Dept. of Electrical Engineering
Oregon Graduate Center
The scanning tunneling microscope (STM) has in recent years provided us with a handle to achieve atomic resolution of surfaces. By replacing the electron source in an STM with a liquid metal ion source (LMIS), essentially a "proximity focused" technique, it has been found possible to do micromachining at close spacings. The current densities attained in such a close-spaced system are up to three orders of magnitude greater than currently available from focused ion beam (FIB) systems. Such high current densities provide the possibility of carrying out nanoscale microfabrication at speeds limited only by the mechanical deflection schemes. Such enormous current densities and the resulting extremely rapid substrate changes led us to study the capability of the LMIS to operate at much lower currents than usual and led to the discovery of a new low current mode of operation of the LMIS. It has further turned out that these low current LMIS have an application to the FIB technology, in terms of attaining the maximum current density possible in the focused beam spot. The desire to image any nanoscale features produced from the proximity focused LMIS led to the investigation to obtain a highly confined beam of electrons from the LMIS by subjecting a frozen-in Taylor cone to a field build-up procedure. It now opens up the use of the LMIS as a dual source of ions and electrons in either a focusing column or in a STM environment. A use has also been found of the field electron emission from the LMIS for determining the source-target separation, based on solving Laplace's equation in cylindrical coordinates or in prolate spheroidal coordinates (the latter by A. M. Russell and Russell Young) and using the form obtained for the field emission voltage to determine the diode separations. Finally the operation of the LMIS in a STM embodiment at emitter-target spacings of 100 nm or less, and also microfabrication features created on various targets will be described. It is seen that the threshold voltage for LMIS operation goes as the square root of the emitter-target spacing as would be expected from the Taylor theory.
Rao, Radhakrishna A., "Nanoscale microfabrication prospects using proximity focused liquid metal ion sources" (1988). Scholar Archive. 280.