9 Managing Metal Density

The metal density in a layout design needs to be managed to ensure that there is uniform metal distribution across the chip for manufacturing. In order to address and manage metal density issues, such as density checks, thin or local analysis, and fixing density errors through slotting, use the Metal Density toolbar. The Metal Density toolbar provides an interactive and user-friendly approach to address and manage the metal density issues. This toolbar is available in Layout XL and higher tiers. Depending on the layout tier, specific number of tokens are required to access the slotting feature. For more information, see Table 1-6 Tokens per Feature (VLS GXL) in the Virtuoso Software and Licensing User Guide.

The Metal Density toolbar currently addresses only maximum density issues.

This chapter covers the following topics:

Metal Density Toolbar

To maintain the integrity of the original shape for as long as possible, slotting lets you add slot shapes to the original shape, such as wires and polygons. For more information on how to convert the original shapes covered by the slot shapes into slotted polygons, see lxConvertSlotToPolygon. Also, the slotted shapes can be edited to solve electromigration violations. In addition, the slot shapes are created as a figGroup to prevent any incorrect manipulation.

Using the Metal Density toolbar, you can perform DRC correct slotting, maxDensity checks, and analyze any kind of shapes. The different scopes provide a smooth and user-friendly experience of the entire design, a specific region, one or more layers, the current window, or by selection. In addition, the toolbar provides an options form for maxDensity checks and local slotting statistic reports. The Metal Density toolbar has the following buttons for metal density checks and slotting.

Check Density

Checks the maximum metal density rules according to the foundry constraints under different scopes. As a prerequisite for the density checks, the maximum density rules need to defined in the technology library.

Command	Lets you
Check Density All	Check the maximum density rule in the entire design.
Check Density By Area	Check the maximum density rule in the area defined by the two specified points in the design.
Check Density Current Window	Check the maximum density rule in the current window (layout canvas limit).

Density Analysis

Reports the local density of each slotted shape, which means that for each slotted shape, its total slotted surface over its associated shape surface is reported.

Command	Lets you
Density Analysis All	Report the local density of each slotted shape in the entire design.
Density Analysis By Area	Report the local density of each slotted shape in the area defined by the two specified points in the design.
Density Analysis Current Window	Report the local density of each slotted shape in the current window (layout canvas limit).
Density Analysis By Selection	Report the local density for all selected shapes in the design. If both nets and shapes are selected in the design, then when Density Analysis By Selection is selected, it filters out the nets. If Density Analysis By Net is selected, it filters out the shapes.
Density Analysis By Net	Report the local density for each slotted shape of the selected nets in the design.

Check Slot Consistency
Checks the consistency of the slot shapes in the design. If conditions do not match, an error marker is displayed in the Annotation Browser.

Command	Lets you
Check Slot Consistency All	Check the consistency of all slot shapes in the design.
Check Slot Consistency By Area	Check the consistency of created slot shapes in the area defined by the two specified points in the design.
Check Slot Consistency Current Window	Check the consistency of created slot shapes in the current window (layout canvas limit).

Fix Density

Adds slot shapes using the metal layer-purpose pair slot on any kind of objects, such as wires, paths, rectangles, or polygons. The geometries of the slot shapes are default. These geometries can also be customized using the Metal Density Options form.

Command	Lets you
Slotting All	Add slot shapes on objects in the entire design.
Slotting By Area	Add slot shapes on objects that are fully or partially enclosed in the area defined by the two specified points in the design.
Slotting Current Window	Add slot shapes on objects in the current window (layout canvas limit).
Slotting By Selection	Add slot shapes on objects of the selected shapes.
Slotting By Selection in Area	Add slot shapes on the selected objects, only in the section delimited by the two points of the area.
Slotting By Net	Add slot shapes on objects of nets selected in the Navigator assistant.

Tools
Lets you delete slots, or convert and reconstruct a polygon to slots.

Delete Slots
Deletes the slot shapes on objects according to the scope of the deletion.

Command	Lets you
Delete Slots	Delete slot shapes on objects in the entire design.
Delete Slots Current Window	Delete slot shapes on objects in the current window (layout canvas limit).
Delete Slots By Area	Delete slot shapes from objects that are fully or partially enclosed in the area defined by the two specified points in the design.
Delete Slots By Selection	Delete slot shapes on objects of the selected shapes.

Convert Slots
Converts the original shapes covered by the slot shapes into slotted polygons.

Command	Lets you
Convert Slots	Convert the original shapes covered by the slot shapes into slotted polygons.
Convert Slots Current Window	Convert the original shapes covered by the slots in the current window (layout canvas limit) into slotted polygons.
Convert Slots By Area	Convert the original shapes covered by the slots defined by an area into slotted polygons.
Convert Slots By Selection	Convert the selected shapes covered by the slots into slotted polygons.

