Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
2002-06-21
2003-11-18
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S321000, C430S322000, C423S490000, C423S489000, C501S003000, C501S040000, C501S905000, C359S352000, C359S350000, C359S361000, C359S642000
Reexamination Certificate
active
06649326
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical projection lithography methods and photolithography, and particularly to fluoride crystals with minimal spatial dispersion for use in optical photolithography systems utilizing ultraviolet light (UV) wavelengths below 200 nm, such as UV lithography systems utilizing wavelengths in the 193 nm region and the 157 nm region.
2. Technical Background
Projection optical photolithography methods/systems that utilize the ultraviolet wavelengths of light below 200 nm provide benefits in terms of achieving smaller feature dimensions. Such methods/systems that utilize ultraviolet wavelengths in the 157 nm and the 193 nm wavelength regions have the potential of improving the manufacturing of integrated circuits with smaller feature sizes. The commercial use and adoption of below 200 nm UV in high volume mass production of integrated circuits hinges on the availability of economically manufacturable optical fluoride crystals with high quality optical performance.
Fluoride crystals for use below 200-nm must have high internal transmission at the use wavelength (>98%/cm), high index of refraction homogeneity (<2 ppm) and low residual stress birefringence (<3 nm/cm). Stress birefringence is a consequence of the manufacturing process and can be minimized through careful annealing of the crystal. While the crystals typically used for these applications are cubic and so exhibit symmetric properties with respect to the crystal axes, they are not isotropic as for example, glass is. This distinction becomes clear when addressing a property called “spatial dispersion”. Spatial dispersion is a property that is described as the presence of birefringence that is dependent on the direction of light propagation. Glass (an isotropic material) has no such dependence. In cubic crystals such as Ge, Si and GaP, however, there is such a dependence that is found to exhibit 1/&lgr;
2
variation with wavelength (Optical Anisotropy of Silicon Single Crystals, by J. Pastrnak and K. Vedam, PHYSICAL REVIEW B, VOLUME 3, NUMBER 8, APR. 15, 1971, p. 2567-2571;
COMPUTATIONAL SOLID STATE PHYSICS
, by Peter Y. Yu and Manuel Cardona, Plenum Press, N.Y., edited by F. Herman, 1972; Spatial Dispersion In The Dielectric Constant of GaAs, by Peter Y. Yu and Manuel Cardona, SOLID STATE COMMUNICATIONS, VOLUME 9, NUMBER 16, Aug. 15, 1971, pp.1421-1424). The effect we are describing, spatial dispersion, is absent from the dielectric response of a cubic crystal in the limit in which the wavelength of light, &lgr;, is much larger than the spacing between atoms. As the wavelength becomes smaller, additional terms in the dielectric response are no longer negligible. In a cubic crystal, inversion symmetry of the crystal structure only allows the first nonzero contribution to occur at order 1/&lgr;
2
and not order 1/&lgr;. There is a mathematical description of dielectric response and crystal symmetry that uses tensors and their transformations to describe how dielectric response (including spatial dispersion) can depend on the direction of light propagation. Dielectric response is described using a rank 2 tensor, denoted &egr;
ij
. The lowest order effects of spatial dispersion can be described by a rank 4 tensor, here denoted &agr;
ijkl
, from the relation
ϵ
ij
(
q
→
)
=
ϵ
ij
(
q
→
=
0
)
+
∑
kl
α
ijkl
q
k
q
l
.
Here the symbol {right arrow over (q)} represents the wavevector of light; it points in the direction of light propagation and its magnitude is
2
π
λ
.
The equation shows that the long-wavelength or {right arrow over (q)}=0 part of the dielectric tensor gets corrected by the sum of elements of the &agr;
ijkl
tensor times the x-, y-, or z-components of the wavevector. (The sum on k and l is a sum over cartesian directions x, y, and z.) This correction term represents the source of spatial dispersion. In the absence of this term, a cubic crystal would have a completely isotropic dielectric tensor &egr;
ij
and hence no spatial dispersion. Of the possible 3×3×3×3=81 terms in the &agr;
ijkl
tensor, only 3 are nonzero and distinct in a cubic crystal with m3m symmetry, such as zincblende or fluorite structure crystals. It is known that rank 4 tensors have 3 tensor invariants. In fully isotropic systems such as glass, the tensor &agr;
ijkl
can only have 2 independent nonzero elements, and obeys the relation
(&agr;
1111
−&agr;
1122
)/2−&agr;
1212
=0.
The independent nonzero elements can be taken as &agr;
1111
and &agr;
1122
. In a cubic system with m3m symmetry, the relation above need not be satisfied, and there are 3 independent nonzero elements of &agr;
ijkl
. These may be taken as &agr;
1111
, &agr;
1122
, and &agr;
1212
. Since the first two tensor invariants are present in isotropic glasses, they cannot impart any anisotropy. Thus all anisotropy from spatial dispersion in cubic crystals is associated with the relation
(&agr;
1111
−&agr;
1122
)/2−&agr;
1212
≠0.
The value of this combination of tensor elements in a cubic system sets the scale for all anisotropic optical properties associated with spatial dispersion. These constants themselves depend on the wavelength of light with a variation that is typical of index dispersion, i.e. much less variation with wavelength than the explicit 1/&lgr;
2
. This invention shows how to design a material in which (&agr;
1111
−&agr;
1122
)/2−&agr;
1212
is minimized or preferably zero at a given wavelength of design.
Calcium fluoride, a potential material for use in UV lithography systems, also exhibits spatial dispersion. Spatial dispersion is an inherent property of the crystal and as such cannot be reduced by processing such as annealing. Stress-induced birefringence and spatial dispersion birefringence can be distinguished by their respective wavelength dependences. The variation of spatial dispersion with wavelength is very strong compared with the variation in index of refraction or stress-induced birefringence with wavelength, with stress birefringence exhibiting roughly the dependence expected for simply the index of refraction and spatial dispersion having 1/&lgr;
2
dependence.
Birefringence, whether it is derived from stress or the spatial properties of the crystal, can have a detrimental effect on high performance optical systems. The formation of multiple images is a major concern. Phase front distortion also presents problems both in terms of imaging and metrology. Given the wavelength dependence of spatial dispersion and the bandwidth of the lasers, dispersion becomes an important issue. It is thus of importance to minimize the amount of birefringence in a material for use in high performance optical imaging systems. As was mentioned previously, stress-related birefringence can be minimized by processing (annealing) while spatial dispersion is an inherent property that must be addressed in a different manner. One approach to the problem is to prepare mixed crystals that have minimized spatial dispersion; this is a single cubic fluoride crystal that contains 2 or 3 different alkaline earth metal cations that can deliver minimized spatial dispersion. This approach recognizes that the spatial birefringence of a given crystal is largely determined by the polarizability of the cation, by analogy with the Si and Ge crystals mentioned earlier. Specifically, we utilize a change in sign of the intrinsic birefringence for SrF
2
, CdF
2
, or BaF
2
relative to CaF
2
based on trends in polarizability.
SUMMARY OF THE INVENTION
The present invention includes an UV lithography method. The lithography method includes providing a radiation source with wavelength below 200-nm. The method includes providing cubic fluoride crystal optical elements having minimal spatial dispersion. The cubic fluoride crystals are comprised of a combination of alkaline earth cations having different optical polarizabilities suc
Allan Douglas C.
Borrelli Nicholas F.
Smith Charlene M.
Sparrow Robert W.
Chacko Davis Daborah
Douglas Walter M.
Huff Mark F.
Murphy Edward F.
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