JP2018142481A - Electron microscope and sample height adjustment method - Google Patents

Abstract

An electron microscope capable of accurately adjusting the Z position of a sample is provided. The electron microscope includes a Z position adjustment unit that adjusts the Z position of the sample so as to achieve just focus when the lens current of the objective lens is a reference current Is, the Z position adjustment unit adjusting the Z position of the sample. A first process for obtaining a first focus current of the objective lens that is in just focus when the position is at the first position, and a difference between the first focus current and the reference current to obtain a change amount of the Z position corresponding to the difference. a second process, a third process of controlling the sample stage so that the Z position of the sample moves from the first position to the second position by the amount of change obtained in the second process, and a third process of controlling the Z position of the sample to the second position. A fourth process of obtaining the second focus current of the objective lens that is in just focus at the position, and the Z position of the sample that is in just focus at the reference current is obtained based on the first focus current and the second focus current. A fifth process is performed. [Selection drawing] Fig. 2

Description

  The present invention relates to an electron microscope and a method for adjusting a sample height.

  A transmission electron microscope (TEM) irradiates a sample with an electron beam, images the electron beam transmitted through the sample with an objective lens, obtains an image, and magnifies the image with an intermediate lens and a projection lens. This is an apparatus for observing an enlarged sample image.

  The sample stage of the transmission electron microscope includes an X moving mechanism for moving the sample in the X direction, a Y moving mechanism for moving the sample in the Y direction, and a Z moving mechanism for moving the sample in the Z direction. (For example, refer to Patent Document 1).

  In a transmission electron microscope, the height (Z position) of a sample must be adjusted before observation or analysis. Specifically, in the transmission electron microscope, it is necessary to adjust the Z position of the sample so as to achieve just focus when the lens current of the objective lens is set to a predetermined current (reference current).

JP 2010-212067 A

  FIG. 8 is a flowchart showing an example (reference example) of a method for adjusting the Z position of the sample.

First, when the sample is located at the Z position Z0, which is the initial position, and the lens current of the objective lens is the reference current Is, the lens current of the objective lens that is just focused (hereinafter also referred to as “focus current I _f ”). Calculate (step S2).

  Next, using the coefficient D (μm / A) representing the change in the Z position of the sample with respect to the change in the lens current of the objective lens and the reference current Is of the objective lens, the change amount ΔZ in the Z position is expressed by the following equation (a): (Step S4).

ΔZ = D × (I _f −Is) (a)

  Next, the sample is moved by the calculated change amount ΔZ (step S6). If the Z position at which the focus is just at the reference current Is is Zs, the Z position Zs is expressed by the following equation (b).

  Zs = Z0 + ΔZ (b)

  Through the above steps, the Z position of the sample can be adjusted.

In the method for adjusting the Z position of the sample shown in FIG. 8, a coefficient D is used to calculate the change amount ΔZ of the Z position as shown in the above equation (a). Here, although the coefficient D is used as a constant, the change in the Z position of the sample with respect to the actual change in the lens current of the objective lens differs depending on the Z position. For this reason, the Z position Zs may not be accurately obtained by the method for adjusting the Z position of the sample using the coefficient D shown in FIG.

  The present invention has been made in view of the above problems, and one of the objects according to some aspects of the present invention is to provide an electron microscope that can accurately adjust the Z position of a sample. It is to provide. Another object of some aspects of the present invention is to provide a method for adjusting the sample height that can accurately adjust the Z position of the sample.

(1) The electron microscope according to the present invention is
An electron beam source;
An irradiation lens for irradiating the sample with an electron beam emitted from the electron beam source;
An objective lens that forms an electron microscope image with an electron beam transmitted through the sample;
A sample stage provided with a Z moving mechanism for moving the sample in the Z direction along the optical axis;
A Z position adjustment unit that adjusts the Z position of the sample by controlling the sample stage so that the focus current is just focused when the lens current of the objective lens is a reference current;
Including
The Z position adjusting unit is
A first process for obtaining a first focus current that is a lens current of the objective lens that is in a just focus when the Z position of the sample is the first position;
A second process for obtaining a change amount of the Z position corresponding to the difference from the difference between the first focus current and the reference current;
A third process for controlling the sample stage so that the Z position of the sample becomes a second position moved from the first position by the amount of change obtained in the second process;
A fourth process for obtaining a second focus current which is a lens current of the objective lens which is in a just focus when the Z position of the sample is the second position;
A fifth process for determining a Z position of the sample that is just focused at the reference current based on the first focus current and the second focus current;
A sixth process for controlling the sample stage so that the sample is positioned at the Z position obtained in the fifth process;
I do.

  In such an electron microscope, the Z position of the sample is adjusted by calculating the Z position that is just focused when the lens current of the objective lens is the reference current from the combination of the Z position of the sample and the focus current at the Z position. Therefore, for example, as shown in FIG. 8, the Z position that is the just focus at the reference current is calculated more accurately than the case where the Z position that is the just focus at the reference current is calculated using the coefficient D. can do. Therefore, in such an electron microscope, the Z position of the sample can be accurately adjusted.

(2) In the electron microscope according to the present invention,
In the first process, the Z position adjusting unit may
A process for setting the first focus current obtained by obtaining the lens current of the objective lens;
A process for obtaining the first focus current again in a state where the lens current of the objective lens is set to the first focus current obtained;
May be repeated until the difference between the first focus current obtained at the nth time and the first focus current obtained at the (n + 1) th time is equal to or less than a predetermined value.