Environment Variable: hideConvertToPolygon

Reconstruct Slots
Reconstructs a slotted polygon to a regular polygon that has no holes and also covers the slot shapes.

Command	Lets you
Reconstruct Slots	Reconstruct a slotted polygon to a regular polygon that has no holes and also covers the slot shapes in the entire design.
Reconstruct Slots Current Window	Reconstruct a slotted polygon to a regular polygon that has no holes and also covers the slot shapes in the current window (layout canvas limit).
Reconstruct Slots By Area	Reconstruct a slotted polygon to a regular polygon that has no holes and also covers the slot shapes, which is fully or partially enclosed in the area defined by the two specified points in the design.
Reconstruct Slots By Selection	Reconstruct a slotted polygon, which is selected, to a regular polygon that has no holes and also covers the slot shapes.

Environment Variable: hideReconstruct

Create Fillet (Virtuoso RF Solution Only)
Creates fillets. A fillets is an extra etch added to prevent acid traps on a design, which in turn improves manufacturability. In Virtuoso, fillets are created as curved polygons..

Command	Lets you
Create Fillet All	Create fillets across the entire design.
Create Fillet By Area	Create fillets in a specified area of the design.
Create Fillet Current Window	Insert fillets in the area defined by the size of the current window.
Create Fillet by Net	Insert fillets on selected nets.
Delete Fillet All	Delete fillets across the entire design.
Delete Fillet By Area	Delete fillets from the specified area of the design.
Delete Fillet Current Window	Delete fillets in the area defined by the size of the current window.
Delete Fillet by Net	Delete fillets on selected nets.

Options
Opens the Metal Density Options form, which contains all the geometric and style settings for the generation of the slot shapes. In addition, it contains valid layers that can be considered by each shape for checking, analysis, and fixing metal density.

Performing Metal Density Checks for Shapes

You can check, analyze, and fix metal density for various routing shapes, such as for wires and polygons. To perform metal density checks for shapes, perform the following steps:

Click the arrow next to the Check Density icon on the Metal Density toolbar. A drop-down menu appears.
Choose the scope that you want from the drop-down list. The following figure shows the result of the metal density check when the Check Density All option is selected.
.
Click the Options button on the Metal Density toolbar.
The Metal Density Options form appears.
Select a material to be processed from the Material drop-down list. By default, metal is selected.
From the Layers list box, select a layer, for example Metal3. The selected layers are used for checking, analyzing, and slotting.
The fields in the other tabs of the form are pre-populated with the values defined in the environment variables in the .cdsenv file. You can edit the options needed to define the size and spacing of the slot shapes in the Slot Geometry tab of the Metal Density Options form. Also, you can edit the advanced slotting settings in the Slotting Style tab.
For a polygon shape, select square from the Slots Orientation drop-down list in the Slotting Style page.
Select the consistency checks that you want to run for the slot shapes in the Slot Consistency page.
Click Close to close the form.
Click the arrow next to the Fix Density icon on the Metal Density toolbar. A drop-down menu appears.
Choose the scope that you want from the drop-down list to perform slotting. The following figure shows the result when the Slotting All option is selected to slot the shapes and fix metal density.
Click the arrow next to the Density Analysis icon on the Metal Density toolbar. A drop-down menu appears. You can either perform density analysis of each slotted shape or check for the correctness and consistency of the slot shapes.
1. To report the local density of each slotted shape, choose the scope that you want from the drop-down list. The following figure shows the report of the local density check when the Density Analysis All option is selected.
2. To check the consistency and correctness of the slot shapes, select the required option in the Slot Consistency tab of the Metal Density Options form. For example, to check for a floating slot shape, select the Floating Slot option.
  
  The following figure shows the report of the consistency check along with the markers being displayed in the Annotation Browser, when the Check Slot Consistency All option is selected.
Click the arrow next to the Tools icon on the Metal Density toolbar. A drop-down menu appears. You can either delete the slots on the polygon or convert and reconstruct the slots on the polygons.
1. To delete a slot shape on a polygon, choose the scope that you want from the drop-down list. The following figure shows the result when the Delete Slots by Area option is selected to delete the slots by area.
2. To convert the original shape covered by the slot shapes as slotted polygons, choose the scope that you want from the drop-down list. The following figure shows the result when the Convert Slots option is selected to convert a polygon to slot shapes.
3. To reconstruct a slotted polygon as a regular polygon that has no holes and also covers the slot shapes, choose the scope that you want from the drop-down list. The following figure shows the result when the Reconstruct Slots option is selected to reconstruct a slotted polygon shape into a regular polygon.
(Virtuoso RF Solution Only) Click the arrow next to the Fillet icon on the Metal Density toolbar. A drop-down menu displays.

