JPH0444380B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for observing a sample with high resolution using a transmission imaging electron microscope.

In an electron microscope, high-resolution images are observed by irradiating a thin film sample with electrons. Electrons irradiated onto atoms or small groups of atoms constituting a sample pass through them without being absorbed. Therefore, electron microscope images do not have contrast due to electron absorption.

Electrons irradiated onto a sample are scattered by atoms or small groups thereof that make up the sample. The electron wave scattered by this small object spreads, but this spreading wave interferes with the wave that travels without being refracted, forming an interference pattern behind the small object depending on the phase difference. This phase difference causes the image contrast of single atoms and small groups thereof. Therefore, if you focus the lens on the atom you want to observe,
That is, if the object plane position of the objective lens is made to coincide with the sample position, there will be no phase shift, and no contrast will be obtained. Therefore, it is necessary to effect photography by shifting the focal point by a certain amount so that the object plane of the objective lens is located below or above the position of the atoms. However, the amount of shift required to obtain the highest resolution image varies depending on the sample, as explained below. Therefore,
It is necessary to shift the focus by the optimum amount depending on the sample and take the image. Therefore, conventionally,
The defocus amount Δf, which is the amount of defocus between the sample position and the object surface position of the objective lens, is gradually changed, and a photograph is taken each time. I am trying to select photos and it is very complicated.

Moreover, even if one chooses the highest-resolution photograph from among these photographs, the differences between them are slight and require a lot of effort.

SUMMARY OF THE INVENTION An object of the present invention is to solve these conventional drawbacks and provide an electron microscope that allows even an unskilled operator to easily observe a high-resolution electron microscope.

Therefore, in the first invention, λ is the wavelength defined from the accelerating voltage value of the electron beam, Δf is the defocus amount, and Cs is calculated based on the accelerating voltage of the electron beam, the excitation current Io of the objective lens, and the sample position. Function signal generating means for generating a signal representing a phase contrast transfer function T (ξ, η) of an objective lens given by the following formula as a function of spatial frequencies ξ, η, when it is a spherical aberration coefficient; )=sin {πλΔf(ξ ² +η ² ) −(π/2)λ ³ Cs(ξ ² +η ² ) ² } Means for detecting a diffraction image formed by an imaging lens system including the objective lens. and display means for displaying the signal from the detection means and the position where the phase contrast function from the function signal generation means becomes zero on the same display screen, and the display means displays the position where the phase contrast function is zero from the function signal generation means. The present invention is characterized by an electron microscope configured to display a position corresponding to the diffraction image and distinguishing it from the diffraction image.

In the second invention, λ is the wavelength defined from the accelerating voltage value of the electron beam, Δf is the defocus amount, and Cs is the spherical surface calculated based on the accelerating voltage of the electron beam, the excitation current Io of the objective lens, and the sample position. Function signal generating means for generating a signal representing a phase contrast transfer function T (ξ, η) of an objective lens given by the following formula as a function of spatial frequencies ξ, η, when it is an aberration coefficient; = sin {πλΔf(ξ ² +η ² ) −(π/2)λ ³ Cs(ξ ² +η ² ) ² } The electron microscope is characterized by an imaging lens system including the objective lens.

The operation of the present invention will be explained below.

Assuming that the structure in the xy plane perpendicular to the optical axis of a thin film sample observed by an electron microscope is represented by a function of O(x, y), the transfer function T(ξ,
Function i (x, y) of the microscope image formed on the fluorescent screen by the imaging lens system including the objective lens η)
is expressed by the following functional formula (1). Here, ξ and η are spatial frequencies in the x and y directions, respectively.

i(x, y)=∫〓 _- 〓∫〓 _- 〓[T(ξ, η)∫ ^a _-a ∫ ^b
_-b O(x′+
y′) exp{−2πi(ξx′+ηy′)}dx′dy′]exp{2πi
(ξx+
ηy)dξdη...(1) The integral in [ ] in equation (1) is the sample structure O(x,y)
The result obtained by Fourier transformation (Fourier transformed image) is expressed as (ξ, η), and the Fourier transformed form of image i (x, y) is expressed as I (ξ , η), the following equation (2) is obtained.

