Particle-optical component

1. Field of the Invention

The present invention relates to an objective lens arrangement for use in particle-optical systems. In addition, the invention relates to a particle-optical beam system as well as a particle-optical inspection system.

The invention may be applied to charged particles of any type, such as electrons, positrons, muons, ions (charged atoms or molecules) and others.

2. Brief Description of Related Art

The increasing demand for ever smaller and more complex microstructured devices and the continuing demand for an increase of a throughput in the manufacturing and inspection processes thereof have been an incentive for the development of particle-optical systems that use multiple charged particle beamlets in place of a single charged particle beam, thus significantly improving the throughput of such systems. The multiple charged particle beamlets may be provided by a single column using a multi-aperture array, for instance, or by multiple individual columns, or a combination of both, as will be described in more detail below. The use of multiple beamlets is associated with a whole range of new challenges to the design of particle-optical components, arrangements and systems, such as microscopes and lithography systems.

A conventional particle-optical system is known from U.S. Pat. No. 6,252,412 B1. The electron microscopy apparatus disclosed therein is used for inspecting an object, such as a semiconductor wafer. A plurality of primary electron beams is focused in parallel to each other on the object to form a plurality of primary electron spots thereon. Secondary electrons generated by the primary electrons and emanating from respective primary electron spots are detected. For each primary electron beam a separate electron beam column is provided. The plurality of separate electron beam columns is closely packed. A density of the primary electron beam spots formed on the object is limited by a remaining footstep size of the electron beam columns forming the electron microscopy apparatus. Thus, the number of primary electron beam spots, which may be formed simultaneously on the object, is also limited in practice, resulting in a limited throughput of the apparatus when inspecting semiconductor wafers of a high surface area at a high resolution.

From U.S. Pat. No. 5,892,224, US 2002/0148961 A1, US 2002/0142496 A1, US 2002/0130262 A1, US 2002/0109090 A1, US 2002/0033449 A1, US 2002/0028399 A1, electron microscopy apparatus are known which use a plurality of primary electron beamlets focused onto the surface of the object to be inspected. The beamlets are generated by a multi-aperture plate having a plurality of apertures formed therein, wherein an electron source generating a single electron beam is provided upstream of the multi-aperture plate for illuminating the apertures formed therein. Downstream of the multiple-aperture plate a plurality of electron beamlets is formed by those electrons of the electron beam that pass the apertures. The plurality of primary electron beamlets is focused on the object by an objective lens having an aperture, which is passed by all primary electron beamlets. An array of primary electron spots is then formed on the object. Secondary electrons emanating from each primary electron spot form a respective secondary electron beamlet, such that a plurality of secondary electron beamlets corresponding to the plurality of primary electron beam spots is generated. The plurality of secondary electron beamlets also pass the objective lens, and the apparatus provides a secondary electron beam path such that each of the secondary electron beamlets is supplied to a respective one of a plurality of detector pixels of a CCD electron detector. A Wien-filter is used for separating the secondary electron beam path from a beam path of the primary electron beamlets.

Since one common primary electron beam path comprising the plurality of primary electron beamlets and one common secondary electron beam path comprising the plurality of secondary electron beamlets is used, one single electron-optical column may be employed, and the density of primary electron beam spots formed on the object is not limited by a foot step size of the single electron-optical column.

The number of primary electron beam spots disclosed in the embodiments of the above-mentioned documents is in the order of some ten spots. Since the number of primary electron beam spots formed on the object at the same time limits the throughput, it is desirable to increase the number of primary electron beam spots in order to achieve a higher throughput. It has been found, however, that it is difficult to increase the number of primary electron beam spots formed at the same time, or to increase a primary electron beam spot density, employing the technology disclosed in those documents while maintaining a desired imaging resolution of the electron microscopy apparatus.

What has been described above with reference to electrons applies in a similar manner to other charged particles.

It is therefore an object of the present invention to provide an objective lens arrangement as well as a particle-optical system having improved particle-optical properties.

The present invention is applicable to particle-optical systems using multiple beamlets of charged particles; the present invention is, however, not limited in the application to systems using multiple beamlets, but is equally applicable to particle-optical systems using only one single beam of charged particles.

