Chemistry of inorganic compounds – Halogen or compound thereof – Binary fluorine containing compound
Reexamination Certificate
2001-11-20
2003-12-30
Kunemund, Robert (Department: 1765)
Chemistry of inorganic compounds
Halogen or compound thereof
Binary fluorine containing compound
C117S081000, C117S082000, C117S083000, C117S940000, C430S321000, C355S053000, C355S077000
Reexamination Certificate
active
06669920
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to optical lithography, and particularly to optical microlithography crystals for use in optical photolithography systems utilizing vacuum ultraviolet light (VUV) wavelengths below 193 nm, preferably below 175 nm, more preferably below 164 &mgr;m, such as below 160 nm VUV projection lithography refractive systems utilizing wavelengths in the 157 nm region.
Semiconductor chips such as microprocessors and DRAM's are manufactured using a technology called “Optical Lithography”. An optical lithographic tool incorporates an illuminating lens system for illuminating a patterned mask, a light source and a projection lens system for creating an image of the mask pattern onto the silicon substrate.
The performance of semiconductors have been improved by reducing the feature sizes. This in turn has required improvement in the resolution of the optical lithographic tools. In general, the resolution of the transferred pattern is directly proportional to the numerical aperture of the lens system and inversely proportional to the wavelength of the illuminating light. In the early 1980's the wavelength of light used was 436 nm from the g-line of a mercury lamp. Subsequently the wavelength was reduced to 365 nm (I-line of mercury lamp) and currently the wavelength used in production is 248 nm obtained from the emission of a KrF laser. The next generation of lithography tools will change the light source to that of an ArF laser emitting at 193 nm. The natural progression for optical lithography would be to change the light source to that of a fluoride laser emitting at 157 nm. For each wavelength different materials are required to fabricate lenses. At 248 nm the optical material is fused silica. For 193 nm systems there will be a combination of fused silica and calcium fluoride lenses. At 157 nm fused silica does not transmit the laser radiation. At present the preferred material for use at 157 nm is pure calcium fluoride crystal.
Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light below 193 nm provide benefits in terms of achieving smaller feature dimensions. Such systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits with smaller feature sizes. Current optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have progressed towards shorter wavelengths of light, such as the popular 248 nm and 193 nm wavelengths, but the commercial use and adoption of vacuum ultraviolet wavelengths below 193 nm, such as 157 nm has been hindered by the transmission nature of such vacuum ultraviolet wavelengths in the 157 nm region through optical materials. For the benefit of vacuum ultraviolet photolithography in the 157 nm region such as the emission spectrum VUV window of a F
2
excimer laser to be utilized in the manufacturing of integrated circuits there is a need for optical lithography crystals that have beneficial optical properties below 164 nm and at 157 nm.
The present invention overcomes problems in the prior art and provides a fluoride optical lithography crystal that can be used to improve the manufacturing of integrated circuits with vacuum ultraviolet wavelengths.
SUMMARY OF THE INVENTION
One aspect of the present invention is a below 160 nm optical lithography barium fluoride crystal. The barium fluoride crystal has a refractive index wavelength dispersion dn/d&lgr;<−0.003 at 157 nm.
In another aspect, the present invention includes a dispersion management optical lithography crystal. The dispersion management crystal is an isotropic barium fluoride crystal. Preferably the barium fluoride crystal has a 157.6299 nm refractive index wavelength dispersion dn/d&lgr; less than −0.003 and a 157.6299 nm refractive index n>1.56.
In a further aspect, the present invention includes a below 160 nm optical lithography method which comprises providing a below 160 nm optical lithography illumination laser, providing a calcium fluoride crystal optical element, providing a barium fluoride crystal optical element having a below 160 nm refractive index wavelength dispersion dn/d&lgr;<−0.003, and transmitting the below 160 nm optical lithography light through the calcium fluoride optical element and the barium fluoride optical element to form an optical lithography pattern.
In another aspect, the present invention includes a method of making a dispersion managing optical lithography element. The method includes providing a barium fluoride source material, melting the barium fluoride source material to form a precrystalline barium fluoride melt, solidifying the barium fluoride melt into a barium fluoride crystal and annealing the barium fluoride crystal to provide an isotropic barium fluoride crystal with a 157 nm refractive index wavelength dispersion dn/d&lgr;<−0.003.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described herein, including the detailed description which follows and the claims.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview of framework for understanding the nature and character of the invention as it is claimed.
Wherein reducing the wavelength of the illuminating light for lithography processes is necessary to achieve higher resolution, the illuminating light laser emission has a finite bandwidth. To achieve the resolution required at the 100 nm node, the optical lithography tool manufacturer using an all refractive optical design can either use a very highly line narrowed laser (to less than 2 pm) or can use two optical materials which have dispersion properties that compensate for the bandwidth of the laser.
In a preferred embodiment the invention includes providing isotropic optical lithography crystalline materials for color correction for VUV lithography in general but especially in the region of 157 nm to enable refractive lenses to be constructed to make use of the light from a fluoride excimer laser that has not been line narrowed to below 2 pm. The invention includes a range of fluoride crystalline materials that provide benefits to 157 nm optical lithography. In the preferred embodiment the dispersion managing optical lithography crystal is utilized in conjunction with a 157 nm optical lithography illumination laser with a bandwidth not less than 0.2 picometers.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The refractive index of a material varies with the wavelength of energy passing through it and this is called the dispersion of the material. Hence if light passing through a lens system, constructed of one optical material, has a range of wavelengths then each wavelength would be brought to a different focus so reducing resolution. This effect can be overcome by using a second optical material with different dispersion characteristics. This technique is known as color correction. To be of use as a color correction material, there are specific criteria that have to be met, namely the material must transmit at the wavelength of operation, it must be isotropic and must have optimum dispersion characteristics. At 157 nm, the only material that has had its dispersion characteristics measured to the necessary degree of accuracy is calcium fluoride. Applying the criteria of 157 nm transmission and of being isotropic, the following materials can be used as color correction materials.
I. Materials Based on Alkali Metal Fluorides:
Lithium fluoride, sodium fluoride, potassium fluoride, and materials of the formula: MRF
3
in which M is either Li, Na or K and R is either Ca, Sr, Ba or Mg. Examples of such materials include but are not limited to: K
Corning Incorporated
Douglas Walter M.
Murphy Edward F.
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