  In such an electron microscope, the first focus current can be accurately obtained.

(3) In the electron microscope according to the present invention,
In the fourth process, the Z position adjusting unit may
A process of setting the second focus current obtained from the lens current of the objective lens;
A process for obtaining the second focus current again in a state where the lens current of the objective lens is set to the second focus current obtained;
May be repeated until the difference between the second focus current obtained at the nth time and the second focus current obtained at the (n + 1) th time is equal to or less than a predetermined value.

  In such an electron microscope, the second focus current can be accurately obtained.

(4) In the electron microscope according to the present invention,
A deflection unit for deflecting an electron beam emitted from the electron beam source and changing an incident angle of the electron beam with respect to the sample;
In the first process, the Z position adjusting unit may
A process of acquiring a plurality of image shift vectors representing image shifts before and after changing the incident angle of the electron beam to the sample by a predetermined angle by changing the lens current of the objective lens;
Based on a plurality of the image shift vectors, a process for calculating a minimum image shift vector that minimizes an image shift amount before and after changing the incident angle of the electron beam to the sample by the predetermined angle;
A process for obtaining the first focus current based on the minimum image shift vector;
May be performed.

  In such an electron microscope, since the first focus current is obtained based on the minimum image shift vector that minimizes the image shift amount before and after changing the incident angle of the electron beam to the sample by a predetermined angle, the first focus current is obtained. Can be obtained accurately.

(5) The method for adjusting the sample height according to the present invention includes:
In a transmission electron microscope, a sample height adjustment method for adjusting a Z position, which is a position in the Z direction along an optical axis of a sample, so that the focus current is just focused when a lens current of an objective lens is a reference current. ,
A first step of obtaining a first focus current which is a lens current of the objective lens which is in a just focus when the Z position of the sample is the first position;
A second step of determining a change amount of the Z position corresponding to the difference from the difference between the first focus current and the reference current;
A third step of moving the Z position of the sample from the first position to a second position moved by the amount of change obtained in the second step;
A fourth step of obtaining a second focus current which is a lens current of the objective lens which is in a just focus when the Z position of the sample is the second position;
Based on the first focus current and the second focus current, a fifth step of determining the Z position of the sample that is just focused at the reference current;
A sixth step of moving the sample to the Z position determined in the fifth step;
including.

  In such a method for adjusting the sample height, the Z position where the focus is just focused when the lens current of the objective lens is the reference current is calculated from the combination of the Z position of the sample and the focus current at the Z position. In order to adjust the Z position, for example, as shown in FIG. 8, when the reference position is used to calculate the Z position that is the just focus at the reference current using the coefficient D, the focus is accurately adjusted at the reference current. The Z position can be calculated. Therefore, in such a method for adjusting the sample height, the Z position of the sample can be accurately adjusted.

The figure which shows the electron microscope which concerns on this embodiment typically. The flowchart which shows an example of the process which adjusts the Z position of the sample in the control processing part of the electron microscope which concerns on this embodiment. The graph for demonstrating the method of calculating | requiring Z position. 6 is a flowchart showing an example of processing for calculating a focus current in the control processing unit of the electron microscope according to the present embodiment. The figure which shows a 1st inclination image and a 2nd inclination image typically. The figure for demonstrating image shift vector (xi, yi). The graph showing the formula y = ax + b which approximates image shift vector (x0, y0), (x1, y1), (x2, y2). The flowchart which shows an example (reference example) of the adjustment method of the Z position of a sample.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. The embodiments described below do not unduly limit the contents of the present invention described in the claims. In addition, not all of the configurations described below are essential constituent requirements of the present invention.

1. First, an electron microscope according to the present embodiment will be described with reference to the drawings. FIG. 1 is a diagram schematically showing an electron microscope 100 according to the present embodiment. In FIG. 1, an X axis, a Y axis, and a Z axis are illustrated as three axes orthogonal to each other. The Z axis is an axis (parallel axis) along the optical axis of the optical system (for example, the objective lens 16) of the electron microscope 100.

  The electron microscope 100 is an apparatus that observes the sample S using the electron beam EB. The electron microscope 100 is a transmission electron microscope (TEM).

  As shown in FIG. 1, the electron microscope 100 includes an electron beam source 10, an irradiation lens 12, a deflection coil 14 (an example of a deflection unit), an objective lens 16, a sample stage 18, a sample holder 19, and imaging. Apparatus 20, electron beam source control unit 22, lens control unit 24, deflection control unit 26, imaging control unit 28, sample stage control unit 30, and control processing unit 32 (an example of a Z position adjustment unit) The display unit 34 is included.

  The electron beam source 10 generates an electron beam EB. The electron beam source 10 is, for example, a known electron gun.

  The irradiation lens 12 focuses the electron beam EB emitted from the electron beam source 10 and irradiates the sample S. The irradiation lens 12 is composed of a plurality of (three in the illustrated example) condenser lenses.

  The deflection coil 14 deflects the electron beam EB focused by the irradiation lens 12. When the deflection coil 14 deflects the electron beam EB, the incident angle of the electron beam EB with respect to the sample S (the tilt angle (Tilt) of the electron beam EB) can be changed. The deflection coil 14 can also shift the electron beam EB by deflecting the electron beam EB. For example, the deflection coil 14 is disposed between the irradiation lens 12 and the objective lens 16.

