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Recently an innovative mSpeaker technology, based on a novel kind of CMOS compatible small gap electrostatic actuators for large deflection, has been introduced [1]. In this paper, we devise methods to model such speakers in terms of lumped parameters using the deflection of the beam centre as sole dynamic variable. Optical, acoustical and finite element methods (FEM) are used to validate the non-linear lumped parameter model (LPM). The experimental evaluation and LPM simulations of the total harmonic distortion reveal deeper insights into the mechanisms governing linearity.
In der Arbeitsgruppe integrierte Siliziumsysteme des Fraunhofer IPMS werden am Standort Cottbus innovative Anwen-dungen eines neuartigen nanoskopischen elektrostatischen Antriebs (nanoskopic electrostatic drive – NED) untersucht. Eine der vielversprechendsten Anwendungen sind Lautsprecher. Vollständig aus Silizium gefertigte MEMS-Lautsprecher auf Basis von NED-Biegeaktoren werden vor den akustischen Messungen mechanisch und elektrisch cha-rakterisiert. Beidseitig eingespannte elektrostatisch betriebene in-plane NED-Aktoren werden dabei über einen Fre-quenzbereich von 20 Hz bis 20 kHz angeregt. Dabei sind die Strukturen oft verdeckt, was ihre Charakterisierung er-schwert. Es werden zerstörungsfreie optische Methoden präsentiert, mit deren Hilfe die Bestimmung der mechanischen Schwingungseigenschaften einzelner Aktoren ermöglicht wird sowie die Beweglichkeit im verdeckten Zustand nach-gewiesen werden kann.
MEMS-Loudspeaker - A Novel Class of Electroacoustic Transducers for Mobile Audio Applications
(2018)
Wireless earphones require a tremendous degree of integration: In addition to the electroacoustic transducer at least the power supply and a radio module have to be accommodated, supplemented by several microphones for hands-free applications or for noise reduction, as well as position and bio sensors, memory and CPU. With increasing integration, the pressure on the individual components increases in terms of size, energy efficiency and costs. In the field of microphones, MEMS technology has enjoyed great popularity for almost 10 years, and due to their benefits (size, cost, integration) MEMS microphones have almost completely supersede other microphone technologies in mobile audio devices and opened up new fields of applications.
For electrostatic actuators, the pull-in marks an upper limit for the operation range. Once reached, the electrodes come into contact and are shorted without further protection. A non-destructive measurement technique to predict this failure mode is of high interest to allow, e.g. fabrication monitoring or reliability studies. To this end, we develop a surprisingly simple nonlinear lumped parameter model (LPM) for a rather complex electrostatic actuator, designed for an in-ear loudspeaker application. It turns out that a single degree-of-freedom model with only one parameter is sufficient. Our key approach is to experimentally determine this free model parameter by analysing harmonic distortions at low frequencies. Harmonic distortions are a very sensitive tool for nonlinearities. Our method is suggested by simulations with a 2D stationary finite element method (FEM), demonstrating how the analysis of harmonic distortions for voltages far below the pull-in can predict not only the DC pull-in but also the quasi-static AC pull-in voltages at different working points. The distortion analysis of electrostatic actuator ensembles therefore seems a viable route for their non-destructive characterization in the nonlinear domain.
For several years, Fraunhofer IPMS has been developing a new type of an electrostatic MEMS actuator, called nanoscopic electrostatic drive - NED. Since the first publication in 2015, the project group at IPMS has succeeded in providing lateral actuators, which serve as innovative drives in applications such as micro-positioners or micro-speakers. These applications require a high long-term stability of the drive in order to ensure that the system operates as desired. For this purpose, the reliability of such novel lateral electrostatic actuators was evaluated. Long-term studies of NED actuators have shown that different degradation phenomena can occur during operation. The experiments indicate that high electric fields in the range of several MV/m are one of the most important triggers of the degradations observed. On the one hand, they lead to anodic oxidation occurring at the boundary layers between the NED electrodes and neighbored insulators, which results in corrosion of the silicon electrodes. On the other hand, geometry-based field enhancement far from the insulator interfaces lead to permanent field emission, which causes dark discharges in the electrode gap. This in turn leads to cold plasma oxidation and the growth of oxide b ulges on the electrode surfaces. In the course of the experiments, it was shown that corrosion does not take place in a vacuum and can be avoided with the help of passivation layers. Non silicon oxides, e.g. Al2O3, are well suited as passivation layers. Although they bring additional flexural rigidity to the overall structure, which changes the mechanical behavior of the actuators, they increase the effective relative permittivity in the electrode gap, which in turn makes the actuators more effective. Thanks to the passivation coating,the lifespan of the NED actuators was increased and they can become even more effective by choosing a layer with suitable properties.
