R. A. Synowicki

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...

Thin Film Materials Exposure to Low Earth Orbit Aboard Space Shuttle

To study the effects of Atomic Oxygen on various thin film materials, fourteen thin film samples were exposed to the corrosive environment of low Earth orbit. Total exposure was 42 hours, resulting in a nominal atomic oxygen fluence of 2.2 X 1020 atoms/cm2. The films included aluminum, diamondlike carbon, diamond, and multilayer stacks. Included are experimental details of sample preparation, exposure, and post-flight results. Pre-flight characterization techniques included Variable Angle Spectroscopic Ellipsometry, optical reflectance and transmittance, Atomic Force Microscopy, and Raman scattering. Post-flight analysis repeated pre-flight characterization. Aluminum films resisted degradation. Surface contaminants were identified using Auger Electron Spectroscopy. Contaminants were SiC>2, fluorine, and sulfur which most likely result from degradation of cargo bay lining, waste water dumps, and outgassing. Diamondlike carbon films were completely etched away during exposure. Polycrystalline diamond films were extremely resistant to atomic oxygen degradation, showing no post-flight structural, compositional, or mass changes. Aluminum films 23.5 nm thick simultaneously protect silver reflecting layers from oxidation and increase the ultraviolet reflectance of the stack. Decreasing the aluminum thickness to 7.5 nm resulted in complete oxidation during exposure and failure as a protective coating.

Low Earth Orbit Effects on Indium Tin Oxide and Polyester and Comparison With Laboratory Simulations

Abstract Laboratory simulation of the low Earth orbit (LEO) environment using oxygen plasma ashers are discussed. Their effectiveness as space simulators are compared with LEO through analysis of indium tin oxide (ITO) thin films and bulk polyester exposed to both environments. Spectrophotometry and atomic force microscopy have been used to characterize optical and microstructural changes as a result of exposure to the simulated (oxygen plasma asher) and the actual space environment aboard shuttle flight STS-46. Results show that the low Earth orbit space environment is much harsher than the plasma asher on the optical properties of ITO as well as the surface roughness of polyester. On space-exposed samples, a significant shift in the ITO absorption edge is seen for fluences of 2 × 10 20 atoms cm -2 but not on films exposed in the asher. The surface roughness of polyester exposed in the asher increase by a factor of 5.5, while that of polyester exposed in space increases by a factor of 20 for the same atomic oxygen fluence. The directional nature and higher kinetic energy of atomic oxygen in LEO serves to erode polyester more than in the asher. The different results obtained in the asher for both ITO and polyester bring into question the suitability of using plasma ashers as space simulators for these materials.

Low Earth Simulation and Materials Characterization

Oxygen plasma ashers and an electron cyclotron resonance (ECR) sources are currently being used for low Earth orbit (LEO) simulation. The suitability of each of these simulation techniques is considered. Thin film coatings are characterized by optical techniques, including variable-angle spectroscopic ellipsometry, optical spectrophotometry, and laser light scatterometry. Atomic force microscopy (AFM) has been used to characterize the surface morphology of thin aluminum films as a function of substrate temperature during deposition. Results on diamondlike carbon (DLC) films show that DLC degrades with simulated atomic oxygen (AO) exposure at a rate comparable to Kapton polyimide. Since DLC is not as susceptible as Kapton to environmental factors such as moisture absorption, it could potentially provide more accurate measurements of AO fluence on short space flights.

Oxygen plasma ashing effects on aluminum and titanium space protective coatings

Using variable angle spectroscopic ellipsometry and atomic force microscopy (AFM), the surface roughness and oxidation of aluminum and titanium thin films have been studied as a function of substrate deposition temperature and oxygen plasma exposure. Increasing substrate deposition temperatures affect film microstructure by greatly increasing grain size. Short exposures to an oxygen plasma environment produce sharp spikes rising rapidly above the surface as seen by AFM. Ellipsometric measurements were made over a wide range of plasma exposure times, and results at longer exposure times suggest that the surface is greater than 30% void. This is qualitatively verified by the AFM images.

Degradation of thin films: comparison between low Earth orbit experiments and laboratory simulations of the space environment

Abstract The low Earth orbit (LEO) environment exposes spacecraft materials to atomic oxygen, UV light, meteroid impact and thermal cycling. The purpose of this paper is to report on progress towards evaluating damage done to candidate space materials, and ways to protect materials on future long-term space missions in LEO. Specifically, we prepared and characterized sets of samples for flights on the US Space Shuttle missions STS-46 and STS-51, and evaluated samples returned from STS-46. In addition, laboratory simulations of the LEO environment are shown to present interesting problems and challenges.

