All About Calma's GDSII Stream Format [6]

Steve DiBartolomeo
Applications Manager
© 2011 Artwork Conversion Software, Inc.

The Array Reference

In addition to the SREF (structure reference) which is a single placement of a structure (cell), GDSII has an AREF (array reference) which enables placement of 1D and 2D arrays. This is very useful for many types of IC layouts that include memory; identical memory cells are often arrayed in two dimensions.

An AREF record has a number of arguments:

  1. Name of Structure to Array
  2. Mirror Flag (M0 or M1)
  3. Scale Factor
  4. Rotation
  5. Number of Columns
  6. Number of Rows
Then, there are three sets of coordinates that must be supplied:
  1. the Array Reference Point
  2. a position displaced from the Reference Point by the inter column spacing x the number of columns
  3. a position displaced from the Reference Point by the inter row spacing x the number of rows

Let's look at a 5 x 5 array of cells that have an extents of 50 um x 30 um and a gap of 2 um between them.

AREF coordinates

We are going to write an ASCII "equivalent" of a small GDSII file using this array.

0 0
50000 0
50000 -30000
0 -30000
0 0
AREF RECT M0 1.0 0.0 5 5 0 0 260000 0 0 -160000

If we use Artwork's ascii2gds program to convert this to GDSII here is what we see:

Qckvu3 of array_example1.gds

Rotating the Array

One would think that rotating this array by 30 degrees would be a simple matter of changing the rotation parameter, i.e.:

AREF RECT M0 1.0 30.0 5 5 0 0 260000 0 0 -160000

However this won't work! One actually has to compute the second and third pair of coordinates of the rotated array and enter those. I have no idea why this is the case but it is. So for the same array (but with reference point 0 20000 (instead of 0 0 ) we would have the following parameters in our AREF.

AREF RECT M0 1.0 30.0 5 5 0 200000 225167 330000 80000 61436

You can see in the illustration below how the locations for the second and third pair of coordinate parameters are calculated:

computing coordinates for rotated AREF parameters

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