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%.
We have used the glancing angle deposition technique to fabricate highly porous nanostructured optical thin films that
act as humidity sensors. The responsiveness and repeatability of these sensors has been investigated for samples stored
under different environmental conditions. It has been found that samples stored in air have a more stable performance
than those stored in a dry nitrogen environment. It has also been found that annealing impacts the responsiveness of the
optical thin film sensors.
Transparent electrically-conductive nanoporous thin films can be used as electrodes to attract dye ions from solution to
modulate the reflectance of a surface. Here we demonstrate that this technique can be used to create diffraction gratings
that can be modified by the application of a small electrical potential with a selected spatial distribution. Using
nanoporous ITO films fabricated by Glancing Angle Deposition, we have produced variable diffraction gratings,
fabricated by a variety of methods, including lithography combined with etching techniques or direct patterning using
focused ion beam etching. We demonstrate modulation of the diffraction pattern by employing electric force to attract
the dye ions into the nanoporous electrode, thereby introducing a substantial local change in the effective refractive
index value and thus altering the resultant diffraction pattern and, in some cases, yielding diffractive orders that lie
between those associated with the underlying grating. These new orders are easily distinguished and their intensity can
be substantially modified by controlling the applied voltage. Because this technique can work with very small pitch
gratings, this approach has the potential to enable new applications that may not be readily achieved using conventional
liquid crystal technology.
We present a novel method of modulating total internal reflection (TIR) from an optical surface using a solution of dye ions in combination with a nanostructured electrode. Previous work using the electrophoretic movement of pigment particles to modulate TIR was limited by agglomeration of the pigment over time. Dye ions do not suffer from this limitation, but because of their small size they have significantly smaller absorption cross-section per unit charge than pigment particles which are generally two orders of magnitude larger. This significantly limits the maximum absorption caused by electrostatic attraction of the ions to a transparent conductive electrode. This can be overcome by using a transparent conductive nanoporous thin film as the electrode in which the porosity increases the effective surface area, allowing more dye ions to move into the evanescent wave region near the nanoporous transparent electrode and thus substantially increases the amount of absorption. In this paper, we demonstrate the modulation of TIR by observing the time-dependent variation of the reflectance as the dye ions are moved into and out of the evanescent wave region. This approach may have applications in reflective displays and active diffractive devices.
Titanium dioxide thin films were formed by electron-beam evaporation onto fused silica substrates using serial bideposition (SBD). The SBD technique combines rapid substrate rotation and oblique-angle physical vapor deposition (PVD) to create optical coatings that are composed of nanostructured columns which exhibit large birefringence values in the plane of the substrate. In this study, post-deposition annealing was used to crystallize amorphous TiO2 thin films formed by SBD to improve birefringence without significantly increasing optical absorption or scattering. Birefringent thin films were fabricated at deposition angles ranging from 60° to 75° and annealed in air at temperatures ranging from 200°C to 900°C to form anatase and rutile TiO2. Changes in the optical properties, crystallinity, and nanostructure were characterized by ellipsometry, x-ray diffraction, atomic force microscopy, and scanning electron microscopy. It was found that optical anisotropy increases strongly upon formation of anatase, yielding in-plane birefringence values that doubled from 0.11 to 0.22 in the case of TiO2 thin films deposited at 60° and annealed at 400°C. Raising the annealing temperature to 900°C to form rutile thin films increased the thin film birefringence further but also led to low optical transparency due to increased absorption and diffuse scattering.
Thin films with chiral or helical microstructures exhibit circular birefringence effects. Glancing angle deposition (GLAD) is a fabrication method capable of producing chiral thin films with controllable porosity and microstructure. In this paper, the effects of porosity on the circular birefringence exhibited by helical TiO2 films are presented. Transmittance measurements reveal two optimal film growth angles: one corresponding to a maximum in form birefringence and another corresponding to strong anisotropic scattering. Reflectance data support the transmittance measurements in the regime where scattering is minimized.
