A common theme here on Planet Analog has been that digital circuits and signals are really analog circuits and signals to which we’ve added an abstraction -- a layer of significance or meaning. The digital abstraction has proven itself to be among the most powerful constructs the field of electronics has ever invented -- up there, I’d assert, with superposition and communications theory.
Of course, as is true with all such constructs, the digital abstraction holds only so long as the underlying circuit’s parametric (read analog) behavior performs sufficiently to maintain the illusion. Perhaps the clearest demonstration of this fact is the eye diagram (Figure 1).
An open eye.
(Image courtesy Tektronix)
Here, symbol uncertainty or, if you prefer, error-probability rises with a range of parametric challenges including increasing phase and amplitude noise, multipath interference, and limitations on channel bandwidth (Figure 2).
A stressed eye.
(Image courtesy Tektronix)
Getting information rapidly on and off chip, be it for communication or data processing applications, is a significant concern because, in a world of deep submicron processes, I/O and associated media often form the throughput bottlenecks. I mention this because an announcement that slipped under my radar (and may have done the same with yours) last December concerns technology advances that can significantly broaden I/O bandwidth.
The technology -- called silicon nanophotonics -- integrates optical components with electrical circuits on a monolithic device. IBM has succeeded in fabricating silicon nanophotonic devices for the first time on a sub-100nm semiconductor process (Figure 3).
This image depicts a device IBM fabricated on a 90-nm process that integrates a photodetector (red feature on left), a modulator (blue feature on right), silicon transistors, and eight metal-interconnect layers.
The work builds on initial proof-of-concept efforts in 2010, but the more recent effort demonstrates IBM’s ability to transfer silicon nanophotonic technology to a semiconductor process running in a commercial foundry (Figure 4).
IBM silicon nanophotonic chip showing blue optical waveguides and copper conductors for
high-speed electrical signals.
IBM described the work in a paper presented at the last IEEE IEDM (International Electron Devices Meeting) held December 10 through 12, 2012 (Reference 1). Enhancements to a standard 90nm high-performance process allow integration of optical modulators and germanium photodetectors. The resultant process is optimized for analog functionality and supports monolithic fabrication of power-efficient multichannel WDM (wavelength division multiplexed) 25 Gbps transceivers. The 90nm process with 63nm gate lengths produces NMOS devices with Ft greater than 150 GHz with high transconductance and good matching characteristics.
Amongst the passive nanophotonics features IBM integrated without additional mask layers were optical waveguides, waveguide crossings, directional couplers, lateral fiber couplers, wide-temperature range (±30°C) WDM filters, and vertical grating couplers. With minimal add-on mask layers, the technology can implement active nanophotonic functions including modulators, E/O (electrical-to-optical) and O/E (optical to electrical) converters, and waveguide-integrated photodetectors.
- Assefa, Solomon, et al., A 90nm CMOS integrated nanophotonics technology for 25-Gbps WDM optical communications applications, IEDM, Dec 2012.