Modular integration of CMOS and SOI-MEMS using "plug-up" concept

This paper reports a novel method for monolithic integration of electronics with micromechanical devices. The fabrication sequence differs from those previously described. Even though some of the elements of CMOS and MEMS fabrication are intermixed and make use of each other, they still are modular packages, which can be sequentially fabricated in separate specialized factories if both of the technologies are not available at one site. Standard or tailored SOI wafers can be used for the substrate material. Vacuum cavities are then made at predetermined regions of the buried oxide using a special technique, "Plug-up". We have demonstrated that cavity wafers of this kind can progress through the entire bipolar enhanced CMOS cycle without complications. In the end, MOS and bipolar transistors operate almost identically to reference devices on bulk silicon wafers and MEMS membranes resonate mechanically as anticipated. The proposed modular process sequence is tabulated in Fig. 1.