Hannu Kattelus

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.

Implementing ALD layers in MEMS processing

Layers manufactured by the ALD technique have many interesting applications in microelectromechanical systems (MEMS), for example as protective layers for biocompatible coating, highdielectric-constant layers, or low-temperature conformal insulating layers. Before an ALD process can be successfully implemented in MEMS processing, several practical issues have to be solved, starting from patterning the layers and characterizing their behaviour in various chemical and thermal environments. Stress issues may not be forgotten. We have recently implemented two ALD processes, namely the trimethylaluminium/water process to deposit Al2O3 and the titanium tetrachloride/water process to deposit TiO2 in our MEMS processing line and carried out the necessary characterization, details of which are reported here. For us, ALD has been a truly enabling technology in the processing of a three-dimensional micromechanical compass based on the Lorentz force, where Al2O3 acted as a pinhole-free electrical insulation grown at low temperature.

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.

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.

Stress control of sputter-deposited Mo-N films for micromechanical applications

Sputter-deposited metallic thin films are attractive materials for micromechanics but they suffer from large stress variations within the batch or even a single wafer. The origin of such variations is in the microcrystalline structure, and specifically its dependence on sputtering geometry. It might be assumed that getting rid of the long-range atomic order in the deposited film would help in obtaining improved uniformity. The purpose of this study is to amorphise molybdenum films by nitrogen and to characterize the resulting Mo-N film properties. A partial cure for the nonuniformity of stress is, indeed, realized since film-plane stress variations are eliminated by nitridation. A vertical gradient still remains, bending relieved micromechanical beams strongly upwards. This behavior is believed to be due to imperfect amorphisation-the existence of columnar nano-scale crystallites.

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.

Atomic Layer Deposition in MEMS Technology

Atomic layer deposition can be defined as a film deposition technique that is based on the sequential use of self-terminating gas–solid reactions. ALD can offer significant advantages in MEMS processing compared to traditional film deposition methods. This chapter describes atomic layer deposition and its different processes and applications. It also explains the basic operation principles of the ALD technique and briefly introduces the already developed ALD materials and processes. ALD is a cyclic process based on repeated reaction cycles that consist of self-terminating reaction steps followed by a purge or evacuation step. The operation principle based on separate, self-terminating reactions, which means that means that the reactions continue as long as there are suitable reactive sites on the substrates. There are two main limitations of ALD, which are the slowness of the process and the limited material and process selection. This chapter helps us to get some idea on the ALD processes. This is detailed with the help of headings such as general requirements for the reactants, metal reactants, nonmetal reactants, materials made by ALD, multi-element films by ALD etc. This chapter is explains in detail the characteristics of ALD processes and films. It gives some idea on the growth modes possible in ALD. The demerits like roughness of ALD films, stress and pinholes of ALD films are explained. The stability of ALD films in different chemical environments are also briefed in this chapter. ALD reactors, which is used to create conditions of LAD processes are detailed in this chapter.

Plug-up-a new concept for fabricating SOI MEMS devices

This paper reports a novel process sequence for fabricating micromechanical devices on silicon-on-insulator (SOI) wafers. Among the merits of the described process are its improved immunity to stiction and elimination of conductor metal endurance problems during sacrificial etching in hydrofluoric acid. With this novel process one can controllably embed vacuum cavities within SOI substrates. Further processing of such cavity wafers enables realization of a wide variety of micromechanical devices based on single crystalline silicon or even integrated read-out circuitry.

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.