Thin film effects in the crystallization of CoFeB and CoFeB-based alloys

The devitrification of CoFeB thin films, where BCC α-FeCo crystallizes from the amorphous matrix, is critically important to the fabrication of successful CoFeB/MgO-based magnetic tunnel junctions (MTJs), which in turn are the leading candidates for scalable MRAM. Future perpendicular MRAM requires MTJs with much thinner CoFeB electrodes (~1-2nm), which may yield significant changes to the reaction kinetics and resulting diffusion profiles upon annealing as compared to in-plane devices. In this study, we have analyzed the crystallization processes of sputtered thin films of Co₂₀Fe₆₀B₂₀ and Co₂₀Fe₆₀B₂₀-TM alloys (TM = Ta, Hf) using the increase in saturation magnetization (M_s) that accompanies the formation of α-FeCo. Although annealing with in-situ measurement of M_s is a common method of analysis in the study of bulk amorphous and nanocrys-talline magnetic materials, it is not well studied in thin films. We demonstrate thin film-specific effects on crystallization energetics including changes due to different nucleation surfaces and film thicknesses as well as effects analogous to those seen in bulk materials such as alloying with large transition metal species. M_s measurements represent a high throughput method for the determination of crystallization temperature which can be used to screen potential composite electrodes where a layer is placed adjacent to CoFeB in order to enhance perpendicular magnetic anisotropy (PMA) or spin transfer torque (STT). While these properties are necessary to have stable and switchable devices, respectively, CoFeB must still crystallize on the MgO surface in order to produce the large tunneling magnetoresistance (TMR) required by practical devices. Adjacent layers with lower crystallization temperature than those induced by the MgO interface may therefore need to be avoided.