# LDS Realization of High-Q SIW Millimeter Wave Filters with Cyclo-Olefin Polymers
## 1 Introduction  
Filters occupy a key place in microwave communication systems. Their roles include suppression of unwanted signals or isolation of certain parts of a system. There is a demand to develop compact, efficient and low cost filters, for which substrate integrated waveguide (SIW) technology [1] offers a good compromise between high performance but bulky waveguides and compact planar lines such as microstrip. SIW technology was first implemented on classic Printed Circuit Board (PCB) [2], then on ceramic substrates, for which laser cutting or drilling is necessary [3; 4]. In this paper, we present SIW filters based on cyclo-olefin polymers (COP). The goal was to combine high and low thicknesses to associate high quality factor cavities with microstrip accesses compatible with monolithic microwave integrated circuits (MMIC). The objective is to minimize the length of the bonding wires between the components, allowing an optimal integration of the filter in a global system. Cyclo-olefin polymers offer many interesting properties for the design of microwave passive devices in the millimeter wave range. First, their permittivity is low and very stable over a large frequency range and their dielectric losses are very low, even at high frequencies. They have been used for antenna applications [5] and even for terahertz applications [6]. When designing conventional SIW passive devices, one of the main limitations is the substrate height, which conditions their electrical performance and compatibility with the other components of the system. This limitation leads to the need to find a compromise. To overcome this constraint, we propose the innovative solution of using a material compatible with a standard molding process. Molding the device makes it possible to vary its thickness between different zones, therefore achievinga higher quality factor in the thicker part and 50 \(\Omega\) microstrip lines in the thinner access parts. Once the dielectric structure is molded, the laser direct structuring (LDS) technique is used to selectively activate the piece before using an autocatalysis bath to metalize the LDS zones. Just before the metallization steps, we use mechanical drillings to make a set of via holes, defining the expected electrical response. The aim of this paper is to show that it is possible to build high performance millimeter wave filters with COP by combining molding, mechanical drilling and 3D laser direct structuring metallization. The paper is organized as follows: In the first part, we present the main particularities of the chosen COP and the benefits of using them to design millimeter resonant cavities. Then, we present the design steps taken to realize two bandpass filters. Figure 1 presents the exploded view of the proposed topology divided in two different parts: The SIW inductive post filter core, and two transitions with planar microstrip access. As each part was designed independently, we present the ideal filter synthesis, the manufacturing specifications to be taken into account, and the design of the transition in turn. The microstrip-to-SIW transition is designed to work over a wide bandwidth. We conceived two narrow-band filters (w \(<\) 10%), to obtain a good dynamic for the evaluation of the insertion losses (IL), and thus of the quality factors of the cavities. Finally, we present the experimental data and compare them with simulations. In choosing a central frequency of around 38 GHz for the filters, the goal was to demonstrate the efficiency up to millimeter frequency range of the selected thermoplastics coupled with industrial technologies. For the design of millimeter SIW filters, it must be mentioned that other materials can be used. To our knowledge, the solutions realized with RO5880 are state-of-the-art [7]. At the end of the paper, our results are compared with such solutions [7,8]. Due to their very good electrical properties at very high frequency, benzocyclobutene (BCB) and liquid crystal polymer (LCP) are also considered by some authors.