According to a first aspect, the present invention provides an objective lens arrangement having an object plane and an axis of symmetry and comprising first, second and third pole pieces which are substantially rotationally symmetric with respect to an axis of symmetry and which are disposed on a same side of the object plane. The first, second and third pole pieces extend towards the axis of symmetry such that radial inner ends of the first, second and third pole pieces each define a bore which is to be traversed by a beam path of one or more beams of charged particles. A radial inner end of the first pole piece is disposed at a distance from a radial inner end of the second pole piece to form a first gap between them, and a radial inner end of the third pole piece is disposed at a distance from the radial inner end of the second pole piece to form a second gap between them.

The axis of symmetry referred to above generally coincides with the optical axis of a particle-optical system the objective lens arrangement is comprised in, such that the two terms are used to the same effect herein. The objective lens arrangement may also be described as having a central axis which may or may not also be an axis of symmetry, which central axis generally coincides with the optical axis of a system the objective lens arrangement is comprised in and thus also be used synonymously to the term optical axis.

A first excitation coil is provided for generating a magnetic field in a region of the first gap, and a second excitation coil is provided for generating a magnetic field in a region of the second gap. A first power supply is provided for supplying an excitation current to the first excitation coil, and a second power supply is provided for supplying an excitation current to the second excitation coil. The first and second power supplies may be two portions of a same power supply. The first and second power supplies are configured to supply currents to the first and second excitation coils and thus generate excitation currents such that a magnetic flux generated by the first excitation coil in the second pole piece is oriented in the same or a different direction as a magnetic flux generated by the second excitation coil in the second pole piece.

Generally, the first excitation coil is disposed between the first and second pole pieces and the second excitation coil disposed between the second and third pole pieces.

Depending on a shape, configuration and position of the pole pieces, a position and configuration of the excitation coil and an excitation current, the magnetic field generated in a region of the gap may have different magnetic field strengths and different dimensions. For instance, the magnetic field may extend only over a region close to the gap or may extend as far as the object plane. Since magnetic lenses are usually employed in inspection optical systems to provide a focusing effect, a magnetic focusing field generally extends as far as the object plane in order to achieve a good focusing effect, avoid defocusing before the object plane and avoid particle-optical aberrations.

The objective lens arrangement according to the first aspect of the present invention allows adjusting the magnetic fields in the first and second gaps such that the magnetic field in the first gap provides a focusing effect on the one or more beams of charged particles traversing the focusing magnetic field whilst the magnetic field generated in the second gap is configured to compensate for the focusing magnetic field extending from the first gap to locations on or at least close to the object plane.

The first and second gaps may be disposed at an angle to one another, for instance. The angle formed between the first and second radial gaps may be in a range of from 10 to about 170 degrees, for instance, and may be in a range of from 45 to 135 degrees or from 60 to 120 degrees, by way of example. In other words, in this exemplary embodiment, the first gap is disposed at an angle to the axis of symmetry that is different from an angle formed between the second gap and the axis of symmetry. For purposes of determining an angle between gaps, a straight line connecting the radial inner ends of the respective pole pieces forming the gap may be employed to represent the gap.

In an exemplary embodiment, the first gap is oriented substantially in an axial direction, i.e. substantially parallel or at a relatively small angle to the axis of symmetry, and thus forms an axial gap. An axial gap does not necessarily imply that radial innermost ends of the pole pieces forming the gap need to have the same distance from the axis of symmetry, but also encompasses those embodiments wherein the innermost ends have different distances from the axis of symmetry and wherein the gap formed between points on radial inner ends of the pole pieces that are disposed closest to one another form an angle of less than 45°, for instance less than 30° or less than 15° to the axis of symmetry. The second gap may be oriented substantially in a radial direction with respect to the objective lens arrangement, i.e. orthogonal to the axis of symmetry and thus form a radial gap. Radial gaps also encompass those embodiments wherein the gap defined by a closest distance (straight line along the closest distance) between the inner radial ends of the pole pieces is disposed at an angle of from about 50° to 90° to the axis of symmetry, such as from about 80° to 90° to the axis of symmetry.

According to an exemplary embodiment, the focusing magnetic field (generated in the first gap) is compensated for by the magnetic field generated in the second gap to such an extent that a total magnetic field in a region on the object plane and about the optical axis is substantially zero, in other words, the compensating magnetic field substantially cancels the focusing magnetic field in a region on the object plane.