Diffusion and migration of elements are commonly observed in the fabrication of multilayer thin-film devices, including those of STT-RAM. The CoFeB/MgO/CoFeB tri-layer thin-film stack has been widely used in the design of STT-RAM devices as the functional magnetic-tunnel-junction (MTJ) structure. Such issues faced in the fabrication of these devices have been extensively researched from the stand point of engineering the materials property and structure to achieve the best MTJ performance. In this work, we conducted a detailed examination of the chemical-state change of the Ta and B in a CoFeB/MgO/CoFeB/Ta film stack by using x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry. We showed that the chemical-state change of Ta and B is a result of the Ta diffusion phenomena through the CoFeB/MgO/CoFeB tri-layer structure. In particular, we report the evidences of the formation of TaB x O y compound at some considerable depth away from the Ta layer. Also of value to XPS spectroscopy, the Ta binding energy for such TaB x O y compound is reported for the first time.studied Ta diffusion through a CoFeB/MgO/CoFeB tri-layer. The technology of magnetic-tunnel-junction (MTJ) is the enabler for a number of important applications such as the magnetic recording head and the magnetic RAM devices. The fundamental function of a MTJ structure comes from a threelayer thin-film structure. The top and the bottom layers are magnetic films while the center layer is an insulating film. Spinpolarized electrons can tunnel through this insulating barrier differently when the magnetic states of the top and bottom layers are in parallel or anti-parallel. CoFeB/MgO/CoFeB trilayer-based MTJ has shown to achieve high tunnel magnetoresistance (TMR), and has been the topic of many research works (Ref 1). This tri-layer structure is typically grown on a template that would reduce roughness and promote favorable