Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-05-18
2003-05-13
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492100, C250S492200, C250S492220, C250S492230, C250S492300, C250S3960ML, C315S005310
Reexamination Certificate
active
06563125
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projection-exposure) of a pattern, defined by a reticle or mask, onto a sensitive substrate such as a semiconductor wafer using a charged particle beam such as an electron beam or ion beam. Microlithography is a key technology used in the fabrication of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to charged-particle-beam (CPB) microlithography apparatus and methods in which image defocusing and distortion caused by Coulomb effects are reduced.
BACKGROUND OF THE INVENTION
As the degree of integration of semiconductor integrated circuits has increased in recent years, more intricate circuit patterns have been demanded. As the desired minimum linewidth in circuit patterns has fallen to 0.1 &mgr;m and below, the inability of optical microlithography to provide acceptable resolution is apparent. Consequently, substantial research currently is being devoted to the development of a practical charged-particle-beam (CPB) microlithography apparatus that can provide the desired pattern resolution at a satisfactory throughput.
Initial efforts in this regard were directed to CPB microlithography systems that performed exposure of an entire chip pattern (“die”), or even multiple dies, in one “shot.” Such systems currently are impractical because of the extreme difficulty of preparing a reticle that can be exposed in one shot and of maintaining aberrations below necessary levels within a CPB-optical field sufficiently large to accommodate an entire die or multiple dies.
As an alternative to “one-shot” CPB microlithography systems, so-called “divided-reticle” CPB microlithography systems are being investigated actively. In a divided-reticle system, rather than exposing an entire die or multiple dies at one time, a die as defined on the reticle is divided into small regions (e.g., “subfields”) each measuring about several hundred &mgr;m square, and the small regions are exposed sequentially in an ordered manner to effect exposure of an entire die. During exposure of each small region, correction of exposure parameters can be made as required, such as correction of focus and distortion of the image being formed on the substrate (“wafer”). Thus, exposure can be performed with good resolution, accuracy, and precision over an even wider area than obtainable using a one-shot system.
CPB microlithography systems are subject to certain problems such as defocusing and distortion of the image due to Coulomb effects. A Coulomb effect is caused whenever neighboring charged particles in the charged particle beam repel one another sufficiently to disturb the trajectories of the charged particles in the beam. Particles having disturbed trajectories degrade the image. Coulomb effects are also exhibited by divided-reticle systems. Current attempts to reduce Coulomb effects include adjusting focus by altering the electrical current supplied to a refocus lens, etc. However, Coulomb effects cause not only shifts in focal point but also distortion of the image as projected onto the wafer.
An example of a CPB-optical system exhibiting reduced distortion caused by Coulomb effects and that includes three focus lenses and two stigmators is disclosed in Japan Kôkai Patent Application No. Hei 11-87208 (1999). Whereas this CPB-optical system exhibits reduced lower-order distortion, higher-order distortion is not corrected satisfactorily. Also, because this system performs correction of focus and distortion for each subfield as projected, this system is complicated. Furthermore, the subfield-by-subfield corrections require substantial time for performing the necessary measurements and calculations.
According to another conventional approach to reducing Coulomb effects, distortion that otherwise would be created by Coulomb effects is estimated or measured in advance and the reticle pattern is distorted deliberately so as to achieve, when the subfields are exposed, an offsetting distortion. However, as with the other approach summarized above, the Coulomb distortion has to be calculated or measured in advance, which adds complexity and processing time.
Another conventional approach that offers prospects of overcoming certain of the lingering problems involves shaping the charged particle beam to have a ring-shaped transverse profile (i.e., a “hollow beam”). This approach is disclosed in Japanese Kôkai Patent Application No. Hei 11-297610 (1999). For example, FIGS.
4
(
a
)-
4
(
b
) herein depict this approach for forming a hollow beam. An annular aperture
24
is provided by defining segmented voids
25
B into a plate
25
A made of molybdenum or tungsten. The segmented voids
25
B collectively define an essentially annular void. The axis of the annular void is coincident with the CPB axis (i.e., in the middle of the beam
21
). As the beam
21
strikes the plate
25
A, portions of the beam incident at a segmented void
25
B are transmitted through the plate
25
A; all other portions of the beam are blocked (scattered and/or absorbed) by the plate
25
A. The portions
22
of the beam passing through the segmented voids
25
B are used for illuminating the reticle (located downstream) and for transferring the pattern. The annular aperture
24
is disposed at a crossover plane A at which the beam is narrowly constricted.
A conventional CPB-optical system (specifically an electron-beam system) utilizing an annular aperture
24
as summarized above is shown in FIG.
5
. The
FIG. 5
system is essentially as disclosed in the JP 11-297610 reference cited above. An electron beam
21
emitted from an electron source
26
passes through an “illumination-optical system” comprising illumination lenses
27
,
28
, and strikes the annular aperture
24
. A field-limiting aperture
29
is used to shape the beam
21
so as to illuminate a desired shape (e.g., square) of subfield on the reticle
33
. The field-limiting aperture
29
is situated at an axial location that is conjugate with the electron-emission surface of the source
26
, with respect to the illumination-optical system. An image of a first crossover
30
(near the source
26
) is formed by the illumination lenses
27
,
28
on the annular aperture
24
. I.e., the annular aperture
24
is disposed at a crossover position. The hollow beam
22
passing through the annular aperture
24
passes through a third illumination lens
32
. The third illumination lens
32
forms an image of the electron-emission surface of the source
26
on the reticle
33
, thereby illuminating the reticle
33
. The field-limiting aperture
29
is situated at an axial position that is conjugate with the reticle
33
, with respect to the lens system comprised of the illumination lenses
28
,
32
. An image of the illuminated portion (subfield) of the reticle
33
is formed on the wafer
36
by projection lenses
34
,
35
collectively constituting an “projection-optical system.” A contrast aperture
37
is situated so as to block particles of the beam that are scattered by passage through the reticle
33
.
Coulomb effects are diminished using the
FIG. 5
system as a result of shaping the beam
21
into a hollow beam
22
before the hollow beam
22
irradiates the reticle
33
. However, manufacturing the annular aperture
24
for use in the
FIG. 5
system is problematic. Namely, the diameter of the crossover at which the annular aperture
24
is placed normally is several hundred &mgr;m. Whenever an annular aperture (defined by a plate several hundred &mgr;m thick) is placed at such a position, the temperature of the plate can reach several thousand degrees C during use. Hence, the material used to make the annular aperture
24
is limited to high-temperature metals such as molybdenum or tungsten. It is extremely difficult to fabricate the necessary voids, having dimensions in micrometers, in molybdenum or tungsten stock that is several hundred &mgr;m thick. Also, the high operating temperatures experienced in the vicinity of the annular aperture during use causes detrimental the
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
Souw Bernard E.
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