We investigated and compared the structural and magnetic properties of MgO/FeCoB based out-of-plane magnetized tunnel junctions at thin film level as well as the magneto-transport properties of corresponding patterned STT-MRAM cells comprising either Ta 1 nm or W2 /Ta1 nm cap layers for different annealing temperatures up to 455°C. W material in the cap was found to improve the structural stiffness of the perpendicular magnetic tunnel junctions and most importantly prohibits Fe diffusion from the FeCoB storage layer to the cap layer, remarkably improving the thermal robustness and magneto-transport properties of the stacks and of the corresponding patterned memory cells. As a result, the interfacial anisotropy constant of the MgO/FeCoB interfaces is improved by 17-29% compared to Ta cap. The STT-MRAM cells fabricated from the pMTJ stacks with W/Ta cap reveal a significant improvement of tunneling magnetoresistance and thermal stability factor, which are respectively 120% and 52 as compared to 70% and 35 for the stack with Ta cap. This improvements are ascribed to the enhancement of MgO crystallinity upon higher temperature annealing (425°C) as well as prohibition Fe out-diffusion.