First Results on a Scanning Ion Microprobe Equipped with an EHD-Type Indium Primary Ion Source

Abstract

Electrohydrodynamic ion sources have recently received increased attention in secondary ion mass spectrometry, owing to their potential of producing extremely small primary ion beams at relatively high focused ion currents. KROHN and RINGO were the first to suggest the use of such a source as primary ion gun in an ion microprobe [1,2,3] and have reported on ion gun tests using Ga⁺ and Cs⁺ ions [4]. The only paper so far actually describing experiments with an EHD source coupled to a secondary ion mass spectrometer was published by PREWETT and JEFFERIES [5]; these authors obtained 0.5 µm diameter primary beams of Ga⁺ at currents of 0.2 nA and were able to record secondary electron and mass separated secondary ion distribution maps from microelectronic circuits. LIEBL [6] has shown that spot currents (at spot sizes below 1 µm) are limited by chromatic aberration and calculated a limiting focusable current of 1.6.10⁻¹⁰ A at a spot size of 200 Å (ca. 50 A/cm²). Values approaching these theoretical limits have actually been obtained with a Ga⁺ ion gun designed for applications in ion lithography [7,8].