Field emission sensing for non-contact probe recording

摘要

In probe recording an array of thousands of nanometer-sharp probes is used to write and read on a storage medium. By using micro-electromechanical system technology (MEMS) for fabrication, small form factor memories with high data density and low power consumption can be obtained. Such a system is expected to offer a promising route towards extremely high-density recording, with bits of several nanometer or even atomic size. To reach these densities, individual control over the position of the probes is essential, to be able to operate the probes in non-contact as is for instance done in scanning tunneling microscopy (STM). At the MESA+ Institute for Nanotechnology at the University of Twente, we currently investigate the possibilities of probe recording using a magnetic medium. In the micro scanning probe array memory (μSPAM) concept, an array of magnetic probes is used to write and read on a patterned recording medium. In such a probe storage system, there is also a need to position individual probes at several nanometers above the recording medium, to be able to detect the small magnetic forces. For this non-contact operation, individual z-feedback should be achieved by integration of an actuator, proximity sensor and feedback loop for each probe of the probe array. As reported in literature, the fabrication of probe arrays and integration of actuators and logic circuitry have already been proven to be attainable, however current research still lacks a proximity sensor with sufficient lateral resolution that can be integrated in each probe. The objective of this thesis is to investigate whether field-emission can be used as an integrated method to control probe-medium distance for non-contact probe recording. Field emission can be used as a proximity sensing method, since the emission current varies exponentially with the electric field, which in turn is proportional to the electrode gap. The lateral resolution is determined by the probe tip radius, on the same order as the targeted bit size of about 10 nm. The signal to noise ratio is not affected by the small sensing area, which is an important advantage over other sensing methods. Moreover it provides an elegant solution for the problem of sensor integration in each probe of the array, since only one wire per probe is needed to connect to the field emitting tip.