Electrical through-wafer interconnect technologies such as vertical through-silicon vias (TSVs) are essential in order to maximize performance, optimize usage of wafer real estate, and enable three-dimensional packaging in leading edge electronic and microelectromechanical systems (MEMS) products. Although copper TSVs have the advantage of low resistance, highly doped polysilicon TSVs offer designers a much larger range of processing options due to the compatibility of polysilicon with high temperatures and also with the full range of traditional CMOS processes. Large stresses are associated with both Cu and polysilicon TSVs, and their accurate measurement is critical for determining the keep-out zone (KOZ) of transistors and for optimizing downstream processes to maintain high yield. This report presents the fabrication and stress characterization of 400-μm deep, 20-Ω resistance, high aspect ratio (25:1) polysilicon TSVs fabricated by deep reactive ion etching (DRIE) followed by low-pressure chemical vapor deposition (LPCVD) of polysilicon with in-situ boron doping. Micro-Raman imaging of the wafer surface showed a maximum stress of 1.2 GPa occurring at the TSV edge and a KOZ of ∼9 to 11 μm. For polysilicon TSVs, the stress distribution in the TSVs far from the wafer surface(s) was not previously well-understood due to measurement limitations. Raman spectroscopy was able to overcome this limitation; a TSV cross section was examined and stresses as a function of both depth and width of the TSVs were collected and are analyzed herein. An 1100°C postanneal was found to reduce average stresses by 40%.
Optical anisotropy is an inherent property of columnar dielectric films, such as those fabricated by the glancing
angle deposition (GLAD) technique. This process utilizes physical vapor deposition combined with computer-controlled
substrate motion to finely tune the direction of column growth and vital morphological parameters
such as column cross-section and inter-columnar spacing. Control over the anisotropic properties of the porous
film provides an opportunity to design polarization-selective photonic devices and films with improved band
gap properties. Anisotropic defects in multilayer films also result in a polarization-sensitive position of resonant
transmission modes. We employed the finite-difference time-domain and frequency-domain methods to
theoretically analyze and design columnar films with unique band-gap properties. The following morphologies
were considered: (i) S-shaped columnar films with polarization-dependent band-gap position and width. Using
numerical simulations we have shown that the competitive effect of different sources of anisotropy can be used
to engineer photonic band gaps with strong selectivity to linearly-polarized light; (ii) Rugate thin films with an
anisotropic defect, which exhibit resonant mode splitting. Optical devices were fabricated using titanium dioxide
because it has good transparency in the visible range of the optical spectrum and a large bulk refractive index.
Experimental results were compared to simulations to verify the designs and understand the limitations of the
fabrication process.
Oblique evaporation of inorganic materials has long been used to induce alignment in liquid crystals, often for
the purpose of controlling the pretilt angle in a liquid crystal cell. These alignment layers are relatively dense,
keeping the liquid crystals above the surface of the inorganic layer. By evaporating at increasingly oblique angles
(> 80°), the alignment layer can be made porous, allowing liquid crystals to infiltrate the film and to align to
individual nanostructures. By coupling simultaneous computer controlled substrate motion during evaporation,
a process known as glancing angle deposition (GLAD), the nanostructures can be grown in a variety of useful
shapes, including helices, polygonal spirals, zigzags and periodically bent S-shaped columns. Alone, these films
exhibit properties such as linear and circular polarization selective Bragg reflection, and full three-dimensional
photonic bandgaps. By infiltrating liquid crystals into the voids of the film, one can align liquid crystals in three
dimensions, as well as tune and switch the film's optical properties. Additionally, the GLAD film can be used
to template polymerizable liquid crystals for subsequent monomer infiltration. In this work, using spectroscopic
ellipsometry, we examine the effects of liquid crystal infiltration on various film structures made from a variety
of metal oxides, for both varying film thickness and deposition angle. Techniques for filling a porous film with a
known volume of liquid crystals are also presented. Additionally, we examine the switching behaviour for these
films under applied electric fields. Finally, we compare experimental and simulated results used to predict and
optimize the optical properties of these hybrid films.
Porous thin films of dielectric materials have been deposited using e-beam evaporation onto substrates held at highly oblique angles (> 80o), coupled with simultaneous computer controlled substrate motion about two independent axes. This technique, known as glancing angle deposition (GLAD), enables the formation of shaped, isolated nanostructures, including vertical posts, zig-zags, and both helical and polygonal spirals, which exhibit chiral optical properties. GLAD films form the backbone of liquid crystal (LC) hybrid optical materials and devices, and afford key advantages. The
porous nature of the GLAD structures allows LCs to uniformly penetrate the film and modify its optical properties. Addition of LCs to GLAD films improves the properties of the films by reducing optical scattering, enhancing transmission, and accentuating existing chiral and linear optical anisotropies. Further, by mixing a dichroic dye with the LCs, polarization selective optical properties can be introduced into the film which can be used to augment the functionality of GLAD films. It has been found that addressing hybrid GLAD films with an electric field reorients the LCs, allowing one to switch the optical properties of the composite film. This behaviour extends to LCs mixed with dichroic dye, allowing one to switch the selective polarization properties with an applied voltage. Using results based on spectroscopic ellipsometry, we will examine the optical properties and switching behaviour of LC/dichroic dye hybrid
GLAD films and discuss how the results allow one to infer the alignment of LCs in GLAD films, as well how the addition of dichroism to the film affects the selective transmission of both linearly and circularly polarized light.
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