An apparatus includes a first tier and a second tier. The second tier is above the first tier. The first tier includes a first cell. The second tier includes a second cell and a third cell. The third cell includes a first ILV to couple the first cell in the first tier to the second cell in the second tier. The third cell further includes a second ILV, the first ILV and the second ILV are extended along a first direction. The first tier further includes a fourth cell. The second tier further includes a fifth cell. The second ILV of the third cell is arranged to connect the fourth cell of the first tier with the fifth cell of the second tier. In some embodiments, the second tier further includes a spare cell including a spare ILV for ECO purpose.