While traditional microwave packages place MMICs in a box that is hermetically sealed (to keep moisture out), these schemes require connectors and DC feed-throughs exhibiting losses at high frequencies (e.g., Ka-band), which defeats the purpose of the low-loss device inside. Therefore, the approach followed in [18] involved etching an approximately 0.007-in cavity in a lid made out of glass material and bonding the edge of the cavity to the chip using nonconductive epoxy (Figure 5.7). Glass was utilized as the lid material because of its low dielectric constant (which minimizes the perturbation or electromagnetic coupling between the lid and the circuit), its optical transparency (which allows the visual observation of the MEM switches while actuating), and its coefficient of thermal expansion (CTE) (which was close to that of silicon). The performance of the packaged phase shifter is shown in Table 5.1. The phase shifter operated in the 30- to 38-GHz band and exhibited insertion loss between 1.8 dB at 34, for the shortest state, and 3 dB for the longest state, with an average loss of 2.25 dB. The return loss was greater than 15 dB for all states.
An analysis of the losses determined that, with a line loss of 0.7 dB/cm at 34 GHz, the average loss per switch was 0.25 dB. Lessons Learned. The packaging of the phase shifters using the glass lid introduced two sources of loss: that due to the additional line length, concomitant with the area occupied by the lid, and that due to the transition beneath the lid.
The additional line length was about 0.2 cm on each side and was determined by the room necessary for proper alignment and automated bonding. The accompanying loss was about 0.3 to 0.35 dB. On the other hand, the dielectric loading that results when the glass lid is placed over the microstrip using a 1- to 2-mil-thick layer of epoxy was measured to be about 0.2 dB per transition, for a total of loss contribution to the phase shifter of 0.4 dB.
Thus, packaging introduced an extra loss of 0.7 to 0.75 dB. (2) The proximity of the glass lid over the phase shifter (Figure 5.7) is expected to introduce dielectric loading as well. In this case, the effect of the 7-mil glass cavity was minimal, causing insignificant changes in the insertion loss, and changing the resonant frequency of the phase shifter down by 0.4 GHz.
