Multi-part Paths

Introduction

When connecting devices together electrically, it is recommended that a multi-part path (MPP) be used for the conducting lines. The multi-part path should contain all of the layers listed in Table 1. The minimum widths for the various parts of the path are also listed in Table 1.

These widths are based on the PolyMUMPs design rules [1]. An additional restriction is that the P1P2VIA path be 4um wide. If this layer is allowed to be its minimum width, then the topography will prevent proper metallization. This will create conducting lines with relatively high resistances [1]. For additional safety, all widths could be increased by 2um.

All layers should be included. Ignoring layers, even for short distances, can compromise the mechanical strength of the conducting path. We have observed short bridges (<10um length) that cracked during wire bonding. The method suggested in [1] is to anchor POLY2 directly by using ANCHOR2. However, it is felt that including all layers in Table 1 eases later design issues.

Drawing all of the layers separately is tedious and error prone. Fortunately, Cadence has the facility to draw multi-part paths (MPP) [2]. With a MPP, the points defining the path are entered to create a poly-line. Cadence, using definition data that needs to be entered, then creates all of the other required paths in the appropriate layers.

To avoid having to repeated enter all of the definitions, the definition can be entered once and saved as a MPP template. Later drawing involves simply selecting the appropriate template before continuing.

Set-up

Automatic

The MPP templates can be set-up by merging an ascii technology file containing the right lxRules into the library. The technology file provided contains definitions not only for the PolyMUMPs line described above, but also for anchored POLY1 and POLY2 lines, which contain ANCHOR1 and ANCHOR2 respectively.

Table 2: Steps for automatic creating of multi-part paths.

Step	Description
Step 1:	Download the file mumps.lxRules.tf.
Step 2:	From the ICFB, open the Technology File menu and select Load.
Step 3:	Type in the filename to the saved technology file for the first field, "ASCII Technology File".
Step 4:	Although the file contains only lxRules, to be safe make sure that the only box checked is the one for lxRules.
Step 5:	Select the library into which you want the MPP templates to be saved
Step 6:	If you wish to replace any existing templates, select Replace, otherwise select Merge.
Step 7:	Hit OK
Step 8:	When exiting Cadence, you may be prompted to save changes to the binary technology file associated with the library. The changes must be saved to have the MPP templates present the next time you run Cadence.

When using the layout editor, you should now find three templates available when creating multi-part paths: mumps_line, poly1_line, and poly2_line.

Manual

The MPP templates can also be setup manually by following these instructions. The specific instructions here are to create a conducting line, as specified in Table 1. However, you could also use these instructions to create other types of MPP templates.

When using the Cadence's Virtuoso layout editor, the first step is to select METAL as the active layer. This can be done by using the "Layer Select Window" (LSW). To create the path, select Create->Multi-part Path from the editor's menu. The first parameter, which is MPP Template, should be set to new. This allows you to create a new definition. Set the width of the multi-part path to 6. At this point, you have a simple path with a width of 6um.

Now you need to define the sub-parts. Click on the button Subpart. The following table contains the steps necessary to define the sub-parts. These sub-parts will take care of the work of defining the all the layers other than metal.

Table 3: Steps for manually creating of multi-part paths.

Step	Description
Step 1:	Select "Enclosure Subpath"
Step 2:	Set "Layer" to POLY1. If you have the physical rules for the technology properly set-up, then some of the following values may be entered automatically.
Step 3:	Set "Begin Offset" to 7
Step 4:	Set "End Offset" to 7
Step 5:	Set "Enclosure" to -7
Step 6:	Click "Add"
Step 7:	Repeat steps 2 through 6, except use ANCHOR1 for the layer, and 3um and -3um for the distances.
Step 8:	Repeat steps 2 through 6, except use POLY2 for the layer, and 3um and -3um for the distances.
Step 9:	Repeat steps 2 through 6, except use P1P2VIA for the layer, and -1um and 1um for the distances.
Step 10:	Click "OK"

At this point, you can start drawing the path in the layout window. However, to avoid the above work, click the button Save Template. The name I've been using is mumps_line. Now, whenever you wish to create a new conducting line, you can change the MPP Template to mumps_line. To make this template available permanently, you need to save the technology file. This can be done by using the menu in the ICFB window. From the menu, select Technology File->Save.

Use

To create a conducting line, select Create->Multi-part Path from the editor's menu. Select mumps_line as the MPP template. The default width for the MPP will be 6um, but you can change that value before continuing. Returning to the editor window, click on the points defining the path in turn to lay out the path. Double clicking on the last point will end the path.

Making changes to the path is relatively easy as well. Begin by opening the properties dialog for the MPP. If you change the width, all of the layers in the path will change to match, although you may need to force a redraw of the screen. You can also edit the list of points from this dialog.

Connections

For devices of POLY1 or POLY2, connecting to the path is easy. Simply connect a bridge of the appropriate width to the path. Connecting to the METAL layer is also easy, even if a POLY2 bridge is also required upon which to place the METAL.

To connect using POLY0, however, the recommended method is to have the POLY0 pass under the path and extend 4um beyond. Having the POLY0 pass clear through the line reduces the amount of topological changes present in the line. As excess topological features can interfere with the formation of a continuous layer of METAL, topology can raise the resistance of the line.

If space is really short, then the POLY0 can stop in the middle or the path, but the METAL may not be continuous. Since the poly-silicon has a really low resistivity in the MUMPS process, this may not be a problem. Without the METAL layer, a path with the dimensions given in Table 1 should have a resistance of approximately 0.34Ωum-1.

References

[1] David Koester, Allen Cowen, Ramaswamy Mahadevan, Mark Stonefield, and Busbee Hardy. PolyMUMPS Design Handbook: 9th Ed. MEMSCAP, Bernin, France, 2003.

[2] Virtuoso Layout Editor User Guide. Cadence Design Systems.