Silicon Photonics

Fueling the Next Information Revolution

Daryl Inniss, Roy Rubenstein

• Publisher: Morgan Kaufmann
• Year: 2016
• Print ISBN:
  9780128029756
• eBook ISBN:
  9780128029923
• Topic: Computer Science
• Subtopic: Hardware
• Index: Computer Science

Silicon photonics uses chip-making techniques to fabricate photonic circuits. The emerging technology is coming to market at a time of momentous change. The need of the Internet content providers to keep scaling their data centers is becoming increasing challenging, the chip industry is facing a future without Moore's law, while telcos must contend with a looming capacity crunch due to continual traffic growth. Each of these developments is significant in its own right. Collectively, they require new thinking in the design of chips, optical components, and systems. Such change also signals new business opportunities and disruption. Notwithstanding challenges, silicon photonics' emergence is timely because it is the future of several industries. For the optical industry, the technology will allow designs to be tackled in new ways. For the chip industry, silicon photonics will become the way of scaling post-Moore's law. New system architectures enabled by silicon photonics will improve large-scale computing and optical communications. Silicon Photonics: Fueling the Next Information Revolution outlines the history and status of silicon photonics. The book discusses the trends driving the datacom and telecom industries, the main but not the only markets for silicon photonics. In particular, developments in optical transport and the data center are discussed as are the challenges. The book details the many roles silicon photonics will play, from wide area networks down to the chip level. Silicon photonics is set to change the optical components and chip industries; this book explains how. - Captures the latest research assessing silicon photonics development and prospects - Demonstrates how silicon photonics addresses the challenges of managing bandwidth over distance and within systems - Explores potential applications of SiP, including servers, datacenters, and Internet of Things

Chapter 1

Silicon Photonics

Disruptive and Ready for Prime Time

Abstract

This chapter introduces silicon photonics and addresses its importance. Silicon photonics is not just another optical technology for high-speed communications—it will ultimately benefit both photonics and electronics. It is also a strategically important systems technology, reflected by the spate of vendor acquisitions of silicon photonics startups. This chapter also looks at the status of silicon photonics, whether it has reached its tipping point, and notable market opportunities. Lastly, the question of whether silicon photonics is a disruptive technology is answered.

Keywords

Silicon photonics; Moore’s law; indium phosphide; vertical-cavity surface-emitting lasers; disruptive technology; systems; disaggregated servers

This is the interesting thing about technology, you never really know how successful it will be.

Vladimir Kozlov [1]

There is a difference between a viable technology and the commercial application of it.

Mario Paniccia [2]

1.1 Introduction

In the 1950s the world welcomed the rise of the electronic transistor, which ultimately led to the popularization of the computer. In the 1990s optical technology enabled the exponential growth of data transmission, connecting computers globally, which gave rise to the democratization of the Internet and the World Wide Web. The next step in the journey of the digital economy—the application of optics to electronic processes and vice versa—is silicon photonics.

Optics—the use of light to send signals through a transparent path—is playing an increasingly important role in communications across a vast scale of distances. For two decades or more, it has allowed the networks of the telecommunications operators—the telcos—to cope with huge annual growth in data traffic. Now photonics is also playing a central role in the data center, where Internet data is received, processed, and distributed.

Silicon photonics can be viewed in several ways. From an optical component industry perspective, it is the most recent technology to join several established technologies used to make optical devices. This is a valid but narrow viewpoint, because silicon photonics is much more than that.

Silicon photonics enables optical devices to be made on a silicon substrate and fabricated in a chip facility. The resulting devices are starting to be adopted by the optical industry, but the technology’s commonalities with the much larger semiconductor industry is raising its profile among the chip giants. It is a technology the chip industry recognizes it will need, to tackle input–output bottlenecks in its more complex chips. The adoption of silicon photonics by the semiconductor industry will have far-reaching consequences, as is explained in the book.

This chapter introduces silicon photonics and addresses its significance. The status of silicon photonics and whether it has reached its tipping point are also discussed. Two other points are tackled briefly: its market opportunities, and whether silicon photonics is disruptive. This chapter highlights key issues and themes that are expanded upon in the book.

1.2 Silicon Photonics: An Introduction

Silicon photonics luminary Professor John Bowers of the University of California, Santa Barbara describes silicon photonics as bringing CMOS processing to optics.

CMOS—short for complementary metal-oxide semiconductor—has been the bedrock technology of integrated circuits for decades, and chip-making is one of the most advanced mass volume manufacturing processes ever developed.

Hundreds or thousands of chips, millimeters in size, are processed in parallel on a single silicon wafer measuring 300 mm (12 in.) in diameter. The bigger the wafer, the more devices can be made on it and the better the economics of chip-making. Silicon wafers are processed in chip fabrication plants, known as fabs, that run 24 hours a day, 365 days a year. Modern chip fabrication plants are hugely expensive factories, costing billions of dollars.

CMOS transistors made on wafers in these plants now have feature sizes as small as 14 nm, a fraction of the width of a human hair. Feature size refers to a key dimension of a transistor. The continual reduction in feature size by the chip industry has enabled ever more transistors to be crammed onto a chip, the consequences of which are described by Moore’s law. Based on an observation by Gordon E. Moore, the law states that the complexity of integrated circuits doubles every 18 months (later amended to every 24 months).

Silicon photonics aims to piggyback on the huge semiconductor industry—its know-how and the vast investments it has made over decades. Silicon photonics is not the same as the CMOS process used to make chips. Silicon photonics involves creating, processing, and detecting light and, not surprisingly, has its own manufacturing requirements that differ from those used to make electronic chips. These processes include not just the wafer processing to make the photonic circuits but also custom circuit testing equipment and device packaging.

