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Nanophotonics Accessibility and Applicability 3 Enabling Technologies The development of enabling technologies, along with significant infrastructure, will be essential for the implementation of commercial and military applications of nanophotonics. This chapter describes essential enabling technologies, including the synthesis, growth, and fabrication of nanomaterials and nanostructures; modeling and simulation; characterization techniques for nanophotonics; and the packaging and integration of nanophotonics devices. The development of these enabling technologies in a country is often an important indicator of the state of maturity of nanophotonics in that country. REALIZING HIERARCHICAL SYNTHESIS, GROWTH, AND FABRICATION STRUCTURES AT THE NANOSCALE Introduction Traditionally, synthesis, growth, and fabrication have been separately identified stages in the development of functional devices. An example is a semiconductor laser: the synthesis is in developing the nearly defect-free substrate (gallium arsenide [GaAs], for example) and in developing the feed materials for the epitaxial crystal overgrowth. The growth stage as related to a semiconductor laser is the epitaxial formation of an optical cavity and gain structure for the appropriate confinement of both the photons and the electronic carriers using molecular-beam epitaxy or metal-organic chemical vapor deposition (MOCVD), typically on a wafer scale. The fabrication stage uses processes adapted largely from integrated circuit manufacturing to batch fabricate diode lasers with contacts for electrical input and optical output as required for the device application. In the nanoscale era, these distinctions among synthesis, growth, and fabrication are blurring. The steps outlined above are being employed in mix-and-match ways to produce novel functional nanostructured materials and to arrange them with the necessary hierarchical organization to produce new functionalities. This subsection describes the traditional categories of synthesis, growth, and fabrication, and then comments on the blurring of the boundaries among these stages. As the terms are used here, synthesis includes not only the starting materials, but also the formation of isolated nanostructures, such as metal
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Nanophotonics Accessibility and Applicability and dielectric nanospheres and quantum dots, and fabricated multilayer combinations. Additionally, the chemical synthesis of starting materials such as copolymers and liquid crystals is included. Growth as used here is specifically limited to the epitaxial growth of semiconductor materials; it also includes self-assembled arrays via nanoparticle formation, such as Stranski-Krastanov formation of quantum dots by the interplay between strain and surface tension. Fabrication refers to the creation of ensembles of nanostructures—for example, photonic crystals composed of nanoparticles. Traditional top-down processing derived from the integrated circuit industry is reaching to scales of direct relevance to nanophotonics and beyond and certainly will be an important component of any nanophotonics fabrication suite. This is so in part as a result of the strong ability of traditional top-down processing to engineer hierarchical structures incorporating multiple, disparate length scales and the proven mass production capabilities of batch wafer processing. Self-assembly is bottom-up processing; a simple example is the assembly of colloidal nanoparticles into photonic crystal arrangements. A related approach is the use of multiphase systems, such as block copolymers1 along with surfactant-covered nanoparticles, to spontaneously form complex patterns driven by an external forcing function such as evaporation. Increasingly, techniques are being developed that combine top-down and bottom-up approaches and blur the distinctions of the categories described above. One example is nanoscale crystal growth, in which a pattern is defined by fabrication and a subsequent growth process results in an array of nanoscale semiconductor structures. The fabrication can occur either by self-assembly (as, for example, porous anodization of a continuous metal film) or by traditional lithographic pattern definition and etching. As opposed to the subtractive etching of a large-area semiconductor film (the growth described above), this sequence allows much more flexibility and often produces materials with fewer defects and improved functionality. Generically, this combination is referred to as directed self-assembly. Figure 3-1 shows a lithographic template that serves to guide the self-organizing crystals, block polymers, or colloids to pack and orient into a predetermined pattern, thus bringing the ultrasmall length scales achievable by molecular self-organization into play with the somewhat larger engineered structures to create hierarchical devices. Synthesis Nanoparticles The need for improved optical materials has driven researchers in chemistry, materials science, and chemical engineering to create new synthetic pathways to afford better control over the composition, size, and shape of nanoparticles in order to produce new types of supramolecular building blocks. As mentioned in Chapter 2, metallic nanoparticles, such as gold nanostars, can be useful for localized surface plasmon resonance. Thus, it is important to control not only particle size but particle shape. Moreover, it is absolutely critical to have monodispersity in size and shape. Researchers have found that absorbed surfactants can influence the relative growth rate of various facets (surfactant binding and size-dependent facet surface energies) resulting in the growth of complex-shaped nanoparticles. For example, Alivisatos and colleagues found that they could create tetrapod gold (Au) or cadium selenium (CdSe) nanoparticles. The Au particles are composed of a set of (111) twin 1 Block copolymers are made up of two or more homopolymer subunits linked by covalent bonds. Block copolymers with two (or three) distinct blocks are called diblock (or triblock) copolymers.
