We have investigated a THz detection scheme based on mixing of electrical signals in a voltage-de... more We have investigated a THz detection scheme based on mixing of electrical signals in a voltage-dependent capacitance made out of suspended graphene. We have analyzed both coherent and incoherent detection regimes and compared their performance with the state of the art. Using a high-amplitude local oscillator, we anticipate potential for quantum limited detection in the coherent mode. The sensitivity stems from the extraordinary mechanical and electrical properties of atomically thin graphene or graphene-related 2D materials.
2004 Conference on Precision Electromagnetic Measurements
The design and characterization of a high stability capacitive MEMS device intended for an AC vol... more The design and characterization of a high stability capacitive MEMS device intended for an AC voltage reference at 100 kHz or higher frequencies is presented. Preliminary results from a first prototype device show that the pull-in voltage of the device is stable to within 10 ppm over 60 hours. We discuss an optimised device design which is expected to show greater stability.
The view of VTT (Technical Research Centre of Finland) on the development of selected information... more The view of VTT (Technical Research Centre of Finland) on the development of selected information and communication technologies is presented. The views are given as technology roadmaps in the areas of interoperability and mobility in future networks, micromechanical radio frequency systems, service architectures and smart human environments. The work is a part of a theme called Future Communications Technologies, one of VTT's strategic technology themes. The purpose of the work is to help the planners and players direct their activities towards better competitiveness in this rapidly developing field.
In this paper, finite element method (FEM) simulations are used to model mechanical properties of... more In this paper, finite element method (FEM) simulations are used to model mechanical properties of MEMS resonators. Using a static displacement analysis the effective spring constant and mass are calculated. Nonlinearity of the mechanical restoring force is simulated to analyze large amplitude behavior of the resonator. Equivalent electrical circuit modeling is used for the simulating the frequency response of the resonators by APLAC RF-circuit simulator [1]. The FEM-calculated effective mass and spring constant are used to calculate equivalent electric circuit values. FEM and circuit simulation results are compared with the measured response of two example resonator structures; as an example we show detail analysis of clamped-clamped beam resonator operating at 1.6 MHz frequencies with Qvalue 30000. Mechanical non-linearities are included in the circuit simulation.
Several RF-MEMS devices can benefit from metallic micromachining processes in comparison to the c... more Several RF-MEMS devices can benefit from metallic micromachining processes in comparison to the conventional silicon-based MEMS technologies for various reasons. A variety of metallic materials and their fabrication techniques have indeed been tested in the laboratory but commercial volume manufacturing processes for RF-MEMS devices are almost non-existent. Good conductors such as copper and gold do not perform optimally as structural materials during extended cycling. This report describes a new group of materials for RF-MEMS: amorphous metals. In terms of electrical conductivity amorphous metals are intermediate between elemental metals and semiconductors but they can fulfil the mechanical function of the devices despite of their conductivity limitation.
The long-term stability of single-crystal silicon microresonators is evaluated. The vacuum-encaps... more The long-term stability of single-crystal silicon microresonators is evaluated. The vacuum-encapsulated length-extensional mode resonators (f r ∼ 13 MHz) are demonstrated to be capable of a ppm-level aging rate for a follow-up period of 42 days. Comparison with unsealed devices reveals that the silicon microresonators are sensitive to water contamination from the ambient humidity.
TRANSDUCERS '03. 12th International Conference on Solid-State Sensors, Actuators and Microsystems. Digest of Technical Papers (Cat. No.03TH8664), 2003
The fundamental performance limit of single-crystal silicon resonators set by device nonlineariti... more The fundamental performance limit of single-crystal silicon resonators set by device nonlinearities is characterized. Using Leeson's model for near carrier phase noise, the nonlinearity is shown to set the scaling limit in miniaturizing oscillators. A circuit model based on discretization of distributed mass and nonlinear elasticity is introduced to accurately simulate the large amplitude vibrations. Based on published data for the third-order silicon stiffness tensor, the fundamental material nonlinearity limit is estimated. This theoretical limit is compared to measured nonlinearities in bulk acoustic wave (BAW) micromechanical resonators. The material set and measured nonlinearities are of same order-ofmagnitude showing that the maximum vibration amplitude of studied BAW microresonators is near the fundamental limit. The maximum strain for single-crystal silicon resonators set by hysteresis limit is estimated to be 2 • 10 -3 (fracture limit 10 -2 ), which corresponds to the maximum energy density of E m /V = 3 • 10 5 J/m 3 . This value is at least two ordersof-magnitude higher than for shear-mode quartz resonators, which partially compensates for the small size of MEMS components.
