Study On The Microfabrication Technology And Characteristics Of RF MEMS Microinductors Microcapacitors

With the development of the wireless communication systems towards minitype, low-voltage, low-power, multi-function, high-performance and high-frequency, traditional components and systems fabricated by current semiconductor processes can not meet the demands of high-performance and miniaturization of radio frequency communication components, leading to the current wireless communication systems have large volume, high cost and low-operation frequency. Radio frequency microelectromechanical systems (RF MEMS) study and fabricate the wireless communication components and systems and their integration using by MEMS technologies. The components and systems fabricated by RF MEMS have smaller volume, easier integration and higher performance than traditional RF devices. MEMS technologies provide good solutions for realizing miniature, low-power and high-frequency wireless communication systems. Among RF MEMS devices, inductor and capacitor are important components and vital parts of filters and resonators, affecting the performance of the resonant circuit, impedance matching network, amplifier and voltage-controlled oscillator (VCO). So with the greater requirement of electronic devices working at high frequency, domestic and overseas researchers gradually focus on developing and improving the high-frequency performance of MEMS inductors and capacitors. Traditional MEMS microinductors are planar spiral inductors, which have low inductance, large parasitic losses and occupy large on-chip area. Compared to planar spiral inductor, three-dimensional (3D) solenoid inductor has improved the quality (Q) factor because the main electromagnetic flux of the solenoid inductor is parallel to the substrate surface,thus eddy current loss is potentially reduced then that of the planar spiral inductor whose main electromagnetic flux is vertical to the substrate surface, and smaller overlap area of the conductors with the substrate results in lower parasitic losses which is caused by the parasitic capacitance and parasitic resistance. In order to improve the performance of the solenoid inductors and simplify the fabrication process, the lumped parameter circuit model was studied and RF solenoid inductors were designed and fabricated. Moreover, the RF circuit also needs variable capacitors with high tunable range and Q-factor so that wide band tuning and related circuit function can be realized. Compared to the traditional varactors, the micromechanical variable capacitor fabricated by MEMS technologies has no static current, low signal lasses, high Q-factor, wide tunable range and low losses at radio frequency, so that it can decrease circuit losses and improve the circuit merit as an alternative of off-chip voltage-controlled varactors to realize monolithic integration of RF VCOs and tuned filters. In this dissertation, Al2O3 was used as the sacrificial layer, the variable high-performance capacitors were fabricated by MEMS process which is similar to that of the fabrication of the solenoid inductors.In order to meet the demand of microinductors and microcapacitors in RF circuit combined with the current research status and the fabricated devices, in this work, high-performance microinductors and microcapacitors were fabricated by MEMS technologies including UV-LIGA, dry etching, wet etching, fine polishing and electroplating technique. In a word, the main work and the conclusions of this dissertation are as follows:(1) The modeling and simulation of solenoid inductorsThe solenoid inductors were modeled, designed and optimized according to the Greenhouse theory of calculating inductance and the theory of calculating Q-factor equations by using a two-portπ-network physical model and equivalent circuit of on-chip microinductor. The MATLAB software was programmed and used to get the performance of the modeled and designed microinductor compared with the experiment results. All these can be used for the guidance to design and optimize the solenoid inductors. The effect of structure parameters of the solenoid inductors such as coil turns, conductor length, conductor width and conductor thickness on the performance of microinductor was modeled, analyzed and optimized. Meanwhile, the performance of the solenoid inductors were simulated by the high frequency structure simulator (HFSS) software, and the calculated/simulated results were matched well with the experimental results. So the microinductors can be designed and optimized according to the demands of practical applications in the future, then the experiments can be validated and carried out to meet with the applications.(2) Fabrication and test of the air-core solenoid inductorsThe effect of the geometrical parameters on the air-core solenoid inductors were considered and analyzed. The MEMS technologies were used to fabricate different geometrical structure inductors on different substrates, including sputtering, coating, lithography, electroplating, dry etching technique, wet etching technique and fine polishing, etc. During the fabrication process, the following key issues have been studied: 1) by appropriate designs for mask and double-side mask alignment marks, the problem of double-side mask alignment photolithography was solved, and the photoresist mold with high aspect ratio was formed;2)The coil windings and link windings were realized by deep electroplating and high aspect ratio micro- scale blind holes electroplating were resolved through adding some additives in the electroplating solution. All the coils of the air-core inductors were copper in order to decrease the series resistance. The testing results indicate that the microindcutors have high Q-factor over wide range of operating frequency on glass substrate. The peak Q-factors at 6GHz is 38 and the corresponding inductance at peak-Q frequency is 1.81nH. The microindcutors fabricated on silicon substrate also have high Q-factor over wide range of operating frequency. The peak Q-factors at 2.3GHz is 32.8 and the corresponding inductance at peak-Q frequency is 1.79nH. The HFSS simulated results were approximate to the measured results. The simulated peak Q-factors at 2.3GHz is 34.8 and the corresponding inductance at peak-Q frequency is 1.68nH.(3) Fabrication and test of the variable capacitorsSimilar to the fabrication process of the solenoid inductors, the electrostatically driven RF variable micromechanical capacitors were fabricated by MEMS technologies. The most advantage of the fabrication is that using Al2O3 as the sacrificial layer so that it is easy to control the air gap between the movable plate and the fixed plate; moreover, Al2O3 can be easily etched by KOH solution. Electroplated nickel were used to form the structure because nickel has low stress at room temperature which will benefit to the reliability of the capacitors. The surface profile and displacement of the variable capacitor at different values of the applied voltage is measured by using WYKO NT1100 optical surface profiler. The measured results show that the pull-in voltage of the variable square capacitor with the effective length 500μm was 13.5V, the capacitance and Q-factor at 1GHz are 0.792pF and 51.6, and the tuning ratio of the capacitor is more than 1.31:1.(4) Deformation and dynamic analysis of the variable capacitorsBased on the traditional theory of the parallel plate variable capacitor and according to the practical situation, the deformed parameters were introduced. The deformation of the movable plate was properly analyzed, and the initial capacitance and the pull-in voltage of the capacitor were modified in some degree, giving their deviation for specific states. Successful MEMS devices rely not only on the fabrication technologies but also on the knowledge of device behaviors, based on which a favorable structure of the device can be forged. A well-known phenomenon of the electrostatic actuators or microbeams is pull-in. In order to achieve equilibrium, the electrostatic force must balance with spring force. However, as the applied voltage increases or the initial gap decreases, the electrostatic force increases much faster than the linear spring force. The stability of the equilibrium is broken and pull-in occurs when the applied voltage exceeds a threshold value. As for the mass–spring–damper system of the parallel plate variable capacitor, by approximately expanding the governing equation of motion for the dynamics of the system and combining with the boundary conditions, the dynamic parameters such as displacement and the velocity of the variable capacitor will be achieved. When a voltage is applied, the suspended plate moves to the fixed plate and finally arrives at the equilibrium position, meanwhile, an instability zone occurs around the stability zone (line). As voltage increases, the displacement of the equilibrium position increases and the instability zone deviates from the stability zone (line) even extrudes. When the applied voltage exceeds the pull-in voltage, the electric force may become higher than the spring restoring force and the movable plate will stick to the fixed plate.