Application of brightness of scanning electron microscope images to measuring thickness of nanometer-thin SiO2 layers on Si substrates

When a surface of a silicon wafer, on which a nanometer-thin SiO2 layer is selectively formed, is observed by a scanning electron microscope (SEM), th e brightness of the SEM image at the SiO2 portion is stronger than that of the bare Si portion, and the brightness increases with the increasing thick ness of the SiO2 layer. This paper reports experimental and theoretical stu dies relating to whether the brightness difference between the SiO2 and the bare Si portions is applicable to measuring the thickness of nanometer-thi n SiO2 layers. The results suggest that the brightness of the SEM image mon otonically increases with the SiO2 thickness and saturates when the thickne ss reaches 12 nm; thus, it is applicable to evaluate relative thickness in a range smaller than 4 nm (directly applicable) or 10 nm (with calibration based upon the theory).