by L. C. Kimerling

he evolution of the information mance and affordability of the net- turn of a single knob—the shrinking of age is being paced by the intro- work. A key frontier is the large scale device dimensions. What is known today duction of new materials and integration and manufacturing of pho- as “technology shrink” has increased components more than by the tonic components to enable the distrib- computational speed due to shorter gate design of systems, software, and ution of high bit rate optical streams to widths, and increased functionality networks. A rebuilding of the the individual information appliance. through higher levels of integration and Tworld’s information infrastructure is decreased costs by producing more tran- taking place to give instantaneous The Interconnection sistors/chip and more chips/wafer. The availability of data, voice, and video. Bottleneck driving forces for electronics tech- Electrons transmitted through metal nology have been higher computational wires have an information carrying It is now one-half century since the speed and functionality at lower cost. capacity that is limited by the resis- invention of the transistor and the The increase in integrated circuit tance and capacitance of the cable and advent of solid-state electronics. Through performance has been exponential in the terminating electronic circuits. unparalleled gains in functionality at rel- time at rates of more than 100/decade. transmitted through fiber are atively constant cost, integrated circuits Such a pace represents the over- capacity-limited only by the dispersion have enabled telecommunications, com- whelming impact of a “killer tech- of the medium. Each network node putation, and manufacturing to move to nology.”1 However, as critical that requires transduction from pho- the leading edge of societal change. This dimensions have shrunk, the metal tonics to electronics limits the perfor- revolution has been conducted with the interconnection lines that provide the communication paths between devices have become smaller and more closely 40 spaced to limit performance. This con- Interconnect straint is reflected in the increasing gap Delay between the gate delay (switching 30 speed) of a transistor and the propaga- tion delay between transistors of an Gate integrated circuit (see Fig. 1).2 The 20 Delay introduction of copper lines and low Delay (ps) Delay FIG. 1. Trends in transistor dielectric constant insulating layers gate delay (switching time) 10 gives only a factor of four potential and interconnect delay improvement at great risk to manufac- (propagation time for the turing yield and cost. This limit to chip Al/SiO system) with inte- 2 performance is termed the “intercon- grated circuit fabrication 0 technology. The cross over nection bottleneck.” As Fig. 1 shows, a point represents the start 0.65 0.5 0.35 0.25 0.18 0.13 0.1 factor of four will delay the bottleneck of the “interconnection for at most two generations to the 0.13 bottleneck.” Line width (µm) µm technology. During the last two decades a new 1014 killer technology has emerged in the Multi-channel telecommunications field. This pho- 1012 (WDM) OPTICAL tonic technology uses optical fibers for 1010 FIBER interconnection, and has delivered an SYSTEMS Single channel 8 exponential increase with time of 10 (ETDM) information carrying capacity to the 106 Communication industry (see Fig. 2).3 has Satellites 4 Advanced provided unparalleled bandwidth to 10 coaxial and FIG. 2. Trend in informa- microwave systems the backbone of the discrete, point-to- 2 tion carrying capacity of a 10 Early coaxial cable links point long distance (wide area net- single line (wire or optical Carrier Telephony first used 12 voice Relative Information Capacity (bit/s) 0 work, WAN) telephone line fiber) with time and 10 channels on one wire pair technology. (WDM: Telephone lines first constructed technology. However, the circuit archi- 10-2 wavelength division tecture (central switching office) that 1880 1900 1920 1940 1960 1980 2000 2020 2040 multiplexing; TDM: time worked so well for voice communica- division multiplexing). Year tions now limits the universal access

