MEMS
The manufacture of Micro-Electro-Mechanical Systems or MEMS uses many of the same techniques developed originally for silicon semiconductors. This is due to the requirement of MEMS to have extremely small features. MEMS devices can have numerous moving parts, can include multiple activities on the same chip and can have extremely high aspect ratios. Often in MEMS silicon is used both a a mechanical substrate, that is adapted to achieve fluid flow dynamics, as well as a semiconductor, acquiring electrical signals and passing them on to more standard data processors. The nature of MEMS makes it difficult to standardize production.
Aspect ratios for some MEMS devices can reach 50:1. This is unsuitable for CMP, or even standard reflow techniques. Sheer forces can destroy these types of features. Also, MEMS often require unusual (for semiconductors) materials that have more in common with BCB than with SiO2. Planarization of these structures and materials is perhaps more difficult than standard semiconductor structures and materials.