MEMS-based micro speakers are attractive candidates as sound transducers for smart devices, particularly wearables and hearables. For such devices, high sound pressure levels, low harmonic distortion and low power consumption are required for industrial, consumer and medical applications. The ability to integrate with microelectronic circuitry, as well as scalable batch production to enable low unit costs, are the key factors benchmarking a technology. The Nanoscopic Electrostatic Drive based, novel micro speaker concept presented in this work essentially comprises in-plane, electrostatic bending actuators, and uses the chip volume rather than the its surface for sound generation. We describe the principle, design, fabrication, and first characterization results. Various design options and governing equations are given and discussed. In a standard acoustical test setup (ear simulator), a MEMS micro speaker generated a sound pressure level of 69 dB at 500 Hz with a total harmonic distortion of 4.4%, thus proving the concept. Further potential on sound pressure as well as linearity improvement is outlined. We expect that the described methods can be used to enhance and design other MEMS devices and foster modeling and simulation approaches.
In this paper, an in-plane reciprocating displacement micropump for liquids and gases which is actuated by a new class of electrostatic bending actuators is reported. The so-called “Nano Electrostatic Drive” is capable of deflecting beyond the electrode gap distance, enabling large generated forces and deflections. Depending on the requirements of the targeted system, the micropump can be modularly designed to meet the specified differential pressures and flow rates by a serial and parallel arrangement of equally working pumping base units. Two selected, medium specific micropump test structure devices for pumping air and isopropanol were designed and investigated. An analytical approach of the driving unit is presented and two-way Fluid-Structure Interaction (FSI) simulations of the micropump were carried out to determine the dynamic behavior. The simulation showed that the test structure device designed for air expected to overcome a total differential pressure of 130 kPa and deliver a flow rate of 0.11 sccm at a 265 Hz driving frequency. The isopropanol design is expected to generate 210 kPa and pump 0.01 sccm at 21 Hz. The device is monolithically fabricated by CMOS-compatible bulk micromachining processes under the use of standard materials only, such as crystalline silicon, silicon dioxide and alumina.
A contribution to the expansion of the applicability of electrostatic forces in micro transducers
(2017)
A recently introduced novel actuator class, called the nano electrostatic drive (NED), uses the elec-trostatic actuation to generate large deflections of elastic structures. The NED principle was recently successfully applied to create an all silicon loudspeaker based on micro-electro-mechanical systems (MEMS) technology. Such MEMS audio transducers cover the full frequency range required for high fidelity audio applications. High fidelity audio reproduction also demands minimizing harmonic distortions substantially below 1 %. A major advance in this direction is combining the NED principle with a push-pull driving scheme in a balanced design (BNED), eliminating even harmonics. The practical implementation of a BNED design is however demanding. The nature of the Coulomb force, the impact of stress stiffening and the large deformations required for generating high sound pressures, to name a few aspects, potentially contribute to the harmonic distortion and therefore need advanced experimental methods and simulation models to allow for an apt design. In this paper, we report first results of an experimental technique, combining an optical microscope with a high-speed camera, capable of analyzing the local details of the actuator movement at frame rates of 50,000 frames per second. Dynamic features, such as the excitation of harmonics and intermodulations become clearly visible. These experimental results are then used to scrutinize and refine our multi physics FEM simulations.
