3.3. Capacitors

This page documents the capacitor primitives currently exposed through the SG13G2 schematic libraries.

3.3.1. Capacitor Overview

The current primitive set includes MIM capacitors, RF MIM capacitors, a parasitic-capacitance symbol, and the high-voltage SVaricap device.

Delivered capacitor symbols

Symbol	Model	Notes
cap_cmim	cap_cmim	scalable MIM capacitor
cap_rfcmim	cap_rfcmim / LVS model rfcmim	RF-oriented MIM capacitor with body node
cap_cpara	cparasitic	explicit parasitic-capacitance symbol
sg13_svaricap	sg13_hv_svaricap	high-voltage variable-capacitance device

3.3.2. Capacitor User Parameters

User-visible capacitor parameters

Parameter	Meaning	Used by	Notes
w	capacitor width	cap_cmim, cap_rfcmim, sg13_svaricap	geometry parameter used directly in netlisting
l	capacitor length	cap_cmim, cap_rfcmim, sg13_svaricap	geometry parameter used directly in netlisting
m	multiplicity	cap_cmim, cap_rfcmim	scales the displayed capacitance expression
wfeed	feed width	cap_rfcmim	present in RF MIM symbol netlisting
Nx	multiplier	sg13_svaricap	replicated variable-capacitance fingers
C	explicit capacitance value	cap_cpara	used directly without geometry derivation

3.3.3. Capacitor Default Symbol Templates

Default capacitor symbol values

Symbol	Default values
cap_cmim	w=7.0e-6, l=7.0e-6, m=1
cap_rfcmim	w=10.0e-6, l=10.0e-6, wfeed=5.0e-6, m=1
cap_cpara	C=10f
sg13_svaricap	w=3.74u, l=0.3u, Nx=1, body=sub!

3.3.4. Displayed Capacitance Expressions

For the MIM capacitor symbols, the displayed capacitance is computed in the symbol text from the geometry parameters.

3.3.4.1. cap_cmim

The symbol displays:

\[C_{cmim} = m \left( w l \cdot 1.5 \times 10^{-3} + 2(w+l) \cdot 40 \times 10^{-12} \right)\]

3.3.4.2. cap_rfcmim

The RF MIM symbol displays the same capacitance expression:

\[C_{rfcmim} = m \left( w l \cdot 1.5 \times 10^{-3} + 2(w+l) \cdot 40 \times 10^{-12} \right)\]

3.3.4.3. cap_cpara

The parasitic capacitor symbol does not derive its value from geometry. It uses the user-entered capacitance directly:

\[C_{cpara} = C\]

3.3.5. Interleaved MoM Capacitor

In addition to the MIM-based capacitor family, SG13G2 also has development data for an interleaved metal-oxide-metal capacitor built from stacked MOM layers. The source used here is MOM model development notes v4 by Volker Muehlhaus, dated 09 November 2022.

3.3.5.1. Geometry and scaling

The documented MOM structure is built from repeating unit cells of size 840 nm x 890 nm.

The field plots in the model notes indicate that the inner unit cells have a nearly homogeneous electric-field distribution, so the active-area capacitance scales approximately linearly with the number of cells.

3.3.5.2. Active-area capacitance

The notes distinguish between nominal geometry and active area. Active area is the portion of the MOM array that is paired with a counter-electrode; outer edge rows are not counted into this area term.

For the model values recommended from the study, the active-area capacitance is:

Interleaved MoM active-area capacitance density

MOM layer count	Active-area capacitance density	Note
3 layers	about 0.88 fF/um^2	based on Empire extraction of active-area scaling
4 layers	about 1.17 fF/um^2	based on Empire extraction of active-area scaling
5 layers	about 1.36 fF/um^2	corrected for true via size after comparing fast and accurate models

This gives the core capacitance term:

\[C_{active} = C_{dens} \cdot A_{active}\]

where C_dens is chosen from the table above.

3.3.5.3. Feed capacitance

The model notes distinguish two feed configurations.

3.3.5.3.1. Same-side feed

For same-side feed, the total capacitance includes a feed-overlap contribution.

The extracted feed-capacitance coefficients are:

Single-sided MoM feed capacitance

MOM layer count	Feed capacitance coefficient
3 layers	about 0.97 fF/um of feed width
4 layers	about 1.28 fF/um of feed width
5 layers	about 1.46 fF/um of feed width

This gives the single-sided feed term:

\[C_{feed,single} = k_{feed} \cdot W_{feed}\]

3.3.5.3.2. Opposite-side feed

For opposite-side feed, the additional feed capacitance is much smaller and is treated as negligible in the notes.

The documented conclusion is that feed capacitance is less than 2 fF for the tested cases and can be approximated as zero:

\[C_{feed,opposite} \approx 0\]

3.3.5.4. Total capacitance model

Using the extracted terms above, the MOM model notes recommend:

\[C_{total} = C_{active} + C_{feed}\]

with C_feed chosen according to the feed geometry.

3.3.5.5. Series resistance notes

The notes also extract series-resistance behavior.

For single-sided feed, two solvers were compared and both gave an approximately linear dependence on length / width, but with noticeably different fitted coefficients.

The notes explicitly mark this difference as still requiring investigation, so these single-side resistance fits should be treated as model-development data rather than final compact-model equations.

For two-sided feed, the notes report linear fits of the form:

\[R_{series} \approx a + b \cdot \frac{L}{W}\]

with extracted coefficients:

• 3-layer MOM: a = 0.10 ohm, b = 0.31 ohm

• 4-layer MOM: a = 0.14 ohm, b = 0.27 ohm

• 5-layer MOM: a = 0.15 ohm, b = 0.26 ohm

The same notes also discuss SRF and inductive behavior for the single-sided configuration.

3.3.5.6. Status in this documentation

This subsection documents the extracted model-development results for the interleaved MOM capacitor. It should be read as characterization guidance for a MOM device family, not yet as a direct replacement for the existing capacitor symbols already delivered through the current schematic libraries.

3.3.6. Capacitor Simulator Library Mapping

3.3.6.1. Capacitors in ngspice

• corner selector: cornerCAP.lib

• family model files: capacitors_mod.lib and capacitors_stat.lib

3.3.6.2. Capacitors in Xyce

• corner selector: cornerCAP.lib

• family model files: capacitors_mod.lib and capacitors_stat.lib

3.3.6.3. Capacitors in Gnucap

The Gnucap tree includes capacitor model sources such as cornerCAP.va, capacitor_paramset.va, and capacitor.va.

3.3.7. Capacitor Examples

The Xschem test library already includes AC, transient, S-parameter, and Monte Carlo examples for MIM and parasitic capacitor usage.