Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-04-03
2002-06-11
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S296000
Reexamination Certificate
active
06403268
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to microlithography performed using a charged particle beam such as an electron beam. More specifically, the invention pertains to reticles as used for such microlithography.
BACKGROUND OR THE INVENTION
In recent years the progressive miniaturization of semiconductor integrated circuit elements has led to current efforts to develop a practical projection-exposure system (microlithography system) that utilizes either an X-ray or a charged particle beam (e.g., electron beam or ion beam) as an energy beam. The reason is because optical microlithography (i.e., microlithography using light, especially ultraviolet light) has resolution limits that make it extremely difficult to impossible to resolve circuit elements as small as currently desired. Electron-beam microlithography offers prospects of achieving the currently desired pattern-element resolution (0.1 &mgr;m or less) because an electron beam can be focused to a diameter of a few nanometers.
Conventional electron-beam exposure systems “write” a pattern onto a wafer or other substrate one line at a time. Hence, the finer the pattern the more focused the electron beam must be. Also, the finer the pattern the longer the time required to draw the pattern. In fact, the time required to draw a pattern line-by-line is so long that the electron-beam-drawing technique cannot be used to expose wafers for mass production.
In view of the low “thoughput” (number of wafers processed per unit time) and hence low cost-efficiency of electron-beam drawing technology, considerable effort currently is being expended to develop a practical electron-beam projection-microlithography system in which an image of a pattern, defined by a reticle, is projected (rather than written) from a pattern-defining reticle onto the wafer. The projected image typically is “reduced” or “demagnified”, by which is meant that the image is smaller (usually by an integer factor) than the corresponding pattern on the reticle. The image is projected onto the wafer using a projection lens.
To perform projection microlithography of a circuit pattern, a transfer mask (“reticle”) is required upon which the circuit pattern is formed (i.e., the reticle “defines” the pattern). A first representative conventional reticle is a scattering-membrane reticle
31
as shown in FIG.
3
(
a
). In the scattering-membrane reticle
31
, the pattern is defined by a corresponding arrangement of “scattering bodies”
34
formed on a membrane
32
. The scattering bodies
34
are respective portions of a layer of a material (e.g., heavy metal) that scatters incident electrons. The membrane
34
is relatively transmissive to the electron beam irradiating the upstream-facing surface of the reticle, whereas the scattering bodies
34
tend to scatter electrons incident on the reticle. A second representative conventional reticle is a scattering-stencil reticle
41
as shown in FIG.
3
(
b
). In the scattering-stencil reticle
41
, the pattern is defined by a corresponding arrangement of through-holes (“voids”)
44
defined in a membrane
42
. The membrane
42
is typically thicker than the membrane
34
in the scattering-membrane reticle
31
so as to exhibit substantial scattering of electrons in a beam incident on the upstream-facing surface of the reticle
41
.
Due to the current impossibility of simultaneously exposing an entire reticle at one instant using a charged particle beam, conventional CPB-microlithography reticles typically are “divided” or “segmented” into multiple small regions (“subfields” or “exposure units”). In FIG.
3
(
c
), each subfield on a scattering-membrane reticle
34
is denoted by the reference numeral
32
a
, and each subfield on a scattering-stencil reticle
41
is denoted by the reference numeral
42
a
. Each subfield
32
a
,
42
a
defines a respective portion of the overall pattern defined by the respective membrane
32
,
42
. A representative subfield
32
a
is shown in FIG.
3
(
a
). The subfields are separated from one another by boundary regions
35
in which no pattern features are defined. Extending from each boundary region
35
is a support strut (item
33
in FIG.
3
(
a
)) that provides physical support for the membrane
32
. Reference is also made to
FIG. 4
showing support struts
43
on a scattering-stencil reticle
41
. The support struts
33
,
43
form a criss-cross network on the respective reticle, wherein the subfields
32
a
,
42
a
are located between the support struts
33
,
43
.
In a conventional scattering-stencil reticle the membrane
42
typically is a silicon membrane about 2&mgr;m thick. As noted above, the membrane
42
defines the through-holes that are transmissive to the incident electron beam.
Conventionally, the area of the reticle that can be exposed at any instant by the incident electron beam is about 1 mm square. Hence, each subfield must define a respective portion of the overall pattern to be transferred to a particular region (“die”) on the wafer, wherein a die corresponds to the area occupied by a “chip” as formed on the wafer.
As indicated in FIG.
3
(
c
), pattern transfer is conventionally performed by illuminating the subfields
32
a
,
42
a
with the incident charged particle beam. The subfields
32
a
,
42
a
are typically exposed sequentially in a stepwise manner. As each subfield is illuminated for exposure, the corresponding portion of the pattern is demagnified and transferred to the “sensitive substrate” (wafer)
37
by a projection-optical system (not shown). The images of the subfields
32
a
,
42
a
are formed on the wafer
37
in respective locations in which the images are properly “stitched together” (contiguously arranged) with no intervening boundary regions.
In a conventional segmented reticle as described above, each support strut typically has a width of approximately 180 &mgr;m. The cumulative effect of having to dedicate a substantial portion of the reticle to non-pattern-defining struts is an excessively large reticle. Furthermore, during manufacture of such a reticle in which the struts are formed by etching, it is difficult to satisfactorily control the width of such support struts.
Moreover, the resulting large reticle must be mounted on and conveyed by a correspondingly large reticle stage. A suitably large reticle stage has a substantial mass that requires correspondingly large and robust stage-actuating mechanisms for moving the reticle as required for exposure.
One conventional approach for reducing the size of a segmented reticle is to arrange groups of subfields into rows, wherein each row of subfields is separated from other rows by support struts. Thus, each row contains multiple subfields situated side-by-side. (Such a reticle is regarded as having a “slot” configuration.) In order to scan a row of subfields, an electron beam is deflected in a lateral sweeping manner.
The positional accuracy of such scanning desirably is 0.5 &mgr;m or less. Unfortunately, maintaining such positional accuracy is not possible from the perspective of achieving adequate digital-to-analog (DAC) conversion of energizing signals routed to the respective deflectors in the electron-optical system. Also, the variation in positional accuracy of the electron beam is not uniform in conventional practice, resulting in double-exposed portions or non-exposed portions of the pattern as projected onto the wafer. These problems are manifest as “stitching” errors of the pattern as projected onto the wafer. Also, the continuously scanning electron beam must be rigorously controlled during exposure so as to achieve accurate stitching and to compensate for variations in pattern-element density and shape configurations from one subfield to the next. That is, the electron-optical system must be controlled in a manner allowing continuous high-speed processing. However, achieving such control is conventionally extremely problematic.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide, inter alia,
Klarquist & Sparkman, LLP
Nikon Corporation
Rosasco S.
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