Electrical passivation of B-doped Si through thin films used in VLSI fabrication

Atomic hydrogen diffusion through the thin films used in VLSI fabrication is investigated by measuring the electrically passivated boron (B) profile in silicon (Si) substrate under the films. After hydrogen plasma treatment, carrier concentration profiles in Si are measured using a spreading resistance profiler, and translated to the electrically passivated B profiles. The CVD oxide gave rather small hydrogen diffusivity in comparison with thermal oxide. Effective hydrogen diffusivities in Si under thermal oxides were evaluated at 150, 200, and 250°C. The effective hydrogen diffusivity in 0.01 μ cm B-doped Si was determined as 1.2 x 10−12 cm2 s−1 at 200°C. The difference in the hydrogen behaviour between the thermal oxide and the CVD oxide is analyzed numerically by a trap-included diffusion model, by fitting it to the passivated B profile in Si. Hydrogen trap densities in the thermal oxide and the CVD oxide were estimated to be NT ≤ 1.0 × 1017 and NT ⋍ 1.0 × 1019 cm−3, respectively. The diffusion through CVD oxide was slow because of the high density traps in the film. CVD oxide is more effective than thermal oxide as a retardation film against atomic hydrogen penetration into the VLSI devices in the dry cleaning process using hydrogen plasma. The passivations of B in Si under Si3N4, aluminum, and poly Si films were not observed. These films might be much more effective than CVD oxide films in the protection of devices in the short term, typically 3 h, during hydrogen surface cleaning in VLSI processes.