a metalworking process for producing very fine surface finishes
Various micro electro mechanical systems
High-aspect-ratio microstructure technologies
Bulk micromachining is a process
used to produce micro machinery or micro electro mechanical systems (MEMS).
Unlike surface micromachining,
which uses a succession of thin film deposition and selective etching, bulk
micromachining defines structures by selectively etching inside a substrate.
Whereas surface micromachining creates structures on top of a substrate,
bulk micromachining produces structures inside a substrate.
Usually, silicon wafers are used
as substrates for bulk
micromachining, as they can be anisotropically wet etched,
forming highly regular structures.
Wet etching typically uses
alkaline liquid solvents, such as potassium hydroxide (KOH) or
tetramethylammonium hydroxide (TMAH) to dissolve silicon which has been left
exposed by the photolithography masking step. These alkali solvents dissolve
the silicon in a highly anisotropic way, with some crystallographic
orientations dissolving up to 1000 times faster than others. Such an approach
is often used with very specific crystallographic orientations in the raw
silicon to produce V-shaped grooves. The surface of these grooves can be
atomically smooth if the etch is carried out correctly, and the dimensions and
angles can be precisely defined.
Bulk micromachining starts with a
silicon wafer or other substrates which is selectively etched, using
photolithography to transfer a pattern from a mask to the surface. Like surface
micromachining, bulk micromachining can be performed with wet or dry etches,
although the most common etch in silicon is the anisotropic wet etch. This etch
takes advantage of the fact that silicon has a crystal structure, which means
its atoms are all arranged periodically in lines and planes. Certain planes
have weaker bonds and are more susceptible to etching. The etch results in pits
that have angled walls, with the angle being a function of the crystal
orientation of the substrate. This type of etching is inexpensive and is
generally used in early, low-budget research.
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.
HARMST is an acronym for High
Aspect Ratio Microstructure Technology that
describes fabrication technologies, used to create high-aspect-ratio
microstructures with heights between tens of micrometers up to a centimeter and
aspect ratios greater than 10:1. Examples include the LIGA fabrication process,
advanced silicon etch, and deep reactive ion etching.