Prof. Dr.-Ing. Martin Hoffmann

Microsystem Technology
Ruhr-Universität Bochum


  • 3-Bit digital-to-analog converter with mechanical amplifier for binary encoded large displacements
    Schmitt, L. and Schmitt, P. and Hoffmann, M.
    Actuators 10 (2021)
    We present the design, fabrication, and characterization of a MEMS-based 3-bit Digitalto-Analog Converter (DAC) that allows the generation of large displacements. The DAC consists of electrostatic bending-plate actuators that are connected to a mechanical amplifier (mechAMP), enabling the amplification of the DAC output displacement. Based on a parallel binary-encoded voltage signal, the output displacement of the system can be controlled in an arbitrary order. Considering the system design, we present a simplified analytic model, which was confirmed by FE simulation results. The fabricated systems showed a total stroke of approx. 149.5 ± 0.3 μm and a linear stepwise displacement of 3 bit correlated to 23 ≙ eight defined positions at a control voltage of 60 V. The minimum switching time between two input binary states is 0.1 ms. We present the experimental characterization of the system and the DAC and derive the influence of the mechAMP on the functionality of the DAC. Furthermore, the resonant behavior and the switching speed of the system are analyzed. By changing the electrode activation sequence, 27 defined positions are achieved upgrading the 3-bit systems into a 3-tri-state (33) system. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/act10080182
  • 40 W SESAM-modelocked Ho:YAG thin-disk laser at 2090 nm
    Tomilov, S. and Hoffmann, M. and Heidrich, J. and Alaydin, B.O. and Golling, M. and Wang, Y. and Keller, U. and Saraceno, C.J.
    2021 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2021 (2021)
    High power ultrafast lasers in the short-wave mid-IR wavelength region (2-3m) are of great interest for a large number of applications in science and technology. Finding paths to increase the average power of ultrafast laser systems directly emitting in this wavelength region has seen particularly strong interest, due to the potential of these sources as direct drivers for the generation of XUV, mid-IR and THz. Among different power-scalable technologies, the development of thin-disk lasers (TDLs) at 2m is very promising for power and energy scaling of ultrafast lasers in this wavelength range, but only very few results have so far conclusively shown this potential [1] , [2]. Recently, we have shown the potential of Ho:YAG for further power scaling by demonstrating a cw, single fundamental-mode 2-m TDL with >100 W and 54% optical-to-optical efficiency, representing the highest power TDL around 2m [3]. In the present work, we report first high-power modelocking SESAM-modelocking of this system, reaching an average power of 40-W in ps-pulses, which respresents the highest average power from a modelocked oscillators in the wavelength region. © 2021 IEEE.
    view abstract10.1109/CLEO/Europe-EQEC52157.2021.9542367
  • Analysis of THz generation using the tilted-pulse-front geometry in the limit of small pulse energies and beam sizes
    Wulf, F. and Hoffmann, M. and Saraceno, C.J.
    Optics Express 29 (2021)
    Optical rectification in lithium niobate using the tilted-pulse-front geometry is one of the most commonly used techniques for efficient generation of energetic single-cycle THz pulses and the details of this generation scheme are well understood for high pulse energy driving lasers, such as mJ-class, kHz-repetition rate Ti:Sa amplifier systems. However, as modern Yb-based laser systems with ever increasing repetition rate become available, other excitation regimes become relevant. In particular, the use of more moderate pulse energies (in the few ìJ to multi-10 ìJ regime), available nowadays by laser systems with MHz repetition rates, have never been thoroughly explored. As increasing the repetition rate of THz sources for spectroscopy becomes more relevant in the community, we present a thorough numerical analysis of this regime using a 2+1-D numerical model. Our work allows us to confirm experimental trends observed in this unusual excitation regime and shows that the conversion efficiency is naturally limited by the small pump beam sizes as a consequence of spatial walk-off between the pump and THz beams. Based on our findings, we discuss strategies to overcome the current limitations, which will pave the way for powerful THz sources approaching the watt level with multi-MHz repetition rates. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.
    view abstract10.1364/OE.426228
  • Configuration of a MEMS-based Terahertz Reflectarray Using a Genetic Algorithm
    Liu, X. and Schmitt, L. and Kolpatzeck, K. and Hoffmann, M. and Balzer, J.C. and Czylwik, A.
    International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2021-August (2021)
    In this work, we demonstrate the configuration of a MEMS-based terahertz reflectarray using a genetic algorithm for continuous beam steering with the capability of manipulating the beam pattern. Two different optimization goals are considered: maximizing the directivity and minimizing the sidelobe level. As preliminary results, we show calculated and simulated radiation patterns where the beam is successfully steered to 45° at 0.3 THz without grating lobes. For the first optimization goal, we achieve a directivity of 16.7 dBi at the desired angle, whereas we suppress the sidelobe level from -13.7 dB to -16 dB for the second optimization goal. © 2021 IEEE
    view abstract10.1109/IRMMW-THz50926.2021.9567102
  • Design and characterization of an electrostatic constant-force actuator based on a non-linear spring system
    Thewes, A.C. and Schmitt, P. and Löhler, P. and Hoffmann, M.
    Actuators 10 (2021)
    In recent years, tissue engineering with mechanical stimulation has received considerable attention. In order to manipulate tissue samples, there is a need for electromechanical devices, such as constant-force actuators, with integrated deflection measurement. In this paper, we present an electrostatic constant-force actuator allowing the generation of a constant force and a simultaneous displacement measurement intended for tissue characterization. The system combines a comb drive structure and a constant-force spring system. A theoretical overview of both subsystems, as well as actual measurements of a demonstrator system, are provided. Based on the silicon-on-insulator technology, the fabrication process of a moveable system with an extending measurement tip is shown. Additionally, we compare measurement results with simulations. Our demonstrator reaches a constant-force of 79 ± 2 µN at an operating voltage of 25 V over a displacement range of approximately 40 µm, and the possibility of adjusting the constant-force by changing the voltage is demonstrated. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/act10080192
  • Design and fabrication of MEMS reflectors for THz reflect-arrays
    Schmitt, L. and Liu, X. and Czylwik, A. and Hoffmann, M.
    2021 4th International Workshop on Mobile Terahertz Systems, IWMTS 2021 (2021)
    We present the design and fabrication of MEMS reflectors intended to be part of a mechanical terahertz (THz) beam steering reflect-array. Each reflector is shifted by a bidirectional stepwise electrostatic actuator system. The design and functionality of actuator system and reflect-array limit and determine the reflector design. In this paper, we adapt a dicing-free Silicon-on-Insulator (SOI) fabrication process to the requirements of a THz MEMS reflector. We present the partial release of the handle layer from the SOI substrate on wafer level, leaving the remaining connected part as a functional system component. To realize an open interface for the interaction of the reflector with the THz beam, the device layer and the handle layer frame surrounding the SOI chip are partially released during hydrofluoric (HF) vapor etching making a mechanical destruction of the surrounding frame obsolete. © 2021 IEEE.
    view abstract10.1109/IWMTS51331.2021.9486804
  • Development and Implementation of a Rotating Nanoimprint Lithography Tool for Orthogonal Imprinting on Edges of Curved Surfaces
    Supreeti, S. and Schienbein, R. and Feßer, P. and Fern, F. and Hoffmann, M. and Sinzinger, S.
