Photocopying – Projection printing and copying cameras – Focus or magnification control
Reexamination Certificate
1999-12-28
2001-08-21
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Focus or magnification control
C355S053000, C355S067000
Reexamination Certificate
active
06278514
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an exposure apparatus for projecting a pattern, formed on a reticle or a photomask, for example, onto a photosensitive substrate, for example, through a projection optical system.
Exposure apparatuses such as described above are used in a lithographic process for the manufacture of semiconductor devices, for example. More specifically, they are used in a procedure for transferring, through a projection lens, an image of a circuit pattern, for example, formed on a reticle or a photomask (hereinafter, simply a “mask”), onto a semiconductor wafer, for example, having a photosensitive material coating thereon.
In such exposure apparatuses, it is required that a pattern on a mask is accurately transferred onto a wafer with a predetermined magnification (reduction scale). In order to meet this requirement, it is important to use a projection lens of good imaging performance, with reduced aberration. Particularly, because of recent needs for further miniaturization of a semiconductor device, in many cases, a pattern beyond an ordinary imaging performance of an optical system has to be transferred (printed). As a result of it, a pattern to be transferred becomes more sensitive to aberration of an optical system.
On the other hand, it has been required for a projection lens to enlarge its exposure area and numerical aperture (NA). This makes the aberration correction more difficult to attain.
In these situations, it is strongly desired to measure the imaging performance of a projection lens, particularly, wavefront aberration, in a state that the projection lens is mounted in an exposure apparatus, that is, a state that it is actually used for exposure.
An example which may meet such a requirement is a phase restoration method. The phase restoration method has been used mainly for resolution improvement in an electron microscope or an astronomical telescope, for example, having large aberration. In this method, a phase distribution of an image is detected on the basis of image intensity distributions at plural positions such as an image plane, a pupil plane, and a defocus position, for example. From the phase distribution, wavefront aberration of the optical system can be calculated.
FIG. 5
illustrates an ordinary algorithm of the phase restoration method. Initially, by using an intensity distribution of light upon an image plane measured, an arbitrary phase is given. Thereafter, through Fourier transform, a complex amplitude distribution upon a pupil plane is detected. Then, while a phase portion of the thus given complex amplitude distribution is kept as it is, only an absolute value of an intensity portion is replaced by a value corresponding to an actually measured value (i.e., a square root of the intensity on the pupil plane). The resultant is taken as a complex amplitude distribution. The thus determined complex amplitude distribution is inversely Fourier transformed, whereby a complex amplitude distribution upon an image plane is determined. Again, while keeping its phase portion, the intensity is replaced by an actually measured value.
The above-described calculation is repeated, by which complex amplitude distributions on the image plane and the pupil plane are calculated. From the phase distribution of the complex amplitude distribution on the pupil plane, wavefront aberration of the lens can be obtained.
If measurement of an intensity distribution on a pupil plane is difficult to accomplish, as in the case of a photolithographic projection optical system, the transform and the inverse transform may be repeated between an image plane and a defocus plane and through a pupil plane such as shown in
FIG. 6
, to calculate complex amplitude distributions on the image plane and the defocused plane, respectively, and, from the results, the phase distribution on the pupil, that is, the wavefront aberration of the projection lens may be determined (see J.J.A.P. Vol. 36, 1997, pp. 7494-7498, or Japanese Laid-Open Patent Application, Laid-Open No. 284368/1998).
FIG. 7
illustrates an exposure apparatus having a mechanism for calculating the wavefront aberration of a projection lens
71
in accordance with the phase restoration method. In this apparatus, a pattern of a reticle
72
is illuminated with an illumination light flux IL, and an image thereof is imaged on a light intensity detecting system
78
, by which the intensity distribution of the light is measured. Subsequently, a stage
74
is moved in an optical axis AX direction by means of a stage driving system
75
, such that the pattern of the reticle
72
is placed defocused upon the light intensity detecting system
78
. Then, the intensity distribution in this state is measured.
By using the results of these two intensity distributions, Fourier transform and inverse transform are repeated in a data processing system
81
, by which wavefront aberration of the projection lens
71
is calculated.
the major factor which determines the precision in calculation of wavefront aberration based on the phase restoration method described above is how to accurately measure a light intensity distribution upon an image plane, a pupil plane or a defocus plane.
On the other hand, in a projection lens of a semiconductor exposure apparatus, the produced aberration is inherently very small. This raises a problem that, if the wavefront aberration of the projection lens is calculated on the basis of the phase restoration method, since a calculation error resulting from an error in the measurement of intensity distribution, for example, is relatively large as compared with the amount of wavefront aberration to be detected, accurate calculation of the wavefront aberration is not attainable as a consequence.
Further, as described above, due to increasing miniaturization of an exposure pattern, the influence of aberration of a projection lens becomes large. Therefore, the need for accurate detection of the wavefront aberration is increasing.
Moreover, in conventional exposure apparatuses, for measurement of a light intensity distribution, a light intensity detector such as at
78
in
FIG. 7
is provided on a stage
74
. This means that a heavy weight is mounted on the stage
74
which is driven at high speed. As a result, the stage design becomes more difficult, and the stage driving speed has to be decreased, causing reduction of throughput.
Particularly, when an enlargement optical system is used for more precise measurement of the intensity distribution, such as shown in
FIG. 8
, an enlargement optical system
80
has to be mounted on the stage
74
. This makes the weight problem more critical.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a unique and improved arrangement for an exposure apparatus by which at least one of the problems described above can be solved or reduced.
In accordance with a first aspect of the present invention, there is provided an exposure apparatus wherein a wavefront aberration of a projection optical system is detected on the basis of a light intensity distribution of an image of a mask, formed by light passed through the projection optical system plural times.
In accordance with a second aspect of the present invention, there is provided an exposure apparatus for forming an image of a circuit pattern on a photosensitive substrate, placed on a stage, through a projection optical system, wherein the stage includes a reflecting portion, and wherein a wavefront aberration of the projection optical system is detected on the basis of a light intensity distribution of an image of a mask, formed by light passed through the projection optical system, reflected by the reflecting portion, and then passed again through the projection optical system.
More specifically, light from an image of a mask being formed with light which has been emitted from a light source, then passed through the mask, then entered into a projection optical system, then emitted from the projection optical system, then reflected by a reflective member or a re
Adams Russell
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Hung Henry
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