3D Silicon Nanophotonics
In this thesis we aim to provide a platform to develop 3D silicon nanophotonics.
The field of nanophotonics is steadily expanding in recent years but most of the current nanostructures remain planar or two dimensional (2D).
It is important to explore opportunities of three dimensional nanostructures in order to answer fundamental questions that arise when the material is sculptures in three dimensions such as an existence of a full photonic band gap – a range of frequencies for which light is forbidden to propagate inside a photonic crystal for any direction and any polarization. The interest to 3D nanostructure arises not only from fundamental science but also from industrial needs. Current state-of-the-art integrated circuits already have a 3D architecture despite being produced by multiple stacking of 2D layers. Stacking approach requires multiple alignment steps and thus it remains a challenge to fabricate 3D structures monolithically.
In Chapter 2 of this thesis we propose and demonstrate a novel single-step etch mask to fabricate 3D nanostructures by patterning oblique planes in one step. The desired pattern is written on two adjacent surfaces as a single image thus the alignment of the features located in different planes is ensured by design to circumvent the problem of overlay in present semiconductor processing. We show several the examples of etch masks fabricated using our single-step etch mask. The masks that are shown in the chapter are designed for fabrication of 3D photonic crystals. We show that the alignment between apertures written in different planes is about 3 to 5 nm, which is mostly given by the resolution of scanning electron microscopy.
In Chapter 3 we propose and demonstrate how to expand single-step etch mask to deep UV lithography and expose the apertures on the inclined slope of the silicon wafer. Having mask apertures on the oblique slope is a key to achieve a 3D structure after etching. We present the first experiments on the homogeneous coverage of the inclined slope with the photoresist layer. We show that with a single UV exposure it is possible to expose apertures up to 1.18 μm deep on the slope.
In Chapter 4 we discuss the etching of deep pores in Si. We start from considering the deep reactive ion etching process parameters for the etching of ultra-high aspect ratio pores in silicon. The process that we design allows to etch nano pores in silicon with aspect ratio as high as 42. We next consider etching of two perpendicular sets of pores in Si to form a 3D structure and the necessary adjustments to the etching process. We suggest to add the oxygen plasma cleaning step between the etching of the two sets of pores in order to remove polymer residuals left from the earlier processing steps. We fabricate and present 3D photonic band gap crystals with and without embedded defects.
In Chapter 5 we present the application of the zoom tomography to the noninvasive imagine of 3D photonic nanostructures. We provide the experimental scheme as well as theoretical description of phase retrieval algorithms needed for the reconstruction. We apply zoom tomography to 3D photonic band gap crystals and obtain 3D material density distribution throughout the fabricated crystals with ultimate 20 and 10 nm resolution. We also present a binary model that describes tomography data and allows to use them as an input for ab initio numerical models in order to compute optical properties free from assumptions.
In Chapter 6 we study the optical properties of 2D and 3D photonic crystals by means of broadband microscopic reflectivity measurements. We investigated polarization behavior of light reflected from the 2D photonic crystals with different configuration of polarizer and analyzer and observe for the first time a large birefringence within the stop bands. We interpret the polarization behavior of 2D photonic crystals with a model of a semi-infinite birefringent slab with the effective refractive index tensor . We obtain the components of the effective refractive index by fitting the model to the experimental data and therefore obtain the anisotropy of the effective refractive index. We study reflectivity of 3D photonic crystal structures with and without embedded defects. From the stop bands observed in a reflectivity we obtain structural properties of photonic crystal that we compare to the ones obtained from tomography experiment and find an excellent match. In the reflectivity spectra taken on the 3D photonic crystals with embedded defects we observe as expected a trough within the calculated band gap. We find good correspondence of the trough in the reflectivity peak with the spatial position of the cavity defect in the crystal.
The results obtained during this work provide a platform for further development of 3D silicon nanophotonics.
