RF MEMS

The RF MEMS acronym stands for radio frequency microelectromechanical system, and refers to components of which moving sub-millimeter-sized parts provide RF functionality. RF functionality can be implemented using a variety of RF technologies. Besides RF MEMS technology, ferrite, ferroelectric, GaAs, GaN, InP, RF CMOS, SiC, and SiGe technology are available to the RF designer. Each of the RF technologies offers a distinct trade-off between cost, frequency, gain, large scale integration, lifetime, linearity, noise figure, packaging, power consumption, power handling, reliability, repeatability, ruggedness, size, supply voltage, switching time and weight.

Microfabrication

An RF MEMS fabrication process allows for integration of SiCr or TaN thin film resistors (TFR), metal-air-metal (MAM) capacitors, metal-insulator-metal (MIM) capacitors, and RF MEMS components. An RF MEMS fabrication process can be realized on a variety of wafers: fused silica (quartz), borosilicate glass, LCP, sapphire, and passivated silicon and III-V compound semiconducting wafers. As shown in Fig. 1, RF MEMS components can be fabricated in class 100 clean rooms using 6 to 8 optical lithography steps with a 5 μm contact alignment error, whereas state-of-the-art monolithic microwave integrated circuit (MMIC) and radio frequency integrated circuit (RFIC) fabrication processes require 13 to 25 lithography steps. The essential microfabrication steps are:

Deposition of the bias lines (Fig. 1, step 3)
Deposition of the electrode layer (Fig. 1, step 4)
Deposition of the dielectric layer (Fig.1, step 5)
Deposition of the sacrificial spacer (Fig. 1, step 6)
Deposition of seed layer and subsequent electroplating (Fig. 1, step 7)
Beam definition, release and critical point drying (Fig. 1, step 8)

RF MEMS fabrication processes, unlike barium strontium titanate (BST) or lead zirconatetitanate (PZT) ferroelectric and MMIC fabrication processes, do not require electron beam lithography, molecular beam epitaxy (MBE), or metal organic chemical vapor deposition (MOCVD). With the exception of the removal of the sacrificial spacer, the fabrication steps are compatible with a CMOS fabrication process.

Fig. 1: RF MEMS fabrication process

Nanyoly Mendez

CAF