Mika Koskenvuori

Square-extensional mode single-crystal silicon micromechanical RF-resonator

A micromechanical 13.1 MHz bulk acoustic mode (BAW) silicon resonator is demonstrated. The vibration mode can be characterized as a 2-D plate expansion that preserves the original square shape. The prototype resonator is fabricated of single-crystal silicon by reactive ion etching a silicon-on-insulator (SOI) wafer. The measured high quality factor (Q=130000) and current output (i/sub MAX/ /spl ap/ 160 /spl mu/A) make the resonator suitable for reference oscillator applications. An electrical equivalent circuit based on physical device parameters is derived and experimentally verified.

Temperature measurement and compensation based on two vibrating modes of a bulk acoustic mode microresonator

A method to detect the temperature of a micromechanical resonator by measuring the frequency- shifts of two different eigenmodes is presented. The resonator is a square-extensional bulk-acoustic-resonator with two fundamental modes of vibration. The modes feature different temperature coefficients of the frequency and therefore by measuring the frequency change of both modes of vibration the temperature information can be deduced. The measured temperature is used to mathematically compensate for the temperature induced drift of the second eigenmode. The advantage of the demonstrated in-situ temperature measurement is that as no external temperature sensor is needed, the resonator temperature is accurately measured and hysteresis arising from different temperature time constants for heating and cooling of the sensor and resonator is eliminated.

Micromechanical bulk acoustic wave resonator

We describe the use of bulk acoustic mode in micromechanical silicon resonators operating at radio frequencies. Based on measured data from the fabricated resonator (f/sub r//spl sim/14 MHz, Q>100 000) we analyze the characteristic impedance and signal levels in such microdevices and compare the values with conventional quartz crystals. We find that the high impedance level of microresonators can be met with integration of the readout electronics and that silicon can accommodate significantly larger vibration energy densities than quartz. Based on the results, we anticipate a wide application range for the micromechanical bulk acoustic wave structures in future wireless communication devices and microsensors.

Atomic layer deposition enhanced rapid dry fabrication of micromechanical devices with cryogenic deep reactive ion etching

A fast, dry microfabrication process combining atomic layer deposition, electron beam lithography and cryogenic deep reactive ion etching is presented. The process exploits the extremely high selectivity of atomic layer deposited amorphous Al2O3 (alumina) to silicon in cryogenic etching by using an ultra-thin (t ≤ 5nm) Al2O3 film as a mask. The process rules and limitations are carefully analyzed and a thorough understanding of the limiting factors is reached, and the effect of the limitations on the critical output current of a dc-biased clamped–clamped beam is studied. To test the process, multiresonant tuning fork resonators are fabricated and found to exhibit Q ≈ 8000 at fr ≈ 11.4 MHz. Both values are the highest reported for resonators fabricated with the dry process and comparable with values achieved with existing silicon micromachining processes.

Reducing the Effect of Parasitic Capacitance on MEMS Measurements

The use of micromechanical resonant structures in RF electronics possesses often a problem caused by a very low signal amplitude. In order to alleviate the influence of parasitic capacitance we propose here the use of the differential amplifier and demonstrate its use here on the processed electrostatically driven resonators. The component used in verifying the use of differential amplifier is a clamped-clamped beam resonator with Q=8000 and resonant frequency of f 0 = 12.3 MHz. A low-noise high input-impedance amplifier was used as a reference.

Towards Micromechanical Radio: Overtone Excitations of a Microresonator Through the Nonlinearities of the Second and Third Order

A micromechanical resonator with eigenfrequencies in the megahertz-range is excited by signals having frequencies from tens of megahertz to gigahertz. The high-frequency excitation voltage is downconverted to mechanical force at the lower resonance frequency by the second-order force-voltage nonlinearity. The conversion is either assisted by additional local-oscillator signal or it is intrinsic due to an amplitude-modulated (AM) input signal. A circuit-simulator model is tested against measurements and an excellent agreement and thorough interpretation of the results is found. The third-order intercept point is measured and simulated to study the strength of the capacitive third-order nonlinearity. Finally, various nonlinear contributions are compared and further improvements for the device are suggested based on the simulations.

Low noise silicon micromechanical bulk acoustic wave oscillator

A 180-nm gap micromechanical resonator biased at 20 V and full custom integrated electronics are used to implement a 13-MHz oscillator that has noise floor of -147 dBc/Hz and power consumption of 240 µW including both the loop amplifier and the buffer to a 10-pF load. The design of Pierce type MEMS oscillator is discussed in terms of noise, power, and oscillator stability. Mixing of 1/f-noise is simulated and nonlinear electrostatic coupling is identified as a significant noise aliasing mechanism.

Silicon Micromechanical Resonators for RF-Applications

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.

Parametrically amplified microelectromechanical mixer

The down-conversion performance of a multimodal microelectromechanical mixer-filter is improved over 30 B by parametric amplification. The input signal is an AM-modulated signal with a carrier frequency of 0.5 Hz. The obtained amplification is shown to depend on the particular eigenmode in predetermined way due to the configuration of the electrodes used to excite and amplify the resonance.

Chapter Twelve – Electrostatic and RF-Properties of MEMS Structures

Publisher Summary This chapter discusses the electrostatic and RF-properties of MEMS structures in detail. Well known examples of micromechanical sensors are accelerometers, pressure sensors and cantilevers that are used as fluid sensors and in various microscopes. In designing and modeling of nano and micromechanical systems the various aspects should be taken into account. A most simplified way of modeling a micromechanical moving system is to form a lumped element model that contains a minimum number of physical parameters. Often in practice an electrical circuit simulator is used to obtain detailed information of the behavior of the mechanical model as a part of an electronic circuit. If the biasing voltage used is a DC voltage UDC, the electrostatic force can be calculated as a negative gradient of energy. Narrow gaps between the electrodes can be used to increase the electromechanical coupling. The oscillating MEMS resonator can be sensed using a DC-biased electrode. The pull-in behavior is observed when the mechanical and electrical forces (and simultaneously also their derivatives) cancel each other. The capacitive coupling has a significant drawback. The parasitic current is often much stronger than the induced motional current. Electrostatic nonlinearities discussed can generate harmful side-effects in capacitively coupled devices. RF-properties are studied from switching point of view. Some aspects related to the RF-properties are discussed here. The RF-properties are usually characterized by concepts as isolation (ISOL), insertion loss, (IL) and reflection. Both the isolation and insertion loss describe the forward power transmission, S21, of the switch.