Hints, Tips and Solutions

Volume 11, Number 6, June 2000

Q: To simplify netlist extraction, I specify one kind of diffusion resistors in my layout by means of a special resistor definition layer. To my surprise, the extracted value for one such resistor always exactly twice the value I expected, no matter how I stretch this resistor. Other resistors (of the same type) are extracted by Maverick exactly as expected. Moreover, in my previous version of technology which defined diffusion resistors by means of intricate DRC operations all values were good.

A: The most probable reason is that you have duplicated shapes exactly overlapping each other, so that you do not see the duplication. Such shapes might result from erroneous copying. However quite often they are produced by overlapped cell instances.

When you defined resistors by means of DRC operations, the resulting device recognition layer could be merged, e.g., during logical operations, and hence could not contain overlaps. However when device definition layers are drawn manually, a common mistake to leave such layers "as is" for device extraction. Even for such "simple" device (and pin!) layers you must always perform merge operation, to avoid the problem discussed here.

If you don't want to add merge operation (sure, it slows down the LVS), a good idea is to use the Cell>>Info command and compare the number of shapes in the device layer and the number of the corresponding extracted devices.

The puzzle with stretch resolves easily as well: most probably, you pick object to stretch by box, so both overlapping shapes stretch exactly in the same way.

Q: Are there any means to compare two layouts in Expert?

A: You may perform layout comparison tailored exactly according to your needs by means of xi-scripts. An example of such scripts is given in Expert's distribution. The first script (compare.xis) defines the comparison command. The second one (compproj.xis) is for data input. These scripts are included into Expert's distribution. They are easy to understand, and you may easily modify them, adding more flexibility and fine tuning to your needs.

You need to run the first script only once per Expert's session, e.g., from the autostart xi-script (see Setup>>General>>Auto run scripts).

!! script: COMPARE.xis

! This script allows you to compare two eld, gds or apl files,

! even of different formats

! The comparison is performed cellwise and layerwise by means of XOR

! bFlat parameter selects whether to perform comparison of flattened cells

! or of cell's own shapes only.

Q: After each upgrading Expert to a new version I keep receiving messages, like: "Polygonal font file "C:\Silvaco\lib\expert\3.0.9.R\x86-NT\polgfont.gds not found. Please update your settings". How can I get rid of them once and for all?

A: This file contains drawings for characters used when you are creating "manufacturable" texts (To do this, use Edit>>Create>>Text command with Edit>>Numeric Input mode on and "Polygonal text" option checked on the Text panel). A polygonal font file is an ordinary gds file, and you may replace font drawings by your favorite ones. You may even have several font files and switch between them.

The mentioned message appears when you de-install the previous version of Expert and this default file is deleted. To get rid of it, put your polygonal font file into a permanent place on your disk and select it into Expert's Setup>>General.