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Scanning Microscopy

Abstract

A new high resolution scanning ion microprobe (SIM) is described which combines laser non-resonant multiphoton ionization (MPI) and time of flight (ToF) spectroscopy. The proposed instrument is designed to overcome limitations of the conventional secondary ion mass spectrometry (SIMS) method. A pulsed ion probe (with current 1 - 100 pA) is extracted from a liquid metal ion source (LMIS). This beam is purified by a Wien filter, focused to a spot (15 - 150 nm), and scanned across a sample in a raster pattern (512 x 512 pixels). A high power (200 mJ, 193 nm, 500 Hz) ArF pulsed laser -an off-axis ellipsoidal reflector is planned to boost its power density - intercepts the sputtered neutrals, ionizing a large fraction for detection. The resultant ions are collected by a spherical sector energy analyzer and mass resolved by either a reflectron or Poschenrieder type ToF spectrometer. The laser pulse defines the time base for the spectrometer; mass resolution of more than 3000 is feasible. Detailed calculations of the neutrals' sputtering and photoionization yields are given. In particular, an analytical expression for two-photon ionization of sputtered atoms, which optimizes the photon-neutrals overlap, is derived and results plotted. This technique, post-ionization of sputtered atoms (PISA), is shown to permit several high statistical accuracy, high mass and lateral resolution images to be obtained simultaneously, even for elements with high ionization potential or low electron affinity, elements difficult to examine with SIMS. Compared to SIMS, PISA greatly reduces the range of the variation in detected signal as a function of atomic number, facilitating quantification.

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