Early Analysis is Key – Parasitic-Aware Design

Decreasing geometries and increasing design complexity are making the task of designing custom ICs very difficult (not that it was easy before). One of the main issues designers grapple with is the issue of parasitics and their effect on design specifications and yield estimates. With increasing cost pressures and decreasing ASPs, meeting yield targets could decide the commercial success of the chip.

In an era of ever shortening design cycles, waiting for layout engineers to provide designers with a complete, optimized layout and then perform post-layout verification is not always preferable, and in many cases it's inefficient. Having a methodology to consider the effects of parasitics in the early stages of design cycle is crucial to the success of the project. This is precisely where a parasitic-aware design flow comes to the help of designers.  Parasitic-aware design is a design methodology that allows designers to understand and analyze the effect of parasitics and help take corrective actions.

There are two aspects to parasitic-aware design flow. One part of parasitic-aware design deals with the front-end design, and the other with the physical implementation (see parasitic-aware design flow diagram here). In this blog we will focus on design and verification phases of design, cover important features of parasitic-aware design flow, and show how these features add value to designers in their day-to-day design activities.

In the front-end, the industry leading tools such as the Virtuoso Schematic Editor and Virtuoso Analog Design Environment allow designers to capture design intent and characterize the design for a multitude of operating conditions. A schematic editor with features like Canvas Magnifier, Custom Checks Creator, and HTML Publisher for Schematics will ensure reduced mouse clicks and increased productivity.  Analog Design Environment XL’s Multi-Test environment has statistical analysis (Corners, Monte Carlo) capabilities that allow designers to use hardware resources efficiently, and comprehensively verify the design.

Native RelXpert integration with Analog Design Environment XL will enable designers to perform reliability analysis and better understand age and stress related effects on their designs. Design documentation capabilities provide designers with a simple but powerful way to capture all the necessary information and electronically tie this information to the IP for improved intra- and inter- design team communication.

Parasitic estimation and parasitic stitching to incorporate parasitics in early stages of the design are key steps in the parasitic-aware design flow.  These techniques enable designers to leverage the prior knowledge and experience they have about parasitics on similar designs, and to incorporate this knowledge into the design they are working on. Analog Design Environment GXL provides designers with an easy to use GUI to accomplish this task.  Designers can estimate parasitics by manually adding R, C, L and K to the schematic, such that the estimated parasitics are representative of those coming from the final layout.

Alternatively, users can stitch parasitics from previously designed modules into the design they are working on. Since the majority of the designs are reused in one form or another, parasitic stitching could be a more accurate form of parasitic estimation. Designers can then use Analog Design Environment XL to simulate and compare results with schematic design to analyze and understand parasitic effects. Designers can subsequently filter, refine, and customize these parasitics to meet their design specific needs and analyze their effects on various figures of merit.

Designers can iterate through this process multiple times to clearly understand parasitic effects in the early stages of design cycle, and do so without a final layout.  The availability of various Cadence simulators (such as Spectre, SpectreRF,  UltraSim, Accelerated Parallel Simulator, AMS Designer), all using common device models, seamlessly integrated into a single GUI, give designers freedom to choose simulation tools that meet their accuracy, capacity and performance needs. Finally, designers can take advantage of Analog Design Environment GXL optimization to take corrective action and optimize their designs to mitigate parasitic effects.

NOTE: To access “Quick Start” videos that show step-by-step instructions on how to accomplish various tasks outlined above, please contact your local AE or Sales Engineer.

In summary, the parasitic-aware design flow is not a panacea for all the parasitic issues that are plaguing your design, nor is it a single tool that is going to perform miracles. Parasitic-aware design, on the other hand, is a design methodology that allows designers to systematically analyze and understand effects of parasitics during early stages of design cycle and help take corrective actions.

Since designers know more about their designs than anyone else, parasitic-aware design provides a consistent methodology to capture this knowledge in a systematic way, helping designers to maintain design intent throughout the design process. By providing designers with a faster and better way to bring parasitics back into the design, parasitic-aware design allows designers to avoid over-designing and instead focus their energies on fine tuning their design to increase the yield.

Rama Jupalli