  The objective lens 16 is a first-stage lens for forming a transmission electron microscope image (hereinafter also referred to as “TEM image”) with the electron beam EB transmitted through the sample S. The objective lens 16 changes its lens action (lens focal length and magnification) in accordance with the supplied lens current (excitation current).

  The sample stage 18 holds the sample S. In the illustrated example, the sample stage 18 holds the sample S via the sample holder 19. The sample S can be positioned by the sample stage 18. In the illustrated example, the sample stage 18 is a side entry type sample stage in which the sample holder 19 is inserted in the horizontal direction (lateral) with respect to the pole piece of the objective lens 16. The sample stage 18 may be a top entry type sample stage in which the sample S is inserted from above the pole piece of the objective lens 16.

  The sample stage 18 includes an X moving mechanism that moves the sample S in the X direction, a Y moving mechanism that moves the sample S in the Y direction, and a Z moving mechanism that moves the sample S in the Z direction. The height of the sample S (that is, the Z position) can be changed by the Z moving mechanism of the sample stage 18. Here, the height of the sample S is not the height of the sample S itself (the vertical size of the sample S), but the position of the sample S in the height direction of the electron microscope, that is, the Z axis of the sample S in the electron microscope. The position in the direction along the line (Z position).

  Although not shown, the electron microscope 100 may include an intermediate lens and a projection lens. The intermediate lens and the projection lens enlarge the image formed by the objective lens 16 and form it on the imaging device 20. The objective lens 16, the intermediate lens, and the projection lens constitute an imaging system of the electron microscope 100.

  The imaging device 20 captures a TEM image formed by the imaging system. The imaging device 20 is a digital camera such as a CCD camera, for example. Image data of a TEM image taken by the imaging device 20 is output to the control processing unit 32 via the imaging control unit 28. The TEM image captured by the imaging device 20 is stored as an image file by the control processing unit 32 in a storage device (not shown) and displayed on the display unit 34.

  The display unit 34 displays an image generated by the control processing unit 32, and its function can be realized by an LCD, a CRT, or the like. The display unit 34 displays a GUI (Graphical User Interface) for operating the electron beam source control unit 22, the lens control unit 24, the deflection control unit 26, the imaging control unit 28, and the sample stage control unit 30. The display unit 34 displays a TEM image taken by the imaging device 20.

  The control processing unit 32 controls the electron beam source control unit 22, the lens control unit 24, the deflection control unit 26, the imaging control unit 28, and the sample stage control unit 30. The control processing unit 32 is, for example, a control for controlling the electron beam source control unit 22, the lens control unit 24, the deflection control unit 26, the imaging control unit 28, and the sample stage control unit 30 based on a GUI operation or the like. A signal is generated and these control units are controlled.

  The control processing unit 32 performs a process of adjusting the Z position of the sample S as will be described later.

  The function of the control processing unit 32 may be realized by executing a program with various processors (CPU or the like), or may be realized with a dedicated circuit such as an ASIC (gate array or the like).

  The electron microscope 100 includes an operation unit that acquires an operation signal according to an operation by a user, and the control processing unit 32 performs an electron beam source control unit 22, lens control based on the operation signal from the operation unit. The unit 24, the deflection control unit 26, the imaging control unit 28, and the sample stage control unit 30 may be controlled. The operation unit may be, for example, a button, a key, a touch panel display, a microphone, or the like.

  The electron beam source control unit 22 controls the electron beam source 10 based on the control signal generated by the control processing unit 32. The lens control unit 24 controls the irradiation lens 12 and the objective lens 16 based on the control signal generated by the control processing unit 32. The lens control unit 24 may further control the intermediate lens and the projection lens based on the control signal generated by the control processing unit 32. The deflection control unit 26 controls the deflection coil 14 based on the control signal generated by the control processing unit 32. Further, the imaging control unit 28 controls the imaging device 20 based on the control signal generated by the control processing unit 32. The sample stage control unit 30 controls the sample stage 18 based on the control signal generated by the control processing unit 32.

2. Operation of electron microscope 2.1. Next, an operation of the electron microscope 100 according to the present embodiment will be described. Here, a process of adjusting the Z position (the height of the sample S) of the sample S in the control processing unit 32 of the electron microscope 100 will be described. The adjustment of the Z position of the sample S means that the Z position of the sample S is adjusted so that the focus is just when the lens current of the objective lens 16 is the reference current Is.

  The reference current Is is an optimum current value at which the performance of the objective lens 16 can be most exerted, and is a current value unique to the objective lens 16. The just focus refers to a state where the sample S is in focus.

  FIG. 2 is a flowchart illustrating an example of a process for adjusting the Z position of the sample S in the control processing unit 32 of the electron microscope 100 according to the present embodiment.

  For example, the control processing unit 32 determines whether or not the user has given a start instruction based on a GUI operation, and starts the Z position adjustment process when it is determined that the start instruction has been given. It is assumed that the Z position of the sample S when the processing is started is the Z position Z0, and the lens current of the objective lens 16 when the processing is started is the reference current Is.

