Monolithic micromachined planar spiral transformer

Micromachined microwave passive devices have been successfully fabricated in a standard GaAs HEMT MMIC technology, through a straightforward, low-cost, maskless front-side bulk micromachining. Planar spiral inductors with the strips suspended individually have been possible because of the small 'open area' dimensions, needed to etch away some portions of the bulk material, and present numerous advantages with respect to the commonly used membrane-supported device, such as the reduced etching time and the elimination of fringing parasitic capacitances. In this paper, a two interleaved spiral inductor structure, in a 1:1 transformer-like configuration, has been fabricated and characterized up to 15 GHz, in order to demonstrate the features of this novel inductor. Moreover, heating and mechanical characteristics associated with the suspended devices are also briefly investigated.