Using advanced lithography to pattern nano-optic devices
09/01/2005
Nano-optics - a class of highly compact, high-performance optical components that are readily customized and easily integrated with other optical devices and electronics - are of significant interest to both electro-optic circuit designers and manufacturers. Nano-optics obtain their unique properties from the physical interaction of light with nanoscale structures - gratings or similar structures with key dimensions much smaller than the wavelength of light being controlled. For visible light (used for digital imaging and display applications) and near-infrared (IR) wavelengths (used for some optical storage, sensor, and communications applications), this requires a capability of creating structures with dimensions on the order of tens to a few hundreds of nanometers with an accuracy of 10nm or less.
Fabricating physical devices with such fine-scale structures in a way that can be flexibly applied to various structural shapes presents the challenge of developing nanolithography techniques that support high-fidelity pattern replication with accuracies of a single nanometer. In general, materials can be formed into nanoscale structures by either bottom-up methods (built or grown molecule by molecule) or top-down methods (by etching the pattern into a deposited material).
 Figure 1. Illustration of a typical multistep, semiconductor-like nanopattern transfer process for a nanostructure-based optical device. |
The most flexible method in commercial application is a top-down approach referred to as “nanopattern transfer,” which uses contact lithography to replicate 2D nanostructures. Figure 1 shows the following basic process:
- A substrate is prepared by depositing a thin layer (usually 50-500nm thick) of a target (dielectric, metal, or polymer) material for nanostructuring.
- A pliable polymer resist is spun on top of this surface.
- A mold inscribed with the complement of the desired nanoscale pattern is brought into contact with the polymer layer, causing the polymer to conform to the shape of the mold.
- The polymer resist is set, using ultraviolet (UV) or heat techniques, and the mold is removed.
- The patterned resist is used as an etching mask for transferring the pattern to the target material.
The commercialization of this manufacturing process has required innovation in mask creation, polymer resist formulation, and lithographic patterning, as well as compatible deposition and etching processes, for a variety of materials including dielectrics, metals, plastics, and thin films. In particular, other, more basic techniques must be used to create the original mold. Electron-beam lithography, interference lithography, or complex multistep processes are commonly used for pattern transfer.
One benefit of nanopattern transfer is that these complex, expensive, and sensitive techniques need only be applied once to make the master mold - there is no need for them to be applied to each wafer being processed. For commercial applications, the original or master mold is not used in wafer processing; instead, it is replicated to create clone molds for use in manufacturing, thus extending the useful life of the master.
The polymer resist acts as an intermediary in the transfer of the pattern to the target material, so the masking step is relatively independent of the target material. This is in contrast to direct embossing methods, for instance. The formulation of the polymer resist is critical. It determines the pressure, temperature, and duration of the pattern transfer operation from the mold to the resist. Desirable attributes include near-liquid flow properties, relatively low temperature or UV setting, high uniformity, and an ability to support relatively high aspect-ratio etching. The latter is important because it is desirable to transfer the nanostructure pattern to the resist with a modest relief compared to the one in the target material after etching. The result is a simpler, more rapid pattern transfer to the polymer layer.
Because of the nanostructures’ scale, to achieve uniform, defect-free optical performance and surface quality, the target layer material must be deposited at a depth of only a few hundred nanometers with a high degree of homogeneity across the substrate wafer. It must also be etched with similar requirements for uniformity. These critical considerations can be achieved in volume production with proper process selection and control.
While nanopatterning allows the replication of many nanostructure shapes, additional processing steps are required to create a commercially usable optical device. Complete optical design uses a thin-film layer under and over the nanostructure layer to provide index matching, antireflection coatings, and, possibly, other optical functionality. The materials used for the nanostructures and their design are specified to produce uniform optical performance, as well as to optimize manufacturing yield in the face of slight variations in the nanostructure dimensions, material properties, and etching speed. By using conformal deposition to fill air channels in the grating and by selecting materials to match the coefficient of thermal expansion between the nanostructure layer and the substrate, robustness and environmental stability of the nanostructure layer are improved. The resulting general nano-optic device structure is shown in Fig. 2.
 Figure 2. A general nano-optic device is a sandwich of nanostructured layers, possibly arrays, and thin-film layers. |
Overall, the materials and design choices discussed above allow all basic passive and dynamic (or tunable) optical functions to be realized across wavelength ranges from deep UV through the visible spectrum, up to long-wavelength IR. Functions that can be implemented include polarizers, wave plates or retarders, bandpass and narrowband spectral filters, lenses, and antireflection or diffusive coatings.
By fabricating nano-optics using wafer-scale, semiconductor-like manufacturing processes, the devices become intrinsically integratable. Complex optical functions can be created by monolithic self-integration. For example, nanostructured optics can be stacked to serially process light and concatenate multiple optical functions or filters, and they can be arranged in pixelated arrays to produce different effects depending on where the light enters the optic. The latter is useful for display, multiwavelength switching, and digital imaging applications. Hybrid integration of nano-optic layers with other optical materials and electronic circuits allows for application of thin nanostructures to the surface of other materials or directly in electro-optic circuits.
Innovative nanoscale, nanostructure manufacturing brings the benefits of smaller size, lower cost, improved reliability, and improved performance to applications spanning CD/DVD optical drives, digital imaging, projection display, and optical communications. Although this article has focused on nanostructure-enabled op-tics, the manufacturing methods that create nanostructured materials have further applications in realizing hybrid materials, microfluidics, and semiconductors. These new manufacturing methods are creating new opportunities for electro-optic product design, device integration, and manufacturing efficiency, with exciting potential that continues to be explored.
For more information, contact Hubert Kostal at NanoOpto Corp., 1600 Cottontail Ln., Somerset, NJ 08873; ph 732/627-0808, e-mail [email protected].