Vacuum Ultraviolet Exposure Research Articles

Degradation Property of Commercially Available Si-containing Polyimide in Simulated Atomic Oxygen Environments for Low Earth Orbit

Real-time measurement of the erosion rate of a commercially available Si-containing polyimide (BSF30) under hyperthermal atomic oxygen (AO) beam exposure condition, which simulates the AO environment in low Earth orbit (LEO), was performed. It was found that the erosion rate of BSF30 decreased with increasing AO fluence and it reached as low as 4% of the standard PMDA-ODA polyimide. X-ray photoelectron spectroscopy confirmed that the surface of AO-exposed BSF30 was covered by a SiO2 layer which functioned as a protective coating. In contrast, an SiO2 surface layer thick enough to protect bulk BSF30 was not formed by thermal AO, which was generated by vacuum ultraviolet exposure in an O2 atmosphere. Exposure to hyperthermal AO collision in LEO can also form an SiO2 layer which enables the surface to be self-healing and is desirable for a polyimide that would be used in LEO.

Vacuum-ultraviolet-induced charge depletion in plasma-charged patterned-dielectric wafers

Plasma-induced charging of patterned-dielectric structures during device fabrication can cause structural and electrical damage to devices. In this work, we report on vacuum-ultraviolet (VUV) radiation-induced charge depletion in plasma-charged patterned-silicon-oxide dielectric wafers. Charge depletion is studied as a function of photon energy and the aspect ratio of hole structures. The wafers were charged in a plasma and subsequently exposed to monochromatic-synchrotron-VUV. Surface-potential measurements after VUV exposure showed that photon energies less than 11 eV were beneficial in depleting the plasma-induced charge from the patterned-dielectric wafers. In addition, for a given photon-flux density and for photon energies less than 11 eV, VUV-induced charge depletion decreases with increasing hole aspect ratio. The results are explained with a physically plausible equivalent-circuit model, which suggests that both electron photoinjection from Si into the oxide and oxide surface conductivity play an important role in the charge-depletion process.

Surface modification of poly(tetrafluoroethylene-co-hexafluoropropylene) with vacuum UV radiation from rotating helium dc arc plasmas

Treatment of poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) with vacuum ultraviolet (VUV) radiation from high pressures of helium in rotating dc arc plasmas was investigated. X-ray photoelectron spectroscopy (XPS) detected defluorination and appearance of the functional groups (C=O, O–C=O and C–O) on the surface. Fourier transform infrared (FT-IR) spectroscopy showed the appearance of a band at 1884 cm−1 indicating the formation of the carboxylic acid fluoride moiety, –(C=O)–F. Improvement in film wettability was observed by contact angle measurements while SEM micrographs showed an increased amount of cracking on the surface with VUV exposure. Adhesion measurements of Cu sputter-coated onto the photo-modified surfaces resulted in failure within the fluoropolymer (cohesive failure) and not at the Cu-fluoropolymer interface. Cohesive failure occurred with shorter treatment times than for VUV exposure downstream from low pressure Ar microwave plasmas.

Synergistic effect of hyperthermal atomic oxygen beam and vacuum ultraviolet radiation exposures on the mechanical degradation of high-modulus aramid fibers

The synergistic effect of atomic oxygen and vacuum ultraviolet (VUV) radiation exposures was investigated on the mechanical properties of high-modulus aramid fibers. Tensile tests were performed on single fibers. It was found that both tensile strength and Young's modulus decreased significantly due to the simultaneous exposures of atomic oxygen and vacuum ultraviolet radiation. In contrast, the surface of aramid fiber exposed to ultraviolet alone showed no change in tensile strength and Young's modulus. X-ray photoelectron spectroscopy showed that atomic oxygen exposure leads to oxidization and decomposition of amid groups, that only VUV exposure does not change the fiber surface, and that VUV exposure decreased oxidized components on the oxidized fiber surface exposed to atomic oxygen and ultraviolet simultaneously. Scanning electron microscopy revealed that the diameter of the fiber was not influenced so much by atomic oxygen and ultraviolet exposures, but the fiber surface became substantially rougher than the aramid fiber surface exposed to atomic oxygen alone.

