A Micromachined Reconfigurable Attenuator

As the essential power control and adjustment component, microwave power attenuator has been widely applied in spectrum analyzer, network analyzer, receivers, and other microwave instrument systems. There has been significant interest in developing miniaturized microwave devices with small geometric dimension for commercial microwave test systems. Traditional attenuator with large size is difficult to integrate with IC due to its cumbersome mechanical relay or switches. Radio frequency microelectromechanical system (RF MEMS) switches present many advantages such as less insertion loss, higher isolation, better linearity, and lower power consumption and more importantly, its small size. It has absorbed great attention for the purpose of miniaturization of attenuator. Recent research shows step attenuator based on RF MEMS switches meet well the state-of-the-art requirements of miniaturized reconfigurable attenuation devices with high precision and broadband performance. In this chapter, we introduce a compact 3-bit step attenuator based on RF MEMS switches with 0 ~ 70 dB attenuation at 10 dB intervals up to 20GHz. The on-chip attenuator consists of 12 ohmic MEMS switches, 3 π-type resistive attenuation networks, and microwave compensate structures. To optimize the attenuation characteristics within the broadband, theoretical analysis and 3D modelling were performed. The device was obtained using MEMS process combined with polysilicon integrated circuit (IC) process.

In section “Structure Design,” the structure of the step attenuator was proposed and three modules were designed independently including: (1) Resistive attenuation network. Two approaches are presented and discussed. (a) Polysilicon thin film resistor with symmetric topology is applied to realize the high-precision π-type resistive attenuation modules (10, 20, 40 dB). (b) Distributed TaN single thin film resistor (STFR) equivalent to π-type resistive network was designed with a more compact structure (2/3 compared to traditional network). It can realize 5 ~ 30 dB attenuation with less parasite effects. (2) RF MEMS switches. Toggling part is the determining factor to reduce the footprint of the entire attenuator. SP2T switch including 2 ohmic Au contact single-cantilever MEMS switches was customized for the attenuator. (3) CPW and microwave compensate structures. Precise design of the transmission path connecting each module is significant to suppress the inherent insertion loss and improve the matching performance. Right-angle bends and T-junctions were analyzed and designed. Based on the results of these three blocks, the overall attenuator was assembled and optimized with consideration of matching performance.

In section “Fabrication,” we describe the fabrication method of RF MEMS attenuators. The 3-bit attenuator was manufactured with surface micromachining process and polysilicon IC techniques on 4 inch wafers and features 11 lithography steps. Key processes are emphasized including high-precision resistance and excellent ohmic contact between polysilicon and Au CPW was realized with experimental research. The photosensitive PI was used as sacrificial layer and the patterning process was simplified. The CPW and upper electrodes of the switch which were manufactured by low stress Au electroplating process have small roughness and without evident buckling after the sacrificial layer released. Based on these process studies, the 3-bit attenuator is successfully fabricated.

In section “Characterization,” characterization of the obtained RF MEMS attenuator is presented. The measurement results show that the driven voltage of RF MEMS switch is 32 ~ 42 V and it can operate more than 3 × 10⁸ times (cold switched mode). When toggling between the different transmission paths using MEMS switches, the 3-bit attenuator can realize target attenuation in range of 0 ~ 70 dB at 10 dB intervals up to 20 GHz. The accuracy and error of all the attenuation states is better than ±1.88 dB and 2.11 dB. The return loss of the 3-bit switched attenuator is better than 11.95 dB. Micromachined reconfigurable attenuator based on RF MEMS switch is demonstrated and can be further applied in particular microwave systems.