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.

Atomic Layer Deposited Titanium Dioxide in Optical Waveguiding Applications

Tapani Alasaarela

Doctoral dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Aalto University School of Electrical Engineering for public examination and debate in the lecture hall AS1 of the TUAS building (Otaniementie 17, Espoo, Finland) at the Aalto University School of Electrical Engineering on the 29th of June 2011 at 12 o'clock.

Overview in PDF format (ISBN 978-952-60-4180-3)   [5234 KB]
Dissertation is also available in print (ISBN 978-952-60-4179-7)


This thesis introduces the use of atomic layer deposited (ALD) amorphous titanium dioxide (TiO2) in various waveguiding applications. The ALD of amorphous TiO2 is done by sequential pulsing of titanium tetrachloride (TiCl4) and water vapors into a nitrogen carrier gas flow at relatively low deposition temperatures of below 120°C and in a medium vacuum. The optical properties of the amorphous TiO2 slab waveguides are studied in the visible and near-infrared wavelengths, and a propagation loss of < 1 dB/cm at 1.55 micrometer wavelength is measured. The conformal growth mode of ALD is used to narrow down silicon slot and to smoothen waveguide edges with an amorphous TiO2 layer, which enables fabrication of silicon slot waveguides with thin air slots and relatively low losses of down to 7 dB/cm.

ALD-TiO2 is also used to successfully fabricate novel absorbing polarization selective resonant gratings, and semi-wide bandwidth resonant waveguide reflectors. A novel way of fabricating a resonant waveguide grating by applying an ALD-TiO2 layer on a holographically written surface relief grating on an azobenzene polymer complex is introduced. Amorphous ALD-TiO2 shows an excellent performance in all the studied applications, and will very probably see use in various photonic devices in the future. The low growth temperature allows the fabrication of ALD-TiO2 waveguides on top of processed microchips, so ALD-TiO2 might also turn out to be a suitable material in building the optically interconnected microchips of the future.

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

  1. T. Alasaarela, A. Säynätjoki, T. Hakkarainen, and S. Honkanen. Feature size reduction of silicon slot waveguides by partial filling using atomic layer deposition. Optical Engineering, vol. 48, no. 8, pp. 080501-1-3, August 2009.
  2. A. Säynätjoki, T. Alasaarela, A. Khanna, L. Karvonen, P. Stenberg, M. Kuittinen, A. Tervonen, and S. Honkanen. Angled sidewalls in silicon slot waveguides: conformal filling and mode properties. Optics Express, vol. 17, no. 23, pp. 21066-21076, November 2009.
  3. T. Alasaarela, D. Korn, L. Alloatti, A. Säynätjoki, A. Tervonen, R. Palmer, J. Leuthold, W. Freude, and S. Honkanen. Reduced propagation loss in silicon strip and slot waveguides coated by atomic layer deposition. Optics Express, vol. 19, no. 12, pp. 11529-11538, June 2011.
  4. T. Alasaarela, T. Saastamoinen, J. Hiltunen, A. Säynätjoki, A. Tervonen, P. Stenberg, M. Kuittinen, and S. Honkanen. Atomic layer deposited titanium dioxide and its application in resonant waveguide grating. Applied Optics, vol. 49, no. 22, pp. 4321-4325, August 2010.
  5. A. Lehmuskero, I. Vartiainen, T. Saastamoinen, T. Alasaarela, and M. Kuittinen. Absorbing polarization selective resonant gratings. Optics Express, vol. 18, no. 26, pp. 27270-27279, December 2010.
  6. T. Saastamoinen, T. Alasaarela, A. Lehmuskero, I. Vartiainen, N. Heikkilä, and M. Kuittinen. Resonance waveguide reflectors with semi-wide bandwidth at the visible wavelengths. Optics Express, vol. 19, no. 3, pp. 2126-2132, January 2011.

Keywords: atomic layer deposition, waveguide, silicon photonics, grating, titanium dioxide

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Last update 2011-07-12