GB2140196A - Particle beam lenses - Google Patents

Abstract

A rotationally symmetric magnetic or electrostatic lens for focusing a particle beam includes two pole pieces or electrodes, the inner surface (12) of the pole piece or electrode (10) closest to the focus being shaped such that its radius R4 at the side adjacent the focus (16) is smaller than its radius R3 at the side adjacent the other pole piece or electrode. <IMAGE>

Description

SPECIFICATION Particle beam lenses The present invention relates to particle beam lenses, and, more particularly to magnetic and electrostatic lenses with rotationally symmetric pole pieces or electrodes for focusing electron or ion beams.

Various types of apparatus, for example, electron microscopes, electron probe microanalysers and electron or ion beam microlithographic equipment have a requirement for the precise focusing of electron or ion beams, hereinafter referred to as particle beams. It is a common requirement that the particles should be focused in a region which is unaffected by the presence of a significant magnetic or electric field.

The level of electric or magnetic field which is considered significant is dependent upon the precise requirements of the particular application but is commonly very much smaller than the field required to produce the focusing action.

Lenses which have hitherto been used for focusing particle beams consist of rotationally symmetric pole pieces constructed of a magnetically permeable material (such as iron) for magnetic lenses, or rotationally symmetric electrodes, constructed of a conducting material (such as stainless steel) for electrostatic lenses.

Such magnetic and electrostatic lenses normally suffer from a number of lens defects. One such defect, known as spherical aberration, in which off-axis particle beams are focused closer to the lens than particle beams having trajectories close to the axis, is commonly important in determining the ultimate performance of apparatus in which such lenses are employed. it has been shown by 0. Scherzer in Z. Physik, Vol.

101 Pages 593 to 603 (1936) that this aberration can never be totally eliminated in any system of rotationally symmetric particle lenses.

The problem which the present invention seeks to solve is to provide a particle beam lens which maintains spherical aberration within acceptable limits and also focuses the beam in a region which is not subject to excessive electric or magnetic fields.

The present invention accordingly provides a particle beam lens comprising first and second pole pieces or electrodes which are rotationally symmetric about an axis, said pole pieces or electrodes being axially spaced and arranged to focus a particle beam passing through the lens from the first to the second pole piece or electrode at a focus beyond said second pole piece or electrode, characterised in that said second pole piece or electrode has an inner surface facing the axis which has a greater radius at the side adjacent said first pole piece or electrode than at the side adjacent said focus.

The use of a smaller radius adjacent the focus prevents the magnitude of the electric or magnetic field at the focus becoming excessive, while the larger radius adjacent the first electrode minimises spherical aberration. It will be appreciated that appropriate selections of the magnitudes of the inner and outer radii of the second pole piece or electrode can be chosen that either have an advantageous effect on both the magnitude of the electric or magnetic field at the focus and the degree of spherical aberration, or provide improvements in either the spherical aberration or the magnitude of the field at the focus without impairing the other property of the lens.

The invention also provides a particle beam lens comprising a plurality of pole pieces and/or electrodes which are rotationally symmetric about an axis of the lens, the inner surface facing said axis of at least one of said pole pieces and/or electrodes being provided with a surface portion of smaller radius at its side adjacent a region where the magnitude of the electrostatic and/or magnetic field is to be limited.

Some particle beam lenses of both prior art design and representing embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 represents a section through a rotationally symmetric magnetic particle beam lens of the prior art, showing only half the lens from the axis to the periphery, Figure 2 is a section similar to Figure 1, of a prior art electrostatic lens, and Figures 3 and 4 represent sections through the second electrode or pole piece of an electrostatic or magnetic lens as shown in Figures 1 and 2, modified in accordance with the present invention.

Figures 1 and 2 are simplified diagrammatic representations of prior art magnetic and electrostatic particle beam lenses respectively.

In the lens illustrated in Figure 1 the required magnetic field is produced by a coil 2, a halfsection of which is shown in the drawing, which surrounds a core 4, which is rotationally symmetric about an axis 6. The core 4 has two pole pieces 8 and 1 0. The first pole piece 8 has an inner cylindrical surface facing the axis of radius R1. The second pole piece 10 has an inner cylindrical surface 1 2 of a smaller radius R2. The first and second pole pieces 8 and 10 are axially separated by a distance S. The magnetic field produced by this arrangement of coil 2, core 4 and pole pieces 8 and 10 is such as to cause a charged particle passing through the lens to follow a trajectory as indicated by reference numeral 14 so that a beam of such particles is brought to a focus 16 on the axis 6.The required magnetic field may alternatively be produced by the use of permanent magnets to provide the field between the pole pices 8, 10.

