Accurate capacitance extraction is critical in high-speed and RF design. While simple formulas can provide a good starting point, they often fall short in capturing the effects of fringing fields and layout-dependent parasitics. In this blog, we demonstrate how Ansys Q3D Extractor can be used to compute capacitance values, starting with a basic parallel plate capacitor and then moving to a cutout from a MEMS die to examine the impact of the pads and RF lines on the overall capacitance.
Capacitance Extraction of Parallel Plates
This parallel plates example demonstrates the Q3D workflow for extracting capacitance. Begin by drawing a 1.4 mm X 1.7 mm rectangle, then duplicate it with a 0.1 mm spacing to create the parallel plates.
To assign copper to the created objects, select them and choose Assign Boundary > Thin Conductor. Set the material to copper, then click OK.
Next, identify the nets by right-clicking on Nets in the project manager and selecting Auto Identify Nets.
To add a ground net reduced matrix, right-click Reduce Matrix in the project manager, select Ground Net, choose one of the identified nets, then click Save and Close. The reduced matrix references the defined ground net instead of an infinite ground.
In the project manager, right-click on Analysis, select Add Solution Setup, and choose Capacitance/Conductance.
Right-click on Setup1 and select Analyze to run the simulation. Once the simulation is complete, right-click on Setup1 and select Matrix. In the matrix viewer, uncheck Conductance and select Ground Net Matrix to view the capacitance results.
Pads and RF Lines Capacitance
In the following comparison, we use Q3D to extract the capacitance matrix from a MEMS die cutout in two configurations: one with pads only, and another with pads connected to RF lines.
Pads only
Pads connected to RF lines
Following the workflow described above, the capacitance of the pads was extracted, and the resulting capacitance matrix is shown below.
When the RF lines are added, the updated capacitance matrix is:
This comparison shows that the pads are the primary contributors to the overall capacitance, while the RF lines introduce a small increase.
The video below walks through these examples in detail, and the models shown are available under the downloadable resources.
Downloadable Resources
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