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Micro/Nano Lithography

Nanostructures benefit telecom

Subwavelength optical elements offer economical, easily manufacturable, compact realization of telecom components.

From oemagazine August 2002

31 August 2002, SPIE Newsroom. DOI: 10.1117/2.5200208.0004

Enabled by nano-imprint lithography fabrication, a new generation of optical components incorporating subwavelength optical elements (SOEs) offers size and cost/performance advantages over existing bulk optic components. Nano-imprint lithography and SOEs facilitate the efficient creation of integrated optical components, promising significant cost reductions in current optical networking systems and opening a path to new levels of overall integration and functionality.

cutting cost

Non-wireless communications service providers worldwide face the imperative to reduce their operating costs and service prices while simultaneously accommodating ever-growing traffic demands. While optical networking provides the structure and capacity to handle the traffic, systems must surmount significant technology challenges to achieve the cost reductions that communications service providers require.

Optical networking is expensive in large part because optical components themselves are expensive. Today's optical devices lag far behind their semiconductor counterparts in integration, density, diversity, manufacturability, availability, and reliability. There are several key reasons for this developmental disparity. As of now, multiple technologies are needed to provide the various optical effects required in optical networking. This in turn requires multiple, discrete manufacturing technologies, which limits production efficiencies and drives up individual device costs. To make matters worse, in contrast to the high flexibility and reusability of semiconductor design elements, the natural optical properties of many discrete optical elements are relatively fixed, which limits options for the component designer and reduces design transferability.

The general lack of integrated functionality at the device level also drives up the cost of optical-component and system design as well as assembly. While electronics manufacturing has evolved from physical assembly of discrete components to the high-volume manufacture of multi-million-transistor devices, optical devices are generally still assembled from discrete components, a comparatively slow, laborious, costly, and low-yield process.

The two approaches currently used for optical-component integration—hybrid and monolithic—both display distinct limitations. Hybrid integration is a packaging-based approach, bringing together carefully selected, compatible optical components and integrating them using a precise structure, such as a silicon optical bench, to create sophisticated integrated optical modules. Monolithic integration is more manufacturing-based: A single device technology is self-integrated, usually within a wafer-based manufacturing process, to create integrated functionality. Hybrid integration can take advantage of the "best of the best" in individual devices and combine them with great flexibility but at a high cost in terms of complexity of design and manufacturing. Monolithic integration offers the advantages of simplified manufacturing and high component density but faces limitations in the diversity of optical functions that can be achieved through a single technology.

Taking integrated optical-component design to the next level of density, cost, and reliability will require an enhanced set of optical element "building blocks" that can provide a broad range of tailored optical processing functionality. These discrete building blocks must have individually useful optical properties and easily integrate with other optical materials in a broad range of configurations. In addition, these building blocks should self-integrate to allow increased flexibility in optical-component design while reducing parts counts and increasing reliability in optical components and systems.

SOEs possess exactly these properties and are now being commercially introduced. These SOEs are the realization of nanotechnology applied to optical elements. Through manipulation of the size, shape, and period of nanostructures in the optical beam path, SOEs deliver a broad range of useful optical effects.

design of SOEs

Figure 1. A basic 1-D SOE interacts with light based on a subwavelength grating structure. Manufactured discrete SOEs (inset) are optical chips that can be sized to meet application requirements. Those shown here are 1.4 mm x 1.4 mm x 0.5 mm.

SOEs consist of nanoscale structures with critical dimensions much smaller than the wavelength of the light passing through them, arranged in periodic patterns on an optical substrate (see figure 1). For applications at the wavelengths used for optical transmission, the useful dimensions of these structures range from 10 to several hundred nanometers. To achieve their intended optical effects, SOEs need be only a fraction of a wavelength thick. By contrast, bulk optical elements typically involve structures many times larger than the wavelengths of light, both to provide the desired optical behavior and to facilitate manufacturability.

