Browsing by Subject "Microelectromechanical systems."

Now showing 1 - 4 of 4

Open Access
Design and fabrication of micromachined radio-frequency cavity resonators
(2006) Arslan, Cihan Hakan
Resonators are used almost in every wireless communications applications and play an important role in the performance of these systems. At radio frequencies, for high performance applications, realization of high-Q resonators is required. Furthermore, in the near future, integration of RF resonators with rest of the system is intended. This thesis describes the design and fabrication of a type of radio-frequency MEMS cavity resonator operating in the frequency range of 2-3 GHz. The fabricated resonators are small in size so that they allow the integration of a whole system on a single-chip. The cavity is realized by selectively removing (etching) silicon substrate using standard MEMS techniques. The resonator is based on creating a low-loss inductor by enclosing the inductor in a metal-coated cavity and then resonating it with either a fixed or tunable high-Q capacitor. In this thesis, formulas for the inductance and the Q-factor of the cavity are derived and a number of resonators are fabricated and measured. The Q-factors of the measured cavities were found to be in the range going up to 25- 30. The obtained results are promising and showed that on-chip resonators with Q-factors higher than 30 can be realized based on this design and fabrication technique at this frequency range.
Open Access
Design, fabrication and characterization of liquid-solid microelectromechanical DC-contact switches
(2012) Çağatay, Engin
From critical applications such as life support systems and massive data communication centers to every day necessities like mobile phones and street lighting, people depend on the performance of electrical switches. Today, the vast majority of electrical switches are of solid-state semiconductor type. They are fast, with nanosecond response times, have the advantage of being produced by CMOS-compatible microfabrication processes, and unlike their major competitor counterparts, i.e., microelectromechanical systems (MEMS) switches, they do not suffer from mechanical contact issues like contact bounce, stiction, or contact degradation, due to the absence of solid-to-solid mechanical contacts. Therefore, electrical switches exhibit extremely long life times with superior reliability performance. However, they have a high ON-state resistance of 2-6 , low open-state impedance on the order of 105-107 , and show relatively low power-handling capabilities. In addition, their temperature and radiation-sensitive performance, limits their range of operating environment. The major counterpart switching technology, solid-to-solid DC-contact MEMS switches, on the other hand, transmit current when the two surfaces, namely dimple and contact pad, make contact. Although MEMS switches show very good RF performance, the required solid-to-solid contact is often quite non-ideal. This stems from the fact that the two contact surfaces have certain micro or nano-scale roughness including nano-scale asperities and thus cannot conform perfectly onto each other. Consequently, the actual contact area is only a small fraction of the apparent one. These devices suffer from mechanical problems such as switch bouncing, microwelding, adhesion and contact degradation, and as a result show degraded switching performance over time with long-term reliability issues. At this point, an alternative switching technology might be the proposal of liquid-to-solid MEMS (LS-MEMS) switches using movable liquid metal droplets. This promising concept enables electrical switches with higher isolation and lower insertion loss, much like conventional solid-solid MEMS switches. Moreover, since they do not have fragile moving solid parts, LS-MEMS switches potentially do not suffer from mechanical fatigue problems increased contact resistance and stiction/adhesion problems. Our aim in this study is to design and fabricate LS-MEMS switches, whereby we can characterize and examine the actuation of metallic liquid droplets, namely eutectic Ga-In (EGaIn), Gallium Indium Tin alloy (Galinstan), and mercury (Hg) using electrowetting on dielectric (EWOD) principle. We have investigated the effect of different actuation electrode geometries like rectangular, interdigitated fingers and crescent-shaped electrodes on the droplet actuation. With the application of 30-100 V voltage difference across the actuation electrode and the ground electrode, the metallic liquid droplets were observed to be actuated. With further optimization, LS-MEMS device structures demonstrated in this work might have potential applications as alternative high-performance electrical switches.
