X-ray or gamma ray systems or devices – Source
Reexamination Certificate
1999-09-30
2002-01-15
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Source
C378S143000
Reexamination Certificate
active
06339634
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a soft X-ray generating apparatus and a soft X-ray lithography apparatus using this soft X-ray generating apparatus. More specifically, the present invention concerns a soft X-ray generating apparatus that can generate a large quantity of soft X-rays, and a soft X-ray lithography apparatus using this soft X-ray generating apparatus.
2. Discussion of the Related Art
Conventionally, in exposure equipment used in semiconductor manufacture, equipment with an exposure and transfer system has been widely used in which a photo-mask (hereafter referred to as a “mask”) is irradiated by means of an irradiation optical system so that circuit patterns formed on the mask surface are projected and transferred onto a substrate such as a wafer, etc., via an image-focusing device. The substrate is coated with a resist, and the resist becomes photosensitive as a result of being exposed so that a resist pattern is obtained.
The resolving power w of an exposure apparatus is determined mainly by the exposure wavelength &lgr; and the numerical aperture NA of the image-focusing optical system, and is expressed by the following equation:
w=k&lgr;/NA
(1)
k: constant
Meanwhile, the focal depth DF is determined by the following equation:
DF=&lgr;/
2(
NA
)
2
(2)
As is clear from Equation (1), it is necessary to reduce the constant k, increase the numerical aperture NA, or reduce the wavelength &lgr; of the light source in order to reduce the dimensions of the minimum pattern that can be exposed.
K is a constant that is determined by the projection optical system and process, and ordinarily has a value of approximately 0.5 to 0.8. Methods for reducing this constant k are referred to as super-resolution in a broad sense. In the past, methods such as improvement of the projection optical system, deformed illumination and phase shift mask methods, etc., have been proposed; however, there are difficulties in terms of the patterns that can be used, so that the range of application of such methods is limited. Meanwhile, as is clear from Equation (1), the minimum pattern dimensions can be reduced if the numerical aperture, NA, is increased. At the same time, however, as is clear from Equation (2), this leads to the problem of a reduced focal depth. Accordingly, there are limits to the extent to which the NA value can be increased, and a value of approximately 0.5 to 0.6 is ordinarily appropriate.
Accordingly, the most effective method of reducing the minimum pattern dimensions is to shorten the wavelength &lgr; of the light used for exposure, and to simultaneously reduce the NA as well, since shortening the exposure wavelength alone will reduce the focal depth.
Currently, in the manufacture of semiconductor integrated circuits, a method is widely used in which an extremely fine pattern formed on a mask is projected in a reduced form and transferred onto a silicon wafer (coated with a resist) by means of visible light or ultraviolet light. However, as pattern sizes become finer, the diffraction limit is being approached even in the case of ultraviolet light, so that reduction and projection type exposure using soft X-rays with wavelengths even shorter than those of ultraviolet light, i. e., wavelengths of 13 nm or 11 nm, has been proposed.
In cases where soft X-rays with a wavelength of 13 nm or 11 nm are used, one conceivable candidate for the light source (soft X-ray source) used is a laser plasma X-ray source (hereafter referred to as “LPX”). When pulsed light emitted from a laser device is focused and directed onto a substance, if the irradiation intensity exceeds 10
10
W/cm
2
, electrons are stripped from the atoms of the substance by the intense electric field so that the substance is converted into a plasma, and soft X-rays are radiated from this plasma. The brightness of the soft X-rays radiated from this plasma is extremely high, and a large quantity of soft X-rays can be obtained by generating the plasma at a high repetition rate. Furthermore, an LPX is extremely compact as an apparatus compared to synchrotron radiation facility, etc. Accordingly, LPXs show great promise, not only in the area of soft X-ray lithography, but also as radiation sources for X-ray microscopes and analysis devices, etc.
In cases where an LPX is used in soft X-ray lithography, the quantity of soft X-rays obtained from the light source is important. Since soft X-rays are strongly absorbed by all substances, ordinary lenses and reflective mirrors cannot be used. Accordingly, in order to obtain a high throughput in soft X-ray lithography, optical systems are constructed from reflective mirrors in which a multi-layer film is formed on the reflective surface. There is an intimate relationship between the combination of substances making up such a multi-layer film and the wavelengths reflected by the multi-layer film. In the case of an Mo/Si multi-layer film, a high reflectivity is obtained in the vicinity of a wavelength of 13 nm, while in the case of an Mo/Be multi-layer film, a high reflectivity is obtained in the vicinity of 11 nm. Accordingly, these wavelengths may be cited as candidates for the wavelengths used in soft X-ray lithography. However, even in the case of reflective surfaces formed using such multi-layer films, the limit of the reflectivity obtained for soft X-rays is approximately 70%. Assuming that approximately 10 reflective surfaces are used for reduction and projection type exposure, the transmissivity (reflectivity) of the optical system as a whole is extremely low, i. e., a few percent. Accordingly, in order to obtain a sufficient treatment speed (throughput) for a projection exposure apparatus, it is desirable that the quantity of soft X-rays generated by the light source be as large as possible.
In the case of an LPX using clusters formed by causing a gas to jet into a vacuum vessel as a target material, it is reported that the efficiency of conversion to a wavelength region of 13 nm or 11 nm (2.5% BW) is approximately 1 to 2%. The development of an LD-excited solid laser, which has an output of 1.5 kW as an exciting laser light source, has been promoted in order to obtain a sufficient throughput at this conversion efficiency. In the case of soft X-ray lithography, a method in which an annular band-form region is scanned is used in order to obtain a broad exposure area. When such scanning is performed, it is desirable that a continuous light source with a stable intensity be used in order to prevent the occurrence of irregularities in brightness within the exposure region. However, even in the case of an LPX, which is a pulsed light source, there is no problem if the repetition rate is on the order of kHz.
However, in order to increase the output of a conventional solid laser so that an output of 1 kW or greater is obtained, it is necessary to have a repetition rate on the order of kHz, and to increase the energy of one pulse to a high value. The development of such laser devices is currently being pursued; however, the development of a laser device which has such a large output, and which can operate stably over a long period of time, is not easy. Furthermore, even if such a laser device is developed, the resulting device would be extremely expensive.
Accordingly, there is a demand for a soft X-ray generating apparatus, which produces an output exceeding 1 kW, which is easy to manufacture, and which is equipped with an inexpensive pulsed laser light-generating device as an exciting pulsed light source, for use as an exciting light source that excites the laser plasma used in a soft X-ray lithography apparatus.
In cases where an LPX is used in a soft X-ray lithography apparatus, several optical systems for the purpose of projecting and exposing patterns of 0.1 &mgr;m or less have been proposed. In these optical systems, only an out-of-axis circular-arc-form good-image region is utilized, and only a circular-arc-form region on the mask is projected. Accordingly, the overall pattern of
Kandaka Noriaki
Kondo Hiroyuki
Ohtsuki Tomoko
Owa Soichi
Morgan & Lewis & Bockius, LLP
Nikon Corporation
Porta David P.
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