28 The Electrochemical Society Interface • Summer 2000 that is required for Internet and data directly integrated discrete devices the constraints posed by carrier trans- communications. A single optical fiber, (sources, detectors, and waveguides) in a port and lifetime, and electric field with several hundred gigabits/second of point-to-point architecture. Optical design criteria. One is concerned with capacity, is limited by electronic pro- sources ( ), whose the control of scattering sites, recombi- cessing at each circuit node. To avoid power, spectral purity, and temporal dis- nation centers and localized sources this problem direct optical connections tortion performance have dominated of dielectric breakdown and leakage cur- are required: optical cross-connects and WAN architecture design, may become rent. focuses concern optical add/drop multiplexers. To pro- the least critical component. Consider on creation, propagation, and vide full functionality, these compo- the optical bus architecture shown in Fig. optoelectronic transduction. The figures nents must be integrated at densities 4. A photon bus supplies photons to the of merit for photonics are optical power compatible with current microelec- circuit form a central source in much the insertion loss, efficiency of tronic integration. This microphotonics same way that a traditional power supply photonic/electronic transduction, and platform represents not only a solution provides electrons. These photon streams quality factor, Q to information access, but it can also can be switched, modulated and wave- (photon lifetime). Photon absorption solve the problems of bandwidth, pin- length-converted to provide clock sig- and scattering are sources of loss. Local- out density, reliability, and complexity nals, to encode data and to route signals. ized dipole coupling determines the effi- that threaten to end the advance of the In addition, they can be wavelength ciency of photon generation and silicon integrated circuit technology. multiplexed to reduce pin count. Thus, detection. Because optical modes and waveguides, modulators, optical filters resonances are dimensionally deter- Microphotonics and switches and optoelectronic detec- mined, precise dimensional control at The key enabler for microphotonics tors rise in relative importance to distrib- length scales of the photon wavelength is confinement of the optical uted, discrete sources. For silicon in the medium is more critical for pho- carrier to submicron device dimensions. microphotonics, a minimum materials tonics than for electronics. Optical confinement is analogous to the set of Si, Ge, and SiO2 can perform most A good overall figure of merit for the particle-in-a-box problem in quantum of these functions. performance of microphotonic inte- mechanics. The high potential walls of grated circuits, relative to all electronic the box are equivalent to a high index Silicon Microphotonics or hybrid counterparts, is (speed)/(power difference between the optical wave- x area). In addition, cost reduction and guide core and its cladding. The Silicon microphotonics utilizes sil- reliability are expected benefits integra- industry standard carrier wavelength is icon-based materials that are process tion. For example, an evaporated inter- λ = 1.55 µm. In an optical this compatible with standard integrated cir- connect on a complex microelectronics λ cuit fabrication methods. This approach chip has better reliability than a copper dimension is /nr, where nr is the refrac- tive index of the guide. Thus, high has the support of the huge imbedded cable for telecommunications at about -12 index waveguide media are necessary. materials and processing knowledge 10 of the cost! The following sections This requirement is contrary to fiber base that supports the integrated circuit will review principles and prototypes of microelectronics industry. Microelec- silicon microphotonics media, and optic technology, where low index SiO2 tronics engineering is structured to meet active and passive components. (nr = 1.5) is the medium. Transmission loss by light scattering is roughly pro- ∆ 2 portional to ( nr) . For fiber tech- ∆ polySi nology nr ~ 0.01 to minimize loss, ∆ whereas high confinement requires nr > 1 to minimize size and enable large 0.2 µm FIG. 3. Cross- scale integration. As a rule of thumb, SiO sectional view microphotonic device dimensions scale 2 of a polycrys- ∆ ∆ talline silicon with nr. Thus, for nr = 1 a photonic optical wave- 2 device can shrink to an area of (0.01) or guide designed 10-4 the size of a comparable fiber optic Si for a single device. For simplicity this paper will mode propaga- tion of λ = 1.5 concentrate on the use of silicon, nr = 3.5, as the high index medium. Figure 3 µm light. The standoff from shows a typical silicon waveguide cross- 0.5 µm the silicon sub- section for single mode transmission at strate is 0.7 µm. λ = 1.55 µm. Most of the commonly used , GaAs, InP, and Ge off-chip source (1.5 µm) have similar large values of nr. with modulation Microphotonic Systems and Components waveguides (polySi) There are no physical limits to the splitters and bends introduction of an all optical intercon- nection technology platform. In fact, the photodetectors (Ge) FIG. 4. Schematic optical platform may be the only one diagram of an optical bus that is scalable to an orderly evolution of architecture future generations. It is unlikely that the for global clock microphotonic platform will consist of distribution.