First, the control processing unit 32 calculates a focus current _If1 when the Z position of the sample S is the Z position Z0 (step S10). The focus current _If is the lens current of the objective lens 16 that is in a state where the sample S is in focus (just focus). For example, the focus current when the Z position of the sample S is the Z position Z0 is the lens current of the objective lens 16 that is just focused when the sample S is at the Z position Z0. A method for calculating the focus current _If will be described later.

Next, the control processing unit 32 sets the lens current of the objective lens 16 to the focus current _If1 obtained in the process of step S10 (step S12). As a result, the lens current of the objective lens 16 becomes the focus current _If1 obtained in the process of step S10. Note that the Z position of the sample S remains at the Z position Z0.

Next, the control processing unit 32 calculates a focus current _If2 when the Z position of the sample S is the Z position Z0 (step S14). That is, the control processing unit 32 again focuses when the Z position of the sample S is the Z position Z0 in a state where the lens current of the objective lens 16 is set to the focus current _If1 obtained in the process of step S10. The current _If2 is calculated.

Next, the control processing unit 32 calculates a difference ΔI _f = I _f2 −I _f1 between the focus current _If1 calculated in Step S10 and the focus current _If2 calculated in Step S14 (Step S16).

Next, the control processing unit 32, a difference [Delta] I _f it is determined whether it is less than a predetermined value Ia (step S18). Here, the predetermined value Ia can be appropriately set according to the required accuracy of the Z position of the sample S.

Control processing unit 32, when the difference [Delta] I _f is not equal to or smaller than the predetermined value Ia (NO in step S18), and again returns to step S12, sets the focus current _{I f2} to determine the lens current of the objective lens 16 and (step S12), the calculated focus current _{I f3} (step S14), and calculates the difference ΔI _f = _I f3 _{-I f2} (step S16).

Control processing unit 32, until the difference [Delta] I _f is determined to be equal to or lower than a predetermined value Ia, the process of setting the focus current _{I fn} obtained lens current of the objective lens 16 (step S12), the process of calculating the focus current _{I fn + 1} (Step S14), the process of calculating the difference ΔI _f = I _{fn + 1} −I _fn (Step S16) is repeated.

  Here, the case where the focus current is calculated until the difference ΔIf becomes equal to or smaller than the predetermined value Ia has been described. However, for example, the number of repetitions (for example, the repetition ends when n = 2) may be set in advance. Good.

Control processing unit 32, when the difference [Delta] I _f is determined to below a predetermined value Ia (YES in step S18), and the focus current _{I fn + 1} samples S obtained is to focus current when the Z position Z0. Next, the control processing unit 32 determines whether or not the number m of the processes for obtaining the focus current is 2 (m = 2) (step S20), and determines that m = 2 is not satisfied (step S20). That is, when m = 1 (NO in step S20), a change amount ΔZ of the Z position corresponding to the difference is obtained from the difference between the obtained focus current _{Ifn + 1} and the reference current Is (step S22).

  Specifically, the change amount ΔZ can be obtained by the following equation (1).

ΔZ = D × (I _{fn + 1} −Is) (1)

  Here, the coefficient D is a coefficient (μm / A) representing a change in the Z position of the sample S with respect to a change in the lens current of the objective lens 16. Here, although the coefficient D is used, since the coefficient D does not affect the finally obtained Z position Zs, an arbitrary value can be set instead of the coefficient D.

  Next, the control processing unit 32 sets the sample stage 18 so that the Z position of the sample S becomes the Z position Z1 (= Z0 + ΔZ) moved from the Z position Z0 of the current sample S by the change amount ΔZ obtained in step S22. (Z moving mechanism) is controlled (step S24). As a result, the sample S moves from the Z position Z0 to the Z position Z1 (= Z0 + ΔZ).

The control processing unit 32 obtains the focus current _If (Z0) when the Z position of the sample S is the Z position Z0 and sets the Z position to Z1 (steps S10 to S24, where m = 1). If (NO in step S20)) is performed, the process returns to step S10 again (m = m + 1), and the process of obtaining the focus current _If (Z1) when the Z position of the sample S is the Z position Z1 (step S10). Step S18, where m = 2) is performed.

Specifically, in the process of obtaining the focus current Ifn _{+ 1} when the Z position of the sample S is the Z position Z1 (step S10 to step S18, where m = 2), the control processing unit 32 first determines the sample S The focus current _If1 when the Z position is the Z position Z1 is calculated (step S10), and the lens current of the objective lens 16 is set to the obtained focus current _If1 (step S12).

Next, the control processor 32, in a state where the lens current of the objective lens 16 is set to the focus current I _f1, calculates a focus current I _f2 when Z position of the sample S is Z position Z1 (step S14 ), A difference ΔI _f = I _f2 −I _f1 between the focus current _If1 calculated in step S10 and the focus current _If2 calculated in step S14 is calculated (step S16).

Next, the control processing unit 32, a determination difference [Delta] I _f of whether equal to or less than a predetermined value Ia (step S18), and when the difference [Delta] I _f is not equal to or smaller than the predetermined value Ia (NO in step S18 ) returning to step S12, and sets the focus current _{I f2} to determine the lens current of the objective lens 16 (step S12), the calculated focus current _{I f3} (step S14), and the difference [Delta] I _f = _{I f3} calculating a -I _f2 (step S16).

