Influence of Morphology and Domain Structure on Mechanical Stress in High, Electroplated Co-Fe Flux Guides
Authors: Kruppe Rahel Maria , Rissing Lutz 
Published: February 2020

The most notable advantage electroplated binary Co-Fe alloys offer for MEMS applications like sensors and actuators is the high saturation flux density, which may reach values of 2.4 T in a composition range of 50 – 70 % Fe. In contrast, there are certain disadvantages limiting the use of these alloys, e. g. high film stress, high corrosion sensitivity and high magnetostriction. This triggered a lot of research aiming at the minimization of these unfavorable properties (1-4, 6).

In previous studies (5), we examined very high (20 µm) ring-shaped Co-Fe flux guide structures for a 3D, ultra-thin magnetic field sensor. Due to the GMR sensors underneath the flux guides, a thermal treatment at temperatures above 150°C to reduce film stress was not possible. Other measures were taken instead, like the use of saccharin, reverse pulse plating and the application of a magnetic field during deposition (2, 6). Thus, saturation flux densities above 2.1 T with drastically lowered film stress and good corrosion behavior were achieved, which may be attributed for a great part to the magnetohydrodynamic (MHD)-effect generated by the applied magnetic field. The gained results still necessitate further investigation of the connection between the process conditions and the mechanical and magnetic properties of the films. The goal is to maximize BS and the squareness of the hysteresis loop γ as well as to further reduce film stress σ and the coercivity Hc.

The mechanical and magnetic properties of Co-Fe films influence each other strongly. They are determined by the films’ microstructure, given by the film texture, grain size and morphology, structural defects and impurities. In this study, the effect of process conditions like current density, pulse duration, duty cycle, pH of the plating bath and application of a constant magnetic field parallel to the electrode surface on film stress, microstructure and magnetic properties are examined. The data for film stress, calculated on the basis of profilometry and x-ray diffraction (XRD) measurements, and the data for magnetic properties (saturation flux density, coercivity, hysteresis loop squareness) are linked together and will be referenced with the associated parameters for film texture, grain size and morphology – determined with XRD, scanning electron microscopy (SEM) and atomic force microscopy (AFM) – as well as magnetic domain structure, observed with Kerr microscopy.

Figure 1: a) Top view of electroplated flux guide b) B-H-loops of flux guides c) domain pattern of flux guide

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