Balun

A method for manufacturing a balun, comprising the steps of:

• providing a dielectric substrate, wherein the dielectric substrate has a first surface and a second surface opposite one to another;
• forming a first metallic layer, wherein the first metallic layer comprises:
• a first spiral line, comprising an unbalanced I/O end and a first connection end;
• and
• a second spiral line, comprising a second connection end and an open-circuit end;
• forming a first dielectric layer on the first metallic layer;
• forming a second metallic layer on the first dielectric layer, wherein the second metallic layer comprises:
• a third spiral line, corresponding to the first spiral line and comprising a third connection end and a first balanced I/O end;
• and
• a fourth spiral line, corresponding to the second spiral line and comprising a fourth connection end and a second balanced I/O end;
• forming a second dielectric layer on the second metallic layer;
• forming a third metallic layer on the second dielectric layer, wherein the first connection end is electrically connected to the second connection end via the third metallic layer;
• and
• forming a fourth metallic layer, wherein the fourth metallic layer has a first conductive portion;
• wherein the third connection end is electrically connected to the first conductive portion, and the fourth connection end is electrically connected to the first conductive portion;
• wherein the fourth metallic layer is disposed on the first surface by thin film processing, and the first metallic layer, the second metallic layer, the first dielectric layer, the third metallic layer and the second dielectric layer are disposed on the second surface by thin film processing.