Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2001-09-10
2003-08-12
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S296000, C430S942000, C250S492200, C250S492300
Reexamination Certificate
active
06605398
ABSTRACT:
FIELD
This disclosure pertains to, inter alia, microlithography (lithographic transfer-exposure of extremely fine patterns using an energy beam). Microlithography is a key technology used in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, thin-film magnetic pickup heads, micromachines, and the like. More specifically, this disclosure pertains to microlithography as performed using a charged particle beam (e.g., an electron beam or ion beam) under conditions in which beam blur is minimized.
BACKGROUND
Certain aspects of an exemplary conventional charged-particle-beam (CPB) microlithography apparatus
10
are depicted in FIG.
4
. The depicted apparatus
10
utilizes an electron beam as the charged particle beam. The electron beam is produced by an electron-beam source
11
(i.e., “electron gun”). The electron beam from the source
11
propagates in a downstream direction (vertically downward in the figure) through an illumination-lens assembly
12
, a beam-shaping aperture
13
, and an aperture stop
14
to a reticle
15
. The reticle
15
defines a pattern to be projection-transferred to a substrate
18
(e.g., semiconductor wafer having an upstream-facing surface coated with a suitable resist). The electron beam propagating from the source
11
to the reticle
15
is termed an “illumination beam” IB and the electron-optical components located between the source
11
and the reticle
15
collectively constitute an “illumination-optical system” IOS that extends along an optical axis Ax. From the reticle
15
, the electron beam passes through a projection-lens assembly
16
and an aperture stop
17
to the substrate
18
. The electron beam propagating from the reticle
15
to the substrate
18
is termed a “patterned beam” or “imaging beam” PB, and the electron-optical components situated between the reticle
15
and substrate
18
collectively constitute a “projection-optical system” POS that extends along the optical axis Ax. The illumination-optical system IOS and projection-optical system POS collectively are termed the “CPB-optical system.”
The illumination beam IB is manipulated by the illumination-optical system IOS so as to illuminate a selected region (e.g. a selected “subfield”) on the reticle
15
in a uniform manner. An image of the illuminated region of the reticle
15
is formed on the substrate
18
by the projection-optical system POS. So as to be imprinted with the image, the upstream-facing surface of the substrate
18
is coated with a suitable resist. Such a substrate is termed “sensitive” to the patterned beam PB. The aperture stops
14
,
17
trim the illumination beam IB and patterned beam PB, respectively, so as to limit the angular aperture of the respective beam. Situated at a location that is optically conjugate to the reticle
15
is the beam-shaping aperture
13
, which limits the size and shape of the region on the reticle
15
that is illuminated by the illumination beam IB.
A well-known phenomenon associated with electron-beam microlithography is the “Coulomb Effect” in which repulsion between individual electrons in the beam results in a downstream shift of the focal point. This shift causes blur and distortion of the image as formed on the substrate
18
. In conventional electron-beam microlithography systems that operate according to the well-known variable-shaped beam and cell-projection-exposure schemes, it is desirable that the respective upper limits for beam-spread half-angle and exposure beam current be set so as to minimize a “total” blur (i.e., blur resulting from both the Coulomb Effect and geometrical aberrations of the CPB-optical system).
As used herein, the term “blur” generally refers to the maximum blur evident in an exposed region, on the sensitive substrate, corresponding to a single illuminated region (generally termed a “subfield”) on the reticle. As a result of blur, the edge of a pattern element as exposed on the resist does not exhibit an abrupt change from 100% relative beam-current density to 0% relative beam-current density. Rather, the fall-off in relative beam-current density at the edge exhibits a sloped distribution profile. As a result of blur, the respective distributions of relative beam-current density associated with, for example, opposing edges of adjacent lines of the pattern can extend across the intervening space sufficiently to interfere with the proper exposure of the lines, possibly causing a bridging exposure between the lines and/or an undesired change in profile of the lines as exposed on the substrate.
In this regard, two types of beam half-angle are of interest: the beam-spread half-angle (as noted above) and the beam-cutoff half-angle. The “beam-spread half-angle” is a half-angle (also termed “distribution half-angle”) corresponding to half the width of the distribution of beam-current density within this range of ±50% relative beam-current density (i.e., width between the apex of the distribution of beam-current density and the + or −50% value of relative beam-current density, as shown in FIG.
5
). The “beam-cutoff half-angle” is a half-angle (also termed “cutoff half-angle”) corresponding to half the width of the distribution of beam-current density within the range bounded by the + and − cutoff points on the distribution curve at which respective tails of the distribution are trimmed off by an aperture stop
14
or
17
.
Using the foregoing concepts, a conventional determination of blur is performed based on edge profiles of a projected test pattern element. The test element normally is rectangular and is substantially larger than the estimated beam blur. Measurements are obtained of the beam-current density across the projected pattern element, including the edges of the projected element. Two threshold values (lower and upper) are set within the determined distribution of beam-current density across the element, and blur is defined as the distance between the two thresholds. The threshold values normally are set at 12% and 88%, or 10% and 90%, of the maximum value (apex) of the distribution curve. The following description will refer to the example of FIGS.
6
(
a
)-
6
(
c
), in which the lower and upper thresholds are set at 12% and 88%, respectively.
FIG.
6
(
a
) is a contour drawing of the beam-current distribution for an exemplary, projected rectangular pattern element. The distribution of beam current exhibits a high plateau (nominally 100% relative intensity) in the center of the projected element, with a rapid decline (cutoff) at the edges of the projected element. FIGS.
6
(
b
) and
6
(
c
) show beam cutoff (edge profiles) of sections A-A′ and B-B′, respectively, of the projected element. In this example, blur is defined as the distance between the 12% and 88% thresholds of relative intensity in each edge profile, as indicated in FIGS.
6
(
b
) and
6
(
c
). Selecting an optimum beam-spread half-angle for minimizing this blur is determined by actual measurement or simulation.
A conventional method for determining beam-spread half-angle is described with reference to FIG.
7
. To simplify the description, it is assumed that the cutoff half-angle and distribution half-angle (
FIG. 5
) are equal to each other. In
FIG. 7
, curve (a) represents beam blur due to geometrical aberration of the electron-optical system at maximum lateral deflection of the beam; curve (b) represents beam blur due to the Coulomb Effect for a given exposure beam current at maximum deflection; and curve (c) represents total blur, which is a composite of curve (a) and curve (b).
Blur due to geometrical aberration (curve (a)) increases with increases in the beam-spread half-angle. Blur due to the Coulomb Effect (curve (b)), on the other hand, decreases with increases in the beam-spread half-angle. Hence, there is a beam-spread half-angle at which total blur (curve (c)) is at a minimum.
In
FIG. 7
, the horizontal line A denotes a specified blur tolerance (maximum allowable blur). If the total blur (curve (c)) has a minimum ex
Klarquist & Sparkman, LLP
Nikon Corporation
Young Christopher G.
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