X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Reexamination Certificate
2002-03-18
2003-09-23
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
C378S085000, C378S145000
Reexamination Certificate
active
06625251
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a laser plasma x-ray generation apparatus used for an x-ray reduction projection lithography apparatus, an apparatus for highly sensitively analyzing the chemical state, the chemical composition, and the concentration of an impurity, especially a light element, of various types of materials such as semiconductor materials, and the like.
A laser x-ray generation apparatus used in the application purpose described above generally is constituted to irradiate a target with x-rays from a laser plasma x-ray source, which is arranged at a remote position, through a reflecting mirror.
In this case, to improve the resolution, an x-ray source with a short wavelength is used. When, however, the wavelength is equal to or shorter than several hundred Å, the reflectance of the reflecting mirror is extremely lowered and practically becomes zero.
For example, as illustrated in
FIG. 1
, two kinds of substance layers
2
and
3
are reciprocally and regularly layered with constant thickness on a substrate
1
made of a material such as quartz or silicon, a direct incidence reflecting mirror (multilayer reflecting mirror) with high reflectance utilizing Bragg diffraction can be actualized.
Especially, since as high as sixty and some % of direct incidence reflectance of a soft x-ray with approximately 130 Å wavelength can be obtained using a Mo/Si multilayer film produced by regularly and reciprocally layering at least several ten Mo layers and Si layers with independently several nm thickness, such a multilayer film has widely been used for x-ray reduction exposure and for a reflecting mirror of an x-ray microscope, an astronomical telescope and the likes.
In this case, synchrotron (SR) radiation light, a laser plasma x-ray (LPX), and a gas pinch plasma (GPP) are employed as an x-ray source and among them, the synchrotron radiation light requires a large-scale and costly apparatus using an accelerator and the gas pinch plasma is a light source with luminance low by some figures, 50 that the laser plasma x-ray, which is a relatively compact and has luminance as high as 1012 w/cm
2
, is expected to be a practically usable light source.
Nevertheless, as illustrated in
FIG. 2
, a target material and a fine particle are evaporated by using the laser x-ray source excited by pulsed laser light, so that if the evaporated material and particle adhere to an x-ray optical system, of which the x-ray reflecting mirror is representative, the function of the system is deteriorated.
In order to inhibit such deterioration, a mechanically synchronized shutter or a buffer gas is inserted between the laser x-ray source and the reflecting mirror or the reflecting mirror is positioned at a remote position for converging generated x-rays and using the converged x-rays.
However, in any case, since a reflecting mirror has to be positioned at a position far from a laser plasma x-ray source, which is a point source, the reflecting mirror has a disadvantageous point of a low use coefficient of x-rays and its practical application has been hindered.
Note that, in
FIG. 2
,
21
shows an incidence laser,
22
shows a target,
23
shows the reflected rays which are created when the incidence laser
21
is reflected by the target
22
,
24
shows a carbon thin film-like filter,
25
shows a reflecting mirror,
25
a
shows a reflecting surface, and
26
shows debris.
FIGS. 3A and 3B
independently illustrate a constitution example of a conventional laser plasma x-ray generation apparatus, and represent a paraboloid of revolution and an ellipsoid of revolution used as light concentration reflecting mirrors, respectively. Since a reflecting mirror with a surface of revolution is used in these constitutions, a condensing solid angle used for condensing x-rays from a laser plasma x-ray source serving as a point source is larger than that in FIG.
2
. However, laser light is incident on the reflecting mirror along the rotation axis of the reflecting mirror, so that, as will be described later, relatively intense x-rays irradiated in a direction of a normal (vertical direction) to a target material run outside through an aperture which is made on the rotation axis for laser incidence. Alternately, even if the x-rays are reflected by the rotating reflecting mirror, a target material
32
with a finite size reabsorbs the x-rays. Therefore, this poses a problem of low light concentration efficiency.
On the other hand, it has been required to use a soft x-ray with wavelength shorter than approximately 130 Å in order to further improve the resolution in those application cases. Nevertheless, in the case a Mo/Si multilayer film is used at wavelength 124 Å (L absorption edge of Si) or shorter, the direct incidence reflectance is considerably lowered on the order of several to about ten % to make it impossible to practically use the multilayer film at the wavelength.
A reflecting surface using Mo in combination with B4C has been proposed in order to increase the reflectance of the reflecting surface
25
a
, and even such a surface can provide reflectance 30% at the highest and has been unsuitable for practical use.
Recently, the multilayer structure type reflecting surface
25
a
comprising reciprocally layered Mo and Be layers has been proposed and it is found that higher than 60% of direct incidence reflectance of wavelength just at absorption edge of Be (just at 111 Å) can be obtained by the multilayer film type x-ray reflecting mirror and resolution improvement is expected to be achieved by obtaining high reflectance even of a soft x-ray with a short wavelength in the foregoing application.
Also recently, a cryotarget is invented (Patent No. 2614457) using a chemically inert rare gas element in a liquid or solid state at a low temperature or in a low temperature gas state at vapor density near that in liquid state as a target material irradiated with laser light, thereby eliminating adhesion of the target particle to an x-ray optical system such as a reflecting mirror or others. It can be said that practical application of laser plasma x-ray source (LPX) is just started.
Normally, since a plurality of multilayer reflecting mirrors are employed in an x-ray image-forming optical system as illustrated in
FIG. 1
, the x-ray concentration and transmittance is generally considerably lowered. For that, each reflecting mirror is required to have a reflectance higher even by a little.
In the case of Mo/Be combination, when a film is actually formed, the interfaces or respective layers become rather rough since Mo and Be are aggregated and owing to the effect of the roughness, there occurs a problem that the actual reflectance is lower than the theoretical reflectance of the multilayer film with an ideal structure by about 20%.
Further, in the case of Mo/Be combination, since the melting point of Be is as low as about 1270° C., the reflectance is sharply decreased by use of soft x-rays with high luminance and by increase of the ambient temperature to be high just like the case of the Mo/Si multilayer film type x-ray reflecting mirror and there also occurs a problem of insufficient heat resistance for stable use. Moreover, since the reflectance is extremely lowered at 111 Å or shorter wavelength in the case of a multilayer film x-ray reflecting mirror using Mo and Be in combination, there occurs another problem that the reflecting mirror can not be usable at the wavelength or shorter.
On the other hand, it was found by Mochizuki, one of inventors of the present invention, that a Xe (xenon) cryotarget laser plasma x-ray source, which is a typical cryotarget, has the most intense emission spectrum near 108 Å wavelength (A. Shimoura, S. Amano, S. Miyamoto, and T. Mochizuki, “X-ray generation in cryogenic targets irradiated by 1 &mgr;m pulse laser”, Applied Physics Letters 72, PP. 164-166 (1998)). So far, there exits no reflecting mirror capable of providing sufficiently high reflectance (50% or higher) at the wavelength. Furthermore, though a ref
Mochizuki Takayasu
Takenaka Hisataka
Barber Therese
Blakely & Sokoloff, Taylor & Zafman
Dunn Drew A.
NTT Advanced Technology Corporation
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