A wet chemical procedure has been elaborated to measure the thickness of thin silicon dioxide layers. The procedure is based on the etching of the SiO 2 layer by HF and the determination of Si concentration in the microgram per liter range in the HF containing etch solutions. Two analytical techniques were optimized for this purpose: a spectrophotometric technique, the so-called molybdenum blue method and inductively coupled plasma mass spectrometry (ICP-MS). In the first method a detection limit of 3.3 g/L Si could be achieved with a sensitivity of (780 Ϯ 8.7) ϫ 10 Ϫ6 /(g/L Si). Interference by HF up to 0.1% v/v (volume/volume %) HF could be eliminated by adding boric acid to the solution. In the second method Si was determined by ICP-MS using the 28 Si isotope. The detection limit in bidistilled water was 1.2 g/L Si with a sensitivity of (5807 Ϯ 98) cps/(g/L Si). The presence of HF increased the background signal of Si due to the etching of the quartz plasma torch. In 0.005% v/v HF a detection limit of 5.9 g/L Si could be achieved. For silicon dioxide layers below 1 nm, a reproducibility better than 5% was obtained.
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