    Nanomanufacturing and Metrology 4 (2021)
    Uniform molding and demolding of structures on highly curved surfaces through conformal contact is a crucial yet often-overlooked aspect of nanoimprint lithography (NIL). This study describes the development of a NIL tool and its integration into a nanopositioning and nanomeasuring machine to achieve high-precision orthogonal molding and demolding for soft ultraviolet-assisted NIL (soft UV-NIL). The process was implemented primarily on the edges of highly curved plano-convex substrates to demonstrate structure uniformity on the edges. High-resolution nanostructures of sub-200-nm lateral dimension and microstructures in the range of tens of microns were imprinted. However, the nanostructures on the edges of the large, curved substrates were difficult to characterize precisely. Therefore, microstructures were used to measure the structure fidelity and were characterized using profilometry, white light interferometry, and confocal laser scanning microscopy. Regardless of the restricted imaging capabilities at high inclinations for high-resolution nanostructures, the scanning electron microscope (SEM) imaging of the structures on top of the lens substrate and at an inclination of 45° was performed. The micro and nanostructures were successfully imprinted on the edges of the plano-convex lens at angles of 45°, 60°,and 90° from the center of rotation of the rotating NIL tool. The method enables precise imprinting at high inclinations, thereby presenting a different approach to soft UV-NIL on curved surfaces. © 2021, The Author(s).
    view abstract10.1007/s41871-021-00114-6
  • DRIE Si Nanowire Arrays Supported Nano-Carbon Film for Deriving High Specific Energy Supercapacitors On-Chip
    Lu, P. and Chen, X. and Ohlckers, P. and Halvorsen, E. and Hoffmann, M. and Müller, L.
    Journal of Physics: Conference Series 1837 (2021)
    Supercapacitor is a promising solution to storage of pulsed energy generated by MEMS energy harvesting systems, relying on its faster charging/discharging capability than secondary battery. To improve the energy density of on-chip supercapacitor which shows potential for integration with MEMS devices, in this paper we first present a successful electrode design for high specific energy pseudo-supercapacitors on the basis of deep reactive ion etched Si nanowire array supported nano-carbon matrix. Widely used pseudo-capacitive manganese oxide active material is facilely deposited into the conductive nano-carbon matrix by a chemical bath deposition. The derived electrode exhibits a remarkable capacitance increase (around 4.5x enhancement) compared with the nano-carbon matrix benefiting from the contribution of pseudo-capacitive manganese oxide. Assembled sandwich prototype on-chip supercapacitors with a symmetric configuration offer a high specific capacitance of 741.6 mF cm-2 when discharged at 1 mA cm-2, and the energy density can attain as high as 51.5 ?Wh cm-2. The achieved high specific energy makes such on-chip supercapacitors attractive in the field of energy collection when cooperated with micro-or nano-energy generators. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/1837/1/012005
  • Frequency Coded Retroreflective Landmark for 230 GHz Indoor Self-Localization Systems
    Kadera, P. and Jimenez-Saez, A. and Schmitt, L. and Schusler, M. and Hoffmann, M. and Lacik, J. and Jakoby, R.
    15th European Conference on Antennas and Propagation, EuCAP 2021 (2021)
    This paper presents a planar Luneburg lens frequency-coded retroreflector based on a high-resistive silicon (HR-Si) photonic-crystal (PhC) high-Q tag with two resonators working at 237 GHz and 243 GHz. A characterization of the retroreflector, including the readout of the resonance frequency of the resonators, is performed at 30 cm over 130-degree angular range in azimuthal plane. The measured gain of the lens antenna is 15.9 dBi at a frequency of 240 GHz and the radar-cross section (RCS) of the frequency-coded tag landmark achieves -30 dBsm at 237 GHz. © 2021 EurAAP.
    view abstract10.23919/EuCAP51087.2021.9410973
  • High Power THz Generation Using Tilted Pulse Fronts with Low Pump Pulse Energies
    Wulf, F. and Vogel, T. and Mansourzadeh, S. and Hoffmann, M. and Saraceno, C.J.
    2021 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2021 (2021)
    Optical rectification using tilted pulse fronts in lithium niobate (LN) is currently the method of choice for the generation of strong-field THz pulses for THz-TDS. The success of this technique owes mostly to the high conversion efficiencies achievable, reaching the percent level at room temperature [1]. So far, this method has been mostly used with amplified laser systems, with repetition rates up to the kHz range and corresponding pulse energies of more than 1 mJ. To increase the DR and SNR of today's THz sources, there is an increasing demand for high average power, high repetition rate THz sources. Recently, we have demonstrated up to 66 mW THz average power, driven by a >100-W average power mode-locked thin-disk oscillator with 13.3 MHz repetition rate and pulse energies on the 10J level [2] , enabling us to achieve the highest average power of a laser-driven THz source at MHz repetition rates. Despite this promising first achievement, the conversion efficiency of 610 -4 was significantly lower than the record conversion efficiencies obtained with lower repetition rates and higher pulse energies. Furthermore, the scaling laws in this unusual excitation regime remain so far unexplored. Here we present an in-depth investigation of this excitation regime using a 2+1D model including pump beam depletion. It is shown that a combination of spatial walk-off and pump beam break-up is responsible for a reduction in efficiency at small beam sizes. Furthermore, we discuss possibilities to overcome the current limitations and predict that watt-level THz sources at MHz repetition rates will become available in the very near future. © 2021 IEEE.
    view abstract10.1109/CLEO/Europe-EQEC52157.2021.9542675
  • High-Power Lensless THz Imaging of Hidden Objects
    Mansourzadeh, S. and Damyanov, D. and Vogel, T. and Wulf, F. and Kohlhaas, R.B. and Globisch, B. and Schultze, T. and Hoffmann, M. and Balzer, J.C. and Saraceno, C.J.
    IEEE Access 9 (2021)
    The potential of pulsed THz radiation for time-of-flight imaging applications is well recognized. However, advances in this field are currently severely limited by the low average power of ultrafast THz sources. Typically, this results in impractically long acquisition times and a loss in resolution and contrast. These limitations make imaging of the objects in real-life scenarios impossible. Here, conclusively, the potential of state-of-the-art high-average power THz time-domain spectrometer (TDS), driven by a 100-W class, one-box ultrafast oscillator for imaging applications is shown by demonstrating lensless THz imaging in reflection mode of a dielectric sample with low reflectivity. Images obtained with our home-built 20-mW average power THz-TDS system show a significant contrast enhancement compared to a state-of-the-art commercial THz-TDS with less than 200~mu text{W} of average power. Our unique setup even allows us to obtain images of such an object with high-contrast hidden inside a medium-density fiberboard (MDF) box. This opens the door to THz time-of-flight imaging of concealed objects of unknown shape and orientation in various real-life scenarios which were so far impossible to realize. © 2013 IEEE.
    view abstract10.1109/ACCESS.2020.3048781
  • Highly Anisotropic Fluorine-Based Plasma Etching of Ultralow Expansion Glass
    Weigel, C. and Phi, H.B. and Denissel, F.A. and Hoffmann, M. and Sinzinger, S. and Strehle, S.
    Advanced Engineering Materials (2021)
    Deep etching of glass and glass ceramics is far more challenging than silicon etching. For thermally insensitive microelectromechanical and microoptical systems, zero-expansion materials such as Zerodur or ultralow expansion (ULE) glass are intriguing. In contrast to Zerodur that exhibits a complex glass network composition, ULE glass consists of only two components, namely, TiO2 and SiO2. This fact is highly beneficial for plasma etching. Herein, a deep fluorine-based etching process for ULE 7972 glass is shown for the first time that yields an etch rate of up to 425 nm min−1 while still achieving vertical sidewall angles of 87°. The process offers a selectivity of almost 20 with respect to a nickel hard mask and is overall comparable with fused silica. The chemical surface composition is additionally investigated to elucidate the etching process and the impact of the tool configuration in comparison with previously published etching results achieved in Zerodur. Therefore, deep and narrow trenches can be etched in ULE glass with high anisotropy, which supports a prospective implementation of ULE glass microstructures, for instance, in metrology and miniaturized precision applications. © 2021 The Authors. Advanced Engineering Materials published by Wiley-VCH GmbH
    view abstract10.1002/adem.202001336
  • Highly Selective Guiding Springs for Large Displacements in Surface MEMS
    Schmitt, P. and Schmitt, L. and Tsivin, N. and Hoffmann, M.
