Martin O. Jensen

Periodically structured glancing angle deposition thin films

Thin films fabricated using the glancing angle deposition technique have a porous microstructure consisting of freestanding columns. Many promising applications of such thin films require that the columns be arranged in periodic arrays using substrate topographies-so-called seed layers-that enforce controlled film nucleation. In this paper, we present the optimized design, fabrication, and characteristics of periodically structured thin films, achieving highly uniform periodic film morphologies. We derive geometrical rules for designing substrate seed layers, and explain how to fabricate large area seed patterns with submicrometer features. Using negative-resist electron-beam lithography and laser direct write lithography, we have reached extremely high pattern densities. An experimental analysis is provided of seed-enforced nucleation and thin-film growth, showing that the elimination of film growth between seeds is crucial, and that the substrate seed layer geometry must match the intended film microstructure. Finally, we discuss the enhanced properties of periodically structured oblique angle thin films and their applications.

High-resolution pattern generation using the epoxy novolak SU-8 2000 resist by electron beam lithography

We report the fabrication of high-resolution sub 50 nm patterns by electron beam lithography using the epoxy novolak SU-8 2000 resist formulation. The minimum linewidth achieved is on the order of 30 nm and corresponds to a threefold reduction in minimum linewidth over previous reports describing similar resist chemistries. Our results also show that it is possible to fabricate dense linear grating elements without proximity correction. The dry etch resistance of native SU-8 2000 was found to be nearly twice that of poly(methylmethacrylate), making it ideal for applications that require pattern transfer. These studies are intended to explore the feasibility of SU-8 2000 as an electron beam resist for pattern generation on length scales below 50 nm.

Square spiral 3D photonic bandgap crystals at telecommunications frequencies

We present evidence of complete, three-dimensional photonic bandgaps in obliquely deposited thin films with a porous microstructure of tetragonally arranged square spirals. We further present a capability to engineer the bandgap center to wavelengths as low as 1.65 mum, with bandgap widths of up to 10.9%. Using new deposition methods that provide detailed control over the photonic crystal dimensions and morphology, this approach allows advanced photonic crystal architectures to be realized over large scales with uncomplicated fabrication technology.

Embedded air and solid defects in periodically structured porous thin films

Highly porous thin films with columnar microstructures are applicable to many optical, chemical, and electronic devices, and can be fabricated using the glancing angle deposition method for physical vapour deposition onto tilted substrates. In a recent advancement of this method, it was shown that decoupling of the vapour incidence direction from the column growth direction provides significant flexibility to engineer the pore structure of the films. Here we elaborate on the decoupling principle by applying it to chiral thin films with a periodic microstructure, and demonstrating how it leads to improved film uniformity and the elimination of column broadening. We also show the effects of such depositions onto substrate seed layers with intentional defects. Substrate based defects normally transfer to thin films as air filled defects, but here we present for the first time that a simple adjustment of the deposition parameters can invert the normally air filled defects to become solid, evaporant filled defects. This defect engineering capability provides new opportunities for the deployment of glancing angle deposition thin films to photonic bandgap crystals and microfluidic devices.

Three-dimensional square spiral photonic crystal nanostructures by glancing angle deposition

We demonstrate the fabrication of a three-dimensional photonic band gap nanostructure using the unique Glancing Angle Deposition (GLAD) thin film technique. This innovative photonic structure is based on a tetragonal, square spiral symmetry found in FCC and diamond lattices and can be implemented in a virtual single-step fabrication process using GLAD. A highly porous film with engineered nanostructure results from the implementation of advanced substrate motion during the deposition, and three-dimensional periodicity required for a photonic crystal can be obtained when a square spiral staircase is fabricated on a tetragonally seeded substrate. We present successful fabrication of three-dimensional square spiral structures along with some examples of the unique capabilities of these nanostructured thin films.

Fabrication of Periodic Arrays of Nanoscale Square Helices

We demonstrate fabrication of periodic arrays of nanometre scale square helices, with potential applications in three-dimensional photonic bandgap (PBG) materials. Processing is performed using a thin film deposition method known as Glancing Angle Deposition (GLAD). Through advanced substrate motion, this technique allows for controlled growth of square helices in a variety of inorganic materials. Organization of the helices into periodic twodimensional geometries is achieved by prepatterning the substrate surface using electron beam lithography. The regular turns of the helices yield periodicity in the third dimension, perpendicular to the substrate. We present studies of tetragonal and trigonal arrays of silicon helices, with lattice constants as low as 300 nm. By deliberately adding or leaving out seeds in the substrate pattern, we have succeeded in engineering line defects. Our periodic nanoscale structure closely matches an ideal photonic band gap architecture, as recently proposed by Toader and John. While our fabrication technique is simpler than most suggested PBG schemes, it is highly versatile. A wide range of materials can be used for GLAD, manipulation of lattice constant and helix pitch ensures optical tunability, and the GLAD films are robust to micromachining.

