Modelithics has developed a proprietary methodology to create accurate and scalable models for SLC single-layer microwave capacitors.
Single-layer capacitors (SLCs) are considered the most basic type of electrostatic capacitor.
They consist of a single layer of dielectric material with conductive electrode material on both sides (Fig. 1). Given that SLCs have the highest self-resonant frequency (SRF) in comparison to other capacitor types, high frequency applications have long been utilizing these components.
With that being said, let’s now turn our attention to simulation models for capacitors. Modelithics offers a vast collection of highly scalable models for multi-layer ceramic capacitors (MLCCs) and other passive components.
These models, known as Microwave Global Models™, represent an early and popular innovation from the company.
Having said that, one might assume that developing models for SLCs would be an easy task. However, creating scalable models for SLCs has proven to be more challenging than originally anticipated.
The modeling challenges include properly addressing the bondwire connections, accounting for various configurations, and the complexity of many of the dielectric materials.
The good news is that due to a recent internal research initiative, Modelithics has developed a proprietary methodology to create accurate and scalable models for SLCs. As a result, Modelithics now offers both equivalent-circuit and 3D electromagnetic (EM) geometry models for an initial set of SLCs. These models are available in the Modelithics COMPLETE Library™.
Let’s dive into these models by looking at the CAP-TCD-CMSK280-001 microwave ceramic capacitor model, which is an equivalent-circuit model for Tecdia’s Type C (Class 1) borderless SLC series (Fig. 2). The CAP-TCD-CMSK280-001 is a 1-port model intended for shunt configurations. This initial model includes two values: 1.0 and 4.7 pF. The CAP-TCD-CMSK280-001 model offers the benefit of broadband performance—it’s validated from DC to 60 GHz.
To develop this model, the Modelithics lab team performed measurement validations in a 1-port shunt configuration. The modeling methodology also involved 3D EM simulations performed in Ansys HFSS. Figure 3 shows the measurement configuration used to characterize these components.
In Figure 3, we see a bond wire attached to the top of the SLC. The other end of the bond wire is attached to a 50-Ω microstrip line on a 5-mil-thick alumina substrate fixture. Specifically, this substrate fixture is known as the Probe Point™ 0503, which is included in the line of Probe Point fixtures that Modelithics
offers as standard products. Modelithics also offers the associated CM05 calibration substrate.
Let’s now talk about some of the features of the CAP-TCD-CMSK280-001 model. The model offers a great deal of flexibility, particularly in regard to the bond wire. Let’s start with the “BW_Removal” parameter, which makes it possible for users to include or remove the bond wire from the model. By default (BW_Removal = 0), the bond wire is included, meaning the reference plane is located at the end of the bond wire (Fig. 3, again). Setting BW_Removal to 1 removes the bond wire from the model and places the reference plane at the top of the capacitor.
The model’s flexibility goes far beyond the option to simply include or remove the bond wire. When the bond wire is included, several other parameters allow the user to precisely adjust the relevant settings. For example, the “BW_Stride” parameter lets you adjust the distance between bonding points. Another parameter is “BW_ContactH,” which makes it possible to set the height of the contact point above ground (i.e., the substrate height). The “BW_PeakH” parameter lets you adjust the peak height of the bond wire, while the “BW_PeakMode” parameter lets you adjust its peak location.Finally, the “BW_Diameter” parameter is used to set the diameter of the bond wire. Users can refer to the model information datasheet to see a portrayal of these different parameters.
In addition to the parameters mentioned, the CAP-TCD-CMSK280-001 model includes a “Cap_Size” parameter that allows you to adjust the length and width of the capacitor. When the value is set to 1.0 pF, Cap_Size defaults to 10. What this means is that the capacitor is set to a size of 10 × 10 mils. Note that Cap_Size can be adjusted within a range of ±20% from the default size. Cap_Size is included because the value of each capacitor is affected by the manufacturer’s dicing tolerance. When an SLC is larger than the nominal size, the capacitance value will be greater and the fundamental internal resonance will be lower in frequency. The opposite is true when an SLC is smaller than the nominal size. The user can then evaluate the circuit performance for the range of delivered parts.
Let’s now quickly show the CAP-TCD-CMSK280-001 model in action. We mentioned at the beginning that SLCs have the highest SRF when compared to other capacitor types. Figure 4 shows the impedance curves for the model when the value is set to 1.0 pF. Shown are the results when the the bond wire is included in the model (BW_Removal = 0) and when the bond wire is removed (BW_Removal = 1). When the bond wire is included, the SRF is 10.43 GHz. Without the bond wire, we see a SRF of 22.82 GHz. All other parameters are set to the default values.
Figure 5 shows the modeled and measured S11 when the value is 4.7 pF. The results are shown for frequencies all the way to 60 GHz.
In this blog post, we’ve investigated the CAP-TCD-CMSK280-001 equivalent-circuit model. Keep in mind that this SLC model is not the only one currently available. Modelithics offers models for other SLCs from Tecdia as well as SLCs from other manufacturers. And be on the lookout for more of these models in the future.
In Part 2, we will focus on Modelithics 3D EM geometry models for SLCs. 3D EM geometry models make it possible to capture coupling interactions that can occur when components are located very close to other components or objects.2 On a final note, we are always interested in feedback from designers. In addition, we look forward to partnering with SLC manufacturers to expand Modelithics libraries by adding accurate and flexible models for SLC product lines.
References
- V. Lu, “SLCs vs. MLCCs: Which Capacitor Type is Right for My Application,” Knowles Precision Devices blog, February 2022.
- C. DeMartino, “Capitalize on Full-Wave Electromagnetic Simulations with 3D EM Component Models,” Microwave Journal, April 2024.