But the benefits the semiconductor industry can bring to photonics are unquestionable. The chip-making process can be used to make efficient light pipes—waveguides—that direct the light between optical functions. The precision manufacturing of chip-making improves photonic device optical performance and device yields, and large silicon wafers benefit the economics of component making. The chip industry also brings packaging and testing benefits, as well as a sophisticated design tool environment.

Silicon has a key shortfall, however: it does not lase because it does not give off light—photons—when driven with electrons, a consequence of its electronic structure. Here, optical materials such as indium phosphide and gallium arsenide—known as III-V compounds based on the columns in the periodic table—are needed to provide a silicon photonic circuit’s light source, a topic discussed in Chapter 3, The Long March to a Silicon-Photonics Union.

The same applies to detecting light—converting photons to electrical current using a photodetector circuit. Silicon needs help, and here the element germanium is used. The chipmakers have already been down this path of adding materials to advance CMOS, so it is not a deal-breaker for silicon photonics. But as will be explained in Chapter 3, The Long March to a Silicon-Photonics Union, it is not trivial either, and several approaches are being pursued by the silicon photonics players.

Another distinction of silicon photonics is that, unlike chip-making, it uses much larger feature sizes. The minimum size of silicon photonics waveguides—the optical equivalent of wires—is governed by the light’s transmission wavelength. The light is in the infrared part of the electromagnetic spectrum; its wavelength is several orders of magnitude larger than an electron, which means that much larger feature sizes and hence older CMOS processing nodes—at 130, 90, and 65 nm—are sufficient to make the optical waveguides.

Compared to today’s 14-nm CMOS processes, these larger CMOS sizes are archaic. CMOS manufacturing equipment already mothballed has been given a new lease of life, thanks to silicon photonics. And such processes, no longer state of the art, are far cheaper to operate. In turn, the optical masks used to pattern and construct the chips are a lot cheaper to make for these older processes.

In summary, while silicon photonics is driving its own requirements, being able to use a chip fabrication plant brings huge advantages such as precision manufacturing, device yield, and volume manufacturing, which ultimately promises cheaper chips.

Silicon photonics is not about using silicon for everything. That misses the point, says Professor Bowers. The key element is using silicon as a substrate—on 12-in. wafers rather than the smaller 2-,3- and 6-in. wafers used for indium phosphide or gallium arsenide optical devices—and having all the process capability of a modern silicon CMOS facility.

1.2.1 The Application of Silicon Photonics

There are several battle lines between the different technologies when it comes to communicating data. And these battle lines are shifting as data rates continue to grow.

One competitive battle is between optics and copper wire. Optical technology has long secured the role of sending core network traffic over long distances. The amounts of data are too vast and distances too great—hundreds and even thousands of kilometers—for this to be done using copper wire. That is because copper’s capacity-reach product is orders of magnitude lower than that of optical fiber.

The current battleground between copper and optics is over distances of several meters to a few kilometers. Copper wire is still used to deliver data to the home in the form of telephone wires and broadband services, but when it comes to data centers and the higher gigabits-per-second links used to connect equipment, copper starts to run out of steam after a few meters. The underlying trend is that optics continues to advance, slowly pushing copper’s use to ever shorter distances.

There is also competition within optics, between the three main technologies used to implement communications. Indium phosphide has secured the long reach while vertical-cavity surface-emitting lasers (VCSELs), made using gallium arsenide, are used for tens of meters to a few hundred meters. And like copper, the reach of VCSELs diminishes as data rates continue to rise.

Silicon photonics is the newest of the three optical technologies. Being silicon-based, the technology suits being used ever closer to electronic chips. Silicon photonics has also been shown to work within an electronic chip, sending and receiving data on and off the chip. Such an application of silicon photonics is still some way off commercially, but it is coming. Fig. 1.1 shows the breadth of silicon photonics’ reach.

Figure 1.1 The reach of silicon photonics and other interconnect technologies. Based on “On-board Optical Interconnect,” CTR III TWG Report #3, MIT Microphotonics Center, April 2013, figure 2.

The issues of bandwidth and reach for the different optical technologies and for copper are discussed in Chapter 2, Layers and the Evolution of Communications Networks. Note that Fig. 1.1 segments distances into layers, a classification we introduce and expand upon in Chapter 2, Layers and the Evolution of Communications Networks.

1.3 The Significance of Silicon Photonics

The central tenet of silicon photonics is that it has an edge based on its ability to exploit the huge investment made over decades in the mass production of semiconductor chips. This is not something that established optical technologies such as indium phosphide and gallium arsenide can benefit from to the same degree. But while the basic premise is sound, the early reality of silicon photonics has proved more complex.

Silicon photonics uses silicon-on-...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• Acknowledgments
• Chapter 1. Silicon Photonics: Disruptive and Ready for Prime Time
• Chapter 2. Layers and the Evolution of Communications Networks
• Chapter 3. The Long March to a Silicon-Photonics Union
• Chapter 4. The Route to Market for Silicon Photonics
• Chapter 5. Metro and Long-Haul Network Growth Demands Exponential Progress
• Chapter 6. The Data Center: A Central Cog in the Digital Economy
• Chapter 7. Data Center Architectures and Opportunities for Silicon Photonics
• Chapter 8. The Likely Course of Silicon Photonics
• Appendix 1. Optical Communications Primer
• Appendix 2. Optical Transmission Techniques for Layer 4 Networks
• Index