Study On Fabrication And Reconfigurable Technologies Of High-Performance RF MEMS All-Silicon-Cavity/Substrate-integrated Filters

In this dissertation,high-performance RF microelectromechanical systems(RF MEMS)filters focusing on their fabrication and reconfigurable technologies have been studied.All-silicon cavity evanescent-mode tunable bandpass filters(BPFs)based on silicon-micromachining technology and reconfigurable substrate-integrated filters employing commercially-available RF MEMS devices have been expounded,respectively.The research contents are devided and dissertated in the following Sections.Section ?: A K–Ka-band RF MEMS all-silicon cavity evanescent-mode two-pole tunable BPF based on silicon-micromachining technology is presented for the first time.Also proposed for the first time are novel microcorrugated diaphragms(MCDs)that are manufactured using a gold(Au)-based sputtering technique and are utilized as frequency-tuning elements of the BPF.The coupled evanescent-mode cavity resonators of the BPF were fabricated on a(700 ± 25)-mm thick low-resistivity(5–10 ?·cm,the same below)silicon substrate using a tetramethylammonium-hydroxide(TMAH)-based wet etching technique.Its surface was coated with a 1-mm thick layer of sputtered Au.The microcorrugations of the MCDs were also wet-etched in TMAH on a(300 ± 25)-mm thick low-resistivity silicon substrate,the surface of which was coated with a 1-mm thick sputtered-Au layer.The MCDs were dry-released in xenon difloride gas.The proposed filter features a continuous(analog)center-frequency tuning of its passband at 23–35 GHz,a filter quality factor(Q)of 530–750,and a low electrostatic actuation voltage of less than 140 V.For realizing the same frequency-tuning range,the MCD exhibits no hysteresis in its actuation voltage versus deflection relationship which on the contrary is prominent for a commercially-available piezo actuator disk,and furthermore an electrostatic actuation voltage far less than that of the latter.The electrostatic actuation voltage of the MCD in this design is over 2× lower than previous demonstrations where conventional planar diaphragms with identical physical dimensions and materials were used.Section ?: Creep-resistant nanocrystalline gold-vanadium(Au-V)alloyed MCDs based on a dual-metal-target(gold and vanadium)co-sputtering technique are presented,and the creep-resistant property of the Au-V MCD has been studied for the first time.The research was motivated to improve creep of the conventional sputtered-Au MCD in the applications of widely frequency-tunable RF MEMS all-silicon cavity filters.With the introduction of a small amount of vanadium(2.2 atomic percentage(at.%)of vanadium)into the sputtered-Au thin film,creep-resistant property of the thin film can be significantly enhanced as a result of highly fined grains and an increased yield strength of the material,which is in agreement with the grain-boundary-strengthening mechanism for metal thin films.Creep-resistant property of the Au-V MCD can be further improved by a 2-hour annealing at 300 °C in argon(Ar).In this study,the 1-mm thick Au-V MCD was co-sputtered on a(300 ± 25)-mm thick low-resistivity silicon substrate.X-ray photoelectron spectroscopy(XPS)was used to characterize the vanadium concentration in the Au-V thin film.Field emission scanning electron microscopy(FESEM)was used to characterize the surface morphology of the thin film and measure its average surface grain size.Creep-resistant property of the MCD was characterized and quantified by its stress relaxation behavior which was measured on a microforce probe station.Compared to the conventional Au MCD,the proposed Au-V(2.2 at.% V)MCD exhibits over 50% smaller average surface grain size,a ~2× and a ~10× lower stress relaxation rates respectively at the third and twelfth hour of the steady-state stress relaxation,and a 13.4% less decay of the measured restoring force in the 12-hour test.The developed Au-V thin films demonstrate excellent process compatibility and significantly improved creep-resistant property.As a result,they are very promising in enhancing the mechanical reliability of diaphragm-enabled RF MEMS devices under long-term operation senarios.Section ?: Effects of the 2-hour Ar annealing at 300 °C and varying vanadium concentration to mechanical and electrical properties of the Au-V thin films have been studied for the first time.In this Section,Au-V MCDs with identical physical dimensions and fabrication technologies but with different vanadium concentrations of 0.7 and 6.8 at.