A program has been started at NIST to make high-accuracy measurements of the infrared (IR) index properties of technologically important IR materials, in order to provide the IR optics community with updated values for the highest quality materials now available. For this purpose, we designed and built a minimum-deviation-angle refractometry system enabling diffraction-limited index measurements for wavelengths from 0.12 μm to 14 μm. We discuss the apparatus and procedures that we use for IR measurements. First results are presented for germanium for the wavelength range from 2 μm to 14 μm, with standard uncertainties ranging from 2 × 10-5 near 2 μm to 8 × 10-5 near 14 μm. This is an improvement by about an order of magnitude of the uncertainty level for index data of germanium generally used for optic design. A Sellmeier formula fitting our data for this range is provided. An analysis of the uncertainty is presented in detail. These measurements are compared to previous measurements of Ge.
We have developed a number of second-generation high-index candidate immersion fluids that exceed the 1.6 refractive index requirement for immersion lithography at 193 nm to replace the water used in first-generation immersion systems. To understand the behavior and performance of different fluid classes, we use spectral index measurements, based on the prism minimum deviation method, to characterize the index dispersion. In addition to fluid absorbance and index requirements, the temperature coefficient of the refractive index is a key parameter. We have used a laser-based Hilger-Chance refractometer system to determine the thermo-optic coefficient (dn/dT) by measuring the index change versus temperature at two different laser wavelengths, 632.8 and 193.4 nm. Also, we determined the batch-to-batch (within a 6-month period), before and after irradiation (at 193.4 nm), before and after air exposure, and before and after resist exposure (image printing test) variations of index and n/. The optical properties of these second-generation immersion fluids mostly compare favorably to water; the ratio of index of refraction at 193.4 nm is 1.644/1.437, the dispersion from d-line (n193-d) is 0.160 versus 0.103 and dn/dT at 193.4 is −550×10−6/K vs. −93×10−6/K, respectively.
We discuss the birefringence issues associated with use of crystalline sapphire, with uniaxial crystal structure, as a last
lens element for high-index 193 nm immersion lithography. Sapphire is a credible high-index lens material candidate
because with appropriate orientation and TE polarization the ordinary ray exhibits the required isotropic optical
properties. Also, its material properties may give it higher potential to meet the stringent optical requirements compared
to the potential of the principal candidate materials, cubic-symmetry LuAG and ceramic spinel. The TE polarization
restriction is required anyway for hyper-NA imaging, due to TM-polarization contrast degradation effects. Further, the
high uniaxial-structure birefringence of sapphire may offer the advantage that any residual TM polarization results in a
relatively-uniform flare instead of contrast degradation. One issue with this concept is that spatial-dispersion-induced
effects should cause some index anisotropy of the ordinary rays, in a way similar to the intrinsic birefringence (IBR)
effects in cubic crystals, except that there is no ray splitting. We present the theory of this effect for the trigonal crystal
structure of sapphire and discuss its implications for lithography optics. For this material the spatial-dispersion-induced
effects are characterized by eight material parameters, of which three contribute to index anisotropy of the ordinary
rays. Only one gives rise to azimuthal distortions, and may present challenges for correction. To assess the
consequences of using sapphire as a last element, neglecting any IBR effects, we use lithography simulations to
characterize the lithographic performance for a 1.7 NA design, and compare to that for LuAG.
The progress of optical lithography has approached the sub-30 nm regime using 193nm excimer lasers as the
exposure sources. To increase the numerical aperture (NA) further, many issues, especially those related to materials,
need to be addressed. In this paper, we present the analytical and experimental results of oblique two-beam lithography
with sapphire (Al2O3) as the optical material. At 193nm, the index of sapphire is 1.92 while the typical index of a
photoresist is near 1.70. Classical theory predicts that, ignoring the absorbance in the photoresist, once the NA is greater
than the photoresist refractive index, no energy will be transmitted across the sapphire/photoresist boundary due to total
internal reflection. However, it can be shown that the absorbance in the resist prevents a "critical angle" and total internal
reflection will not occur. Photoresist exposure can result even when NA is greater than the photoresist refractive index.
The image profile is strongly affected by the real and imaginary parts of the photoresist refractive index. Optimization of
photoresist optical properties is necessary for good image profile. Lutetium aluminum garnet (Lu3Al5O12 or LuAG with
an index 2.14 at 193 nm) is also investigated as an alternative lens material.
