Formation of Nanoscale Protrusions on Polymer Films after Atomic Oxygen Exposure: Observations with Positron Annihilation Lifetime Spectroscopy.

Atomic oxygen (AO) is one of the dominant components of the residual atmosphere in low Earth orbit. AO collides with spacecraft with a translational energy of 5 eV, forming nanoscale protrusions on polymeric materials. To clarify the influences of a polymer's chemical structure on the formation of AO-induced microstructures, this study investigated the size of free-volume holes and the layer thickness that interacted with AO for polyethylene (PE), polypropylene (PP), and polystyrene (PS) by positron annihilation lifetime spectroscopy. The injection energies of positrons varied from 1.3 to 10 keV to adjust the injection depth (range) into the polymers (40 nm-1.6 µm). For the pristine films, the lifetime of ortho-positronium (o-Ps, τ3) was longer in the order of PS, PP, and PE regardless of the injection energy of positrons, showing the different sizes of free-volume holes with radii of 0.29, 0.31, and 0.32 nm, respectively. The fraction of the decay component corresponding to o-Ps in all decay components (relative intensity of o-Ps, I3) was used to investigate the chemical change induced by AO exposure. The I3 values for the three polymers were decreased by AO exposure of (2-5) × 1018 atoms/cm2 or more at a depth of 40-48 nm, obtained by 1.3 keV positrons. This indicates that AO formed polar groups (i.e., an oxidized layer) on the polymer surfaces. The maximum depths of such chemical change for PE and PP were deeper than that for PS. The different sizes of free-volume holes would affect the diffusion or ballistic penetration of AO, resulting in the difference in the oxidized layers' thicknesses and surface morphologies.

Formation of Nanoscale Protrusions on Polymer Films after Atomic Oxygen Irradiation: Changes in Morphologies, Masses, and FT-IR Spectra.

Atomic oxygen (AO) is the main component of the residual atmosphere in a low Earth orbit. AO with a translational energy of 5 eV colliding with artificial satellites forms nano- and microscale protrusions on polymeric materials. This study investigated the influences of AO (fluence and velocity distribution) and a polymer's chemical structure on such surface morphologies. The correlations between samples' mass losses and positions in the irradiation field of an AO beam were analyzed with polyimide (Kapton) films, a standard reference material for AO fluence measurements. The characterizations of polyethylene (PE), polypropylene (PP), and polystyrene (PS) films were studied using gel permeation chromatography and X-ray diffraction. The sample surfaces were observed using a field emission scanning electron microscope. Nanoscale protrusions were formed on all the samples and were larger but fewer with increasing AO fluence. The numerical density of protrusions formed on PE and PP was lower than that on PS. However, the erosion yields and functional groups of PE, PP, and PS were similar per FT-IR spectra.

Erosion of FEP Teflon and PMMA by VUV radiation and hyperthermal O or Ar atoms.

A combination of beam-surface-scattering, quartz-crystal-microbalance, and surface-recession experiments was conducted to study the effects of various combinations of O atoms [in the O((3)P) ground state], Ar atoms, and vacuum ultraviolet (VUV) light on fluorinated ethylene-propylene copolymer (FEP) Teflon and poly(methyl methacrylate) (PMMA). A laser-breakdown source was used to create hyperthermal beams containing O and O(2) or Ar. A D(2) lamp provided a source of VUV light. O atoms with 4 eV of translational energy or less did not react with a pristine FEP Teflon surface. Volatile O-containing reaction products were observed when the O-atom energy was higher than 4.5 eV, and the signal increased with the O-atom energy. Significant erosion of FEP Teflon ( approximately 20% of Kapton H) was observed when it was exposed to the hyperthermal O/O(2) beam with an average O-atom energy of 5.4 eV. FEP Teflon and PMMA that were exposed to VUV light alone exhibited much less mass loss. Collision-induced dissociation by hyperthermal Ar atoms also caused mass loss, similar in magnitude to that caused by VUV light. There were no observed synergistic effects when VUV light or Ar bombardment was combined with O/O(2) exposure. For both FEP Teflon and PMMA, the erosion yields caused by simultaneous exposure to O/O(2) and either VUV light or Ar atoms could be approximately predicted by adding the erosion yield caused by O/O(2), acting individually, to the erosion yield caused by the individual action of either VUV light or Ar atoms.

Atomic beam-induced fluorination of polyimide and its application to site-selective Cu metallization.

Metallization methods of polyimide by hyperthermal atomic oxygen and atomic fluorine beams were developed. An atomic fluorine beam with a translational energy of 6.2 eV modified the wettability of polyimide surfaces to provide an advancing water contact angle of 132 degrees. It was confirmed that in-air storage for 2 months did not alter the hydrophobic property created by the atomic fluorine beam. This stable beam-induced surface fluorination technique was then applied to site-selective electroless Cu plating on polyimide. It was demonstrated that changing the exposure sequence could create both positive- and negative-type plating processes.