X-ray or gamma ray systems or devices – Source
Reexamination Certificate
2000-11-28
2003-01-07
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Source
C378S145000
Reexamination Certificate
active
06504903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser-excited plasma light source, and an exposure apparatus. More particularly, the present invention relates to a laser-excited plasma light source which generates an energy beam by irradiating an energy-ray generating material released from a nozzle with a laser beam to excite the energy-ray generating material to a plasma state, an exposure apparatus incorporating the laser-excited plasma light source and its making method, and a device manufacturing method for manufacturing micro-devices such as semiconductor devices using the exposure apparatus.
2. Description of the Related Art
EUV (Extreme Ultraviolet) exposure apparatus using EUV light having an wavelength within a range from 5 nm to 20 nm, for instance, 13 nm or 11 nm, as an exposure light, is currently being developed for use in a lithographic process for manufacturing semiconductor devices, as a generation after the next exposure apparatus which transfers, to a substrate (a wafer), a circuit pattern having a practical minimum line width (device rule) of 100 nm to 70 nm. Proposed as a first candidate for the exposure light source of the EUV exposure apparatus is a laser-excited plasma light source.
In conventional laser-excited plasma light sources chiefly employed, EUV light radiating material such as a copper tape is used as an energy-ray generating material (hereinafter referred to as a “target” as appropriate). And, a high-energy laser beam is condensed and directed onto the target to excite it to a plasma state to generate an energy beam such as an EUV light beam. Since, in spite of its compact size, the laser-excited plasma light source provides luminance as bright as that by undulators, the laser-excited plasma light source currently attracts attention as a light source for X-ray equipment such as an X-ray analysis apparatus or an X-ray exposure apparatus.
In the laser-excited plasma light source, besides energy beams, resulting from the destruction of the materials forming the target on which the plasma and the laser beam are converged, ions, atoms, and microscopic fragments are released. These sputtered particles, so-called debris, are stuck onto or deposited onto optical elements arranged close to the plasma (including a lens for condensing a laser beam, a collecting mirror for reflecting an energy beam generated from the plasma, i.e., an X ray, and a filter for transmitting the X ray generated from the plasma while cutting off visible light), thereby reducing the performance of the optical elements (reflectance and transmittance). The reduction of the sputtered particles is thus a major concern in the utilization of the laser-excited plasma light source.
To substantially reduce the sputtered particles, a gas cluster jet laser-excited plasma light source has been proposed (U.S. Pat. No. 5,577,092), in which a material (a high-density gas), remaining in a gaseous state at normal temperature, for instance, xenon (Xe), krypton (Kr), nitrogen, or carbon dioxide, is used and released through a nozzle, as a target, and the jet of this target gas or cluster is irradiated with a laser beam. Since the target remains gaseous at normal temperature, the target is not deposited in debris on the optical elements, and the performance of the optical elements is thus free from degradation.
The laser-excited plasma light source does not need a high level of vacuum (as high as 10
−9
Torr (10
−7
Pa) or so), and the vacuum level is sufficient if the laser beam does not perform gaseous release in residual gas prior to reaching the target and if the energy beam generated from the plasma is free from high absorption prior to reaching an object to be irradiated. Specifically, a vacuum of several tens of Torr to 0.1 Torr (8,000 Pa-10 Pa) is acceptable. For this reason, a low-cost evacuator, such as a rotary pump, works, and is thus reasonable for use.
In the gas cluster jet laser-excited plasma light source referenced above, the jet of the gas from the nozzle freely expands in the vacuum, and the density of the gas rapidly drops as the gas distances from the nozzle. To increase the dose of the energy beam from the plasma, the plasma has to be generated close to the nozzle (within a distance of several tenths mm to several mm) where the density of the gas (cluster) is still high. When the plasma is generated close to the nozzle, fast atoms, ions and electrons emitted from the plasma collide with nearby components, and erode these components. Atomic or fragmental debris eroded from the nozzle and components peripheral to the nozzle (hereinafter referred to as sputtered particles) scatter in all directions, and are adhered and deposited on the optical elements arranged close to the plasma, causing a drop in the performance of the optical elements.
To increase an energy conversion efficiency from the laser beam to the energy beam such as an X ray in the gas cluster jet laser-excited plasma light source, the plasma has to be located somewhat closer to the nozzle. But locating the plasma close to the nozzle increases sputtered particles from the nozzle and the components close to the nozzle. This method in the laser-excited plasma light source cannot satisfy the two requirements at the same time, namely the improvement in the energy conversion efficiency and the reduction in the sputtered particles.
Also, according to recent studies, even with a minimum distance of several mm between the laser converging point and the end of the nozzle, the plasma becomes extremely hot and erodes and damages the spout at the end of the nozzle, scattering heavy metal which constitutes the end of the nozzle. It then becomes some sort of debris, and the debris is deposited on the collecting mirror, degrading the reflectance of the collecting mirror.
Presumably, the same phenomenon takes place in the laser-excited plasma light source disclosed in U.S. Pat. No. 5,577,091 and U.S. Pat. No. 5,459,771, in which ice crystals and snow flakes are used as a target.
Although it is a small amount, oil used in the rotary pump is reversed to the vacuum system, and is adhered and deposited on the optical elements during long time of use, thereby gradually decreasing the performance of the optical elements (i.e., reflectance, transmittance, and diffraction efficiency) in time. To cope with this problem, the apparatus needed to be disassembled to replace an affected optical element with a new one, or an affected optical element had to be cleaned and put back in place.
In an EUV exposure apparatus using the above-referenced gas cluster jet laser-excited plasma light source, the life of a collecting mirror is shortened by the generation of debris or the reverse flow of the oil, and a maintenance job, such as the replacement of the collecting mirror, has to be frequently performed. For each maintenance job, the apparatus needs to be stopped.
SUMMARY OF THE INVENTION
The present invention has been developed in view of the above problems, and it is a first object of the present invention to provide a laser-excited plasma light source which reduces a drop in the reflectance of a collecting mirror due to the generation of debris.
A second object of the present invention is to provide an exposure apparatus which reduces the frequency of maintenance jobs, such as the replacement of the collecting mirror, and improves the production yield of devices.
The present invention in a first aspect lies in a first laser-excited plasma light source which generates energy beams by irradiating an energy-ray generating material with a laser beam to excite the energy-ray generating material to a plasma state, the light source comprising: a nozzle which releases the energy-ray generating material, and at least a surface layer of an end portion of the nozzle is formed of a material including a specific material, the specific material having a transmittance larger than that of a heavy metal to an energy beam which has a wavelength to be utilized, the energy beam being among the generated beams; an
Kandaka Noriaki
Kondo Hiroyuki
Nikon Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Porta David P.
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