Nanofabrication with Atomic Force Microscopy

One particularly promising SPM-based nanostructuring process is the local oxidation of metal and semiconductor surfaces. Atomic force microscopy (AFM) in particular has emerged as a highly versatile tool for this type of surface modification, with a variety of applications including device fabrication and data storage.

Local oxidation is performed by applying a voltage between the AFM tip and the sample. The electric field is enhanced due to the nanoscale dimensions of the tip and sufficient electric field strength results in water adsorbed on the sample surface instantaneously forming a capillary neck between the tip and sample. This water bridge acts as an electrolyte, providing sources of O^- and OH^- oxidising species. Oxidation occurs within the confines of the water neck, resulting in an expansion of the surface due to the change in crystal structure.

Schematic of local oxidation process.

Although much attention has been directed to the mechanism of local oxidation, there has been relatively little discussion of the parameters relating to existing lithographic methods. Arguably the two most important factors are the time resolution, the shortest time required to create single nanostructures, and throughput, the speed at which large areas can be patterned.

Experiments are conducted using a Dimension 3100 AFM, with Nanoscope IV controller (Veeco Instruments). Short timescale tip bias signals were generated using an arbitrary waveform generator (National Instruments). Fast scanning speeds were achieved using a resonant scanning stage, based on the motion of a quartz crystal tuning fork, enabling relative tip-sample speeds in the cms^-1 regime.

Investigation of time resolution for local oxidation of silicon.

We find that the local oxidation, using an AFM in intermittent-contact mode, can occur on timescales as low as 500 ns for single tip bias events, resulting in oxide nanostructures with lateral dimensions as small as 15 nm. Despite such short timescales, existing models for oxide growth remain valid, at least until limitations imposed by AFM hardware interfere. Furthermore, we have confirmed that oxidation can be produced at scan speeds in excess of 2 cms^-1, enabling micron-size patterns to be fabricated in tenths of seconds[1,2].

This work has recently been extended to perform simultaneous imaging and patterning in order to perform real-time nanofabrication.

Large area oxidation patterned on silicon using a resonant scan stage to generate speeds of 2 cms^-1.

AFM image of ~ 1 μm square gold depositions.

Line profile through gold structures.