Study Of Microinductor With Magnetic Core Based On MEMS Fabrication Processes

As one of the most important passive devices in the field of radio frequency integrated circuit,passive inductor can be used to achieve a variety of functions,such as circuit tuning,impedance matching and signal filtering.By applying microfabrication process technique,the conventional inductors are going to be chips and films,the volume and the weight of the inductors can be reduced greatly.However,both the inductance and quality factor of microinductor show a relatively low value,which seriously restricts the development of radio frequency integrated circuit technology.Integrating soft magnetic thin film core materials into microinductor is an effective way to improve both the inductance and quality factor,but the magnetic hysteresis loss due to the large coercive force and the eddy current losses derived from magnetic core materials will result in a major magnetic loss.Moreover,the inherent low ferromagnetic resonance(FMR)frequency restricts the improvement of operating frequency.Confined by these factors,the overall performance of microinductor can't be fully improved.On the other hand,the magnetic core is easily oxidated when the curing process of polyimide is conducted in a high temperature,which causes a negative influence on the overall performance of microinductor.Based on the analysis of microinductors with magnetic core,in this dissertation,an optimization scheme about the topological structure of magnetic core was designed.Furthermore,microfabrication processes of microinductor with magnetic core compatible with MEMS technology were studied.Aiming to reduce the eddy current loss and improve the high frequency characteristics of magnetic core,the dissertation carried out some investigations and discussions to the relevant issues.The main innovation points of this dissertation are presented as follows:(1)A method to evaluate high frequency characteristics and permeability of rectangular magnetic core by the related parameters extracted from static hysteresis curve and “double ten” regulation respectively was proposed.The FMR frequency and permeability are two crucial factors to determine the operating frequency and inductance of microinductor with magnetic core.In the present studies,the frequency dependence of complex permeability curve for magnetic core is measured by three main approaches including cavity resonance method,pick-up coil method,and transmission line method,then the FMR frequency and permeability of magnetic core can be characterized.The above three methods are demanding to the requirements of equipment,and the procedure is very complicated and time-consuming.Based on the rectangular soft magnetic thin film core,the method of evaluating the high frequency characteristics and permeability was proposed using the related parameters extracted from static hysteresis curve and “double ten” regulation,respectively.The FMR frequency of rectangular magnetic core can be estimated by saturation flux density and anisotropic field,which are extracted from static hysteresis curve.The permeability of rectangular magnetic core can be assessed by the “double ten” regulation.Three Fe-based alloy cores were fabricated to confirm the validity of proposed method.Compared with the existing methods,the proposed method is simple and convenient to select appropriate magnetic core materials according to the specific requirements of microinductor with magnetic core.(2)A method to effectively reduce the coercive force of Fe/Co and Co/Fe bilayers was proposed.The Fe/Co and Co/Fe bilayers are two multilayers which are deposited with different order by the magnetron sputtering technique.If the two bilayers are used as magnetic cores for microinductor,it will show a relatively large coercive force,which results in a larger magnetic loss,then the performance of microinductor will be deteriorated.It is reported that nitrogen ion implantation and heat treatment within nitrogen atmosphere are two ways to reduce the coercive force,but the effect is not quite good.In this dissertation,we proposed an effective method to reduce the coercive force of two bilayers.The two bilayers deposited on silicon substrates were annealed by a series procedures including rapid heating,constant temperature for a certain time followed by natural cooling.The test results indicate that the coercive force of as-deposited Fe/Co and Co/Fe bilayers is 90.8Oe and 91.6Oe,respectively,and it shows a great decrease after heat treatment.When annealed at 300? with duration time of 4 hours,the Fe/Co bilayer reaches a minimum coercive force 36.8Oe and the coercive force is reduced by 59%;whereas the Co/Fe bilayer realizes a minimum coercive force 22.4Oe and the coercive force shows a 75% drop when it annealed at 350? with duration time of 5 hours.It is also observed that the coercive force of Fe/Co bilayer is always lower than that of Co/Fe bilayer for the same annealing condition.(3)A microinductor with closed magnetic core by laminating,slitting and taking finger-shaped magnetic vias was designed.Based on this topological structure,the microfabrication processes of microinductor with magnetic core were investigated.During the fabrication procedure for microinductor,the performance of microinductor could be improved by changing the topological structure of magnetic core.However,a single topological structure is usually adopted at present studies,with which the performance of microinductor can't be fully improved.In this dissertation,a microinductor with laminated,slitted and finger-shaped magnetic vias' magnetic core was designed,which is an optimized topological structure of magnetic core realized by patterning techniques and aims to improve microinductor's overall performance.Compared with existing magnetic microinductor,the laminated magnetic core can suppress the skin effect and increase the permeability;the slitted magnetic core can not only realize a higher FMR frequency by enlarging the demagnetization factor,but also prevent the forming of in-plane eddy current;the finger-shaped magnetic vias can reduce the eddy current loss of magnetic vias' area.Meanwhile,it is helpful to increase FMR frequency.The fabrication processes of microinductor were developed on the basis of optimized topological structure of magnetic core,which is completely compatible with MEMS microfabrication processes and it is benefit to prepare a magnetic microinductor with high quality.(4)A microfabrication process of depositing a chromium thin film as a seed layer to grow magnetic vias sidewalls with antioxidant property was proposed.A regular fabrication processes for preparing magnetic vias core are that magnetic vias are grown on a conductive seed layer,followed by spin and cured polyimide insulation layer and then polishes the polyimide by chemical mechanical polishing devices.The method is difficult to ensure the soundness and smoothness of magnetic vias,and more importantly,magnetic core is easily oxidized during the high temperature curing process of polyimide.In this dissertation,a chromium thin film seed layer deposited to grow magnetic vias was proposed,it can ensure the soundness and smoothness and avoid magnetic core surface oxidation.The chromium thin film seed layers were respectively deposited on the bottom magnetic core and sidewalls of trench to grow magnetic vias.Compared with the existing methods,the proposed method can not only strengthen the adhesiveness and soundness between the magnetic vias and bottom magnetic core,but also give a solution to stop the oxidizing of magnetic vias and plain magnetic core.