Surface Cleaning on Aluminum for Ultrahigh Vacuum Using Supercritical Fluid CO₂ with O₂

Abstract

Supercritical fluid (SCF) CO₂ has been employed for cleaning of organic contamination on the surface. However, the cleaning rate was 2% at 25 MPa in SCFCO₂ alone. Oxygen (O₂) was added to base SCFCO₂ to improve the rate by utilizing oxidizing effect. Aluminum sample is φ5 × t1. After a sample was cleaned in the fluid with 02, the surface was analyzed using x-ray photoelectron spectroscopy (XPS). Assuming that C1s peak in XPS spectra represents organic contamination, the contamination rate is defined as the ratio of area of C1s peak and summation of the area of C1s, O1s, and Al2p peaks. The cleaning rate at 55°C for 30 minute cleaning was 25% at 2030 MPa in SCFCO₂with O₂ of 1.5 MPa. The cleaning at the constant ratio of O₂ pressure against SCFCO₂ pressure showed that cleaning rate is approximately proportional to the oxygen molecule number. The addition of oxygen is primarily effective on cleaning of the organic contamination, where base SCFCO₂ showed secondary effect on cleaning.