JP3040245B2 - Vienna filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Wien filter used for obtaining an energy filter image or the like in an electron microscope or the like.

[0002]

2. Description of the Related Art A Wien filter applies an electric field E and a magnetic field B perpendicular to each other in a plane perpendicular to the traveling direction of an electron beam, so that only an electron beam having a specific speed travels straight. The condition for the electron beam to travel straight is that the direction of the electric field is x direction and the direction of the magnetic field is y direction.
When the direction is used, E ₁ −vB ₁ = 0 using the primary component E ₁ of the electric field, the primary component B ₁ of the magnetic field, and the velocity v of the electron, which is called the Wien condition. Since the Wien filter has such properties, it is used not only for energy analysis but also for a monochromator for energy analysis, a beam separator for a low-speed electron microscope, and the like in various devices that handle charged particle beams.

[0003]

The Wien condition must be satisfied in all regions where an electric field and a magnetic field work. However, it is very difficult to construct such a Wien filter. The distribution of the magnetic field and the electric field is usually different at the edges of the entrance and exit of the filter, and this difference in distribution means that the relationship between the electric field and the magnetic field at the edge deviates from the Wien condition. This means that in this case the electron beam is deflected, causing large aberrations.

[0004] On the other hand, the applicant of the present invention firstly made the distribution of the electric field and the magnetic field of the fringe field at the edge portion coincide with each other,
In the Wien filter having the configuration shown in FIG.
_1, 1 and the gap S _e between ₂ was proposed to equalize the gap S _m between the magnetic poles 2 _1, 2 _2.

As a result, the electric field distribution and the magnetic field distribution of the fringe field are as shown in FIG. 4B, and it becomes possible to improve the degree of coincidence between the electric field and the magnetic field distribution at the edge portion as compared with the conventional case. However, the electric field distribution and the magnetic field distribution cannot be matched in the fringe field.

[0006] The reason is that the first, using a non-magnetic metal such as copper as the electrode 1 _1, 1 _2, when a ferromagnetic material such as iron as the magnetic pole 2 _1, 2 _2, the electrode 1 _1, 1 ₂ does not contribute anything like for the occurrence of the magnetic field, the magnetic poles 2 _1, 2 ₂ for the generation of electric fields is because due to the fact that would give ground potential to an electrical conductor, the second electrode 1
_1, 1 ₂ and, in order to pole 2 _1, 2 ₂ of the shape different from the effect electrode ₁ 1, 1 ₂ on the fringe field, the magnetic poles 2 _1,
2 ₂ and the impact on the fringe field is because due to different things. Of course, it is possible to make the electrode surface and the magnetic pole surface the same shape, but since the electrode surface and the magnetic pole surface are primarily intended to generate a uniform electric field and a uniform magnetic field, , Cannot have the same shape in order to make the distribution of the fringe fields the same.

The Wien filter is required not only to make the electric field distribution and the magnetic field distribution of the fringe field coincide with each other, but also to be able to perform astigmatic imaging and to have a small secondary aberration. The conditions for carrying out the stigmatic imaging, it is known that it satisfies the _{E 2 / E 1 -B 2 /} B 1 = -1 / 4R C. Here, E ₂ is the primary gradient of the electric field, B ₂ is the primary gradient of the magnetic field, R _C is the cyclotron radius, and in order to satisfy this condition, the magnetic poles are inclined or a quadrupole is formed.
It has also been proposed to have a first order gradient in the electric or magnetic field.

On the other hand, as for the secondary aberration, E ₃ / E _{1 is obtained with} respect to the secondary gradient E ₃ of the electric field and the secondary gradient B _{3 of the} magnetic field.
It is known that the minimum is obtained when −B ₃ / B ₁ = 0 holds. Therefore, the structure of the electrode and the magnetic pole is expressed as E ₃ = B ₃
It is understood that the secondary aberration can be minimized by setting = 0 or E ₃ / E ₁ = B ₃ / B ₁ .

As a Wien filter satisfying all three conditions described above, an octupole structure is proposed, but the mechanical structure and the electric structure are very complicated, and it is very difficult to realize. Difficult.

The present invention has been made to solve the above-mentioned problem, and can not only make the electric field distribution and the magnetic field distribution in the fringe field coincide with each other, but also perform astigmatic imaging, and perform secondary image formation. It is an object of the present invention to provide a Wien filter having small aberration.