    Journal of Microelectromechanical Systems 30 (2021)
    In this paper we introduce the concept, modelling and analysis of triangular and sinusoidal springs intended for large in-plane translational displacements for MEMS-guiding applications. The proposed spring systems combine the advantages of minimal space requirement, low stiffness in the axial direction and high mechanical resistance in off-axis directions. An analytical model for the description of the force-displacement characteristic of triangular springs is derived considering typical mechanical constraints. Based on the model, geometrical parameters of the springs influencing linearity and selectivity with respect to the in- and off-axis stiffness are analyzed. The validity of the models is demonstrated by finite element analysis and experimental verification realized by silicon-on-insulator demonstrators. [2020-0360] © 1992-2012 IEEE.
    view abstract10.1109/JMEMS.2021.3074822
  • Large stepwise discrete microsystem displacements based on electrostatic bending plate actuation
    Schmitt, L. and Hoffmann, M.
    Actuators 10 (2021)
    We present the design, fabrication, and experimental characterization of microsystems achieving large and stepwise discrete displacements. The systems consist of electrostatic bending plate actuators linked in a chain with increasing electrode gaps to allow a stepwise system dis-placement. A derived analytic transfer function permits to evaluate the influence of the system components on both the total and the stepwise system displacement. Based on calculation and simulation results, systems featuring 5, 8, 10, 13, and 16 steps are modeled and fabricated using a dicing-free SOI-fabrication process. During experimental voltage-and time-dependent system characterization, the minimum switching speed of the electrostatic actuators is 1 ms. Based on the guiding spring stiffness and the switching time, step-by-step and collective activations of the microsystems are performed and the system properties are derived. Furthermore, we analyze the influence of the number of steps on the total system displacement and present 16-step systems with a total maximum displacement of 230.7 ± 0.9 µm at 54 V. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/act10100272
  • Moving towards high-power thin-disk lasers in the 2 µm wavelength range
    Tomilov, S. and Hoffmann, M. and Wang, Y. and Saraceno, C.J.
    JPhys Photonics 3 (2021)
    Thin-disk lasers (TDLs) have made spectacular progress in the last decades both in continuous-wave (CW) and ultrafast operation. Nowadays, single thin-disk oscillators with >16 kW of CW-power have been demonstrated and ultrafast amplifiers have largely surpassed the kilowatt milestone with pulse energies in the multi-100 mJ range. This amazing development has been demonstrated in the 1 µm wavelength range, using Yb-doped materials and supported by industrially available components. Motivated by both strong scientific and industrial applications, interest in expanding this performance to longer wavelength regions continues to increase. In particular, TDLs emitting directly in the short-wave mid-infrared (SW-MIR) region (2-3 µm) are especially sought after, and although many early studies have been reported, most remained in the proof-of-principle stage and the potential for multi-100 W operation remained undemonstrated. Here, we report on our recent results of a single fundamental-mode CW Ho:YAG thin-disk oscillator with >100 W of power, surpassing previous single-mode TDLs by a factor of >4, and marking a first milestone in the development of high-power SW-MIR TDLs. In optimized conditions, our laser system emitting at ≈2.1 µm reaches an output power of 112 W with 54.6% optical-to-optical efficiency and an M2 = 1.1. This system is ideally suited for future direct modelocking at the 100 W level, as well as for ultrafast amplification. We start the discussion with a review of the state-of-the-art of TDLs emitting directly in the vicinity of 2 µm, and then discuss difficulties and possible routes both towards ultrafast operation and next possible steps for power scaling. © 2021 The Author(s). Published by IOP Publishing Ltd
    view abstract10.1088/2515-7647/abdd81
  • Advanced broadband MEMS infrared emitter based on high-temperature-resistant nanostructured surfaces and packaging solutions for harsh environments
    Biermann, S. and Magi, A. and Sachse, P. and Hoffmann, M. and Wedrich, K. and Müller, L. and Koppert, R. and Ortlepp, T. and Baldauf, J.
    Proceedings of SPIE - The International Society for Optical Engineering 11279 (2020)
    An advanced infrared emitter, consisting of a non-periodic silicium-microstructure and a platinium-nano-composition, which enables extraordinary highly emission intensities is presented. A spectral broadband emission coefficient ϵ of nearly 1 is achieved. The foundation of the emitter is a MEMS hot plate design containing a high temperature stable molybdenum silicide resistance heater layer embedded in a multilayer membrane consisting of silicon nitride and silicon oxide. The temperature resistance of the silicon-platinum micro-nanostructure up to 800 °C is secured by a SiO2 protection layer. The long-term stability of the spectral behavior at 750 °C has been demonstrated over 10,000 h by FTIR measurements. The low thermal mass of the multilayer MEMS membrane leads to a time constant of 28 ms which enables high chopper frequencies. A precondition for long term stability under rough conditions is a real hermetic housing. High temperature stable packaging technologies for infrared MEMS components were developed. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2545119
  • Direct Binary Encoding of Displacements on the Nano-Scale
    Schmitt, P. and Hoffmann, M.
    Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 2020-January (2020)
    We present a passive force and displacement sensor based on a compliant mechanical amplifier combined with a micromechanical analog-to-digital converter that allows to amplify and convert a displacement signal in the nanometer range directly into an electrically readable binary code without the need of electrical energy for the conversion itself. The presented compliant amplifier achieves a displacement amplification ratio of about 100 within an input range of 1.3μ m, which enables a resolution of 40 nm steps by using a 5-bit mechanical analog-to-digital converter. Furthermore, a method is proposed to implement a pre-defined transfer function into the mechanical A/D converter in order to linearize or benchmark a non-linear primary displacement signal. © 2020 IEEE.
    view abstract10.1109/MEMS46641.2020.9056116
  • Engineering a Compliant Mechanical Amplifier for MEMS Sensor Applications
    Schmitt, P. and Hoffmann, M.
    Journal of Microelectromechanical Systems 29 (2020)
    In this paper, we introduce a compliant mechanical amplifier (mechAMP) suitable for the sensitivity enhancement in MEMS sensor applications. The design, fabrication and characterization of a planar compliant amplifier mechanism is presented with special focus on the kinematic and static system modelling of the displacement and force amplification. We show that the proposed system can also be applied as mechanical stiffness transformer for the adaption of mechanical signals or as mechanical transformer for MEMS actuators. Based on a kinematic model, a compact mechanism with a system size of 930 $\mu \text{m}\,\,\times2080\,\,\mu \text{m}$ and a displacement amplification ratio of 200 with an output displacement in a range of 100 $\mu \text{m}$ was designed. The fabrication of the system was carried out using silicon-on-insulator (SOI) technology. Experimentally, we could verify an amplification ratio of 197.9 for the designed and fabricated system which corresponds to the analytic model by a deviation of about 1%. [2019-0228]. © 1992-2012 IEEE.
    view abstract10.1109/JMEMS.2020.2965260
  • Frequency-Coded mm-Wave Tags for Self-Localization System Using Dielectric Resonators
    Jiménez-Sáez, A. and Alhaj-Abbas, A. and Schüßler, M. and Abuelhaija, A. and El-Absi, M. and Sakaki, M. and Samfaß, L. and Benson, N. and Hoffmann, M. and Jakoby, R. and Kaiser, T. and Solbach, K.