The electrostatic lens illustrated in Figure 2 is of similar construction to the magnetic lens of Figure 1 and the same reference numerals are used for corresponding parts and features. The lens comprises first and second electrodes 1 8 and 20 which each have an annular form centred on the axis 6. Each electrode has an inner cylindrical surface facing the axis, which has a radius R1 for the first electrode 1 8 and a radius R2 for the second electrode 20. The electrodes 1 8 and 20 are axially spaced by distance S. The electrodes 18, 20 are connected to an appropriate power supply 22 in order to produce an electrostatic field inside the lens which causes a charged particle to follow a trajectory indicated by reference numeral 24 to a focus 16 on the axis 6.The electrodes 1 8, 20 may alternatively be connected to a resistance network energised by some other power supply, such as that used to accelerate the particle beam.

Conventionally, it has been considered that the construction of the lens can be largely described by the three dimensions R1, R2 and S. Of these three dimensions only R2 has a major effect in determining the axial extension of the focusing electrostatic or magnetic field towards the focus 1 6. The larger the value of R2 the greater the axial extension of the field. The requirement to limit the axial extension of the field thus sets an upper limit on the value which may be given to R2. However, it is known that increasing the value R2 usually reduces the spherical aberration of the lens.

It will be appreciated that, apart from the components of the magnetic and electrostatic lenses shown in Figures 1 and 2, other components are normally required, e.g. for providing mechanical support, insulation and the maintenance of a vacuum. These are not fundamental to the operation of the lens and have therefore not been illustrated for the sake of clarity. It will further be appreciated that the basic lens elements described are frequently used in combination, particularly in the case of electrostatic lenses, and that it is also possible to use magnetic and electrostatic lenses in combination to produce the required effect.

In accordance with a feature of the invention, in order to reduce the field at the focus of the lens and/or to reduce the spherical aberration of the lens, the inner cylindrical surface 1 2 of the second pole piece 10 or electrode 14 is modified as shown for example in Figures 3 and 4, so that the radius of the surface at the side of the pole piece 10 or electrode 20 closest to the first pole piece 8 or electrode 1 8 is greater than the radius R4 of the surface closer to the focus 1 6.

In the embodiment of Figure 3, the surface 12 has a stepped form and includes two cylindrical surface portions 26 and 28. The surface 26 having the larger radius R3 is closest to the first electrode or pole piece, whereas the surface 28 having the smaller radius R4 is closest to the focus 1 6.

If R3 is chosen to be greater than the R2 of the prior art lens and R4 to be less than R2, then it is found that there is some improvement in both the spherical aberration and the magnitude of the field at the focus. However, values of R3 and R4 may be chosen which improve the spherical aberration whilst maintaining the previous field at the focus.

To achieve this R4 may be chosen equal to the previous R2 or somewhat less, whilst R3 is chosen to be larger than R2. Likewide, if R4 is chosen to be less than R2 whilst R3 is substantially equal to R2, then the spherical aberration will be largely unaffected, whilst the magnitude of the field at the focus will be reduced.

In the embodiment illustrated in Figure 4, the inner surface 12 has closest to the focus, a cylindrical surface portion 30 of radius R4, which adjoins a frusto- conical surface portion 32, which widens to a final radius R3 at the edge of the electrode or pole piece closest to the first pole piece or electrode 8, 1 8.

It will be appreciated that multiple steps or tapers or combinations of steps or tapers, may be introduced into the inner surface 1 2 in order to provide a desired effect, provided that the radius of the surface adjacent the focus is smaller than the radius of the surface closer to the other electrode or pole piece.

In certain cases it is also required to limit the extent of the field on the side of the lens element away from the focus, in which case, the inner surface of the first pole piece 8 or electrode 1 8 may be suitably modified by the introduction of a surface portion which has a radius smaller than R1 on the side away from the other pole piece 10 or electrode 20.

The above described modifications to the inner surfaces of the pole pieces or electrodes may be applied to lenses of rotational symmetry having superimposed magnetic and electric fields, in which case the variable bore construction may be applied to any or all of the pole pieces and electrodes responsible for producing the superimposed fields.

Claims (6)

1. A particle beam lens comprising first and second pole pieces or electrodes which are rotationally symmetric about an axis, said pole pieces or electrodes being axially spaced and arranged to focus a particle beam passing through the lens from the first to the second pole piece or electrode at a focus beyond said second pole piece or electrode, said second pole piece of electrode having an inner surface facing the axis which has a greater radius at the side adjacent said first pole piece or electrode than at the side adjacent said focus.

2. A lens as claimed in claim 1, wherein the inner surface of said second pole piece or electrode includes a portion inclined relative to the lens axis.

3. A lens as claimed in claim 1, wherein the inner surface of said second pole piece or electrode includes at least one step.

4. A particle beam lens comprising a plurality of pole pieces and/or electrodes which are rotationally symmetric about an axis of the lens, the inner surface facing said axis of at least one of said pole pieces and/or electrodes being provided with a surface portion of smaller radius at its side adjacent a region where the magnitude of the electrostatic and/or magnetic field is to be limited.

5. A lens as claimed in claim 4, wherein the surface portion of smaller radius is defined in the pole piece or electrode closest to the focus of the lens, the smaller radius surface portion being on the side adjacent said focus.

6. A particle beam lens substantially as herein described with reference to Figure 3 or 4 of the accompanying drawings.