It is common that we use higher metal layer to fix, this is something to do that lower metal layers will be formed than higher. after fabrication of every metal layer there is planarization process which takes care of removing collected charge on metal layers. if we use higher layer charges will be less as extra charges are removed. if we use lower layers to fix antenna there might be extra charge deposited to lower level which will find the path downward to flow that charge may break gate oxide, jog to lower layers might not help as it will increase net resistance and hence delay.

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