    Journal of Infrared, Millimeter, and Terahertz Waves 41 (2020)
    The paper describes the development of passive, chipless tags for a novel indoor self-localization system operating at high mm-wave frequencies. One tag concept is based on the low-Q fundamental mode of dielectric resonators (DR) which exhibits peak scattering at its resonance frequency. As the radar cross-section (RCS) of DRs at mm-wave frequencies is far too low for the intended application, arrays of DRs and combinations with dielectric lens and corner reflectors are investigated to boost the RCS while keeping the scattering retro-directive over wide-angle incidence. Satisfactory results are demonstrated experimentally in W-band with metal corner reflectors combined with planar arrays of DRs; the tags produce a high RCS level over a moderately broad angular range and a wide frequency range where they exhibit a notch at the resonance frequency of the dielectric resonators. These designs suffer from low coding range of 3 to 6 bit, degradations of RCS in angular range, and a difficult separation of the tag response from strong clutter. Both the suppression of large clutter interference by using time gating of the tag response and a larger coding range are promised by a chipless tag concept based on multiple high-Q resonators in photonic crystal (PhC) technology. Experimental samples are characterized as transmission resonators and as retro-directive tags at the 230 GHz band. As a concept to boost the retro-directive RCS with a truly wide-angle response, the integration of PhC resonators with a Luneburg lens is discussed. © 2020, The Author(s).
    view abstract10.1007/s10762-020-00707-0
  • Terahertz beam steering concept based on a MEMS-reconfigurable reflection grating
    Liu, X. and Samfaß, L. and Kolpatzeck, K. and Häring, L. and Balzer, J.C. and Hoffmann, M. and Czylwik, A.
    Sensors (Switzerland) 20 (2020)
    With an increasing number of applications of terahertz systems in industrial fields and communications, terahertz beamforming and beam steering techniques are required for high-speed, large-area scanning. As a promising means for beam steering, micro-electro-mechanical system (MEMS)-based reflection gratings have been successfully implemented for terahertz beam control. So far, the diffraction grating efficiency is relatively low due to the limited vertical displacement range of the reflectors. In this paper, we propose a design for a reconfigurable MEMS-based reflection grating consisting of multiple subwavelength reflectors which are driven by 5-bit, high-throw electrostatic actuators. We vary the number of the reflectors per grating period and configure the throw of individual reflectors so that the reflection grating is shaped as a blazed grating to steer the terahertz beam with maximum diffraction grating efficiency. Furthermore, we provide a mathematical model for calculating the radiation pattern of the terahertz wave reflected by general reflection gratings consisting of subwavelength reflectors. The calculated and simulated radiation patterns of the designed grating show that we can steer the angle of the terahertz waves in a range of up to ± 56.4∘ with a maximum sidelobe level of −10 dB at frequencies from 0.3 THz to 1 THz. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/s20102874
  • A microoptical sidestream cuvette based on fast passive gas exchange for capnography
    Weigel, C. and Grewe, A. and Sinzinger, S. and Hoffmann, M.
    Sensors and Actuators, A: Physical 276 (2018)
    In the case of clinical ventilation, sensors are needed that analyze the carbon dioxide concentration during the exhalation cycle. For devices capable to individually control and adjust parameters for each patient, the analysis of the breathing conditions including carbon dioxide concentration is of critical importance. We present a miniaturized system that is optimized for a fast passive gas exchange with low dead volumes in the side stream. The presented system which is also suitable for neonates shows high dynamics and no influence on the breathing behavior due to small systems sizes. Cost reduction is achieved through a fully passive gas flow that does not require active elements such as valves and pumps. The cuvette has been designed for optimized optical and fluidic properties for surveillance in a wide range of breathing conditions. © 2018 Elsevier B.V.
    view abstract10.1016/j.sna.2018.04.022
  • Deep etched and released microstructures in Zerodur in a fluorine-based plasma
    Weigel, C. and Sinzinger, S. and Hoffmann, M.
    Microelectronic Engineering 198 (2018)
    In this paper, we demonstrate the dry-etch release of free-standing mechanical elements with a high aspect ratio from a Zerodur wafer. Our previous research was focussed on deep plasma etching of Zerodur and the determination of characteristic process parameters such as selectivity, sidewall angle and etch rate. Nevertheless, the releasing of structures is more challenging. The achievable sidewall angle in fluorine-based plasma of 71° and a low selectivity limits the etch depth. Here, we show a double-sided process, which allows a high aspect ratio and structure heights of 150 μm and above. We implement a process that allows a damage-free finishing of the micromechanical elements. The chemical sidewall composition of the released structures was carefully investigated. A strong build up of non-volatile products at specified positions of the structures results from the two-directional process and the structure geometry. This is discussed in the article and can be applied to other glasses and glass ceramics with complex composition, too. © 2018 Elsevier B.V.
    view abstract10.1016/j.mee.2018.07.004
  • Deep etching of Zerodur glass ceramics in a fluorine-based plasma
    Weigel, C. and Schulze, M. and Gargouri, H. and Hoffmann, M.
    Microelectronic Engineering 185-186 (2018)
    The etching of glass is still much more challenging than the deep etching of silicon. But in contrast to pure silica, most glasses are complex alloys of serval oxides including aluminium oxide. For this reason, it is quite difficult to find suitable high-rate deep dry etching processes and related masking materials. For extremely temperature-insensitive micromechanical systems it is of interest to use zero-expansion glass ceramics such as Zerodur. But the microstructure of Zerodur consists of crystalline and amorphous phases and shows a high percentage of Al2O3-bonds. This makes plasma etching challenging. Here, deep etching of Zerodur only in a fluorine-based plasma for micro-technical applications is investigated. Different process parameters such as the physical power and gas mixtures of the ICP-RIE-process have been varied. Etch rates of about 250 nm/min and sidewall angles of approximately 71° were reached with a nickel mask and the etch gas SF6. The achieved total etching depth is as large as 150 μm resulting in a release of microelements such as springs and gears from a Zerodur wafer. © 2017 Elsevier B.V.
    view abstract10.1016/j.mee.2017.10.013
  • Image inverting, topography and feature size manipulation using organic/inorganic bi-layer lift-off for nanoimprint template
    Si, S. and Hoffmann, M.
    Microelectronic Engineering 197 (2018)
    A fast and cost efficient approach to fabricate multiple NIL templates with inverse image tone, modified topography and tunable feature sizes is presented. The nanopatterns from the negative master is inverted to positive structures in the produced NIL templates using a UV-curable bi-layer lift-off process which excludes high temperature baking. The bi-layer consisting of a 150 nm sacrificial layer of pure organic resist and a 200 nm patterning resist of inorganic/organic composite is employed. The topography on the new NIL templates are in square layout which is generated from a single master with circular nanoholes. The feature sizes on the master are shrunken down to sub-200 nm (120/200/250/300 nm) in a preciously controllable manner. Nanostructures up to 150 mm wafer scale are transferred by soft UV-NIL using an ambient center-to-edge scheme. The feature sizes of openings in the patterning layer can be precisely and controllably tuned taking advantage of an intermediate template with nanopyramids that is produced from the master. The organic sacrificial layer is descummed and underetched by oxygen plasma. Furthermore, 40 nm Chromium is evaporated and the sacrificial layer along with the patterning layer is lifted off by wet chemical stripper TechniStrip P1316. Silicon etching using the Chromium etch-mask is engaged in for smooth and vertical sidewalls for NIL templates. © 2018 Elsevier B.V.
    view abstract10.1016/j.mee.2018.05.005
  • A monolithic micro-optical interferometer deep etched into fused silica
    Weigel, C. and Markweg, E. and Müller, L. and Schulze, M. and Gargouri, H. and Hoffmann, M.