%,respectively,were experimented.Similarly,the vanadium concentration and the average surface grain size of these thin films were measured using XPS and FESEM,while their stress relaxations were measured on the microforce probe station.Neither of the Au-V(0.7 and 6.8 at.% V)MCDs exhibits significant difference in its surface average grain size as compared to the Au-V(2.2 at.% V)MCD,but a great reduction as compared to the pure Au MCD.The grain-boundary-strengthened Au-V(0.7 and 2.2 at.% V)MCDs show trivial difference in their 12-hour steady-state stress relaxation responses,but a significant improvement as compared to the pure Au MCD.The solid-solution-strengthened Au-V(6.8 at.% V)MCD demonstrates the best creepresistant property among all tested materials,even much better than the aforementioned grain-boundary-strengthened Au-V MCDs.It exhibits only a 6% decay of the measured restoring force in the 3-hour steady-state stress relaxation,~4× and ~3× less than that,respectively,for the pure Au and Au-V(2.2 at.% V)MCDs tested in the same period.It also exhibits a ~9× and a ~5× lower stress relaxation rates at the third hour than that of the latters,respectively.It is also found that the stress relaxation of the pure Au MCD has been improved after annealing as a result of a strong recrystallization in gold,which on the other hand appears to be much less prominent for the Au-V thin films.The four-port terminal sensing technique was used to evaluate the electrical property of the Au-V thin films in the first place.The sheet resistance Rs of the 500-nm thick Au and Au-V thin films was measured before and after a 2-hour Ar annealing at 300 °C,and the electrical resistivity ? and the electrical conductivity ? of the thin films were then calculated.Secondly,the RF-property evaluation of the Au and Au-V(2.2 at.% V)thin films was experimentally implemented at 20–40 GHz by measuring the transmission performance of a grounded coplanar waveguide(GCPW)transmission line before and after annealing.The Au and Au-V GCPW transmission lines were designed and fabricated based on a(500 ± 50)-mm thick quartz substrate.The attenuation factor ? of the GCPW transmission line was then calculated for each material.An excellent agreement for the electrical conductivity of the thin film has been experimentally validated through DC(four-port terminal sensing)and RF(S-parameters)measurements.It is found that the 2-hour Ar annealing at 300 °C contributes to improving electrical conductivity of the Au thin film,but on the other hand appears to be much less effective for improving that of the Au-V thin film.The electrical property was specified as follows for the annealed Au-V(2.2 at.% V)thin film: Rs = 339.10 m?/?,? = 5.9 MS/m,and ? = 0.327–0.410 dB/mm.The electrical resistivity of the Au-V(2.2 at.% V)thin film is demonstrated ~5× greater than that of the pure Au thin film,which is attributed to the increased density of the electron scattering at the grain boundaries as a result of the highly reduced grain size.Section ?: A compact L-band BPF with RF MEMS-enabled reconfigurable narrowband deep notches is presented in this Section.The filter is based on commercially-available RF MEMS switches and a bandpass-to-absorptive-bandstop series-cascaded filter topology,where the 1.575-GHz third-order static narrowband BPF was realized using highly-miniaturized hairpin-line resonators,while the frequencyswitching capability of the two series-cascaded quasi-absorptive bandstop filters(QABSFs)for reconfiguring the notches were enabled by using Omron's ohmic RF MEMS switches.Three reconfigurable states have been achieved: a 1.575-GHz passband only,a 1.575-GHz passband with 1.525-GHz low-side notches,and a 1.575-GHz passband with 1.625-GHz high-side notches.Furthermore,a notch attenuation of greater than 90 dB has been achieved with the use of QABSFs,which on the other hand is hardly implementable for conventional reflective bandstop filters.Section ?: A 0.95/2.45-GHz switched-frequency BPF using commercially-available RF MEMS tuning elements is presented in this Section.The filter is composed of a 0.95-GHz fourth-order lumped-element BPF that is parallel-cascaded with a 2.45-GHz fourth-order microstrip BPF using highly-miniaturized hairpin-line resonators.The filter passband was intrinsically switched by detuning two resonators of each BPF with the employment of WiSpry's RF MEMS digitally-tunable capacitors.Four reconfigurable filter states have been achieved: 0.95-and 2.45-GHz dual passbands,a 0.95-GHz passband only,a 2.45-GHz passband only,and no passband(both BPFs switched off).The filter features switching of the reconfigurable constant-bandwidth passband with a frequency-tuning ratio of over 2.5:1 and the functionality of being intrinsically switched on and off.