Strategic Thinking on Open-Source PDK
In 1990, NHK hailed Japan as an “electronic powerhouse,” spotlighting the semiconductor industry. Now, three decades later, the spotlight has swung back onto semiconductors — though the star this time is cutting-edge manufacturing technology. In this piece, however, I’d like to shift the focus to the design side of semiconductors. This article follows in the footsteps of several earlier posts: “The Tale of PDKs, Past and Present,” posted December 3, 2023; “A Qualitative Cost Analysis of the Semiconductor Business,” posted August 1, 2024; and “Semiconductors We Want to Make, Semiconductors We Want to Use” posted December 23, 2024. I’m grateful that these pieces still receive steady traffic, and I hope they’ve helped broaden my understanding of the design aspects of semiconductors for those in the industry.
This time, under the title “Strategic Thinking on Open-Source PDK (Process Design Kit),” I aim to explain key points about open-source PDKs clearly. The discussion doesn’t stop at semiconductor designers — it also addresses perspectives relevant to foundries (semiconductor manufacturing service providers) and those planning semiconductor-related businesses. I hope you’ll find it an engaging read and a helpful resource.
Note: The views expressed here are my own, based on past work experience, and do not represent any organization.
What Is Open-Source Silicon?
The term “open-source silicon” refers to an ecosystem comprising the following three elements:
1. Open-Source Design Tools (EDA)
Designs are created using open-source EDA tools, and the design source files and scripts can be published and shared in a community setting so that third parties can verify, improve, and replicate them.
2. Open-Source Process Information (PDK)
Designs are created using an open-source PDK, and netlists, GDSII data, RTL code, and other design assets can be published and shared in a community so that third parties can verify, improve, and replicate them.
3. Foundry Services for Open-Source Chip Designs
A contract manufacturer (foundry) can fabricate the open-source designs generated under points 1 and 2, ensuring the hardware’s operation can be physically verified and tested.
Below, we’ll explore why this open-source silicon ecosystem is needed and its strategic business implications.
What Is a PDK (Process Design Kit)?
In my earlier piece, “The Tale of PDKs, Past and Present,” I provided a detailed historical overview. However, “PDK” is often used as a buzzword between those who offer design services and those who use them, implying “a toolbox for design.”
This interpretation is convenient for business conversations and isn’t entirely inaccurate. Yet, by viewing a PDK purely as a “design toolbox,” we sometimes hear arguments like, “We can’t possibly disclose our design toolbox!” or “Our design toolbox is brimming with secrets!” This mindset can be one of the reasons companies hesitate to open up.
As noted earlier, an open-source PDK is a key requirement for creating an open-source silicon ecosystem. In light of that, let’s look at what goes into a PDK.
Categorizing PDK by Level
Let’s take a closer look at PDK by organizing it into distinct tiers, as outlined below.
Level 1 is commonly referred to as the “Technology File” layer.
Level 2 is the layer of essential “Library Files,” indispensable for logic LSI design.
If you’re familiar with the C programming language, you can think of the Technology File as analogous to the system call functions of an operating system, while the Cell Library corresponds to the standard library (stdlib). Just as there are many C libraries catering to different practical applications, the PDK also provides various higher-level layers (Level 3 and beyond) hosting a wide range of IP (design assets). Because these assets differ depending on factors like process generation and the specific EDA tools used — and are often proprietary to a company or a commercial vendor (for example, ARM’s ISA) — they are frequently protected and thus difficult to release as open source.
Now, look deeper at the information categorized under Level 1 in the PDK.
SPICE Models
SPICE is a circuit simulator originally developed at UC Berkeley. Although development at UC Berkeley has ceased, open-source EDA projects like Ngspice and Zyce continue to progress, and several free SPICE tools — most notably LTspice — are widely available.
Device model files compatible with SPICE are needed for an open-source PDK. A device model describes, in mathematical form, the current-voltage characteristics and other behaviors of fundamental devices like transistors. The device model file itself contains a set of parameters tied to that device model.
Since parameter values vary based on manufacturing processes and materials, companies can plausibly classify them as “proprietary.” Moreover, if the extraction of SPICE model parameters is outsourced, contractual obligations may prohibit public disclosure. On the other hand, discrete semiconductor manufacturers often provide SPICE models for their products online. This practice is common even among major Japanese manufacturers such as Toshiba Devices, ROHM, and Renesas.
