Jyrki Kiihamäki

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.

PATTERN SHAPE EFFECTS AND ARTEFACTS IN DEEP SILICON ETCHING

Deep silicon etching in an inductively coupled plasma (ICP) reactor offers a high etch rate (7 μm/min), nearly vertical profile with simple oxide masking. Test structures with patterns of different sizes (from a few microns to over 100 μm) and shapes (square and circular holes and trenches of variable width/length) have been etched to depths up to 500 μm. Long narrow features are etched faster than wide short features, indicating the three-dimensional nature of the reactive ion etching lag. Experiments have been done for many different etch times in order to understand aspect ratio dependence of deep etching. Simple flow conductance model explains most of the observed aspect ratio and feature size dependence.

Stability of wafer level vacuum encapsulated single-crystal silicon resonators

Abstract 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 demonstrated by the sample temperature–frequency coefficients differing from the bulk silicon value.

Depth and profile control in plasma etched MEMS structures

We have achieved uniform etched depth regardless of feature size by employing a combination of anisotropic plasma etching in inductively coupled plasma (ICP) followed by wet etching. In our approach, the original feature is divided into small elementary features in a mosaic-like pattern. These individual small features are all the same size and thus exhibit identical etch rates and sidewall profiles. Final patterns are completed by wet etching: the ridges between the elementary features are removed in TMAH. In this paper, we present the results obtained using this dry/wet etching sequence. The benefits and limitations of this method are described. Extensions to more complex multidepth structures are discussed.

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.

Low noise, low power micromechanical 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 /spl mu/W 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.

Nonlinearities in single-crystal silicon micromechanical resonators

The fundamental performance limit of single-crystal silicon resonators set by device nonlinearities in characterized. Using Leesons 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-of-magnitude 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/spl middot/10/sup -3/ (fracture limit 10/sup -2/), which corresponds to the maximum energy density of E/sub m//V=3/spl middot/10/sup 5/ J/m/sup 3/. This value is at least two orders-of-magnitude higher than for shear-mode quartz resonators, which partially compensates for the small size of MEMS components.

Fabrication of single crystal silicon resonators with narrow gaps

This paper reports a novel method for fabrication of micromechanical resonators with very narrow gaps for electrostatic actuation. Vertical 50-180 nm wide gaps are realized using sacrificial etching of oxide sandwiched between APCVD deposited epipoly and patterned single crystal silicon structure layer of SOI wafer.

Reducing stiction in microelectromechanical systems by rough nanometer-scale films grown by atomic layer deposition

Stiction during device operation remains one of the mechanisms leading to permanent failure of operating silicon-based MEMS devices (MicroElectroMechanical Systems). The goal of this work was to investigate, whether stiction between parallel, smooth silicon surfaces can be decreased by thin inorganic films grown by atomic layer deposition (ALD). Test structures based on the cantilever-beam-array (CBA) method were fabricated and coated with ALD layers varying in chemical nature and roughness. Rough crystalline TiO2 decreased the adhesion energy orders of magnitude as compared to Si and other smooth films, indicating that TiO2 and other crystalline ALD films are candidates for anti-stiction layers in MEMS.

Low-temperature processes for MEMS device fabrication

The high temperatures typical in semiconductor and conventional MEMS fabrication limit the material choices in MEMS structures. This paper reviews some of the low-temperature processes and techniques available for MEMS fabrication and describes some characteristics of these techniques and practical process examples. The techniques described are plasma-enhanced chemical vapour deposition, atomic layer deposition, reactive sputtering, vapour phase hydrofluoric acid etching of low-temperature oxides, and low-temperature wafer bonding. As a practical example of the use of these techniques, the basic characteristics of a MEMS switch and other devices fabricated at VTT are presented.