Open Access
Design, fabrication and characterization of surface plasmon resonance based MEMS displacement sensors
(2009) Güner, Hasan
Strong dependence of surface plasmon resonance (SPR) on coupling parameters offers new varieties of sensing mechanisms in nano and micro-scale engineering fields. In this study, design, fabrication and characterization of MEMS displacement sensors that utilize angular dependence of grating coupled SPR condition are explored. Several surface plasmon polariton (SPP) excitation mechanisms are reported in the academic literature. One of them which is quite adaptable to microelectromechanical systems is grating coupling scheme. In this scheme, thin metallized grating structures are particularly designed depending on the desired wavelength and the angle of incidence of the SPP excitation light. Geometric parameters like periodicity, surface profile, depth and duty cycle of the grooves and material parameters like dispersion and thickness of the top metal layer have to be chosen with care in order to reach sharp SPR curves in the reflected intensity spectra with respect to either wavelength or angle of incidence. As the first step, geometric and material parameters of SPR gratings are numerically optimized using rigorous coupled-wave analysis (RCWA). Angular quality factors on the order of tens are shown to be achievable. Various lithographic techniques (nanoimprint, electron beam and optical lithography) are used to nanofabricate those certainly defined gratings. It is observed that p-polarized reflected intensity measurements using spectroscopic ellipsometry are in quite good agreement with those numerically calculated. Spectroscopic scan measurements are also provided to show the polarization dependence of SPP excitation. All effort to obtain high angular Q-factor grating structures is aimed at enhancing the sensitivity of angular displacement detection scheme. In this scheme, angular position of the grating structure in the polarization plane is detected through the reflected intensity response of the photodetector. Dependence of sensitivity on excitation light source wavefront parameters and photodetector noise are analyzed. MEMS displacement sensor designs relying on the principle of angular displacement detection scheme are developed. Simply, SPR grating structures are transferred on conventional micromembranes. Two types of such particular designed micromembranes are introduced: corrugated microcantilevers (singly clamped) and corrugated microbridges (doubly clamped). They are fabricated through well-known surface micromachining processes in addition to SPR grating nanofabrication procedures. Mechanical resonance frequencies, flexural mode shapes and effective spring constants are analytically, numerically and experimentally obtained. In addition, a MEMS accelerometer design with plasmonic readout with nano-G noise floor is presented. An experimental configuration for micromechanical displacement sensing is investigated. According to the results of this work, novel arrayed sensors combining the sensitivities of plasmon resonance and micromembrane type sensors may provide unprecedented performance.
Open Access
Lumped element modeling of circular CMUT in collopsed mode
(2014) Aydoğdu, Elif
Capacitive micromachined ultrasonic transducer is a microelectromechanical device, which serves as an acoustic signal source or sensor, in a variety of applications including medical ultrasound imaging, ultrasound therapy, airborne applications. It has a suspended membrane with an electrode inside, and at the underlying substrate there is another electrode, so that the membrane can be deflected by the electrical field formed between the electrodes. Similarly, any mechanical disturbance on the membrane can be sensed as a change in the capacitance of the two electrodes. CMUT is a nonlinear device which has a distributed mechanical operation. Although, it is a mass-spring system basically, the nonlinear electrical force and the radiation force makes it impossible to solve CMUT through analytical equations. In order to predict its behavior, and design a CMUT towards the needs of a specific application, either finite element analysis or equivalent electrical circuit modeling should be utilized. Finite element analysis (FEA) can predict the distributed CMUT operation with high accuracy, but its usage is limited to designs employing low number of CMUTs because of the computation cost. Recently, advances in equivalent circuit modeling, made it possible to simulate arrays employing very high number of CMUTs, with high accuracy. These models assume uncollapsed mode operation and except collapsed mode operation as it is highly nonlinear. This thesis focuses on obtaining an accurate equivalent circuit model for a circular CMUT in collapsed mode. The outcome is a parametric circuit model, that can simulate a CMUT of any physical and material parameters, under an arbitrary electrical or mechanical excitation. In collapsed mode, the compliance of the membrane is no longer constant as in uncollapsed mode, and it increases with increasing contact radius. Similarly, the capacitance, the electrical force and the radiation impedance all have new behavior regarding the contact radius. As there is no analytical solution for those parameters, we perform numerical calculations and extract expressions for each of them. First, we calculate the collapsed membrane deflection, utilizing the exact electrical force distribution in the analytical formulation of membrane deflection. Then we use the deflection profile to calculate the capacitance, electrical force, and compliance. Performing a set of calculations for different CMUT dimensions, different pressure and voltage levels, we obtain dependencies of those parameters on rms deflection. Then we develop a lumped element model of collapsed membrane operation, expressing the parameters as functions of rms deflection. The radiation impedance for the collapsed mode is also included in the model. The model is then merged with the uncollapsed mode model to obtain a simulation tool that handles all CMUT behavior, in transmit or receive. Large- and smallsignal operation of a single CMUT can be fully simulated for any excitation regime. The results are in good agreement with FEM simulations.