An important aspect of the development of electromagnetic microactuators is the search for suitable materials as well as the development of the respective deposition and patterning processes. Within the Collaborative Research Center 516 “Design and Fabrication of Active Microsystems”, it is the task of the subproject B1 “fabrication of magnetic thin films for electromagnetic microactuators” to perform these investigations. The materials of interest can be divided into two groups: hard magnetic materials and soft magnetic materials. Materials with optimized properties and fabrication processes have been developed within both groups. An example is Samarium–Cobalt (SmCo), which can either be deposited using magnetron sputtering as Sm2Co17 with a very high energy product or in the SmCo5 phase using gas flow sputtering with very high deposition rates. In the area of soft magnetic materials, investigations on Nickel-Iron (NiFe) especially NiFe81/19 were followed by the evaluation of NiFe45/55, which features a higher saturation flux density B s and relative permeability μ r. Furthermore, current investigations focus on Cobalt-Iron (CoFe) and its further increased saturation flux density B s and relative permeability μ r. Current tasks include the stabilization of the fabrication processes to achieve good material properties (i.e. electroplating of CoFe) or a shortening (e.g. by using heated substrates during deposition) by using process alternative not used so far. Another topic is the integration into fabrication processes, i.e. the investigation of process stability and compatibility.

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.

When developing linear micro motors, the synchronous drive scheme is well suited since it offers relatively high driving forces while keeping the design, fabrication, and assembly relatively simple. After the feasibility of the application of the synchronous drive scheme in a micro motor was proven, a smaller version of the motor was developed to investigate this drive scheme’s miniaturization potential. Apart from scaling down the motor dimensions, further optimizations were applied using the results of FEM simulations. The micro motor was fabricated, assembled, and successfully tested. The results demonstrate that the synchronous linear micro motor was successfully scaled down. Furthermore, the results indicate that even a further miniaturization seems feasible.

Flux guides made of soft magnetic thin-film material are an important component of micro actuators based on electromagnetic transducer principles. One essential property of soft magnetic films is the relative permeability µr. To achieve a low magnetic resistance (reluctance), a high relative permeability µr is required. While the permeability in patterned flux guides located in the wafer plane typically can be determined rather easily (it typically is similar to the permeability of non-patterned films) the permeability in patterns with a flux perpendicular to the wafer plane quite often is unknown. However, for the magnetic behavior of the whole thin-film core it is of utmost importance. Flux guides often feature a thickness of some ten micrometers, but lateral dimensions of hundreds of micrometers or even some millimeters and therefore are subject to demagnetization effects. Therefore, any permeability measurement on patterned magnetic films has to take the influence of the flux guide geometry into account. This paper describes an approach to determine the real average relative permeability µr along the main axes, i.e. parallel and perpendicular to the wafer surface, by using special multiple probes, subjecting the probes to a Vibrating Sample Magnetometer (VSM) test to determine the apparent relative permeability µr,app of a probe, and calculating the real relative permeability µr,real by taking the demagnetization factor into account.

There are two step motor principles for building up linear microactuators: variable reluctance (VR) and hybrid. While the VR motors use only soft magnetic material for guiding the magnetic flux, hybrid motors use permanent magnets as a source of static magnetic flux, as well. The benefit is a greater driving force, while the drawback is a more difficult fabrication process. The working principle of the hybrid motors requires the flux created by the permanent magnets to be of the same magnitude as the flux created by the excited coils. Therefore, it was investigated how to ensure that the two fluxes match. This paper describes the approach taken and the resulting advances in the fabrication of the micromotor

We present here the application of an exchange spring multilayer system in an on-chip microwave device. The microwave devices were made in a coplanar geometry using a [SmCo/NiFe]10 sputtered multilayer structure as the active material. At low fields we find an up shift of the operational frequency by more than 15 GHz for the multilayer system compared to the NiFe alone. For higher fields (above 2 kOe) the increase in operational frequency is about 8-10 GHz. In contrast to previous results using an oriented SmCo film, we find in our polycrystalline film that there is not a large difference between frequencies measured with positive magnetic field compared to those measured with negative magnetic field. We studied multilayer systems with different thicknesses of NiFe. Magneto-optical Kerr effect measurements show a distinct uniaxial anisotropy for structures with 30-nm NiFe. Thinner NiFe films did not result in a clear anisotropy. Nonetheless, a substantial frequency shift was measured for all the samples. These measurements indicate that exchange coupled structures can substantially increase the frequency of signal processing devices

For designing and fabricating electromagnetic microactuators as pursued within the collaborate research center “Design and Fabrication of Active Microsystems” (Sonderforschungsbereich 516), soft and hard magnetic materials are required to create and guide magnetic flux. The investigations on the development of suitable materials and their deposition technologies are presented in this paper. In the area of soft magnetic materials, the application of Cobalt–Iron (CoFe) as an alternative to Nickel–Iron (NiFe, Permalloy) was investigated. The benefit of CoFe over NiFe is its greater saturation flux density. The technology utilized for the deposition was electroplating. In the area of hard magnetic materials, gas flow sputtering was applied for depositing Samarium–Cobalt (SmCo). This technology enables the deposition of pure SmCo layers at high deposition rates and without high vacuum. Furthermore, the dependence of the magnetic properties of the SmCo on the film composition was examined.

To replace NiFe alloys used as flux guides in magnetic microelectromechanical systems, electroplated CoFe thin films were examined. The films were deposited using pulse plating. The results show a strong influence of the reverse current Ireverse on the deposition rate and the saturation flux density Bs. The relative permeability μr is influenced both by the forward current Iforward and by reverse current Ireverse. Based on the examinations an optimized deposition was performed. A film thickness of 15μm was reached during a deposition time of 1h. This film exhibited a saturation flux density Bs of 1.4T, a coercivity Hc of 1130kA∕m, and a relative permeability μr of 290. At this thickness, no delamination occurred and the film showed only minor signs of corrosion, a phenomenon CoFe films are typically prone to.