X-ray or gamma ray systems or devices – Specific application – Telescope or microscope
Reexamination Certificate
2000-08-11
2003-02-18
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Telescope or microscope
C378S034000, C378S070000
Reexamination Certificate
active
06522717
ABSTRACT:
This application claims the benefit of Japanese Applications No. 11-227005, filed in Japan on Aug. 11, 1999, and No. 2000-173427, filed in Japan on Jun. 9, 2000, both of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflective-type soft X-ray microscope in which reflected images of samples are observed using soft X-rays. The present invention also relates to a mask inspection device in which reflective masks used in soft X-ray reduction projection exposure are inspected for defects using the above-mentioned reflective-type soft X-ray microscope. In addition, the present invention relates to a reflective mask manufacturing method using the above-mentioned mask inspection device.
2. Discussion of the Related Art
In recent years, as semiconductor integrated circuit elements have become progressively smaller, reduction projection lithographic techniques using soft X-rays of an even shorter wavelength instead of the conventional ultraviolet light have been developed in order to improve the resolution of optical systems, which has been limited by the diffraction limit of light. Soft X-rays with a wavelength of 10 nm to 15 nm are used as the exposure light source in such techniques.
Since there are no available substances that are transparent in the aforementioned wavelength range, reflective masks have been used in soft X-ray reduction projection lithographic techniques instead of the conventional transmission-type masks. In such reflective masks, to reflect soft X-rays, a reflective film is formed of a multi-layer film on the surface of a substrate having sufficient mechanical strength and surface smoothness. A specified circuit pattern is formed on this reflective film by a layer formed of a substance that absorbs soft X-rays. In soft X-ray reduction projection exposure techniques, an image of the circuit pattern formed on the aforementioned reflective mask is focused on the wafer, which is coated with a photoresist, by means of a projection image-focusing optical system constructed of a plurality of optical elements, such as multi-layer film reflective mirrors, etc. The image is thus transferred to the photoresist on the wafer. Furthermore, since soft X-rays are attenuated by absorption in the atmosphere, the entire light path is maintained at a specified degree of vacuum.
Besides foreign objects adhering to the mask surface and defects in terms of external appearance such as missing portions and excess portions of the circuit pattern, defects in the aforementioned reflective mask include defects in the reflective film itself. As described above, the reflective film is a multi-layer film, and a high overall reflectivity can be obtained by aligning the phases of weak reflected light at the interfaces between the layers laminated in the multi-layer configuration. If local step differences are generated in the reflective film as a result of a bumpy surface profile of the substrate itself, which may exist prior to the formation of the aforementioned reflective film, and/or as a result of foreign objects accidentally incorporated during the process of lamination of the multi-layer film, etc., portions having deviations in periodic structures are formed. Consequently, the phase relationship of the light inside the reflective film is deviated from the design relationship at such portions, resulting in local decrease in reflectivity. Here, a phase difference of 2&pgr; corresponds to a used wavelength of 10 to 15 nm. Accordingly, local step differences on the order of nanometers cause the reflectivity defects. It has been extremely difficult to observe such extremely small step differences with microscopes using visible light or ultraviolet light, or with electron beam microscopes, etc.
If the actual exposure wavelength used in a soft X-ray reduction projection exposure apparatus is used, all defects, which are transferred to the photoresist on the wafer, can be detected. Thus, an inspection of such a reflective mask for defects should preferably be performed using the same wavelength as the exposure wavelength. Consequently, it is necessary to use a reflective-type soft X-ray microscope in order to inspect a reflective mask for defects.
Transmission-type microscopes and reflective microscopes exist for soft X-ray microscopes. In the reflective-type soft X-ray microscopes, Schwarzschild optical systems are widely used. As shown in
FIG. 4
, a Schwarzschild optical system is an optical system that generally is formed of two spherical-surface mirrors with concentric spherical surfaces, i.e., a concave mirror
2
and a convex mirror
1
. A hole
7
that allows light to pass through is formed in the center of the concave mirror
2
. In cases where such an optical system is used in the soft X-ray region as described above, the reflective surfaces are coated with a multi-layer film coating that reflects soft X-rays.
Conventionally, in order to construct a reflective-type microscope using such a Schwarzschild optical system, as shown in
FIG. 4
, it has been necessary to incline the normal of sample
3
with respect to the optical axis
0
(the central axis of rotation of the optical system) so that mechanical interference of illuminating light beam
5
and optical system mirror tube
19
is prevented. Furthermore, when the sample is inclined relative to the optical axis
0
, the visual field that can be observed is limited by the depth of focus of the optical system.
It is known that the diffraction-limit resolution (R) and depth of focus (DOF) of the optical system are determined by the numerical aperture (NA) and wavelength (&lgr;), and are given by the following equations:
R=
0.61
&lgr;/NA
DOF=&lgr;/NA
2
For example, if a resolution of 70 nm is to be obtained with a light source having a wavelength of 13 nm, NA is 0.11, and the depth of focus (DOF) in this case is approximately 1 &mgr;m. Here, if the sample is inclined 45°, the width of the visual field limited by the depth of focus is a mere 1.4 &mgr;m. If the sample is installed perpendicular to the optical axis, a visual field of at least several tens of microns can be ensured, although this also depends on the dimensions and magnification of the optical system.
Imaging is also accomplished to some extent even in the regions outside the visual field limited by the depth of focus; however, since blurring of the image is severe, the diffraction-limit resolution cannot be obtained. For example, an experiment performed with a soft X-ray microscope using a Schwarzschild optical system with such an inclined sample arrangement has been reported in Optics Letters, Vol. 17, (1992) p. 157. In this experiment, an image of only about #200 mesh (pitch: 127 &mgr;m) was visible, so that the results did not even remotely approach the resolution (0.1 &mgr;m) expected from the wavelength (18.2 nm) and NA (0.1) of the optical system.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a reflective-type soft X-ray microscope that substantially obviates the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a reflective-type soft X-ray microscope which makes it possible to observe images with the sample installed perpendicular to the optical axis and with a reflective-type arrangement, using an image-focusing system consisting of a concave mirror and a convex mirror, as typified by a Schwarzschild optical system.
Another object of the present invention is to provide a defect inspection device for reflective masks used in soft X-ray reduction projection exposure, which uses such a soft X-ray microscope.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof a
Kondo Hiroyuki
Murakami Katsuhiko
Dunn Drew A.
Ho Allen C.
Morgan & Lewis & Bockius, LLP
LandOfFree
Reflective-type soft x-ray microscope does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Reflective-type soft x-ray microscope, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Reflective-type soft x-ray microscope will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3175451