# LDS Realization of High-Q SIW Millimeter Wave Filters with Cyclo-Olefin Polymers
## 4 Simulations and Results  
The prototypes of the two filters were built by the French technical center Innovation Plasturgie Composites (15), which has all the necessary facilities to perform molding, machining and laser direct structuring for metallization. The two prototype filters are presented in Figures 9 and 10. One important advantage of the LDS technique is that it makes it possible to obtain thick and robust metallization by chemical electroless deposition. The downside is the high degree of surface roughness, due to the action of the laser beam during activation [16; 17]. Metal was deposited using commercial electroless solutions of copper, nickel, then gold. Deposition was examined with a stylus profilometer, showing surface roughness (Ra) equal to 5 \(\upmu\)m and root mean square roughness (Rq) of 6.2 \(\upmu\)m. To illustrate this roughness, Figure 11 presents the profile of one of the microstrip lines of a filter under an optical microscope. Measurements were made on the filters using a VNA E8364a (0-50 GHz) from Agilent(r) and a universal test fixture 3680 V from Arnitsu(r). The electrical responses of the two filters are shown in Figures 9 and 10, where the experimental data are compared with simulations. Despite differences observed between simulated and measured responses, the results validate the approach taken, as in each case an electrical response was obtained with all of the six poles in the bandwidth. Several differences can be observed in the experimental responses. First, there is a slight shift of the central frequency by about 1% that can be attributed to technological dispersions on the dimensions of the plastic piece, to the mechanical drillings, and at the first order to the uncertainty of the relative dielectric permittivity value of the thermoplastic. Second, the transmission levels in the attenuated bands are higher, due to the fact that parasitic couplings between the access microstrip lines of the filter are accentuated by the test fixture. It should be remembered that the filters are very short in length, less than 5 cm, thus promoting such couplings. Third, the matching levels are a few decibels higher than expected, hence impacting the level of insertion losses. This can be explained by the lower electrical conductivity of the LDS metallization, composed of copper but also of a thin layer of nickel/phosphorus, in comparison with a typical PCB copper layer. It also can be explained by the non-uniform roughness and the presence of some discrepancies in the dimensions: thickness of the microstrip access part, width of the microstrip lines, and positioning of the inductive posts. In our measurement conditions, the insertion losses are higher for the 2.5% and 4.5% relative bandwidth filters by about 1 and 0.6 dB, respectively. However, we are able to extract, following [14], a correct estimation of the quality factor of the fabricated filters. By considering 0.35 dB losses for each transition, we obtain a value of around 300 for both filters. These results can be compared with the results of other solutions, extracted from the literature. Table 4 summarizes the features of two relevant solutions realized at 35 and 83 GHz, based on the well-known RO5880, whose electrical characteristics are very close to the ones of the cyclo-olefin polymer. As shown in Table 4, we obtain similar or better electrical performances with dimensions quite comparable. However, the access lines are very different. The RO5880 working at 35 GHz has a thick coplanar waveguide (CPW) access line with a strip width of around 1.6 mm. For the RO5880 working at 83 GHz, a coupling aperture interface structure is used as WR10-SIW transition. In our case, there is a thin microstrip access line with a strip width of around 900 \(\upmu\)m. This configuration minimizes the losses and the parasitic effects when connecting the filter with bonding wires, making the integration of the COP filter easier in a global system.