Graphite furnace atomic absorption spectrometric studies for the quantification of trace and ultratrace impurities in the semiconductor grade organic chemicals such as triethylborate, tetraethylorthosilicate and trimethylphosphate

Abstract

The paper describes the direct determination of impurities by graphite furnace atomic absorption spectrometry (GF AAS) in high (7 N, i.e. 99.99999%) purity organic compounds used as precursors for the deposition of dielectric films by chemical vapour deposition (CVD). The impurity analysis is required to assess the suitability of the compounds that include triethyl borate (TEB), tetraethyl orthosilicate (TEOS) and trimethyl phosphate (TMPO), used as precursors for the deposition of compositionally good quality films. Such features as high sensitivity and limits of detection of GFAAS are exploited for the direct determination of twenty elements (Ag, Al, Bi, Ca, Cd, Cr, Co, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, Pb, Sn, Sb, Zn) in μg L−1 levels in the three compounds. The analysis was preceded by a comprehensive study on the quantitative recovery of spiked elements using clean room procedures. Specially designed sample holders made of polytetrafluoroethylene (PTFE) were used for the analysis. The routinely used polystyrene sample holders are attacked by these chemicals and are thus rendered unsuitable for analysis. GFAAS operating parameters such as drying, pyrolysis and atomisation temperatures were optimized for each analyte in each organic sample for the removal of the matrix effects on the analyte and for quantitative recovery of the analytes. A fourth drying step was introduced in the furnace program. The accuracy of the method has been established by spike recovery tests in the absence of certified reference materials for high purity chemicals. The method is free from any interference and has a precision of <10%. Studies on the effect of polypropylene (PP) and perfluoroalkhoxy (PFA) storage containers on the impurities of these chemicals were performed and the results were discussed. The method can be effectively utilized for the quality control and quality assurance of these important class of chemicals used extensively in semiconductor industry.