Optics: measuring and testing – Lamp beam direction or pattern
Reexamination Certificate
2000-09-26
2004-02-17
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Lamp beam direction or pattern
C250S201900, C356S124000
Reexamination Certificate
active
06693704
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for measuring a wave front aberration, a method for measuring a wave front aberration using the apparatus, and a method for manufacturing a projection lens by using thereof.
2. Description of Related Art
Recently, a projection lens of a projection exposure apparatus has been required to have higher resolving power in order to cope with the recent improvement of the density and the minuteness of a semiconductor device.
An optical performance of a projection lens has been evaluated by a wave front aberration because the wave front aberration is the sole measure capable of foreseeing resolution of any pattern.
There are several methods for measuring a wave front aberration. Among them, a method that measures a wave front aberration by a phase retrieve method based on point-spread functions of an object at a focused position and a defocused position and known information has been proposed in following references: J. Maeda at al. “
Retrieval of wave aberration from point
-
spread function or optical transfer date
” Applied Optics vol. 20, p274-279, D. L. Misell “
An examination of an iterative method for solution of the phase problem in optics and electron optics
” Test calculations Journal of physics D: Applied Physics vol. 6, p2200-2216.
However, when an apparatus for measuring a wave front aberration is realized based on the theory, since it is impossible to directly detect the point-spread function by a CCD on account of limited number of pixels of the CCD, it is inevitable for the apparatus f or measuring a wave front aberration to have an magnifying projection optical system for magnifying the point-spread function.
In this case, it is necessary to eliminate aberrations of the magnifying projection optical system. Thus the aberrations of the magnifying projection optical system have to be measured. This is fit for calibration. In the phase retrieve method from a point image, calibration method for the apparatus has not been found out.
Moreover, deformation of a point image may be caused not only by a wave front aberration of a test lens but also by an unnecessary element caused by a construction of the point image. When the deformation of a point image includes other elements than the wave front aberration of the test lens, the wave front aberration of the test lens cannot be measured correctly.
FIG. 10
is a drawing showing a construction of an ordinary point source
71
. The point source
71
is constructed by a pinhole
74
arranged on a shield film
73
formed on a glass substrate
72
, a lamp
75
for illuminating the pin hole
74
with a parallel light L
11
from the shield film
73
side, and an illumination lens
76
.
In the point source
71
, a light L
12
passed through the pinhole
74
is exit as a measuring light L
13
after passing through the glass substrate
72
. Then, the light L
13
is converged by a test lens (not shown), and forms a deformed point image (an image of the pinhole
74
).
In this case, a light L
12
just passed through the pin hole
74
is approximately a spherical wave. However, a light incident to the test lens (not shown) is the measuring light L
13
affected with aberrations (spherical aberration and coma) caused by the glass substrate
72
while passing through the glass substrate
72
. Accordingly, the point image obtained by converging the measuring light L
13
inevitably includes deformation in accordance with aberrations caused by the glass substrate
72
.
Further, the parallel light L
11
illuminating the pinhole
74
has an angular divergence &thgr; determined by the focal length of the illumination lens
76
and the aperture diameter d of the lamp
75
. Since the angular divergence &thgr; appears directly on an angular divergence &psgr; of the measuring light L
13
after passing through the glass substrate
72
, the point image contains deformation in accordance with the angular divergence &psgr; (which is equal to the angular dispersion &thgr; of the parallel light L
11
) of the measuring light L
13
.
Furthermore, when a flare light L
14
is incident to the pinhole
74
, since an unnecessary light caused by the flare light
14
is mixed with the measuring light L
13
, deformation caused by the flare light
14
is added to the point image.
Thus when the wave front aberration of the test lens is measured by using the point source
71
having the construction shown in
FIG. 10
, deformation of the point image includes influence of unnecessary elements (aberrations in the glass substrate
72
, an angular dispersion &psgr; of the measuring light L
13
, a flare light L
14
), so that the wave front aberration of the test lens cannot be measured precisely.
Moreover, although influence of aberrations of the glass substrate
72
can be corrected after the calculation using the phase retrieve method, it has not been easy because the thickness and the inclination of the glass substrate
72
have to be measured precisely.
Further, although the influence of the angular dispersion &psgr; of the measuring light L
13
can be relieved by making the angular dispersion &thgr; of the parallel light L
11
smaller by using smaller aperture diameter d, it is not desirable that the light quantities of the parallel light L
11
as well as the measuring light L
13
decrease in correspondence with decrease in the aperture diameter d.
SUMMARY OF THE INVENTION
The present invention is made in view of the above-mentioned problems and has an object to provide a method and apparatus for measuring a wave front aberration of a projection lens with high precision and a calibration method of the apparatus for measuring a wave front aberration.
According to an aspect of the present invention, an apparatus for measuring a wave front aberration includes: either a light source and an element that produces a first point source in combination with the light source and that is removably or movably arranged or a first point source generating part; a holding mechanism that holds a test object; an magnifying projection optical system that projects and enlarges a point image of the first point source projected by the test object; a detector that detects the magnified point image projected and magnified by the magnifying projection optical system; a supporting member that supports the magnifying projection optical system and the detector and that can be moved along the optical axis and in a plane perpendicular to the optical axis; a calculating part that calculates a wave front aberration by means of a phase retrieve algorithm based on a point-spread function detected by the detector and known information input in advance; and either a second point source producing element that is removably or movably arranged and that produces a second point source on the image plane of the test object by means of the test object in combination with any one of the light source, the light source and the element, and the first point source generating part, or a second point source generating part that produces the second point source on the image plane of the test object and that is removably or movably arranged.
The point image of the first point source is projected by the test object in the image plane of the test object.
The image is deformed by the aberration of the test object relative to an ideal point image.
A point image which is further projected and magnified image by the magnifying projection optical system is further deformed by an aberration of the magnifying projection optical system.
Accordingly, the point image formed on the CCD includes the aberration of the test object superimposed by the aberration of the magnifying projection optical system.
Therefore, in order to measure the wave front aberration of only the test object, it is necessary that the aberration of only the magnifying projection optical system is measured first, and, then, the aberration of the test object and that of the magnifying projection optical system is subtracted by that of only the magnifying projection opti
Noda Tomoya
Ooki Hiroshi
Nikon Corporation
Oliff & Berridg,e PLC
Pham Hoa Q.
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