Micromachines are mechanical objects that are fabricated in the same general manner as integrated circuits. They are generally considered to be between 100 nanometres to 100 micrometres in size, though that is debatable. The applications of micromachines include accelerometers that detect when a car has hit an object and trigger an airbag. Complex systems of gears and levers are another application.
The fabrications of these devices is usually done by one or both of two techniques: surface micromachining and bulk micromachining.
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.
Bulk micromachining starts with a silicon wafer or other substrate and selectively etches into it, again 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 in lines and planes. Certain planes have weaker bonds and are more susceptible to etching. The etch results in pits that have angled walls instead of vertical walls. This type of etching is inexpensive and is generally used in early and low-budget research.
Most micromachines fall into one of two categories: sensors and actuators.
Sensors convert information from the environment into interpretable electrical signals. One example of a micromachine sensor is a resonant chemical sensor. A lightly damped mechanical object vibrates much more at one frequency than any other, and this frequency is called its resonant frequency. A chemical sensor is coated with a special polymer that attracts certain molecules, such as anthrax, and when those molecules attach to the sensor, its mass increases. The increased mass alters the resonant frequency of the mechanical object, which is detected with circuitry.
Actuators, also called transducers, convert electrical signals and energy into motion of some kind. The three most common types of actuators are electrostatic, thermal, and magnetic. Electrostatic actuators use the force of electrostatic energy to move objects. Two mechanical elements, one that is stationary (the stator) and one that is movable (the rotor) have two different voltages applied to them, which creates an electric field. The field competes with a restoring force on the rotor (usually a spring force produced by the bending or stretching of the rotor) to move the rotor. The greater the electric field, the farther the rotor will move. Thermal actuators use the force of thermal expansion to move objects. When a material is heated, it expands and amount depending on material properties. Two objects can be connected in such a way that one object is heated more than the other and expands more, and this imbalance creates motion. The direction of motion depends on the connection between the objects. This is seen in a "heatuator", which is a U-shaped beam with one wide arm and one narrow arm. When a current is flown through the object, heat is created. The narrow arm is heated more than the wide arm due to the fact that they have the same current density. Since the two arms are connected at the top, the stretching hot arm pushes in the direction of the cold arm. Magnetic actuators used fabricated magnetic layers to create forces.
See also: MEMS