Control processing unit 32, until the difference [Delta] I _f is determined to be equal to or lower than a predetermined value Ia, the process of setting the focus current _{I fn} obtained lens current of the objective lens 16 (step S12), the process of calculating the focus current _{I fn + 1} (Step S14), the process of calculating the difference ΔI _f = I _{fn + 1} −I _fn (Step S16) is repeated.

Control processing unit 32, when the difference [Delta] I _f is determined to below a predetermined value Ia (YES in step S18), and the focus current _{I fn + 1} samples S obtained is to focus current when the Z position Z1.

  Next, the control processing unit 32 determines whether or not m = 2 (step S20). If it is determined that m = 2 (YES in step S20), the focus is adjusted when the reference current Is. The Z position Zs that is the Z position to be calculated is calculated (step S26).

Specifically, the control processing unit 32 calculates the Z position Zs based on the focus current _If (Z0) at the Z position Z0 and the focus current _If (Z1) at the Z position Z1. A method for calculating the Z position Zs will be described later.

  Next, the control processing unit 32 controls the sample stage 18 (Z moving mechanism) so that the Z position of the sample S becomes the Z position Zs calculated in the process of Step S26 (Step S28). In addition, the control processing unit 32 sets the lens current of the objective lens 16 to the reference current Is. As a result, the Z position of the sample S becomes the Z position Zs, and the lens current of the objective lens 16 becomes the reference current Is.

  With the above processing, the Z position of the sample S can be adjusted.

  In the above example, when m = 2, that is, two combinations of the Z position and the focus current at the position are obtained, but three or more combinations of the Z position and the focus current at the position may be obtained. Good (that is, m may be 3 or more). In this case, the Z position Zs can be calculated using a quadratic or higher curve (function).

  Next, a method for obtaining the Z position Zs will be described. FIG. 3 is a graph for explaining a method for obtaining the Z position Zs.

When the lens current of the objective lens 16 is the reference current Is, the Z position Zs that is in the just focus is the focus current I _f (Z0) at the Z position Z0 and the Z position Z0, and the focus current I at the Z position Z1 and the Z position Z1. _{From f} (Z1), an expression representing the relationship between the Z position and the focus current _If is obtained, and the Z position Zs is obtained from the expression.

An expression representing the relationship between the Z position and the focus current _If can be represented by, for example, y = ex + f. Therefore, from (x, y) = (Z0, _If (Z0)) and (x, y) = (Z1, _If (Z1)), “
”And“ f ”. “E” and “f” can be obtained by the following equations, respectively.

In this way, when an expression representing the relationship between the Z position and the focus current _If is obtained, the Z position Zs can be obtained from the reference current Is using the following expression.

2.2. Next, a method for calculating the focus current _If will be described. In the process of adjusting the Z position of the sample S shown in FIG. 2 described above, the focus current _If is calculated in the process of step S10 and the process of step S14.

  FIG. 4 is a flowchart illustrating an example of processing for calculating the focus current in the control processing unit 32 of the electron microscope 100 according to the present embodiment.

First, the control processing unit 32 calculates a change step ΔI _OL of the lens current I _OL of the objective lens 16 and a change step ΔI _TILT of the coil current I _TILT of the deflection coil 14 at the current magnification (step S100). For example, the change step ΔI _OL is calculated by dividing a preset value by the magnification of the electron microscope, and the preset value is used as it is for the change step ΔI _TILT .

Next, the control processing unit 32 sets the lens current of the objective lens 16 to IN = I _OL − (N−1) ΔI _OL (where N is an integer equal to or greater than 2), and sets the coil current of the deflection coil 14 to I. _TILT + _{ΔI TILT,} is set to _{I TILT} -.DELTA.I _TILT, it acquires the image (step S102~ step S116). That is, for each of the lens currents I 0, I 1,..., IN of the objective lens 16, the first tilted image in which the coil current of the deflection coil 14 is I _TILT + ΔI _TILT and the coil current of the deflection coil 14 is I _TILT −ΔI. _A second tilted image obtained as _TILT is acquired. Hereinafter, a case where N = 2 will be described.

Specifically, the control processing unit 32 first, the lens current of the objective lens 16 _I0 = I OL - set to _{(0-1) ΔI OL = I OL} + ΔI OL (N = 0) ( step S104). Then, the coil current of the deflection coil 14 is set to I _TILT + ΔI _TILT (step S106), and a first tilt image is taken (step S108). Next, the control processing unit 32 sets the coil current of the deflection coil 14 to I _TILT −ΔI _TILT (step S110), and takes a second tilt image (step S112). The photographed first tilted image and second tilted image are stored in a storage device (not shown).

  FIG. 5 is a diagram schematically showing the first tilted image 1 and the second tilted image 2. By changing the incident angle of the electron beam EB with respect to the sample S by the deflection coil 14, the image is shifted as shown in FIG. The first tilted image 1 and the second tilted image 2 can be said to be images before and after changing the incident angle of the electron beam EB by a predetermined angle.

Next, the control processing unit 32 determines whether or not N = 2 (step S114). If N ≠ 2 is determined (No in step S114), 1 is added to N. Te (step S116), the lens current of the objective lens _{16 I1 = I OL - (1-1} ) ΔI is set to _{OL = I OL (N = 1} ) ( step S104). And the process of step S106-step S112 is performed, and the 1st inclination image 1 and the 2nd inclination image 2 when the lens current of the objective lens 16 is I1 are acquired. The control processing unit 32 repeatedly performs the processing from step S104 to step S116 according to the preset number N.

