STM Lateral Micropositioning: A Tool for Nanofabrication

Abstract

A concentric tube scanning tunneling microscope (STM) has been fitted with a lateral micropositioning device in order to allow imaging of surfaces over a range of a few millimeters. The probe is mounted on a stage which uses the principle of piezoelectric inertial translation to produce a controlled stepping motion along two axes. The stage is driven by signals already supplied to the STM, is insensitive to vibration, and is UHV compatible. A position sensor can be attached to the stage to provide a coordinate readout of its position and has been used to study the dynamics of the stage motion. The micropositioning capability has been demonstrated by using it to locate semiconductor device structures.

Patterning by STM of H-passivated Si has been investigated as a means of fabricating nanoscale device structures. It was found that by scanning repeatedly in a given pattern under appropriate bias conditions, several monolayers of oxide could be selectively grown on the surface. The patterned features could be imaged by SEM and remained on the surface after thermal oxidation and metallization. This technique was used to write a grid structure in the channel region of a Si MOSFET prior to completion of the device. The completed device contains a modulation of the gate oxide thickness in the patterned area. Two different modifications of the device electrical characteristics are predicted, depending on the orientation of the grid. Although the predicted behavior has not yet been verified experimentally, the feasibility of fabricating device structures with the STM has been demonstrated.