated. The movement amount and the degree of coincidence may be calculated using the methods (a) and (b) described later. At this time, calculation may be performed using normalized correlation. In step S108, it is determined whether or not the degree of coincidence is greater than 50%. Here, 50% is used as the threshold value of the degree of coincidence.

  If the degree of coincidence is greater than 50%, the process proceeds to step S111. If the degree of coincidence is not greater than 50%, the process proceeds to step S109, and the tilt angle of the electron beam is halved. In step S110, the current value of the objective lens is changed, and the process returns to step S107.

  Here, in order to increase the degree of coincidence, the inclination angle of the electron beam was halved. As shown in FIGS. 17 and 18, the smaller the inclination angle of the electron beam, the larger the correction accuracy and the correction range. Because.

  If the degree of coincidence is greater than 50%, it is determined in step S111 whether or not the inclination angle of the electron beam is a preset value α. If the tilt angle of the electron beam is not α, the process returns to step S106 to set the tilt angle of the electron beam to α.

  Thus, by repeating step S106 to step S111, the inclination angle of the electron beam is α and the degree of coincidence is greater than 50%.

  In step S112, the defocus amount is calculated using the inclination angle α, and is fed back to the objective lens current. The calculation of the defocus amount is performed using an expression described later in (f).

  When autofocus is thus completed, the image I2 is captured and stored in the RAM 46 in step S113. Image I2 is an image after autofocus. A luminance distribution LUT2 of the image I2 is obtained and stored in the RAM 46. In step S114, the brightness of the image before and after autofocus is compared. That is, the correlation between the luminance distribution LUT1 of the image I1 before autofocus and the luminance distribution LUT2 of the image I2 after autofocus is calculated.

  A method for calculating the correlation of the luminance distribution will be described later in (d). That is, as described later in (a) and (b), the degree of coincidence using image correlation may be calculated.

  In step S115, brightness adjustment is performed using the result of the correlation calculation. The brightness adjustment is performed by controlling the current value of the converging lens coil. Brightness adjustment methods include emission current control, CCD camera gain adjustment, exposure time adjustment, and the like. The brightness adjustment method may be input on the screens shown in FIGS. In step S 116, the image I 3 is captured and stored in the RAM 46. The image I3 is an image after brightness adjustment. A luminance distribution LUT3 of the image I3 is obtained and stored in the RAM 46.

  In step S117, the brightness of the image before autofocus and the image after brightness adjustment are compared. That is, the degree of coincidence between the luminance distribution LUT1 of the image I1 and the luminance distribution LUT3 of the image I3 is obtained, and it is determined whether or not it is greater than 50%. Here, 50% is used as the threshold value of the degree of coincidence. As the matching degree threshold value, the input value of the search accuracy in step S101 may be used, or may be input separately.

  A method of calculating the degree of coincidence of the luminance distribution will be described later in (d). That is, as described later in (a) and (b), the degree of coincidence using image correlation may be calculated.

  If the degree of coincidence is greater than 50%, the process proceeds to step S118. If the degree of coincidence is not greater than 50%, the process returns to step S115 and the brightness is adjusted again. When the degree of coincidence becomes greater than 50%, the process proceeds to step S118. The degree of coincidence is displayed on the screens shown in FIGS.

  In step S118, pattern matching is performed to search for particles. In step S119, the image I3 subjected to pattern matching and its luminance distribution LUT3 are stored in the RAM 46. The correlation value between the luminance distribution LUT1 of the image I1 and the luminance distribution LUT3 of the image I3 is stored in the RAM 46. As shown in FIG. 6, the number of retrieved particles and the retrieval accuracy (matching degree) can be displayed.

  In step S120, the visual field is moved. In step S121, it is determined whether the particle search has been completed for all fields of view. If the particle search has not been completed for all fields of view, the process returns to step S103. When the particle search is completed for all fields of view, this process is terminated.

  In this example, the luminance distribution LUT1 of the image I1 before autofocus is stored in step S105, and the luminance distribution LUT1 of the image I1 before autofocus and the luminance distribution LUT2 of the image I2 after autofocus are stored in step S114. Calculate the correlation. In step S117, the degree of coincidence between the luminance distribution LUT1 of the image I1 before autofocus and the luminance distribution LUT3 of the image I3 after brightness adjustment is obtained. However, in these steps, the peak position of the luminance distribution or the contrast of the image may be used instead of the luminance distribution of the image.

