SEARCH · Engineering Papers
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
XeF2 is Cyanogen Chloride-derived structured and crystallizes in the tetragonal I4/mmm space group. The structure is zero-dimensional and consists of two XeF2 clusters. Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.08 Å. F is bonded in a single-bond geometry to one Xe atom.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
Fine-pitch, sub-collimating X-ray grids have been developed for an instrument in the High Energy Solar Spectroscopic Imager , a proposed NASA mission. In addition to high-energy x- rays, the instrument requires collimation of photons with energies of less than 4 keV such that free-standing grids are required that have no material between the grid slats. Fabrication of these grids is described.
Pure Aluminum (Al) exhibits intrinsic high reflectance over the proposed Large UV/Optical/IR (LUVOIR) Surveyor observatory target spectral range (90-2500 nm). However, Al coatings have to be protected from oxygen exposure in order to prevent the formation of the naturally occurring Al2O3 oxide layer, which limit performance for use only above 160 nm. Aluminum protected with fluorides such as LiF or MgF2 have been the most commonly used solutions. But below 102 nm and down to 90 nm, no transparent material is available to protect Al and coating mirror reflectance stays below 40%. But even above 102 nm, the reflectance of protected Al is limited by the residual absorption of the fluoride overcoats. Hence, this paper will report on recent advances in producing enhanced Al-based mirror coatings with the highest possible far-ultraviolet (FUV) reflectance, while exhibiting a more stable LiF protection layer. The process starts with a bare optically smooth glass substrate that is coated with Al in an ultra-high vacuum chamber by using the physical vapor deposition (PVD) process, which has been shown to provide the best aluminum thin-films when compared to other approaches. The next step is done by in-situ exposure of the freshly made Al film to a reactive XeF2 gas that will grant a thin AlF3 overcoat (2-3 nm) to the Al film that prevents further oxidation. This fluorinated Al film is then coated with a final layer of a LiF metal-fluoride overcoat. The coating process is finalized with a second exposure to the XeF2 precursor gas. The preparation of these mirror coatings will be studied and analyzed as a function of the XeF2 exposure time and deposition rate of the LiF layer during the PVD process. This paper will present and discuss characterization of a number of Al+XeLiF witness coupons produced with this reactive PVD (rPVD) process. These studies include characterization of specular reflectance in the 90-2500 nm spectral range, micro-roughness, long-term stability, as well as polarization characteristics in the visible and near-infrared spectral regions. These studies have been performed in order to demonstrate the improved reflectance performance, longer durability, and less hygroscopic nature of protected Al mirrors produced with the rPVD process (through exposure to XeF2) and in comparison with the standard PVD process.
Previously, it was difficult to fabricate deformable mirrors made by piezoelectric actuators. This is because numerous actuators need to be precisely assembled to control the surface shape of the mirror. Two approaches have been developed. Both approaches begin by depositing a stack of piezoelectric films and electrodes over a silicon wafer substrate. In the first approach, the silicon wafer is removed initially by plasmabased reactive ion etching (RIE), and non-plasma dry etching with xenon difluoride (XeF2). In the second approach, the actuator film stack is immersed in a liquid such as deionized water. The adhesion between the actuator film stack and the substrate is relatively weak. Simply by seeping liquid between the film and the substrate, the actuator film stack is gently released from the substrate. The deformable mirror contains multiple piezoelectric membrane layers as well as multiple electrode layers (some are patterned and some are unpatterned). At the piezolectric layer, polyvinylidene fluoride (PVDF), or its co-polymer, poly(vinylidene fluoride trifluoroethylene P(VDF-TrFE) is used. The surface of the mirror is coated with a reflective coating. The actuator film stack is fabricated on silicon, or silicon on insulator (SOI) substrate, by repeatedly spin-coating the PVDF or P(VDFTrFE) solution and patterned metal (electrode) deposition. In the first approach, the actuator film stack is prepared on SOI substrate. Then, the thick silicon (typically 500-micron thick and called handle silicon) of the SOI wafer is etched by a deep reactive ion etching process tool (SF6-based plasma etching). This deep RIE stops at the middle SiO2 layer. The middle SiO2 layer is etched by either HF-based wet etching or dry plasma etch. The thin silicon layer (generally called a device layer) of SOI is removed by XeF2 dry etch. This XeF2 etch is very gentle and extremely selective, so the released mirror membrane is not damaged. It is possible to replace SOI with silicon substrate, but this will require tighter DRIE process control as well as generally longer and less efficient XeF2 etch. In the second approach, the actuator film stack is first constructed on a silicon wafer. It helps to use a polyimide intermediate layer such as Kapton because the adhesion between the polyimide and silicon is generally weak. A mirror mount ring is attached by using adhesive. Then, the assembly is partially submerged in liquid water. The water tends to seep between the actuator film stack and silicon substrate. As a result, the actuator membrane can be gently released from the silicon substrate. The actuator membrane is very flat because it is fixed to the mirror mount prior to the release. Deformable mirrors require extremely good surface optical quality. In the technology described here, the deformable mirror is fabricated on pristine substrates such as prime-grade silicon wafers. The deformable mirror is released by selectively removing the substrate. Therefore, the released deformable mirror surface replicates the optical quality of the underlying pristine substrate.