Electrostatic micromechanical actuators have numerous applications in science and technology. In many applications, they are operated in a narrow frequency range close to resonance and at a drive voltage of low variation. Recently, new applications, such as microelectromechanical systems (MEMS) microspeakers (µSpeakers), have emerged that require operation over a wide frequency and dynamic range. Simulating the dynamic performance under such circumstances is still highly cumbersome. State-of-the-art finite element analysis struggles with pull-in instability and does not deliver the necessary information about unstable equilibrium states accordingly. Convincing lumped-parameter models amenable to direct physical interpretation are missing. This inhibits the indispensable in-depth analysis of the dynamic stability of such systems. In this paper, we take a major step towards mending the situation. By combining the finite element method (FEM) with an arc-length solver, we obtain the full bifurcation diagram for electrostatic actuators based on prismatic Euler-Bernoulli beams. A subsequent modal analysis then shows that within very narrow error margins, it is exclusively the lowest Euler-Bernoulli eigenmode that dominates the beam physics over the entire relevant drive voltage range. An experiment directly recording the deflection profile of a MEMS microbeam is performed and confirms the numerical findings with astonishing precision. This enables modeling the system using a single spatial degree of freedom.
This article presents a circuit model that is able to capture the full nonlinear behavior of an asymmetric electrostatic transducer whose dynamics are governed by a single degree of freedom. Effects such as stress-stiffening and pull-in are accounted for. The simulation of a displacement-dependent capacitor and a nonlinear spring is accomplished with arbitrary behavioral sources, which are a standard component of circuit simulators. As an application example, the parameters of the model were fitted to emulate the behavior of an electrostatic MEMS loudspeaker whose finite-element (FEM) simulations and acoustic characterisation where already reported in the literature. The obtained waveforms show good agreement with the amplitude and distortion that was reported both in the transient FEM simulations and in the experimental measurements. This model is also used to predict the performance of this device as a microphone, coupling it to a two-stage charge amplifier. Additional complex behaviors can be introduced to this network model if it is required.
The push-pull principle: an electrostatic actuator concept for low distortion acoustic transducers
(2022)
Electrostatic actuators are of particular interest for microsystems (MEMS), and in particular for MEMS audio transducers for use in advanced true wireless applications. They are attractive because of their typically low electrical capacitance and because they can be fabricated from materials that are compatible with standard complementary metal-oxide semiconductor (CMOS) technology. For high audio performance and in particular low harmonic distortion (THD) the implementation of the push-pull principle provides strong benefits. With an arrangement of three electrodes in a conjunct moving configuration on a beam, we demonstrate here for the first time a balanced bending actuator incarnating the push-pull principle operating at low voltages. Our first design already exhibits a harmonic distortion as low as 1.2% at 79 dB using a signal voltage of only 6 Vp and a constant voltage of only ±10 Vdc in a standard acoustic measurement setup. Thus, exceeding our previously reported approach in all three key performance indications at the same time. We expect that our novel electrode configurations will stimulate innovative electrostatic actuator developments for a broad range of applications. In this paper we report the basic theory, the fabrication and the performance of our novel actuator design acting as an audio transducer.
Proof of concept of an air-coupled electrostatic ultrasonic transducer based on lateral motion
(2022)
An alternative implementation of an electrostatic MUT (Micromachined Ultrasonic Transducer), relying on multiple beams that displace along the chip’s surface instead of a single membrane displacing perpendicular to it, is presented in this work. With this approach, a design requiring a low bias voltage (24 V) and occupying a small area (3.3×3.3 mm², 2D/λ≈0.77) was shown to generate a sound pressure level of 82 dB (re. 20 µPa-rms) at 40 kHz and a distance of 8.9 cm. The high level of damping allows this transducer to operate in a wide frequency range (35-63 kHz). The operation of this device as an ultrasonic receiver was also proven. An implementation of this transducer as a rangefinder requires a strong reduction in the noise level, particularly coming from radio-frequency interference, in order to increase its detection range.