Silicon MMIC Design with Cadence Virtuoso Studio RF Platform

Monolithic microwave integrated circuits (MMICs) combine passive and active components onto a single semiconductor substrate to operate at microwave frequencies (300MHz to 300GHz). Often packaged into discrete surface mount components for PCB-based systems or integrated along with other ICs in an RF front-end module, MMICs are critical components commonly found in wireless communication, satellite systems, radar, and military electronics.

While MMICs are commonly fabricated using semiconductor materials such as gallium arsenide (GaAs) and gallium nitride (GaN), silicon (Si) MMICs are increasingly attractive due to their cost-effectiveness and mature silicon manufacturing processes. With the advent of nanoscale technologies, silicon MMICs and RFICs have seen remarkable performance improvements, making them competitive alternatives to traditional III-V (GaAs, GaN) semiconductor MMICs. However, silicon MMIC designers require EDA solutions that can tackle the increasing complexities associated with higher operating frequencies, smaller geometries, and increased integration density.

The Virtuoso Studio RF platform from Cadence addresses these challenges with proven silicon modeling and simulation technology, foundry-approved 3D EM simulation, integrated thermal analysis, and robust multi-technology capabilities. It operates from within a single environment optimized for RF and microwave design methodologies.

The Challenges of Silicon MMIC Design

Designing MMICs, especially high-frequency silicon, poses many challenges. The high-frequency operation necessitates careful consideration of parasitic effects, electromagnetic interference, thermal concerns, and signal integrity. At higher frequencies, parasitic capacitances and inductances become more significant, affecting circuit performance and making accurate modeling difficult. These parasitic effects are highly dependent on the layout of the circuit. Even slight variations in models and layouts can significantly impact design outcomes in high-frequency ICs.

In addition to providing the infrastructure and capabilities for fabricating chips, semiconductor foundries play a crucial role in the success of any MMIC design by validating model accuracy and ensuring the quality of their process design kits (PDKs). To deliver the expected correlation between circuit simulation and measured results, they focus on incorporating parasitic elements, process variance, element mismatch across operating conditions, and modes of operation into their PDKs and simulation models. The simulation and manufacturing IP from the foundry is passed along to the MMIC designer through the PDKs and supporting EDA tools.

A design process based on multiple iterations of design, simulation, and prototyping can be too time-consuming and costly for today's development cycles. The need for accurate modeling of the silicon substrate and the interconnects further complicates the design process. Foundry validated PDKs combined with proven in-design EM analysis provide the necessary accuracy to reduce the likelihood of design respins.

The Virtuoso Studio RF platform offers several distinct capabilities to address these challenges. Virtuoso Studio RF leverages foundry-validated Spectre simulation models, Virtuoso PDKs, and the proven accuracy of EMX 3D Planar Solver's electromagnetic (EM) modeling engine to offer advanced parasitic extraction and accurate modeling of layout-dependent effects. By incorporating EM analysis directly into the design flow, designers can predict and mitigate undesired parasitic behavior early in the development process. Virtuoso Studio RF provides access to these proven Cadence custom IC technologies from within a powerful RF/microwave design environment to support silicon MMIC design flows.

Support for RF Design Methodologies

High-speed analog designs approach interconnects as parasitics—unavoidable elements that degrade performance. A typical high-speed design flow performs parasitic extraction to include these effects in post-layout circuit simulation and design verification. Conversely, in RF and microwave design, interconnects are critical design elements that can be shaped, tuned, and optimized to enhance system performance. This approach to physical design (layout) is essential to RF design methodologies, accurate circuit simulation, and achieving optimal performance.

The Virtuoso Studio RF platform employs a unified database to store all project data, including schematics, layouts, EM structures, data files, and system diagrams. This unified database supports concurrent electrical and physical design, whereby changes made in the schematic are automatically reflected in the layout, and vice versa, accelerating the design process and reducing errors. The unified data model also facilitates in-design multiphysics (EM and thermal) analysis and design verification, directly incorporating analysis results into circuit simulations without the need to export layout definition and import extraction data.

PDKs – The Essential Building Blocks for IC Design

PDKs provide the necessary data files from semiconductor foundries to ensure IC manufacturability and simulation models for design. They include technology files that define layer information and other process-specific parameters, SPICE or similar device models that accurately represent the electrical behavior of transistors and other components for circuit simulation, parameterized cells (pcells) for customizable layouts, and verification tools like design rule checks (DRC) and layout-versus-schematic (LVS) decks.

