Technology Used For MEMS

Sculpturing using processes originating from the semiconductor industry is a new technology that enables fabrication of very small devices. Micromachining is today sufficiently mature to turn industry driven, except for some more advanced process steps such as integrating electronics on the sensor chip.

Surface micromachining

Micromachining has, since its introduction in the 1960s, developed its own arsenal of fabrication tools based on processes similar to those developed for the semiconductor industry. Its toolbox now contains many powerful tools based on etching, deposition and bonding. Some characteristics are:

The size of the fabricated structures range from submicrometer for the smallest details up to several millimeters for complex devices and systems.
The first production steps are performed on a wafer level. This enables batch production in which a large number of identical devices are manufactured simultaneously.
Joining of pieces (bonding) is often done without conventional adhesives. Anodic bonding uses a very thin intermediate layer (e.g. glass) that is melted at high temperature, and silicon-to-silicon fusion bonding does not rely on any intermediate material at all. Bonding is often done on a wafer level, and can be used to create cavities, to package the sensor chips, etc.
Wet and dry etching is often used to remove material and to sculpture detailed features. Crystalline materials are anisotropic which result in a complex dependence of the etch speed on etch direction, etchant and temperature.
Thin films of various materials, characteristics and thickness can be depositing on the surface. Free-floating structures can be created by etching away thin embedded ‘sacrificial’ layers (see figure).
Patterning is done with various masking techniques (photolithography) that often involve covering selected areas with materials, such as photoresist and oxide, that are resistant to the liquid etchant or to the particle bombardment encountered in dry etching.
Combining etching, deposition and bonding makes it possible to form more complex three-dimensional structures and complete sensor systems that can include several MEMS materials.
For some micromachining materials, such as silicon, it is possible to integrate electronics onto the same chip as the sensor element.

Micromachining comes in two varieties, bulk and surface micromachining (BM and SM). Both date back to the 1960’s, and were developed in parallel. Although some process steps are common for both, there are notable differences. Combinations of the two techniques exist. Both BM and SM are commonly used for silicon while BM is the predominant technique used for quartz.

BM tailors the substrate wafer itself into the desired geometric shape, while SM is based on deposition and etching of thin layers on one side of the wafer. With SM it is difficult to create thicker structures, while both thick and thin structures can be formed with BM. Due to thermal mismatch and to the films being deposited at elevated temperatures, SM often generates layers and structures that bend due to the resulting internal mechanical stress.

The material selectivity during etching is essential for SM in the same way as BM relies on the anisotropy of the etching. Both BM and SM use ordinary photolithographic methods for patterning the shapes. The toolbox for SM has been more complicated to develop and has taken longer to mature into commercial usage. In general, SM is more compatible with electronics and enables freely movable structures, but it involves more complex and expensive process steps.

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