Unlike Bulk micromachining, where a silicon substrate (wafer) is selectively etched to produce structures, surface micromachining builds microstructures by deposition and etching of different structural layers on top of the substrate. Generally polysilicon is commonly used as one of the layers and silicon dioxide is used as a sacrificial layer which is removed or etched out to create the necessary void in the thickness direction. Added layers are generally very thin with their size varying from 2-5 Micro metres. The main advantage of this machining process is the possibility of realizing monolithic microsystems in which the electronic and the mechanical components(functions) are built in on the same substrate. The surface micromachined components are smaller compared to their counterparts, the bulk micromachined ones.
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 panel 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.
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 an 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.
Complicated components, 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, which is then selectively removed at the locations where the future beams must be attached to the substrate (i.e. the anchor points). The structural layer is then deposited on top of the polymer and structured to define the beams. Finally, the sacrificial layer is removed to release the beams, using a selective etch process that will not damage the structural layer.
There are many possible combinations of structural/sacrificial layer. The combination chosen depends on the process. For example it is important for the structural layer not to be damaged by the process used to remove the sacrificial layer.
Surface Micromachining can be seen in action in the following MEMS products:
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- Boser, B.E.; R.T. Howe (March 1996). "Surface Micromachined Accelerometers". IEEE Journal of Solid State Circuits 31 (3): 366–375. doi:10.1109/4.494198.
- Takeuchi, Shoji; Takafumi Suzuki, Kunihiko Mabuchi and Hiroyuki Fujita (October 2003). "3D Flexible Multichannel Neural Probe Array". Journal of Micromachines and Microengineering.