  A case where the peak position of the luminance distribution is used will be described. The peak position of the luminance distribution can be obtained from the luminance distribution (histogram) of the image as shown in FIG. In step S114, the correlation between the peak position pLUT1 of the luminance distribution of the image I1 before autofocus and the peak position pLUT2 of the luminance distribution of the image I2 after autofocus is calculated. A method for calculating the correlation of the peak positions of the luminance distribution will be described later in (e). That is, the ratio or difference between two peak positions is used.

  In step S117, the degree of coincidence between the peak position pLUT1 of the luminance distribution of the image I1 and the peak position pLUT3 of the luminance distribution of the image I3 is obtained, and it is determined whether or not it is greater than 50%. A method for calculating the degree of coincidence of the peak positions of the luminance distribution will be described later in (e). That is, the ratio of two peak positions is used.

  A case where the contrast of an image is used will be described. As shown in FIG. 8, the contrast is expressed as a ratio of the intensity I1 of the particle image to be searched to the background intensity I0. In step S114, the correlation between the contrast C1 of the image I1 before autofocus and the contrast C2 of the image I2 after autofocus is calculated. A method for calculating the correlation of contrast will be described later in (h). That is, the ratio, difference, etc. of the two trusts are used.

  In step S117, the degree of coincidence between the contrast C1 of the image I1 and the contrast C3 of the image I3 is obtained, and it is determined whether or not it is greater than 50%. A method for calculating the degree of coincidence of contrast will be described later in (h). That is, the ratio of the two contrasts is used.

  A second example of a method for searching for microparticles using a transmission electron microscope according to the present invention will be described with reference to FIG. In the first example of FIG. 2, the luminance distribution of the image is used, but in this example, the current value of the CCD camera is used. A method for measuring the current value of the CCD camera is shown in FIG. FIG. 22 will be described later.

  Steps S201 to S204 are the same as steps S101 to S104 in FIG. In step S 205, the image I 1 is captured and stored in the RAM 46. The image I1 is an image before autofocus. The current value A1 of the CCD camera is measured and stored in the RAM 46.

  The autofocus process from step S206 to step S212 is the same as the process from step S106 to step S112 in FIG. When the autofocus is finished, the image I2 is captured and stored in the RAM 46 in step S213. Image I2 is an image after autofocus. Further, the current value A 2 of the CCD camera is measured and stored in the RAM 46. In step S214, the brightness of the image before and after autofocus is compared. That is, the correlation between the current value A1 before autofocus and the current value A2 after autofocus is calculated. As an example of the calculation method of the correlation between the current values, the ratio or difference between the two current values is calculated as will be described later in (g). In step S215, brightness adjustment is performed using the result of the correlation calculation.

  In step S 216, the image I 3 is captured and stored in the RAM 46. The image I3 is an image after brightness adjustment. The current value A3 of the CCD camera is measured and stored in the RAM 46. In step S217, the brightness of the image before autofocus and the image after brightness adjustment are compared. That is, the degree of coincidence between the current value A1 and the current value A3 is obtained, and it is determined whether or not it is greater than 50%.

  As an example of a method for calculating the degree of coincidence between two current values, the ratio or difference between the two current values is calculated as will be described later in (g).

  If the coincidence is greater than 50%, the process proceeds to step S218. If the degree of coincidence is not greater than 50%, the process returns to step S215, and brightness adjustment is performed again. When the degree of coincidence becomes greater than 50%, the process proceeds to step S218.

  In step S218, pattern matching is performed to search for particles. In step S219, the image I3 subjected to pattern matching and the current value A3 are stored in the RAM 46. The correlation value between the current value A1 of the image I1 and the current value A3 of the image I3 is stored in the RAM 46. As shown in FIG. 6, the number of retrieved particles and the retrieval accuracy can be displayed.

  The processes in steps S220 and S221 are the same as the processes in steps S120 and S121 in FIG.

  A third example of a method for searching for microparticles using a transmission electron microscope according to the present invention will be described with reference to FIG. Steps S301 to S312 are the same as steps S101 to S112 in FIG. That is, it is the same as the example of FIG. 1 until the autofocus is completed.