We report on our comprehensive survey of high-index UV optical materials that may enable extension of immersion lithography beyond a numerical aperture of 1.45. Band edge, refractive index, and intrinsic birefringence (IBR) at 193 nm determine basic viability. Our measurements of these properties have reduced the list of potential candidates to: ceramic spinel, lutetium aluminum garnet, and a class of germanium garnets. We discuss our measurements of the intrinsic properties of these materials and assess the present status of their material quality relative to requirements. Ceramic spinel has no significant IBR, but transmission and scatter for the best samples remain at least two orders of magnitude from specifications. Improving these would require a major development effort. Presently available lutetium aluminum garnet has material quality much closer to the specifications. However, the IBR is about three times the required value. The germanium garnets offer the possibility of a lower IBR, but a suitable candidate material has yet to be established.
For the next-generation immersion lithography technology, there is a growing interest in the immersion fluids having a refractive index larger than 1.5 and low absorbance at 193nm wavelength. In this paper, we report our effort in identifying new immersion fluid candidates. The absolute refractive index values and thermo-optic coefficients, dn/dT, were measured with 1x10-4 and 1x10-5 accuracy respectively at 193nm wavelength. The results showed promising candidates having refractive index ranging from 1.5 to 1.65 with low absorbance at 193nm wavelength. Preliminary imaging results with a new immersion fluid gave good 65nm Line/Space patterns. However, the minimum exposure time of 20sec is about ten times as needed for water, indicating the need to further reduce the absorbance of the immersion fluid.
193 nm immersion lithography optical projection systems using conventional UV optical materials and water as the immersion fluid, with planar lens/fluid interfaces, have a practical numerical aperture (NA) limit near 1.3. The bottleneck for pushing the NA further is the refractive index of the final lens element. Higher-index immersion fluids cannot alone give much improvement, because the NA is limited by the lowest material index. In this paper we consider the possibility of using novel high-index materials in the last lens element to get around this bottleneck and to push the NA limit to at least 1.5, while containing the lens system size and complexity. We discuss three classes of high-index (n>1.8), wide-band-gap, oxide-based materials that have the potential for being fabricated with optical properties appropriate for lithography optics: group-II oxides, magnesium-aluminum-spinel-related materials, and ceramic forms of spinel. We present theoretical calculations and experimental measurements of the optical properties of these materials, including intrinsic birefringence, and we assess their prospects.
Immersion fluids for 157 nm and 193 nm immersion lithography have been measured over the spectral range from 156 nm to 1700 nm in a nitrogen purged environment. The refractive index n and k of several candidate fluids have been measured using the prism minimum deviation technique implemented on a commercial Variable Angle Spectroscopic Ellipsometer (VASE) system. For measurement the liquids were contained in a triangular prism cell made with fused silica windows. The refractive index of high-purity water at 21.5° C measured over the spectral range 185 nm to 500 nm. was checked against values measured on high accuracy prism minimum deviation equipment by NIST and agreement with NIST has been found to be good. The refractive index at a nominal temperature of 32°C for four fluorinated fluids in the range of n=1.308 to 1.325 at 157 nm are also reported. It was found to be extremely important to correct for temperature differences among different instruments using the thermo-optic coefficient of each liquid. The 157 nm results on fluorinated fluids are compared with measurements at NIST using a VUV Hilger-Chance Refractometer, which measured both the refractive index and the thermo-optic coefficient. In all cases results agree well.
We present accurate measurements of several important optical constants of high-purity water for wavelengths near 193.39 nm at 21.50°C: the absolute refractive index n = 1.436 62(2), the dispersion dn/dλ = −2.109(17)×10−3 nm−1, and the thermo-optic coefficient dn/dT = −1.00(4)×10−4 (°C)−1. We determine these values by two independent approaches, the minimum deviation prism method and an interferometric technique, and find that they give mutually consistent results.
We discuss our approaches for measuring the absolute index (n), and its dependencies on wavelength (dn/dλ) and temperature (dn/dT), of high-purity water for wavelengths near 193 nm, using the minimum deviation prism method and an interferometric technique. We present preliminary results for these quantities measured by the minimum deviation method.
