In order to launch spin transfer torque-based magnetoresistive random access memory into mass production below the 40 nm technology node, high performance perpendicularly magnetized magnetic tunnel junctions (pMTJs) are crucial. One of the key issues for pMTJs is to ensure compatibility with conventional back-end-of-line (BEOL) heating process, where thermal tolerances over 350 $^{}{\rm C}$ for standalone memory and 400 $^{\circ}{\rm C}$ for embedded memory are typically required. In this work, we successfully demonstrated high thermal tolerance in CoFeB-based pMTJs with a synthetic antiferromagnet (SAF) pinned structure while keeping stray fields on the free layer low. The annealing temperature $(T_{\rm anneal})$ dependence of spin torque switching (STS) and TMR characteristics revealed that the reduction of $I_{{\rm c}0}$ , the increase of $\Delta$ , and the increase of TMR ratio were achieved by elevating $T_{\rm anneal}$ from 250 $^{\circ}{\rm C}$ up to 380 $^{\circ}{\rm C}$ for 30 min. Thus, we attained high STS efficiency $(I_{{\rm c}0}/\Delta)=0.88$ ( $I_{{\rm c}0}=69\ \mu{\rm A}$ , $\Delta=78$ ), ${\rm TMR}\ {\rm ratio}=120\%$ , $RA=12\ \Omega\mu{\rm m}^{2}$ at $T_{\rm anneal}=380\, ^{\circ}{\rm C}$ for the pMTJs with dimensions of 44–46 nm in diameter. Moreover, by using specially tailored pMTJs with a SAF pinned structure, we achieved high thermal tolerance up to 400 $^{\circ}{\rm C}$ for 30 min while keeping $I_{{\rm c}0}/\Delta=1.12$ ( $I_{{\rm c}0}=56\ \mu{\rm A}$ , $\Delta=50$ ), ${\rm TMR}\ {\rm ratio}=150\%$ , $RA=13\ \Omega\mu{\rm m}^{2}$ .

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