Jeremy C. Sit

Thin Film Microstructure Control Using Glancing Angle Deposition by Sputtering

Thin films with microstructures controlled on a nanometer scale have been fabricated using a recently developed process called glancing angle deposition (GLAD) which combines oblique angle evaporation with controlled substrate motion. Critical to the production of GLAD thin films is the requirement for a narrow angular flux distribution centered at an oblique incidence angle. We report here recent work with low-pressure, long-throw sputter deposition with which we have succeeded in fabricating porous titanium thin films possessing “zig-zag,” helical, and “pillar” microstructures, demonstrating microstructural control on a level consistent with evaporated GLAD. The use of sputtering for GLAD simplifies process control and should enable deposition of a broader range of thin film materials.

Gradient-index narrow-bandpass filter fabricated with glancing-angle deposition

Glancing-angle deposition (GLAD) is a fabrication method capable of producing thin films with engineered nanoscale porosity variations. GLAD can be used to create optical thin-film interference filters from a single source material by modification of the film refractive index through control of film porosity. We present the effects of introducing a layer of constant low density into the center of a rugate thin-film filter fabricated with the GLAD technique. A rugate filter is characterized by a sinusoidal refractive-index profile. Embedding a layer of constant refractive index, with a thickness equal to one period of the rugate index variation, causes a narrow bandpass to appear within the filters larger stop band. Transmittance measurements of such a gradient-index narrow-bandpass filter, formed with titanium dioxide, revealed an 83% transmittance peak at a vacuum wavelength of 522 nm, near the center of the stop band, with a FWHM bandwidth of 15 nm.

Surface Area Characterization of Obliquely Deposited Metal Oxide Nanostructured Thin Films

The glancing angle deposition (GLAD) technique is used to fabricate nanostructured thin films with high surface area. Quantifying this property is important for optimizing GLAD-based device performance. Our group has used high-sensitivity krypton gas adsorption and the complementary technique of cyclic voltammetry to measure surface area as a function of deposition angle, thickness, and morphological characteristics for several metal oxide thin films. In this work, we studied amorphous titanium dioxide (TiO(2)), amorphous silicon dioxide (SiO(2)), and polycrystalline indium tin oxide (ITO) nanostructures with vertical and helical post morphologies over a range of oblique deposition angles from 0 to 86 degrees. Krypton gas sorption isotherms, evaluated using the Brunauer-Emmettt-Teller (BET) method, revealed maximum surface area enhancements of 880 +/- 110, 980 +/- 125, and 210 +/- 30 times the footprint area (equivalently 300 +/- 40, 570 +/- 70, and 50 +/- 6 m(2) g(-1)) for vertical posts TiO(2), SiO(2), and ITO. We also applied the cyclic voltammetry technique to these ITO films and observed the same overall trends as seen with the BET method. In addition, we applied the BET method to the measurement of helical films and found that the surface area trend was shifted with respect to that of vertical post films. This revealed the important influence of the substrate rotation rate and film morphology on surface properties. Finally, we showed that the surface area scales linearly with film thickness, with slopes of 730 +/- 35 to 235 +/- 10 m(2) m(-2) microm(-1) found for titania vertical post films deposited at angles from 70 to 85 degrees. This characterization effort will allow for the optimization of solar, photonic, and sensing devices fabricated from thin metal oxide films using GLAD.

Optical properties of porous helical thin films

Porous dielectric thin films, composed of isolated helical columns, are fabricated by the glancing angle deposition technique. The selective reflection of circularly polarized light and the optical rotation of linearly polarized light are investigated as a function of film material and helical morphology. The strongest chiral optical response is observed for titanium-dioxide films because of its large refractive index. Optical rotatory powers as high as 4.5 degrees are observed in 830-nm-thick helical films. By tailoring the pitch of the helical columns, the wavelength dependence of the circular reflection band is tuned to preferentially reflect red, green, or blue light, a promising quality for display applications.

Double-handed circular Bragg phenomena in polygonal helix thin films

Oblique-incidence physical vapor deposition has been used to create optical thin films with a polygonal helix-shaped nanostructure. A series of titanium dioxide thin films are investigated, including triangle, square, pentagon, and star-shaped polygonal helices. Experimental optical measurements reveal a double-handed circular Bragg response: at one frequency band a polygonal helix reflects left-handed circularly polarized light, and at a second frequency band reflects right-handed circularly polarized light. The relative wavelength dependence of each reflection band is determined by the physical structure of the polygonal helix, a property that is set during the thin-film deposition process. Spectral-hole polarization filters, produced by adding twist and spacing layer defects to polygonal helix thin films, are also reported.

Impact of morphology on high-speed humidity sensor performance

Capacitive-based humidity sensors were fabricated using unique nanostructured aluminum-oxide thin films. These sensors exhibited extremely fast desorption response times as short as 42 ms. In this paper, we present the effects of varying the thin-film porosity on sensor performance. Specifically, we look at the capacitive response and the desorption response time of the sensors. It was found that increased porosity tends to decrease the desorption response time and increase the relative humidity where the devices become sensitive.

