Yesterday, today and tomorrow are "HOT" "silicon light"
It was said that it was popular yesterday because silicon-based optoelectronic technology began to develop as early as 30 years ago. In 1985, Richard Scoref et al. believed that monocrystalline silicon can be used as an optical waveguide material for microelectronic applications and optical signal transmission. A series of theoretical foundations for laying out silicon light technology have emerged, such as Sore waveguide theory, laser amplification theory, silicon doping modulation, etc., but the development of devices has begun to be slow. Until 2004, Intel successfully developed a 1Gbps silicon light modulator, marking a breakthrough in silicon optoelectronics research for many years.
Say it's hot now, take a look at the many big-level players in the industry chain diagram below, you need a small K to say more? As Professor Chu Tao of the School of Information and Electronic Engineering of Zhejiang University said in his speech entitled “Ubiquitous Silicon Light Chip Technology” in September 2018:
No one can guarantee that silicon light can't make money,
But no company,
Can withstand the loss caused by not doing silicon light.






Figure 2: From Yole Development, April 2019


In the current shipment of silicon optical module products, there are two main categories:


■ Data Center Optical Module: Manufacturers represented by Intel can supply 100G CWDM4 and PSM4 silicon optical modules in batches.
■ Medium and long-distance coherent optical modules: for DCI connection between data centers or long-distance communication for telecom metropolitan area backbone networks, Acacia, Macom, SiPhotonics, and the National Information Optoelectronics Innovation Center led by Xunxun have launched coherent silicon light. Chip products, in which silicon optical coherent modules such as Acacia and Macom have been mass-produced.
And it will be popular. In the figure below, we can see that the forecast data from Yole shows that the compound annual growth rate of silicon light from 4018 to 2024 is 44.5%.


“The future of silicon photons will have the highest compound annual growth rate of 44.5%”
“The market will grow from $455 million in 2018 to $4 billion in 2024.”
--Yole analyst Eric Mounier


Figure 3: From Slicon Photonics and Photonics Integrated Circiuts 2019, Yole Development






The advantages of disilicon light and industrialization challenges
At present, the data centers of major Internet giants have adopted 100G interconnection technology and will upgrade to 400G interconnection. Therefore, the implementation technology of 400G optical modules has become the focus of the industry. At the OFC exhibitions in 2018 and 2019, major companies at home and abroad exhibited 400G QSFP-DD and OSFP products, and in the silicon light industry chain, the only one across the Epi wafer, fabless, foundry Intel, a company in the field of transceiver, has mass-produced and shipped a number of 100G PSM4 and CWDM4 silicon products, and also disclosed on the 2019 interconnect day that 400G silicon products will be mass-produced by the end of 2019.






Figure 4: From Hong Hou Intel Silicon Photonics 400G DR4
From the above figure, you can see the laser used for the light input and output of the gray area on the right side of the module. The modulator and detector parts are integrated on the silicon optical chip, 4 channels per channel 100G, and finally achieve 400G rate. Although it seems that the process of industrialization of silicon light is almost the same as that of traditional optical modules, from the analysis report of lightCounting, another market research company in the field of optical communication, silicon optical devices currently occupy only 10% of the entire transmission market. Share, the biggest competition it faces is still from the traditional InP and GaAs discrete and integrated devices/modules. The traditional products are more mature than the technical route, the market time is longer, and the application is more popular, so the market scale is far greater than silicon light. market. The long road ahead of silicon light also relies on CMOS-compatible processes to achieve mass production, share costs, and ultimately compete in wafer-level manufacturing, packaging and test technology, and become the leading technology force in the post-Moore era.
Figure 5: From LightCounting
At the Keysight World Shanghai 2019 conference, Dr. Chen Ben from Hengtong Rockley also admitted that the current challenges of silicon industrialization:


The table is from 2019 Keysight World Shanghai Dr. Chen Ben's speech
Maybe there will be a small partner who wants to ask at this time:
"Since all are doing 100G, 400G products, what is the difference between silicon-based optical products and discrete and integrated? Why do you have to invest in the silicon direction since you are faced with so many challenges?"
Before answering this question, we still have to review together what silicon-based light is. Here, we directly quote the definition of Professor Zhou Zhiping from Peking University:


