Micromachinery

The micro-mechanics is the field of microsystems technology, which deals with design, manufacture and application of mechanical components with dimensions of a few to several 100 microns. A distinction is made simple structures (eg grid, holes, channels), sensors, actuators ( such as relays, switches, valves, pumps ) and microsystems (micro motors, pressure heads). For the production technologies are used in microchip manufacturing used to come (for example galvanic processes, etching, laser technology ), but there are also the photolithography, thin-film, screen-printing and LIGA technique used. The direct tool-free manufacture of micromechanical plastic components with the patented RMPD techniques ( RMPD = Rapid Micro Product Development, see Fast product development ) is possible.

Silicon bulk mechanical

Free-standing mechanical structures are obtained by one or both sides etching of a silicon wafer. They result from the dependent on the crystal orientation etching of silicon in alkaline solutions (usually potassium hydroxide solution, KOH solution ); this process is often referred to as an anisotropic wet etching, which does not accurately reflect the nature of the etching process. The anisotropic nature of the etching process due to the different etching rate of different crystal directions of the silicon (see Miller indices ). The two main crystal planes are in this case the Si {100} and the {111} Si levels. RR is the etch rate depending on the process parameters RR about {111 }: RR = 1:100 {100}, that is, the {100} Si layers to be etched 100 times faster than the {111} Si levels. The reason for this difference is the number of located on the respective surface atoms.

For the production of structures, it is necessary to mask areas of the substrate surface, so that here, the etchant can not attack. Typical etching masks are layers of silicon nitride or silicon dioxide, which have significantly lower etch rate in KOH solution in comparison to silicon. Etched, only the part which is not covered by an etching mask ( unmasked regions ).

The resulting structures are of the substrate or the substrate orientation and the masking -dependent. Starting from a { 110} silicon wafer ( right figure ) are formed in the etching trenches having vertical walls, the {111} Si surfaces forming the substantially lower etch rate, a kind of natural etch stop. Is etched on the other hand a { 100} Si wafer ( left figure ), established initially trapezoidal ditches. The oblique side walls are here again the {111} Si surfaces. The tilt angle of 54.74 ° is obtained from the diamond structure of the crystallized silicon. At sufficiently long etching time, the two sides of the V-shaped trench touch now.

Silicon surface micromachining

Mechanical structures are obtained through a plurality of deposition and etch processes at the wafer surface. The particular advantage of this technique is that it allows the micromechanical structures unite together with electrical circuits on a microchip; sometimes even common process steps between the mechanical and electric part are possible. This integration can not only reduce production costs, but also implement solutions that would not be feasible in the spatial separation of electrical and mechanical components, such as due to parasitic capacitances on electrical connections between components.

Among the already realized micromechanical systems include electromechanical switches for high frequency applications, mechanically tunable capacitors and inertial sensors.

Production steps

Using the example of a capacitive acceleration sensor, the possible process steps are to be clarified in the silicon surface micromachining:

On the top silicon layers of the electrical process was first a sacrificial layer of a material that can be removed later in a wet etching process again deposited. This sacrificial layer is then etched away at the places where later supports for the mechanical structure to be built up to the silicon layer.

In the resulting gaps in the subsequent process step, the silicon is deposited, so that a polycrystalline silicon layer is formed which is connected to the lower layer by means of supports.

After the newly created layer was patterned by a further etching process (eg, anisotropic dry etching), the sacrificial layer can be removed under the polysilicon layer in a wet etching process, so that a roughly structured in any layer.

In the case of an acceleration sensor, this layer containing a large surface ( reference mass) and thin webs, with anchored (ie by column associated with the lower layer ) connects these surface structures. These ridges act as a beam spring, so that the reference mass is movable under the influence of forces.

If the entire chip accelerates away in the chip plane, a force acts on the reference mass, so that it is deflected from its rest position. Thus, the distances and thus the capacitances between said movable member and the adjacent fixed structures in the plane change. This usually very weak changes in capacitance can now be evaluated by the CMOS circuit on the same chip.

To make the changes in capacitance as large as possible, reference ground and immovable parts are usually designed as a structure of interlocking combs: The deeper the tines of the combs plunge into each other, the higher the capacity.