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A Process Design Kit (PDK) is a library of basic photonic components generated by the foundry to give open access to their generic process for fabrication.

Designers can design a wide variety of photonic integrated circuits (PICs) using the photonic components of the foundry, which are technically and geometrically represented in their Process Design Kits. 

A PDK can be compared to a set of building blocks, where each photonic component in the library is a separate block. A designer can use these blocks to build many types of photonic circuits for various applications. A generic technology is useful for reducing costs when the designer is using predefined, tested photonic components on the material platform of their choice.

A designer can also create his or her own building blocks, but the designer must follow the fabrication rules of the foundry to be able to use a custom component from a particular foundry.  Among others, the rules usually include:

  • Material stack (types of layers and thickness)
  • Minimum distance between optical components (like gaps between waveguides)
  • Maximum etching depths
  • Metallization and electrical probes (how to place the metal, metal layers allowed)
  • Feature size (size of waveguides, holes, active areas, etc.)
What is a Process Design Kit? | Synopsys

How does a process design kit work, and why is it so important?

In photonics, the basic building blocks in a PDK are waveguides, phase shifters, active sections (semiconductor optical amplifiers or SOAs and lasers), and rotators (polarizers). These resemble the basic components in electronics, which are wires/resistors, inductors, capacitors, and transistors, shown in the following figure.

Electronic integrated circuits vs. Photonic integrated circuits | Synopsys

To model a photonic component, you need to consider material and optical properties. At the device level, these properties are usually:

  • The refractive index of the material stack versus the wavelength, represented by the real and imaginary part of it (also known as optical performance)
  • Values of pre-characterized components or validated components 
  • Analytical models to predict the performance (when real data are not available)

The material and optical properties for the photonic component can be represented as a building block by the S-matrix; the S-matrix describes the signal transfer between the ports of the component from the device level to the circuit/layout level to simulate the PIC.

These building blocks are represented as white and black boxes. The black box is represented by a geometric shape, usually a rectangle with input and output ports, and it contains linear and non-linear models (or values from pre-characterized components) that represent the performance of the component; they are called black boxes because the designer can see only the input and output ports, while the foundry owns the relevant information of the component for fabrication. This information is loaded into the white box component when the foundry is assembling the mask (putting together all the photonic integrated circuit layout designs in the wafer) for fabrication. White and black boxes protect the foundry’s intellectual property because its process flow (masks, layers, and materials) cannot be seen by the designer.

Foundries use different material platforms, such as silicon photonics, InP, LiNbO3, polymer, and glass. Each material platform and foundry has (up to now) its own process flow or fabrication process which, based on the material and optical properties, determines the performance of the PICs at the physical level.

Designing a photonic integrated circuit (PIC) is not straight forward; its performance is linked to material and optical properties, which in turn are linked to geometrical shapes (light travels more or less efficiently in different geometries). The ability to design an efficient PIC comes with experience. There is a vast library of examples and papers that describe performance and design properties to help improve PIC designs. The following figure shows how a designer can create a very complex integrated circuit with hundreds of components from the basic and composite building blocks of a PDK. The layout shown on the right is a MDM (Mode Division Multiplexer).

Photonic PDK and Mode Division Multiplexer layout | Synopsys

Where do process design kits fit in the design process?

Process design kits need to be integrated into the design flow when designing PICs. Synopsys’ OptSim from the Photonic Solutions portfolio enables both PDK-driven and custom design, and it supports the widest portfolio of foundries offering Multi Project Wafer (MPW) Run access.  

With OptSim, you can use the industry’s best electrical circuit simulators in conjunction with the most comprehensive photonic circuit simulator on the respective portions of the same chip within the familiar OptoCompiler platform. 

Learn more about Synopsys Photonic Solutions

Synopsys Photonic Solutions supports Photonic IC foundries in the industry, with PDKs for photonic processes such as silicon, silicon nitride, indium phosphide, polymers, and silica-on-glass.  We support all technologies:

  • Silicon photonics

  • InP/III-V

  • TriPleX

  • SiO2/SiN technologies, including polymers, silica and more

Learn how to set up your own PDK or how to obtain PIC foundry PDKs by visiting our website.

View some of our application notes to see how our tools support PDKs from major Photonic IC foundries:

Explore Synopsys Photonic Solutions