Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2001-07-09
2003-09-16
Tran, Huan (Department: 2861)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492300
Reexamination Certificate
active
06621090
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to high-emittance electron-beam sources for use in any of various electron-beam devices, particularly electron-beam microlithography apparatus and related exposure apparatus. Electron-beam microlithography is a type of charged-particle-beam microlithography, which represents one of several “next-generation” microlithography technologies currently experiencing intensive development effort due to its potential for achieving substantially greater resolution than obtainable using conventional optical microlithography technology. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, thin-film magnetic pickup heads, micromachines, and the like.
BACKGROUND OF THE INVENTION
A conventional thermionic electron-beam source (also termed an “electron gun”) is shown in FIG.
7
. The depicted source includes a cathode
1
(serving as the electron-emitting surface), a Wehnelt electrode
3
, and an “extraction electrode” (anode)
4
arranged along an axis Ax. The cathode
1
normally is heated by a heating means (not shown but well understood in the art) to cause the cathode to emit hot electrons. The emitted electrons are formed into a beam
2
by the Wehnelt electrode
3
and anode
4
. Specifically, the anode
4
extracts electrons emitted from the cathode
1
and urges them to propagate in a downstream direction (to the right in the figure) from the cathode to the anode
4
and beyond. Electrons of a beam
2
emitted from the cathode
1
and propagating initially parallel to the axis Ax (i.e., axially propagating electrons) are subjected to respective lens actions by the respective voltages applied to the Wehnelt electrode
3
and anode
4
. The electrons of the beam
2
converge at a gun crossover
5
. The axial position of the gun crossover
5
is a function of the respective voltages applied to the Wehnelt electrode
3
and anode
4
(e.g., a higher negative voltage applied to the Wehnelt electrode
3
will tend to move the gun crossover
5
to the left in the figure). In a microlithography context, the beam
2
propagating downstream of the anode
4
is acted upon by a downstream electron-optical system (not shown) that shapes and conditions the beam for use as an illumination beam for illuminating a desired region of a reticle or mask or other object (not shown).
In certain types of electron-beam apparatus the emittance of the electron gun is critical, especially if the apparatus is used for making one-shot lithographic exposures of respective portions of a pattern, or for making reduced (demagnified) transfer-exposures of a pattern. “Emittance” is a quantitative expression of the ability of the beam to achieve uniform irradiation of a defined surface, and is expressed as the range of uniform beam current in an area irradiated by the electron beam
2
multiplied by the aperture half-angle of the beam at the irradiated region.
In electron microscopes and microlithography apparatus utilizing an electron beam having a transversely Gaussian distribution but configured as a spot beam for pattern drawing, emittance normally is not a significant variable. This is because the area illuminated by a spot beam at any instant in time is only 1 to 10 nm in diameter, which is effectively at the apex of the distribution. In contrast, in microlithography apparatus utilizing a reticle divided into subfields that are exposed individually with demagnification, merely forming the beam to irradiate a spot is insufficient for achieving proper pattern transfer because the area illuminated by the electron beam is substantially larger than 1-10 nm across. Rather, it is necessary to achieve uniform irradiation in an area measuring 10 &mgr;m square (typical of one-shot partial pattern block exposures) to 1 mm square (typical of one-shot reduced transfer exposures from respective subfields of a divided reticle). These latter areas encompass not only the apex of the Gaussian distribution but also the tails (distal or outlying portions) of the distribution. In addition, the aperture half-angle in these latter two cases is several mrad. As a result, to achieve the required uniform illumination over the desired one-shot area, high emittance from the electron gun is necessary.
To improve the transverse uniformity of the energy of the electron beam emitted from the electron gun used in apparatus for performing partial pattern block exposures and reduced transfer exposures from a divided reticle, the cathode normally is made transversely wide and planar as shown in
FIG. 7. A
wide planar cathode also improves the uniformity of beam current as incident on a substrate such as a semiconductor wafer when forming a microlithographic image of the cathode on the substrate. However, whenever electrons are emitted from a wide cathode surface, beam current tends to be excessive. Hence, various means conventionally are employed to prevent emission of extraneous electrons from the cathode. Exemplary conventional means include fabricating the cathode of a material having a high work function or applying a substance having a high work function to portions of the cathode surface located off axis.
If the cathode is a thermionic-emission type, the electron gun generally exhibits a relationship between emission-current density J
c
and anode voltage V
a
as shown in FIG.
10
. In the figure T
c
is cathode temperature. For example, the relationship of J
c
versus V
a
for T
c3
is indicated by the solid-line curve. The region where the relationship of J
c
versus V
a
is nearly according to J
c
∝V
a
2/3
is termed a “space-charge-limited” region. The more distal region is a “temperature-limited” region.
As the temperature of a thermionic cathode rises, beam current can become excessively high, and operation of the electron gun becomes space-charge limited. Whereas operation of the gun in a space-charge-limited manner can be performed in a stable manner, the presence of a high-charge field at or adjacent the cathode surface can cause the emitted electrons to lose characteristics reflective of the cathode surface from which they were emitted. If the electric field is substantially non-uniform, then electron emission from the cathode surface is not uniform. Under such conditions, the uniformity of current at the cathode surface conventionally cannot be utilized. Hence, there is a need for a way in which to utilize the electron gun in a temperature-limited region having a relatively low temperature and low beam current at the cathode.
Meanwhile, the distribution of the aperture angle is determined by the transverse energy distribution of the beam, which is determined by the cathode temperature of the cathode as electrons are being emitted from the cathode. The trajectories of electrons, emitted from the cathode
1
at a point of intersection of the optical axis Ax with the cathode surface, and emitted at an angle relative to the optical axis Ax are shown in FIG.
8
. Near-axis trajectories
6
determine the configuration and dimensions of a crossover
7
formed at the crossover point
5
. The spatial intensity of the beam at the crossover
7
is a function of the distribution of electron emission at the cathode surface, which (as discussed above) usually is a Gaussian distribution.
The emittance at a surface irradiated by the beam is determined not only by the uniformity of beam current and aperture angle at the cathode of the electron gun, but also by aberrations generated by lens actions generated by respective voltages applied to the electrodes of the electron gun. Whereas emittance can be preserved if the downstream optical system is free of aberrations, emittance is degraded by an optical system exhibiting significant aberrations. If emittance has deteriorated, it generally cannot be restored by the downstream optical system.
In conventional electron guns, substantial aberrations are imparted to the image at the crossover point
5
by the respective lens actions of the Wehnelt electrode
3
and the anode
4
, a
Klarquist & Sparkman, LLP
Nikon Corporation
Tran Huan
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