How does Sonnet calculate dielectric layer loss?

The dielectric loss calculations in Sonnet are virtually exact, given the substrate really has a frequency independent conductivity, and/or loss tangent. Our web site has a lossy conductivity benchmark you can perform on any electromagnetic solver (or measurement system). See chapter 8 of the Sonnet benchmarking section of the web.

The dielectric loss is calculated in Sonnet at the beginning stages of the analysis. The method Sonnet uses starts with the calculation of a sum of waveguide modes. The exact solution requires an infinite sum of modes, but Sonnet truncates this sum to some reasonable value (the truncation has never been a source of error). So, for each mode, if there is a lossy dielectric, the calculation involves complex numbers instead of just real numbers. This is NOT a discretized function - it is a continuous function. Therefore, the dielectric loss calculation can be thought of as exact (only limited to the precision of the machine).

A more reasonable source of error is in the assumption that the conductivity is constant with frequency. All real dielectrics have frequency-dependant loss (some smaller than others). Sonnet supplies you with two parameters (Loss Tan and Diel Cond) to control this frequency dependency. The equation Sonnet uses to calculate the TOTAL loss is given in the Sonnet manual. There are some dielectrics with more complicated frequency dependencies, but this equation works for most dielectrics. Of course, this requires that you know the frequency dependency of your dielectric. If you have a method of measuring the loss as a function of frequency (or published data which you can trust), and if it is constant over your range of frequencies, then dielectric loss is probably not a source of error. Be careful, however, of published loss data. Verify that the data is valid over your frequency range.

See the ''Metalization and Dielectric Layer Loss,'' chapter in the Sonnet User's Guide for a detailed discussion of dielectric loss. You may access the manuals in PDF format by clicking on the Manuals button on the Sonnet task bar or by selecting Help => Manuals from any Sonnet application.