  The objective lens current value is set to a new value by autofocus. In step S313, the current value of the converging lens coil is set to a value corresponding to the objective lens current. FIG. 15 shows conditions for the current value of the objective lens and the current value of the converging lens so that the brightness of the image does not change. From the curve in FIG. 15, the current value of the convergent lens corresponding to the objective lens current value obtained by autofocus is read and used as a set value. Thus, in this example, the brightness is adjusted by setting the current value of the converging lens coil to a predetermined value.

  In step S 314, the image I 2 is captured and stored in the RAM 46. Image I2 is an image after brightness adjustment. A luminance distribution LUT2 of the image I2 is obtained and stored in the RAM 46. In step S315, the brightness of the image before autofocus is compared with the brightness of the image after brightness adjustment. That is, the correlation between the luminance distribution LUT1 of the image I1 before autofocus and the luminance distribution LUT2 of the image I2 after brightness adjustment is calculated.

  A method for calculating the correlation of the luminance distribution will be described later in (d). That is, as described later in (a) and (b), the degree of coincidence using image correlation may be calculated.

  In step S316, the degree of coincidence between the luminance distribution LUT1 of the image I1 and the luminance distribution LUT2 of the image I2 is obtained, and it is determined whether or not it is greater than 50%.

  A method of calculating the degree of coincidence of the luminance distribution will be described later in (d). That is, as described later in (a) and (b), the degree of coincidence using image correlation may be calculated.

  If the degree of coincidence is greater than 50%, the process proceeds to step S317. If the degree of coincidence is not greater than 50%, the process ends. In this example, as the brightness adjustment, the current value of the converging lens coil is set to a predetermined value. Therefore, the brightness adjustment is not performed again.

  In step S317, pattern matching is performed to search for particles. In step S318, the pattern I-matched image I2 and its luminance distribution LUT2 are stored in the RAM 46. The correlation value between the luminance distribution LUT1 of the image I1 and the luminance distribution LUT2 of the image I2 is stored in the RAM 46. As shown in FIG. 6, the number of retrieved particles and the retrieval accuracy can be displayed.

  The process of step S319 and step S320 is the same as the process of step S120 and step S121 of FIG.

  In this example, the luminance distribution LUT1 of the image I1 before autofocus is stored in step S305, the luminance distribution LUT2 of the image I2 after brightness adjustment is stored in step S314, and the autofocus is processed in step S315. The correlation between the luminance distribution LUT1 of the previous image I1 and the luminance distribution LUT2 of the image I2 after brightness adjustment is calculated. However, in these steps, the peak position of the luminance distribution or the contrast may be used instead of the luminance distribution of the image.

  Further, in these steps, the current value of the CCD camera may be used instead of the luminance distribution of the image.

  FIG. 5 shows an example of an input screen for particle search. A sample mesh is displayed on the left side of the input screen. In the lower right of the input screen, fields for setting the search accuracy, the diameter of the search target particle, and the ellipticity of the search target particle are displayed. The user first inputs search accuracy, particle diameter, and ellipticity. The search accuracy may be set in advance. Next, a place where particle search is performed on the sample mesh is set.

  FIG. 6 shows an example of a screen showing search accuracy and detection results. In the example of FIG. 6, when the correlation (degree of coincidence) between the luminance distribution LUT1 of the image I1 before autofocus and the luminance distribution LUT2 of the image I2 after brightness adjustment is set to 80%, one particle to be searched is detected. Indicates that On the right side of the screen, an image when particles are detected is displayed. FIG. 7 shows an example of the contrast, the number of detected particles, and the search accuracy. The higher the contrast, the higher the search accuracy. As shown in FIG. 8, the contrast is expressed as a ratio of the intensity I1 of the particle image to be searched to the background intensity I0.

  FIG. 9 shows the relationship between the defocus amount and the current ratio of the CCD camera. The horizontal axis represents the defocus amount, and the vertical axis represents the ratio of the current value I1 after defocusing to the current value I0 when the defocus amount is zero. As shown, when the defocus amount increases from negative to positive, the current value of the CCD camera increases. Therefore, the defocus amount can be known by detecting the current value of the CCD camera.

  With reference to FIG. 10, it will be described that the brightness of an image changes by changing the objective lens current value. When the objective lens current value is changed, the pre-objective magnetic field and the post-objective magnetic field change. FIG. 10 shows only the pre-objective magnetic field. When the objective lens current value is changed, the pre-objective magnetic field is changed, the electron beam irradiation region is changed, and the electron beam density on the sample is changed, as indicated by a broken line. As a result, the brightness of the image changes. Note that the focal position changes as the post-objective magnetic field changes.

