Accurate Modeling of Residual recoil-mixing during SIMS Measurements
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Accurate Modeling of Residual recoil-mixing during SIMS Measurements Ming Hong Yang and Robert Odom Charles Evans and Associates, Sunnyvale, CA 94086, USA
Abstract Secondary ion mass spectrometry (SIMS) is an effective and powerful analytical technique, widely used in accurately determining dopant distributions (depth profiles). However, primary ion beam induced mass transport (ion mixing), especially the residual effect during SIMS profile measurements, greatly limits the accuracy at nanometer depth resolutions by displacing and broadening the measured depth profile. In this paper, we present a simple deconvolution algorithm based on the general characteristics of the experimentally observed SIMS response function to reduce this broadening effect, thereby providing more accurate depth profiles. The results for several specific applications of this approach are presented and its strengths and limitations are discussed. INTRODUCTION Secondary ion mass spectrometry (SIMS) is an effective and powerful analytical technique, widely used in accurately determining dopant distributions. However, primary ion beam induced mass transport (recoil ion mixing) during SIMS measurements greatly limits the accuracy at nanometer depth resolutions by displacing and broadening the measured depth profile. A common yet dramatic illustration of this ion mixing effect is shown in Figure 1a for the case of arsenic and boron implants into silicon. Recoil ion mixing effects in SIMS profiles are traditionally described [1] by an experimentally determined response function measured on a delta doped layer as illustrated in Figure 1b. The response function is typically anisotropic with an exponential tail extending deep into the sample. The peak is also shifted slightly toward beam direction by a distance δ. Littmark and Hofer [2] showed that the SIMS response function can be described basically by cascade recoil ion mixing. Wilson et al.[3] further pointed out that the symmetric portion of the response function (Region I) corresponds to the direct removal of overlayer species from the surface, either by sputtering or by ion mixing to the subsurface layers and often is approximated by Gaussian distributions. The exponential decay (Region II) corresponds to the residual effect of continual recoil mixing and erosion of sample surface during SIMS depth profiling. This residual effect is shown more clearly in Figure 1c. For the simplicity we use ‘direct mixing’ and ‘residual recoil mixing’ to represent these two unique characteristics of the response function. Theoretically the true depth profiles can be calculated by deconvolution if the response function is known. Many deconvolution methods have been proposed in the SIMS literature [4-6]. However they are not used widely, partly because SIMS response functions from delta doped standards are often not available. Instead, most efforts are focused on lowering the primary beam energy to reduce the recoil ion mixing [7]. We recognize that though the detailed response for direct ion mixing is often not
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