N. Wang, E. J. O’Sullivan, P. Herget, B. Rajendran, L. E. Krupp, L. T. Romankiw, B. C. Webb, R. Fontana, E. A. Duch, E. A. Joseph, S. L. Brown, X. Hu, G. M. Decad, N. Sturcken, K. L. Shepard, and W. J. Gallagher, “Integrated on-chip inductors with electroplated magnetic yokes” (invited), J. Appl. Phys. 111, 07E732 (2012), DOI:10.1063/1.3679458
Thin-film ferromagnetic inductors show great potential as the energy storage element for integrated circuits containing on-chip power management. In order to achieve the high energy storage required for power management, on-chip inductors require relatively thick magnetic yoke materials (several microns or more), which can be readily deposited by electroplating through a photoresist mask as demonstrated in this paper, the yoke material of choice being Ni45Fe55, whose properties of relatively high moment and electrical resistivity make it an attractive model yoke material for inductors. Inductors were designed with a variety of yoke geometries, and included both single-turn and multi-turn coil designs, which were fabricated on 200mm silicon wafers in a CMOS back-end-of-line (BEOL) facility. Each inductor consisted of electroplated copper coils enclosed by the electroplated Ni45Fe55 yokes; aspects of the fabrication of the inductors are discussed. Magnetic properties of the electroplated yoke materials are described, including high frequency permeability measurements. The inductance of 2-turn coil inductors, for example, was enhanced up to about 6 times over the air core equivalent, with an inductance density of 130 nH/mm2 being achieved. The resistance of these non-laminated inductors was relatively large at high frequency due to magnetic and eddy current losses but is expected to improve as the yoke material/structure is further optimized, making electroplated yoke-containing inductors attractive for dc-dc power converters.