Struggles ahead for corner-case, statistical simulations

July 26, 2016 // By Brandt Braswell
Brandt Braswell, a distinguished member of the technical staff at NXP Semiconductor, discusses some of the complexities of simulation with regard to fine geometry analog and mixed-signal design.

So I have been thinking about the design environment for analog designers and the challenges that plague mixed-signal design engineers in the new technology space as we design on process nodes that have significant layout-dependent effects (LDEs) as well as strong correlation among devices.

The challenge from a design perspective is complicated. Typically in the past the designer relied on corner simulations to provide feedback into how a circuit will behave in silicon that meets required design for manufacturability goals set by the company. So the question is what happens when the designer in these new technologies cannot rely on corner models but must run extracted simulations even at the block level because of all the LDE effects and correlations and interdependencies amongst devices?

I co-authored a paper with some colleagues that addresses the concern about corner models that hits on part of the problem I will discuss in this article. The article is “ Corner Models: Inaccurate at Best, and it Only Gets Worst… ” (Proc. IEEE CICC, 2013). Here is a brief statement from the conclusions drawn in the article that highlight some of the issues: corner models "cannot accurately bracket +/- 3σ variation in every performance measure for every circuit; “appropriate” corner models are not just circuit-dependent, they also vary with the device sizes and biases used within a single circuit, and can be different for different measures of circuit performance for the same circuit."

This paper mentioned highlights some of the reasons why corner models will not work for complex mixed-signal design but why. Look at this image from a paper I presented at a modeling conference (Figure 1).

Figure 1: Effects on global and local parameters as the technology shrinks.

As shown in the image, as we shrink the technology, the global variations and local mismatch merge and it is no longer valid to just run corners. Furthermore, if we look at the electrical performance