Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
2002-10-11
2003-12-30
Dinkins, Anthony (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
C361S816000
Reexamination Certificate
active
06670547
ABSTRACT:
FIELD
This disclosure pertains to magnetic shielding of an enclosure such as a room containing equipment that is sensitive to the effects of external magnetic fields, and to enclosures shielded in such a manner. Exemplary field-sensitive systems that can be contained in such a magnetically shielded enclosure include systems for performing microlithography using a charged particle beam.
BACKGROUND
The growth of modern processing and analytical technology has included more extensive use of techniques that employ a charged particle beam (e.g., electron beam or ion beam). Accompanying more extensive use of these techniques generally has been a demand for progressively more accurate and precise performance from the systems that perform the techniques. For example, increasingly greater image resolution is being demanded from electron microscopy. Also, increasingly greater pattern-transfer resolution and accuracy is being demanded from charged-particle-beam (CPB) microlithography, which is a key “next-generation lithography” technology being actively developed for fabricating microelectronic devices.
Obtaining greater pattern-transfer accuracy from a CPB microlithography system requires application of more stringent measures to prevent the charged particle beam from being influenced uncontrollably by stray external and internal magnetic and electrical fields. External stray magnetic fields include magnetic fields produced by the earth, events in outer space, and by nearby man-made equipment such as power equipment, power cables, and elevators, for example. Similarly, external stray electrical fields can be produced by any of various sources, both natural and man-made. “Internal” stray fields usually are produced by components of the CPB microlithography system located, for example, inside the “lens column” (vacuum chamber that houses the CPB optical system) and/or inside the “substrate chamber” (vacuum chamber that houses the substrate stage and peripheral components). Even if the magnitude or fluctuation amplitude of a stray field is very small, the field nevertheless can cause an undesired change in the trajectory and/or position of the charged particle beam sufficient to destroy any prospect of achieving a desired accuracy and precision of pattern transfer. For example, if the charged particle beam is being used to transfer a pattern having linewidths of, e.g., 70 nm, the importance of reducing the effect of a stray magnetic and/or electrical field, even an extremely small-magnitude field, on the beam is readily appreciated.
As noted above, potentially troublesome fields can be electrical or magnetic, static or dynamic (fluctuating), strong or weak, man-made or natural, internal or external. An example of an internal field is a field generated by a component of the system, such as a stray magnetic field produced by an electron lens or deflector or by a reticle stage or substrate stage. An example of an external field is a field produced by the earth or by nearby industrial activity.
For shielding purposes, conventional CPB microlithography systems usually include one or more magnetic shields located inside the lens column and inside the substrate chamber. For example, shielding may be associated with certain peripheral components located in or near these chambers, such as wafer loaders, reticle loaders, electromagnetic lenses, stage motors, vacuum pumps, etc. Another conventional manner of shielding CPB microlithography lens columns and substrate chambers is the application to the chambers of a single, double, or triple coating of a material having a high initial magnetic permeability such as Permalloy. Alternatively or in addition, the lens columns and substrate chambers themselves are made of a material having high initial magnetic permeability, such as Permalloy.
Reference is made to
FIG. 6
that schematically depicts exemplary conventional magnetic shielding used in association with a CPB microlithography system. The subject system
100
comprises an electron gun
1
that generates an electron beam that propagates in a downstream direction (downward in the figure). A substrate stage
24
includes a “wafer chuck” on which the lithographic substrate is mounted for exposure by the electron beam. The electron gun
1
and an electron-optical system (not detailed) extending along the trajectory of the electron beam are contained in a “lens column”
31
, and the substrate stage
24
is contained in a substrate chamber
33
. The lens column
31
, typically made of invar or soft iron, is connected via a duct
37
to a vacuum pump (not shown). The substrate chamber
33
typically is made of aluminum or non-magnetic stainless steel. The lens column
31
and substrate chamber
33
are conjoined and thus communicate with each other, allowing their respective internal spaces to be shared. Although not detailed, the microlithography system
100
includes one or more condenser lenses that direct the electron beam onto a reticle, one or more beam-trimming apertures, a reticle stage, a projection-lens assembly that demagnifies and projects the electron beam (propagating downstream of the reticle) onto a lithographic substrate, and one or more beam deflectors for beam positioning and aberration correction.
The microlithography system
100
is enclosed within a shielded enclosure
21
. The enclosure
21
is effectively a chamber made of a material having high initial magnetic permeability. The enclosure
21
houses the entire lens column
31
and substrate chamber
33
of the system
100
.
The enclosure
21
defines various openings. For example, a vacuum duct
37
extends through the wall of the enclosure
21
to allow evacuation of the lens column
31
and substrate chamber
33
. Other openings
22
in the enclosure
21
correspond with respective feed-through apertures
39
in the lens column
31
to allow passage of wires and the like to and from the lens column
31
. Another opening (not shown) corresponds with a respective opening in the lens column
31
through which the reticle is moved to and from the reticle stage. Although not detailed in
FIG. 6
, yet another opening in the enclosure
21
allows passage of the lithographic substrate through a respective opening in the substrate chamber
33
through which the substrate is moved to and from the substrate stage
24
. In addition, a gap
40
usually is associated with the location of a connecting member coupling the lens column
31
to the substrate chamber
33
. Since the enclosure
21
typically is not unitary, other gaps in the enclosure
21
typically are provided at respective conjunctions of shield segments.
Openings and gaps in the magnetically permeable material of the enclosure
21
usually reduce the magnetic-shielding performance of the enclosure, sometimes to a level at which the shielding effect is inadequate. Consequently, an aperture or gap is provided in the enclosure usually only when necessary. To offset the consequences of providing apertures and gaps in the enclosure
21
, it frequently is necessary to shield the walls of the room containing the enclosure
21
(with the CPB microlithography system
100
or other field-sensitive system inside). The shielded room can be “passively” shielded, wherein the room walls simply are covered with a magnetic-shielding material. Alternatively or in addition, the room can be “actively” shielded, wherein the room walls include respective coils that generate respective magnetic fields extending usually in a selected direction normal to the plane of the wall. The coil-containing walls typically are separated by a distance from the system enclosed in the room (e.g., separated from the lens column and substrate chamber). By appropriate energization of one or more of the coils, a portion of an external magnetic field leaking into the room is cancelled by a countervailing magnetic field generated from by the coil(s). This technique is termed “active cancellation,” and the coils are termed “active cancellers.”
A conventional shielded room
81
including active cancellers is shown in
FIG.
Dinkins Anthony
Klarquist & Sparkman, LLP
Oliva Carmelo
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