Trace surface analysis: 30 ppb analysis with removal of less than a monolayer. Fe and Ti impurities in the first atomic layer of Si wafers

Abstract

Recent trends, exemplified by the stringent demands of the semiconductor industry, demonstrate a need for surface analysis at progressively lower impurity levels. Because the total number of impurity atoms available for measurement in the first atomic layer is severely limited, a successful trace analysis technique must efficiently utilize the impurity atoms available as well as achieve good signal to noise. We report here the results of a new Surface Analysis by Resonant Ionization of Sputtered Atoms (SARISA) apparatus which has a demonstrated ability to measure Ti and Fe impurities in the surface atomic layer of a Si wafer at the 30 ppb level. The Fe results are particularly instructive since Secondary Ion Mass Spectrometry (SIMS), because of the isobaric overlap of Fe and Si 2, has achieved no better than 200 ppb with removal of nearly 100 atomic layers. The measurements were made with a 2 mm 2 ion spot. A measured collection efficiency of 1 Fe atom counted per 100 Fe atoms sputtered could also be achieved. Apparatus improvements will be described which should increase the collection efficiency to 1 atom counted per 10 sputtered.