There are several different broad categories of MEMS
technologies, outlined below:
Bulk micromachining is a fabrication technique which builds
mechanical elements by starting with a silicon wafer, and then etching away
unwanted parts, and being left with useful mechanical devices. Typically,
the wafer is photo patterned, leaving a protective layer on the parts of the
wafer that you want to keep. The wafer is then submersed into a liquid etchant,
like potassium hydroxide, which eats away any exposed silicon. This is a
relatively simple and inexpensive fabrication technology, and is well suited for
applications which do not require much complexity, and which are price
Today, almost all pressure sensors are built with Bulk
Micromachining. Bulk Micromachined pressure sensors offer several
advantages over traditional pressure sensors. They cost less, are highly
reliable, manufacturable, and there is very good repeatability between devices.
All new cars on the market today have several micromachined
pressure sensors, typically used to measure manifold pressure in the engine.
The small size and high reliability of micromachined pressure
sensors make them ideal for a variety of medical applications as well.
While Bulk micromachining creates devices by etching into a
wafer, Surface Micromachining builds devices up from the wafer layer-by-layer.
A typical Surface Micromachining process is a repetitive
sequence of depositing thin films on a wafer, photopatterning the films, and
then etching the patterns into the films. In order to create moving, functioning
machines, these layers are alternating thin films of a structural material
(typically silicon) and a sacrificial material (typically silicon dioxide).
The structural material will form the mechanical elements, and the sacrificial
material creates the gaps and spaces between the mechanical elements. At the end
of the process, the sacrificial material is removed, and the structural
elements are left free to move and function.
For the case of the structural level being silicon, and the
sacrificial material being silicon dioxide, the final "release" process is
performed by placing the wafer in Hydrofluoric Acid. The Hydrofluoric Acid
quickly etches away the silicon dioxide, while leaving the silicon undisturbed.
The wafers are typically then sawn into individual chips, and
the chips packaged in an appropriate manner for the given application.
Surface Micromachining requires more fabrication steps than
Bulk Micromachining, and hence is more expensive. It is able to create
much more complicated devices, capable of sophisticated functionality. Surface
Micromachining is suitable for applications requiring more sophisticated
LIGA is a technology which creates small, but relatively high
aspect ratio devices using x-ray lithography. The process typically starts with
a sheet of PMMA. The PMMA is covered with a photomask, and then exposed to high
energy x-rays. The mask allows parts of the PMMA to be exposed to the
x-rays, while protecting other parts. The PMMA is then placed in a
suitable etchant to remove the exposed areas, resulting in extremely precise,
microscopic mechanical elements.
LIGA is a relatively inexpensive fabrication technology, and
suitable for applications requiring higher aspect ratio devices than what is
achievable in Surface Micromachining.
Deep Reactive Ion Etching:
Deep reactive ion etching is a type of Bulk Micromachining
which etches mechanical elements into a silicon wafer. Unlike traditional Bulk
Micromachining, which uses a wet chemical etch, Deep Reactive Ion Etching
micromachining uses a plasma etch to create features. This allows
greater flexibility in the etch profiles, enabling a wider array of mechanical
elements. The fabrication tools needed to perform Deep Reactive Ion etching are
somewhat expensive, to this technology is typically more expensive than
traditional Bulk Micromachining based on wet etching.
Integrated MEMS Technologies
Since MEMS devices are created with the same tools used to
create integrated circuits, in some cases it is actually possible to fabricate
Micromachines and Microelectronics on the same piece of silicon.
Fabricating machines and transistors side by side enables machines that can have
intelligence. A number of exciting products are already taking advantage
of this capability.