JP3139484B2 - Charged particle beam microscopy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning charged particle microscope and similar devices, and more particularly to a charged particle optical system having high resolution in a low acceleration region and suitable for high secondary electron detection efficiency.

[0002]

2. Description of the Related Art In order to improve the resolution of a scanning electron microscope, an optical system as described in JP-A-61-294746 is used. That is, a field emission (FE) electron gun having a high brightness and a small energy width;
This is a combination of an in-lens type objective lens in which a sample is arranged inside a lens and aberration is minimized. Even in such an optical system, the resolution is reduced in the low acceleration region.

On the other hand, in order to reduce chromatic aberration, Japanese Patent Publication No.
An optical system as described in 3-34588 has been proposed.

This optical system has a high accelerating voltage until just before an electron beam irradiates a sample, and decelerates to a low accelerating voltage when irradiating the sample. In this case, since the energy of the electron beam when passing through the lens is high, lens aberration can be reduced. That is, high resolution can be achieved.

From the above viewpoints, it is possible to obtain higher resolution than before in the low acceleration region by combining the above two optical systems. That is, the sample may be placed inside the lens, and a negative voltage may be applied to the sample to reduce the speed.

However, the problem in this case is the detection of secondary electrons. In the conventional case where the sample is outside the lens, as shown in JP-B-63-34588, the electric field of the secondary electron detector is used until the secondary electrons are accelerated by the deceleration electric field of the primary electron beam. It should have been configured to detect secondary electrons. However, in the in-lens type, in which the sample is placed inside the lens, the magnetic field of the lens is so strong that not only the secondary electrons are strongly bound by this magnetic field, but also that the secondary electron detector cannot be placed inside the lens. Problems arise.

[0007] Japanese Patent Application Laid-Open No. Sho 62-31933 discloses that a primary electron beam and a secondary electron are separated only by a magnetic field.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide an electron optical system capable of achieving high resolution in a low acceleration region and obtaining high sensitivity for detecting secondary electrons.

[0009]

A first charged particle beam is extracted from a charged particle source at a predetermined acceleration voltage, and the extracted first charged particle beam is charged.
Passing through a lens for narrowing the charged particle beam to scan and deflect to a predetermined position on the sample;
Irradiating the sample with a deceleration electric field for decelerating the charged particle beam of the sample, and the second charged particle beam and the first charged particle beam generated from the sample by irradiating the sample with the first charged particle beam. And a method of correcting the influence on the first charged particle beam generated in the separation step by using the second filter.

First, it can be seen from the prior art that if the electron beam is decelerated immediately before sample irradiation, high resolution can be obtained even at a low acceleration voltage.

On the other hand, regarding secondary electron detection, since an E × B type filter is used between the sample and the detector, secondary electrons having different energies can be obtained by moving the primary electron beam straight. You will be naturally deflected. That is,
As shown in FIG. 5, with respect to the acceleration voltage V ₀ of the electron beam 2,
If E and B are applied so as to satisfy the following expression, the trajectory of the electron beam 2 is not affected.

[0012]

(Equation 1)

At this time, since the energy of the secondary electrons 8 to be detected is the deceleration voltage VR and the direction is opposite to that of the electron beam 2, the deflection angle θ of the secondary electrons 8 is

[0014]

(Equation 2)

## EQU1 ##

If this deflection direction is made to coincide with the direction of the detector, the secondary electrons travel toward the detector, so that the detection efficiency can be improved.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to FIG.

The electron beam 2 emitted from the electron gun 1 is narrowed down by a number of lenses (accelerating lens 3, condenser lens 4, and objective lens 5 in this embodiment) to irradiate the sample 6 onto the sample. The electron beam 2 is two-dimensionally scanned on the sample 6 by the deflector 7. Also, secondary electrons 8 coming out of sample 6
Is detected by the secondary electron detector 9 and becomes a video signal.

Here, a negative voltage VR is applied to the sample 6 to decelerate the electron beam 2.

At this time, the emitted secondary electrons are accelerated in reverse by the deceleration voltage VR, and cannot be sufficiently deflected toward the detector 9 only by the electric field of the detector 9.

Therefore, a deflector may be arranged to deflect the emitted secondary electrons 8 toward the detector 9. However, the electric field E and the magnetic field B are changed so as not to affect the trajectory of the electron beam 2. An orthogonal so-called E × B filter 10 is arranged between the objective lens 5 and the detector 9. At this time, if E and B are applied as in the expression (1), only the secondary electrons 8 can be deflected to the detector without affecting the trajectory of the electron beam 2, and the detection efficiency can be improved. .

