Magnetic thin films with large magnetostriction constant and low intrinsic stress are demanded for magnetically actuated MEMS and NEMS [ ]. Amorphous Fe80B20 and Fe-B-N thin films are adequate candidates for MEMS operating in liquids, due to their anisotropy and magnetostriction, together with their excellent adhesion, stability and corrosion resistance properties. However, residual deposition stress in sputtered amorphous thin films tends to be large and causes undesirable bending or distortion of cantilevers and actuators. On the other hand, resulting strain modifies strongly magnetic anisotropy and consequent magnetic field dependent properties. Therefore, a study on the sputtering conditions dependence of Fe-B-N amorphous system strain has been carried out using an in-situ, real time, highly sensitive optical method monitoring substrate curvature.
Results allow us to optimize mechanical and magnetic properties by controlling deposition parameters. Fe-B-N amorphous alloy films have been grown by DC triode sputtering from a nominally Fe80B20 commercial target under an Ar + N2 atmosphere both on glass and on Si<100> substrates . This system allows to work under a wide range of sputtering pressures (2- 25x10-3 mbar) and cathode voltages (0.5- 2kV), thus allowing to determine the influence of impinging atoms energy on thin film stress evolution during deposition. Main intrinsic stresses were compressive and related to local distortion induced by energetic particles striking the film (ion peening). We also observe a significant stress generated at the film-substrate interface during the early stages of growth (initial 2.5nm). Finally, it is shown how film strain can be controlled by varying Fe80B20 target potential or by energetic particles thermalization by gas-phase collisions under gas pressure. A 20% increase on the magnetization, Ms , and on the magnetostriction constant is found for films incorporating ~ 8% atomic nitrogen compared to those for pure Fe80B20 amorphous films. A parameter of importance for magnetostrictive actuators, the rate, D, of elongation change with applied field, dλ/dH, reached 4x106 Oe-1 for low values of Hk, corresponding to 1.5x10-2mbar working pressure.
Ref [ ] K.L.Ekinci , Small 1, 786 (2005)
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