The Electrochemical Society Interface • Summer 2000 29 Silicon Microphotonic Waveguides dimensional integrity and microelec- equivalent to the Q factor of the pho- tronic compatibility. tonic microcavity. These materials and The main challenge of silicon We have fabricated micron-sized, photonic defects have been engineered microphotonics lies in reducing device polycrystalline silicon bends and split- to produce the world’s smallest sizes to dimensions comparable to inte- ters based on submicron waveguide microphotonic components. grated circuit electronics while utilizing cross-section dimensions with low loss. Figure 5 shows the structure and CMOS compatible processes. Typical With Y-splitters and bends, we fabri- transmission spectrum of a one dimen- optical fiber and planar waveguide struc- cated the first 1x4 and 1x16 fanout sional PBG structure with a photonic tures feature ∆n = 0.01 by doping of the optical power distribution using poly Si defect.6 The structure is composed of a SiO2 waveguide core with Ge or P. This waveguides. Our 1x16 fanout optical 0.5 µm wide silicon waveguide with a relatively weak confinement limits not system occupies an area as small as series of periodically spaced air holes to only device size, but also the ability to 0.0001 cm2, which is the smallest such create the photonic band gap. A missing navigate photons around the sharp fanout system ever built.4 A 1x8 splitter air hole in the center is the defect. The turns required for intrachip optical inter- based on a multimode interference bandgap was designed to span the connection. Turn radii of less than a mil- (MMI) design has yielded our smallest amplification spectrum of the industry limeter yield high radiative loss. and most efficient 1x16 splitters to date. standard SiO2:Er fiberoptic amplifier. Alternately, a Si core clad by SiO2 wave- These nanowaveguide structures also The defect acts as a photon tunneling guide structure exhibits a ∆n=2, and the allow construction of microring res- state with the width of the pass band linear scaling factor of 200 allows turn onator devices that act as wavelength being inversely proportional to the Q radii of one micrometer. The Si/SiO2 division multiplex (WDM) filters and factor of the defect microcavity. This materials system with n(Si) = 3.5 and routers.5 Data from a micro-ring cascade device functions as a channel drop filter n(SiO2) = 1.5 meets all the requirements for WDM demultiplexing have shown for WDM applications. The device per- for microphotonic waveguides. Silicon is excellent channel separation, made pos- formance closely follows the PBG transparent for λ = 1.3-1.5 µm photons, sible by wide free spectral range of design, and the measured Q of 250 is and the index match to semiconductor micrometer-dimensioned ring res- capable of fitting 128 different wave- emitters and detectors is ideal for low onators. The miniaturization and inte- length channels within the amplifier insertion loss. However, high index con- gration of these optical routing devices spectrum. The optical mode volume of trast structures present a process and systems constitutes the basic foun- the device, 0.0552 µm3, is the smallest problem, because performance is limited dation for the microphotonics platform. ever created. Photonic crystals represent a new sil- by scattering loss from surface rough- Photonic Crystals ness. icon process challenge. The periodicity Silicon-on-insulator (SOI) platforms Photonic crystals are composite of the microstructure is the major per- are required to prevent power loss to the materials that consist of a periodic lat- formance constraint. Air holes and silicon substrate. High performance strip tice of high and low Si/SiO2 multilayers in one dimension, waveguides (see Fig. 3) are dimensioned materials. The refractive index variation post arrays in two dimensions, and dis- at 0.5 x 0.2 µm for single mode trans- acts in a similar fashion to atomic lat- placed checkerboards in three dimen- λ mission in the = 1.3-1.5 µm range. A tice potentials in semiconductor mater- sions have been addressed by a variety 7000 Å SiO2 cladding isolation from the ials. In photonic crystals