High performance thick copper inductors in an RF technology

With the emergence of wired and wireless communication technologies, on-chip inductors find applications in a variety of high performance radio frequency (RF) circuits. In this work, we present two approaches for high-performance copper inductors in an RF technology. In the first approach (Type I), we lower ohmic losses to realize a high Q-factor. This is achieved by using, for the first time in a manufacturable technology, 4 mum thick copper spirals alone, with a 4 mum thick copper underpass on high-resistivity substrates (75 Omega-cm). The underpass is connected to the spirals with a 4 mum tall copper via, which lowers spiral to underpass capacitance. For further lowering the capacitive losses, an additional 6.1 mum thick interlayer dielectric separates the underpass from the substrate. In the second approach (Type II), we utilize a novel one-mask CMOS-compatible micromachining scheme to eliminate substrate losses. This is achieved by completely removing the silicon substrate from directly below the inductors. For a 1.1nH inductor, peak-Q shows an impressive two-fold improvement from 26.6 at 3.8 GHz for Type I inductor to 52.8 at 8.2 GHz for Type II inductor after silicon micromachining. The resonant frequency increases from 18 GHz to 27 GHz after substrate micromachining.