clock shielding
- usually, strongly driven clock net sare noise sources (attacker). Shieldig can avoid other stronger noise and isolate itself.
- some people use double spacing instead of shielding. Less coupling capacitance. Less RC delay.

wire spreading就是把wire density平均化，一般design rule会有min density和max density，min自然不用说，max density主要出于两个目的：
      1. 减少critical area。critical area被定义为容易受杂质影响的区域，容易产生short或者open。某个区域的metal density越大，受杂质影响的几率越大。
      2. 在CMP过程中，由于metal的材料相对隔离介质较软，容易产生dishing，当metal density太大或者metal太宽的时候，dishing进一步严重产生erosion。erosion的影响就比较大了，RC变差，电流能力变差，IR-drop变大，可能产生open等等。所以对于宽metal要加slot，除了liftoff的考虑，erosion也是原因之一。

wire spreading
  - It is a feature for better DFM (what vendors claim). In general, it is also good for better timing due to less long wire coupling.

A few new routers choose routing layer balance (vertical) and wire spreading (horizontal).

Solution in Encounter:




Spare cell for ECO

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