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PublicationSubstrate-dependent differences in ferroelectric behavior and phase diagram of Si-doped hafnium oxide( 2021)Non-volatile memories based on ferroelectric hafnium oxide, especially the ferroelectric field-effect transistor (FeFET), have outstanding properties, e.g. for the application in neuromorphic circuits. However, material development has focused so far mainly on metal-ferroelectric-metal (MFM) capacitors, while FeFETs are based on metal-ferroelectric-insulator-semiconductor (MFIS) capacitors. Here, the influence of the interface properties, annealing temperature and Si-doping content are investigated. Antiferroelectric-like behavior is strongly suppressed with a thicker interface layer and high annealing temperature. In addition, high-k interface dielectrics allow for thicker interface layers without retention penalty. Moreover, the process window for ferroelectric behavior is much larger in MFIS capacitors compared to MFM-based films. This does not only highlight the substrate dependence of ferroelectric hafnium oxide films, but also gives evidence that the phase diagram of ferroelectric hafnium oxide is defined by the mechanical stress.
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PublicationAtomic layer deposition of textured Li4Ti5O12: A high-power and long life-cycle life anode for lithium-ion thin-film batteries( 2021)The "zero-strain" Li4Ti5O12 is an attractive anode material for 3D solid-state thin-film batteries (TFB) to power upcoming autonomous sensor systems. Herein, Li4Ti5O12 thin films fabricated by atomic layer deposition (ALD) are electrochemically evaluated for the first time. The developed ALD process with a growth per cycle of 0.6 Å cycle−1 at 300 °C enables high-quality and dense spinel films with superior adhesion after annealing. The short lithium-ion diffusion pathways of the nanostructured 30 nm films result in excellent electrochemical properties. Planar films reveal 98% of the theoretical capacity with 588 mAh cm−3 at 1 C. Substrate-dependent film texture is identified as a key tuning parameter for exceptional C-rate performance. The highly parallel grains of a strong out-of-plane (111)-texture allow capacities of 278 mAh cm−3 at extreme rates of 200 C. Outstanding cycle performance is demonstrated, resulting in 97.9% capacity retention of the initial 366 mAh cm−3 after 1000 cycles at 100 C. Compared to other deposition techniques, the superior performance of ALD Li4Ti5O12 is a breakthrough towards scalable high-power 3D TFBs.
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PublicationA Fully Integrated Ferroelectric Thin-Film-Transistor - Influence of Device Scaling on Threshold Voltage Compensation in Displays( 2021)Thin-film transistors (TFTs) based on amorphous indium-gallium-zinc-oxide (a-IGZO) have attracted vast attention for use in organic light-emitting diode (AMOLED) displays due to their high electron mobility and large current on off ratio. Although amorphous oxide semiconductors show considerably less threshold voltage (Vth) variation than poly-silicon, large-area processing and degradation effects can impede the characteristic parameters of a-IGZO TFTs, which manifests in an uneven brightness distribution across the display panel. Such Vth variations are usually reduced by additional compensation circuits consisting of TFTs and capacitors. Herein, a new approach to compensate such variabilities is demonstrated: the integration of a programmable ferroelectric (FE) film in the gate stack o f the TFT. This simplifies the complexity of the pixel cell and potentially minimizes the need for compensation circuits, which is crucial for transparent displays. To test this new approach, fully integrated FE-TFTs (i.e., with vias contacting a structured bottom gate electrode from the top) based on a-IGZO and FE hafnium-zirconium oxide (HZO) are developed. A single low-temperature post-fabrication treatment at 350 °C for 1 h in air is used to simultaneously crystallize the HZO film in the FE phase and to reduce the number of defects in the a-IGZO channel. The structural and electrical characterizations provide comprehensive guidance for the design of effective FE-TFT gate stacks and device geometries. An accurate control of the polarization state and linear switching between multiple intermediate states is shown by using programming pulses of various amplitudes and widths. Furthermore, a direct correlation between the channel length and the applied pulse width for programming is observed.