Developing a comprehensive and accurate PDK can take a considerable amount of time, spanning from several months to years, especially for advanced process nodes. Foundries invest heavily in developing their PDKs, leveraging their deep knowledge of process technology, device physics, and manufacturing capabilities. Foundries perform rigorous validation and testing of their PDKs to ensure accuracy and compliance with design rules and process specifications. Kit development is iterative, involving ongoing updates and refinements based on design house feedback and evolving process capabilities.

Foundries collaborate closely with EDA vendors to ensure that PDKs are compatible with EDA design tools and flows. With the time and effort invested in kit development and given how widely adopted Cadence Virtuoso is for analog, mixed-signal, and custom digital IC design, foundry PDKs are primarily developed for Virtuoso, leveraging Cadence's proprietary SKILL language for customization, Pcells, and automation.

Interoperable PDKs (iPDKs) based on a common database (OpenAccess) and standard languages like Tcl and Python for customization have been developed from Virtuoso PDKs to support multiple EDA vendors. However, compared to Virtuoso PDKs optimized for a single environment, iPDKs require additional effort from the foundry to ensure full compatibility and performance across different tools.

Image courtesy of Kumar, A., & Bae, J., "Deep Dive in Foundry Process Design Kits (PDKs)," Proceedings of the 2024 Design Automation Conference (DAC), 2024.

PDKs developed for Virtuoso can be enabled for use in Virtuoso Studio RF, ensuring consistent accuracy and efficiency in RF design. PDKs developed and validated by foundries that have been enabled for use in Virtuoso Studio RF incorporate the same pCells, callbacks, and simulation models used in the Virtuoso Studio platform.

Through collaboration between Cadence and the foundry, enablement is simply a matter of adding Virtuoso Studio RF-specific housekeeping files to the existing PDK and does not require any modification to the original Virtuoso PDK. Since Virtuoso Studio RF and Virtuoso use the same PDK files, circuit simulation engine (Spectre), and EM analysis tool (EMX), accurate and consistent simulation results are achieved across either platform.

Virtuoso Studio RF, Spectre RF, EMX, and PVS

The Cadence Spectre RF simulator provides RF analyses for the design and verification of RFIC and MMIC designs such as mixers, transceivers, power amplifiers, and high-speed analog designs, including dividers, switched capacitors, filters, and phased-lock loops (PLLs). With Virtuoso Studio RF, the Spectre RF simulator also offers RF designers a library of microwave design elements, piece-wise harmonic balance analyses, X-parameters, large signal stability analyses, and harmonic load/source pull analyses.

The Cadence EMX Planar 3D Solver allows designers to model complex layouts accurately. Offering uncompromised accuracy, coupled with capacity, speed, and design flow automation, the EMX Solver has been chosen by many foundries as the preferred EM analysis tool supporting process nodes down to 2nm. Its seamless integration with the Virtuoso platform and Virtuoso Studio RF provides consistent extraction of passive components and MMIC interconnects.

Cadence Physical Verification System (PVS) provides in-design and back-end physical verification, constraint validation, and reliability checking. PVS is integrated into the Virtuoso Studio RF platform, providing Si MMIC design signoff checks (DRC and LVS) with comprehensive debug tools to reduce debug time and increase productivity.

Summary of Virtuoso Studio RF Features

• Comprehensive design environment: Designers can leverage a unified environment that integrates schematic capture, layout editing, and electromagnetic analysis. This ensures consistency and accuracy throughout the design process.
• Virtuoso PDK enablement: This allows engineers to develop MMIC devices using leading silicon RF processes with foundry-validated simulation models, PCells, and callbacks simulated with the Spectre Simulation Platform to ensure accurate performance predictions. The platform also supports in-design analysis with EMX Planar 3D Solver for parametric optimization of on-chip passives and interconnects with best-in-class EM analysis for silicon ICs.
• Advanced simulation and modeling: The platform offers state-of-the-art simulation with Spectre RF to accurately model the behavior of RF components and the silicon substrate. This includes support for harmonic balance analysis, circuit envelope simulation, and advanced RF analyses, including harmonic source/load pull and nonlinear stability.
• Automation and optimization: The platform includes powerful automation features for layout generation, parasitic extraction, and design rule checking. These tools help reduce design time and optimize performance.

Conclusion

The introduction of Cadence's Virtuoso Studio RF platform marks a significant milestone in the field of silicon MMIC design. By providing a comprehensive and integrated suite of tools, the platform addresses the complexities of RF design and enables designers to create high-performance MMICs with greater efficiency and accuracy. As the demand for advanced communication systems continues to grow, the Virtuoso Studio RF platform will play a crucial role in driving innovation and shaping the future of RF technology.

Learn more about the Cadence Virtuoso Studio RF Platform.