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Nanophotonics Accessibility and Applicability FIGURE 3-1 A top-down lithographically defined template can guide the bottom-up self-assembly of materials, creating a system with several length scales: a structural hierarchy. SOURCE: Cheng et al. (2006b). Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. Au crystals (Manna et al., 2002). Additionally, hollow nanoparticles can be produced by the oxidation of metallic nanocrystals and directional diffusion of species (the Kirkendall effect) (Yin et al., 2004). Traditional solution synthesis has recently been extended to microfluidic reactors, in which on-chip in situ monitoring of the growing entities and the ability to add chemical feed streams and to adjust residence times provide opportunities for dynamic adjustment and tuning of the targeted materials. An example is shown in Figure 3-2, where an overcoating of zinc sulfide (ZnS) onto CdSe quantum dots (QDs) increases the quantum yield from ~10 percent to ~40 percent along the reactor pathway. The meandering synthesis channel is fabricated in silicon and is thermally isolated from the room-temperature inlets and outlet. ZnS overcoating reagents enter through the side inlets to enable core-shell particle growth without secondary nucleation. Advances in microfluidic reactors will increasingly impact the ability to engineer many types of nanoparticles because, instead of batch reactions with a Schlenk line apparatus with multiple step-function injections of reactants, one can tailor concentration-time schedules and use gradients and can separate and remix-recycle, impinge, and sinter or stack the growing targets to achieve desired structures and properties. Currently most particles are homogeneous or core-shell and either spherical
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Nanophotonics Accessibility and Applicability FIGURE 3-2 Microfluidic reactor for the synthesis of zinc sulfide (ZnS) overcoated cadium selenide (CdSe) quantum dots. The brightness in the lower right image increases from bottom to top, indicative of the increased quantum efficiency as the ZnS overcoat layer forms on the CdSe dots. SOURCES: Yen (2006), Yen et al. (2005). Reproduced with permission. or cylindrical; however, multicomponent, complex-geometry particles are possible and can provide structural and functional anisotropy. Layered-Nanoparticle Fabrication Techniques Layered nanoparticles with a core-shell geometry in which the core and shell layers consist of either a dielectric material or a noble metal are of scientific interest, as the optical properties of these particles can be controlled by adjusting the thickness of each layer. The most common types of core-shell particles are nanoshells, which consist of a silica core coated with a thin layer of gold or silver (Oldenburg et al., 1998). The opposite particle consisting of a gold or silver colloid as the core particle coated with a silica shell of varying thickness can also be made (Liz-Marzan et al., 1996). The optical properties of these particles are controlled by the ratio of the core radius to the total particle radius. Similar to these, but nonspherical in shape, are spindle-shaped iron oxide particles coated with a thin layer of gold (Wang et al., 2006b). Core-shell semiconductor particles are also important—for example, a thin (<1 nm) ZnS outer shell passivates CdSe quantum dots. The silica core particles are typically synthesized using the Stöber process (Stöber et al., 1968). The Stöber method involves the base catalyzed hydrolysis and condensation of tetraethylorthosilicate (TEOS). Typically, TEOS and ammonium hydroxide (NH4OH) are mixed in varying ratios in ethanol to produce silica nanoparticles in the 80 nm to 500 nm diameter size regimes. After the silica particles are synthesized, the surface is functionalized with a silane, such as 3-aminopropyltriethoxysilane (APTES).
