Etching a substrate: processes – Forming or treating electrical conductor article
Reexamination Certificate
2002-07-16
2004-10-12
Hassanzodet, P. (Department: 1763)
Etching a substrate: processes
Forming or treating electrical conductor article
C029S602100
Reexamination Certificate
active
06802986
ABSTRACT:
FIELD
This disclosure pertains to charged-particle-beam (CPB) optical systems and to systems, such as CPB microlithography systems, incorporating such optical systems. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, magnetic-recording heads, displays, and micromachines. More specifically, the disclosure pertains to deflectors used in CPB optical systems and to methods for manufacturing such deflectors. Compared to conventional deflectors, the deflectors disclosed herein exhibit reduced aberration, produce strong beam-deflecting fields at low excitation currents being applied to the deflectors, exhibit low fluctuation of their deflecting magnetic fields with changes in temperature, and are relatively little influenced by eddy currents.
BACKGROUND
As the limitations of optical microlithography have become more apparent, a large research and development effort in recent years has been directed to charged-particle-beam (CPB) microlithography as a primary candidate for “next generation” lithography technology. By using a charged particle beam (e.g., an electron beam), CPB microlithography offers prospects of improved pattern-transfer resolution, compared to optical microlithography, for reasons similar to the reasons for which electron microscopy produces better imaging resolution than optical microscopy. Thus, CPB microlithography offers the prospect of producing microelectronic devices (e.g., semiconductor integrated circuits) having smaller and more densely packed active-circuit elements than can be produced by conventional means. In CPB microlithography, exemplified by electron-beam projection microlithography, a pattern is defined on an “EB reticle,” from which the pattern is “transferred” with reduction (demagnification) to a “sensitive” substrate using a projection-optical system of a CPB optical system. “Sensitive” means that the surface of the substrate is coated with a substance, termed a “resist,” that is imprintable with an image of the pattern as carried to the surface by the beam.
A reticle suitable for use in CPB microlithography typically is fabricated from a silicon wafer having a diameter of, for example, about 200 mm. The exposure field of an electron-optical system is only about 250 &mgr;m wide, rendering most full-die exposures from a reticle currently impossible. Consequently, exposure of an entire pattern from the reticle to the substrate involves defining the pattern on a reticle that is “segmented” into a large number of portions (usually termed “subfields”) each defining a respective portion of the overall pattern. The subfields are exposed in respective exposure “shots” to the substrate, on which the subfield images are placed to form a contiguous pattern on the substrate. Thus, large dies can be exposed, including dies having dimensions of tens of millimeters square on the substrate. An exemplary technique in this regard is disclosed in U.S. Pat. No. 4,376,249, incorporated herein by reference.
With any of various types of CPB microlithography systems, an ability to deflect the charged particle beam laterally with respect to the optical axis is absolutely essential for having a functional system. Consequently, to such end, all CPB microlithography systems comprise multiple deflectors.
One type of deflector frequently employed in electron-beam optical systems is a “saddle” deflector. A saddle deflector is produced by winding an electrical coil around a square bobbin, then bending the coil part way in a saddle manner around the outer surface of a cylinder. Unfortunately, this method of forming a saddle coil yields inaccurately configured coil windings and poor precision from one deflector to the next. Manufacturing difficulties also are encountered while positioning deflector cores relative to the coil windings.
Another type of deflector frequently used in electron-beam optical systems is denoted a “vane-yoke” type of toroidal deflector, as shown in FIGS.
16
(
a
)-
16
(
b
). A coil
34
of such a deflector is made by cutting (e.g., wire-cutting) a copper sheet to form a planar coil. A separate coil
34
is positioned on and applied to each side of a rigid, planar, insulative substrate
33
(e.g., quartz). Thus, each substrate
33
is provided with a “clockwise” coil
34
and a “counter-clockwise” coil
34
. The clockwise coil
34
is applied to one side of the substrate
33
, and the counter-clockwise coil is applied to the opposite side of the substrate. The respective inner termini of the coils
34
are electrically connected together, and the respective outer termini are connected to a power supply. Each such planar assembly is a respective “vane.” The vanes
32
are radially positioned relative to each other about an optical axis
36
to form the deflector
31
.
In an electron beam, the constituent propagating electrons repel each other. Consequently, an image carried by and formed by the beam can exhibit distortion and/or blur, especially at higher beam currents. This phenomenon is commonly known as the “Coulomb effect.” If the beam current is reduced in an effort to decrease the Coulomb effect, then exposure time of an electron-beam microlithography system is lengthened, which can reduce the throughput of the system. “Throughput” is the number of workpieces (e.g., wafers) that can be processed (e.g., lithographically exposed) by the system per unit time.
Another way in which the Coulomb effect can be reduced is by decreasing the length of the column containing the electron-beam optical system. In a shorter column, the distance of beam propagation is correspondingly reduced, which reduces the time during which the electrons of the propagating beam are near each other sufficiently to repel each other. However, a shorter beam column usually results in the beam being deflected, by a given deflector, a shorter distance from the optical axis than experienced in a longer column. Hence, in a shorter column, achieving a desired lateral deflection of the beam requires that the deflector coil be energized with a higher electrical current than an otherwise similar deflector in a longer column. The elevated electrical current results in more heat being generated in the coil. Unless this heat is rapidly and efficiently dissipated from the deflector coil, the deflector itself is heated. Thus, the deflector exhibits a greater variation in temperature, which produces a correspondingly greater variation in performance.
Reducing the column length of a CPB optical system also requires that each deflector be made smaller than would be allowable in a longer column. As a result, the deflectors in a short column are very close to other components of the column, thereby concentrating heat in a smaller area around the deflector. Achieving sufficient cooling of the deflector for more accurate and precise operation is correspondingly more difficult. As a result, the deflectors tend to experience greater temperature fluctuation during operation, yielding correspondingly greater thermal expansion and contraction of the deflectors. As a deflector expands, the magnetic field generated by the deflector increases in magnitude, which increases the magnitude of beam deflection, at an applied current, imparted by the deflector. As the imaging position of the beam fluctuates with temperature changes of the deflector, the accuracy and precision with which the subfield images are stitched together on the substrate correspondingly fluctuates.
One way in which to increase the magnitude of the deflection field produced by a deflector energized with a relatively small electrical current is to configure the coil as being wound around a magnetic “core.” Because deflectors usually generate high-frequency magnetic fields, ferrite often is used for the core because of its high electrical resistance, which is important for reducing eddy currents in the deflector. This type of deflector is able to create a relatively strong deflecting field in response to a relatively low current applied to the coil, and is therefore utilized in m
Culbert Roberts
Hassanzodet P.
Klarquist & Sparkman, LLP
Nikon Corporation
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