Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-05-16
2002-08-06
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06428937
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projection-transfer) in which a pattern, defined on a mask or reticle, is projected (and thus “transferred”) to a suitable substrate using a charged particle beam such as an electron beam. This type of microlithography has especial utility in the fabrication of integrated circuits and displays. More particularly, the invention pertains to “reticle blanks” from which reticles for charged-particle-beam (CPB) microlithography can be made, reticles made from such reticle blanks, and to methods for making such reticle blanks and 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 inscribing 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.
3
(
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.
3
(
b
), in which pattern features are defined by beam-transmissive through-holes
34
in a membrane
32
that tends to scatter particles in the beam. The membrane
32
is normally 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 (items
25
in FIG.
3
(
a
)) that do not define any pattern features. In order to provide the membrane
22
,
32
with sufficient mechanical strength and rigidity, support struts
23
,
33
extend from the boundary regions
25
.
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 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.
3
(
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.
Certain steps of a conventional method for fabricating a reticle are depicted in FIGS.
4
(
a
)-
4
(
d
), respectively. In a first step, an “SOI” (Silicon On Insulator) substrate
40
is made from a silicon support substrate
41
, a silicon oxide layer
42
, and an SOI layer
43
, using conventional fabrication techniques (FIG.
4
(
a
)). The silicon oxide layer
42
is formed on a first major surface
50
of the silicon support substrate
41
, and the SOI layer
43
is formed on the silicon oxide layer
42
. In a second step, a dry-etching mask
46
, made from a thick film of silicon oxide or resist, is formed on or applied to, respectively, the second major surface
51
of the silicon support substrate
41
(FIG.
4
(
b
)). The dry-etching mask
46
defines openings
45
in which corresponding regions of the second major surface
51
are exposed. The silicon support substrate
41
exposed in the openings
45
is dry-etched (FIG.
4
(
c
)).
The silicon support substrate
41
is dry-etched depthwise to the silicon oxide layer
42
that acts as an etch-stop barrier. In the resulting structure, the silicon oxide layer
42
and the SOI layer
43
are supported by a silicon peripheral frame
41
b
and silicon struts
41
a
contiguous with the peripheral frame
41
b
. Each strut
41
a
is approximately several hundreds of &mgr;m wide. The struts
41
a
are spaced apart from one another so as to leave open areas between the peripheral frame
41
b
and the struts
41
a
, and between individual struts
41
a.
As evident in FIG.
4
(
c
), the silicon oxide layer
42
is exposed in the openings between struts. The exposed silicon oxide
42
is removed using a mixture of HF+NH
3
F, leaving the SOI layer
43
to become a reticle membrane
43
a
(FIG.
4
(d)), thereby completing manufacture of a reticle “blank.”
In the foregoing method, dry-etching must be performed to a depth equal to the thickness of the silicon support substrate
41
of the SOI substrate
40
. For example, in the case of an SOI substrate
40
having a diameter of 3 inches, dry-etching is performed to a depth of at least 300 &mgr;m. In the case of an SOI substrate
40
having a diameter of 8 inches, dry-etching is performed to a depth of at least 700 &mgr;m.
Conventionally, if dry-etching is to be performed to a depth of at least 700 &mgr;m, then a silicon oxide layer used as a dry-etching-mask desirably is at least approximately 10 &mgr;m thick. (This is because the etching selectivity for silicon relative to silicon oxide is about 100:1. Hence, 700 &mgr;m/100=7 &mgr;m, which is approximately 10 &mgr;m.) A resist layer used as a dry-etching mask conventionally is 50 &mgr;m to 60 &mgr;m thick. (The etching selectivity for silicon relative to resist is about 20:1. Hence, 700 &mgr;m/20=35 &mgr;m, which is approximately equal to 50 &mgr;m.)
The dry-etching technique summarized above is performed according to the well-known sidewall-protecting plasma dry-etching technique. Sidewall-protecting plasma dry-etching protects the sidewalls of the cavities
Klarquist & Sparkman, LLP
Nikon Corporation
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