    Microelectronic Engineering 174 (2017)
    For free-space micro-optical systems, the alignment of the components is still a challenging task in manufacturing. Alternatively, a monolithic integration can overcome this problem, but especially for in-plane optical elements in the visible wavelength range, the optical surfaces have to fulfill critical demands. Here, we show a fabrication process that allows the deep-reactive ion etching (RIE) of fused silica with high optical quality. We achieve vertical sidewalls with etch depths of about 100 μm with an arithmetic mean roughness of about 7.2 nm. By using this process, a new in-plane monolithic, free-space interferometer is demonstrated that reaches a resolution of 20 nm with our current setup. © 2017 Elsevier B.V.
    view abstract10.1016/j.mee.2017.01.002
  • Consecutive imprinting performance of large area UV nanoimprint lithography using Bi-layer soft stamps in ambient atmosphere
    Si, S. and Hoffmann, M.
    Microelectronic Engineering 176 (2017)
    For UV nanoimprint lithography (UV-NIL) using polymer soft stamps, imprinting at ambient atmosphere brings additional challenges due to evaporated solvents and possible byproducts resulting from the interaction between the UV light, oxygen and the polymer-based material. Moreover, the Laplace pressure may impact differently on the capillary filling for both positive and negative patterns at atmospheric pressure compared to that in the vacuum. Twenty consecutive imprints using bi-layer Polydimethylsiloxane (PDMS), PDMS/toluene-diluted PDMS, PDMS/X-PDMS, PDMS/vvsPDMS stamps have been tracked and inspected. The imprinting employs a center-to-edge scheme in ambient atmosphere. The results show that high reusability and imprint uniformity can be achieved for at least twenty consecutive imprints using the pure PDMS (PDMS/PDMS) and PDMS/toluene-diluted PDMS. These stamps can overcome the challenges of the interaction between the UV light, oxygen and the polymer-based materials. The Laplace pressure under atmosphere does not hinder the resist filling for such consecutive imprints. © 2017 Elsevier B.V.
    view abstract10.1016/j.mee.2017.01.032
  • Low-cost fabrication of nanoimprint templates with tunable feature sizes at a constant pitch
    Si, S. and Dittrich, L. and Hoffmann, M.
    Microelectronic Engineering 170 (2017)
    Nanoimprint lithography (NIL) templates are in general costly, especially for large area and small feature sizes. With a simple shrinking technique using a serial of well-known technologies, it has been feasibly realized to produce high-quality soft NIL templates with widely tunable feature sizes at a constant pitch from a single master at very low-cost and in a short processing cycle. The master featuring at sub-micron range has been replicated to new silicon NIL templates with feature sizes at, but not limited to, sub-200 nm. soft UV-NIL combined with a dry etch process, were employed to fabricate an intermediate template of nanocone patterns that is further used for an imprint on the final substrate. The feature size of the SiO2 etch mask on the final substrate can be adjusted by varying the duration of mask formation etching. The final patterning of the template is realized by cryogenic etching based on SF6/O2 chemistry. Silicon NIL templates featuring nanopillar patterns with diameters of 150 nm, 200 nm and 250 nm, respectively, have been fabricated on wafer level from the same master with 450 nm feature size. The presented process flow avoids the time-consuming and cost-intensive electron beam writings and gives more flexibility in the fabrication of nanopatterns. The fabrication cycle for such NIL working templates with tunable feature sizes is maintained short and at a low cost. Moreover, the technique allows the fabrication of wafer level products at a constant pitch, which is of importance as well for the stacked large-area imprintings with varying feature sizes. © 2016 Elsevier B.V.
    view abstract10.1016/j.mee.2016.12.023
  • Material dependence of the contact behavior of oscillating microprobes-modeling and experimental evidence
    Bohm, S. and Goj, B. and Dittrich, L. and Dressler, L. and Hoffmann, M.
    Journal of Micro and Nano-Manufacturing 5 (2017)
    Oscillating microprobes avoid high stress and the sticking effect during contact between microprobe and measured surface. The full performance and application scope of oscillating microprobes can be explored and utilized once the reliable prediction of the microprobe contact behavior is understood. Here, an improved contact model considering adhesion forces, surface roughness, and viscoelastic damping for oscillating microprobes is presented and it is validated by exemplary measurements utilizing a uniaxially oscillating electrostatic microprobe. These results show that the nondestructive identification of material classes seems to be feasible by evaluating the phase shift between the sinusoidal signals of sensor and actuator, respectively. © 2017 by ASME.
    view abstract10.1115/1.4035619
  • Stress-modulated tilt actuator for tunable optical prisms
    Leopold, S. and Paetz, D. and Sinzinger, S. and Hoffmann, M.
    Sensors and Actuators, A: Physical 266 (2017)
    A tunable optical prism MOEMS based on the deformation of a liquid droplet is presented. An aluminum-nitride membrane is tilted by a novel type of thermo-mechanical actuator. The actuation principle is based on a thermo-mechanical modulation of the intrinsic stress in aluminum-nitride beams. Based on an analytical model, the key parameters of the actuator are optimized. Furthermore, the influence of the intrinsic stress on the actuator properties is investigated. These dependencies and the model are verified by mechanical characterization of samples. Operation in air and with ambient fluid has been confirmed. An image shift of 30 mm is found in a microscopic setup which corresponds to 19 % of the field of view. © 2017 Elsevier B.V.
    view abstract10.1016/j.sna.2017.09.021
  • The NanoTuFe — Fabrication of large area periodic nanopatterns with tunable feature sizes at low cost
    Si, S. and Dittrich, L. and Hoffmann, M.
    Microelectronic Engineering 180 (2017)
    Fabricating nanopatterns for large area and in small feature sizes is in general cost-intensive. In this paper, a cost-efficient process chain is demonstrated to fabricate periodic nanopatterns with tunable feature sizes (NanoTuFe) utilizing a series of well-known technologies such as soft UV Nanoimprint Lithography (NIL), dry and wet etching. An intermediate template featuring nanocone and nanopyramid is introduced to bridge the original and the final patterns. The feature size on the final fabricated substrate can be widely tuned and controlled. Wafers in diameter of 100 mm full of square nanocavities with 130/160/190/220 nm feature sizes, respectively, have been fabricated from a single master wafer containing nanocavities with 350 nm diameter. Nanopillar patterns with diameters of 150/200/250 nm, respectively, have been produced on wafer level from a single master with 450 nm feature size. Cryogenic silicon etching based on SF6/O2 chemistry is employed to create the final shrunk nanopatterns with smooth and vertical profiles. The fabrication processes cope without costly electron beam or laser beam direct writings, achieving the nanopatterns with tunable feature sizes in a cost-efficient manner. In addition, the process chain grants the wafer-level production at a constant pitch, which is of importance for the stacked alignment of layers with varying feature sizes. The generated periodic nanopatterns can be applied for NIL templates, photonic crystals, optics, energy conversion and storage, sensing elements, etc. © 2017 Elsevier B.V.
    view abstract10.1016/j.mee.2017.06.002
  • Linearized control of an uniaxial micromirror with electrostatic parallel-plate actuation
    Weinberger, S. and Nguyen, T.T. and Lecomte, R. and Cheriguen, Y. and Ament, C. and Hoffmann, M.
