Microwave synthesis of superparamagnetic cobalt-ferrite thin films for RF CMOS applications
In the era of 5G technologies, the emphasis is on getting all the devices, active or passive to work in high frequencies. Higher efficiency chips can be obtained by combining both the active and passive devices on a single chip. This will avoid the noise caused by unwanted parasitics due to the external connection of the passive devices. The miniaturization trend seen in active electronic devices needs to be extended to the development and integration of passive components. Inductors typically occupy considerable “chip real estate”. Downscaling inductors can reduce the quality factor considerably. This can be countered by raising inductance density, i.e., inductance per unit area. Transition from air core to magnetic core is one of the ways to improve the inductance density. Our idea is to coat the inductor with a material which has high relative permeability to increase inductance, high resistivity to reduce eddy currents, low coercivity to reduce hysteresis losses, and frequency-independent magnetic permeability to work in higher frequencies. Spinel ferrites were chosen as the magnetic core material of the on chip RF-CMOS inductor. In the past many deposition methods have been used to deposit a thin film of the spinel ferrites but all of them either had a processing temperature which was not compatible with CMOS processing (> 400 oC) or had low saturation magnetization or high coercivity which is not very suitable as the magnetic core of an on chip RF-CMOS inductor. In this work, we use the microwave irradiation method for depositing the spinel ferrite thin film. The maximum temperature reached in this synthesis is ~190 oC and the processing time is not more than 20 mins. The precursors and solvents used do not emit any poisonous by-products so this process is environment friendly, cost effective, fast and effective. Bulk Cobalt ferrite shows all the properties needed for a magnetic core except for having high coercivity which further results in hysteresis losses. In this work, we demonstrate that nanostructured cobalt ferrite shows superparamagnetism at room temperature which results in very low coercivity while retaining the high saturation magnetization. We use a low temperature (< 200 oC) microwave synthesis method to deposit the cobalt ferrite thin film. This process uses molecular heating and hence can reach very high localised temperatures although the total temperature is maintained around 190 oC. XRD, SEM, FIB, XPS, TEM, neutron diffraction and SQUID techniques have been used to understand the structure, oxidation states, composition, and magnetic properties of the as-deposited thin films. SEM and FIB images show a conformal and uniform cobalt ferrite thin film deposition. XRD, XPS and neutron diffraction give the composition and the spinel inversion. The spinel inversion seems to be lesser than the bulk values. This is directly related to the reduction in saturation magnetization measured using SQUID. Although the saturation magnetization is little lesser than the bulk cobalt ferrite, it is still large enough to use as a magnetic core. The nanocrystallinity helps make the cobalt ferrite superparamagnetic which means it shows very less coercivity which is again measured using SQUID. Further, we investigate the effect of post-annealing time, precursor concentration and microwave exposure time on the saturation magnetization and coercivity. We realise that on post-annealing the thin film at 300 oC for 5 and 10 mins increases the crystallite size and also changes the spinel inversion. This, in turn, increases the saturation magnetization (Ms) and coercivity (Hc). Similarly, different samples with different precursor concentration and microwave exposure time are processed and characterized to get the optimised process flow for the deposition of the cobalt ferrite thin film, in order to be used in the application we choose. Bulk cobalt ferrite is known to have high electrical resistivity. The dielectric properties of a material acting as a core, play an important role in deciding the eddy current losses and hence affect the Q-factor. So it is important to measure the dielectric properties of the nanocrystalline thin films we have obtained. MIS capacitors have been fabricated to understand the dielectric properties of these films. We correlate the effect on magnetic properties with varying degree of inversion in the spinel structure and the grain size. The dielectric properties in terms of frequency dispersion of capacitance are analysed and the effect of processing condition on the permittivity is reported. The DC and AC resistivity of the films is also characterized using I-V measurement. We also investigate the effect of substituting the cobalt ferrite thin film with nickel and zinc to observe the effects of inversion on the magnetic and dielectric properties. We use three samples – cobalt-nickel ferrite (CNFTF), cobalt-zinc ferrite (CZFTF) and cobalt-nickel-zinc ferrite (CNZFTF) thin films with a specified composition for each. Again XRD, SEM, FIB, XPS, TEM, neutron diffraction and SQUID techniques were used to understand the structure, oxidation states, composition, and magnetic properties of the as-deposited thin films. We realise uniform and conformal films of each. Post-annealing is done on all the thin films and their characterization has been done to realise that post-annealing increases saturation magnetization while not affecting the coercivity much. Further MIS capacitors are made using these three samples to measure the dielectric properties of all the three samples. Finally, the optimised cobalt ferrite thin films and the substituted cobalt ferrite thin films were deposited on an on-chip RF-CMOS inductor to evaluate the performance of the inductor. The ferrite films are deposited all over the inductor acting as the magnetic core. We demonstrate a substantial enhancement in inductance density, without compromising the Q-factor. While the as-prepared cobalt ferrite thin film acting as the magnetic core results in an 18% increase in inductance density, the Q-factor doubles at 5 GHz. As-prepared CNFTF shows an 8% increase in inductance density and a 5% increase in Q-factor. As-prepared CZFTF shows a 4% increase in inductance density and 16% increase in Q-factor. While the as-prepared CNZFTF shows an 11% increase in inductance density and 69% increase in Q-factor, the annealed film shows a 44% increase in inductance density and the Q-factor reduces by half. We realise that the inductance density and Q-factor of the inductor can be tuned based on the choice of the above ferrites used as the magnetic core.