I (ξ, η) = T (ξ, η)・(ξ, η) ...(2) As mentioned above, high-resolution electron microscope images at high magnification are mostly formed based on phase contrast effects. Therefore, the following explanation will be limited to those related to phase contrast. In this case, O(x, y) mentioned above represents the phase structure based on the potential distribution in the sample, and T(ξ, η) represents the phase contrast transfer function (PCTF).
η) is expressed as the following equation (3).

T (ξ, η) = sin {πλΔf (ξ ² + η ² ) − (π/2) λ ³ Cs (ξ ² + η ² ) ² } ...(3) Here, λ is calculated from the accelerating voltage value of the electron beam. The defined wavelength, Δf is the aforementioned defocus amount, and Cs is the spherical aberration coefficient of the objective lens. Equation (3) can be calculated using the accelerating voltage of the electron beam in the electron microscope, the excitation current Io of the objective lens, and the sample position. When displayed visually, it looks like the one shown in Figure 1, for example. In Figure 1, ξ represents the spatial frequency ξ axis, η represents the spatial frequency η axis, and O
represents the origin.

Furthermore, T can also be expressed as in equation (4) by setting θ=λ(ξ ² +η ² ) ^1/2 .

T(θ)=sin {(π/λ) Δfθ ² − (π/2λ) Csθ ⁴ } ...(4) In this case, θ (or preset d
1/d), and T(θ) is plotted on the vertical axis to illustrate T(θ) as shown in FIG.

As is clear from equation (2) above, T(ξ, η)
is a function that expresses the ratio at which (ξ, η) is transmitted to I (ξ, η) by the objective lens, and the larger the absolute value of T (ξ, η), the greater the (ξ, (ξ, η) in η) is transmitted and contributes to the sample image. As mentioned above, this phase contrast transfer function is the amount of defocus (defocus amount)
There is a relationship between Δf etc. and equation (3), and by adjusting these, the phase contrast transfer function T
(ξ, η) can be changed. A high-resolution electron microscope image is the phase structure of the sample O(x,y)
Each spatial frequency component, especially a high spatial frequency region component, of the Fourier transformed image (ξ, η) of is sufficiently transmitted by the imaging lens system including the objective lens and contributes to imaging. This is because the components of the Fourier transform image (ξ, η) in the high spatial frequency region are involved in the fine structure of the microscopic image finally obtained. Since the diffraction image obtained by the imaging lens system corresponds to the Fourier transform image, it is conceivable to compare the diffraction image with the display of the phase contrast function.

Therefore, in the present invention, the diffraction image, which is an image formed by the imaging lens system including the objective lens and represents a Fourier transformed image of the phase structure of the sample, and the position where the phase contrast function becomes zero are provided. The position where the phase contrast function becomes zero is displayed on the same display screen, and at this time, the position where the phase contrast function is zero is displayed based on the output signal from the function signal generating means, and the position is made to correspond to and separate from the diffraction image. I'm trying to display them separately.

Since the diffraction image represents the Fourier transform image (ξ, η) of the phase structure O(x, y) of the sample, the Fourier transform image (ξ, η) is adjusted by adjusting the defocus amount while observing this displayed image. ) The shape of the phase contrast transfer function is changed so that most of the Fourier transform image (ξ, η) can contribute to image formation without the peak of the phase contrast transfer function matching the position where the phase contrast transfer function becomes zero. For example, even an unskilled operator can easily adjust the defocus amount to obtain a high-resolution image.

Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 3 is a schematic diagram showing the main parts of an apparatus according to an embodiment of the present invention. In FIG. 3, numeral 25 represents a crystalline sample irradiated with the electron beam 2 at a constant acceleration voltage. In actual equipment, the sample 25 is often placed in the magnetic field of a strongly excited objective lens, and an astigmatism correction device is installed to cancel out the magnetic field components asymmetric to the optical axis that occur in the imaging lens system. 9 is provided in most devices. The sample is moved with high precision by the sample moving mechanism 3 along the optical axis 4 of the lens system, and its position is displayed by the sample position display means 5. The electron beam transmitted through the sample is passed through the objective lens 6 and another imaging lens 7 to a fluorescent screen 8.
An electron beam diffraction line is imaged on top. Excitation lens power supply 1
The supply of excitation current from 6 to the objective lens 6 is controlled by an objective lens excitation current operating means 17. The fluorescent screen 8 has micro holes. In order to two-dimensionally scan the electron beam diffraction image with respect to the hole, a deflection coil 23 and its scanning power source 24 are provided. Reference numeral 22 denotes a detector for detecting the electron beam that has passed through the microhole, and an output signal from the detector 22 is supplied to the display device 15 via the amplifier 12. Reference numeral 26 indicates a control power source for the imaging lens system that controls the size of the diffraction image formed on the fluorescent screen. A part of the output signal of the control power source 26 is used as a control signal for the variable amplifier 27 provided between the scanning power source 24 and the deflection coil 23, and is used as a control signal for the variable amplifier 27 provided between the scanning power source 24 and the deflection coil 23.
This is used to match the coordinate units in the phase contrast transfer function display shown in 5 with the coordinate units in the diffraction image display. A scanning signal is generated from the scanning power source 24, the electron diffraction image is two-dimensionally scanned by the deflection coil 23, and the electron beam diffraction image is scanned two-dimensionally by the deflection coil 23.
The resulting signal is displayed on the display means 15 in a brightness modulation display. The electron beam diffraction image shows the phase structure O(x,
Since it represents the Fourier transform image (ξ, η) of .

On the other hand, a part of the output signal from the objective lens excitation operating means 17 that controls the power supply 16 of the objective lens 6 is applied to the function signal generating means 19 as a signal representing the excitation current value Io of the objective lens. A signal related to the acceleration voltage is applied to the function signal generating means 19 from the electron beam acceleration voltage control means, and a signal representing the deviation of the sample position from the positive focus position, that is, the amount of defocus, is applied from the sample moving mechanism 3. Corresponding to the combination of these input signals, the function signal generating means 19 generates a signal representing the above-mentioned phase contrast transfer function and supplies it to the display means 15. Therefore, the display means 15 displays the transfer function (PCTF) value of the objective lens two-dimensionally as a brightness modulation image, and also displays the phase structure O of the sample 25.
A diffraction image corresponding to a Fourier transform image of (x, y) is displayed in correspondence and distinctly.

FIG. 4 shows the display device 15 in the device shown in FIG.
The black dots correspond to diffraction points, and the boundary line between the shaded area and other areas represents the coordinates where the phase contrast transfer function value becomes zero. In FIG. 4, ξ and η represent the aforementioned spatial frequency axes.

Adjustment of the defocus amount Δf using the apparatus shown in FIG. 3 is performed as follows. That is, the display device 15
Observe whether the position where the phase contrast transfer function becomes zero coincides with the main peak position appearing in the Fourier transform image. If there is such a coincidence, a large portion of the Fourier transform components of the sample that can contribute to a high-resolution image will not be sufficiently transmitted by the imaging lens system including the objective lens. Therefore, while watching the display on the display device 15, the sample 1 is moved by the sample moving mechanism 3, and the defocus amount is adjusted so that the position where the phase contrast transfer function becomes zero does not coincide with the peak position in the Fourier transform image. Adjust. If the electron microscope is set to image observation mode in this state, the sample can be observed as a high-resolution image. In addition, if the periodic structure that you particularly want to observe is known in advance, the excitation of the objective lens and the amount of defocus can be adjusted so that the peak of the phase contrast transfer function value is at the corresponding spatial frequency coordinate of the display device 15. All you have to do is observe the sample. In the example of FIG. 4, the diffraction spot 30 corresponding to the peak of the Fourier transform image coincides with the zero value portion of the phase contrast transfer function representation, which is not desirable. Therefore, it is necessary to change the phase contrast transfer function by changing the excitation intensity of the objective lens.

In the case of such a diffraction image, if one-dimensional display is performed, the signal distribution along the ξ axis, η axis, or straight line l in Figure 4 cannot be observed at the same time, and the line scanning direction must be switched for observation. No.