Running SPICE on a local PC — often using free or open-source tools — has become a standard procedure for confirming how a device will function within a user’s circuit, aiding in purchasing decisions. However, when you talk with people from Japanese semiconductor fabs, you often hear statements like, “We can’t disclose our device model files without an NDA because the transistor’s performance would become evident.” Indeed, the parameters embedded in a SPICE model are considered vital trade secrets for cutting-edge devices in single-nanometer (nm) processes (like GAA).
As we see, with numerous discrete semiconductor manufacturers publishing their SPICE models, offering open PDK files for legacy processes in a manufacturing services (foundry) context may be a valuable marketing asset for attracting potential clients.
Another point to consider is that even if the device model file itself is identical, responsibility differs between a model released as part of an open-source PDK and one released under a manufacturing service contract. In the case of open-source PDK models, the user bears the responsibility for any use. Conversely, when a model is provided under a manufacturing service contract, the foundry bears responsibility for management and is obligated to address support and modification requests. Put differently, users have no right to demand support or changes for the open-source PDK model, whereas the foundry must respond to such requests when the model is shared under a formal contract.
DRC/LVS Rules
DRC (Design Rule Check) rules — used to check design constraints such as minimum line widths — are indispensable when designing LSI layouts. In the PCB world, hobbyists can now design layouts on their computers using free software like KiCad. PCB contract manufacturers (such as P-ban.com, JLCPCB, and PCBgogo) make their DRC rules publicly available.
Similarly, LVS (Layout Versus Schematic) rules — used to verify that a layout matches the schematic — are also vital for LSI layout design. In many PCB layout tools, schematic entry is already bundled and offers a feature called “Schematic-Driven Layout,” which lets you design without worrying about LVS. In semiconductor design, commercial EDA tools have recently adopted a similar functionality. However, unlike PCB layout (which essentially involves wiring devices together), LSI layout requires rules that recognize active elements like transistors (resistors and capacitors).
When speaking with people from Japanese semiconductor fabs, you sometimes hear, “We can’t publish our DRC/LVS rule files without an NDA because it would reveal our fab’s capabilities or mask configurations.” Indeed, for cutting-edge devices at the single-nanometer (nm) level (like GAA), the parameters embedded in DRC and LVS rules could be extremely sensitive trade secrets, given the fierce competition in that space.
That said, many PCB contract manufacturers make their DRC rules public. In the context of legacy processes, making such rules public can serve as crucial marketing material, enabling potential clients to evaluate the feasibility of manufacturing services. The same logic applies to LVS rules.
Depending on the EDA tool, the file formats for DRC and LVS rules may be protected as intellectual property by commercial EDA vendors. Even so, for open-source PDKs, what matters are the manufacturing details and numerical data that the fab owns. By converting those parameters — described in the fab’s documentation — into a file format readable by open-source EDA tools, you can successfully release DRC and LVS rules as part of an open-source PDK.
Level 2
In principle, information categorized under Level 2 can be used to carry out designs with open-source EDA tools, provided you already have the Level 1 data. Strictly speaking, making Level 2 information publicly available under an open-source PDK is not mandatory. Trailblazers such as Skywater and GlobalFoundries have also chosen to release Level 2 data. Making Level 2 PDK information accessible not only spares designers from reinventing the wheel but also suggests that much of the data in Level 2 is no longer deemed confidential.
Fab companies generally provide Level 2 PDK data free of charge to their contract manufacturing customers under NDA. Alternatively, if you pay for a commercial IP vendor’s license, you can design with a standard cell library with a smaller cell area. From a fab’s perspective, Level 2 is part of the no-cost service offering to contract manufacturing clients, making it a strong candidate for open release in the form of an open-source PDK.
No Warranty
Generally speaking, the SPICE models that discrete semiconductor manufacturers make publicly available come with no warranty. In other words, the manufacturer assumes no responsibility for how you use these models to simulate circuits before you purchase their products — which makes sense. In open-source PDKs, the widely adopted Apache 2.0 License is often employed. Under this license, the published information is provided “as is” (no warranty, no liability), and users can redistribute it under these terms.
When considering the open sourcing of a PDK, it’s crucial to understand how open-source licensing works and how it differs from commercial licenses.
