Monitoring of Ultra-Trace Contaminants on Silicon Wafers for ULSI by a Novel Impurity Extraction and AC Surface Photovoltage Methods

Abstract

Monitoring of detrimental micro-contamination coming from various sources has been strived to upgrade process cleanliness in silicon (Si) ultra-large-scale-integration (ULSI), and to improve the device performance and chip yield. Processed wafers were investigated by means of chemical analytical and nondestructive ac surface photovoltage (SPV) methods. In this investigation, the pack extraction method (PEM), where sample wafers are enclosed in cleaned Teflon bags with aqueous acid solutions, were expanded to extract impurities. The PEM collect anions (SO₄²⁻, NO₃⁻, Cl⁻, F⁻, etc.), cations (NH₄⁺, Na⁺, K⁺, P⁵⁺, B³⁺, etc.) and detrimental metal impurities in Si wafers of both front and back surfaces, in contrast to the previously proposed technique. The PEM combined with ion chromatography and/or high resolution inductively coupled plasma mass spectroscopy facilitated to detect surface impurities to the level of 10¹²–10¹³ atoms/m² which is equivalent to ppt in actual sample volume. Monitoring harmful impurities was successful in upgrading the cleanliness in Si wafers and device manufacturing processes. Based on the chemical analysis of SO₄²⁻ at wafer surfaces having inverted layers, ac SPV in n-type Si was found to decrease in proportion to increase in SO₄²⁻ concentration. SO₄²⁻ causes interface traps in native oxide, thus reducing the ac SPV. Hence, this nondestructive method is a useful technique for characterizing SO₄²⁻ as well as other metal impurities as proposed in the previous report.