# LDS Realization of High-Q SIW Millimeter Wave Filters with Cyclo-Olefin Polymers
## 2 Thermoplastics and SIW Filters  
Cyclo-olefin polymers or copolymers (COP/COC) have excellent optical and microwave characteristics and are widely used in a broad range of applications, including packaging of films, lenses, displays, and medical devices. They also offer other chemical and physical advantages, and are low in cost. Topas(r) (Frankfurt-Hochst, Germany), Mitsui Chemicals(r) (Tokyo, Japan), Arton(r) (Tokyo, Japan), and Zeon(r) (Tokyo, Japan) are the main manufacturers. Among these, Zeon(r) was the first to propose a COP, RS420-LDS, compatible with the LDS metallization technique under patent and commercialized by the company LPKF(r). To make it possible to exploit this property, a pure COP is loaded with organometallic particles, which implies some changes to the electrical and optical properties. Inevitably, the dielectric losses increase slightly, and the materials become opaque and fully black, hence changing the optical properties of the polymer. Table 1 compares the dielectric permittivity and loss tangent of the loaded COP RS420-LDS with the conventional COP 480R. In [12], we used a cavity-perturbation method and a Ka-band waveguide method to extract dielectric properties, at 7 GHz and in the 26-40 GHz frequency band respectively. Figure 1: Exploded view of the Substrate Integrated Filter (SIW) filter. The accuracy of waveguide method is known to be too low to extract loss tangent values lower than \(10^{-3}\). However, both methods give similar values of permittivity from 7 to 40 GHz. To illustrate the difference in appearance, Figure 2 shows pictures of the two filters after molding and before drilling. The black and the transparent filters are the loaded and the conventional, and are respectively presented in Figure 2a,b. Each picture is a side view of the filter, showing the vertical injection sprue which must be suppressed. As the electromagnetic fields are entirely confined in the dielectric, the loss tangent is a significant parameter for the development of SIW filters. Figure 3 shows (in blue) the evolution at 38 GHz of the unloaded quality factor of a parallelepiped cavity for a TE101 resonant mode in COP RS420-LDS substrate. The Q factor increases significantly with the height of the cavity. We also plotted the same curve for COP RS420-LDS (in red), in order to show the impact of a better loss tangent. Indeed, the curves are limited by the value of the dielectric quality factor (Qd), equal to \(1/\tan\delta\). For COP RS420-LDS and COP 480R, the values of Qd are around 1300 and 3000, respectively. Therefore, this encourages developing filters on low loss material with high thickness substrates. The limit is to avoid a resonant mode other than the desired one appearing in the cavity. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \(\varepsilon_{\mathbf{r}}\) & **tan \(\delta\)** \\ \hline
**COP 480 R** & 2.32 & \(3.2\times 10^{-4}\) \\ \hline
**COP RS420-LDS** & 2.48 & \(7.6\times 10^{-4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrical characteristics of two COP at 7 GHz. Figure 3: Influence of cavity height on the unloaded quality factors (Q) for COP RS420-LDS and COP 480R. Figure 2: Picture of the filters after molding (**a**) COP RS420-LDS and (**b**) COP 480R. Figure 4 shows the width of microstrip line against characteristic impedance, calculated for a permittivity of 2.3, for the COP 480R. As the dielectric constants of the two materials are not very different, 2.3 against 2.48, very similar values and conclusions would be obtained and drawn for the COP RS420-LDS. Depending on the substrate characteristics (height, permittivity, metallization limitations), some line impedances can be correctly realized and others not. For our 38 GHz filters, the width of the microstrip-line must be between 100 \(\upmu\)m and 1600 \(\upmu\)m. The lower bound, equal to 100 \(\upmu\)m, was chosen to limit the sensitivity to technological dispersions when using the LDS technique for metallization.