Embracing Multi-Die Systems and Photonics for Aerospace and Government Applications

If you were to open an aircraft from 30 years ago, the stark contrast in technological capabilities when compared to a modern aircraft would be readily apparent to even those unfamiliar with the aerospace industry. Traditional aircraft once relied on copper wires for transmitting electrical signals and data. However, with the emergence of new chip architectures and the growing benefits of fiber optics, as well as the replacement of metals such as aluminum with carbon fiber, the way that today’s government, aerospace, and defense systems are designed has changed dramatically.

In particular, two technologies are taking the industry by storm with their impressive growth and development trajectory: 3D heterogeneous integration and photonics. 

Before we delve into how these technologies are driving innovation and enabling more advanced and efficient solutions from ground to space, let’s start by establishing some important definitions.

6 Terminologies to Keep Top of Mind

• Die: A digital, analog, or mixed-signal circuit that is either made from a single semiconductor material (silicon, germanium, etc.) or a compound material (indium phosphide, gallium arsenide, gallium nitride, silicon carbide, etc.) and implements analog or digital processing functions such as CPU, GPU, memory, RF, photonics, or sensing.
• III-V Materials: A class of compound semiconductors made from elements belonging to Group III (such as aluminum, gallium, and indium) and Group V (such as nitrogen, phosphorus, and antimony) of the periodic table.
• Multi-die System: Multiple heterogeneous dies that are integrated in a package to create a single, more complex system. Each die contains different types of circuits, and the different dies are interconnected or stacked using an interposer and techniques such as through-silicon vias (TSVs) and microbumps.
• Interposer: An active or passive circuit layer that facilitates the connection between dies with different pitches (spacing between the input/output connections) to the package substrate. Interposers can be made using various materials, including silicon, polyimide, ceramic, glass, and diamond, and can include interconnect fabric and embed other devices within them.
• 2.5D Heterogeneous Integration (HI): Heterogeneous dies integrated side-by-side, on a single plane (e.g., silicon interposer), with horizontal die-to-die interconnections. 
• 3DHI: Heterogeneous dies integrated with multiple tiers, with vertical die-on-die and horizontal die-to-die interconnections.

3DHI: Paybacks and Challenges

Modern government, aerospace, and defense systems that perform automation and cognitive processing require a high computation density and, often, processing in a small form factor. For example, when it comes to high-performance computing (HPC) designs, monolithic SoCs are no longer capable of providing the scalability and yield that designers are looking for. Since almost all aspects of system performance are limited by power and electrical input/output (I/O), size and weight reduction are crucial objectives for designers. 

By vertically or horizontally integrating different types of chips, 3DHI makes it possible to shrink big systems into small packages, leading to better computation as well as size, weight, and power (SWaP) of components — solving an important concern for space-constrained platforms (especially in harsh environments like the ocean, desert, and outer space). Smaller chips tend to yield better results and can be reused in different applications, creating new design possibilities. Moreover, owing to the proximity of these chips, 3D packages enable high-bandwidth, ultra-short-latency, and incredibly power-efficient bit transfers — a significant advantage over traditional 2D designs. 

Increasing integration density has been a great benefit for realizing more capable and portable systems, but it has also raised serious multi-physics challenges. As demonstrated by Moore’s law, the geometric size of components on a chip cannot be reduced indefinitely, and sooner or later, the number of transistors that can be integrated on a chip will reach its limit. While we have yet to reach that stage, each technological miniaturization can take longer and be very costly. 

The design and manufacturing complexity of 3DHI for challenging environments also translates to advancements in packaging technologies, interconnect solutions, and thermal management. This makes it more arduous for teams to ensure the reliable and efficient operation of integrated components for various external environments, including considerations for aircraft design assurance (DO-254/DO-178) and testing for the rigors of defense and aerospace with MIL-STD-883.

Unlike legacy EDA solutions that start from the PCB stage, Synopsys tools are unique because they can help co-design the die/package together, along with system signoff analysis. Finally, with the additional complexities around digital and analog processing, compound semiconductors, and supporting storage, memory, and photonics advancements, we see material and process engineering solutions playing a vital role in navigating integration intricacies with compound semiconductors as well as supporting existing defense designs and government research with a full tool flow for multi-die system designs.

The Parallel Rise in Photonics and Fiber Optics

A complement to our advanced multi-die system architectures is using light to communicate data and to perform functions traditionally accomplished by electronics. The emergence of photonics has revolutionized what’s possible with communication and sensing capabilities for aerospace systems. On average, a modern commercial aircraft has anywhere between 70 to 300 miles of copper cables, weighing between 1,750 and 7,000 pounds. With the dramatic data explosion from sensor networks, the need for ultra-high-resolution aerial imaging, real-time simulation, and secure communication, the bandwidth allowance of RF and copper-based infrastructure are no longer enough. 