FIG. 5 is a diagram showing another solution, in which the fluorescent screen for forming the diffraction image in FIG. 3 is used as a means for displaying the phase contrast transfer function. That is, the display device 15 in the embodiment shown in FIG. 3 is not used. The entire surface of the fluorescent screen 8b is used, and a large number of light emitting elements (for example,
LED) 31 is embedded, and the light emitting element 31 located near the coordinate where the output 32 of the function signal generating means 19 is zero is configured to emit light.
As shown in the display means 15, the relationship between the diffraction image (Fourier transform image of the sample) and the phase contrast transfer function can be roughly observed in correspondence, and the same effect as shown in FIG. 3 can be achieved. .

Note that the diffraction image and the phase contrast transfer function may be displayed in different colors.

As explained in detail above, in the device of the present invention,
The diffraction image representing the Fourier transform image of the phase structure of the imaged image and the position where the phase contrast function becomes zero are displayed on the same display screen in a corresponding and distinct manner. , while observing this display image, if you adjust the defocus amount etc. so that the peak part of the Fourier transform image (ξ, η) does not coincide with the position where the phase contrast transfer function becomes zero, it will be possible to improve your skills. Even an operator who is not familiar with the system can easily obtain a high-resolution image, and it is extremely easy to adjust the amount of defocus, etc. Therefore, even unskilled operators can easily observe samples, and it is expected to make a significant contribution to research on physical properties using electron microscopy.

[Brief explanation of drawings]

1 and 2 are schematic diagrams each showing an example of the phase contrast transfer function of an objective lens, FIG. 3 is a diagram showing an embodiment of the present invention, and FIG. 4 is a diagram showing the embodiment of FIG. 3. FIG. 5 is a diagram showing a display example of the display device 15 in FIG. 5, and FIG. 5 is a diagram showing still another embodiment of the present invention. 2: Electron beam, 3: Sample moving mechanism, 5: Sample position display means, 6: Objective lens, 7: Imaging lens,
8, 8b: Fluorescent screen, 9: Astigmatism correction device, 1
5: Display device, 16: Objective lens power supply, 17: Objective lens excitation operation means, 19: Function signal generation means, 22: Detector, 23: Deflection coil, 24: Scanning power supply, 25: Sample, 26: Imaging lens System control power supply, 27: variable amplifier.

Claims (1)

[Claims] 1. λ is the wavelength defined from the accelerating voltage value of the electron beam, Δf is the defocus amount, and Cs is the spherical surface calculated based on the accelerating voltage of the electron beam, the excitation current Io of the objective lens, and the sample position. When taken as an aberration coefficient, the phase contrast transfer function T (ξ, η) of the objective lens is given by the following formula as a function of spatial frequencies ξ, η.
function signal generating means for generating a signal representing T(ξ, η)=sin {πλΔf(ξ ² +η ² ) −(π/2)λ ³ Cs(ξ ² +η ² ) ² }; and the objective lens. A means for detecting a diffraction image formed by an imaging lens system, and a position where the signal from the detecting means and the phase contrast function from the function signal generating means are zero are displayed on the same display screen. An electron microscope comprising a display means configured to display a position where the phase contrast function becomes zero in correspondence with the diffraction image and distinguishing it from the diffraction image. 2 When λ is the wavelength defined from the accelerating voltage value of the electron beam, Δf is the defocus amount, and Cs is the spherical aberration coefficient calculated based on the accelerating voltage of the electron beam, the excitation current Io of the objective lens, and the sample position, The phase contrast transfer function T(ξ, η) of the objective lens is given by the following formula as a function of the spatial frequencies ξ, η
function signal generating means for generating a signal representing T(ξ, η)=sin {πλΔf(ξ ² +η ² ) −(π/2)λ ³ Cs(ξ ² +η ² ) ² }; and the objective lens. A fluorescent screen on which a diffraction image is formed by an imaging lens system, a large number of light emitting elements embedded in this fluorescent screen along several straight lines passing through the center, and a light emitting element from the function signal generating means. An electron microscope comprising driving means for causing a light emitting element at a coordinate position indicating zero to emit light in accordance with an output signal corresponding to a diffraction image.