R.F. magnetron sputtering of thick platinum coatings on glass microspheres

Abstract

Thick platinum coatings on glass microspheres are needed for proposed Laser Fusion targets. The spherical nature of these substrates coupled with the small dimensions (M.00 ym OD) make it difficult to achieve a smooth and uniform coating. Coating problems encountered include a rough surface and porous microstructure from the oblique incidence and lack of temperature and bias control, "clumping" of the microspheres causing non-uniformities, and particle accumulation causing "cone" defects.Sputtering parameters significantly affecting the coatings include total pressure, DC substrate bias, and the addition of doping gases. Using an ultrasonic vibrating screened cage and RF magnetron Sputtergun, we have successfully batch coated microspheres with up to 6 um of Pt, with a surface roughness of 200 nm, thickness non-concentricity of 300 nm, and density greater than 98% of bulk Pt.

Magnetron sputter deposition onto fluidized particle beds

“…Vacuum welding may therefore be contributing to coating roughness on the particles. Meyer [22] also observed a tendency of the fluidized particles to clump together during the sputter coating process, similar to that described in the experimental section above. He attributed this agglomeration to a vacuum welding mechanism.…”

“…Clumping was not observed in the 35 μm samples. In order to prevent clumping of the 230 μm microspheres during deposition, pure oxygen was periodically introduced via needle valve into the chamber (similar to [22]). Specifically, plasma deposition was stopped, the 1 mTorr argon atmosphere in the chamber was replaced with 5 mTorr of oxygen, the sample was held in the oxygen atmosphere for approximately 2 min, the oxygen was replaced with 1 mTorr argon, and sputtering resumed.…”

“…Over time, the atoms build up to form a film on the substrate. Early studies used "bouncing" techniques, with electromagnetic shakers [21] or piezoelectrics [22] within the vacuum chamber providing the vibratory action. Concerns about coating roughness and particle breakage led to an interest in less aggressive randomization techniques, including "tapping" a static sample pan once every 5-10 s [24] or rolling the microparticles in a tilted pan [25,26].…”

“…Prior researchers have combined magnetron sputter deposition with mechanically agitated beds to produce coatings on hollow glass microspheres 100 μm to 2 mm in diameter for inertial confinement fusion (ICF) nuclear reactions [21][22][23][24][25][26][27]. Magnetron sputtering is a line-of-sight deposition technique that uses a plasma to bombard a target material, ejecting atoms of the target material onto a substrate.…”

Deposition kinetics on particles in a dusty plasma reactor

“…In the 1980s, the need for coated glass microspheres as laser fusion targets spurred a series of coatings experiments using ͑PE͒CVD and physical vapor deposition ͑PVD͒ on particles levitated by a gas flow or bounced around by mechanical vibration. [22][23][24][25] In every case, the particles sizes were in the high micron to low millimeter range. Some research groups have synthesized nanoparticles in situ via PECVD for further thermal or plasma treatment.…”