According to one aspect of the present invention, a method is provided for forming a microelectronic assembly. The method comprises forming first and second trenches on a semiconductor substrate, filling the first and second trenches with an etch stop material, forming an inductor on the semiconductor substrate, forming an etch hole in at least one of the etch stop layer and the semiconductor substrate to expose the substrate between the first and second trenches, isotropically etching the substrate between the first and second trenches through the etch hole to create a cavity within the substrate, and forming a sealing layer over the etch hole to seal the cavity.