Klayout Vs Xschem: Resolving W_min Discrepancy

by Mireille Lambert 47 views

Hey everyone! Today, we're diving deep into a fascinating discrepancy that came up during our discussions, particularly thanks to the insightful observations from @d-m-bailey. It's all about the differences in the minimum width (W_min) parameter between Klayout and Xschem, and how it aligns (or doesn't!) with the ngspice models definition. This is super crucial for ensuring our designs behave as expected in simulation and layout, so let's get into it!

Understanding the W_min Discrepancy

When we talk about W_min, we're referring to the smallest allowable width for a transistor in our design. This parameter is incredibly important because it directly impacts the transistor's performance, its manufacturability, and the overall reliability of our integrated circuit. Using a width smaller than W_min can lead to unpredictable behavior, fabrication issues, and even device failure. Therefore, it’s vital that our design tools and models agree on this value.

So, here’s the core of the issue: we've noticed that the W_min values presented in Klayout don't always match what we see in Xschem (and the underlying ngspice models). This inconsistency can create a real headache. Imagine designing a circuit in Xschem, simulating it, and everything looks great. But then, when you go to lay it out in Klayout, you find that the tool is flagging your transistor widths as too small! This is precisely the kind of scenario we want to avoid, and understanding the root cause of this discrepancy is the first step.

The Numbers: Xschem vs. Klayout

Let's look at the specific numbers that sparked this investigation. For the 3.3V devices in our design, here's what we're seeing:

Xschem:

  • NMOS 3.3V:
    • Lmin = 2.8e-007 meters
    • Wmin = 2.2e-007 meters
  • PMOS 3.3V:
    • Lmin = 2.8e-007 meters
    • Wmin = 2.2e-007 meters

These values seem consistent and are what we'd expect based on the process technology we're using. Now, let's see what Klayout is showing us.

Klayout:

  • NMOS 3.3V:
    • Lmin = 2.8e-007 meters
    • Wmin = 4.2e-007 meters (or 3.6e-007 meters if bulk is set to none)
  • PMOS 3.3V:
    • Lmin = 2.8e-007 meters
    • Wmin = 4.2e-007 meters (or 3.6e-007 meters if bulk is set to none)

Whoa! That's a significant difference, especially for the default W_min value. In Klayout, the W_min is almost double what Xschem indicates. Even when we try to tweak the bulk setting to