Abstract: Vias or feedthroughs in the order of 0.5 mil diameter or less, for interlayer communication in multilevel interconnected microelectronics, such as large scale integration of active devices and circuits on a wafer, are formed by pyrolytically deposited silicon dioxide covered by RF-sputtered silicon dioxide, or vice-versa. The pyrolytically deposited silicon dioxide can be made sufficiently thin to enable the formation of 0.5 mil diameter or smaller vias therein by conventional etching techniques. The RF-sputtered silicon dioxide is deposited at several times the thickness of the pyrolytically deposited silicon dioxide to bear the main burden of isolation between layers and the vias therein are formed by the use of mushroommasks, such as by the techniques disclosed in U.S. Pat. No. 3,700,510 to form a beveled edge around the via opening to assure continuity of subsequently deposited metal. Two or more small vias may be grouped within a single large via of convenient size and shape.