Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-04-19
2003-03-18
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C428S014000, C219S765000, C219S603000
Reexamination Certificate
active
06534222
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography in which a pattern, defined on a mask or reticle, is 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 semiconductor integrated circuits and displays. More particularly, the invention pertains to reticles for charged-particle-beam microlithography and to methods for making such reticles.
BACKGROUND OF THE INVENTION
In recent years, as semiconductor integrated circuits increasingly have become miniaturized, the resolution limitations of optical microlithography (i.e., microlithography performed using ultraviolet light as an energy beam) increasingly have become 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 drawing system that literally “draws” a pattern on a substrate using an electron beam. In such a system, no reticle is used. 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 few nanometers. However, with such systems, the more intricate the pattern, the more focused the electron beam must be in order to render 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) and 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.
11
(
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.
11
(
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, 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.
11
(
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.
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.
11
(
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.
Referring now to
FIG. 12
, a conventional scattering-stencil reticle
1
is shown. The reticle
1
comprises a reticle membrane
13
b
, a silicon oxide layer
12
a
, and supporting struts
11
a
made of silicon. In order to increase the strength and rigidity of the reticle
1
, it has been proposed to bond the reticle
1
to a support frame
4
. To such end, the reticle
1
comprises a peripheral frame
11
b
that is essentially a wide strut extending circumferentially around the reticle. In
FIG. 12
, the underside of the peripheral frame
11
b
is bonded circumferentially to the upper surface of the support frame
4
. The support frame
4
also facilitates easier handling of the reticle, especially while the reticle is being transported to and from a reticle stage in the CPB microlithography system.
The support frame
4
conventionally is made of glass such as borosilicate glass (containing mobile ions). The peripheral frame
11
b
, on the other hand, typically is made of silicon. One conventional manner of bonding the support frame
4
to the peripheral frame
11
b
is by anodic welding. The coefficient of thermal expansion of the glass in the support frame
4
is approximately 3 ppm/° C. within the temperature range of room temperature to 500° C., while the coefficient of thermal expansion of the silicon in the peripheral frame
11
b
is approximately 2 ppm/° C. within the same temperature range. Anodic welding of silicon to glass normally is performed by applying a voltage of 500 Vdc to 1000 Vdc while heating to 300° C. to 450° C. Unfortunately, under such conditions, distortion of the weld joint usually occurs, due to the stated differences in coefficients of thermal expansion, as the welds return to room temperature after completion of welding. This distortion also frequently extends to the arrangement of pattern features on the reticle membrane. Membrane distortion due to welding under such conditions can be on the order of 1 &mgr;m, which exceeds tolerance limits for CPB microlithography (typically 20 nm or less).
Another conventional manner of bonding the support frame to the peripheral frame of a reticle is by eutectic bonding, which is preferred from the viewpoint of preventing charging of more electrically conductive materials of the reticle. An example of such a reticle assembly
40
is shown in FIGS.
13
(
a
)-
13
(
b
). The reticle assembly
40
comprises a reticle portion
41
bonded to a support frame
42
. As shown in FIG.
13
(
b
), the reticle portion
41
comprises a reticle membrane
46
, supporting struts
47
, and peripheral frame
48
. Between the peripheral frame
48
and the support frame
42
is a layer
43
of silicon oxide. To perform eutectic bonding of the support frame
42
to the reticle portion
41
, both the peripheral frame
48
and the support frame
42
are coated with a thin film of aluminum. The aluminum is used to create a bond in regions of contact of the support frame
42
with the peripheral frame
48
. The eutectic point of aluminum-silicon is approximately 577° C., which requires a high eutectic-bonding temperature (approximately 650° C. to 700° C.). Having to attain such temperatures for bonding purposes is problematic because of the time required to reach bonding temperature.
Also, this technique requires robust and costly equipment. An additi
Huff Mark F.
Klarquist & Sparkman, LLP
Mohamedulla Saleha R.
Nikon Corporation
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