Solid state reaction of metals and semiconductors (Si, Ge, SiGe, GeSn) probed by in-situ X-ray diffraction Rapid thermal processing of metals deposited on Si (or Ge, SiGe, GeSn) is a common step in microelectronics, photonics and photovoltaics device fabrication. The so-called silicide (or germanide, germano-silicide) alloys forming during solid-state reaction are usually used as contact because of their numerous advantage, especially when the device size is reaching several nanometers scale (FDSOI CMOS for example). Understanding the phase transformation during this solid-state reaction is tremendous important for process optimization. The use of Synchrotron light source and a 2D detector enables in-situ monitoring of this rapid thermal processing within a reasonable time (tens of minutes), whereas this study is usually done ex-situ with a lab-source X-ray diffraction. In this talk we will present an example of in-situ X-ray diffraction study of solid-state reaction of Ni and GeSn on Si substrate. Smooth phase transformation is observed during the annealing and some kinetic parameters could be withdrawn.