  If it is determined that N = 2 (Yes in step S114), the control processing unit 32 causes the first tilted image 1 and the second observed in the lens currents I0, I1, and I2 of the objective lens 16, respectively. An image shift vector (xi, yi) (i = 0, 1, 2) representing the shift of the image with respect to the tilted image 2 is calculated (step S118). The unit of (xi, yi) is a pixel.

  FIG. 6 is a diagram for explaining the image shift vector (xi, yi). The image shift vector is a vector representing an image shift (positional shift) between the first tilted image 1 and the second tilted image 2. The image shift vector (xi, yi) can be calculated using, for example, a cross correlation function.

  Next, the control processing unit 32 linearly approximates the calculated image shift vectors (x0, y0), (x1, y1), (x2, y2) with the equation y = ax + b (step S120). That is, the control processing unit 32 calculates the value of “a” and the value of “b” from the image shift vectors (x0, y0), (x1, y1), (x2, y2), and obtains an approximate expression.

  FIG. 7 is a graph showing an expression y = ax + b that approximates the image shift vectors (x0, y0), (x1, y1), (x2, y2). In FIG. 7, an approximate expression for approximating the image shift vector is represented by a solid line.

The control processing unit 32 coordinates (x _min , y) that makes the distance L between the calculated approximate expression y = ax + b and the origin (0, 0) of the graph representing the image shift vector shown in FIG. 7 the shortest. The value x _min of ( _min ) is calculated (step S122).

The value of x _min of the coordinates (x _min , y _min ) can be obtained from the following equation.

The coordinates (x _min , y _min ) at which the distance L between the approximate expression y = ax + b and the origin (0, 0) is the shortest are the coordinates of the image between the first tilted image 1 and the second tilted image 2. This corresponds to an image shift vector (hereinafter also referred to as “minimum image shift vector”) in which the shift amount (position shift amount) is the smallest.

Next, the control processing unit 32 calculates the focus current _If of the objective lens 16 based on the minimum image shift vector (step S124).

Specifically, the control processing unit 32 expresses vectors (x0, I0), (x1, I1), and (x2, I2) representing the relationship between the value xi of the image shift vector and the lens current of the objective lens 16. A straight line approximation is performed with I = cx + d (that is, the values of “c” and “d” are obtained). Next, x = x _min is substituted into the calculated I = cx + d (see the following formula).

I _f = _{cx min} + d

With the above processing, the focus current _If of the objective lens 16 can be obtained.

  In the above example, the case of N = 2, that is, the case where the total number of N is 3, is not particularly limited as long as the value of N is 2 or more. The value of N can be set arbitrarily.

  In the above example, the control processing unit 32 has described an example in which the image shift vectors (x0, y0), (x1, y1), (x2, y2) are linearly approximated by the equation y = ax + b. The vectors (x0, y0), (x1, y1), (x2, y2) may be approximated by an arbitrary function. For example, the image shift vector may be approximated by a quadratic curve or approximated by a cubic or higher curve. In the above example, the total number of N is 3, but when the total number of N is increased, the image shift vectors (x0, y0), (x1, y1),..., (Xn, yn) are changed. , It can be approximated by an arbitrary function y = f (x) applicable in that case.

  In the above example, the control processing unit 32 has described an example in which (x0, I0), (x1, I1), (x2, I2) are linearly approximated by the formula I = cx + d, but (x0, I0) , (X1, I1), (x2, I2) may be approximated by an arbitrary function. For example, (x0, I0), (x1, I1), (x2, I2) may be approximated by a quadratic curve, or may be approximated by a cubic or higher curve. In the above example, the total number of N is 3, but when the total number of N is increased, (x0, I0), (x1, I1),. Can be approximated by an arbitrary function I = f (x) applied to

  The electron microscope 100 has the following features, for example.

In the electron microscope 100, the control processing unit 32 includes a first focus current I _f (Zm) that is a lens current of the objective lens 16 that is just focused when the Z position of the sample S is the Z position Zm−1 (first position). first processing obtains -1) (step S10~ step S18), and the variation of Z position corresponding to the difference [Delta] I _f from the difference [Delta] I _f the first focusing current _I f (Zm-1) and the reference current is A second process (step S22) for obtaining ΔZ, and a Z position Zm (first step) in which the Z position of the sample S is moved from the Z position Zm-1 (first position) by the change amount ΔZ obtained in the second process (step S22). Lens current of the objective lens 16 that is in the just focus when the Z position of the sample S is the Z position Zm (second position). Fourth processing obtains a certain second focusing current _I f (Zm) (step S10~ step S18), and based on the first focus current _I f (Zm-1) and a second focusing current _I f (Zm), the reference The fifth stage (step S26) for obtaining the Z position Zs of the sample S that is just focused at the current Is, and the sample stage 18 so that the sample S is positioned at the Z position Zs obtained in the fifth process (step S26). 6th process (step S28) which controls this is performed.

