Improvements in the chemistry of secondary ion mass spectrometry - negative ion techniques

P. Williams, R. K. Lewis, C. A. Evans, P. R. Hanley

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

Abstract

Secondary ion mass spectrometry is the most sensitive of the surface analytical technique currently available and is the only such technique suited to the routine characterization of dopant and impurity depth profiles in semiconductors. The chemistry of the sputtered surface has been shown to have a profound effect upon secondary ion yields. Oxygen ion bombardment gives much higher yields of positive and negative secondary ions than does oxygen argon ion bombardment. Oxygen also forms a multitude of oxide polymer ions which can superimpose upon and mask signals from analytically important species. Significant success has been achieved in the depth profiling of light electropositive species (e.g. B, Al) for which positive ion yields are high and interferences unimportant. Conversely, the n-dopant elements in silicon (P, As, Sb) and in gallium gallium arsenide (S, Se, Te) have high ionization potentials (∼ 10eV) and thus have low positive ion yields. The silicon n-dopants also have low electron affinities (< 1 eV) and therefore have had low negative ion yields under conventional oxygen ion bombardment. The low yields and/or interference problems (e.g. S-O2-) have resulted in poor detection limits (1018-1020 atoms/cm3) for these species. It has been known for some time that cesium enhances negative ion yields, but the lack of availability of a Cs+ ion source for ion probes has meant that the application of negative ion spectroscopy has been significantly restricted. In order to realize the intrinsic sensitivity of the SIMS technique for the electronegative elements we have adapted an ion accelerator Cs+ ion source ("Hiconex"TM, General Ionex Corporation) to our ion microprobe. Under Cs+ ion bombardment negative ion yields are increased by two to three orders of magnitude and certain molecular ion species e.g. AsSi- are formed in particularly high yield. Moreover with the removal of oxygen, oxide interference problems have been greatly reduced. Together, the yield enhancements and oxide interference reductions lead to detection limits for electronegative species two to three orders of magnitude lower than any previously reported. High negative ion yields of H, C, O and the halogens are also obtained. Residual vacuum quality is the major concern when analysing for these elements. An additional benefit of Cs+ ion bombardment is high sputtering rates leading to a reduction in the time required for depth profiling analyses. This paper will briefly describe the ion source hardware, then detail the analytical values of its use and demonstrate its efficacy with several ion implant applications previously inaccessible to the SIMS technique. The research supported in part by NSF Grants DMR-76-01058, CHE-74-05745 and CHE-76-03694 and by General Ionex Corporation.

Original languageEnglish (US)
Number of pages1
JournalNuclear Instruments and Methods
Volume149
Issue number1-3
DOIs
StatePublished - Jan 1 1978
Externally publishedYes

ASJC Scopus subject areas

  • Medicine(all)

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