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The performance of a miniature 60 GHz branch-line coupler (BLC) based on a slow-wave coplanar waveguide (SW-CPW), implemented in the back-end-of-the-line of a low-cost 90 nm digital CMOS technology, is reported. Design insights addressing the coupler structural dimensions, artificial dielectric, and loss are discussed. The BLC achieves a measured insertion loss of 2.8 dB, isolation of 25 dB at 60 GHz, and return losses (all four ports) of greater than 10 dB from 46 GHz to at least 67 GHz. The measured amplitude imbalance and phase difference are 0.8 dB and 90.38 at 60 GHz, respectively. By utilising the slow-wave technique, an area reduction of .75% compared to a conventional CPW design has been accomplished with a compact coupler core size of only 290 × 285 mm 2 (0.083 mm 2 ).Introduction: Recent innovations in the 60 GHz unlicensed band (57 to 64 GHz) designs have been well advanced by the multi-gigabit shortrange applications such as wireless HDMI [1]. At millimetre-wave (mm-wave) frequencies, power-efficient quadrature generation based on a passive approach has been widely adopted in I/Q direct-conversion transceivers [2] and balanced amplifiers. However, the enormous size of the quadrature couplers limited by the quarter-wave (l/4) transmission lines is a key concern to the growing chip cost. Numerous techniques towards miniaturisation of passive quadrature generation including the loaded elevated-centre coplanar waveguide (EC-CPW) with a 73% size reduction [3], the meander unfolded Lange coupler [4], and the mixed lumped-distributed 908 power splitter [5] at mm-wave have been demonstrated with reasonably good amplitude and phase mismatches. In this Letter, we present a miniature 60 GHz slow-wave coplanar waveguide (SW-CPW) branch-line coupler (BLC) in a 90 nm digital CMOS technology with a significant size reduction of .75% compared to a conventional CPW-based coupler design.