In the radio frequency (RF) and microwave technology domain there is a need to integrate passive components including inductors, capacitors, switches and filters on an integrated circuit (IC) chip to lower device size and manufacturing cost and to improve performance and reliability. The integration of these passive devices into an RF IC will provide high value for such applications as voltage-controlled-oscillator (VCO), phase-locked-loop (PLL) and other RF functionality required for advanced telecom systems.
Efforts have been focused on improving the RF performance of silicon IC technology, in part due to its low cost, dielectric compatibility, and micromachining properties. The on-chip integration of relatively high-Q fixed and variable inductors with silicon IC technology, however, has been problematic due to the parasitic effects of low-conductivity metallization as well as lossy substrate interactions. As a result, the quality factor Q of inductors fabricated using silicon IC technology is less than about 10 at a frequency of 2 GHz. Therefore, to achieve high performance, most RF IC applications still require the use of off-chip inductors. However, drawbacks of the off-chip inductors include significant parasitic effects, prohibitive size, and large losses due to board-level flip-chip and/or surface-mount interconnections.
Inductors with disk-shaped coils wound about an axis which is substantially perpendicular to an underlying semiconductor substrate are lossy due to eddy currents induced in the closely underlying substrate, and also due skin effects which restrict an alternating current (AC) in the coil to a small skin depth at the edge of the coil thereby substantially increasing the AC resistance of the coil. Efforts to overcome the drawbacks of disk-shaped coil inductors with the above-cited alignment have sought to raise the inductors off the substrate (see e.g. U.S. Pat. Nos. 6,184,755; 6,621,141; and 6,922,127).
The present invention provides an advance over the prior art by providing a tunable inductor having a pair of coils of substantially the same size which are arranged about a central axis which is substantially parallel to the substrate, and with the pair of coils being electrostatically unrolled in tandem to change the inductance.
The tunable inductor of the present invention comprises multi-turn coils which can be partially or completely unrolled to provide a wide variation in inductance.
The electrical power required to tune the inductor of the present invention is low since tuning is accomplished electrostatically, and not by resistive current heating.
The tunable inductor of the present invention can also be digitally tuned by shaping the coils and/or underlying electrodes, or alternately by using a segmented electrode beneath the pair of coils.
These and other advantages of the present invention will become evident to those skilled in the art.