A co-axial structure includes a substrate, a first conductive structure, a second conductive structure, and an insulating layer. The substrate includes a first surface. The first conductive structure includes a first circuit deposited on the first surface and a first via penetrating the substrate. The second conductive structure includes a second circuit deposited on the first surface and a second via penetrating the substrate. The first via and the second via extend along a first direction. The first circuit and the second circuit extend along a second direction, and the second direction is perpendicular to the first direction. The insulating layer is located between the first via and the second via. The insulating layer includes a filler. The first conductive structure and the second conductive structure are electrically insulated. The first circuit and the second circuit are coplanar.