a band of of approaches. The multilayer structure underlying silicon substrate is required. energies exist for which no photons can research closely parallels the develop- A single crystal silicon waveguide can be propagate (similar to the electronic ment of 3-D SOI integration of 7 fabricated by wafer bonding approaches bandgap of semiconductors and insula- electronic circuits. Wafer bonding, to give flexibility in layer thicknesses tors). The higher is the index contrast, chemical vapor deposition (CVD), and with a high quality, silicon transmission the larger is the bandgap. The bandgap sputter deposition have been successful, medium. Polycrystalline silicon offers represents a frequency range of perfect because the high index contrast of the the maximum flexibility in layer posi- reflection. A defect in the photonic materials system requires only four layer tioning as well as thickness. The mater- crystal, such as a vacancy or missing pairs (GaAs/AlGaAs requires many tens ials engineering challenges are reduction high index component on a lattice site, of layer pairs) for a sufficiently high of sidewall roughness to limit optical constitutes a deep level that traps pho- cavity Q. The critical dimension for λ loss and process integration to maintain tons. The binding energy of the trap is light of = 1 µm is of the order of 0.1 µm for silicon with a tolerance of ± 0.01 µm. This dimensional regime is highly desirable for microphotonics, and the 10 — Measured Data (scaled) capability to achieve these dimensions 08 — Calculated with routine silicon fabrication-line Tr processes is likely within the next five an 06 years. sm iss 04 Detectors ion 02 Transduction of photons to electrons is required for the integration of 00 microphotonics with microelectronics. 1200 1300 1400 1500 1600 1700 1800 Because photons of energy greater than Wavelength (nm) the silicon band gap can be absorbed throughout the integrated circuit, and

FIG. 5. Structure and performance of a 1D resonator. The structure consists of air holes in a 0.5 inject spurious signals, “sub-gap” ener- µm wide silicon waveguide. gies are preferred. Materials with

30 The Electrochemical Society Interface • Summer 2000 bandgaps less than the photon energy SiO2 and polystyrene spheres with contract 97-SC-309, SRC/NSF Center for must be used for detection. The primary micron and submicron dimensions have Environmentally Benign Semiconductor candidate for monolithic integration been employed.11 Manufacturing, MIT-MRSEC supported with silicon is the SiGe alloy. Band Block copolymers phase segregate by the National Science Foundation structure calculations have defined the into periodic microstructures with under contract DMR-9400334, NREL design window for these alloys.8 For length scales determined by the polymer under contract XD-2-11004-4, the high speed and high levels of integra- segment lengths. By adding SiWEDS Consortium and AF Rome Labs tion small detector sizes and short homopolymer, the optical response can under contract F19628.95.C-0049. absorption lengths are desired. Pure Ge be controlled by swelling of the periodic can provide absorption coefficients for λ microstructure. Systematic changes have References = 1.3-1.5 µm photons that are compa- been observed in the reflectivity of a 1. C. H. Fine and L. C. Kimerling, in OIDA rable to direct gap compound semicon- polyisoprene/polystyrene block Future Vision Program, p. 1, Washington, DC ductors. The defect engineering copolymer as styrene and isoprene (1997). challenge is accommodation of the 4% homopolymers are added.12 For block 2. The National Technology Roadmap for Semi- lattice misfit between the Si substrate copolymers with one block possessing a conductors 1997 Edition, Semiconductor and the Ge layer. Planar misfit disloca- silicon backbone and the other a carbon Industry Association, San Jose, CA, (1997). tions are acceptable in a passive region backbone, oxidation leads to a periodic 3. L. C. Kimerling, Opt. Photon. News, 19 (Oct 1998). of a photodetector, but threading dislo- SiO2 lattice as the carbon is volatilized to 4. K. K. Lee, D. R. Lim, A. Agarwal, H. C. Luan, 13 cations that