In the simple, 1-D subwavelength grating structure, adjusting the size, shape, and spacing of the grating structure varies the effect on the transmitted light to create a broad range of optical effects. SOEs can also be constructed with nanostructures of two or three dimensions. Depending on the configuration of the nanostructures and the constituent materials used in their construction, SOEs can function as polarizers, polarization beam splitters and combiners, waveplates, filters, photodetectors, phase modulators, microlenses, and more.

When subwavelength structures interact with light, the equations describing conventional optical behavior do not fully cover the resultant phenomena because both single-electron and quantum mechanical effects can come into play. For most applications, subwavelength structures are subwavelength diffraction gratings whose behavior is described by rigorous application of diffraction grating theory and Maxwell's equations.

Refraction, for example, is a key property exploited in optical components. Usually, engineers must use different materials to obtain different refractive indices. In SOEs, however, different refractive indices can be obtained with the same material by adjusting the grating filling factor. For a diffraction grating, if a is the grating period, and t is the grating width, then the effective refractive indices of the TE wave nte (the electric vector parallel to the grating grooves) and the TM wave ntm (the electric vector normal to the grating groove) are expressed as:

nte2 = f n12 +(1–f ) n22

ntm2 = n12 n22 / ( f n22 +(1–f ) n12 ),

where n1 is the dielectric constant of the grating material, n2 is the dielectric constant of the fill material, and f is the grating fill factor defined as f = t/a. Adjusting these physical parameters yields an effective birefringence that is much larger that those achieved with standard components. The practical application is a polarization beam splitter that achieves 180° of separation with a grating layer of less than 500 nm.

nano-imprint lithography

Generally speaking, nanoscale structures can be created by a number of methods, including electron-beam lithography, holographic lithography techniques, and others. Each of these approaches has practical limits for commercial application, including cost, complexity, yield, volume, and limited application to the production of diverse structures. To build SOEs for high-volume, low-cost commercial applications, the challenge has been to develop a fabrication technology that approaches the efficiency of semiconductor fabrication. Nano-imprint lithography answers this need.

Figure 2. Nano-imprint lithography is a highly repeatable, wafer-scale process for creating SOE devices.

The process involves making a silicon or silicon-dioxide mold inscribed with the complement of the desired nanostructure, impressing this mold into a resist-coated wafer, separating the mold, and selectively removing the resist with reactive ion etching to transfer the nanopattern to the target material (see figure 2). Post-imprint processing may be used to add metal layers to enhance performance, and to provide a protective overcoat for ease of handling in a standard manufacturing environment. Testing and dicing yields the final product. Appropriate selection of the material for the substrate—usually transparent, optical-quality glass to allow selective transmission through the grating structure, grating material, metal layers, and overcoat material—allow the optical performance of the SOE to be further manipulated.

Various lithography techniques, including electron-beam lithography, are used to create the desired negative image mold of the nanostructure. Because the mold can be replicated and reused, the complex multistep and multiprocess methods used to create the desired nanostructure mold are amortized over the full production run of a particular SOE. Using different molds with different nanostructure patterns allows the same manufacturing process to create the full range of SOEs.

Monolithic integration of SOEs is achieved by multipass nano-imprint lithography that stacks SOE layers, thereby creating aggregated optical effects. Combining SOEs with optically active layers allows designers to build optical control circuits with chip-level integration. University research has demonstrated the use of SOEs to integrate a polarizer layer with a photo-detector layer to yield a high-speed feedback device for determining the polarization state of the incident light. An interesting area for further exploration is manufacturing-level integration of SOE devices and structures with other wafer-based optical element technologies.

SOEs constitute a highly flexible building block technology for integrated optical components. Because the optical properties of SOEs are realized through local interactions with light, on a subwavelength scale, all these devices share very thin, small form factors, with specific dimensions driven by application requirements rather than optical limitations. Moreover, due to the local nature of the nanoscale optical effects, multiple optical functions can be physically close-coupled within SOEs as they cannot be in bulk optics. This, combined with nano-imprint lithography, opens the door to building SOEs that integrate multiple optical functions. oe

Hubert Kostal

Hubert Kostal is vice president of marketing and sales, NanoOpto Corp., Somerset, NJ.