Chapter 8: Creating New Technologies
8-4: The Layer Cells

Creating and Deleting Layer Cells

Layers are used to construct primitive nodes and arcs in a technology. Because of this, the layers must be edited before the nodes and arcs. To edit an existing layer, select it from the cell explorer or the Edit Cell... command (in menu Edit).

Figure 8.24
To create a new layer, use the context menu on the "TECHNOLOGY LAYERS" entry of the cell explorer and choose "Add New Layer". A layer can be deleted simply by deleting its cell. A layer can be renamed by renaming its cell, but remember to use the name "layer-" in front (i.e. the old name is "layer-metal" and the new name is "layer-metal-1"). Finally, you can rearrange the order in which the layers will be listed with the "Reorder Layers" command from the context menu.

Editing Special Layer Information

There are many pieces of information in a layer, most of which can be updated by double-clicking on them. There is a 16x16 stipple pattern, a large square of color above that, and a number of pieces of textual information along the right side.
Figure 8.3

The stipple pattern can be changed by double-clicking on any grid squares. You can also do operations on the entire stipple pattern ("Clear Pattern", "Invert Pattern", "Copy Pattern", and "Paste Pattern") by double-clicking on their name below the pattern area.

Figure 8.4
The color of the layer can be changed by double-clicking on the "Color" entry. The dialog lets you choose a color, opacity, and foreground factor for the layer. Opacity ranges from 1.0 (fully opaque) to 0 (transparent). The foreground flag is "on" to indicate that the non-opaque colors can be combined with others.

Transparency lets a layer have a unique appearance where it overlaps other layers. The overlap is defined in the technology's color map. You can double-click on the "Transparency" entry to assign this factor to a layer. Non-transparent layers (with "Transparency: none") are opaque, so they obscure anything under them when drawn. In general, the most commonly used layers should be transparent. See Section 4-6-1 for more information on transparency.

The "Style" entry on the right can be "solid" or "patterned", with varying outline types around the pattern ("None", "Solid", "Solid-Thick", "Solid-Thicker", "Dotted-Close", "Dotted-Far", "Dashed-Short", "Dashed-Long", "Dotted-Dashed-Short", "Dotted-Dashed-Long", "Dotted-Close-Thick", "Dotted-Far-Thick", "Dashed-Thick", "Dotted-Close-Thicker", "Dotted-Far-Thicker"). The "Style" can also specify printer patterns "PRINTER-Solid" and "PRINTER-Patterned". When using "solid" styles, the 16x16 stipple pattern is ignored (except for hardcopy). Transparent layers should be solid because they distinguish themselves in the color map. Layers with opaque colors should probably be patterned so that their combination is visible.

Many of the entries on the right side of the layer cell provide correspondences between a layer and various interchange standards. The "CIF Layer" entry is the string to use for CIF I/O (see Section 7-3-2). The "GDS-II layer" entry can be as simple as a single layer number, but it can also be two numbers separated by a "/" (the layer number and its type). You can also add a comma and then another layer/type pair with the letter "t" (for text) or "p" (for pin) at the end (see Section 7-3-3).

Another set of options on the right side of the layer cell is for Spice parasitics. You may assign a resistance, capacitance, and edge capacitance to the layer for use in creating Spice simulation decks (see Section 9-10-1).

The "3D Height" and "3D Thickness" are used when viewing a chip in 3-dimensions. The height and thickness are arbitrary values which describe the location and thickness in the third axis (out of the screen). For example, to show how poly and diffusion interact, the poly layer can be at height 21 and the diffusion layer at height 20, both with 0 thickness. This will appear as two ribbons, one over the other. See Section 4-10-2 for more information on 3D display.

The last option on the right side of the layer cell specifies the minimum coverage percentage (see Section 9-2-4 for more).

Layer Function

The "Function" entry allows a general-purpose description to be attached to the layer. Metal layers can have "-C1", "-C2", or "-C3" appended to them to describe multiple patterns (colors 1, 2, or 3).

A function consists of a single base description plus optional additional modifiers. The additional modifiers are found in the last entries of the function list.
Figure 8.5

These additional modifiers can be added to the base function:

For example, you can double-click the function entry many times, selecting "Diffusion", "p-type", and "heavy" to indicate a Diffusion layer that is heavily-doped p-type. To clear the layer function, set it to "unknown."

A number of rules apply to the selection of layer functions. There must be a "pseudo" layer for every layer used to build arcs. This is because every arc needs a pin, and pins are constructed from "pseudo" layers. The "pseudo" layers are virtual geometry that do not appear in the fabrication output. It is important that every "pseudo" layer have an associated real layer, with similar descriptive fields. The technology editor will issue a warning if pins are not constructed from pseudo-layers.

Note that the layer functions must be treated carefully as they form the basis of subsequent arc and node definitions. One consideration to note is the use of "Wells" and "Substrates". If the technology requires a separate contact to the well, then it will typically contain a metal layer, and a piece of heavily doped material under the metal to make ohmic contact to the well; i.e. p++ in a P-well. This will have the same doping as the well, unlike a device diffusion, which is of opposite type to the well in which it is located.

Two rules apply here:

  1. There must be a separate diffusion layer for the p++ or n++ used as a contact in a P-well or N-well, respectively; it cannot be the same layer that is used for diffusions in active devices.
  2. A p++ or n++ layer that is used to make a contact in a well of the same semiconductor type (for example p++ in a P-well) must not be defined with the layer function Diffusion; it must be declared as "Well". In the well contact shown below, both the p++ layer and the P-well layer will be defined with the layer function "Well, P-type".

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