Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
1998-12-21
2001-06-12
Anderson, Bruce C. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML
Reexamination Certificate
active
06246058
ABSTRACT:
The invention relates to a particle-optical apparatus which includes:
a particle source for producing a beam of electrically charged particles which travel along an optical axis of the apparatus in order to irradiate an object to be irradiated in the apparatus by means of the particle beam,
a focusing lens for focusing the beam of electrically charged particles,
and a correction device for correcting lens aberrations of the focusing lens,
which correction device comprises pole faces for producing a uniform electrical field and a uniform magnetic field which extends perpendicularly thereto, both dipole fields also extending perpendicularly to the optical axis of the apparatus,
which correction device also comprises pole faces for producing an electrical quadrupole field, which pole faces extend substantially parallel to the optical axis of the apparatus.
The invention also relates to an assembly which consists of a focusing lens for focusing a beam of electrically charged particles and of correction means for correcting lens aberration of the focusing lens and is intended for use in such an apparatus.
A correction device of this kind, intended for use in such an apparatus, is known from European patent No. 0 373 399.
Generally speaking, particle-optical apparatus, such as electron microscopes or apparatus for electron lithography, are arranged to irradiate an object to be studied or worked by means of a beam of electrically charged particles (usually an electron beam) which is produced by means of a particle source such as a thermal electron source or an electron source of the field emission type. Irradiation of the object may be aimed at imaging such objects to be studied in such apparatus (specimens in electron microscopes) or at forming very small structures on the object, for example for microelectronics (electron lithography apparatus). In both cases focusing lenses are required to focus the electron beam.
The electron beam can in principle be focused in two ways. According to the first method, a specimen to be examined is more or less uniformly exposed to the electron beam and an enlarged image of the specimen is formed by means of the focusing lens. The focusing lens is in that case the objective lens of an imaging lens system; the resolution of the objective lens then governs the resolution of the apparatus. Apparatus of this kind are known as Transmission Electron Microscopes (TEM). According to a second method of focusing, the emissive surface of the electron source, or a part thereof, is imaged, be it usually strongly reduced, on the specimen to be examined (in the Scanning Electron Microscope or SEM) or on an object on which the desired microstructure is to be formed (in lithography apparatus). The image of the electron source (the “spot” which is displaced across the object by means of, for example deflection coils) is again formed by means of an imaging lens system. In the latter case the focusing lens is the objective lens of the spot-forming lens system; the resolution of this objective lens governs the spot size of the beam, and hence the resolution of the apparatus.
The lenses used in all such apparatus are usually magnetic lenses, but may also be electrostatic lenses. Both types of lens are practically always rotationally symmetrical lenses. The behavior of such lenses inevitably is not ideal, i.e. they exhibit lens aberrations, among which the so-called spherical aberration and the chromatic aberration are usually decisive in respect of resolution of the lens; these lens aberrations thus determine the limit of the resolution of the known electron optical apparatus. According to a fundamental theorem of particle optics, such lens aberrations cannot be eliminated by compensation utilizing rotationally symmetrical electrical or magnetic fields.
In contemporary electron optical apparatus, notably in scanning particle-optical apparatus comprising a spot-forming objective lens (the so-called Scanning Electron Microscope or SEM), there is a tendency to select the acceleration voltage of the electron beam so as to have a value which is lower than was customary thus far, i.e. of the order of magnitude of from 0.5 kV to 5 kV instead of the previously customary voltage of the order of magnitude of 30 kV or more. The reason for doing so is that at such comparatively low acceleration voltages the charging of non-conductive specimens (such as photoresist material in the case of manufacture of electronic integrated circuits) is substantially reduced; moreover, at these low voltages the so-called topographic contrast can be substantially enhanced. At such low acceleration voltages the chromatic aberration is the major lens aberration, and hence the decisive factor in respect of resolution of the particle-optical apparatus. (This can be readily understood by considering the fact that chromatic aberration is proportional to &Dgr;U/U, in which the &Dgr;U is the non-variable energy spread in the electron beam and U is the nominal acceleration voltage; this factor, therefore, increases as U is decreased.)
In order to enhance the resolution of the particle-optical apparatus nevertheless, the cited European patent No. 373 399 proposes to reduce said lens aberrations by means of a correction device having a non-rotationally symmetrical structure. This structure is formed by a Wien-type corrector, that is to say a structure in which a uniform electric field and a uniform magnetic field which extends perpendicularly thereto are both oriented perpendicularly to the optical axis of the apparatus. For the correction of spherical aberration as well as chromatic aberration, this corrector is provided with a number of multipoles, i.e. an electrical and a magnetic quadrupole, an electrical and a magnetic hexapole, and an electrical and/or a magnetic octupole. (Thus, in this known correction device it may occur that only the electrical field or only the magnetic field of the octupole fields is present.)
An embodiment of the correction device according to the cited European patent (described with reference to FIG.
5
and denoted therein by the reference numeral
20
) enables correction of the chromatic aberration. This embodiment consists of a multipole unit which is formed by a number of electrical and magnetic poles whose pole faces are axially oriented, i.e. they extend parallel to the optical axis of the apparatus. Each of said poles can be separately excited; by suitably choosing the individual excitations, therefore, a multipole unit thus constructed can form, as desired, a uniform electrical field extending perpendicularly to the optical axis and a uniform magnetic field which extends perpendicularly thereto, both fields extending perpendicularly to the optical axis; thereon superposed electrical and magnetic quadrupole fields, hexapole fields and an electrical and/or a magnetic octupole field can be formed.
In such a comparatively complex correction device it is extremely difficult to find the correct electrical and magnetic adjustment for the (very accurate) generation of said multipole fields. This difficulty becomes more serious as the number of multipole fields to be generated is greater, because each of these fields must have and retain exactly the adjusted correct value. Therefore, it is of essential importance to minimize the number of multipole fields required.
It is an object of the invention to provide a particle-optical apparatus of the kind set forth in which correction for the chromatic aberration of the focusing lens takes place and in which the requirements in respect of electrical and magnetic adjustment and reproducibility can be more readily satisfied.
To this end, the particle-optical apparatus according to the invention is characterized in that the strengths of the electrical and the magnetic fields in the correction device are such that in the correction device the electrically charged particles travel along a sinusoidal trajectory which consists of i full sine periods (i=0, 1, 2, . . . ) plus a sine line whose length does not deviate more than 10% from a
Anderson Bruce C.
Fulbright & Jaworski LLP
Philips Electron Optics B.V.
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