Introduction to RF & Microwave Substrates & Base Materials

The term “substrate” can be somewhat confusing the context of RF/Microwave devices, components, and applications. A substrate can generally be anything from the insulative material used to develop a semiconductor or film (thick-film or thin-film circuits or components), or the structural material of printed circuit boards (PCBs). These electrically insulating materials are the backbone of virtually all RF/Microwave electronics, and are therefore essential in the construction of devices, components, integrated circuits, PCBs, assemblies, and most RF/Microwave systems.

Typically though, when speaking of RF/Microwave substrates, there are either hard or soft substrates. A hard substrate is also known as a base material, and is some type of ceramic or insulator used to develop semiconductors, devices, components, or films on, but can also be used to make larger devices, such as amplifier pallets. A soft substrate, or simply microwave substrate, is typically a material used to develop a RF/Microwave PCB.

Key Parameters

Regardless of the nomenclature, RF/Microwave substrates have several key electrical and mechanical properties that determine their suitability for a given application. This includes dielectric properties, dissipation (loss tangent), the physical roughness of the surface, thermal conductivity, and the ability of the insulative material to withstand high voltage (dielectric strength). It is important to note that some materials, such as GaN, Sapphire, and titanium dioxide are electrically anisotropic, which means that the permittivity of the material depends on the exact direction of the electric field in respect to the crystal axis of the materials 3D matrix. This can be either parallel (∥) or perpendicular (⟂).

Dielectric Permittivity

The dielectric permittivity, or relative permittivity, is a measure of the dielectric properties of a material in reference to that of air at room temperature. The dielectric permittivity determines the behavior of electric fields that pass through the material, and is a function of frequency. For high frequency applications, a low dielectric is desirable, as this minimizes the parasitic capacitances developed between traces and conductive structures. This factor also determines the geometries of planar structures, such as microstrip and stripline transmission lines and antenna. Hence, more compact structures can be formed on materials with lower permittivity values.

Loss Tangent

The dielectric loss tangent, or dissipation factor, is a measure of the amount of electromagnetic energy absorbed within the material and dissipated as heat. Understandably, materials with low loss tangents are desirable for high performance and high power applications, as this would minimize the amount of loss within a transmission line or along a structure. The loss tangent of a material is also a function of frequency.

Surface Roughness

The surface roughness of a material is gauge of how smooth a surface is. For some applications a precise surface roughness is desirable, where either a too smooth or too rough surface can lead to performance degradation of the material. This typically has to do with adhesion characteristics and resolution of layers deposited on the material.

Thermal Conductivity

Thermal conductivity is a measure of how well a material conducts heat through it’s bulk. For high power applications, this is an extremely important characteristic, as substrates are generally used to separate the conductive base of a power device and a heat sink. In these cases, a poor thermally conductive substrate could significantly reduce the overall thermal conductivity of the stack and limit the device’s performance.

Dielectric Strength

This parameter is a gauge of the necessary electric field strength to start stripping electronics from the dielectric material. For high voltage and high power applications, this is an important parameter as this could be a limiting factor in a devices operation.