Microlocalization during ion beam analysis

Abstract

We have investigated three techniques for identifying microregions of the sample examined by an ion beam during analysis. These techniques involve 1) the detection of secondary electrons ejected from the sample during ion bombardment; 2) the detection of light emitted by a phosphor placed behind a thin sample; and 3) the detection of ion tracks in a track recording material placed behind a thin sample. First, we studied the feasibility of obtaining topographic information by detecting bursts of secondary electrons as individual ions strike the sample. The efficiency of detection was sensitive to sample/detector geometry and to detector operation parameters. Under the best operating conditions for our system, a change corresponding to a 95 um sample height difference was detected, and a minimum detectable change corresponding to a 30-40 um sample height difference was estimated. However, we were unable to reproduce the operating conditions under which we obtained these results. Second, we wanted to determine the location where each ion passed through the sample by detecting the ions with a phosphor coupled with a position sensitive detector. Initially, we used a photomultiplier tube (PMT) to detect light from the phosphor. The PMT was sufficiently sensitive both to detect the light emitted upon bombardment of the phosphor by a single ion and to provide a start signal for particle induced desorption time-of-flight mass spectrometry. Next, we attempted to use a silicon-intensified target multichannel detector to detect the light. However, this detector did not demonstrate single ion detection capability. Third, we evaluated two track recording materials for locating where the ions passed through the sample. The polycarbonate track detector recorded the integrated beam spot image, which was revealed with post-analysis etching and SEM observation. The fluorophlogopite track detector recorded tracks which could be observed by TEM without etching. This capability suggests the incorporation of a TEM into the ion beam analysis setup to observe single ion events in real time and to obtain submicrometer spatial resolution.