Electron microprobe analysis of impurity heterogeneities in thermally grown silicon oxide

The electron probe X-ray microanalyzer is a powerful tool for studying impurity distribution and motion in thin films. This analytical instrument is capable of detecting metallic impurities present in areas as small as 1 × 10
^-6
mm
²
and in concentrations of greater than 1 × 10
¹⁹
atoms/cm
³
. The analysis requires no sample preparation and is essentially a nondestructive test. This instrument was used to examine unoxidized and oxidized silicon surfaces and a finished microcircuit. With the electron microprobe, aluminum-bearing regions approximately one microns in diameter were detected on the bare surface of mechanically polished silicon slices. These aluminum-rich regions are believed to be alumina abrasive used in polishing. If these regions are not removed by chemical etching they will generate oxide defects during oxidation. These defects were found to contain Al (1 × 10
²¹
atoms/cm
³
and Na (1 × 10
²⁰
atoms/cm
³
). Other oxide defects, i.e., pinholes, generated during oxidation varied in size from 0.5 to 5.0 microns and were found to contain Na (1×10
²¹
atoms/cm
³
) and K (5×10
²¹
atoms/cm
³
). Mg and Ca (1 × 10
²⁰
atoms/cm
³
) were occasionally observed in these defects. After oxidation, all these impurities could be removed with a hot hydrochloric acid and deionized water rinse; surprisingly, this treatment reduced the silicon surface charge in the MOS structure (
X_{0} \cong 1500
Å) by approximately 1.4 × 10
¹¹
charges/cm
²
. The surface charge could be further reduced by heating the oxidized wafer at 900°C in a silicon nitride tube.