Memory Effects in Quadrupole SIMS

Abstract

Semiconductor assessment studies often require the measurement of dopant concentrations at the 10¹⁴cm⁻³ level and to achieve this detection sensitivity with SIMS it is necessary to reduce sources of background interferences [1]. One source of background is the cross contamination or memory which occurs when samples of different atom composition are analysed. It should be remembered that most of the sample sputtered by the primary beam is deposited on adjacent mechanical surfaces and some of this material is sputtered back onto the target surface by reflected primary ions [2] and energetic secondary ions [3]. Thus when the sample matrix is changed, for example GaAs to Si, a sensitive SIMS system will show a memory due to Ga and As and both elements will apparently be present in the Si sample. As the magnitude of the memory effect depends on the experimental conditions [3] and on the system configuration in the target region, it is important to characterise particular types of SIMS instruments with respect to this effect. Such measurements have been reported for the As and Si cross-contamination occurring in an ion microscope system [3]. This paper gives details of a quantitative assessment of the memory present in a high sensitivity quadrupole system.