Development of Primary Reference Gas Mixture of CF₄ and SF₆ in N₂ as CRM for Measurements of Their Emission from Semiconductor Process Tool

Abstract

National Metrology Institute of Japan (NMIJ) has developed certified reference materials (CRMs) of CF₄ and SF₆ in N₂. These CRMs correspond to primary reference gas mixtures in Japan. The nominal concentration of CF₄ or SF₆ is 5 mmol/mol and its concentration corresponds to concentration level of gas analyses at the system of semiconductor process tools. In the first procedure for developing this CRM, the five primary gas mixtures were prepared using high-accuracy electronic mass comparator (maximum capacity, 15 kg, readability, 1 mg) with handmade automatic cylinder exchanger, for gravimetric blending method. The prepared mixtures were verified by analysis adopting a gas chromatograph with a thermal conductivity detector (GC-TCD), a quality control cylinder, and, Deming's least squares method for linear regression analysis. This verification (internal consistency test) confirmed that there was no mistake in the process of the preparation within the accuracy of GC-TCD. The relative standard uncertainty of the response of GC-TCD was ∼0.03%. After that, the long-term stability test for 47 months was performed and the dependence of inner gas pressure in a cylinder from 11 MPa to 1 MPa was evaluated. The relative expended uncertainty of this CRM became 0.2 to 0.3%, which included the uncertainties estimated from results of the preparation, the internal consistency test, a long-term stability test, and, an inner pressure dependence test. This expanded uncertainty is sufficiently small for users of this CRM.