The present invention relates to an extreme ultraviolet (“EUV”) light spectrum measuring apparatus and an EUV light intensity calculating method used to evaluate an EUV light source (having a wavelength between 10 and 15 nm) for use with a projection exposure apparatus in a photolithography in manufacturing semiconductor devices.
Reduction projection exposures using ultraviolet have been conventionally employed to manufacture such fine semiconductor devices as a semiconductor memory and a logic circuit in the photolithography method. The critical dimension to be transferred by the reduction projection exposure is proportionate to a wavelength of light used for transfer, and inversely proportionate to the numerical aperture (“NA”) of a projection optical system. In order to transfer a finer circuit pattern, a shorter wavelength of used ultraviolet (“UV”) light has been promoted from an ultra-high pressure mercury lamp i-line with a wavelength of about 365 nm to a KrF excimer laser with a wavelength of about 248 nm and an ArF excimer laser with a wavelength of about 193 nm.
However, the lithography using the UV light has the limits to satisfy the rapidly promoting fine processing to semiconductor devices, and a reduction projection exposure apparatus using the EUV light with a wavelength of about 10 to 15 nm much shorter than that of the ultraviolet has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
A development of the EUV light source for the reduction projection exposure apparatus is promoted parallel to a development of the reduction projection exposure apparatus. One illustrative EUV light source is, for example, a laser excited EUV light source as disclosed in Japanese Patent Application, Publication No. 2002-174700 (corresponding to U.S. Pat. No. 6,324,256). The laser excited EUV light source irradiates a highly intensified pulse laser beam to a target material put in a vacuum chamber, and generates the high-temperature plasma. Among the lights having various wavelengths emitted from thus generated plasma, the EUV light utilizes the light having a wavelength, for example, of about 13 nm. The target material uses a metallic thin film, an inert gas, a droplet, etc., and is supplied to a vacuum chamber by such a means as a gas jet. The EUV light emitted from the plasma is generally condensed into a condensing point by a rotationally elliptic condenser mirror, is introduced into a projection exposure apparatus after diverging from the condensing point, and illuminates a mask uniformly via an illumination optical system of the projection exposure apparatus. Alternatively, the EUV light may be introduced, as light collimated by the rotationally elliptical condenser mirror, into the projection exposure apparatus.
In the exposure apparatus using the EUV light, an optical element in the optical system that introduces the EUV light mainly uses an oblique-incidence total reflection mirror and a Si/Mo multilayer coating mirror as a mirror having an incident angle close to normal incidence. Since the multilayer coating mirror for the normal incidence has a high reflectance to the EUV light having a wavelength around 13.5 nm, the EUV light for projection exposure generally uses a wave range between 13.365 nm and 13.635 nm around the wavelength of 13.5 nm among the lights emitted from the EUV light source. The throughput of the projection exposure apparatus depends upon the absolute value of the EUV light intensity in the wave range between 13.365 nm and 13.635 nm, and an exposure apparatus has a higher productivity as the absolute value increases.
The EUV light source needs to emit the EUV light at a high intensity in the wave range between 13.365 nm and 13.635 nm among the lights having various wavelengths. The light intensity in the non-exposure wave range should be low, because this light turns to the heat after absorbed in the optical element in the optical system, and deteriorates the of the optical system.
For these reasons, it is important to previously recognize a spectrum of the light emitted from the EUV light source for the exposure apparatus.
One illustrative, conventional EUV light spectrum measuring apparatus is disclosed, for example, in Japanese Patent Application, Publication No. 10-073698. According to this reference, a diffraction grating forms a spectrum of part of the EUV light (or referred to as “soft-X-ray” in this reference) emitted from the plasma as a light emitting point, making the light intensity of each wavelength measurable.
However, the above prior art can measure the spectrum of the light only in a specific divergent direction and cannot structurally measure the spectra in all the divergent directions. When the spectra differ according to divergent directions, the prior art cannot measure the spatial spectrum distribution or the intensity in the predetermined wave range, such as between 13.365 nm and 13.635 nm, in the entire divergent directions. In addition, the prior art disadvantageously cannot measure a total intensity in the predetermined wave range emitted from the light source.