  As described above, in the electron microscope 100, the Z position is adjusted by calculating the Z position Zs that is just focused when the lens current of the objective lens 16 is the reference current Is from the combination of the Z position and the focus current at the Z position. To do. Therefore, in the electron microscope 100, for example, compared to the case where the Z position Zs is calculated using the coefficient D shown in FIG. 8, the obtained Z position Zs and the Z position that is just focused when the reference current Is is actually set. The difference can be reduced. That is, in the electron microscope 100, the Z position Zs can be accurately calculated, and the Z position of the sample S can be adjusted accurately.

  In this embodiment, the coefficient D is used in the process of step S22, but the coefficient D does not affect the finally obtained Z position Zs. In the electron microscope 100, it is also possible to calculate the Z position Zs using an arbitrary constant instead of the coefficient D (that is, without using the coefficient D).

In the electron microscope 100, the control unit 32, in the first process (step S10~ step S18, for example, m = 1), processing of setting the first focus current _{I fn} obtained lens current of the objective lens 16 (step S12 a), in a state where the lens current of the objective lens 16 is set to a first focus current I _fn obtained, again, the first focusing current I _{fn + 1} to determine processing (step S14), and was determined to n-th 1 until the difference between the focus current _{I fn} and (n + 1) th to the first focusing current _{I fn + 1} obtained is equal to or less than a predetermined value Ia, repeated. Therefore, the electron microscope 100 can accurately determine the first focus current If (Zm−1) = I _{fn + 1} .

For example, if the Z position of the current sample S such as when largely deviated (e.g. more than 100 [mu] m) from the Z position Zs, the lens currents of the current objective lens has deviated from the focus current I _f, the focus current I _{In some} cases, _f cannot be obtained accurately. In the electron microscope 100, as described above, until the difference between the first focus current I _{fn + 1} obtained in the first focus current I _fn and n + 1 th obtained in the n-th is equal to or less than a predetermined value Ia, repeated. Therefore, the first focus current can be accurately obtained without causing the above problem.

In the electron microscope 100, the control processor 32, the fourth process (step S10~ step S18, for example, m = 2) in the process of setting the second focusing current _{I fn} obtained lens current of the objective lens 16 (step S12 ) And the process of _obtaining the second focus current _{Ifn + 1} (step S14) in the state where the lens current of the objective lens 16 is set to the obtained second focus current _Ifn , the second focus obtained for the nth time. until the difference between the second focusing current _{I fn + 1} obtained in the current _{I fn} and n + 1 th is equal to or smaller than a predetermined value Ia, repeated. Therefore, the electron microscope 100 can accurately obtain the second focus current If (Zm) = I _{fn + 1} .

In the electron microscope 100, the control processing unit 32 changes the lens current of the objective lens 16 by changing the lens current of the objective lens 16 before and after changing the incident angle of the electron beam EB with respect to the sample S by a predetermined angle. A process of obtaining, a process of calculating a minimum image shift vector that minimizes an image shift amount before and after changing the incident angle of the electron beam EB by the predetermined angle based on a plurality of image shift vectors, and a minimum image shift Based on the vector, a process of obtaining the focus current _If of the objective lens 16 is performed.

In this way the electron microscope 100, the control processor 32 is an electron beam EB incident angle based on the minimum image shift vector shift amount is smallest in the image before and after changing a predetermined angle focusing current I _f for the sample S Therefore, the focus current _If can be accurately obtained.

For example, when the direction of the image shift vector representing the image shift between the first tilted image and the second tilted image is determined using the inner product, the calculated focus current _If and the actual focus on the sample S are determined. In some cases, there was a large difference from the lens current of the objective lens 16 in the state where there was. On the other hand, as described above, the electron microscope 100 obtains the focus current _If based on the minimum image shift vector. Therefore, the difference between the calculated focus current _If and the lens current of the objective lens 16 when the sample S is actually focused is compared with the case where the direction of the image shift vector is determined using the inner product. The focus current _If can be accurately obtained.

In the electron microscope 100, the control processing unit 32 performs image shift vector (x ₀ , y ₀ ), (x ₁ , y ₁ ),..., (X _n , y _n ) in the process of calculating the minimum image shift vector. ) Is approximated by a predetermined function y = f (x), and the minimum image shift vector (x _min ,) that minimizes the distance between the predetermined function y = f (x) and the origin (0, 0). y _min ) is calculated. In addition, the control processing unit 32 determines (x ₀ , I ₀ ), (x ₁ , I ₁ ),..., (X _n , I _n ) as predetermined _{values in the} process for obtaining the lens current of the objective lens 16. approximated by a function I = f (x), by substituting _{x min} to the predetermined function I = f (x), determining the focus current _{I f} of the objective lens 16. Therefore, according to the electron microscope 100, the focus current _If can be accurately obtained.

In the method for adjusting the Z position of the sample S according to the present embodiment, the first focus current that is the lens current of the objective lens 16 that is in a just focus when the Z position of the sample S is the Z position Zm-1 (first position). I _f the first step of obtaining a (Zm-1) (step S10~ step S18), Z corresponding to the difference [Delta] it _f from the difference [Delta] I _f the first focusing current _I f (Zm-1) and the reference current is A second step (step S22) for obtaining a change amount ΔZ of the position, and a Z position Zm (the Z position of the sample S moved from the Z position Zm-1 (first position) by the change amount ΔZ obtained in the second step). The second focus current I, which is the lens current of the objective lens 16 that is in the just focus when the Z position of the sample S is the Z position Zm (second position). _f (Zm) Based on the fourth step (step S10 to step S18) to be obtained, the first focus current _If (Zm-1) and the second focus current _If (Zm), the sample S that is just focused at the reference current Is. A fifth step (step S26) for obtaining the Z position of the first sample, and a sixth step (step S28) for moving the sample S to the Z position Zs obtained in the fifth step.

