Radiant energy – With charged particle beam deflection or focussing – With detector
Reexamination Certificate
2001-01-19
2003-08-19
Anderson, Bruce (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
With detector
C250S491100, C250S492210
Reexamination Certificate
active
06608313
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to,
inter alia
, charged-particle-beam (CPB) optical systems as used in CPB microlithography. (Microlithography is projection-transfer of a pattern, defined by a reticle or mask, onto a sensitive substrate using an energy beam. Microlithography is a key technique used in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, and the like.) CPB optical systems typically include various CPB lenses, deflectors, and apertures. More specifically, the invention pertains to devices and methods for aligning an aperture-angle-limiting aperture with an optical axis of the CPB optical system.
BACKGROUND OF THE INVENTION
Charged-particle-beam (CPB) microlithography is a candidate new-generation microlithography technique offering prospects of better image resolution than currently obtainable with optical microlithography. A CPB microlithography apparatus includes a CPB optical system and a CPB source. The CPB source produces a suitable charged particle beam, such as an electron beam or ion beam, for use as a microlithographic energy beam. The CPB optical system typically includes CPB lenses, deflectors, and apertures. One type of aperture limits the angle at which charged particles are incident on the reticle or substrate, and hence is termed herein an “aperture-angle-limiting aperture.”
A conventional CPB optical system as used in a conventional CPB microlithography system is shown in FIG.
3
. The
FIG. 3
system is shown and described in the context of forming and using an electron beam as a representative charged particle beam, and in the context of employing a reticle defining a pattern that is projected onto a sensitive substrate.
The
FIG. 3
system includes a source
1
, an illumination-optical system IOS, and a projection-optical system POS. The source
1
produces an electron beam
3
that propagates in a downstream direction. The illumination-optical system IOS comprises components situated downstream of the source
1
and upstream of a reticle
9
. The projection-optical system POS comprises components situated downstream of the reticle
9
and upstream of a sensitive substrate (or “wafer”)
12
. By “sensitive” is meant that the upstream-facing surface
12
s
of the substrate
12
is coated with a suitable material (termed a “resist”) that responds in an image-imprinting way to exposure by the charged particle beam. Exposure of the resist with an image of a region (e.g., a “subfield”) of the reticle
9
causes “transfer” of an image of the respective pattern portion to the upstream-facing surface
12
s
. Extending through the illumination-optical system IOS and projection-optical system POS is an optical axis A.
The electron beam
3
emitted from a cathode of the source
1
forms a beam crossover
2
on the optical axis A. The beam
3
propagating downstream of the beam crossover
2
is an “illumination beam” that passes through a first illumination lens
4
. The first illumination lens
4
forms an image of the cathode on a beam-shaping aperture
5
(defining typically a rectangular opening
5
a
). The beam-shaping aperture
5
trims the transverse profile of the illumination beam, according to the profile of the opening
5
a
, as appropriate for illuminating the desired shape and size of individual subfields or other exposure units on the reticle. Meanwhile, the first illumination lens
4
forms an image of the beam crossover
2
on an aperture-angle-limiting aperture
7
. A maximal aperture angle of the beam
3
(as incident on the upstream-facing surface
12
s
located in the imaging plane) is imposed on the beam by the aperture-angle-limiting aperture
7
.
After establishing the desired transverse dimensions of individual exposed subfields and the desired range of the aperture angle, as described above, an image of the cathode is formed on the reticle
9
by a second illumination lens
8
. Portions of the illumination beam passing through a selected subfield on the reticle
9
constitute a “patterned beam” that forms an image of the illuminated subfield on the upstream-facing surface
12
s
of the substrate (“wafer”)
12
. Actual imaging is performed by a first projection lens
10
and a second projection lens
11
of the projection-optical system POS.
The reticle
9
defines the pattern to be exposed. In one type of conventional reticle
9
(termed a “stencil” reticle), openings are defined in a thin film or membrane (made of a silicon membrane or the like). The openings versus surrounding regions in the thin film define the pattern elements (i.e., the openings are transmissive to charged particles of the illumination beam and the membrane tends to block incident charged particles). In another type of conventional reticle
9
, termed a “scattering-membrane” reticle, pattern elements are defined by corresponding regions of a heavy-metal layer (that exhibits a high level of scattering of incident charged particles) situated on a CPB-transmissive membrane.
With a stencil reticle, as noted above, incident charged particles of the illumination beam not passing through an opening tend to be blocked (and absorbed) by the membrane portion of the reticle
9
. This absorption causes membrane heating, especially if the membrane is thick, which results in reticle instability. Consequently, the reticle membrane usually is made sufficiently thin to transmit (with scattering) at least some of the incident charged particles. Since incident charged particles are scattered widely by such a membrane (but not by the openings in the membrane), an aperture normally is situated downstream of the reticle
9
to absorb the scattered electrons and thus prevent them from propagating to the substrate. By absorbing these scattered charged particles, appropriate contrast is obtained of the image as formed on the substrate
12
.
In a conventional CPB microlithography apparatus, the center of the aperture-angle-limiting aperture
7
is located on the optical axis A. It is desirable that the propagation axis of the illumination beam be aligned with the optical axis A at the aperture-angle-limiting aperture
7
. Significant misalignment causes the distribution of beam angle on the substrate to be asymmetric, which causes substantial aberration of an image as projected onto the upstream-facing surface
12
s.
To avoid or minimize Coulomb effects, a recent innovation is to configure the aperture-angle-limiting aperture
7
as an annular aperture, which produces a “hollow” illumination beam. In this regard, reference is made to Japan
Kôkai
Patent Document Nos. 11-297610, filed Apr. 8, 1998, 2000-012454, filed Jun. 25, 1998, and 2000-100691, filed Sep. 21, 1998. With an annular aperture-angle-limiting aperture, misalignment of the propagation axis of the illumination beam, the optical axis A, and the center of the aperture-angle-limiting aperture
7
with each other causes marked asymmetry in the transverse distribution of beam current. Such asymmetry of beam-current density causes, in turn, a corresponding asymmetry of the Coulomb effect, making controlled reductions of the Coulomb effect especially difficult. These problems cause substantial problems with aberrations.
In
FIG. 3
, a deflector
6
normally is used to align the propagation axis of the illumination beam with the center of the aperture-angle-limiting aperture
7
. To such end, the deflection center of the deflector
6
normally is set to the position of the beam-shaping aperture
5
to prevent the image of the beam-shaping aperture
5
from shifting laterally as the deflector
6
is energized. By energizing the deflector
6
, the illumination beam is shifted laterally relative to the aperture-angle-limiting aperture
7
. While energizing the deflector
6
, the beam current incident to the aperture-angle-limiting aperture
7
is read using an ammeter
17
. The propagation axis of the illumination beam is regarded as aligned with the center of the aperture-angle-limiting aperture
7
whenever the measured current is at a minimum, indicating completion of
Anderson Bruce
Klarquist & Sparkman, LLP
Nikon Corporation
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