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Nanophotonics Accessibility and Applicability This chemical functionalization of the silica surface provides an amine moiety at the surface, which is used to attach ultrasmall gold colloid. Ultrasmall gold colloid particles of 1 nm to 3 nm in size are synthesized by reducing chloroauric acid using tetrakis (hydroxymethyl) phosphonium chloride (THPC) as a reducing agent. The ultrasmall gold attached to the silica surface act as nucleation sites for the electroless deposition of Au (or silver [Ag]) to form a complete shell on the silica core. The technique for depositing gold layers on spindle-shaped hematite particles is similar to depositing a gold layer on silica particles. The growth of silica layers on gold or silver colloidal particles follows an inverse process. A solution of gold particles with an average diameter of around 15 nm and 10 percent polydispersity is used to form the core of these layered particles. A freshly prepared aqueous solution of APTES (2.5 milliliters [ml], 1 millimeter [mm]) is added to 500 ml of the gold solution under vigorous magnetic stirring. The mixture of APTES and gold dispersion is allowed to stand for 15 minutes to ensure complete complexation of the amine groups with the gold surface. A solution of active silica is prepared by lowering the pH of a 0.54 weight percent sodium silicate solution to 10 to 11 by the progressive addition of cation exchange resin. Twenty milliliters of active silica are then added to 500 ml of the surface-modified gold solution, again under vigorous magnetic stirring. The resulting dispersion (pH ~8.5) is then allowed to stand for at least 1 day, so that the active silica polymerizes onto the gold particle surface. The silica shell thickness is about 2 nm to 4 nm after 24 hours. The particles can then be transferred into ethanol if further growth or chemical modification of the silica layer is intended. At this point, thicker silica shells can be grown using the Stöber method. Another class of these layered particles consists of monolayers of silica or polystyrene nanoparticles coated with a layer of metal (Dieringer et al., 2006). This geometry has also been used for plasmonic applications such as surface-enhanced spectroscopy. The fabrication process begins with the self-assembly of monodisperse nanospheres to form a two-dimensional colloidal crystal. A substrate is prepared so that the nanospheres freely diffuse until they reach their lowest energy configuration. This is achieved by chemically modifying the nanosphere surface with a negative charge that is electrostatically repelled by a negatively charged substrate such as chemically treated glass. As the solvent (water) evaporates, capillary forces draw the nanospheres together, and they crystallize in a hexagonally close-packed pattern on the substrate. Following self-assembly of the nanosphere mask, a metal (typically silver) is deposited by physical vapor deposition from a collimated source normal to the substrate through the nanosphere mask to a controlled mass thickness. The resulting surface is referred to as a metal (e.g., Ag) film over nanosphere (AgFON) surface. Nanorods and Nanowires Nanorods and nanowires are another promising type of nanoparticle that can be made a variety of ways. Nanowires are quasi−one-dimensional single-crystalline structures and, in contrast to nanorods, they have a much larger length-to-radius aspect ratio, with diameters as small as a few nanometers and lengths up to several hundreds of micrometers. Sized controlled nanorods (Hu et al., 2001) can be made by thermal decomposition of organometallic precursors in a coordinating organic solvent. The discovery that various ligands and co-ligands can promote anisotropic growth has led researchers to create a number of nanorods that afford uniform size and shape and good electrooptical properties, controlled by the molar ratio of the components to the coordinating solvent as well as annealing time. Nanowires are typically grown using chemical vapor deposition (CVD) by means of a catalyst-mediated vapor-liquid-solid (VLS) mechanism. The growth is initiated by the dissolution of gaseous reactants in nanosized liquid particles, followed by nucleation and growth on the substrate. In this way,
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Nanophotonics Accessibility and Applicability the nanowire diameter is predefined by the size of catalytic particles, length depends on the growth time, and often there is an epitaxial relationship between the nanowire and the substrates allowing control of the nanowire growth direction (Lu and Lieber, 2006). In addition, nanowires can be grown in place by a number of techniques, allowing an ensemble of wires to be grown all having the same crystal orientation. One technique uses VLS growth with a low concentration of nickel nitrate catalyst on a sapphire substrate to produce dense arrays of gallium nitride (GaN) nanowires, well aligned to each other and having a vertical orientation (Wang et al., 2006a). Another extremely promising technique is that of using conventional MOCVD growth to produce uniformly oriented and ordered arrays of GaN nanowires by growing through a nanopatterned mask material of silicon nitride or silicon dioxide onto a GaN