Research and development of silicon-based large-scale integration technology using photons and electrons as information carriers. The core content is to study how to "small", "siliconize" photonic devices and integrate them with nanoelectronic devices, that is, using silicon or other materials compatible with silicon, using silicon technology, simultaneously fabricated on the same silicon substrate. A number of micro-nano-scale information function devices based on photons and/or electrons form a complete new large-scale integrated chip with integrated functions.
——Zhou Zhiping, Silicon-based Optoelectronics
It can be seen that several key points of silicon-based light are also the advantages of adopting this technology:


■The optical signal has small attenuation and high transmission bandwidth during transmission, and can obtain super fast rate and high anti-interference characteristics.
■Using existing microelectronics technology to achieve advantages in large-scale CMOS integration, low power consumption, low cost, etc.
■ The process of integrating optical transmission channels on silicon chips is relatively difficult
■ CMOS integration of silicon light, electricity, and other materials (mainly III-V) using silicon as a substrate
Then I went back to the questions of the friends at the beginning, why the industry is doing 100G, 400G products, and still have to do 100G, 400G products based on silicon-based light? The root cause is due to the low cost, low power consumption, high integration and high transmission bandwidth of silicon light technology. As can be seen from the above diagram of LightCounting's Transceiver market trend chart, the share of discrete devices is flat and the share of integrated devices and the share of silicon products are increasing. It can be seen that integrating more and more devices is the trend of the times, and everyone is optimistic about the silicon light market for the following three reasons:


1. The optical module architecture is mainly composed of a light source, a modulator, a fiber/waveguide, a detector, etc. The conventional process needs to sequentially package an electric chip, an optical chip, a lens, an alignment component, an optical fiber end face, etc., and finally realizes a modulator. Highly integrated with receivers and passive optics. Compared with traditional discrete devices, the optical module under the silicon light technology is based on the CMOS manufacturing process, and the etching process can be quickly processed on the silicon substrate, so that the volume is greatly reduced, the material cost, the chip cost, and the packaging cost are further optimized. Silicon light technology can be tested in batches by methods such as wafer testing, and test efficiency is significantly improved.




Figure 6: From OFWEEK
2. The traditional optical communication module is mainly packaged by III-V semiconductor chips, high-speed circuit chips, passive optical components and optical fibers. However, as transistor sizes continue to shrink, electrical interconnects face many limitations. The industry has found that Moore's Law is no longer applicable, and 50Gbps is close to the limits of traditional copper circuits. The "light into the copper back" inside the data center and at the chip level becomes inevitable. Silicon light, that is, using a laser beam instead of an electronic signal to transmit data, integrating the optical device and the electronic component in a separate microchip, and replacing the copper wire as an information transmission medium on the silicon wafer to improve the connection between the chip and the chip. speed.
3. The reduction in cost and power consumption for large-scale integration is also obvious. Take the Intel 100G short-distance PSM4 optical module as an example. In the traditional 100G PSM4 solution, four 25G lasers are used to separately modulate four signals to transmit 100G signals through four optical fibers. Intel's silicon optical 100G optical module is highly integrated with a modulator. And passive optical path, using only one 25G laser, to achieve modulation and transmission of four signals, very cost advantage.


Figure 7: From Intel Silicon Photonics 100G PSM4 QFSP28 Transceiver report
The key point here is that silicon is an indirect bandgap semiconductor, and it is difficult to become a light source material. To integrate a silicon-based integrated optical path to generate a light source, it is necessary to heterogeneously integrate the III-V family and the silicon material. This way of combining several materials can not only use the III-V material to achieve on-chip active functions, but also take advantage of the silicon-based integration process, so silicon light is not just a silicon.


