A method of forming a semiconductor device includes forming a lower coil segment in a first dielectric layer over a substrate, forming a second dielectric layer over the lower coil segment and the first dielectric layer, anisotropically etching a top portion of the second dielectric layer to form an opening over the lower coil segment, depositing magnetic material in the opening to form a magnetic core, forming a third dielectric layer over the magnetic core and the second dielectric layer, forming vias extending through the second dielectric layer and the third dielectric layer, and after forming the vias, forming an upper coil segment over the third dielectric layer and the magnetic core, wherein the vias connect the upper coil segment with the lower coil segment.