# Design and Fabrication of a MEMS Bandpass Filter with Different Center Frequency of 8.5-12 GHz
## 3 Fabrication and Measurement of the Filter  
The fabrication of the filter process involved using a five-layer mask, and was completed by five executions of photolithography. The device was fabricated on a high-resistance silicon substrate, and the complete filter processing flow is described below in eight steps. The image of the sample is shown in Figure 8.  
(1) Preparation, cleaning, and quartz glass substrate cleaning: the glass was placed in a NH\({}_{3}\cdot\)H\({}_{2}\)O (ammonia) + H\({}_{2}\)O\({}_{2}\) mixture (according to a volume ratio of 30%NH\({}_{3}\cdot\)H\({}_{2}\)O: 30%H\({}_{2}\)O\({}_{2}\): H\({}_{2}\)O = 1:1:5) and boiled for 5 min, then rinsed and dried repeatedly with hot and cold deionized water. (2) The fabrication of CPW: A 2 \(\upmu\)m Al film was sputtered as a CPW. The photoresist AZ5214 was spin-coated on the Al film at a speed of 4000 rad/min. The photolithography plate was used as a mask for exposure and development. After that, O\({}_{2}\) plasma was used to apply the base film for 5 min. A 30% phosphoric acid water bath was used to heat it to 50 \({}^{\circ}\)C and corrote it for 12 to 15 min. Then, the water was flushed with acetone to remove the photoresist in it. (3) The fabrication of the dielectric layer: 400 nm of plasma enhanced chemical vapor deposition (PECVD) Si\({}_{3}\)N\({}_{4}\) was patterned and etched by RIE (Reactive Ion Etching) as the dielectric layer. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Center** & **Width of Top** & **Insertion** & **Return Loss** & **Stopband** \\  & **Frequency** & **Electrode** & **Loss (dB)** & **(dB) (S11)** & **Rejection** \\  & **(GHz)** & **(\(\upmu\)m)** & **(S21)** & **(dB) (S21)** \\ \hline
1 & 8.5 & 260 & \(-1.27\) & \(-17.59\) & \(-25.83\) \\
2 & 9 & 240 & \(-1.12\) & \(-17.23\) & \(-25.42\) \\
3 & 9.5 & 220 & \(-1.17\) & \(-21.21\) & \(-35.11\) \\
4 & 10 & 200 & \(-1.14\) & \(-15.68\) & \(-38.57\) \\
5 & 10.5 & 190 & \(-1.13\) & \(-21.28\) & \(-25.03\) \\
6 & 11 & 180 & \(-1.03\) & \(-16.54\) & \(-25.55\) \\
7 & 11.5 & 170 & \(-1.24\) & \(-15.41\) & \(-29.12\) \\
8 & 12 & 140 & \(-1.10\) & \(-15.69\) & \(-26.76\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The list of the S parameters of MEMS filter. Figure 8: The fabrication processes of the MEMS switch: (**a**) preparation, cleaning, and quartz glass substrate cleaning; (**b**) the fabrication of CPW; (**c**) the fabrication of dielectric layer; (**d**) the fabrication of resonant ring; (**e**) sacrificial layer preparation; (**f**) making anchor through holes; (**g**) the fabrication of top electrode; (**h**) sacrifice layer release. (4) The fabrication of resonant ring: 3 \(\upmu\)m Al film was sputtered as a resonant ring. The photoresist AZ4620 was spin-coated on the Al film at a speed of 4000 rad/min. The photolithography plate was used as a mask for exposure and development. After that, O\({}_{2}\) plasma was applied to the base film for 5 min. Then, a 30% phosphoric acid water bath was used to heat it to 50 \({}^{\circ}\)C and corrode it for 18 to 22 min. Finally, the water was flushed with acetone to remove the photoresist on the wafer. (5) Sacrificial layer preparation: Here, 2.9 \(\upmu\)m thick polyimide (PI) was spun-cast, solidified, and patterned to define a sacrificial layer. (6) Making anchor through holes: AZ4620 photoresist was spin-coated on the precured PI film. The speed of the glue was 4300 rpm, the glue time was 30 s, and the thickness of the photoresist was 7-8 \(\upmu\)m. A pre-baked AZ4620 was set to a temperature of 85 \({}^{\circ}\)C for 45 min at an exposure time of about 2 min. After the exposed photoresist was removed by using the photoresist developing solution, the PI was further etched, and the anchor through the hole was etched for 3-4 min. PI is insoluble in acetone, so the AZ4620 photoresist was removed with acetone to obtain a patterned PI film. The residual AZ4620 on the PI surface and the residual PI at the bottom of the pit were removed by an oxygen plasma stripper for 5 min.