Blocking layers 486, 487 of Al2O3, TiN or other materials are conveniently formed by CVD, PECVD, ALD, sputtering, or evaporation with thicknesses desirably in the range of about 10 to 500 Angstrom units and more preferably in the range of about 10 to 100 Angstrom units for each of layers 486, 487, but other thicknesses may also be used in various embodiment.