C. Shearwood

Applications of polyimide membranes to MEMS technology

Abstract The fabrication of polyimide membranes on GaAs (001) substrates by bulk micro-machining is described. These membranes have diameters in the range of 1 mm to 8 mm and a thickness of 7 μm. The predicted and measured performance of a representative number of membranes is presented; their application is illustrated with reference to a “micro-speaker” for use in a hearing aid.

Initial fabrication of a micro-induction gyroscope

A planar coil design capable of levitating an aluminium rotor by electromagnetic induction is described. The coil is fabricated from Au onto a Si(001) substrate with a footprint size of 700 μm, and coil thickness and typical width of 1.3 μm and 50 μm respectively. The rotors are made from Al with a thickness of 12 μm and diameter of 400 μm. A maximum levitation height of 10 μm is measured with a coil excitation current of 800 mApk. This height is increased to 25 μm by the incorporation of a high permeability amorphous magnetic backing plane below the planar coils. Excellent quantitative agreement is observed between the predictions of modelling and experimental measurements of levitation height versus coil excitation current. The application of this levitated rotor to a wide range of applications, including a micromachined rotating gyroscope is described.

Growth and patterning of amorphous FeSiBC films

Abstract We report the growth of amorphous FeSiBC films on a range of substrates including oriented semiconductor, polycrystalline metal, and polyimide. The films vary in thickness from 2.6 μm down to 20 nm and were characterised by the Magneto-Optic Kerr Effect (MOKE), and for the thicker films a combination of MOKE and a high resolution inductive magnetometer. These measurements imply a skin-depth of 30 nm, at the frequency of the MOKE laser. A representative amorphous FeSiBC film of thickness 0.3 μm was patterned into wires of nominal width 10 μm, separation 5 μm, and length 10 mm by conventional microelectronic fabrication techniques. The switching of the wire-array was investigated by MOKE.

Extrinsic magnetoresistance and resistance relaxation in and films and heterostructures

The magnetoresistance and resistance relaxation after a discontinuous field change of (LCMO) and thin film structures was investigated. We compare the magnetoresistance and resistance relaxation of epitaxial films, polycrystalline films, mechanically induced grain boundaries and heterostructures in current-perpendicular-to-plane geometry. We show that the low-field magnetoresistance of various LCMO structures containing strongly scattering interfaces is significantly larger than the intrinsic magnetoresistance of epitaxial LCMO films. On the other hand, the corresponding structures made from magnetite show only a small enhancement of the magnetoresistance compared with epitaxial films. The resistance relaxation of the LCMO heterostructures is found to increase with increasing magnetoresistance, whereas no relaxation was observed in the heterostructures. We propose that the extrinsic magnetoresistance in LCMO structures is due to the formation of a spin-glass-like state at highly resistive interfaces.

Brillouin light scattering studies of iron-cobalt multilayers

Abstract Spin wave excitations have been observed in a series of sputtered iron-cobalt (FeCo) multilayer structures using Brillouin light scattering. The substrate in all cases was GaAs (001) and the total FeCo thickness was maintained at a nominal 170 nm; non-magnetic spacer layers of Ag and Cu with nominal thicknesses of 2 nm and an amorphous magnetic FeSiBC layer of 4 nm thickness were used. The saturation inductions B s were found to be in the range of 1.5–2.1 T, i.e. smaller than the bulk value of 2.44 T. The disparity between the bulk and multilayer values is attributed to microstructural features which suggest that the multilayers are stacks of bilayers consisting of a film with bulk-like properties in contact with a ‘composite’ film with a smaller magnetisation. Each bilayer can then be treated as a single layer with a volume-averaged magnetisation. In general the multilayer structures showed no observable uniaxial anisotropy and any cubic anisotropy fields were of a small magnitude ∼ 0.03 T. The results illustrate the use of Brillouin light scattering as a diagnostic tool in inhomogeneous systems.