JPH05217537A - Photographing device for transmission type electron microscope - Google Patents

Abstract

PURPOSE:To efficiently take in information having high resolving power by using the through focus function, in photographing a transmission type electron microscope image. CONSTITUTION:As for a transmission type electron microscope which possesses the through focus function for taking photograph by varying the defocus quantity in stepwise form, taking a through focus photograph is carried out, setting the defocus condition to (mcslambda)<1/2> (m: odd number, cs: spherical aberration coefficient, lambda: wave length). Accordingly, a through focus photograph which the condition for making the phase contrast constant can be obtained for the space frequency in a wide range, and the image information having the higher resolving power can be obtained by combining these photographs.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission electron microscope photographing apparatus for obtaining a high resolution image.

[0002]

2. Description of the Related Art In a photographic device of a transmission electron microscope,
In particular, the contrast of a high-magnification, high-resolution image is a phase contrast transfer based on a slight phase shift of an electron beam passing through an object.
function). When the phase contrast is B, B is given by the equation (1).

B = Bc · Gα · Gε (1) where Bc is a coherent transfer function, Gα is a coherence coefficient, and Gε is a chromatic aberration coefficient. Coherent transfer function B
c is given by the equation (2).

Bc = −sin (π / 2λ) · (c _S θ ⁴ −2Δf θ ² ) = − sin φ (θ) (2) where c _S is the spherical aberration coefficient of the lens, Δf is the defocus amount, θ is a spatial frequency.

Incidentally, the coherence coefficient Gα, the chromatic aberration coefficient Gε, and the coherent transfer function B with respect to the spatial frequency θ.
The c and the phase contrast B show the characteristics as shown in FIG. As can be seen from this characteristic, Gα which is the envelope function of the opening angle of the sample irradiation electron beam and G which is the envelope function of the chromatic aberration with respect to the high pressure fluctuation
The attenuation of ε is relatively fast, and the phase contrast B is rapidly attenuated from the point where the coherent transfer function Bc first becomes zero. The resolution corresponds to the spatial frequency θ, and a high resolution image means that a predetermined phase contrast can be obtained even for a large spatial frequency θ. The resolution of the transmission electron microscope must be discussed within the range where this contrast can be obtained, because the image cannot be confirmed unless the phase contrast is obtained. In this respect, the conventional high-resolution image is obtained by searching for a Schulzer focus condition, that is, a point at which the coherent transfer function Bc becomes 0 first, from among photographs taken in through focus described later, That was the limit.

What is generally used as the theoretical resolution of a transmission electron microscope at present is that the phase contrast transfer function becomes 0 first when Δf = 1.2 (c _S λ) ^1/2. ,
That is, it is the point P in FIG. 5, and when the photograph was taken, the focus condition was selected and searched for. One of the methods is through focus, which is considered to be the Scherzer focus condition. In the vicinity of what seems to be the Scherzer focus condition, the Sherzer focus condition is included in some photographs in which the focus position of the objective lens is changed in a certain focus step. It takes a few photos manually or automatically with the expectation that there will be something that meets the requirements.

This will be described with reference to FIGS. 7 and 8.
In FIG. 7, an electron beam 2 from an electron gun 1 is focused on a focusing lens 3
The sample 6 is irradiated with the light through the, the sample image is formed by the objective lens 4, and the image is projected on the fluorescent plate 7 through the projection lens 5. At this time, the exciting current varying mechanism 8 changes the exciting current to the objective lens 4 stepwise as shown in FIG. 8 to change the defocus amount stepwise, and an image is taken at each step. With the through focus function like this, you can take a picture of only the number of steps,
I tried to find the one that meets the Sherzer focus condition.

[0008]

The use of field emission type emitters and the advent of ultra-sensitive recording devices (imaging plates) have realized an aperture angle of the sample irradiation electron beam that is one digit smaller and an energy width of less than half. It became possible.
As a result, as shown in FIG. 6, the limit at which the phase contrast defined by the attenuation of Gα or Gε can be obtained is about the first folding point of the coherent transfer function Bc which is the Sherzer focus condition in the related art. However, it has been improved so much that it does not affect the contrast reduction even in the region of large spatial frequency. In such a case, it is possible to obtain information with higher resolution by combining various focus images in addition to simply obtaining a photograph that satisfies the Sherzer focus condition as in the past.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an imaging apparatus for a transmission electron microscope capable of efficiently taking in information of a sample with higher resolution.

[0010]

According to the present invention, a defocus condition is set to (mc _S λ) ^1/2 (in a transmission electron microscope having a through focus function for taking a photograph while changing a defocus amount in steps. m: odd number, c _S : spherical aberration coefficient,
It is characterized in that through-focus photography is performed as (λ: wavelength).

