Dexian Ye

Designing nanostructures by glancing angle deposition

Three-dimensional nanostructures can be fabricated by the glancing angle deposition technique. By rotating the substrate in both polar and azimuthal directions, one can fabricate desired nanostructures, such as nano-rod arrays with different shapes, nano-spring arrays, and even multilayer nanostructures. This method offers a fully three-dimensional control of the nanostructure with additional capability of self-alignment. There is almost no limitation on materials that can be fabricated into desired nanostructures. In this presentation, we will discuss the current status of the glancing angle deposition technology, its potential applications, and its future challenges.

β-phase tungsten nanorod formation by oblique-angle sputter deposition

We report the creation of an unusual simple cubic β-phase W(100) nanorods with a pyramidal tip having four (110) facets using an oblique-angle sputter deposition technique with substrate rotation (also known as glancing-angle deposition). During the oblique-angle deposition, both β-phase W(100) and α-phase W(110) islands exist at the initial stages of growth. The β-phase W(100) islands grow taller due to the lower adatom mobility on these islands. The taller islands survive in the competition and form isolated nanorods in the later stages of growth. This is in contrast to the sputter deposition at normal incidence, where only the thermodynamically stable bcc α-phase W(110) polycrystalline films were formed when the film grows to a certain thickness.

Metal-coated Si springs: Nanoelectromechanical actuators

We demonstrated a nanoscale electromechanical actuator operation using an isolated nanoscale spring. The four-turn Si nanosprings were grown using the oblique angle deposition technique with substrate rotation, and were rendered conductive by coating with a 10-nm-thick Co layer using chemical vapor deposition. The electromechanical actuation of a nanospring was performed by passing through a dc current using a conductive atomic force microscope (AFM) tip. The electromagnetic force leads to spring compression, which is measured with the same AFM tip. The spring constant was determined from these measurements and was consistent with that obtained from a finite element analysis.

Very low-refractive-index optical thin films consisting of an array of SiO 2 nanorods

The refractive-index contrast in dielectric multilayer structures, optical resonators, and photonic crystals is an important figure of merit that creates a strong demand for high-quality thin films with a low refractive index. A SiO2 nanorod layer with low refractive index of n = 1.08, to our knowledge the lowest ever reported in thin-film materials, is grown by oblique-angle electron-beam deposition of SiO2. A single-pair distributed Bragg reflector employing a SiO2 nanorod layer is demonstrated to have enhanced reflectivity, showing the great potential of low-refractive-index films for applications in photonic structures and devices.

Uniform Si nanostructures grown by oblique angle deposition with substrate swing rotation

Slanted nano-columns and square nano-springs made of amorphous silicon (a-Si) were fabricated on bare Si and patterned substrates by oblique angle deposition with a back–forth substrate swing rotation mode. Scanning electron microscopy was used to characterize the grown nanostructures. The tilt angle of slanted nano-rods is determined by the incident angle of deposition flux and the azimuthal swing rotation angle of a substrate. The controlled substrate rotation affects the uniformity and the shape of the nanostructures. On the patterned substrate, the broadening of the size of individual nano-columns is greatly reduced and the nano-columns are not connected as they grow. A simple model based on decomposing the deposition flux is used to describe the effect of substrate rotation on tilt angle, uniformity, and the top-end shape of nanostructures. The feasibility of fabricating separated and well aligned nanostructures by our swing rotation method provides an effective and controllable way to fabricate nano-devices.

Low temperature melting of copper nanorod arrays

We report the melting of nanorod arrays of copper at temperatures much lower than its bulk melting point (1083°C). The Cu nanorods were produced by an oblique angle sputter deposition technique through a physical self-assembly mechanism due to the shadowing effect. The as-deposited nanorods were ∼2300nm in length, ∼100nm in diameter, and separated from each other with gaps varying between ∼10 and ∼30nm. The melting process was investigated through the analysis of scanning electron microscopy, transmission electron microscopy, and x-ray diffraction measurements. Start of premelting (or surface melting) has been observed to occur at annealing temperature ∼400°C under vacuum (10−6Torr). As the temperature was raised the arrays of Cu nanorods started to coalesce and formed a dense continuous film with a (111) texture at ∼550°C. The results of this work may be useful for low temperature soldering applications.

