Microfabricated spiral inductors were employed for nanoscale displacement detection, suitable for use in implantable pressure sensor applications. We developed a variable inductor sensor consisting of two coaxially positioned planar coils connected in series to a measurement circuit. The devices were characterized by varying the air gap between the coils hence changing the inductance, while a Colpitts oscillator readout was used to obtain corresponding frequencies. Our approach shows significant advantages over existing methodologies combining a displacement resolution of 17 nm and low hysteresis (0.15%) in a 1 × 1 mm2 device. We show that resolution could be further improved by shrinking the device's lateral dimensions.
The alternative would be a reversal of the connections such that the field is enhanced (the negative signs in Eq. (1) would become positive). In both instances, the effect of bringing the inductors together yields the same inductance change (given by Eq. (3)), however, by having opposing magnetic fields the change in the frequency is greater than having enhancing fields.