GDSII Net Annotation

A GDSII file created by NETEX-G has had all of the physical connectivity traced out and sorted into structures (cells.) However this may not be the way that some CAD tools expect to find the net information.

Certain CAD tools depend on a special GDSII record type known as a PROPERTY record.

About the Property Record

A PROPERTY record can be attached to either SREFs (structure references or insertions) or to geometries such as BOUNDARY and PATH. The record has two parts: an index and a value.

The index is a number from 1 to 127 that is used to identify the property.

The value is a string of text.

One can see that it is possible to simple define a record (say #6) whose value is a net name string. One then attaches this to all geometric entities that belong to that net.

This doesn't work well with most GDSII files since those originating from chip designs generally re-use structures many times. An entity cannot have a single net name attached to it. However with the NETEX-G output there is no structure re-use so we can safely label every single entity with a PROPERTY that assigns its net.

Example

Consider the section of a 2 layer IC package as shown below. There are three signal nets that start with a bump pad on layer M1, drop to layer M2 through a via on layer D1 where the net ends on a bump pad on layer M2.

The GDSII for such a package will look like its ascii representation below:

BGNLIB SAMPLE.DB
UNITS UM RESOLUTION 1000
BGNSTR
TOP
SREF NET1
PROP 30: NET1       < the user chose to use PROP 30 for SREFs  
SREF NET2
PROP 30: NET2
SREF NET3
PROP 30: NET3
ENDSTR

BGNSTR NET1
BOUNDARY LAYER 1
XY XY XY XY XY
PROP 6: NET1        <-- user chose PROP 6 for BOUNDARY/PATH
BOUNDARY LAYER 2
XY XY XY XY XY
PROP 6: NET1
BOUNDARY LAYER 3
XY XY XY XY XY
PROP 6: NET1
ENDSTR

BGNSTR NET2
BOUNDARY LAYER 1
XY XY XY XY XY
PROP 6: NET2
BOUNDARY LAYER 2
XY XY XY XY XY
PROP 6: NET2
BOUNDARY LAYER 3
XY XY XY XY XY
PROP 6: NET2
ENDSTR

BGNSTR NET3
BOUNDARY LAYER 1
XY XY XY XY XY
PROP 6: NET3
BOUNDARY LAYER 2
XY XY XY XY XY
PROP 6: NET3
BOUNDARY LAYER 3
XY XY XY XY XY
PROP 6: NET3
ENDSTR

ENDLIB