Measurement strategy for dielectric ultra-thin film characterization by vacuum ultra-violet reflectometry

In this work, vacuum ultra violet reflectometry (VUV-R) is applied to measure the film thickness of ultra-thin dielectric layers of, e.g., SiO₂ and Al₂O₃. The objective of these measurements was to determine and to decouple several effects which can affect the measurement accuracy and precision of the measurements in the VUV region. Systematic studies were performed applying series of gridded and repeated measurements to take contamination effects, effects due to film modification by the VUV radiation, and drift of the measurement system into account. It is shown, that potentially due to the high sensitivity of the measurement in the VUV part of the wavelength region, the measurement can be influenced by any of the aforementioned effects. Based on the results of the investigations optimized measurement conditions were defined and the results were compared to those obtained using the default settings as well as to reference measurements by ellipsometry and x-ray reflectometry. It was shown that applying the optimized measurement strategy, stable measurement conditions can be obtained and good agreement to the reference methods is achieved.