Optics: measuring and testing – By alignment in lateral direction
Reexamination Certificate
1999-02-25
2001-02-20
Kim, Robert (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
C356S401000
Reexamination Certificate
active
06191858
ABSTRACT:
This application is based on Japanese patent application HEI 10-45508 filed on Feb. 26, 1998, the whole contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to a position detecting apparatus and a position detecting method for position alignment. More particularly, the invention relates to a position detecting apparatus using an observation optical system having an oblique optical axis relative to the surface of an object, and a position detecting method using such a position detecting apparatus.
b) Description of the Related Art
A vertical detection method is known as a method of detecting the positions of marks on a wafer and a mask by using an aligner having a lens system combined with an image processing system. The vertical detection method observes position detecting marks along a direction perpendicular to the mask surface.
A chromatic bifocal method is known as a focussing method used by the vertical detection method. The chromatic bifocal method observes a wafer mark formed on a wafer and a mask mark formed on a mask by using light of different wavelengths and chromatic aberrations of the lens system, and focuses the images of the marks on the same flat plane. A wafer mark and a mask mark are hereinafter collectively called an alignment mark. An absolute precision of position detection by the chromatic bifocal method can be made high because the optical resolution of the lens system can be set high in principle.
However, since an alignment mark is observed vertically, a part of the optical system enters the exposure area. Since the optical system shields exposure light, it is necessary to retract the optical system from the exposure area when exposure light is applied. A time required for retracting the optical system lowers throughput. The alignment mark cannot be observed during the exposure, which is one of the reasons of lowering an alignment precision during the exposure.
An oblique detection method capable of solving the above-described problem associated with the vertical detection method is disclosed in Japanese Patent Laid-open Publication HEI 9-27449.
FIG. 8
is a schematic perspective view of a position detecting apparatus for the oblique detection method disclosed in the Publication HEI 9-27449. This position detecting apparatus is constituted of a wafer/mask holder unit
110
and an optical system
120
.
The wafer/mask holder unit
110
is constituted of a wafer holder
115
and a mask holder
116
. When position alignment is performed, a wafer
111
is held on an upper surface of the wafer holder
115
and a mask
112
is held on a lower surface of the mask holder
116
. The wafer
111
and mask
112
are disposed facing each other with a predetermined gap being set between the upper surface (exposure surface) of the wafer and the lower surface (mask surface) of the mask. Wafer marks for position detection are formed on the exposure surface of the wafer
111
, and a mask mark for position detection is formed on the mask surface of the mask
112
.
The wafer mark
113
and mask mark
114
have edges from which incidence light is scattered. When light is incident upon these marks, light incident upon the marks are scattered whereas light incident upon another area is regularly reflected.
The optical system
120
is constituted of an image detector
121
, a lens
122
, a half mirror
123
, and a light source
124
.
The optical system
120
is disposed in such a manner that the optical axis
125
thereof is oblique relative to the exposure surface of the wafer
111
. Illumination light radiated from the light source
124
is reflected by the half mirror
123
in a direction of the optical axis
125
, passes through the lens
122
, and becomes obliquely incident upon the exposure surface. The light source
124
is positioned at the focal point on the image side so that illumination light radiated from the light source
124
is collimated and becomes parallel light fluxes. The intensity of illumination light of the light source
124
is made adjustable.
Of light fluxes scattered at the edges of the wafer marks
113
and mask mark
114
, the light fluxes incident upon the entrance pupil of the lens
122
is converged by the lens
122
and focussed on a light reception surface of the image detector
121
. Since the optical axes of the illumination optical system and the observation optical system are disposed obliquely, it is not necessary to dispose each optical system just above an exposure area of the exposure surface. Therefore, exposure can be performed without retracting the exposure system from above the exposure area. It is also possible to observe alignment marks during exposure.
In the position detecting apparatus shown in
FIG. 8
, illumination light is applied to the alignment marks
113
and
114
after being reflected by the half mirror
123
and penetrating through the lens
122
. The intensity of the illumination light decreases by about a half when the half mirror
123
reflects it. Although most of the illumination light transmits through the lens
122
, a fraction of the illumination light is reflected by the surface of the lens
122
. This reflected light produces flare so that the background level of light incident upon the light reception surface of the image detector
121
is raised. A contrast is therefore lowered between the background and an image formed by light scattered from the alignment marks.
Another problem is that the intensity of light propagating toward the image detector
121
is lowered by about a half by the half mirror
121
, because light scattered by the alignment marks reaches the image detector
121
after passing through the half mirror
123
. The intensity of light scattered and reached the image detector
121
may be raised by making the intensity of illumination light high. With this method, however, flare of illumination light increases at the same time. Therefore, this method is not effective for increasing the contrast between the background and an image formed by light scattered and reached the image detector
121
.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a position detecting apparatus capable of increasing the contrast between a background and an image formed by light scattered from alignment marks and making it easy to detect positions.
It is an object of the present invention to provide a position detecting method capable of increasing the contrast between a background and an image formed by light scattered from alignment marks and making it easy to detect positions.
According to one aspect of the present invention, there is provided a position detecting apparatus comprising: an illumination optical system for applying illumination light to a wafer mark and a mask mark both for scattering incidence light for position alignment, the wafer mark being formed on an exposure surface of a wafer, the mask mark being formed on a mask surface of an exposure mask, and the illumination light being applied along an optical axis which is oblique relative to the exposure surface; an observation optical system having a light reception surface on which scattered light from the wafer mark and the mask mark is focussed, an optical axis of the observation optical system being slanted in a direction opposite to a direction of an optical axis of the illumination optical system, relative to a normal direction to the exposure surface of the wafer, and regular reflection light of the illumination light applied to the wafer and the mask from the illumination optical system being set so as not to become incident upon the light reception surface; and control means for controlling to detect a positional relation between the wafer and the mask in accordance with images formed by the observation optical system by using the scattered light from the wafer mark and the mask mark.
According to another aspect of the present invention, there is provided a position detecting method comprising the steps of: disposing a wafe
Arent Fox Kintner & Plotkin & Kahn, PLLC
Kim Robert
Sumitomo Heavy Industrie's, Ltd.
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