Sodium distribution in thermal oxide on silicon by radiochemical and MOS analysis

Direct comparison has been made between Na distribution determined radiochemically and electrically in 6000 Å thermal oxide on 10Ω-cm n-type Si. Na within
\siml 1000
Å of Si/SiO
2
interface correlates in most cases with MOS flat band voltage shift, i.e. N
Na
(
\siml 1000
Å) ≈ +ΔQ
MOS
. Appreciable Na, which does not appear to affect Si surface potential, is found through the bulk of the oxide. Neutron activation of "clean" oxide (grown in wet or dry oxygen) showed ∼1 ppm Na throughout, ∼10 ppm at free surface. Gold was also identified at ppm concentrations but did not correlate with ΔQ
MOS
. The U-shaped profile seen after diffusion (300 to 1000°C in dry N
2
) or in-drift (∼200°C, 5 × 10
⁵
V/cm) is believed due to rate limitation at the free surface; electrostatic binding plus enhanced solubility at the Si/SiO
2
interface. Time dependence was observed in diffusion at 500 and 800°C, which is not expected from the fast diffusion of Na. This is also attributed to the surface rate limitation. The anion significantly affects Na diffusion kinetics and distribution in contamination experiments with radiotracer Na
²⁴
OH, Na
²⁴
Cl, and Na
²⁴
Br
⁸²
. Br is found to accompany Na diffusion, suggesting anion neutralization of Na within the bulk oxide. P
2
O
5
. SiO
2
is seen to "getter" Na from the oxide. These results support the suggestion of Snow et al. that Na is responsible for uncontrolled drift of surface potential in oxidized Si devices.