High-Q micromachined three-dimensional integrated inductors for high-frequency applications

Three-dimensional micromachined inductors are fabricated on high-resistivity (10 kΩ cm) and low-resistivity (10 Ω cm) Si substrates using a stressed metal technology. On high-resistivity Si substrate with low-k dielectric material (SU-8™), this technology achieves a quality factor Q of 75 for a 1 nH inductor at frequencies around 4 GHz and a self-resonant frequency f_sr above 20 GHz. Using Si bulk micromachining to etch away the low-resistivity Si substrate with a combination of deep reactive ion etching and tetramethyl ammonium hydroxide etching methods, a 1.2 nH inductor achieves a peak quality factor Q of 140 at a frequency of 12 GHz with a self-resonant frequency f_sr above 40 GHz. The dependence of high-frequency performance on the inductor’s variables, such as the number of turns, turn-to-turn gap, and substrate type, has been investigated. Excellent performance is achieved by removing the substrate due to the complete elimination of substrate losses and the reduction of the parasitic capacitance. This technology is simple and provides high performance integrated inductors on Si or compound-semiconductor platforms.