Stability of wafer level vacuum encapsulated micromechanical resonators is characterized. The res... more Stability of wafer level vacuum encapsulated micromechanical resonators is characterized. The resonators are etched of silicon-on-insulator (SOI) wafers using deep reactive ion etching (DRIE) and encapsulated with anodic bonding. Bulk acoustic wave (BAW) resonator show drift better than 0.1 ppm/month demonstrating that the stability requirements for a reference oscillator can be met with MEMS. The drift of flexural resonators range from 4 ppm/month to over 500 ppm/month depending on resonator anchoring. The large drift exhibited by some flexural resonator types is attributed to packaging related stresses.
VTC Spring 2009 - IEEE 69th Vehicular Technology Conference, 2009
This paper presents building blocks for a proposed automotive intermodulation radar for detecting... more This paper presents building blocks for a proposed automotive intermodulation radar for detecting and identifying vulnerable road users (VRUs). The intermodulation radar transmits at two frequencies and it has receivers both for normal reflections and for reflections from VRUs at the intermodulation frequency. A possible radar architecture is presented and modulation techniques are discussed. Different mixing elements of radar reflectors are studied with simulations. An antenna structure enabling a wearable reflector that can be integrated into clothes is designed and characterized. The simulated and measured performance of the radar reflector is used to estimate the achievable detection range.
We consider a chain of coupled micromechanical resonators as a delay line for radio-frequency sig... more We consider a chain of coupled micromechanical resonators as a delay line for radio-frequency signals. Wave propagation in the chain is generated and detected using capacitive transducers. Analytical results, numerical simulations and rst measurements for the response of a prototype device are presented. The delay line is shown to have a bandpass response and a very low signal group velocity of the order of only few meters per second. Weaknesses of the rst prototypes are identi ed through simulations and a more optimal design is suggested.
We have developed a programmable Josephson voltage standard based on an array of superconductor-i... more We have developed a programmable Josephson voltage standard based on an array of superconductor-insulator-superconductor junctions. The junctions are damped by an external frequency-dependent shunt circuit, which allows operation at Shapiro steps with nϾ1. We derive optimization criteria for the design parameters for a fast and stable array with low power consumption. An array able to generate 1.0 V RMS ac voltage signal with sub-ppm accuracy is experimentally demonstrated. Theoretically it is also shown that a fast programmable 10 V array can be realized.
At the pull-in point, a capacitive MEMS sensor becomes infinitely sensitive to applied force as t... more At the pull-in point, a capacitive MEMS sensor becomes infinitely sensitive to applied force as the effective spring constant goes to zero because of electromechanical feedback We show that this phenomenon can be used to fully eliminate the noise contribution of readout electronics. Experimentally, we show that the electronics noise and interference contribution to system resolution could be suppressed by an order of magnitude, reaching the intrinsic resolution of the MEMS microphone. Experiments are in good agreement with a theory based on a small signal model of a harmonic MEMS oscillator. The technique allows the use of standard integrated electronics with noise-critical MEMS sensors, such as microphones, pressure sensors and accelerometers.
The increased electrostatic stability of MEMS sensors enables new application areas for the senso... more The increased electrostatic stability of MEMS sensors enables new application areas for the sensors, and decreases the manufacturing costs of existing products. Especially in the applications where the MEMS component is operated under bias voltage close to the pull-in point, the undesired instability phenomenon becomes a major source of inaccuracy. We demonstrate that biasing the sensor to the pull-in point using AC voltage is significantly more stable than the conventionally used DC voltage biasing.
