Figure 6 Test data for the 2.9 GHz fusion-bonded, surface-mount filter.
Utilizing a fusion-bonded approach, a single-layer filter is easily extended to a stacked configuration using EM software to model the layer transitions and fusion bonding to implement the via-to-via interconnections. In this case, the port connections of the upper filter are required to transition through an interstitial ground plane to realize the surface-mount footprint. An HFSS model of this transition geometry is shown in Figure 7, along with the return loss of this transition.
Figure 7 HFSS model of an internal via transition through an interstitial ground plane.
Figure 8 HFSS model showing the internal structure of a stacked fusion-bonded surface-mount filter.
The full HFSS model of a stacked filter architecture in S-Band is shown in Figure 8. For this model, the two stacked filters occupy the same surface area as the single filter. A photo of this stacked filter is shown in Figure 9. The outline dimensions of the filter are 0.75 × 0.75 × 0.13 in. Similarly, this approach can be used to stack up to four filters in the same surface area.
Figure 9 Stacked fusion-bonded surface-mount filter.
Figure 10 Stacked fusion-bonded switched filter banks with four filters.
Figure 11 Four filter stacked fusion-bonded switched filter banks data.
STACKED, SWITCHED FILTERS
Figure 12 3D EM model of stacked fusion-bonded switched filter bank with eight filters. (b) Measured results of stacked fusion-bonded switched filter bank with eight filters.
Stacked filters greatly reduce the area required for RF filtering in frequency converters and other integrated microwave modules. For applications requiring switched filters, the stacked filter component can be mounted to the parent board of the integrated microwave module to interface with the switches that are also contained within the module. An alternate approach is to integrate the switches within the stacked filters. In this case, the switches and associated circuitry are mounted inside or on top of the stacked filter component. Several fusion-bonded switched filter banks have been fabricated as six-layer and 12-layer designs in L-, S- and C-Bands. Figure 10 shows two different switched filter banks using four filters. In these realizations, the input and output SP4T switches are mounted internally within the stacked filter component. The outline dimensions of the switched filter banks shown are 1.0 × 2.0 x 0.31 in. and 0.9 × 2.6 × 0.15 in. Another fabricated C-Band switched filter bank measured 0.8 × 1.1 × 0.15 in. Test data is shown for two different stacked switched filter banks in Figure 11. This stacked switched filter approach has been extended to banks of as many as eight filters as shown in the 3D EM model of Figure 12a with the measured results shown in Figure 12b. This approach has been used in multiplexers with as many as 12 channels.
CONCLUSION
This article has demonstrated a stacking strategy for interdigital filters using a fusion bonding approach that enables size reduction in a common footprint. The examples show how fusion bonding facilitates the interconnections of the inner structure, providing a homogeneous component. Since temperature-stable materials are used in this process, the performance of the resulting components is also stable over temperature extremes and able to withstand the rigors of MIL-STD screening. Several of the filter examples have been qualified for both airborne and space applications. Multi-Mix® is being used in many applications to integrate filters along with various functional components within multilayer microwave assemblies. Multi-Mix® technology has proven attractive because of size and weight reduction, power handling and superior electrical and reliability performance and devices using this technology have been space-qualified on multiple programs. The examples in this article demonstrate the ability of Multi-Mix® technology to be used as a differentiator allowing designers greater flexibility for integration.