(XeF2)4(AsF6)2O2 crystallizes in the tetragonal I4/mcm space group. The structure is zero-dimensional and consists of four water molecules, four AsF6 clusters, and eight XeF2 clusters. In each AsF6 cluster, As is bonded in an octahedral geometry to six equivalent F atoms. All As–F bond lengths are 1.78 Å. F is bonded in a single-bond geometry to one As atom. In each XeF2 cluster, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.09 Å. F is bonded in a single-bond geometry to one Xe atom.
XeF2IF5 is Heusler-derived structured and crystallizes in the tetragonal I4/m space group. The structure is zero-dimensional and consists of four IF5 clusters and four XeF2 clusters. In each IF5 cluster, I is bonded in a square pyramidal geometry to five F atoms. There is one shorter (1.87 Å) and four longer (1.93 Å) I–F bond length. There are two inequivalent F sites. In the first F site, F is bonded in a single-bond geometry to one I atom. In the second F site, F is bonded in a single-bond geometry to one I atom. In each XeF2 cluster, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.09 Å. F is bonded in a single-bond geometry to one Xe atom.
BaSb2(XeF5)4XeF2 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional and consists of two XeF2 clusters and one BaSb2(XeF5)4 framework. In each XeF2 cluster, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.07 Å. F is bonded in a single-bond geometry to one Xe atom. In the BaSb2(XeF5)4 framework, there are three inequivalent Xe sites. In the first Xe site, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.08 Å. In the second Xe site, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.08 Å. In the third Xe site, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.08 Å. Ba is bonded to twelve F atoms to form BaF12 cuboctahedra that share corners with four equivalent SbF6 octahedra. The corner-sharing octahedral tilt angles are 22°. There are a spread of Ba–F bond distances ranging from 2.79–3.08 Å. Sb is bonded to six F atoms to form SbF6 octahedra that share corners with two equivalent BaF12 cuboctahedra. There is four shorter (1.92 Å) and two longer (1.93 Å) Sb–F bond length. There are five inequivalent F sites. In the first F site, F is bonded in a single-bond geometry to one Sb atom. In the second F site, F is bonded in a distorted single-bond geometry to one Ba and one Sb atom. In the third F site, F is bonded in a bent 150 degrees geometry to one Xe and one Ba atom. In the fourth F site, F is bonded in a distorted single-bond geometry to one Xe and one Ba atom. In the fifth F site, F is bonded in a distorted single-bond geometry to one Xe and one Ba atom.
XeF2XeF4 is Copper structured and crystallizes in the monoclinic P2_1/c space group. The structure is zero-dimensional and consists of two XeF2 clusters and two XeF4 clusters. In each XeF2 cluster, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.08 Å. F is bonded in a single-bond geometry to one Xe atom. In each XeF4 cluster, Xe is bonded in a square co-planar geometry to four equivalent F atoms. All Xe–F bond lengths are 2.04 Å. F is bonded in a single-bond geometry to one Xe atom.
BaNb2(XeF5)4XeF2 crystallizes in the orthorhombic Fmmm space group. The structure is three-dimensional and consists of four XeF2 clusters and one BaNb2(XeF5)4 framework. In each XeF2 cluster, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.07 Å. F is bonded in a single-bond geometry to one Xe atom. In the BaNb2(XeF5)4 framework, there are two inequivalent Xe sites. In the first Xe site, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.08 Å. In the second Xe site, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.07 Å. Ba is bonded to twelve F atoms to form BaF12 cuboctahedra that share corners with four equivalent NbF6 octahedra. The corner-sharing octahedral tilt angles are 21°. There are a spread of Ba–F bond distances ranging from 2.83–3.10 Å. Nb is bonded to six F atoms to form NbF6 octahedra that share corners with two equivalent BaF12 cuboctahedra. There is four shorter (1.94 Å) and two longer (1.96 Å) Nb–F bond length. There are four inequivalent F sites. In the first F site, F is bonded in a bent 150 degrees geometry to one Xe and one Ba atom. In the second F site, F is bonded in a distorted single-bond geometry to one Xe and one Ba atom. In the third F site, F is bonded in a distorted single-bond geometry to one Ba and one Nb atom. In the fourth F site, F is bonded in a single-bond geometry to one Nb atom.
