Investigation on a NiFe alloy with a coefficient of thermal expansion matching the one of silicon

Abstract

Thin-film materials with a high relative permeability μr play an important role in the design of magnetic microactuators. Since the relative permeability μr is affected by external parameters such as film thickness and internal stress, investigations were carried out to determine the influence of film stress caused by different coefficients of thermal expansion of substrate and magnetic film, especially in combination with an annealing process at high temperatures. When using a soft magnetic material with a coefficient of thermal expansion close to the one of the substrate, the stress can be minimized. A suitable combination is NiFe35∕65 and Si substrates. Investigations on the thermal match of the two materials, as well as the relative permeability μr, the coercivity Hc, and the saturation flux density BS, were carried out as functions of the deposition temperature tdep. The relative permeability increases from 200 to greater than 1000 at temperatures above 400°C, while the coercivity Hc decreases from 2000to1000A∕m.