A Self Servo Writing Scheme for a MEMS Storage Device with Sub-Nanometer Precision

In the probe-based storage concept being pursued by IBM, a MEMS based micro-scanner is used to position the storage medium relative to the read/write probes. To achieve repeatable positioning over a large storage area, it is necessary to have medium-derived position information. Dedicated servo-fields are typically employed to obtain medium-derived position information. Sub-nanometer positioning resolutions are desirable while writing these servo-fields. Such precise positioning at acceptable bandwidth using a global position sensor requires the directed design of the closed-loop noise sensitivity transfer function so as to minimize the impact of sensing noise. This paper describes one such control architecture based on resonant controllers where the impact of measurement noise on positioning is minimal while providing sufficient damping and hence satisfactory tracking performance. It is estimated that the positioning error due to sensing noise is a remarkably low 0.25 nm. Experimental results are also presented that show error-free operation of the device at high densities.