Electroless Deposition for Nanoscale Applications: Challenges and Opportunities
Authors: O'Sullivan Eugene J. , Camagong C , Lavoie C. , Jordan-Sweet J. , Muir D. , Hopstaken M , Krishnan Mahadevaiyer 
Published: December 2020

We recently developed a maskless, electroless, high-P-content, Ni(P) process to protect the final Cu bitline wiring level in our STTM MRAM test vehicles to enable functional testing in an air atmosphere at elevated temperatures for evaluation of MRAM device memory state retention. The process was developed as a replacement for the final aluminum level, and drastically shortened wafer processing cycle time. We demonstrated an electroless Ni(P) that selectively deposited on Cu bit lines with effective spacing approaching 200 nm without shorting, for coating thicknesses up to ca. 50 - 60 nm, with excellent bitline coverage (no pinholes). This was achieved through optimization of Cu surface cleanliness, surface catalyzation using an acidic Pd solution, and finally electroless Ni(P) deposition (1).
Testing (electrical resistance and magnetoresistance (R & MR)) of Ni(P)-coated wafers, showed virtually unchanged R & MR for MRAM 4Kb arrays encompassing a large range of device critical dimensions (CDs). Figure 1 shows a topdown SEM image of a region of electroless Ni(P)-coated Cu bitlines.
While electroless deposition is capable of depositing only a limited number of metals and alloys compared to electrodeposition, it is often easier to obtain coatings of uniform thickness and composition, since one does not have the current density uniformity problem of electrodeposition. Although it is not always possible to obtain pure materials using electroless deposition, nevertheless, materials with unique properties, such as Ni(P) (corrosion resistance), and Co(P) (magnetic properties) and related alloys e.g., Co(W)(P), are readily obtained by electroless deposition methods. These materials may be crystalline or amorphous, and the crystallization kinetics of the latter type may be strongly dependent on film thickness, and have a two-dimensional character due to interfacial effects.
It is important to understand how such materials transform upon heating, given that back-end-of-line (BEOL) processing may involve temperature excursions to 400 ⁰C. We will discuss in detail the kinetics of crystallization of our high-P-content, amorphous, electroless Ni(P) films on Cu thin films using synchrotron X-Ray diffraction analysis (2). For 1 um thick Ni-P films deposited on Cu/Ta/TaN underlayer, as we increase the rates at which the temperature is raised during ramp anneals, crystallization temperatures vary from about 360C to 400C from which we extract an activation energy of 3.4 eV for this crystallization. The in-situ data also shows a gradual reduction in lattice constant for the underlying Ta layer. We will complement the synchrotron X-Ray analysis study results with TEM, SIMS and X-ray fluorescence (XRF) results of Ni(P) film composition for unpatterned and patterned (sub-micron) features.
In addition to the subtlety of its mechanisms and range of solution formulations, electroless deposition has much to offer in terms of niche applications, which explains why it is often turned to first by researchers seeking creative deposition methods in the still emerging field of nanofabrication. Given the wide range of materials involved in electroless deposition, whether it is the final product, or the choice of a reasonable reducing agent, the mechanism of electroless deposition tends to be complex. This talk will include a discussion of the mechanism of electroless deposition, and whether the sought after, one-mechanism-fits-all approach, is tenable.
[1]. E. J. O'Sullivan et al 2019 Meet. Abstr.
MA2019-02 916; https://doi.org/10.1149/MA2019-02/15/916

[2]. Canadian Light Source Inc., 44 Innovation Boulevard Saskatoon, SK S7N 2V3, Canada.

Acknowledgements

The authors gratefully acknowledge the efforts of the staff of the Microelectronics Research Laboratory (MRL) at the IBM T. J. Watson Research Center, where some of the fabrication work described in this talk was carried out. Part of the research described in this paper was performed at the Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation (CFI), the Natural Sciences and Engineering Research Council (NSERC), the National Research Council (NRC), the Canadian Institutes of Health Research (CIHR), the Government of Saskatchewan, and the University of Saskatchewan.