film. By using a “pulsed” growth mode in which precursor gases are introduced alternately in sequence, the diameter of the quantum wires can be made extremely uniform along their length (Hersee et al., 2006). Additional description of the latter technique is provided later in this chapter. Semiconductor nanowires can simultaneously act as interconnects and as active components and therefore have emerged as attractive building blocks for applications ranging from nanoelectronics to photonics or sensing. Through the rational growth of nanowire heterostructures with controllable doping, the functionality of such devices has been enhanced, and nanowire-based light-emitting devices, high-performance field-effect transistors (FETs), and arrays of single-nanowire silicon FETs for sensing have been demonstrated (Xiang et al., 2006). Central to future applications will be the ability to assemble and position such nanoscale devices at different length scales, as well as the ability to address individual elements in high-density arrangements. For example, microfluidic flows can be used for aligning an array of nanowires in devices (see Figure 3-3), while for the manipulation of individual nanowires several techniques have been proposed: optical tweezers, or manipulation using electrical and magnetic fields. Organic Materials Organic nanophotonic materials are more difficult to control with the structural precision of inorganic materials, but chemists can create intricate molecules and supramolecular assemblies with extremely highly specific interactions with chemicals and biological agents. Also, organics have an enormous range of electrooptical properties and are flexible hosts for metallic and inorganic materials to create multifunctional devices. In addition, their huge tunability across the ultraviolet (UV) to infrared (IR) spectral regions by means of a host of stimuli (e.g., temperature [Wiersma and Cavalieri, 2001], mechanical stress, pH, ionic strength, specific binding to biological agents, response to electric fields, and so on) makes them excellent sensors (Gaillot et al., 2007; Moreira et al., 2004).2 Many industrial processes have been developed for the deposition of uniform organic films. Spin casting is the most familiar and produces films with nanometer-thickness resolution and control. Dry film-deposition processes have also been commercialized. For many device applications—for example, organic light-emitting diodes (OLEDs)—this approach has been very successful. Other device concepts require a precision placement and/or contacting of individual molecular moieties. A challenge for solution-based synthesis is to be able to place and to orient, contact, and interconnect these individual molecular moieties into nanoscale 2 Certainly, chromonic liquid crystals are being used (two companies are developing this scheme) for the detection of specific biological agents. For more information, see (http://dept.kent.edu/biology/woolver.htm), (http://www.blackwell-synergy.com/doi/pdf/10.1111/j.1472-765X.2006.01916.x). The current schemes are not PBG-based, but certainly can be and very likely will be.
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Nanophotonics Accessibility and Applicability FIGURE 3-3 Use of microfluidics to flow-align nanowires and form crossed arrays by using a polydimethylsiloxane (PDMS) stamp and successive directional flows. SOURCE: Lu and Lieber (2006). Reproduced with permission from Journal of Physics D: Applied Physics. devices. The use of nanoimprint lithography3 to create surface patterns with variable chemical affinity is emerging as a viable pathway to direct the assembly of organics. Co-assembly Instead of the self-organization of a single type of molecule during solvent evaporation, co-assembly encompasses the idea that multiple types of entities—various molecules, quantum dots, and so on—can be simultaneously organized from a liquid mixture as the temperature is lowered or as the solvent evaporates to form more-complex and often multiscale, multifunctional structures. An example is a two-size colloidal assembly to form Laves phases. The structure depends on the size and molar ratio of the small and large spheres and basically seeks to achieve maximum packing density. This idea has been used 3 “Conventional nanoprint lithography often involves exposing the patterned polymer to high temperatures, UV exposure and etching processes. These processes result in a harsh environment that potentially degrades the electrical properties of the polymeric semiconductor…. Still other techniques use a surface-energy pattern on a substrate to pattern a polymer. C. R. Kagan et al. in Appl. Phys. Lett. 79 (21) 3536 (2001) describes patterning self-assembled monolayers using such a surface-energy pattern. Such patterns are typically generated using surface energy modulation. However, use of such a system in electronic device fabrication is restricted to surfaces on which a self-assembled monolayer can be deposited (typically the noble metals such as gold or palladium). An additional coating step, typically accomplished through dip-coating the surface-energy pattern of the substrate over the entire substrate area is complex and slow, lowering throughput and yield” (see http://www.freepatentsonline.com/20060279018.html).
