Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1998-12-16
2003-09-30
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492230
Reexamination Certificate
active
06627905
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to charged-particle-beam (CPB) exposure devices for transferring a pattern from a self-supporting thin-film mask onto a substrate, such as a wafer.
BACKGROUND OF THE INVENTION
Mask-based charged-particle-beam (CPB) exposure devices use a charged-particle beam to transfer a desired pattern from a mask to a substrate. The substrate is typically in the form of a charged-particle-sensitive substrate such as a photo-resist-coated wafer or the like.
An example of a conventional CPB exposure device is shown schematically in FIG.
6
. The X axis, Y axis, and Z axis are defined as shown in the figure.
In this conventional CPB exposure device, a charged-particle beam in the form of an electron beam EB is emitted from an electron gun
1
. The electron beam EB is formed into a constant cross-sectional size and shape by an aperture
3
situated between the electron gun
1
and a mask
2
. A position-selection deflector
4
deflects the shaped electron beam EB to irradiate a selected sub-field at a selected position on the mask. The electron beam EB, which has been patterned by passing through the selected sub-field, is then passed through a lens
5
to form a reduced image of the patterned beam at a selected position on a substrate such as a semiconductor wafer
6
. The selected position on the wafer
6
is thus exposed, at a particular reduction ratio, with the pattern from the selected sub-field of the mask
2
, and the pattern image of the selected sub-field on the mask
2
is transferred to a corresponding position on the wafer
6
.
The various other sub-fields on the mask
2
are then sequentially and selectively irradiated by the shaped electron beam EB and exposed on the wafer
6
at respective desired locations. This is accomplished by deflecting the beam with position-selection deflector
4
, and by shifting the wafer
6
, in the X- and Y-directions, with a substrate stage
7
, and by shifting the mask
2
in the X- and Y-directions with a mask stage
8
. Patterns formed by irradiation of various individual sub-fields at the mask are positioned on the wafer so as to be aligned and joined or “stitched” together, so that a pattern that is divided into various sub-fields on the mask
2
is formed as a single pattern on the wafer
6
. A controller
9
controls the various elements
1
,
4
,
5
,
7
, and
8
described above, and a deflector
12
, in order to accomplish the pattern transfer.
Note that the substrate stage
7
can generally be moved not only in the X- and Y-directions, but also the Z-direction as well.
Prior to or during the exposure operation described above, device calibrations are performed, such as calibrating the image rotation, image magnification, and the like, in the image-transfer field. This device calibration is typically accomplished by irradiating the electron beam EB onto marks formed at a specific location on the mask
2
. An image of the marks on the mask is formed on the wafer
6
. The image, formed by the charged-particle beam (electron beam) EB, causes back-scattered electrons and/or other radiation to emanate from corresponding marks on the wafer
6
. A detector
10
is employed to detect the electrons and/or other radiation emanating from the corresponding marks on the wafer
6
. The controller
9
then calculates control parameters and the like for the lens
5
, the deflector
12
, and/or similar exposure-device components, based on the detected radiation from the marks on the wafer
6
. The control parameters are then saved and/or used to control the wafer stage
7
and/or the mask stage
8
.
Also prior to or during the normal exposure operation described above, the mask
2
either is initially installed to the position for exposure, or is swapped and loaded by a mask loader
11
. The mask
2
is then initially aligned (also referred to as rough alignment), using alignment marks previously formed on the mask, by a process similar to the process used for device calibration. Namely, the position-selection deflector
4
deflects the electron beam EB so as to irradiate one or more locations, at the plane of the mask
2
, where the alignment marks should be positioned. The detector
10
is then used to detect back-scattered electrons and/or other radiation emanating from corresponding marks on the wafer
6
or on the wafer stage
7
. The controller
9
then controls the mask stage
8
and/or the wafer stage
7
, based on the detected radiation, so as to eliminate any positional shift at the wafer between the image of the alignment marks and the marks on the wafer or wafer stage, thereby rough-aligning the mask.
Note that description of other typical elements generally employed in the illumination system of the electron-beam exposure device to illuminate the aperture
3
and the mask
2
have been omitted above for simplicity. Such elements are known in the art and need not be reviewed here.
In a conventional mask-based CPB exposure device like the example described above, the beam is always shaped to a given constant cross-sectional size and shape by the aperture
3
. The constant size and shape of the beam is selected so as to match the size of the sub-fields to be irradiated at the wafer.
SUMMARY OF THE INVENTION
Although an exposure device using a mask and having a constant beam-shape, determined in the manner described above, has no particular detriment during actual pattern exposure and during use with some types of masks, the inventor has found that problems can arise during use of a self-supporting thin-film type mask. These problems will be explained below, with respect to the use of a self-supporting thin-film type mask as shown in FIG.
7
.
FIG. 7
shows an example of a known self-supporting thin-film type mask. FIG.
7
(
a
) is a schematic plan view. FIG.
7
(
b
) is a cross-sectional view taken along the line A—A′ in FIG.
7
(
a
). The mask is constructed from a thin film
21
which scatters electrons at a large scattering angle, and a thick grid-shaped frame
22
which supports the thin film
21
at its bottom surface. Since the frame
22
is thick, when the charged-particle beam irradiates the frame
22
, the frame absorbs a significant portion of the incident particles. The process of absorbing incident particles heats the frame. Heating the frame deforms the mask, decreasing the transfer precision of the pattern on the mask.
Pattern-formation fields (transfer fields)
32
are smaller than, and positioned within, respective square fields
31
defined by the frame
22
. An irradiation field
33
(shown by the cross-hatching) corresponds to the beam cross-sectional size and shape, and is predetermined to be larger than the pattern-formation fields
32
, but smaller than the square fields
31
. The electron beam, which has been shaped by the aperture
3
of
FIG. 6
, sequentially irradiates multiple irradiation fields
33
in the mark of FIG.
7
(
a
) (only one irradiation field
33
is shown). While it is not shown in the figure, a desired pattern to be transferred to the wafer
6
is formed by openings or low-scattering-angle areas in each pattern formation field
32
of the thin film
21
.
When this type of mask is used in an exposure device, an aperture is provided (not shown in
FIG. 6
) that blocks the electrons that have been scattered by the non-open and/or non-low-scattering areas of the thin film
21
. (Such an aperture would also be used, though not shown, in the devices shown in
FIGS. 1
,
3
, and
4
to be described below.)
If the self-supporting thin-film type mask shown in
FIG. 7
is used in the conventional CPB exposure device shown in
FIG. 6
, there will generally be no problems during normal exposure. However, the inventor has found that the following problems can arise during the device calibration and mask alignment processes discussed above.
First, the problems arising during device calibration will be explained, with reference to FIG.
8
.
FIG. 8
is a schematic plan view of another portion of the mask shown in
FIG. 7
, showing irradiation of the electron beam dur
Klarquist & Sparkman, LLP
Nguyen Kiet T.
Nikon Corporation
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