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Layer Block Keywords

This section describes the keyword entries which appear in layer blocks which relate to extraction. These define the conductor layers which are involved in grouping, identify vias between conductors, etc. Further, geometrical electrical data can be assigned to layers, which will be used for parameter extraction. All of these settings can be entered with the Edit Extraction command in the Extract menu and then written to disk with the Save Tech command in the Attributes Menu, or be entered with a text editor directly into the technology file.

Some of the keywords below use layer expressions, as were described in 10.1. A layer expression in its simplest form is a layer name. More generally, it consists of an expression involving layer names, the intersection operator (&), the union operator (|), and the inversion operator (!). Parentheses can be used to enforce precedence. These are the same type of expressions as used in the DRC tests. The expression is ``true'' at points where the expression would return opacity.

Conductor [Exclude expression]
This keyword indicates that the present layer is to be included in conductor net grouping. If the keyword Exclude and a following layer expression are given, the regions of the current layer under which the expression is true are clipped out for grouping purposes. For example, in CMOS technology a transistor is formed by a strip of CAA (active area) bisected by a CPG (polysilicon) gate. If ``Conductor Exclude CPG'' is given in the CAA layer block, the two pieces of CAA will be given separate group numbers, which is necessary to keep the transistor source and drain separate.

Routing
This keyword implies that the layer is a conductor used for connecting between cells. The Conductor keyword is implied, so that the Conductor keyword does not also have to be supplied, unless there is an Exclude directive. Only layers with the Routing keyword given will be considered by the extraction system for connecting between cells, and cell formal terminals will only be assigned to Routing layers. This is not absolute, however. The extraction system will place formal terminals on Conductor layers under some circumstances, if necessary.

$\textstyle \parbox{4in}{\raggedright
{\sf GroundPlane}\\
{\sf GroundPlaneDark} (alias)\\
}$
This keyword indicates that the present layer is to be treated as a clear-field ground plane. The layer is given the Conductor attribute. If the keyword ``Global'' appears, then every object on the layer will be assigned to the ground group 0. This would be appropriate if the layer represents a diffusion rather than a metallic ground plane. The default is to treat this level as a normal conductor, except that when this layer is grouped in the top-level cell, the group with the largest area is assigned to the ground group.

If ``Global'' is given, the GroundPlaneGlobal variable, which activates the mode, will be set.

Only one of the ground plane keywords can appear in the technology file. Conductor group 0 is used only if a ground plane has been specified. The ground plane layer can be referenced in Via and Contact lines just as any Conductor.

$\textstyle \parbox{4in}{\raggedright
{\sf GroundPlaneClear} [{\sf MultiNet} [0...
...vert$2]]\\
{\sf TermDefault} [{\sf MultiNet} [0$\vert$1$\vert$2]] (alias)\\
}$
This keyword indicates that the present layer is to be treated as a dark-field ground plane. These keywords imply DarkField. Giving GroundPlane (or GroundPlaneDark) and DarkField is equivalent to GroundPlaneClear without MultiNet.

Only one of the ground plane keywords can appear in the technology file. Conductor group 0 is used only if a ground plane has been specified.

Without the MultiNet keyword, connections to this layer (as specified with the Via and Contact keywords), where this layer does not appear, are considered as connections to ground (group 0). Although this approach may work for simple cells, it can lead to trouble. Suppose that an island of ground plane metal is used as part of the metalization for the chip pads. This would appear as a hole in the displayed representation of the ground plane layer. Then each pad will be extracted as shorted to ground!

There is provision for more intelligent handling of the GroundPlaneClear layer, allowing the layer to be included in paths and groups. If the MultiNet keyword appears, the inverse of the layer is computed, and that (temporary) layer is used in the grouping. However, it can take quite a lot of behind-the-scenes computation if the GroundPlaneClear layer has complex patterning. Inversion is also done if the !set variable GroundPlaneMulti is given (note: this variable was formerly named HandleTermDefault). The temporary layer is treated as a clear-field ground plane, and all references to the ground plane will be applied to the temporary layer during grouping and extraction.

The name of the internal layer created is ``$GPI''. By default, this layer is invisible. It should not be directly edited by the user. The inverse layer is an internal layer and is never written to a file during conversion or a save. During extraction the GroundPlaneClear layer is ignored, and the inverse, which is a Conductor, is used to establish connectivity.

To establish connectivity for the commands in the Extract Menu, the inverse layer is created according to one of the algorithms described below. An optional integer 0-2 may follow the MultiNet keyword, which indicates the algorithm used for inversion. The algorithm can also be selected by setting the variable GroundPlaneMethod to an integer in the same range, with the !set command.

0 The inverted layer is created for each cell in the hierarchy by computing
$GPI = !GP & !$$
i.e., for each cell the ground plane is inverted and the areas over subcells are removed (recall that ``$$'' is a pseudo-layer representing subcell boundaries). This is the default.
1 The inverted layer is created only in the top cell in the hierarchy, and is the inverse of a flat representation of the ground plane layer from all cells in the hierarchy. The extraction algorithm will add virtual contacts from this layer to the appropriate places in the subcells.
2 The inverted layer is created in each cell of the hierarchy by creating a flat inverse of all of the ground plane found in the cell or lower in the hierarchy.

The default (0) method is the most efficient computationally, but the method will probably fail if sibling subcells overlap. In general, it is good practice to avoid cell overlap.

