High aspect ratio spiral microcoils fabricated by a silicon lost molding technique

In this paper, a silicon lost molding process is described for fabricating high aspect ratio microcoils with high Q factors. Deep reactive ion etching, electroplating and XeF₂ silicon etching are utilized in this process. Microcoils with an aspect ratio of 16 and inner diameters from 80 νm to 200 νm are fabricated. The electrical characteristics are measured using a network analyzer and a two-terminal radio frequency probe. For a microcoil with an inner diameter of 130 νm, three windings and an outer diameter of 240 νm, the quality factor is 85 at a frequency of 1.6 GHz. The proximity effect and parasitic capacitance are found to be the key issues for limiting the maximum Q factor of the microcoil at high frequencies. The high Q values of the microcoils make them attractive for high resolution micro-MRI (magnetic resonance imaging) applications.