Open-source licenses are broadly accepted and integrated into business models in the software world. For instance, companies with open-source roots (such as MySQL) often operate under a dual-licensing strategy. The open-source license version helps the product gain widespread adoption and benefits from community feedback. Meanwhile, a commercial license version provides user support and product warranties, allowing customers to use MySQL in a commercial setting with peace of mind.
Sign-Off
As noted earlier, completing the open-source silicon ecosystem requires a foundry willing to manufacture open-source designs and verify the resulting hardware’s functionality. Yet when speaking with people from Japanese semiconductor fabs, you may hear concerns like, “Can we accept simulation results obtained with an open-source SPICE model or layout data checked using open-source DRC/LVS for contract manufacturing?”
Under normal circumstances, a fabless design house submits design data (in GDSII format) that is confirmed error-free via the fab’s approved commercial sign-off EDA tool and its corresponding technology file. The fab also runs acceptance checks using the same commercial sign-off tool and technology file before proceeding to mask generation. However, both the designer (submitting the final data) and the fab (receiving it) rely on the same sign-off environment to confirm the design is error-free.
So how do pioneers like Google, efabless, and Skywater handle sign-off verification in their open-source projects? The figure below provides a concise overview of each entity’s role in the OpenMPW project, illustrating how they manage and validate open-source designs.
How Does OpenMPW Handle Sign-Off?
• Google negotiated with Skywater to develop a PDK suitable for open-source EDA at efabless, covering shuttle (prototyping) costs.
• Google promotes OpenMPW, hosting a GitHub repository where users can push their design data.
• efabless collects the user-submitted design data from the repository, consolidating multiple designs into a single batch for manufacturing at Skywater.
• Skywater receives the combined design data from efabless, verifies it, and then fabricates test wafers on its production line.
• After fabrication, the wafers are shipped to efabless, and Skywater receives payment. At the same time, efabless handles packaging, PCBs, and final chip delivery to each user.
In other words:
• efabless acts as a proxy for users (including individuals) who wish to have their designs manufactured at Skywater.
• Google is the central GitHub repository's funding source, leading promoter, and manager.
• Any discrepancies between open-source EDA sign-off and the fab’s commercial EDA acceptance process — such as new rule additions — are handled by efabless.
Through OpenMPW, any sign-off issues are worked out between efabless and individual users, meaning the fab can treat efabless just like any other contract-manufacturing customer. If the fab can eventually perform acceptance checks using open-source sign-off tools directly, that could further reduce reliance on a proxy, potentially lowering costs for the entire ecosystem.
Key Points
Below is a summary of the crucial elements for establishing an open-source silicon ecosystem based on the preceding discussion:
1. Don’t Treat the PDK as a Monolith
Carefully consider exactly what information needs to be open-sourced. Determine whether or not that information requires NDA protection.
2. Understand “No Warranty”
Recognize that open-source PDKs are provided “as is” without liability. Leverage this concept to incorporate open-source PDKs into new business models proactively.
3. The Role of a Proxy
In an open-source silicon ecosystem, a proxy entity is essential for interacting with the fab on behalf of designers and other stakeholders.
4. Choose the Right License
Use an open-source license (e.g., Apache 2.0) with well-established legal interpretations in the software world. This ensures clarity and reduces uncertainty in rights and responsibilities.
Why Go Open Source?
Let’s explore the strategic significance of adopting an open-source PDK.
A Powerful Sales Tool
When discrete semiconductor manufacturers publish SPICE model files, they do so because it’s a critical service to attract customers. Likewise, increasing the customer base is essential for legacy fabs looking to expand and revitalize their business actively. To achieve that, they need effective “sales tools” — an open-source PDK can be exactly that.
Let's look at how open-source software often leverages dual licensing to broaden its user base while expanding its commercial business. It’s a valuable point of reference for anyone designing new business strategies.
For many small and medium-sized companies, signing an NDA at an early stage — when it’s not even certain if a product will go into mass production — can be a barrier to entry. And if we want to nurture startups, we need to include individuals who might only have a good idea. Academics can also be a highly promising market segment if the goal is to expand the future customer base. In these cases, forcing NDAs on individuals or academic institutions can be cumbersome: carefully evaluating what you’re trying to protect and what assurances you can realistically provide.
