Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-07-21
2002-10-01
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S505100
Reexamination Certificate
active
06459090
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projection-transfer of a pattern, defined by a reticle or mask, onto a sensitive substrate such as a semiconductor wafer). Microlithography is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography using a charged particle beam (electron beam or ion beam) as an energy beam. Even more specifically, the invention pertains to methods for making reticles as used in charged-particle-beam (CPB) microlithography, to reticles made using such methods, and to CPB microlithography methods performed using such reticles.
BACKGROUND OF THE INVENTION
In recent years, as semiconductor integrated circuits have become increasingly miniaturized, the resolution limits of optical microlithography (i.e., projection-transfer of a pattern performed using ultraviolet light as an energy beam) have become increasingly apparent. As a result, considerable development effort currently is being expended to develop microlithography methods and apparatus that employ an alternative type of energy beam that offers prospects of better resolution than optical microlithography. For example, considerable effort has been directed to use of X-rays. However, a practical X-ray system has not yet been developed because of many technical problems with that technology. Another candidate microlithography technology utilizes a charged particle beam, such as an electron beam or ion beam, as an energy beam.
A current type of electron-beam pattern-transfer system is an electron-beam system that literally “draws” a pattern on a substrate using an electron beam. In such a system, no reticle is used. Rather, the pattern is drawn line-by-line. These systems can form intricate patterns having features sized at 0.1 &mgr;m or less because, inter alia, the electron beam itself can be focused down to a spot diameter of several nanometers. However, with such systems, the more intricate the pattern, the more focused the electron beam must be in order to draw the pattern satisfactorily. Also, drawing a pattern line-by-line requires large amounts of time; consequently, this technology has very little utility in the mass production of semiconductor wafers where “throughput” (number of wafers processed per unit time) is an important consideration.
In view of the shortcomings in electron-beam drawing systems and methods, charged-particle-beam (CPB) projection-microlithography systems have been proposed in which a reticle defining the desired pattern is irradiated with a charged particle beam. The portion of the beam passing through the irradiated region of the reticle is “reduced” (demagnified) as the image carried by the beam is projected onto a corresponding region of a wafer or other suitable substrate using a projection lens.
The reticle is generally of two types. One type is a scattering-membrane reticle
21
as shown in FIG.
15
(
a
), in which pattern features are defined by scattering bodies
24
formed on a membrane
22
that is relatively transmissive to the beam. A second type is a scattering-stencil reticle
31
as shown in FIG.
15
(
b
), in which pattern features are defined by beam-transmissive through-holes
34
in a particle-scattering membrane
32
. The membrane
32
normally is silicon with a thickness of approximately 2 &mgr;m.
Because, from a practical standpoint, an entire reticle pattern cannot be projected simultaneously onto a substrate using a charged particle beam, conventional CPB microlithography reticles are divided or segmented into multiple “subfields”
22
a
,
32
a
each defining a respective portion of the overall pattern. The subfields
22
a
,
32
a
are separated from one another on the membrane
22
,
32
by boundary regions
25
,
35
, in which no pattern elements are defined. In order to provide the membrane
22
,
32
with sufficient mechanical strength and rigidity, support struts
23
,
33
extend from the boundary regions
25
,
35
.
Each subfield
22
a
,
32
a
typically measures approximately 1-mm square. The subfields
22
a
,
32
a
are arrayed in columns and rows across the reticle
21
,
31
. For projection-exposure, the subfields
22
a
,
32
a
are illuminated in a step-wise or scanning manner by the charged particle beam (serving as an “illumination beam”). As the illumination beam passes through each subfield, the beam becomes “patterned” according to the configuration of pattern elements in the subfield. As depicted in FIG.
15
(
c
), the patterned beam propagates through a projection-optical system (not shown) to the sensitive substrate
27
. (By “sensitive” is meant that the substrate is coated on its upstream-facing surface with a material, termed a “resist,” that is imprintable with an image of the pattern as projected from the reticle.) The images of the subfields have respective locations on the substrate
27
in which the images are “stitched” together (i.e., situated contiguously) in the proper order to form the entire pattern on the substrate.
Conventionally, reticles of the types summarized above are manufactured using semiconductor-fabrication technology. Fabrication begins with a silicon reticle substrate (typically having a thickness of 1 mm or less). The reticle membrane, subfields, and support struts are fabricated from the reticle substrate. The reticle conventionally is attached circumferentially to a peripheral frame typically having a thickness of about 10 mm. The peripheral frame, normally also made of silicon, strengthens the reticle for routine handling and during use of the reticle in the CPB projection-microlithography apparatus.
A conventional scattering-stencil reticle mounted to a peripheral frame is shown in FIGS.
16
(
a
)-
16
(
b
). FIG.
16
(
a
) depicts a reticle assembly
39
comprising a stencil-reticle portion
41
that includes a pattern-defining region
45
and a peripheral region
44
. The pattern-defining region
45
includes multiple subfields
42
(each with a respective membrane portion) and support struts
43
. The membrane portions have a thickness of about 2 &mgr;m and define respective portions of the reticle pattern, as described above. If the stencil-reticle portion
41
has an outer diameter of about 8 inches, then the thickness of the peripheral region
44
is about 700 &mgr;m. The edge region
46
of the stencil-reticle portion
41
is attached to a peripheral frame
40
having a thickness of about 10 mm.
Unfortunately, with reticles made by conventional technology, attachment of the stencil-reticle portion
41
to a peripheral frame
40
generates a stress throughout the stencil-reticle portion
41
that tends to cause warping (deformation) of the pattern-defining region
45
. The warping extends to the subfields
42
and thus to the respective pattern portions defined by the subfields
42
. This warping is especially a problem if the stencil-reticle portion
41
is attached to the peripheral frame
40
after the pattern has been formed on the pattern-defining region
45
. The warping prevents attainment of sufficiently accurate pattern transfer.
Hence, there is a need for a reticle (for CPB microlithography) that is attached to a peripheral frame
40
but that exhibits substantially reduced warp in the pattern-defining region
45
, compared to conventional reticles.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide reticles in which pattern warp is substantially reduced or reducible.
To such end and according to a first aspect of the invention, reticles are provided, for charged-particle-beam (CPB) microlithography, that comprise a reticle portion. In an embodiment, the reticle portion comprises a pattern-defining region, an inner supporting part, and an outer supporting part. The pattern-defining region comprises multiple subfields separated from one another by support struts. Each subfield defines a respective portion of a pattern defined by the reticle. The inner supportin
Klarquist & Sparkman, LLP
Nguyen Kiet T.
Nikon Corporation
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