Electron beam lithography system

Details Electron beam lithography system Electron beam lithography system

1999-05-21

2004-12-07

Lee, John R. (Department: 2881)

Radiant energy

Irradiation of objects or material

Irradiation of semiconductor devices

C250S492300, C250S492220

Reexamination Certificate

active

06828573

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an electron beam lithography system for rendering fine patterns using an electron beam.
Generally, electron beam lithography systems are known for such characteristics as (1) an ability to form fine patterns, (2) possession of a pattern generating function, and (3) a capability of correcting distortion. With these features in their arsenal, the systems have been used extensively in developing LSIs and fabricating masks for optical exposure. While excelling in fine working, the electron beam lithography systems are nevertheless confronted with a physically unavoidable bottleneck known as the proximity effect experienced when VLSI patterns are rendered on the submicron order.
FIG. 16
is a schematic view illustrating the principle of the proximity effect. In
FIG. 16
, an electron beam
72
is incident on a substrate
70
over which an electron beam resist
71
has been applied. The proximity effect is caused primarily by two kinds of electron scattering: forward scattering and backward scattering. Electrons emitted by an electron gun and incident on the electron beam resist
71
first collide with component atoms of the resist
71
and are scattered thereby continuously while discharging their energy. The process is called forward scattering. Most of the incident electrons penetrate the substrate, get scattered thereby and return partially to the resist
71
. This process is called backward scattering.
These processes result in the incident energy being accumulated not only in the irradiated (i.e., exposed) region but also elsewhere. Hence an insufficient dose (exposure) likely to occur in rendering fine patterns. Conversely, excess energy tends to accumulate in space between figures of a large pattern. Consider a case where an electron beam is used to render a pattern made of a plurality of figures. In that case, if the figures are numerous and are clustered in close proximity, they tend to be rendered thick; if the figures are sparsely distributed and isolated from one another, they are likely to be rendered thin. The phenomenon is called the proximity effect.
One technique for correcting the proximity effect is disclosed illustratively in Japanese Published Unexamined Patent Application No. Hei 03-225816. The disclosed technique involves calculating a rendering area of each of regions divided by rendering data and thereby creating a dose (exposure) map. The exposure map is referenced for each shot. The exposure is increased for patterns of low rendering densities and lowered for patterns of high rendering densities, whereby each pattern is rendered at an optimum exposure level in keeping with the rendering density in effect.
Other techniques have been proposed to correct the proximity effect. For example, U.S. Pat. No. 4,463,265 discloses a so-called ghost method whereby a defocused electron beam is used to render a monochromatically reversed pattern of a target figure for correcting the proximity effect. Japanese Published Examined Patent Application No. Sho 63-14866 describes a method for obtaining levels of exposure at opposing ends of a dimensionally corrected pattern so that the pattern ends will have intensity ratios lower than a predetermined value. Japanese Published Examined Patent Application No. Sho 62-46059 discloses a method for acquiring a ratio of an irradiation density at which a resist pattern of the same size as that of a target rendering pattern is obtained, to an irradiation density at which a predetermined residual film is provided, so that the target pattern is rendered at a suitable irradiation density and in a suitable size for the residual film to be formed.
The technique disclosed in Japanese Published Unexamined Patent Application No. Hei 03-225816 is characterized in that it initially carries out mock rendering to create an exposure map and then performs actual rendering by referring to the exposure map thus created.
FIG. 17
is a schematic block diagram illustrating the concept of the above technique. Rendering data that are input to an input unit
81
comprise exposure data as well as position and shape data about each of shots computed by a figure disassembling and reassembling correction unit located upstream of the input unit
81
. The exposure map is created by first carrying out mock rendering. When rendering data (shot shapes and positions) are received from the preceding stage, an area value computing unit
82
calculates the area of each shot.
As shown in
FIG. 18
, the rendering data are divided into square meshes having a side length of &agr; each. To a mesh containing a center
91
of each shot
90
, the area of the shot in question is added cumulatively. The result is stored temporarily in a memory portion called a partial memory
83
. The process is repeated. The area value held in the partial memory
83
is transferred to an exposure map memory
84
. The data retained in the exposure map memory
84
are subjected to such processes as data smoothing by a smoothing filter
85
. The processed data are placed back into the exposure map memory
84
.
Actual rendering is then performed as follows: the same rendering data are again received from the preceding stage. Given the shot position and shape data, an address computing unit
86
calculates an address in the exposure map. The area value for the calculated address in the exposure map memory
84
is converted simultaneously to a level of exposure by an exposure converting unit
87
. An adder
88
adds or subtracts the exposure level to or from the exposure of each shot. The output of the adder
88
is forward downstream through an output unit
89
. In this manner, shots included in large-area meshes are assigned low levels of exposure while those in small-area meshes are given high levels of exposure, whereby desired correction is supposed to be accomplished.
Where the exposure map is created by adding the area of each shot to a mesh containing the center of the shot in question, as described above, problems occur if the shot center is located close to a mesh boundary.
FIG. 19
is a schematic view outlining such problems experienced illustratively when three meshes contain a number of shots.
In
FIG. 19
, shots
1
and
3
are significantly close in size to their corresponding meshes while a shot
2
is appreciably smaller than its corresponding mesh. The shot
1
straddles the meshes
1
and
2
. The center C
1
of the shot
1
is included in the mesh
1
close to the boundary between the meshes
1
and
2
. The shot
2
together with its center C
2
is included in the mesh
2
. The shot
3
straddles the meshes
2
and
3
. The center C
3
of the shot
3
is included in the mesh
3
close to the boundary between the meshes
2
and
3
.
In the above makeup, the proximity effect works in such a manner that space between shots tends to be narrowed. Thus to correct the adverse effect requires reducing the exposure of the shot
2
. With the above conventional method, however, the area value is accumulated at the center of each shot so that, as shown in the lower part of
FIG. 19
, all area value of the shot
1
is added to the mesh
1
and the entire area value of the shot
3
is added to the mesh
3
.
Meanwhile, the mesh
2
that needs to have the largest possible area value is only supplemented by the area value of the shot
2
. As a result, the meshes
1
and
3
are given large area values while the mesh
2
is assigned a small area value. When this kind of exposure map is used for exposure correction, the shot
2
in the mesh
2
is corrected to have a raised level of exposure and the shots
1
and
3
are corrected to have lowered levels of exposure. This type of exposure correction is contrary to what is expected of the corrective process implemented. Exposure correction remains unavailable.
FIGS. 20 and 21
illustrate specific simulations for purpose of illustration.
FIG. 20
shows rendering data representing a 60 &mgr;m×60 &mgr;m square.
FIG. 21
depicts a three-dimensional exposure map created by dividing the

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