JPS5837939B2 - electronic lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electronic lens having multiple magnetic pole gaps in a single magnetic circuit.

, scanning electron microscopes, etc., a crossover image formed directly below the electron gun is reduced and formed onto the sample surface using a lens system.

Generally, a two-stage lens system or a three-stage lens system is used as this lens system.
For purposes such as shortening the length of mirror strips, a three-stage lens system is used.

The three-stage lens system consists of three independent lenses, and the other consists of a so-called double-gap lens, which has two gaps in a single magnetic circuit, and an independent lens. There are some.

In other words, a double gap lens of a three-stage lens system, such as the latter, is constructed so that the lens strength of each gap is the same.

Therefore, when trying to obtain a high-resolution sample image by maximizing the lens excitation intensity, which is an essential operation in scanning electron microscopes, etc., you can maximize the excitation intensity of the lens to narrow down the electron beam to the maximum, and vice versa. A problem arises when trying to expand the maximum electron beam current to obtain an image of the sample.

The optical diagrams in each case are shown in FIGS. 1a and 1b, respectively.

(However, 1 in the figure is a crossover image formed directly below the electron gun,
2, 2' are image forming points of the front stage lens of the double gap lens existing on line A1, 3, 3' are image forming points of the rear stage lens of the double gap lens existing on line A2,
4 and 4' are the imaging points of the objective lens existing on the line B, and 5 is the sample) That is, the distance d1 between the imaging points 3 and 3' on the optical axis of the double gap rear lens is extremely large.

Therefore, the excitation strength of the objective lens placed between the double gap lens and the sample is adjusted depending on the operation of the double gap lens, and the image of the imaged point on the optical axis of the double gap rear lens is adjusted. The focus was on the sample surface.

The present invention was made to solve these drawbacks and provides a novel electronic lens.

FIG. 2 is a schematic diagram of a double gap lens showing an embodiment of the present invention, in which 10 is a yoke, 11 is an excitation coil, and 12 is a schematic diagram of a double gap lens showing an embodiment of the present invention.
．． Reference numerals 13 denote an upper magnetic pole piece and a lower magnetic pole piece of the front lens, respectively, and these two magnetic pole pieces are integrated by a spacer 14.

Reference numerals 15 and 16 are upper and lower magnetic pole pieces of the rear lens, respectively, which are integrated by a spacer 17.

Now, a double gap lens equivalent circuit having two gaps in a single magnetic circuit is as shown in FIG.

(However, the magnetomotive force of the magnetic circuit around Eo, E, and E2 are the magnetomotive forces generated in the gap between the front and rear lenses, respectively, and R
1 and R2 are the magnetic resistances of the front and rear lenses, respectively) The focal lengths of the front and rear lenses of the double gap lens are f1 and f2, respectively, and the upper magnetic pole piece O and the lower magnetic pole are When the gap of one piece U is Q, the hole diameter is P, and the upper surface area of the lower magnetic pole piece is S, magnetic resistance R
is proportional to the gap Q and inversely proportional to the area S.

Further, if the magnetomotive force of this magnetic circuit is constant, the lens focal length f is proportional to the hole diameter P and the gap Q, respectively.

In the present invention, by utilizing the above-mentioned relationship, the lens strength of the rear lens is approximately 3 times to 1 times that of the front lens.
It is characterized by being constructed so that it becomes 0 times.

That is, the hole diameter of the front lens is made larger than the hole diameter of the rear lens so that the above-mentioned lens strength ratio can be obtained.

By doing so, the focal length f2 of the rear lens becomes approximately 3 to 10 times smaller than the focal length f1 of the front lens.

That is, the lens strength of the rear lens is approximately 3 to 10 times greater than the lens strength of the front lens.

Similarly, if the hole diameter of the front lens is increased, the top surface area of the upper magnetic pole piece will become smaller, and the magnetic resistance R1 will increase, but the degree of decrease in this area with respect to the increase in the hole diameter is very small, so The increase is negligible.

Furthermore, if the gap of the front lens is made several times larger than the gap of the rear lens, the magnetic resistance R1 of the front lens will increase, and as is clear from the equivalent circuit, the lens strength of the rear lens will also become weaker. Both the gaps between the stage lenses become smaller.

When the excitation intensity of the so-called three-stage lens system is maximized and the electron beam is narrowed down to its maximum intensity in a so-called three-stage lens system consisting of a double-gap lens configured as described above and an objective lens placed below it,
On the other hand, when the excitation intensity is minimized and the electron beam is widened to its maximum extent, the optical diagrams become as shown in the fifth [ka, b, respectively.

(However, 1 in the figure is a crossover image formed directly below the electron gun, and 5
is the sample, 20.20' is the imaging point of the front lens of the double gap existing on line A1, 30,30' is the imaging point of the rear lens of the double gap lens existing on line A2, 40.40' is the imaging point of the objective lens existing on line B.

In other words, the imaging point 3 on the optical axis of the double gap rear lens
The distance d2 of 0.30' is extremely small.

Therefore, the degree of day focus of the objective lens image on the sample can be ignored, and at least it is no longer necessary to adjust the excitation strength of the objective lens dependently on the operation of the double gap lens, which greatly simplifies the operation. shall be taken as a thing.

If the lens strength of the rear lens is made 10 times or more greater than the lens strength of the front lens, magnetic saturation will occur and the lens will not function properly.

Also, if it is 3 times or less, the distance d2 between the imaging points 30.30' on the optical axis of the double-gap rear lens is the same as the distance d2 of the double-gap lens of the conventional three-stage lens system.
1 and the effect is weak.

[Brief explanation of the drawing]

114a and 114b are schematic optical diagrams of the conventional three-stage lens system including a double gap lens when the electron beam is narrowed down to the maximum extent and when it is widened to the maximum extent, respectively. 3 is a magnetic equivalent circuit thereof, FIG. 4 is a partially enlarged view of a lens used to explain the principle of the present invention, and FIG. This is an optical diagram of the stage lens system when the electron beam is narrowed down to the maximum and when it is widened to the maximum. 1... Crossover image, 5... Sample,
10... Yoke, 11... Excitation coil, 12... Upper magnetic pole piece of the front stage lens of the double gap lens, 13... Lower part of the front stage lens of the double gap lens. Magnetic pole piece, 14... Spacer, 1
5... Upper magnetic pole piece of the rear lens of the double gap lens, 16... Lower magnetic pole piece of the rear lens of the double gap lens, 17... Spacer.

Claims (1)

[Claims] 1. A two-stage electron lens used in a lens system for narrowly focusing an electron beam generated from an electron gun on a sample is constructed using a single magnetic circuit. 1. An electronic lens characterized in that two magnetic pole gaps are provided so that the strength of the lens due to the magnetic field generated in the gap in the rear stage is approximately 3 to 10 times greater than the strength of the lens due to the magnetic field generated in the gap in the rear stage.