Enhanced Control of Morphology in Thin Film Nanostructure Arrays

Glancing angle deposition (GLAD) was used to grow thin films of silicon and titanium dioxide slanted post nanostructures onto periodically patterned substrates. The patterned substrates consisted of tetragonal arrays of small hillocks with periodicities of 100, 200, and 300 nm. An advanced substrate rotation algorithm called PhiSweep was used during the deposition. The PhiSweep algorithm consists of rotating the substrate back and forth such that the arriving vapour flux direction alternates from either side of desired column tilt direction. This reduces the anisotropy of the shadowing conditions, which diminishes column fanning. The tilt angle of the columns is affected by the PhiSweep parameters, which is important in applications such as square spiral photonic crystals. This relation is derived and confirmed with tilt angle measurements of the slanted post films. The films grown using the PhiSweep method were compared with similar films grown using traditional GLAD. The PhiSweep technique produced films which conformed to the initial periodic pattern much better than the films grown with traditional GLAD, enabling the growth of nanostructure arrays with smaller periodicities.

Microfabrication of chiral optic materials and devices

Chiral thin films have been demonstrated to have significant optical activity and device applications for gratings, filters, retarders and optical switches. These helically nanostructured films may be microfabricated onto silicon or other substrates utilizing the Glancing Angle Deposition (GLAD) technique with various nanostructures such as helices, chevrons, or polygonal spirals. GLAD is a simple one-step process that enables ready integration of these structures onto optical chips. As proposed by Toader and John, the GLAD technique can be used to fabricate large bandwidth photonic crystals based on the diamond lattice. This structure yields a predicted photonic bandgap as much as 15% of the gap center frequency. Moreover, the corresponding inverse square spiral structure is predicted to have a photonic bandgap as much as 24% of the gap center frequency. We report the details of basic chiral thin film fabrication and calibration. We will also discuss optical characteristics of the chiral films such as the optical rotatory power. Finally, we present the results of our efforts to fabricate square spiral and inverse square spiral structures.

Defect and bandgap engineering in square spiral photonic crystals

The existence of three-dimensional photonic bandgaps in square spiral thin films, made using the Glancing Angle Deposition (GLAD) method, was recently verified. We further demonstrate the flexibility of the GLAD process to fabricate silicon photonic bandgap crystals with customizable bandgap centre frequencies. GLAD combines physical vapor deposition at highly oblique flux incidence angles with dual axis substrate motion control, creating porous thin films with three-dimensional submicrometer topographies. This makes it a near-ideal approach for diamond lattice based photonic crystal fabrication, with manipulation of the photonic properties through the deposition parameters. We have produced a range of different square spiral thin films, and present characterization results indicating bandgaps at wavelengths close to 2 micrometers. Such low wavelength bandgaps have not previously been achieved for square spiral architectures. Ongoing work towards optimization of the process holds the promise of square spiral photonic crystals with even lower bandgap centre wavelengths, approaching the telecommunications windows. In addition to its flexibility and mass-production suitability, we present how GLAD can be used to engineer defects inside the photonic crystals during the one-step growth process. Such defects may potentially be employed as stand-alone waveguides, or as elements of more complex photonic device and circuitry designs involving subsequent micromachining of the GLAD thin films.

Dye Sensitized Solar Cells Using Nanostructured Thin Films of Titanium Dioxide

Dye sensitized solar cells (DSSCs) were fabricated using porous thin films of TiO 2 . These films were deposited by electron beam evaporation and an advanced substrate motion technique called PhiSweep. PhiSweep, an extension of glancing angle deposition (GLAD), allows for greater control over the surface area of nanostructured thin films than is possible with traditional GLAD. The as-deposited films were amorphous, so the films were annealed to improve their crystal structure. The films were sensitized with a photoactive dye and implemented into a DSSC configuration as the electron collecting electrode. It was expected that the higher surface area of the films produced using the PhiSweep method would improve the cell performance compared with cells made using traditional GLAD films of TiO 2 . However, the performance of the cells prepared using PhiSweep films was likely hindered by higher internal resistance of the films compared to the films prepared by traditional GLAD. The highest photoelectric conversion efficiency of the dye sensitized solar cells produced using the PhiSweep method was 1.5%.