CapacitanceHowto.txt
in the directory share/doc/howto.
CAPACITANCE EXTRACTION HOWTO
============================
Introduction
============
This file serves as a rough guide to get you started on extracting
capacitive parasitics from a layout.
First, we briefly explain how to setup a new process; then we create a
simple test-cell, and show how to extract capacitive parasitics, first
using the 3d extraction mode, and then using the faster 2d mode.
Setting up a process
====================
We will describe here the steps necessary to setup a simple process, with
information suitable for capacitance extraction.
Note that the software provides two ways of creating a process. The first
method is to write the necessary configuration files manually. The second
method is to use the graphical tool "spock" (see [4]). We will use the first
method here. One advantage of this method is that it provides a better
understanding of the structure and the interdependencies of the software.
Note also that we will not discuss the description of device rules.
For details about adding devices, see [1], or see [4].
Now, to begin, create a directory for your process:
% mkdir test_process
% cd test_process
The next step is to define the masks used in your process. This is done by
editing the file "maskdata". Basically, this file defines only the names of
the masks, and some color (and style) information necessary to plot the
masks in various tools. Thus, the information in the maskdata file is not
extremely important, except of course that the masknames should be right.
Below is a simple example of the contents of a maskdata file.
"test_process" "simple process used as an example"
#---------------------------------------------+---------------------
# Layer | PG-Tape | CMask | Dali | Plot | Comment
#-----------+---------+-------+-------+-------+---------------------
# name | job | color | color | pen |
# | type | | type | | fill| | fill| | fill|
# | | | | | | | | | | | | | | |
# 1 2 | 3 4 | 5 6 | 7 8 | 9 10 |
#-v----v----+-v--v----+-v--v--+-v--v--+-v--v--+---------------------
cpg 1 1 1 1 1 1 1 1 0 "polysilicon"
caa 1 2 1 2 1 2 1 2 0 "active area"
cmf 1 3 1 3 1 4 1 4 0 "metal"
cms 1 4 1 5 1 3 2 6 0 "metal2"
cca 0 5 1 8 1 0 1 8 0 "contact metal to caa"
ccp 0 7 1 8 1 0 1 8 0 "contact metal to cpg"
cva 0 8 1 6 1 7 1 6 0 "contact metal to cms"
cwn 0 9 1 4 1 5 0 5 0 "n-well"
csn 0 10 1 5 1 6 2 3 0 "n-channel implant"
cog 0 12 1 6 1 6 0 6 0 "contact to bondpads"
cx 2 0 1 0 0 7 0 8 0 "bounding box"
Place the above lines in the file "maskdata" in the "test_process"
directory. The syntax of the file should be obvious from this example:
different fields are separated by one or more spaces, and you can
use a "#"-mark to start a comment. The first line contains a short,
descriptive name for the process, as well as a longer description.
For the meaning of the different fields in the above example, you can type
% icdman maskdata
You should make appropriate changes to the "maskdata" file, to let it
represent the masks in your own technology; add, change or delete lines
where necessary. In the examples that follow, we will assume that the file
is left untouched.
Now, add a file "default_lambda", which specifies the default unit distance
in your layouts. For this example, put the following number in the file:
0.20
This number means that a distance of 1 in the layout editor corresponds
to 0.2 microns in reality (the layout editor, and in fact the complete cacd
database, can only cope with integer coordinates, hence this scaling factor).
Now, we also need an element definition file, which is called "space.def.s".
This file describes the "elements" used in the extraction process. Elements
are, for example, conductors, transistors, capacitance-rules, etc. Since
we focus here on capacitance extraction only, we will define elements for
conductors only. We will use a special preprocessing tool to generate
capacitance-rules for these conductors. Capacitive effects originating from
semiconductor junctions will not be considered here. See [1] for details.
