Understanding the Correlation of SurfaceSIMS and TXRF Measurements of Surface Metal Contamination on Silicon Wafers
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CONTAMINATION ON SILICON WAFERS STEPHEN P. SMITH and JENNY METZ Charles Evans and Associates, 301 Chesapeake Drive, Redwood City, CA 94063 ABSTRACT A comparison of SurfaceSIMS measurements of surface metal contamination on
silicon wafers with TXRF analyses of the same wafers is presented. These two important surface analysis techniques are characterized by differing analytical areas and sampling depths. For Fe, agreement between SurfaceSIMS and TXRF results is good (generally
within a factor of two) for silicon wafers from a variety of sources with contamination levels ranging from 8x109 to 2x1015 atoms/cm2 .
Differences between SurfaceSIMS and TXRF
results occur for other elements (such as W and Cu) when the contamination is not uniform across the surface of the wafer (particles are present), or extends a significant depth below the surface of the wafer (as is frequently the case for ion-implanted wafers).
INTRODUCTION The measurement of the surface metal contamination on silicon wafers is an essential part of yield improvement during IC processing. Total Reflection X-Ray Fluorescence (TXRF) and Secondary Ion Mass Spectrometry (SIMS) are widely used analytical tools used to monitor contamination on semiconductor wafers. This paper presents TXRF and SIMS data on the same samples to investigate the correlation between the two measurement techniques. TXRF AND SurfaceSIMS ANALYSIS TECHNIQUES Total Reflection X-Ray Fluorescence (TXRF) is a nondestructive method of analyzing
flat, smooth samples. Wafer surfaces are illuminated by a primary x-ray beam that excites surface atoms to produce secondary x-rays. Each element produces x-rays with photon energy characteristic of that element. The number of photon counts is proportional to the amount of element present. Secondary x-rays from the sample surface are measured by a Li-drifted silicon solid state detector of conventional design but large area. Monochromatic primary x-ray beams (e.g. W Lo or Mo Ka) are used to excite different ranges of elements. TXRF quantification is achieved by analyzing a sample with a known contamination level of one element and calculating areal densities for unknown samples using sensitivity factors related to well known relative fluorescence yields for the elements. Quantification for these analyses is based on a measurement of Cu. Correlation of TXRF with Rutherford backscattering spectrometry (RBS) and Vapor Phase Decomposition followed by Atomic Absorption Spectrometry (VPD/AAS) has validated the quantitation approach. Secondary Ion Mass Spectrometry (SIMS) is characterized by very high sensitivity and good depth resolution for the measurement of trace contaminants in semiconductors. SurfaceSIMS is an adaptation of SIMS optimized for the accurate analysis of contaminants on the surface and near-surface (0 to 20 nm) of silicon wafers. Analyses are carried out with oxygen primary ion bombardment to sputter the sample and generate positive secondary ions. During the analysis, a jet of oxygen gas is directed at the sample surface (0-flood)
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