extend through the active CO2. The above self-assembly H. H. Fujimoto, M. Morse, and L. C. Kimer- region are sources of leakage current and processes offer promising paths for the ling, Proceedings of the SPIE Photonics East recombination that reduce responsivity growth of photonic crystals at low cost. Symposium, Boston, MA, (Sept 1999). and introduce noise to the detector per- 5. B. E. Little, J. S. Foresi, G. Steinmeyer, E. R. formance. We have fabricated and Summary Thoen, S. T. Chu, H. A. Haus, E. P. Ippen, L. C. Kimerling, and W. Green, IEEE Photon. tested heterojunction Ge photodetec- The microelectronics age has been Technol. Lett., 10, 549 (1998). tors based on Ge epitaxially grown on Si characterized by an exponential growth 6. J. S. Foresi, P. R. Villeneuve, J. Ferrera, E. R. (001) using a two-step ultra-high in the performance of electronic systems Thoen, G. Steinmeyer, S. Fan, J. D. Joannopoulos, L. C. Kimerling, H. I. Smith, vacuum-CVD process followed by cyclic at relatively constant cost. The benefits and E. P. Ippen, Nature, 390, 143 (1997). thermal annealing. For selective area have been primarily in the form of growth of 100% Ge on Si, the multi-step 7. K. Wada, H. Aga, K. Mitani, T. Abe, M. faster, more available computation hard- Suezawa, and L. C. Kimerling, Proceedings of deposition and anneal sequence pro- ware. The information age has been ush- the 1998 International Conference on Solid- duces threading dislocation-free Ge ered into existence by this high quality State Devices and Materials, Vol. 30, p. 382, 9 material. As shown in Fig. 6, the detec- database. The future of the information Hiroshima, Japan (1998). 8. L. Giovane, Ph.D. Thesis, Massachusetts tors exhibit typical responsivity of 550 age lies in the networking of databases Institute of Technology, Cambridge, MA mA/W at 1.32 µm and 250 mA/W at for universal availability, that in turn 1.55 µm. Response times shorter than (1998). will require a mating of microelectronics 9. H. C. Luan, D. R. Lim, K. K. Lee, K. M. 10 850 ps were measured at 1.32 µm. and fiberoptic technology with Chen, J. G. Sandland, K. Wada, and L. C. Novel process schemes such as these are microphotonic interconnection. ■ Kimerling, Appl. Phys. Lett., 75, 2909 (1999). critical to the introduction of a mono- 10. G. Masini, L. Colace, G. Assanto, K. Wada, lithic microphotonics technology plat- Acknowledgments and L. C. Kimerling, Electron. Lett., 35, 1467 form. (1999). The energy and creativity of the fac- 11. K. M. Chen, X. P. Jiang, L. C. Kimerling, New Materials and Processing ulty, research staff and students refer- and P. T. Hammond, Langmuir, In press. Paths for Microphotonics enced below are responsible for the 12. A. Urbas, R. Sharp, Y. Fink, E. Thomas, M. Xenidou, and L. Fetters in Turnable Block development and prototyping of the A range of new processes are being Copolymer/Homopolymer Photonic Crystals, concepts described in this paper. Related Submitted. considered to meet the stringent dimen- research was sponsored by SRC under 13. E. L. Thomas, Private Communication. sional constraints of microphotonics. Photonic crystals offer the ultimate in control of photonic functions, but the need for an artificial, periodic composite 100 microstructure makes pattern transfer V = 1V approaches difficult. Self-assembly tech- bias niques are the most promising alterna- tive path to generate such structures. Recently, colloid aggregation and phase 10-1 separation in block copolymers have + been adapted to the needs of micropho- - tonics. Ag Colloidal aggregation can lead to a Ge close packed lattice of spheres that can 10-2 be the building block for a three dimen- Responsivity [A/W] sional photonic crystal. In the limiting FIG. 6. Performance of a Si case of a two dimensional structure p-i-n germanium on light made with this approach, electrostatic silicon photodetector. The device was adhesion, promoted by charged poly- 10-3 fabricated by direct 1000 1200 1400 1600 1800 electrolytes, is used to create a selective, growth with low stable array of dielectric spheres. Both dislocation density. Wavelength [nm]

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