[0011]

In the present invention, the through focus function allows Δf =
By taking a photograph while changing the defocus amount stepwise under the condition of (mc _S λ) ^1/2 (m: odd number),
It is possible to obtain a through-focus photograph that satisfies the condition that the phase contrast is substantially constant in a wider range of spatial frequencies, and by combining these, it is possible to obtain higher resolution image information.

[0012]

EXAMPLES Examples of the present invention will be described below. As described above, since Bc that substantially determines the phase contrast B is expressed by the equation (2), the reason why a region where the phase contrast is constant occurs is that φ, which is a function of θ, has a small θ. In the region, the term −2Δfθ ² is large and therefore decreases monotonically, and the term of c _S θ ⁴ increases as θ increases, and becomes 0 at φ = −π / 2−nπ (n is a natural number). It can be seen that this is caused by a monotonic increase.

Therefore, when φ = φ (θ) is examined, Δf = k (c _S λ) ^{1/2 in} φ (θ) = (π / 2λ)  (c _S θ ⁴ -2Δf θ ² ). And variable conversion (c _S λ
= (Constant), φ (θ) = (π / 2λ) · (c _S θ ⁴ −2k (c _S λ) ^1/2 · θ ² ) φ ′ (θ) = (2π / λ) · (c _S θ ³ −k (c _S λ) ^1/2 · θ) = (2π / λ) · c _S θ (θ ² −k (λ / c _S ) ^1/2 ) Therefore, φ ′ (θ) = 0 The satisfied θ is θ = k ^1/2 · c _S ^-1/4 · λ ^1/4 , and it can be seen that the monotonic decrease turns into a monotonic increase at this time.

Then, by substituting θ = k ^1/2 · c _S ^−1/4 · λ ^1/4 , φ (θ) = (π / 2) · k ² , and a function of only k is obtained. Therefore φ = −π / 2−n
It can be seen that k satisfying π is k = (m) ^1/2 (m is an odd number). Therefore, if a photograph is taken while changing the defocus amount under the condition of Δf = (mc _S λ) ^1/2 (m is an odd number), it is possible to obtain a through-focus photograph satisfying the condition that the phase contrast is almost constant. Become.

FIG. 1 shows the phase contrast in the case of m = 1, and the information obtained in the region of θ of 0 to 0.53 has all the phases, which satisfies the Sherzer focus condition. FIG. 2 shows the case where m = 5. In the area A1 (θ is 0.45 to 0.7) in the figure, an area with a constant phase contrast is obtained, and higher resolution information with a large spatial frequency is obtained. Be done. In FIG. 3, m =
9 and the area A (θ is 0.55 to 0.7).
In 8), the phase contrast becomes almost constant, and higher resolution information can be obtained. FIG. 4 shows the case where m = 13, and the phase contrast is almost constant in the region A3 (θ is 0.62 to 0.83), and higher resolution information can be obtained.

Thus, at (mc _S λ) ^1/2 , m
By making the value an odd number and increasing the value, areas where the phase contrast is almost constant on the higher resolution side can be obtained, so by taking photographs of these areas and synthesizing them, the conventional Sherzer condition is satisfied. It is possible to obtain image information with higher resolution than that of the image information.

[0017]

As described above, according to the present invention, Δf =
When a field emission type emitter or an imaging plate is used by performing through focus under a defocus condition of (mc _S λ) ^1/2 (m is an odd number),
Through-focus photographs that satisfy the condition that the phase contrast is constant over a wider range of spatial frequencies can be obtained, and by combining these, it is possible to efficiently capture information of higher resolution samples. ..

[Brief description of drawings]

FIG. 1 is a diagram showing a phase contrast when m = 1.

FIG. 2 is a diagram showing a phase contrast when m = 5.

FIG. 3 is a diagram showing a phase contrast when m = 9.

FIG. 4 is a diagram showing a phase contrast when m = 13.

FIG. 5 is a diagram for explaining Schelzer focus conditions.

FIG. 6 is a diagram showing the relationship between the attenuation of Gα and Gε and the phase contrast.

FIG. 7 is a diagram illustrating a through focus function.

FIG. 8 is a diagram illustrating a through focus function.

[Explanation of symbols]

B ... phase contrast, Bc ... coherent transfer function,
Gα ... Coherence coefficient, Gε ... Chromatic aberration coefficient, c _S ... Spherical aberration coefficient, θ ... Spatial frequency.

Claims (1)

[Claims] 1. In a transmission electron microscope having a through-focus function for taking a photograph by changing the defocus amount in steps, the defocus condition is (mc _S λ) ^1/2 (m: odd number, c _S : spherical surface). Aberration coefficient,
An image pickup apparatus for a transmission electron microscope, which is configured to perform through-focus photography as (λ: wavelength).