Illumination – Light source and modifier – Plural serial lens elements or components
Reexamination Certificate
1999-04-27
2001-05-29
Cariaso, Alan (Department: 2875)
Illumination
Light source and modifier
Plural serial lens elements or components
C362S019000, C362S259000, C362S301000, C362S346000, C359S629000
Reexamination Certificate
active
06238063
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to an exposure apparatus for fabrication of semiconductor devices, and in particular pertains to an illumination optical apparatus that can be used in an exposure apparatus.
BACKGROUND OF THE INVENTION
Conventional prior art illumination optical apparatus include, for example, that disclosed in Japanese Laid-Open Patent Application (Kokai) No. H1[1989]-198759, shown in FIG.
21
. With reference to
FIG. 22
, a light beam b
1
from a laser light source
400
is incident a first triangular prism
401
serving as an optical delay element. Prism
401
has an incident surface
401
S. A portion of light beam b
1
incident triangular prism
401
at surface
401
S is reflected therefrom at a location L
1
without entering the interior thereof. The remaining portion of the light beam enters the interior of prism
401
travels a triangularly shaped delay optical path DOP
1
, and thereafter returns to location L
1
.
The portion of light beam b
1
that returns to point L
1
exits therefrom along the same optical path as the portion of light beam b
1
reflected from surface
401
S. The remaining portion of the beam within prism
401
once again passes through delay optical path DOP
1
, and returns to location L
1
.
In this way, light beam b
1
from laser light source
400
is temporally split into a plurality of light beams (theoretically, an infinite number of light beams) at optical delay element
401
. Any two temporally consecutive light beams will have an optical path length difference equal to the optical path length of delay optical path DOP
1
. Furthermore, the optical path length of delay optical path DOP
1
is set to be not less than the temporal coherence length of light beam b
1
from laser light source
400
.
In this manner, a light beam b
2
is formed from light beam b
1
Light beam b
2
is incident a second triangular prism
402
serving as a second optical delay element. Triangular prism
402
has a constitution similar to that of triangular prism
401
, the only fundamental difference being that the optical path length of the triangularly shaped delay optical path DOP
2
thereof is set to be twice DOP
1
. Accordingly, a light beam passing through first optical delay element
401
is thereafter temporally split into a plurality of light beams at second optical delay element
402
. Any two temporally consecutive light beams exiting from second optical delay element
402
are imparted with an optical path length difference that is twice the optical path length difference of first optical delay element
401
.
In this manner, a light beam b
3
is formed from light beam b
2
. The former is incident a fly-eye lens
403
, which forms a secondary light source image SL
1
comprising a multiplicity of light source images at the rear focus thereof. A light beam b
4
from secondary light source image SL
1
passes through a condenser lens
404
and illuminates, in superimposed fashion, a mask
405
set at a plane of illumination P
1
.
As described above, the conventional illumination optical apparatus shown in
FIG. 22
permits reduction of coherence even when a coherent light source is employed. This is achieved by creating a series of light beams successively generated using a first optical delay element
401
and a second optical delay element
402
with optical path length differences not less than the coherence length.
Considering the peak width at half height of laser light source
400
to be D
1
and wavelength to be &lgr;, the temporal coherence length tc is in general given by:
tc=&lgr;
2
/D
1
.
If &lgr;=248 nm and D
1
=0.8 pm, then tc=77 mm; if &lgr;=248 nm and D
1
=0.6 pm, then tc=103 mm.
In principle, the light within an optical delay element could make an infinite number of passes over the delay optical path before exiting the optical delay element, based on half mirror reflectance, reflecting member reflectance, and so forth. However, half mirror reflectance is typically set at on the order of between 33% and 50%. Setting the reflectance at such a value allows the light to exit the optical delay element after between roughly two or three passes, assuming down to on the order of 1% of the light energy is to be used.
As described above, the value employed for coherence length has conventionally been a function of the entire spectral distribution of the light source. However, upon using an excimer laser and attempting to obtain uniform illumination therefrom, it has been found that the expected uniform illumination cannot be obtained despite the use of coherence reduction means such as described above.
Furthermore, with a conventional illumination optical apparatus such as described above, the delay optical path of the optical delay element is triangular in shape. Thus, the light beam incident the optical delay element can become offset in parallel fashion from the reference optical axis due to the influence of, for example, vibration of the apparatus or the like. In this case, the light beam entering the interior of the optical delay element will no longer return to the location at which it was originally incident. As a result, the optical path of the light beam reflected from the surface of the optical delay element and will no longer be coincident with the optical path of the light beam that enters the interior of the optical delay element, makes just one pass through the delay optical path thereof, and exits therefrom. Consequently, the optical paths of the series of light beams successively generated by way of the two optical delay elements will no longer mutually coincide. Instead, they will move progressively farther away from the reference optical axis. Thus, with the conventional illumination optical apparatus as described above, instabilities arise with respect to vibration. Further, it is difficult to carry out optical adjustments on the apparatus.
SUMMARY OF THE INVENTION
The present invention pertains to an exposure apparatus for fabrication of semiconductor devices, and in particular pertains to an illumination optical apparatus that can be used in an exposure apparatus.
A goal of the present invention is to provide an illumination optical apparatus that adequately permits adequate reduction of coherence even when an excimer laser light source is used, and an exposure apparatus employing such an illumination optical apparatus. A further goal of the present invention is to provide an illumination optical apparatus that is highly stable with respect to vibration and that permits easy optical adjustment, and an exposure apparatus equipped with such an illumination optical apparatus.
Accordingly, a first aspect of the invention is an illumination optical apparatus comprising a coherent light source capable of providing a first light beam with a coherence length along a first optical path. The apparatus further includes a first optical delay element capable of splitting from the first light beam a second light beam having a first delay optical path. The first path length is not less than the coherence length of the first light beam. Also, the first delay optical path returns to the first optical path at a first deflection angle.
A second aspect of the invention is the illumination optical apparatus as described above, further including (m−1) additional optical delay elements, for a total of m optical delay elements, wherein m is an integer equal to 2 or greater. The apparatus further includes a depolarizer arranged between the light source and a most light-source-wise optical delay element. The depolarizer has an S-polarized delay length L associated with S-polarized light. Also, the first optical delay element has a delay optical path of 2n×L, where n is an integer, the second optical delay element has a delay optical path of 6n×L, and the mth optical delay element has a delay optical path of 2×m
2
×L.
A third aspect of the invention is an exposure apparatus for exposing a pattern present a mask onto a substrate. The apparat
Kanayamaya Nobumichi
Shibuya Masato
Tanitsu Osamu
Cariaso Alan
Nikon Corporation
Oliff & Berridg,e PLC
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