[0011]

To achieve the above object, a Wien filter according to the present invention is a Wien filter having an eight-pole ferromagnetic conductor having a shape constituting a part of a cylindrical surface, Four of the eight poles are arranged at 90 ° intervals with an opening angle of approximately 30 ° when viewed from the central axis, and the remaining four poles are formed with an opening angle of approximately 60 ° when viewed from the central axis. When viewed from the central axis, three poles within an opening angle of 120 ° centered on the x-axis are opposed to each other among the eight poles. A magnetic field is generated with the other magnetic poles facing each other on the y-axis as opposite magnetic poles.
An electric field is generated by setting three poles within an opening angle of 120 ° centered on the y-axis as viewed from the central axis as facing electrodes, and each other pole on the x-axis as other facing electrodes. It is characterized by making it.

[0012]

The Wien filter according to the present invention has a shape whose surface constitutes a part of a cylindrical surface.
It has a polar ferromagnetic conductor. The four poles of the eight poles are arranged at 90 ° intervals with an opening angle of about 30 ° when viewed from the central axis, and the remaining four poles are formed with an opening angle of about 60 ° when viewed from the central axis. And the opening angle is approximately 3 when viewed from the central axis.
It is arranged at the middle position between the four poles of 0 °.

In the magnetic field, three poles within an opening angle of 120 ° centered on the x axis as viewed from the central axis out of the eight poles are opposed magnetic poles, and the other magnetic fields are on the y axis. Each pole is generated as another magnetic pole facing each other, and the electric field is generated by setting three poles facing each other within an opening angle of 120 ° centered on the y-axis as viewed from the central axis, as opposed electrodes, and the other x-axis. Each upper pole is generated as another opposing electrode.

That is, the eight poles function not only as electrodes but also as magnetic poles. Since the electrodes and the magnetic poles are made of the same material and have the same shape, the electric field distribution of the fringe field and the magnetic field The distribution can be matched, and the condition for astigmatic imaging and the condition for minimizing the secondary aberration can be satisfied.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description of the embodiments, the principle of the present invention will be described first. Now, in the fringe field, E
_It can be easily understood from the above that the electrodes and the magnetic poles may be made to have the same shape using the same material in order to make the distributions of ₁ and B1 coincide. for that purpose,
The electrode may be made of a conductor having ferromagnetism. However, in this case, since a ferromagnetic material as an electrode is inserted into the gap between the magnetic poles, the uniformity of the magnetic field is significantly impaired, B, which is the condition for minimizing the secondary aberration
It is very difficult to satisfy the condition of ₃ = 0.

By the way, as shown in FIG. 1, it constitutes a quadrupole having a shape constituting a part of a cylindrical surface with a conductor having a ferromagnetic, each of the electrodes _{1 1, 1 2 (V 1} + V x) ,
(V ₁ -V _x) giving, in case the electrode 1 _3, 1 ₄ both gave V ₂ can be generated a uniform electric field, i.e. electric field becomes E ₃ = 0, and 4 it is known that it is possible to provide a quadrupole component E _2. In FIG. 1, open angle from the electrode 1 _1, 1 ₂ of the central axis facing on the x-axis are both 120 °, the electrode 1 ₃ opposed on the y-axis,
1 ₄ open angle from the central axis of the are both 60 °.

The same applies to the magnetic poles. As shown in FIG. 2, four poles having a shape that forms a part of a cylindrical surface with a ferromagnetic conductor are formed, and the magnetic poles 2 ₁ and 2 ₂ have Both give −2 NI _B ampere turns, and the magnetic poles 2 ₃ and 2 ₄ have (NI _B + NI _y ) ampere turns and (NI _B
It is known that when a -NI _y ) ampere turn is applied, a uniform magnetic field, that is, a magnetic field in which B ₃ = 0 can be generated, and a quadrupole component B ₂ can be applied. In FIG. 2, the opening angle of the magnetic pole 2 _1, 2 ₂ of the central axis facing on the x-axis are both 60 °,
pole 2 _3, 2 ₄ opening angle from the central axis of the opposing on the y-axis are both 120 °.

Therefore, if the electrode structure shown in FIG. 1 and the magnetic pole structure shown in FIG. 2 are formed in the same plane, the Wien filter satisfying the above three conditions, that is, the electric field distribution and the magnetic field distribution in the fringe field match. It can be seen that it is possible to perform astigmatic imaging, and to configure a Wien filter with small secondary aberration.

