X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
1999-02-16
2001-05-29
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S205000, C378S206000
Reexamination Certificate
active
06240158
ABSTRACT:
This application claims the benefit of Japanese Applications No. 10-035108, filed in Japan on Feb. 17, 1998, No. 10-037616, filed in Japan on Feb. 19, 1998, and No. 10-037617, filed in Japan on Feb. 19, 1998, all of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an X-ray projection exposure apparatus, and more particularly, to an X-ray projection exposure apparatus which is suitable for transferring a circuit pattern formed on a mask (also referred to as “reticle”) onto a substrate, such as a wafer, via a reflective type focusing X-ray optical system using a mirror projection scheme or the like.
2. Discussion of the Related Art
Conventionally, in exposure apparatus used for semiconductor manufacture, circuit patterns formed on a mask (photo-mask) used as an object surface are projected and transferred onto the surface of a photosensitive substrate such as a wafer or substrate for forming a mask, etc., via a focusing optical system. The photosensitive substrate is coated with a resist. The resist is exposed with exposing light to form a rest pattern.
The resolving power W of the exposure apparatus is determined mainly by the wavelength &lgr; of the exposing light and the numerical aperture NA of the focusing optical system, and is expressed by the following equation:
W=k1&lgr;/NA (k1:constant) (1)
Accordingly, in order to improve the resolving power, it is necessary to shorten the wavelength and/or increase the numerical aperture. Currently, exposure apparatus used in the manufacture of semiconductor devices uses mainly the i-line having a wavelength of 365 nm, and a resolving power of 0.5 &mgr;m is obtained at a numerical aperture of about 0.5. Since increasing the numerical aperture is difficult due to various constraints in optical design, it will be necessary in the future to shorten the wavelength of the exposing light. Excimer lasers are examples of exposing light that has a wavelength shorter than the i-line. The wavelengths are 248 nm for the KrF excimer laser and 193 for the ArF excimer laser, respectively. A resolving power of 0.25 &mgr;m is obtained in the case of the KrF excimer laser, and a resolving power of 0.18 &mgr;m is obtained in the case of ArF. Furthermore, if X-rays with an even shorter wavelength are used as exposing light, a resolving power of 0.1 &mgr;m or less should be possible at a wavelength of 13 nm, for example.
The main components of the conventional exposure apparatus are a light source, an illumination optical system, and a projection focusing optical system. The projection focusing optical system is constructed from a plurality of lenses or reflective mirrors, etc., so as to focus the mask pattern on the mask onto a substrate, such as a wafer.
To obtain a desired resolving power, it is necessary that at least the focusing optical system be essentially free from aberration. If aberration is present in the focusing optical system, the sectional profile of the resist pattern deteriorates, and as a result, adverse effects on the processes following the exposure and/or the problem of image distortion may arise.
In the conventional exposure apparatus for manufacturing semiconductor devices or the like, a position detection device (also referred to as “alignment device”) is provided so that a resist pattern can be formed at a predetermined position on the wafer with respect to an existing circuit patterns on the wafer. The alignment device detects the positions of the mask and wafer, and the respective detected positions of the wafer and the mask are adjusted by a wafer stage and a mask stage so that a reduced image of the mask pattern is focused at a prescribed position on the wafer.
An example of the alignment device is an optical detection device. This type of device detects alignment marks on the wafer by illuminating the marks and detecting the light reflected from the alignment marks through a photo-detector, for example. When the wafer position changes, the signal output from the photo-detector also changes, thereby enabling the detection of the wafer position. Similarly, the position of the mask can be detected by illuminating the alignment marks on the mask with illuminating light, and then detecting the light reflected from the alignment marks through a photo-detector, for example.
Such an alignment device can detect the positions of the respective marks on the wafer and the mask with high accuracy. Accordingly, alignment of the mask with respect to the wafer can accurately be performed. In the conventional exposure apparatus, the alignment devices are disposed between the focusing optical system and the wafer and between the focusing optical system and the mask.
Furthermore, in the conventional exposure apparatus, a high resolving power can be obtained in the vicinity of the focal position of the projection focusing optical system. Accordingly, the position of the surface of the wafer that is being exposed must be located in the vicinity of the focal position of the projection focusing optical system. The range in the direction of the optical axis in which the projection focusing optical system exhibits a high resolving power is called the “depth of focus(DOF).” The depth of focus, DOF, is determined mainly by the wavelength &lgr; of the exposing light and the numerical aperture NA of the focusing optical system, and is expressed by the following equation:
DOF=k2&lgr;/NA
2
(k2:constant) (2)
For example, if the numerical aperture is 0.5 and the constant K2 is 1 at a wavelength of 365 nm, then the DOF is 1.5 &mgr;m.
In order to expose the wafer surface while the wafer surface is positioned within the range of the depth of focus, a device for detecting the position of the wafer surface in the direction of the optical axis of the projection focusing optical system (also referred to as “focal point detection device,” because the device detects the vertical position of the wafer in order to position the wafer at the focal point) is installed in the exposure apparatus. Through this device, the position of the wafer in the direction of the optical axis is detected, and the position of the wafer in the direction of the optical axis is adjusted by the wafer stage so as to position the wafer surface at an appropriate position within the DOF.
FIG. 12
schematically shows an example of such a focal point detection device. The detection scheme illustrated in
FIG. 12
is generally referred to as the triangulation method. In this method, wafer
6
is illuminated with illuminating light
91
, which is obliquely incident on the wafer
6
through mirror
95
, and light
92
reflected from the wafer is detected by a photo-detector
96
through mirror
95
. When the wafer position changes, the optical path of the reflected light changes, which in turn changes the position of the reflected light at the photo-detector
96
. Thus, by detecting such position changes at the photo-detector
96
, the position of the wafer can be measured. A one-dimensional or two-dimensional position detection sensor is used as the photo-detector.
Such a focal point detection device is advantageous because the position on the wafer at which the focal point detection device detects the position of the surface (i.e., the position illuminated by detection light) can be set inside the area being exposed or in the vicinity thereof. In the conventional exposure apparatus, the focal point detection device is installed between the focusing optical system and the wafer.
FIGS. 13 and 14
are schematic diagrams illustrating examples of conventional exposure apparatus that uses the i- line. This apparatus is constructed mainly from a light source and illumination optical system (not shown in the figures), a stage
15
for holding mask
14
, a projection focusing optical system
13
, a stage
17
for holding wafer
16
, alignment devices
18
and
18
′ (FIG.
13
), and a focal point detection system
18
″ (FIG.
14
). The mask
14
has a mask pattern thereon, whic
Bruce David V.
Ho Allen C.
Morgan & Lewis & Bockius, LLP
Nikon Corporation
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