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Thin Film Resistor Distributed Lossy Line Model (Closed Form): TFR2

Symbol

Parameters

Name Description Unit Type Default
ID Name Text TL1
W Resistor width Length W[1]um
L Resistor length Length L[1] um
RS Sheet (surface) resistivity   50
F Frequency for scaling sheet resistivity Frequency 0
MSUB Substrate definition Text MSUB[2]

[1] User-modifiable default. Modify by editing under $DEFAULT_VALUES in the default.lpf file in the root installation directory.

[2] If only one MSUB is present in the schematic, this substrate is automatically used. If multiple MSUB substrate definitions are present, you must specify which to use.

Implementation Details

This circuit component models a thin film resistor as a lossy microstrip line with the complex characteristic impedance. TFR2 is intended to replace the TFR model. The TFR model has a pad overlap of the resistive material by 3um on each side, so the layout draws 6um smaller than actually specified. This is a small error in PCB or other similar processes, but the error is large for integrated circuit processes. The new model draws the resistive material the exact length specified in the model. Additional layout only parameters control how the pad for the resistors draw.

The parameters W (width), and L (length) are dimensions entered in the default length units. The MSUB parameter specifies microstrip substrate element, which defines additional cross-sectional parameters of the transmission line. If blank, a default is used.

If parameter F is set to zero, then sheet resistance RS stays constant with respect to frequency. This is true if the thickness of the resistive film that makes the resistor body is less than skin depth.

If the thickness of the resistive film exceeds skin depth, the sheet resistance RS may be scaled with frequency in accordance with the formula:

You can activate this scaling by setting F≠0. In this case you should evaluate F in advance as

where σ - bulk conductivity of the resistive film in S/m and t - thickness of the resistive film in microns. If F is set in accordance with this formula, scaling of RS is implemented only for F >f; for f<F RS stays constant.

Note that you must enter the actual value of parameter F in default frequency units.

Generally, frequency F is very large (for example, hundreds of gigahertz) and setting F=0 is the best choice.

TFR2 accounts for losses in metal and in substrate dielectric. Dispersion is included.

Layout

The layout for this cell has its model layers hard-coded. To draw the layout, you must add layers named "NiCr" and "Metal1" to your model layer list (if they don't already exist). After these are defined in the model layer, you can assign them to draw on any draw layer. The resistor material is drawn on the "NiCr" model layer and the resistor pad is drawn on the "Metal1" model layer.

For more information on making new model layers, see “Layer Mapping”.

This model has several layout only parameters that control how the pad draws. You access these parameters by selecting the item in the layout, right-clicking and choosing Shape Properties, then clicking the Parameters tab.

Name Description Unit Type Default
PADL Total length of the overlap pad. Layout 6um
PADEXT Pad width extension, draws this much wider than the W of the resistor on each side. Layout 3um
PADOV Total length that resistor material overlaps the pad. Layout 3um

References

[1] E. Hammerstad and O. Jensen, "Accurate Models for Microstrip Computer-Aided Design," IEEE MTT-S International Microwave Symposium Digest, 1980, pp. 407-409.

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