Enabling STT-MRAM in High Volume Manufacturing for LLC Applications

There is a growing interest in using Spin Transfer Torque Magnetic Random-Access Memory (STT-MRAM) technology for SRAM replacement, particularly for Last Level Cache (LLC) applications. Besides LLC requirements of high performance of the MRAM cell, and array patterning, MRAM remains challenging for high volume manufacturing (HVM) because of the process complexity: deposition and growth of up to 50 layers with sub-Ångstrom precision required to optimize device performance. In this paper, we demonstrate systematic process monitoring and optimization of magnetic tunnel junction (MTJ) stack, pattering unit-processes, and fabrication hardware and capabilities to address these challenges. We present our novel solution for precision process control required for HVM: On-board Metrology (OBM™), which is integrated into the Applied Materials Fndura® PVD System used for MTJ stack deposition. OBM technology, for the first time, truly enables volume production of STT-MRAM by providing commercial in-vacuum monitoring of MTJ film properties on product wafers in real-time. OBM is a novel optical technique, which demonstrates <0.2Å sensitivity and repeatability on the key MRAM layers without impacting process throughput. OBM helps to accelerate learning in R&D and provide precise process control in HVM. We also present a study of perpendicular MTJ (pMTJs) with resistance-area product (RA) of less than 3 Ohm-
$\mu\mathrm{m}^{2}$
, having world-class tunneling magnetoresistance
$(\mathbf{TMR}) \ \ > 150{\%}$
with stable RA, TMR, and magnetic properties post 400°C anneal. We present our most recent device data with patterned MTJ diameter below 40 nm and pitch, down to 88 nm, with well-behaved switching distributions for write pulses as short as 5 ns and with excellent device stability against 400°C back-end-of-line (BEOL) baking up to 3hr.