ASITIC Documentation: Session

Contents:

• Autocell Automatic Mesh Generation
• Cat List File Contents
• Ccell Cell Size (Capacitance)
• Cell Cell Size (Inductance)
• Chip Change Layout (Chip) Size
• Eddy Toggle Eddy Current Flag
• Exec Execute Script
• Gmd Control Mutual Inductance Approximation
• Help Help
• Input Redirect Input to File
• Listsegs List Segments (Wires) in Spiral
• Load Load
• Maxnw Upper Limit on Width Meshing
• Modifytechlayer Modify Technology Parameters
• Opengl Toggle 3D Viewport
• Pause Pause ASITIC
• Save Save
• Savemat Save Matrix Data
• Set Set/Display Environmental Variables
• Tekio Toggle FFT Disk Storage
• Time Display a Timer
• Units Set Length and Frequency Scale Units
• Ver ASITIC Version
• Verbose Toggle Verbose Flag
• Who List Resident Spirals

ACELL .01 .5 # Use auto cell generation with (alpha,beta) = (.01,.5)

The AutoCell command is used for automatic cell size generation. When in auto cell mode, the cell size is adjusted according to two parameters, <alpha> and <beta>. These parameters control the cell size in the following fashion: The length of a segment is sub-partitioned if it is larger than <alpha>*lambda and the width/thickness of a segment are partitioned into sub-cells if they exceed <beta>*delta. Lambda is the wavelength calculated in the dielectric oxide layer at the particular frequency of interest. Delta is the skin depth calculated in the metal segment at the particular frequency of interest. The segment metal is assumed to be non-magnetic so the magnetic permeability is assumed as unity. For capacitance calculations, the cell size must be larger than Chip_Size/FFT_Size, and this is automatically satisfied by selecting the smallest possible cell (see fftsize).

Auto cell generation is very convenient as it frees the user from worrying about the cell size to select. By default, <alpha>=.05 and <beta>=.8; these values give consistent results for most simulations. In other words, decreasing the cell size beyond these values does not lead to significantly different results.

To return to manual cell size generation, use the cell and ccell commands.

Note: At present time uniform cell size is used for the calculation.

CCELL 10 1 # Set cell size to 10u by 1u

Without any arguments, print the current cell size. With arguments, set the maximum segment size mesh accordingly.

• <max length>: Maximum length of sub-cell
• <max width>: Maximum width of sub-cell

See also the autocell command for automatic cell size generation.

Note: The thickness does not enter into the capacitance calculation as all metals are assumed infinitely thin. This approximation is valid if the charge distribution does not vary along the thickness of a conductor. This is the case when metallization is much wider and longer than thick. This assumption, though, is increasingly less valid for new metallization processes that allow relatively thick metal layers with a pitch smaller than the thickness (say a .4u pitch with 1u thickness). If this introduces a significant error, two or more adjacent metal layers can be used to construct the structure by using the joinshunt command.

Note: This command only alters sub-sectioning for the capacitance matrix. To set the cell size for the inductance matrix, use the cell command.

Note: If calculating `pi' parameters, the <max length> is determined by a combination of the <max length> parameter from the capacitance and inductance cell size by choosing the minimum of the two. This constraint is introduced in order to solve the node equations to derive the 2-port parameters of the device.

CELL 50 2 1 # Set cell size to 50u by 2u by 1u

Meshing is performed automatically by ASITIC if the autocell option is invoked (default behavior). However, in some circumstances, it is important to have control over the meshing routines.

Without any arguments, print the current cell size. With arguments, set the maximum segment size mesh accordingly.

• <max length>: Maximum length of sub-cell
• <max width>: Maximum width of sub-cell
• <max thickness>: Maximum thickness of sub-cell

The dimensions of the arguments are in units of units, microns by default. This mesh size affects how many matrix entries are generated in the partial inductance matrix and can thus have an adverse effect on performance and accuracy.

See also the autocell command for automatic cell size generation.

Note: This command only alters meshing for the partial inductance matrix. To set the cell size for the capacitive matrix, use the ccell command.

Warning: Thickness meshing is currently under construction and gives unreliable results for thick metals. Please avoid meshing the thickness of the conductor and instead use high frequency equivalent sheet resistance for thick metals loss calculation.

If the eddy current flag is set, then structures with Manhattan geometries will pass through the eddy current calculation engine to include the effects of substrate induced eddy current losses (but not reflected inductance). The eddy current engine assumes a single or two layer semi-infinite conductive substrate in performing the calculation. The results of substrate losses are added to the Inductance matrix. Vias are ignored in the calculation.

With the geometric distance approximation, the averaging of the mutual inductance is accomplished without numerical integration by assuming the coupling is occurring in a logarithmic fashion (essentially a two-dimensional approximation).

Specify <normal> for to use the geometric mean distance approximation (GMD), and <none> to use the filamental approximation where all segments are assumed to couple as equivalent filaments of current running along the length of the segment (from port to port). As segments are sub-divided into more sub-segments, this approximation is increasingly valid.

If <integrate> is specified, then the mutual inductance is averaged over the cross section of the conductors by using numerical integration (this option is not currently available).

With the <ignore_far> option, the GMD is only computed for structures geometrically in close proximity and a filamental approximation is used for distant coupling. This is equivalent to saying that for far coupling the detailed current distribution does not significantly alter the interaction of segments.

Load design1 # Load all spirals from file `design1' into memory

To see a list of spirals residing in a particular file, use the cat command.

SetMaxNW 5  # Allow a maximum of 5 sub-divisions along the width

For very wide metallization, or at high frequencies where the skin depth is much smaller than the physical width of a conductor, ASITIC generates very large partial inductance matrices. In many cases the extra complexity is not accompanied by a corresponding improvement in accuracy. To avoid unnecessary sub-divisions by the autocell meshing routines, set this parameters accordingly.

Note: For best accuracy, use odd numbers.

To avoid unnecessary disk activity, turn off the tekio flag to prevent writing new FFT data to disk.

Save design3 sp3 # save spiral sp3 into the file `design3'

Save fails if the named spiral does not exist in memory, or if a spiral by the same name already appears in <filename>. Another condition for failure relates to the user's ability to save files in the current directory based on write permissions.

When computing N-port parameters of a structure, such as with the `pix' command, ASITIC computes the capacitance and partial inductance matrix. Normally this data is discarded after the necessary information has been extracted. With this option, users can save these matrices to disk in ASCII format for further examination.

set eddy=true gmd=normal

User defined environmental variables can also be defined and used as arguments to many commands. For instance, as a shortcut, we can define a long and descriptive name to a short variable:

set a=square_100_10_5_3

and use $a in place of the 'name' argument for a 'square' command.

Since many users repeat commands at the same frequency, this is a useful feature that can save considerable time since the FFT computation needs to be performed only once for a given technology file. On the other hand, if the technology file is not constant or not needed in future sessions, such as changes made with the modifytechlayer command, then this options prevents unnecessary disk IO by skipping the save step.

If the timer is engaged, then ASITIC computations are timed. This is useful for gauging how long computations take as the cell size is varied.

Without any arguments, the current unit scale factors are displayed.

Optional Arguments:

• <len_units> Length units, i.e. 1e-6 for microns
• <frq_units> Frequency units, i.e. 1e9 for GHz

Given an argument, the geom command is invoked to display specific information about the device.