Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-09-22
2003-10-21
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492100
Reexamination Certificate
active
06635891
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projection-transfer of a pattern, as defined on a reticle or mask, to a substrate). Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as an energy beam. Yet more specifically, the invention pertains to charged-particle-beam (CPB) optical systems used in CPB microlithography apparatus, and to apertures used in such optical systems to form a hollow beam.
BACKGROUND OF THE INVENTION
The progressive reduction of the sizes of circuit elements in microelectronic devices has led to the development of microlithography apparatus that use an energy beam other than ultraviolet light, so as to achieve finer resolution than obtainable using optical microlithography (i.e., microlithography performed using light). One promising approach has centered on the use of a charged particle beam (e.g., electron beam) as a microlithographic energy beam. Because the rectilinearity (and hence the resolution) of an electron beam tends to be better than of a light beam, microlithography apparatus using an electron beam have the potential of accurately exposing a pattern having smaller pattern elements than is possible using optical microlithography.
Charged-particle-beam (CPB) microlithography apparatus at their current state of development tend to exhibit low throughput (number of wafers or substrates that can be processed microlithographically per unit time). One way in which to increase throughput is to increase the beam current of the charged particle beam. However, increasing the beam current causes an accompanying increase in the particle density within the beam, which tends increasingly to aggravate the “Coulomb effect.” The Coulomb effect arises from electrostatic (Coulombic) repulsion between particles of like charge in the beam. The Coulomb effect causes, inter alia, beam blur, which substantially degrades the achievable pattern-transfer resolution obtained with CPB microlithography.
U.S. Pat. No. 5,834,783 discloses an exemplary technology for reducing the Coulomb effect in electron-beam microlithography. Specifically, the electron beam is made hollow (i.e., is configured to have a ring-shaped or annular transverse sectional profile) by passing the beam through a hollow-beam aperture. A hollow beam greatly reduces Coulombic repulsion between the electrons in the beam and, as a result, reduces the Coulomb effect. A typical hollow-beam aperture comprises an electron-absorbing plate defining a substantially ring-shaped (annular) through-hole and a circular center portion. The annular through-hole typically has multiple struts extending radially across it that serve to support the center portion. The center portion desirably absorbs electrons incident on it as other electrons pass directly through the annular through-hole. The hollow-beam aperture normally is situated on an optical axis at a location in the electron-optical system where electrons emitted by an upstream source converge (this location is termed a “crossover”).
The diameter of the beam at the crossover of a CPB-microlithography apparatus typically is approximately 100 &mgr;m. Hence, the size of a hollow-beam aperture placed at the crossover must be very small. Specifically, the diameter of the circular center portion of the hollow-beam aperture should be about 60 &mgr;m, and the radial width of the annular through-hole should be about 20 &mgr;m. The hollow-beam aperture should be made of a material that effectively blocks (absorbs) charged particles (except for particles passing through the annular through-hole) and has a high melting point. Molybdenum is a particularly useful material for this purpose.
The conventional method for fabricating a hollow-beam aperture includes machining arc-shaped openings in a molybdenum sheet using an end mill or analogous cutting tool, as shown in FIG.
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. However, this method is incapable of cutting an annular through-hole having a very narrow radial width. A more suitable alternative method is electric-discharge machining (EDM), in which an electrode is situated very near the workpiece (molybdenum plate) where the aperture is to be formed. For example, the electrode is situated 20 &mgr;m from the workpiece. High-voltage pulses are applied between the electrode and the workpiece to form an electrical arc across the gap between the electrode and the workpiece. The narrowest aperture that can be formed using EDM is equal to the diameter of the electrode plus 40 &mgr;m (20-&mgr;m gap on each side of the electrode). In other words, it is impossible to cut a 20-&mgr;m wide annular opening using EDM. Therefore, no practical method currently exists for making a hollow-beam aperture having a desired width for placement at a beam crossover.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art summarized above, an object of the invention is to provide hollow-beam apertures, for use in CPB microlithography, having very small radial widths, such as a radial width of approximately 20 &mgr;m. Another object is to provide methods for manufacturing such hollow-beam apertures and to provide CPB microlithography apparatus comprising such hollow-beam apertures.
To such ends, and according to a first aspect of the invention, hollow-beam apertures are provided for incorporation and use in a charged-particle-beam (CPB) microlithography apparatus. A first embodiment of such an aperture comprises a first member, multiple second members, and a circular center member made of a CPB-absorbing material. The center member is supported relative to the first member by support bars extending radially from the first member to the center member. The second members are situated between the support bars and the first member, and are displaceable relative to the first member radially toward the center member. The second members each have a distal edge. The distal edges are configured to engage the support bars whenever the second members are displaced maximally toward the center member. The distal edges each define a cutout having a respective edge configured as an arc having a radius greater than the radius of the center member. Whenever the second members are displaced maximally toward the center member, an annular aperture is defined between the center member and the cutouts. The aperture is “substantially annular,” i.e., annular except for the support bars extending across the aperture to the center member.
A respective spring can be situated relative to the first member and extending to each of the second members. Each spring is configured to urge the respective second member toward the center member. The springs desirably are contiguous with the first and second members, thereby connecting the respective second members to the first member.
The first member desirably defines angled edges in a region between the second members. In such a configuration, each second member defines angled edges that conform to and contact corresponding angled edges of the first member in a manner, whenever the second members are displaced maximally toward the center member, serving to maintain concentricity of the cutouts relative to the center portion. In such a configuration, the angled edges of the second members effectively “fit” into respective spaces defined by the angled edges of the first member. As the second members are urged closer to the center body, the engaging angled edges of the first and second members cause the second members to self-align relative to the first member and the center member, thereby assuring concentricity of the cutouts in the distal edges with the center member.
In a second embodiment of a hollow-beam aperture according to the invention, a first member defines a cutout having a radial dimension. A cylindrical beam-absorbing member is disposed relative to the first member. The beam-absorbing member has an axis
Nakamura Junji
Nakano Katsushi
Kalivoda Christopher M.
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
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