LMAIS development and testing

Introduction

Liquid Metal Ion Sources (LMIS) are ion sources with a very high brightness and a point like emission area, which make them very suitable for use in focused ion beam application.  There are needle and capillary type LMIS. The application of a strong electric field in front of the emitter tip leads to the formation of a so called Taylorcone from which the ion emission occurs. This process is called field evaporation. As source material pure metals (Ga, In, ... - LMIS) or eutectic alloys (AuSi, CoNd, MnGe, ... - LMAIS) can be used .

Required properties of these materials:

• high concentration of the wanted component in the alloy
• low melting point
• low vapour pressure at that temperature
• good wetting behaviour to the needle
• no chemical reactions or phase transitions with the emitter

Fabrication

The FIB laboratory of the Institute has the experience and the equipment for the development, fabrication and analysis of new needle type alloy LMIS, which are summarized in the Table:

Material	Melting Point (°C)	Applications
Au73Ge27	365	Au:Sputtering, nanocluster; Ge: Doping, SixGe1-x alloy
Au77Ge14Si9	365	Si:contamination free processing, imaging
Au82Si18	365	dto.
Co36Nd64	566	Co:CoSi2 ; IBS, Nd: optical + magnetic application
Co27Ge73	817	Co:magnetic application
Er69Ni31	765	Er:optical application
Er70Fe22Ni5Cr3	862	Fe,Ni,Cr:  magnetic application
Sn74Pb26	183	Sn:doping, academic
In14Ga86	14.2	Academic, source emission investigation
Mn45Ge55	720	Mn implantation in compound semiconductors
Ga38Bi62	222	Ga: acceptor; Bi: shallow donor in Si
Ga35Bi60Li5	250	Li: source for analysis and ion beam lithography
Ga	29.7	All classical FIB applications
Bi	271.5	Surface modification by monomers and clusters

Testing

For testing and characterisation of new LMAIS emitters, the following ion source parameters can be measured:

• Current - Voltage - characteristics, stability of emission, life time and temperature behaviour
• Mass spectra (below shown for GaBi LMAIS)
• Distribution and angular intensity by means of a rotating Faraday cup

(click to enlarge)

Ion energy distribution and energy deficit of each emitting component of the source using a mass filter and a retarding field analyzer

(click to enlarge)

FWHM of the energy distribution for different ion species with different masses and charge states at an emission current of 15 µA.