Linear tape drives use the timing-based servo (TBS) technique to derive position and velocity information of the tape relative to the head, using servo patterns written in dedicated regions of the tape. The servo patterns are written to tape during cartridge manufacturing using a special servo write head, see Figure 1. For the servo pattern used in LTO drives, the servo write head features two write gaps arranged in a 'V' shape.
The pattern is written by applying short unipolar current pulses to the head, such that the write gaps only write for short time periods. Two important factors that determine the quality (sharpness or abruptness) of a magnetic transition written to tape are the write gap width and the current switching time. A narrower write gap results in a sharper transition because of a larger gradient in the magnetic write field produced by the write gap.
The accuracy of a TBS servo channel is limited in part by the accuracy with which the relative timing of the transitions can be estimated. Sharper transitions result in better timing estimates and hence a more accurate position estimate.
Conventional servo write heads for servo formatting magnetic tape have a large inductance and require a large write current (>5 A). This makes it challenging to obtain the nanosecond-scale switching times required for writing sharp transitions at high tape velocities. Moreover, it is challenging to fabricate heads with very narrow write gaps.
We are developing a planar servo write head, shown in Figures 2 and 3, that is fabricated using thin-film microfabrication technology. We call it a planar head because the tape bearing surface is in the plane of the wafer on which the devices are fabricated, in contrast to data read/write head whose tape bearing surface is perpendicular to the wafer. Microfabrication technology makes it possible to create a servo write head with much smaller dimensions, overcoming the limitations of conventional servo writers. Current rise times (0 to 100 mA) of 3 ns have been obtained, and preliminary results show that this rise time can be further reduced. The obtainable write gap width is much narrower than for conventional servo write heads. The reduced switching time and the narrower write gap result in an improved quality of the transitions written to tape, and also facilitate an increase in tape format velocity [2012-4].