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Nanophotonics Accessibility and Applicability to make novel superlattices of metallic nanoparticles, magnetic nanoparticles, and semiconductor nanoparticles (Shevchenko et al., 2006). Co-assembly of a block copolymer (BCP) and one or more types of nanoparticles can produce interesting hierarchically structured materials. The high-molecular-weight BCP acts as a nanostructured host matrix with a periodicity on the order of the wavelength of light, and the sequestration of various types of the much smaller nanoparticles (see the following subsection) can be used to locally alter the refractive index of the larger block structures. Synthesis of Block Polymers The synthesis of polymers is being extended to an ever-greater range of monomers to create unique diblock and multiblock polymers with both linear and highly branched architectures. The ability to combine two or more polymeric species into a block polymer presents the possibility for multifunctionality of properties owing to the self-assembly of the blocks into equilibrium one-, two-, and three-dimensional periodic nanophases and microphases having a variety of geometries. BCPs can also be used to sequester nanoparticles where, by appropriate choice of the surface ligand on the nanoparticles, the particles co-assemble with the BCP from solution into the targeted microdomains (Bockstaller et al., 2005). Incorporation of metallic and inorganic (semiconducting) particles can modify the dielectric properties of the assembly to provide, for example, increased dielectric contrast for photonic crystals. Liquid Crystals Liquid crystals spontaneously form modulated phases that are photonic band-gap materials and hence are interesting and useful for nanophotonics applications. The structures of the phases that are formed are determined by the properties of the molecules that are the building blocks of the liquid crystal. Rod-like, chiral low-molecular-weight molecules with aromatic cores and flexible alkyl chains form cholesteric and ferroelectric phases, which are periodic in one dimension. These are photonic band-gap materials, with a pitch that can be made to vary from tenths to tens of microns. Such structures can also be polymerized to make rigid cholesteric plastics, as well as weakly cross-linked to make mechanically deformable cholesteric “rubbers.” One application is the resulting tunable rubber distributed feedback (DFB) lasers shown in Figure 3-4. Liquid-crystal elastomers may also be used as photoactuators and nano-optomechanical system elements. Chiral molecules also exhibit the cholesteric “blue” phases, which are periodic in three-dimensions. Although in the past these have only existed in a very narrow temperature range, recent work (Coles et al., 2006) enabled the realization of materials that are stable over a range of 50°C. Such robustness is expected to enable the synthesis of easily processible large-area flexible plastic photonic band-gap materials in the near future, although of lower index contrast (n2 − n1 < 0.3). Examples of such self-assembled structures are the helical phases of cholesteric liquid crystals, which are periodic in one-dimension and show strong Bragg reflection for a range of wavelengths and one polarization mode and mirrorless lasing at the band edge (see Figure 3-5). As shown in Figure 3-6, liquid crystals that are periodic in three-dimensions exist as well. These are the cholesteric and smectic blue phases, twisted grain-boundary phases, forming self-assembled three-dimensional photonic band-gap structures. Other relevant new materials developments are chromonic liquid crystals, in which the mesogenic building blocks are molecular aggregates without covalent bonding, and “banana” liquid crystals of achiral bent-core molecules that form periodic chiral phases.
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Nanophotonics Accessibility and Applicability FIGURE 3-4 Optically pumped mirrorless lasing by a dye-doped cholesteric liquid- crystal elastomer. The lasing wavelength is tuned by stretching the rubber laser (which results in compression of the one-dimensional Bragg reflector in the direction of the film thickness). (a) Demonstration of tunable lasing by deformation (DCM dye optically pumped at 532 nm); (b) changes in normal incidence reflectivity of cholesteric rubber structure due to biaxial deformation. SOURCE: Finkelmann et al. (2001). Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. Nanoparticles with functionalized surfactants and tethers have the potential to self-assemble into a tremendous variety of structures. One example, predicted by numerical simulations and observed experimentally, is the spontaneous formation of flat sheets of cadmium telluride (CdTe) nanocrystals without the presence of templating surfaces. Figure 3-7 shows experimental demonstration of this type of structure formation, with CdTe nanocrystals forming into nanometer-thin sheets.
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Nanophotonics Accessibility and Applicability FIGURE 3-5 Self-assembled liquid crystals can form one-dimensional reflectors and exhibit band-edge lasing. SOURCES: (a, c) Palffy-Muhoray et al. (2006), reproduced with permission of SPIE; and (b) provided by committee member Peter Palffy-Muhoray.
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Nanophotonics Accessibility and Applicability FIGURE 3-6 The blue phase is composed of an ordered array of linear defects that pack on a cubic lattice. SOURCE: Reprinted by permission from Cao et al. (2002). Copyright 2002 by Macmillan Publishers Ltd. FIGURE 3-7 Self-assembly of cadmium telluride nanocrystals (left) into free-floating sheets (right). SOURCE: Tang et al. (2006). Reprinted with permission of AAAS. Recent research shows that materials, whose building blocks are not molecules, but nanoparticles, also show liquid-crystal phases. Examples are clays and mineral suspensions, such as gibbsite platelets and goethite nanorods, as well as synthesized semiconductor nanorods and viruses such as tobacco mosaic virus. The resulting anisotropy provides an additional degree of freedom in the design of the optical properties of these metamaterials. Advances in the synthesis of molecules and of nanoparticles that aggregate to form hierarchical structures are certain to play a key role in the development of materials for emerging nanophotonics device applications.