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PublicationNonlinearity of balanced mems loudspeakers: Optical experiments and numerical modeling using time-harmonic signals( 2021)A recently introduced novel actuator class, called the nano electrostatic drive (NED), uses the electrostatic 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.
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PublicationAnalog Spatial Light Modulators Based on Micromirror Arrays( 2021)The Fraunhofer Institute for Photonic Microsystems (IPMS) has been developing and manufacturing micromirror arrays for more than 20 years. While originally focusing on applications related to microlithography and therefore mainly for light in the deep ultraviolet range, the range of applications has been expanded since, including applications in the visible and near-infrared range. This paper gives an overview of the devices and their designs, fabrication, and characterization.
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PublicationEmpowering Robots for Multimodal Tactile Gripping using Capacitive Micromachined Ultrasonic Transducers( 2021)
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PublicationWafer-level vacuum-packaged translatory MEMS actuator with large stroke for NIR-FT spectrometers( 2020)We present a wafer-level vacuum-packaged (WLVP) translatory micro-electro-mechanical system (MEMS) actuator developed for a compact near-infrared-Fourier transform spectrometer (NIR-FTS) with 800-2500 nm spectral bandwidth and signal-nose-ratio (SNR) > 1000 in the smaller bandwidth range (1200-2500 nm) for 1 s measuring time. Although monolithic, highly miniaturized MEMS NIR-FTSs exist today, we follow a classical optical FT instrumentation using a resonant MEMS mirror of 5 mm diameter with precise out-of-plane translatory oscillation for optical path-length modulation. Compared to highly miniaturized MEMS NIR-FTS, the present concept features higher optical throughput and resolution, as well as mechanical robustness and insensitivity to vibration and mechanical shock, compared to conventional FTS mirror drives. The large-stroke MEMS design uses a fully symmetrical four-pantograph suspension, avoiding problems with tilting and parasitic modes. Due to significant gas damping, a permanent vacuum of <3.21 Pa is required. Therefore, an MEMS design with WLVP optimization for the NIR spectral range with minimized static and dynamic mirror deformation of <100 nm was developed. For hermetic sealing, glass-frit bonding at elevated process temperatures of 430-440 °C was used to ensure compatibility with a qualified MEMS processes. Finally, a WLVP MEMS with a vacuum pressure of <0.15 Pa and Q > 38,600 was realized, resulting in a stroke of 700 µm at 267 Hz for driving at 4 V in parametric resonance. The long-term stability of the 0.2 Pa interior vacuum was successfully tested using a Ne fine-leakage test and resulted in an estimated lifetime of >10 years. This meets the requirements of a compact NIR-FTS.
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PublicationInkjet printed adhesives for advanced M(O)EMS packaging( 2019)In the quest of low-stress M(O)EMS packaging, in this study, low-temperature adhesive bonding methodologies facilitated by inkjet printing technology were investigated. M(O)EMS devices contain fragile components and are very susceptible to external damages as well as thermal stresses. As to alleviate these stresses, integration of M(O)EMS devices at low processing temperature have been one of the critical requirements in the development of MOEMS packages. Given that, room or low-temperature die-attachments by using UV-curing for transparent substrates or UV-assisted hybrid approaches such as B-staging and shadow curing for opaque substrates are gaining significance. Inkjet printing of these adhesives is proposed here as a promising technique for M(O)EMS die-attachment; while controlled amount of adhesives jetted by a digital printer can mitigate several bonding-induced defects. Bond-line engineering of M(O)EMS die-attachments via inkjet printing is also introduced and further discussed.
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PublicationFunctional integration - structure-integrated wireless sensor technology targeting smart mechanical engineering applications( 2018)Functional integration on the micro/nano scales enables smart functionalities in mechanical engineering systems. Here, exemplarily shown for a ball screw drive, a structure-integrated wireless sensor technology is implemented into a manufacturing system for advanced process control and status monitoring - even at machine components being not yet accessible or difficult to access. This includes also a miniaturized, networked and energy-efficient information and communication technology (ICT) integrated into the machine.
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PublicationElectrostatically Driven In-Plane Silicon Micropump for Modular Configuration( 2018)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.
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