Thermally Robust Perpendicular STT-MRAM Free Layer Films Through Capping Layer Engineering

In order to enable the spin-transfer torque (STT) magnetoresistive random access memory technologies, it is essential to control the thermal budget, perpendicular magnetic anisotropy, and magnetic damping parameter of perpendicular magnetic tunnel junction (pMTJ) thin films. This paper demonstrates the enhancement of pMTJ free layer (FL) properties through selective engineering of capping materials placed between a CoFeB FL and the top electrode. By introducing a capping layer of Mg or MgO, thermal budget and tunneling magnetoresistance (TMR) ratio are significantly improved over the conventional FL schemes due to reduced atomic intermixing at interfaces. In full-stack pMTJ films, thin Mg above the FL enables the TMR above 170% after 400 °C annealing. Capping via MgO increases the interface anisotropy, allowing for the use of thicker CoFeB FLs with magnetic damping constants as low as 0.003. We discuss the implications of these capping layer improvements and suggest that Mg and MgO show the potential for optimizing the FLs with high-thermal budget and good STT efficiency.