X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
1999-10-27
2001-10-23
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S119000
Reexamination Certificate
active
06307913
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for providing a shaped radiation source or field in the ultraviolet, extreme ultraviolet, soft x-ray and other emission spectra, and to a method and apparatus for performing lithography using the emitted radiation, such as is useful in integrated circuit manufacturing.
BACKGROUND OF THE INVENTION
In photolithography, the wavelengths of radiation sources have progressed from the visible spectrum to the deep ultraviolet (approximately 365 nanometers to approximately 100 nanometers). The reduction in wavelength is dictated by the requirement for smaller circuit feature sizes and the particular wavelengths are determined by the availability of high power radiation sources.
For advanced photolithography, there is a need for short wavelength radiation sources to produce smaller and higher performance integrated circuits. Wavelengths of 157 or 126 nanometers in the deep ultraviolet spectrum and 13 or 10 nanometers in the soft x-ray spectrum (sometimes characterized as the extreme ultraviolet spectrum) are being considered for advanced photolithography systems. The presently known photolithography apparatus suffers deficiencies in producing high power radiation at such wavelengths at or below 157 nanometers in an efficient, reliable and economical fashion.
One example of a proposed soft x-ray or extreme ultraviolet projection lithography apparatus using an arc-shaped illumination field, called a “ring field” is described in Ceglio, Hawryluk and Sommargren, “Front-End Design Issues In Soft-X-Ray Projection Lithography,”
Applied Optics
, vol. 32, pp. 7050-7056 (Dec. 1, 1993). Plasma emitting radiation in a desired wavelength is created by striking a target with an optical laser beam focused to a small spot. In one such system, it is proposed that the optical laser beam be scanned across the target in an arc or ring field pattern (i.e. creating a scanned point-type source). In another such system, an arc or ring field pattern is generated from a point-source of radiation by condenser optics, creating a narrow ring field. Ultimately a mask and wafer (typically coated with a photoresist) is illuminated with the arc or ring field pattern. Because the pattern does not illuminate the entire mask or wafer, the pattern is also scanned to illuminate the entire mask or wafer. One disadvantage is that scanning the laser beam to produce a ring field of extreme ultraviolet radiation increases the exposure time and generates inefficiencies and can result in a non-uniform field, which is not desirable in photolithography. Other disadvantages are that known ring field condenser optics are complex, difficult to properly align and expensive. Known condenser optics that use point-like radiation sources typically do not provide a sufficiently high amount of light and provide an undesirably high level of coherence for optimal mask illumination for photolithography applications.
There are also various techniques for shaping laser beams. For example, creating a line focus is known, as described in I. N. Ross et al., “Design and Performance of a New Line Focus Geometry For X-Ray Laser Experiments,”
Applied Optics
, Vol. 25, No. 9, pp. 1584-87 (May 1, 1997).
Accordingly, there is a need for a system that provides a shaped illumination field, without resorting to scanning a series of points from a point source in creating arc shapes or relatively complex condenser optics in the creation of the shaped radiation field.
SUMMARY OF THE INVENTION
The present invention alleviates to a great extent the disadvantages of the known lithography systems and methods using shaped plasma discharges as sources of x-ray, soft x-ray, extreme ultraviolet and ultraviolet radiation. In one embodiment, a laser source (preferably such as used in a laser-plasma source system) provides an output beam (such as a laser-plasma source illumination) at a desired wavelength (&lgr;
1
), power level and beam quality in order to generate such a shaped plasma source. This laser source ultimately can impart in whole or part a shape to a plasma discharge from a target that emits radiation (at a wavelength &lgr;
2
) when illuminated by the illumination field of the laser source. Alternatively, a shaped plasma discharge is created by other apparatus, such as an electric discharge system, as described more fully below. The shaped plasma discharge preferably is directed to illuminate a mask/wafer combination as used in a photolithography system.
In the shaped laser source embodiment, the output beam (at a wavelength of &lgr;
1
) is shaped into a desired profile using shaping optics. Such a shaped laser beam can be formed into any beam profile, such as a line, arc or array of focused spots. In one embodiment, the shaping optics include a lens or a set of lenses that produce the desired shaped laser beam illumination field (which in a preferred embodiment produces a shaped plasma source for a shaped plasma radiation illumination field (at &lgr;
2
)). Alternatively, the shaping optics includes a compound or holographic lens, which produces the desired shaped illumination field. In another embodiment, the shaping optics includes one or more mirrors and optionally one or more lenses. Any combination of these optical components may be used. All or a portion of the shaping optics may be a part of the laser source, or they may be separate from the laser source.
In one embodiment, plural laser pulses are provided substantially at the same time, such as by using plural laser sources or splitting mirrors. The plural pulses are fed to plural shaping optics, which in turn generate plural shaped illumination fields. In this embodiment, for example, each pulse can be shaped into an arc, and the arcs can be combined in any desired fashion.
The shaped illumination field hits a plasma generating target downstream of the shaping optics. The ionized plasma emits radiation in the desired wavelength (&lgr;
2
). Any target may be used which generates the desired radiation emission. In one embodiment a solid material is used. For example, ice or solid xenon may be used to emit in the extreme ultraviolet spectrum (as used here, “UV” is an abbreviation for “ultraviolet” and “EUV” is an abbreviation for “extreme ultraviolet”; “EUV” and “soft x-ray” will be used synonymously). Examples of ice targets include a thin sheet or cylindrical block of ice. In use the target is illuminated by the shaped output laser beam (&lgr;
1
). The ice preferably is cooled by a heat pump, such as including liquid nitrogen reservoir placed in proximity to or in contact with the ice. In another embodiment, a metallic strip or band is provided as the target material. Alternatively, a liquid target may be provided, such as water or liquid xenon (or liquid forms of other gases) emitted from a nozzle in a stream. The liquid may be treated with additives such as zinc chloride, to adjust the emission spectrum. Likewise the solid component may be increased in this stream to the point where the stream comprises solid micropellets or clusters. For example, micropellets of tin or other suitable substances may be provided via a nozzle in a fluid (gas or liquid) stream.
In one embodiment, an electrical discharge is applied along with the shaped laser discharge. The shaped laser discharge is used to shape a channel of ionized material in the target. Then, electrical energy is applied to the target material, converting to plasma the target material within the ionization channel. Thus, the laser discharge determines and stabilizes the position, shape, and volume of the electrical discharge plasma. As a result of this technique, the same power plasma can be produced with a lower intensity laser input, or higher power plasma as achieved with the same laser source.
In another embodiment, an electrical energy source creates the plasma discharge that emits a radiation field (i.e. the plasma radiation source). When an electrical current is passed through a material between two electrodes, an arc discharge is formed. Plasma is formed in the t
Foster Richard M.
Morris James H.
Rieger Harry
Sasian Jose M.
Turcu Edmond
Baker & McKenzie
Ho Allen
JMAR Research, Inc.
Kim Robert H.
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