Below are the contents of the example "space.defs.s" file.
unit resistance 1 # ohm
unit c_resistance 1e-12 # ohm um^2
unit a_capacitance 1e-6 # aF/um^2
unit e_capacitance 1e-12 # aF/um
unit capacitance 1e-15 # fF
unit vdimension 1e-6 # um
unit shape 1e-6 # um
colors :
cpg red
caa green
cmf blue
cms gold
cca black
ccp black
cva black
cwn glass
csn glass
cog glass
@sub pink
conductors :
# name : condition : mask : resistivity : type
cond_mf : cmf : cmf : 0.045 : m # first metal
cond_ms : cms : cms : 0.030 : m # second metal
cond_pg : cpg : cpg : 40 : m # poly interconnect
contacts :
# name : condition : lay1 lay2 : resistivity
cont_s : cva cmf cms : cmf cms : 1 # metal to metal2
cont_p : ccp cmf cpg : cmf cpg : 100 # metal to poly
vdimensions :
v_cpg_of_caa : cpg !caa : cpg : 0.60 0.50
v_cpg_on_caa : cpg caa : cpg : 0.35 0.70
v_cmf : cmf : cmf : 1.70 0.70
v_cms : cms : cms : 2.80 0.70
dielectrics :
# Dielectric consists of 5 micron thick SiO2
# (epsilon = 3.9) on a conducting plane.
SiO2 3.9 0.0
air 1.0 5.0
For detailed information on the format of the "space.def.s" file,
you may type:
% icdman tecc
or see [1].
The "unit" section describes the units used for the various numbers used
later on. The "colors" section describes the colors used by Xspace for each
mask. Typically, you will choose approximately the same colors as used in
the "maskdata" file. Note however, that color names are used here instead of
numbers. You can find an overview of color names (and their respective RGB
values) in the file "cacd/share/lib/rgb.txt".
Conductors are specified in the "conductors" section. Here we define only
the conductive behavior of the "cmf", "cms" and "cpg" masks.
Note that you should specify the sheet-resistivity in the fourth column;
the unit of sheet-resistivity is given in the "unit" section above,
and is set to 1 ohm in this example.
The "contacts" section define the conditions under which a contact (via) is
assumed between the conductors. Here, a contact between "metal" and "metal2"
(masks "cmf" and "cms", resp.) is made at locations where the masks "cmf",
"cms" and "cva" are present in the layout. Similarly, a contact is made
between "metal" and "polysilicon" (masks "cmf" and "cpg", respectively),
under the condition that "cmf", "cpg" and "ccp" are present.
The "vdimensions" section contains the vertical dimensions of the
conductors. The third field contains the name of the conducting mask.
The fourth field contains the vertical position, as well as the thickness
of the conductor. The values are in microns, as specified in the "units"
section (under "vdimensions"). These values are particularly important
for capacitance extraction.
The "dielectrics" section contains a description of the dielectric layers.
The first column contains a name for the dielectric, the second column
contains the relative permittivity (epsilon-r), and the third column
contains the y-position at which the dielectric layer starts.
Note that at most 3 layers may be currently defined (this limitation
will be removed in the near future).
When you have entered the "space.def.s" file, type the following command:
% tecc -m maskdata space.def.s
This will compile the element file into a compiled version, suitable for
use by the extractor. A new file will be written to the process directory,
under the name "space.def.t".
Finally, we will need a default parameter file for the extractor. This file
is called "space.def.p". Use your text-editor to create this file, and
give it the following contents:
min_art_degree 3
min_degree 4
min_res 100 # ohm
max_par_res 20
no_neg_res on
min_coup_cap 0.05
lat_cap_window 6.0 # micron
max_obtuse 110.0 # degrees
equi_line_ratio 1.0
BEGIN cap3d # Data for 3D capacitance extraction
be_mode 0c
be_window 2.0
max_be_area 1.0
omit_gate_ds_cap on
END cap3d
The extractor will take its parameters from this file by default, but you
can override the parameters from the command-line, as we will see later on.
See [2] if you are interested in the meaning of the different parameters.
Creating a project
==================
We will now create an example project, which will use the process defined
in the previous section.
Perform the following commands:
% cd ..
% mkpr -p ./test_process test_project
Now, cd into the test project directory:
% cd test_project
Creating a test-cell
====================
Now, we will place a test-cell into the database of the current project.
Normally, we would import a cell using the "cgi" tool, or we would create a
cell by using the layout editor "dali". However, in this example, we will
create a cell by entering its geometry in a textual format. Use your text-
editor to create a file named "squares.ldm", and give it the following
contents:
ms squares
term cmf 0 250 0 250 a
term cms 0 250 0 250 b
me
This defines two square terminals of 250x250 units, consisting of masks
"cmf" and "cms", respectively, and named "a" and "b", respectively.
Since we set lambda to 0.20 microns, the terminals are of dimensions
50x50 microns in reality.
Now, place the cell in the database, by issuing the following command:
% cldm squares.ldm
Type "dblist" to verify that the cell has been entered into the database.