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Nanophotonics Accessibility and Applicability Two very important implications can be gleaned from these general semiconductor technology trends. First, because of power limitations and statistical variability, the semiconductor industry is undergoing a major paradigm shift to multicore processing with increased on-chip communication requirements. This shift is quite likely to accelerate the introduction of (nano)photonic solutions, which provide an attractive alternative to traditional electronics. Second, the continuously increasing technological challenges associated with exploding development and fabrication costs for the next-generation technology (which has already led to the formation of large international alliances in the semiconductor industry) (Isaac, 2003), in combination with more modular design strategies, will make it more likely that the industry seeks cost reduction by outsourcing its design and (possibly) fabrication work to foreign countries. Packaging and Integration Technologies In response to the realization that future enhancements of chip performance require major innovations and more heterogenous technology components, it is expected that the focus on integration and packaging will continue to increase, which is consistent with the view of the ITRS (see Figure 3-23). In Figure 3-23, monolithic system-on-chip (SoC) and polylithic system-in-package (SiP) technologies are viewed to be critical to enabling the continued growth of information technology beyond Moore’s FIGURE 3-23 The minimum feature sizes of integrated circuit manufacturing have already passed the required scales for nanophotonic components, at least at telecommunication wavelengths, making the integration of photonics and electronics readily achievable using standard integrated circuit nanofabrication tooling. NOTE: CMOS, complementary metal oxide semiconductor; CPU, central processing unit; RF, radio frequency; HV, high voltage. SOURCE: Semiconductor Industry Association (2005b). Reproduced with permission of SEMATECH.
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Nanophotonics Accessibility and Applicability law (i.e., “more Moore”). Figure 3-24 illustrates the balance between SoC and SiP solutions, which is naturally governed by the respective cost function. While certain system complexities and heterogeneousness can be integrated cost-effectively using a modular SoC approach, the integration of highly complex and heterogenous systems—possibly including nanophotonic devices, but also microelectromechanical systems (MEMS) and bioelectronics—is more economically addressed by SiP technologies with a higher degree of miniaturization and flexibility. Because nanophotonics is still an infant technology, it is arguably a very difficult task to understand which packaging and integration technology is most relevant for its enablement. The complicated interplay between limitations of existing technologies, the potential enablement of new computing architectures by optics and nanophotonics, the appropriate level of integration, and the advances of the different nanophotonic technology platforms is still an ongoing research topic. However, it is likely that the accessibility of nanophotonics will be significantly influenced by the way that computer manufacturers decide to integrate optics and nanophotonics (i.e., How close does optics have to get to the device and microprocessor to harness the benefits of optics cost-effectively?). The committee recommends the close monitoring of these trends in the integration of optics and nanophotonics by computer manufacturers because they will most likely determine the direction of massive investments. If indeed nanophotonics can be integrated cost-effectively and monolithically with high-performance CMOS, the respective suitable nanophotonic technology platforms will be more limited (Si-based and CMOS-compatible nanophotonics). Special attention has to be directed toward CMOS-compatible nanophotonics research and development activities because the nanophotonic technology platform will have to be attuned with CMOS processes and materials. However, if optics will be integrated polylithically or heterogeneously, then the nanophotonic technology platforms can be much more diverse, and a corresponding technology watch would have to encompass a much wider range of activities. In the latter case, the role of packaging will be even more central. FIGURE 3-24 Cost versus complexity for system-on-chip, system-in-package (SiP), and 3D Packaging integration approaches. NOTE: MEMS, microelectromechanical systems; 3D, three dimensional. SOURCE: Semiconductor Industry Association (2005a). Reproduced with permission of SEMATECH.
