Development of magnetic tunnel junctions (MTJ) of low RA and high TMR

Optimization of the deposition parameters using the LETI/DCOS IBS sputtering deposition tool from SPTS to realize high quality magnetic tunnel junction oscillators of different device configurations.

LETI/DCOS (Marie Claire Cyrille)
CEA/SPEC Saclay (Olivier Klein)
CNRS/THALES (Vincent Cros, Julien Kermorvant)

ANR-09-NANO-037 Milestone (2010-2013)
ANR-2011-NANO-016 (2011-2015),
FP7-ICT-2011 MOSAIC (2013-2016)

Integration of STNOs into conventional RF circuitry means that the STNO has has to deliver 0dBm to drive conventional microwave components. This is achievable only with magnetic tunnel junctions of high TMR in conjunction with device configurations that lead to large oscillation amplitudes. The challenge is that the resistance area (RA) product has to be well below RA=5Ωµm², since the critical current (or voltage) to induce the steady state oscillations has to be well below the breakdown voltage of the tunnel barrier. Low RA means an extremely thin MgO barrier while maintaining good structural properties. Application of a large continuous (DC) current for STNO operation can degrade the barrier and lead to pinhole formation.

Latest results
Using the SPTS tool, MTJ materials have been developed whose current performances are TMR = 70% at RA= 2 Ωµm² (CAPRES wafer level characterization), see Fig. 1, a for standard planar configuration (in-plane magnetized free layer and polarizer)

Using our PTA platform we have realized nanopillars of 100 nm. Preliminary dynamic studies show that that the steady state regime is reached with a microwave emission linewidth of 40 MHz, see Fig. 1. However, these results also indicate pinhole formation.

Further materials optimization is in progress as well as the development of non-standard MTJs such as double SAFs, a perpendicular polarizer and vortex oscillators.

Ursula Ebels (SPINTEC), Marie Claire Cyrille (LETI), Cécile Manoury (LETI), Marité Delaye (LETI), Christophe Dieudonné (LETI & SPINTEC), Jiafeng Feng (SPINTEC & LETI)


Figure 1

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Left : TMR vs RA for different MTJ processes developed at LETI (yellow) and uing other deposition tools (literature)
Right : Microwave emission spectrum of a 100nm nanopillar realized at PTA, with a linewidth of 40 MHz.

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