Magnetic Inductors and Their Increasing Importance in the Development of on-Chip Power Conversion

Major strides in on-chip power conversion have been made by IBM in case of their POWER8^TM processor (1), and by Intel in case of their 4th generation (Haswell) processor (2), although neither company’s new on-chip voltage regulator (VR) uses an integrated inductor. After outlining why on-chip power conversion is so desirable, this talk will highlight the important role of magnetic inductors in power conversion, and will review recent developments in, and prospects for, magnetic materials for inductors, especially those formed by plating methods.

Granular DC-DC power delivery, consisting of fast dynamic voltage scaling for each load through use of a dedicated VR, can significantly improve energy efficiency. Traditionally, the large size of the inductor component has impeded efforts to fabricate the VR in one module. The preferred approach to shrinking the inductor is to increase the switching frequency. The downside to higher frequency operation is reduced efficiency and increased heat dissipation. Thus, shrinking the inductor involves a compromise of space vs. performance, and placing the VRM with an integrated inductor on the processor using thin-film fabrication processes is a natural, if challenging, progression.

In thin-film ferromagnetic inductors, yoke material and thicknesses (typically 1 - 3 microns) can be tailored to reach desired inductance values (e.g., 10 – 40 nH), while endeavoring to maintain high enough operating frequency (e.g., 50 – 100 MHz).  Considerable efforts have gone into developing new magnetic alloys with higher resistivity (> 100 µΩcm) to reduce yoke eddy currents. A notable example produced by sputtering is amorphous Co_91.5Zr₄Ta_4.5 (CZT) [3, 4], which seems to  have become the standard against which other materials are compared. This has been used as the yoke material in inductors fabricated on top of 90 nm CMOS structures by Gardner et al. [5], for example.

Electroplating has been a standard technique for the deposition of thick metal films due to its high deposition rate, conformal coverage and low cost. It was an enabling technology for the thin-film magnetic recording head, and was thus used for yoke fabrication for our inductors (6, 7). For the latter, Ni₄₅Fe₅₅ was chosen over Ni₈₀Fe₂₀ for its higher magnetic moment (1.6T), high anisotropy field, and higher electrical resistivity (40 µΩcm).    

Plated Co-based materials are attractive alternatives to Ni-Fe as yoke materials, e.g. due to their higher moment, especially if their resistivity can be made to approach or exceed 100 µΩcm. We are exploring the use of electrolessly-plated, Co-W-P films for inductor applications (8). The electroless Co-W-P films show excellent magnetic properties, with good magnetic anisotropy, and coercivity of less than 0.1 Oe (Fig. 1). The resistivity of the films is about 90-100 µΩcm, which is close to that of most amorphous Co-based alloys. 

[1] Z. Toprak-Deniz et al., ISSCC Digest, 112 (2014).

[2] E. A. Burton, et al., Proc. Applied Power Electronics and Expositions (IEEE-APEC), p.  432-439 (2014)

[3] K. Hayashi et al., J. Appl. Phys., 61, 2983 (1987).

[4] D. S. Gardner et al., IEEE Trans. Magn. 43, 2615 (2007).

[5] D. S. Gardner et al., J. Appl. Phys., 103, 07E927 (2008).

[6] N. Wang et al., J. Appl. Phys., 111, 07E732 (2012).

[7] N. Sturcken et al., ISSCC, 48, 244 (2013).

[8] N. Wang et al., MMM-Intermag, paper HG-11, 2013

Fig. 1.  Hysteresis loop (left) and SEM Xsection (right) of an electrolessly-plated Co-W-P films.

This work was supported in part by Lawrence Livermore National Laboratory subcontract No. B601996 under prime contract DE-AC52-07NA27344 from the U.S. Government. Inductor fabrication was carried out in the Microelectronics Research Laboratory (MRL) at IBM’s T. J. Watson Research Center.