Comment by tarlinian

To be clear, you want to decrease the permittivity that adds capacitance where the accumulation of charge serves no purpose (i.e., in the interconnects) and increase permittivity where we want to have a lot of charge (i.e., on the actual transistor gates so we can produce a large enough electric field to switch the transistor), which is why gate dielectrics use high-k materials like HfO2 and interconnect layers use low-k dielectrics like carbon-doped oxides.)