Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-05-17
2003-03-11
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000, C430S296000, C430S942000
Reexamination Certificate
active
06531251
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (transfer of a pattern, defined by a reticle or mask, to a “sensitive” substrate) using a charged particle beam (e.g, electron beam or ion beam). Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, thin-film magnetic pickup heads, and micromachines. More specifically, the invention is directed to reducing proximity effects as manifest on the pattern as transferred to the substrate. Even more specifically, the invention pertains to methods for calculating exposure dose at specified regions of the sensitive substrate so as to determine expected respective proximity effects at the specified regions. The invention also pertains to methods for fabricating a reticle, taking into account the results of the proximity effect determinations, that produces less proximity effects during transfer of the reticle pattern to the substrate.
BACKGROUND OF THE INVENTION
Essentially all contemporary methods for fabricating microelectronic devices involve microlithography steps. In the microlithography step, a pattern defined on a reticle or mask is transferred to a “sensitive” substrate such as a semiconductor wafer or the like. The devices are formed on the substrate as respective “chips” that are separated later from each other by “dicing” the wafer. “Sensitive” means that the substrate is coated with a substance, termed a “resist,” that can be imprinted with a pattern exposed onto the resist using an energy beam. Exemplary energy beams used for microlithography include light, X-rays, and charged particle beams.
A typical pattern includes a large number of pattern elements or features. As the pattern is exposed onto the substrate, the pattern elements are formed by differential exposure of the substrate, i.e., certain areas of the resist receive a relatively high exposure dose and other areas receive a relatively low exposure dose. After exposure, areas of resist where the exposure dose (cumulative exposure-irradiation energy) exceeds a threshold value are removed (in the case of a positive resist) or left on the substrate (in the case of a negative resist) by developing the resist. To form a pattern element having a respective desired shape profile on the sensitive substrate, it is necessary to calculate whether the exposure dose at the location on the substrate where the pattern element is exposed is higher than a specified threshold. It also is necessary to configure the pattern element on the reticle such that portions of the pattern element corresponding to areas in which the localized exposure dose on the substrate exceeds the threshold nevertheless form the pattern element with the desired shape profile on the substrate.
In charged-particle-beam (CPB) microlithography, proximity effects arise under conditions in which actual localized exposure doses (e.g., exposure doses at single pattern elements) vary according to the nearness, respective profiles, and distribution of neighboring pattern elements, due to scattering of electrons in the resist and from the substrate. More specifically, proximity effects arise due to the scattering of charged particles, incident upon the surface of the sensitive substrate, at small angles that reduce the exposure dose at specified locations. These small-angle scattering events are termed “forward-scattering.” Proximity effects also arise due to the scattering of charged particles at wide angles that contribute exposure energy to neighboring unexposed areas. These wide-angle scattering events are termed “back-scattering.” Whenever a proximity effect occurs, the exposure dose at a respective location on the sensitive substrate differs from what is expected or desired at the location. As a result, the pattern element that is formed at the location on the substrate usually has a profile that undesirably is different from the desired profile.
Conventional methods for reducing proximity effects generally involve making localized exposure doses closer to desired respective doses. For example, certain methods involve changing and adjusting localized exposure doses by changing beam intensity (dose modulation); others involve changing the profiles of pattern elements as defined on the reticle (“local resizing”).
On the substrate, a planar distribution of exposure dose in the resist from irradiation, by a charged particle beam, of a point (x, y) on the surface of the resist can be expressed as the sum of a Gaussian distribution (i.e., a double Gaussian distribution):
E
(
x
,
y
)
=
(
1
1
+
η
)
(
1
πσ
f
2
)
exp
[
-
(
x
2
+
y
2
)
σ
f
2
]
+
(
η
1
+
η
)
(
1
πσ
b
2
)
exp
[
-
(
x
2
+
y
2
)
σ
b
2
]
The standard-deviation terms, &sgr;
f
and &sgr;
b
, are broadening terms known as the “forward-scattering diameter” and “back-scattering diameter,” respectively; and &eegr; is a ratio of exposure energy from back-scatter to exposure energy from forward-scatter (i.e., the “back-scatter fraction”). If defocusing in the projection-optical system of the microlithography apparatus is taken into consideration, then the sum of squares of the magnitude of defocusing and the forward-scattering diameter is calculated and substituted as a new &sgr;
f
.
The following discussion refers to mathematical expressions based on back-scattering diameter &sgr;
b
. Expressions based on forward-scattering diameter &sgr;
f
or optical-system defocusing can be set forth in a similar manner, wherein the back-scattering term is substituted with the forward-scattering term or optical-system-defocusing term.
If a pattern of reference figures (reference elements) is configured as N rectangles each having diagonal apices at the coordinates (x
1j
, y
1j
), (x
2j
, y
2j
) (where j=1, 2, 3, . . . , N), then the back-scattering energy E
b
(x, y) at a location (x, y) can be expressed by integrating the E(x, y) expression above, yielding the following:
E
b
(
x
,
y
)
=
∑
j
[
erf
(
(
x
-
x
1
j
)
σ
b
)
-
erf
(
(
x
-
x
2
j
)
σ
b
)
]
×
[
erf
(
(
y
-
y
1
j
)
σ
b
)
-
erf
(
(
y
-
y
2
j
)
σ
b
)
]
wherein “erf” denotes an error function.
This calculation yields a sum corresponding only to the specific number of reference figures to which reference is being made. Consequently, a problem with this calculation is that, as the number N of reference figures increases (with an increase in the density and/or complexity of circuit elements in the pattern), the calculation time increases commensurately.
A conventional method for addressing this problem involves the use of “representative figures,” as exemplified in FIGS.
9
(A)-
9
(B). In FIG.
9
(A), a pattern region
91
is depicted containing multiple reference figure
93
. The region
91
is divided (along dashed lines) into multiple sub-regions
92
. Each sub-region
92
is small relative to the back-scatter diameter &sgr;
b
and serves as the fundamental unit of pattern area on which calculations of local exposure dose are based. By performing exposure-dose calculations based on the contents of specified sub-regions, the number of reference figures used for calculating back-scatter energy is reduced (with a concomitant reduction in calculation time) compared to making calculations based on each individual actual pattern element of the pattern. In FIG.
9
(B), a single respective representative
figure 94
is derived for each sub-region
92
. Each representative
figure 94
has the same total surface area and centroid as the respective reference
figure 93
in the respective sub-region.
However, in certain instances (e.g., with a pattern for a LSI device) an actual pattern element
103
can have a marked dimensional bias. For example,
FIG. 10
depicts a region
102
of a pattern portion
101
(in
FIG. 10
, the dashed lines denote respective coordinate axes). As shown in the upper portion of
FIG. 10
, the pattern element
103
extends across the upper portio
Klarquist & Sparkman, LLP
Nikon Corporation
Young Christopher G.
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