Radiation damage in silicon dioxide films exposed to reactive ion etching

Abstract

The enhanced electron-trapping characteristics and the location of this trapped charge in an SiO2 layer exposed to either CF4, O2, or Ar plasmas in a reactive ion etching (RIE) system are reported. Capacitance-voltage (C-V) and photocurrent-voltage (photo I-V) techniques were used to monitor charge trapping and location after the samples were incorporated in metal-oxide-semiconductor (MOS) capacitors. Using a CF 4 plasma which etches SiO2, trapping sites caused by penetrating radiation were observed. These traps were removed by annealing at temperatures ≥600 °C for 30 min in N2 prior to metallization. These bulk SiO2 trapping sites showed no strong dependence on whether the samples were placed on the anode or cathode in the RIE chamber, implying no preferred directionality in the photons which are believed to generate them. With an O2 or an Ar plasma which does not etch SiO2, an additional trapping layer within about 100 Å of the exposed SiO 2 surface caused by penetration of energetic ions (≤400 eV) was detected. Trapping in this layer was greatly reduced by a 1000 °C anneal in N2 for 30 min, and almost entirely removed by a buffered HF dip which etched about 100 Å of SiO2. Samples placed on the anode (ground plane) of the RIE system which would see energetic secondary electrons, but only relatively low-energy ions (approximately 35 eV), also had electron-trapping sites near the exposed surface; however, the trapping in this layer was greatly reduced compared with samples placed on the cathode.