Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Plasma generating
Reexamination Certificate
2003-12-19
2004-11-09
Wells, Nikita (Department: 2881)
Electric lamp and discharge devices: systems
Discharge device load with fluent material supply to the...
Plasma generating
C315S111710, C315S111010, C313S231310, C250S304000, C250S365000
Reexamination Certificate
active
06815900
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a radiation source for generating extreme ultraviolet (EUV) radiation based on a hot, dense plasma generated by gas discharge, particularly for generating high average EUV radiation outputs.
b) Description of the Related Art
In the last 35 years, semiconductor chip producers have achieved considerable growth rates and increases in output by continuously reducing transistor sizes from the micrometer range to the nanometer range. Since its formulation in 1965, Moore's law has been steadily corroborated in the semiconductor lithography industry by a gradual reduction of the wavelength in the utilized radiation. At present, the industry is making the transition from the ArF excimer laser with a wavelength of &lgr;=193 nm to the F
2
laser with a wavelength of &lgr;=157 nm. There is a conviction that because of the transmission limits of lens systems radiation at &lgr;=157 nm will be the smallest radiation ever used in semiconductor lithography which utilizes transmission optics or catadioptric systems.
However, the increase in the operating speed of a microprocessor predicted for the end of this decade by Moore's law could stagnate if the resolution limit of exposure equipment given by R~&lgr;/NA for a resolvable structure spacing R is reached. This equation shows that the structure resolution can only be improved by reducing the wavelength &lgr; and/or increasing the numerical aperture NA of the optics. Since the theoretical limit of the numerical aperture NA is 1 and the industry already uses values up to NA=0.8, the sole possibility for reducing the resolution limit and, therefore, further reducing transistor size is a further reduction in wavelength.
Therefore, it can be stated at the present time that a further substantial increase in the numerical aperture of optics is impossible and that no transmission optics or catadioptric system permits the use of wavelengths substantially smaller than 157 nm. Accordingly, there was reason to fear that the development predicted by Moore's law would stagnate in coming years if no alternative possibilities were found for overcoming the problem. Fortunately, the development of multilayer mirrors with a 70-% reflection factor in the range of 10 to 15 nm offered the semiconductor industry a new prospect for the use of EUV radiation in this wavelength range and accordingly provided new hope that current lithographic chip fabrication will remain for another decade as dynamic as it has been thus far.
Although radiation sources based on plasma generated by gas discharge as well as laser-generated plasmas have shown adequate potential to emit EUV radiation in the desired wavelength range of 10 to 15 nm, these sources are still far from being used as commercial high-output radiation sources such as are required in chip fabrication for exposure machines with output powers of several hundred watts. With the greatest possible conversion efficiency that can be achieved for a plasma generated by gas discharge estimated at about 1%/2&pgr;·sr, an input power of 20 kW would be required to collect 100-watt EUV radiation in a solid angle of &pgr;sr. Further, it must be kept in mind that the majority of this enormous power for converting into plasma must be transmitted over discharge surfaces of a few square centimeters. It can easily be imagined that these small surfaces will not be stable over a long duration, so that radiation sources based on a gas discharge appear unsuitable for stable long-term use due to the fact that they must work in continuous operation for upwards of at least twenty hours and more at repetition frequencies of between 2 and 10 kHz for commercial use in chip lithography.
OBJECT AND SUMMARY OF THE INVENTION
Therefore, it is the primary object of the invention to find a novel possibility for the realization of an EUV radiation source which achieves a high average radiation output in the EUV region and remains stable for a sufficiently long period of time.
According to the invention, in a radiation source for the generation of extreme ultraviolet (EUV) radiation based on a dense, hot plasma generated by gas discharge containing two electrodes which are electrically separated from one another by insulators which are resistant to breakdown and at the same time form rotationally symmetric electrode housings for parts of a vacuum chamber, wherein a gas discharge for plasma generation is provided between a first electrode housing and a second electrode housing within the vacuum chamber and an exit or outlet opening for the radiation emitted by the plasma is provided in the first electrode housing, further containing a gas supply unit for generating a flow of working gas through the vacuum chamber, a high-voltage module for providing high-voltage pulses at the electrodes and a preionization unit for generating preionization of the working gas prior to the gas discharge triggered by the high-voltage pulse, the above-stated object is met, according to the invention, in that the second electrode housing has a narrowed portion and an electrode collar which adjoins the latter and which is enclosed concentrically by the first electrode housing, wherein a concentric insulator layer is provided in this area of concentric overlapping between the first electrode housing and the electrode collar of the second electrode housing in order to shield the concentric surface regions of the two electrode housings, which concentric insulator layer extends in the direction of the outlet opening of the first electrode such that the gas discharge takes place substantially only parallel to the axis of symmetry of the electrode housing, and the electrode collar is stepped radially relative to the concentric insulator layer in such a way that at least one end region of the electrode collar is at a distance from the concentric insulator layer such that a concentric gap is formed.
The outlet opening in the first electrode housing advantageously has the shape of a circular narrowed portion coaxial to the axis of symmetry of the electrode housing and the first electrode housing is expanded conically following the narrowed outlet opening, so that the gas discharge is ignited between the two electrodes in the interior of the first electrode housing and the dense, hot plasma is formed within the conical expansion after the outlet opening of the first electrode housing.
For purposes of suitable orientation of the gas discharge in the interior of the first electrode housing, the electrode collar of the second electrode housing projecting into the first electrode housing preferably has the shape of a hollow cylinder with a plurality of steps.
In this connection, it can be advantageous that the electrode collar is a hollow cylinder with two outer and one inner step, wherein the second outer step forms a transition from the electrode collar to the base body of the second electrode housing. Further, it is useful when at least one of the steps of the hollow cylinder has a conical transition in order to improve heat dissipation and the stability of the electrode collar relative to the base body of the second electrode housing.
The base body of the electrode housing is advantageously produced from one of the metals, copper, tungsten, molybdenum or a tungsten-copper alloy in a desired mixture ratio, wherein at least highly loaded zones of the electrode collar of the second electrode housing are produced from an alloy of tungsten with one of the materials, titanium, tantalum, zirconium, rhenium, lanthanum, lanthanum oxide, nickel, iron, nickel-iron compounds or zirconium-oxygen compounds in a desired mixture ratio, or the highly loaded zones comprise an alloy of molybdenum with one of the materials, titanium, tantalum, zirconium, rhenium, lanthanum, lanthanum oxide, nickel, iron, nickel-iron compounds or zirconium-oxygen compounds in a desired mixture ratio.
Zones of the electrode housing upon which the radiation flow acts particularly intensively, particularl
Ahmad Imtiaz
Goetze Sven
Kleinschmidt Juergen
Schriever Guido
Stamm Uwe
Reed Smith LLP
Wells Nikita
Xtreme technologies GbmH
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