Wednesday, 27 November 2019

Metal Slotting


In very deep-submicron VLSI, certain manufacturing steps – notably optical exposure, resist development and etch, chemical vapor deposition and chemical-mechanical polishing (CMP)– have varying effects on device and interconnect features depending on local characteristics of the layout. To make these effects uniform and predictable, the layout itself must be made uniform with respect to certain density parameters. Traditionally, only foundries have performed the post-processing needed to achieve this uniformity, via insertion (“filling”) or partial deletion (“slotting”) of features in the layout. Today, however, physical design and verification tools cannot remain oblivious to such foundry post-processing. Without an accurate estimate of the filling and slotting, RC extraction, delay calculation, and timing and noise analysis flows will all suffer from wild inaccuracies. Therefore, future placeand-route tools must efficiently perform filling and slotting prior to performance analysis within the layout optimization loop.

A wider wire results in smaller current density and, hence, less likelihood of electromigration. Also, the metal grain size has influence; the smaller grains, the more grain boundaries and the higher likelihood of electromigration effects. However, if you reduce wire width to below the average grain size of the wire material, grain boundaries become "crosswise", more or less perpendicular to the length of the wire. The resulting structure resembles the joints in a stalk of bamboo. With such a structure, the resistance to electromigration increases, despite an increase in current density. This apparent contradiction is caused by the perpendicular position of the grain boundaries; the boundary diffusion factor is excluded, and material transport is correspondingly reduced.

However, the maximum wire width possible for a bamboo structure is usually too narrow for signal lines of large-magnitude currents in analog circuits or for power supply lines. In these circumstances, slotted wires are often used, whereby rectangular holes are carved in the wires. Here, the widths of the individual metal structures in between the slots lie within the area of a bamboo structure, while the resulting total width of all the metal structures meets power requirements.

What is the Need of Metal slotting

  1. To maintain the reliability during manufacturing
  2. To overcome mechanical and thermal stress during manufacturing

How Metal slotting is done

  1. Determining long and wide wires, based on foundry rules and foundry capability to manufacture
  2. Place and route tools perform metal filing and slotting with utmost optimization


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