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Nanophotonics Accessibility and Applicability The following subsections first discuss monolithic (i.e., Si photonics) and then polylithic/heterogenous integration and packaging approaches (three-dimensional Si, Si-carrier). For purposes of discussion, the committee distinguishes between monolithic and polylithic (heterogeneous) integration, but it realizes that this boundary is somewhat blurred with the introduction of new integration and packaging technologies such as three-dimensional silicon. Monolithic Integration: Silicon Photonics Monolithic integration using silicon photonics drives toward the seamless fabrication of CMOS and photonic circuits on the same circuit layer, creating an SoC-like chip with an on-chip communication layer (Reed and Knights, 2004). The United States is a leader in this field, driven by its dominant semiconductor industry, but there are also significant and technically relevant activities in Europe (in particular, at IMEC in Belgium, COM [Communications, Optics, and Materials] Research in Denmark, RWTH Aachen University, Germany); in Japan (at NTT); and also more recently in China, Korea, and India. The promise of Si photonics lies in the exploitation of the maturity and precision of CMOS technology as well as in the synergy of ultracompact integration of optical and electrical functions on the same chip. Silicon is transparent in the range of optical telecommunication wavelengths (at 1.55 micrometers [μm]) and has a high refractive index that allows for the fabrication of densely packed nanophotonic structures. The general concept of Si as a waveguide and of modulating its index using electrons and holes had been proposed for quite some time; only recently, however, has significant progress been made, in which CMOS-compatible waveguides (and other passive devices such as splitters), modulators, deflection switches, wavelength demultiplexers, delay lines, and detectors have been demonstrated (Bogaerts et al., 2005; Soref and Bennett, 1987; Soref and Lorenzo, 1986). In addition, complete chip-level demonstrations have been also been reported, which have implemented monolithic integration between Si photonics and CMOS using a standard CMOS fabrication line (Gunn, 2006; Jalali et al., 2005). While these demonstrations are clearly major stepping-stones, the monolithic integration between “high-performance” CMOS in the form of a microprocessor or application-specific circuits has not been done yet—partially because of integration issues (as discussed in more detail below) but also because the required optical components are not available yet. Waveguides and Passives The recent progress in silicon-based waveguides has been helped by the general availability of silicon on insulator (SOI) wafers, which can be readily used to fabricate Si waveguides supported by the buried oxide (BOX). SOI-based single mode (SM) and multimode (MM) waveguides have been fabricated with cross-sectional dimensions of approximately 0.3 × 0.3 μm2 and 0.3 × 2.0 µm2, respectively (Bogaerts et al., 2004; Vlasov and McNab, 2004). Despite the fact that propagation losses increase drastically with an increasing index of refraction contrast ratio the unprecedented accuracy of CMOS fabrication capabilities (by fabricating waveguides with extremely small roughness) has yielded ultradense (pitch of less than 2.0 µm with negligent cross-talks) waveguide structures with quite low losses of less than 1.6 dB/cm and 0.2 dB/cm for SM and MM, respectively (Lee et al., 2001; Vlasov and McNab, 2004). Despite this progress, it remains an ongoing challenge because any losses directly impact the link budgets, which can be very tight in a power-limited design environment. Consequently, the optical properties of Si waveguides are still an active research topic, because propagation (as well as coupling and/or insertion losses) is dependent sensitively on group index, BOX thicknesses, exact wave-
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Nanophotonics Accessibility and Applicability guide dimensions, wavelengths, optical modes, distance to electrical contacts, and other interconnects. Following the successful fabrication of these waveguides, low-loss splitters, bends, directional couplers, and other passives such as wavelength demultiplexers and delay lines have been fabricated (Dumon et al., 2006; Xia et al., 2006; Xia et al., 2007). The dispersion properties of SM Si waveguides are shown to have a dispersion length of several centimeters at data rates of larger than 200 Gbps, which is suitable for on-chip communications (Dulkeith et al., 2006). It is also established that these waveguides can carry up to 20 dBm of power, which is limited by nonlinear effects in the Si waveguides, fundamentally constraining the link budgets (Hsieh et al., 2006). Modulators Several recent demonstrations have shown the feasibility of compact modulators on SOI with up to 10 Gbps bandwidth (e.g., 6.5 Gbps and 1 milliwatt [mW] power dissipation) (Almeida et al., 2005). Fundamentally, there are two simple mechanisms (thermooptic and carrier injection) for modulating the refractive index in silicon, which can be exploited for building modulators. Modulators with both mechanisms have been demonstrated in several optical device configurations, such as Mach-Zehnder interferometers and ring resonators (Almeida et al., 2005; Espinola et al., 2003). The disadvantage with thermooptic modulators (typically realized in a Mach-Zehnder configuration) is the limited bandwidth, which is governed by the typically slow thermal diffusion times. While carrier-injection-based modulators can be very fast (limited by carrier lifetimes), they can suffer from poor overlap between the optical mode and the injected carriers, which require high Q resonator devices (to boost the sensitivity) and which are very temperature-sensitive. At this stage it remains a research topic to