This work investigates the millimeter-wave loss performance of grounded coplanar waveguides (GCPWs) on doped and HR Si substrates, both fabricated on 4-inch wafers. Identical 2146_µm-long GCPWs are implemented on HR Si (p > 1kΩ·cm, thickness = 155 µm) and doped Si (p < 10 Ω·cm, thickness = 205 µm). Ground electrodes are tightly grounded via a dense TSV array to suppress higher-order modes. TSVs are filled with annular Al structures to mitigate residual thermal stress from CTE mismatch. DC measurements confirm a TSV series resistance on the order of 1 Ω. Small-signal millimeter-wave on-wafer measurements show the coplanar interconnects fabricated on HR Si have an insertion loss of 0.6 dB/mm at 40 GHz, representing more than a fivefold improvement over the same coplanar interconnects fabricated on doped Si. The return loss of the coplanar interconnects fabricated on HR Si is always greater than 27 dB between 1 and 40 GHz. S-parameter measurements from 1– 40 GHz demonstrate the superior performance of HR Si interposers for mmWave heterogeneous integration.