For the development of highly integrated narrow-band filters in a planar environment, resonators of smaller size and weight than conventional ones made by metallic rectangular or cylindrical waveguides are needed. One approach to providing such structures is the use of metallized micromachined cavities underneath microstrip lines on top of a wafer, coupled by slots in the metallization layer of the cavities. These structures meet the requirement of less weight and space usage and are less expensive to produce while providing a better quality factor (Q) than traditional resonators.
As can be seen in Figure 10, the resonators are built by bonding three different wafers of 500 µm thickness together in order to achieve a metallized cavity underneath the transmission line. The topmost wafer (wafer 1) contains the gold electroplated microstrip lines and a metallization layer on its bottom. In some distinct areas, the substrate is left free from metallization to create slots for coupling the cavity underneath to the waveguide. The cavity is created by anisotropically etching all the way through wafer 2 and subsequent evaporation of the metallization film. The cavity is then covered at its bottom by applying metallized wafer 3.
Figure 10: Components of a micromachined resonator. (a) The three different wafers with respective microstrip lines, cavity and metallization. (b) Bonding of wafers 2 and 3, and application of an additional metallization layer for sealing. (c) The completed resonator structure.
In the process, first the bottom two wafers are bonded together to complete the cavity, using silver epoxy as the bonding layer. Silver epoxy is applied to the surfaces of the two wafers that are to be bonded. No alignment procedure is necessary, since the bottommost wafer contains no specific structure and only serves the purpose to close the cavity to its bottom. After placing the bonded wafers between two glass plates and on top of a hotplate, the bonding is completed by heating and applying gentle pressure (see Table 3 for details). A cover of Al and Ti is then evaporated onto the cavity for sealing. During this step, the metallized crosses on top of wafer 2 that serve as alignment marks for the next bonding have to be protected by attaching small pieces of Si with photoresist.
Afterwards, the bonding layer is applied to wafer 1 and the top of the wafer 2 cavity. The alignment necessary for the exact placement of the slots to the cavity is done using small openings etched through the topmost wafer (so-called "vias"). Through these vias, alignment marks on the surface of the middle wafer can be lined up, using the positioning facility of an electrostatic bonding station.
It is conceivable not to etch all the way through the middle wafer but only part of it, thus making a third wafer unnecessary.
Table 3 (opening in new window) shows the manufacturing process steps for a micromachined filter.