Highlights

Development of Techniques to Prepare Atom Probe Samples of Thin Film Oxide Heterostructures with FIB

Scientific Achievement

Procedures were developed on the Zeiss 1540 XB FIB to fabricate atom probe (AP) specimens of magnetic tunnel junctions (MTJs), following the process developed elsewhere [Miller et al., Ultramicrosc. 102 287 (2005)].  AP analyses of these MTJ specimens were unsuccessful.  In contrast to 5 kV used elsewhere, the 15 kV ion-beam energy used to clean-up the specimens was too high to remove surface ion-beam damage from the needle specimens.  The partial AP data collected suggest that development of techniques that produce higher shank angle needles is also needed to reduce heating effects during analysis.  We were able to produce high quality MTJ specimens for high resolution electron microscopy (HREM).  Some HREM images, as well as AP analysis from specimens fabricated at Imago Scientific Instruments were presented orally at the Microscopy and Microanalysis meeting in August 2007 [Microsc. Microanal. 13 (Suppl. 2) 620-621 (2007)].  These results showed more chemical intermixing at the oxide barriers/CoFeB interface than at the CoFeB/oxide interface for a MTJ structure seeded with Ta.  Also, that there is depletion of boron near the barrier / electrode interfaces, with the level of depletion being asymmetric for the two interfaces.  HREM images of a CoFeB/MgO(001)/CoFeB MTJ seeded on Cr showed the high quality of the MTJ with a flat, continuous and crystalline barrier.  In the as-grown sample, HREM confirmed that the CoFeB electrodes are nanocrystalline and wet both the MgO barrier and Cr seed layers.  Annealing yielded partial crystallization of the bottom electrode, with crystallization initiating from the MgO interface toward the seed layer. In contrast, the top electrode remains nanocrystalline after this anneal.  This difference in crystallinity may result from a difference in chemical intermixing between the top and bottom barrier interface.

Significance

Three-dimensional atom probe and HREM can provide atomic-scale information about local chemistry and morphology, and are therefore very effective at analyzing layers and interfaces in MTJs.  MTJs are of interest because of the tunneling magnetoresistance (TMR) that they display, with potential application as magnetic sensors and in magnetic random access memory.  The TMR phenomenon relies on spin-dependent tunneling across an ultra thin oxide tunnel barrier less than 2 nm in thickness sandwiched between two ferromagnetic electrodes.  It has been shown that amorphous CoFeB reduces interfacial roughness because of the absence of grains found in crystalline electrodes. For CoFeB/MgO(001)/CoFeB MTJs, a significant enhancement in TMR has been reported following annealing, which may result from crystallization of the electrodes.  Further research is planned to correlate the transport behavior to the microstructure of as-grown and annealed CoFeB/MgO(001)/CoFeB MTJ specimens imaged with HREM.  

Performers