  FIG. 11 shows an example of an image when the brightness is not corrected after the autofocus as in the conventional technique. The upper left image is a positive focus image, the right image is an image that is shifted to the negative side by autofocus, and the lower image is an image that is shifted to the positive side. An image is formed by focusing. FIG. 12 shows an example of an image when brightness is corrected after autofocus according to the present invention. The upper left image is a positive focus image, the right image is an image that is shifted to the negative side by autofocus, and the lower image is an image that is shifted to the positive side. An image is formed by focusing. In the image of FIG. 12, the brightness does not change before and after autofocus. Therefore, according to the present invention, particles can be accurately detected even when autofocusing is performed.

  FIG. 13 shows an example of the luminance distribution (histogram) and peak position of an image. The vertical axis represents the number of pixels, and the horizontal axis represents the gradation. The peak of the luminance distribution curve is the peak position, which is the center of the luminance distribution. FIG. 13A shows the luminance distribution before autofocus, and FIG. 13B shows the luminance distribution after autofocus. As can be seen by comparing the two, in this example, the brightness of the image increases after autofocus.

  FIG. 15 shows conditions for the current value of the objective lens and the current value of the converging lens so that the brightness of the image does not change. The horizontal axis represents the current value of the objective lens, and the vertical axis represents the current value of the convergent lens. Even if the current value of the objective lens changes, a change in the brightness of the image can be avoided by setting the current value of the convergent lens to a value obtained from the curve of FIG.

  FIG. 16 shows the relationship between the current value of the objective lens and the current value on the sample surface. The horizontal axis represents the current value of the objective lens, and the vertical axis represents the current value on the sample surface. The current value on the sample surface is the current value of the CCD camera. As the current value of the objective lens increases from negative to positive, the current value on the sample surface decreases. By detecting the current value on the sample surface, the current value of the objective lens can be known.

  FIG. 17 shows the relationship between magnification and correction accuracy. The horizontal axis represents magnification, and the vertical axis represents correction accuracy (defocus amount). As the magnification increases, the correction accuracy decreases rapidly. The relationship between the magnification and the correction accuracy was examined by changing the inclination angle α (0.5 to 0.1 degree) of the electron beam. The correction accuracy decreases as the inclination angle α of the electron beam increases.

  FIG. 18 shows the relationship between the magnification and the correction range. The horizontal axis is the magnification, and the vertical axis is the correction width (defocus amount). As the magnification increases, the correction width decreases rapidly. The relationship between the magnification and the correction width was examined by changing the inclination angle α (0.5 to 0.1 degree) of the electron beam. The correction width decreases as the tilt angle α of the electron beam increases.

  FIG. 19 shows an example of an autofocus setting screen. This screen has a field for inputting the tilt angle α of the electron beam.

  FIG. 20 shows an example of a setting screen when the brightness correction is performed by controlling the exposure time. This screen includes a field for selecting one of the convergence lens, electron gun emission current, and the default value corrected by the convergence lens as a method for controlling the exposure time, and a field for displaying the obtained search accuracy (degree of coincidence). Have.

  FIG. 21 shows an example of a setting screen when brightness correction is performed by controlling the current value of the CCD camera. In this screen, as a method for controlling the current value of the CCD camera, a field for selecting one of a convergence lens, an electron gun emission current, a gain of the CCD camera, and an exposure time of the CCD camera, and the obtained search accuracy (degree of coincidence) Has a field to display. This screen further has a field for displaying the luminance distribution.

  An example of a method for measuring the current value of the CCD camera will be described with reference to FIG. In this example, a scintillator 54 having a current detector is provided for the CCD camera.

  23 and 24 show an example of a visual field moving method. In the example of FIG. 23, the visual field is moved using a sample stage 53 provided with a driving device 53a in the X direction, a driving device 53b in the Y direction, and a driving device 53c in the Z direction. In the example of FIG. 24, the visual field is moved using the deflection coils 4 and 5.

  Hereinafter, examples of a movement amount calculation method, a coincidence calculation method, a contrast calculation method, a defocus amount calculation method in autofocus, and various search accuracy calculation methods will be described.

