Santa Clara, Calif.
Even the semi industry's familiar Moore's Law has its own supercharged analog in photonics. Photonic bandwidth — the volume of information transmitted per second over optical fibers — has been growing at four times the 18-month doubling of densities foreseen by Moore for ICs. This began when researchers developed ways of sending multiple wavelengths, each containing their own stream of data, down a single fiber. Wavelength division multiplexing (WDM) can now superimpose up to 800 individual channels on a fiber strand and, in so doing, has significantly cut the cost of communications over relatively long distances. The same technology will soon prove indispensable for communication over even miniscule distances within an integrated circuit.
Photonics manufacturing is evolving at warp speed. Packaging-automation techniques have improved yields. Initiatives to optimize throughput have been effective as well. One problem has been a lack of suitable packaging automation tools. Existing pick-andplace robotics solutions were incompatible with photonics-device tolerances, which are tighter than for even the latest semiconductor processes.
Similarly, available robotics could not address the angular alignment processes required to package devices like Dense Wavelength Division Multiplexing (DWDM) components, Micro-ElectroMechanical System (MEMS) switching arrays and arrays of microlithographically produced optical waveguides. Meanwhile, settling times associated with mechanical movements conspire with device tolerances to present a bottleneck to throughput in packaging and fabrication processes.
Currently, manual packaging methodologies dominate in photonics, with banks upon banks of technicians twiddling micrometers to position the tiny components within their packages. This is no more sustainable or scalable than manual operations were in the earliest semifabs.
Unlike the wire loops that often bridge chip contact pads to leads, photonic interconnects require alignment of the optical active areas of the fibers, lasers, micro-optics or other components. Alignments must be accurate to 100 nm and cannot be performed via video recognition approaches since the coupling cannot be visualized in the way a wire-pad contact can be. And, increasingly, photonic packaging alignments must take place in five or six degrees of freedom.
Happily, recent innovations let photonics manufacturers address tough packaging-automation needs in several ways. Methods range from quasiintegrated systems based on assemblies of high-precision motion stages to compact, industrial microrobots based on a novel, compact hexapod configuration. The latter approach provides the necessary submicron spatial resolutions while allowing tip/tilt alignments to be automated about any point in space, such as at the focal point of a microlens, the polished face of an optical fiber, or the optical axis of a laser diode. The hexapod design also eliminates troublesome moving cables and results in a much stiffer structure than is available by stacking conventional motion axes.
Meanwhile, processes requiring nanometer-scale precision pose other process bottlenecks. Fabrication of Fiber Bragg Gratings (FBGs) is a good example. FBGs are important elements of optical networks that separate out an individual wavelength from those traveling through a multiplexed fiber. The FBG is a diffraction grating fashioned deep within an optical fiber, often requiring nanometer-precision scanning motions of optics in the exposure apparatus. The process is exquisitely demanding, but the results are tremendous: a simple, compact, rugged, entirely integrated device capable of precisely isolating a single wavelength.
The selectivity of FBGs must keep pace with the trend toward cramming more and more information channels on a fiber. In FBG manufacturing, inaccuracies arising from structural vibrations in tools become more important. Positioners in the tooling must perform to nanometer-scale levels, and there must be measures to suppress all unwanted motions in tooling components. Air-isolation tables address ambient disturbances, but resonances driven by the scanning can still take hundreds of milliseconds to damp out — an unacceptable throughput penalty.
An understanding of the problem comes from a review of vibrational physics. After a motion, the amplitude of the resonant ringing of each element in a structure scales as e-t/ τ ,
where τ = time constant for each element's inherent resonance. For structures with damping qualities typical of precision motion subassemblies
τ ≈ ( ω n ζ )-1
where ω n = resonant angular frequency and ζ = damping ratio for the resonance. ζ is commonly defined as the ratio of the damping for the resonance to critical damping ( ζ = C/Cc) and varies from 0 (no damping) to 1 (critical damping).
The implication of these relationships is that resonances more severely degrade process throughputs as device tolerances tighten. Fortunately, damping is not the only tool for eliminating structural resonances. A patented, real-time feedforward technology called Input Shaping was developed based on research at the Massachusetts Institute of Technology and commercialized by Convolve Inc. (New York City). It is now an integrated option for the latest digital piezo controllers. It essentially prevents movements in a system from exciting structural resonant frequencies. (For a comprehensive description of this technology, see MACHINE DESIGN's Semiconductor Manufacturing Equipment Supplement, 6/17/99, pg. S48.)
An interesting trend in photonics is that the technology has been applied to progressively smaller and smaller distances. The first widespread use of optical fiber was in transcontinental and transoceanic telephony. In contrast, today's focus is on stringing highbandwidth photonic media between the central office and the home.
The trend toward spanning smaller distances is likely to continue with photonic interconnects between chips and ultimately between circuits residing on a single chip. As clock rates rise and chip features grow smaller, it becomes more difficult to route information on and between chips using conductive traces and wires. One problem is that signal delays caused by interconnections increase with the square of the reduction in feature size. In addition, a progressively larger percentage of chip real estate must get devoted to interconnect as circuit features shrink.
Fortunately, multiplexed photonic connections have no such difficulties. In that regard, photonics represents the destiny and salvation of semiconductors. Without photonics, Moore's Law may eventually be repealed. With photonics, circuit densities can continue to double every 18 months for quite some time.