For the development of highly integrated circuits for microwave applications, so-called "monolithic microwave integrated circuit" (MMIC), the miniaturization of the components used is of great importance in order to reduce the weight and space occupied by the completed system. Micromachined transmission lines made by metal layers on top of a passivated surface of a Si wafer offer a means to fulfill these criteria and provide easy fabrication and a potential for low cost.
Different designs for micromachined transmission lines have been developed. The following picture shows two of the approaches used:
Figure 11: Different designs for microfabricated planar transmission lines: (a) single microstrip line on top of a wafer with metallized bottom. (b) coplanar waveguide (CPW) line.
Usually, the transmission line on top of the wafer can be fabricated using a photoresist lift-off process, i. e., exposing and developing the photoresist in a way that opens the area in which the metal layer should be applied. After evaporation of the desired metallization, the photoresist is lifted of, taking away the unneeded metal. The ground plane shown in (a) can be evaporated after or before installation of the microstrip line on top of the wafer.
In order to reduce unwanted coupling between the waveguides making the structure usable at higher frequencies, further shielding around the lines is necessary. Different approaches are used in this respect, most of which involve bonding two wafers onto each other, building so-called "self packaged circuits". One wafer usually carries the transmission line, being covered by a second wafer with a micromachined metallized cavity in the areas that cover the waveguide. Figure 12 shows one possible design. The thin passivation layer between the waveguide and the surface of the Si wafer (usually thermally grown SiO2) is not shown because it is very thin. The transmission line is a coplanar waveguide, with the ground lines next to the transmission line clearly visible.
Figure 12: Shielded coplanar waveguide with cavity wafer on top. The transmission line is shown in red, the yellow parts are metallized areas for shielding purposes.
The manufacturing process for a microshielded circuit of this kind is described in the Table 4.