Etching with Gold Enables Improved Deep-UV Antireflective Optics
Transmission and reflection characteristics get a boost without coatings.
Lynn M. Savage
The ability to diffuse or eliminate reflections is an important part of optics involving ultraviolet and deep-UV wavelengths. Antireflection coatings, which form interfering structures of low and high refractive indices, are ubiquitous. However, such coatings can be mechanically unstable and, because there are a limited number of refractive indices available to choose from in coating materials, it is difficult to find pairs that work in important wavelength bands.
As an alternative to antireflection coatings, techniques such as electron-beam writing and mask lithography are used to inscribe interference patterns directly onto optical materials. This process avoids some of the pitfalls of coatings but can be slow, expensive, applicable over only small areas of optical material and nearly impossible to execute on nonflat optics, such as lenses — especially for deep-UV applications, where very small features must be etched.
A schematic illustrates a method developed to improve antireflection in a fused-silica optic. A polymer mixed with toluene and tetrachloroaurate is used to coat the substrate, thereby spreading gold nanoparticles across the optical surface via micellular structures (A). A hydrogen plasma treatment ablates the polymer, leaving behind the gold particles (B). Finally, a reactive ion etching process ablates the silica itself, pushing the goldparticles into the substrate (C). Images courtesy of the American Chemical Society.
Recently, investigators at Max Planck Institute for Metals Research in Stuttgart and their colleagues at Carl Zeiss AG in Jena and in Oberkochen, all in Germany, developed a method that uses gold nanoparticles as a lithographic mask to inscribe a pattern onto a fused-silica substrate. The group, led by the institute’s Joachim P. Spatz, used a technique called block copolymer micelle nanolithography to drive the etching process.
The researchers began by mixing a polystyrene-based block copolymer with toluene to form spherical micellular structures. To this mixture, they added tetrachloroaurate to introduce gold particles about 7 nm in diameter. They then coated fused-silica substrates with the solution by dipping them and withdrawing them slowly. The micelle structures ensured that the gold particles were spaced homogeneously across the substrate.
Next, the group ablated the polymer with a hydrogen-plasma treatment. After the polymer completely wore away, the researchers used a reactive ion processing step to etch the substrate itself. During this process, the gold particles sank into the silica, forming hollow conelike pillars as, eventually, the gold wore away. The hollow portion of each pillar was about half the total height of the structures.
The array of conelike pillars remaining after the gold nanoparticles are completely eroded gradually changes the refractive index between the air and the solid fused-silica substrate, enabling antireflective properties to be built into the optic directly.
The substrate gains antireflective properties because the pillars enact a gradual change in refractive index between the air above and the solid silica below.
Atomic force microscopy imaging of the substrates showed that the pillars were 116 ±8 nm tall and spaced 110 ±7 nm apart. Tests with larger particles resulted in smaller gaps between the pillars as well as broader rims. It also made the gold harder to eliminate. Smaller particles resulted in a reduced structural aspect ratio and less hollow pillars.
To test the technique, the investigators performed transmission and reflection measurements of the treated flat fused-silica substrates, using a PerkinElmer spectrometer and an ellipsometer made by J.A. Woollam Co. Inc. of Lincoln, Neb. They found that maximum transmittance occurred at 400 nm and that the reflectivity was held to 0.7 percent. They also found good results using the technique on the convex side of a fused-silica lens. For UV applications, they measured increases in transmission of about 5 percent at 193 nm.
According to the researchers, the method is suitable for a variety of optical materials but only with CaF2, MgF2, fused silica and quartz for deep-UV applications.
Nano Letters, ASAP Edition, April 16, 2008, doi: 10.1021/nl080330y.
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