You can verify the layout in the layout-editor by typing:
% dali squares
For information on how to use this layout-editor, see [3]. Use the "+"
and "-" keys to zoom in to your layout, and use the arrow keys to change
the view-position.
Performing a 3d capacitance extraction
======================================
We will now perform a 3d capacitance extraction of the test-cell.
The simplest way of doing this is by issuing the following command.
% space3d -3C squares
The -3 option specifies that the three-dimensional extraction mode should
be active, and -C specifies that coupling capacitances should be extracted.
If you wish, you can add the -v option to raise the verbosity level.
Type "icdman space3d" for more detailed information, or see [2].
After the extraction is finished, you can inspect the resulting circuit
for parasitic capacitances. You can do this by typing:
% xspice -a squares
This gives you a spice netlist. For the example above, the output will
contain something similar to the following.
...
c1 b a 222.0366f
c2 b GND 11.81104f
c3 a GND 55.57874f
...
It is important to know that When the extractor is run, it will consider
only capacitive coupling within a certain window. For maximum precision,
the size of this window can be set to infinity, but in practice this is
not necessary. A smaller window size means that the extractor can divide
the problem into smaller pieces, so that less processing time is necessary.
You can change the window size by specifying "-Scap3d.be_window=value" at
the command-line. The value is in microns.
For example:
% space3d -3C -Scap3d.be_window=1 squares
You will notice that the extraction will be somewhat faster. This is because
the default value for this parameter is 2 microns. (We specified this in the
file "space.def.p" above). When running "xspice -a squares" again, we get:
...
c1 b a 223.1784f
c2 b GND 11.47019f
c3 a GND 45.85488f
...
As you see, decreasing the window size introduces some inaccuracy, but the
results are still acceptable in this case. When you are just starting with a
new kind of process, you should experiment with this parameter to see what
value gives accurate results in a reasonable amount of time.
For more information on 3d capacitance extraction, see [2].
Defining the 2d capacitance rules
=================================
Because full 3d extraction of capacitances is more time-consuming, space
also offers a method of extraction known as "2d extraction". This method
uses a "database" of capacitance rules to perform the extraction. Thus
instead of performing tedious computations, space will simply "look up"
capacitance values using a set of rules. Of course, this method is
inherently less accurate than full 3d extraction, but it can provide
results in much less time.
This means that 2d extraction is suitable for realistic vlsi designs,
while 3d extraction is suitable only for small to middle-size test-cases.
In the past, it was necessary to generate capacitance rules manually, but
we have recently developed a tool to automate this task. You can simply
invoke this tool by changing to your project directory, and typing:
% tabs -L 2 -S 0.4
The value specified after -L is the number of conductor layers which should
be taken into account. In many cases, it is reasonable to use the value 1,
and in some cases you might need to use the value 2. Higher values may not
be useful in most situations.
The value after -S is the nominal lateral spacing in microns between two
conductors. Of course, the lateral distance between conductors can be
different in an actual design, but the capacitance rules will be "optimized"
for the given value.
Running "tabs" will take some time. Internally, tabs invokes the space 3d
extractor many times to extract data for the various capacitance rules.
You may use the -v option to see what is going on.
When "tabs" is finished, you may want to inspect the file "space.def.s".
This file will now also contain the capacitance rules.
You should run "tecc" once again to compile the file.
% tecc ../test_process/space.def.s
Do not forget this step (!)
If you wish to have more information on the tool "tabs", type the command
"icdman tabs".
Performing a 2d capacitance extraction
======================================
After the capacitance rules have been generated, you can perform a 2d
extraction. Enter the following command:
% space3d -l squares
You will notice that this time, extraction is very fast. Inspect the
results by typing
% xspice -a squares
The output will now be:
...
c1 b a 220.3094f
c2 a GND 50.78f
...
Notice that the results deviate somewhat from the results obtained by 3d
extraction, but that they are still reasonable. No coupling between terminal
"b" and "GND" is present, since there is a distance of more than 2 layers
between them (layers "cmf" and "cpg" are between the top of the substrate
and "cms"), and we specified "-L 2" when running "tabs".
For more information on 2d extraction, see [1].
References
==========
[1] Space User's Manual.
http://space.tudelft.nl/documentation
[2] Space 3D Capacitance Extraction User's Manual.
http://space.tudelft.nl/documentation
[3] Dali User's Manual.
http://space.tudelft.nl/documentation
[4] Building Technology Files Using Spock.
(part of the user documentation released by Optem)