X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
2002-04-11
2004-12-21
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Lithography
Reexamination Certificate
active
06834098
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This application is a continuation of copending U.S. application Ser. No. 08/917,373, filed Aug. 26, 1997, which was allowed on Feb. 12, 2002.
This invention relates to an X-ray illumination optical system and an X-ray reduction exposure apparatus using the same and, in another aspect, it relates to a device manufacturing method using such an X-ray reduction exposure apparatus.
An X-ray reduction projection exposure process is used for the manufacture of microdevices such as semiconductor circuit devices, having fine patterns. In such a process, a mask having a circuit pattern formed thereon is illuminated with X-rays and an image of the pattern of the mask is projected, in a reduced scale, on the surface of a wafer. A resist on the wafer surface is sensitized, whereby the pattern is transferred and printed thereon.
FIG. 12
is a schematic view of a known example of an X-ray reduction projection exposure apparatus,
FIG. 13
is a schematic and perspective view of a reflection type integrator, and
FIG. 14
is a schematic view for explaining an illumination region upon the surface of a mask. In these drawings, denoted at
1001
is a light emission point for X-rays, and denoted at
1002
is an X-ray beam. Denoted at
1004
is a first rotational parabolic surface mirror, and denoted at
1005
is a reflection type integrator. Denoted at
1006
is a second rotational parabolic surface mirror, and denoted at
1007
is a mask. Denoted at
1008
is a projection optical system, and denoted at
1009
is a wafer. Denoted at
1010
is a mask stage, and denoted at
1011
is a wafer stage. Denoted at
1012
is an arcuate aperture, and denoted at
1013
is a laser light source. Denoted at
1014
is a laser collecting optical system, and denoted at
1015
is an illumination region defined on the surface of the mask. Denoted at
1016
is an arcuate region within which the exposure is performed.
An X-ray light source may comprise a laser plasma or an undulator. In the illumination optical system, X-rays from the light source are collected by means of the first rotational parabolic surface mirror
1004
, and the collected X-ray beam is projected on the reflection type integrator
1005
, whereby secondary light sources are formed. X-rays from these secondary light sources are collected by means of the second rotational parabolic surface mirror
1006
, to illuminate the mask
1007
.
The mask
1007
comprises a multilayered film reflection mirror on which non-reflective portions are defined by use of an X-ray absorptive material, for example, whereby a transfer pattern is formed thereon. X-rays reflected by the mask
1007
are imaged by the projection optical system
1008
upon the surface of the wafer
1009
. The projection optical system
1008
is designed so as to provide good imaging performance with respect to a narrow arcuate region off the optical axis. In order that the exposure is performed only by use of this narrow arcuate region, an aperture
1012
having an arcuate opening is disposed just before the wafer
1009
. For the pattern transfer to the whole surface of the mask having a rectangular shape, the exposure process is performed while scanningly moving the mask
1007
and the wafer
1009
simultaneously.
The reflection type integrator
1005
may comprise a fly's-eye mirror having a number of small spherical surfaces arrayed two-dimensionally, as is best seen in
FIG. 13
, which are adapted to define a number of secondary light sources.
Here, if the spatial extension of the secondary light source group is d, the angular extension of X-rays emitted from each secondary light source is &thgr;, and the focal length of the second parabolic surface mirror
1006
is f, then the size of the illumination region
1015
on the mask
1007
surface is fx&thgr; and the angular extension of X-rays illuminating a single point on the mask is d/f.
As a parameter which represents the characteristic of the illumination optical system, there is a coherence factor &sgr;. If the mask-side numerical aperture of the projection optical system
1008
is NAp1, and the mask-side numerical aperture of the illumination optical system is NAi, the coherence factor can be defined as follows:
&sgr;
NAi/NAp
1.
The optimum value of &sgr; is determined by the required resolution and contrast. Generally, if the factor &sgr; is too small, an interference pattern appears at the edge portion of an image of a fine pattern as projected on the wafer
1009
. If the factor &sgr; is too large, the contrast of the projected image reduces.
If &sgr; is zero, it is called coherent illumination. Regarding the transfer function OTF of the optical system, a constant value will be shown up to a particular spatial frequency as can be given by NAp2/&lgr; where NAp2 is the wafer-side numerical aperture of the projection optical system and &lgr; is the wavelength of the X-ray beam. For a higher frequency above the particular frequency, it becomes equal to zero, and resolving is not attainable.
If, on the other hand, &sgr; is equal to 1, it is called incoherent illumination. The transfer function OTF reduces with an increase in the spatial frequency, but it does not become equal to zero unless a particular spatial frequency given by 2×NAp2/&lgr; is reached. Thus, a more fine pattern can be resolved. In X-ray exposure, an optimum value of &sgr; may be selected in accordance with the shape or size of a pattern to be transferred, or the characteristic of a resist process to be adopted. Usually, a value such as &sgr;=0.1−1.0 may be set.
SUMMARY OF THE INVENTION
There is a problem to be solved, in conventional X-ray reduction projection exposure apparatuses. That is, as shown in
FIG. 14
, the illumination region
1015
on the mask surface has a rectangular shape or an elliptical shape, including an arcuate region
1016
through which the exposure is actually made. Thus, the region outside the exposure region is illuminated with many X-rays. These X-rays are not contributable to the exposure process, and they are wasteful. The loss of X-ray light quantity is large and it leads to prolongation of the exposure time. Thus, the throughput is low.
It is accordingly an object of the present invention to provide an X-ray reduction exposure apparatus and/or an X-ray illumination optical system therefor, by which the loss of light quantity can be very small, the exposure time can be shortened, and the throughput can be improved.
It is another object of the present invention to provide a device manufacturing method based on such an X-ray reduction exposure apparatus.
In accordance with an aspect of the present invention, there is provided an X-ray illumination optical system, comprising: a reflection type integrator having cylindrical surfaces, for reflecting an X-ray beam; and a concave mirror for reflecting the X-ray beam reflected by said integrator and for illuminating an object with the X-ray beam.
The system my further comprise a second concave mirror for projecting a parallel X-ray beam onto said integrator.
Said integrator may provide a secondary light source and said concave mirror may have a focal point which is disposed at the position of said secondary light source.
Said concave mirror may have a reflection surface of a rotational parabolic shape.
An axis of said cylindrical surface of said integrator and an axis of the X-ray beam impinging on said integrator may be placed on the same plane.
There may be an illumination region defined on the object, having an arcuate shape.
Said integrator may have a reflection surface on which a multilayered film is formed.
The illumination of the object may substantially satisfy the conditions for Koehler illumination.
The X-ray beam may comprise one of a beam emitted from a laser plasma X-ray source and a beam emitted from an undulator X-ray source.
In accordance with another aspect of the present invention, there is provided an X-ray reduction exposure apparatus, comprising: an X-ray illumination optical system as discussed above, for illuminatin
Miyake Akira
Tsukamoto Masami
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