  In the sample height adjusting method according to the present embodiment, the Z position Zs that is just focused when the lens current of the objective lens 16 is the reference current Is from the combination of the Z position of the sample S and the focus current at the Z position. For example, as shown in FIG. 8, the reference current is more accurately compared with the case where the Z position that is just focused at the reference current is calculated using the coefficient D as shown in FIG. It is possible to calculate the Z position that is the just focus at the time of Is. Therefore, in such a method for adjusting the sample height, the Z position of the sample S can be adjusted accurately.

The present invention includes configurations that are substantially the same as the configurations described in the embodiments (for example, configurations that have the same functions, methods, and results, or configurations that have the same objects and effects). In addition, the invention includes a configuration in which a non-essential part of the configuration described in the embodiment is replaced. In addition, the present invention includes a configuration that exhibits the same operational effects as the configuration described in the embodiment or a configuration that can achieve the same object. Further, the invention includes a configuration in which a known technique is added to the configuration described in the embodiment.

DESCRIPTION OF SYMBOLS 10 ... Electron beam source, 12 ... Irradiation lens, 14 ... Deflection coil, 16 ... Objective lens, 18 ... Sample stage, 19 ... Sample holder, 20 ... Imaging device, 22 ... Electron beam source control part, 24 ... Lens control part, 26 ... Deflection control unit, 28 ... Imaging control unit, 30 ... Sample stage control unit, 32 ... Control processing unit, 34 ... Display unit, 100 ... Electron microscope

Claims (5)

An electron beam source;
An irradiation lens for irradiating the sample with an electron beam emitted from the electron beam source;
An objective lens that forms an electron microscope image with an electron beam transmitted through the sample;
A sample stage provided with a Z moving mechanism for moving the sample in the Z direction along the optical axis;
A Z position adjustment unit that adjusts the Z position of the sample by controlling the sample stage so that the focus current is just focused when the lens current of the objective lens is a reference current;
Including
The Z position adjusting unit is
A first process for obtaining a first focus current that is a lens current of the objective lens that is in a just focus when the Z position of the sample is the first position;
A second process for obtaining a change amount of the Z position corresponding to the difference from the difference between the first focus current and the reference current;
A third process for controlling the sample stage so that the Z position of the sample becomes a second position moved from the first position by the amount of change obtained in the second process;
A fourth process for obtaining a second focus current which is a lens current of the objective lens which is in a just focus when the Z position of the sample is the second position;
A fifth process for determining a Z position of the sample that is just focused at the reference current based on the first focus current and the second focus current;
A sixth process for controlling the sample stage so that the sample is positioned at the Z position obtained in the fifth process;
Do the electron microscope. In claim 1,
In the first process, the Z position adjusting unit may
A process for setting the first focus current obtained by obtaining the lens current of the objective lens;
A process for obtaining the first focus current again in a state where the lens current of the objective lens is set to the first focus current obtained;
The electron microscope is repeatedly performed until the difference between the first focus current obtained at the nth time and the first focus current obtained at the (n + 1) th time becomes a predetermined value or less. In claim 1 or 2,
In the fourth process, the Z position adjusting unit may
A process of setting the second focus current obtained from the lens current of the objective lens;
A process for obtaining the second focus current again in a state where the lens current of the objective lens is set to the second focus current obtained;
The electron microscope is repeatedly performed until the difference between the second focus current obtained at the nth time and the second focus current obtained at the (n + 1) th time is equal to or less than a predetermined value. In any one of Claims 1 thru | or 3,
A deflection unit for deflecting an electron beam emitted from the electron beam source and changing an incident angle of the electron beam with respect to the sample;
In the first process, the Z position adjusting unit may
A process of acquiring a plurality of image shift vectors representing image shifts before and after changing the incident angle of the electron beam to the sample by a predetermined angle by changing the lens current of the objective lens;
Based on a plurality of the image shift vectors, a process for calculating a minimum image shift vector that minimizes an image shift amount before and after changing the incident angle of the electron beam to the sample by the predetermined angle;
A process for obtaining the first focus current based on the minimum image shift vector;
Do the electron microscope. In a transmission electron microscope, a sample height adjustment method for adjusting a Z position, which is a position in the Z direction along an optical axis of a sample, so that the focus current is just focused when a lens current of an objective lens is a reference current. ,
A first step of obtaining a first focus current which is a lens current of the objective lens which is in a just focus when the Z position of the sample is the first position;
A second step of determining a change amount of the Z position corresponding to the difference from the difference between the first focus current and the reference current;
A third step of moving the Z position of the sample from the first position to a second position moved by the amount of change obtained in the second step;
A fourth step of obtaining a second focus current which is a lens current of the objective lens which is in a just focus when the Z position of the sample is the second position;
Based on the first focus current and the second focus current, a fifth step of determining the Z position of the sample that is just focused at the reference current;
A sixth step of moving the sample to the Z position determined in the fifth step;
A method for adjusting the sample height, including

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