(A) Calculation Method of Movement Amount A calculation method of the movement amount of an image will be described using an example of image correlation shown in FIG. An image obtained by cutting out a part of the transmission image 1 is referred to as a transmission image 3. This is recorded as a registered image f1 (m, n) in the storage device with the number of pixels of M × N. Next, an image captured after the recording mode is set as a transmission image 2. This is recorded as a reference image f2 (m, n) in the storage device with the number of pixels of M × N. However, both are natural images, and m = 0, 1, 2,... M-1 and n = 0, 1, 2,.

Discrete Fourier images F1 (m, n) and F2 (m, n) of f1 (m, n) and f2 (m, n) are defined by equations (1) and (2), respectively.
F1 (u, v) = A (u, v) ejθ (u, v) (1)
F2 (u, v) = B (u, v) ejφ (u, v) (2)
However, u = 0,1,2,... M-1 and v = 0,1,2,. A (u, v) and B (u, v) are amplitude spectra, and θ (u, v) and φ (u, v) are phase spectra.

  In phase correlation, when there is parallel movement of an image between two images, the position of the correlation peak is shifted by the amount of movement. A method for deriving the movement amount will be described below.

First, f4 (m, n) = f2 (m + r ′, n) assuming that the original image f2 (m, n) has moved by r ′ in the x direction. Equation (2) is transformed into Equation (3).
F4 (u, v) = ΣΣ f2 (m + r ', n) e -j2π (mu / M + nv / N)
= B (u, v) ej (φ + 2πr'u / M) (3)

By setting the amplitude spectrum B (u, v) as a constant, a phase image independent of the contrast of the image is obtained. The phase image F′4 (u, v) of the image f4 is expressed by Equation (4).
F4 '(u, v) = ej (φ + 2πr'u / M) (4)

By multiplying the phase image F′1 (u, v) by the complex function of F′2 (u, v), the composite image H14 (u, v) is expressed by Equation (5).
H14 (u, v) = F'1 (u, v) (F'2 (u, v)) * = ej (θ-φ-2πru / M) (5)

Correlation strength image G14 (r, s) is expressed by equation (6) by performing inverse Fourier transform on synthesized image H14 (u, v).
G14 (r, s) = ΣΣ (H14 (u, v)) ej2π (ur / M + us / N)
= ΣΣ (ej (θ-φ-2πr'u / M)) ej2π (ur / M + us / N) = G12 (r-r ') (6)

  From Expression (6), when there is a positional deviation amount r ′ in the X direction between two images, the peak position of the correlation intensity image is shifted by −r ′. In addition, since the correlation calculation is performed using the phase component, the movement amount can be calculated even if there is a difference in brightness or contrast between the two images. When there is a positional shift amount in the X direction between two images, a peak occurs at a position of ΔG (pixel) from the center of the correlation strength image. For example, if there is a shift of 2 pixels in the X direction between two images, the composite image becomes a two-cycle wave. When this is subjected to inverse Fourier transform, a correlation intensity image is obtained, and a peak is generated at a position shifted by 2 pixels from the center.

This ΔG (pixel) corresponds to the amount of movement on the light receiving surface of the detector. Therefore, ΔG is converted into a movement amount Δx on the sample surface. Assuming that the diameter L of the light receiving surface of the detector, the magnification M of the transmission electron microscope on the light receiving surface, and the number of pixels Lm of the light receiving surface of the detector are given by equation (7).
Δx ＝ ΔG (pixel) × L / Lm (pixel) / M (7)
Δx is the amount of movement on the sample surface between the two images.

(B) Method for calculating the degree of coincidence A method for calculating the degree of coincidence using image correlation will be described. In the correlation calculation using only the phase component, since only the phase is used mathematically, the peak appearing in the correlation strength is the δ peak.

  For example, if the image is shifted by 1.5 pixels between two images, the composite image becomes a wave of 1.5 cycles. When this is subjected to inverse Fourier transform, a δ peak appears at a position shifted by 1.5 pixels from the center of the correlation intensity image, but since there is no 1.5 pixel, the value of the δ peak is assigned to the first pixel and the second pixel.

Here, taking the center of gravity of the pixel having a high degree of coincidence and calculating the true δ peak position from the assigned value, the calculation result can be obtained with an accuracy of about 1/10 pixel. Further, since the correlation intensity image is δ peak, the similarity between the two images is evaluated based on the peak height of the correlation intensity image. When the image f1 (m, n) and the peak height Peak (pixel) are assumed, the degree of coincidence (%) is shown in the equation (8).
Agreement (%) = (Peak) / (m × n) × 100 (8)
For example, when the number of processed pixels is 128 pixels × 128 pixels and the peak is 16384 (pixels), the degree of coincidence = (16384) / (128 × 128) × 100 = 100 (%).