    Microsystem Technologies 22 (2016)
    Electrostatic parallel-plate actuation is a common method for driving micromirrors with analog deflection control. This actuation enables high dynamics, low power consumption, compact design, large mirror deflection and a fabrication with MEMS-technology. The drawback is the highly nonlinear behavior of the angle vs. voltage curve by using the common single-ended or differential control. This paper presents an advanced control method. A four-electrode arrangement is used to drive the mirror. An “actuating electrode” which is placed close to the rotation axis and an “outer electrode” on each side are used. For the actuation, the electrodes from only one side are used. The outer electrode voltage is in dependence on the driving voltage which is applied at the actuating electrode. This dependence is described by a control function. This one allows realizing a nearly linear angle vs. driving voltage curve, by increasing or decreasing the additional actuating torque caused by the outer electrode. To show the suitability of this method, a laser beam is deflected by a micromirror and is detected by a position-sensitive device (PSD) which is mounted on a moving stage. The PSD is used as feedback sensor and the mirror is actuated using a linear PID-controller. Stage movements with a speed of up to 80 mm/s have been tracked over an angular range between 0.8° and 12.3° for tilting the mirror with an angular velocity of about 14.7°/s. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00542-015-2535-2
  • MEMS gas ionization sensor with palladium nanostructures for use at ambient pressure
    Müller, L. and Mehner, H. and Hoffmann, M.
    Journal of Physics: Conference Series 757 (2016)
    A microfabricated gas ionization sensor with integrated field enhancing silicon- palladium nanostructures is presented. The sensor can be used to determine the ambient gas by its specific breakdown voltage. Technological details of the fabrication and the assembly of the sensor are presented too. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/757/1/012023
  • Micro-Venturi injector: Design, experimental and simulative examination
    Degenhardt, S. and Cheriguen, Y. and Geiling, T. and Hoffmann, M.
    Journal of Physics: Conference Series 757 (2016)
    This paper reports the first factorial design of a micro Venturi injector completed by a simulative investigation of the device. For the first time, a comprehensive correlation between the point of the maximum vacuum pressure generated by the Venturi nozzle and the variation of the inlet pressure is shown. The device reported in this contribution enables a new solution for robust low-pressure generation in parallel fluidic channels. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/757/1/012027
  • Nano fabricated silicon nanorod array with titanium nitride coating for on-chip supercapacitors
    Lu, P. and Ohlckers, P. and Müller, L. and Leopold, S. and Hoffmann, M. and Grigoras, K. and Ahopelto, J. and Prunnila, M. and Chen, X.
    Electrochemistry Communications 70 (2016)
    We demonstrate high aspect ratio silicon nanorod arrays by cyclic deep reactive ion etching (DRIE) process as a scaffold to enhance the energy density of a Si-based supercapacitor. By unique atomic layer deposition (ALD) technology, a conformal nanolayer of TiN was deposited on the silicon nanorod arrays as the active material. The TiN coated silicon nanorods as a supercapacitor electrode lead to a 6 times improvement in capacitance compared to flat TiN film electrode. © 2016 Elsevier B.V.
    view abstract10.1016/j.elecom.2016.07.002
  • Process flow to integrate nanostructures on silicon grass in surface micromachined systems
    Mehner, H. and Müller, L. and Biermann, S. and Hänschke, F. and Hoffmann, M.
    Journal of Physics: Conference Series 757 (2016)
    The process flow to integrate metallic nanostructures in surface micromachining processes is presented. The nanostructures are generated by evaporation of microstructured silicon grass with metal. The process flow is based on the lift-off of a thin amorphous silicon layer deposited using a CVD process. All steps feature a low temperature load beneath 120 °C and high compatibility with many materials as only well-established chemicals are used. As a result metallic nanostructures usable for optical applications can be generated as part of multilayered microsystems fabricated in surface micromachining. © Published under licence by IOP Publishing Ltd.
    view abstract10.1088/1742-6596/757/1/012022
  • Scanning micromirror for large, quasi-static 2D-deflections based on electrostatic driven rotation of a hemisphere
    Bunge, F. and Leopold, S. and Bohm, S. and Hoffmann, M.
    Sensors and Actuators, A: Physical 243 (2016)
    A micromirror (mirror area 3.1 mm2) for laser tracking applications is presented. The mirror, which is based on a hemisphere, is designed to achieve large quasi-static deflection around two rotational axes by adapting the principle of ultrasonic motors. Here, the deflection of the mirror is achieved by a periodic momentum transfer from a stage with electrostatically driven oscillations. Due to the periodic hemisphere-stage-contact, the system has multiple degrees of freedom and is non-linear. A simple model of stage-hemisphere-interaction is presented and verified in order to identify design rules and adequate excitation regimes. The actuator is fabricated in standard SOI-technology. The final system is excited as well in a non-resonant (2000 Hz) as in a resonant mode (2900 Hz). Thus excitation frequencies over a wide range are possible. For a resonant operation of the stage, a maximum quasi-static deflection of the mirror of up to +/-35.2°with a maximum angular velocity of 732°/s is demonstrated. In this case, the crosstalk (movement perpendicular to desired direction) is less than 22%. For the non-resonant operation the crosstalk is reduced significantly (less than 10%). In this case, a quasi-static deflection of +/-10.5°is found. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.sna.2016.02.031
  • 2D stepping drive for hyperspectral systems
    Endrödy, C. and Mehner, H. and Grewe, A. and Sinzinger, S. and Hoffmann, M.
    Journal of Micromechanics and Microengineering 25 (2015)
    We present the design, fabrication and characterization of a compact 2D stepping microdrive for pinhole array positioning. The miniaturized solution enables a highly integrated compact hyperspectral imaging system. Based on the geometry of the pinhole array, an inch-worm drive with electrostatic actuators was designed resulting in a compact (1 cm2) positioning system featuring a step size of about 15 μ m in a 170 μ m displacement range. The high payload (20 mg) as required for the pinhole array and the compact system design exceed the known electrostatic inch-worm-based microdrives. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0960-1317/25/7/074002
  • A passive microsystem for detecting multiple acceleration events beyond a threshold
    Mehner, H. and Weise, C. and Schwebke, S. and Hampl, S. and Hoffmann, M.
    Microelectronic Engineering 145 (2015)
    A fully mechanical microsystem for detecting multiple acceleration threshold violations is presented. It combines a seismic mass as a part of a ratcheting mechanism so that discrete positions are enabled depending on the external acceleration forces. A numerical model describing the dynamic system behavior has been developed. The fabricated system demonstrator is investigated utilizing impact tests, and the numerical model is verified by comparing the model with the measurement values. Various acceleration thresholds between 5g and 30g could safely be measured. Additionally, the counter can be re-initialized utilizing an electrostatic actuator for releasing the ratcheting mechanism. This recording of acceleration events does not require electrical energy for detection nor for storage; hence it is suitable for monitoring of sparse but critical acceleration or impact events over large time periods. The system design is prepared for an electrical read-out by, e.g. RFID transponders or comparable systems that transmit the position of the seismic mass within the ratcheting mechanism. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.mee.2015.03.023
  • Chromatic confocal matrix sensor with actuated pinhole arrays
    Hillenbrand, M. and Weiss, R. and Endrödy, C. and Grewe, A. and Hoffmann, M. and Sinzinger, S.
    Applied Optics 54 (2015)
    We present two versions of a chromatic confocal matrix sensor for the snapshot acquisition of three-dimensional objects. The first version contains separate illumination and detection pinhole arrays, while the second version uses a single pinhole array in double pass. The discrete lateral measurement points defined by the illumination and detection pinhole arrays are evaluated in parallel with a hyperspectral detection module. As this approach enables the spectrometric evaluation of all lateral channels, multilayer objects can be analyzed. To increase the lateral resolution the pinhole arrays are moved by micromechanical actuators. The paper includes a quantitative evaluation of the chromatic confocal module and proof-of-principle experiments with the full sensor system. © 2015 Optical Society of America.
    view abstract10.1364/AO.54.004927
  • Design and characterization of a resonant triaxial microprobe
    Goj, B. and Dressler, L. and Hoffmann, M.
