## 2 Fabrication Process and Filter Design
### Au-Au Thermocompression Bonding  
The implementation of our strip-structured RF MEMS filters, shown in Figure 3, required the development of a sophisticated Au-Au thermocompression bonding process to create a robust mechanical and electrical interface between the cap and bottom wafers. This bonding approach was selected after careful evaluation of multiple potential technologies, with gold thermocompression bonding offering distinct advantages for our specific application requirements.  
The selection of gold as the bonding medium was based on its superior characteristics relative to alternative metals commonly used in thermocompression processes, such as copper and aluminum. Gold provides the significant advantage of a lower bonding temperature requirement, reducing thermal stress on the fragile MEMS structures during the bonding process. Furthermore, unlike copper and aluminum, which readily form surface oxides that interfere with bonding quality, gold maintains an oxide-free surface during atmospheric exposure and processing [19]. This inherent resistance to oxidation typically eliminates the need for aggressive surface cleaning procedures immediately preceding the bonding operation, simplifying the process flow and reducing potential contaminant introduction. Additionally, the gold-plated surfaces enabled subsequent bare die gold chip attachment and wire bonding processes, facilitating integration into higher-level assemblies.  
Our bonding process utilized an SB8 Gen2 system with a carefully optimized two-step bonding protocol developed through extensive experimental characterization. The first  
Figure 2: Overall fabrication process flow: (**A**) CMP, (**B**) photoresist coating (PR), (**C**) Deep Reactive Ion Etching (DRIE), (**D**) PR removal and seed layer deposition (Ti: 100 nm, Cu: 1 \(\upmu\)m), (**E**) application of dry film resist (DFR), (**F**) Cu electroplating (7–8 \(\upmu\)m), (**G**) Au electroplating (1–2 \(\upmu\)m), (**H**) DFR removal, (**I**) seed layer removal, and (**J**) wafer-to-wafer bonding.  
bonding step operated at 340 \({}^{\circ}\)C under 6000 mbar of applied pressure for 60 min, initiating the atomic diffusion and bond formation processes while allowing for stress relaxation in the metal layers. This initial bonding step was immediately followed by a second bonding step at a slightly elevated temperature of 350 \({}^{\circ}\)C, still maintaining 6000 mbar pressure for an additional 60 min. This two-step approach was developed to enhance bond uniformity by allowing atomic diffusion to progress further after initial contact establishment, significantly increasing the bond interface quality and mechanical strength.  
It is important to note that while the design target was an ideal stripline, the fabricated L3 device is more accurately described as a suspended stripline structure, where the top and bottom conductors are separated by the silicon substrate and a thin air gap resulting from the bonding process. The L1 configuration, lacking a top conductor, functions as a microstrip-like structure. These distinctions are critical for understanding the RF performance.