Fabrication of Hyperbolic Metamaterials using Atomic Layer Deposition

This thesis describes the technological development, design and fabrication of hyperbolic metamaterials (HMMs) - one of the most unusual classes of articial electromagnetic subwavelength structures. The thesis begins with the review of optical metamaterials. Starting with Maxwell's equations the concept of hyperbolic medium is explained. Metamaterial design, implementation as well as possible applications are reviewed. Electrodynamically, HMMs are described by a dielectric permittivity tensor ε with components of opposite signs (e.g. ε_x = ε_y < 0, ε_z > 0). HMMs possess unusually high wavevector, optical density of states, and anisotropy, leading to a wide variety of potential applications such as broadband enhancement in the spontaneous emission for a single photon source, sub-wavelength imaging, sensing, thermal engineering, and steering of optical signals. HMMs have a potential to be a robust and versatile multi-functional platform for nanophotonics in the broad range of operating wavelengths from visible to THz regions and even at microwave region. Despite the proposed architecture of hyperbolic media, which geometry includes simple metal/deielctric multilayers and metallic wires incorporated in dielectric host, the fabrication is still challenging, since ultrathin, continuous, pinhole free nanometer-scale coatings are desired. The required high-quality thin layers have been fabricated using atomic layer deposition (ALD). It is a relatively new, cyclic, self-limiting thin film deposition technology allowing thickness control on atomic scale. As the deposition relies on a surface reaction, conformal pinhole free films can be deposited on various substrates with advanced topology. This method has been a central theme of the project and a core fabrication technique of plasmonic and dielectric metamaterial components. 
The deposition characteristics of the simplest and most studied ALD processes, Al₂O₃ and TiO₂ were studied as a starting point. Later, the growth and characterization of ZnO semiconductor and Cu metal have been explored. 
The ability to reproducibly deposit conformal TiO₂ and Al₂O₃ dielectric coatings has been implemented in optical experiment where the effective medium approximation theory (EMA) was tested. Flat dielectric multilayers with strict thicknesses of either 10 or 20 nm of each individual layer, were deposited on Si substrates and characterized using x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM).
The next milestone was a technology development where the ALD dielectric coatings could be placed as individual, separate units in one or two dimensional lattices (gratings or pillars). This was achieved by combining ALD, deep ultra-violet (UV) stepper lithography and advanced silicon deep reactive ion etching (DRIE) techniques. Three different high aspect ratio, freestanding Al₂O₃ and TiO₂ structures have been successfully manufactured: gratings, pillars and poly-cylindrical arrays. The challenge of Si template fabrication using DRIE and isolation of the final structures using selective Si etch were addressed.
Fabrication of HMMs requires the implementation of plasmonic components, and traditionally noble metals are used for such purposes due to their abundant free electrons in the conduction band. However, their large real and imaginary parts of the permittivity, especially in the infrared range, result in high loss and weak connement to the surface. Additionally, the most implemented metals in plasmonics such as Au and Ag are diffcult to pattern at nanoscale due to their limited chemistry, adhesion or oxidation issues. Therefore the implementation of alternative plasmonic materials suitable for certain wavelength range has been the focus of this work.
Transparent conductive oxides such as Al-doped ZnO (AZO) have attracted significant attention as alternative plasmonic materials, due to their low loss and metallic behavior in the near/mid infrared range. One more advantage of AZO is the possibility of tuning the permittivity by design, by deciding the dopants or the ratio of different components, thus constituting an advantage over metals having fixed permittivity values. AZO was chosen since the Cu ALD showed up to be far less successful in terms of reproducibility and conformality requirements. AZO has been grown on different substrates in the temperature range 150-250 °C and optical, electrical and physical properties have been clarified.
Finally, HMMs with two different geometries has been realized, AZO trenches and AZO pillars standing in a dielectric host (air or Si). Furthermore, it has been proposed that high aspect ratio grating structures with AZO lamellas in a silicon matrix function as a versatile platform supporting both surface and volume infrared waves. By selective etching of Si the performance of the whole structure can be reconfigured. In other words a bi-slab HMM has been suggested, where the effective properties of the structure are controlled by the thickness of the top slab (etching depth).