Some of the recently shipped hard disk drives have a new technology to actively control the flying height between slider and disk. The slider to disk spacing is controlled by thermal protrusion actuation using a small heater coil which is located close to the read write element at the trailing end of the slider. By applying an electric current to the heater coil, the slider's trailing end protrudes towards the disk and can be driven into contact with sufficiently high heating power. The contact force and the thermal protrusion efficiency is mainly controlled by air bearing design. In this paper we want to discuss the trade offs in air bearing design to achieve low contact force and high thermal actuation efficiency. We have done both numerical simulation and experimental measurements to investigate contact force and air bearing stiffness. Typically a softer air bearings will produce less contact force but usually exhibit worse flying height tolerances We have found a nonlinear clearance change with applied heater power. At closer spacings, the pressure peak increases dramatically leading to reduced actuation efficiency. The actuation efficiency may also vary at different skew angles. For "calibration purpose slider to disk touchdown requires contact. Due to different actuation efficiencies at different radii different contact forces are estimated.
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