(C) Contrast C Calculation Method As shown in FIG. 8, the contrast C is calculated from equation (9) using the background intensity I0 and the intensity I1 indicating the height of the sample from the line profile.

  Contrast C = (I0) / (I1) × 100 (9)

(D) Method of calculating search accuracy using correlation calculation As shown in FIG. 13, the luminance distribution is considered as an image, and the amount of movement or matching using the image correlation of (a) or (b) is used as the search accuracy. .

(E) Method of calculating luminance distribution peak position and search accuracy (coincidence) As shown in FIG. 13, the peak position of the luminance distribution is obtained by equation (10). The search accuracy is obtained by equation (11).
Peak position = Peak position 1-Peak position 2 (10)
Search accuracy = (Peak position 1) / (Peak position 2) x 100 (11)

(F) Method for automatically correcting focus (autofocus)
The movement amount ΔX on the sample calculated by Expression (7) is put into Expression (12) to calculate the defocus amount Δf.
Δf = ΔX / (α × M) −CS × α2 (12)
ΔX is the amount of movement caused by the tilt of the electron beam, M is the magnification of the transmission electron microscope, CS is the spherical aberration coefficient of the objective lens, and α is the tilt angle of the electron beam.

(G) Method of calculating current value position and search accuracy (degree of coincidence) from current value The current value position is obtained by equation (13), and the search accuracy is obtained by equation (14).
Current value position = current value 1-current value 2 (13)
Search accuracy = (Current value 1) / (Current value 2) x 100 (14)

(H) Method of calculating position and search accuracy (coincidence) from contrast The contrast position is obtained by equation (15), and the search accuracy is obtained by equation (16).
Contrast position = Contrast 1-Contrast 2 (15)
Search accuracy = (Contrast 1) / (Contrast 2) x 100 (16)

  The example of the present invention has been described above, but the present invention is not limited to the above-described example, and various modifications can be easily made by those skilled in the art within the scope of the invention described in the claims. Will be understood.

It is a figure which shows the structure of the transmission electron microscope of this invention. It is a figure which shows the 1st example of the method of searching a microparticle with the transmission electron microscope of this invention. It is a figure which shows the 2nd example of the method of searching a microparticle with the transmission electron microscope of this invention. It is a figure which shows the 3rd example of the method of searching a microparticle with the transmission electron microscope of this invention. The example of an input screen when performing an automatic search in the transmission electron microscope of this invention is shown. The example of the screen which displayed the number of particle | grain detection and search accuracy which are search results in the transmission electron microscope of this invention is shown. It is a figure which shows the relationship between contrast, the number of particle detections, and search precision in the transmission electron microscope of this invention. It is a figure which shows the calculation method of contrast in the transmission electron microscope of this invention. It is a figure which shows the relationship between the defocus amount and the current ratio of a CCD camera in the transmission electron microscope of this invention. It is a figure which shows the relationship between a pre-objective magnetic field and an irradiation area | region in the transmission electron microscope of this invention. It is a figure which shows the image which does not correct | amend brightness after autofocus in the transmission electron microscope of this invention. It is a figure which shows the image which the brightness | luminance was correct | amended after the autofocus in the transmission electron microscope of this invention. It is a figure which shows the example of the luminance distribution calculated | required from the image in the transmission electron microscope of this invention. It is a figure for demonstrating the example of an image correlation in the transmission electron microscope of this invention. It is a figure which shows the relationship between the objective-lens electric current value and the convergence lens coil electric current value whose brightness does not change in the transmission electron microscope of this invention. It is a figure which shows the relationship between the objective-lens electric current value and the electric current value of a CCD camera in the transmission electron microscope of this invention. It is a figure which shows the correction | amendment precision of an autofocus in the transmission electron microscope of this invention. It is a figure which shows the correction range of an autofocus in the transmission electron microscope of this invention. It is a figure which shows the example of the input screen which performs the setting of an autofocus in the transmission electron microscope of this invention. It is a figure which shows the example of the input screen which sets the exposure time in the transmission electron microscope of this invention. It is a figure which shows the example of the input screen which sets a CCD camera in the transmission electron microscope of this invention. It is a figure which shows the example of the measuring method of the electric current value of a CCD camera in the transmission electron microscope of this invention. It is a figure which shows the example of the sample stage for moving a visual field in the transmission electron microscope of this invention. It is a figure which shows the example of the electron beam deflection | deviation system for moving a visual field in the transmission electron microscope of this invention.

