The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
Aalto

Fabrication of SOI Micromechanical Devices

Jyrki Kiihamäki

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 15th of April, 2005, at 12 o'clock noon.

Overview in PDF format (ISBN 951-38-6436-7)   [1701 KB]
VTT Publications 559, ISSN 1455-0849

Dissertation is also available in print (ISBN 951-38-6435-9)
Copyright © 2005 VTT Technical Research Centre of Finland
VTT Publications 559, ISSN 1235-0621
VTT-PUBS-559
TKK-DISS-1983

Abstract

This work reports on studies and the fabrication process development of micromechanical silicon-on-insulator (SOI) devices. SOI is a promising starting material for fabrication of single crystal silicon micromechanical devices and basis for monolithic integration of sensors and integrated circuits. The buried oxide layer of an SOI wafer offers an excellent etch stop layer for silicon etching and sacrificial layer for fabrication of capacitive sensors. Deep silicon etching is studied and the aspect ratio dependency of the etch rate and loading effects are described and modeled. The etch rate of the deep silicon etching process is modeled with a simple flow conductance model, which takes into account only the initial etch rate and reaction probability and flow resistance of the etched feature. The used model predicts qualitatively the aspect-ratio-dependent etch rate for varying trench widths and rectangular shapes. The design related loading can be modeled and the effects of the loading can be minimized with proper etch mask design.

The basic SOI micromechanics process is described and the drawbacks and limitations of the process are discussed. Improvements to the process are introduced as well as IR microscopy as a new method to inspect the sacrificial etch length of the SOI structure.

A new fabrication process for SOI micromechanics has been developed that alleviates metallization problems during the wet etching of the sacrificial layer. The process is based on forming closed cavities under the structure layer of SOI with the help of a semi-permeable polysilicon film.

Prototype SOI device fabrication results are presented. High Q single crystal silicon micro resonators have potential for replacing bulky quartz resonators in clock circuits. Monolithic integration of micromechanical devices and an integrated circuit has been demonstrated with the developed process using the embedded vacuum cavities.

This thesis consists of an overview and of the following 6 publications:

  1. Kiihamäki, J. Deceleration of silicon etch rate at high aspect ratios. J. Vac. Sci. Technol. A, Vol. 18, No. 4, (2000), pp. 1385-1389. © 2000 American Vacuum Society. By permission
  2. Karttunen, J., Kiihamäki, J., Franssila, S. Loading effects in deep silicon etching. Proc. SPIE, Vol. 4174, (2000), pp. 90-97. © 2000 International Society of Optical Engineering (SPIE). By permission
  3. Kiihamäki, J. Measurement of oxide etch rate of SOI structure using near IR microscopy. Physica Scripta, Vol. T101, (2002), pp. 185-187.
  4. Kaajakari, V., Mattila, T., Oja, A., Kiihamäki, J., Seppä, H. Square-extensional mode single-crystal silicon micromechanical resonator for low-phase-noise oscillator applications. IEEE Electron Device Letters, Vol. 25, No. 4, (2004), pp. 173-175. © 2004 IEEE. By permission
  5. Kiihamäki, J., Dekker, J., Pekko, P., Kattelus, H., Sillanpää, T., Mattila, T. 'Plug-Up' – A new concept for fabricating SOI MEMS devices. Microsystem Technologies, Vol. 10, No. 5, (2004), pp. 346-350. © 2004 Springer-Verlag. By permission
  6. Kiihamäki, J., Ronkainen, H., Pekko, P., Kattelus, H., Theqvist, K. Modular integration of CMOS and SOI-MEMS Using 'Plug-Up' concept. Digest of Technical Papers The 12th International Conference on Solid-State Sensors, Actuators and Microsystems, Vol. 2. Institute of Electrical and Electronics Engineers, (2003), pp. 1647-1650. © 2003 IEEE. By permission

Keywords: silicon-on-insulator, SOI, micromechanics, MEMS, microfabrication, HARMST, DRIE, etching, vacuum cavities, resonators, monolithic integration

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© 2005 Helsinki University of Technology

Last update 2011-05-26