Magnetic properties of Co nanocolumns fabricated by oblique-angle deposition

The magnetic properties of columnar Co films on SiO2 substrates fabricated by oblique-angle incident thermal evaporation at room temperature were systematically examined by multiple techniques, including magnetic force microscopy (MFM), magneto-optical Kerr effect (MOKE), and scanning electron microscopy (SEM). Films with thickness ranging from 50 to 500 nm were deposited at the incident angles θ (with respect to substrate normal) from 0° to 85°. For films with thickness of ∼500 nm, the SEM shows the column tilt angle β increases as the θ angle increases and β 60° . The result implies that for θ>60°, the axis parallel to the incident beam behaves more like the in-plane easy axis. These magnetic anisotropies are correlated to the angular-dependent columnar structure of Co films.The magnetic properties of columnar Co films on SiO2 substrates fabricated by oblique-angle incident thermal evaporation at room temperature were systematically examined by multiple techniques, including magnetic force microscopy (MFM), magneto-optical Kerr effect (MOKE), and scanning electron microscopy (SEM). Films with thickness ranging from 50 to 500 nm were deposited at the incident angles θ (with respect to substrate normal) from 0° to 85°. For films with thickness of ∼500 nm, the SEM shows the column tilt angle β increases as the θ angle increases and β<θ. The MFM images show that for 55°<θ<75°, stripe domains are formed and are nearly parallel to the direction of incident vapor beam. The hysteresis loops obtained from MOKE show that along the direction perpendicular to the incident vapor beam the coercivity Hc stays almost constant for all θ angles and the squareness decreases as the θ increases. This is in contrast to the increase of Hc and the increase of squareness in the direction parallel to ...

Texture of Ru columns grown by oblique angle sputter deposition

Ru films were sputter deposited on native oxide p-Si(100) substrates under normal incidence and oblique angle incidence with and without substrate rotation. We characterized the crystalline texture and morphology of the Ru films by x-ray diffraction, transmission electron microscopy, and scanning electron microscopy. For the case of normal incidence, a smooth, uniform surface layer was observed, and pole figure analysis showed coexisting {101¯0}, {0002}, and {101¯1} normally oriented textures. For oblique angle incidence, we found that the films grown by uniform substrate rotation consist of isolated, vertical columnar structures with a clear pyramidal-shaped apex and display a normal {101¯0} fiber texture. Individual vertical columns were found to possess a single-crystal structure. In comparison, Ru films grown without substrate rotation possess a slanted columnar structure. They mainly show a tilted {101¯1}{101¯0} two-orientation (II-O) texture, with non-negligible {101¯0}{112¯0} and {0002} {112¯0} II-...

Physical properties of nanostructures grown by oblique angle deposition

Isolated three-dimensional nanostructures were grown on templated or flat substrates by oblique angle deposition with or without substrate rotation where the physical shadowing effect dominates and controls the structures. The mechanical and electromechanical properties of Si springs and Co coated Si springs were measured by atomic force microscopy. The electrical property of β-phase W nanorods were measured by scanning tunneling microscopy. Examples of measurements of the elastic property of springs, electromechanical actuation, field emission of electrons, and field ionization of argon gas are presented. Potential applications and improvements of growth of uniform nanostructures are discussed.

Molecular Caulking A Pore Sealing CVD Polymer for Ultralow k Dielectrics

Porosity has been introduced in existing low-k interlayer dielectrics to further reduce their dielectric constant. It is desirable to deposit a metallic layer on top of the porous dielectric by chemical vapor deposition (CVD). However this presents the challenge of preventing the precursor from penetrating into the porous dielectric and depositing metal within this insulating layer. In the present paper a low-k CVD polymer capping (Molecular Caulking) is deposited at room temperature onto the porous ultralow k dielectric methyl silsesquioxane. Experiments show that the Molecular Caulking prevents precursor penetration during subsequent metallorganic CVD. In addition, while the Molecular Caulking itself slightly penetrates into the methyl silsesquioxane, it does not appreciably increase surface roughness or film dielectric constant.