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Surface micromachining is a process used to produce micromachinery or MEMS.
Unlike Bulk micromachining, where a silicon substrate (wafer) is selectively etched to produces structures, surface micromachining is based on the deposition and etching of different structural layers.
As the structures are built on top of the substrate and not inside it, the substrate's properties are not as important as in bulk micromachining, and the expensive silicon wafers can be replaced by cheaper substrates, such as glass or plastic. The size of the substrates can also be much larger than a silicon wafer, and surface micromachining is used to produce TFTs on large area glass substrates for flat pannel displays. This technology can also be used for the manufacture of thin film solar cells, which can be deposited on glass, but also on PET substrates or other non-rigid materials.
Fabrication Process
Surface micromachining starts with a silicon wafer or other substrate and grows layers on top. These layers are selectively etched by photolithography and either a wet etch involving an acid or a dry etch involving a ionized gas, or plasma. Dry etching can combine chemical etching with physical etching, or ion bombardment of the material. Surface micromachining can involve as many layers as is needed with a different mask (producing a different pattern) on each layer. Modern integrated circuit fabrication uses this technique and can use dozens of layers, approaching 100. Micromachining is a younger technology and usually uses no more than 5 or 6 layers. Surface micromachining uses developed technology (although sometimes not enough for demanding applications)which is very repeatable for volume production.
Sacrificial Layers
Complexe structures, such as movable parts, are built using a sacrificial layer. For example, a suspended cantilever can be built by depositing and structuring a sacrificial layer, such as a polymer. During the structuration of this layer, the polymer is selectively removed at the locations where the future beams must be attached to the substrate. The structural layer (Aluminium for example) is then deposited on top of the polymer and structured to define the beams. Finally, the sacrificial layer is removed (In an oxygen plasma for the polymer)to release the beams.
Aluminium and a polymer is one of many possible combinations of structural/sacrificial layer. The combination chosen depends of the process. For exemple it is important for the structural layer not to be damaged by the process used to remove the sacrificial layer.
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