Wiring substrate and method of manufacturing the same

A wiring substrate, comprising:

• a first insulating layer, having a first opening;
• a first conductive layer, formed on a sidewall of the first opening;
• a second insulating layer, having a second opening;
• a second conductive layer, formed on a sidewall of the second opening;
• a high-frequency wiring layer, sandwiched between the first insulating layer and the second insulating layer, wherein the high-frequency wiring layer comprises:
• a high-frequency trace, having a first surface and a second surface opposite to the first surface, the first opening exposing the first surface, and the second opening exposing the second surface, wherein the first surface has a first smooth surface which is not covered by the first insulating layer, and the second surface has a second smooth surface which is not covered by the second insulating layer,
• wherein both the first smooth surface and the second smooth surface have a roughness ranging between 0.1 μm and 2 μm apiece;
• a first wiring layer, comprising a first return layer;
• a second wiring layer, comprising a second return layer and a signal wiring layer, wherein the signal wiring layer is located at a peripheral region of the second return layer, and both the first insulating layer and the second insulating layer are located between the first wiring layer and the second wiring layer,
• wherein the first opening and the second opening are located between the first return layer and the second return layer and form a hollow cavity,
• wherein the high-frequency trace is located in the hollow cavity, and the high-frequency trace, the first return layer and the second return layer overlap and do not touch each other;
• and
• a plurality of conductive columns, disposed in the second insulating layer, wherein the conductive columns are connected to the high-frequency trace and the signal wiring layer, not connected to the second return layer.