Birefringence enhancement in annealed TiO2 thin films

Postdeposition thermal annealing is used to enhance the form birefringence of nanostructured TiO2 thin films grown by electron-beam evaporation using the serial bideposition technique. Thin films were grown on fused silica substrates using oblique deposition angles between 60° and 75° and repetitive 180° substrate rotations to produce birefringent thin films that are structurally anisotropic. Postdeposition annealing in air, between 200 and 900°C, was used to increase the form birefringence of the films by changing the TiO2 phase from the as-deposited amorphous state to a polycrystalline state that exhibits a greater inherent density and larger bulk refractive index. The optical properties, microstructure, and crystallinity were characterized by Mueller matrix ellipsometry, scanning electron microscopy, atomic force microscopy, and x-ray diffraction. It was found that the in-plane birefringence increased significantly upon thermal annealing, in some cases yielding birefringence values that doubled in magn...

Alignment and switching of nematic liquid crystals embedded in porous chiral thin films

Porous thin films with engineered microstructures have been fabricated using glancing angle deposition (GLAD). GLAD films with chiral microstructures have been previously shown to exhibit unique chiral optical response. The pores of these films were embedded with (non-chiral) nematic liquid crystals (LCs) to produce a new composite optical material wherein the GLAD film induces chiral nematic-like LC orientation. We demonstrate here reversible electro-optic switching of the LC component of these hybrid films. Unaddressed, cells of GLAD/LC hybrid films show enhanced chiral optic response compared with the unfilled GLAD film. When addressed, the chiral optic response vanishes.

Reactive Ion Etching of Columnar Nanostructured

A CF4 dry etch recipe for TiO2 was optimized for nanostructured thin films. The impact of our etching process and ultraviolet irradiation of nanostructured relative humidity (RH) sensors was studied. Reactive ion etching of titanium dioxide decreased device adsorption response time by opening high diffusivity channels while retaining the high surface area and high dynamic range of the interdigitated electrode device. The full electrical response (impedance and phase) and response time of our sensors was studied as a function of etch duration. Using a TiO2 etch recipe consisting of CF4 produced large changes to RH sensor electrical response and introduced a large hysteresis. As a result of significant microstructural change, the adsorption response time of the RH sensors is greatly improved from ap 150 ms to an instrument-limited 50 ms. The adsorption times are at least six times faster than previous, thinner sensors. However, the current sensors do not recover as well as previous sensors, possibly due to nodular defects observed here and which are absent in previous devices. Although sensor step desorption times improved from an unetched ap 130 ms to the instrument limit of 50 ms, full recovery times increased beyond ap 3 s. When the etch treatment was followed by a 48 h ultraviolet treatment, the hysteresis introduced by the CF4 etch was significantly reduced, without reducing the improvement in response time.

{\rm TiO}_{2}

Optimization of optical properties in liquid crystal (LC) devices requires control over the long-range orientation of the LC molecules. A variety of techniques are used to produce alignment in LC devices such as the familiar liquid crystal display (LCD). One well-known technique involves surface treatment of cell substrates by coating with thin polymer films such as polyimides that are subsequently rubbed. These treatments are used to produce a known orientation of the LC molecules at the surface. For example, rubbed layers may be used to align rod-like nematic LC molecules near the substrate with their long axes parallel to the direction of rubbing. The twisted nematic LCD, as an example, has the rubbing direction of the two substrates of the display cell perpendicular to each other to generate a 90 twist in the nematic orientation aided by a small amount of chiral dopant. A second technique for controlling LC orientation is the use of obliquely deposited thin films as alignment layers which generate a certain atilto of the LC molecules at the substrates. These types of orientation layer techniques influence LC orientation near the substrate surfaces only and are reliant on the LC molecules asettlingo into some minimum potential energy arrangement which gives the desired properties. The major drawback of these surface alignment layer techniques is that they are unable to provide significant influence on LC orientation further from the substrate surfaces. Maintenance of the desired LC orientation in thicker switching cells, particularly with chiral or cholesteric LCs (CLC), becomes especially difficult. This leads for instance to irreversible switching when the LCs do not settle back into their original arrangement under influence of the alignment layer after having been addressed. For better control over LC alignment, a technique which induces LC alignment throughout the cell is required, rather than influencing the LC orientation near the substrate surfaces only. This can be accomplished, for instance, by phase separation in polymer-dispersed liquid crystals (PDLC) or photopolymerization to afreezeo in the desired LC alignment. These techniques, however, may come at the sacrifice of the ability to switch the LC. A recent innovation in LC alignment was made by Robbie, Broer, and Brett who embedded LCs into porous thin films with engineered microstructures. These porous thin films are fabricated using glancing angle deposition (GLAD), a physical vapor deposition technique which allows the fabrication of highly porous thin films (Fig. 1) with columnar microstructure controllable on the sub-micrometer scale. GLAD uses highly oblique or glancing angle deposition (typically at vapor incidence angles of a > 75 , measured relative to the substrate normal) to accentuate the atomic shadowing effects, leading to thin films with porosities tunable from 10 % to 90 %. Computer control of substrate motion during