Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2001-02-16
2003-05-20
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S3960ML
Reexamination Certificate
active
06566663
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projecton-transfer of a pattern, defined by a reticle or mask, onto a sensitive substrate using an energy beam). Microlithography is a key technique used in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, micromachines, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam, such as an electron beam or ion beam, as the energy beam. Even more specifically, the invention pertains to charged-particle-beam (CPB) microlithography apparatus and methods exhibiting reduced aberrations such as image displacement caused by temperature changes in the column containing the CPB optical system of such apparatus.
BACKGROUND OF THE INVENTION
Advancement of microelectronics technology has been accompanied by a relentless drive toward increased miniaturization and higher density of circuit integration. Higher integration and circuit density require correspondingly smaller linewidths of constituent circuit elements. Currently, the required linewidths of integrated circuits are so small that optical microlithography (microlithography performed using ultraviolet light) often cannot provide the necessary pattern resolution. This situation has led to investigations into alternative microlithography technologies offering prospects of substantially better resolution than obtainable with optical microlithography.
One alternative microlithography technology receiving considerable attention is charged-particle-beam (CPB) microlithography, which utilizes a charged particle beam (e.g., electron beam or ion beam) as a pattern-transfer energy beam. Several CPB microlithography approaches have been the subject of intensive research and development effort, and each approach has respective advantages and disadvantages. One approach offering prospects of reasonably good resolution and throughput is the so-called “divided reticle” projection-transfer approach.
Divided-reticle projection transfer involves dividing the pattern, as defined on a reticle, into multiple individual exposure units usually termed “subfields.” Each subfield is exposed individually by projection onto a respective region on the wafer. The subfield images are transferred to the wafer so that, after exposing all the subfields, the subfield images are “stitched” together in a contiguous manner to form the entire chip pattern. As each subfield is exposed, corrections can be made to achieve proper focus and reduction of aberrations (e.g., distortion) for the particular subfield. Divided-reticle projection transfer allows exposures to be made over an optically wide field with much better resolution and accuracy than obtainable by projection-exposing the entire reticle in one shot using a charged particle beam.
Certain aspects of divided-reticle projection transfer are shown in
FIGS. 9 and 10
.
FIG. 9
depicts a wafer showing the intended sites of multiple “chips” or “dies.” As exposed, each chip comprises multiple “stripes,” and each stripe comprises multiple subfields arranged in rows. This same divided arrangement of stripes and subfields is used to define the pattern on the reticle.
FIG. 10
depicts an actual exposure. For exposure, the reticle and wafer are mounted on respective stages (not shown but well understood in the art) configured to move the reticle and wafer horizontally (in the figure) as required for exposure. During exposure of a stripe (a portion of which is shown), the reticle stage and wafer stage both move along the longitudinal line of the respective stripes. Movements of the reticle and wafer are at constant respective velocities (but in opposite directions) in accordance with the demagnification ratio of the projection lens. Meanwhile, the charged particle beam incident on the reticle (the beam upstream of the reticle is termed the “illumination beam” and passes through an “illumination-optical system” to the reticle) illuminates the subfields on the reticle row-by-row and subfield-by-subfield within each row. The rows extend perpendicularly to the movement directions of the reticle and wafer. As each subfield is illuminated in this manner, the portion of the illumination beam passing through the respective subfield (now termed the “patterned beam” or “imaging beam”) passes through a projection-optical system (including the projection lens) to the wafer.
During exposure of a stripe, to expose the subfields within each row of the stripe in a sequential manner, the illumination beam is deflected at right angles to the movement direction of the reticle stage, and the patterned beam is deflected at right angles to the movement direction of the wafer stage. After completing exposure of each row, the illumination beam is deflected in the opposite direction, as shown in
FIG. 10
, to expose the subfields in the next row of the stripe. This exposure technique reduces extraneous deflections of the beam and improves throughput.
In a divided reticle, as noted above, each subfield defines a respective portion of the pattern. Usually, each subfield is surrounded by “struts” (relatively thick structural members that collectively strengthen and provide rigidity to the reticle) and by a “skirt” (a thin non-patterned zone adjacent to the struts and surrounding the patterned region of the subfield). The struts extend from non-patterned portions of the reticle located between the subfields. The skirt helps to provide illumination isolation for each subfield to avoid illumination of adjacent subfields whenever a particular selected subfield is being illuminated.
In a conventional CPB projection-microlithography apparatus, a portion located between the reticle
3
and substrate
4
is shown in FIG.
11
. Specifically shown are first and second projection lenses
1
,
2
, respectively, arranged on an optical axis
6
. Associated with the first projection lens
1
is a deflector
7
and a ferrite stack
9
, wherein the ferrite stack
9
is located in a radial space between the first projection lens
1
and the deflector
7
. Associated with the second projection lens
2
is a deflector
8
and ferrite stack
10
, wherein the ferrite stack
10
is located in a radial space between the second projection lens
2
and the deflector
8
. Also shown is a scattering aperture
5
centered on the axis
6
and located where the beam (“patterned beam”) forms a crossover. An exemplary beam trajectory
11
from the reticle
3
to the substrate
4
is shown.
Each ferrite stack
9
,
10
typically comprises alternating rings of non-magnetic ferrite and of high-permeability ferrite stacked atop one another coaxially with the axis
6
. Hence, the ferrite stacks
9
,
10
are each symmetrical to the same axis as the optical axis
6
. The inside diameter, outside diameter, thickness, etc., are determined so as to satisfy the desired parameters provided. In some configurations, the non-magnetic ferrite is not included in the ferrite stack, but rings of the non-magnetic ferrite generally are included with the rings of high-permeability ferrite to improve the positional accuracy of the high-permeability ferrite.
The reticle
3
is irradiated by an electron “illumination beam” passing through an “illumination-optical system” (not shown but understood to be located upstream of the reticle
3
). Hence, the portion shown in
FIG. 11
is the “projection-optical system.” A combination of an illumination-optical system and projection-optical system as used in a CPB projection-microlithography apparatus is referred to herein as a CPB optical system. As projected onto the substrate
4
, the image of the illuminated portion of the reticle
3
is “demagnified” by which is meant that the image as formed on the substrate
4
is smaller (usually by a factor that is the reciprocal of an integer) than the corresponding reticle portion illuminated by the illumination beam.
By “sensitive” is meant that the substrate
4
is coated with a suitable “resist” material that responds, to exposure by the patterned b
Kamijo Koichi
Kojima Shin-ichi
Nakano Katsushi
Okamoto Kazuya
Klarquist & Sparkman, LLP
Nguyen Kiet T.
Nikon Corporation
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