Laboratory Simulation of Low Earth Orbit

Thin films of several inorganic protective coatings were prepared in duplicate by sputtering and evaporation. Pure oxygen plasmas were used as a source of atomic oxygen and UV radiation degradation, and in-situ spectroscopic ellipsometry used as a sensitive diagnostic of surface damage or film deposition. After both plasma and LEO exposure on the shuttle (three separate flights) detailed materials degradation evaluations were made using ellipsometry, weight loss, Auger spectroscopy, and reflectrometry. Plasma chambers often contaminate the ground-based experiments and sensitive ways to detect this are discussed. Likewise, contamination together with degradation take place simultaneously during shuttle flights.

Oxygen plasma asher contamination: An analysis of sources and remedies

The low Earth orbit ~LEO! environment is commonly simulated using oxygen plasma ashers to determine the effects of LEO on spacecraft materials. However, plasma ashers can also contaminate samples during plasma exposure, making them less than ideal for space simulation. This study results from attempts to minimize or eliminate contamination. Optical methods of variable angle spectroscopic ellipsometry and reflectance spectrophotometry were used to quantify contaminant stoichiometry and deposition rate. Auger electron spectroscopy identified deposited contaminants and their surface coverage. Contamination results from etching of the rubber chamber seals by the plasma. The deposited contaminant was nearly indistinguishable from fully stoichiometric SiO 2 . Contaminant deposition rates up to 0.27 nm/min have been observed, and these layers effectively passivate the surface by depositing an overcoat of SiO x . Placing metal into the path of the plasma before it can reach the chamber seals greatly reduces contamination. A newly designed chamber confines the plasma to a small volume away from the chamber seals. For fluences as high as 3.5310 22 atoms/cm 2 , equivalent to 7.5 years of space exposure for the International Space Station, the redesigned asher showed less than one monolayer of deposited contaminant.

Spectroscopic Ellipsometry Applications in Advanced Lithography Research

Spectroscopic ellipsometry (SE) is an optical metrology technique widely used in the semiconductor industry. For lithography applications SE is routinely used for measurement of film thickness and refractive index of polymer photoresist and antireflective coatings. While this remains a primary use of SE, applications are now expanding into other areas of advanced lithography research. New applications include immersion lithography, phase‐shift photomasks, transparent pellicles, 193 and 157 nm lithography, stepper optical coatings, imprint lithography, and even real‐time monitoring of etch development rate in liquid ambients. Of recent interest are studies of immersion fluids where knowledge of the fluid refractive index and absorption are critical to their use in immersion lithography. Phase‐shift photomasks are also of interest as the thickness and index of the phase‐shift and absorber layers must be critically controlled for accurate intensity and phase transmission. Thin transparent pellicles to protec...

Microstructural characterization of SiOx surface contaminants on ashed aluminum thin films

Abstract Thin films of aluminum were exposed in short increments to a plasma environment using a semiconductor plasma asher. The process gas was a mixture of air and oxygen. The total exposed oxygen fluence was 1.2 × 10 21 oxygen atoms cm −2 accumulated over 34 h. Heavy contamination of film surfaces resulted from plasma etching of the chamber seals. The surface microstructure of this deposited contaminant was studied with atomic force microscopy (AFM). The contaminants appear to nucleate and grow at protruding surface features. These features appear in the AFM data as dome-shaped growths rising rapidly from the surface. The density of these observed growths is much higher on sputtered films than on electron-beam evaporated films. These differences are most likely due to the microstructure of the as-deposited films. The as-deposited sputtered films are rougher than their evaporated counterparts and provide more sites for nucleation of the observed contaminant growths. The surface roughness of the contaminated films increases quickly by a factor of 3–6 within 300 min of exposure and saturates thereafter. It is postulated that the surface roughness increases until a continuous, but rough, contaminant layer is formed. After the formation of a continuous layer, additional material deposits while maintaining the existing surface microstructure. The growth of rough contaminant features has a significant effect on the specular reflectance of the films. Sputtered films show larger losses in specular reflectance than evaporated films after asher exposure.