In yet another embodiment, the blocking layer comprises mainly silicon dioxide, deposited by PECVD at a power of about 250 W to about 350 W, at a temperature of about 400??? C. or less, and at a pressure of 1 Torr or less, while feeding silane (SiH4) at a rate of about 50 sccm or less and nitrous oxide (N2 O) at a rate of about 50 sccm or less.