Enter fiber optic cables. These cables are made of thin strands of glass or plastic and transmit signals using light pulses. Because these particles of light (photons) travel much faster than electrons, they have low latency, consume less energy, and produce no heat. In addition, optical fibers offer a much larger data-carrying capacity (i.e., bandwidth) than copper. Although fiber optics cannot completely replace all copper cables, substituting them with fiber and photonics can significantly reduce weight and size — all while improving thermal, energy, and spectral efficiencies.

Specifically, single-mode fibers are rapidly replacing copper to provide a smaller footprint, more resiliency to eavesdropping/breaches, and the ability to operate in an unregulated optical spectrum. While this enables an efficient communication stream between aircraft components and systems, it also means that customers don’t need to worry about spectral jamming or additional licensing when deploying new systems. 

The figure below gives a side-by-side comparison of other factors that make fiber optics a more attractive option compared to copper.

The Prospects vs. Obstacles of Integrating Silicon Photonics at Scale

Aerospace and defense systems often operate in limited physical spaces with strict weight limitations. By getting the light source closer to the system and integrating photonics with electronics, it’s possible for design teams to overcome existing I/O and power limitations at high speed (think tens of Gbps per I/O) and enable scale to tens of Tbps per I/O while managing heat dissipation requirements, energy, and cost. 

Conversely, due to the prevalent RF-heavy ecosystem, changing characteristics of many photonic devices when exposed to pressure/strain, lack of military-grade standards, and early adoption entry barriers for photonic systems, managing SWaP requirements for mission-critical systems can take time and effort. 

Synopsys offers the industry’s broadest photonic design products portfolio that can be categorized into three functional groups:

• Photonic Device Design: Synopsys RSoft™ Photonic Device Tools offer simulation for active and passive waveguides.
• Photonic IC Design: Synopsys OptoCompiler™ is the industry’s only unified electronic and photonic IC design platform.
• Photonic Systems Design: Synopsys OptSim™ is an award-winning fiber-optic and free-space optical systems and circuit simulator for designing and simulating optical communication systems at the signal propagation level.   
• Co-Packaged Optics: Synopsys 3DIC Compiler is a state-of-the-art solution for 3DHI and multi-die systems with integrated photonics. 

Recognizing the fast-growing silicon photonics market, we announced a joint venture last year with Juniper Networks, Inc. to launch OpenLight, providing the industry with the world’s first open silicon photonics platform with integrated lasers. We believe this investment will reduce existing system construction barriers and increase the adoption of integrated photonics, including systems with integrated light sources.

Multi-die Systems Foster Greater Photonic-Electronic Integration

Growing Interest from Government and Industry Organizations

While the overall adoption of photonics in aerospace and government applications is still gaining traction, the favorable economics of energy efficiency, weight, reliability, and cost has made photonics integration vital to national security.

Focused investments from governments and universities have helped increase awareness and promote research in photonics over the past decade. For instance, the U.S. Department of Defense (DoD) initiated a public-private investment program in 2015 called the American Institute for Manufacturing Integrated Photonics (AIM Photonics) to serve as a collaborative research and development hub for advanced integrated photonic technologies and manufacturing processes. 

An example of Synopsys’ work on photonics in advanced electronics packaging is the OUSD KANAGAWA program. In this program, Synopsys is a sub-contractor to Lockheed Martin to enable them to build state-of-the-art multi-die in package solutions, which include co-packaged optics (CPO) in a 2.5D/3D design. Such industry collaborations can help advance the commercialization of integrated photonics technologies and strengthen the nation's manufacturing readiness across applications.

As we progress from each successive process node, the complexity of chips and volume of unknowns will increase substantially, making it mission-critical to create solutions that are secure, safe, and reliable no matter where they are (land, sea, air, or space!). 

With multi-die systems’ ability to integrate photonics, power, and thermal analysis, our portfolio of tools such as Synopsys 3DIC Compiler coalesces numerous transformative, multi-die design capabilities to offer a complete architecture-to-signoff platform. Solutions like Synopsys ZeBu® emulation systems can handle mixed-signal designs using real numbers and scale capacity for 3D systems in addition to in-chip environmental monitoring and testing with silicon lifecycle management technologies. And our complete UCIe IP solution provides reliable, low-latency, and secure die-to-die connectivity across heterogeneous dies in a multi-die system. 

A combination of a broad array of tools and capabilities working cohesively through proven data exchange platforms will be imperative to successfully integrate photonics with the existing signal processing chain. Going forward, we envision multi-die designs and silicon photonics offering significant advantages over traditional electrical systems. Synopsys will continue playing a crucial role in investing in these technologies to shape the future of aerospace and government applications.