An apparatus for use in an optical disc includes an optical disc and an optical branching unit irradiating a main light beam and a sub-light beam, wherein the sub-light beam includes a optical aberration in a direction on a track of the optical disc. The circuit board has a first side, an opposed second side having a circuitry provide thereon, and a positioning hole formed on the circuit board. After forming the conductive bump, the barrier layer may be removed from the metal layer thereby exposing the metal layer while maintaining a portion of the barrier layer between the conductive bump and the substrate. Each plug includes a cylindrical body portion having a flat end surface, a bowl-shaped protruding portion extending from the end surface and having a top surface away from the end surface, and a buffering portion formed in the top surface.