Conductors

Conductor materials are used for Planar and Via Tech Layers and for the top and bottom covers of the Analysis Box. When starting from a blank project, Sonnet provides a single conductor material called "Perfect Conductor". You may add more conductor materials to your project by selecting Circuit Settings > [Materials] : [Conductors] and clicking Add. This opens the Conductor Properties dialog box.

The Conductor Properties dialog box may also be used to edit conductors that are already defined. If you edit the properties of a conductor, all uses of that conductor material will be updated.

Conductor Properties

The choices for the Conductor Properties dialog box are described below.

Name: The name must be unique, is case sensitive, and cannot contain any of the following characters:

    < > "  :  '  ;  [  ]  {  }  *  ?  | 

Loss Type: The Loss Type determines the method of specifying the loss of the conductor.

  • Conductivity, Resistivity, Sheet Resistance at DC: If you specify Conductivity, Resistivity, or Sheet Resistance at DC, Sonnet will use the value you entered and the conductor thickness to calculate the frequency-dependent loss of the conductor. These loss types may be used for Planar or Via Tech Layers.
  • Surface Impedance: Surface Impedance is measured in ohms per square. This option should be used for conductors that do not behave like normal conductors. This loss type may be used for Planar Tech Layers but is not recommended for Via Tech Layers. See the Surface Impedance Loss Type section below for more details.
  • Resistance per Via (RPV): When using the Volume via metal model, Resistance per Via represents the total vertical (Z) resistance in ohms of an individual via polygon. When using a Array via metal model, Resistance Per Via represents the vertical (Z) resistance in ohms of an individual via within the via array. This resistance is constant with frequency. If the via polygon extends over more than one dielectric layer in the Z-direction, then the total specified resistance is apportioned to each layer according to the thickness of each layer. If Specify RPV Reference Area is enabled, you enter an RPV Area value. Any via with an area equal to the RPV Area has a resistance equal to the RPV Value. The resistance of all other vias are scaled according to their area. For example, if the via has an area that is twice the RPV Area, the resistance of that via will be half of the RPV value.

All conductor properties support using INF to represent an infinite value.

Surface Impedance Loss Type

The Surface Impedance loss type provides the most flexibility in defining conductor loss. It is usually used for special cases such as resistors, superconductor metal, magnetic materials, or compatibility with legacy Sonnet projects. You may specify one or more of the following values:

Rdc

Rdc is defined as the surface resistance at DC in ohms per square. If Rdc is specified and Rrf, Xdc, and Ls are all zero, then the surface impedance will be completely real and equal to Rdc at all frequencies. This value is often used to create a thin film resistor. When the conductivity and thickness are known, Rdc can be calculated as follows:

Rdc = 1 σt

where σ is the conductivity (S/m) and t is the thickness (meters) of the material. See Creating a Thin Film Resistor for details.

Rrf

Rrf is the skin effect coefficient. The EM solver multiplies this number by the square root of the frequency (in Hertz) to yield the ohms per square value at very high frequency. The EM solver also properly models the transition between electrically thin (low frequency) and electrically thick (high frequency) conductors [6] [7]. The transition frequency is (Rdc/Rrf)2. At this frequency, and a about an octave around it, both coefficients are important.

Rrf can be calculated as follows:

Rrf = Skin Effect Coefficient = K × π μ σ

where:

σ = Bulk conductivity in Siemens/meter

t = Conductor thickness in meters

μ = μ0 x μr

μr = relative permeability of your metal, which is typically 1.0 for most metal materials. Magnetic materials, such as nickel, have a μr greater than 1 and therefore significantly decrease the skin depth.

K: For Thick Metal and TrueVolume metal models, K = 1.0. For Thin Metal, K is a value between 0.5 and 1.0. and depends on the current ratio. For a detailed discussion of current ratio see Current Ratio. Shown below is a table of K values for typical applications when using Thin Metal:

Thin Metal Application Recommended K values
Symmetrical Stripline 0.5
Coplanar Waveguide signal lines 0.6
Microstrip signal lines 0.6
Polygon ground planes 1.0
Analysis box covers 1.0

Xdc

Xdc is defined as the surface reactance at DC in ohms per square. If Xdc is specified and Rdc, Rrf, and Ls are all zero, then the surface impedance will be completely imaginary with a magnitude of Xdc at all frequencies.

If you wish to create your own surface reactance as a function or frequency, you can create a variable which is a custom function of frequency, using the FREQ constant. Then set the value of Xdc to the variable.

Ls

Ls is the surface inductance in pH/square and models the surface reactance as linearly proportional to frequency. If both Xdc and Ls are specified, then the total reactance is the sum of Xdc and the reactance from Ls at each frequency.

Ls can be used to include the kinetic inductance, Lk, of a superconductor. The value of Lk for a given superconductor is based on several variables including temperature and film thickness. The best approach is to work with your fabrication group to determine an appropriate value for your application. For a list of references related to superconductors, please see the Superconductors section of the Sonnet References page.

A change in Ls will influence the Z0 and Eeff values calculated by Sonnet because of the change in phase velocity.

Temperature Dependency

You may optionally specify a temperature dependency of your conductor. To do so, expand the Temperature Dependency section of the Conductor Properties dialog box and enter the temperature coefficients for your conductor. You enter temperature coefficients, T0, TC1, and TC2. The equation used to calculate the temperature dependent values is based on the loss type selected and is shown above the temperature coefficients.

Whenever a value of TC1 or TC2 exists for any conductor, a new variable, "TEMP" is automatically added to your list of variables and is set to 25° C. If you sweep the TEMP variable, the loss of the conductor will change according to the equation shown above the temperature coefficients. See Parameter Sweeps for details on sweeping parameters.

Surface Roughness

The surface roughness of a metal trace is a property of the Tech Layer to which it is assigned. This was done because often the same conductor material could have different surface roughness values in the same circuit, but at different locations in the circuit. For example, a ground plane may often be rougher than a signal layer. See Roughness for more details.