Clean aluminum oxide formation on surface of aluminum cylinder in an ultraclean gas-sampling system

Abstract

Because it is difficult to obtain enough space for a gas-analysis system in semiconductor manufacturing lines, gas purity is usually confirmed by an ex situ analysis of gas sampled inside the gas-sampling cylinders (samplers). In order to analyze trace impurities, it is necessary to significantly suppress contaminants generation in samplers and to fabricate surfaces and/or materials with extremely low outgasing rates. We have produced a sampler made of pure aluminum (Al) carried out plasma oxidation in 3% oxygen $({\mathrm{O}}_{\mathrm{2}})$ /argon (Ar) after the extrusion-lathing process. Nevertheless, the hydrogen $({\mathrm{H}}_{\mathrm{2}})$ concentration in nitrogen $({\mathrm{N}}_{\mathrm{2}})$ or Ar sealed at 0.58 MPa in the sampler increased from below 1 to 8 ppb after 168 h. The carbon monoxide (CO) and carbon dioxide $({\mathrm{CO}}_{\mathrm{2}})$ concentration in ${\mathrm{O}}_{\mathrm{2}}$ also increased. Wet cleaning was carried out in the sampler by deionized water for 72 h at a flow rate of 2 L/min. After wet cleaning, the sampler was annealed at 423 K for 72 h in ${\mathrm{N}}_{\mathrm{2}}$ without exposure to air. We have observed that the ${\mathrm{H}}_{\mathrm{2}}$ concentration in ${\mathrm{N}}_{\mathrm{2}}$ was maintained below 1 ppb, the detection limit of the gas chromatograph for 168 h. The CO and ${\mathrm{CO}}_{\mathrm{2}}$ concentrations in ${\mathrm{O}}_{\mathrm{2}}$ were also sufficiently low. We have confirmed that amorphous γ-aluminum oxide $({\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}})$ film with a thickness over 0.5 μm was formed on the inner surface of the sampler using cross-sectional transmission electron microscopy observation. These results suggest that the amorphous $\mathrm{\gamma -}{\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ film formed by a series of treatments mentioned above functions as a gas-barrier film with an anticatalytic property.