A New Horizon in Semiconductor Design Education
It’s practically impossible for individual hobbyists or students to secure commercial PDKs and EDA tools independently, given the NDAs (Non-Disclosure Agreements) these tools require. In Japan, universities can turn to VDEC — a system that grants access to commercial EDA tools strictly for on-campus research and development —. Still, that privilege comes with a pledge prohibiting usage for any other purpose. Even if you design a fascinating chip, you can’t simply pop it onto Kickstarter or Make: and sell it. While there has been some movement to ease these constraints, we’re still far from a world where anyone can freely and limitlessly use these commercial resources.
Enter open-source EDA tools and PDKs, which are completely free of NDAs and runnable on your PC. This open ecosystem makes semiconductor design accessible to high school students, technical college students, and independent enthusiasts alike. Moreover, most open-source licenses do not forbid commercial use. As a result, once you’ve produced your chip, you’re free to sell it or even share your design or IP under an open-source license while offering commercial support through a dual-licensing model.
In software, leveraging open source for PoC (Proof of Concept) projects is already standard practice — sparking new startups or securing funding by showcasing technical originality. We believe the same principle applies to semiconductor development. For entrepreneurs and startups eager to make their mark as fresh faces in the semiconductor industry, an open-source silicon ecosystem is not just a handy tool — it’s the fertile soil that helps new players take root and flourish.
Fostering Open-Source IP and Economic Security
Process Design Kits (PDKs) typically include a host of commercial IPs classified as “Level 3,” each tailored to specific applications. These can range from interface IPs such as DDR2/3/4/5, USB2/3, and PCIe3/4/5 to wireless communication IPs like Wi-Fi and Bluetooth and even MPU cores such as ARM32/64. In most cases, these IP blocks require licensing fees at the time of adoption and during mass production.
Now, if we have open-source PDKs at Levels 1 and 2, it becomes possible to design your Level 3 IP and make it open-source. Borrowing a page from the open-source software world, a vibrant ecosystem could emerge if individuals and small or medium-sized enterprises can freely develop Level 3 IP. This ecosystem might foster startups while allowing cost-sensitive projects to harness powerful IP without prohibitive upfront licensing fees.
In Europe, there is a growing movement to establish a system in which Level 3 IP designed using open-source PDKs can be independently circulated within the region. Behind this initiative lies a recognition that reliance on commercial IP vendors — largely dominated by U.S. manufacturers — constitutes an economic security risk. Even if a region has semiconductor manufacturing facilities, a sudden halt in IP supply could jeopardize the design of strategically critical chips.
Summary
The emergence of the “Open-Source Silicon” trend creates an excellent opportunity to think strategically about questions such as:
“What kinds of information can be open-sourced?” ”How can open-source PDKs be leveraged as a business tool?” “How do we nurture and expand a new generation of semiconductor designers?” “How do we safeguard economic security in the semiconductor space?”
In recent years, tech giants like GAFA have designed custom semiconductors for their products and services. This shift from software-based solutions to hardware (chips) is a strategic response to demands for reduced costs (e.g., lower power consumption) and higher processing speeds.
Similarly, CPU makers like Intel and AMD have released their AI chips for the same reason. Relying on a standard processor plus software is no longer competitive enough; custom AI-enabled silicon allows them to differentiate their offerings.
Both trends reflect a slowdown in the performance gains once achieved through the miniaturization of general-purpose processors. As a result, custom hardware solutions — once the exclusive domain of large corporations — are now gaining renewed attention.
The significant challenge for designing specialized chips for small and medium enterprises and long-tail products has been the high initial costs and relatively low production volumes. An open-source silicon ecosystem promises to lower these barriers. By leveraging open-source design flows and IP, companies can implement their unique know-how directly in silicon — ensuring both differentiation and confidentiality in ways that would be impossible if sticking solely to off-the-shelf processors and software.
Traditionally, “semiconductor engineers” are considered specialists who design and integrate circuits with sub-micron precision on silicon wafers. Yet, the real game-changer is the “system designer” — someone who can envision where a custom chip in a system’s architecture can provide real differentiation and select integrated circuit implementation as another tool in the design toolbox. A prime example is Apple’s use of “Apple Silicon” to differentiate its Mac, iPad, and iPhone products.
The underlying fabrication process itself is secondary since the choice of process is driven by functionality and cost. By tapping into the open-source silicon ecosystem, we can spark many fresh business endeavors and develop new talent in semiconductor design and systems engineering. Let’s work together to nurture the next wave of innovation — where custom silicon is no longer out of reach.
Jun
2025/Jan/17th