    Journal of Micromechanics and Microengineering 25 (2015)
    A new trend for tactile microprobes leads to oscillating microprobes in order to overcome the drawbacks resulting from high Hertzian stress and disturbing surface forces. Thin water films on the measurement surface result in the so-called sticking effect which causes measurement faults such as snap-back and false triggering. This leads to measurement errors and low measurement speeds. We present an innovative oscillating triaxial microprobe which safely avoids sticking in all Cartesian measurement directions. The system design as well as the characterization of the microprobe are presented in this work. The low number of coupling elements, the batch-capable design and the low contact forces are the key features of the microprobe. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0960-1317/25/12/125011
  • Linear micromechanical stepping drive for pinhole array positioning
    Endrödy, C. and Mehner, H. and Grewe, A. and Hoffmann, M.
    Journal of Micromechanics and Microengineering 25 (2015)
    A compact linear micromechanical stepping drive for positioning a 7×5.5 mm2 optical pinhole array is presented. The system features a step size of 13.2 μm and a full displacement range of 200 μm. The electrostatic inch-worm stepping mechanism shows a compact design capable of positioning a payload 50% of its own weight. The stepping drive movement, step sizes and position accuracy are characterized. The actuated pinhole array is integrated in a confocal chromatic hyperspectral imaging system, where coverage of the object plane, and therefore the useful picture data, can be multiplied by 14 in contrast to a non-actuated array. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0960-1317/25/5/055009
  • Radio frequency microelectromechanical system-platform based on silicon-ceramic composite substrates
    Fischer, M. and Gropp, S. and Nowak, J. and Capraro, B. and Sommer, R. and Hoffmann, M. and Mülle, J.
    Journal of Microelectronics and Electronic Packaging 12 (2015)
    In the last few years, several low-temperature coefficient of expansion of low temperature cofired ceramic (LTCC) materials have been developed for direct wafer bonding to silicon. BGK, a sodium-containing LTCC, was originally developed for anodic bonding of the sintered LTCC, whereas BCT (bondable ceramic tape) was tailored for direct silicon bonding of green LTCC tapes to fabricate a quasi-monolithic, silicon ceramic compound substrate. This so-called silicon-on-ceramic (SiCer) technique is based on homogeneous nanostructuring of a silicon substrate, a lamination step of BCT and Si, and a subsequent pressure-assisted sintering. We present a new approach for an integrated radio frequency (RF)-platform setup combining passive, active, and mechanical elements on one SiCer substrate. In this context, RF parameters of the Si-adapted LTCC tapes and the use of commercial metal pastes on BCT with respect to bondability and solderability are investigated. We show first technological results of creating cavities at the SiCer interface for SiCer-specific contacting options (e.g., exposed contact pads at the interface), as well as windows in the ceramic layer of the SiCer substrate for additional Si processing (e.g., Si backside thin-film wiring, plasma etching). A further investigated platform technology is deep reactive-ion etching of the SiCer composite substrate. The etching behavior of Si and BCT is demonstrated and discussed. With the SiCer technique, it is possible to reduce the Si content at the setup of RF microelectromechanical system to a minimum (low signal damping). © 2015 International Microelectronics Assembly and Packaging Society.
    view abstract10.4071/imaps.442
  • Self-sufficient sensor for oxygen detection in packaging via radio-frequency identification
    Weigel, C. and Schneider, M. and Schmitt, J. and Hoffmann, M. and Kahl, S. and Jurisch, R.
    Journal of Sensors and Sensor Systems 4 (2015)
    A new disposable radio-frequency identification (RFID) sensor for detecting oxygen in packages with a protective atmosphere is presented. For safety reasons and system costs in consumer packages, no battery or energy harvesting devices can be used. Each part of a package, especially in food packaging, must be completely safe even if it is swallowed. Several materials have been investigated that safely react with oxygen and thus change electrical parameters without the need of an additional energy supply. In particular linseed oil was tested, because it is known to react in oxygen-containing atmosphere from liquid to solid. Linseed oil is used not only as food but also as a key part in ecological paint coatings. A significant relative change of capacity was observed during linseed oil drying, which results in 20% after 5 h and 38% after 30 h at an oxygen concentration of 20.5 and 50% relative humidity, respectively. Pure unsaturated fatty acids were also tested in an oxygen-containing atmosphere and showed similar behaviour. The reaction speed is partially dependent on the level of unsaturation of fatty acids. The oxygen sensor is coupled with an RFID front end with an internal charge time measurement unit for capacity determination. The combination of sensor element, sensitive material and RFID allows for biocompatible and save systems that indicate the presence of oxygen within a package.
    view abstract10.5194/jsss-4-179-2015
  • Infrared emitting nanostructures for highly efficient microhotplates
    Müller, L. and Käpplinger, I. and Biermann, S. and Brode, W. and Hoffmann, M.
    Journal of Micromechanics and Microengineering 24 (2014)
    A highly emissive Si-based microhotplate based on self-organizing nanostructures is presented. The silicon was structured by a self-masking deep reactive ion etching process resulting in needle-like non-periodical microstructures. Evaporated platinum settles in a kind of glancing angle deposition as well-defined nanocrystals on the silicon microstructures. Finite-difference time-domain simulation allowed the evaluation of the ideal platinum thickness for maximized infrared absorption and emission. We measured the hemispherical spectral transmittance and reflectivity of the fabricated surfaces and found the hemispherical spectral absorbance to be up to 0.97 in the investigated wavelength range. To demonstrate the advantages of these micro-nano-structures, we present the fabrication and characterization of a thermal infrared hotplate-emitter. With integrated Pt-on-Si-needles, the emitter shows a 2.6 times higher IR emission without wavelength-dependent interference patterns as compared to an uncoated Si-based emitter at the same membrane temperature. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0960-1317/24/3/035014
  • Quasi-static micromirror with enlarged deflection based on aluminum nitride thin film springs
    Weinberger, S. and Nguyen, T.T. and Ament, C. and Hoffmann, M.
    Sensors and Actuators, A: Physical 210 (2014)
    Uniaxial electrostatically driven micromirrors with very large non-resonant rotation angles are presented. The mirrors achieve an analog deflection of about ±12 at voltages between 200 and 320 V. At pull-in, a digital tilt angle of approx. ±21 is found. A mirror control for a nearly linear characteristic curve, by using a counter torque is approved. The key for large quasi-static deflections are novel aluminum nitride based thin film springs. The high mechanical strength of AlN enables the fabrication of thin but stable torsion springs. The mirror has a size of 1 mm × 1.2 mm with a bending of less than 0.2 μm and high surface quality (Ra < 2 nm). The rotational eigenfrequencies are found to be between 37 and 73 Hz and the eigenfrequencies of the vertical mirror movement are between 1235 and 1751 Hz. © 2014 Elsevier B.V.
    view abstract10.1016/j.sna.2014.02.017
  • Semi-contact measurements of three-dimensional surfaces utilizing a resonant uniaxial microprobe
    Goj, B. and Dressler, L. and Hoffmann, M.
    Measurement Science and Technology 25 (2014)
    Recent development trends in the area of tactile probing systems led to miniaturized highly accurate probes that allow measurements of small structures with a high aspect ratio. Additionally, small contact forces are aspired to achieve measurements without damage to the specimen. Thus, more and more challenges arise because of an increasing influence of scaling effects. Sticking is one crucial effect that generates measurement faults such as snap-back and false triggering. This work presents investigations on a highly accurate electrostatic probing system, which operates at resonant motion to minimize the influence of capillary forces. The unique feature of the uniaxial microprobe is the fully integrated design in a silicon-on-insulator substrate, which minimizes the number of coupling connections and improves the measurement accuracy. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0957-0233/25/6/064012
  • MOEMS tunable microlens made of aluminum nitride membranes
    Leopold, S. and Polster, T. and Paetz, D. and Knoebber, F. and Ambacher, O. and Sinzinger, S. and Hoffmann, M.