Optical studies of porous nano-engineered thin film materials fabricated using Glancing Angle Deposition (GLAD) have been a focal point of research since the inception of the GLAD technique over ten years ago. As the sophistication of porous nano-engineered thin film designs has increased over the years, photonic device applications of these materials have been explored. We will review some of our recent advancements in the study and fabrication of porous nano-engineered thin films for optical applications including our group's work with helical films and devices, square spiral photonic crystal films, and graded-index (GRIN) films and devices. Initial optical studies of helical films focused upon the circular Bragg effects and optical rotatory dispersion exhibited by such structures. In recent years, the exploration of different materials and the fabrication of liquid crystal (LC) cells using these films have brought the prospect of using such film-LC hybrids in display applications much closer. Helical films made from luminescent materials have also been investigated and were found to emit partially circularly-polarized light. Our work with square spiral structures focuses upon the fabrication of periodic arrays of such structures in order to yield a three-dimensional photonic bandgap. Our techniques also enable the formation of designed defects in the array with relative ease, opening the door to a myriad of potential applications. Finally, we will discuss graded-index structures which are made by varying the porosity of the film structure during film growth. Films of this nature have been designed and fabricated for use as wide-band antireflection coatings, rugate filters, spectral-hole filters, and optical humidity sensors.
Titanium dioxide was evaporated onto rotating substrates at highly oblique deposition angles to create thin films exhibiting a nanostructure which resembles a polygonal helix. Abrupt, periodic rotations of the substrate were used to create triangle, square, pentagon, and star-shaped film morphologies. Experimental optical measurements show that polygonal-helix thin-films exhibit double-handed circular Bragg phenomena. Unlike a standard chiral filter, a polygonal-helix thin-film reflects left-handed circularly polarized light at one frequency band and right-handed circularly polarized light at a second frequency band. The relative wavelength-dependence of the reflection bands is controlled by the angular rotation between arms of a polygonal helix. Spectral-hole polarization filters, produced by adding twist and layer defects to a polygonal helix, are also reported. Twist-defects tend to produce a narrow passband within both circular Bragg reflection bands of a polygonal helix, while a spacing layer defect can be used to produce a passband within only one of the reflection bands.
Porous thin films of TiO2 exhibit interesting and useful optical properties when the glancing angle deposition (GLAD) technique is used to impart controlled structural variations on the nanometer scale. Specifically, helically structured thin films possess optical properties sensitive to the polarization state of incoming light, including selective reflection of circular polarizations and optical rotation of the vibration ellipse of light as it passes through the film. By adjusting the deposition parameters, the helical structures can be transformed into vertically aligned columns with nanometer diameter variations. These films possess a continuously varying refractive index along the substrate normal. This index profile can be tailored so that it varies sinusoidally along the substrate normal to form a rugate interference filter. With the addition of a constant index layer of thickness equal to the sine period located in the center of the film, a narrow bandpass appears within the filter’s larger reflectance band.
Porous thin films have been fabricated by physical vapor deposition at an extremely oblique angle of incidence (85°). This deposition technique, called glancing angle deposition (GLAD), was used to create thin films composed of isolated helical columns. By investigating a variety of dielectrics, we found that helical GLAD films fabricated from titanium dioxide produce the strongest chiral optical response because of its large refractive index. Further improvements were made by using post-deposition annealing to form anatase and rutile polycrystalline phases of TiO2. By tailoring the pitch of the helical structures, the circular Bragg reflection band was tuned to preferentially reflect red, green, and blue light. The high porosity of a GLAD film (>50%) permits liquid crystals (LC) to be incorporated into the pores of the helical nanostructure, which creates chiral alignment in otherwise non-chiral LCs. This technique improves circular Bragg reflection and can create addressable hybrid materials with potential applications to high-efficiency reflective displays.
Porous thin film structures have been fabricated by physical vapor deposition at an incident flux angle that was typically greater than 80°. This deposition technique, often called glancing angle deposition (GLAD), was used to create thin films composed of isolated helical columns. Modification of the deposition parameters was used to control the porosity, the handedness, and the pitch of the helical structure. The high porosity of the GLAD film (>50%) permits fluids, and in particular liquid crystals (LC), to be incorporated into the voids of the nanostructure. We present the results of a study assessing the effect of film material, chiral morphology, and liquid crystalline material on the optical performance of helical GLAD films. Films fabricated from TiO2, a high refractive index material, exhibited strong optical rotation of linearly polarized light and selective reflection of circularly polarized light. By increasing the number of turns of the helix the chiral optical response was enhanced, and by tailoring the pitch of the helical columns, the wavelength-dependence of the reflection band was tuned to preferentially reflect red, green, or blue light.