Microfabrication
An RF MEMS fabrication process is based on surface micromachining techniques, and 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: III-V compound semi-insulating, borosilicate glass, fused silica (quartz), LCP, sapphire, and passivated silicon wafers. As shown in Fig. 4, 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 MMIC and RFIC fabrication processes require 13 to 25 lithography steps.
As outlined in Fig. 4, the essential microfabrication steps are:
- Deposition of the bias lines (Fig. 4, step 1)
- Deposition of the electrode layer (Fig. 4, step 2)
- Deposition of the dielectric layer (Fig. 4, step 3)
- Deposition of the sacrificial spacer (Fig. 4, step 4)
- Deposition of seed layer and subsequent electroplating (Fig. 4, step 5)
- Beam patterning, release and critical point drying (Fig. 4, step 6)
With the exception of the removal of the sacrificial spacer, which requires critical point drying, the fabrication steps are similar to CMOS fabrication process steps. RF MEMS fabrication processes, unlike BST or PZT ferroelectric and MMIC fabrication processes, do not require electron beam lithography, MBE, or MOCVD.
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