Photocopying – Projection printing and copying cameras – Focus or magnification control
Reexamination Certificate
2002-09-12
2004-07-13
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Focus or magnification control
C356S624000
Reexamination Certificate
active
06762825
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a focal point position detecting method and detecting system suitably usable in a reduction projection exposure apparatus for the manufacture of semiconductor devices, for measuring a focal point position of a projection optical system.
In semiconductor manufacturing exposure apparatuses, the focal point position of a projection optical system is detected generally as follows. In
FIG. 6
, denoted at RS is a reticle stage, and denoted at R is a reticle. Denoted at RM is a reticle mark, and denoted at W is a wafer, which is a substrate to be exposed. Denoted at
1
is a projection optical system. Also, denoted at S is a focal point position detecting optical system, and denoted at
2
is focal point position detecting illuminating means. Denoted at
3
is a beam splitter, and denoted at
4
and
5
are imaging optical systems. Denoted at
6
is image pickup means, and denoted at
7
is analog-to-digital (A/D) converting means. Denoted at
8
is integration means, and denoted at
9
is contrast calculating means. Denoted at
10
is stage driving means, and denoted at
11
is a stage which is movable three-dimensionally. Denoted at
12
and
13
are imaging optical systems, and denoted at
14
is Z-axis position measuring illuminating means. Denoted at
15
is image pickup means, and denoted at
16
is analog-to-digital (A/D) converting means. Denoted at
17
is integration means, and denoted at
18
is Z-axis position measuring means. The components from the imaging optical system to the Z-axis position measuring means
18
, inclusive, are used for Z-axis position measurement.
The process for detecting the focal point position is as follows.
First, the stage is moved to a position where a focal point position detecting mark provided on the stage can be observed. Initially, by use of a light beam emitted from the focal point position detecting illuminating means
2
, which projects exposure light, the focal point position detecting mark (hereinafter, reference mark SM) is illuminated through the beam splitter
3
, the reticle R and the projection optical system
1
.
FIG. 2A
shows an image of the reference mark SM, and, in this example, it comprises plural patterns of the same shape. Light reflected by the reference mark SM goes through the projection optical system
1
and the reticle R once again, and it impinges on the beam splitter
3
and is reflected thereby. Then, through the imaging optical system
5
, an image of the reference mark SM is formed on an image pickup surface Wp of the image pickup means
6
. The image of the reference mark SM is then photoelectrically converted by the image pickup means
6
. Thereafter, the A/D converting means
7
processes its output into a two-dimensional digital signal train. Denoted
8
in the drawing is integration means which performs integration processing with respect to the Y direction in FIG.
2
A and functions to transform a two-dimensional signal into a one-dimensional digital signal train S(x). By using the thus transformed digital signal train, the contrast calculating means
9
measures the contrast within the range of a reference mark measuring window WSM at the reference mark portion. The operation described above is carried out plural times, while changing the position of the stage within a Z-axis range being expected to include the focus of the projection optical system
1
. Then, a graph is plotted, as shown in
FIG. 4A
, while taking the axis of the abscissa for the Z-axis position and the axis of the ordinate for the reference mark contrast level. Around a maximum contrast level portion, the Z-axis position which provides the largest contrast level is detected by means of curve fitting or gravity center calculation, for example. The thus detected position is taken as the focal point position (best focus) of the projection optical system.
The focal point position detecting method described above is particularly effective in an apparatus wherein precise detection of focal point position is required.
However, when images of the reference mark are picked up at different stage positions, the intensity of illumination light may not be constant as shown in FIG.
3
A. With the elapse of time within the image storage time, the intensity might change slightly or, alternatively, it might fluctuate (FIG.
3
B). If this occurs, the contrast is influenced by the variation in light intensity, such that the graph of Z-axis position versus contrast level cannot be produced correctly. As a result, the detected focal point position contains an error.
In order to avoid such inconveniences, there may be a method for improving the stability of the light source and a method in which a system for monitoring the light quantity is incorporated into the illumination system. However, in order to increase the light source stability, the precision of the illumination system has to be improved, and it makes the system very complicated and expensive. On the other hand, the method for monitoring the light quantity makes the measuring system very complicated, in the point that it requires a light quantity monitor in addition to a mark image-taking camera and that the measurements should be done at the same timing.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a focal point position detecting method and a focal point position detecting system by which, even if the intensity of illumination light changes when a reference mark is to be image-taken at different positions of a stage, the influence upon the detection of focal point position is reduced and also the measurement can be done simply without complications.
In accordance with an aspect of the present invention, there is provided a method of detecting a focal point position, comprising the steps of: moving an object having a mark, in an optical axis direction of a projection optical system; illuminating, with respect to different movement positions of the object, the mark of the object through the projection optical system and taking an image of the mark to produce imagewise information; correcting the imagewise information in accordance with an intensity of light illuminating the mark; and detecting a focal point position of the projection optical system on the basis of the imagewise information corrected with respect to each of the different movement positions of the object.
In one preferred form of this aspect of the present invention, the object is provided with plural marks each being as aforesaid.
The light intensity may be measured by use of light quantity measuring means.
The light intensity may be based on light reflected by a mark separate from the mark of the object.
The light quantity measuring means may be provided on a stage for carrying the object thereon.
The imagewise information may contain a contrast level of the mark image.
The contrast level of the mark image may be corrected in accordance with [contrast level]/[light intensity].
The focal point position detecting process may be repeated again when the light intensity at a movement position of the object is out of a predetermined range.
At an initial movement position of the object, the image-taking for the mark image and the measurement of the light intensity may be carried out plural times, and, after dispersion or variance of the light intensity is converged to a predetermined range, the focal point position detection may be initiated.
In accordance with another aspect of the present invention, there is provided a focal point position detecting system, comprising: moving means for moving an object having a mark, in an optical axis direction of a projection optical system; producing means for illuminating, with respect to different movement positions of the object, the mark of the object through the projection optical system and taking an image of the mark to produce imagewise information; correcting means for correcting the imagewise information in accordance with an intensity of light illuminating
Hayashi Nozomu
Tanaka Hiroshi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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