I. Introduction
The servo track writers (STW) are critical equipments in hard disk drive manufacturing. In the servo writing process, a servo wedge is written on the blank recording media or the surface of disk platter, which acts as the servo benchmarks for voice-coil motion (VCM) to do seeking and tracking. In modern embedded servo schemes, servo wedge quality is directly related to disk servo performance. A well-written servo pattern may improve TMR and reduce adjacent data track encroachment.