Mastering RF design challenges: The role of capacitors and context-aware PEX Tools

By Claudia Relyea and Sandeep Koranne

Updated January 17, 2025

The increasing complexity of modern RF circuits demands precise modeling of parasitic effects to ensure optimal performance. Fifth-generation (5G) networks promise ultra-fast speeds, lower latency, and enhanced service flexibility, while the Internet of Things (IoT) is driving demand for efficient integration, improved power management, and exceptional RF/mmWave performance. Together, these innovations are reshaping industries from transportation to entertainment.

However, as exciting as 5G and IoT applications are, they also present unique design hurdles, including stringent power management and high linearity requirements. Designers must carefully select process technologies such as fin field-effect transistors (finFETs) or fully-depleted silicon-on-insulator (FDSOI) to balance performance, cost, and complexity.

Within this context, capacitors—particularly metal-insulator-metal (MIM) and metal-oxide-metal (MOM) types—play a vital role in analog and RF design. These components power critical applications like oscillators, bandpass filters, and charge pumps, making their precision and efficiency essential for design success.

This blog delves into the critical role of capacitors in RF design and explores how advanced PEX tools like Calibre xACT can help engineers overcome these challenges.

The vital role of capacitors in analog and RF design

Capacitors are passive devices that store electrical energy in an electrostatic field. Their design and application are central to ensuring the reliability and efficiency of RF circuits. For instance:

MIM capacitors: Used in oscillators, coupling circuits, and bypass capacitance due to their highly linear nature and wide dynamic range.
MOM capacitors: Constructed with inter-digitated metal layers, these capacitors offer higher capacitance density and reduced costs by eliminating the need for additional masks.
Vertical natural capacitors (VNCAPs): A type of MOM capacitor with stacked inter-digitated layers connected by vias, delivering higher capacitance per unit area and excellent Q-factor.

These capacitors are integral to circuits like analog-to-digital converters (ADCs), tuners, and digitally controlled oscillators (DCOs), where precision in capacitance and parasitic extraction (PEX) accuracy are critical.

A deep dive into this topic is also available in our technical paper Parasitic extraction of MIM/MOM capacitor devices in analog/RF designs

Addressing the challenges of capacitor design and PEX

The complexity of analog and RF design demands rigorous attention to capacitor performance metrics, including:

Capacitance matching: Variations can significantly impact ADCs and charge pumps.
Symmetry and density: Essential for ensuring design consistency and efficiency.
Parasitic effects: Coupling capacitance, impedance, and equivalent series resistance must be accurately modeled.

The accuracy of these designs hinges on parasitic extraction tools that account for the detailed interactions between capacitors and their surrounding routing. Conventional rule-based PEX tools, though fast, often lack the accuracy required for advanced nodes. Full-wave field solvers provide unparalleled precision but are prohibitively slow for large-scale designs.

Advanced PEX with the Calibre xACT Platform

The Calibre xACT platform addresses these challenges by blending the speed of rule-based PEX tools with the precision of a 3D field solver. Its key features include:

Integrated field solver accuracy: Handles complex fringe and coupling capacitances, ensuring attofarad-level precision.
Context-aware functionality: Models non-local effects like density and double-patterning mask shifts to improve accuracy.
Efficient netlist generation: Creates distributed RC netlists for post-layout simulations, streamlining verification.

This hybrid approach enables designers to achieve precise parasitic modeling while meeting tight delivery schedules.

Bonus content! Access our Calibre xACT Fact Sheet for more details

Best practices for RF designers: Getting the most out of PEX

To maximize design accuracy and efficiency, RF designers can adopt these best practices:

Capacitor design considerations
Select the right capacitor type: Evaluate trade-offs between MIM and MOM capacitors, such as:
1. MIM: Lower parasitic effects, better for high-frequency applications.
1. MOM: Higher capacitance density, more cost-effective for large designs.

Figure 2. Figure 2 Comparison of MIM/MOM characteristics

Account for design context: Use context-aware PEX tools to model interactions with adjacent routing and layers.

Figure 3. Parasitic coupling capacitance between MOM capacitor cell and adjacent routing.

Parasitic extraction strategies
Leverage hybrid PEX tools:
- Use field solvers for critical regions with complex geometries or high sensitivity.
- Employ rule-based extraction for less critical areas to speed up processing.
Model coupling effects accurately:
- Address fringe and line-end capacitance for MOM designs.

*Figure 4. Coupling capacitance (within yellow outline) in VNCAP*

Use silicon-verified models for enhanced reliability in manufacturing.

Why context-aware PEX matters for advanced nodes

*Figure5. Parasitic fringe capacitances between metal comb structures in MOM capacitor device cell.*

Advanced process technologies bring new challenges, such as double-patterning shifts and density effects, which significantly influence capacitance accuracy. For example:

MIM capacitors: Often located in upper metal layers, their performance is impacted by routing beneath the device. Accurate PEX modeling ensures reliable simulation and design optimization.
MOM capacitors: While generally less affected by layout context, designers must still model fringe capacitance accurately to avoid performance degradation.

The Calibre xACT platform excels in these scenarios, delivering precise parasitic modeling even for advanced node designs.

Key advantages of the Calibre xACT Platform

The Calibre xACT platform is a game-changer for analog and RF designers, offering benefits such as:

Speed and scalability: Handles large designs efficiently without sacrificing accuracy.
Flexibility: Adapts to various PEX requirements, from simple rule-based extraction to detailed 3D modeling.
Reliability: Ensures that designs meet stringent performance and manufacturing specifications.

*Figure 6. Symmetric device matching comparison between field solver and rule-based extraction.*

By integrating these capabilities, the platform empowers designers to innovate with confidence, delivering robust solutions for 5G, IoT, and beyond.

Watch this short video discussing the whole family of circuit verification including parasitic extraction tools here

Summary: Paving the way for next-generation RF design

The demands of 5G and IoT are pushing the boundaries of RF design, making precise parasitic extraction an indispensable part of the process. With tools like the Calibre xACT platform, designers can achieve the balance of speed, accuracy, and flexibility required to tackle modern challenges. By understanding the nuances of capacitor design and leveraging advanced PEX solutions, engineers can ensure their designs meet performance goals while staying on schedule.

So great news… with innovations like these, the future of RF design looks brighter than ever!