关于tsmc 28nm工艺库中cfmom_wo的疑惑？

晬婴

小弟最近刚接触tsmc 28nm工艺库，发现里面有cfmom_wo器件，含义是FMOM without poly-shielding。可是小弟打开它的layout，发现cfmom_wo的版图的金属层下面有dummy的PO和OD。

问题1：按我的理解有这两个层不就会加上PO和OD的dummy了吗？那这样金属层下面不就有poly了？那怎么还能叫without poly-shielding？

问题2：为什么要在金属层下面加上PO和OD的dummy呢？这有什么用吗？
问题3：既然工艺库提供了FMOM without poly-shielding的cfmom_wo，那为什么不提供FMOM with poly-shielding呢？或者说有poly-shielding的FMOM长什么样子呢？
谢谢大家！

xdmicro

#在这里快速回复#with poly-shielding means that a large poly connected to VDD/VSS under the mom capacitor. PO和OD的dummy is used to pass drc (density).

晬婴

   

xdmicro 发表于 2025-10-5 18:23
#在这里快速回复#with poly-shielding means that a large poly connected to VDD/VSS under the mom capac ...

非常感谢您的解答！那既然PO和OD的dummy如果加在mom下面是为了过DRC，那加上PO和OD的dummy难道不会对mom的容值产生影响吗？如果会产生影响，岂不是会破坏原来的设计预期？

xdmicro

   

晬婴 发表于 2025-10-5 21:09
非常感谢您的解答！那既然PO和OD的dummy如果加在mom下面是为了过DRC，那加上PO和OD的dummy难道不会对mom ...

Typically, the absolute capacitance value has a 10% to 20% tolerance due to process variations. While the presence of dummy PO and OD does affect the absolute capacitance value, the impact is expected to be minimal. However, this does not limit its application, as we generally place greater emphasis on capacitance matching accuracy, or alternatively, employ calibration techniques to compensate for deviations in absolute capacitance precision.

晬婴

   

xdmicro 发表于 2025-10-5 23:44
Typically, the absolute capacitance value has a 10% to 20% tolerance due to process variations. Wh ...

非常感谢！