three
Application field and development direction of silicon light
Cloud computing, Internet of Things (IoT), streaming video, and 5G force data centers to grow exponentially and have new demands on energy consumption, latency, security and reliability of the entire communication network. All these factors are a turning point in the industrialization of silicon light. Silicon light is expected to bring a new wave of chips, modules, and systems to meet the growing demand for faster, more data.
Compared to traditional circuits and systems, silicon-based systems use light to transmit more data, with higher efficiency, faster transmission speeds, and lower power consumption. Silicon light technology provides a solution that enables significant and more efficient bandwidth transfer within and between data centers. Optical communication-based systems use light to transmit data over fiber optic lines faster and more efficiently than systems that transmit electrical signals over copper wires. Simultaneous multi-level pulse amplitude modulation (PAM4), advanced optical modulation formats such as Quadrature Amplitude Modulation (QAM) and Orthogonal Frequency Division Multiplexing (OFDM), and coherent detection techniques improve spectral efficiency. Silicon light coverage ranges from short-range chips to chips to local area networks (LANs) and wide area networks (WANs) over 100 kilometers. For the telecommunications field, silicon technology increases the amount of data transmitted over the same fiber line without the need to extend the fiber optic cable. For data center interconnects that are typically less than 80 kilometers away, silicon light carries higher capacity and more efficient data transfer. As new technologies continue to evolve, silicon photonics technology will enable new designs for semiconductors, chips, optical components, and entire data systems. These new optical components will provide higher bandwidth, higher power efficiency and faster speeds than traditional electronic systems.


Figure 8: Application areas of silicon light
At present, most of the silicon optical modules are used in the data center and metro backbone networks, accounting for more than 90%. With the approach of 5G commercial scale, there are about 50 million 25G/50G optical modules required for 5G forward. 5G is the most basic requirement for transmission rate, ultra-low latency and high stability. These are undoubtedly silicon. The area where light technology is best at. It is generally believed in the industry that the highly integrated features of silicon light technology will have greater demand in the more sensitive consumer areas, such as smart sensing, smart driving, laser radar, quantum communication and other fields.
However, as the entire industrial chain is still in the early stages of development, the overall shipments are low, the cost effect of CMOS mass production cannot be realized, and the improvement of yield and the optimization of manufacturing processes cannot be supported. In terms of cost, power consumption and performance, it does not show a subversive advantage compared to traditional optical module products. However, the overall optical module industry is expanding at a compound annual growth rate of 10% per year, and the share of silicon light is also rising, gradually improving.
The advantage of silicon light is integration. So is the ultimate goal to integrate active devices, passive components, and circuits in current optical modules? In the 2018 OFC, we saw a silicon optical chip exhibited by Huntong Rockley: laser external, integrated optical transceiver function on the switch chip, equivalent to bringing the distance between the optical module and the switch chip infinitely close together.


Figure 9: From the OFC 2018 Hengtong Rockley Show


Co-packaging has many benefits, as Pennies Consulting pointed out in the 2019 OFC's PIC workshop:


"It can realize high-speed, high-bandwidth design with separate circuit and optical path. It can realize automatic mass production through the combination of integrated connector and optical path part, multi-channel expansion and can use advanced circuit and heat-dissipation packaging technology in electric package. ”


——Pennies Consulting






Figure 10: Speech from Pennies Consulting's 2019 OFC's PIC workshop
However, you should also consider the challenges of the test and verification that will be faced after the Co-package. The first thing to bear is the signal integrity and power integrity issues. After the integration is high, you need to consider the detection problems during the test, the heat dissipation of the module and the integration. Testing, etc.:






Conclusion


Although the silicon optical chip industry faces many difficulties, it cannot be denied its broad market prospects. The series of technical problems facing now are slowly being resolved. The breakthrough in the first commercial "100G silicon optical transceiver chip" in China indicates that the "light into copper retreat" at the chip level will be the general trend, and the commercialization of silicon light technology will be expected in the future. Some mainstream companies have begun to deploy silicon photonic chips and devices in various ways. One is the foreseeable market opportunity, and the other is to optimize their product structure to form a competitive advantage. However, faced with the critical issues of production, yield, production process, etc., we will also find that silicon light is not a disruptor. Maybe it will coexist with III-V for a long time in the next 5-10 years, but The great influence of silicon light on the development of the optical communication industry is unquestionable.
Finally, with regard to silicon-based active, passive chips, devices and modules, Keysight can provide a corresponding test solution. At the OFC show in March 2019, Keysight also teamed up with Form Factor to showcase the real on-wafer silicon test site.
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