Fabrication of an RF-MEMS-Switch on a hybrid Si-Ceramic substrate

The integration of MEMS sensors, microelectronics and RF circuits including RF-MEMS is still a challenging task but becomes crucial for the Internet of Things. A wafer-level silicon-ceramic composite substrate (called SiCer, Silicon-on-Ceramics) allows new options in smart system integration. SiCer substrates combine the benefits of two different worlds of materials. The silicon substrate is a suitable material to build active MEMS devices such as switches and resonators. The ceramic substrate, a Low Temperature Cofired Ceramic (LTCC), is well-known for RF circuit integration including resistors, capacitors and coils. Both materials are co-sintered into a monolithically composite substrate. Chemical and physical modification of the silicon interface allows a low-pressure sintering and therefore new techniques for generating buried cavities at the bond interface. A carbon paste is applied on the LTCC via screen printing. After sintering, this results in a defined cavity. To demonstrate the advantages of the buried cavities within SiCer substrates the fabrication process of a RF-MEMS switch is shown. The switch is intended for a switching matrix to select frequency bands in a mobile LTE receiver. A parallel-plate electrostatic actuation with in-plane movement has been selected. This type of switch allows a large displacement range and a low actuation voltage can be achieved.