A high magnetic coupling, low loss, stacked balun in digital 65nm CMOS

We present a high magnetic coupling, low loss, stacked balun using a thick aluminum bonding metal layer over a thick copper layer. The thick copper is used to realize a differential primary input winding that resides directly underneath a single ended spiral winding using the aluminum. The spiral forms the single ended secondary output of the balun and is rotated by 90deg so as to prevent any metal shorting for its cross under. Occupying an area of 0.078 mm₂ on a digital 65 nm process, a 5 turn primary with a 3 turn secondary has a measured coupling of 0.94 and a total balun loss of 1.55 dB at 1845 MHz.