Sputter cleaning and etching of crystal surfaces (Ti, W, Si) monitored by Auger spectroscopy, ellipsometry and work function change

Abstract

Surfaces of Ti, W and Si have been cleaned in an ultra high vacuum system by ion bombardment with Cs + and Ar +. Also a thin film (220 Å) of Si 3N 4 on Si has been etched off by sputter-etching with Cs + at 500V. The surface composition was monitored with Auger spectroscopy, film thickness with ellipsometry and work function changes with surface potential difference measurements. Surface contamination on Si, Ti and W samples is mostly C, probably from hydrocarbons used for cleaning or as adsorbed CO. The C and O contamination (∼ 12 Å) on Si 3N 4/Si can be removed with about 6.2 μA-min/cm 2 of bombardment with Cs + at 500 V. At 800 °C no Cs is left on the sample but if the sample is near room temperature, Cs is left embedded in the sample. C and O contanination can be removed from the surface of W and Ti by ion bombardment with Cs + or Ar +. However, at low temperatures these ions are left embedded in the Ti substrate. The embedded Cs or Ar change the work function and ellipsometer values and therefore must be removed by annealing at high temperature. Ti presents the special problem that during the anneal Cl and S segregate and cover the surface even though the bulk concentration is ∼0.1 ppm. A minimum of contamination (<0.1 monolayer) by Ar, Cl and S is obtained by bombarding with Ar + at about 525 °C. Analysis of the composition of thin films can be made by monitoring the surface composition by Auger spectroscopy as the film is sputter-etched away. The sputter-etching efficiency for 500V Cs + to remove Si 3N 4 from Si 3N 4/Si is between 0.55 and 0.73 Å(μA-min/cm 2) or about 4 atoms/ion. Since the ion current ∼ 1 μA/cm 2, it takes about 15 min to remove 10 Å. The conductivity of the SiO 2 is much larger than for the Si 3N 4 and much larger ion currents and sputter-etching rates could be obtained.