Changes in Properties and Structure of Polyethylene Terephthalate Film Under Vacuum Ultraviolet

The effect of vacuum ultraviolet (VUV) radiation on polyethylene terephthalate (PET) film was investigated on a gas-jet VUV source with a wavelength of 5‐200 nm. The experimental results show that under VUV irradiation both tensile fracture strength and elongation decrease slightly. The spectral absorbance of the PET film increases noticeably with increasing dose. An absorption band mainly forms in the ultraviolet-to-visible region. X-ray photoelectron spectroscopy, Fourier transform infrared, and electron spin resonance analyses indicate that in the skin layer of PET film under VUV radiation, the C‐O bonds are broken and decarbonylation occurs, leading to the formation of free radicals as well as a trend to carbonification. The scission of the macromolecule chains, the increase in radical concentration, and the carbonification are considered to be major contributors to the change of optical properties for PET film under VUV exposure.

Fabrication of a Cu nanodot array based on electroless plating employing a diblock copolymer nanotemplate

An array of copper (Cu) nanodots was fabricated onto a Si substrate covered with native oxide (SiO2∕Si) using a diblock copolymer as a resist film. First, polystyrene/polymethylmethacrylate (PS/PMMA) diblock copolymer was spin-coated on a SiO2∕Si substrate and then annealed for 24h in a vacuum oven maintained at 170°C to achieve good phase separation. To prepare a nanotemplate, this phase-separated diblock copolymer film was photodecomposed selectively using 172nm vacuum ultraviolet (VUV) light. Due to the difference in photoetching rate between PS and PMMA domains, the latter regions were preferentially decomposed. By controlling the VUV irradiation conditions, that is, exposure time and atmospheric pressure, we were able to prepare a diblock copolymer template comprised of a nanoporous PS matrix on the SiO2∕Si surface. Next, the nanopore regions were site-selectively modified through a vapor phase chemisorption of an amino-terminated organosilane. Palladium (Pd[II]) particles were then immobilized onto the amino-terminated nanopore regions through donor and accepter reactions and, subsequently, Cu was selectively electroless-plated onto these Pd-activated regions. As evidenced by atomic force microscopy, due to the elimination of the residual PS nanotemplate by VUV irradiation, we fabricated an array of Cu nanodots 3.0to∼4.3nm in height and 10to∼25nm in diameter on the entire 10×10mm2 area of the SiO2∕Si substrate.

Monte Carlo simulation of the effects of vacuum-ultraviolet radiation on dielectric materials

Radiation-induced damage during plasma processing of semiconductor materials can adversely affect device reliability. However, it has been shown that vacuum ultraviolet (VUV) radiation (8–20 eV) can beneficially deplete previously deposited charge on the surface of dielectrics by temporarily increasing their conductivity. Incident VUV photons can cause photoemission and form electron-hole pairs in the dielectric thus producing the desired increased conductivity. To verify this, statistical information obtained from a Monte Carlo simulation is used to model VUV exposure of dielectrics. The simulation calculates the surface potential on the dielectric produced by electron photoemission, which compares favorably with experimental surface-potential measurements made using a Kelvin probe.

Synergistic Study on Atomic Oxygen-Induced Erosion of Polyethylene with Vacuum Ultraviolet

Ground-based experimental results on atomic oxygen-induced erosion of polyethylene with simultaneous vacuum ultraviolet (VUV) are reported. In situ mass-loss measurement was made using a quartz crystal microbalance with amass resolution of 2 ng. A laser detonation atomic oxygen beam source was used to simulate collision energy of atomic oxygen with materials in low Earth orbit. To change relative intensity of atomic oxygen and VUV, the sample was rotated with an axis perpendicular both to the axes of atomic oxygen and VUV. It was observed that the erosion rate of polyethylene was increased 30-100% by a simultaneous VUV exposure depending on the incident angle of atomic oxygen. No contribution of temperature increase was confirmed in the polyethylene erosion. The experimental results suggested that two reaction pathways exist in the simultaneous atomic oxygen and VUV exposure conditions. It is suggested that the synergistic effect on polyethylene erosion is emphasized in the exposure condition where relative atomic oxygen flux is low against VUV flux.

Measuring vacuum ultraviolet radiation-induced damage

During plasma processing of semiconductors, ultraviolet (UV) and vacuum ultraviolet (VUV) radiation are present, but their effects can be difficult to separate from those due to charged particles incident on the wafer. The contribution of VUV photon irradiation to gate-oxide damage, and damage to dielectric materials in general, was examined using two measurement techniques that can predict the possibility of damage. They are (1) surface potential measurements and (2) electrically erasable read-only memory transistors (CHARM-2 wafers). To isolate the radiation effects, unpatterned oxide-coated wafers and CHARM-2 wafers were exposed to VUV synchrotron radiation. VUV exposure of dielectrics and conductors results in an accumulation of positive charge due to photoemission. As a result, it can become difficult to distinguish the photoemitted from the plasma-deposited charge. In addition, it was determined that the UV monitors on CHARM-2 wafers did not respond to VUV radiation.