Explanation of symbols

1: electron gun, 2: first focusing lens coil, 3: second focusing lens coil, 4: first deflection coil, 5: second deflection coil, 6: objective lens coil, 7: first electromagnetic sample image movement Coil: 8: second electromagnetic sample image moving coil, 9: first intermediate lens coil, 10: second intermediate lens coil, 11: first projection lens coil, 12: second projection lens coil, 13-23 : Excitation power source, 24-34: DAC, 35: Microprocessor, 36: Storage device, 37: Arithmetic device, 38: CRT controller, 39: CRT, 40-41: I / F, 42: Rotary encoder for switching magnification 43: Input rotary encoder, 44: Keyboard, 45: Mouse, 46: RAM, 47: ROM, 48: Image capture interface, 49: TV control unit, 50: TV, 51: Optical recording 'S, 52: CCD, 53: sample stage 54: scintillator

Claims (6)

A focusing lens that irradiates a sample with an electron beam from an electron gun, an objective lens that focuses the electron beam on the sample, an image data acquisition device that acquires image data of a transmission image, and an autofocus mechanism that performs autofocusing when, by the auto-focus have a, and brightness adjustment mechanism for adjusting the brightness of the image so that the current value is the brightness of the sample image when the change is not change of the objective lens,
The brightness adjustment mechanism calculates a correlation between the luminance distribution of the image before the autofocus and the luminance distribution of the image after the autofocus, and uses the result of the correlation calculation to calculate the current of the convergent lens. Control the value to adjust the brightness of the image after autofocus,
The degree of coincidence between the luminance distribution of the image before the autofocus and the luminance distribution of the image after the brightness adjustment is obtained. If the degree of coincidence is not greater than a preset value, the brightness adjustment is performed again. A transmission electron microscope characterized in that the degree of coincidence is obtained and the brightness adjustment is performed until the degree of coincidence becomes greater than the preset value .   The transmission electron microscope according to claim 1, further comprising a monitor that displays an image after the brightness adjustment.   2. The transmission electron microscope according to claim 1, wherein the monitor displays a degree of coincidence between an image before autofocus and an image after the brightness adjustment, and a luminance distribution of the image. electronic microscope. Performing autofocus using the autofocus mechanism of a transmission electron microscope;
When the current value of the objective lens changes due to the autofocus,
The correlation between the luminance distribution of the image before autofocus and the luminance distribution of the image after autofocus is calculated, and the current value of the converging lens is controlled by using the result of the correlation calculation. Adjust the brightness of the above image,
The degree of coincidence between the luminance distribution of the image before the autofocus and the luminance distribution of the image after the brightness adjustment is obtained. If the degree of coincidence is not greater than a preset value, the brightness adjustment is performed again. A method for adjusting the brightness of a transmission electron microscope image, wherein the brightness adjustment is performed until the matching degree is larger than the preset value . A converging lens for irradiating a sample with an electron beam from an electron gun, an objective lens for converging the electron beam on the sample, a CCD camera for acquiring image data of a transmission image, and a transmission image obtained by the CCD camera , A pattern matching mechanism that performs pattern matching, an autofocus mechanism that performs autofocus, and the brightness of the sample image does not change when the current value of the objective lens changes due to autofocus. and brightness adjustment mechanism to adjust the brightness, the possess,
The brightness adjustment mechanism calculates a correlation between the luminance distribution of the image before the autofocus and the luminance distribution of the image after the autofocus, and uses the result of the correlation calculation to calculate the current of the convergent lens. Control the value to adjust the brightness of the image after autofocus,
The degree of coincidence between the luminance distribution of the image before the autofocus and the luminance distribution of the image after the brightness adjustment is obtained. If the degree of coincidence is not greater than a preset value, the brightness adjustment is performed again. A pattern search apparatus characterized in that the matching degree is obtained by performing the brightness adjustment until the matching degree becomes larger than the preset value . 6. The pattern search apparatus according to claim 5, wherein the monitor displays a screen having a field for inputting a shape of a search target pattern and a field for setting a field of view for the search target.

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