Ultra-soft magnetic films for high-frequency inductors (GWN.4561)

There is a growing need to reduce the size and the costs of power converters widely used in portable/wireless devices,like mobile phones, messaging devices, palmtop terminals, and organisers, i.e. versatile cornmunication tools. For this the use of high frequencies (e.g. 101000 MHz) in combination with thin magnetic materials is desirable. The use of magnetic films allows the integration of transformers and inductors into silicon IC circuitry in a postprocessing step. Soft magnetic films are also widely used in modem electromagnetic devices as a high-frequency (>100 MHz) field-amplifying component, e.g. in read-write heads for magnetic disk memories for computers, and as a magnetic shielding material, e.g. in tuners.

The main requirements for the film material are: a high saturation magnetisation, combined with a low coercivity and a small but finite anisotropy field. In addition, the material should have a reasonably high specific electrical resistivity to reduce eddy currents, and appropriate mechanical properties. To obtain a low coercivity, the film material should be amorphous or polycrystalline with a grain size of less than 10 nin, should have little strain, and a very small magnetrostriction. At present, the materials of choice [Via]] are thin sputtered films with Fe and N as main constituents, with afew at% ofan alloy element such as A 1, Ti, Cr, Zr, Hf, Ta.

Here we propose a novel approach to the synthesis of ultra-soft magnetic films, applying a lowtemperature gaseous nitriding process to self-supporting Fe films, for which we have applied a patent. In this process the Fe to be nitrided is covered with a ~ 25 nin thick Ni layer, before being exposed to a mixture of NH3 and H2 at an unusual low temperature of <400 OC. At these low temperatures a pore-free nitride can be obtained. Under these conditions, in pure Fe the nucleation of nitrides occurs homogeneously throughout the layer, at least in the first few hundreds of nm, i.e. no nitridation front is formed. The nucleation can be influenced by providing nucleation centres. For this purpose an alloying element (Al, Ti, Al, Cr, Zr, Hf) will be admixed with the Fe to a concentration of a few at%. Small ( z_ 1 nm) precipitates of nitrides of these elements can be formed if a low nitriding potential is applied. They act as nucleation centres for the Fe nitrides to be formed in a second step. At low processing temperatures a polycrystalline material with grain sizes in the order of 10 run can be obtained. With our present experience we expect that homogeneous nitride layers with a thickness of one to several ~im's can be produced. The strain will be small, because the layers are homogeneous and have no substrate. The desired electro-magnetic and mechanical properties can be tuned by choosing the processing conditions. For applications (like reading heads) for which thicker layers are required, stacks of thin layers, separated by insulating films, can be used.

It is our aim to study the nitriding process and the formation of nanocrystallites with 'foreign' nitrides as a kernel. The conditions requiredfor the synthesis offilms with a high saturation magnetisation, very low strain and magnetostriction, and with appropriate mechanical properties, will be established The magnetic properties will be measured as a function of frequency (0 Hz -1 GHz). For this we will use a large variety of methods and measuring techniques. The major part of these were developed to a high degree of perfection in one or more of the participating groups. A survey is given under point 8. At the end of the 4-years period we expect to be able to produce ultra-soft magneticfilms using cheap techniques, that in principle can be applied on an industrial scale.

The instrumentation for this project has been acquired and it is tested. The instrumentation includes an analyser to measure the impedance of magnetic films at high (GeV) frequencies, a vibrating rod magnetometer, ovens for nitriding samples, a dc sputter-deposition facility. The group has access to advanced equipment for SEM (Scanning Electron Microscopy) and TEM (Transmission Electron Microscopy) characterisations, to XRD (X-ray diffraction) spectrometers and ion beam analysis facilities in Groningen and Utrecht. Via Delft there is access to characterisation positron annihilation and to small-angle neutron scattering.
Two metallic alloys were investigated: Fe94Ti2Ni4 and Fe93Cr3Ni4, in order to establish their microstructure, composition and homogeneity. The alloys were obtained by melting the constituents in an oven in an Ar atmosphere. The microstructure of the two alloys corresponds to the primary structure after rapid solidification. Films with a thickness of 2-4 mm were produced by rolling thin slices of the material.

Er zijn 2 bedrijven en 1 andere universiteit bij dit project betrokken.