# Spice utility addresses process variations

SAN JOSE, Calif. - Process variations are becoming a significant problem for sub-0.25-micron chip design, but designers traditionally have not had an accurate way to predict them. A technology-CAD (T-CAD) company, PDF Solutions, is trying to change that by aiming its new Circuit Surfer tool at designers.

Developed with Texas Instruments Inc., the Circuit Surfer utility sits on top of Avanti's HSpice or Cadence Design Systems' Spectre. It lets these simulators incorporate statistical Spice models, manages statistical-simulation runs and provides post-simulation visualization and optimization.

While transistor sizing traditionally has caused most process variations, other factors such as doping, implantation and gate-oxide growth are entering the picture, said Kimon Michaels, vice president at PDF.

And it's no longer adequate, he said, to try to predict these effects with "corner case" simulations that try best- and worst-case combinations. "Just trying two or four corners doesn't ensure that a circuit works with real fab variability," said Michaels. "And because the corner models are fairly conservative, you end up with less performance or you throw away chip area. People want to do more-accurate statistical design so they can operate more closely to the fringes of their process capability."

**Dual analysis**

Circuit Surfer aims primarily at analog and mixed-signal designers, although it can be used to analyze both analog and digital blocks. Since it's a Spice-based tool, it's typically used for blocks of tens to hundreds of transistors. It can be used by chip designers, library designers, or process and manufacturing personnel.

A designer inputs a Spice net-list, a definition of performances and process-specific statistical models. Michaels said Circuit Surfer can be used before chip layout to look at architectural trade-offs. What users get out of Circuit Surfer is an estimation of the statistics of circuit performance, and if the user chooses, optimized transistor lengths and widths.

The tool can help users evaluate the effects of design changes. For instance, it can see what happens if the designer increases the width of differential pair transistors.

Circuit Surfer does not include the statistical models. These will have to come from the user or be created by PDF Solutions on a consulting basis.

While HSpice or Spectre can run statistical models on their own, Circuit Surfer can capture correlations between parameters and models, said Michaels. "We can develop parameterized equations, where your Spice cards, instead of just being lists of parameter values, are actually equations. So Circuit Surfer can then, for each run, evaluate a complicated Spice model, or pick a model from a library and insert it into Spice."

Circuit Surfer is available now on Unix workstations starting at $75,000.