Luminescence from Vacuum‐Ultraviolet–irradiated Cosmic Ice Analogs and Residues

Here we report a study of the optical luminescent properties for a variety of vacuum-ultraviolet (VUV)- irradiated cosmic ice analogs and the complex organic residues produced. Detailed results are presented for the irradiated, mixed molecular ice: H2O:CH3OH:NH3:CO (100:50:1:1), a realistic representation for an interstellar/precometary ice that reproduces all the salient infrared spectral features associated with interstellar ices. The irradiated ices and the room-temperature residues resulting from this energetic processing have remarkable photoluminescent properties in the visible (520-570 nm). The luminescence dependence on temperature, thermal cycling, and VUV exposure is described. It is suggested that this type of luminescent behavior might be applicable to solar system and interstellar observations and processes for various astronomical objects with an ice heritage. Some examples include grain temperature determination and vaporization rates, nebula radiation balance, albedo values, color analysis, and biomarker identification.

The effects of contamination from silicones and a modified-tefzel insulation on critical surfaces of the international space station

The purpose of this study was to assess the contamination resulting from extensive usage of silicone-insulated and modified Tefzel-insulated electrical cables and a silicone-based thermal control coating, S-13GP/LO-1, used on the International Space Station (ISS). The outgassing condensation rates of these materials were characterized per ASTM 1559-93 as a function of contamination source and receiver hardware temperatures. The results of MOLFLUX analyses for both the silicone-insulated cables and the silicone-based thermal control coating showed that these materials did not meet the program requirements. The silicone-insulated cables and the antenna and associated hardware painted with S-13GP/LO-1 were vacuum baked and the outgassing condensation rates remeasured. These products now met the program requirements. The condensable portion of the outgassed products from modified-Tefzel cable insulation at two temperatures was deposited onto the white, inorganic thermal control coating, Z-93P. These contaminants were exposed to various space environments that included near ultraviolet (UV) radiation, vacuum UV (VUV) radiation, and atomic oxygen (AO). The contaminant darkened more rapidly with VUV exposure than a silicone contaminant of the same thickness. It was eroded by AO and bleached in air after VUV. It was shown that the ISS hardware provided would perform satisfactorily.

Effects of vacuum and vacuum-ultraviolet radiation on sulfuric acid anodized aluminum oxide coatings

Porous aluminum oxide coatings processed by sulfuric acid anodizing (SAA) of aluminum have been of interest in recent years for use as thermal control surfaces in space applications and see use in other applications as protective layers against Al corrosion. For space applications, porous Al oxide coatings must be able to withstand the effects of atomic oxygen, the interaction with contaminants, electron bombardment, and vacuum-ultraviolet (VUV) irradiation. This study focuses on the effects of vacuum and VUV exposure on SAA aluminum. Porous anodic coatings exposed to VUV exhibit optical property degradation, i.e. an increase in solar absorptance ({alpha}) without commensurate increase in infrared emittance ({epsilon}). In this study, samples were vacuum/VUV-exposed and optically measured; their composition, structure and morphology were investigated using electron probe micro-analysis (EPMA), x-ray diffraction (XRD) and optical microscopy. The optical property degradation, associated with water loss and cracking, was observed on the sealed coatings exposed to either vacuum or VUV radiation.

Recovery of vacuum ultraviolet irradiated metal-oxide-silicon systems

This article is concerned with the instability and recovery of the large negative midgap voltage induced in metal-thermally grown SiO2-silicon capacitors with transparent Al gate upon vacuum ultraviolet exposure by a Kr lamp (hν=10 eV). Evidence is presented from high-frequency capacitance–voltage measurements, showing that this midgap voltage instability is caused by the generation of unstable fast donor states, stable and unstable slow donor states, and drifting ions. Two fundamentally different mechanisms contribute to the gradual recovery of the samples: the disappearance of the unstable centers (fast and slow donor states and mobile ions) and hole emission from slow states and from mobile ions arriving at the substrate or at the gate. The annealing of unstable fast donor centers is inhibited when the states are charged positively. It is argued that the unstable fast and slow donor states and the drifting ions have an identical microscopic nature: all are associated with a H atom attached to an O atom in the network. The unstable fast donor states are attributed to H atoms trapped at or very close to the Si substrate, the unstable slow donor states to H trapped somewhat further away. Annealing of these unstable slow states is explained from the gradual disappearance of the atomic H from the oxide. It is shown that the stable slow states are related to near-interfacial O vacancies, activated by first trapping a hole.