Min K. Yang

Optical properties of Teflon ® AF amorphous fluoropolymers

The optical properties of three grades of Teflon ® AF— AF1300, AF1601, and AF2400—were investigated using a J.A. Woollam VUV-VASE spectroscopic ellipsometry system. The refractive indices for each grade were obtained from multiple measurements with different film thicknesses on Si substrates. The absorbances of Teflon ® AF films were determined by measuring the transmission intensity of Teflon ® AF films on CaF2 substrates. In addition to the refractive index and absorbance per cm base 10, the extinction coefficient k, and absorption coefficient per cm base e, Urbach parameters of absorption edge position and edge width, and two-pole Sellmeier parameters were determined for the three grades of Teflon ® AF. We found that the optical properties of the three grades of Teflon ® AF varied systematically with the AF TFE/PDD composition. The indices of refraction, extinction coefficient k, absorp- tion coefficient , and absorbance A increased, as did the TFE con- tent, while the PDD content decreased. In addition, the Urbach edge position moved to a longer wavelength, and the Urbach edge width became wider.

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...

Second generation fluids for 193nm immersion lithography

Water is the first generation immersion fluid for 193 nm immersion lithography. With a fluid refractive index of 1.436 and an optical absorbance of 0.01/cm at 193 nm, water immersion technology can enable optical lithography for the ITRS’ 65 nm half-pitch node. However, to achieve numerical apertures above 1.35 and to go beyond the 45 nm node, low absorbance fluids with indices higher than 1.6 are needed for the second generation of immersion lithography. We have developed a number of Gen. 2 high index fluids for immersion lithography at 193 nm. These highly transparent fluids have 193 nm indices up to 1.67. 32 nm 1:1 line space imaging has been demonstrated using two of our Gen 2 candidate fluids, representing major advance in optical lithography. To understand the behavior and performance of different fluid classes, we use spectral index measurements to characterize the index dispersion, coupled with Urbach absorption edge analysis, and Lorentz oscillator modeling.

Optical Properties and van der Waals–London Dispersion Interactions of Polystyrene Determined by Vacuum Ultraviolet Spectroscopy and Spectroscopic Ellipsometry

The interband optical properties of polystyrene in the vacuum ultraviolet (VUV) region have been investigated using combined spectroscopic ellipsometry and VUV spectroscopy. Over the range 1.5–32 eV, the optical properties exhibit electronic transitions we assign to three groupings, E1 ,E 2, and E3, corresponding to a hierarchy of interband transitions of aromatic (π → π*), non-bonding (n → π*, n → σ*), and saturated (σ → σ*) orbitals. In polystyrene there are strong features in the interband transitions arising from the side-chain π bonding of the aromatic ring consisting of a shoulder at 5.8 eV (E � ) and a peak at 6.3 eV (E1), and from the σ bonding of the C–C backbone at 12 eV (E � ) and 17.1 eV (E3). These E3 transitions have characteristic critical point line shapes associated with one-dimensionally delocalized electron states in the polymer backbone. A small shoulder at 9.9 eV (E2) is associated with excitations possibly from residual monomer or impurities. Knowledge of the valence electronic excitations of a material provides the necessary optical properties to calculate the van der Waals–London dispersion interactions using Lifshitz quantum electrodynamics theory and full spectral optical properties. Hamaker constants and the van der Waals–London dispersion component of the surface free energy for polystyrene were determined. These Lifshitz results were compared to the total surface free energy of polystyrene, polarity, and dispersive component of the surface free energy as determined from contact angle measurements with two liquids, and with literature values. The Lifshitz approach, using full spectral Hamaker constants, is a more direct determination of the van der Waals–London dispersion component of the surface free energy of polystyrene than other methods.

