# LDS Realization of High-Q SIW Millimeter Wave Filters with Cyclo-Olefin Polymers
## 3 Filter Design
### Manufacturing Specifications  
A thermoplastic of this type cannot withstand high temperatures. For example, the glass transition temperature of COP RS420-LDS is 136 \({}^{\circ}\)C. For this reason, low temperature selective metallization techniques were needed. The one used here was laser direct structuring (LDS), under patent and commercialized by the company LPKF.  
The principle is to activate the substrate locally, using a laser beam. The COP RS420-LDS used here, loaded with additional organometallic particles, permits the activation, and subsequently the electroless metal deposition, of the activated surfaces. The addition of the particles to the polymer  
Figure 4: Impedance and width of microstrip line at 38 GHz for \(\epsilon_{\tau}\) = 2.3 at different thicknesses of substrate.  
modifies some of the characteristics of the material. It becomes fully black. In addition, the relative dielectric permittivity and the dielectric losses increase. One constraint of this technique is that it does not allow vertical walls to be activated. We thus had to rework the shape of walls and vias. We substituted cylindrical vias with hourglass shaped vias, which had an alpha angle of 70\({}^{\circ}\).  
The molding process also has its own specifications. One of these is the capability of the injection press to correctly fill the entire mold, especially in low thickness areas. In fact, the structure is easier to mold with low relative differences of thickness.  
To better fit with standard MMIC dimensions, with which the filter could be interfaced in a system, we chose an access thickness of 300 \(\upmu\)m. Moreover, the injection mold is in two parts, creating a surface joint, which has to be kept at the same height as the access line. To illustrate the overall changes, Figure 5 compares the modified filter with inclined walls (in grey) with the traditional rectangular SIW (in dotted red lines). Moreover, all the shape modifications were made, taking care to keep the same resonant modes and, obviously, the same electrical response. The diameters of the inductive posts depend on the electrical requirements and also the characteristics of the waveguide. One critical point here was the diameters of posts 1 and 7, at the extremities of the filter, which were too tiny to be drilled. To overcome this issue, one solution is to substitute each of these posts with a pair of posts, as shown in Figure 6.  
All these technological requirements inevitably modify the electrical response of the filter, and require the structure to be re-adjusted in order to retrieve the original electrical response given by an ideal rectangular SIW filter. The work of re-adjustment of the electrical response is necessarily made with the commercial electromagnetic software ANSYS HFSS. Without any experience, this can be time consuming and it is not easy to obtain exactly the same response. Some differences, such as modification of the bandwidth, can be difficult to eliminate. In this case, the problem needs to be foreseen when designing the prototype. This work was done for the modified SIW filters in our study (dimensions given in Table 2). The responses of the two filters with relative bandwidths of 2.5% and 4.5%, before and after modifications, are shown in Figure 7a,b, respectively. As mentioned above, we can obviously observe a slight increase of the bandwidth, but it is important to note that the insertion losses in both cases remain nearly unchanged, meaning that the quality factor was not significantly impacted by the technological modifications. The next step is to design a low loss microstrip-to-waveguide transition.  
Figure 5: Modifications of the shape of the guide and vias due to LDS process constraints.  
Figure 6: Layout of a modified 6 th-order inductive post SIW filter.