(Invited) Electrochemistry: Adventures in Metallization
Authors: O'Sullivan Eugene J. 
Published: November 2022

Microelectronics has benefited enormously from electrochemistry, particularly in metallization. Metallizing through-holes in multilevel printed circuit boards was a major, successful application of electroless Cu (1). Electroless Co-based magnetic films deposited on non-magnetic electroless nickel films on rigid aluminum disks propelled the magnetic storage industry for years.
A decade or more ago, it looked as if electroless Co(W)(P) was the ideal candidate to replace PVD Ta-based liners for CMOS back-end-of-line (BEOL) builds (2). Its cost undid it, however, despite meeting selectivity, diffusion barrier and reliability requirements. Electrolytic Cu has been an outstanding success for CMOS BEOL interconnect metallization, mostly because of its submicron feature superfilling ability (3).
Following such success, electrolytic and electroless deposition methods have never been far from microelectronics researchers’ interest. In this talk, I will describe examples of electrochemical metallization in chip level, power conversion and MEMS areas that I have worked on.

MRAM Final Interconnect Level Capping

We recently developed a maskless, electroless, high-P-content, Ni(P) capping process for the final Cu bitline wiring level in our STTM MRAM 200 mm wafer test vehicles. This replaced a two litho mask, final aluminum metal interconnect level, drastically shortening process time.
This novel protective layer enables functional testing of MRAM device memory state retention in an air atmosphere at elevated temperatures (4). The Ni(P)-coated wafers show virtually unchanged device resistance and magnetoresistance (MR) for MRAM 4Kb arrays.

Magnetic Inductor Fabrication

Magnetic inductors are increasing in importance in the ongoing development of integrated, on-chip power conversion. The latter is critical for realizing the dream of granular, DC-DC power delivery using dedicated voltage regulators (VR). Traditionally, the large size of the inductor component has impeded efforts to fabricate the VR in one module.
We explored potentially manufacturable processes for magnetic-core inductors with enhanced inductance using through-mask electrodeposited Ni45Fe55 (Fig. 1) (5) and electroless Co(W)(P) layers (6). Electroless Co(W)(P) yoke material performed best overall, showing excellent magnetic properties, good magnetic anisotropy and coercivity of less than 0.1 Oe (6). The resistivity of the Co(W)(P) material was about 90-100 µΩcm; a value of 100 µΩcm is desired to limit yoke eddy current loss at high frequencies.
Device scaling has finally brought magnetic inductor fabrication within reach of BEOL CMOS fabs.

Magnetic Minimotor Fabrication

High-aspect-ratio optical or X-ray lithography (LIGA) and electrodeposition processes were used to fabricate variable-reluctance, nearly planar, integrated minimotors with 6-mm-diameter rotors on silicon wafers (7). The motors comprised six electrodeposited Ni81Fe19 (Permalloy) horseshoe-shaped cores that surrounded the rotor. We formed copper coils around each core. LIGA processing provided vertical wall profiles, which were important for the rotor and stator core pole tips (see stator pole tip, feature D, in Fig. 2).
We fabricated the rotors separately and slipped them onto the shaft after releasing them from the substrate wafer. Shaft fabrication via electrodeposition occurred as part of the stator fabrication process.
The LIGA fabricated minimotor (100 μm thick Permalloy core with 40 μm thick rotor) represented the successful integration of aligned X-ray exposures and planarizing dielectric into a MEMS fabrication process, producing a working, five-layer magnetic motor. I will show some minimotor operational data.
[1]. See papers in IBM J. Res. Develop.,
28(6) (1984), available online.
[2]. See, e.g., Y. Shacham-Diamand et al., J. Electrochem. Soc., 148 (2001) C162.
[3]. P. C. Andricacos et al., IBM J. Res. Develop.,
42, 567 (1998).
[4]. E. J. O'Sullivan et al., 2019 Meet. Abstr.
MA2019-02 916; doi: 10.1149/MA2019-02/15/916.
[5]. E. J. O'Sullivan et al.,
ECS Transactions 50(10):93-105, doi: 10.1149/05010.0093ecst.
[6]. N. Wang et al., MMM-Intermag, paper HG-11, 2013.
[7]. E. J. O'Sullivan et al., IBM J. of Res. Develop., 42, 681 (1998).

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.

Figure 1