Hereinafter, an embodiment of the present invention will be described with reference to FIG. In FIG. 3, eight poles P ₁ , P ₂ , P
_{3, P 4, P 5,} P 6, P 7, P 8 are iron, nickel
It is composed of a ferromagnetic conductor such as permalloy.
The opening angles of the poles P ₂ , P ₄ , P ₆ and P _{8 from} the central axis are 30.
° and are arranged at 90 ° intervals around the central axis. The opening angles of the poles P ₁ , P ₃ , P ₅ and P _{7 from} the central axis are 60 °, and 90 ° around the central axis at an intermediate position between the poles P ₂ , P ₄ , P ₆ and P _8. ° are arranged at intervals.
The pole P ₁ and pole P ₅ have the same potential since they are formed integrally, these poles function as one electrode, respectively. The poles P ₂ , P ₃ , P ₄ are of the same potential since they are integrally formed, and thus these three poles function as one electrode as a whole. Similarly, since the poles P ₆ , P ₇ , and P ₈ are also integrally formed, they have the same potential, respectively, and these three poles function as one electrode as a whole.

Therefore, V ₂ is given to the poles P ₁ and P ₅ , (V ₁ + V _x ) is given to the poles P ₂ , P ₃ and P ₄ , and the poles P ₆ and P ₅ are given.
When (V ₁ −V _x ) is given to P ₇ and P ₈ , the electrode configuration becomes the same as that shown in FIG. 1, so that a uniform electric field having a quadrupole component E ₂ can be generated.

In FIG. 3, the coil C ₁ has a pole P
_1, the P ₂ and P ₈ are wound collectively. Therefore, the poles P ₁ , P ₂ , and P ₈ function as one magnetic pole as a whole. Similarly coil C _3, since wound pole P _4, P ₅ and P ₆ collectively these three poles serves as one magnetic pole as a whole. Coils C ₂ and C _4, respectively, electrode so P ₃ and is wound only in P _7, these poles serves as one magnetic pole, respectively.

Therefore, the magnetic pole composed of the pole P ₃ and the pole P
Both give -2NI _B ampere-turns in the magnetic pole constituted by _7, the pole consists of three poles of P _1, P _2, P ₈ gave (NI _B + NI _y) ampere-turns, P _4, P
_5, the pole consists of three poles of P ₆ (NI _B -NI
_y ) If an ampere turn is given, the magnetic pole configuration is the same as that shown in FIG. 2, so that a uniform magnetic field having a quadrupole component B ₂ can be generated. In the configuration shown in FIG. 3, since the magnetic poles and the electrodes are completely symmetrical in the plane surrounding the central axis, the electric field distribution and the magnetic field distribution in the fringe field match.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of an electrode used in the present invention.

FIG. 2 is a diagram illustrating a configuration of a magnetic pole used in the present invention.

FIG. 3 is a diagram showing a configuration of one embodiment of a Wien filter according to the present invention.

FIG. 4 is a diagram showing a configuration example of a conventional Wien filter.

[Explanation of symbols]

 1 ... electrode, 2 ... magnetic pole, P ... pole, C ... coil.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-79138 (JP, A) JP-A-5-275057 (JP, A) JP-A-1-503422 (JP, A) (58) Survey Field (Int.Cl. ⁷ , DB name) H01J 49/48 H01J 37/05

Claims (1)

(57) [Claims] 1. A member having a shape that forms a part of a cylindrical surface.
A Wien filter comprising a polar ferromagnetic conductor, wherein
The four poles of the poles are arranged at 90 ° intervals with an opening angle of approximately 30 ° as viewed from the central axis, and the remaining four poles are formed with an opening angle of approximately 60 ° as viewed from the central axis. The opening angle is located at the middle position between the four poles of about 30 °,
12 of the eight poles centered on the x-axis when viewed from the center axis
Three poles within an opening angle of 0 ° are opposed magnetic poles,
A magnetic field is generated as other magnetic poles with each pole on the y-axis facing each other, and three poles within an opening angle of 120 ° centered on the y-axis as viewed from the central axis. A Wien filter characterized in that an electric field is generated as an electrode, and each of the other electrodes on the x-axis is used as another electrode facing each other.