Planar Metalization Loss
Planar Metalization loss is specified in the project editor in the Planar metals tab defined on the Metals/Brick page of the Circuit Settings dialog box which is opened by selecting Circuit - Settings. You click on the Add Planar button to add a planar metal. Planar metal types may be assigned to circuit metal, top covers and ground planes. Sidewalls are always assumed to be perfect conductors.
A common misconception is that only one type of planar metalization is allowed on any given level. In fact, different metalizations (i.e., different losses) can be mixed together on any and all levels. For example, it is possible to have a thin film resistor next to a gold trace on the same level.
Sonnet allows you to use predefined planar metals, such as gold and copper, using the global library. The global library allows you to define your own metal types as well. There is also a local metal library which can be created for an individual or to share between users. There are both planar and via metals available in the metal libraries.
Sonnet provides many choices for how to model the loss of your metal. Most of them are based on the concept of surface impedance, measured in Ohms per square. This concept allows planar EM Simulators, such as Sonnet's em, to model real 3-dimensional metal in two dimensions.
If you are unfamiliar with this concept, please refer to any classic textbook such as Fields and Waves in Communication Electronics by Simon Ramo, John R. Whinnery and Theodore Van Duzer, John Wiley & Sons, New York, 1994.
This technique models the loss of the true 3-dimensional metal fairly accurately, but does not model any change in field distribution due to the metal thickness. This approximation is valid if the metal thickness is small with respect to the width of the line, the separation between lines, and the thickness of the dielectric. If the true 3-dimensional effect of the metal is important, then you should consider using the Thick Metal Model metal type as discussed in Thick Metal.
Sonnet properly models conductors which are electrically thin (low frequency), electrically thick (high frequency), and the transition between electrically thin and electrically thick. See reference [26] in Sonnet References for a detailed description of the theory used by Sonnet. See reference [93] for the equations actually used in the Sonnet model.
Another aspect of loss is that the surface impedance of a good conductor has an imaginary part which is equal to the real part. This reactive surface impedance is physically due to the increased surface inductance caused by the current being confined closer to the surface of the conductor. This surface reactance is included in the Sonnet loss.
Sonnet's planar metal loss model is very accurate if accurate values are used. In practice, however, there are many aspects of metal loss that cannot easily be accounted for. For example, metal purity, metal porosity, etc. cannot easily be measured and included in an all-encompassing loss model. In addition, most software programs, Sonnet included, do not allow you to enter all of the parameters that determine metal loss. Many users like to use the ideal values as a starting point and add a little of their own “real-world” loss. But how much should be added to the ideal models?
The best method to determine proper loss values is to build and measure a simple structure of the desired metalization. The structure should be sensitive enough to loss so that measurement errors do not significantly affect the results. Then analyze the same structure in Sonnet and adjust the loss values until the calculated loss matches the measured loss. This may take several iterations before success, but then you can use these values for similar circuits. You are now effectively using measured values for the loss parameters.
When you create a new planar metal type, you need to decide which loss model for planar metal loss to use.The following is a list of the available loss models:
Normal: Used for most circuit metal. Metal loss is based on the conductivity, resistivity or Sheet Resistance at DC, and thickness of the conductor. Metal is modeled as having zero thickness, where the entered thickness value only affects the calculated loss.
Thick Metal Model: Used to model physically thick planar metal. The Normal metal type (described above) is modeled as having zero thickness where the entered thickness value only affects the calculated loss. The Thick Metal model allows you to insert physically thick metal in your circuit. This is the most accurate way to model thick metal, but it uses more computer resources.
Resistor: Used for most resistors where you know the surface resistance.
Rdc/ Rrf: Used for compatibility with previous Sonnet releases. This type allows you to include the effects of mr the magnetic permeability.
Rough Metal: Used when rough metal surfaces have a significant effect. Generally, conductors with rough surfaces and narrow trace widths, that operate at high frequency (above 5 GHz) are good candidates for the Rough Metal model.
General: Used for unique circumstances such as superconductor metals.
Sense Metal: Used to view tangential electric fields.