Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-02-04
2002-08-06
Anderson, Bruce (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S397000, C250S398000
Reexamination Certificate
active
06429441
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to microlithography apparatus and methods using a charged particle beam. With such apparatus and methods, a pattern as defined on a mask or reticle is transferred to a sensitive substrate (e.g., semiconductor wafer) using a charged particle beam (e.g., electron beam or ion beam). The invention more specifically relates to such apparatus and methods in which the velocity of the charged particle beam incident to the sensitive substrate can be made different from the velocity of the charged particle beam incident to the reticle.
BACKGROUND OF THE INVENTION
In an ongoing effort to develop practical microlithography apparatus that can achieve better resolution than optical microlithography, microlithography using a charged particle beam (e.g., electron beam) has received much attention. However, a practical charged-particle-beam (CPB) microlithography system has not yet been realized due to many technical problems such as satisfactory aberration control and acceptable throughput (number of semiconductor wafers that can be processed microlithographically per unit time).
In a typical CPB microlithography system as exemplified by an electron-beam system, an “illumination beam” is produced by an electron gun and passed through multiple condenser lenses (constituting an “illumination-optical system”) to illuminate a region on a reticle. The portion of the illumination beam passing through the reticle becomes a “patterned beam” that passes through multiple projection lenses (constituting a “projection-optical system”) to form an image, on the wafer, of the illuminated region of the reticle.
Japanese Kokai (laid-open) patent document no. Hei 8-124834 discloses electron-beam microlithography apparatus in which a decelerating electric field is established between the reticle and the wafer (i.e., within the projection-optical system) to reduce the velocity of the charged particle beam incident to the wafer relative to the beam velocity incident to the reticle. Maintaining a relatively high beam velocity at the reticle reportedly yields better contrast and electron-beam transmission through the reticle, even when using a scattering contrast reticle. A relatively high beam velocity at the reticle also reportedly reduces electron absorption by the reticle (which reduces reticle heating due to electron absorption by the reticle) and reduces chromatic aberration. A relatively low beam velocity incident to the wafer reportedly helps prevent loss of resist sensitivity and reduces heating of the resist and of the wafer.
However, apparatus and methods according to JP 8-124834 pose the following problems:
(a) The beam-decelerating electric field is produced by imposing a high voltage to a “liner tube” in a lens of the projection-optical system. Application of such a high voltage to the liner tube produces a localized beam-decelerating electric field that produces a corresponding localized electrostatic lens action between the wafer and the liner tube (or between the wafer and a shield of the projection-optical system, as shown in
FIG. 2
of that reference). The lens action generates a new aberration for which no corrective action is disclosed or contemplated by the reference.
(b) Application of a high voltage to a liner tube of a lens causes other problems leading to aberrations and beam blur. This reference provides no information on how to solve such problems or how to correct for aberrations arising from irregularities in the surface planarity of the wafer.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art summarized above, the present invention was devised to achieve one or more of the following:
(1) In a simple manner, control parasitic aberrations arising from passing the charged particle beam through a beam-decelerating electric field.
(2) Provide the charged particle beam with high energy at the reticle using a relatively low-voltage power supply.
(3) Exploit a change in the beam half-angle of the charged particle beam within a projection lens and at the surface of the wafer that arises from subjecting the beam to a beam-decelerating electric field. For a particular beam half-angle, a relatively small beam half-angle in the lens yields reduced geometric aberrations and chromatic aberrations. (The reduction in chromatic aberration is especially pronounced.)
(4) Avoid problems with lenses of the projection-optical system that arise due to application of high voltage to the respective liner tubes.
(5) Achieve improved beam adjustment and registration.
(6) Avoid deleterious effects of a non-planar surface around the reticle and/or wafer.
To achieve the ends listed above, and according to a first aspect of the invention, charged-particle-beam (CPB) microlithography apparatus are provided. In such apparatus according to the invention, an illumination-optical system is configured and situated to illuminate an “illumination beam” onto a desired region on a reticle defining a pattern to be transferred to a sensitive substrate (“wafer”). A projection-optical system is configured and situated to project, onto a desired corresponding region on the wafer, a “patterned beam” created by passage of the illumination beam through the desired region on the reticle. A first beam-decelerating electric field is established between the reticle and the projection-optical system, and a second beam-decelerating electric field is established between the projection-optical system and the wafer. Aberrations arising from a convex lens action of the first beam-decelerating electric field and aberrations arising from a concave lens action of the second beam-decelerating electric field at least partially cancel each other. Thus, aberrations parasitic to the beam-decelerating electric fields are controlled relatively simply. (The lens actions of the first and second beam-decelerating electric fields can be opposite to what is described above.)
In a first representative embodiment of an apparatus according to the invention, the projection-optical system comprises a lens having a magnetic pole that is rotationally symmetric about an optical axis and an excitation coil. A liner tube is situated inside the inside diameter (ID) of the magnetic pole and its excitation coil. A first voltage (electrical potential) difference is imposed between the liner tube and the reticle, and a second potential difference is imposed between the liner tube and the wafer. These potential differences are utilized for forming the respective beam-decelerating electric fields.
In a second representative embodiment, an electron beam is used as the charged particle beam. The projection-optical system comprises a lens having a magnetic pole that is rotationally symmetric about the optical axis and an excitation coil. A first liner tube is situated inside the ID of the magnetic pole and its excitation coil. Similarly, the illumination-optical system comprises a lens having a magnetic pole that is rotationally symmetric about the optical axis and an excitation coil. A second liner tube is situated inside the ID of the magnetic pole and its excitation coil. A high negative voltage (V
k
) is applied to a cathode of an electron gun (serving as the source of the electron beam), and a high positive voltage (V
i
) is applied to the first liner tube (i.e., liner tube of the lens in the illumination-optical system). A high positive voltage (V
m
), wherein V
m
≧V
i
, is applied to the reticle; a positive voltage (V
p
), wherein V
p
<V
i
, is applied to the second liner tube (i.e., liner tube of the lens in the projection-optical system). The wafer is either electrically grounded, or a voltage (V
w
) is applied thereto, wherein V
w<V
p
.
In the second representative embodiment, not only can a high acceleration voltage (|V
k
|+V
m
) be established between the cathode of the electron gun and the reticle, but also the respective absolute values of V
k
and V
m
can be kept low. Also, a first beam-decelerating electric field can be realized between the reticle and the sec
Anderson Bruce
Klarquist & Sparkman, LLP
Nikon Corporation
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