The discovery of a significant spatial-dispersion-induced birefringence (intrinsic birefringence) in CaF2 at ultraviolet wavelengths has had a major impact on the design of 157 nm lithography systems, requiring complete redesign of the optics to take account of the imaging aberrations resulting from the birefringence and the accompanying index anisotropy. This intrinsic birefringence phenomena results from a symmetry-breaking effect of the finite wave vector of the photon on the symmetry of the light-matter interaction in fluorite-structure cubic crystals. As a follow-up to our original concise report of measurements and theory of the effect in CaF2 and BaF2, we present here a more detailed analysis of the theory, focusing on the symmetry and its consequences. We also provide the full directional dependence of the effect in useful closed forms. We analyze the implications for precision optical design with CaF2 optical elements, and discuss qualitatively the approaches being considered to compensate for it.
We have made accurate measurements near 157 nm of the absolute index of refraction, the index of refraction, the index dispersion, and the temperature dependence of the index, for the cubic-symmetry, group-II fluorides: calcium fluoride, strontium fluoride, and barium fluoride. Accurate measurements of these quantities for calcium fluoride are needed for designs of lens systems for 157 nm excimer-laser- based exposure tools for photolithography. Measurements of these isotropic materials high ly transmissive near 157 nm, such as strontium fluoride and barium fluoride, are needed for designs which include secondary materials in addition to calcium fluoride to correct for chromatic aberrations. These optical properties were measured with precision gonimeters on prisms of the materials in a nitrogen atmosphere using the minimum deviation method. The relative refractive indices were converted to absolute indices by using values of the index of nitrogen obtained by independent interferometric measurement near 157 nm. Values of the absolute refractive indices for these materials were obtained with an accuracy of 6 X 10-6. A key result is that the dispersion of barium fluoride near 157 nm is approximately 68 percent larger than that of calcium fluoride, which makes barium fluoride a potentially useful material for combination with calcium fluoride for chromatic aberration correction near 157 nm.
In the minds of many, Fourier transform spectrometry is restricted to applications in the infrared. In the ultraviolet, the increasingly severe demands on optical, data acquisition, and motion control systems of the interferometer diminish the effectiveness of the technique. However, with recent advances in ultraviolet optics, data acquisition and sampling techniques for Fourier transform spectrometers, these stringent demands are easier to meet at vacuum ultraviolet wavelengths and significantly reduce the cost of Fourier transform spectrometers. The FT700 spectrometer at NIST can operate at wavelengths as low as 140 nm, limited by the short wavelength cut-off of the calcium fluoride optics. We illustrate the capabilities of the FT700 spectrometer in the ultraviolet with several recent results in atomic emission spectrometry, plasma diagnostics, and refractometry.
We have made accurate measurement near 157 nm of the relative index of refraction, its dispersion, and its temperature dependence for two grades of calcium fluoride in N2 gas. Accurate measurements of these quantities are needed for the design of lens system for 157 nm F2 excimer-laser-based exposure tools for photolithography. These optical properties were measured with precision goniometer on prisms of the materials in a N2 atmosphere using the minimum deviation method. The dispersion was determined using line emission radiation from a deuterium lamp at several wavelengths near 157 nm. Values of the relative refractive index for two grades of calcium fluoride in N2 gas corrected to a temperature of 20 degrees C and a pressure of one standard atmosphere are well within our 7 ppm estimated uncertainty forth measurements. The temperature of the samples and the surrounding medium were controlled to 0.05 degrees C, which enabled accurate measurements of the temperature dependencies of the indices around room temperature near 157 nm.
We discuss a procedure for making accurate measurement of the index of refraction, its dispersion, and its temperature dependence, in the deep ultraviolet (near 193 nm), using precision goniometric spectrometers and the minimum deviation method. Measurements of the indices of fused silica and calcium fluoride near 193 nm, with a fractional accuracy of 7 ppm, are discussed. These measurements revealed differences in the indices between different grades of fused silica. Accurate values of the temperature dependencies were determined from measurements of the indices at several temperatures in a 20 degree Celsius range about 20 degrees Celsius. A procedure to measure the index of calcium fluoride in the vacuum ultraviolet region (157 nm) using a N2 purge housing is discussed.
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