Defects in Plasma-Activated Amorphous SiO ₂ Probed Using Positron Annihilation Spectroscopy

Defects introduced by plasma activation in amorphous SiO2 were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured as a function of the incident positron energy for SiO2 deposited from tetraethylorthosilicate using plasma-enhanced chemical vapor deposition on Si substrates. After N2 plasma activation, electron (and/or positron) trapping centers were primarily introduced in the subsurface region (≤6 nm). The defect concentration decreased after postplasma processes: deionized water rinsing and additional H2O plasma activation, which was associated with the formation of silanol groups at the surface. However, a certain number of defects existed in the subsurface region, and they were not annealed out at the typical annealing temperature for bonding SiO2 layers (400 °C). Because these defects can cause softening of the top surface region and attract water trapped between two wafers or in SiO2, they are considered to contribute to the wafer-bonding process.