Method 1 will work if subcells overlap. However, since there is no local ground plane in the subcells, generating a netlist while in a Push (subedit) will not yield correct results.

Method 2 is the least efficient computationally, but each cell has a local ground plane.

Via layer1 layer2 [expression]
This keyword indicates that the present layer may provide connection points between conductor nets on layer1 and layer2. The layer1 and layer2 are names of layers each of which have the Conductor, Routing, or one of the GroundPlane keywords specified. In extraction, it is assumed that the via is formed by dark area on the present layer, and vias are completely covered by layer1 and layer2. A connection is indicated if the expression (which is a layer expression) is true at any point within the via. The Via keyword implicitly assigns DarkField. The recognition logic is as follows:

for each region of the Via layer {
if (there exists an object on layer1 that overlaps region)
if (there exists an object on layer2 that overlaps region)
if (there is no expression, or the area where expression is true in region is nonzero)
then the via indicates a connection between the two objects
}

If the expression is not given, it is always taken as ``true''.

Examples:

Via M1 M2 !RES
A via is indicated if part of the via object on the present layer which is being evaluated is not covered by objects on RES.

Via M1 M2 I2
A via is indicated if the via object on the present layer is partially or completely covered with I2.

Via M1 M2 (!I2)&(!RES)
A via is indicated if part of the via object is not covered by I2 or by RES.

Contact layer [expression]
This keyword specifies that the present layer may be in contact with layer, which has the Conductor attribute, and is to be grouped accordingly in the wire net extraction. The expression (which is a layer expression), if given, must be true in the overlap region between the object and the objects on layer for contact to be established.

The purpose is to account for a contact metalization which is applied over the normal wiring layers, which may itself be used for making connections occasionally. The Contact keyword implies Conductor. The Contact keyword should be given in the layer block of the contact metal layer. It is not necessary (or desirable) to include a reciprocal Contact specification in the referenced layer's block.

DarkField
This keyword indicates that the layer polarity on the chip is the reverse of that shown on-screen. This is usually the case for via layers, for example, which are rendered as small squares to indicate the contact location, which is actually a hole in an insulating layer. At present, the only command that uses this keyword is the Cross Section command in the View Menu. Layers with the keyword applied will be shown as on-chip in the cross sectional view. This keyword is implicitly assigned by both Via and GroundPlaneClear.

The keyword has a secondary effect if used in conjunction with the GroundPlane (or the equivalent GroundPlaneDark) keyword. The combination is equivalent to GroundPlaneClear.

Symbolic
If this keyword is present in a layer block, the layer will not be included in the cross section displays produced with the Cross Section command (in the View Menu).

The following keywords can appear only in physical layer fields, and they specify the computation of electrical parameters associated with the layer. These parameters can be displayed while boxes are created (with the boxes command), and are used in the Compute Params command. The results from these models should be considered as an approximation. No account is taken of the ``real'' geometry, just the simple assumption of a non-featured dielectric and ground plane underneath the conductor, for the capacitor and transmission line models.

Resis[tance] ohms_per_square
The single parameter is a floating point number giving the ohms per square value of the material. This is used in extracting the resistance of resistor devices.

Cap[acitance] units_per_sq_micron [units_per_micron]
This enables computation of the capacitance of a conductor group. The first parameter is a floating point number giving capacitance per square micron. The optional second parameter (default 0) is the edge capacitance, per micron. The extracted capacitance is the conductor group area multiplied by the first parameter, plus the conductor group perimeter length multiplied by the second parameter, if given. The capacitance for each wire net is computed during extraction, and will be printed (if enabled) in the physical netlist output file.

Tran[line] line_thick line_pene gp_thick gp_pene diel_thick diel_const
This keyword will invoke a microstrip model which computes the transmission line parameters along the long dimension of a newly created box. The parameters are all floating point numbers, and dimensions are in microns, except for the dielectric constant which is dimensionless. The penetration depths are included (the model is actually for a superconducting microstrip line) and can be set to some small value for normal metals. The algorithm is the same as used in the public domain sline program available as source code from the Whiteley Research web site. These parameters are used for inductance extraction. The extraction assumes that the layer is over a continuous ground plane, separated by a constant-thickness dielectric.

The following keywords are used by the FastCap/Fasthenry extraction interface. Each keyword takes a single real value argument.

Thickness thickness
This keyword supplies the film thickness of the corresponding deposited film. The thickness is given in microns.

At most one of the following two keywords (Rho and Sigma) should be used. These apply to layers with the Conductor attribute given explicitly or implicitly.

Rho resistivity
This keyword supplies the resistivity, in MKS units, of the corresponding conducting film.

Sigma conductivity
This keyword supplies the conductivity, in MKS units, of the corresponding conducting film. This is converted to resistivity (1.0/conductivity) internally.

EpsRel diel_constant
This keyword supplies the relative dielectric constant of insulating layers. An insulating layer must also have the Via keyword given, or must be a passivation layer. A passivation layer is a single layer that must be above all Via layers which has the EpsRel given and a nonzero Thickness given.

Lambda pene_depth
This keyword specifies the London penetration depth of superconducting conductors, in microns. When Lambda is given, Rho/Sigma (if given) represents the conductivity due to unpaired electrons from the two-fluid model.


next up previous contents index
Next: Device Blocks Up: Extraction Setup and Configuration Previous: Extraction Setup and Configuration   Contents   Index
Stephen R. Whiteley 2006-10-23