However, in this case, chromatic aberration caused by the filter 10 becomes a problem. The deflection angle β due to this chromatic aberration is

[0023]

(Equation 3)

## EQU2 ## Here, ΔV is the energy width of the electron beam 2.

That is, as shown in FIG. 2, this chromatic aberration causes the spread of Sβ at the object point 12, and when the magnification of the objective lens is M, the spread of MSβ occurs on the sample. As a typical example of specific numerical values, θ = 30
_{°, ΔV = 0.3eV, V 0} = 1kV, β for V _R as is as shown in FIG. From this figure, β is largely estimated to be 5 × 10 ⁻⁵ , S = 200 mm, M = 1/5
If it is set to 0, the spread will be 0.2 μm. This value is much larger than the desired value (〜 nm) of the electron beam 2.

Therefore, in the present invention, as shown in FIG. 4 and FIG. 1, an E × B type filter 11 is arranged so that this chromatic aberration can be self-erased. That is, as can be understood from FIG. 4, the filter 11 is operated such that the electron beam 2 having the energy of ΔV spreads as if it came out of one point of the object point 12. The deflection angle β ′ of this filter 11 is

[0027]

(Equation 4)

[0028]

As described above, it is possible to deflect only the secondary electrons 8 toward the detector 9 without increasing the diameter of the electron beam 2. That is, high resolution and high detection efficiency of secondary electrons can be obtained even in a low acceleration region.

One example of the result of implementing the present invention shown in FIG. 1 is shown below. Filter 11 is the object point 12 and filter 1
0 and the action length L between the electric field E and the magnetic field B
Was set to about 20 mm, and V ₀ was fixed to 1 kV, and VR was changed to _{0 to} 900 V. At this time, the strengths of E and B of the filters 10 and 11 are set to E = 0 to E = 0.
25 V / mm, 0 to 50 V / mm, B = 0 to 14 Gauss, 0 to 28 Gauss, and when changed in conjunction with VR, a high resolution of 4 to 6 nm was realized.

According to the present invention, at a low accelerating voltage of 1 kV or less, nm
Although the filter is provided in two stages for the purpose of obtaining the resolution of the order, it is described in the present embodiment that the efficiency of secondary electron detection can be increased even if the filter is formed in one stage depending on the purpose. As expected.

In this embodiment, the sample is arranged inside the lens. However, the present invention can be applied to an optical system having a structure arranged outside the lens. In this case, it goes without saying that the secondary electron detector may be located between the sample and the objective lens, or may be located above the objective lens as shown in FIG. In short, it can be realized if there is an E × B type filter between the sample and the secondary electron detector.

Furthermore, the present invention has been described with respect to a scanning electron microscope. However, the present invention is not limited to this, but can be applied to similar electron beam application devices in general, and further to general charged particle beam application devices such as ion beams. Needless to say. However, in the case of a charged particle beam having a positive charge, the deceleration voltage needs to be a positive value.

[0034]

According to the present invention, the secondary charged particles can be deflected toward the detector without increasing the charged particle diameter even in the low acceleration region, so that the secondary charged particles can be highly resolved and the secondary charged particles can be deflected. There is an effect that a high particle detection efficiency can be obtained.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a charged particle optical system according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating chromatic aberration of an E × B filter.

FIG. 3 is a graph showing a relationship between a deflection angle caused by chromatic aberration of an E × B filter and a deceleration voltage applied to a sample.

FIG. 4 is a longitudinal sectional view of a basic optical system for self-cancelling chromatic aberration of a filter.

FIG. 5 is an explanatory diagram showing trajectories of primary electron beams and secondary electrons by an E × B filter.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 electron gun 2 electron beam 3 acceleration lens 4 condenser lens 5 objective lens 6 sample 7 deflector
8 secondary electron, 9 secondary electron detector, 10, 11 Ex
B-type filter, 12 ... object point.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) H01J 37/28 H01J 37/05 H01J 37/244

Claims (4)

(57) [Claims] 1. A first charged particle beam is extracted from a charged particle source at a predetermined acceleration voltage, passed through a lens for narrowing the extracted first charged particle beam, and scan-deflected to a predetermined position on a sample. Irradiating the sample through a deceleration electric field for decelerating the deflected first charged particle beam, and a second charged particle beam generated from the sample by irradiating the sample with the first charged particle beam Separating the first charged particle beam and the first charged particle beam by a first filter, and correcting the influence on the first charged particle beam generated in the separating step by using the second filter. A charged particle beam microscopy method characterized by comprising: 2. A first charged particle beam is extracted from a charged particle source at a predetermined acceleration voltage, passed through a lens for narrowing the extracted first charged particle beam, and scan-deflected to a predetermined position on a sample. Irradiating the sample with the deflected first charged particle beam at a low acceleration voltage lower than the acceleration voltage, and irradiating the sample with the first charged particle beam to generate second charged particles. Separating the first charged particle beam from the first charged particle beam using a first filter, and correcting the influence on the first charged particle beam generated in the separating step using the second filter. A charged particle beam microscopy method comprising: 3. The charged particle beam microscopy method according to claim 2, wherein 1 kV or less is used as said low acceleration voltage. 4. A charged particle beam microscopic method in accordance with claim 1, 2, 3, characterized in that the filter using electric and magnetic fields as the filter.