Optical: systems and elements – Single channel simultaneously to or from plural channels
Reexamination Certificate
1999-07-01
2001-06-19
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
C359S649000
Reexamination Certificate
active
06249382
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an illumination optical system and a projection exposure apparatus using same, and more particularly relates to an illumination optical system used in the manufacture of semiconductor integrated devices and the like, and a projection exposure apparatus that uses same.
BACKGROUND OF THE INVENTION
Conventionally, a projection exposure apparatus that projects a mask pattern onto a wafer is constructed so that the optical axis of the illumination optical system and the optical axis of the projection optical system coincide. Initially, unevenness in the transmittance of the projection optical system and inclinations of the principal rays at the mask surface arise rotationally symmetric to the optical axis, at least from the standpoint of design. In addition, the illumination intensity distribution of the illumination optical system and the inclinations of the principal rays are also rotationally symmetric with respect to the optical axis. Accordingly, to achieve a high degree of exposure uniformity and spatial coherence uniformity, the optical axes of the projection optical system and the illumination optical system are generally made to coincide.
Projection optical systems that use a concave mirror have been under development in recent years for use in projection exposure apparatuses. Such systems allow for a reduced number of lenses. With reference to
FIG. 1
, there is shown a prior art projection optical system
10
comprising, in order along a folded optical axis AZ, a mask M having a pattern thereon (not shown), three lenses
14
,
16
,
18
, a concave mirror
20
, a first fold mirror MI, a fourth lens
24
, a second fold mirror M
2
, an aperture stop AS, a fifth lens
26
, and a wafer W.
A light beam L from an illumination optical system (not shown) is incident mask M. Light beam L emerging from mask M is transmitted through lenses
14
,
16
,
18
and is incident concave mirror
20
. Light beam L reflected by concave mirror
20
is transmitted again through lens
18
and is incident mirror Ml. Light beam L reflected by mirror MI is transmitted through lens
24
and is incident mirror M
2
. Light beam L reflected by mirror M
2
is transmitted through aperture stop AS and lens
26
, and projects an image of the mask pattern onto wafer W. Light beam L incident concave mirror
20
and the light beam reflected therefrom must be separated in a projection optical system that uses a concave mirror. Accordingly, optical axis AZ cannot constitute part of the exposure region. With reference to
FIG. 2
, exposure region ER is shifted from optical axis AZ of projection optical system
10
, and is only a portion of the entire possible exposure region
32
.
In such prior art projection exposure apparatus having a projection optical system that uses a concave mirror, such as system
10
, a comparatively high exposure uniformity and spatial coherence uniformity are easy to achieve from the viewpoint of design if the optical axes of the projection optical system and the illumination optical system are made to coincide. With reference now to
FIG. 3
, since the possible illumination region
36
of the illumination optical system is centered on optical axis AZ, illumination efficiency is extremely low. Thus, the entire possible illumination region
36
is extremely large compared with illumination region IR.
To increase illumination efficiency, the optical axis of the illumination optical system should be made to coincide with the center of illumination region IR. In other words, the optical axis AZ of projection optical system
10
and the optical axis of the illumination optical system should be shifted. However, as discussed earlier, this is difficult from the viewpoint of design.
The necessity to make the optical axes of the illumination optical system and the projection optical system coincide is now explained in greater detail.
With reference now to FIG.
4
and projection optical system
40
with principal rays B
1
, spherical aberration exists at the entrance pupil EP. Consequently, even if principal rays B
1
are extended at mask M, they do not meet at one point, as can be seen by the broken lines in the figure. This type of spherical aberration is called pupil spherical aberration.
With reference now to
FIG. 5
, if projection optical system
40
is made telecentric on the mask M side, all of principal rays B I are not parallel to optical axis AZ due to the effect of pupil spherical aberration. Moreover, the degree of inclination from parallelism with optical axis AZ differs depending on the image height h.
With reference now to
FIG. 6
, projection optical system
40
used in combination with an illumination optical system
44
with an optical axis AI, an entrance pupil EP′ and a condenser lens
46
to form a projection exposure apparatus. Because a high degree of spatial coherence uniformity is required in a projection exposure apparatus, exit pupil EP′ must be such that for all image heights, h′ an image of entrance pupil EP′ is formed at the center of entrance pupil EP of projection optical system
40
. Therefore, the exit pupils EP and EP′ must be conjugate and also aberration free (e.g., no pupil spherical aberration). To satisfy these conditions, the principal ray at each image height h′ of illumination optical system
44
viewed from mask M must be inclined in relation to the inclination of the principal ray of projection optical system
40
.
Generally, since an optical system is designed to be rotationally symmetric about its optical axis, the inclinations of the principal rays, discussed earlier, are also set rotationally symmetric about the optical axis. Accordingly, if optical axis Al of illumination optical system
44
and optical axis AZ of projection optical system
40
do not coincide (
FIG.6
) then it becomes difficult, from the viewpoint of design, to make the inclination of the principal ray at each image height coincide.
If pupil spherical aberration can be corrected in the design, then optical axes AI and AZ do not necessarily need to coincide. However, it is required that projection optical system
40
be strictly conjugate with respect to mask M and wafer W, namely that it be aberration free. In addition, the correction of pupil spherical exacerbates difficulties in the design of the projection optical system. Furthermore, the manufacture of such a projection optical system is extremely difficult. As it is, the cost of the projection optical system is already high within the total cost of the projection exposure apparatus, and the above-described design issues unfortunately further increase the cost.
SUMMARY OF THE INVENTION
The present invention relates to an illumination optical system and a projection exposure apparatus using same, and more particularly relates to an illumination optical system used in the manufacture of semiconductor integrated devices and the like, and a projection exposure apparatus that uses same.
The present invention has the objective to provide an illumination optical system with high illumination efficiency and uniform exposure and spatial coherence, even for a projection optical system wherein the effective exposure range is shifted from the optical axis, and a projection exposure apparatus that uses same.
Accordingly, a first aspect of the invention is an illumination optical system for illuminating a surface. The apparatus comprises a predetermined conjugate plane optically conjugate with respect to the surface to be illuminated, a first optical system arranged between the light source and the conjugate plane and having a first optical axis, and a second optical system arranged between the conjugate plane and the surface to be illuminated and having a second optical axis. The second optical axis is shifted with respect to the first optical axis, at the conjugate plane.
A second aspect of the invention is an illumination optical system as described above, and further includes an optical integrator arranged within the first optical system an
Epps Georgia
Nikon Corporation
Oliff & Berridg,e PLC
Seyrafi Saeed
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