XeF2Sb2F11 crystallizes in the triclinic P-1 space group. The structure is zero-dimensional and consists of two Sb2F11 clusters and two XeF2 clusters. In each Sb2F11 cluster, there are two inequivalent Sb sites. In the first Sb site, Sb is bonded to six F atoms to form corner-sharing SbF6 octahedra. The corner-sharing octahedral tilt angles are 12°. There are a spread of Sb–F bond distances ranging from 1.90–2.08 Å. In the second Sb site, Sb is bonded to six F atoms to form corner-sharing SbF6 octahedra. The corner-sharing octahedral tilt angles are 12°. There are a spread of Sb–F bond distances ranging from 1.90–2.06 Å. There are eleven inequivalent F sites. In the first F site, F is bonded in a single-bond geometry to one Sb atom. In the second F site, F is bonded in a single-bond geometry to one Sb atom. In the third F site, F is bonded in a single-bond geometry to one Sb atom. In the fourth F site, F is bonded in a single-bond geometry to one Sb atom. In the fifth F site, F is bonded in a single-bond geometry to one Sb atom. In the sixth F site, F is bonded in a linear geometry to two Sb atoms. In the seventh F site, F is bonded in a single-bond geometry to one Sb atom. In the eighth F site, F is bonded in a single-bond geometry to one Sb atom. In the ninth F site, F is bonded in a single-bond geometry to one Sb atom. In the tenth F site, F is bonded in a single-bond geometry to one Sb atom. In the eleventh F site, F is bonded in a single-bond geometry to one Sb atom. In each XeF2 cluster, Xe is bonded in a linear geometry to two equivalent F atoms. Both Xe–F bond lengths are 2.03 Å. F is bonded in a single-bond geometry to one Xe atom.
Microscopic circuit structures prepared for probing. Xenon difluoride removes relatively large amounts of silicon from integratedcircuit or solar-cell structures while leaving SiO2, Si3N4, Al2O3, and other compounds intact. In Etching Apparatus, solid XeF2 sublimated in vacuum, then allowed to flow over sample at controlled rate and pressure. Wafer etched from back to expose SiO2 and Al layers for spectroscopic analysis of SiO2/Al interface. Using XeF2 technique, silicon wafer with oxide layer reduced in thickness from standard 300 micrometer to as little as 10 nanometer without adversely affecting oxide.
X-ray photoelectron spectroscopy has been used to study the composition of 100-A thermally grown SiO2 films that have been thermally nitrided in ammonia. The SiO(x)N(y)/Si interface was studied both by chemical depth profiling of the oxynitride and by removal of the Si substrate with XeF2. It is found that N is distributed throughout the film, but with the concentration higher at the surface and in a region centered 25 A from the film/substrate interface. The interface region itself is found to be oxygen-rich relative to the rest of the film. Possible models which can explain these results are discussed.
The interfacial structure of silicon/dielectric and silicon/metal systems is particularly amenable to analysis using a combination of surface spectroscopies together with a variety of chemical structures of Si/SiO2, Si/SiO2Si3N4, Si/Si2N2O, Si/SiO2/Al, and Si/Native Oxide interfaces using high resolution (0.350 eV FWHM) X ray photoelectron spectroscopy. The general structure of these dielectric interfaces entails a monolayer chemical transition layer at the Si/dielectric boundary. Amorphous Si substrates show a wide variety of hydrogenated Si and Si(OH) sub x states that are not observed in thermal oxidation of single crystal material. Extended SiO2 layers greater than 8 A in thickness are shown to be stoichiometric SiO2, but to exhibit a wide variety of local network structures. In the nitrogen containing systems, an approach to stoichiometric oxynitride compounds with interesting impurity and electron trapping properties are seen. In native oxides, substantial topographical nonuniformity in oxide thickness and composition are found. Analysis of metal/oxide interfacial layers is accomplished by analytical removal of the Si substrate by UHV XeF2 dry etching methods.
The first nondestructive measurement of the chemical and physical characteristics of the interface between bulk SiO2 and thick aluminum films is presented. Both X-ray photoelectron spectroscopy (XPS) and electrical measurements of unannealed resistively evaporated Al films on thermal SiO2 indicate an atomically abrupt interface. Postmetallization annealing (PMA) at 450 C induces reduction of the SiO2 by the aluminum, resulting in the layer ordering SiO2/Al2O3/Si/Al. The XPS measurement is performed from the SiO2 side after removal of the Si substrate after etching with XeF2 gas and thinning of the SiO2 layer with HF:ETOH. This represents a powerful new approach to the study of metal-insulator and other interfaces.
The nondestructive measurement of the chemical and physical characteristics of the interface between bulk SiO2 and thick aluminum films is reported. Both X-ray phototelectron spectroscopy (XPS) and electrical measurements of unannealed, resistively evaporated Al films on thermal SiO2 indicate an atomically abrupt interface. Post metallization annealing at 450 C induces reduction of the SiO2 by the aluminum, at a rate consistent with the bulk reaction rate. The XPS measurement is performed from the SiO2 side after the removal of the Si substrate with XeF2 gas and thinning of the SiO2 layer with HF:ETOH. This represents a powerful new approach to the study of metal-insulator and related interfaces.
This paper will describe efforts at developing broadband mirror coatings with high performance that will extend from infrared wavelengths down to the Far-Ultraviolet (FUV) spectral region. These mirror coatings would be realized by passivating the surface of freshly made aluminum coatings with XeF2 gas in order to form a thin AlF3 overcoat that will protect the aluminum from oxidation and, hence, realize the high-reflectance of this material down to its intrinsic cut-off wavelength of 90 nm. Improved reflective coatings for optics, particularly in the FUV region (90-120 nm), could yield dramatically more sensitive instruments and permit more instrument design freedom.