balance bandwidth, power consumption, losses, extinction ratio, temperature stability, and footprint requirements for these types of modulators. The committee notes that recently Si-based electrooptical modulators have also been demonstrated using quantum wells, although still with a low extinction ratio (Kuo et al., 2005). Detectors Compared with III/V detectors, Si-based receivers are lacking performance, although recently a very high bandwidth germanium-on-silicon-on-insulator (Ge-on-SOI) photodetector was demonstrated with a very small footprint (Dehlinger et al., 2004; Schow et al., 2006). CMOS-compatible detector design will be governed by a trade-off between CMOS compatibility (materials, processes, process flow), required bandwidth, wavelength range, sensitivity (quantum field, absorption depth), voltage requirements, and packaging approach. The detector will be closely integrated with the waveguide structure with minimum footprint to limit capacitance (Dehlinger et al., 2004; Schow et al., 2006). Other complementary detector approaches comprise tunnel junctions such as metal-insulator-metal, where nanoscopic optical antennae may be used to increase the coupling and detection efficiency (Zia et al., 2006). Light Sources or Gain Elements Despite intensive research, an Si laser or light source is still the central missing piece needed to enable the complete monolithic integration of Si photonics. As is well known, the reason for the difficulties in engineering a suitable light source lies in the fact that Si is an indirect band-gap material, which makes nonradiative processes more probable than the actual light emission. Heterogeneous approaches involve silicon evanescent amplifiers, which employ silicon waveguides and an evanescent tail extending into indium gallium aluminum arsenide (InGaAlAs) quantum wells. These quantum wells, which
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Nanophotonics Accessibility and Applicability are deposited by low-temperature, oxygen plasma-enhanced bonding, are very promising because these devices can be quite small, with low power consumption (Fang et al., 2006). The committee believes that nanophotonics will play an important role in the potential enablement of an Si-based light source and/or gain element. In the absence of an on-chip light source, light needs to be coupled on and off the chip, which is a technically difficult task owing to the high index of refraction contrast of the Si waveguides. However, significant progress has recently been made on two fronts. First, several groups managed to mode-match the different optical profiles between a large single-mode fiber and the Si waveguides by using a smart waveguide taper coupling the light adiabatically into the submicron waveguide (Day et al., 2003; Salib et al., 2004). Second, passive alignment techniques for the external optical fiber with respect to the waveguide have been demonstrated (by creating lithographically defined structures on the silicon surface in order to align the fiber to the waveguide aperture); these techniques remove the need for a closed-loop optimization/coupling scheme. In summary, the emergence of Si photonics is likely to have a profound impact in large distance-communication systems. As Si photonics relates to the enablement of chip-scale supercomputers (enabling the unmatched communication between many cores), it is currently not clear how Si photonics will be integrated with the high-performance CMOS for many reasons and issues ranging from BOX thickness, metallization, circuit and device integration, and process compatibility. Heterogeneous Integration: Silicon Carrier, Three-Dimensional Silicon The monolithic integration of Si photonics with high-performance CMOS circuitry could be eventually the most cost-effective integration way; it may turn out, however, that the optimization of electrical and optical components leads to such different competing requirements that only heterogeneous packaging approaches can rectify this. As discussed, these heterogeneous approaches will provide openings for more diverse technology platforms. Although the general trend to use Si as a package material somewhat favors Si photonic-based approaches (even if they are not entirely compatible with high-performance CMOS): specifically, an Si chip carrier can provide via densities, which make heterogeneous assembly approaches more feasible, where one needs very high-interconnect densities—formerly not supported by traditional carriers (Knickerbocker et al., 2005). The Si carrier technology components are manifold (Si through micro-C4, high-density wiring, and many supporting testing, assembly, and fabrication technologies) and have been demonstrated recently. In essence, an Si carrier provides a general way of very tightly integrating different chip technologies and achieving SoC performances with a system on a package (SoP) solution, thereby creating a “virtual” chip. This technology can play a key role in the enablement of nanophotonics—especially in conjunction with high-performance CMOS—without the burden of being “fully” CMOS-compatible. For example, a chip including nanophotonic devices (e.g., a receiver chip) can be readily integrated and packaged with a microcontroller or microprocessor using SoP technologies. Figure 3-25 illustrates a roadmap for this technology, with a trend toward increasing integration from two to three dimensions. It also shows that the Si carrier is the first important step toward even tighter three-dimensional integration, which is a trend consistent in the industry’s advancing chip-stacking techniques. OVERARCHING RECOMMENDATION Recommendation 3-1. The committee recommends that the intelligence technology warning community be especially aware of countries and groups that have access to a combination of advanced packaging and integration technologies and nanophotonics.
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