Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
2000-06-09
2002-07-16
Berman, Jack (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S492220, C250S3960ML
Reexamination Certificate
active
06420714
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention is directed to a lithographic process for device fabrication in which charged particle energy is used to delineate a pattern in an energy sensitive material. The pattern is delineated by projecting the charged particle energy onto a patterned mask, thereby projecting an image of the mask onto the energy sensitive material.
2. Art Background
In device processing, an energy sensitive material, denominated a resist, is coated on a substrate such as a semiconductor wafer (e.g., a silicon wafer), a ferroelectric wafer, an insulating wafer, (e.g. a sapphire wafer), a chromium layer supported by a substrate, or a substrate having a combination of such materials. An image of a pattern is introduced into the resist by subjecting the resist to patterned radiation. The image is then developed to produce a patterned resist using expedients such as a solution-based developer or a plasma etch to remove one of either the exposed portion or the unexposed portion of the resist. The developed pattern is then used in subsequent processing, e.g. a mask to process, i.e. etch, the underlying layer. The resist is then removed. For many devices, subsequent layers are formed and the process is repeated to form overlying patterns in the device.
In recent years, lithographic processes in which a charged particle beam is used to delineate a pattern in an energy sensitive resist material have been developed Such processes provide high resolution and high throughput. One such process is the SCALPEL® (scattering with angular limitation projection electron beam lithography) process. The SCALPEL® process is described in U.S. Pat. No. 5,260,151 which is hereby incorporated by reference.
Referring to
FIG. 1
, a doublet lens system
15
is used in the lithography tool for the SCALPEL® process. A first lens system (not shown) is used to direct and focus incident radiation
10
from the radiation source (not shown) onto the mask
20
. The mask
20
is used to pattern particle beam
10
. The entire mask
20
is not illuminated at once. Mask
20
, as shown, consists of a membrane
13
, which is transparent to the particle beams incident thereon, and blocking regions
14
.
The developed image of the mask pattern is defined by blocking regions
14
, which scatter the particle beams
10
incident thereon. Unblocked illumination, illustrated as beams
12
, is transmitted through the membrane regions
13
. Blocked illumination, illustrated as beams
11
is caused to converge by means of a first electromagnetic/electrostatic projector lens
30
in lens system
15
. Filter
19
is an aperture scatter filter. The aperture scatter filter
19
is designed so that the unscattered radiation (beams
12
) passes through the aperture
21
therein. The scattered radiation
11
is blocked by the aperture scatter filter
19
, which is located in the mutual focal plane of the lenses
30
and
31
.
Second projector lens
31
of lens system
15
is of such configuration and so powered as to bring the unscattered beams
12
into an approximately parallel relationship. The action of the lens
31
is sufficient to direct beams
12
into orthogonal incidence onto wafer
24
.
Lens system
15
consists of two lenses. Consequently, the lens system is referred to as a doublet electromagnetic lens arrangement. Such a doublet electromagnetic lens arrangement is described in Waskiewicz, W., et al., “Electron-optics method for High-Throughput in a SCALPEL system: preliminary analysis,” Microelectronic
Engineering
, Vol. 41/42, pp. 215-218 (1998). The doublet electromagnetic lens system described in Waskiewicz et al. provides telecentric reduction imaging from the mask to the wafer. Such an arrangement uses two lenses of similar construction. The lenses are laid out sequentially and separated by a distance equal to the sum of their two focal lengths. Referring again to
FIG. 1
, the object (i.e. the mask
20
) is located in the back focal plane of the first projector lens
30
of lens system
15
. An image of the mask
20
is formed at the front focal plane (i.e. the layer of energy sensitive material
23
on wafer
24
) of the second projector lens
31
of second lens system
15
. The magnification provided by the lens system is determined by the ratio of the focal length of lens
30
to the focal length of lens
31
. The bore (D) to gap (S) ratio for both lenses are identical and the excitations (NI) are set equal but opposite.
When designed properly, the doublet lens not only substantially eliminates the rotation introduced into the image by an individual lens in the doublet, but also eliminates rotation-related aberrations in the image. These aberrations are primarily chromatic aberrations. Removing these aberrations provides the lowest total image blur. Doublet lens systems are described in Heritage, M. B., “Electron-projection microfabrication system,”
J. Vac. Sci. Technol
., Vol. 12, No. 6, pp. 1135-1140 (1975), which is hereby incorporated by reference.
In the classic magnetic doublet design, the first and second lenses are separated along their common optical axis to ensure that there is a space between the lenses that is field-free. The field-free space is a space that is not affected by the magnetic field generated by the lenses. Typically, both lenses have a common focal length (F) within this field-free space. Such an arrangement is illustrated in FIG.
2
.
FIG. 2
illustrates the magnetic flux as a function of distance along the optical axis relative to the position of magnetic lenses
30
and
31
. In the region between lenses
30
and
31
the magnetic flux is zero. This is the desired field-free space.
However, in certain applications, design constraints do not permit the spacing between the first and second lenses that provides for a field-free space. In the lithography tool for the SCALPEL® process, for example, the mutual focal plane of lenses
30
and
31
is at the apertured scatter filter
19
. Furthermore, in order to increase the speed at which the image is written (and thereby to achieve the desired throughput from the tool) the electron beam scans about the optical axis. In order to control the off-axis aberrations, e.g. astigmatism, that result from off-axis scanning, the bore of the doublet lens is increased while the axial separation between the two lenses either remains the same or is shortened to control space-charge blur. Consequently, the magnetic fields of the doublet lens overlap. This problem is illustrated in FIG.
3
. In
FIG. 3
, the magnetic flux of each lens is affected by this overlap. This is observed with reference to dashed line
50
in FIG.
3
. Observe that, due to the proximity between lenses
30
and
31
, the flux as a function of axial position for lens
31
on one side of line
50
is not a mirror image of flux as a function of axial position on the other side of line
50
. Thus, the desired axial magnetic field symmetry for lens
31
(and for lens
30
) in
FIG. 3
is not preserved.
This overlap causes field distortion. Also, the apertured scatter filter
19
is immersed in the magnetic field of lenses
30
and
31
. Since this field overlap compounds aberrations and total blur growth and also causes projection magnification changes. Consequently, a solution to the magnetic field overlap of the projection lens doublet that is compatible with the SCALPEL® tool design is sought.
SUMMARY OF INVENTION
The present invention is directed to a magnetic doublet lens system in which the spacing between the two lenses is such that their magnetic fields overlap. The magnetic doublet lens system is equipped with magnetic clamps that effect substantial separation of the magnetic fields. The magnetic clamps are made of a ferromagnetic material. The present invention is also directed to an apparatus for electron beam lithography that has a magnetic doublet lens system that is equipped with magnetic clamps to effect substantial separation of the magnetic fields between the two lenses. Substantial separation, in the context of the present in
Katsap Victor
Munro Eric
Rouse John Andrew
Waskiewicz Warren K
Zhu Xieqing
Berman Jack
Botos Richard J.
Fernandez K
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