    Journal of Micro/Nanolithography, MEMS, and MOEMS 12 (2013)
    We present tunable lenses based on aluminum nitride membranes. The achievable tuning range in the refractive power is 0 to 25 dpt with an external pressure load of ≤20 kPa. The lenses are manufactured using MOEMS technology. For 500-nm-thick membranes with a diameter of 3 mm, a spherical deflection profile is found. The system provides good long-term stability showing no creep or hysteresis. A model for the refractive power versus applied pressure is derived and validated experimentally. Based on this model, design guidelines are discussed. One essential parameter is the residual stress of the aluminum nitride layer that can be controlled during deposition. © 2013 Society of Photo-Optical Instrumentation Engineers.
    view abstract10.1117/1.JMM.12.2.023012
  • Online monitoring of the passivation breakthrough during deep reactive ion etching of silicon using optical plasma emission spectroscopy
    Leopold, S. and Mueller, L. and Kremin, C. and Hoffmann, M.
    Journal of Micromechanics and Microengineering 23 (2013)
    We present optical emission spectroscopy (OES) as a technique for process optimization of the etch step during deep reactive ion etching of silicon. For specific process steps, the spectrum of optical plasma emission is investigated. Two specific wavelengths are identified (fluorine at 703.8 nm and CS compounds at 257.6 nm), which significantly change intensity during the etch step. Their intensity drop is used for the recognition of the passivation layer breakthrough. Thus, the net silicon etch time can be measured. This time can be used for process optimization. A structural analysis of the passivation layer shows its fragmentation during its breakthrough. The plasma-surface interaction and their correlation with the plasma emission are described. Within an application example, the passivation breakthrough is investigated in detail. For different process regimes, the residues of the fragmented passivation layer are analyzed by scanning electron microscopy. Residue densities of 14-38 μm -2 are fabricated. For silicon grass generation, the OES technique offers a versatile tool for the process optimization of the mask generating process within the first cycles. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0960-1317/23/7/074001
  • Resonant probing system comprising a high accurate uniaxial nanoprobe and a new evaluation unit
    Goj, B. and Dressler, L. and Hoffmann, M.
    Journal of Micromechanics and Microengineering 23 (2013)
    The increasing demand to tactilely measure micro-electro-mechanical systems featuring high aspect ratios leads to probing systems with low stiffness and small touching elements (e.g. ruby balls). Thus, more challenges arise because of an increasing influence of the so-called micro world effects. Sticking is one crucial effect which generates measurement faults like snap back and false triggering. This paper presents a highly accurate electrostatic probing system which operates at resonant motion to minimize the influence of capillary forces. The unique feature of the uniaxial nanoprobe is the fully integrated design in a silicon-on-insulator substrate which minimizes the number of coupling connections and improves the measurement accuracy. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0960-1317/23/9/095012
  • Variation of the intrinsic stress gradient in thin aluminum nitride films
    Mehner, H. and Leopold, S. and Hoffmann, M.
    Journal of Micromechanics and Microengineering 23 (2013)
    The intrinsic stress gradient variation of thin aluminum nitride (AlN) films is the central objective in this paper. For the first time, significant influence parameters on the stress gradient are identified and varied during the deposition process. The process power induced in the plasma and the gas flow ratio of the sputter gases argon and nitrogen are the two major parameters for controlling the stress gradient of deposited AlN films. The controlled avoidance as well as the controlled generation of positive and negative gradients is shown. The stress gradient was investigated by analysis of released one-side clamped cantilever test structures. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0960-1317/23/9/095030
  • Microforming process for embossing of LTCC tapes
    Bartsch, H. and Albrecht, A. and Hoffmann, M. and Müller, J.
    Journal of Micromechanics and Microengineering 22 (2012)
    Embossing of low-temperature cofired ceramics (LTCC) enables the fine patterning of these multilayer materials in the green state and thus allows the fabrication of smart ceramic microsystems even for moderate quantities or prototypes. To understand the embossing process, mechanical properties such as the viscoelastic modulus, yield strain and densification are investigated for commercially available LTCC tapes. The forming and shrinkage behaviour are compared for large cavities as well as for fine patterns. The results are discussed and a comprehensive explanation of the forming mechanism is worked out. Relevant material properties are identified and the microforming of different tapes is explained under consideration of their mechanical properties. This paper therefore gives an essential guide to understanding the main forming influences and failures for LTCC tapes. © 2012 IOP Publishing Ltd.
    view abstract10.1088/0960-1317/22/1/015004
  • Formation of silicon grass: Nanomasking by carbon clusters in cyclic deep reactive ion etching
    Leopold, S. and Kremin, C. and Ulbrich, A. and Krischok, S. and Hoffmann, M.
    Journal of Vacuum Science and Technology B:Nanotechnology and Microelectronics 29 (2011)
    Initial cluster formation on silicon surfaces in cyclic deep reactive ion etching (c-DRIE) using c -C4 F8 / SF6 plasma is investigated. These clusters act as a nanomask for the fabrication of nanostructured surfaces such as silicon grass. Different wafer preconditioning regimes and subsequent x-ray photoelectron spectroscopy show that no wafer or process contaminations are the reason for nanomasking in c-DRIE. Furthermore, no Si-containing compounds, such as SiFx Oy, SiO x, or SiC, are detected. The clusters consist of residues of the fluorinated carbon layer deposited in c-DRIE. Experimental process analysis using design of experiments shows the dependence of nanomask morphology on passivation time and power. The results indicate that the properties of the nanomask, in particular, density, are determined during passivation. © 2011 American Vacuum Society.
    view abstract10.1116/1.3521490
  • Integration of 3-D cell cultures in fluidic microsystems for biological screenings
    Witte, H. and Stubenrauch, M. and Fröber, U. and Fischer, R. and Voges, D. and Hoffmann, M.
    Engineering in Life Sciences 11 (2011)
    A life support system for the cultivation of adherent 2-D and scaffold-based 3-D cell cultures in a microfluidic device, a Bio-Micro-Electro-Mechanical System (BioMEMS) is presented. The miniaturization level and system set-up allow incubator-independent operation modes and long-term experiments with real-time microscope observation. A dedicated seeding procedure for adherent cells into the microstructures is one key issue of the BioMEMS developed. Several seeding methods for the cells were evaluated. Biocompatibility of all materials, surfaces and methods could be demonstrated. First experiments with several cell types show the feasibility of the approach employing standard laboratory protocols. At present, the modular design and set-up offer a broad application spectrum as well as its future extension to e.g. cultivation of other cell types, coupled cultivation chambers and the implementation of other manipulation or analysis components. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elsc.201000045
  • Liquid-crystalline elastomer microvalve for microfluidics
    Sánchez-Ferrer, A. and Fischl, T. and Stubenrauch, M. and Albrecht, A. and Wurmus, H. and Hoffmann, M. and Finkelmann, H.
    Advanced Materials 23 (2011)
    Microactuators are an essential component in microsystems or microdevices, and in applications that include artificial muscles, pumps, valves, or switchers. Liquid-crystalline elastomers are a new class of actuator material in the field of microsystem technologies, which can be used in standard processes. This newly developed actuator provides new possibilities in microfluidics because of its dimensional changes activated by the increase in temperature. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adma.201102277
  • actuators

  • micromechanics

  • microsystems

  • nanostructures

  • sensors

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