A novel approach for measuring ultra‐thin, buried SiO2 film thickness

Accurate measurement of physical thickness of thin SiO 2 films is of great interest to the semiconductor industry. Existing inspection techniques are subject to large error when the oxide thickness falls below 2 nm. This work explored a new approach with improved accuracy to the measurement of thickness of thin oxide films. The new method utilizes dynamic SIMS with a primary ion beam of one isotope of oxygen ( 16 O or 18 O) at normal incidence and detecting negative secondary ions of another isotope ( 18 O or 16 O, respectively) inside SiO 2 . The experiment was performed by using an 16 O 2 + primary beam and detecting 18 O ― as characteristic secondary ions for SiO 2 . We substantiated that the matrix effect was eliminated during profiling through the SiO 2 /Si interface in a poly Si/SiO 2 /Si stack with an O 2 + beam at normal incidence, which is crucial for reliable quantification of oxygen amount inside SiO 2 . The high ion yield of 18 O ― and negligible contribution from the mass interference of 16 OH 2 ― ensured measurement of the total amount of oxygen inside an SiO 2 film with good sensitivity. By assuming that the silicon oxide is in the form of stoichiometric SiO 2 , which is the case for those layers grown with dry oxidation, the measured amount of oxygen can be readily converted into the thickness of SiO 2 . This technique provides reproducible measurement of the thickness of SiO 2 films and potentially a good accuracy if a reference material is well calibrated.