Choose the scope you want to create fillets. The following figure shows how the fillet is created when the Create Fillet All option is selected.

For more information on fillet creation, see Fillet Creation (Virtuoso RF Solution Only).
Click Slotting on the Metal Density Options form. For more information, see Create Packaging Bus Form.

Deleting Fillets

To delete a fillet:

Click the arrow next to the Fillet icon on the Metal Density toolbar. A drop-down menu displays.
To delete a fillet on a curved shape, choose the scope you want, from the drop-down list.
The following figure shows the result when the Delete Fillet by Area option is selected.

Specification Cells

To handle contextual situations and to apply directives per layer, you can specify a specification cell. The specification cell is a cellview that contains, for each specified layer, the representation of the directional slot shapes and the turn slot shapes. The check box next to the Specification Cell field is used to enable or disable the field. The syntax to specify it in the Metal Density Options form is libName/cellName/ViewName.

When a specification cell is specified, all the geometric information for each layer is derived from the specified cell. For each layer, the following information is derived.

A physical shape that represents the directional slot and the turn slot (optional) geometries. It implicitly defines the following slot parameters by its bounding box:
- slotLength
- slotWidth
Optionally, some of the parameters in the specification cell are defined using:
- The cellview properties
  - slotStaggered
  - excludePins
- The directional slot shape properties
  - slotToEdge
  - lSpacing
  - wSpacing
  - widthThreshold
  - lengthWidthThreshold

In case a list of contexts is defined using the sltSetContextSpecificationCells SKILL function, the Context field is available in the Slot Geometry tab of the Metal Density Options form. Instead of the string field for the Specification Cell, a drop-down list is available, which lists the defined contexts.

When a context is selected from the Context drop-down list, the slot geometry parameters are disabled. All the disabled parameters must be defined in the specification cell corresponding to the selected context. Else, the slotting cannot be completed and the status of the Slotting button is updated and the color is changed to Red.

The following figures shows the cellview and the slot shape properties.

The parameters derived from the specification cell take precedence and override the values that are available in the Metal Density Options form. If one of the above properties is not defined for the specification cell, then the values from the Metal Density Options form is used.
The lower-left corner should be (0,0) for the directional slot shape and the upper-right corner should be (0,0) for the turn slot shape.
The slot shape should be drawn in the direction of the current (from left to right).
If the turn slot is not defined, then the width of the direction slot is used to adjust the size of the shape at turn (square).
The purpose slot should be used.

The following figure shows an example of a specification cell.

The following figure shows the result of slotting when the Slotting All option is selected. As you can see, slotting is different on the two shapes.

Fillet Creation (Virtuoso RF Solution Only)

Fillets are an extra etch added to prevent acid traps on a design, which in turn improves manufacturability. In Virtuoso, fillets are created as curved polygons.

Some considerations for fillet creation are as follows.

Consider that all wires are always center aligned to the target objects. Also, for a square or rectangle target, the connection can be aligned to the corner.
Offset distance is computed from the object side and the intersection between the wire edges and the fillet shape, as shown in the following figure. Also, fillet shapes are symmetric.
When inserting a fillet, the object side is completely covered by the fillet shape. In addition, the desired angle is respected as long as the actual offset is less than the specified maximum offset.

In cases where the maximum offset is not respected while trying to get the desired angle to fully cover the side object, there are two possible options:
- Object side can still be completely covered. This implies that the actual angle is greater than the desired angle.
- Object side cannot be completely covered. In this case, maximum offset and maximum angle are used.

Specifying the Fillet Options

You can specify the following in the Fillet tab of the Metal Density Options form.

The unassigned shapes (without any connectivity) to be filtered out.
The type of fillet that is to be created, linear or curved.
The parameters used to create the fillet shape.
The type of objects that a fillet can be created for: Paths, Bond Fingers, Vias, Pins, Pad without drills, or T connections.
The type of fillet shape to be created (along with the maximum size to control the shape creation): Circle, Square, Rectangle, or Oblong.

Support for Slot Shapes in DRD and EAD

The added slot shapes are supported both by the DRD checker and EAD. For DRD, the maximum density checks support the slot shapes.

In EAD, the R extraction as well as the EM analysis consider the slot shapes using the layoutEAD.pex doSlotting environment variable.

Virtuoso Interactive and Assisted Routing User Guide
Product Version IC23.1, September 2023

9

Managing Metal Density

Metal Density Toolbar

Related Topics