Micromachined microwave planar spiral inductors and transformers

A new micromachined planar spiral inductor, with the strips suspended indiv idually, has been fabricated in standard GaAs high electron-mobility transi stor monolithic-microwave integrated-circuit technology through maskless fr ont-side bulk micromachining. The electronic compatibility, the use of indu strial integrated-circuit production lines, the straightforward and low-cos t additional procedure for structure releasing, and the very short etching time required to do such are the principal features related to such a novel inductor structure. Moreover, the air-gap layer created underneath the dev ice and between the strips significantly reduces shunt and fringing parasit ic capacitances, consequently increasing the performance and operating freq uency range. Experimental measurements, carried out up to 15 GHz, before an d after micromachining, showed for a 12-nH inductor an increase of the maxi mum Q factor from 5 (at 3 GHz) to about 20 (at 7 GHz), while the self-reson ant frequency was shifted from 5 to 13 GHz. Furthermore, a structure with t wo interleaved spiral inductors, in a 1:1 transformer-like configuration, w as also fabricated, and its performance was verified as well in order to de monstrate the promising performance improvements provided by the proposed d evice. Finally, heating and mechanical characteristics associated with free standing microstructures are briefly evaluated using finite-element method simulations.