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Micro/Nano Lithography

Fabrication and metrology of micro- and nano-optics

New techniques aim to improve fabrication and simplify the metrology of optical devices.
6 November 2007, SPIE Newsroom. DOI: 10.1117/2.1200710.0900

New applications of optical components and systems in many industries have necessitated improved methods for making and integrating micro- and nano-optics. At the same time, advanced fabrication tools have enabled optical scientists and engineers to develop smaller, more functional devices and modules at lower cost.

Research projects at the Optics Center of the University of North Carolina at Charlotte address both the fabrication and characterization of these optical devices. Here we highlight a few examples of research projects focused on micro and nano-optical fabrication and metrology.

Figure 1. Tungsten nanogratings obtained by chemical-vapor deposition using different laser exposure and scanning conditions: (a,d) Static exposure, (b,e) transverse grating, (c,f) longitudinal grating. Power and exposure time (scanning speed) are varied. (Courtesy of T. Her).

Tsing-Hua Her and his team have developed a new direct-write laser-assisted chemical-vapor-deposition process to generate feature sizes of λ/5 and to process multiple features simultaneously.1 Inside a vacuum chamber at room temperature, a 400-nm, 150-femtosecond-pulse laser beam is focused onto a surface containing a tungsten hexacarbonyl [W(CO)6] precursor. One-dimensional, grating-like structures of tungsten spontaneously form on the substrate without using techniques such as holography or masking.

Examples of these tungsten nanogratings (TNGs) are shown in Figure 1. Atomic force microscopy images confirm that they are surface-relief features arising from tungsten deposition. Linear scanning of the beam relative to the substrate produces TNGs with excellent long-range order.

Figure 2. Surface texturing to improve light-emitting diode efficiency: a) pseudo-random nano-texturing in sapphire, b) electron-beam lithography on sapphire, c) nano-imprint lithography template, d) sample made using the nano-imprint template. (Courtesy of T. Suleski)

Figure 3. Measurements of the same lens using liquids with different indices of refraction. The top row shows the interferometric fringes and the bottom row shows the corresponding form measurements. n: index of refraction. m: measurement.

In an effort to increase light-extraction efficiency from high brightness ultra-violet light-emitting diodes (LEDs), Thomas Suleski and his group are developing methods of nano-texturing sapphire, as shown in Figure 2. The external quantum efficiency is limited largely by internal reflection of light at the interface between the LED material and the surrounding medium.2,3 The light-extraction efficiency can be enhanced by incorporating, on the LED surface, sub-wavelength-scale structures that reduce the effective refractive index of the layer and enable it to act as an anti-reflection layer. Possible design approaches include both one- and two-dimensional gratings, as well as two-dimensional photonic-crystal geometries.

Although fabrication techniques such as electron-beam lithography or diamond turning machines may not be scalable to large volume production, they can be used to make an individual mold template for nano-imprint lithography. Using this template in "step-and-repeat" mode across large wafer areas enables manufacturing of devices that would not otherwise be practical.

As fabrication technology improves, measuring the resulting micrometer- and nanometer-scale optical features becomes more challenging. Currently, most characterization of micro- and nano-optical devices is done one component at a time. A research program at the Optics Center aims to develop techniques to characterize many optical components in parallel.

Matthew Davies and Faramarz Farahi have developed a simple way to use a plane wave to measure simultaneously the overall geometry and surface-figure errors of an area containing many micro- and nano-optics devices. Defects on individual micro lenses in a large array were identified using a liquid with a specific refractive index next to the micro-lens array to control the degree of refraction at the lens surface. This technique is particularly effective for devices with high aspect ratios and large slopes.4

Faramarz Farahi
Physics and Optical Science
University of North Carolina at Charlotte
Charlotte, NC

Faramarz Farahi is a Professor and the chair of Department of Physics and Optical Science at the University of North Carolina and a member of Center for Optoelectronics and Optical Communications. Dr. Farahi has more than 20 years of experience in applied optics. His current research interests include integrated optical devices and systems and optical metrology.