The small size and integrability make the silicon micromechanical rf-resonators attractive compon... more The small size and integrability make the silicon micromechanical rf-resonators attractive components for future wireless communication devices. In particular, we show that using the microresonators one can construct oscillators exhibiting low phase noise and good long-term stability. Such compact solutions challenge conventional quartz crystals in frequency reference applications.
A prototype of SQUID read-out for current measurements at the output of a normal metal hot-electr... more A prototype of SQUID read-out for current measurements at the output of a normal metal hot-electron microbolometer has been developed and tested. The system is based on serial VTT dc SQUID and input l-metal core solenoid transformer. The achieved current resolution is 300 fA/Hz 1=2 . Johnson noise of metal resistor and shot noise of tunnel junction were used for current calibration of SQUID read-out. The current noise spectra of 35 kX SIN tunnel junction measured at different bias voltages are presented.
We have designed and manufactured a micromachined moving plate capacitor to be used as an AC volt... more We have designed and manufactured a micromachined moving plate capacitor to be used as an AC voltage reference in electrical metrology. The reference is based on the characteristic AC current -voltage curve of the component having a maximum, the value of which depends on the geometry of the component and material properties of single crystalline silicon. The electrode surface stability is essential in this application and hence a new fabrication process has been developed to metallize both surfaces of an electrostatically actuated micromachined structure. The stability of the AC reference voltage with the frequency of 100 kHz and the RMS value of 6.4 V was measured to be ± 60 ppm over 14 hours.
Journal of Micromechanics and Microengineering, 2011
Single crystal silicon based micromechanical resonators are developed for mass sensing in dissipa... more Single crystal silicon based micromechanical resonators are developed for mass sensing in dissipative media. The design aspects and preliminary characterization of the resonators are presented. For the suggested designs, quality factors of about 20000 are typically measured in air at atmospheric pressure and 1000-2000 in contact with liquid. The performance is based on a wine-glass type lateral bulk acoustic mode excited in a rectangle resonator plate. The mode essentially eliminates the radiation of acoustic energy into the sample media leaving viscous drag as the dominant fluid-based dissipation mechanisms in the system. For a mass loading distributed over the central areas of the resonator a sensitivity of 27 ppm/ng is measured exhibiting good agreement with the results of the finite element method based simulations. It is also shown that the mass sensitivity can be somewhat enhanced, not only by the proper distribution of the loaded mass, but also by introducing shallow barrier structures on the resonator.
Journal of Micromechanics and Microengineering, 2006
A slow-wave microelectromechanical delay line, composed of a chain of coupled resonators, is intr... more A slow-wave microelectromechanical delay line, composed of a chain of coupled resonators, is introduced. The delay line has a bandpass response and, depending on structural details, signal group velocity can be as low as ∼ 10 m/s that is over 100 times smaller than for acoustical SAW or BAW delay lines. Properties of the delay line are analyzed theoretically and the theory is verified in measurements.
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2010
This paper presents a passive wireless resonating sensor that is based on a ferroelectric varacto... more This paper presents a passive wireless resonating sensor that is based on a ferroelectric varactor. The sensor replies with its data at an intermodulation frequency when a reader device illuminates it at 2 closely located frequencies. The paper derives a theoretical equation for the response of such a sensor, verifies the theory by simulations, and demonstrates a temperature sensor based on a ferroelectric varactor.
IEEE Transactions on Microwave Theory and Techniques, 2003
A microelectromechanical system (MEMS) sound waveguide is considered as a transmission line for R... more A microelectromechanical system (MEMS) sound waveguide is considered as a transmission line for RF signals. We analyze a device geometry of a straight one-dimensional microsize silicon rod, where a longitudinal acoustic wave is generated and detected using capacitive transducers. Linear, isotropic, and nondispersive acoustic-wave propagation is assumed. Based on the calculation of the electromechanical impedance, an electrical equivalent model is derived for the acoustic transmission line. A numerical example and a comparison to measured properties of a MEMS-transmission-line resonator shows that the characteristic impedance level of the waveguide is typically high, which causes challenges for matched termination. Solutions to overcome the matching problems are discussed.
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Papers by Heikki Seppä