The development and characterization of self-packages using micromachining techniques for high frequency circuit applications (1995)

The development and characterization of micromachined self-packages for high frequency circuit applications is presented. Micromachining techniques are explored to develop packages for microwave circuits based on coplanar waveguide transmission lines. Half and completely shielded package topologies are realized as self-packaged tuning stubs elements and filters to demonstrate the capability of micromachining a microwave circuit design. The concept is extended to develop the first generation conformal package that follows the direction of an individual line path and provide multiple input access at a cross-junction to a specific line path. Lastly, micromachining is used in a microstrip patch antenna to locally reduce the dielectric constant while the feeding line is maintained on the high index material. The package and circuit are fabricated in high and low resistivity silicon substrates, respectively, using standard photolithographic and lift-off processing techniques for pattern definition. Evaporation and electro-plating processes are used for metallization, and wet-chemical anisotropic etching processes are used for cavity formation. The planar transmission line circuits are typically printed on a polished surface of a high resistivity silicon wafer. Lower shielding cavity formation occurs by anisotropically removing silicon material to form the sidewalls of the lower substrate-filled cavity while the upper air-filled cavity is formed by etching the actual cavity into low resistivity silicon using similar processes. Both surfaces are metallized, then the upper cavity is mounted over the circuits to form the completely shielded or self-packaged configuration. This simple two-dimensional shielding geometry extends in either a straight path, referred to as "in-line", or into bending paths, referred to as "conformal". Passive circuit designs implemented in micromachined self-packages are experimentally characterized to evaluate the effect of this monolithic shielding on the reduction of parasitic radiation, substrate mode excitation, and package resonances. The "in-line" and "conformal" self-packaged designs are characterized using on-wafer probing techniques from 2 to 40 GHz. Basic microwave components such as series open/short tuning stubs, stepped impedance filters, and right angle bends are evaluated since these represent standard elements to many high frequency designs such as filters, switches, couplers, RF chokes, and phase shifters. The main contribution of this work is the characterization of a monolithically integrated micromachined package to planar circuits that offers reduced parasitic radiation as a result of the shielding topology. Poor performance due to the excitation of parallel plate modes in grounded coplanar waveguide circuits is eliminated with the incorporation of the lower shielding cavity. In antenna problems, this technique has been used to locally reduce the dielectric constant of the antenna while maintaining the feeding line on the high index material. This micromachining approach impacts HF packaging designs, complex circuit designs with RF circuitry and radiating elements, as well as vertical and horizontal circuit integration at microwave and millimeter wave frequencies.