John H. Burnett

Periodically poled BaMgF 4 for ultraviolet frequency generation

Ferroelectric domain inversion has been demonstrated in BaMgF(4) . Transparency has been measured to <140nm, and no change in transmission was measured under 157-nm irradiation for >1.1x10(9) shots at 2mJ/cm(2) per pulse. First-order quasi-phase-matched generation of 157 nm is predicted by use of grating periods as long as 1.5mum. This material should permit shorter-wavelength chi((2)) frequency-mixing processes than with any other crystalline material.

Fluid refractive index measurements using rough surface and prism minimum deviation techniques

Two techniques are presented for measuring the refractive index of fluids. The first is a reflective technique where liquid is applied to a rough surface to hold the liquid during measurement. Ellipsometric psi and delta data are acquired and analyzed to determine the fluid refractive index. The second technique is refractive and uses a hollow prism cell to contain the liquid. The fluid index is then determined using the prism minimum deviation technique. Both techniques have been applied over a very wide spectral range from the vacuum ultraviolet to the infrared and have been implemented on a research spectroscopic ellipsometer system (VUV-VASE®) with continuously variable angle of incidence. The refractive index of several candidate immersion fluids for 157 and 193nm immersion lithography are reported over the spectral range from 156to1700nm in a nitrogen-purged environment. The advantages and disadvantages of both techniques are discussed. Results were checked against values measured on very accurate p...

Symmetry of spatial-dispersion-induced birefringence and its implications for CaF2 ultraviolet optics

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.

Measurement of the refractive index and thermo-optic coefficient of water near 193 nm

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.

Absolute refractive indices and thermal coefficients of fused silica and calcium fluoride near 193 nm

The refractive indices of several fused silica and calcium fluoride samples from different suppliers were measured with the minimum deviation method in the deep UV between 191 and 196 nm with a standard uncertainty of 7 ppm. For both materials the dispersion dn/dlambda near 193 nm and 20 degrees C was determined. In addition, we measured the thermal coefficients of the refractive index near 193 nm and between 15 and 25 degrees C.

High-index materials for 193 nm immersion lithography

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.

Absolute Refractive Indices and Thermal Coefficients of CaF2, SrF2, BaF2, and LiF Near 157 nm

We present high-accuracy measurements for wavelengths near 157 nm of the absolute index of refraction, the index dispersion, and the temperature dependence of the index for the ultraviolet optical materials with cubic symmetry: CaF2, SrF2, BaF2, and LiF. Accurate values of these quantities for these materials are needed for designs of the lens systems for F2 excimer-laser-based exposure tools for 157-nm photolithography. These tools are expected to use CaF2 as the primary optical material and possibly one of the others to correct for chromatic aberrations. These optical properties were measured by the minimum deviation method. Absolute refractive indices were obtained with an absolute accuracy of 5 x 10(-6) to 6 x 10(-6).

Refractivity of nitrogen gas in the vacuum ultraviolet

We have measured the refractivity of nitrogen gas in the ultraviolet and the vacuum ultraviolet, using a Fourier-transform spectrometer. A new two-term Sellmeier formula for the standard refractivity between 145 and 270 nm is derived.

Optical properties of fluids for 248 and 193 nm immersion photolithography

We present measured values of the refractive index, thermo-optic coefficient, and absorption coefficient of a number of common organic solvents and aqueous inorganic solutions that may have application in immersion photolithography at 248 or 193 nm wavelengths. The measurements were performed with a laser-based Hilger-Chance refractometer system whose design and operation are described. The optical properties of the sample fluids are compared with those of water, the currently favored immersion medium, and we discuss the potential for finding higher-index fluids that will be suitable for this application.

Immersion fluid refractive indices using prism minimum deviation techniques

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.