Electroless Deposition of Ferromagnetic CoxFe1-x alloys Using Metal Ion Reducing Agent

21^st century computing applications such as big-data analytics, cloud-based computing, etc., require enormous memory capacity.  Shuttling large amounts of information between numerous levels of interconnects and memory hierarchies that are necessary for these kind of applications incurs substantial latency and energy costs which is not suitable for today’s low-power, high performance devices.  A different approach that is being considered involves inserting low-power non-volatile memory cells closer to logic/processor for faster and more efficient processing¹ in so-called ‘monolithic integration’.  Due to its low power and high endurance, spin-based (spin-torque transfer, spin Hall effect) ^{2, 3} memories have recently gained attention.  In spin-torque transfer memories, information is stored in the spin orientation of a soft ferromagnetic layer, typically a CoFe or CoFeB alloy. A spin-polarized current is used to change the spin orientation in the ferromagnetic layer either ‘up’ or ‘down’, providing the binary states.  This memory stack is usually formed by a subtractive process. Individual stacks are deposited by physical vapor deposition followed by reactive ion etching to obtain the memory cells.  Alternatively, an additive process where the ferromagnetic layers are selectively deposited can be proposed.  The additive scheme does not have some of the negative consequences associated with subtractive processes such as non-uniform profile, re-deposition of etch by-products and difficulties in scaling. Here, we demonstrate a selective deposition process for Co_xFe_1-x (x=0-1) ferromagnetic alloy enabled by electroless deposition.  Trivalent TiCl₃ is used as a reducing agent which facilitates growth of crystalline and contaminant-free (such as B or P) film even at room temperature.  Electrochemical characterization of the oxidation and reduction half-reactions are presented and are used to rationally design the electrolyte.  X-ray photoelectron spectroscopy (XPS) characterization indicates the presence of metallic Co and Fe in the bulk of the deposit. Finally, the magnetic properties of the CoFe film are investigated using vibrating sample magnetometer (VSM).  Electroless deposited CoFe thin films demonstrate magneto-crystalline anisotropy with distinct easy and hard axes and high saturation moment.  Effect of heat treatment and surface capping on magnetic properties are also discussed.  Finally, some challenges involving integration of such an additive process is highlighted along with possible solutions.

Figure 1: XPS characterization of the CoFe film deposited on a PVD Cu substrate.  Carbon and oxygen are mainly present at the surface and not shown for clarity.  Chemical state analysis indicates presence of metallic Co and Fe in the bulk film.

References:

1. H.-S. P. Wong and S. Salahuddin, Nat. Nanotech. 10 (2015)
2. S. Ikeda, K. Miura, H. Yamamoto et al, Nat. Mater. 9 (2010)
3. L. Liu, C-F. Pai, Y. Li et al, Science 336 (2012)