Imaging of 32-nm 1:1 lines and spaces using 193-nm immersion interference lithography with second-generation immersion fluids to achieve a numerical aperture of 1.5 and a k 1 of 0.25

Water-based immersion lithography using ArF illumination is able to provide optical solutions as far as the 45-nm node, but is not able to achieve the 38- or 32-nm nodes as currently defined. Achieving these lithographic nodes will require new, higher refractive index fluids to re- place the water used in first-generation immersion systems. We have developed a number of such second-generation high-index fluids for im- mersion lithography at 193 nm. These highly transparent fluids have 193-nm indices up to 1.664. To understand the behavior and perfor- mance of different fluid classes, we use spectral index measurements to characterize the index dispersion, coupled with Urbach absorption edge analysis and Lorentz Oscillator modeling. Interference imaging printers have long been available, and they now have a new use: a rapid, cost- effective way to develop immersion lithography, particularly at extremely high resolutions. Although interference printers will never replace classi- cal lens-based lithography systems for semiconductor device production, they do offer a way to develop resist and fluid technology at a relatively low cost. Their simple image-forming format offers easy access to the basic physics of advanced imaging. Issues such as polarization of the image-forming light rays, fluid/resist interaction during exposure, topcoat film performance, and resist line edge roughness LER at extremely high resolutions, can all be readily studied. 32-nm 1:1 line/space L/S imaging is demonstrated using two of the second-generation fluids. These resolutions are well beyond current lens-based system capabili- ties. Results on the performance of various resists and topcoats are also reported for 32-nm L/S features.

Optical properties of materials for concentrator photovoltaic systems

As part of our research on materials for concentrator photovoltaics (CPV), we are evaluating the optical properties and solar radiation durability of a number of polymeric materials with potential CPV application. For optical materials in imaging or non-imaging optical systems, detailed knowledge of the wavelength dependent complex index of refraction is important for optical system design and performance. Here we report the index of refraction, optical absorbance and haze results of various polymers of interest for CPV systems. Fluoropolymers such as polyvinylfluoride (Tedlar? PVF Film), which has wide application in crystalline silicon (c-Si) flat plate PV modules, as well as poly(tetrafluoroethylene-co-hexafluoropropylene) (Teflon? FEP Film) and poly(ethylene-co-tetrafluoroethylene) (Tefzel? ETFE Film) have desirable optical and physical properties for optical applications such as CPV. Hydrocarbon polymers such as polyvinylbutyral (PVB) sheet such as DuPont? PV5200, and the ethylene copolymers such as poly(ethylene-co-vinyl acetate) (EVA) such as Elvax? PV1400, poly(ethylene-co-methacrylic acid metal salt) ionomer sheet such as DuPont? PV5300 have applications as encapsulant in c-Si and other flat plate PV applications. These materials have both a wide variety of polymer compositions and also additive packages, which affect their optical properties such as the UV absorption edge. Even materials such as Kapton? polyimide films, which are used behind the PV cell for their electrically insulating properties, have optical requirements, and we characterize these materials also. The detailed optical properties of these materials will be useful for CPV system design of the geometrical optics, optimization of the systems optical throughput, and also provide insights into the systems optical absorption, for example in the UV, where this absorption can impact the radiation durability of the materials.

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.

Materials for Concentrator Photovoltaic Systems: Optical Properties and Solar Radiation Durability

Concentrator photovoltaic (CPV) systems are designed to operate over a wide range of solar concentrations, from low concentrations of ∼1 to 12 Suns to medium concentrations in the range from 12 to 200 Suns, to high concentration CPV systems going up to 2000 Suns. Many transparent optical materials are used for a wide variety of functions ranging from refractive and reflective optics to homogenizers, encapsulants and even thermal management. The classes of materials used also span a wide spectrum from hydrocarbon polymers (HCP) and fluoropolymers (FP) to silicon containing polymers and polyimides (PI). The optical properties of these materials are essential to the optical behavior of the system. At the same time radiation durability of these materials under the extremely wide range of solar concentrations is a critical performance requirement for the required lifetime of a CPV system. As part of our research on materials for CPV we are evaluating the optical properties and solar radiation durability of var...